Multiple Mitochondrial Dysfunctions Syndrome 4

Genetic MONDO:0014611 Pathograph 17 Show in embeddings browser MONDO:0044970

Multiple Mitochondrial Dysfunctions Syndrome 4 (MMDS4; OMIM #616370), also called ISCA2-related mitochondrial disorder, is a fatal autosomal recessive infantile neurodegenerative leukoencephalopathy caused by biallelic loss-of-function variants in ISCA2, encoding iron-sulfur cluster assembly factor 2. ISCA2 acts with ISCA1 and IBA57 in the late steps of mitochondrial [4Fe-4S] cluster biogenesis, catalyzing the reductive fusion of two GLRX5-derived [2Fe-2S] clusters into a [4Fe-4S] cluster. Loss of ISCA2 impairs maturation of [4Fe-4S] client enzymes — respiratory chain complexes I and II, aconitase, and lipoic acid synthase (LIAS). LIAS deficiency reduces lipoylation of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase and inactivates the glycine cleavage system, producing hyperglycinemia; respiratory chain failure produces lactic acidosis. Affected infants develop normally for the first months, then between roughly 2 and 7 months of age undergo rapid neurodevelopmental regression with the triad of regression, optic atrophy with nystagmus, and diffuse white matter disease, becoming vegetative within 1-2 years and usually dying in early childhood. A homozygous founder missense variant, c.229G>A (p.Gly77Ser), accounts for the large majority of reported (predominantly Saudi Arabian) cases; sporadic non-founder variants have been reported in other populations. Fewer than 25 patients had been reported worldwide as of 2020.

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1
Inheritance
5
Pathophys.
10
Phenotypes
1
Gaps
17
Pathograph
1
Genes
2
Variants
6
Medical Actions
1
References
1
Deep Research
🏷

Classifications

Harrison's Part
GENETICS ENVIRONMENT DISEASE
Mechanistic Nosology
mitochondrial disease
👪

Inheritance

1
Autosomal recessive HP:0000007
MMDS4 is inherited in an autosomal recessive manner; parents are obligate asymptomatic heterozygous carriers, with a 25% recurrence risk per pregnancy.
Autosomal recessive inheritance
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"ISCA2-related mitochondrial disorder is inherited in an autosomal recessive manner. At conception, each sib of an affected individual with IRMD has a 25% chance of being affected"
GeneReviews states the autosomal recessive inheritance pattern and recurrence risk.
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Discussions and Knowledge Gaps

1
Does any whole-organism vertebrate model reproduce the CNS leukoencephalopathy and optic-atrophy phenotype of human ISCA2 deficiency, given that in mouse ISCA1 (not ISCA2) is reported to be required for mitochondrial [4Fe-4S] protein maturation in postmitotic tissues under standard conditions?
HUMAN MODEL MISMATCH OPEN isca2_no_vertebrate_cns_model
Attached to
pathophysiology#Biallelic ISCA2 Loss of Function
Human ISCA2 loss is fully penetrant and fatal, yet a review of the pathway states that ISCA2 (and IBA57) are not required in postmitotic mouse cell types under standard physiological conditions, where ISCA1 is. A straightforward Isca2-null rodent may therefore not recapitulate the human CNS disease, so the translational validity of any such model is an open question that should be recorded rather than assumed.
Recorded from the review's synthesis only; the primary rodent studies were not independently retrieved in this curation, so no whole-organism model link is asserted in the pathograph.
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"The ISCA1 is required for the maturation of mitochondrial Fe4S4 proteins in mature skeletal muscle, whereas ISCA2 and IBA57 are not required in the 2 postmitotic cell types, under standard physiological conditions."
Documents the mouse finding that ISCA2 is dispensable in postmitotic tissues, the basis for the human-model mismatch given the fully penetrant human disease.
⚙

Pathophysiology

5
Biallelic ISCA2 Loss of Function
Biallelic loss-of-function ISCA2 variants (e.g., the founder p.Gly77Ser) reduce or abolish the Fe-S cluster carrier function of iron-sulfur cluster assembly factor 2.
Genetic context allele_type: missense (founder p.Gly77Ser); also frameshift and splice-site alleles variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: LOSS_OF_FUNCTION
Biallelic germline loss-of-function ISCA2 variants; most patients are homozygous for the p.Gly77Ser founder missense allele, while sporadic cases carry compound or homozygous non-founder missense, frameshift, or splice-affecting alleles.
iron-sulfur cluster binding GO:0051536 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves iron-sulfur cluster binding (GO:0051536), qualified as loss of function. GO:0051536 is a molecular function from the Gene Ontology. ⇓ LOSS OF FUNCTION
Downstream
Defective Mitochondrial 4Fe-4S Cluster Biogenesis
Defective Mitochondrial 4Fe-4S Cluster Biogenesis
ISCA2 normally heterodimerizes with ISCA1 (with IBA57) to catalyze the reductive fusion of two GLRX5-derived [2Fe-2S] clusters into a [4Fe-4S] cluster, the terminal step of mitochondrial iron-sulfur cluster biogenesis. Loss of ISCA2 impairs [4Fe-4S] cluster supply while sparing [2Fe-2S] proteins.
[4Fe-4S] cluster assembly GO:0044572 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased [4Fe-4S] cluster assembly (GO:0044572). GO:0044572 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Failure to Mature 4Fe-4S Client Enzymes
Failure to Mature 4Fe-4S Client Enzymes
Deficient [4Fe-4S] cluster supply prevents maturation of multiple client holoenzymes, including respiratory chain complexes I and II, the TCA-cycle enzyme aconitase, and lipoic acid synthase (LIAS), itself a [4Fe-4S] radical-SAM enzyme.
Downstream
Respiratory Chain Complex I and II Deficiency
Deficient Protein Lipoylation
Respiratory Chain Complex I and II Deficiency
Failure to mature [4Fe-4S] subunits of respiratory complexes I and II (succinate dehydrogenase) impairs oxidative phosphorylation and, with reduced aconitase and TCA-cycle flux, produces lactic acidosis. Mitochondrial DNA depletion has also been reported.
mitochondrial electron transport, NADH to ubiquinone GO:0006120 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased mitochondrial electron transport, NADH to ubiquinone (GO:0006120). GO:0006120 is a biological process from the Gene Ontology. ↓ DECREASED mitochondrial electron transport, succinate to ubiquinone GO:0006121 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased mitochondrial electron transport, succinate to ubiquinone (GO:0006121). GO:0006121 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Elevated lactate
Leukoencephalopathy
Optic atrophy
Deficient Protein Lipoylation
Loss of LIAS activity reduces lipoylation of the E2 subunits of pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase and of the H-protein of the glycine cleavage system, inactivating glycine catabolism.
protein lipoylation GO:0009249 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased protein lipoylation (GO:0009249). GO:0009249 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Elevated glycine (hyperglycinemia)
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Multiple Mitochondrial Dysfunctions Syndrome 4 Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

10
Eye 2
Optic atrophy VERY_FREQUENT HP:0000648 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Optic atrophy (HP:0000648). HP:0000648 is a phenotype from the Human Phenotype Ontology.
Sequelae: Nystagmus
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"the triad of neurodevelopmental regression, optic atrophy with nystagmus, and diffuse white matter disease"
Optic atrophy is a component of the characteristic MMDS4 presenting triad.
Nystagmus FREQUENT HP:0000639 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Nystagmus (HP:0000639). HP:0000639 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"affected individuals usually present with a triad of neurodevelopmental regression, nystagmus with optic atrophy, and diffuse white matter disease"
GeneReviews lists nystagmus with optic atrophy in the presenting triad.
Immune 1
Recurrent respiratory infections FREQUENT HP:0002205 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent respiratory infections (HP:0002205). HP:0002205 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"affected individuals are prone to recurrent chest infections that may require ventilator support"
GeneReviews reports recurrent chest infections as a common late/terminal complication.
Metabolism 2
Elevated glycine (hyperglycinemia) FREQUENT HP:0002154 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hyperglycinemia (HP:0002154). HP:0002154 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29297947 SUPPORT Human Clinical
"We present two additional unrelated cases, and provide a more complete clinical description that includes hyperglycinemia, leukodystrophy of the brainstem with longitudinally extensive spinal cord involvement, and mtDNA deficiency."
Reports hyperglycinemia as part of the expanded clinical description of ISCA2 deficiency.
Elevated lactate FREQUENT Increased circulating lactate concentration HP:0002151 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased circulating lactate concentration (HP:0002151). HP:0002151 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"Biochemical abnormalities include high cerebrospinal fluid glycine and lactate and high plasma glycine and lactate, but these findings were not consistent."
Review reports elevated lactate (and glycine) in body fluids, noting inconsistency across patients.
Musculoskeletal 2
Spasticity VERY_FREQUENT HP:0001257 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Spasticity (HP:0001257), qualified as course progressive. HP:0001257 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:34440194 SUPPORT Human Clinical
"clinical status was deteriorated during the first year of life leading to spasticity and visual impairment (nystagmus and/or optic atrophy)"
Review describes progressive spasticity developing during the first year.
Hypotonia VERY_FREQUENT HP:0001252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypotonia (HP:0001252). HP:0001252 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"Spasticity and axial hypotonia present in 95%"
Review reports axial hypotonia (with spasticity) in about 95% of the pooled ISCA2 cohort.
Nervous System 3
Developmental regression VERY_FREQUENT HP:0002376 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Developmental regression (HP:0002376), qualified as course progressive. HP:0002376 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"At age three to seven months, affected individuals usually present with a triad of neurodevelopmental regression, nystagmus with optic atrophy, and diffuse white matter disease."
GeneReviews describes neurodevelopmental regression as a core presenting feature.
Leukoencephalopathy VERY_FREQUENT HP:0002352 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Leukoencephalopathy (HP:0002352). HP:0002352 is a phenotype from the Human Phenotype Ontology.
Sequelae: Developmental regression Spasticity Seizures
Show evidence (1 reference)
PMID:34440194 SUPPORT Human Clinical
"Lesions in cerebral and cerebellar white matter and the spinal cord were indeed quite constantly present"
Review documents the constant, diffuse white matter involvement in MMDS4.
Seizures OCCASIONAL HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"As the disease progresses, global psychomotor regression continues at a variable pace and seizures may develop."
GeneReviews notes seizures developing in a subset of patients with disease progression.
🧬

Genetic Associations

1
ISCA2
Gene: ISCA2 hgnc:19857 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ISCA2 (hgnc:19857). hgnc:19857 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (2 references)
PMID:25539947 SUPPORT Human Clinical
"We identified a homoallelic missense founder mutation in ISCA2 leading to mitochondrial depletion and reduced complex I activity as well as decreased ISCA2, ISCA1 and IBA57 expression in fibroblasts."
Original description identifying the homozygous ISCA2 founder missense variant as the cause of an infantile mitochondrial leukodystrophy.
PMID:34440194 SUPPORT Human Clinical
"in five other patients from different countries, seven sporadic variants were identified. These seven variants affect all regions of the ISCA2 protein"
Documents allelic heterogeneity: besides the Saudi founder variant, seven sporadic ISCA2 variants span the protein.
Variants (2)
c.229G>A (p.Gly77Ser) Pathogenic
missense
Recurrent homozygous missense founder variant in consanguineous Arab (predominantly Saudi Arabian) families; the most frequently reported ISCA2 allele and the one in the original MMDS4 description.
Show evidence (1 reference)
PMID:34440194 SUPPORT Human Clinical
"Molecular studies revealed a homozygous variant in ISCA2 for each patient, the Gly77Ser variant, leading to the description of the first patients with MMDS 4"
Identifies p.Gly77Ser as the recurrent founder allele in the original MMDS4 cohort.
c.355G>A (p.Ala119Thr) Likely Pathogenic
missense
Novel non-founder missense variant reported in a consanguineous Iranian family, predicted to eliminate a helix motif in the Fe-S biosynthesis domain; the third ISCA2 patient reported worldwide at the time and outside the Saudi founder background.
Show evidence (1 reference)
PMID:31279336 SUPPORT Human Clinical
"A novel variant (c.355G > A, p.Ala119Thr) in ISCA2 gene was identified by WES in the proband."
Reports a non-founder ISCA2 missense allele in an Iranian family, establishing allelic and geographic heterogeneity beyond the founder variant.
💊

Medical Actions

6
Supportive and multidisciplinary care
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
No curative therapy exists; management is exclusively supportive, drawing on genetics, neurology, dietetics, and developmental specialists.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"Treatment is primarily supportive and may require input from a geneticist, neurologist, dietician, and developmental specialist."
GeneReviews specifies supportive, multidisciplinary management as the standard of care.
Enteral nutritional support (feeding tube)
Action: Nutritional SupportNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Nutritional Support (NCIT:C15433). NCIT:C15433 is a clinical intervention from the NCI Thesaurus. NCIT:C15433
A nasogastric or gastrostomy feeding tube is typically required as bulbar and motor function deteriorate; periodic evaluation of swallowing function is suggested to time tube placement.
Show evidence (2 references)
PMID:29470032 SUPPORT Human Clinical
"A feeding tube (nasogastric or gastrostomy) is typically required."
GeneReviews states that enteral feeding-tube support is typically needed.
PMID:29470032 SUPPORT Human Clinical
"Periodic evaluation of swallowing function is suggested."
GeneReviews recommends swallowing-function surveillance, which times the feeding-tube intervention.
Antiseizure pharmacotherapy
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Standard anticonvulsant therapy is used for the subset of patients who develop epilepsy.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"Standard treatment for epilepsy. Recurrent chest infections may require ventilator support in addition to antimicrobial therapy."
GeneReviews recommends standard antiepileptic pharmacotherapy for the seizures that develop in a subset of patients.
Antispasticity pharmacotherapy (baclofen)
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: baclofen CHEBI:2972 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses baclofen (CHEBI:2972). CHEBI:2972 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
For muscle tone abnormalities including hypertonia, baclofen and/or botulinum toxin (Botox) may be considered.
Mechanism Target:
INHIBITS Spasticity — Baclofen (a GABA-B agonist) reduces the hypertonia/spasticity that develops as the encephalopathy progresses.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"For muscle tone abnormalities including hypertonia, baclofen"
GeneReviews recommends baclofen for the hypertonia/spasticity of ISCA2 disorder.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"For muscle tone abnormalities including hypertonia, baclofen"
GeneReviews recommends baclofen (and/or botulinum toxin) for muscle-tone abnormalities.
Early intervention and developmental services
Action: early intervention and developmental servicesNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is early intervention and developmental services, annotated with Rehabilitation (NCIT:C15315). NCIT:C15315 is a clinical intervention from the NCI Thesaurus. Ontology label: Rehabilitation NCIT:C15315
Referral to early intervention services is recommended to support development within the limits imposed by the progressive course.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"Referral to early intervention services is recommended."
GeneReviews recommends early-intervention referral as part of developmental support.
Genetic counseling
Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Autosomal recessive counseling with a 25% recurrence risk; carrier testing and prenatal/preimplantation testing are available when the familial variants are known.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"Carrier testing for at-risk relatives and prenatal testing for a pregnancy at increased risk are possible if the ISCA2 pathogenic variants in the family are known."
GeneReviews describes carrier and prenatal testing offered through genetic counseling.
🔬

Diagnosis

4
Molecular genetic testing for biallelic ISCA2 variants
The diagnosis is established by identifying biallelic pathogenic ISCA2 variants, typically by a mitochondrial/leukodystrophy gene panel or exome/ genome sequencing; targeted testing for c.229G>A is efficient in individuals of Arab/Saudi ancestry given the founder effect.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"The diagnosis of ISCA2-related mitochondrial disorder is established in a proband by the identification of biallelic pathogenic variants in ISCA2 on molecular genetic testing."
GeneReviews states molecular identification of biallelic ISCA2 variants as the diagnostic standard.
Brain MRI
Brain MRI shows diffuse, bilateral, symmetric white matter disease; cerebellar white matter is abnormal in about 75% of patients and the spinal cord is involved in about 55%, with a lactate peak on MR spectroscopy in a subset.
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"Magnetic resonance imaging findings include 75% of patients have cerebellar white matter abnormalities, and the spinal cord was affected in 55%."
Review characterizes the MRI white-matter pattern used diagnostically.
Plasma and CSF amino acids and lactate
Plasma and CSF amino-acid analysis may show elevated glycine, and plasma/CSF lactate may be elevated; these biochemical findings are supportive but inconsistent and do not by themselves establish the diagnosis.
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"Biochemical abnormalities include high cerebrospinal fluid glycine and lactate and high plasma glycine and lactate, but these findings were not consistent."
Review lists elevated CSF/plasma glycine and lactate as supportive biochemical findings.
Respiratory chain enzymology (complex II/IV)
Respiratory-chain enzymology in patient cells/tissue shows deficient complex II and IV activity, reflecting failure to mature the [4Fe-4S] respiratory-complex subunits.
Show evidence (1 reference)
PMID:29297947 SUPPORT In Vitro
"activity of ETC complexes II and IV"
Patient-cell studies show reduced complex II and IV activity, the biochemical signature assayed in ISCA2 deficiency.
📈

Progression

1
Infantile onset and regression
Normal antenatal history and early development, followed by rapid deterioration between about 2 and 7 months of age with the presenting triad, progression to a vegetative state within 1-2 years, and death usually in early childhood. The course is uniformly progressive with no reported remission.
Show evidence (1 reference)
PMID:29470032 SUPPORT Human Clinical
"Affected children become vegetative within one to two years. During their vegetative state, which may persist for years, affected individuals are prone to recurrent chest infections that may require ventilator support."
GeneReviews describes the progression to a vegetative state and terminal respiratory complications.
📊

Prevalence

1
Worldwide
Cases In Literature Not yet documented
Ultra-rare. Approximately 20-24 affected individuals from ~18 families had been reported as of 2019-2020, most Saudi Arabian and homozygous for the p.Gly77Ser founder variant. No formal population prevalence or incidence estimate exists.
Show evidence (1 reference)
PMID:31106229 SUPPORT Human Clinical
"To date, 20 cases have been reported: 13 females and 7 males from 18 different families. All cases are from Saudi Arabia except those from one Italian family."
Review tabulates the reported case count and predominant population as of 2019.
{ }

Source YAML

click to show 
name: Multiple Mitochondrial Dysfunctions Syndrome 4
category: Genetic
creation_date: "2026-09-05T00:00:00Z"
synonyms:
- MMDS4
- ISCA2-related mitochondrial disorder
- ISCA2 deficiency
- Fatal infantile leukoencephalopathy due to ISCA2 deficiency
description: >
  Multiple Mitochondrial Dysfunctions Syndrome 4 (MMDS4; OMIM #616370), also called
  ISCA2-related mitochondrial disorder, is a fatal autosomal recessive infantile
  neurodegenerative leukoencephalopathy caused by biallelic loss-of-function
  variants in ISCA2, encoding iron-sulfur cluster assembly factor 2. ISCA2 acts
  with ISCA1 and IBA57 in the late steps of mitochondrial [4Fe-4S] cluster
  biogenesis, catalyzing the reductive fusion of two GLRX5-derived [2Fe-2S]
  clusters into a [4Fe-4S] cluster. Loss of ISCA2 impairs maturation of [4Fe-4S]
  client enzymes — respiratory chain complexes I and II, aconitase, and lipoic
  acid synthase (LIAS). LIAS deficiency reduces lipoylation of pyruvate
  dehydrogenase and 2-oxoglutarate dehydrogenase and inactivates the glycine
  cleavage system, producing hyperglycinemia; respiratory chain failure produces
  lactic acidosis. Affected infants develop normally for the first months, then
  between roughly 2 and 7 months of age undergo rapid neurodevelopmental
  regression with the triad of regression, optic atrophy with nystagmus, and
  diffuse white matter disease, becoming vegetative within 1-2 years and usually
  dying in early childhood. A homozygous founder missense variant, c.229G>A
  (p.Gly77Ser), accounts for the large majority of reported (predominantly Saudi
  Arabian) cases; sporadic non-founder variants have been reported in other
  populations. Fewer than 25 patients had been reported worldwide as of 2020.
disease_term:
  preferred_term: Multiple mitochondrial dysfunctions syndrome 4
  term:
    id: MONDO:0014611
    label: multiple mitochondrial dysfunctions syndrome 4
parents:
- MONDO:0044970
classifications:
  mechanistic_category:
  - classification_value: mitochondrial disease
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
genetic:
- name: ISCA2
  gene_term:
    preferred_term: ISCA2
    term:
      id: hgnc:19857
      label: ISCA2
  relationship_type: CAUSATIVE
  notes: >
    Biallelic loss-of-function variants in ISCA2 (iron-sulfur cluster assembly
    factor 2) cause MMDS4. The recurrent homozygous missense founder variant
    c.229G>A (p.Gly77Ser) is found in consanguineous Arab (predominantly Saudi
    Arabian) families and accounts for most reported cases; several sporadic
    non-founder variants have been reported in patients of other ancestries and
    affect all regions of the ISCA2 protein.
  variants:
  - name: c.229G>A (p.Gly77Ser)
    description: >
      Recurrent homozygous missense founder variant in consanguineous Arab
      (predominantly Saudi Arabian) families; the most frequently reported ISCA2
      allele and the one in the original MMDS4 description.
    type: missense
    clinical_significance: PATHOGENIC
    evidence:
    - reference: PMID:34440194
      reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Molecular studies revealed a homozygous variant in ISCA2 for each patient, the Gly77Ser variant, leading to the description of the first patients with MMDS 4"
      explanation: >
        Identifies p.Gly77Ser as the recurrent founder allele in the original
        MMDS4 cohort.
  - name: c.355G>A (p.Ala119Thr)
    description: >
      Novel non-founder missense variant reported in a consanguineous Iranian
      family, predicted to eliminate a helix motif in the Fe-S biosynthesis
      domain; the third ISCA2 patient reported worldwide at the time and outside
      the Saudi founder background.
    type: missense
    clinical_significance: LIKELY_PATHOGENIC
    evidence:
    - reference: PMID:31279336
      reference_title: "A novel ISCA2 variant responsible for an early-onset neurodegenerative mitochondrial disorder: a case report of multiple mitochondrial dysfunctions syndrome 4."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "A novel variant (c.355G > A, p.Ala119Thr) in ISCA2 gene was identified by WES in the proband."
      explanation: >
        Reports a non-founder ISCA2 missense allele in an Iranian family,
        establishing allelic and geographic heterogeneity beyond the founder
        variant.
  evidence:
  - reference: PMID:25539947
    reference_title: "ISCA2 mutation causes infantile neurodegenerative mitochondrial disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified a homoallelic missense founder mutation in ISCA2 leading to mitochondrial depletion and reduced complex I activity as well as decreased ISCA2, ISCA1 and IBA57 expression in fibroblasts."
    explanation: >
      Original description identifying the homozygous ISCA2 founder missense
      variant as the cause of an infantile mitochondrial leukodystrophy.
  - reference: PMID:34440194
    reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "in five other patients from different countries, seven sporadic variants were identified. These seven variants affect all regions of the ISCA2 protein"
    explanation: >
      Documents allelic heterogeneity: besides the Saudi founder variant, seven
      sporadic ISCA2 variants span the protein.
inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >
    MMDS4 is inherited in an autosomal recessive manner; parents are obligate
    asymptomatic heterozygous carriers, with a 25% recurrence risk per pregnancy.
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "ISCA2-related mitochondrial disorder is inherited in an autosomal recessive manner. At conception, each sib of an affected individual with IRMD has a 25% chance of being affected"
    explanation: >
      GeneReviews states the autosomal recessive inheritance pattern and
      recurrence risk.
pathophysiology:
- name: Biallelic ISCA2 Loss of Function
  description: >
    Biallelic loss-of-function ISCA2 variants (e.g., the founder p.Gly77Ser)
    reduce or abolish the Fe-S cluster carrier function of iron-sulfur cluster
    assembly factor 2.
  biological_scale: MOLECULAR
  genetic_context:
    functional_impact_category: LOSS_OF_FUNCTION
    variant_origin: GERMLINE
    zygosity: HOMOZYGOUS
    allele_type: missense (founder p.Gly77Ser); also frameshift and splice-site alleles
    description: >
      Biallelic germline loss-of-function ISCA2 variants; most patients are
      homozygous for the p.Gly77Ser founder missense allele, while sporadic
      cases carry compound or homozygous non-founder missense, frameshift, or
      splice-affecting alleles.
  molecular_functions:
  - preferred_term: iron-sulfur cluster binding
    term:
      id: GO:0051536
      label: iron-sulfur cluster binding
    modifier: LOSS_OF_FUNCTION
  downstream:
  - target: Defective Mitochondrial 4Fe-4S Cluster Biogenesis
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:29297947
      reference_title: "Loss-of-function mutations in ISCA2 disrupt 4Fe-4S cluster machinery and cause a fatal leukodystrophy with hyperglycinemia and mtDNA depletion."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We specifically tested the impact of loss of ISCA2 on 2Fe-2S proteins versus 4Fe-4S proteins and observed deficits in the functioning of 4Fe-4S but not 2Fe-2S proteins."
      explanation: >
        Knockdown and patient-cell studies show ISCA2 loss selectively impairs
        [4Fe-4S] protein maturation.
- name: Defective Mitochondrial 4Fe-4S Cluster Biogenesis
  description: >
    ISCA2 normally heterodimerizes with ISCA1 (with IBA57) to catalyze the
    reductive fusion of two GLRX5-derived [2Fe-2S] clusters into a [4Fe-4S]
    cluster, the terminal step of mitochondrial iron-sulfur cluster biogenesis.
    Loss of ISCA2 impairs [4Fe-4S] cluster supply while sparing [2Fe-2S]
    proteins.
  biological_scale: MOLECULAR
  biological_processes:
  - preferred_term: "[4Fe-4S] cluster assembly"
    term:
      id: GO:0044572
      label: "[4Fe-4S] cluster assembly"
    modifier: DECREASED
  downstream:
  - target: Failure to Mature 4Fe-4S Client Enzymes
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:34440194
      reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "which results in a deficiency of mitochondrial respiratory complexes and the impaired function of lipoic-acid-dependent enzymes"
      explanation: >
        Review states that impaired [4Fe-4S] cluster maturation is what leaves
        the downstream client enzymes (respiratory complexes and lipoic-acid
        enzymes) deficient.
- name: Failure to Mature 4Fe-4S Client Enzymes
  description: >
    Deficient [4Fe-4S] cluster supply prevents maturation of multiple client
    holoenzymes, including respiratory chain complexes I and II, the TCA-cycle
    enzyme aconitase, and lipoic acid synthase (LIAS), itself a [4Fe-4S]
    radical-SAM enzyme.
  biological_scale: MOLECULAR
  downstream:
  - target: Respiratory Chain Complex I and II Deficiency
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:34440194
      reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "most of them, including the subunits of the respiratory complexes I and II and the lipoic acid synthase (LIAS) involved in the critical modifications of the pyruvate dehydrogenase (PDH) and of the alpha-ketoglutarate dehydrogenase (αKGDH) complexes, require dedicated late-acting ISC targeting factors"
      explanation: >
        Complexes I/II and LIAS are [4Fe-4S] clients whose maturation fails when
        the late ISC pathway is disrupted.
  - target: Deficient Protein Lipoylation
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:34440194
      reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Lipoylated proteins (PDH, KGDH) were also decreased in the fibroblasts of patients"
      explanation: >
        In ISCA2-deficient patient fibroblasts, lipoylated PDH and KGDH are
        reduced, evidencing failure of the LIAS-dependent lipoylation branch.
- name: Respiratory Chain Complex I and II Deficiency
  description: >
    Failure to mature [4Fe-4S] subunits of respiratory complexes I and II
    (succinate dehydrogenase) impairs oxidative phosphorylation and, with
    reduced aconitase and TCA-cycle flux, produces lactic acidosis. Mitochondrial
    DNA depletion has also been reported.
  biological_scale: CELLULAR
  biological_processes:
  - preferred_term: mitochondrial electron transport, NADH to ubiquinone
    term:
      id: GO:0006120
      label: mitochondrial electron transport, NADH to ubiquinone
    modifier: DECREASED
  - preferred_term: mitochondrial electron transport, succinate to ubiquinone
    term:
      id: GO:0006121
      label: mitochondrial electron transport, succinate to ubiquinone
    modifier: DECREASED
  downstream:
  - target: Elevated lactate
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:29297947
      reference_title: "Loss-of-function mutations in ISCA2 disrupt 4Fe-4S cluster machinery and cause a fatal leukodystrophy with hyperglycinemia and mtDNA depletion."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Loss of ISCA2 diminished mitochondrial membrane potential, the mitochondrial network, basal and maximal respiration, ATP production, and activity of ETC complexes II and IV."
      explanation: >
        ISCA2 loss reduces respiration and ETC complex activity, the metabolic
        basis for lactate accumulation.
  - target: Leukoencephalopathy
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:25539947
      reference_title: "ISCA2 mutation causes infantile neurodegenerative mitochondrial disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Our data demonstrate that ISCA2 deficiency leads to a hereditary mitochondrial neurodegenerative white matter disease in infancy."
      explanation: >
        Links the mitochondrial respiratory defect to the infantile white matter
        (leukoencephalopathy) disease.
  - target: Optic atrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:31106229
      reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "the triad of neurodevelopmental regression, optic atrophy with nystagmus, and diffuse white matter disease"
      explanation: >
        Optic atrophy is a constant feature of the ISCA2 respiratory/energy
        failure, but the tissue-selective mechanism linking the bioenergetic
        defect to optic-nerve degeneration is not established, so the edge is
        indirect.
- name: Deficient Protein Lipoylation
  description: >
    Loss of LIAS activity reduces lipoylation of the E2 subunits of pyruvate
    dehydrogenase and 2-oxoglutarate dehydrogenase and of the H-protein of the
    glycine cleavage system, inactivating glycine catabolism.
  biological_scale: MOLECULAR
  biological_processes:
  - preferred_term: protein lipoylation
    term:
      id: GO:0009249
      label: protein lipoylation
    modifier: DECREASED
  downstream:
  - target: Elevated glycine (hyperglycinemia)
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:31106229
      reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This occurs due to defects in the iron–sulfur clusters pathway that leads to the inhibition of lipoic acid synthase that is vital for the glycine cleavage system, an impairment of which results in hyperglycinemia."
      explanation: >
        Directly traces impaired lipoic acid synthase, and thus glycine cleavage
        system failure, to the hyperglycinemia of ISCA2 deficiency.
phenotypes:
- name: Developmental regression
  category: Phenotypic
  description: >
    After apparently normal early development, affected infants undergo rapid
    neurodevelopmental regression, typically between 2 and 7 months of age,
    progressing to a vegetative state within 1-2 years.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At age three to seven months, affected individuals usually present with a triad of neurodevelopmental regression, nystagmus with optic atrophy, and diffuse white matter disease."
    explanation: >
      GeneReviews describes neurodevelopmental regression as a core presenting
      feature.
- name: Leukoencephalopathy
  category: Phenotypic
  description: >
    Diffuse, bilateral, symmetric white matter disease is a defining feature;
    cerebral and cerebellar white matter and the spinal cord are frequently
    involved, and some cases show cavitation. The white matter destruction is the
    proximate driver of the psychomotor regression, spasticity, and seizures.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Leukoencephalopathy
    term:
      id: HP:0002352
      label: Leukoencephalopathy
  sequelae:
  - target: Developmental regression
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:29470032
      reference_title: "ISCA2-Related Mitochondrial Disorder."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "As the disease progresses, global psychomotor regression continues at a variable pace and seizures may develop."
      explanation: >
        Progressive white matter disease tracks with the continuing psychomotor
        regression; the link is clinical association rather than a demonstrated
        mechanism.
  - target: Spasticity
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:34440194
      reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "clinical status was deteriorated during the first year of life leading to spasticity and visual impairment (nystagmus and/or optic atrophy)"
      explanation: >
        Progressive CNS white matter disease is followed by spasticity; the
        corticospinal-tract mechanism is inferred.
  - target: Seizures
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:29470032
      reference_title: "ISCA2-Related Mitochondrial Disorder."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "global psychomotor regression continues at a variable pace and seizures may develop"
      explanation: >
        Seizures develop as the encephalopathy advances; the epileptogenic
        mechanism is not specified.
  evidence:
  - reference: PMID:34440194
    reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Lesions in cerebral and cerebellar white matter and the spinal cord were indeed quite constantly present"
    explanation: >
      Review documents the constant, diffuse white matter involvement in MMDS4.
- name: Optic atrophy
  category: Phenotypic
  description: >
    Optic atrophy, part of the presenting triad, develops with disease
    progression and is nearly universal in the founder-variant cohort; nystagmus
    typically accompanies it.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
  sequelae:
  - target: Nystagmus
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:29470032
      reference_title: "ISCA2-Related Mitochondrial Disorder."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "affected individuals usually present with a triad of neurodevelopmental regression, nystagmus with optic atrophy, and diffuse white matter disease"
      explanation: >
        Nystagmus co-presents with optic atrophy in the visual-pathway
        involvement; the two are reported together rather than as a demonstrated
        causal sequence.
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the triad of neurodevelopmental regression, optic atrophy with nystagmus, and diffuse white matter disease"
    explanation: >
      Optic atrophy is a component of the characteristic MMDS4 presenting triad.
- name: Nystagmus
  category: Phenotypic
  description: >
    Nystagmus commonly accompanies optic atrophy and is often an early sign.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "affected individuals usually present with a triad of neurodevelopmental regression, nystagmus with optic atrophy, and diffuse white matter disease"
    explanation: >
      GeneReviews lists nystagmus with optic atrophy in the presenting triad.
- name: Spasticity
  category: Phenotypic
  description: >
    Progressive spasticity and hypertonia develop as the disease advances;
    spasticity or axial hypotonia is present in about 95% of reported patients.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:34440194
    reference_title: "A Review of Multiple Mitochondrial Dysfunction Syndromes, Syndromes Associated with Defective Fe-S Protein Maturation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "clinical status was deteriorated during the first year of life leading to spasticity and visual impairment (nystagmus and/or optic atrophy)"
    explanation: >
      Review describes progressive spasticity developing during the first year.
- name: Hypotonia
  category: Phenotypic
  description: >
    Axial hypotonia is common and, together with spasticity, is reported in
    about 95% of patients; hypotonia may precede the later spasticity.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Spasticity and axial hypotonia present in 95%"
    explanation: >
      Review reports axial hypotonia (with spasticity) in about 95% of the
      pooled ISCA2 cohort.
- name: Seizures
  category: Phenotypic
  description: >
    Seizures develop in a subset of patients as the disease progresses and are
    treated with standard anticonvulsants.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As the disease progresses, global psychomotor regression continues at a variable pace and seizures may develop."
    explanation: >
      GeneReviews notes seizures developing in a subset of patients with disease
      progression.
- name: Recurrent respiratory infections
  category: Phenotypic
  description: >
    During the prolonged vegetative state, affected individuals are prone to
    recurrent chest infections, a frequent terminal complication that may require
    ventilator support.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Recurrent respiratory infections
    term:
      id: HP:0002205
      label: Recurrent respiratory infections
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "affected individuals are prone to recurrent chest infections that may require ventilator support"
    explanation: >
      GeneReviews reports recurrent chest infections as a common late/terminal
      complication.
- name: Elevated glycine (hyperglycinemia)
  category: Laboratory
  description: >
    Elevated plasma and CSF glycine, secondary to glycine cleavage system
    failure from the lipoylation defect, is a characteristic (though not
    invariant) biochemical finding.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Hyperglycinemia
    term:
      id: HP:0002154
      label: Hyperglycinemia
  evidence:
  - reference: PMID:29297947
    reference_title: "Loss-of-function mutations in ISCA2 disrupt 4Fe-4S cluster machinery and cause a fatal leukodystrophy with hyperglycinemia and mtDNA depletion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We present two additional unrelated cases, and provide a more complete clinical description that includes hyperglycinemia, leukodystrophy of the brainstem with longitudinally extensive spinal cord involvement, and mtDNA deficiency."
    explanation: >
      Reports hyperglycinemia as part of the expanded clinical description of
      ISCA2 deficiency.
- name: Elevated lactate
  category: Laboratory
  description: >
    Elevated lactate in blood and CSF, and a lactate peak on MR spectroscopy in
    a subset of patients, reflects the respiratory chain defect.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Increased circulating lactate concentration
    term:
      id: HP:0002151
      label: Increased circulating lactate concentration
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Biochemical abnormalities include high cerebrospinal fluid glycine and lactate and high plasma glycine and lactate, but these findings were not consistent."
    explanation: >
      Review reports elevated lactate (and glycine) in body fluids, noting
      inconsistency across patients.
diagnosis:
- name: Molecular genetic testing for biallelic ISCA2 variants
  description: >
    The diagnosis is established by identifying biallelic pathogenic ISCA2
    variants, typically by a mitochondrial/leukodystrophy gene panel or exome/
    genome sequencing; targeted testing for c.229G>A is efficient in individuals
    of Arab/Saudi ancestry given the founder effect.
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of ISCA2-related mitochondrial disorder is established in a proband by the identification of biallelic pathogenic variants in ISCA2 on molecular genetic testing."
    explanation: >
      GeneReviews states molecular identification of biallelic ISCA2 variants as
      the diagnostic standard.
- name: Brain MRI
  description: >
    Brain MRI shows diffuse, bilateral, symmetric white matter disease; cerebellar
    white matter is abnormal in about 75% of patients and the spinal cord is
    involved in about 55%, with a lactate peak on MR spectroscopy in a subset.
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Magnetic resonance imaging findings include 75% of patients have cerebellar white matter abnormalities, and the spinal cord was affected in 55%."
    explanation: >
      Review characterizes the MRI white-matter pattern used diagnostically.
- name: Plasma and CSF amino acids and lactate
  description: >
    Plasma and CSF amino-acid analysis may show elevated glycine, and plasma/CSF
    lactate may be elevated; these biochemical findings are supportive but
    inconsistent and do not by themselves establish the diagnosis.
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Biochemical abnormalities include high cerebrospinal fluid glycine and lactate and high plasma glycine and lactate, but these findings were not consistent."
    explanation: >
      Review lists elevated CSF/plasma glycine and lactate as supportive
      biochemical findings.
- name: Respiratory chain enzymology (complex II/IV)
  description: >
    Respiratory-chain enzymology in patient cells/tissue shows deficient
    complex II and IV activity, reflecting failure to mature the [4Fe-4S]
    respiratory-complex subunits.
  evidence:
  - reference: PMID:29297947
    reference_title: "Loss-of-function mutations in ISCA2 disrupt 4Fe-4S cluster machinery and cause a fatal leukodystrophy with hyperglycinemia and mtDNA depletion."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "activity of ETC complexes II and IV"
    explanation: >
      Patient-cell studies show reduced complex II and IV activity, the
      biochemical signature assayed in ISCA2 deficiency.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: NOT_YET_DOCUMENTED
  notes: >
    Ultra-rare. Approximately 20-24 affected individuals from ~18 families had
    been reported as of 2019-2020, most Saudi Arabian and homozygous for the
    p.Gly77Ser founder variant. No formal population prevalence or incidence
    estimate exists.
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To date, 20 cases have been reported: 13 females and 7 males from 18 different families. All cases are from Saudi Arabia except those from one Italian family."
    explanation: >
      Review tabulates the reported case count and predominant population as of
      2019.
progression:
- phase: Infantile onset and regression
  notes: >
    Normal antenatal history and early development, followed by rapid
    deterioration between about 2 and 7 months of age with the presenting triad,
    progression to a vegetative state within 1-2 years, and death usually in
    early childhood. The course is uniformly progressive with no reported
    remission.
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Affected children become vegetative within one to two years. During their vegetative state, which may persist for years, affected individuals are prone to recurrent chest infections that may require ventilator support."
    explanation: >
      GeneReviews describes the progression to a vegetative state and terminal
      respiratory complications.
treatments:
- name: Supportive and multidisciplinary care
  description: >
    No curative therapy exists; management is exclusively supportive, drawing on
    genetics, neurology, dietetics, and developmental specialists.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Treatment is primarily supportive and may require input from a geneticist, neurologist, dietician, and developmental specialist."
    explanation: >
      GeneReviews specifies supportive, multidisciplinary management as the
      standard of care.
- name: Enteral nutritional support (feeding tube)
  description: >
    A nasogastric or gastrostomy feeding tube is typically required as bulbar
    and motor function deteriorate; periodic evaluation of swallowing function is
    suggested to time tube placement.
  treatment_term:
    preferred_term: Nutritional Support
    term:
      id: NCIT:C15433
      label: Nutritional Support
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A feeding tube (nasogastric or gastrostomy) is typically required."
    explanation: >
      GeneReviews states that enteral feeding-tube support is typically needed.
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Periodic evaluation of swallowing function is suggested."
    explanation: >
      GeneReviews recommends swallowing-function surveillance, which times the
      feeding-tube intervention.
- name: Antiseizure pharmacotherapy
  description: >
    Standard anticonvulsant therapy is used for the subset of patients who
    develop epilepsy.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Standard treatment for epilepsy. Recurrent chest infections may require ventilator support in addition to antimicrobial therapy."
    explanation: >
      GeneReviews recommends standard antiepileptic pharmacotherapy for the
      seizures that develop in a subset of patients.
- name: Antispasticity pharmacotherapy (baclofen)
  description: >
    For muscle tone abnormalities including hypertonia, baclofen and/or
    botulinum toxin (Botox) may be considered.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: baclofen
      term:
        id: CHEBI:2972
        label: baclofen
  target_mechanisms:
  - target: Spasticity
    treatment_effect: INHIBITS
    description: >
      Baclofen (a GABA-B agonist) reduces the hypertonia/spasticity that develops
      as the encephalopathy progresses.
    evidence:
    - reference: PMID:29470032
      reference_title: "ISCA2-Related Mitochondrial Disorder."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "For muscle tone abnormalities including hypertonia, baclofen"
      explanation: >
        GeneReviews recommends baclofen for the hypertonia/spasticity of ISCA2
        disorder.
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "For muscle tone abnormalities including hypertonia, baclofen"
    explanation: >
      GeneReviews recommends baclofen (and/or botulinum toxin) for muscle-tone
      abnormalities.
- name: Early intervention and developmental services
  description: >
    Referral to early intervention services is recommended to support
    development within the limits imposed by the progressive course.
  treatment_term:
    preferred_term: early intervention and developmental services
    term:
      id: NCIT:C15315
      label: Rehabilitation
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Referral to early intervention services is recommended."
    explanation: >
      GeneReviews recommends early-intervention referral as part of
      developmental support.
- name: Genetic counseling
  description: >
    Autosomal recessive counseling with a 25% recurrence risk; carrier testing
    and prenatal/preimplantation testing are available when the familial variants
    are known.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:29470032
    reference_title: "ISCA2-Related Mitochondrial Disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Carrier testing for at-risk relatives and prenatal testing for a pregnancy at increased risk are possible if the ISCA2 pathogenic variants in the family are known."
    explanation: >
      GeneReviews describes carrier and prenatal testing offered through genetic
      counseling.
discussions:
- discussion_id: isca2_no_vertebrate_cns_model
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Biallelic ISCA2 Loss of Function
  prompt: >
    Does any whole-organism vertebrate model reproduce the CNS
    leukoencephalopathy and optic-atrophy phenotype of human ISCA2 deficiency,
    given that in mouse ISCA1 (not ISCA2) is reported to be required for
    mitochondrial [4Fe-4S] protein maturation in postmitotic tissues under
    standard conditions?
  rationale: >
    Human ISCA2 loss is fully penetrant and fatal, yet a review of the pathway
    states that ISCA2 (and IBA57) are not required in postmitotic mouse cell
    types under standard physiological conditions, where ISCA1 is. A
    straightforward Isca2-null rodent may therefore not recapitulate the human
    CNS disease, so the translational validity of any such model is an open
    question that should be recorded rather than assumed.
  evidence:
  - reference: PMID:31106229
    reference_title: "Multiple Mitochondrial Dysfunctions Syndrome 4 Due to ISCA2 Gene Defects: A Review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The ISCA1 is required for the maturation of mitochondrial Fe4S4 proteins in mature skeletal muscle, whereas ISCA2 and IBA57 are not required in the 2 postmitotic cell types, under standard physiological conditions."
    explanation: >
      Documents the mouse finding that ISCA2 is dispensable in postmitotic
      tissues, the basis for the human-model mismatch given the fully penetrant
      human disease.
  notes: >
    Recorded from the review's synthesis only; the primary rodent studies were
    not independently retrieved in this curation, so no whole-organism model
    link is asserted in the pathograph.
references:
- reference: PMID:29470032
  title: "ISCA2-Related Mitochondrial Disorder."
  tags:
  - GeneReviews
📚

References & Deep Research

References

1
PMID:29470032
ISCA2-Related Mitochondrial Disorder.
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: Multiple Mitochondrial Dysfunctions Syndrome 4 (MMDS4, ISCA2) · 2026-09-05T16:10:21Z · View source

De novo curation of MMDS4 (ISCA2-related mitochondrial disorder, MONDO:0014611) as a DISEASE entry. Deep research via claude_code literature sweep (research/Multiple_Mitochondrial_Dysfunctions_Syndrome_4-deep-research-claude_code.md; 10/10 report references resolved). Curated: biallelic ISCA2 LoF (hgnc:19857, founder p.Gly77Ser) causal genetics; a conserved pathograph chain (ISCA2 LoF -> defective mitochondrial [4Fe-4S] cluster biogenesis -> failure to mature [4Fe-4S] client enzymes -> branching to complex I/II deficiency + elevated lactate + leukoencephalopathy, and to deficient protein lipoylation -> hyperglycinemia); HP phenotypes (regression, leukoencephalopathy, optic atrophy, nystagmus, spasticity, seizures, hyperglycinemia HP:0002154, hyperlactatemia); supportive treatments (NCIT); autosomal recessive inheritance; ultra-rare prevalence. GeneReviews (PMID:29470032) used as mandatory phenotype baseline and tagged. Evidence from PMID:25539947 (Al-Hassnan original), PMID:29297947 (Alaimo mechanism), PMID:31106229 (Alfadhel review), PMID:34440194 (Lebigot review), all exact-substring verified. Corrected two report term slips: HP:0002153 (Hyperkalemia) -> HP:0002154 (Hyperglycinemia); avoided invalid HP:0007070. Validated: validate, validate-terms, count-verified-snippets 24/24, validate-disorders, check-entity-refs, check-causal-targets, check-duplicate-keys, check-enum-values, check-qualifier-terms, check-snippet-length/title/grading, check-folded-hyphens.

Claude Code ▸
Multiple Mitochondrial Dysfunctions Syndrome 4 (MMDS4): Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 14 citations 2026-09-05T15:50:57.220756

Multiple Mitochondrial Dysfunctions Syndrome 4 (MMDS4): Comprehensive Research Report

Important nomenclature note (read first)

The query specifies MONDO:0014611 / "Multiple Mitochondrial Dysfunctions Syndrome 4". Multiple search results returned information on NFU1 in association with "multiple mitochondrial dysfunctions syndrome," but NFU1 causes MMDS1 (OMIM #605711), not MMDS4. MMDS4 (OMIM #616370) is caused by biallelic pathogenic variants in ISCA2 (OMIM 615317, chromosome 14q24.3) OMIM #616370; GeneReviews NBK481904. This report is built around ISCA2/MMDS4. NFU1 (MMDS1), BOLA3 (MMDS2), IBA57 (MMDS3), and ISCA1 (MMDS5) are related but genetically and, in the germline-vs-cell-biology sense, distinct entries in the same iron-sulfur (Fe-S) cluster biogenesis pathway family — worth flagging for a lump/split decision if a KB entry already exists for "MMDS" broadly, since these five genes produce five genetically distinct disorders with overlapping but not identical phenotypes (§2b of the design-decisions register would apply: keep genetically distinct entities split, cross-reference the shared pathway via a mechanism module).

1. Disease Information

Overview. ISCA2-related mitochondrial disorder (MMDS4) is a fatal, autosomal recessive, infantile-onset neurodegenerative leukoencephalopathy caused by biallelic loss-of-function variants in ISCA2, a gene encoding a late-acting component of the mitochondrial iron-sulfur cluster (Fe-S) assembly machinery. Affected infants develop normally for the first months of life, then between 3 and 7 months of age undergo rapid neurodevelopmental regression, culminating in a vegetative state and death, usually in early childhood (GeneReviews NBK481904; Al-Hassnan et al. 2015, PMID:25539947).

Key identifiers: | Resource | Identifier | |---|---| | OMIM (phenotype) | #616370 | | OMIM (gene, ISCA2) | *615317 | | Orphanet | ORPHA:457406 (Multiple mitochondrial dysfunctions syndrome type 4) | | MONDO | MONDO:0014611 | | HGNC gene | ISCA2 (hgnc:29253) | | Inheritance | Autosomal recessive |

Synonyms/alternative names: MMDS4; ISCA2-related mitochondrial disorder (IRMD); ISCA2 deficiency; multiple mitochondrial dysfunctions syndrome type 4, ISCA2-related; fatal infantile leukoencephalopathy with cavitation (a subset).

Evidence basis: Nearly all clinical knowledge derives from aggregated case series/case reports (~20-24 published individuals across ~18-20 families as of the most recent reviews), not large-cohort epidemiologic or EHR-derived data — consistent with an ultra-rare disorder (Alfadhel 2019 review, PMID:31106229).

2. Etiology

Disease causal factor: Purely genetic — biallelic (homozygous or compound heterozygous) pathogenic variants in ISCA2. No environmental, infectious, or purely mechanistic (non-genetic) causal factor is described.

Genetic risk factors: - Founder variant: c.229G>A (p.Gly77Ser), a homozygous missense founder mutation identified in consanguineous Arab (predominantly Saudi Arabian) families, accounting for the large majority of reported cases (18-19 of ~19-24 probands in different series) (Al-Hassnan et al. 2015, PMID:25539947; PMC8393393). Haplotype analysis dated the founder event to roughly 4,802 years ago and confirmed absence from dbSNP/1000 Genomes and 1,060 ethnically matched control chromosomes at the time of discovery. - Other reported pathogenic variants: c.295delT (p.Phe99LeufsTer18), c.334A>G (p.Ser112Gly), c.355G>A (p.Ala119Thr) — the latter reported in a consanguineous Iranian family with a distinct genetic background, indicating the disorder is not confined to Arab founder-mutation carriers (case report, PMC6612116; additional novel variants described in more recent case reports, e.g. PMC11561297 — a novel missense variant causing aberrant splicing). - Modifier genes: None specifically established; phenotypic variability (see §3, §8) suggests modifiers or hypomorphic-allele effects may exist but are not characterized.

Consanguinity: A major risk factor at the population level — nearly all reported families are consanguineous, consistent with autosomal recessive inheritance of an otherwise rare allele.

Environmental/lifestyle risk factors: None identified; this is a monogenic, congenitally-programmed metabolic/mitochondrial disorder with no known environmental trigger or exacerbating exposure.

Protective factors: None described in the literature (genetic or environmental).

Gene-environment interactions: Not applicable/not described — no evidence of environmental modulation of expressivity.

3. Phenotypes

MMDS4 presents a fairly stereotyped triad: neurodevelopmental regression, nystagmus with optic atrophy, and diffuse (often bilateral, symmetric) white matter disease on MRI (GeneReviews NBK481904).

Phenotype HPO term (suggested) Frequency Onset/course
Optic atrophy HP:0000648 Nearly universal (18/18 in one series) Progressive, from ~3-7 months
Nystagmus HP:0000639 Common (8/12 assessed) Early, often first noticed sign
Developmental regression HP:0002376 Universal Onset 3-7 months, rapid
Spasticity / hyperreflexia HP:0001257 / HP:0001347 Universal (18/18) Progressive
Hypotonia (early) HP:0001252 Common May precede spasticity
Diffuse white matter abnormality on MRI HP:0007070 (cerebral white matter hypoplasia/abnormality) Universal/near-universal Progressive, may cavitate in some cases
Cerebellar white matter involvement HP:0002197 (cerebellar atrophy) or specific white-matter term ~75% (per MRI review) —
Spinal cord involvement — ~55% —
Seizures HP:0001250 ~30% (3/10 in one cohort) Variable, responsive to standard anticonvulsants
Elevated CSF/plasma glycine (hyperglycinemia) HP:0002153 Common Biochemical, tracks with disease activity
Elevated lactate (blood/CSF/MRS) HP:0002151 Common (elevated lactate peak on MRS in ~25%) —
Vegetative state (end-stage) HP:0031988 (or free text) Universal survivors' endpoint Within 1-2 years of onset
Recurrent respiratory infections / ventilator dependence HP:0002205-adjacent Common, end-stage Terminal complication
Dysmorphic features (low-set ears, broad nasal bridge, short 4th metacarpals, cutaneous toe syndactyly) Various HP terms Rare (2/18) Minor, inconsistent

Atypical/expanded phenotypes: A 2020 Neurogenetics report ("Expanding the genotype-phenotype spectrum of ISCA2-related multiple mitochondrial dysfunction syndrome — cavitating leukoencephalopathy and prolonged survival") described cases with cavitating leukoencephalopathy and longer-than-typical survival, broadening the phenotype beyond the uniformly rapidly fatal founder-mutation presentation (DOI 10.1007/s10048-020-00611-8; full text was paywalled during this research session — treat as a lead pending direct verification). One atypical Italian infant showed extremely rapid, neonatal-onset leukoencephalopathy with death at 3 months and without the typical optic atrophy, indicating phenotypic heterogeneity at the severe end as well (GeneReviews NBK481904).

Quality of life impact: Not formally studied with validated instruments (no EQ-5D/SF-36/PROMIS data identified); qualitatively, the disease produces total loss of independent function, feeding, and communication, with a terminal course — consistent with the most severe end of pediatric neurodegenerative disease burden. No disease-specific QOL literature was found.

Evidence source classification: All phenotype data above are HUMAN_CLINICAL (case reports/series), not model-organism or computational.

4. Genetic/Molecular Information

Causal gene: ISCA2 (HGNC gene; hgnc:29253), chromosome 14q24.3; OMIM *615317. Encodes the 154-amino-acid mitochondrial protein "Iron-sulfur cluster assembly 2 homolog, mitochondrial" — an A-type ISC (iron-sulfur cluster) scaffold/carrier protein.

Variant classification and type: - c.229G>A (p.Gly77Ser) — missense; founder pathogenic variant (ClinVar RCV000310400, classified pathogenic for "Fatal multiple mitochondrial dysfunctions syndrome") (ClinVar). - c.295delT (p.Phe99LeufsTer18) — frameshift/nonsense-mediated, presumed loss of function. - c.334A>G (p.Ser112Gly) — missense. - c.355G>A (p.Ala119Thr) — missense; in-silico modeling predicted disruption of a helix motif within the Fe-S biosynthesis domain (case report, PMC6612116). - Additional novel missense variants causing aberrant splicing rather than a direct amino-acid-level mechanism have been reported in recent (2024) case reports, expanding the mutational mechanism spectrum beyond simple missense loss-of-function (PMC11561297).

Allele frequency: The founder c.229G>A variant is absent from dbSNP/1000 Genomes and was not found in >1,000 ethnically matched control chromosomes at time of original description; no gnomAD-specific frequency was retrievable in this research session (flag as a gap to verify directly against gnomAD before citing a number).

Germline vs. somatic: Exclusively germline (constitutional) — this is a classic Mendelian recessive disorder, not a somatic/mosaic condition.

Functional consequence: Loss of function. Experimental knockdown/patient-fibroblast studies show the founder and other pathogenic variants diminish mitochondrial membrane potential, mitochondrial network integrity, basal and maximal respiration, and ATP production, and disrupt [4Fe-4S] cluster assembly machinery specifically (not [2Fe-2S] proteins) (Alaimo et al. 2018, PMID:29297947).

Modifier genes: None specifically validated.

Epigenetic information: None reported for this disorder.

Chromosomal abnormalities: None reported; MMDS4 is caused by intragenic ISCA2 variants, not copy-number or structural chromosomal change.

5. Environmental Information

No environmental factors, lifestyle factors, or infectious triggers are implicated in disease causation. This is a purely monogenic disorder. (One atypical patient's rapid neonatal presentation was noted above but was not attributed to any environmental co-factor.) Left deliberately unresearched further beyond confirming absence of any reported association — this is expected for a highly penetrant recessive inborn error of mitochondrial biogenesis.

6. Mechanism / Pathophysiology

Causal chain (ordered, from molecular lesion to clinical phenotype)

  1. Biallelic loss-of-function variants in ISCA2 (e.g., founder p.Gly77Ser) reduce or abolish the protein's Fe-S cluster carrier function → [demonstrated].
  2. ISCA2 normally heterodimerizes with ISCA1 (and interacts with IBA57) to catalyze the reductive fusion of two GLRX5-derived [2Fe-2S]²⁺ clusters into a [4Fe-4S]²⁺ cluster, using electrons supplied by ferredoxin FDX2 — this is the terminal, "late" step of the mitochondrial ISC (iron-sulfur cluster) biogenesis pathway, downstream of the earlier ISCU/NFS1/ISD11/FDX2 "early" assembly complex and the GLRX5 handoff step → loss of ISCA2 function directly impairs [4Fe-4S] cluster maturation while sparing [2Fe-2S]-dependent proteins (PNAS 2020 mechanism paper; Alaimo et al. 2018, PMID:29297947) — [demonstrated, in vitro/patient fibroblast].
  3. Deficient [4Fe-4S] cluster supply leads to failure to mature multiple client [4Fe-4S]-dependent mitochondrial enzymes, notably: (a) respiratory chain complexes I, II (succinate dehydrogenase), and to a lesser extent III/IV; (b) aconitase (TCA cycle); and (c) lipoic acid synthase (LIAS), itself a [4Fe-4S] radical-SAM enzyme required to synthesize the lipoyl cofactor → [demonstrated].
  4. Loss of LIAS activity results in failure to lipoylate the E2 subunits of pyruvate dehydrogenase (PDH), α-ketoglutarate dehydrogenase (KGDH), and the H-protein of the glycine cleavage system (GCS) → [demonstrated, fibroblast studies].
  5. Glycine cleavage system failure causes impaired glycine catabolism, leading to the characteristic hyperglycinemia (elevated plasma/CSF glycine) seen in MMDS4 patients — mechanistically analogous to (but genetically distinct from) classic nonketotic hyperglycinemia, here secondary to a lipoylation defect rather than a primary GCS gene mutation (PMC5839994 / Alaimo et al. 2018, PMID:29297947) — [demonstrated].
  6. Combined respiratory chain complex I/II/IV dysfunction and TCA cycle impairment (aconitase, KGDH) result in cellular ATP deficiency, elevated lactate (from compensatory glycolysis and impaired pyruvate oxidation), reduced mitochondrial membrane potential, and disrupted mitochondrial network morphology — observed directly in patient fibroblasts and knockdown cell models → [demonstrated, in vitro].
  7. mtDNA depletion has also been observed in patient-derived cells in some studies, plausibly secondary to the broader mitochondrial bioenergetic failure, though the precise mechanistic link (e.g., via disrupted Fe-S-dependent replication/repair enzymes) is less directly established than steps 1-6 → [partially inferred].
  8. Chronic bioenergetic failure disproportionately affects the most metabolically demanding tissues — myelinating oligodendrocytes/white matter and the optic nerve/retinal ganglion cell axons — leading to progressive diffuse white matter disease (leukoencephalopathy, sometimes cavitating) and optic atrophy with nystagmus, the two dominant structural/clinical hallmarks → [inferred from tissue-selectivity pattern common to mitochondrial leukoencephalopathies; not mechanistically proven for MMDS4 specifically why white matter/optic pathway are preferentially vulnerable].
  9. Progressive white matter and upper-motor-neuron pathway involvement produces the clinical picture of developmental regression, spasticity/hyperreflexia, and (in a subset) seizures, culminating in a vegetative state and death, typically from respiratory complications, in early childhood.

Detail by category

  • Molecular pathways: Mitochondrial ISC (iron-sulfur cluster) biogenesis pathway — "late-acting" ISCA1/ISCA2/IBA57 module, downstream of the "early" NFS1-ISCU-ISD11-FDX2-FXN core complex and GLRX5 hand-off (KEGG/Reactome: mitochondrial Fe-S cluster biogenesis).
  • Cellular processes: Mitochondrial respiration/oxidative phosphorylation failure, impaired TCA cycle flux, disrupted mitochondrial network dynamics/morphology, and (indirectly) glycine catabolism failure.
  • Protein dysfunction: Loss of ISCA2's Fe-S cluster carrier/transfer function (not misfolding/aggregation per se, though some variants such as p.Ala119Thr are predicted to disrupt a structural helix within the Fe-S binding domain); downstream loss of function of multiple client [4Fe-4S] holoenzymes (complex I/II, aconitase, LIAS).
  • Metabolic changes: Elevated lactate (impaired oxidative metabolism), hyperglycinemia (impaired glycine cleavage), reduced lipoylated-protein pool (PDH/KGDH E2 subunits).
  • Immune system involvement: Not implicated.
  • Tissue damage mechanisms: Bioenergetic failure/oxidative stress in high-energy-demand CNS tissue (white matter, optic pathway), rather than classical inflammatory or fibrotic injury.
  • Biochemical abnormalities: Deficient respiratory chain complex II and IV activity (most consistently reported; complex I also reduced in some studies); deficient aconitase; deficient LIAS/lipoylation.
  • Molecular profiling: No transcriptomic, proteomic, metabolomic, or single-cell datasets specific to MMDS4/ISCA2 patient tissue were identified in this search; mechanistic data derive from targeted biochemical assays in patient fibroblasts and shRNA-knockdown cell models, not omics screens.

Suggested ontology terms: - GO biological process: GO:0016226 (iron-sulfur cluster assembly); GO:0097428 (protein maturation by iron-sulfur cluster transfer); GO:0022900 (electron transport chain); GO:0006544 (glycine metabolic process). - GO molecular function: GO:0051536 (iron-sulfur cluster binding). - GO cellular component: GO:0005739 (mitochondrion); GO:0005743 (mitochondrial inner membrane, for respiratory complexes). - CL: CL:0000540 (neuron), CL:0000128 (oligodendrocyte) as principally affected cell types (inferred from white-matter/optic-pathway tropism, not directly demonstrated by cell-type-resolved data).

7. Anatomical Structures Affected

Organ level (primary): Central nervous system — cerebral and cerebellar white matter, spinal cord, optic nerve/visual pathway.

Secondary/complication-level: Respiratory system (recurrent infections, ventilator dependence as a terminal complication); musculoskeletal (myofiber atrophy on muscle biopsy in some patients, per Al-Hassnan et al.).

Body systems involved: Nervous system (primary), ophthalmologic system (optic atrophy/nystagmus), respiratory system (secondary/terminal).

Tissue/cell level: Myelinating white matter (oligodendrocyte-associated tracts), optic nerve axons/retinal ganglion cells, skeletal muscle (myofiber size variation, atrophic fibers reported on biopsy in the founder-mutation cohort).

Subcellular level: Mitochondria — specifically the mitochondrial matrix, site of Fe-S cluster biogenesis (GO:0005759) and inner membrane respiratory complexes (GO:0005743).

Suggested UBERON terms: UBERON:0002240 (spinal cord); UBERON:0002037 (cerebellum); UBERON:0002771 (deep white matter); UBERON:0000941 (optic nerve).

Localization/laterality: MRI white matter changes are typically bilateral and symmetric; no laterality preference reported.

8. Temporal Development

Onset: Congenital genetic lesion with delayed clinical onset — normal development for the first months of life, clinical presentation typically at 3-7 months of age (infantile onset). Onset pattern is subacute-to-acute regression after an apparently normal early infancy — not present at birth, not insidious over years.

Progression: Rapidly progressive. Global psychomotor decline at variable rates, spasticity/hyperreflexia developing early, seizures in a subset (~30%), vegetative state within 1-2 years of onset, death typically in early childhood (most cases die before age 5; several before 15 months in atypical/severe presentations). One atypical case had a neonatal-onset, extremely rapid course with death at 3 months. Conversely, the 2020 Neurogenetics report described prolonged survival in association with cavitating leukoencephalopathy and non-founder variants, indicating some genotype-phenotype correlation with severity/pace of progression.

Course pattern: Uniformly progressive/degenerative — no remitting-relapsing pattern described. No spontaneous remission reported.

Critical periods: The 3-7 month window represents the critical period of clinical onset; there is no described "window of therapeutic opportunity" established in the literature (no disease-modifying therapy exists to exploit one).

9. Inheritance and Population

Epidemiology: Ultra-rare. Approximately 20-24 affected individuals from ~18-20 families reported in the literature as of the most recent reviews (2019-2020) (Alfadhel 2019, PMID:31106229; GeneReviews NBK481904). True population prevalence is unknown/not yet documented (Orphanet-style "prevalence class" would be NOT_YET_DOCUMENTED or ULTRA_RARE, given the case-count-only evidence). No formal incidence estimate exists.

Inheritance pattern: Autosomal recessive. Parents are obligate asymptomatic heterozygous carriers.

Penetrance: Appears fully penetrant among reported homozygotes/compound heterozygotes, though ascertainment bias (only clinically apparent/tested cases are reported) limits confidence in this claim.

Expressivity: Variable — the founder mutation cohort shows a relatively stereotyped, "predictable" severe phenotype, while non-founder variants are associated with a broader phenotypic spectrum including cavitating leukoencephalopathy and prolonged survival, and at least one atypical neonatal-onset case lacking the classic optic-atrophy feature.

Genetic anticipation: Not applicable — no repeat-expansion mechanism.

Germline mosaicism: Not specifically reported for ISCA2/MMDS4.

Founder effect: Well-established — the c.229G>A (p.Gly77Ser) founder variant is estimated (via haplotype analysis) to have arisen roughly 4,802 years ago, and is concentrated in consanguineous Arab, predominantly Saudi Arabian, families (Al-Hassnan et al. 2015, PMID:25539947).

Consanguinity role: Central — the overwhelming majority of reported families are consanguineous, both for the founder-mutation cases and several of the non-founder-variant case reports (e.g., the Iranian and other consanguineous families reporting novel variants).

Carrier frequency: Not established in population databases at time of the founder-variant discovery (absent from dbSNP/1000 Genomes and >1,000 ethnically matched control chromosomes); no gnomAD-derived carrier frequency was identified in this session — flag for direct gnomAD lookup before citing a number in a KB entry.

Population demographics: Strong concentration in Arab/Saudi Arabian populations (19/20 cases in the GeneReviews summary), but a documented Italian case and Iranian case (non-founder variants) confirm the disorder is not confined to that population — worldwide distribution with population-specific founder effects.

Sex ratio: In the Al-Hassnan founder-mutation series and Alfadhel's aggregate review, both sexes are affected (e.g., 13 females and 7 males across 20 cases in one count), consistent with autosomal (non-sex-linked) recessive inheritance — no strong sex skew reported.

Age distribution: Uniformly pediatric/infantile — no adult survivors documented as of the most recent reviews.

10. Diagnostics

Clinical suspicion criteria: Progressive developmental regression at 3-7 months, optic atrophy with nystagmus, diffuse bilateral symmetric white matter abnormality on MRI, elevated plasma/CSF glycine, and biochemical evidence of deficient respiratory chain complex II/IV (and often complex I) activity.

Laboratory tests: - Plasma and CSF amino acids: elevated glycine (hyperglycinemia) — a "usual finding." - Plasma/CSF lactate: elevated in a subset. - Muscle/fibroblast respiratory chain enzymology: deficient complex II and IV activity (complex I also reduced in some reports); deficient aconitase activity. - Lipoylation studies (research-level, not routine clinical): reduced lipoylated PDH/KGDH E2 subunits in fibroblasts.

Imaging: Brain MRI — diffuse, bilateral, symmetric white matter abnormality; cerebellar white matter involvement in ~75%; spinal cord involvement in ~55%; some cases show cavitation (cavitating leukoencephalopathy, an expanded-phenotype finding). MR spectroscopy: elevated lactate peak in ~25%, elevated glycine peak in ~10%.

Electrophysiology: Not a primary diagnostic modality here; EEG may be used to characterize seizures in the ~30% of patients who develop them, but no MMDS4-specific EEG signature was identified.

Histopathology/muscle biopsy: Mild-to-moderate myofiber size variation with randomly distributed atrophic fibers reported in the founder-mutation cohort (Al-Hassnan et al.) — a nonspecific mitochondrial myopathy pattern.

Genetic testing (recommended approach per GeneReviews): - Diagnosis established by identification of biallelic pathogenic ISCA2 variants. - First-tier: multigene panel including ISCA2 (and the other MMDS genes NFU1, BOLA3, IBA57, ISCA1 for differential coverage). - Alternative/complementary: comprehensive genomic testing (exome or genome sequencing), given ISCA2 sequence analysis alone detects ~100% of known pathogenic variants; deletion/duplication analysis is available but its detection rate is unknown/unreported. - Population-specific: targeted testing for c.229G>A may be efficient in individuals of Arab/Saudi ancestry with a compatible phenotype, given the founder effect, but should not substitute for broader testing given the demonstrated non-founder-variant cases.

Differential diagnosis: The other MMDS types (MMDS1/NFU1, MMDS2/BOLA3, MMDS3/IBA57, MMDS5/ISCA1) — distinguished by which organ systems are additionally involved (e.g., pulmonary hypertension is characteristic of NFU1/MMDS1, cardiomyopathy of BOLA3/MMDS2) and by specific biochemical/MRI patterns; other leukodystrophies (metachromatic leukodystrophy, Krabbe disease, Canavan disease, Alexander disease); and other white matter disorders such as leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL) and vanishing white matter disease/CACH.

Screening: No population or newborn screening program exists for this ultra-rare disorder; carrier testing and prenatal/preimplantation genetic testing are available once a family's pathogenic variants are identified (standard recessive-disorder genetic counseling pathway).

11. Outcome/Prognosis

Survival/mortality: Uniformly poor. Most affected children die in early childhood (commonly well before age 5; several reports document death within the first 1-2 years, and one atypical case died at 3 months). No adult survivors have been reported in the literature reviewed here. The 2020 Neurogenetics report of "prolonged survival" associated with non-founder variants is the notable exception and should be read as expanding, not overturning, this generally grim prognosis.

Disease course: Progressive loss of function to a vegetative state within 1-2 years of clinical onset; recurrent respiratory infections and ventilator dependence are common terminal complications and a frequent proximate cause of death.

Complications: Recurrent respiratory infections, need for enteral (nasogastric/gastrostomy) feeding due to progressive bulbar/motor dysfunction, constipation (managed with hydration/stool softeners), seizures in a subset.

Recovery potential: None described — the disease is uniformly progressive with no reported spontaneous improvement, and no disease-modifying therapy exists.

Prognostic factors: Genotype appears to correlate loosely with severity — the classic founder variant (p.Gly77Ser) is associated with a stereotyped, rapidly fatal course, while some non-founder variants are associated with a milder/longer course (cavitating leukoencephalopathy with prolonged survival). This is a lead from a single report, not yet a robustly validated genotype-phenotype rule.

Quality of life measures: No validated QOL instrument data identified for this condition.

12. Treatment

Curative therapy: None exists. Management is exclusively supportive/multidisciplinary (genetics, neurology, dietetics, developmental specialists) (GeneReviews NBK481904; Alfadhel 2019 review, PMID:31106229).

Supportive care components: - Nutritional support: nasogastric or gastrostomy tube feeding as swallowing function deteriorates (NCIT candidate: NCIT:C15433 Nutritional Support, or a gastrostomy-specific surgical procedure term for tube placement — NCIT:C15329 Surgical Procedure). - Seizure management: standard anticonvulsant therapy for the ~30% who develop seizures (NCIT: NCIT:C15986 Pharmacotherapy, with the specific anticonvulsant agent, e.g., levetiracetam/valproate, as therapeutic_agent — specific agents were not detailed in the sources reviewed and would need per-case confirmation). - Respiratory support: ventilator support for recurrent respiratory infections/failure (NCIT: a respiratory-support/mechanical-ventilation device term, handled per the device-vs-action convention — bind the clinical action and carry the device as a qualifier). - Rehabilitative therapy: physical therapy for motor dysfunction (NCIT:C15302 Physical Therapy). - Developmental services: early intervention (ages 0-3) and developmental preschool placement (ages 3-5) (NCIT:C15315 Rehabilitation, or a developmental-services-specific term if available). - Symptomatic management of constipation: hydration and stool softeners/laxatives (pharmacotherapy, NCIT:C15986).

Investigational/theoretical mitochondrial cofactor therapy: No MMDS4/ISCA2-specific clinical trial or case report of riboflavin, lipoic acid, CoQ10, or similar "mitochondrial cocktail" supplementation was identified in this search. General mitochondrial-disease literature notes such cofactors (CoQ10, idebenone, riboflavin, dichloroacetate, thiamine, creatine, antioxidants including lipoic acid) are used empirically across mitochondrial disorders broadly, but a Cochrane-level review found little evidence supporting efficacy of any vitamin/cofactor therapy in mitochondrial disease generally, and none of this is disease-specific to MMDS4. Given the pathway (LIAS/lipoic acid deficiency downstream of ISCA2 loss), exogenous lipoic acid or riboflavin supplementation is mechanistically plausible but is not documented as tried or effective specifically in MMDS4 in the literature surveyed — this should be recorded as a knowledge gap / untested hypothesis rather than an established treatment, if curated.

Gene therapy, cell therapy, targeted/immunotherapy, surgical intervention (disease-modifying): None reported for this disorder.

Clinical trials: No MMDS4/ISCA2-specific registered clinical trial (NCT identifier) was identified.

Treatment algorithms/combination therapy/personalized medicine: Not applicable beyond generic multidisciplinary supportive-care pathways common to severe infantile leukodystrophies.

13. Prevention

Primary prevention: Genetic counseling and carrier testing in at-risk (especially consanguineous, Arab/Saudi-ancestry) families once a family's pathogenic variants are known; prenatal diagnosis and preimplantation genetic testing (PGT) are available for at-risk pregnancies once familial variants are identified (GeneReviews NBK481904). No vaccine or exposure-avoidance strategy applies, since there is no environmental causal factor.

Secondary prevention (early detection): No population or newborn screening program exists. Early clinical recognition of the 3-7 month regression pattern with optic atrophy/nystagmus and diffuse white matter change should prompt urgent genetic testing, given the rapid subsequent course, but this is disease detection rather than true secondary prevention.

Tertiary prevention: Anticipatory management of predictable complications — periodic swallowing-function evaluation to time feeding-tube placement, seizure surveillance, respiratory-infection vigilance — as outlined in GeneReviews management recommendations.

Genetic counseling: Central to prevention in this recessive disorder — informing carrier parents of 25% recurrence risk per pregnancy, offering carrier testing to extended family members (especially in consanguineous, founder-population contexts), and offering prenatal/preimplantation testing.

Public health/environmental interventions: Not applicable (no environmental risk factor to intervene on).

14. Other Species / Natural Disease

No naturally occurring MMDS4/ISCA2 disease has been reported in non-human species (companion animals, wildlife, or livestock) in the sources reviewed — this appears to be a human-specific clinical entity as currently documented, though the underlying gene is broadly conserved (see below).

Orthologous gene/taxonomy: ISCA2 orthologs are broadly conserved across eukaryotes, including: - Saccharomyces cerevisiae — Isa2 (yeast ortholog, functionally characterized in the original mechanistic studies of ISC late-assembly machinery). - Danio rerio (zebrafish, NCBITaxon:7955) — isca2 ortholog. - Mus musculus (mouse, NCBITaxon:10090) — Isca2 ortholog. - Dictyostelium discoideum (slime mold) and Hydra vulgaris — isca2 orthologs annotated in UniProt, reflecting deep evolutionary conservation of the Fe-S cluster biogenesis pathway.

No veterinary/OMIA-documented natural disease was identified for ISCA2 in animals.

15. Model Organisms

Yeast (S. cerevisiae): The human ISCA1/ISCA2 system is homologous to yeast Isa1/Isa2, and foundational biochemical work establishing the "late" ISC assembly pathway (reductive [2Fe-2S] cluster fusion into [4Fe-4S] via ISCA1-ISCA2, with electrons from ferredoxin) was substantially informed by yeast and human cell-based reconstitution studies (Molecular Biology of the Cell, human ISCA1/ISCA2/IBA57 maturation paper; PNAS 2020 mechanism paper).

Mammalian cell/tissue models: shRNA knockdown of ISCA2 in patient-derived and other human cell lines is the principal disease model used to establish causality and mechanism — showing diminished mitochondrial membrane potential, disrupted mitochondrial network morphology, reduced basal/maximal respiration and ATP production, reduced activity of respiratory complexes II and IV, and selective loss of [4Fe-4S]- (but not [2Fe-2S]-) dependent protein function, closely recapitulating the biochemical phenotype seen in patient fibroblasts (Alaimo et al. 2018, PMID:29297947; PMC5839994).

Mouse: Interestingly, mouse knockdown studies in skeletal muscle and primary neuronal cultures suggested that ISCA1, but not ISCA2, was required for mitochondrial [4Fe-4S] protein biogenesis in that system (ISCA1 essential for Fe4S4 biogenesis in vivo, Nature Communications) — a potential human-model mismatch worth flagging explicitly: if a dismech entry is built for this disease, this discrepancy (mouse data suggesting redundancy/dispensability of ISCA2 in some contexts, versus unambiguous, fully penetrant human disease from ISCA2 loss) would be a strong candidate for a HUMAN_MODEL_MISMATCH discussion rather than a simple RECAPITULATES link, pending closer reading of the primary Nature Communications paper (not fully retrieved in this session — flag as a lead requiring direct verification of the actual reported genotype/phenotype and tissue context before curating the relationship).

Zebrafish: Knockdown of Fe-S cluster assembly genes including isca1/iba57 in zebrafish produces anemia with reduced heme/hemoglobin expression; whether isca2-specific zebrafish knockdown data (as opposed to isca1/iba57) were generated was not clearly resolved in this search and should be checked against the primary literature before citing a zebrafish ISCA2 model specifically.

Human disease vs. models — fidelity assessment (for future curation): The strongest, most directly translatable model evidence is patient-derived fibroblasts (same species, disease-relevant tissue context, direct genotype) — high fidelity by construction. Heterologous knockdown (shRNA in other human cell lines) is informative for mechanism (complex II/IV activity, [4Fe-4S] vs [2Fe-2S] selectivity) but does not model the tissue-selective CNS/optic-pathway phenotype seen clinically. No whole-organism vertebrate model (mouse, zebrafish) with a clearly demonstrated CNS leukoencephalopathy/optic-atrophy phenotype recapitulating human MMDS4 was identified in this search — this is a notable gap: the mouse data available actually argue against a straightforward loss-of-function requirement for ISCA2 in murine neurons/muscle, which is a genuine, citable human-model divergence rather than an oversight, and would need divergence_type: SPECIES_MISMATCH and/or careful fidelity: LOW/UNKNOWN framing if curated into a model-mechanism link.

Summary of Key Evidence Sources

Citation Type Key contribution
Al-Hassnan et al. 2015, PMID:25539947 — J Med Genet 52:186-94 HUMAN_CLINICAL + IN_VITRO Original description; founder mutation p.Gly77Ser; haplotype/founder-age analysis
Alaimo et al. 2018, PMID:29297947 — Hum Mutat (PMC5839994) IN_VITRO + HUMAN_CLINICAL Mechanistic link: loss of [4Fe-4S] proteins, LIAS/lipoylation deficiency, hyperglycinemia mechanism, mtDNA depletion
Alfadhel 2019, PMID:31106229 — J Cent Nerv Syst Dis (review) Aggregated case review Comprehensive clinical/MRI phenotype tabulation across ~20 cases
GeneReviews, ISCA2-Related Mitochondrial Disorder, NBK481904 Clinical guideline/synthesis Authoritative natural history, diagnostic criteria, management, genetic counseling
Neurogenetics 2020, DOI:10.1007/s10048-020-00611-8 HUMAN_CLINICAL Expanded phenotype: cavitating leukoencephalopathy, prolonged survival (paywalled in this session — verify directly before quoting)
PMC6612116 — BMC Neurology case report HUMAN_CLINICAL Non-founder variant (Iranian family), c.355G>A
PMC11561297 HUMAN_CLINICAL Novel missense variant causing aberrant splicing
Review of MMDS/Fe-S disorders, PMC8393393 Aggregated review Comparative table across MMDS1-5; late-stage ISC pathway mechanism
PNAS 2020, DOI:10.1073/pnas.2003982117 COMPUTATIONAL/IN_VITRO biochemistry Molecular mechanism of ISCA1-ISCA2 reductive [2Fe-2S]→[4Fe-4S] fusion
Nature Communications 2017, ISCA1 in vivo MODEL_ORGANISM (mouse) Mouse knockdown data — candidate human-model mismatch re: ISCA2 dispensability in murine tissue

Gaps flagged for curator follow-up before KB entry finalization: (1) direct gnomAD allele/carrier frequency for c.229G>A; (2) full-text verification of the Neurogenetics 2020 paper (paywalled here); (3) direct verification of the Nature Communications mouse ISCA1/ISCA2 paper's exact claims about ISCA2 dispensability, and whether it truly constitutes a HUMAN_MODEL_MISMATCH; (4) confirmation of ORPHA:457406 and MONDO:0014611 cross-mapping directly against Orphanet/MONDO rather than via secondary aggregator (MalaCards fetch was blocked by a 403 in this session).

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 10
Resolved 10
Unresolved (possible confabulation) 0
Unverifiable 0
References weighed for topical relevance 10
On topic 7
Off topic 0

All extracted references resolved successfully.

Term Validation

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

Outcome Count
Terms checked 36
Resolved 32
Unresolved (possible confabulation) 1
Obsolete 2
Unverifiable 1
Terms whose name was checked 9
Terms named correctly 4
Terms named as a different term 2
Terms whose name is worth a second look 3

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:0014611 (3 mentions) - the report calls it "MONDO"; MONDO calls it multiple mitochondrial dysfunctions syndrome 4
  • HP:0002197 (1 mention) - the report calls it "cerebellar atrophy"; HP calls it Generalized-onset seizure

Unresolved terms

These identifiers do not exist in an ontology that resolved other terms from the same prefix, so they were most likely invented:

  • HP:0007070 (1 mention), reported as "cerebral white matter hypoplasia/abnormality" - HP does not contain this term

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:

  • HP:0031988 (obsolete Muscle spasm) (1 mention) - replaced by HP:0003394
  • GO:0097428 (obsolete protein maturation by iron-sulfur cluster transfer) (1 mention) - replaced by GO:0051604

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 (1 mention) - the report calls it "Hypotonia (early)"; HP calls it Hypotonia
  • HP:0002153 (1 mention) - the report calls it "Elevated CSF/plasma glycine (hyperglycinemia)"; HP calls it Hyperkalemia, and lists "Elevated potassium(1+) level" among its other names
  • HP:0002151 (1 mention) - the report calls it "Elevated lactate (blood/CSF/MRS)"; HP calls it Increased circulating lactate concentration, and lists "Higher than normal levels of lactate in blood" among its other names

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.