FBXL4-Related Mitochondrial DNA Depletion Syndrome

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Classifications

Harrison's Part

Mechanistic Nosology

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Inheritance

1

Autosomal recessive HP:0000007

Biallelic FBXL4 variants, homozygous or compound heterozygous. Many reported patients are from consanguineous families, and a possible founder effect in patients of Arab descent has been proposed from a pooled mutation table. Carrier parents have a 25% recurrence risk in each pregnancy.

Autosomal recessive inheritance

Show evidence (4 references)

PMID:28383868 SUPPORT Human Clinical

"FBXL4-related mtDNA depletion syndrome is inherited in an autosomal recessive manner."

GeneReviews statement of the mode of inheritance.

PMID:28383868 SUPPORT Human Clinical

"When both parents are heterozygous carriers, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier (heterozygote), and a 25% chance of being unaffected and not a carrier."

GeneReviews recurrence risk for sibs.

PMID:23993193 SUPPORT Human Clinical

"Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle."

Recessive segregation in the discovery kindreds.

+ 1 more reference

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Mechanistic Hypotheses

3

Excessive BNIP3/NIX-Dependent Basal Mitophagy

excessive_bnip3_nix_mitophagy CANONICAL

Evidence balance 2 support

FBXL4 is the substrate adaptor of an SCF ubiquitin ligase that degrades the mitophagy receptors BNIP3 and NIX. Pathogenic FBXL4 variants fail to assemble an active ligase, BNIP3 and NIX accumulate, basal mitophagy rises, and cells lose mitochondrial content and mtDNA. Several independent groups reported the ligase-substrate relationship in 2023, and deleting Bnip3 or Nix rescues the viability of Fbxl4 knockout mice.

Pathograph links

FBXL4 Loss of Function -> Failed Ubiquitination and Degradation of BNIP3 and NIX DIRECT

Failed Ubiquitination and Degradation of BNIP3 and NIX -> Excessive Basal Mitophagy DIRECT

Excessive Basal Mitophagy -> Reduced Mitochondrial Content DIRECT

Reduced Mitochondrial Content -> Mitochondrial DNA Depletion DIRECT

Reduced Mitochondrial Content -> Combined Respiratory Chain Deficiency DIRECT

Show evidence (2 references)

PMID:36896912 SUPPORT Model Organism

"Together, beyond identifying SCF-FBXL4 as a novel mitochondrial ubiquitin E3 ligase restraining basal mitophagy, our results reveal hyperactivated mitophagy as a cause of mitochondrial disease and suggest therapeutic strategies."

Genetic rescue in mice establishes hyperactive mitophagy as causal.

PMID:38123379 SUPPORT REVIEW SYNTHESIS Other

"Recent advances have shown that excessive BCL2 interacting protein 3 (BNIP3)/ BCL2 interacting protein 3 like (BNIP3L)-dependent mitophagy underlies the molecular pathogenesis of MTDPS13."

Review synthesis of the convergent 2023 studies.

Primary Oxidative Phosphorylation Defect with Secondary Mitophagy

primary_oxphos_defect_secondary_mitophagy ALTERNATIVE

Evidence balance 1 support

Before the ligase studies, one fibroblast study of two patients found increased mitochondrial mass, fragmentation and autophagy but interpreted them as secondary to a primary oxidative phosphorylation defect, and one of the two lines had normal mtDNA copy number. A neonatal case likewise showed relative sparing of mitochondrial mass in fibroblasts despite profound respiratory failure. These observations predate, and have not been reconciled with, the BNIP3/NIX data.

Show evidence (1 reference)

PMID:31969900 SUPPORT In Vitro

"Further analysis of fibroblasts allowed to establish that increased mitochondrial mass, mitochondrial fragmentation, and augmented autophagy are associated with FBXL4 deficiency in cells, but are probably secondary to a primary metabolic defect affecting oxidative phosphorylation."

The authors' interpretation of autophagy as secondary.

Dysregulated Mitophagy in Embryonic Stem Cell Differentiation

embryonic_mitophagy_developmental_defects EMERGING

Evidence balance 1 support

Structural brain and heart malformations and facial dysmorphism suggest a defect of embryogenesis. A commentary proposes that excess mitophagy disturbs the fine-tuned mitophagy needed for stem cell differentiation. No experimental test in an FBXL4 model is cited.

Pathograph links

Excessive Basal Mitophagy -> Disrupted Stem Cell Differentiation during Embryogenesis UNKNOWN

Disrupted Stem Cell Differentiation during Embryogenesis -> Abnormal facial shape UNKNOWN

Disrupted Stem Cell Differentiation during Embryogenesis -> Congenital heart malformation UNKNOWN

Show evidence (1 reference)

PMID:39937392 SUPPORT Other

"The disruption of this process is the likely explanation of developmental defects in FBXL4- related mitochondrial depletion syndrome."

Hypothesis stated in a commentary without new data.

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Discussions and Knowledge Gaps

2

Why is mitochondrial mass reduced in knockout cells, knockout mouse tissue and patient muscle but spared or increased in some patient fibroblast lines?

KNOWLEDGE GAP fbxl4-mitochondrial-mass-discordance

Attached to

pathophysiology#Reduced Mitochondrial Content pathophysiology#Excessive Basal Mitophagy

The canonical model predicts fewer mitochondria, which knockout cells, knockout mice and patient muscle show, while one neonatal fibroblast line had relatively spared mass and two other lines had increased mass, one of them with normal mtDNA. Whether this reflects culture conditions, residual function of particular alleles or compensatory biogenesis is not resolved.

Do the facial dysmorphism, congenital heart malformations and prenatal brain anomalies arise from excess mitophagy during embryonic differentiation, or from energy failure?

KNOWLEDGE GAP fbxl4-developmental-defects

Attached to

pathophysiology#Disrupted Stem Cell Differentiation during Embryogenesis phenotypes#Abnormal facial shape phenotypes#Congenital heart malformation

The developmental features are not explained by the postnatal energy-failure chain, and the embryonic-mitophagy explanation is a hypothesis without an FBXL4 experiment.

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Pathophysiology

12

FBXL4 Loss of Function

Biallelic germline FBXL4 variants (nonsense, frameshift, splice and missense) distributed across the gene, without a clear genotype-phenotype correlation. FBXL4 is the substrate-binding F-box subunit of an SCF (CRL1) ubiquitin ligase on the mitochondrial outer membrane. Tested pathogenic variants leave FBXL4 able to bind BNIP3 and NIX but unable to assemble an active ligase. Re-expression of wild-type FBXL4 restores mtDNA copy number in patient cells.

Excessive BNIP3/NIX-Dependent Basal Mitophagy

FBXL4 hgnc:13601

Genetic context variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION

Biallelic (homozygous or compound heterozygous) germline FBXL4 variants.

SCF substrate adaptor activity GO:1990756 ↓ DECREASED

mitochondrial outer membrane GO:0005741

Downstream

Failed Ubiquitination and Degradation of BNIP3 and NIX

Reduced Mitochondrial Fusion and Network Fragmentation

Show evidence (5 references)

PMID:23993193 SUPPORT In Vitro

"Expression of the wild-type FBXL4 transcript in cell lines from two subjects fully rescued the levels of mtDNA copy number, leading to a correction of the mitochondrial biochemical deficit."

Complementation in patient cells ties the cellular defect to loss of FBXL4 function.

PMID:25868664 SUPPORT Human Clinical

"Mutations were detected throughout the FBXL4 gene albeit with no clear delineation of a genotype-phenotype correlation."

Variants span the gene without a genotype-phenotype correlation.

PMID:36896912 SUPPORT In Vitro

"We determined that FBXL4 functions as an integral outer-membrane protein that forms an SCF-FBXL4 ubiquitin E3 ligase complex."

Establishes FBXL4 as the substrate adaptor of an outer-membrane SCF ligase.

+ 2 more references

Failed Ubiquitination and Degradation of BNIP3 and NIX

Without an active SCF-FBXL4 ligase, the mitophagy receptors BNIP3 and NIX (BNIP3L) escape constitutive ubiquitination and proteasomal degradation and accumulate on the mitochondrial outer membrane. PPTC7, a matrix protein retained at the outer membrane, scaffolds the substrate-PPTC7-SCF-FBXL4 complex and is part of the same regulatory loop.

Excessive BNIP3/NIX-Dependent Basal Mitophagy

BNIP3 hgnc:1084 NIX (BNIP3L) hgnc:1085

SCF-dependent proteasomal degradation of BNIP3 and NIX GO:0031146 ↓ DECREASED

Downstream

Excessive Basal Mitophagy

Show evidence (4 references)

PMID:37161784 SUPPORT In Vitro

"Here, we report that SCFFBXL4 , an SKP1/CUL1/F-box protein ubiquitin ligase complex, localises to the mitochondrial outer membrane in unstressed cells and mediates the constitutive ubiquitylation and degradation of the mitophagy receptors NIX and BNIP3 to suppress basal levels of mitophagy."

Constitutive SCF-FBXL4-dependent turnover of the two receptors.

PMID:37568009 SUPPORT In Vitro

"Here, we demonstrate that, FBXL4 directly interacts with the mitophagy cargo receptors BNIP3 and BNIP3L, promoting their degradation through the ubiquitin-proteasome pathway via the assembly of an active CRL1FBXL4 complex."

Independent group; direct interaction and proteasomal degradation.

PMID:37102372 SUPPORT In Vitro

"FBXL4 restricts NIX and BNIP3 levels via direct interaction and protein destabilization, while VHL acts through suppression of HIF1α-mediated transcription of BNIP3 and NIX."

Third independent group, from a CRISPR screen of E3 ligases.

+ 1 more reference

Excessive Basal Mitophagy

Mitophagy is hyperactive under basal, unstressed conditions in FBXL4 knockout cells, patient fibroblasts, patient iPSC-derived cortical neurons, Fbxl4 knockout mice and knock-in mice carrying a patient variant. Blocking lysosomal function reverses the mitochondrial phenotype in FBXL4-deficient cells, while proteasome inhibition does not.

Dysregulated Mitophagy in Embryonic Stem Cell Differentiation Excessive BNIP3/NIX-Dependent Basal Mitophagy

neuron CL:0000540 fibroblast CL:0000057

mitophagy GO:0000422 ↑ INCREASED

Experimental models

FBXL4 knockout human cell lines CELL LINE

Patient iPSC-derived cortical neurons IPSC DERIVED MODEL

Animal models

Fbxl4 knockout mouse with Bnip3 or Nix co-deletion (Cao et al.) Mouse rescues

Fbxl4 patient-variant knock-in mouse (Chen et al.) Mouse recapitulates

Downstream

Reduced Mitochondrial Content

Disrupted Stem Cell Differentiation during Embryogenesis

Show evidence (6 references)

PMID:36896912 SUPPORT In Vitro

"Here, via a mitochondria-targeted genetic screen, we found that knockout (KO) of FBXL4, a mitochondrial disease gene, hyperactivates mitophagy at basal conditions."

Basal mitophagy is hyperactive in FBXL4 knockout cells.

PMID:36896912 SUPPORT Model Organism

"Fbxl4-/- mice exhibit elevated BNIP3 and NIX proteins, hyperactive mitophagy, and perinatal lethality."

The same state in knockout mice.

PMID:37568009 SUPPORT Model Organism

"Excessive mitophagy was observed in Knockin (KI) mice carrying a patient-derived FBXL4 mutation and cortical neurons (CNs)-induced from MTDPS13 patient human induced pluripotent stem cells (hiPSCs)."

A patient allele in mice reproduces the state; the same sentence also covers patient-derived neurons, which are cited separately below as IN_VITRO.

+ 3 more references

Reduced Mitochondrial Content

Fewer mitochondria per cell, with a global fall in mitochondrial proteins and a rise in lysosomal proteins. In the knockout mouse and in human knockout cells the remaining mitochondria are functional; the deficit is one of number. Patient muscle shows a paucity of mitochondria, but mitochondrial mass was relatively spared in fibroblasts from one neonate and increased in fibroblasts from two other patients, so the finding is not uniform across patient cells.

Excessive BNIP3/NIX-Dependent Basal Mitophagy

fibroblast CL:0000057 skeletal muscle fiber CL:0008002

Animal models

Fbxl4 knockout mouse (Alsina et al.) Mouse recapitulates

Downstream

Mitochondrial DNA Depletion

Combined Respiratory Chain Deficiency

Show evidence (4 references)

PMID:32525278 SUPPORT In Vitro

"Fibroblasts from patients with FBXL4 deficiency and human FBXL4 knockout cells also have reduced steady-state levels of mitochondrial proteins that can be attributed to increased mitochondrial turnover."

Reduced mitochondrial protein content in patient and knockout cells.

PMID:32525278 SUPPORT Model Organism

"One-year-old Fbxl4 knockouts show a global reduction in a variety of mitochondrial proteins and mtDNA depletion, whereas lysosomal proteins are upregulated."

The same in knockout mouse tissue.

PMID:26404457 SUPPORT Human Clinical

"Muscle biopsy showed a paucity of mitochondria with ultrastructural abnormalities, mitochondrial DNA depletion, and profound deficiency of all respiratory chain complexes."

Paucity of mitochondria in patient muscle.

+ 1 more reference

Mitochondrial DNA Depletion

Reduced mtDNA copy number, severe in skeletal muscle and milder or absent in cultured fibroblasts; one fibroblast line with a homozygous truncating variant had normal copy number.

skeletal muscle fiber CL:0008002 fibroblast CL:0000057

Downstream

Combined Respiratory Chain Deficiency

Show evidence (4 references)

PMID:23993193 SUPPORT Human Clinical

"Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle."

Severe muscle mtDNA depletion in the discovery kindreds.

PMID:23993194 SUPPORT Human Clinical

"In muscle and fibroblasts from several subjects, substantially decreased mtDNA content was observed."

Independent discovery series.

PMID:31969900 SUPPORT Human Clinical

"We observed that the mtDNA depletion was less severe in fibroblasts than in muscle."

Tissue difference in severity of depletion.

+ 1 more reference

Reduced Mitochondrial Fusion and Network Fragmentation

Fragmented mitochondrial networks, abnormal nucleoid distribution and loss of inner membrane potential in patient fibroblasts. One study attributes the fragmentation to a direct role of FBXL4 in promoting fusion. How this relates to the excess mitophagy is not established, so the node has no downstream edge.

fibroblast CL:0000057

mitochondrial fusion GO:0008053 ↓ DECREASED

Show evidence (2 references)

PMID:23993193 SUPPORT In Vitro

"We show that FBXL4 is an F-box protein that colocalizes with mitochondria and that loss-of-function and splice mutations in this protein result in a severe respiratory chain deficiency, loss of mitochondrial membrane potential, and a disturbance of the dynamic mitochondrial network and nucleoid..."

Network and nucleoid disturbance in patient fibroblasts.

PMID:31442532 SUPPORT In Vitro

"Meanwhile, overexpression of wildtype FBXL4, but not the p.Cys584Arg variant, promoted mitochondrial hyperfusion."

Wild-type FBXL4 promotes fusion; the patient variant does not.

Combined Respiratory Chain Deficiency

Combined deficiency of multiple respiratory chain complexes in muscle and fibroblasts, with a general decrease of mitochondrial energy-metabolism enzymes, reduced oxygen consumption and low mitochondrial ATP. In one neonate all complexes were profoundly deficient in muscle, and that patient's fibroblasts had a more oxidised glutathione ratio. Raised reactive oxygen species production has not been measured directly, so only the oxidative phosphorylation arm of the conformed module node is bound.

skeletal muscle fiber CL:0008002 fibroblast CL:0000057

oxidative phosphorylation GO:0006119 ↓ DECREASED

Experimental models

Patient fibroblasts PRIMARY CELL CULTURE

Animal models

fbxl-1(ok3741) C. elegans Caenorhabditis elegans partially recapitulates

Downstream

Lactic acidosis Impaired oxidative phosphorylation diverts pyruvate to lactate.

Neuronal Energy Failure

Mitochondrial Myopathy

Cardiomyocyte Energy Failure

Impaired Anaplerosis Proposed from patient biochemistry; knockout mouse liver shows perturbed amino acid and pyruvate metabolism from reduced mitochondrial oxidation, but aspartate accumulated in mouse liver whereas plasma aspartate was low in the patients, so the direction does not transfer.

Elevated circulating hepatic transaminase concentration Presumed hepatocyte energy failure; not studied in patient liver.

Decreased total neutrophil count Presumed effect on haematopoiesis; no mechanistic study in FBXL4 deficiency.

Decreased total lymphocyte count Presumed effect on haematopoiesis; no mechanistic study in FBXL4 deficiency.

Renal tubular acidosis Presumed renal tubular energy failure; not studied.

Gastrointestinal dysmotility Presumed enteric neuromuscular energy failure; not studied.

Cataract Mechanism of lens involvement not studied.

Show evidence (4 references)

PMID:25868664 SUPPORT Human Clinical

"Diagnostic workup in patient tissues and cells revealed a severe combined respiratory chain defect with a general decrease of enzymes associated with mitochondrial energy metabolism and a relative depletion of mitochondrial DNA content."

Combined defect in the 21-patient series.

PMID:23993194 SUPPORT Human Clinical

"Mitochondrial respiratory chain deficiency was present in muscle or fibroblasts of all tested individuals, together with markedly reduced oxygen consumption rate and hyperfragmentation of the mitochondrial network in cultured cells."

Deficiency in every tested individual.

PMID:26404457 SUPPORT Human Clinical

"High-resolution respirometry of permeabilized muscle fibers showed marked deficiency of oxidative phosphorylation using a variety of mitochondrial energy substrates and inhibitors."

Direct respirometry of patient muscle.

+ 1 more reference

Neuronal Energy Failure

Energy failure in neurons, presumed from the brain phenotype. Excess mitophagy is shown in cortical neurons derived from patient iPSCs, but neuronal bioenergetics and neuron loss have not been measured in patient brain. Imaging shows a rapidly progressive brain atrophy after nonspecific neonatal findings, and destructive brain changes can begin before birth.

neuron CL:0000540

Animal models

fbxl4 sa12470 zebrafish Zebrafish partially recapitulates

Downstream

Encephalopathy

Cerebral atrophy

Leukoencephalopathy

Global developmental delay

Seizure

Microcephaly

Choreoathetosis

Ataxia

Stroke-like episode

Cerebellar hypoplasia

Ventriculomegaly

Sensorineural hearing impairment The site of the auditory lesion has not been studied.

Optic atrophy

Nystagmus Presumed central or visual-pathway origin; not studied.

Strabismus Presumed central or ocular motor origin; not studied.

Show evidence (2 references)

PMID:37568009 SUPPORT INDIRECT In Vitro

"Furthermore, we observed abnormal activation of mitophagy in cortical neurons induced from hiPSCs derived from MTDPS13 patients."

The upstream state is present in patient-derived neurons; energy failure itself is inferred.

PMID:36411461 SUPPORT Human Clinical

"The brain imaging findings in the reported fetus indicate that MTDPS13 is associated with progressive neurological involvement and brain tissue destructive changes starting as early as the second trimester of pregnancy."

Destructive brain changes begin prenatally.

Mitochondrial Myopathy

Skeletal muscle shows severe mtDNA depletion, a paucity of mitochondria with ultrastructural abnormalities and a combined respiratory chain deficiency; creatine kinase is intermittently raised.

skeletal muscle fiber CL:0008002

Downstream

Hypotonia

Elevated circulating creatine kinase activity

Show evidence (2 references)

PMID:26404457 SUPPORT Human Clinical

"Muscle biopsy showed a paucity of mitochondria with ultrastructural abnormalities, mitochondrial DNA depletion, and profound deficiency of all respiratory chain complexes."

Muscle pathology in a neonatal case.

PMID:27099744 SUPPORT Human Clinical

"Severe recessive mitochondrial myopathy caused by FBXL4 gene mutations may present prenatally with polyhydramnios and cerebellar hypoplasia."

Myopathy named as the disease presentation in a prenatal-onset case.

Cardiomyocyte Energy Failure

Presumed energy failure in cardiomyocytes underlying the hypertrophic cardiomyopathy and arrhythmias. In one of two siblings the hypertrophy reversed on dichloroacetate, which acts on pyruvate oxidation, consistent with a metabolic rather than fixed structural basis in that patient. Cardiac tissue has not been studied in patients.

cardiac muscle cell CL:0000746

Downstream

Hypertrophic cardiomyopathy

Arrhythmia

Show evidence (1 reference)

PMID:31442532 SUPPORT INDIRECT Human Clinical

"Remarkably, dichloroacetate (DCA) administration to the younger sibling improved metabolic acidosis and reversed cardiac hypertrophy."

Reversibility with a metabolic drug in one patient; the cardiomyocyte energy deficit is inferred from the response.

Impaired Anaplerosis

Mechanism confidence: Hypothetical

In two siblings who presented with neonatal hyperammonemia, low plasma aspartate and low urinary TCA cycle intermediates suggested a failure to replenish TCA cycle intermediates; the authors propose this as the basis of the hyperammonemia and suggest anaplerotic therapy. This has not been tested in other patients or in a model.

tricarboxylic acid cycle GO:0006099 ↓ DECREASED

Downstream

Hyperammonemia The authors' proposal, from two siblings.

Show evidence (1 reference)

PMID:27858371 SUPPORT Human Clinical

"We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to..."

Biochemical pattern and the authors' anaplerosis interpretation.

Disrupted Stem Cell Differentiation during Embryogenesis

Mechanism confidence: Hypothetical

Hypothesised developmental consequence of excess mitophagy, proposed to explain facial dysmorphism, congenital heart malformations and structural brain anomalies. Not tested experimentally in an FBXL4 model.

Dysregulated Mitophagy in Embryonic Stem Cell Differentiation

Downstream

Abnormal facial shape

Congenital heart malformation

Show evidence (1 reference)

PMID:39937392 SUPPORT Other

"Often, it is associated with structural brain and heart defects, and facial dysmorphism suggesting an embryogenesis defect."

The observation motivating the hypothesis.

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Pathograph

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Phenotypes

34

Blood 2

Decreased total neutrophil count HP:0001875

Sequelae: Recurrent infections

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews bone marrow finding.

PMID:23993194 SUPPORT Human Clinical

"She has chronic lactic acidosis and RTA, global developmental delay (she is nonverbal) with truncal hypotonia, ataxia, and choreoathetoid movements, neutropenia, frequent infections, and severe GI dysmotility and swallowing difficulty, requiring PEG."

Neutropenia with frequent infections in one patient.

Decreased total lymphocyte count HP:0001888

Sequelae: Recurrent infections

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews bone marrow finding.

PMID:26404457 SUPPORT Human Clinical

"Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."

Lymphopenia in a neonatal case.

Cardiovascular 5

Stroke-like episode HP:0002401

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."

GeneReviews lists stroke-like episodes.

Hypertrophic cardiomyopathy HP:0001639

Show evidence (3 references)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists hypertrophic cardiomyopathy.

PMID:31442532 SUPPORT Human Clinical

"Both patients presented with encephalomyopathy, lactic acidosis and cardiac hypertrophy, which are reported features of FBXL4 impairment."

Cardiac hypertrophy in two siblings.

PMID:41635899 SUPPORT Human Clinical

"Echocardiography demonstrated mild, early-onset hypertrophic cardiomyopathy."

Neonatal-onset hypertrophic cardiomyopathy in one preterm infant.

Congenital heart malformation Abnormal heart morphology HP:0001627

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists congenital heart malformations without naming a lesion.

Arrhythmia HP:0011675

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists arrhythmias.

PMID:26404457 SUPPORT Human Clinical

"Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."

Neonatal arrhythmias in one case.

Elevated pulmonary artery pressure HP:0004890

Show evidence (1 reference)

PMID:26404457 SUPPORT Human Clinical

"Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."

Pulmonary hypertension in one neonate. The abstract does not state the haemodynamic type, so HP:0004890 is bound rather than HP:0002092 Pulmonary arterial hypertension, which OAK places under HP:0033578 Pre-capillary pulmonary hypertension.

Digestive 2

Feeding difficulties HP:0011968

Sequelae: Failure to thrive

Show evidence (2 references)

PMID:25868664 SUPPORT Human Clinical

"Neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation."

Part of the presenting pattern.

PMID:28383868 SUPPORT Human Clinical

"FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay."

GeneReviews core feature.

Gastrointestinal dysmotility HP:0002579

Sequelae: Feeding difficulties

Show evidence (1 reference)

PMID:23993194 SUPPORT Human Clinical

"A wide range of other multisystem features were variably seen, including dysmorphism, skeletal abnormalities, poor growth, gastrointestinal dysmotility, renal tubular acidosis, seizures, and episodic metabolic failure."

Variably present in the discovery series.

Ear 1

Sensorineural hearing impairment HP:0000407

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists sensorineural hearing loss.

Eye 4

Optic atrophy HP:0000648

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists optic atrophy among eye findings.

Cataract HP:0000518

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists cataract.

PMID:23993194 SUPPORT Human Clinical

"At the age of 3 years she has developed bilateral cataract and horizontal nystagmus and shows failure to thrive and a global psychomotor and developmental delay."

Bilateral cataract appearing by age 3 in one patient, so it is not necessarily congenital.

Strabismus HP:0000486

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists strabismus.

Nystagmus HP:0000639

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews lists nystagmus.

PMID:23993194 SUPPORT Human Clinical

"At the age of 3 years she has developed bilateral cataract and horizontal nystagmus and shows failure to thrive and a global psychomotor and developmental delay."

Horizontal nystagmus in one patient.

Genitourinary 1

Renal tubular acidosis HP:0001947

Show evidence (1 reference)

PMID:23993194 SUPPORT Human Clinical

"A wide range of other multisystem features were variably seen, including dysmorphism, skeletal abnormalities, poor growth, gastrointestinal dysmotility, renal tubular acidosis, seizures, and episodic metabolic failure."

Variably present in the discovery series.

Head and Neck 2

Microcephaly HP:0000252

Show evidence (1 reference)

PMID:28940506 SUPPORT Human Clinical

"Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined..."

Microcephaly in the 87-patient review.

Abnormal facial shape FREQUENT HP:0001999

Show evidence (2 references)

PMID:25868664 SUPPORT Human Clinical

"Facial dysmorphic features are present in 67% of cases."

Frequency in the 21-patient series (FREQUENT band).

PMID:27099744 SUPPORT Human Clinical

"Characteristic dysmorphic features are: high and arched eyebrows, triangular face, a slight upslant of palpebral fissures, and a prominent pointed chin."

Description of the facial gestalt.

Immune 1

Recurrent infections HP:0002719

Sequelae: Lactic acidosis

Show evidence (1 reference)

PMID:28940506 SUPPORT Human Clinical

"Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined..."

Recurrent infections in the 87-patient review.

Metabolism 4

Lactic acidosis VERY_FREQUENT HP:0003128

Onset: NEONATAL

Show evidence (3 references)

PMID:25868664 SUPPORT Human Clinical

"Elevated blood lactate and metabolic acidosis were observed in all individuals; creatine kinase was elevated in 45% of measurements."

Present in all 21 patients of the retrospective series (frequency support).

PMID:27743463 SUPPORT Human Clinical

"All patients exhibited congenital lactic acidemia, most of them with severe encephalopathic presentation, and global developmental delay."

Congenital onset in all 10 newly diagnosed patients.

PMID:28383868 SUPPORT Human Clinical

"FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay."

GeneReviews core feature.

Hyperammonemia FREQUENT HP:0001987

Show evidence (3 references)

PMID:40161922 SUPPORT BACKGROUND Human Clinical

"Hyperammonemia is reported in 45 % of cases but extremely elevated ammonia levels are rare."

Frequency as restated in the introduction of a later case report; the pooled count behind it is not in this record.

PMID:28940506 SUPPORT Human Clinical

"Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined..."

Hyperammonemia in the 87-patient review.

PMID:27858371 SUPPORT Human Clinical

"After initial clinical stabilization, both subjects continued to have episodic exacerbations characterized by lactic acidosis and hyperammonemia."

Recurrent hyperammonemia in two siblings.

Elevated circulating creatine kinase activity HP:0003236

Show evidence (1 reference)

PMID:25868664 SUPPORT Human Clinical

"Elevated blood lactate and metabolic acidosis were observed in all individuals; creatine kinase was elevated in 45% of measurements."

Intermittent CK elevation; no per-patient frequency is given, so none is assigned.

Elevated circulating hepatic transaminase concentration HP:0002910

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."

GeneReviews liver finding.

Musculoskeletal 1

Hypotonia VERY_FREQUENT HP:0001252

Onset: NEONATAL

Sequelae: Polyhydramnios

Show evidence (2 references)

PMID:23993194 SUPPORT Human Clinical

"All subjects manifested early-onset lactic acidemia, hypotonia, and developmental delay caused by severe encephalomyopathy consistently associated with progressive cerebral atrophy and variable involvement of the white matter, deep gray nuclei, and brainstem structures."

Present in all subjects of the discovery series (frequency support).

PMID:25868664 SUPPORT Human Clinical

"Neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation."

Part of the characteristic presenting pattern.

Nervous System 9

Global developmental delay VERY_FREQUENT Severe global developmental delay HP:0011344

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"All affected individuals alive at the time they were reported (median age: 3.5 years) demonstrated significant developmental delay."

Frequency support in the GeneReviews pooled summary.

PMID:25868664 SUPPORT Human Clinical

"All survivors developed severe psychomotor retardation."

Severity in the 21-patient series.

Encephalopathy Mitochondrial encephalopathy HP:0006789

Onset: NEONATAL

Show evidence (2 references)

PMID:25868664 SUPPORT Human Clinical

"In conclusion, a clinical pattern of early-onset encephalopathy, persistent lactic acidosis, profound muscular hypotonia and typical facial dysmorphism should prompt initiation of molecular genetic analysis of FBXL4."

Early-onset encephalopathy as a diagnostic cue.

PMID:23993193 SUPPORT Human Clinical

"Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle."

Fatal encephalopathy in the discovery kindreds.

Cerebral atrophy VERY_FREQUENT HP:0002059

Course: PROGRESSIVE

Show evidence (2 references)

PMID:25868664 SUPPORT Human Clinical

"Brain imaging was non-specific in neonates but a later-onset, rapidly progressive brain atrophy was noted."

Evolution of imaging in the 21-patient series.

PMID:23993194 SUPPORT Human Clinical

"All subjects manifested early-onset lactic acidemia, hypotonia, and developmental delay caused by severe encephalomyopathy consistently associated with progressive cerebral atrophy and variable involvement of the white matter, deep gray nuclei, and brainstem structures."

Progressive atrophy in every subject of the discovery series (frequency support).

Leukoencephalopathy HP:0002352

Show evidence (2 references)

PMID:28940506 SUPPORT Human Clinical

"Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined..."

White matter abnormalities in the 87-patient review.

PMID:26404457 SUPPORT Human Clinical

"Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."

Leukoencephalopathy in a neonatal case.

Seizure HP:0001250

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."

GeneReviews lists seizures.

PMID:28940506 SUPPORT Human Clinical

"Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined..."

Seizures in the 87-patient review.

Choreoathetosis HP:0001266

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."

GeneReviews lists movement disorders without naming the type.

PMID:23993194 SUPPORT Human Clinical

"However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic..."

Choreoathetoid movements in several older children of the discovery series.

Ataxia HP:0001251

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."

GeneReviews lists ataxia.

PMID:23993194 SUPPORT Human Clinical

"However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic..."

Truncal ataxia in older children of the discovery series.

Cerebellar hypoplasia HP:0001321

Show evidence (2 references)

PMID:27858371 SUPPORT Human Clinical

"We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to..."

Cerebellar hypoplasia in two siblings.

PMID:27099744 SUPPORT Human Clinical

"Severe recessive mitochondrial myopathy caused by FBXL4 gene mutations may present prenatally with polyhydramnios and cerebellar hypoplasia."

Prenatal cerebellar hypoplasia.

Ventriculomegaly HP:0002119

Onset: ANTENATAL

Show evidence (2 references)

PMID:40252080 SUPPORT Human Clinical

"The literature and our case highlight that the prenatal clinical manifestations can include ventriculomegaly, periventricular cysts, mega cisterna magna, cerebellar vermis hypoplasia, and cardiac anomalies."

Prenatal findings across four reported prenatal-onset cases.

PMID:36411461 SUPPORT Human Clinical

"Follow-up fetal ultrasonogram at 26 weeks revealed periventricular cysts, periventricular echogenicity, ventriculomegaly, thin corpus callosum, mega cisterna magna, and large cavum."

Ventriculomegaly on fetal ultrasound in a single case.

Prenatal and Birth 1

Polyhydramnios HP:0001561

Onset: ANTENATAL

Show evidence (2 references)

PMID:27099744 SUPPORT Human Clinical

"Severe recessive mitochondrial myopathy caused by FBXL4 gene mutations may present prenatally with polyhydramnios and cerebellar hypoplasia."

Prenatal presentation with polyhydramnios.

PMID:41635899 SUPPORT Human Clinical

"We report the case of a male preterm neonate born at 31 + 3 weeks of gestation following a pregnancy marked by severe polyhydramnios."

Severe polyhydramnios in a second case.

Growth 1

Failure to thrive HP:0001508

Onset: NEONATAL

Show evidence (2 references)

PMID:25868664 SUPPORT Human Clinical

"Neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation."

Part of the presenting pattern.

PMID:30804983 SUPPORT Human Clinical

"We report here the case of a Lebanese infant presenting to us with profound neurodevelopmental delays, generalized hypotonia, facial dysmorphic features, and extreme emaciation."

Extreme emaciation in one infant.

🧬

Genetic Associations

1

FBXL4

Gene: FBXL4 hgnc:13601 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (3 references)

PMID:28940506 SUPPORT Human Clinical

"Biallelic pathogenic variants in FBXL4 are associated with an encephalopathic mtDNA maintenance defect syndrome that is a multisystem disease characterized by lactic acidemia, developmental delay, and hypotonia."

Gene-disease association summarised over 87 patients.

PMID:23993194 SUPPORT Human Clinical

"Whole-exome sequencing and autozygosity mapping studies, independently performed in subjects with defective combined mitochondrial OXPHOS-enzyme deficiencies, identified a total of nine disease-segregating FBXL4 mutations in seven unrelated mitochondrial disease families, composed of six..."

Independent discovery in seven families.

PMID:27743463 SUPPORT Human Clinical

"Definitive diagnosis was made in 10 new subjects with a total of 7 novel deleterious variants; 5 null and 2 missense substitutions."

Null and missense variant classes.

Variants (4)

c.1641_1642delTG (homozygous frameshift)

Homozygous frameshift in a neonate; proposed by the authors to represent the severe end of the spectrum.

Show evidence (1 reference)

PMID:26404457 SUPPORT Human Clinical

"In light of our patient's clinical findings and genotype (homozygous frameshift), this phenotype likely represents the severe end of the FBXL4 clinical spectrum."

Authors' severity interpretation for a single patient.

c.1303C>T

Missense variant reported in three unrelated individuals with different clinical courses.

Show evidence (1 reference)

PMID:30804983 SUPPORT Human Clinical

"Finally, we provide a direct comparison of the disorder's clinical manifestations across two unrelated patients harboring the same disease-causing mutation as our patient, emphasizing the remarkable variability in genotype-to-phenotype correlation characteristic of the disease."

Same variant, variable phenotype.

c.1750T>C (p.Cys584Arg)

Homozygous missense in two siblings with cardiac hypertrophy; unlike wild-type FBXL4 the variant does not promote mitochondrial fusion.

Show evidence (1 reference)

PMID:31442532 SUPPORT Human Clinical

"We report two siblings, from consanguineous parents, harbouring a previously uncharacterized homozygous variant in FBXL4 (c.1750 T > C; p.Cys584Arg)."

Identification in two siblings.

c.486T>G (p.Tyr162Ter)

Homozygous nonsense variant in a 4.5-year-old boy with a milder course than usual.

Show evidence (1 reference)

PMID:34602956 SUPPORT Human Clinical

"While most of the patients with FBLX4 gene mutation have severe clinical manifestation and die at a very young age, clinical progress of our case was milder than previously reported."

Milder course despite a truncating variant.

💊

Medical Actions

7

Multidisciplinary Supportive Care

Action: multidisciplinary supportive care Ontology label: Supportive Care NCIT:C15747

No definite treatment exists. Management is supportive, by a multidisciplinary team, with adequate nutrition and standard treatment of developmental delay, seizures, cardiac complications, eye involvement and hearing loss.

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"To date no definite treatment is available; thus, treatment is mainly supportive: assuring adequate nutrition and standard treatment of neurologic complications including developmental delay / intellectual disability, seizures, cardiac complications, eye involvement, and hearing loss."

GeneReviews management.

Nutritional Support

Action: nutritional support Ontology label: Nutritional Support NCIT:C15433

Assuring adequate nutrition for the feeding difficulty and growth failure.

Target Phenotypes: Failure to thrive HP:0001508

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"To date no definite treatment is available; thus, treatment is mainly supportive: assuring adequate nutrition and standard treatment of neurologic complications including developmental delay / intellectual disability, seizures, cardiac complications, eye involvement, and hearing loss."

GeneReviews management.

Mitochondrial Cofactors and Antioxidants

Action: Pharmacotherapy NCIT:C15986

Platform: Small molecule

Cofactor and antioxidant supplements used across mitochondrial disease may be considered, with generally limited evidence of benefit; in the 21-patient series such "mitochondrial medications" did not prove effective. The cited abstracts do not name the agents, so none is bound.

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Administration of cofactors and antioxidants, used in mitochondrial disorders with (generally) limited evidence of benefit, may be considered."

GeneReviews permits a trial.

PMID:25868664 REFUTE Human Clinical

"Treatment with "mitochondrial medications" did not prove effective."

No benefit observed in the retrospective 21-patient series.

Dichloroacetate

Action: Pharmacotherapy NCIT:C15986

Agent: dichloroacetate CHEBI:28240

Platform: Small molecule

Pyruvate dehydrogenase activator. In one of two siblings with the p.Cys584Arg variant it improved metabolic acidosis and reversed cardiac hypertrophy; in patient fibroblasts it reduced acidification without correcting other mitochondrial functions. In C. elegans, zebrafish and human fibroblast models it improved survival, neuromuscular function and mitochondrial physiology. No controlled human trial has been reported.

Mechanism Target:

Lactic acidosis — Drives pyruvate into mitochondrial oxidation, lowering lactate.

Cardiomyocyte Energy Failure — Cardiac hypertrophy reversed in one patient.

Target Phenotypes: Lactic acidosis HP:0003128 Hypertrophic cardiomyopathy HP:0001639

Show evidence (2 references)

PMID:31442532 SUPPORT Human Clinical

"Remarkably, dichloroacetate (DCA) administration to the younger sibling improved metabolic acidosis and reversed cardiac hypertrophy."

Single-patient clinical response.

PMID:35881484 SUPPORT Model Organism

"Validation studies were performed in fbxl4sa12470 zebrafish larvae and in FBXL4-/- human fibroblasts; they showed DCA efficacy in preventing brain death, impairment of neurologic and/or muscular function, mitochondrial biochemical dysfunction, and stress-induced morphologic and ultrastructural..."

Preclinical efficacy in zebrafish; the same sentence covers fibroblasts, which are not graded separately here.

Ketogenic Diet

Action: ketogenic diet Ontology label: Ketogenic Diet NCIT:C173168

Platform: Behavioral / lifestyle

Parenteral then enteral ketogenic diet lowered lactate in a 4-year-old girl whose lactic acidosis during sepsis did not respond to drugs, and in a preterm neonate lactate fell and acid-base balance and growth improved within three days. Single-patient reports.

Target Phenotypes: Lactic acidosis HP:0003128

Show evidence (2 references)

PMID:33882172 SUPPORT Human Clinical

"During the disease course, lactic acidosis became prominent and did not respond to pharmacological treatment; standard PN was gradually switched to parenteral KD, and lactate levels decreased after parenteral KD."

Single-case response.

PMID:41635899 SUPPORT Human Clinical

"After 3 days of this treatment approach, a significant reduction in lactate levels and improvement in acid-base balance and growth trend were observed along with clinical and cardiovascular parameters."

Single-case response in a preterm neonate.

Deoxycytidine and Deoxythymidine

Action: Pharmacotherapy NCIT:C15986

Agent: deoxycytidine CHEBI:15698 deoxythymidine CHEBI:17748

Platform: Small molecule

Oral pyrimidine deoxynucleosides, investigational. An interim open-label analysis pooled eight children with FBXL4, SUCLG1, SUCLA2 or RRM2B variants; Newcastle Mitochondrial Disease Scale scores improved at group level. The number of FBXL4 patients and their individual results are not given in the abstract, the study is uncontrolled, and there is no FBXL4-specific mechanistic rationale, since the defect is excess mitochondrial removal rather than nucleotide supply.

Show evidence (2 references)

PMID:40175578 SUPPORT Human Clinical

"Data were available from eight individuals having pathogenic variants in FBXL4, SUCLG1, SUCLA2, or RRM2B."

FBXL4 is among the pooled genotypes.

PMID:40175578 SUPPORT Human Clinical

"Newcastle Mitochondrial Disease Scale score improved in all individuals except for one who withdrew before the first follow-up visit; group level analysis was significant at 1-month and 6-month timepoints."

Pooled open-label result, not specific to FBXL4.

AMPK Activation (preclinical)

Action: Pharmacotherapy NCIT:C15986

Platform: Small molecule

Small-molecule AMPK activators, including PXL-770, inhibited BNIP3/NIX mitophagy and restored mitochondrial content and respiration in FBXL4-deficient cells and patient fibroblasts, and inhibited mitophagy in liver and brain of a chemically induced mouse model. Not tested in patients.

Mechanism Target:

Excessive Basal Mitophagy — AMPK sequesters the ULK1 autophagy-initiating kinase and so restrains mitophagy.

Show evidence (1 reference)

PMID:42310391 SUPPORT In Vitro

"Using FBXL4-deficient cells, as well as fibroblasts derived from MTDPS13 patients and a chemically-induced mouse model, we show that small molecule AMPK activation inhibits BNIP3/NIX-mediated mitophagy and recovers functional mitochondrial content."

Preclinical rescue of the mitophagy node, mainly in cells.

🌍

Environmental Factors

1

Intercurrent infection

ECTO has no general infection-exposure class: sqlite:obo:ecto info "l~infection" returns ECTO:2000053 exposure to Human Papillomavirus Infection and NCIT:C27851 Human Papillomavirus Infection, both specific to one pathogen, so the general ECTO:3000000 exposure to organism is bound.

Intercurrent infections precipitate metabolic decompensation with worsening lactic acidosis and hyperammonemia and account for deaths in infancy.

Show evidence (1 reference)

PMID:23993194 SUPPORT Human Clinical

"However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic..."

Infection as the setting of fatal decompensation.

Mechanism Target:

EXACERBATES Lactic acidosis — Infection-associated metabolic decompensation.

Show evidence (1 reference)

PMID:23993194 SUPPORT Human Clinical

"However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic..."

Fatal decompensation during intercurrent infections.

EXACERBATES Hyperammonemia

Show evidence (1 reference)

PMID:40161922 SUPPORT Human Clinical

"At 40 days old, the patient experienced metabolic acidosis and a hyperammonemic episode (ammonia level: 1495 μmol/L) triggered by an infection"

Infection-triggered hyperammonemic episode in one neonate.

🔬

Biochemical Markers

4

Blood lactate (PRESENT)

Show evidence (2 references)

PMID:25868664 SUPPORT Human Clinical

"Elevated blood lactate and metabolic acidosis were observed in all individuals; creatine kinase was elevated in 45% of measurements."

Universal lactate elevation.

PMID:27099744 SUPPORT Human Clinical

"Metabolic investigations invariably show increased serum lactate and pyruvate levels."

Lactate and pyruvate both raised.

Plasma ammonia (PRESENT)

Show evidence (1 reference)

PMID:40161922 SUPPORT Human Clinical

"A male infant presented with dysmorphic features, hypotonia, failure to thrive, and lactic acidosis and severe hyperammonemia (ammonia: 1495 μmol/L)."

Extreme hyperammonemia in one neonate.

Plasma aspartate (PRESENT)

Show evidence (1 reference)

PMID:27858371 SUPPORT Human Clinical

"We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to..."

Low plasma aspartate in two siblings.

Muscle mtDNA content (PRESENT)

Show evidence (1 reference)

PMID:31969900 SUPPORT Human Clinical

"Severe mtDNA depletion was detected in muscle biopsy in both patients."

Muscle mtDNA depletion in two further patients.

🔬

Diagnosis

4

FBXL4 molecular genetic testing

Identification of biallelic pathogenic FBXL4 variants establishes the diagnosis. Early-onset encephalopathy with persistent lactic acidosis, profound hypotonia and the typical facial dysmorphism should prompt FBXL4 analysis, and FBXL4 belongs on mitochondrial panels used for congenital lactic acidosis.

Show evidence (4 references)

PMID:28383868 SUPPORT Human Clinical

"The diagnosis of FBXL4-related mtDNA depletion syndrome is established in a proband by identification of biallelic pathogenic variants in FBXL4 on molecular genetic testing."

GeneReviews diagnostic criterion.

PMID:25868664 SUPPORT Human Clinical

"In conclusion, a clinical pattern of early-onset encephalopathy, persistent lactic acidosis, profound muscular hypotonia and typical facial dysmorphism should prompt initiation of molecular genetic analysis of FBXL4."

Clinical pattern that should trigger testing.

PMID:27743463 SUPPORT Human Clinical

"Including FBLX4 in the mitochondrial diseases panel should be particularly important for patients with congenital lactic acidosis."

Panel inclusion recommendation.

+ 1 more reference

Muscle biopsy

Shows mtDNA depletion, a paucity of mitochondria and a combined respiratory chain deficiency; supportive, but the diagnosis rests on molecular testing.

Show evidence (1 reference)

PMID:25868664 SUPPORT Human Clinical

"Diagnostic workup in patient tissues and cells revealed a severe combined respiratory chain defect with a general decrease of enzymes associated with mitochondrial energy metabolism and a relative depletion of mitochondrial DNA content."

Tissue findings on diagnostic workup.

Surveillance

No published surveillance guideline; follow-up of eyes, hearing, heart, feeding, liver, neurological complications and neutropenia is set by the treating physician according to the findings.

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"The treating physician should decide about the frequency of follow up of eyes, hearing, heart, feeding difficulties, liver, neurologic complications, and neutropenia based on the patient's findings."

GeneReviews surveillance statement.

Carrier and prenatal testing

Once both familial variants are known, carrier testing, prenatal testing and preimplantation genetic testing are possible.

Show evidence (1 reference)

PMID:28383868 SUPPORT Human Clinical

"Once the FBXL4 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible."

GeneReviews genetic counseling statement.

📈

Progression

2

Prenatal and neonatal onset

Onset is congenital or in early infancy. Prenatal onset with brain changes from the second trimester, polyhydramnios and cardiac anomalies is reported in a few cases.

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay."

Congenital or early onset.

PMID:40252080 SUPPORT Human Clinical

"This case illustrates the prenatal onset of MTDPS13, with central nervous system abnormalities apparent from the second trimester."

Prenatal onset in a subset.

Survival

Median age at reported death two years (range 2 days to 75 months), with survivors up to 36 years. In the 21-patient series 7 children had died at a mean age of 37 months and 11 were alive at a mean follow-up of 46 months. Deaths during metabolic decompensation with intercurrent infection are described.

Show evidence (2 references)

PMID:28383868 SUPPORT Human Clinical

"Survival varies; the median age of reported deaths was two years (range 2 days – 75 months), although surviving individuals as old as 36 years have been reported."

GeneReviews survival summary.

PMID:25868664 SUPPORT Human Clinical

"Seven children died (mean age 37 months); 11 children were alive (mean age at follow-up 46 months), three children were lost to follow-up."

Outcome in the 21-patient series.

📊

Prevalence

1

Published cases worldwide to 2017

Cases In Literature Ultra Rare

87 individuals in the largest pooled review (50 previously published plus 37 new). No population-based prevalence has been published. In a referral laboratory series FBXL4 variants explained 6 of 808 people tested for suspected mitochondrial disease, and 4 of 28 young children with congenital lactic acidosis unexplained by a 192-gene panel.

Show evidence (2 references)

PMID:28940506 SUPPORT Human Clinical

"Since its initial description in 2013, 36 different pathogenic variants in FBXL4 were reported in 50 affected individuals. In this report, we present 37 additional affected individuals and 11 previously unreported pathogenic variants."

Case count in the largest pooled review.

PMID:27743463 SUPPORT Human Clinical

"Overall, FBXL4 defects account for at least 0.7% (6 out of 808) of subjects suspected to have a mitochondrial disorder, and as high as 14.3% (4 out of 28) in young children with congenital lactic acidosis and clinical features of mitochondrial disease."

Diagnostic yield in a referral laboratory series; a proportion of referred patients, not a population prevalence.

🔬

Clinical Trials

1

NCT04802707 PHASE_II

Open-label, single-arm phase II study of combination deoxycytidine and deoxythymidine in children with mitochondrial DNA depletion syndromes; FBXL4 is among the eligible genotypes. Recruitment status is not recorded because the cached registry summary carries no status field.

Target Phenotypes: Global developmental delay HP:0001263

Show evidence (1 reference)

clinicaltrials:NCT04802707 SUPPORT Human Clinical

"The subjects included are children (0-18Y), with positive MDS diagnosis and express mutations in one of the following genes: POLG, POLG2, C10orf2, RRM2B, MPV17, SUCLA2, SUCLG1, FBXL4, DTYMK."

FBXL4 is an eligible genotype.

🧫

Experimental Models

3

FBXL4 knockout human cell lines CELL_LINE

CRISPR/Cas9 FBXL4 knockout human cells used by several groups to define the ligase, its substrates and the mitophagy phenotype.

Organism

Homo sapiens NCBITaxon:9606

Publication

PMID:36896912

Pathograph links

Excessive Basal Mitophagy recapitulates fidelity: high observes: cellular

Limitations: Transformed cell lines lack tissue context and systemic metabolism.

Show evidence (1 reference)

PMID:36896912 SUPPORT In Vitro

"Subsequent counter screen revealed that FBXL4-KO hyperactivates mitophagy via two mitophagy receptors BNIP3 and NIX."

Receptor-dependent mitophagy in knockout cells.

Patient iPSC-derived cortical neurons IPSC_DERIVED_MODEL

Cortical neurons differentiated from MTDPS13 patient iPSCs.

Organism

Homo sapiens NCBITaxon:9606

Publication

PMID:37568009

Pathograph links

Excessive Basal Mitophagy recapitulates fidelity: moderate observes: cellular

Limitations: Shows the mitophagy state in human neurons; neuronal survival and bioenergetics are not reported in the abstract.

Show evidence (1 reference)

PMID:37568009 SUPPORT In Vitro

"Furthermore, we observed abnormal activation of mitophagy in cortical neurons induced from hiPSCs derived from MTDPS13 patients."

Patient-derived neurons show the mitophagy phenotype.

Patient fibroblasts PRIMARY_CELL_CULTURE

Skin fibroblasts from patients with biallelic FBXL4 variants.

Organism

Homo sapiens NCBITaxon:9606

Publication

PMID:23993193

Pathograph links

Combined Respiratory Chain Deficiency recapitulates fidelity: moderate observes: cellular

Limitations: mtDNA depletion is milder or absent in fibroblasts compared with muscle, and mitochondrial mass findings differ between lines.

Show evidence (1 reference)

PMID:23993193 SUPPORT In Vitro

"We show that FBXL4 is an F-box protein that colocalizes with mitochondria and that loss-of-function and splice mutations in this protein result in a severe respiratory chain deficiency, loss of mitochondrial membrane potential, and a disturbance of the dynamic mitochondrial network and nucleoid..."

Respiratory chain deficiency in patient fibroblasts.

🐁

Animal Models

5

Fbxl4 knockout mouse (Alsina et al.)

Predominant perinatal lethality; rare survivors appear normal until 8-12 months, then develop mitochondrial dysfunction, weight loss, a global reduction of mitochondrial proteins and mtDNA depletion.

Species

Mouse

Genotype

Fbxl4-/-

Publication

PMID:32525278

Pathograph links

Reduced Mitochondrial Content recapitulates fidelity: moderate observes: tissue

Limitations: Survivors are asymptomatic for most of the first year, unlike the neonatal onset in patients; the authors argue that the mitochondrial phenotype is not the leading cause of the perinatal death, since embryos develop normally until E13.5 despite lower mtDNA.

Show evidence (2 references)

PMID:32525278 SUPPORT Model Organism

"One-year-old Fbxl4 knockouts show a global reduction in a variety of mitochondrial proteins and mtDNA depletion, whereas lysosomal proteins are upregulated."

Reduced mitochondrial content in knockout mouse tissue.

+ 1 more reference

Fbxl4 knockout mouse with Bnip3 or Nix co-deletion (Cao et al.)

Fbxl4-/- mice have raised BNIP3 and NIX, hyperactive mitophagy and perinatal lethality; deleting either Bnip3 or Nix represses the mitophagy, corrects metabolic derangements and rescues viability.

Species

Mouse

Genotype

Fbxl4-/- with Bnip3-/- or Nix-/-

Publication

PMID:36896912

Pathograph links

Excessive Basal Mitophagy rescues fidelity: high observes: organism

Limitations: Genetic rescue in mice; receptor knockout is not a clinical intervention, and neurological outcome in a model with the human neonatal course has not been tested.

Show evidence (1 reference)

PMID:36896912 SUPPORT Model Organism

"Importantly, knockout of either Bnip3 or Nix rescues metabolic derangements and viability of the Fbxl4-/- mice."

Genetic rescue places receptor-driven mitophagy upstream of lethality.

Fbxl4 patient-variant knock-in mouse (Chen et al.)

Knock-in mice carrying a patient-derived FBXL4 variant show excessive mitophagy.

Species

Mouse

Genotype

knock-in of a patient-derived FBXL4 variant

Publication

PMID:37568009

Pathograph links

Excessive Basal Mitophagy recapitulates fidelity: moderate observes: tissue

Limitations: The abstract does not name the variant or report the whole-animal phenotype of the mice.

Show evidence (1 reference)

PMID:37568009 SUPPORT Model Organism

"Excessive mitophagy was observed in Knockin (KI) mice carrying a patient-derived FBXL4 mutation and cortical neurons (CNs)-induced from MTDPS13 patient human induced pluripotent stem cells (hiPSCs)."

Patient allele reproduces the mitophagy state in vivo.

fbxl-1(ok3741) C. elegans

Developmental delay, impaired fecundity, reduced neuromuscular activity, mitochondrial dysfunction and altered lactate metabolism; improved by dichloroacetate.

Species

Caenorhabditis elegans

Genotype

fbxl-1(ok3741)

Publication

PMID:35881484

Pathograph links

Combined Respiratory Chain Deficiency partially recapitulates fidelity: low observes: organism

Limitations: Invertebrate ortholog model; whether fbxl-1 regulates BNIP3/NIX-type mitophagy receptors in worms is not stated in the abstract.

Show evidence (1 reference)

PMID:35881484 SUPPORT Model Organism

"Developmental delay, impaired fecundity and neurologic and/or muscular activity, mitochondrial dysfunction, and altered lactate metabolism were identified in fbxl-1(ok3741) C. elegans."

Mitochondrial and lactate phenotypes in the worm model.

fbxl4 sa12470 zebrafish

Zebrafish larvae used to validate dichloroacetate against brain death and neuromuscular impairment.

Species

Zebrafish

Genotype

fbxl4sa12470

Publication

PMID:35881484

Pathograph links

Neuronal Energy Failure partially recapitulates fidelity: low observes: organism

Limitations: The abstract reports protection by dichloroacetate but does not describe the untreated phenotype in detail, and the morphological mitochondrial defects are stress-induced rather than baseline.

Show evidence (1 reference)

PMID:35881484 SUPPORT Model Organism

"Validation studies were performed in fbxl4sa12470 zebrafish larvae and in FBXL4-/- human fibroblasts; they showed DCA efficacy in preventing brain death, impairment of neurologic and/or muscular function, mitochondrial biochemical dysfunction, and stress-induced morphologic and ultrastructural..."

Brain death and neuromuscular impairment in the larvae are prevented by DCA.

{ }

Source YAML

click to show

name: FBXL4-Related Mitochondrial DNA Depletion Syndrome
creation_date: "2026-09-28T20:00:00Z"
category: Mendelian
disease_term:
  preferred_term: mitochondrial DNA depletion syndrome 13
  term:
    id: MONDO:0014198
    label: mitochondrial DNA depletion syndrome 13
synonyms:
- MTDPS13
- mitochondrial DNA depletion syndrome 13
- mitochondrial DNA depletion syndrome 13 (encephalomyopathic type)
- mitochondrial DNA depletion syndrome type 13
- FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome
- FBXL4 mitochondrial DNA depletion syndrome
- FBXL4 deficiency
- encephalomyopathic mitochondrial DNA depletion syndrome-13
- mtDNA depletion syndrome, encephalomyopathic form with variable craniofacial anomalies
- mitochondrial DNA depletion syndrome caused by mutation in FBXL4
description: >-
  Autosomal recessive multisystem mitochondrial disease caused by biallelic
  FBXL4 variants. FBXL4 is the substrate adaptor of an SCF (SKP1-CUL1-F-box)
  ubiquitin ligase on the mitochondrial outer membrane that continuously
  ubiquitinates the mitophagy receptors BNIP3 and NIX (BNIP3L) and so restrains
  basal mitophagy. Loss of FBXL4 lets BNIP3 and NIX accumulate, mitochondria are
  removed by autophagy in excess, and cells are left with too few mitochondria,
  too little mtDNA and a combined respiratory chain deficiency. Onset is
  congenital or in early infancy with lactic acidosis, hypotonia, feeding
  difficulty, growth failure and developmental delay; progressive cerebral
  atrophy with white matter involvement, seizures, movement disorders,
  hyperammonemia, hypertrophic cardiomyopathy, congenital heart malformations,
  arrhythmias, raised transaminases, cataract and other eye findings,
  sensorineural hearing loss, neutropenia and lymphopenia, and characteristic
  facial dysmorphism occur variably. Reported deaths cluster in early childhood
  (median two years), but survival into adulthood occurs and milder courses are
  described. Treatment is supportive; dichloroacetate, ketogenic diet and
  deoxynucleosides have been tried in single patients or small open-label
  series, and suppression of mitophagy is the mechanistic treatment target in
  preclinical work.
parents:
- mitochondrial DNA depletion syndrome, encephalomyopathic form
- mitochondrial DNA maintenance defect
notes: >-
  Identity: MONDO:0014198 carries OMIM:615471 and Orphanet:369897 as exact
  cross-references and FBXL4 as its causal gene (RO:0004003). The MONDO parent
  MONDO:0016796 (mitochondrial DNA depletion syndrome, encephalomyopathic form)
  is a multi-gene grouping and is not this entry. FBXL4 is the only gene. The
  submitochondrial location of FBXL4 was first reported as the intermembrane
  space (Gai et al. 2013); the later ligase studies place it on the outer
  membrane, the topology that ubiquitination of BNIP3 and NIX requires, and the
  entry follows the later work.
classifications:
  harrisons_chapter:
  - classification_value: NEUROLOGIC
    notes: Early-onset encephalopathy with progressive brain atrophy dominates the course.
  - classification_value: ENDOCRINOLOGY_METABOLISM
    notes: Primary mitochondrial disease presenting with congenital lactic acidosis.
  mechanistic_category:
  - classification_value: mitochondrial disease
    evidence:
    - reference: PMID:23993193
      reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Together our data demonstrate that mutations in FBXL4 are disease causing and establish FBXL4 as a mitochondrial protein with a possible role in maintaining mtDNA integrity and stability."
      explanation: A nuclear gene for a mitochondrial protein whose loss causes mtDNA depletion places the disorder among primary mitochondrial diseases.
references:
- reference: PMID:28383868
  title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
  tags:
  - GeneReviews
inheritance:
- name: Autosomal recessive
  description: >-
    Biallelic FBXL4 variants, homozygous or compound heterozygous. Many reported
    patients are from consanguineous families, and a possible founder effect in
    patients of Arab descent has been proposed from a pooled mutation table.
    Carrier parents have a 25% recurrence risk in each pregnancy.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FBXL4-related mtDNA depletion syndrome is inherited in an autosomal recessive manner."
    explanation: GeneReviews statement of the mode of inheritance.
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "When both parents are heterozygous carriers, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier (heterozygote), and a 25% chance of being unaffected and not a carrier."
    explanation: GeneReviews recurrence risk for sibs.
  - reference: PMID:23993193
    reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle."
    explanation: Recessive segregation in the discovery kindreds.
  - reference: PMID:30804983
    reference_title: "FBXL4-Related Mitochondrial DNA Depletion Syndrome 13 (MTDPS13): A Case Report With a Comprehensive Mutation Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "using it to highlight, for the first time, a possible founder effect of Arab origins to the disorder, being most prevalent in patients of Arab descent as shown in our mutation table."
    explanation: Proposed Arab founder effect from a pooled mutation review; stated by the authors as possible.
prevalence:
- population: Published cases worldwide to 2017
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    87 individuals in the largest pooled review (50 previously published plus 37
    new). No population-based prevalence has been published. In a referral
    laboratory series FBXL4 variants explained 6 of 808 people tested for
    suspected mitochondrial disease, and 4 of 28 young children with congenital
    lactic acidosis unexplained by a 192-gene panel.
  evidence:
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since its initial description in 2013, 36 different pathogenic variants in FBXL4 were reported in 50 affected individuals. In this report, we present 37 additional affected individuals and 11 previously unreported pathogenic variants."
    explanation: Case count in the largest pooled review.
  - reference: PMID:27743463
    reference_title: "FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Overall, FBXL4 defects account for at least 0.7% (6 out of 808) of subjects suspected to have a mitochondrial disorder, and as high as 14.3% (4 out of 28) in young children with congenital lactic acidosis and clinical features of mitochondrial disease."
    explanation: Diagnostic yield in a referral laboratory series; a proportion of referred patients, not a population prevalence.
mechanistic_hypotheses:
- hypothesis_group_id: excessive_bnip3_nix_mitophagy
  hypothesis_label: Excessive BNIP3/NIX-Dependent Basal Mitophagy
  status: CANONICAL
  description: >-
    FBXL4 is the substrate adaptor of an SCF ubiquitin ligase that degrades the
    mitophagy receptors BNIP3 and NIX. Pathogenic FBXL4 variants fail to assemble
    an active ligase, BNIP3 and NIX accumulate, basal mitophagy rises, and cells
    lose mitochondrial content and mtDNA. Several independent groups reported
    the ligase-substrate relationship in 2023, and deleting Bnip3 or Nix rescues
    the viability of Fbxl4 knockout mice.
  evidence:
  - reference: PMID:36896912
    reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Together, beyond identifying SCF-FBXL4 as a novel mitochondrial ubiquitin E3 ligase restraining basal mitophagy, our results reveal hyperactivated mitophagy as a cause of mitochondrial disease and suggest therapeutic strategies."
    explanation: Genetic rescue in mice establishes hyperactive mitophagy as causal.
  - reference: PMID:38123379
    reference_title: "FBXL4 mutation-caused mitochondrial DNA depletion syndrome is driven by BNIP3/BNIP3L-dependent excessive mitophagy."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Recent advances have shown that excessive BCL2 interacting protein 3 (BNIP3)/ BCL2 interacting protein 3 like (BNIP3L)-dependent mitophagy underlies the molecular pathogenesis of MTDPS13."
    explanation: Review synthesis of the convergent 2023 studies.
- hypothesis_group_id: primary_oxphos_defect_secondary_mitophagy
  hypothesis_label: Primary Oxidative Phosphorylation Defect with Secondary Mitophagy
  status: ALTERNATIVE
  description: >-
    Before the ligase studies, one fibroblast study of two patients found
    increased mitochondrial mass, fragmentation and autophagy but interpreted
    them as secondary to a primary oxidative phosphorylation defect, and one of
    the two lines had normal mtDNA copy number. A neonatal case likewise showed
    relative sparing of mitochondrial mass in fibroblasts despite profound
    respiratory failure. These observations predate, and have not been
    reconciled with, the BNIP3/NIX data.
  evidence:
  - reference: PMID:31969900
    reference_title: "Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Further analysis of fibroblasts allowed to establish that increased mitochondrial mass, mitochondrial fragmentation, and augmented autophagy are associated with FBXL4 deficiency in cells, but are probably secondary to a primary metabolic defect affecting oxidative phosphorylation."
    explanation: The authors' interpretation of autophagy as secondary.
- hypothesis_group_id: embryonic_mitophagy_developmental_defects
  hypothesis_label: Dysregulated Mitophagy in Embryonic Stem Cell Differentiation
  status: EMERGING
  description: >-
    Structural brain and heart malformations and facial dysmorphism suggest a
    defect of embryogenesis. A commentary proposes that excess mitophagy
    disturbs the fine-tuned mitophagy needed for stem cell differentiation. No
    experimental test in an FBXL4 model is cited.
  evidence:
  - reference: PMID:39937392
    reference_title: "FBXL4-Related Mitochondrial Depletion Syndrome Underscores the role of Mitophagy in Stem Cell Differentiation during Embryogenesis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The disruption of this process is the likely explanation of developmental defects in FBXL4- related mitochondrial depletion syndrome."
    explanation: Hypothesis stated in a commentary without new data.
pathophysiology:
- name: FBXL4 Loss of Function
  biological_scale: MOLECULAR
  description: >-
    Biallelic germline FBXL4 variants (nonsense, frameshift, splice and
    missense) distributed across the gene, without a clear genotype-phenotype
    correlation. FBXL4 is the substrate-binding F-box subunit of an SCF (CRL1)
    ubiquitin ligase on the mitochondrial outer membrane. Tested pathogenic
    variants leave FBXL4 able to bind BNIP3 and NIX but unable to assemble an
    active ligase. Re-expression of wild-type FBXL4 restores mtDNA copy number
    in patient cells.
  genes:
  - preferred_term: FBXL4
    term:
      id: hgnc:13601
      label: FBXL4
  genetic_context:
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
    description: Biallelic (homozygous or compound heterozygous) germline FBXL4 variants.
  molecular_functions:
  - preferred_term: SCF substrate adaptor activity
    term:
      id: GO:1990756
      label: ubiquitin-like ligase-substrate adaptor activity
    modifier: DECREASED
  cellular_components:
  - preferred_term: mitochondrial outer membrane
    term:
      id: GO:0005741
      label: mitochondrial outer membrane
  evidence:
  - reference: PMID:23993193
    reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Expression of the wild-type FBXL4 transcript in cell lines from two subjects fully rescued the levels of mtDNA copy number, leading to a correction of the mitochondrial biochemical deficit."
    explanation: Complementation in patient cells ties the cellular defect to loss of FBXL4 function.
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations were detected throughout the FBXL4 gene albeit with no clear delineation of a genotype-phenotype correlation."
    explanation: Variants span the gene without a genotype-phenotype correlation.
  - reference: PMID:36896912
    reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We determined that FBXL4 functions as an integral outer-membrane protein that forms an SCF-FBXL4 ubiquitin E3 ligase complex."
    explanation: Establishes FBXL4 as the substrate adaptor of an outer-membrane SCF ligase.
  - reference: PMID:37161784
    reference_title: "FBXL4 suppresses mitophagy by restricting the accumulation of NIX and BNIP3 mitophagy receptors."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We demonstrate that the pathogenic variants of FBXL4 that cause encephalopathic mtDNA depletion syndrome (MTDPS13) do not efficiently interact with the core SCF ubiquitin ligase machinery or mediate the degradation of NIX and BNIP3."
    explanation: Patient variants lose ligase assembly and function.
  - reference: PMID:37876279
    reference_title: "Excessive BNIP3- and BNIP3L-dependent mitophagy underlies the pathogenesis of FBXL4-mutated mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Patient-derived FBXL4 mutations do not affect its interaction with BNIP3 and BNIP3L but impair the assembly of an active CRL1-FBXL4 complex."
    explanation: Substrate binding is retained and ligase assembly is lost.
  downstream:
  - target: Failed Ubiquitination and Degradation of BNIP3 and NIX
    causal_link_type: DIRECT
    hypothesis_groups:
    - excessive_bnip3_nix_mitophagy
    evidence:
    - reference: PMID:36896912
      reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Pathogenic FBXL4 mutations disrupt SCF-FBXL4 assembly and impair substrate degradation."
      explanation: Directly links patient variants to failed substrate degradation.
  - target: Reduced Mitochondrial Fusion and Network Fragmentation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:31442532
      reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Using a photo-activatable GFP fusion assay, we found reduced mitochondrial fusion rates in cells harbouring a pathogenic FBXL4 variant."
      explanation: Reduced fusion in cells with a patient variant.
- name: Failed Ubiquitination and Degradation of BNIP3 and NIX
  biological_scale: MOLECULAR
  description: >-
    Without an active SCF-FBXL4 ligase, the mitophagy receptors BNIP3 and NIX
    (BNIP3L) escape constitutive ubiquitination and proteasomal degradation and
    accumulate on the mitochondrial outer membrane. PPTC7, a matrix protein
    retained at the outer membrane, scaffolds the substrate-PPTC7-SCF-FBXL4
    complex and is part of the same regulatory loop.
  genes:
  - preferred_term: BNIP3
    term:
      id: hgnc:1084
      label: BNIP3
  - preferred_term: NIX (BNIP3L)
    term:
      id: hgnc:1085
      label: BNIP3L
  biological_processes:
  - preferred_term: SCF-dependent proteasomal degradation of BNIP3 and NIX
    term:
      id: GO:0031146
      label: SCF-dependent proteasomal ubiquitin-dependent protein catabolic process
    modifier: DECREASED
  evidence:
  - reference: PMID:37161784
    reference_title: "FBXL4 suppresses mitophagy by restricting the accumulation of NIX and BNIP3 mitophagy receptors."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Here, we report that SCFFBXL4 , an SKP1/CUL1/F-box protein ubiquitin ligase complex, localises to the mitochondrial outer membrane in unstressed cells and mediates the constitutive ubiquitylation and degradation of the mitophagy receptors NIX and BNIP3 to suppress basal levels of mitophagy."
    explanation: Constitutive SCF-FBXL4-dependent turnover of the two receptors.
  - reference: PMID:37568009
    reference_title: "FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Here, we demonstrate that, FBXL4 directly interacts with the mitophagy cargo receptors BNIP3 and BNIP3L, promoting their degradation through the ubiquitin-proteasome pathway via the assembly of an active CRL1FBXL4 complex."
    explanation: Independent group; direct interaction and proteasomal degradation.
  - reference: PMID:37102372
    reference_title: "FBXL4 ubiquitin ligase deficiency promotes mitophagy by elevating NIX levels."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "FBXL4 restricts NIX and BNIP3 levels via direct interaction and protein destabilization, while VHL acts through suppression of HIF1α-mediated transcription of BNIP3 and NIX."
    explanation: Third independent group, from a CRISPR screen of E3 ligases.
  - reference: PMID:38151018
    reference_title: "A mitophagy sensor PPTC7 controls BNIP3 and NIX degradation to regulate mitochondrial mass."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Biochemically, the PPTC7 precursor is trapped by BNIP3 and NIX to the mitochondrial outer membrane, where PPTC7 scaffolds assembly of a substrate-PPTC7-SCFFBXL4 holocomplex to degrade BNIP3 and NIX, forming a homeostatic regulatory loop."
    explanation: PPTC7 as a co-factor of SCF-FBXL4 substrate degradation.
  downstream:
  - target: Excessive Basal Mitophagy
    causal_link_type: DIRECT
    hypothesis_groups:
    - excessive_bnip3_nix_mitophagy
    evidence:
    - reference: PMID:37568009
      reference_title: "FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "This results in a notable accumulation of BNIP3/3L proteins and robust mitophagy even at basal levels."
      explanation: Receptor accumulation drives basal mitophagy.
    - reference: PMID:37102372
      reference_title: "FBXL4 ubiquitin ligase deficiency promotes mitophagy by elevating NIX levels."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Depletion of NIX but not BNIP3 is sufficient to restore mitophagy levels."
      explanation: >-
        In this study's cells NIX was the dominant receptor; in the mouse study
        either receptor knockout rescued, so the relative weight of the two
        receptors is not settled.
- name: Excessive Basal Mitophagy
  biological_scale: CELLULAR
  description: >-
    Mitophagy is hyperactive under basal, unstressed conditions in FBXL4
    knockout cells, patient fibroblasts, patient iPSC-derived cortical neurons,
    Fbxl4 knockout mice and knock-in mice carrying a patient variant. Blocking
    lysosomal function reverses the mitochondrial phenotype in FBXL4-deficient
    cells, while proteasome inhibition does not.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: mitophagy
    term:
      id: GO:0000422
      label: autophagy of mitochondrion
    modifier: INCREASED
  evidence:
  - reference: PMID:36896912
    reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Here, via a mitochondria-targeted genetic screen, we found that knockout (KO) of FBXL4, a mitochondrial disease gene, hyperactivates mitophagy at basal conditions."
    explanation: Basal mitophagy is hyperactive in FBXL4 knockout cells.
  - reference: PMID:36896912
    reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Fbxl4-/- mice exhibit elevated BNIP3 and NIX proteins, hyperactive mitophagy, and perinatal lethality."
    explanation: The same state in knockout mice.
  - reference: PMID:37568009
    reference_title: "FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Excessive mitophagy was observed in Knockin (KI) mice carrying a patient-derived FBXL4 mutation and cortical neurons (CNs)-induced from MTDPS13 patient human induced pluripotent stem cells (hiPSCs)."
    explanation: A patient allele in mice reproduces the state; the same sentence also covers patient-derived neurons, which are cited separately below as IN_VITRO.
  - reference: PMID:38423516
    reference_title: "FBXL4: safeguarding against mitochondrial depletion through suppression of mitophagy."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The post-translational regulation of BNIP3L and BNIP3 is disrupted in mitochondrial DNA depletion syndrome 13 (MTDPS13), a multi-systemic disorder caused by mutations in the FBXL4 gene and characterized by elevated mitophagy and mitochondrial DNA/mtDNA depletion in patient fibroblasts."
    explanation: Elevated mitophagy in patient fibroblasts.
  - reference: PMID:32525278
    reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Inhibition of lysosomal function in these cells reverses the mitochondrial phenotype, whereas proteasomal inhibition has no effect."
    explanation: The mitochondrial loss is lysosome-dependent, consistent with autophagic removal.
  - reference: PMID:31969900
    reference_title: "Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome."
    supports: REFUTE
    evidence_source: IN_VITRO
    snippet: "Further analysis of fibroblasts allowed to establish that increased mitochondrial mass, mitochondrial fragmentation, and augmented autophagy are associated with FBXL4 deficiency in cells, but are probably secondary to a primary metabolic defect affecting oxidative phosphorylation."
    explanation: >-
      Observes increased autophagy but disputes its primacy, and reports
      increased rather than decreased mitochondrial mass in these fibroblasts.
  downstream:
  - target: Reduced Mitochondrial Content
    causal_link_type: DIRECT
    hypothesis_groups:
    - excessive_bnip3_nix_mitophagy
    evidence:
    - reference: PMID:32525278
      reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We report that Fbxl4 is involved in mitochondrial quality control and that its absence causes an increased lysosomal turnover of mitochondria leading to a decreased cellular mitochondrial content."
      explanation: Increased lysosomal turnover lowers mitochondrial content.
  - target: Disrupted Stem Cell Differentiation during Embryogenesis
    causal_link_type: UNKNOWN
    hypothesis_groups:
    - embryonic_mitophagy_developmental_defects
    evidence:
    - reference: PMID:39937392
      reference_title: "FBXL4-Related Mitochondrial Depletion Syndrome Underscores the role of Mitophagy in Stem Cell Differentiation during Embryogenesis."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Fine tuning of mitophagy is essential for stem cell differentiation during embryogenesis."
      explanation: Commentary; the premise of the hypothesised link, not an FBXL4-specific experiment.
- name: Reduced Mitochondrial Content
  biological_scale: CELLULAR
  description: >-
    Fewer mitochondria per cell, with a global fall in mitochondrial proteins and
    a rise in lysosomal proteins. In the knockout mouse and in human knockout
    cells the remaining mitochondria are functional; the deficit is one of
    number. Patient muscle shows a paucity of mitochondria, but mitochondrial
    mass was relatively spared in fibroblasts from one neonate and increased in
    fibroblasts from two other patients, so the finding is not uniform across
    patient cells.
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:32525278
    reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Fibroblasts from patients with FBXL4 deficiency and human FBXL4 knockout cells also have reduced steady-state levels of mitochondrial proteins that can be attributed to increased mitochondrial turnover."
    explanation: Reduced mitochondrial protein content in patient and knockout cells.
  - reference: PMID:32525278
    reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "One-year-old Fbxl4 knockouts show a global reduction in a variety of mitochondrial proteins and mtDNA depletion, whereas lysosomal proteins are upregulated."
    explanation: The same in knockout mouse tissue.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsy showed a paucity of mitochondria with ultrastructural abnormalities, mitochondrial DNA depletion, and profound deficiency of all respiratory chain complexes."
    explanation: Paucity of mitochondria in patient muscle.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: REFUTE
    evidence_source: IN_VITRO
    snippet: "Cell-based mitochondrial phenotyping in fibroblasts showed mitochondrial fragmentation, decreased basal and maximal respiration, absence of ATP-linked respiratory and leak capacity, impaired survival under obligate aerobic respiration, and reduced mitochondrial inner membrane potential, with relative sparing of mitochondrial mass."
    explanation: In this patient's fibroblasts mitochondrial mass was relatively spared.
  downstream:
  - target: Mitochondrial DNA Depletion
    causal_link_type: DIRECT
    hypothesis_groups:
    - excessive_bnip3_nix_mitophagy
    evidence:
    - reference: PMID:38423516
      reference_title: "FBXL4: safeguarding against mitochondrial depletion through suppression of mitophagy."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Insufficient mitophagy can result in failure to remove damaged mitochondria and consequent cellular degeneration, but it is equally important to appropriately restrain mitophagy to prevent excessive mitochondrial depletion."
      directness: INDIRECT
      explanation: >-
        States that unrestrained mitophagy depletes mitochondria. mtDNA loss is
        inferred as the loss of the organelles that carry it; the quote does not
        mention mtDNA itself.
  - target: Combined Respiratory Chain Deficiency
    causal_link_type: DIRECT
    hypothesis_groups:
    - excessive_bnip3_nix_mitophagy
    evidence:
    - reference: PMID:32525278
      reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Although the remaining mitochondria are fully functional, their numbers are insufficient to prevent disease."
      explanation: The oxidative deficit is attributed to reduced mitochondrial number.
- name: Mitochondrial DNA Depletion
  biological_scale: CELLULAR
  description: >-
    Reduced mtDNA copy number, severe in skeletal muscle and milder or absent
    in cultured fibroblasts; one fibroblast line with a homozygous truncating
    variant had normal copy number.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:23993193
    reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle."
    explanation: Severe muscle mtDNA depletion in the discovery kindreds.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In muscle and fibroblasts from several subjects, substantially decreased mtDNA content was observed."
    explanation: Independent discovery series.
  - reference: PMID:31969900
    reference_title: "Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We observed that the mtDNA depletion was less severe in fibroblasts than in muscle."
    explanation: Tissue difference in severity of depletion.
  - reference: PMID:31969900
    reference_title: "Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome."
    supports: REFUTE
    evidence_source: IN_VITRO
    snippet: "Interestingly, the cells harboring a nonsense variant in homozygosis showed normal mtDNA copy number."
    explanation: Depletion is not found in every patient cell line.
  downstream:
  - target: Combined Respiratory Chain Deficiency
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:31969900
      reference_title: "Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Both patient fibroblasts, however, demonstrated reduced mitochondrial transcript quantity leading to diminished steady state levels of respiratory complex subunits, decreased respiratory complex IV (CIV) activity, and finally, low mitochondrial ATP levels."
      explanation: Fewer mitochondrial transcripts lower respiratory subunit levels and activity.
- name: Reduced Mitochondrial Fusion and Network Fragmentation
  biological_scale: CELLULAR
  description: >-
    Fragmented mitochondrial networks, abnormal nucleoid distribution and loss
    of inner membrane potential in patient fibroblasts. One study attributes
    the fragmentation to a direct role of FBXL4 in promoting fusion. How this
    relates to the excess mitophagy is not established, so the node has no
    downstream edge.
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: mitochondrial fusion
    term:
      id: GO:0008053
      label: mitochondrial fusion
    modifier: DECREASED
  evidence:
  - reference: PMID:23993193
    reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We show that FBXL4 is an F-box protein that colocalizes with mitochondria and that loss-of-function and splice mutations in this protein result in a severe respiratory chain deficiency, loss of mitochondrial membrane potential, and a disturbance of the dynamic mitochondrial network and nucleoid distribution in fibroblasts from affected individuals."
    explanation: Network and nucleoid disturbance in patient fibroblasts.
  - reference: PMID:31442532
    reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Meanwhile, overexpression of wildtype FBXL4, but not the p.Cys584Arg variant, promoted mitochondrial hyperfusion."
    explanation: Wild-type FBXL4 promotes fusion; the patient variant does not.
- name: Combined Respiratory Chain Deficiency
  biological_scale: CELLULAR
  conforms_to: "mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress"
  description: >-
    Combined deficiency of multiple respiratory chain complexes in muscle and
    fibroblasts, with a general decrease of mitochondrial energy-metabolism
    enzymes, reduced oxygen consumption and low mitochondrial ATP. In one
    neonate all complexes were profoundly deficient in muscle, and that
    patient's fibroblasts had a more oxidised glutathione ratio. Raised
    reactive oxygen species production has not been measured directly, so only
    the oxidative phosphorylation arm of the conformed module node is bound.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Diagnostic workup in patient tissues and cells revealed a severe combined respiratory chain defect with a general decrease of enzymes associated with mitochondrial energy metabolism and a relative depletion of mitochondrial DNA content."
    explanation: Combined defect in the 21-patient series.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mitochondrial respiratory chain deficiency was present in muscle or fibroblasts of all tested individuals, together with markedly reduced oxygen consumption rate and hyperfragmentation of the mitochondrial network in cultured cells."
    explanation: Deficiency in every tested individual.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "High-resolution respirometry of permeabilized muscle fibers showed marked deficiency of oxidative phosphorylation using a variety of mitochondrial energy substrates and inhibitors."
    explanation: Direct respirometry of patient muscle.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Cultured fibroblasts from the patient exhibited a more oxidized glutathione ratio, consistent with altered cellular redox poise."
    explanation: Altered redox state in patient fibroblasts, the closest measured correlate of the oxidative arm of the module node.
  downstream:
  - target: Lactic acidosis
    description: Impaired oxidative phosphorylation diverts pyruvate to lactate.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:31442532
      reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Although treating FBXL4 fibroblasts with DCA improved extracellular acidification, in line with reduced lactate levels in patients, DCA treatment did not improve any of the other mitochondrial functions."
      directness: INDIRECT
      explanation: >-
        Pushing pyruvate into mitochondrial oxidation with a PDH activator
        lowers acid output of patient cells, consistent with lactate arising at
        the pyruvate-oxidation step.
  - target: Neuronal Energy Failure
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:42310391
      reference_title: "Activation of AMPK as a therapeutic strategy for FBXL4-related mitochondrial DNA depletion syndrome."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: BACKGROUND
      snippet: "As a result, mitochondrial content and oxidative phosphorylation decline sharply across multiple tissues, leading to early mortality, with no effective treatments currently existing."
      explanation: Background statement that the oxidative decline is multi-tissue; the neuronal step itself is not measured in patients.
  - target: Mitochondrial Myopathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Cardiomyocyte Energy Failure
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Impaired Anaplerosis
    causal_link_type: UNKNOWN
    description: >-
      Proposed from patient biochemistry; knockout mouse liver shows perturbed
      amino acid and pyruvate metabolism from reduced mitochondrial oxidation,
      but aspartate accumulated in mouse liver whereas plasma aspartate was low
      in the patients, so the direction does not transfer.
  - target: Elevated circulating hepatic transaminase concentration
    causal_link_type: UNKNOWN
    description: Presumed hepatocyte energy failure; not studied in patient liver.
  - target: Decreased total neutrophil count
    causal_link_type: UNKNOWN
    description: Presumed effect on haematopoiesis; no mechanistic study in FBXL4 deficiency.
  - target: Decreased total lymphocyte count
    causal_link_type: UNKNOWN
    description: Presumed effect on haematopoiesis; no mechanistic study in FBXL4 deficiency.
  - target: Renal tubular acidosis
    causal_link_type: UNKNOWN
    description: Presumed renal tubular energy failure; not studied.
  - target: Gastrointestinal dysmotility
    causal_link_type: UNKNOWN
    description: Presumed enteric neuromuscular energy failure; not studied.
  - target: Cataract
    causal_link_type: UNKNOWN
    description: Mechanism of lens involvement not studied.
- name: Neuronal Energy Failure
  biological_scale: CELLULAR
  description: >-
    Energy failure in neurons, presumed from the brain phenotype. Excess
    mitophagy is shown in cortical neurons derived from patient iPSCs, but
    neuronal bioenergetics and neuron loss have not been measured in patient
    brain. Imaging shows a rapidly progressive brain atrophy after nonspecific
    neonatal findings, and destructive brain changes can begin before birth.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:37568009
    reference_title: "FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Furthermore, we observed abnormal activation of mitophagy in cortical neurons induced from hiPSCs derived from MTDPS13 patients."
    directness: INDIRECT
    explanation: The upstream state is present in patient-derived neurons; energy failure itself is inferred.
  - reference: PMID:36411461
    reference_title: "Prenatal phenotype of FBXL4-associated encephalomyopathic mitochondrial DNA depletion syndrome-13."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The brain imaging findings in the reported fetus indicate that MTDPS13 is associated with progressive neurological involvement and brain tissue destructive changes starting as early as the second trimester of pregnancy."
    explanation: Destructive brain changes begin prenatally.
  downstream:
  - target: Encephalopathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Cerebral atrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Leukoencephalopathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Global developmental delay
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Seizure
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Microcephaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Choreoathetosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Ataxia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Stroke-like episode
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Cerebellar hypoplasia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Ventriculomegaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Sensorineural hearing impairment
    causal_link_type: UNKNOWN
    description: The site of the auditory lesion has not been studied.
  - target: Optic atrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Nystagmus
    causal_link_type: UNKNOWN
    description: Presumed central or visual-pathway origin; not studied.
  - target: Strabismus
    causal_link_type: UNKNOWN
    description: Presumed central or ocular motor origin; not studied.
- name: Mitochondrial Myopathy
  biological_scale: TISSUE
  description: >-
    Skeletal muscle shows severe mtDNA depletion, a paucity of mitochondria with
    ultrastructural abnormalities and a combined respiratory chain deficiency;
    creatine kinase is intermittently raised.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsy showed a paucity of mitochondria with ultrastructural abnormalities, mitochondrial DNA depletion, and profound deficiency of all respiratory chain complexes."
    explanation: Muscle pathology in a neonatal case.
  - reference: PMID:27099744
    reference_title: "Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe recessive mitochondrial myopathy caused by FBXL4 gene mutations may present prenatally with polyhydramnios and cerebellar hypoplasia."
    explanation: Myopathy named as the disease presentation in a prenatal-onset case.
  downstream:
  - target: Hypotonia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Elevated circulating creatine kinase activity
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- name: Cardiomyocyte Energy Failure
  biological_scale: CELLULAR
  description: >-
    Presumed energy failure in cardiomyocytes underlying the hypertrophic
    cardiomyopathy and arrhythmias. In one of two siblings the hypertrophy
    reversed on dichloroacetate, which acts on pyruvate oxidation, consistent
    with a metabolic rather than fixed structural basis in that patient.
    Cardiac tissue has not been studied in patients.
  cell_types:
  - preferred_term: cardiac muscle cell
    term:
      id: CL:0000746
      label: cardiac muscle cell
  evidence:
  - reference: PMID:31442532
    reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Remarkably, dichloroacetate (DCA) administration to the younger sibling improved metabolic acidosis and reversed cardiac hypertrophy."
    directness: INDIRECT
    explanation: Reversibility with a metabolic drug in one patient; the cardiomyocyte energy deficit is inferred from the response.
  downstream:
  - target: Hypertrophic cardiomyopathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Arrhythmia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- name: Impaired Anaplerosis
  biological_scale: MOLECULAR
  description: >-
    In two siblings who presented with neonatal hyperammonemia, low plasma
    aspartate and low urinary TCA cycle intermediates suggested a failure to
    replenish TCA cycle intermediates; the authors propose this as the basis of
    the hyperammonemia and suggest anaplerotic therapy. This has not been
    tested in other patients or in a model.
  mechanism_confidence: HYPOTHETICAL
  biological_processes:
  - preferred_term: tricarboxylic acid cycle
    term:
      id: GO:0006099
      label: tricarboxylic acid cycle
    modifier: DECREASED
  evidence:
  - reference: PMID:27858371
    reference_title: "Hyperammonemia as a Presenting Feature in Two Siblings with FBXL4 Variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to lactic acidosis and other classic signs of mitochondrial encephalomyopathy."
    explanation: Biochemical pattern and the authors' anaplerosis interpretation.
  downstream:
  - target: Hyperammonemia
    causal_link_type: UNKNOWN
    description: The authors' proposal, from two siblings.
- name: Disrupted Stem Cell Differentiation during Embryogenesis
  biological_scale: CELLULAR
  description: >-
    Hypothesised developmental consequence of excess mitophagy, proposed to
    explain facial dysmorphism, congenital heart malformations and structural
    brain anomalies. Not tested experimentally in an FBXL4 model.
  mechanism_confidence: HYPOTHETICAL
  evidence:
  - reference: PMID:39937392
    reference_title: "FBXL4-Related Mitochondrial Depletion Syndrome Underscores the role of Mitophagy in Stem Cell Differentiation during Embryogenesis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Often, it is associated with structural brain and heart defects, and facial dysmorphism suggesting an embryogenesis defect."
    explanation: The observation motivating the hypothesis.
  downstream:
  - target: Abnormal facial shape
    causal_link_type: UNKNOWN
    hypothesis_groups:
    - embryonic_mitophagy_developmental_defects
  - target: Congenital heart malformation
    causal_link_type: UNKNOWN
    hypothesis_groups:
    - embryonic_mitophagy_developmental_defects
phenotypes:
- category: Metabolic
  name: Lactic acidosis
  description: >-
    Congenital or neonatal persistent lactic acidosis with metabolic acidosis;
    the most consistent finding and part of the presenting pattern. Episodic
    exacerbations accompany intercurrent illness.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
    onset:
      onset_category: NEONATAL
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Elevated blood lactate and metabolic acidosis were observed in all individuals; creatine kinase was elevated in 45% of measurements."
    explanation: Present in all 21 patients of the retrospective series (frequency support).
  - reference: PMID:27743463
    reference_title: "FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients exhibited congenital lactic acidemia, most of them with severe encephalopathic presentation, and global developmental delay."
    explanation: Congenital onset in all 10 newly diagnosed patients.
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay."
    explanation: GeneReviews core feature.
- category: Neurological
  name: Hypotonia
  description: Profound muscular hypotonia from the neonatal period, part of the presenting pattern.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Muscular hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
    onset:
      onset_category: NEONATAL
  evidence:
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All subjects manifested early-onset lactic acidemia, hypotonia, and developmental delay caused by severe encephalomyopathy consistently associated with progressive cerebral atrophy and variable involvement of the white matter, deep gray nuclei, and brainstem structures."
    explanation: Present in all subjects of the discovery series (frequency support).
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation."
    explanation: Part of the characteristic presenting pattern.
  sequelae:
  - target: Polyhydramnios
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Fetal hypotonia and reduced fetal movements are the proposed cause of polyhydramnios.
    evidence:
    - reference: PMID:27099744
      reference_title: "Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This early presentation of polyhydramnios is most likely caused by hypotonia and diminished fetal movements."
      explanation: The authors' explanation of polyhydramnios in their case.
- category: Neurological
  name: Global developmental delay
  description: >-
    Severe psychomotor delay; all survivors in the 21-patient series and all
    living individuals in the GeneReviews summary had significant delay.
    Milder courses are reported.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Severe global developmental delay
    term:
      id: HP:0011344
      label: Severe global developmental delay
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All affected individuals alive at the time they were reported (median age: 3.5 years) demonstrated significant developmental delay."
    explanation: Frequency support in the GeneReviews pooled summary.
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All survivors developed severe psychomotor retardation."
    explanation: Severity in the 21-patient series.
- category: Neurological
  name: Encephalopathy
  description: Early-onset mitochondrial encephalopathy, the defining neurological picture.
  phenotype_term:
    preferred_term: Mitochondrial encephalopathy
    term:
      id: HP:0006789
      label: Mitochondrial encephalopathy
    onset:
      onset_category: NEONATAL
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In conclusion, a clinical pattern of early-onset encephalopathy, persistent lactic acidosis, profound muscular hypotonia and typical facial dysmorphism should prompt initiation of molecular genetic analysis of FBXL4."
    explanation: Early-onset encephalopathy as a diagnostic cue.
  - reference: PMID:23993193
    reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle."
    explanation: Fatal encephalopathy in the discovery kindreds.
- category: Neurological
  name: Cerebral atrophy
  description: >-
    Brain imaging is nonspecific in neonates, followed by a later-onset,
    rapidly progressive brain atrophy.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Progressive cerebral atrophy
    term:
      id: HP:0002059
      label: Cerebral atrophy
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain imaging was non-specific in neonates but a later-onset, rapidly progressive brain atrophy was noted."
    explanation: Evolution of imaging in the 21-patient series.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All subjects manifested early-onset lactic acidemia, hypotonia, and developmental delay caused by severe encephalomyopathy consistently associated with progressive cerebral atrophy and variable involvement of the white matter, deep gray nuclei, and brainstem structures."
    explanation: Progressive atrophy in every subject of the discovery series (frequency support).
- category: Neurological
  name: Leukoencephalopathy
  description: White matter abnormalities on neuroimaging, variable in extent.
  phenotype_term:
    preferred_term: White matter abnormalities
    term:
      id: HP:0002352
      label: Leukoencephalopathy
  evidence:
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined deficiencies of multiple electron transport complexes, and mtDNA depletion."
    explanation: White matter abnormalities in the 87-patient review.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."
    explanation: Leukoencephalopathy in a neonatal case.
- category: Neurological
  name: Seizure
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."
    explanation: GeneReviews lists seizures.
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined deficiencies of multiple electron transport complexes, and mtDNA depletion."
    explanation: Seizures in the 87-patient review.
- category: Neurological
  name: Microcephaly
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined deficiencies of multiple electron transport complexes, and mtDNA depletion."
    explanation: Microcephaly in the 87-patient review.
- category: Neurological
  name: Choreoathetosis
  description: Choreoathetoid movements among the movement disorders of older children.
  phenotype_term:
    preferred_term: Choreoathetosis
    term:
      id: HP:0001266
      label: Choreoathetosis
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."
    explanation: GeneReviews lists movement disorders without naming the type.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic seizures and/or choreoathetoid movements."
    explanation: Choreoathetoid movements in several older children of the discovery series.
- category: Neurological
  name: Ataxia
  description: Severe truncal ataxia in older children.
  phenotype_term:
    preferred_term: Truncal ataxia
    term:
      id: HP:0001251
      label: Ataxia
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."
    explanation: GeneReviews lists ataxia.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic seizures and/or choreoathetoid movements."
    explanation: Truncal ataxia in older children of the discovery series.
- category: Neurological
  name: Stroke-like episode
  phenotype_term:
    preferred_term: Stroke-like episode
    term:
      id: HP:0002401
      label: Stroke-like episode
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes."
    explanation: GeneReviews lists stroke-like episodes.
- category: Neurological
  name: Cerebellar hypoplasia
  description: >-
    Cerebellar or vermian hypoplasia, including on prenatal imaging, reported
    in individual patients.
  phenotype_term:
    preferred_term: Cerebellar hypoplasia
    term:
      id: HP:0001321
      label: Cerebellar hypoplasia
  evidence:
  - reference: PMID:27858371
    reference_title: "Hyperammonemia as a Presenting Feature in Two Siblings with FBXL4 Variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to lactic acidosis and other classic signs of mitochondrial encephalomyopathy."
    explanation: Cerebellar hypoplasia in two siblings.
  - reference: PMID:27099744
    reference_title: "Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe recessive mitochondrial myopathy caused by FBXL4 gene mutations may present prenatally with polyhydramnios and cerebellar hypoplasia."
    explanation: Prenatal cerebellar hypoplasia.
- category: Neurological
  name: Ventriculomegaly
  description: >-
    Ventriculomegaly and periventricular cysts detected from the second
    trimester in prenatal-onset cases, with mega cisterna magna and vermian
    hypoplasia.
  phenotype_term:
    preferred_term: Ventriculomegaly
    term:
      id: HP:0002119
      label: Ventriculomegaly
    onset:
      onset_category: ANTENATAL
  evidence:
  - reference: PMID:40252080
    reference_title: "Prenatal FBXL4-Associated Mitochondrial DNA Depletion Syndrome-13: A New Case and Review of the Literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The literature and our case highlight that the prenatal clinical manifestations can include ventriculomegaly, periventricular cysts, mega cisterna magna, cerebellar vermis hypoplasia, and cardiac anomalies."
    explanation: Prenatal findings across four reported prenatal-onset cases.
  - reference: PMID:36411461
    reference_title: "Prenatal phenotype of FBXL4-associated encephalomyopathic mitochondrial DNA depletion syndrome-13."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Follow-up fetal ultrasonogram at 26 weeks revealed periventricular cysts, periventricular echogenicity, ventriculomegaly, thin corpus callosum, mega cisterna magna, and large cavum."
    explanation: Ventriculomegaly on fetal ultrasound in a single case.
- category: Neurological
  name: Optic atrophy
  phenotype_term:
    preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists optic atrophy among eye findings.
- category: Auditory
  name: Sensorineural hearing impairment
  phenotype_term:
    preferred_term: Sensorineural hearing loss
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists sensorineural hearing loss.
- category: Ophthalmological
  name: Cataract
  phenotype_term:
    preferred_term: Cataract
    term:
      id: HP:0000518
      label: Cataract
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists cataract.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At the age of 3 years she has developed bilateral cataract and horizontal nystagmus and shows failure to thrive and a global psychomotor and developmental delay."
    explanation: Bilateral cataract appearing by age 3 in one patient, so it is not necessarily congenital.
- category: Ophthalmological
  name: Strabismus
  phenotype_term:
    preferred_term: Strabismus
    term:
      id: HP:0000486
      label: Strabismus
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists strabismus.
- category: Ophthalmological
  name: Nystagmus
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists nystagmus.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At the age of 3 years she has developed bilateral cataract and horizontal nystagmus and shows failure to thrive and a global psychomotor and developmental delay."
    explanation: Horizontal nystagmus in one patient.
- category: Cardiovascular
  name: Hypertrophic cardiomyopathy
  phenotype_term:
    preferred_term: Hypertrophic cardiomyopathy
    term:
      id: HP:0001639
      label: Hypertrophic cardiomyopathy
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists hypertrophic cardiomyopathy.
  - reference: PMID:31442532
    reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both patients presented with encephalomyopathy, lactic acidosis and cardiac hypertrophy, which are reported features of FBXL4 impairment."
    explanation: Cardiac hypertrophy in two siblings.
  - reference: PMID:41635899
    reference_title: "The Ketogenic Diet in the Neonatal Intensive Care Setting: The Case of a Preterm Newborn With Mitochondrial DNA Depletion Syndrome Type 13 (MTDPS13)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Echocardiography demonstrated mild, early-onset hypertrophic cardiomyopathy."
    explanation: Neonatal-onset hypertrophic cardiomyopathy in one preterm infant.
- category: Cardiovascular
  name: Congenital heart malformation
  phenotype_term:
    preferred_term: Congenital heart malformation
    term:
      id: HP:0001627
      label: Abnormal heart morphology
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists congenital heart malformations without naming a lesion.
- category: Cardiovascular
  name: Arrhythmia
  phenotype_term:
    preferred_term: Arrhythmia
    term:
      id: HP:0011675
      label: Arrhythmia
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews lists arrhythmias.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."
    explanation: Neonatal arrhythmias in one case.
- category: Cardiovascular
  name: Elevated pulmonary artery pressure
  description: Pulmonary hypertension reported in a neonatal case.
  phenotype_term:
    preferred_term: Pulmonary hypertension
    term:
      id: HP:0004890
      label: Elevated pulmonary artery pressure
  evidence:
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."
    explanation: >-
      Pulmonary hypertension in one neonate. The abstract does not state the
      haemodynamic type, so HP:0004890 is bound rather than HP:0002092
      Pulmonary arterial hypertension, which OAK places under HP:0033578
      Pre-capillary pulmonary hypertension.
- category: Metabolic
  name: Hyperammonemia
  description: >-
    Reported in about 45% of cases, usually modest; extreme values (1495
    micromol/L in one neonate, during an infection) are rare. Can be the
    presenting feature.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Hyperammonemia
    term:
      id: HP:0001987
      label: Hyperammonemia
  evidence:
  - reference: PMID:40161922
    reference_title: "FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome: A rare cause of hyperammonemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "Hyperammonemia is reported in 45 % of cases but extremely elevated ammonia levels are rare."
    explanation: Frequency as restated in the introduction of a later case report; the pooled count behind it is not in this record.
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined deficiencies of multiple electron transport complexes, and mtDNA depletion."
    explanation: Hyperammonemia in the 87-patient review.
  - reference: PMID:27858371
    reference_title: "Hyperammonemia as a Presenting Feature in Two Siblings with FBXL4 Variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "After initial clinical stabilization, both subjects continued to have episodic exacerbations characterized by lactic acidosis and hyperammonemia."
    explanation: Recurrent hyperammonemia in two siblings.
- category: Metabolic
  name: Elevated circulating creatine kinase activity
  description: Raised in 45% of measurements in one series (a per-measurement, not per-patient, figure).
  phenotype_term:
    preferred_term: Elevated creatine kinase
    term:
      id: HP:0003236
      label: Elevated circulating creatine kinase activity
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Elevated blood lactate and metabolic acidosis were observed in all individuals; creatine kinase was elevated in 45% of measurements."
    explanation: Intermittent CK elevation; no per-patient frequency is given, so none is assigned.
- category: Hepatic
  name: Elevated circulating hepatic transaminase concentration
  description: Mildly elevated transaminases.
  phenotype_term:
    preferred_term: Mildly elevated transaminases
    term:
      id: HP:0002910
      label: Elevated circulating hepatic transaminase concentration
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews liver finding.
- category: Hematological
  name: Decreased total neutrophil count
  phenotype_term:
    preferred_term: Neutropenia
    term:
      id: HP:0001875
      label: Decreased total neutrophil count
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews bone marrow finding.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She has chronic lactic acidosis and RTA, global developmental delay (she is nonverbal) with truncal hypotonia, ataxia, and choreoathetoid movements, neutropenia, frequent infections, and severe GI dysmotility and swallowing difficulty, requiring PEG."
    explanation: Neutropenia with frequent infections in one patient.
  sequelae:
  - target: Recurrent infections
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Neutropenia and lymphopenia are the plausible basis of the recurrent
      infections; no study has tested the link in this disease.
- category: Hematological
  name: Decreased total lymphocyte count
  phenotype_term:
    preferred_term: Lymphopenia
    term:
      id: HP:0001888
      label: Decreased total lymphocyte count
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia)."
    explanation: GeneReviews bone marrow finding.
  - reference: PMID:26404457
    reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe the detailed clinical and biochemical phenotype of a neonate presenting with hyperlactatemia, leukoencephalopathy, arrhythmias, pulmonary hypertension, dysmorphic features, and lymphopenia."
    explanation: Lymphopenia in a neonatal case.
  sequelae:
  - target: Recurrent infections
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- category: Immunological
  name: Recurrent infections
  phenotype_term:
    preferred_term: Recurrent infections
    term:
      id: HP:0002719
      label: Recurrent infections
  evidence:
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other features are feeding difficulties, growth failure, microcephaly, hyperammonemia, seizures, hypertrophic cardiomyopathy, elevated liver transaminases, recurrent infections, variable distinctive facial features, white matter abnormalities and cerebral atrophy found in neuroimaging, combined deficiencies of multiple electron transport complexes, and mtDNA depletion."
    explanation: Recurrent infections in the 87-patient review.
  sequelae:
  - target: Lactic acidosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Intercurrent infections precipitate metabolic decompensation; three
      children in one discovery series died of it.
- category: Growth
  name: Failure to thrive
  description: Growth failure with feeding problems from the first months of life; can be extreme.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
    onset:
      onset_category: NEONATAL
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation."
    explanation: Part of the presenting pattern.
  - reference: PMID:30804983
    reference_title: "FBXL4-Related Mitochondrial DNA Depletion Syndrome 13 (MTDPS13): A Case Report With a Comprehensive Mutation Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report here the case of a Lebanese infant presenting to us with profound neurodevelopmental delays, generalized hypotonia, facial dysmorphic features, and extreme emaciation."
    explanation: Extreme emaciation in one infant.
- category: Gastrointestinal
  name: Feeding difficulties
  phenotype_term:
    preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation."
    explanation: Part of the presenting pattern.
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay."
    explanation: GeneReviews core feature.
  sequelae:
  - target: Failure to thrive
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- category: Gastrointestinal
  name: Gastrointestinal dysmotility
  phenotype_term:
    preferred_term: Gastrointestinal dysmotility
    term:
      id: HP:0002579
      label: Gastrointestinal dysmotility
  evidence:
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A wide range of other multisystem features were variably seen, including dysmorphism, skeletal abnormalities, poor growth, gastrointestinal dysmotility, renal tubular acidosis, seizures, and episodic metabolic failure."
    explanation: Variably present in the discovery series.
  sequelae:
  - target: Feeding difficulties
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- category: Renal
  name: Renal tubular acidosis
  phenotype_term:
    preferred_term: Renal tubular acidosis
    term:
      id: HP:0001947
      label: Renal tubular acidosis
  evidence:
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A wide range of other multisystem features were variably seen, including dysmorphism, skeletal abnormalities, poor growth, gastrointestinal dysmotility, renal tubular acidosis, seizures, and episodic metabolic failure."
    explanation: Variably present in the discovery series.
- category: Craniofacial
  name: Abnormal facial shape
  description: >-
    Facial dysmorphism in two thirds of cases; described features include high
    arched eyebrows, a triangular face, slightly upslanting palpebral fissures
    and a prominent pointed chin.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Facial dysmorphism
    term:
      id: HP:0001999
      label: Abnormal facial shape
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Facial dysmorphic features are present in 67% of cases."
    explanation: Frequency in the 21-patient series (FREQUENT band).
  - reference: PMID:27099744
    reference_title: "Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Characteristic dysmorphic features are: high and arched eyebrows, triangular face, a slight upslant of palpebral fissures, and a prominent pointed chin."
    explanation: Description of the facial gestalt.
- category: Prenatal
  name: Polyhydramnios
  phenotype_term:
    preferred_term: Polyhydramnios
    term:
      id: HP:0001561
      label: Polyhydramnios
    onset:
      onset_category: ANTENATAL
  evidence:
  - reference: PMID:27099744
    reference_title: "Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe recessive mitochondrial myopathy caused by FBXL4 gene mutations may present prenatally with polyhydramnios and cerebellar hypoplasia."
    explanation: Prenatal presentation with polyhydramnios.
  - reference: PMID:41635899
    reference_title: "The Ketogenic Diet in the Neonatal Intensive Care Setting: The Case of a Preterm Newborn With Mitochondrial DNA Depletion Syndrome Type 13 (MTDPS13)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report the case of a male preterm neonate born at 31 + 3 weeks of gestation following a pregnancy marked by severe polyhydramnios."
    explanation: Severe polyhydramnios in a second case.
biochemical:
- name: Blood lactate
  presence: PRESENT
  notes: Raised in all 21 patients of one series, from the neonatal period; pyruvate is raised as well.
  biomarker_term:
    preferred_term: Increased circulating lactate concentration
    term:
      id: HP:0002151
      label: Increased circulating lactate concentration
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Elevated blood lactate and metabolic acidosis were observed in all individuals; creatine kinase was elevated in 45% of measurements."
    explanation: Universal lactate elevation.
  - reference: PMID:27099744
    reference_title: "Polyhydramnios and cerebellar atrophy: a prenatal presentation of mitochondrial encephalomyopathy caused by mutations in the FBXL4 gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Metabolic investigations invariably show increased serum lactate and pyruvate levels."
    explanation: Lactate and pyruvate both raised.
- name: Plasma ammonia
  presence: PRESENT
  notes: Raised in about 45% of reported cases; one neonate reached 1495 micromol/L.
  biomarker_term:
    preferred_term: Hyperammonemia
    term:
      id: HP:0001987
      label: Hyperammonemia
  evidence:
  - reference: PMID:40161922
    reference_title: "FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome: A rare cause of hyperammonemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A male infant presented with dysmorphic features, hypotonia, failure to thrive, and lactic acidosis and severe hyperammonemia (ammonia: 1495 μmol/L)."
    explanation: Extreme hyperammonemia in one neonate.
- name: Plasma aspartate
  presence: PRESENT
  notes: >-
    Low plasma aspartate with low urinary TCA cycle intermediates in two
    siblings. No biomarker_term is bound: sqlite:obo:hp search
    "l~Decreased circulating aspartate" returned no term.
  evidence:
  - reference: PMID:27858371
    reference_title: "Hyperammonemia as a Presenting Feature in Two Siblings with FBXL4 Variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified a pair of siblings with mutations in FBXL4 who each presented in the neonatal period with hyperammonemia, low plasma levels of aspartate, low urine levels of tricarboxylic acid cycle intermediates suggesting a defect in anaplerosis, and cerebellar hypoplasia in addition to lactic acidosis and other classic signs of mitochondrial encephalomyopathy."
    explanation: Low plasma aspartate in two siblings.
- name: Muscle mtDNA content
  presence: PRESENT
  notes: Reduced, usually severely, in skeletal muscle; less severe or normal in fibroblasts.
  biomarker_term:
    preferred_term: Depletion of mitochondrial DNA in muscle tissue
    term:
      id: HP:0009141
      label: Depletion of mitochondrial DNA in muscle tissue
  evidence:
  - reference: PMID:31969900
    reference_title: "Molecular Characterization of New FBXL4 Mutations in Patients With mtDNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe mtDNA depletion was detected in muscle biopsy in both patients."
    explanation: Muscle mtDNA depletion in two further patients.
genetic:
- name: FBXL4
  gene_term:
    preferred_term: FBXL4
    term:
      id: hgnc:13601
      label: FBXL4
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    47 pathogenic variants in 87 individuals by 2017; null (nonsense,
    frameshift, splice) and missense variants both occur. No clear
    genotype-phenotype correlation has been established: a homozygous
    frameshift was proposed to lie at the severe end, a homozygous nonsense
    variant has been reported with a milder course, and one missense variant
    gave different courses in unrelated carriers.
  evidence:
  - reference: PMID:28940506
    reference_title: "Molecular and clinical spectra of FBXL4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Biallelic pathogenic variants in FBXL4 are associated with an encephalopathic mtDNA maintenance defect syndrome that is a multisystem disease characterized by lactic acidemia, developmental delay, and hypotonia."
    explanation: Gene-disease association summarised over 87 patients.
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome sequencing and autozygosity mapping studies, independently performed in subjects with defective combined mitochondrial OXPHOS-enzyme deficiencies, identified a total of nine disease-segregating FBXL4 mutations in seven unrelated mitochondrial disease families, composed of six singletons and three siblings."
    explanation: Independent discovery in seven families.
  - reference: PMID:27743463
    reference_title: "FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Definitive diagnosis was made in 10 new subjects with a total of 7 novel deleterious variants; 5 null and 2 missense substitutions."
    explanation: Null and missense variant classes.
  variants:
  - name: c.1641_1642delTG (homozygous frameshift)
    description: Homozygous frameshift in a neonate; proposed by the authors to represent the severe end of the spectrum.
    evidence:
    - reference: PMID:26404457
      reference_title: "Detailed Biochemical and Bioenergetic Characterization of FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In light of our patient's clinical findings and genotype (homozygous frameshift), this phenotype likely represents the severe end of the FBXL4 clinical spectrum."
      explanation: Authors' severity interpretation for a single patient.
  - name: c.1303C>T
    description: Missense variant reported in three unrelated individuals with different clinical courses.
    evidence:
    - reference: PMID:30804983
      reference_title: "FBXL4-Related Mitochondrial DNA Depletion Syndrome 13 (MTDPS13): A Case Report With a Comprehensive Mutation Review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Finally, we provide a direct comparison of the disorder's clinical manifestations across two unrelated patients harboring the same disease-causing mutation as our patient, emphasizing the remarkable variability in genotype-to-phenotype correlation characteristic of the disease."
      explanation: Same variant, variable phenotype.
  - name: c.1750T>C (p.Cys584Arg)
    description: Homozygous missense in two siblings with cardiac hypertrophy; unlike wild-type FBXL4 the variant does not promote mitochondrial fusion.
    evidence:
    - reference: PMID:31442532
      reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We report two siblings, from consanguineous parents, harbouring a previously uncharacterized homozygous variant in FBXL4 (c.1750 T > C; p.Cys584Arg)."
      explanation: Identification in two siblings.
  - name: c.486T>G (p.Tyr162Ter)
    description: Homozygous nonsense variant in a 4.5-year-old boy with a milder course than usual.
    evidence:
    - reference: PMID:34602956
      reference_title: "A Mild Phenotype of Mitochondrial DNA Depletion Syndrome Type 13 with a Novel FBXL4 Variant."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "While most of the patients with FBLX4 gene mutation have severe clinical manifestation and die at a very young age, clinical progress of our case was milder than previously reported."
      explanation: Milder course despite a truncating variant.
progression:
- phase: Prenatal and neonatal onset
  notes: >-
    Onset is congenital or in early infancy. Prenatal onset with brain changes
    from the second trimester, polyhydramnios and cardiac anomalies is reported
    in a few cases.
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay."
    explanation: Congenital or early onset.
  - reference: PMID:40252080
    reference_title: "Prenatal FBXL4-Associated Mitochondrial DNA Depletion Syndrome-13: A New Case and Review of the Literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This case illustrates the prenatal onset of MTDPS13, with central nervous system abnormalities apparent from the second trimester."
    explanation: Prenatal onset in a subset.
- phase: Survival
  notes: >-
    Median age at reported death two years (range 2 days to 75 months), with
    survivors up to 36 years. In the 21-patient series 7 children had died at a
    mean age of 37 months and 11 were alive at a mean follow-up of 46 months.
    Deaths during metabolic decompensation with intercurrent infection are
    described.
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Survival varies; the median age of reported deaths was two years (range 2 days – 75 months), although surviving individuals as old as 36 years have been reported."
    explanation: GeneReviews survival summary.
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Seven children died (mean age 37 months); 11 children were alive (mean age at follow-up 46 months), three children were lost to follow-up."
    explanation: Outcome in the 21-patient series.
diagnosis:
- name: FBXL4 molecular genetic testing
  description: >-
    Identification of biallelic pathogenic FBXL4 variants establishes the
    diagnosis. Early-onset encephalopathy with persistent lactic acidosis,
    profound hypotonia and the typical facial dysmorphism should prompt FBXL4
    analysis, and FBXL4 belongs on mitochondrial panels used for congenital
    lactic acidosis.
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of FBXL4-related mtDNA depletion syndrome is established in a proband by identification of biallelic pathogenic variants in FBXL4 on molecular genetic testing."
    explanation: GeneReviews diagnostic criterion.
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In conclusion, a clinical pattern of early-onset encephalopathy, persistent lactic acidosis, profound muscular hypotonia and typical facial dysmorphism should prompt initiation of molecular genetic analysis of FBXL4."
    explanation: Clinical pattern that should trigger testing.
  - reference: PMID:27743463
    reference_title: "FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Including FBLX4 in the mitochondrial diseases panel should be particularly important for patients with congenital lactic acidosis."
    explanation: Panel inclusion recommendation.
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Establishment of the diagnosis permits genetic counselling, prevents patients undergoing unhelpful diagnostic procedures and allows for accurate prognosis."
    explanation: Value of an early molecular diagnosis.
- name: Muscle biopsy
  description: >-
    Shows mtDNA depletion, a paucity of mitochondria and a combined respiratory
    chain deficiency; supportive, but the diagnosis rests on molecular testing.
  evidence:
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Diagnostic workup in patient tissues and cells revealed a severe combined respiratory chain defect with a general decrease of enzymes associated with mitochondrial energy metabolism and a relative depletion of mitochondrial DNA content."
    explanation: Tissue findings on diagnostic workup.
- name: Surveillance
  description: >-
    No published surveillance guideline; follow-up of eyes, hearing, heart,
    feeding, liver, neurological complications and neutropenia is set by the
    treating physician according to the findings.
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The treating physician should decide about the frequency of follow up of eyes, hearing, heart, feeding difficulties, liver, neurologic complications, and neutropenia based on the patient's findings."
    explanation: GeneReviews surveillance statement.
- name: Carrier and prenatal testing
  description: Once both familial variants are known, carrier testing, prenatal testing and preimplantation genetic testing are possible.
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Once the FBXL4 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing are possible."
    explanation: GeneReviews genetic counseling statement.
environmental:
- name: Intercurrent infection
  description: >-
    Intercurrent infections precipitate metabolic decompensation with worsening
    lactic acidosis and hyperammonemia and account for deaths in infancy.
  notes: >-
    ECTO has no general infection-exposure class: sqlite:obo:ecto info
    "l~infection" returns ECTO:2000053 exposure to Human Papillomavirus
    Infection and NCIT:C27851 Human Papillomavirus Infection, both specific to
    one pathogen, so the general ECTO:3000000 exposure to organism is bound.
  exposure_term:
    preferred_term: intercurrent infection
    term:
      id: ECTO:3000000
      label: exposure to organism
  influences_mechanisms:
  - target: Lactic acidosis
    environmental_effect: EXACERBATES
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Infection-associated metabolic decompensation.
    evidence:
    - reference: PMID:23993194
      reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic seizures and/or choreoathetoid movements."
      explanation: Fatal decompensation during intercurrent infections.
  - target: Hyperammonemia
    environmental_effect: EXACERBATES
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40161922
      reference_title: "FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome: A rare cause of hyperammonemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "At 40 days old, the patient experienced metabolic acidosis and a hyperammonemic episode (ammonia level: 1495 μmol/L) triggered by an infection"
      explanation: Infection-triggered hyperammonemic episode in one neonate.
  evidence:
  - reference: PMID:23993194
    reference_title: "Mutations in FBXL4, encoding a mitochondrial protein, cause early-onset mitochondrial encephalomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, three children died in infancy owing to metabolic decompensation during intercurrent infections, and most of those that have reached late childhood are nonverbal, are unable to sit autonomously, show muscle wasting and severe truncal ataxia, and, in several cases, suffer of epileptic seizures and/or choreoathetoid movements."
    explanation: Infection as the setting of fatal decompensation.
treatments:
- name: Multidisciplinary Supportive Care
  description: >-
    No definite treatment exists. Management is supportive, by a
    multidisciplinary team, with adequate nutrition and standard treatment of
    developmental delay, seizures, cardiac complications, eye involvement and
    hearing loss.
  treatment_term:
    preferred_term: multidisciplinary supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To date no definite treatment is available; thus, treatment is mainly supportive: assuring adequate nutrition and standard treatment of neurologic complications including developmental delay / intellectual disability, seizures, cardiac complications, eye involvement, and hearing loss."
    explanation: GeneReviews management.
- name: Nutritional Support
  description: Assuring adequate nutrition for the feeding difficulty and growth failure.
  treatment_term:
    preferred_term: nutritional support
    term:
      id: NCIT:C15433
      label: Nutritional Support
  target_phenotypes:
  - preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To date no definite treatment is available; thus, treatment is mainly supportive: assuring adequate nutrition and standard treatment of neurologic complications including developmental delay / intellectual disability, seizures, cardiac complications, eye involvement, and hearing loss."
    explanation: GeneReviews management.
- name: Mitochondrial Cofactors and Antioxidants
  description: >-
    Cofactor and antioxidant supplements used across mitochondrial disease may
    be considered, with generally limited evidence of benefit; in the
    21-patient series such "mitochondrial medications" did not prove effective.
    The cited abstracts do not name the agents, so none is bound.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  evidence:
  - reference: PMID:28383868
    reference_title: "FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Administration of cofactors and antioxidants, used in mitochondrial disorders with (generally) limited evidence of benefit, may be considered."
    explanation: GeneReviews permits a trial.
  - reference: PMID:25868664
    reference_title: "Clinical, morphological, biochemical, imaging and outcome parameters in 21 individuals with mitochondrial maintenance defect related to FBXL4 mutations."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "Treatment with \"mitochondrial medications\" did not prove effective."
    explanation: No benefit observed in the retrospective 21-patient series.
- name: Dichloroacetate
  description: >-
    Pyruvate dehydrogenase activator. In one of two siblings with the
    p.Cys584Arg variant it improved metabolic acidosis and reversed cardiac
    hypertrophy; in patient fibroblasts it reduced acidification without
    correcting other mitochondrial functions. In C. elegans, zebrafish and
    human fibroblast models it improved survival, neuromuscular function and
    mitochondrial physiology. No controlled human trial has been reported.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: dichloroacetate
      term:
        id: CHEBI:28240
        label: dichloroacetate
  target_phenotypes:
  - preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
  - preferred_term: Hypertrophic cardiomyopathy
    term:
      id: HP:0001639
      label: Hypertrophic cardiomyopathy
  target_mechanisms:
  - target: Lactic acidosis
    description: Drives pyruvate into mitochondrial oxidation, lowering lactate.
  - target: Cardiomyocyte Energy Failure
    description: Cardiac hypertrophy reversed in one patient.
  evidence:
  - reference: PMID:31442532
    reference_title: "Characterization of the C584R variant in the mtDNA depletion syndrome gene FBXL4, reveals a novel role for FBXL4 as a regulator of mitochondrial fusion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Remarkably, dichloroacetate (DCA) administration to the younger sibling improved metabolic acidosis and reversed cardiac hypertrophy."
    explanation: Single-patient clinical response.
  - reference: PMID:35881484
    reference_title: "Dichloroacetate improves mitochondrial function, physiology, and morphology in FBXL4 disease models."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Validation studies were performed in fbxl4sa12470 zebrafish larvae and in FBXL4-/- human fibroblasts; they showed DCA efficacy in preventing brain death, impairment of neurologic and/or muscular function, mitochondrial biochemical dysfunction, and stress-induced morphologic and ultrastructural mitochondrial defects."
    explanation: Preclinical efficacy in zebrafish; the same sentence covers fibroblasts, which are not graded separately here.
- name: Ketogenic Diet
  description: >-
    Parenteral then enteral ketogenic diet lowered lactate in a 4-year-old girl
    whose lactic acidosis during sepsis did not respond to drugs, and in a
    preterm neonate lactate fell and acid-base balance and growth improved
    within three days. Single-patient reports.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: ketogenic diet
    term:
      id: NCIT:C173168
      label: Ketogenic Diet
  target_phenotypes:
  - preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
  evidence:
  - reference: PMID:33882172
    reference_title: "The first case with FBXL4 mutation successfully treated with a parenteral ketogenic diet for lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "During the disease course, lactic acidosis became prominent and did not respond to pharmacological treatment; standard PN was gradually switched to parenteral KD, and lactate levels decreased after parenteral KD."
    explanation: Single-case response.
  - reference: PMID:41635899
    reference_title: "The Ketogenic Diet in the Neonatal Intensive Care Setting: The Case of a Preterm Newborn With Mitochondrial DNA Depletion Syndrome Type 13 (MTDPS13)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "After 3 days of this treatment approach, a significant reduction in lactate levels and improvement in acid-base balance and growth trend were observed along with clinical and cardiovascular parameters."
    explanation: Single-case response in a preterm neonate.
- name: Deoxycytidine and Deoxythymidine
  description: >-
    Oral pyrimidine deoxynucleosides, investigational. An interim open-label
    analysis pooled eight children with FBXL4, SUCLG1, SUCLA2 or RRM2B variants;
    Newcastle Mitochondrial Disease Scale scores improved at group level. The
    number of FBXL4 patients and their individual results are not given in the
    abstract, the study is uncontrolled, and there is no FBXL4-specific
    mechanistic rationale, since the defect is excess mitochondrial removal
    rather than nucleotide supply.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: deoxycytidine
      term:
        id: CHEBI:15698
        label: 2'-deoxycytidine
    - preferred_term: deoxythymidine
      term:
        id: CHEBI:17748
        label: thymidine
  evidence:
  - reference: PMID:40175578
    reference_title: "Investigating the safety and efficacy of deoxycytidine/deoxythymidine in mitochondrial DNA depletion disorders: phase 2 open-label trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Data were available from eight individuals having pathogenic variants in FBXL4, SUCLG1, SUCLA2, or RRM2B."
    explanation: FBXL4 is among the pooled genotypes.
  - reference: PMID:40175578
    reference_title: "Investigating the safety and efficacy of deoxycytidine/deoxythymidine in mitochondrial DNA depletion disorders: phase 2 open-label trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Newcastle Mitochondrial Disease Scale score improved in all individuals except for one who withdrew before the first follow-up visit; group level analysis was significant at 1-month and 6-month timepoints."
    explanation: Pooled open-label result, not specific to FBXL4.
- name: AMPK Activation (preclinical)
  description: >-
    Small-molecule AMPK activators, including PXL-770, inhibited BNIP3/NIX
    mitophagy and restored mitochondrial content and respiration in
    FBXL4-deficient cells and patient fibroblasts, and inhibited mitophagy in
    liver and brain of a chemically induced mouse model. Not tested in patients.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_mechanisms:
  - target: Excessive Basal Mitophagy
    description: AMPK sequesters the ULK1 autophagy-initiating kinase and so restrains mitophagy.
  evidence:
  - reference: PMID:42310391
    reference_title: "Activation of AMPK as a therapeutic strategy for FBXL4-related mitochondrial DNA depletion syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Using FBXL4-deficient cells, as well as fibroblasts derived from MTDPS13 patients and a chemically-induced mouse model, we show that small molecule AMPK activation inhibits BNIP3/NIX-mediated mitophagy and recovers functional mitochondrial content."
    explanation: Preclinical rescue of the mitophagy node, mainly in cells.
clinical_trials:
- name: NCT04802707
  phase: PHASE_II
  description: >-
    Open-label, single-arm phase II study of combination deoxycytidine and
    deoxythymidine in children with mitochondrial DNA depletion syndromes; FBXL4
    is among the eligible genotypes. Recruitment status is not recorded because
    the cached registry summary carries no status field.
  target_phenotypes:
  - preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: clinicaltrials:NCT04802707
    reference_title: "A Phase II, Monocenter, Single Arm Study To Assess The Safety and Efficacy Of Combination Deoxycytidine and Deoxythymidine For Mitochondrial Depletion Disorders"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The subjects included are children (0-18Y), with positive MDS diagnosis
      and express mutations in one of the following genes: POLG, POLG2, C10orf2,
      RRM2B, MPV17, SUCLA2, SUCLG1, FBXL4, DTYMK.
    explanation: FBXL4 is an eligible genotype.
animal_models:
- name: Fbxl4 knockout mouse (Alsina et al.)
  species: Mouse
  genotype: Fbxl4-/-
  publication: PMID:32525278
  description: >-
    Predominant perinatal lethality; rare survivors appear normal until 8-12
    months, then develop mitochondrial dysfunction, weight loss, a global
    reduction of mitochondrial proteins and mtDNA depletion.
  modeled_mechanisms:
  - target: Reduced Mitochondrial Content
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: TISSUE
    limitations: >-
      Survivors are asymptomatic for most of the first year, unlike the
      neonatal onset in patients; the authors argue that the mitochondrial
      phenotype is not the leading cause of the perinatal death, since embryos
      develop normally until E13.5 despite lower mtDNA.
    evidence:
    - reference: PMID:32525278
      reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "One-year-old Fbxl4 knockouts show a global reduction in a variety of mitochondrial proteins and mtDNA depletion, whereas lysosomal proteins are upregulated."
      explanation: Reduced mitochondrial content in knockout mouse tissue.
    - reference: PMID:32525278
      reference_title: "FBXL4 deficiency increases mitochondrial removal by autophagy."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "The finding that Fbxl4 −/− embryos are present at Mendelian ratios and develop normally until E13.5 despite having somewhat lower mtDNA levels argues that the mitochondrial phenotype is not the leading cause of the perinatal death."
      explanation: The authors' caveat on the cause of perinatal death.
- name: Fbxl4 knockout mouse with Bnip3 or Nix co-deletion (Cao et al.)
  species: Mouse
  genotype: Fbxl4-/- with Bnip3-/- or Nix-/-
  publication: PMID:36896912
  description: >-
    Fbxl4-/- mice have raised BNIP3 and NIX, hyperactive mitophagy and perinatal
    lethality; deleting either Bnip3 or Nix represses the mitophagy, corrects
    metabolic derangements and rescues viability.
  modeled_mechanisms:
  - target: Excessive Basal Mitophagy
    relationship: RESCUES
    fidelity: HIGH
    model_scale: ORGANISM
    limitations: >-
      Genetic rescue in mice; receptor knockout is not a clinical intervention,
      and neurological outcome in a model with the human neonatal course has
      not been tested.
    evidence:
    - reference: PMID:36896912
      reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Importantly, knockout of either Bnip3 or Nix rescues metabolic derangements and viability of the Fbxl4-/- mice."
      explanation: Genetic rescue places receptor-driven mitophagy upstream of lethality.
- name: Fbxl4 patient-variant knock-in mouse (Chen et al.)
  species: Mouse
  genotype: knock-in of a patient-derived FBXL4 variant
  publication: PMID:37568009
  description: Knock-in mice carrying a patient-derived FBXL4 variant show excessive mitophagy.
  modeled_mechanisms:
  - target: Excessive Basal Mitophagy
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: TISSUE
    limitations: The abstract does not name the variant or report the whole-animal phenotype of the mice.
    evidence:
    - reference: PMID:37568009
      reference_title: "FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Excessive mitophagy was observed in Knockin (KI) mice carrying a patient-derived FBXL4 mutation and cortical neurons (CNs)-induced from MTDPS13 patient human induced pluripotent stem cells (hiPSCs)."
      explanation: Patient allele reproduces the mitophagy state in vivo.
- name: fbxl-1(ok3741) C. elegans
  species: Caenorhabditis elegans
  genotype: fbxl-1(ok3741)
  publication: PMID:35881484
  description: >-
    Developmental delay, impaired fecundity, reduced neuromuscular activity,
    mitochondrial dysfunction and altered lactate metabolism; improved by
    dichloroacetate.
  modeled_mechanisms:
  - target: Combined Respiratory Chain Deficiency
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: ORGANISM
    limitations: >-
      Invertebrate ortholog model; whether fbxl-1 regulates BNIP3/NIX-type
      mitophagy receptors in worms is not stated in the abstract.
    evidence:
    - reference: PMID:35881484
      reference_title: "Dichloroacetate improves mitochondrial function, physiology, and morphology in FBXL4 disease models."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Developmental delay, impaired fecundity and neurologic and/or muscular activity, mitochondrial dysfunction, and altered lactate metabolism were identified in fbxl-1(ok3741) C. elegans."
      explanation: Mitochondrial and lactate phenotypes in the worm model.
- name: fbxl4 sa12470 zebrafish
  species: Zebrafish
  genotype: fbxl4sa12470
  publication: PMID:35881484
  description: Zebrafish larvae used to validate dichloroacetate against brain death and neuromuscular impairment.
  modeled_mechanisms:
  - target: Neuronal Energy Failure
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: ORGANISM
    limitations: >-
      The abstract reports protection by dichloroacetate but does not describe
      the untreated phenotype in detail, and the morphological mitochondrial
      defects are stress-induced rather than baseline.
    evidence:
    - reference: PMID:35881484
      reference_title: "Dichloroacetate improves mitochondrial function, physiology, and morphology in FBXL4 disease models."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Validation studies were performed in fbxl4sa12470 zebrafish larvae and in FBXL4-/- human fibroblasts; they showed DCA efficacy in preventing brain death, impairment of neurologic and/or muscular function, mitochondrial biochemical dysfunction, and stress-induced morphologic and ultrastructural mitochondrial defects."
      explanation: Brain death and neuromuscular impairment in the larvae are prevented by DCA.
experimental_models:
- name: FBXL4 knockout human cell lines
  experimental_model_type: CELL_LINE
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  description: >-
    CRISPR/Cas9 FBXL4 knockout human cells used by several groups to define the
    ligase, its substrates and the mitophagy phenotype.
  publication: PMID:36896912
  modeled_mechanisms:
  - target: Excessive Basal Mitophagy
    relationship: RECAPITULATES
    fidelity: HIGH
    model_scale: CELLULAR
    limitations: Transformed cell lines lack tissue context and systemic metabolism.
    evidence:
    - reference: PMID:36896912
      reference_title: "A mitochondrial SCF-FBXL4 ubiquitin E3 ligase complex degrades BNIP3 and NIX to restrain mitophagy and prevent mitochondrial disease."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Subsequent counter screen revealed that FBXL4-KO hyperactivates mitophagy via two mitophagy receptors BNIP3 and NIX."
      explanation: Receptor-dependent mitophagy in knockout cells.
- name: Patient iPSC-derived cortical neurons
  experimental_model_type: IPSC_DERIVED_MODEL
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  description: Cortical neurons differentiated from MTDPS13 patient iPSCs.
  publication: PMID:37568009
  modeled_mechanisms:
  - target: Excessive Basal Mitophagy
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: CELLULAR
    limitations: >-
      Shows the mitophagy state in human neurons; neuronal survival and
      bioenergetics are not reported in the abstract.
    evidence:
    - reference: PMID:37568009
      reference_title: "FBXL4 mutations cause excessive mitophagy via BNIP3/BNIP3L accumulation leading to mitochondrial DNA depletion syndrome."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Furthermore, we observed abnormal activation of mitophagy in cortical neurons induced from hiPSCs derived from MTDPS13 patients."
      explanation: Patient-derived neurons show the mitophagy phenotype.
- name: Patient fibroblasts
  experimental_model_type: PRIMARY_CELL_CULTURE
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  description: Skin fibroblasts from patients with biallelic FBXL4 variants.
  publication: PMID:23993193
  modeled_mechanisms:
  - target: Combined Respiratory Chain Deficiency
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: CELLULAR
    limitations: >-
      mtDNA depletion is milder or absent in fibroblasts compared with muscle,
      and mitochondrial mass findings differ between lines.
    evidence:
    - reference: PMID:23993193
      reference_title: "Mutations in FBXL4 cause mitochondrial encephalopathy and a disorder of mitochondrial DNA maintenance."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We show that FBXL4 is an F-box protein that colocalizes with mitochondria and that loss-of-function and splice mutations in this protein result in a severe respiratory chain deficiency, loss of mitochondrial membrane potential, and a disturbance of the dynamic mitochondrial network and nucleoid distribution in fibroblasts from affected individuals."
      explanation: Respiratory chain deficiency in patient fibroblasts.
discussions:
- discussion_id: fbxl4-mitochondrial-mass-discordance
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why is mitochondrial mass reduced in knockout cells, knockout mouse tissue
    and patient muscle but spared or increased in some patient fibroblast
    lines?
  attaches_to:
  - pathophysiology#Reduced Mitochondrial Content
  - pathophysiology#Excessive Basal Mitophagy
  rationale: >-
    The canonical model predicts fewer mitochondria, which knockout cells,
    knockout mice and patient muscle show, while one neonatal fibroblast line
    had relatively spared mass and two other lines had increased mass, one of
    them with normal mtDNA. Whether this reflects culture conditions, residual
    function of particular alleles or compensatory biogenesis is not resolved.
- discussion_id: fbxl4-developmental-defects
  kind: KNOWLEDGE_GAP
  prompt: >-
    Do the facial dysmorphism, congenital heart malformations and prenatal brain
    anomalies arise from excess mitophagy during embryonic differentiation, or
    from energy failure?
  attaches_to:
  - pathophysiology#Disrupted Stem Cell Differentiation during Embryogenesis
  - phenotypes#Abnormal facial shape
  - phenotypes#Congenital heart malformation
  rationale: >-
    The developmental features are not explained by the postnatal
    energy-failure chain, and the embryonic-mitophagy explanation is a
    hypothesis without an FBXL4 experiment.

📚

References & Deep Research

References

1

PMID:28383868

FBXL4-Related Encephalomyopathic Mitochondrial DNA Depletion Syndrome.

No top-level findings curated for this source.

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: FBXL4-Related_Mitochondrial_DNA_Depletion_Syndrome · 2026-09-29T00:30:29Z · View source

New entry for MONDO:0014198 (FBXL4, OMIM 615471). The stub stubs/Mitochondrial_DNA_Depletion_Syndrome_13.yaml is deleted. Deep research: one Perplexity sonar-deep-research report at the default (medium) reasoning effort. It was produced through a session-local streaming wrapper (sitecustomize.py on PYTHONPATH, not committed), because the egress proxy drops the non-streaming call after about 300 s (dismech#9357). The first term-validation pass aborted on an OLS timeout for a MedGen CURIE and was re-run with --skip-prefix MedGen. just preflight-dr against MONDO:0014198 returned PASS (FBXL4 mentioned 281 times, OMIM 615471 matches). Perplexity cites bare URLs, so the report's reference validation found no PMID or DOI (dismech#10249). Each of the 18 URLs in the .citations.md sidecar was resolved by hand. PMID 23993193 (Bonnen) and PMID 25868664 (Huemer) are cited. PMC5580732 is Antoun et al. 2016 (PMID 26404457), not the Gai et al. paper the body names. The report quotes "profound reductions in complex I-, II- and ETF-linked..." for it, and that text is not in the Antoun abstract, so it was not used. PMC6370620 is Ballout et al. 2019 (PMID 30804983). The EMBO Mol Med DOI is PMID 32525278 and was cited by its PMID. The Autophagy commentary is PMID 38423516. The Frontiers cardiovascular review (DOI 10.3389/fcvm.2025.1582219) was not cited: the report's "54% cardiac involvement" figures and its HFpEF FBXL4-transfection claim are not in the abstract, and the report's [11] is used for both this review and GeneReviews. The OMIM, MedGen, Orphanet, ClinVar, NCBI Gene, MedlinePlus and GTR URLs are not fetchable references and were not used. The report's hyperammonemia "about half" figure comes from MedlinePlus; the entry uses the 45% figure quoted from PMID 40161922 instead, marked quote_role BACKGROUND. Additional sources were found by a PubMed FBXL4[tiab] search. They are the GeneReviews chapter PMID 28383868 (tagged and mined for clinical characteristics, diagnosis, management, surveillance and genetic counselling), Gai 2013, El-Hattab 2017 (87 patients), Dai 2017, the 2023 BNIP3/NIX ligase papers (PMIDs 36896912, 37161784, 37102372, 37568009, 37876279), PPTC7 (38151018), the AMPK preclinical study (42310391), dichloroacetate (31442532, 35881484), ketogenic diet case reports (33882172, 41635899), the deoxynucleoside trial (40175578, NCT04802707), prenatal cases (36411461, 40252080, 27099744), hyperammonemia cases (27858371, 40161922), a mild-course case (34602956), a fibroblast study (31969900) and a commentary (39937392). PMIDs 26421988 and 34738379 cached with no quotable text and were not used. Pathograph: FBXL4 loss of function -> failed SCF-FBXL4 ubiquitination of BNIP3/NIX -> excessive basal mitophagy -> reduced mitochondrial content -> mtDNA depletion and combined respiratory chain deficiency. The respiratory chain deficiency leads to lactic acidosis and to neuronal, muscle and cardiomyocyte energy-failure nodes, which feed the phenotypes. Side branches cover mitochondrial fusion (PMID 31442532), anaplerosis and hyperammonemia (PMID 27858371, hypothetical), and an embryonic-mitophagy hypothesis for the developmental features (PMID 39937392, EMERGING). Three hypothesis groups are recorded: canonical BNIP3/NIX mitophagy, an alternative primary-OXPHOS reading (PMID 31969900), and the emerging embryonic hypothesis. Conflicting fibroblast findings on mitochondrial mass are recorded as REFUTE items and a KNOWLEDGE_GAP discussion. Pulmonary hypertension is left without an upstream edge because no source gives a mechanism. ClinGen gene-disease validity was not cited: just clingen-refresh fails on a checksum mismatch, and repinning was out of scope. Validation: just validate passed; just validate-terms passed; just count-verified-snippets 165/165. check-entity-refs, check-causal-targets, check-duplicate-keys, check-coarse-phenotypes, check-qualifier-terms, check-snippet-length, check-title-snippets, check-snippet-grading (after removing one item that graded a Gai sentence two ways), check-reference-titles, check-folded-hyphens and check-genereviews (TAGGED) all passed. list-disconnected-phenotypes reports 33/34 phenotypes connected.

Perplexity ▸

1. Disease Information

sonar-deep-research 18 citations 2026-09-28T22:43:17.283133

1. Disease Information

1.1 Overview and Disease Definition

FBXL4-related mitochondrial DNA depletion syndrome is classified among the nuclear-gene–encoded mitochondrial DNA maintenance disorders, a subset of mitochondrial DNA depletion syndromes (MTDPS) characterized by a profound reduction in mitochondrial DNA (mtDNA) copy number within affected tissues, leading to impaired oxidative phosphorylation and energy failure.[2][7][11][12] Specifically, MTDPS13 refers to a predominantly encephalomyopathic phenotype arising from biallelic loss-of-function variants in FBXL4 (F-box and leucine-rich repeat protein 4), a gene located on chromosome 6q16.1–q16.2.[1][2][3][8] Individuals typically present with early-onset and often congenital neurologic dysfunction, including hypotonia, encephalopathy, and severe developmental delay, in combination with persistent lactic acidosis, failure to thrive, and variable involvement of other organ systems such as heart, bone marrow, immune system, and craniofacial structures.[2][7][10][11][17] Cells derived from affected patients show combined respiratory chain deficiencies and marked mtDNA depletion, confirming that this disease represents a disorder of mitochondrial maintenance rather than a primary structural or assembly defect of individual respiratory chain complexes.[6][7][10][14] As a Mendelian condition, FBXL4-related MTDPS13 is inherited in an autosomal recessive pattern, with affected individuals harboring pathogenic variants on both alleles of FBXL4 and heterozygous carriers generally being clinically asymptomatic.[1][2][10][11]

The initial molecular delineation of this disorder came from whole-exome sequencing studies in consanguineous families with severe, fatal infantile encephalopathy, lactic acidosis, and mtDNA depletion, which identified recessive nonsense and splice-site mutations in FBXL4 segregating with disease.[6][14] Subsequent clinical reviews and case reports have expanded the phenotype, but the core picture remains that of a severe, early-onset multisystem mitochondrial disease with prominent brain involvement and biochemical evidence of defective oxidative phosphorylation.[7][10][11][12] MedGen, GeneReviews, Orphanet, OMIM, and MedlinePlus consistently describe FBXL4-related MTDPS13 as a “multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay,” with additional neurologic features including seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes in some cases.[2][11][17] Taken together, these aggregated descriptions from curated resources and the primary literature provide a coherent disease concept that integrates clinical, biochemical, genetic, and mechanistic dimensions.

1.2 Key Identifiers and Ontology Mappings

FBXL4-related MTDPS13 is represented in multiple disease ontologies and clinical classification systems, facilitating its integration into genomic databases and clinical decision support tools.[1][2][3][17][18] OMIM assigns the gene FBXL4 the entry number 605654 and the associated mitochondrial DNA depletion syndrome 13 (encephalomyopathic type) the phenotype MIM number 615471.[1][2] Orphanet designates the condition “Mitochondrial DNA depletion syndrome, encephalomyopathic form with variable craniofacial anomalies” under ORPHA:369897.[3] MedGen lists the concept “Mitochondrial DNA depletion syndrome 13 (encephalomyopathic type)” with identifier C3809592 and notes synonyms such as “FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome.”[2][18] The Monarch Initiative maps this disease to MONDO:0014198, corresponding to the Mondo Disease Ontology term used in the user’s query.[2][4] SNOMED CT includes a code 765403009 associated with the FBXL4 gene entry, aligning the genetic defect with clinical terminologies used in electronic health records.[1]

The table below summarizes key identifiers and mappings for this disease based on curated resources.

Resource	Identifier / Term	Notes
OMIM gene	605654 (FBXL4)	Nuclear gene encoding F-box/LRR protein 4[1][8]
OMIM phenotype	615471 (MTDPS13)	“Mitochondrial DNA depletion syndrome 13 (encephalomyopathic type)”[1][2]
Orphanet	ORPHA:369897	“Mitochondrial DNA depletion syndrome, encephalomyopathic form with variable craniofacial anomalies”[3]
MedGen	C3809592	“Mitochondrial DNA depletion syndrome 13 (encephalomyopathic type)”[2][18]
MONDO	MONDO:0014198	FBXL4-related encephalomyopathic mtDNA depletion syndrome[2][4]
SNOMED CT	765403009	Associated with FBXL4 gene; mitochondrial disease context[1]

These identifiers allow cross-linking of FBXL4-related MTDPS13 across resources such as GeneReviews, ClinVar, the Genetic Testing Registry (GTR), and research data repositories, and underpin ontology-based annotation for knowledge bases focused on Mendelian diseases.[3][8][11][18] For example, the GeneReviews chapter “FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome” underlines the OMIM and Orphanet identifiers and provides diagnostic and management guidance, further embedding these mappings within clinical genomics practice.[11][12]

Suggested ontology terms for the disease entity include the Mondo term “FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome” (MONDO:0014198), the Orphanet term noted above, and the Human Phenotype Ontology (HPO) category “Mitochondrial DNA depletion syndrome” (HP:0031486) for grouping with related disorders caused by defects in mtDNA replication and maintenance.[2][3][18]

1.3 Synonyms and Alternative Names

The disease is known under several closely related names that reflect its causal gene, pathologic mechanism, and clinical presentation.[2][3][11][17] MedGen and MedlinePlus list “FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome” as the primary descriptive name, emphasizing both the genetic origin in FBXL4 and the combined brain–muscle involvement typical of encephalomyopathies.[2][17] OMIM uses the standardized designation “Mitochondrial DNA depletion syndrome 13 (encephalomyopathic type)” to distinguish this form from other numbered MTDPS subtypes caused by different nuclear genes.[1][2] Orphanet describes the same condition as “Mitochondrial DNA depletion syndrome, encephalomyopathic form with variable craniofacial anomalies,” highlighting the frequent presence of facial dysmorphic features.[3][10]

Other common synonyms include “FBXL4 deficiency,” “FBXL4-related early-onset mitochondrial encephalopathy,” “MTDPS13,” and “FBXL4-related mitochondrial DNA depletion syndrome.”[2][11][17] GeneReviews adopts “FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome” as its title and cross-references the OMIM numbers for both gene and phenotype.[11][12] These synonymous labels all refer to an identical clinical and genetic entity and differ mainly in emphasis rather than content. For ontology-based annotation, it is important to recognize all of these as alternative labels for MONDO:0014198 and ensure that they point to the same underlying disease concept.[2][3][18]

1.4 Source of Information: Patient-Level vs Aggregated Data

Information about FBXL4-related MTDPS13 in curated resources such as OMIM, Orphanet, MedGen, GeneReviews, and MedlinePlus is derived from aggregated analyses of published case reports, clinical series, and mechanistic studies, rather than directly from raw electronic health records.[1][2][3][10][11][12][17] For instance, OMIM synthesizes findings from the original gene discovery paper by Bonnen et al. (2013), the larger cohort described by Huemer et al. (2015), and additional case reports and biochemical characterizations.[1][6][10][14] Orphanet and MedGen similarly summarize clinical features and genetic associations from multiple published sources, noting that “to date FBXL4-related mtDNA depletion syndrome has been reported in 50 individuals,” a figure based on aggregated case counts up to circa 2017.[2][3]

GeneReviews provides an expert-authored narrative synthesis that integrates clinical, biochemical, genetic, and management information from primary literature and has been updated periodically as new cases and mechanistic insights have emerged.[11][12] MedlinePlus offers a patient-facing summary that distills key points from OMIM, GeneReviews, and NIH genetic databases into accessible language.[17] While individual case descriptions in the literature derive from patient-level data such as clinical histories, imaging, and tissue biopsies, the disease-level resources consulted here represent curated, aggregated knowledge designed for clinical and research use. This distinction is important for the knowledge base, as disease characteristics reported below reflect consensus patterns and ranges gleaned from multiple patients, rather than idiosyncratic features of single individuals.

2. Etiology

2.1 Primary Causal Factors: Genetic Basis in FBXL4

The primary causal factor for FBXL4-related MTDPS13 is the presence of biallelic pathogenic or likely pathogenic variants in the nuclear gene FBXL4, encoding F-box and leucine rich repeat protein 4.[1][2][6][8][10][11] OMIM and MedGen both state that mitochondrial DNA depletion syndrome 13 is an autosomal recessive disorder associated with FBXL4 located at cytogenetic position 6q16.1–q16.2.[1][2] NCBI Gene describes FBXL4 as a protein-coding gene encoding a member of the F-box protein family, characterized by an approximately 40 amino acid F-box motif that mediates interaction with SKP1 and incorporation into SCF (SKP1–Cullin–F-box) E3 ubiquitin ligase complexes, as well as multiple leucine-rich repeats that function in substrate recognition.[8] The gene is ubiquitously expressed across tissues, including thyroid, kidney, and at least 25 other sites, consistent with the multi-organ nature of the disease.[8]

Through whole-exome sequencing of consanguineous kindreds with fatal infantile encephalopathy, lactic acidosis, and severe mtDNA depletion, Bonnen et al. identified recessive nonsense and splicing mutations in FBXL4 segregating with disease, thereby establishing FBXL4 as a mitochondrial protein essential for maintaining mtDNA integrity and stability.[6][14] In their words:

“Through whole-exome sequencing, we identified recessive nonsense and splicing mutations in FBXL4 segregating in three unrelated consanguineous kindreds in which affected children present with a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle.”[6][14]

Subsequent cohorts and case reports have identified a wide range of pathogenic variants across the gene, including truncating frameshift mutations, canonical splice-site deletions, nonsense mutations, and missense substitutions clustering in functionally important domains.[7][10][12][15][16] Huemer et al. reviewed 21 individuals with genetically confirmed FBXL4 deficiency and reported that “mutations were detected throughout the FBXL4 gene albeit with no clear delineation of a genotype–phenotype correlation,” underscoring that loss-of-function is the critical determinant rather than disruption of specific subdomains.[10]

ClinVar entries document several individual pathogenic variants associated with MTDPS13, including NM_001278716.2(FBXL4):c.1698A>G (p.Ile566Met), NM_001278716.2:c.1241T>C (p.Leu414Pro), NM_001278716.2:c.1389+3_1389+6del (splice-site deletion), and NM_001278716.2:c.1232G>A (p.Cys411Tyr).[4][5][15][16] These submissions, based on clinical testing and research studies, classify the variants as pathogenic or likely pathogenic under ACMG guidelines, and explicitly associate them with the clinical entity “Mitochondrial DNA depletion syndrome 13 (encephalomyopathic type)” with identifiers MONDO:0014198, OMIM:615471, and MedGen:C3809592.[4][5][15][16] Thus, the etiological foundation of FBXL4-related MTDPS13 is firmly established as nuclear gene loss-of-function affecting a mitochondrial quality control factor.

From a mechanistic standpoint, the disease reflects a deficiency of FBXL4’s role as the F-box substrate adaptor within an SCF ubiquitin ligase localized to the mitochondrial outer membrane, which normally mediates the ubiquitination and degradation of BNIP3 and BNIP3L/NIX mitophagy receptors to suppress mitophagy.[8][9][13] The Autophagy commentary by Kulkarni et al. succinctly states:

“Here, we discuss our recent discovery that the SKP1-CUL1-F-box (SCF)-FBXL4 (F-box and leucine-rich repeat protein 4) E3 ubiquitin ligase localizes to the mitochondrial outer membrane, where it constitutively mediates the ubiquitination and degradation of BNIP3L/NIX and BNIP3 mitophagy receptors to suppress mitophagy.”[13]

Loss of FBXL4 thus leads to excessive mitophagy, reduced mitochondrial content, mtDNA depletion, and consequent respiratory chain dysfunction, providing a direct link from genetic lesion to pathophysiology.[7][9][13]

2.2 Genetic Risk Factors and Variant Spectrum

Under the autosomal recessive model, the main genetic risk factor for FBXL4-related MTDPS13 is being homozygous or compound heterozygous for pathogenic variants in FBXL4, typically in the context of parental consanguinity or small, isolated populations where founder mutations may be enriched.[6][10][11][16] Bonnen et al. emphasized that all their affected children were from consanguineous families and carried biallelic truncating or splice-site mutations, consistent with a recessive pattern in which carrier parents are unaffected.[6][14] Huemer et al. found that the majority of their 21 patients arose from consanguineous unions and reported that “the genetic diagnosis of FBXL4 deficiency has been established in 28 individuals” worldwide at the time of their publication, suggesting that clustering in certain families and communities may reflect founder effects and increased homozygosity.[10]

ClinVar and related genomic databases provide detailed insight into the variant spectrum of FBXL4 and its clinical classification.[4][5][15][16] For example, the variant NM_001278716.2(FBXL4):c.1389+3_1389+6del is classified as pathogenic based on ACMG guidelines, with submission by the Wong Mito Lab stating that “FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome.”[15] This splice-site deletion is expected to disrupt normal RNA splicing and lead to loss of functional protein, representing a clear loss-of-function allele. Similarly, NM_001278716.2(FBXL4):c.1232G>A (p.Cys411Tyr) is classified as likely pathogenic, having been identified both in homozygous form and as part of compound heterozygous combinations in individuals with MTDPS13.[16] The ClinVar summary notes that although this missense variant is present at low frequency in the general population, its occurrence in affected individuals and functional domain suggests pathogenicity consistent with a recessive carrier model.[16]

Overall, the pathogenic variants in FBXL4 span nonsense, frameshift, splice-site, and missense changes, with truncating mutations generally assumed to cause complete loss of function, while certain missense alterations may severely compromise protein stability, localization, or substrate interactions.[6][7][10][12][15][16] There is currently no strong evidence for dominant-negative or gain-of-function mechanisms; instead, all human data indicate that FBXL4-related MTDPS13 arises when overall FBXL4 activity is markedly reduced or absent due to biallelic loss-of-function, consistent with autosomal recessive inheritance.[6][10][11][13]

2.3 Environmental and Non-Genetic Risk Factors

No specific environmental toxins, infections, or lifestyle factors have been identified as primary causal agents or major risk modifiers for FBXL4-related MTDPS13, reflecting the disease’s origin as a monogenic, nuclear-encoded mitochondrial disorder.[2][10][11] Orphanet and MedlinePlus describe the condition as a genetic syndrome caused by mutations in FBXL4 and do not list environmental exposures as independent risk factors.[3][17] GeneReviews and Huemer et al. emphasize that affected individuals present in the neonatal period or early infancy with severe lactic acidosis and encephalopathy, often in the absence of identifiable external precipitating events, and that the disease course is dominated by intrinsic mitochondrial dysfunction.[10][11]

Nevertheless, clinical experience suggests that common stressors such as intercurrent infections, surgical procedures, or fasting may exacerbate metabolic decompensation and lactic acidosis in affected children, analogous to other mitochondrial disorders.[10][11][12] While these episodes can precipitate acute deterioration and may contribute to mortality, they are best understood as triggers that unmask or worsen underlying mitochondrial failure rather than primary etiologic factors. To date, no epidemiological or mechanistic studies have systematically examined specific environmental exposures, dietary components, or maternal factors as modifiers of disease risk or severity in FBXL4-related MTDPS13, and thus, the knowledge base should regard environmental risk as largely speculative and secondary.

2.4 Protective Factors and Potential Modifiers

Given the rarity and severity of FBXL4-related MTDPS13, there is limited evidence regarding genetic or environmental protective factors that modulate susceptibility or disease course. Heterozygous carriers of pathogenic FBXL4 variants, such as parents of affected children, are generally clinically normal, indicating that one functional allele suffices to maintain mitochondrial quality control under usual conditions.[6][10][11][16] This observation can be interpreted as a form of protection conferred by the intact allele, but it reflects the inherent recessive model rather than a specific variant-based protective effect.

Within affected individuals, variability in phenotype severity and survival suggests that residual FBXL4 function from hypomorphic missense alleles or alternative splicing might modulate disease, but published series have not identified clear genotype–phenotype correlations.[7][10][12] Huemer et al. explicitly reported that “mutations were detected throughout the FBXL4 gene albeit with no clear delineation of a genotype–phenotype correlation,” implying that any modifying effects are subtle or overshadowed by overall loss of function.[10] No modifier genes have been formally identified in human cohorts, though mechanistic work points to BNIP3 and BNIP3L/NIX as functionally downstream targets whose abundance and activity directly affect mitophagy, suggesting that variation in these genes could, in principle, alter disease severity.[9][13] However, this remains hypothetical and has not been demonstrated in clinical populations.

Environmental protective factors, such as optimized nutrition, avoidance of metabolic stress, and early supportive care, likely improve short-term outcomes and quality of life but do not fundamentally alter the underlying mitochondrial defect.[10][11][12] No dietary supplements, vitamins, or pharmacologic agents have proven protective in prospective trials, and Huemer et al. concluded that “treatment with ‘mitochondrial medications’ did not prove effective,” underscoring the absence of established protective therapies.[10] Therefore, for the current knowledge base, it is reasonable to state that no specific genetic or environmental protective factors have been validated for FBXL4-related MTDPS13, beyond the general benefit of standard supportive care in severe pediatric mitochondrial disorders.

2.5 Gene–Environment Interactions

Given the monogenic etiology of FBXL4-related MTDPS13 and the absence of identified external causal agents, gene–environment interactions in this disease are not well characterized and are likely to be indirect. The primary interaction involves the way that environmental stressors—such as infection, fever, fasting, or surgery—increase energy demands and metabolic load on tissues that are already compromised by mitochondrial dysfunction, thereby exacerbating lactic acidosis, encephalopathy, and organ failure.[10][11][12] This pattern, common in many mitochondrial diseases, reflects a generic gene–environment interplay where a genetically determined defect in oxidative phosphorylation diminishes physiologic reserve, making affected individuals vulnerable to otherwise manageable stressors.

Mechanistic studies in cell lines and mice have shown that FBXL4 deficiency leads to increased lysosomal turnover of mitochondria and upregulation of lysosomal proteins, indicating that environmental or pharmacologic factors influencing autophagy or lysosomal function could, in principle, modify disease phenotype.[9][13] For example, the EMBO Molecular Medicine study demonstrated that inhibition of lysosomal function in FBXL4-deficient fibroblasts reversed the mitochondrial phenotype, suggesting that environmental or therapeutic modulation of lysosomal activity might interact with the genetic defect to alter mitochondrial content and function.[9] Kulkarni et al. further discussed how cellular conditions or signaling events that prevent FBXL4-mediated turnover of BNIP3L and BNIP3 would facilitate selective removal of specific mitochondria, implying that stress pathways activating these mitophagy receptors may amplify disease in the absence of FBXL4.[13]

Despite these insights, there are no human data demonstrating specific environmental interventions that consistently ameliorate or exacerbate FBXL4-related MTDPS13 via defined molecular mechanisms. Accordingly, the knowledge base should treat gene–environment interactions as an area of emerging mechanistic interest rather than established clinical doctrine, noting that future work on mitophagy and autophagy modulation may yield more detailed interaction models.

3. Phenotypes

3.1 Global Clinical Phenotype and Age of Onset

FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome presents as a severe, multisystem disorder that almost invariably begins in the neonatal period or early infancy.[2][10][11][17] MedGen and MedlinePlus emphasize that the condition “is a severe condition that begins in infancy and affects multiple body systems,” and that infants have weak muscle tone (hypotonia), difficulty growing and gaining weight, and lactic acidosis.[2][17] Huemer et al., reviewing 21 individuals with genetically confirmed FBXL4 deficiency, reported that “neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive is the characteristic pattern at first presentation,” underscoring the consistency of early severe manifestations.[10] GeneReviews similarly notes that onset is typically in the neonatal period or the first months of life, with very few patients surviving beyond early childhood and all survivors having profound neurodevelopmental disability.[11][12]

Symptom severity is generally marked; MTDPS13 is among the more severe mtDNA depletion syndromes, with high mortality and extensive multisystem involvement.[7][10][11][12] The disease course is progressive, with ongoing neurodegeneration, worsening brain atrophy, and accumulation of metabolic derangements, although some features such as facial dysmorphism and congenital cataracts are static anomalies present from birth.[7][10][12] Clinical variability exists, particularly regarding the presence or absence of cardiac involvement, craniofacial anomalies, and immune or bone marrow dysfunction, but the core encephalomyopathic phenotype and lactic acidosis are nearly universal in reported cases.[7][10][11][17]

Quality of life impact is profound. Surviving children exhibit severe psychomotor retardation, are typically non-ambulatory and non-verbal, require extensive feeding support, and often depend on caregivers for all activities of daily living.[10][11][12] Frequent hospitalizations for metabolic crises, infections, or cardiac events further compromise well-being and impose substantial burden on families. In Huemer’s cohort, all survivors had severe developmental delay, and several had significant feeding problems requiring interventions.[10] Taken together, FBXL4-related MTDPS13 can be characterized as a globally disabling pediatric mitochondrial encephalomyopathy with early onset, severe and progressive course, and major negative impact on quality of life.

Suggested high-level HPO terms for the general disease phenotype include “Encephalopathy” (HP:0001298), “Hypotonia” (HP:0001252), “Global developmental delay” (HP:0001263), “Lactic acidosis” (HP:0003128), and “Failure to thrive” (HP:0001508).[2][10][11][17]

3.2 Neurologic and Developmental Phenotypes

Neurologic involvement is central to FBXL4-related MTDPS13 and largely defines its clinical identity as an encephalomyopathic syndrome.[2][6][7][10][11][12][17] Bonnen et al. described affected children as presenting with “a fatal encephalopathy, lactic acidosis, and severe mtDNA depletion in muscle,” noting features such as microcephaly, severe global developmental delay, hypotonia, and cerebral atrophy on MRI.[6][14] In their series, many children had congenital microcephaly, generalized cerebral atrophy, and progressive neurodegeneration, with age at death typically within the first few years of life.[14] Huemer et al. similarly found that “all survivors developed severe psychomotor retardation,” and that brain imaging in neonates was initially nonspecific but evolved to show rapidly progressive brain atrophy.[10]

Common neurologic phenotypes include generalized hypotonia, global developmental delay, microcephaly, seizures, movement disorders, and abnormalities of tone and reflexes.[7][10][11][12] Hypotonia is often noted at birth and persists throughout life, contributing to delayed motor milestones, poor head control, and inability to sit or walk independently.[10][11][12] Global developmental delay encompasses deficits across gross motor, fine motor, speech, and cognitive domains, with many children lacking meaningful language or interactive behaviors.[10][11] Microcephaly, defined by head circumference below the third centile, is present in a substantial proportion of patients, reflecting underlying brain growth impairment.[6][10][11][14] Seizures, including generalized tonic–clonic and focal events, have been reported in several cases, sometimes associated with cortical atrophy or white matter lesions on MRI.[7][10][12]

Brain imaging findings are notable. Huemer et al. reported that neonatal MRI was often nonspecific, but later imaging showed a pattern of rapidly progressive brain atrophy.[10] In one patient described by Gai et al. and later in Huemer’s series, cystic white matter lesions and marked cerebral atrophy were observed, correlating with severe encephalopathy.[7][10] A case report summarized by Almannai et al. described a girl with FBXL4-related MTDPS13 who developed cerebral atrophy and significant neurodevelopmental delay, with MRI demonstrating progressive cortical thinning.[12] These imaging features suggest selective vulnerability of cortical and white matter neurons to mitochondrial dysfunction and mtDNA depletion.

Quality of life impact of neurologic involvement is extreme. Children are often unable to communicate, ambulate, or participate in age-appropriate activities, and may experience recurrent seizures and distressing episodes of encephalopathy.[10][11][12] Suggested HPO terms for specific neurologic phenotypes include “Encephalopathy” (HP:0001298), “Hypotonia” (HP:0001252), “Global developmental delay” (HP:0001263), “Microcephaly” (HP:0000252), “Seizures” (HP:0001250), “Cerebral atrophy” (HP:0002059), and “White matter abnormalities” (HP:0002500).[6][7][10][11][12][14]

3.3 Muscular, Growth, and Feeding Phenotypes

Muscular weakness and growth failure are prominent features of FBXL4-related MTDPS13 and strongly impact clinical management.[2][7][10][11][17] MedlinePlus notes that infants with FBXL4-related encephalomyopathic mtDNA depletion syndrome “have weak muscle tone (hypotonia) and difficulty growing and gaining weight at the expected rate (faltering weight).”[17] Huemer et al. described “muscular hypotonia, feeding problems and failure to thrive” as part of the characteristic initial pattern, with elevated creatine kinase observed in approximately 45% of measurements, suggesting muscle membrane or metabolic stress.[10] Bonnen et al. likewise reported muscular hypotonia and a severe respiratory chain defect in muscle biopsies, consistent with an underlying myopathy.[6][14]

Failure to thrive manifests as poor weight gain, short stature, and reduced muscle bulk, despite adequate caloric intake, reflecting chronic energy deficit due to impaired oxidative phosphorylation.[10][11][17] Feeding difficulties include poor suck, dysphagia, vomiting, and aspiration risk, often necessitating nasogastric or gastrostomy tube feeding for long-term nutritional support.[10][11][12] These issues contribute to increased healthcare utilization and caregiver burden and create ongoing challenges in maintaining adequate caloric and protein intake in a child with high metabolic vulnerability.

Muscle biopsies in some patients reveal non-specific myopathic changes, combined respiratory chain defects, and mtDNA depletion, identifying a biochemical myopathy underlying the clinical hypotonia and weakness.[6][7][10][14] Functional tests, such as electromyography, have not been systematically reported but are likely to show myopathic patterns. Quality of life impact includes reduced mobility, dependence on assistive devices and caregivers, and frequent feeding interventions.

Suggested HPO terms include “Muscular hypotonia” (HP:0001252), “Myopathy” (HP:0003198), “Failure to thrive” (HP:0001508), “Feeding difficulties” (HP:0011010), and “Elevated serum creatine kinase” (HP:0003236).[7][10][11][17]

3.4 Metabolic Phenotypes: Lactic Acidosis and Hyperammonemia

Metabolic derangements, especially persistent lactic acidosis, are hallmark features of FBXL4-related MTDPS13 and reflect underlying defects in oxidative phosphorylation and energy metabolism.[2][6][7][10][11][17] MedlinePlus states that “All individuals with FBXL4-related encephalomyopathic mtDNA depletion syndrome have a buildup of a chemical called lactic acid in the body (lactic acidosis), and about half of individuals have an accumulation of ammonia in the blood,” emphasizing the near-universal presence of lactic acidosis and frequent hyperammonemia.[17] Huemer et al. found elevated blood lactate and metabolic acidosis in all 21 individuals in their cohort, with creatine kinase elevation in 45% of measurements, and described the lactic acidosis as “persistent” and often severe.[10] Bonnen et al. likewise reported lactic acidosis in affected children and demonstrated severe combined respiratory chain defects, consistent with impaired oxidative phosphorylation driving increased anaerobic glycolysis and lactate accumulation.[6][14]

Hyperammonemia, while not universal, occurs in a substantial subset of patients and may exacerbate encephalopathy and contribute to episodes of acute decompensation.[2][10][17] In MedlinePlus, it is noted that “about half of individuals have an accumulation of ammonia in the blood,” implying a frequency of approximately 50% among reported cases.[17] Elevated ammonia likely reflects secondary effects on hepatic function or urea cycle activity, potentially due to mitochondrial dysfunction in hepatocytes and other tissues.

Other metabolic abnormalities include elevated transaminases, metabolic acidosis, and abnormal acylcarnitine profiles, though these are less consistently reported.[7][10][12] Collectively, these biochemical features confirm that FBXL4 deficiency causes a broad mitochondrial energy metabolism defect, in which decreased mtDNA copy number and respiratory chain function force reliance on glycolysis and perturb intermediary metabolism.

Quality of life impact is considerable, as recurrent metabolic crises necessitate frequent hospitalizations, intravenous bicarbonate or other interventions, and careful management of nutrition and infections.[10][11][12] Suggested HPO terms include “Lactic acidosis” (HP:0003128), “Metabolic acidosis” (HP:0001942), “Hyperammonemia” (HP:0001987), and “Elevated serum lactate” (HP:0002151).[2][10][17]

3.5 Cardiac Phenotypes

Cardiac involvement is increasingly recognized as a prominent feature of FBXL4-related MTDPS13 and contributes significantly to morbidity and mortality.[7][9][10][11] Huemer et al. reported central nervous system and cardiac involvement in their neonate with total FBXL4 deficiency, including cardiomyopathy and arrhythmias, and broader case series have documented congenital heart malformations and pulmonary hypertension.[7][10] A comprehensive review of mitochondrial DNA depletion syndromes and their cardiac manifestations in Frontiers in Cardiovascular Medicine summarized FBXL4-related MTDPS13 as having cardiac involvement in 54% of cases, including cardiomyopathy (27%), congenital heart malformations (19%), arrhythmia (15%), and pulmonary hypertension (11%).[11] In their table, FBXL4 is categorized under “Other pathways: protein homeostasis” with clinical features described as encephalomyopathy and mtDNA depletion.[11]

The review further notes that “Among the MTDPS13, the cardiac involvement was observed in 54% of cases (20/37), with specific manifestations including cardiomyopathy (27%, 10/37), congenital heart malformations (19%, 7/37), arrhythmia (15%, 6/41), and pulmonary hypertension (11%, 4/37).”[11] This quantitative summary underscores that more than half of reported patients have some form of cardiac pathology, and that cardiomyopathy is the single most common cardiac phenotype. Mechanistic work in adult mouse models of heart failure with preserved ejection fraction has shown that transfection of FBXL4 can rescue cardiac geometry and mitochondrial integrity with altered mitochondrial dynamics, providing experimental support that FBXL4 plays a protective role in cardiac mitochondrial homeostasis.[11]

Cardiac phenotypes in FBXL4-related MTDPS13 likely arise from the same mechanistic pathway as neurologic and muscular involvement—namely, excessive mitophagy leading to reduced mitochondrial content, mtDNA depletion, and respiratory chain dysfunction in cardiomyocytes, resulting in energy failure, structural remodeling, and conduction system abnormalities.[9][11][13] Clinically, cardiomyopathy may present as dilated or hypertrophic patterns, with reduced systolic or diastolic function, while arrhythmias can include tachycardias or conduction blocks.[7][10][11] Pulmonary hypertension may result from chronic hypoxia or structural cardiac anomalies.

Quality of life impact includes increased risk of heart failure, sudden cardiac death, limitations in physical activity, and additional treatment burden from medications, device therapies, or hospitalizations.[10][11] Suggested HPO terms include “Cardiomyopathy” (HP:0001626), “Congenital heart malformation” (HP:0001272), “Arrhythmia” (HP:0011675), and “Pulmonary hypertension” (HP:0002093).[7][10][11]

3.6 Craniofacial and Dysmorphic Phenotypes

Craniofacial anomalies and facial dysmorphism are common in FBXL4-related MTDPS13 and contribute to the characteristic clinical gestalt.[7][10][11][12] Huemer et al. reported that “facial dysmorphic features are present in 67% of cases,” indicating that approximately two-thirds of affected individuals show recognizable craniofacial differences.[10] These may include deep-set eyes, high-arched palate, micrognathia, and other subtle anomalies, though precise patterns vary among patients and are not currently linked to specific genotypes.[7][10][12]

Orphanet describes the disease as “encephalomyopathic form with variable craniofacial anomalies,” highlighting that dysmorphism is an important but variable component.[3] GeneReviews and case reports also mention craniofacial dysmorphism, though they focus primarily on functional deficits rather than detailed morphological description.[11][12] In the case study summarized by Almannai et al., the girl with homozygous nonsense FBXL4 mutation had dysmorphic features and cerebral atrophy, further supporting the association.[12]

Quality of life impact of craniofacial anomalies is generally minor compared to the profound neurologic and metabolic impairments, but they can aid in clinical recognition and may have implications for feeding, speech, and airway management.[10][11][12] Suggested HPO terms include “Facial dysmorphism” (HP:0001999), “Craniofacial dysmorphism” (HP:0001999), and more specific terms as needed (e.g., “Micrognathia” HP:0000347) once detailed phenotyping is available.[3][10][11]

3.7 Ophthalmologic Phenotypes

Ophthalmologic involvement, particularly congenital cataracts, has been observed in several patients with FBXL4-related MTDPS13 and may be part of the broader encephalomyopathic phenotype.[7][10][11] Huemer et al. mention “congenital cataract” among the variable clinical findings in their cohort, although exact frequencies are not specified.[7][10] Bonnen et al. reported eye involvement in some subjects, including cataracts and optic nerve abnormalities, aligning with a general pattern of mitochondrial disease affecting high-energy tissues such as the retina and lens.[6][14]

While ocular phenotypes are not as prominent or universal as neurologic, metabolic, or cardiac features, they can contribute to visual impairment and may complicate developmental assessment and rehabilitation.[10][11][12] Suggested HPO terms include “Congenital cataract” (HP:0000519) and “Visual impairment” (HP:0000505), with more specific terms added as detailed case descriptions accumulate.[7][10][11]

3.8 Immune, Hematologic, and Other Systemic Phenotypes

FBXL4-related MTDPS13 can involve multiple additional organ systems, including bone marrow, immune system, and lungs, though these features are less systematically reported.[7][10][11] In the detailed biochemical characterization by Gai et al., a neonate with total FBXL4 deficiency exhibited “CNS, cardiac, and bone marrow involvement, and multicomplex respiratory chain dysfunction associated with mtDNA depletion,” suggesting that hematopoietic tissues may be affected by mitochondrial dysfunction.[7] Huemer et al. noted immunodeficiency in one severely affected patient, with recurrent infections and laboratory evidence of immune compromise.[7][10] Pulmonary hypertension, as noted in the cardiac section, reflects interactions between cardiac and pulmonary vascular systems.[11]

These multisystem features reinforce that FBXL4 is widely expressed and participates in mitochondrial quality control across diverse tissues, so its loss can manifest in any high-energy organ or cell type.[8][9][13] Quality of life and morbidity impacts include recurrent infections, anemia or cytopenias, and respiratory complications, all of which require specialized management and increase overall disease burden.[10][11]

Suggested HPO terms for these features include “Immunodeficiency” (HP:0002721), “Bone marrow failure” (HP:0001876), “Recurrent respiratory infections” (HP:0002205), and “Pulmonary hypertension” (HP:0002093).[7][10][11]

3.9 Phenotype Frequencies and Summary Table

Published series provide approximate frequencies for key phenotypes, although numbers are limited and may evolve as more cases are reported. Huemer et al. described 21 individuals in detail, while the Frontiers in Cardiovascular Medicine review summarized cardiac involvement in 37 cases.[10][11] The table below synthesizes available data on selected phenotypes.

Phenotype	Type	Approximate Frequency among reported cases	Evidence
Neonatal/early-onset lactic acidosis	Metabolic symptom	Nearly 100% (all individuals in Huemer cohort)	Huemer et al. 2015[10]; MedlinePlus[17]
Muscular hypotonia	Clinical sign	Nearly 100%	Huemer et al. 2015[10]; MedGen[2]
Feeding problems/failure to thrive	Symptom	Nearly 100%	Huemer et al. 2015[10]; MedlinePlus[17]
Severe global developmental delay	Symptom	Nearly 100% among survivors	Huemer et al. 2015[10]; GeneReviews[11]
Facial dysmorphic features	Physical manifestation	~67% (14/21)	Huemer et al. 2015[10]
Cardiac involvement (any)	Organ involvement	~54% (20/37)	Frontiers CVMed review[11]
Cardiomyopathy	Organ involvement	~27% (10/37)	Frontiers CVMed review[11]
Congenital heart malformations	Organ involvement	~19% (7/37)	Frontiers CVMed review[11]
Arrhythmia	Organ involvement	~15% (6/41)	Frontiers CVMed review[11]
Pulmonary hypertension	Organ involvement	~11% (4/37)	Frontiers CVMed review[11]
Hyperammonemia	Laboratory abnormality	~50%	MedlinePlus[17]; MedGen[2]

These frequencies should be interpreted cautiously due to small sample sizes and potential ascertainment bias, but they provide useful quantitative anchors for phenotype annotation in the knowledge base.

4. Genetic and Molecular Information

4.1 Causal Gene: FBXL4

FBXL4 (F-box and leucine rich repeat protein 4) is the sole causal gene currently known for MTDPS13 and is central to the disease’s genetic and molecular profile.[1][2][3][6][8][9][11][13] The gene is located on chromosome 6q16.1–q16.2, as mapped by initial cytogenetic studies and later refined by genomic coordinates (GRCh38: 6:98,868,535–98,947,946).[1][3] It encodes a protein that belongs to the F-box family, characterized by an approximately 40 amino acid F-box motif that mediates binding to SKP1 and incorporation into SCF ubiquitin ligase complexes.[8] In addition to the F-box domain, FBXL4 contains at least nine tandem leucine-rich repeats (LRRs), which typically function in substrate recognition and protein–protein interactions.[8]

NCBI Gene summarizes FBXL4’s function as follows:

“This gene encodes a member of the F-box protein family, which are characterized by an approximately 40 amino acid motif, the F-box. F-box proteins constitute one subunit of modular E3 ubiquitin ligase complexes, called SCF complexes, which function in phosphorylation-dependent ubiquitination. The F-box domain mediates protein–protein interactions and binds directly to S-phase kinase-associated protein 1. In addition to an F-box domain, the encoded protein contains at least 9 tandem leucine-rich repeats. The ubiquitin ligase complex containing the encoded protein may function in cell-cycle control by regulating levels of lysine-specific demethylase 4A. Alternative splicing results in multiple transcript variants.”[8]

Recent work has refined this understanding in the mitochondrial context, showing that the SCF-FBXL4 complex localizes to the mitochondrial outer membrane and specifically targets the mitophagy receptors BNIP3L/NIX and BNIP3 for ubiquitination and proteasomal degradation, thereby suppressing mitophagy.[9][13] Gene Ontology annotations in NCBI Gene indicate that FBXL4 enables protein binding and “ubiquitin-like ligase-substrate adaptor activity” and is involved in “SCF-dependent proteasomal ubiquitin-dependent protein catabolic process,” “autophagy of mitochondrion,” and “negative regulation of mitophagy,” among other processes.[8]

Alternative splicing generates multiple transcript variants of FBXL4, some of which may differ in subcellular localization or regulatory properties, though the majority of disease-causing mutations appear to affect exons common to all major isoforms.[8][10][12] FBXL4 is expressed ubiquitously, with detectable levels in thyroid, kidney, and at least 25 other tissues, aligning with the multi-organ nature of FBXL4-related MTDPS13.[8]

Suggested ontology terms for FBXL4 include the HGNC gene symbol “FBXL4” (HGNC:13601), the UniProtKB entry Q9UKA2, and GO terms such as “mitophagy” (GO:0000422), “SCF-dependent proteasomal ubiquitin-dependent protein catabolic process” (GO:0010498), and “mitochondrial outer membrane” (GO:0005741).[3][8][9][13]

4.2 Pathogenic Variants: Types, Classification, and Frequencies

Pathogenic variants in FBXL4 associated with MTDPS13 span a wide spectrum of types, including nonsense mutations, frameshift insertions/deletions, canonical splice-site mutations, and missense variants in conserved domains.[6][7][10][12][15][16] Bonnen et al. identified recessive nonsense and splicing mutations in FBXL4 segregating in affected children, including truncating mutations such as c.1555C>T (p.Gln519*) and splice-site changes that lead to aberrant transcripts and loss of function.[6][14] Huemer et al. reported multiple variants across the gene, with no clear clustering by exon or domain, and noted that genotypes included both homozygous and compound heterozygous combinations.[10]

ClinVar provides detailed classification of several FBXL4 variants, many of which are documented in individuals with MTDPS13.[4][5][15][16] For example, NM_001278716.2(FBXL4):c.1698A>G (p.Ile566Met) is a missense single nucleotide variant located at cytogenetic position 6q16.1, and is associated with MTDPS13; while its clinical significance is not fully elaborated in the summary, it is listed under single nucleotide variant type and linked to the disease concept via MONDO:0014198 and OMIM:615471, implying pathogenic or likely pathogenic classification by submitters.[4] Another variant, NM_001278716.2(FBXL4):c.1241T>C (p.Leu414Pro), is a missense change at genomic location Chr6:98899344 (GRCh38) and is explicitly associated with MTDPS13; multiple submissions classify it under pathogenic categories, and it likely disrupts protein function within the LRR domain or adjacent regions.[5]

Splice-site variants such as NM_001278716.2(FBXL4):c.1389+3_1389+6del are particularly informative. This variant involves deletion of nucleotides at the +3 to +6 positions of the intron following exon 11, disrupting normal splicing signals and causing aberrant transcripts. The Wong Mito Lab classified this variant as pathogenic based on ACMG guidelines and emphasized that “FBXL4 defects are common in patients with congenital lactic acidemia and encephalomyopathic mitochondrial DNA depletion syndrome.”[15] The deletion is cataloged under dbSNP rs1554216735, with genomic coordinates NC_000006.12:g.98880549_98880552del.[15]

The missense variant NM_001278716.2(FBXL4):c.1232G>A (p.Cys411Tyr) has received particular attention. ClinVar summarizes that this variant is classified as likely pathogenic by multiple laboratories, including Baylor College of Medicine, Fulgent Genetics, and the Broad Center for Mendelian Genomics, based on ACMG criteria.[16] It has been observed both in homozygous form and as part of compound heterozygous genotypes in individuals with MTDPS13, and its low allele frequency in population databases is consistent with a recessive carrier model.[16] The ClinVar summary notes that “Computational prediction tools and conservation analyses do not provide strong support for or against an impact to the protein. In summary, although additional studies are required to fully establish its clinical significance, this variant is likely pathogenic,” illustrating the nuanced interpretation of missense variants in critical domains.[16]

In terms of allele frequencies, ClinVar notes that p.Cys411Tyr is present in the general population at a very low frequency, compatible with an autosomal recessive disorder where carriers are rare but do exist.[16] Precise gnomAD frequency data are not provided in the search results, but the overall pattern indicates that pathogenic FBXL4 variants are extremely rare, consistent with the rarity of MTDPS13.[10][11][16] All pathogenic variants described are germline in origin; ClinVar explicitly notes that somatic classification and oncogenicity categories are “None” for these entries, reflecting that FBXL4-related MTDPS13 is a germline Mendelian disease rather than a somatic cancer syndrome.[4][5][15][16]

From a functional perspective, most truncating and splice-site variants are assumed to cause complete loss of FBXL4 function, whether through nonsense-mediated decay, production of truncated proteins lacking critical domains, or aberrant splicing leading to nonfunctional products.[6][7][10][12][15] Missense variants are believed to cause loss of function by destabilizing the protein, disrupting substrate binding, or impairing interactions with SKP1 or other SCF components, though detailed biochemical characterization is available for only a subset.[7][9][13][16] No FBXL4 variants have been reported to cause gain-of-function or dominant-negative effects in humans, and there is no evidence for somatic variants driving cancer or other adult-onset disorders in the current literature.[4][5][15][16]

4.3 Modifier Genes and Genetic Interactions

No definitive modifier genes have been identified for FBXL4-related MTDPS13 in human cohorts, despite variability in phenotype severity and organ involvement among individuals with similar genotypes.[7][10][11][12] Mechanistically, the downstream targets of FBXL4, BNIP3 and BNIP3L/NIX, are likely candidates for genetic interactions, as their abundance and activity directly influence mitophagy and mitochondrial turnover.[9][13] The EMBO Molecular Medicine study demonstrated that FBXL4 deficiency leads to accumulation of BNIP3 and BNIP3L, driving excessive mitophagy via lysosomal pathways and decreasing mitochondrial content.[9] Kulkarni et al. highlighted that FBXL4-mediated ubiquitination and degradation of BNIP3 and BNIP3L is a key suppressive mechanism for mitophagy, and that disruptions to this pathway in MTDPS13 lead to elevated mitophagy.[13]

It is therefore plausible that genetic variation in BNIP3, BNIP3L, or other mitophagy regulators (such as PINK1, PRKN/Parkin, and FUNDC1) might modulate disease severity in FBXL4-deficient individuals, though this remains speculative in the absence of direct evidence.[9][13] Similarly, genes involved in mtDNA replication and maintenance (e.g., POLG, TK2, DGUOK) could interact with FBXL4 deficiency to further impact mtDNA copy number, but again, human data are lacking.[11][12]

For now, the knowledge base should note that no validated modifier genes have been published for FBXL4-related MTDPS13, while acknowledging mechanistic hypotheses that may guide future investigations.

4.4 Epigenetic and Chromosomal Abnormalities

There is currently no evidence that epigenetic alterations, such as DNA methylation or histone modifications at the FBXL4 locus, play a primary role in the etiology of MTDPS13, nor that large-scale chromosomal abnormalities involving 6q16.1–q16.2 are common in affected individuals.[1][2][10][11] OMIM and MedGen characterize the disease as arising from sequence-level mutations within the FBXL4 gene, and neither resource mentions chromosomal rearrangements or epigenetic dysregulation as typical features.[1][2] Huemer et al. and other clinical series likewise focus on point mutations and small indels identified by sequencing, without reporting karyotypic anomalies or imprinting defects.[10][11][12]

DECIPHER and similar structural variant databases are not referenced in the search results, suggesting that no recurrent deletions or duplications encompassing FBXL4 have been firmly linked to MTDPS13. Epigenetic profiling of FBXL4-related MTDPS13 has not been reported in the literature identified here, and thus any epigenetic contributions remain speculative. The pathophysiology appears to be driven primarily by protein-level loss of function due to DNA sequence variants, rather than by regulatory epigenetic changes.

5. Environmental Information

5.1 Environmental Factors

As a classic monogenic mitochondrial maintenance disorder, FBXL4-related MTDPS13 does not have known primary environmental causes, and environmental factors are best viewed as modulators of disease expression rather than etiologic agents.[2][10][11][17] Neither MedGen nor MedlinePlus lists toxins, radiation, pollution, or occupational exposures as causative factors; instead, they emphasize that “As its name suggests, FBXL4-related encephalomyopathic mtDNA depletion syndrome is caused by mutations in the FBXL4 gene.”[2][17] Orphanet similarly categorizes the disease under genetic disorders and does not mention environmental triggers.[3]

Nevertheless, clinical experience in mitochondrial medicine indicates that environmental stressors can exacerbate symptoms and precipitate acute decompensation. In FBXL4-related MTDPS13, infections, fasting, fever, and surgical procedures may increase energy demand and metabolic strain on tissues already compromised by respiratory chain defects, leading to worsening lactic acidosis, encephalopathy, and organ dysfunction.[10][11][12] For example, episodes of severe lactic acidosis in Huemer’s cohort were often associated with intercurrent illnesses or surgical stress, although underlying metabolic vulnerability persists independent of such events.[10] These environmental interactions are nonspecific and common to many mitochondrial disorders, lacking disease-specific evidence in FBXL4 deficiency.

No studies have systematically assessed environmental pollutants, drugs, or dietary components as risk factors or triggers for MTDPS13, and thus the knowledge base should not list specific environmental agents as established contributors. The primary environmental recommendation remains to minimize metabolic stress and avoid exposures known to exacerbate mitochondrial dysfunction, such as certain anesthetic agents or valproic acid, in line with general mitochondrial disease guidelines, but these are extrapolations rather than FBXL4-specific findings.[10][11]

5.2 Lifestyle and Infectious Factors

Lifestyle factors such as smoking, alcohol consumption, and exercise patterns are generally not relevant in the neonatal and early childhood population affected by FBXL4-related MTDPS13, and no studies have linked parental lifestyle to disease risk.[10][11][17] Similarly, infectious agents are not known to cause this disease, although infections may precipitate metabolic crises and contribute to morbidity and mortality.[10][11][12] Immune impairment in some patients may predispose to recurrent infections, but this is a consequence of disease rather than an external risk factor.[7][10]

Given this context, environmental and lifestyle information for FBXL4-related MTDPS13 can be summarized as follows: the disease is primarily genetic, environmental exposures are not known to cause or prevent it, and the main environmental considerations involve supportive management to avoid metabolic stress and promptly treat infections.

6. Mechanism and Pathophysiology

6.1 Ordered Causal Chain from Mutation to Clinical Manifestation

Step 1: Biallelic loss-of-function mutations in FBXL4 lead to deficient or absent FBXL4 protein function in the mitochondrial outer membrane SCF-FBXL4 E3 ubiquitin ligase complex.[6][8][9][13]

Step 2: Loss of FBXL4 activity results in reduced ubiquitination and proteasomal degradation of the mitophagy receptors BNIP3 and BNIP3L/NIX, causing their accumulation on the mitochondrial outer membrane and inferred increased mitophagy signaling.[9][13]

Step 3: Accumulated BNIP3/BNIP3L, together with other mitophagy machinery, leads to excessive autophagic removal of mitochondria via lysosomal pathways, resulting in a global decrease in cellular mitochondrial content; this is demonstrated in patient fibroblasts, FBXL4 knockout mice, and CRISPR/Cas9 FBXL4 knockout human cell lines.[7][9][13]

Step 4: Reduced mitochondrial content is accompanied by depletion of mtDNA copy number and decreased steady-state levels of multiple mitochondrial proteins, including respiratory chain components, resulting in combined multiple respiratory chain complex deficiencies and impaired oxidative phosphorylation.[6][7][9][10][14]

Step 5: Impaired oxidative phosphorylation leads to decreased ATP production, loss of mitochondrial membrane potential, fragmentation of the mitochondrial network, and enlarged mitochondrial nucleoids, as demonstrated in patient fibroblasts, contributing to cellular energy failure and metabolic reprogramming toward glycolysis.[6][7][14]

Step 6: Reliance on glycolysis and reduced oxidative phosphorylation result in systemic lactic acidosis and metabolic acidosis, with elevated lactate and frequent hyperammonemia in affected individuals, as consistently observed in clinical cohorts.[2][10][17]

Step 7: Chronic energy deficit and metabolic derangements in high-demand tissues such as brain, heart, and skeletal muscle cause encephalopathy, hypotonia, cardiomyopathy, failure to thrive, and multi-organ dysfunction, leading to the clinical phenotype of FBXL4-related MTDPS13 and, frequently, early death.[6][7][10][11][12][14]

Step 8: In some patients, additional downstream consequences include immune dysfunction, bone marrow involvement, and pulmonary hypertension, inferred to arise from mitochondrial failure in hematopoietic and vascular tissues, although mechanisms are less directly demonstrated.[7][10][11]

This causal chain integrates demonstrated molecular and cellular mechanisms with inferred tissue-level and clinical outcomes, providing a coherent pathophysiological narrative.

6.2 Molecular Pathways: SCF-FBXL4, Mitophagy, and Autophagy

The core molecular pathway implicated in FBXL4-related MTDPS13 is the SCF-FBXL4 ubiquitin ligase–mediated suppression of mitophagy via BNIP3 and BNIP3L/NIX turnover.[8][9][13] FBXL4 serves as the F-box substrate adaptor within an SCF-type E3 ubiquitin ligase complex composed of SKP1, CUL1, and RBX1, localized to the mitochondrial outer membrane.[8][13] Under normal conditions, this SCF-FBXL4 complex constitutively ubiquitinates BNIP3L/NIX and BNIP3, tagging them for proteasomal degradation and thereby preventing excessive activation of mitophagy.[9][13]

Kulkarni et al. summarize this mechanism:

“Our recent discovery that the SKP1-CUL1-F-box (SCF)-FBXL4 (F-box and leucine-rich repeat protein 4) E3 ubiquitin ligase localizes to the mitochondrial outer membrane, where it constitutively mediates the ubiquitination and degradation of BNIP3L/NIX and BNIP3 mitophagy receptors to suppress mitophagy.”[13]

In FBXL4 deficiency, this pathway is disrupted. EMBO Molecular Medicine’s study of FBXL4 knockout mice, patient fibroblasts, and CRISPR/Cas9 knockout human cells demonstrated that FBXL4-deficient cells have reduced steady-state levels of mitochondrial proteins, mtDNA depletion, and upregulation of lysosomal proteins.[9] Proteomic analyses revealed a global reduction in mitochondrial proteins and concomitant increase in lysosomal components, suggesting that increased autophagic turnover of mitochondria via lysosomes, rather than proteasomal degradation, accounts for the loss of mitochondrial content.[9]

The EMBO study concluded:

“We present data showing that the molecular phenotype instead is explained by increased autophagic removal of mitochondria, leading to a global decrease in cellular mitochondrial content. Inhibition of lysosomal function in these cells reverses the mitochondrial phenotype, whereas proteasomal inhibition has no effect.”[9]

This indicates that the SCF-FBXL4 complex indirectly controls lysosomal mitophagy pathways by regulating the abundance of BNIP3 and BNIP3L, and that FBXL4 deficiency tips the balance toward excessive lysosomal degradation of mitochondria. Gene Ontology terms relevant to this process include “mitophagy” (GO:0000422), “autophagy of mitochondrion” (GO:0000422), “negative regulation of mitophagy” (GO:1901524), and “SCF-dependent proteasomal ubiquitin-dependent protein catabolic process” (GO:0010498).[8][9][13]

6.3 Cellular Processes: Mitochondrial Dynamics, Respiratory Chain, and Energy Failure

At the cellular level, FBXL4 deficiency impacts multiple processes, including mitochondrial dynamics, respiratory chain function, membrane potential, and nucleoid distribution.[6][7][9][14] Bonnen et al. showed that loss-of-function and splice mutations in FBXL4 result in “a severe respiratory chain deficiency, loss of mitochondrial membrane potential, and a disturbance of the dynamic mitochondrial network and nucleoid distribution in fibroblasts from affected individuals.”[6][14] They further demonstrated that expression of wild-type FBXL4 in patient cells fully rescued mtDNA copy number and corrected the mitochondrial biochemical deficit, confirming that FBXL4 plays a critical role in mtDNA maintenance and stability.[6][14]

Gai et al. and Huemer et al. found multiple deficiencies of respiratory chain activities in patient tissues, with severe combined defects across complexes I, II, III, and IV, and associated mtDNA depletion.[7][10] In a neonate with total FBXL4 deficiency, Gai et al. described “profound reductions in complex I-, II- and ETF-linked substrate-dependant respiration, a loss of inner membrane potential, and fragmentation of the mitochondrial network with no parallel decrease in mitochondrial content,” highlighting functional failure even before overt reduction in mitochondrial mass.[7] Later work, including the EMBO study, refined this picture to show that increased mitophagy eventually leads to decreased mitochondrial content, compounding the initial functional defects.[9]

Cellular processes involved include mitochondrial fusion and fission, nucleoid organization, and interactions with autophagosomes and lysosomes. The disturbance of the dynamic mitochondrial network suggests impaired fusion, potentially due to altered expression or function of fusion proteins such as MFN1, MFN2, or OPA1, though FBXL4’s role appears more focused on mitophagy than on direct fusion regulation.[6][7][9][12] Nucleoid distribution anomalies indicate that mtDNA packaging and segregation are affected, likely secondary to altered mitochondrial turnover and network dynamics.[6][14]

Energy failure is a central cellular outcome, as impaired respiratory chain function and decreased mtDNA copy number reduce ATP production and force cells to rely on glycolysis, leading to lactic acid accumulation.[6][7][10][14][17] This is particularly detrimental in high-energy cell types such as neurons (CL:0000540), cardiomyocytes (CL:0000746), skeletal myocytes (CL:0000737), and hepatocytes (CL:0000182), which depend heavily on mitochondrial ATP and thus experience severe dysfunction in FBXL4 deficiency.[7][9][11][13]

6.4 Metabolic Changes and Biochemical Abnormalities

Metabolically, FBXL4-related MTDPS13 is characterized by a shift from oxidative phosphorylation to glycolysis, resulting in lactic acidosis and metabolic acidosis.[2][6][7][10][17] Decreased mtDNA copy number reduces the expression of mtDNA-encoded respiratory chain subunits, leading to diminished electron transport chain activity, reduced oxygen consumption, and decreased ATP generation via oxidative phosphorylation.[6][7][9][14] This energy deficit triggers compensatory upregulation of glycolysis, but glycolytic ATP is insufficient to meet cellular demands in high-energy tissues, and the increased pyruvate production is converted to lactate, causing systemic lactic acidosis.[6][7][10][17]

Biochemical abnormalities observed in patient tissues and cells include combined respiratory chain defects, reduced activities of multiple complexes, and decreased enzyme levels associated with mitochondrial energy metabolism.[7][10] Huemer et al. reported “a severe combined respiratory chain defect with a general decrease of enzymes associated with mitochondrial energy metabolism and a relative depletion of mitochondrial DNA content” in diagnostic workup.[10] Elevated blood lactate and metabolic acidosis were universal in their cohort, and creatine kinase was elevated in 45% of measurements, indicating muscle involvement.[10] Gai et al. described “multicomplex respiratory chain dysfunction associated with mtDNA depletion” in a neonate, confirming the broad impact on energy production.[7]

Metabolic changes extend to other pathways, including potential alterations in fatty acid oxidation, amino acid metabolism, and the urea cycle, as suggested by hyperammonemia in approximately half of patients.[2][10][17] However, detailed metabolomics profiles have not been reported for FBXL4-related MTDPS13, and specific intermediates beyond lactate and ammonia have not been systematically analyzed. HMDB and related metabolomics databases are not referenced in the search results, indicating that metabolomics is an area for future investigation.

Biochemical abnormalities relevant to knowledge base annotation include “enzyme deficiencies” of respiratory chain complexes, “loss of mitochondrial membrane potential,” and “mtDNA depletion” as central defects.[6][7][9][10][14] Suggested GO terms include “oxidative phosphorylation” (GO:0006119), “respiratory electron transport chain” (GO:0022904), and “mitochondrial DNA metabolic process” (GO:0032543).[6][7][9]

6.5 Immune System and Tissue Damage Mechanisms

Immune system involvement in FBXL4-related MTDPS13 is less well characterized but has been reported in individual cases, suggesting that mitochondrial dysfunction may impair immune cell function or hematopoiesis.[7][10] Gai et al. described “immunodeficiency” in a neonate with total FBXL4 deficiency, with recurrent infections and laboratory evidence of immune compromise.[7] Huemer et al. mentioned immunodeficiency as part of the clinical phenotype in some patients, though not quantified.[10] Tissue damage mechanisms likely include oxidative stress, apoptosis, and necrosis in cells unable to maintain energy homeostasis, but detailed immunologic profiling is lacking.

Tissue injury in brain, heart, and other organs manifests as neuronal loss, cardiomyocyte degeneration, and organ atrophy or fibrosis, inferred from imaging and clinical features.[6][7][9][10][11][12][14] In the brain, progressive cerebral atrophy and cystic white matter lesions suggest neuronal and glial cell death, possibly via apoptotic and necrotic pathways triggered by energy failure and oxidative stress.[6][7][10][14] In the heart, cardiomyopathy likely results from cardiomyocyte loss and remodeling, with potential contributions from mitochondrial ROS and inflammatory responses.[7][9][11] In skeletal muscle, myopathy and elevated creatine kinase reflect muscle fiber damage.

Gene Ontology terms relevant to tissue damage include “apoptotic process” (GO:0006915), “oxidative stress” (GO:0006979), and “necrotic cell death” (GO:0070265), although direct evidence for these specific mechanisms in FBXL4-related MTDPS13 is limited and largely inferred from general mitochondrial disease biology.[7][9][10][11]

6.6 Molecular Profiling and Advanced Technologies

Proteomic profiling has played a key role in elucidating FBXL4-related MTDPS13 pathophysiology. EMBO Molecular Medicine’s study used proteomic approaches in FBXL4 knockout mice, patient fibroblasts, and human FBXL4 knockout cells to show “a general decrease in mitochondrial proteins accompanied by an increase in lysosomal proteins,” highlighting the shift in mitochondrial and lysosomal content.[9] This proteomic signature is consistent with increased autophagic removal of mitochondria and compensatory lysosomal expansion.

Transcriptomic profiling and single-cell analyses have not been extensively reported for FBXL4-related MTDPS13 in the available search results, and thus specific gene expression changes, cell-type heterogeneity, and spatial patterns are unknown. Future studies using RNA-seq, single-cell RNA-seq, and spatial transcriptomics could reveal how FBXL4 deficiency impacts transcriptional programs in neurons, cardiomyocytes, and other cell types, but such data are not yet available.[9][13]

CRISPR/Cas9 functional genomics has been used to generate FBXL4 knockout human cell lines that recapitulate patient phenotypes, including reduced mitochondrial protein levels, mtDNA depletion, and increased lysosomal proteins.[9] These in vitro models serve as platforms for high-throughput screens to identify modifiers of mitophagy or mitochondrial content, but published screens specifically targeting FBXL4-deficient cells are not referenced in the search results.[9][13]

Overall, molecular profiling in FBXL4-related MTDPS13 has focused on proteomics and functional genomics, revealing a clear pattern of increased mitophagy and lysosomal turnover of mitochondria, with secondary respiratory chain defects and mtDNA depletion. Integration of multi-omics data remains an opportunity for future research.

7. Anatomical Structures Affected

7.1 Organ-Level Involvement

FBXL4-related MTDPS13 affects multiple organs, with primary involvement of the central nervous system, skeletal muscle, heart, and to a lesser extent liver, bone marrow, and immune system.[6][7][9][10][11][12][17] The brain (UBERON:0000955) is the most severely affected organ, with encephalopathy, cerebral atrophy, microcephaly, seizures, and white matter lesions.[6][7][10][11][12][14] Skeletal muscle (UBERON:0001630) exhibits hypotonia, myopathy, and elevated creatine kinase, reflecting mitochondrial dysfunction in muscle fibers.[7][10] The heart (UBERON:0000948) shows cardiomyopathy, congenital malformations, arrhythmias, and pulmonary hypertension in a substantial subset of patients.[7][10][11] The liver (UBERON:0002107) may contribute to hyperammonemia and metabolic disturbances, although specific hepatopathy is not consistently reported.[2][10][17]

Bone marrow (UBERON:0002371) and immune system organs such as spleen (UBERON:0002106) and lymph nodes (UBERON:0001968) may be involved in cases with immunodeficiency and hematologic abnormalities.[7][10] The lungs (UBERON:0002048) are indirectly affected via pulmonary hypertension and potential respiratory failure.[11] The eyes (UBERON:0000970) can show congenital cataracts and other anomalies, while craniofacial bones and soft tissues (UBERON:0002385) manifest dysmorphic features.[7][10][11][12]

7.2 Tissue and Cell-Level Involvement

At the tissue level, FBXL4-related MTDPS13 primarily affects nervous tissue (UBERON:0001016), skeletal muscle tissue (UBERON:0001134), cardiac muscle tissue (UBERON:0001133), and in some cases hematopoietic tissue (UBERON:0002385).[6][7][9][10][11][12] Within the nervous system, neurons (CL:0000540) and glial cells such as oligodendrocytes (CL:0000128) and astrocytes (CL:0000127) are likely involved, as evidenced by cerebral atrophy and white matter changes.[6][7][10][14] Cardiac involvement implicates cardiomyocytes (CL:0000746), conduction system cells, and vascular smooth muscle cells (CL:0000743).[7][9][11] Skeletal myocytes (CL:0000737) are affected in myopathy and hypotonia.[7][10]

Hematopoietic involvement may affect hematopoietic stem cells (CL:0000037) and related lineages, contributing to bone marrow failure or immunodeficiency.[7][10] Immune cell types such as T lymphocytes (CL:0000084) and B lymphocytes (CL:0000236) may be compromised, though specific cell-type data are sparse.[7][10]

7.3 Subcellular Compartments

FBXL4-related MTDPS13 is fundamentally a mitochondrial disease, and subcellular involvement centers on mitochondria (GO:0005739), the mitochondrial outer membrane (GO:0005741), mitochondrial inner membrane (GO:0005743), mitochondrial matrix (GO:0005759), and lysosomes (GO:0005764).[6][7][8][9][13][14] FBXL4 localizes to the mitochondrial outer membrane, where it participates in SCF-type ubiquitin ligase activity targeting BNIP3 and BNIP3L.[9][13] Mitochondrial nucleoid structures are disrupted, as evidenced by altered nucleoid distribution in patient fibroblasts.[6][14] Mitochondrial membrane potential is reduced, and the dynamic mitochondrial network is fragmented.[6][7][14]

Lysosomes are upregulated and expanded in FBXL4-deficient cells, reflecting increased lysosomal turnover of mitochondria via mitophagy.[9] Autophagosomes (GO:0005776) and autolysosomes participate in the removal of damaged mitochondria. Proteasomes (GO:0000502) are involved in the degradation of ubiquitinated BNIP3/BNIP3L under normal conditions, but in FBXL4 deficiency, lysosomal pathways dominate mitochondrial turnover.[9][13]

7.4 Localization and Lateralization

Anatomical localization of brain lesions in FBXL4-related MTDPS13 is typically diffuse and bilateral, reflecting global cerebral atrophy and widespread white matter abnormalities rather than focal unilateral lesions.[6][7][10][14] MRI studies report generalized cortical thinning, ventricular enlargement, and diffuse white matter changes, without consistent lateralization.[6][10][14] Cardiac involvement affects the whole heart, including ventricles and conduction system, rather than specific localized segments.[7][10][11]

Pulmonary hypertension involves the pulmonary vasculature bilaterally, and skeletal muscle involvement is generalized across limb and axial muscles.[7][10][11] Craniofacial dysmorphism impacts midface, jaw, and cranial vault symmetrically in most reported cases.[10][12] As such, lateralization does not play a major role in FBXL4-related MTDPS13, and anatomical localization is best described as systemic and bilateral.

8. Temporal Development

8.1 Age of Onset and Onset Pattern

FBXL4-related MTDPS13 is a congenital or early infantile-onset disease, with symptoms typically manifesting at birth or within the first few months of life.[2][10][11][17] MedlinePlus notes that the condition “begins in infancy and affects multiple body systems,” and that infants have hypotonia and growth difficulties early on.[17] Huemer et al. described “neonatal/early-onset severe lactic acidosis, muscular hypotonia, feeding problems and failure to thrive” as the characteristic pattern at first presentation, indicating onset in the neonatal period or early infancy.[10] GeneReviews agrees that onset is usually in the first few months of life, occasionally later in infancy or early childhood, but not in adolescence or adulthood.[11][12]

The onset pattern is acute to subacute, with rapid emergence of lactic acidosis and encephalopathy in the neonatal period, followed by chronic progression of developmental delays and organ involvement.[10][11][12] In some cases, lactic acidosis is detected immediately after birth, while in others it develops over days to weeks as feeding and growth difficulties become apparent.[10][11] For modeling purposes, FBXL4-related MTDPS13 can be categorized as a pediatric, early-onset mitochondrial disease.

8.2 Disease Progression, Rate, and Course

Disease progression in FBXL4-related MTDPS13 is generally rapid and progressive, with severe deterioration over months to years, leading to early childhood death in many cases.[6][7][10][11][12][14] Huemer et al. reported that seven children died at a mean age of 37 months, while eleven were alive at a mean follow-up age of 46 months, with three lost to follow-up; all survivors had severe psychomotor retardation.[10] Bonnen et al. described affected children with progressive encephalopathy and lactic acidosis who died in early childhood, often by age 3–4 years.[6][14]

Brain imaging demonstrates a progression from nonspecific findings in neonates to rapidly progressive brain atrophy, indicating ongoing neurodegeneration.[10][14] Cardiac involvement may develop later in infancy or childhood, adding to the risk of heart failure and death.[7][10][11] Metabolic derangements such as lactic acidosis and hyperammonemia can fluctuate with intercurrent illnesses but generally persist as chronic features.[2][10][17]

Disease stages can be conceptualized as early neonatal presentation with lactic acidosis and hypotonia, intermediate childhood with progressive developmental delay and emerging organ involvement, and advanced stage with severe encephalopathy, organ failure, and high mortality risk.[10][11][12] The course is largely progressive and does not exhibit remission or relapsing–remitting patterns; any transient improvements reflect supportive therapy rather than disease reversal.[10][11]

8.3 Critical Periods and Windows of Intervention

The neonatal and early infancy period constitutes a critical window of vulnerability in FBXL4-related MTDPS13, as metabolic crises and encephalopathy early in life can irreversibly damage developing brain and other organs.[10][11][12] Early recognition and supportive management—such as aggressive treatment of lactic acidosis, provision of adequate nutrition, and avoidance of mitochondrial-toxic drugs—may mitigate immediate complications but do not fundamentally alter the disease trajectory.[10][11]

In experimental models, the perinatal period is also critical. EMBO Molecular Medicine’s Fbxl4 knockout mice exhibited predominant perinatal lethality, with only a few animals surviving into adulthood; surviving mice appeared normal until 8–12 months of age, when they gradually developed signs of mitochondrial dysfunction and weight loss.[9] This suggests that developmental timing of mitochondrial quality control is crucial in both humans and mice, and that early FBXL4 deficiency imposes a high risk of perinatal organ failure.

Given the autosomal recessive inheritance, preconception and prenatal windows offer opportunities for genetic prevention via carrier screening and prenatal diagnosis, as discussed in GeneReviews.[11][12] These interventions can prevent the birth of affected children but do not alter disease progression in already-affected individuals.

9. Inheritance and Population Characteristics

9.1 Inheritance Pattern, Penetrance, and Expressivity

FBXL4-related MTDPS13 follows an autosomal recessive inheritance pattern, with affected individuals carrying pathogenic variants on both alleles and heterozygous carriers being asymptomatic.[1][2][6][10][11][16] MedGen explicitly lists “autosomal recessive inheritance” for MTDPS13, referencing Orphanet and OMIM.[2][18] Bonnen et al. reported recessive nonsense and splicing mutations segregating in consanguineous kindreds, with carrier parents unaffected, consistent with autosomal recessive transmission.[6][14] ClinVar entries and GeneReviews also classify FBXL4-related MTDPS13 as autosomal recessive.[11][15][16]

Penetrance appears to be complete among individuals with biallelic loss-of-function variants, as all known homozygous or compound heterozygous carriers of clearly pathogenic FBXL4 variants exhibit clinical disease.[6][10][11][16] Variation in expressivity is present, however, particularly in the severity and spectrum of organ involvement; some patients have prominent cardiac phenotypes, craniofacial anomalies, or immunodeficiency, while others exhibit primarily neurologic and metabolic features.[7][10][11][12] Huemer et al. reported no clear genotype–phenotype correlation, suggesting that expressivity is influenced by factors beyond the specific FBXL4 variant, such as genetic background or environmental exposures.[10]

There is no evidence of genetic anticipation, as the disease does not involve repeat expansions or progressive worsening across generations; instead, disease severity appears similar among affected siblings, conditioned on their shared genotype and environment.[10][11] Germline mosaicism has not been reported in FBXL4-related MTDPS13, but it cannot be entirely excluded; however, the autosomal recessive model predominates, and recurrence risk in families is best estimated based on carrier status rather than mosaicism probabilities.[11][12]

9.2 Epidemiology: Prevalence and Incidence

FBXL4-related MTDPS13 is an extremely rare disorder, with only a few dozen cases reported worldwide in the literature and curated resources.[2][3][10][11][12] MedGen notes that “To date FBXL4-related mtDNA depletion syndrome has been reported in 50 individuals,” summarizing case counts up to approximately 2017.[2] Huemer et al. stated that “to date, the genetic diagnosis of FBXL4 deficiency has been established in 28 individuals,” reflecting the status at the time of their 2015 publication.[10] GeneReviews likely updates these numbers as new cases are reported but still characterizes the disease as very rare.[11][12]

Given these counts and the global population, the prevalence of FBXL4-related MTDPS13 is likely less than 1 per million, possibly closer to 1 per several million, placing it in the Orphanet category of “rare diseases” and supporting its inclusion in orphan disease registries.[3][11] Incidence data are unavailable due to the rarity and lack of population-based screening, but given autosomal recessive inheritance and low carrier frequencies, incident cases are expected to be sporadic and often arise in consanguineous families or small founder populations.[6][10][11]

9.3 Population Demographics, Consanguinity, and Founder Effects

A notable proportion of reported FBXL4-related MTDPS13 cases arise from consanguineous families, particularly in regions where consanguineous marriages are culturally prevalent.[6][10][11][12][16] Bonnen et al. identified affected children in three unrelated consanguineous kindreds, emphasizing that recessive nonsense and splice mutations segregated with disease.[6][14] Huemer et al. mentioned consanguinity in many of their cases, and GeneReviews notes that the disease is more likely to occur in families with consanguineous unions.[10][11][12]

Founder effects have been suggested but not conclusively proven for specific variants. The p.Cys411Tyr missense variant has been reported in multiple unrelated patients, including a compound heterozygous combination with Arg435Gln and a homozygous case, suggesting possible enrichment in certain populations.[16] A Norwegian child with encephalomyopathic MTDPS13 was reported to carry a novel FBXL4 mutation, hinting at geographic clustering of certain alleles.[8] However, detailed population genetics analyses are lacking.

Population demographics such as sex ratio and age distribution are consistent with the autosomal recessive pediatric presentation: affected individuals include both males and females in roughly equal numbers, and all cases present in infancy or early childhood.[6][10][11][12][14] No sex-specific differences in phenotype or outcome have been documented. Geographic distribution of cases appears global, including European, Middle Eastern, and North American families, but precise regional prevalence data are not available.[6][10][11][12]

Carrier frequency estimates are not directly provided in the search results, but given the rarity of disease and the low frequency of known pathogenic variants such as p.Cys411Tyr in population databases, carriers are expected to be rare.[16] GeneReviews suggests carrier testing in at-risk families, but population-level screening is not currently undertaken.[11][12]

10. Diagnostics

10.1 Clinical and Laboratory Testing

Diagnostic evaluation of suspected FBXL4-related MTDPS13 involves a combination of clinical assessment, laboratory testing, imaging, and genetic analysis.[2][7][10][11][12][17][18] Clinically, the presentation of early-onset encephalopathy, hypotonia, failure to thrive, and persistent lactic acidosis, especially in the context of consanguinity or family history of similar illness, should prompt consideration of mitochondrial DNA depletion syndromes and specifically FBXL4 deficiency.[10][11][17][18] Huemer et al. concluded that “a clinical pattern of early-onset encephalopathy, persistent lactic acidosis, profound muscular hypotonia and typical facial dysmorphism should prompt initiation of molecular genetic analysis of FBXL4.”[10]

Laboratory tests focus on metabolic and mitochondrial parameters. Elevated blood lactate and metabolic acidosis are universal in reported cases, and hyperammonemia is present in about half of individuals.[2][10][17] Creatine kinase is elevated in roughly 45% of measurements, indicating muscle involvement.[10] Additional tests may include plasma amino acids, acylcarnitine profiles, pyruvate levels, and liver function tests, which can help exclude other metabolic disorders but are not specific for FBXL4 deficiency.[10][11][12] LOINC codes and SNOMED CT terms corresponding to lactic acid measurement, ammonia levels, and creatine kinase can be associated with these tests.

Muscle or liver biopsy may be performed to assess respiratory chain enzyme activities and mtDNA copy number. Diagnostic workup in patient tissues has revealed “a severe combined respiratory chain defect with a general decrease of enzymes associated with mitochondrial energy metabolism and a relative depletion of mitochondrial DNA content,” as reported by Huemer et al.[10] Biochemical assays demonstrate decreased activities of multiple respiratory chain complexes (I, II, III, IV) and reduced mtDNA content, confirming a mitochondrial maintenance defect.[6][7][10][14]

Histopathologic examination of muscle may show non-specific myopathic changes, with possible ragged-red fibers or fiber atrophy, though these findings are not pathognomonic.[6][10][14] Immunohistochemistry for mitochondrial proteins and mtDNA-encoded subunits can further support the diagnosis. SNOMED CT pathology terms corresponding to “myopathy” and “mitochondrial disease” can be applied.

10.2 Imaging and Electrophysiology

Brain imaging, typically MRI, is critical in diagnosing and characterizing FBXL4-related MTDPS13.[6][7][10][11][12][14] In neonates, MRI may be nonspecific, showing mild abnormalities, but later imaging reveals rapidly progressive brain atrophy, cortical thinning, ventricular enlargement, and white matter lesions.[10][14] Bonnen et al. reported generalized cerebral atrophy and microcephaly in affected children, with MRI imaging confirming widespread cortical and subcortical changes.[6][14] Huemer et al. described “non-specific” neonatal imaging but “later-onset, rapidly progressive brain atrophy,” suggesting that serial imaging is important for tracking disease evolution.[10]

Cardiac imaging, such as echocardiography and cardiac MRI, can reveal cardiomyopathy, congenital malformations, and pulmonary hypertension.[7][10][11] Functional cardiac tests, including electrocardiography (ECG) and Holter monitoring, detect arrhythmias and conduction abnormalities.[10][11] LOINC and RadLex terms corresponding to these imaging and electrophysiologic modalities can be attached in the knowledge base.

Electroencephalography (EEG) may demonstrate diffuse slowing, epileptiform discharges, or other nonspecific encephalopathic patterns, although specific EEG findings in FBXL4-related MTDPS13 are not extensively documented in the available search results.[10][11][12] Similarly, electromyography (EMG) and nerve conduction studies may show myopathic patterns, reflecting muscle involvement.

10.3 Genetic Testing Strategies

Genetic testing is central to definitive diagnosis of FBXL4-related MTDPS13. Initially, gene discovery relied on whole-exome sequencing in consanguineous families, as described by Bonnen et al.[6][14] In contemporary clinical practice, diagnostic strategies include targeted gene panels focused on mitochondrial diseases and mtDNA maintenance disorders, whole-exome sequencing (WES), whole-genome sequencing (WGS), and single-gene testing for FBXL4 in high-suspicion cases.[7][10][11][12]

GeneReviews recommends that for individuals suspected of having FBXL4-related encephalomyopathic mtDNA depletion syndrome based on clinical and biochemical findings, molecular genetic testing should begin with a multigene panel that includes FBXL4 and other genes known to cause mtDNA depletion syndromes (e.g., POLG, TK2, DGUOK, RRM2B).[11][12] If panel testing is inconclusive, WES or WGS can be pursued to identify rare variants in FBXL4 or novel genes.[6][10][11][12] Single-gene testing for FBXL4, using Sanger sequencing or targeted NGS, may be appropriate in families with known pathogenic FBXL4 variants, as part of cascade screening or prenatal diagnosis.[11][12][15][16]

The Genetic Testing Registry (GTR) lists multiple tests for “Mitochondrial DNA depletion syndrome 13,” including panels that assay FBXL4 along with other mitochondrial maintenance genes and single-gene tests for FBXL4.[18] ClinVar entries provide variant interpretations that support clinical decision-making based on genotype.[4][5][15][16]

Chromosomal microarray (CMA), karyotyping, and FISH are generally not diagnostic for FBXL4-related MTDPS13, as the disease arises from sequence-level mutations rather than large-scale structural variants.[1][2][10][11] Mitochondrial DNA sequencing is useful for excluding primary mtDNA mutations but is not sufficient for diagnosing FBXL4-related MTDPS13, which involves nuclear gene defects and secondary mtDNA depletion.[6][7][10][11][14]

10.4 Omics-Based Diagnostics and Molecular Biomarkers

While exome and genome sequencing represent genomic omics approaches used to diagnose FBXL4-related MTDPS13, other omics-based diagnostics such as transcriptomics, proteomics, and metabolomics are not yet standard in clinical practice for this disease.[9][11][13] Proteomic analyses have provided mechanistic insight, as described above, but are not used diagnostically.[9] Metabolomics profiling could, in principle, identify specific signatures of mitochondrial dysfunction, but no published studies have defined such signatures for FBXL4 deficiency in clinical cohorts.

Potential molecular biomarkers include reduced mtDNA copy number in muscle or fibroblasts, combined respiratory chain enzyme deficiencies, and elevated BNIP3/BNIP3L levels in patient cells.[6][7][9][10][13][14] However, BNIP3 and BNIP3L measurement has not been translated into clinical diagnostic tests. FDA biomarker databases and related resources do not currently list specific biomarkers for FBXL4-related MTDPS13.

10.5 Clinical Diagnostic Criteria and Differential Diagnosis

Formal standardized diagnostic criteria for FBXL4-related MTDPS13, akin to DSM or ICD guidelines, have not been published, but clinical experts and GeneReviews provide practical criteria based on core features.[10][11][12] Huemer et al. emphasized that early-onset encephalopathy, persistent lactic acidosis, profound muscular hypotonia, facial dysmorphism, and combined respiratory chain defects with mtDNA depletion should prompt molecular testing of FBXL4.[10] GeneReviews similarly lists key findings including failure to thrive, neurodevelopmental delays, encephalopathy, cerebral atrophy, hypotonia, and persistent lactic acidosis as the clinical hallmarks, combined with genetic confirmation of biallelic FBXL4 variants.[11][12]

Differential diagnosis includes other mtDNA depletion syndromes caused by defects in nuclear genes such as POLG (MTDPS1), C10orf2/Twinkle (MTDPS7), TK2 (MTDPS2), DGUOK (MTDPS3), and RRM2B (MTDPS8), among others.[2][6][10][11][12] Clinically, these conditions may present with overlapping features of encephalopathy, lactic acidosis, hypotonia, and developmental delay, but they may have distinct organ involvement patterns, such as predominant hepatic failure in DGUOK deficiency or myopathic presentations in TK2 deficiency.[11][12] Genetic testing panels that include FBXL4 and these genes can help distinguish among them. Other conditions to consider include primary mtDNA point mutations, inherited metabolic disorders, urea cycle defects, and non-metabolic causes of encephalopathy and lactic acidosis.

10.6 Screening

Population-based screening for FBXL4-related MTDPS13, such as newborn screening, is not currently implemented, given the rarity of the disease and limited treatment options.[11][12][17] However, carrier screening and prenatal diagnosis are recommended in families with known FBXL4 pathogenic variants. GeneReviews advises that carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk should be offered once the familial FBXL4 variants are identified.[11][12] Preimplantation genetic diagnosis (PGD) may be considered in couples at high risk, allowing selection of embryos without biallelic FBXL4 mutations.[11]

Cascade screening of siblings and extended family members can identify carriers and early affected individuals, though the disease’s early onset means that most affected children will already be symptomatic by the time genetic diagnosis is made in an index case.[11][12] Risk stratification based on genotype can inform reproductive decisions, but does not yet influence treatment strategies for existing patients.

11. Outcome and Prognosis

11.1 Survival, Mortality, and Life Expectancy

FBXL4-related MTDPS13 carries a poor prognosis, with high mortality in infancy and early childhood and limited survival into later childhood.[6][7][10][11][12][14] Huemer et al. reported that seven of 21 children died at a mean age of 37 months, while eleven were alive at a mean follow-up age of 46 months.[10] Bonnen et al. described affected children who died between ages 1.5 and 4 years, often from complications of encephalopathy, lactic acidosis, or cardiac disease.[6][14] GeneReviews notes that most individuals with FBXL4-related MTDPS13 die in early childhood, though a few survive longer with severe disability.[11][12]

Life expectancy is therefore significantly reduced compared to the general population. While precise 5-year or 10-year survival rates are not available due to small sample sizes, a rough estimate from existing data would suggest that fewer than half of affected individuals survive beyond age 5, and even fewer beyond age 10.[10][11][12] Mortality is directly attributable to the disease in most cases, due to neurological deterioration, metabolic crises, cardiac failure, or multi-organ dysfunction.[6][7][10][11][12][14]

11.2 Morbidity, Disability, and Quality of Life

Morbidity in FBXL4-related MTDPS13 is severe and pervasive. Survivors have profound psychomotor retardation, significant hypotonia, feeding difficulties, and often multiple organ system involvement, resulting in major disability and near-total dependence on caregivers.[10][11][12] Huemer et al. reported that “all survivors developed severe psychomotor retardation,” and described persistent lactic acidosis, failure to thrive, and ongoing neurologic and cardiac issues.[10] GeneReviews emphasizes that developmental outcome is poor, with most survivors unable to walk or speak, and many requiring gastrostomy feeding and assistive devices.[11][12]

Quality of life is compromised across domains of mobility, self-care, pain, and emotional well-being. Children may experience recurrent hospitalizations, distress from seizures and metabolic crises, and limited ability to engage in social and educational activities.[10][11][12] Family quality of life is also heavily impacted, as caregivers face emotional, financial, and physical burdens in managing complex medical needs over years.

Standardized quality of life measures such as EQ-5D or SF-36 have not been systematically applied to FBXL4-related MTDPS13, but extrapolation from similar severe pediatric mitochondrial disorders suggests extremely low scores in physical functioning, role limitations, and general health domains.[10][11]

11.3 Disease Course, Complications, and Recovery Potential

The disease course is progressive and characterized by accumulating complications over time. Neurological deterioration leads to cerebral atrophy, seizures, and loss of milestones, while cardiac involvement adds risk of heart failure and arrhythmias.[6][7][9][10][11][14] Metabolic crises, including severe lactic acidosis and hyperammonemia, can cause acute encephalopathy and organ failure, often leading to death.[2][10][17] Additional complications may include infections due to immunodeficiency, bone marrow failure, and pulmonary hypertension.[7][10][11]

Recovery potential is limited. Supportive care can stabilize acute crises and improve nutritional status, but does not reverse underlying mitochondrial dysfunction or restore neurodevelopmental capacity.[10][11][12] No disease-modifying therapies have been proven effective, and “mitochondrial medications” such as coenzyme Q10 and vitamins have not altered long-term outcomes.[7][10][11] Huemer et al. concluded that “Treatment with ‘mitochondrial medications’ did not prove effective,” underscoring the lack of recovery-oriented interventions.[10]

11.4 Prognostic Factors and Biomarkers

Prognostic factors in FBXL4-related MTDPS13 include age at onset, severity of lactic acidosis, presence of cardiac involvement, and degree of neurodevelopmental impairment.[6][7][10][11][12][14] Early, severe lactic acidosis and encephalopathy may predict worse outcomes, as they reflect profound mitochondrial failure and structural brain damage.[10][11] Cardiac phenotypes such as cardiomyopathy and arrhythmias are associated with increased mortality risk, given the potential for sudden cardiac death or progressive heart failure.[7][10][11]

Genotype–phenotype correlation is limited, and specific prognostic biomarkers such as mtDNA copy number thresholds or BNIP3/BNIP3L expression levels have not been validated in clinical practice.[7][9][10][13][14] Nevertheless, combined respiratory chain enzyme activities and mtDNA depletion degree could be considered surrogate markers of severity, as more pronounced biochemical defects likely correlate with more severe clinical disease.[7][10][14]

In summary, prognosis in FBXL4-related MTDPS13 is generally poor, with limited survival and substantial disability, and available prognostic factors are largely clinical rather than molecular.

12. Treatment

12.1 Pharmacotherapy and Supportive Medical Management

Currently, there are no disease-specific pharmacologic treatments that correct the underlying mitochondrial maintenance defect in FBXL4-related MTDPS13, and management is predominantly supportive.[7][10][11][12] Standard “mitochondrial medications,” including coenzyme Q10, riboflavin, L-carnitine, and various vitamins and antioxidants, have been empirically used in many mitochondrial diseases, but Huemer et al. reported that in their cohort, “treatment with ‘mitochondrial medications’ did not prove effective,” indicating that these supplements do not significantly alter disease course in FBXL4 deficiency.[10] GeneReviews echoes that no proven pharmacologic therapy exists to modify disease progression.[11][12]

Supportive pharmacotherapy focuses on symptom management and prevention of complications. For lactic acidosis, intravenous bicarbonate and careful fluid management can mitigate acute metabolic acidosis.[10][11][12] For seizures, antiepileptic drugs are used, with caution to avoid mitochondrial-toxic agents such as valproic acid.[10][11] Cardiac medications, including ACE inhibitors, beta-blockers, or diuretics, may be indicated for cardiomyopathy and heart failure.[7][10][11] Antibiotics and immunoglobulin therapy may be used for infections and immunodeficiency, though these are not specific to FBXL4 deficiency.[7][10]

NCIT (NCI Thesaurus) clinical intervention terms relevant to pharmacotherapy include “Mitochondrial disease supportive therapy,” “Antiepileptic therapy,” “Cardiomyopathy management,” and “Metabolic acidosis therapy.” These can be linked to specific drug classes in the knowledge base.

12.2 Advanced Therapeutics and Experimental Approaches

Advanced therapeutics such as gene therapy, cell therapy, and RNA-based interventions have not yet been applied clinically to FBXL4-related MTDPS13, but mechanistic insights suggest potential strategies.[9][11][13] Given that FBXL4 deficiency leads to excessive mitophagy via BNIP3/BNIP3L accumulation and lysosomal degradation of mitochondria, interventions aimed at suppressing mitophagy or modulating lysosomal activity could theoretically restore mitochondrial content and improve function.[9][13]

EMBO Molecular Medicine showed that inhibition of lysosomal function in FBXL4-deficient cells reverses the mitochondrial phenotype, whereas proteasomal inhibition does not, indicating that lysosomal pathways are key targets.[9] Kulkarni et al. suggested that “interventions to stop the increased mitochondrial turnover should be considered as a potential treatment for this disease,” pointing toward pharmacologic or genetic modulation of mitophagy receptors or lysosomal pathways.[9][13] For example, small molecules or RNA-based therapies that reduce BNIP3/BNIP3L expression or block their function could restore mitochondrial abundance.

Gene therapy, involving delivery of functional FBXL4 via viral vectors, could, in principle, correct the defect at its source. The success of FBXL4 transfection in rescuing cardiac geometry and mitochondrial integrity in a mouse model of heart failure with preserved ejection fraction suggests that FBXL4 replacement can restore mitochondrial dynamics in adult tissues.[11] However, translating this to a pediatric mitochondrial disease poses challenges, including delivery to multiple organs and the need to intervene early in life.

Cell therapy, such as transplantation of healthy hematopoietic stem cells, is unlikely to address the global mitochondrial defect affecting neurons and cardiomyocytes. RNA-based therapies, such as antisense oligonucleotides (ASOs) or siRNA targeting BNIP3/BNIP3L, may be more feasible but remain hypothetical.[9][13]

ClinicalTrials.gov and related registries do not list active clinical trials specifically for FBXL4-related MTDPS13 in the available search results, indicating that advanced therapeutics are still at the preclinical stage.

12.3 Surgical and Interventional Care

Surgical interventions in FBXL4-related MTDPS13 are primarily supportive and focused on managing complications. Gastrostomy tube placement for enteral feeding may be necessary in children with severe feeding difficulties and aspiration risk.[10][11][12] Cardiac device implantation, such as pacemakers or defibrillators, could be considered in cases with life-threatening arrhythmias, though evidence is anecdotal.[7][10][11] Surgical correction of congenital heart malformations may be attempted, but surgical risk is higher due to metabolic vulnerability and anesthesia concerns.[10][11]

NCIT terms relevant to surgical interventions include “Gastrostomy tube placement,” “Cardiac pacemaker insertion,” and “Congenital heart defect repair.”

12.4 Rehabilitation and Supportive Care

Rehabilitation and supportive care are essential components of treatment, aiming to optimize function and quality of life despite irreversible neurologic and metabolic deficits.[10][11][12] Physical therapy can help maintain joint mobility, prevent contractures, and support motor function within the constraints imposed by hypotonia and weakness.[10][11] Occupational therapy can assist with activities of daily living, adaptive equipment, and environmental modifications.[10][11] Speech therapy may address feeding and communication issues, although many children have limited speech due to severe developmental delay.[10][11][12]

Nutritional support, including specialized diets and enteral feeding, is critical to maintain adequate caloric intake and prevent further failure to thrive.[10][11][12] Palliative care services may be involved to support families and manage symptoms such as pain, dyspnea, and distress. NCIT terms relevant to supportive and rehabilitative care include “Physical therapy,” “Occupational therapy,” “Speech therapy,” “Nutritional support therapy,” and “Palliative care.”

12.5 Treatment Outcomes and Personalized Medicine

Treatment response in FBXL4-related MTDPS13 is limited. Supportive therapy can reduce acute complications and stabilize metabolic status but does not reverse neurodevelopmental impairment or cure the disease.[10][11][12] No pharmacologic agent has demonstrated clear efficacy in controlled studies, and the use of “mitochondrial medications” has not improved long-term outcomes.[7][10][11]

Personalized medicine approaches currently focus on genetic counseling and reproductive planning, rather than individualized treatment based on genotype, as no genotype-specific therapies exist.[11][12][16] However, understanding the precise FBXL4 variant may influence prognosis and decisions about intensity of care, particularly when variants are predicted to retain partial function versus complete loss-of-function.

Future personalized therapies could involve targeting specific pathways affected by FBXL4 deficiency, such as BNIP3/BNIP3L-mediated mitophagy, but these remain under investigation.[9][13]

13. Prevention

13.1 Primary, Secondary, and Tertiary Prevention

Primary prevention of FBXL4-related MTDPS13 focuses on avoiding the birth of affected children through genetic counseling, carrier testing, and reproductive options such as prenatal diagnosis and preimplantation genetic diagnosis.[11][12] GeneReviews recommends that once the familial FBXL4 pathogenic variants are known, carrier testing for at-risk relatives and prenatal testing for pregnancies at risk should be offered.[11][12] Couples with a history of FBXL4-related MTDPS13 can use PGD to select embryos without biallelic FBXL4 mutations, thereby preventing disease in offspring.[11][12]

Secondary prevention—early detection and treatment of disease to reduce severity—is limited by the disease’s congenital nature and lack of disease-modifying therapies. However, early diagnosis via genetic testing can prevent unnecessary diagnostic procedures and facilitate early supportive care, which may mitigate complications and improve quality of life.[10][11][12] Huemer et al. noted that establishing the diagnosis “permits genetic counselling, prevents patients undergoing unhelpful diagnostic procedures and allows for accurate prognosis,” highlighting the secondary preventive value of timely diagnosis.[10]

Tertiary prevention aims to prevent complications and reduce disability in individuals with established disease. This includes aggressive management of lactic acidosis, cardiac monitoring, infection control, nutritional support, and rehabilitation.[10][11][12] These measures do not prevent disease occurrence but can reduce the impact of complications and improve functional outcomes within the limits set by the underlying mitochondrial defect.

13.2 Immunization, Screening, and Counseling

Immunization strategies for FBXL4-related MTDPS13 follow standard pediatric vaccine schedules, with attention to preventing infections that could trigger metabolic crises.[10][11][12] There is no disease-specific vaccine. Newborn screening programs do not currently include FBXL4-related MTDPS13, given its rarity and the absence of effective treatments.[11][12][17]

Genetic screening plays a central role in prevention. Carrier screening in populations with high consanguinity or known FBXL4 mutations can identify couples at risk of having affected children.[6][10][11][12] Risk stratification based on carrier status informs decisions about prenatal testing and PGD. Counseling by genetics professionals, guided by NSGC and ACMG guidelines, is essential to help families understand inheritance patterns, recurrence risks, and available reproductive options.[11][12][16]

Behavioral interventions such as lifestyle modifications have limited direct impact on disease risk, as FBXL4-related MTDPS13 is primarily genetic and early-onset. However, counseling can address family planning, psychological support, and management of caregiver stress.

Public health interventions are not specific to FBXL4 deficiency, but general awareness of rare genetic diseases can support resource allocation and research funding.

14. Other Species and Natural Disease

14.1 Species and Orthologs

Orthologous genes to human FBXL4 exist in other species, including mouse (Fbxl4) and other vertebrates, reflecting evolutionary conservation of the F-box/LRR protein family and mitochondrial quality control mechanisms.[8][9] NCBI Gene lists FBXL4 in Homo sapiens, and related articles discuss Fbxl4 knockout mice used for mechanistic studies.[8][9] The Alliance of Genome Resources and HomoloGene likely map FBXL4 orthologs in mouse, zebrafish, and other model organisms, though specific identifiers are not provided in the search results.

14.2 Natural Disease in Animals and Comparative Pathology

No naturally occurring FBXL4-related mitochondrial DNA depletion syndromes have been reported in companion animals or livestock in the available literature, and OMIA does not list such a disease in the search results.[9] Veterinary relevance of FBXL4 deficiency remains primarily theoretical, based on the presence of orthologous genes and the importance of mitochondrial quality control in animal health.

Comparative pathology focuses on similarities between human FBXL4-related MTDPS13 and phenotypes observed in Fbxl4 knockout mice. EMBO Molecular Medicine’s mouse model demonstrates perinatal lethality, delayed onset mitochondrial dysfunction in survivors, mtDNA depletion, and global reduction in mitochondrial proteins, paralleling human disease features.[9] Weight loss and signs of mitochondrial disease in adult mice resemble chronic energy failure in humans, although neurologic and behavioral phenotypes in mice are less detailed.[9]

Evolutionary conservation of the SCF-FBXL4–BNIP3/BNIP3L pathway suggests that mitochondrial quality control via mitophagy suppression is a conserved mechanism across mammals, and its disruption can cause mitochondrial disease in multiple species.[9][13]

14.3 Transmission and Zoonotic Potential

FBXL4-related MTDPS13 is a non-infectious, hereditary disease and has no zoonotic potential. It is not transmissible between individuals via pathogens and does not involve cross-species susceptibility beyond shared genetic mechanisms. Transmission occurs vertically via autosomal recessive inheritance within human families.[6][10][11][12]

15. Model Organisms

15.1 Fbxl4 Knockout Mouse Model

The most informative model organism for FBXL4-related MTDPS13 is the Fbxl4 knockout mouse described in the EMBO Molecular Medicine study.[9] Researchers generated homozygous Fbxl4 knockout mice and found that they displayed predominant perinatal lethality, with only a few animals surviving into adulthood.[9] Surprisingly, the surviving animals were apparently normal until the age of 8–12 months, when they gradually developed signs of mitochondrial dysfunction and weight loss.[9] One-year-old Fbxl4 knockouts showed a global reduction in mitochondrial proteins and mtDNA depletion, whereas lysosomal proteins were upregulated.[9]

The EMBO study reports:

“We generated homozygous Fbxl4 knockout mice and found that they display a predominant perinatal lethality. Surprisingly, the few surviving animals are apparently normal until the age of 8–12 months when they gradually develop signs of mitochondrial dysfunction and weight loss. One-year-old Fbxl4 knockouts show a global reduction in a variety of mitochondrial proteins and mtDNA depletion, whereas lysosomal proteins are upregulated.”[9]

Fibroblasts from patients with FBXL4 deficiency and human FBXL4 knockout cells also showed reduced steady-state levels of mitochondrial proteins, attributed to increased mitochondrial turnover.[9] Inhibition of lysosomal function reversed the mitochondrial phenotype, while proteasomal inhibition had no effect, confirming that lysosomal mitophagy drives mitochondrial loss in FBXL4 deficiency.[9]

This mouse model recapitulates key features of human disease at the molecular level, including mtDNA depletion, decreased mitochondrial proteins, and upregulated lysosomal proteins, and demonstrates that FBXL4 plays a critical role in mitochondrial quality control.[9] However, the timing and severity of phenotypes differ: mice exhibit perinatal lethality and late-onset mitochondrial dysfunction in survivors, whereas humans typically show early infantile onset.[9][10][11] Neurologic and cardiac phenotypes in the mouse model are less well characterized, and behavioral outcomes are not extensively described.[9]

15.2 Human Cell Line Models and Patient Fibroblasts

Human cell-based models have been crucial for mechanistic studies. CRISPR/Cas9 knockout human cell lines lacking FBXL4 replicate patient fibroblast phenotypes, including reduced mitochondrial proteins, mtDNA depletion, increased lysosomal proteins, and heightened mitophagy.[9][13] Patient-derived fibroblasts from individuals with FBXL4 deficiency exhibit severe respiratory chain deficiency, loss of mitochondrial membrane potential, fragmentation of the mitochondrial network, enlarged nucleoids, and disturbed mtDNA distribution.[6][7][14]

These cellular models allow precise experimental manipulation and high-throughput screening of mitophagy modifiers, making them valuable tools for preclinical therapeutic development.[9][13] They also enable detailed study of SCF-FBXL4–BNIP3/BNIP3L interactions, autophagic flux, and mitochondrial dynamics under controlled conditions.[9][13]

15.3 Model Limitations and Applications

Model organisms and cell lines for FBXL4-related MTDPS13 capture many molecular and cellular features of the human disease, such as mtDNA depletion, respiratory chain defects, increased mitophagy, and altered mitochondrial–lysosomal balance.[6][7][9][13][14] However, they have limitations in reproducing the full spectrum of clinical phenotypes, including severe early-onset encephalopathy, craniofacial dysmorphism, and multi-organ involvement seen in human patients.[9][10][11][12]

The mouse model’s late-onset mitochondrial dysfunction contrasts with human early-onset disease, suggesting species differences in developmental timing and mitochondrial resilience.[9][10][11] Behavioral and cognitive phenotypes in mice are not well described, and the model may not fully replicate human neurodevelopmental outcomes. Cell line models lack tissue context and cannot capture organ-level interactions or systemic metabolic responses.[9][13]

Despite these limitations, model organisms and cell lines are highly valuable for studying the molecular pathophysiology of FBXL4 deficiency, testing interventions that modulate mitophagy or lysosomal function, and exploring the role of FBXL4 in other mitochondrial diseases. Applications include screening small molecules that inhibit BNIP3/BNIP3L, assessing gene therapy vectors for FBXL4 delivery, and investigating interactions between FBXL4 and other mitochondrial maintenance genes.[9][13]

Conclusion

FBXL4-related mitochondrial DNA depletion syndrome (MTDPS13) is a paradigmatic example of a severe, early-onset, nuclear-encoded mitochondrial maintenance disorder that integrates genetic, molecular, cellular, and clinical complexities into a coherent disease entity. Biallelic loss-of-function variants in FBXL4, a mitochondrial outer-membrane F-box and leucine-rich repeat protein, disrupt the SCF-FBXL4 E3 ubiquitin ligase’s ability to ubiquitinate and degrade BNIP3 and BNIP3L/NIX mitophagy receptors, leading to excessive mitophagy, lysosomal removal of mitochondria, and global reduction in mitochondrial content and mtDNA copy number.[6][8][9][13][14] The resulting combined respiratory chain defects and energy failure manifest clinically as neonatal or early-infantile encephalopathy, persistent lactic acidosis, hypotonia, failure to thrive, severe developmental delay, and multi-organ involvement, including cardiac disease, craniofacial dysmorphism, and occasional immunodeficiency.[2][6][7][9][10][11][12][14][17]

Curated resources such as OMIM, Orphanet, MedGen, GeneReviews, and MedlinePlus provide consistent disease definitions and identifiers, including OMIM 615471, ORPHA:369897, MedGen C3809592, and MONDO:0014198, and emphasize the autosomal recessive inheritance, high mortality, and severe disability associated with FBXL4-related MTDPS13.[1][2][3][10][11][12][17][18] Clinical series, notably Huemer et al.’s 21-patient cohort, quantify phenotype frequencies and outcomes, showing near-universal lactic acidosis and hypotonia, frequent facial dysmorphism, and cardiac involvement in over half of cases, with mean death around 3 years in those who succumb and severe psychomotor retardation in survivors.[10][11] Mechanistic studies, including Bonnen et al.’s gene discovery work and EMBO Molecular Medicine’s mouse and cell line models, elucidate the central role of FBXL4 in mitochondrial quality control and mtDNA maintenance, while the Autophagy commentary clarifies FBXL4’s specific function in suppressing mitophagy via BNIP3/BNIP3L turnover.[6][9][13][14]

From a diagnostic perspective, FBXL4-related MTDPS13 should be suspected in infants with early-onset encephalopathy, lactic acidosis, hypotonia, failure to thrive, and combined respiratory chain defects with mtDNA depletion, particularly in consanguineous families.[2][6][7][10][11][14][17] Molecular genetic testing via multigene panels, WES, or WGS is essential for definitive diagnosis, and ClinVar provides detailed variant interpretations for numerous pathogenic and likely pathogenic FBXL4 alleles.[4][5][15][16] Treatment remains largely supportive, with no proven disease-modifying pharmacotherapy, and “mitochondrial medications” have not shown efficacy.[7][10][11] Management focuses on metabolic stabilization, cardiac care, nutritional support, rehabilitation, and palliative care, while genetic counseling and reproductive planning offer avenues for primary prevention.[10][11][12][17]

Future directions in FBXL4-related MTDPS13 research and care include further elucidation of modifier genes and gene–environment interactions, development of targeted therapies that modulate mitophagy or lysosomal function, exploration of gene therapy to restore FBXL4 activity, and integration of multi-omics profiling to refine disease mechanisms and biomarkers.[9][11][13] Improved model organisms and cell-based systems will facilitate these efforts, but translation to clinical benefit will require careful consideration of timing, organ targeting, and safety in a vulnerable pediatric population. Until such advances materialize, FBXL4-related MTDPS13 remains a devastating mitochondrial disease that poses significant challenges for affected families and healthcare systems, and comprehensive knowledge base entries—grounded in current evidence and ontology frameworks—are essential for supporting diagnosis, counseling, and future therapeutic innovation.

Reference Validation

No PMID or DOI references were found in this report.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome	Count
Terms checked	72
Resolved	66
Unresolved (possible confabulation)	0
Obsolete	1
Unverifiable	5
Terms whose name was checked	37
Terms named correctly	19
Terms named as a different term	7
Terms whose name is worth a second look	11

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

MONDO:0014198 (8 mentions) - the report calls it "if available", "MONDO", "FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome"; MONDO calls it mitochondrial DNA depletion syndrome 13
HP:0031486 (1 mention) - the report calls it "Mitochondrial DNA depletion syndrome"; HP calls it Vascular malformation of the lip
HP:0011010 (1 mention) - the report calls it "Feeding difficulties"; HP calls it Chronic
HP:0001626 (1 mention) - the report calls it "Cardiomyopathy"; HP calls it Abnormality of the cardiovascular system
HP:0001272 (1 mention) - the report calls it "Congenital heart malformation"; HP calls it Cerebellar atrophy
HP:0002093 (2 mentions) - the report calls it "Pulmonary hypertension"; HP calls it Respiratory insufficiency
HP:0001876 (1 mention) - the report calls it "Bone marrow failure"; HP calls it Pancytopenia

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

GO:0070265 (obsolete necrotic cell death) (1 mention)

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

HP:0001252 (3 mentions) - the report calls it "Hypotonia", "Muscular hypotonia"; HP calls it Hypotonia, and lists "Muscular hypotonia" among its other names
HP:0002500 (1 mention) - the report calls it "White matter abnormalities"; HP calls it Abnormal cerebral white matter morphology, and lists "White matter abnormalities" among its other names
HP:0002151 (1 mention) - the report calls it "Elevated serum lactate"; HP calls it Increased circulating lactate concentration, and lists "Increased serum lactate" among its other names
HP:0001999 (2 mentions) - the report calls it "Facial dysmorphism", "Craniofacial dysmorphism"; HP calls it Abnormal facial shape, and lists "Facial dysmorphism" among its other names
HP:0000519 (1 mention) - the report calls it "Congenital cataract"; HP calls it Developmental cataract, and lists "Cataract, congenital" among its other names
GO:0000422 (3 mentions) - the report calls it "mitophagy", "autophagy of mitochondrion"; GO calls it autophagy of mitochondrion, and lists "mitophagy" among its other names
GO:0010498 (2 mentions) - the report calls it "SCF-dependent proteasomal ubiquitin-dependent protein catabolic process"; GO calls it proteasomal protein catabolic process
GO:1901524 (1 mention) - the report calls it "negative regulation of mitophagy"; GO calls it regulation of mitophagy
GO:0032543 (1 mention) - the report calls it "mitochondrial DNA metabolic process"; GO calls it mitochondrial translation, and lists "mitochondrial protein anabolism" among its other names
GO:0006979 (1 mention) - the report calls it "oxidative stress"; GO calls it response to oxidative stress
GO:0070265 (1 mention) - the report calls it "necrotic cell death"; GO calls it obsolete necrotic cell death

Terms named inconsistently

The report gives these identifiers more than one name of its own:

MONDO:0014198 - called "if available", "MONDO", "FBXL4-related encephalomyopathic mitochondrial DNA depletion syndrome"
HP:0001252 - called "Hypotonia", "Muscular hypotonia"
HP:0001999 - called "Facial dysmorphism", "Craniofacial dysmorphism"
GO:0000422 - called "mitophagy", "autophagy of mitochondrion"

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA, OMIM, Chr6.

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Discussions and Knowledge Gaps

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Diagnosis

Progression

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References & Deep Research

References

Deep Research

1. Disease Information

1.1 Overview and Disease Definition

1.2 Key Identifiers and Ontology Mappings

1.3 Synonyms and Alternative Names

1.4 Source of Information: Patient-Level vs Aggregated Data

2. Etiology

2.1 Primary Causal Factors: Genetic Basis in FBXL4

2.2 Genetic Risk Factors and Variant Spectrum

2.3 Environmental and Non-Genetic Risk Factors

2.4 Protective Factors and Potential Modifiers

2.5 Gene–Environment Interactions

3. Phenotypes

3.1 Global Clinical Phenotype and Age of Onset

3.2 Neurologic and Developmental Phenotypes

3.3 Muscular, Growth, and Feeding Phenotypes

3.4 Metabolic Phenotypes: Lactic Acidosis and Hyperammonemia

3.5 Cardiac Phenotypes

3.6 Craniofacial and Dysmorphic Phenotypes

3.7 Ophthalmologic Phenotypes

3.8 Immune, Hematologic, and Other Systemic Phenotypes

3.9 Phenotype Frequencies and Summary Table

4. Genetic and Molecular Information

4.1 Causal Gene: FBXL4

4.2 Pathogenic Variants: Types, Classification, and Frequencies

4.3 Modifier Genes and Genetic Interactions

4.4 Epigenetic and Chromosomal Abnormalities

5. Environmental Information

5.1 Environmental Factors

5.2 Lifestyle and Infectious Factors

6. Mechanism and Pathophysiology

6.1 Ordered Causal Chain from Mutation to Clinical Manifestation

6.2 Molecular Pathways: SCF-FBXL4, Mitophagy, and Autophagy

6.3 Cellular Processes: Mitochondrial Dynamics, Respiratory Chain, and Energy Failure

6.4 Metabolic Changes and Biochemical Abnormalities

6.5 Immune System and Tissue Damage Mechanisms

6.6 Molecular Profiling and Advanced Technologies

7. Anatomical Structures Affected

7.1 Organ-Level Involvement

7.2 Tissue and Cell-Level Involvement

7.3 Subcellular Compartments

7.4 Localization and Lateralization

8. Temporal Development

8.1 Age of Onset and Onset Pattern

8.2 Disease Progression, Rate, and Course

8.3 Critical Periods and Windows of Intervention

9. Inheritance and Population Characteristics

9.1 Inheritance Pattern, Penetrance, and Expressivity

9.2 Epidemiology: Prevalence and Incidence

9.3 Population Demographics, Consanguinity, and Founder Effects

10. Diagnostics

10.1 Clinical and Laboratory Testing

10.2 Imaging and Electrophysiology

10.3 Genetic Testing Strategies

10.4 Omics-Based Diagnostics and Molecular Biomarkers

10.5 Clinical Diagnostic Criteria and Differential Diagnosis

10.6 Screening

11. Outcome and Prognosis