Multiple Mitochondrial Dysfunctions Syndrome 5

Genetic MONDO:0033282 Pathograph 30 Show in embeddings browser MONDO:0017338

Multiple mitochondrial dysfunctions syndrome 5 (MMDS5; OMIM #617613) is an autosomal recessive infantile neurodegenerative disorder caused by biallelic loss-of-function variants in ISCA1, the A-type carrier protein that mediates the late, [4Fe-4S]-generating step of mitochondrial iron-sulfur cluster biogenesis. Losing that step does not disable one enzyme but a whole client class: the respiratory complexes that carry [4Fe-4S] cofactors (complex I and complex II) and lipoic acid synthase, itself a [4Fe-4S] enzyme. The result is a severe leukodystrophy with either no attainment of developmental milestones or very early loss of them, infantile seizures, spasticity, nystagmus and lactic acidosis, with death usually in early childhood. MMDS5 sits in a series of disorders that share this final common lesion - MMDS1 (NFU1), MMDS2 (BOLA3), MMDS3 (IBA57) and MMDS4 (ISCA2) - and the reported phenotype is described by its discoverers as similar to all four. It is nevertheless curated as its own entry rather than as a subtype, because the molecular lesion is a distinct, non-redundant node of the pathway (ISCA1 is required for [4Fe-4S] biogenesis where ISCA2 is not, in the same knockdown experiment) and because neuronal migrational abnormality, reported in both children of the index family, had not been described in the other MMDS subtypes.

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1
Inheritance
7
Pathophys.
13
Phenotypes
3
Gaps
30
Pathograph
1
Genes
9
Medical Actions
4
Differentials
1
Models
1
References
1
Deep Research
🏷

Classifications

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

Inheritance

1
Autosomal recessive HP:0000007
All reported families segregate biallelic ISCA1 variants; heterozygous parents and siblings are unaffected carriers.
Autosomal recessive inheritance
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"ISCA1-related multiple mitochondrial dysfunctions syndrome is inherited in an autosomal recessive manner."
GeneReviews states the inheritance mode directly.
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Discussions and Knowledge Gaps

3
Has non-ketotic hyperglycinemia actually been measured in a patient with biallelic ISCA1 variants, or is it inferred from the shared lipoylation lesion in the other MMDS subtypes?
KNOWLEDGE GAP isca1_glycine_branch_unmeasured
Attached to
pathophysiology#Lipoic Acid Synthase Failure
Hyperglycinemia is a defining feature of the MMDS group and follows mechanically from loss of the lipoyl-dependent glycine cleavage system, which the ISCA1 patient's impaired lipoic acid synthesis would predict. But the sources cited here report hyperglycinemia as a property of MMDS in general, not as a measurement in an ISCA1 patient, and the index families' biochemical investigations are described as normal apart from creatine kinase. Curating the glycine branch as an established patient finding would therefore overstate the evidence, so it is curated as a mechanism node with a gap attached rather than as a phenotype.
Proposed experiments
Plasma and CSF glycine measurement in ISCA1 patients
isca1_glycine_measurement
Measure plasma and cerebrospinal fluid glycine, and lipoylation of the pyruvate dehydrogenase E2 subunit and the glycine cleavage system H protein by immunoblot in fibroblasts, in any newly ascertained ISCA1 patient.
Readouts
Plasma glycine concentration
pathophysiology#Lipoic Acid Synthase Failure
Direction: INCREASED
Interpretation: An elevated glycine would confirm the glycine cleavage branch operates in ISCA1 disease as it does in the other MMDS subtypes.
Does ISCA1 loss also attenuate mitochondrial protein synthesis by leaving the mitoribosome assembly factor METTL17 without its [4Fe-4S] cluster, and if so does that branch contribute to the disease?
KNOWLEDGE GAP isca1_mettl17_translation_branch
Attached to
pathophysiology#Failure of Mitochondrial [4Fe-4S] Cluster Assembly
A third client branch is proposed in the literature and is deliberately not curated as a node. METTL17 is a mitoribosome biogenesis factor carrying a [4Fe-4S] cluster, and the paper that reports it proposes, in its own words, that the cluster is inserted via an ISCA1-NFU1 node. The measured consequence in that paper is in the sibling genes: fibroblasts from BOLA3 and NFU1 patients show attenuated mitochondrial translation. Nobody has measured it in ISCA1 cells, and the deep-research report that raised this branch says so in its own limitations. So the entry carries the question rather than the mechanism. If it holds, the respiratory chain deficit in this disease is partly a translation defect and not only a failure to mature individual complexes, which would change what a rescue experiment should target.
Proposed experiments
Mitochondrial translation and METTL17 maturation in ISCA1 patient cells
isca1_mitochondrial_translation_measurement
Measure mitochondrial protein synthesis by metabolic labelling, and small mitoribosomal subunit assembly, in ISCA1 patient fibroblasts against the BOLA3 and NFU1 cells where the defect is already established.
Readouts
Mitochondrial protein synthesis rate
pathophysiology#Failure of Mitochondrial [4Fe-4S] Cluster Assembly
Direction: DECREASED
Interpretation: Attenuated synthesis in ISCA1 cells would establish the translation branch in this disease rather than only in its siblings.
Show evidence (2 references)
PMID:37823603 SUPPORT In Vitro
"which we propose is inserted via the ISCA1-NFU1 node"
The authors state the ISCA1 involvement as a proposal, which is exactly the epistemic status this gap records.
PMID:37823603 SUPPORT In Vitro
"fibroblasts from subjects suffering from 'multiple mitochondrial dysfunction' syndrome due to mutations in BOLA3 or NFU1 display previously unrecognized attenuation of mitochondrial protein synthesis"
The translation defect is measured in BOLA3 and NFU1 patient cells, not ISCA1 ones, which is why this is a gap rather than a curated branch.
Why does an ISCA1 defect produce a neuronal migration abnormality when the sibling MMDS subtypes, sharing the same final lesion, do not?
KNOWLEDGE GAP isca1_pachygyria_subtype_specificity
Attached to
phenotypes#Pachygyria
Pachygyria was seen in both affected children of the index family and the authors state it had not previously been described in other MMDS subtypes. If that holds up in a larger series it is a genuine mechanistic divergence, since migration is a prenatal event and the shared respiratory lesion is not obviously developmental. The alternative is ascertainment: four children in one family imaged in one centre is a thin base for a claim of subtype specificity, and no later report has replicated it.
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Pathophysiology

7
Biallelic ISCA1 Loss of Function
ISCA1 encodes a 129-amino-acid mitochondrial A-type carrier protein with a conserved C-terminal HESB domain. The founder p.(Glu87Lys) substitution sits in that domain at a highly conserved residue and abolishes a salt bridge to Lys49, destabilizing the protein; the independent p.V10G substitution lies in the uncleaved presequence and impairs both mitochondrial import and stability, reducing ISCA1 protein to a fraction of normal. Both routes converge on loss of functional ISCA1 in the mitochondrial matrix.
ISCA1 hgnc:28660 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ISCA1 (hgnc:28660). hgnc:28660 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: LOSS_OF_FUNCTION
Downstream
Failure of Mitochondrial [4Fe-4S] Cluster Assembly ISCA1 is one of the two non-redundant A-type proteins that carry out the second, [4Fe-4S]-generating step of the pathway.
Pachygyria ISCA1 loss is the only lesion in this entry that precedes the neuronal migration abnormality, but no source identifies an intervening mechanism between [4Fe-4S] biogenesis failure and migration -- see the attached knowledge gap on why this subtype alone shows it.
Show evidence (2 references)
PMID:29767723 SUPPORT In Vitro
"The mutation located in the uncleaved presequence severely affected both mitochondrial import and stability of ISCA1."
Identifies the molecular consequence of the p.V10G allele as loss of imported, stable ISCA1 protein rather than a catalytic change.
PMID:28356563 SUPPORT Computational
"Analyses of protein stability via I-Mutant (score 7) and Eris web-server (score 0.64 ΔΔG kcal/mol) are also consistent in predicting that this variant leads to destabilization of ISCA1 protein."
Supports destabilization as the mechanism of the founder allele. Graded COMPUTATIONAL because the claim rests on in silico stability prediction, not on a measurement in patient material.
Failure of Mitochondrial [4Fe-4S] Cluster Assembly
Mitochondrial Fe-S biogenesis runs in two steps. The core machinery builds a [2Fe-2S] cluster and hands it to GRX5; a second, separately staffed step converts that into [4Fe-4S] clusters and inserts them into client apoproteins. ISCA1 is the organizing node of that second step, interacting with both ISCA2 and NFU1 (which do not interact with each other) and forming the transient ternary complex through which the cluster is handed on. Losing ISCA1 therefore stalls maturation of every mitochondrial [4Fe-4S] client at once, which is why a single gene defect produces a multi-enzyme phenotype. The client class is wider than the two downstream nodes name: purified ISCA1 also matures apo-aconitase, so aconitase deficiency belongs to this lesion even though the human reports measure complex I, complex II and lipoylation specifically.
iron-sulfur cluster assembly GO:0016226 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased iron-sulfur cluster assembly (GO:0016226). GO:0016226 is a biological process from the Gene Ontology. ↓ DECREASED
4 iron, 4 sulfur cluster binding GO:0051539 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased 4 iron, 4 sulfur cluster binding (GO:0051539). GO:0051539 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Respiratory Chain Complex I and II Deficiency
Lipoic Acid Synthase Failure
Show evidence (4 references)
PMID:33711344 SUPPORT In Vitro
"ISCA1 works as the promoter of the interaction between ISCA2 and NFU1 being able to determine the formation of a transient ISCA1-ISCA2-NFU1 ternary complex."
NMR structural work establishing why ISCA1 is the non-redundant node of this step, which is the reason its loss affects the whole client class.
PMID:28492233 SUPPORT Model Organism
"knockdown experiments in mouse skeletal muscle and in primary cultures of neurons suggest that ISCA1, but not ISCA2, is required for mitochondrial Fe4S4 proteins biogenesis"
Shows the requirement is specific to ISCA1 within the same experiment, which is what distinguishes this lesion from the ISCA2 one (MMDS4).
PMID:28492233 SUPPORT In Vitro
"ACO2 activity was progressively increasing and reached more than 70% and 80% of the activity of a chemically reconstituted ACO2"
Purified ISCA1 transfers its cluster to apo-aconitase and restores most of its activity, which is what puts aconitase in the client class alongside the respiratory chain complexes. Graded IN_VITRO because it is a reconstitution with purified proteins, not the in vivo knockdown quoted above.
+ 1 more reference
Respiratory Chain Complex I and II Deficiency
Complex I carries eight [4Fe-4S] clusters and complex II carries one; both therefore fail to mature when the late assembly step is lost. Electron flow through the respiratory chain falls, oxidative phosphorylation is impaired, and pyruvate is shunted to lactate.
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
Animal models
Neuron-specific Isca1 knockout rat Rat recapitulates The knockout reproduces the biochemical lesion: reduced respiratory chain complex protein and reduced ATP production, with mitochondrial fragmentation and cristae fracture on electron microscopy.
Downstream
Lactic Acidosis
Leukodystrophy Sources assert white matter degeneration as the consequence of the respiratory defect without identifying the intervening steps, so the edge is typed as indirect rather than direct.
Elevated circulating creatine kinase concentration Complex I and II are ubiquitously expressed, including in skeletal muscle, so the same bioenergetic defect is a plausible driver of the isolated creatine kinase elevation reported in one patient; the source does not report a muscle biopsy or state the mechanism directly, so the edge is typed as indirect.
Show evidence (1 reference)
PMID:29767723 SUPPORT Human Clinical
"Here, we describe a patient with a severe early onset leukodystrophy, multiple defects of respiratory complexes and a severe impairment of lipoic acid synthesis."
Reports multiple respiratory complex defects in the patient carrying the ISCA1 variant.
Lipoic Acid Synthase Failure
Lipoic acid synthase is itself a [4Fe-4S] enzyme, so a defect in the assembly machinery also removes the cofactor from the enzyme that makes lipoate. Proteins that depend on a lipoyl group - pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and the glycine cleavage system - lose activity as a second, parallel consequence. This is the branch responsible for the non-ketotic hyperglycinemia reported across the MMDS series.
lipoate biosynthetic process GO:0009107 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased lipoate biosynthetic process (GO:0009107). GO:0009107 is a biological process from the Gene Ontology. ↓ DECREASED 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 glycine catabolic process GO:0006546 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased glycine catabolic process (GO:0006546). GO:0006546 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Lactic Acidosis Loss of the pyruvate dehydrogenase lipoyl cofactor contributes to lactate accumulation alongside the respiratory chain defect.
Show evidence (1 reference)
PMID:29767723 SUPPORT Human Clinical
"Multiple mitochondrial dysfunction syndromes (MMDS) comprise a group of severe autosomal recessive diseases characterized by impaired respiration and lipoic acid metabolism, resulting in infantile-onset mitochondrial encephalopathy, non-ketotic hyperglycinemia, myopathy, lactic acidosis and early death."
States that impaired lipoic acid metabolism is one of the two defining lesions of the MMDS group and names hyperglycinemia as its consequence.
Leukodystrophy
Extensive cerebral and cerebellar deep white matter disease with ventricular dilatation is the imaging hallmark, and in the index family was accompanied by pachygyria, a neuronal migration abnormality that had not been reported in the other MMDS subtypes.
oligodendrocyte CL:0000128 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves oligodendrocyte (CL:0000128). CL:0000128 is a cell type from the Cell Ontology. neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
Downstream
Progressive Neurological Deterioration
Ventriculomegaly Ventricular dilatation is reported alongside the white matter disease in the same patients, consistent with passive (ex-vacuo) enlargement as white matter volume is lost, but the source reports the co-occurrence rather than stating that mechanism, so the edge is typed as indirect.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Brain imaging of both children showed pachygyria, extensive cerebral and cerebellar white matter disease and dilated cerebral ventricles."
Documents the imaging phenotype in the index family.
Lactic Acidosis
Blood lactate is elevated and an increased lipid-lactate peak is seen on brain MR spectroscopy, reflecting the block in oxidative phosphorylation.
Downstream
Increased circulating lactate concentration Raised blood lactate is the systemic readout of this node's biochemical lesion; the lipid-lactate peak on brain MR spectroscopy is its CNS correlate.
Lactic acidosis Blood lactic acidosis is the systemic biochemical readout of the same block in oxidative phosphorylation this node describes.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy."
GeneReviews records the biochemical finding and its imaging correlate.
Progressive Neurological Deterioration
The clinical endpoint of the chain: absent or lost milestones, early infantile seizures, spasticity with exaggerated reflexes, nystagmus and hearing loss, ending in death in early childhood.
Animal models
Neuron-specific Isca1 knockout rat Rat partially recapitulates Developmental retardation, epilepsy and memory impairment reproduce the developmental arrest and seizures of the human disease.
Downstream
Developmental regression
Seizure
Spasticity
Nystagmus
Sensorineural hearing impairment
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"ISCA1-related multiple mitochondrial dysfunctions syndrome (ISCA1-MMDS) is a severe neurodegenerative condition typically characterized by either no attainment of developmental milestones or very early loss of achieved milestones, seizures in early infancy, development of spasticity with..."
The GeneReviews Clinical Characteristics sentence, used as the phenotype baseline for this entry.
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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 5 Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

13
Digestive 2
Feeding difficulties VERY_FREQUENT HP:0011968 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Feeding difficulties (HP:0011968). HP:0011968 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Both P1 and P2 had normal birth and antenatal history followed by inconsolable cry and feeding difficulties since newborn period."
Documents feeding difficulty from the newborn period.
Constipation OCCASIONAL HP:0002019 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Constipation (HP:0002019). HP:0002019 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:31580634 SUPPORT Human Clinical
"Monitor constipation, developmental progress, growth parameters, and family needs at each visit."
GeneReviews lists constipation among the manifestations to be monitored at each surveillance visit.
PMID:31580634 SUPPORT Human Clinical
"Adequate hydration, stool softeners, and laxatives may help to prevent severe constipation."
Names severe constipation as a complication that management should prevent.
Ear 1
Sensorineural hearing impairment OCCASIONAL HP:0000407 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sensorineural hearing impairment (HP:0000407). HP:0000407 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"risk for sensorineural hearing loss"
GeneReviews frames hearing loss as a risk, which is why the frequency band is OCCASIONAL rather than VERY_FREQUENT.
Eye 1
Nystagmus VERY_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:31580634 SUPPORT Human Clinical
"nystagmus, and risk for sensorineural hearing loss"
GeneReviews lists nystagmus among the defining clinical characteristics.
Metabolism 3
Increased circulating lactate concentration FREQUENT 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 (2 references)
PMID:28356563 SUPPORT Human Clinical
"Increased lipid lactate peak was seen on magnetic resonance spectroscopy of brain in P1"
Documents the raised brain lactate on spectroscopy.
PMID:31580634 SUPPORT Human Clinical
"Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy."
GeneReviews reports elevated blood lactate, the circulating measure this phenotype names.
Lactic acidosis FREQUENT HP:0003128 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Lactic acidosis (HP:0003128). HP:0003128 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"P3 had lactic acidosis and white matter disease was documented in medical records of P3 on computed tomography of brain."
Documents lactic acidosis in a reported patient.
Elevated circulating creatine kinase concentration OCCASIONAL Elevated circulating creatine kinase activity HP:0003236 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated circulating creatine kinase activity (HP:0003236). HP:0003236 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Normal hematological and biochemical investigations were noted except for elevated creatinine phosphokinase in P2."
Documents the isolated CK elevation and, in the same sentence, that other routine investigations were normal.
Musculoskeletal 1
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:31580634 SUPPORT Human Clinical
"development of spasticity with exaggerated deep tendon reflexes"
GeneReviews lists spasticity among the defining clinical characteristics.
Nervous System 5
Developmental regression OBLIGATE 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:28356563 SUPPORT Human Clinical
"Both of them did not achieve any other developmental milestone except partial head control at 3 months of age."
Documents the developmental ceiling in the two index patients.
Seizure OBLIGATE 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:28356563 SUPPORT Human Clinical
"Seizures developed at the age of 3 months and 2 months in P3 and P4 respectively following which neurologic deterioration was noted in them."
Gives the onset ages in the second family.
Leukodystrophy OBLIGATE HP:0002415 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Leukodystrophy (HP:0002415). HP:0002415 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Further brain MRI findings may include extensive cerebral and cerebellar deep white matter hyperintensities, marked dilatation of the cerebral ventricles, and pachygyria."
GeneReviews lists the MRI findings including the white matter disease.
Ventriculomegaly FREQUENT HP:0002119 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Ventriculomegaly (HP:0002119). HP:0002119 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"P4 had extensive leukodystrophy involving cerebral and cerebellar white matter with dilated ventricles."
Documents ventricular dilatation alongside the white matter disease.
Pachygyria OCCASIONAL HP:0001302 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Pachygyria (HP:0001302). HP:0001302 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Neuronal migrational abnormalities seen in both children in Family 1 has not been previously described in other subtypes of MMDS."
Records both the finding and the claim that it is not shared with the sibling MMDS forms.
🧬

Genetic Associations

1
ISCA1
Gene: ISCA1 hgnc:28660 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ISCA1 (hgnc:28660). hgnc:28660 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:28356563 SUPPORT Human Clinical
"We describe two independent families, both with two affected children each, with a severe neurodevelopmental disorder associated with a homozygous c.259G>A variant in ISCA1."
Establishes the gene-disease relationship with segregation in two families.
PMID:28356563 SUPPORT Human Clinical
"After examining the homozygous regions around the ISCA1 variant in both the probands, generated by the exome sequencing data, we identified only one overlapping region of homozygosity (ROH) spanning 3.3 Mb in chromosome 9 (Chr9: 85613354-88925774) flanking the variant"
Supports the founder-allele interpretation of the recurrent variant.
PMID:32092383 SUPPORT Human Clinical
"We report a patient carrying a novel pathogenic variant p.(Tyr101Cys) in ISCA1 leading to MMDS type 5."
A third independent allele, which is what takes the gene-disease relationship beyond the single founder haplotype.
💊

Medical Actions

9
Supportive 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 disease-modifying therapy exists. Management is supportive. This entry is the umbrella statement; the individual measures GeneReviews names are curated as their own treatments below so that each one reaches the manifestation it addresses.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Treatment of manifestations: Treatment is primarily supportive."
GeneReviews states that management is supportive.
Enteral Feeding Support
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 tube may be required where oral intake becomes unsafe or insufficient.
Target Phenotypes: Feeding difficulties HP:0011968 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Feeding difficulties (HP:0011968). HP:0011968 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"A feeding tube (nasogastric or gastrostomy) may be required."
GeneReviews recommends enteral feeding support.
Genetic Counseling and Carrier Testing
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 recurrence risk is 25% per pregnancy. Carrier testing and prenatal testing are available where the familial variants are known, which in the reported families was the reason for referral. This addresses reproductive risk rather than a disease manifestation, so it carries no `target_phenotypes` link.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Carrier testing for at-risk relatives and prenatal testing for a pregnancy at increased risk are possible if the pathogenic ISCA1 variants in the family are known."
GeneReviews states the availability of carrier and prenatal testing.
Antiseizure Therapy
Action: antiseizure pharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is antiseizure pharmacotherapy, annotated with Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. Ontology label: Pharmacotherapy NCIT:C15986
Seizures are treated with standard antiseizure regimens. GeneReviews names no agent and no MMDS-specific protocol, so no therapeutic_agent is bound here.
Target Phenotypes: Seizure HP:0001250 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Standard treatment for spasticity, seizures, abnormal vision, and hearing loss."
GeneReviews directs standard antiseizure treatment, which is what joins this treatment to the obligate seizure phenotype.
Spasticity Management
Action: standard management of spasticityNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is standard management of spasticity, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Spasticity is managed by standard means. GeneReviews specifies no modality, so the binding is the generic therapeutic action rather than a drug or a physical therapy claim the source does not make.
Target Phenotypes: Spasticity HP:0001257 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Spasticity (HP:0001257). HP:0001257 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Standard treatment for spasticity, seizures, abnormal vision, and hearing loss."
GeneReviews directs standard treatment of spasticity, which is what joins this treatment to the spasticity phenotype.
Constipation Prevention
Action: prevention of severe constipationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is prevention of severe constipation, annotated with Preventive Intervention (NCIT:C15843). NCIT:C15843 is a clinical intervention from the NCI Thesaurus. Ontology label: Preventive Intervention NCIT:C15843
Hydration, stool softeners and laxatives are used to prevent severe constipation, which GeneReviews lists as a secondary complication rather than a presenting feature.
Target Phenotypes: Constipation HP:0002019 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Constipation (HP:0002019). HP:0002019 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Adequate hydration, stool softeners, and laxatives may help to prevent severe constipation."
The GeneReviews prevention-of-secondary-complications recommendation.
Neurologic, Nutritional and Respiratory Surveillance
Action: surveillance physical examination at each visitNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is surveillance physical examination at each visit, annotated with Physical Examination (NCIT:C20989). NCIT:C20989 is a clinical intervention from the NCI Thesaurus. Ontology label: Physical Examination NCIT:C20989
Assessment at every visit for new neurologic manifestations, safe oral intake, adequate nutrition, and respiratory insufficiency or aspiration. This is the schedule against which deterioration is detected, and aspiration risk is the proximate cause of the intercurrent illness that ends most reported cases.
Target Phenotypes: Feeding difficulties HP:0011968 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Feeding difficulties (HP:0011968). HP:0011968 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Assessment for new neurologic manifestations, safety of oral intake, adequate nutrition, and evidence of respiratory insufficiency and aspiration at each visit."
The GeneReviews surveillance recommendation, stated per visit.
Annual Ophthalmologic Evaluation
Action: ophthalmologic evaluationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is ophthalmologic evaluation, annotated with Eye Examination (NCIT:C38060). NCIT:C38060 is a clinical intervention from the NCI Thesaurus. Ontology label: Eye Examination NCIT:C38060
Eye examination annually or on clinical suspicion. Nystagmus is among the defining features, so this is surveillance of a manifestation the disease is expected to produce rather than screening for an unrelated risk.
Target Phenotypes: Nystagmus HP:0000639 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Nystagmus (HP:0000639). HP:0000639 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Ophthalmologic and audiologic evaluations annually or based on clinical suspicion."
GeneReviews sets the ophthalmologic surveillance interval.
Annual Audiologic Evaluation
Action: audiologic evaluationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is audiologic evaluation, annotated with Overall Hearing Assessment (NCIT:C204585). NCIT:C204585 is a clinical intervention from the NCI Thesaurus. Ontology label: Overall Hearing Assessment NCIT:C204585
Hearing assessment annually or on clinical suspicion, against the stated risk of sensorineural hearing loss.
Target Phenotypes: Sensorineural hearing impairment HP:0000407 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Sensorineural hearing impairment (HP:0000407). HP:0000407 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"Ophthalmologic and audiologic evaluations annually or based on clinical suspicion."
GeneReviews sets the audiologic surveillance interval.
🔬

Biochemical Markers

2
Respiratory chain complex I and II activity in cultured fibroblasts (DECREASED)
Context: Enzyme activities measured in patient-derived skin fibroblasts. Together with impaired lipoylation this is the biochemical fingerprint that points at the MMDS group before sequencing, though it does not distinguish ISCA1 from the sibling genes.
Pathograph Readouts
Readout Of Respiratory Chain Complex I and II Deficiency Negative Diagnostic
Reduced complex I and II activity reports the [4Fe-4S] client failure directly, since both complexes depend on [4Fe-4S] subunits.
Show evidence (1 reference)
PMID:32092383 SUPPORT In Vitro
"Biochemical analysis of patient fibroblasts revealed impaired lipoic acid synthesis and decreased activities of complex I and II of respiratory chain."
Reports the complex I and II activity measurement in patient fibroblasts.
Lipoic acid synthesis in cultured fibroblasts (DECREASED)
Context: Impaired lipoylation measured in patient-derived fibroblasts, the second arm of the same fingerprint and the branch that predicts the glycine cleavage defect the entry does not assert.
Pathograph Readouts
Readout Of Lipoic Acid Synthase Failure Negative Diagnostic
Reduced lipoic acid synthesis reports failure of lipoyl synthase, itself a [4Fe-4S] enzyme, which is why the defect is self-amplifying.
Show evidence (1 reference)
PMID:32092383 SUPPORT In Vitro
"Biochemical analysis of patient fibroblasts revealed impaired lipoic acid synthesis and decreased activities of complex I and II of respiratory chain."
Reports the impaired lipoic acid synthesis in patient fibroblasts.
🔬

Diagnosis

1
ISCA1 molecular genetic testing (PRESENT)
Diagnosis rests on biallelic pathogenic ISCA1 variants in a proband with suggestive clinical and imaging findings; in every reported case the gene was reached by exome rather than targeted testing.
Show evidence (1 reference)
PMID:31580634 SUPPORT Human Clinical
"The diagnosis of ISCA1-MMDS is established in a proband with suggestive findings and/or biallelic pathogenic variants in ISCA1 identified by molecular genetic testing."
GeneReviews states the diagnostic criterion.
📈

Progression

1
Infantile onset and rapid neurodegeneration
Reported children are normal at birth and for the first weeks to months, then fail to acquire or lose milestones, develop seizures between two and five months, and deteriorate progressively with spasticity. Death has been reported between eleven months and five years.
Show evidence (2 references)
PMID:28356563 SUPPORT Human Clinical
"P1 and P2 developed seizures at the age of 5 months and 4 months respectively followed by progressive neurological deterioration and spasticity."
Documents the onset window and the sequence of seizure then deterioration.
PMID:31580634 SUPPORT Human Clinical
"Prognosis is poor and most individuals succumb to an intercurrent illness in early childhood."
GeneReviews states the outcome.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Fewer than ten patients have been reported. The founder p.(Glu87Lys) allele accounts for the Indian families; a separate p.V10G allele was reported in an Italian patient. No population prevalence estimate exists.
Show evidence (1 reference)
PMID:30105122 SUPPORT Human Clinical
"We now report the third patient with the same phenotype and variant, further supporting the possibility of a founder event."
Establishes that the reported case series was three patients at the time of this report, and that they share one founder allele.
🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Multiple Mitochondrial Dysfunctions Syndrome 5:

Multiple mitochondrial dysfunctions syndrome 1 MONDO:0011582
Overlapping Features NFU1 encodes a late-acting [4Fe-4S] scaffold, so the lesion converges on the same client class from a different step.
Distinguishing Features
  • Caused by biallelic NFU1 variants rather than biallelic ISCA1 variants.
  • Pulmonary hypertension is a recognised feature of the NFU1 form and is not reported in ISCA1 disease.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
States directly that the MMDS5 phenotype overlaps this sibling syndrome, which is why the differential cannot be made clinically.
Multiple mitochondrial dysfunctions syndrome 2 MONDO:0013675
Overlapping Features BOLA3 partners the same late [4Fe-4S] step, so lipoylation and respiratory chain deficits are shared.
Distinguishing Features
  • Caused by biallelic BOLA3 variants rather than biallelic ISCA1 variants.
  • Distinguished by the gene; no clinical feature reported in the ISCA1 series separates the two.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
States directly that the MMDS5 phenotype overlaps this sibling syndrome, which is why the differential cannot be made clinically.
Multiple mitochondrial dysfunctions syndrome 3 Not Yet Curated MONDO:0014132
Overlapping Features IBA57 acts with ISCA1 and ISCA2 in [4Fe-4S] maturation, giving the same biochemical fingerprint.
Distinguishing Features
  • Caused by biallelic IBA57 variants rather than biallelic ISCA1 variants.
  • Distinguished by the gene; no clinical feature reported in the ISCA1 series separates the two.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
States directly that the MMDS5 phenotype overlaps this sibling syndrome, which is why the differential cannot be made clinically.
Multiple mitochondrial dysfunctions syndrome 4 MONDO:0014611
Overlapping Features ISCA2 is ISCA1's obligate heterodimer partner at the step this disease breaks, so this is the closest of the four.
Distinguishing Features
  • Caused by biallelic ISCA2 variants rather than biallelic ISCA1 variants.
  • The only reported separator is mechanistic rather than clinical. In the same knockdown experiment ISCA1 is required for mitochondrial [4Fe-4S] protein biogenesis and ISCA2 is not.
Show evidence (1 reference)
PMID:28356563 SUPPORT Human Clinical
"Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
States directly that the MMDS5 phenotype overlaps this sibling syndrome, which is why the differential cannot be made clinically.
🐁

Animal Models

1
Neuron-specific Isca1 knockout rat
A CRISPR-Cas9 conditional knockout restricting Isca1 loss to neurons. It is the only in vivo model of this disorder, and it was built specifically to model MMDS5 rather than repurposed from another question.
Species
Rat
Genotype
Isca1 flox/flox; NeuN-Cre (neuron-specific conditional knockout)
Publication
PMID:37140997
Pathograph links
Respiratory Chain Complex I and II Deficiency recapitulates fidelity: moderate observes: cellular The knockout reproduces the biochemical lesion: reduced respiratory chain complex protein and reduced ATP production, with mitochondrial fragmentation and cristae fracture on electron microscopy.
Limitations: Neuron-restricted rather than constitutive, so it cannot report the systemic consequences of the lesion, and it does not distinguish complex I from complex II involvement the way the human fibroblast assays do.
Readouts
Respiratory chain complex protein content and ATP production
Direction: DECREASED
Interpretation: Establishes that neuronal Isca1 loss alone is sufficient for the bioenergetic defect.
Show evidence (1 reference)
PMID:37140997 SUPPORT Model Organism
"mitochondrial fragmentation, cristae fracture, reduced content of respiratory chain complex protein, and reduced production of ATP"
Reports the measured bioenergetic and ultrastructural readouts.
Show evidence (1 reference)
PMID:37140997 SUPPORT Model Organism
"This study established the disease model of MMDS5 in the nervous system for the first time"
Establishes the model as purpose-built for this disorder, which is what makes it informative for these nodes rather than incidentally relevant.
Progressive Neurological Deterioration partially recapitulates fidelity: moderate observes: organism Developmental retardation, epilepsy and memory impairment reproduce the developmental arrest and seizures of the human disease.
Limitations: The knockout rats survive to about eight weeks, whereas the human disorder is fatal in infancy or early childhood, so the model does not reproduce the lethality that dominates the human course. The authors present that longer survival as an advantage for testing therapy, which it is, but it means the model's natural history is not the disease's. The knockout is also neuronal only, so the lactic acidosis and hyperglycinemia of the systemic human disease are outside what it can report.
Readouts
Neuronal death and dendritic spine density
Direction: DECREASED
Interpretation: Massive neuronal death with reduced Nissl bodies and dendritic spines is the histological correlate of the human neurodegeneration.
Show evidence (1 reference)
PMID:37140997 SUPPORT Model Organism
"massive neuronal death, reduced number of Nissl bodies and dendritic spines"
Reports the histological readout behind this link.
Show evidence (1 reference)
PMID:37140997 SUPPORT Model Organism
"the rats suffered from developmental retardation, epilepsy, memory impairment"
The behavioural phenotype that maps onto the human developmental arrest and infantile seizures.
The paper reports that Isca1 knockout causes neuronal oncosis specifically, rather than apoptosis. That is a mechanistic claim about the mode of cell death with no counterpart in the human literature, so it is recorded here rather than curated as a pathophysiology node.
{ }

Source YAML

click to show 
name: Multiple Mitochondrial Dysfunctions Syndrome 5
category: Genetic
creation_date: "2026-09-05T00:00:00Z"
synonyms:
- MMDS5
- ISCA1-related multiple mitochondrial dysfunctions syndrome
- ISCA1-MMDS
- ISCA1 deficiency
description: >-
  Multiple mitochondrial dysfunctions syndrome 5 (MMDS5; OMIM #617613) is an
  autosomal recessive infantile neurodegenerative disorder caused by biallelic
  loss-of-function variants in ISCA1, the A-type carrier protein that mediates the
  late, [4Fe-4S]-generating step of mitochondrial iron-sulfur cluster biogenesis.
  Losing that step does not disable one enzyme but a whole client class: the
  respiratory complexes that carry [4Fe-4S] cofactors (complex I and complex II)
  and lipoic acid synthase, itself a [4Fe-4S] enzyme. The result is a severe
  leukodystrophy with either no attainment of developmental milestones or very
  early loss of them, infantile seizures, spasticity, nystagmus and lactic
  acidosis, with death usually in early childhood.

  MMDS5 sits in a series of disorders that share this final common lesion -
  MMDS1 (NFU1), MMDS2 (BOLA3), MMDS3 (IBA57) and MMDS4 (ISCA2) - and the reported
  phenotype is described by its discoverers as similar to all four. It is
  nevertheless curated as its own entry rather than as a subtype, because the
  molecular lesion is a distinct, non-redundant node of the pathway (ISCA1 is
  required for [4Fe-4S] biogenesis where ISCA2 is not, in the same knockdown
  experiment) and because neuronal migrational abnormality, reported in both
  children of the index family, had not been described in the other MMDS
  subtypes.
disease_term:
  preferred_term: multiple mitochondrial dysfunctions syndrome 5
  term:
    id: MONDO:0033282
    label: multiple mitochondrial dysfunctions syndrome 5
parents:
- MONDO:0017338
classifications:
  mechanistic_category:
  - classification_value: mitochondrial disease
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
references:
- reference: PMID:31580634
  title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
  tags:
  - GeneReviews
inheritance:
- name: Autosomal recessive
  description: >-
    All reported families segregate biallelic ISCA1 variants; heterozygous
    parents and siblings are unaffected carriers.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "ISCA1-related multiple mitochondrial dysfunctions syndrome is inherited in an autosomal recessive manner."
    explanation: GeneReviews states the inheritance mode directly.
prevalence:
  - population: Worldwide
    measure_type: CASES_IN_LITERATURE
    prevalence_class: ULTRA_RARE
    notes: >-
      Fewer than ten patients have been reported. The founder p.(Glu87Lys) allele
      accounts for the Indian families; a separate p.V10G allele was reported in an
      Italian patient. No population prevalence estimate exists.
    evidence:
    - reference: PMID:30105122
      reference_title: "Report of the Third Family with Multiple Mitochondrial Dysfunctions Syndrome 5 Caused by the Founder Variant p.(Glu87Lys) in ISCA1."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We now report the third patient with the same phenotype and variant, further supporting the possibility of a founder event."
      explanation: >-
        Establishes that the reported case series was three patients at the time of
        this report, and that they share one founder allele.
progression:
  - phase: Infantile onset and rapid neurodegeneration
    notes: >-
      Reported children are normal at birth and for the first weeks to months, then
      fail to acquire or lose milestones, develop seizures between two and five
      months, and deteriorate progressively with spasticity. Death has been reported
      between eleven months and five years.
    evidence:
    - reference: PMID:28356563
      reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "P1 and P2 developed seizures at the age of 5 months and 4 months respectively followed by progressive neurological deterioration and spasticity."
      explanation: Documents the onset window and the sequence of seizure then deterioration.
    - reference: PMID:31580634
      reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Prognosis is poor and most individuals succumb to an intercurrent illness in early childhood."
      explanation: GeneReviews states the outcome.
pathophysiology:
- name: Biallelic ISCA1 Loss of Function
  biological_scale: MOLECULAR
  description: >-
    ISCA1 encodes a 129-amino-acid mitochondrial A-type carrier protein with a
    conserved C-terminal HESB domain. The founder p.(Glu87Lys) substitution sits in
    that domain at a highly conserved residue and abolishes a salt bridge to Lys49,
    destabilizing the protein; the independent p.V10G substitution lies in the
    uncleaved presequence and impairs both mitochondrial import and stability,
    reducing ISCA1 protein to a fraction of normal. Both routes converge on loss of
    functional ISCA1 in the mitochondrial matrix.
  genes:
  - preferred_term: ISCA1
    term:
      id: hgnc:28660
      label: ISCA1
  genetic_context:
    functional_impact_category: LOSS_OF_FUNCTION
    zygosity: HOMOZYGOUS
    variant_origin: GERMLINE
  evidence:
  - reference: PMID:29767723
    reference_title: "ISCA1 mutation in a patient with infantile-onset leukodystrophy causes defects in mitochondrial [4Fe-4S] proteins."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The mutation located in the uncleaved presequence severely affected both mitochondrial import and stability of ISCA1."
    explanation: >-
      Identifies the molecular consequence of the p.V10G allele as loss of imported,
      stable ISCA1 protein rather than a catalytic change.
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Analyses of protein stability via I-Mutant (score 7) and Eris web-server (score 0.64 ΔΔG kcal/mol) are also consistent in predicting that this variant leads to destabilization of ISCA1 protein."
    explanation: >-
      Supports destabilization as the mechanism of the founder allele. Graded
      COMPUTATIONAL because the claim rests on in silico stability prediction, not
      on a measurement in patient material.
  downstream:
  - target: Failure of Mitochondrial [4Fe-4S] Cluster Assembly
    causal_link_type: DIRECT
    description: >-
      ISCA1 is one of the two non-redundant A-type proteins that carry out the
      second, [4Fe-4S]-generating step of the pathway.
    evidence:
    - reference: PMID:29767723
      reference_title: "ISCA1 mutation in a patient with infantile-onset leukodystrophy causes defects in mitochondrial [4Fe-4S] proteins."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "In contrast, the ISCA1 p.V10G mutant protein only partially complemented the defects, closely resembling the biochemical phenotypes observed for ISCA1 patient fibroblasts."
      explanation: >-
        A complementation experiment: wild-type ISCA1 restores the assembly defect
        and the patient allele does not, which is what makes this an edge rather
        than a correlation between two separately supported nodes.
  - target: Pachygyria
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      ISCA1 loss is the only lesion in this entry that precedes the neuronal
      migration abnormality, but no source identifies an intervening mechanism
      between [4Fe-4S] biogenesis failure and migration -- see the attached
      knowledge gap on why this subtype alone shows it.
    evidence:
    - reference: PMID:28356563
      reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Neuronal migrational abnormalities seen in both children in Family 1 has not been previously described in other subtypes of MMDS."
      explanation: >-
        Reports the migration abnormality in patients carrying the ISCA1 variant,
        without describing a mechanism connecting it to the [4Fe-4S] lesion.
- name: Failure of Mitochondrial [4Fe-4S] Cluster Assembly
  biological_scale: MOLECULAR
  description: >-
    Mitochondrial Fe-S biogenesis runs in two steps. The core machinery builds a
    [2Fe-2S] cluster and hands it to GRX5; a second, separately staffed step
    converts that into [4Fe-4S] clusters and inserts them into client apoproteins.
    ISCA1 is the organizing node of that second step, interacting with both ISCA2
    and NFU1 (which do not interact with each other) and forming the transient
    ternary complex through which the cluster is handed on. Losing ISCA1 therefore
    stalls maturation of every mitochondrial [4Fe-4S] client at once, which is why
    a single gene defect produces a multi-enzyme phenotype. The client class is
    wider than the two downstream nodes name: purified ISCA1 also matures
    apo-aconitase, so aconitase deficiency belongs to this lesion even though the
    human reports measure complex I, complex II and lipoylation specifically.
  molecular_functions:
  - preferred_term: 4 iron, 4 sulfur cluster binding
    term:
      id: GO:0051539
      label: 4 iron, 4 sulfur cluster binding
    modifier: DECREASED
  biological_processes:
  - preferred_term: iron-sulfur cluster assembly
    term:
      id: GO:0016226
      label: iron-sulfur cluster assembly
    modifier: DECREASED
  evidence:
  - reference: PMID:33711344
    reference_title: "ISCA1 Orchestrates ISCA2 and NFU1 in the Maturation of Human Mitochondrial [4Fe-4S] Proteins."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "ISCA1 works as the promoter of the interaction between ISCA2 and NFU1 being able to determine the formation of a transient ISCA1-ISCA2-NFU1 ternary complex."
    explanation: >-
      NMR structural work establishing why ISCA1 is the non-redundant node of this
      step, which is the reason its loss affects the whole client class.
  - reference: PMID:28492233
    reference_title: "ISCA1 is essential for mitochondrial Fe(4)S(4) biogenesis in vivo."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "knockdown experiments in mouse skeletal muscle and in primary cultures of neurons suggest that ISCA1, but not ISCA2, is required for mitochondrial Fe4S4 proteins biogenesis"
    explanation: >-
      Shows the requirement is specific to ISCA1 within the same experiment, which
      is what distinguishes this lesion from the ISCA2 one (MMDS4).
  - reference: PMID:28492233
    reference_title: "ISCA1 is essential for mitochondrial Fe(4)S(4) biogenesis in vivo."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "ACO2 activity was progressively increasing and reached more than 70% and 80% of the activity of a chemically reconstituted ACO2"
    explanation: >-
      Purified ISCA1 transfers its cluster to apo-aconitase and restores most of its
      activity, which is what puts aconitase in the client class alongside the
      respiratory chain complexes. Graded IN_VITRO because it is a reconstitution
      with purified proteins, not the in vivo knockdown quoted above.
  - reference: PMID:32092383
    reference_title: "Expanding the phenotype of mitochondrial disease: Novel pathogenic variant in ISCA1 leading to instability of the iron-sulfur cluster in the protein."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Biochemical analysis of patient fibroblasts revealed impaired lipoic acid synthesis and decreased activities of complex I and II of respiratory chain."
    explanation: >-
      An independent patient, a third allele, and the same paired lesion measured
      in fibroblasts: impaired lipoylation alongside complex I and II deficiency.
      That reproducibility is what makes the two downstream branches a property of
      the gene rather than of one family. Graded IN_VITRO because the measurement
      is an enzyme assay on cultured cells, not an observation of the patient.
  downstream:
  - target: Respiratory Chain Complex I and II Deficiency
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:29767723
      reference_title: "ISCA1 mutation in a patient with infantile-onset leukodystrophy causes defects in mitochondrial [4Fe-4S] proteins."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Here, we describe a patient with a severe early onset leukodystrophy, multiple defects of respiratory complexes and a severe impairment of lipoic acid synthesis."
      explanation: >-
        Reports the two downstream branches together in the same patient, which is
        what links the assembly failure to the respiratory defect.
  - target: Lipoic Acid Synthase Failure
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:29767723
      reference_title: "ISCA1 mutation in a patient with infantile-onset leukodystrophy causes defects in mitochondrial [4Fe-4S] proteins."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "a severe impairment of lipoic acid synthesis"
      explanation: >-
        Names the second client branch in the patient carrying the ISCA1 variant.
- name: Respiratory Chain Complex I and II Deficiency
  biological_scale: CELLULAR
  description: >-
    Complex I carries eight [4Fe-4S] clusters and complex II carries one; both
    therefore fail to mature when the late assembly step is lost. Electron flow
    through the respiratory chain falls, oxidative phosphorylation is impaired, and
    pyruvate is shunted to lactate.
  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
  evidence:
  - reference: PMID:29767723
    reference_title: "ISCA1 mutation in a patient with infantile-onset leukodystrophy causes defects in mitochondrial [4Fe-4S] proteins."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe a patient with a severe early onset leukodystrophy, multiple defects of respiratory complexes and a severe impairment of lipoic acid synthesis."
    explanation: >-
      Reports multiple respiratory complex defects in the patient carrying the
      ISCA1 variant.
  downstream:
  - target: Lactic Acidosis
    causal_link_type: DIRECT
  - target: Leukodystrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Sources assert white matter degeneration as the consequence of the respiratory
      defect without identifying the intervening steps, so the edge is typed as
      indirect rather than direct.
  - target: Elevated circulating creatine kinase concentration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Complex I and II are ubiquitously expressed, including in skeletal muscle, so
      the same bioenergetic defect is a plausible driver of the isolated creatine
      kinase elevation reported in one patient; the source does not report a muscle
      biopsy or state the mechanism directly, so the edge is typed as indirect.
    evidence:
    - reference: PMID:28356563
      reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Normal hematological and biochemical investigations were noted except for elevated creatinine phosphokinase in P2."
      explanation: Documents the isolated CK elevation in a patient carrying the ISCA1 variant.
- name: Lipoic Acid Synthase Failure
  biological_scale: MOLECULAR
  description: >-
    Lipoic acid synthase is itself a [4Fe-4S] enzyme, so a defect in the assembly
    machinery also removes the cofactor from the enzyme that makes lipoate. Proteins
    that depend on a lipoyl group - pyruvate dehydrogenase, alpha-ketoglutarate
    dehydrogenase, and the glycine cleavage system - lose activity as a second,
    parallel consequence. This is the branch responsible for the non-ketotic
    hyperglycinemia reported across the MMDS series.
  biological_processes:
  - preferred_term: lipoate biosynthetic process
    term:
      id: GO:0009107
      label: lipoate biosynthetic process
    modifier: DECREASED
  - preferred_term: protein lipoylation
    term:
      id: GO:0009249
      label: protein lipoylation
    modifier: DECREASED
  - preferred_term: glycine catabolic process
    term:
      id: GO:0006546
      label: glycine catabolic process
    modifier: DECREASED
  evidence:
  - reference: PMID:29767723
    reference_title: "ISCA1 mutation in a patient with infantile-onset leukodystrophy causes defects in mitochondrial [4Fe-4S] proteins."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multiple mitochondrial dysfunction syndromes (MMDS) comprise a group of severe autosomal recessive diseases characterized by impaired respiration and lipoic acid metabolism, resulting in infantile-onset mitochondrial encephalopathy, non-ketotic hyperglycinemia, myopathy, lactic acidosis and early death."
    explanation: >-
      States that impaired lipoic acid metabolism is one of the two defining lesions
      of the MMDS group and names hyperglycinemia as its consequence.
  downstream:
  - target: Lactic Acidosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Loss of the pyruvate dehydrogenase lipoyl cofactor contributes to lactate
      accumulation alongside the respiratory chain defect.
- name: Leukodystrophy
  biological_scale: TISSUE
  description: >-
    Extensive cerebral and cerebellar deep white matter disease with ventricular
    dilatation is the imaging hallmark, and in the index family was accompanied by
    pachygyria, a neuronal migration abnormality that had not been reported in the
    other MMDS subtypes.
  cell_types:
  - preferred_term: oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain imaging of both children showed pachygyria, extensive cerebral and cerebellar white matter disease and dilated cerebral ventricles."
    explanation: Documents the imaging phenotype in the index family.
  downstream:
  - target: Progressive Neurological Deterioration
    causal_link_type: DIRECT
  - target: Ventriculomegaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Ventricular dilatation is reported alongside the white matter disease in the
      same patients, consistent with passive (ex-vacuo) enlargement as white matter
      volume is lost, but the source reports the co-occurrence rather than stating
      that mechanism, so the edge is typed as indirect.
    evidence:
    - reference: PMID:28356563
      reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "P4 had extensive leukodystrophy involving cerebral and cerebellar white matter with dilated ventricles."
      explanation: Reports ventricular dilatation co-occurring with the white matter disease in the same patient.
- name: Lactic Acidosis
  biological_scale: ORGANISM
  description: >-
    Blood lactate is elevated and an increased lipid-lactate peak is seen on brain
    MR spectroscopy, reflecting the block in oxidative phosphorylation.
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy."
    explanation: GeneReviews records the biochemical finding and its imaging correlate.
  downstream:
  - target: Increased circulating lactate concentration
    causal_link_type: DIRECT
    description: >-
      Raised blood lactate is the systemic readout of this node's biochemical
      lesion; the lipid-lactate peak on brain MR spectroscopy is its CNS
      correlate.
    evidence:
    - reference: PMID:31580634
      reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy."
      explanation: >-
        GeneReviews reports elevated blood lactate, which is what this phenotype
        measures, together with the brain spectroscopy peak.
    - reference: PMID:28356563
      reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Increased lipid lactate peak was seen on magnetic resonance spectroscopy of brain in P1"
      explanation: The CNS correlate of the same lactate elevation, in one reported patient.
  - target: Lactic acidosis
    causal_link_type: DIRECT
    description: >-
      Blood lactic acidosis is the systemic biochemical readout of the same block
      in oxidative phosphorylation this node describes.
    evidence:
    - reference: PMID:28356563
      reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "P3 had lactic acidosis and white matter disease was documented in medical records of P3 on computed tomography of brain."
      explanation: Documents lactic acidosis in a reported patient carrying the ISCA1 variant.
- name: Progressive Neurological Deterioration
  biological_scale: ORGANISM
  description: >-
    The clinical endpoint of the chain: absent or lost milestones, early infantile
    seizures, spasticity with exaggerated reflexes, nystagmus and hearing loss,
    ending in death in early childhood.
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "ISCA1-related multiple mitochondrial dysfunctions syndrome (ISCA1-MMDS) is a severe neurodegenerative condition typically characterized by either no attainment of developmental milestones or very early loss of achieved milestones, seizures in early infancy, development of spasticity with exaggerated deep tendon reflexes, nystagmus, and risk for sensorineural hearing loss."
    explanation: >-
      The GeneReviews Clinical Characteristics sentence, used as the phenotype
      baseline for this entry.
  downstream:
  - target: Developmental regression
    causal_link_type: DIRECT
  - target: Seizure
    causal_link_type: DIRECT
  - target: Spasticity
    causal_link_type: DIRECT
  - target: Nystagmus
    causal_link_type: DIRECT
  - target: Sensorineural hearing impairment
    causal_link_type: DIRECT
phenotypes:
- category: Neurological
  name: Developmental regression
  frequency: OBLIGATE
  description: >-
    Either no developmental milestone is ever attained, or the few that are
    attained are lost very early. In the index family the affected children
    achieved only partial head control at three months.
  phenotype_term:
    preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both of them did not achieve any other developmental milestone except partial head control at 3 months of age."
    explanation: Documents the developmental ceiling in the two index patients.
- category: Neurological
  name: Seizure
  frequency: OBLIGATE
  description: >-
    Seizures begin in early infancy, reported between two and five months of age in
    the four children of the two index families, and mark the start of overt
    neurological deterioration.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Seizures developed at the age of 3 months and 2 months in P3 and P4 respectively following which neurologic deterioration was noted in them."
    explanation: Gives the onset ages in the second family.
- category: Neurological
  name: Spasticity
  frequency: VERY_FREQUENT
  description: Progressive spasticity with exaggerated deep tendon reflexes.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "development of spasticity with exaggerated deep tendon reflexes"
    explanation: GeneReviews lists spasticity among the defining clinical characteristics.
- category: Neurological
  name: Nystagmus
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "nystagmus, and risk for sensorineural hearing loss"
    explanation: GeneReviews lists nystagmus among the defining clinical characteristics.
- category: Neurological
  name: Sensorineural hearing impairment
  frequency: OCCASIONAL
  description: >-
    GeneReviews describes this as a risk rather than a constant feature, so it is
    banded conservatively.
  phenotype_term:
    preferred_term: Sensorineural hearing impairment
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "risk for sensorineural hearing loss"
    explanation: >-
      GeneReviews frames hearing loss as a risk, which is why the frequency band is
      OCCASIONAL rather than VERY_FREQUENT.
- category: Neurological
  name: Leukodystrophy
  frequency: OBLIGATE
  description: >-
    Extensive cerebral and cerebellar deep white matter hyperintensity, the imaging
    hallmark of the disorder.
  phenotype_term:
    preferred_term: Leukodystrophy
    term:
      id: HP:0002415
      label: Leukodystrophy
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Further brain MRI findings may include extensive cerebral and cerebellar deep white matter hyperintensities, marked dilatation of the cerebral ventricles, and pachygyria."
    explanation: GeneReviews lists the MRI findings including the white matter disease.
- category: Neurological
  name: Ventriculomegaly
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Ventriculomegaly
    term:
      id: HP:0002119
      label: Ventriculomegaly
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "P4 had extensive leukodystrophy involving cerebral and cerebellar white matter with dilated ventricles."
    explanation: Documents ventricular dilatation alongside the white matter disease.
- category: Neurological
  name: Pachygyria
  frequency: OCCASIONAL
  description: >-
    Reported in both affected children of the first family. The authors note this
    neuronal migration abnormality had not previously been described in the other
    MMDS subtypes, which is part of the case for MMDS5 as a distinct entity.
  phenotype_term:
    preferred_term: Pachygyria
    term:
      id: HP:0001302
      label: Pachygyria
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Neuronal migrational abnormalities seen in both children in Family 1 has not been previously described in other subtypes of MMDS."
    explanation: >-
      Records both the finding and the claim that it is not shared with the sibling
      MMDS forms.
- category: Metabolic
  name: Increased circulating lactate concentration
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Increased circulating lactate concentration
    term:
      id: HP:0002151
      label: Increased circulating lactate concentration
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Increased lipid lactate peak was seen on magnetic resonance spectroscopy of brain in P1"
    explanation: Documents the raised brain lactate on spectroscopy.
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy."
    explanation: >-
      GeneReviews reports elevated blood lactate, the circulating measure this
      phenotype names.
- category: Metabolic
  name: Lactic acidosis
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "P3 had lactic acidosis and white matter disease was documented in medical records of P3 on computed tomography of brain."
    explanation: Documents lactic acidosis in a reported patient.
- category: Musculoskeletal
  name: Elevated circulating creatine kinase concentration
  frequency: OCCASIONAL
  description: >-
    Reported in one of the four children of the index families; otherwise
    haematological and biochemical investigations were normal.
  phenotype_term:
    preferred_term: Elevated circulating creatine kinase activity
    term:
      id: HP:0003236
      label: Elevated circulating creatine kinase activity
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Normal hematological and biochemical investigations were noted except for elevated creatinine phosphokinase in P2."
    explanation: >-
      Documents the isolated CK elevation and, in the same sentence, that other
      routine investigations were normal.
- category: Gastrointestinal
  name: Feeding difficulties
  frequency: VERY_FREQUENT
  description: >-
    Feeding difficulty from the newborn period; a nasogastric or gastrostomy tube
    may be required.
  phenotype_term:
    preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both P1 and P2 had normal birth and antenatal history followed by inconsolable cry and feeding difficulties since newborn period."
    explanation: Documents feeding difficulty from the newborn period.
- category: Gastrointestinal
  name: Constipation
  frequency: OCCASIONAL
  description: >-
    Listed among the disorder's recognized secondary complications; GeneReviews
    directs monitoring for it at every visit rather than reporting a rate among
    the small reported cohort.
  phenotype_term:
    preferred_term: Constipation
    term:
      id: HP:0002019
      label: Constipation
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Monitor constipation, developmental progress, growth parameters, and family needs at each visit."
    explanation: >-
      GeneReviews lists constipation among the manifestations to be monitored at
      each surveillance visit.
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adequate hydration, stool softeners, and laxatives may help to prevent severe constipation."
    explanation: >-
      Names severe constipation as a complication that management should
      prevent.
genetic:
- name: ISCA1
  gene_term:
    preferred_term: ISCA1
    term:
      id: hgnc:28660
      label: ISCA1
  relationship_type: CAUSATIVE
  presence: PRESENT
  variant_origin: GERMLINE
  notes: >-
    Biallelic ISCA1 variants are the cause. Two alleles are reported: the Indian
    founder c.259G>A p.(Glu87Lys), homozygous in three families sharing a 3.3 Mb
    region of homozygosity on chromosome 9, and c.29T>G p.(Val10Gly) in an
    unrelated Italian patient.
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe two independent families, both with two affected children each, with a severe neurodevelopmental disorder associated with a homozygous c.259G>A variant in ISCA1."
    explanation: Establishes the gene-disease relationship with segregation in two families.
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "After examining the homozygous regions around the ISCA1 variant in both the probands, generated by the exome sequencing data, we identified only one overlapping region of homozygosity (ROH) spanning 3.3 Mb in chromosome 9 (Chr9: 85613354-88925774) flanking the variant"
    explanation: Supports the founder-allele interpretation of the recurrent variant.
  - reference: PMID:32092383
    reference_title: "Expanding the phenotype of mitochondrial disease: Novel pathogenic variant in ISCA1 leading to instability of the iron-sulfur cluster in the protein."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a patient carrying a novel pathogenic variant p.(Tyr101Cys) in ISCA1 leading to MMDS type 5."
    explanation: >-
      A third independent allele, which is what takes the gene-disease relationship
      beyond the single founder haplotype.
treatments:
- name: Supportive Care
  description: >-
    No disease-modifying therapy exists. Management is supportive. This entry is the
    umbrella statement; the individual measures GeneReviews names are curated as
    their own treatments below so that each one reaches the manifestation it
    addresses.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Treatment of manifestations: Treatment is primarily supportive."
    explanation: GeneReviews states that management is supportive.
- name: Enteral Feeding Support
  description: >-
    A nasogastric or gastrostomy tube may be required where oral intake becomes
    unsafe or insufficient.
  treatment_term:
    preferred_term: Nutritional Support
    term:
      id: NCIT:C15433
      label: Nutritional Support
  target_phenotypes:
  - preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A feeding tube (nasogastric or gastrostomy) may be required."
    explanation: GeneReviews recommends enteral feeding support.
- name: Genetic Counseling and Carrier Testing
  description: >-
    Autosomal recessive recurrence risk is 25% per pregnancy. Carrier testing and
    prenatal testing are available where the familial variants are known, which in
    the reported families was the reason for referral. This addresses reproductive
    risk rather than a disease manifestation, so it carries no `target_phenotypes`
    link.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    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 pathogenic ISCA1 variants in the family are known."
    explanation: GeneReviews states the availability of carrier and prenatal testing.
- name: Antiseizure Therapy
  description: >-
    Seizures are treated with standard antiseizure regimens. GeneReviews names no
    agent and no MMDS-specific protocol, so no therapeutic_agent is bound here.
  treatment_term:
    preferred_term: antiseizure pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_phenotypes:
  - preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Standard treatment for spasticity, seizures, abnormal vision, and hearing loss."
    explanation: >-
      GeneReviews directs standard antiseizure treatment, which is what joins this
      treatment to the obligate seizure phenotype.
- name: Spasticity Management
  description: >-
    Spasticity is managed by standard means. GeneReviews specifies no modality, so
    the binding is the generic therapeutic action rather than a drug or a physical
    therapy claim the source does not make.
  treatment_term:
    preferred_term: standard management of spasticity
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  target_phenotypes:
  - preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Standard treatment for spasticity, seizures, abnormal vision, and hearing loss."
    explanation: >-
      GeneReviews directs standard treatment of spasticity, which is what joins this
      treatment to the spasticity phenotype.
- name: Constipation Prevention
  description: >-
    Hydration, stool softeners and laxatives are used to prevent severe
    constipation, which GeneReviews lists as a secondary complication rather than a
    presenting feature.
  treatment_term:
    preferred_term: prevention of severe constipation
    term:
      id: NCIT:C15843
      label: Preventive Intervention
  target_phenotypes:
  - preferred_term: Constipation
    term:
      id: HP:0002019
      label: Constipation
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adequate hydration, stool softeners, and laxatives may help to prevent severe constipation."
    explanation: >-
      The GeneReviews prevention-of-secondary-complications recommendation.
- name: Neurologic, Nutritional and Respiratory Surveillance
  description: >-
    Assessment at every visit for new neurologic manifestations, safe oral intake,
    adequate nutrition, and respiratory insufficiency or aspiration. This is the
    schedule against which deterioration is detected, and aspiration risk is the
    proximate cause of the intercurrent illness that ends most reported cases.
  treatment_term:
    preferred_term: surveillance physical examination at each visit
    term:
      id: NCIT:C20989
      label: Physical Examination
  target_phenotypes:
  - preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Assessment for new neurologic manifestations, safety of oral intake, adequate nutrition, and evidence of respiratory insufficiency and aspiration at each visit."
    explanation: >-
      The GeneReviews surveillance recommendation, stated per visit.
- name: Annual Ophthalmologic Evaluation
  description: >-
    Eye examination annually or on clinical suspicion. Nystagmus is among the
    defining features, so this is surveillance of a manifestation the disease is
    expected to produce rather than screening for an unrelated risk.
  treatment_term:
    preferred_term: ophthalmologic evaluation
    term:
      id: NCIT:C38060
      label: Eye Examination
  target_phenotypes:
  - preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ophthalmologic and audiologic evaluations annually or based on clinical suspicion."
    explanation: >-
      GeneReviews sets the ophthalmologic surveillance interval.
- name: Annual Audiologic Evaluation
  description: >-
    Hearing assessment annually or on clinical suspicion, against the stated risk
    of sensorineural hearing loss.
  treatment_term:
    preferred_term: audiologic evaluation
    term:
      id: NCIT:C204585
      label: Overall Hearing Assessment
  target_phenotypes:
  - preferred_term: Sensorineural hearing impairment
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ophthalmologic and audiologic evaluations annually or based on clinical suspicion."
    explanation: >-
      GeneReviews sets the audiologic surveillance interval.
animal_models:
- name: Neuron-specific Isca1 knockout rat
  species: Rat
  genotype: Isca1 flox/flox; NeuN-Cre (neuron-specific conditional knockout)
  publication: PMID:37140997
  description: >-
    A CRISPR-Cas9 conditional knockout restricting Isca1 loss to neurons. It is the
    only in vivo model of this disorder, and it was built specifically to model
    MMDS5 rather than repurposed from another question.
  modeled_mechanisms:
  - target: Respiratory Chain Complex I and II Deficiency
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: CELLULAR
    description: >-
      The knockout reproduces the biochemical lesion: reduced respiratory chain
      complex protein and reduced ATP production, with mitochondrial fragmentation
      and cristae fracture on electron microscopy.
    limitations: >-
      Neuron-restricted rather than constitutive, so it cannot report the systemic
      consequences of the lesion, and it does not distinguish complex I from complex
      II involvement the way the human fibroblast assays do.
    readouts:
    - name: Respiratory chain complex protein content and ATP production
      target: Respiratory Chain Complex I and II Deficiency
      direction: DECREASED
      interpretation: >-
        Establishes that neuronal Isca1 loss alone is sufficient for the
        bioenergetic defect.
      evidence:
      - reference: PMID:37140997
        reference_title: "A neuron-specific Isca1 knockout rat developments multiple mitochondrial dysfunction syndromes."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "mitochondrial fragmentation, cristae fracture, reduced content of respiratory chain complex protein, and reduced production of ATP"
        explanation: Reports the measured bioenergetic and ultrastructural readouts.
    evidence:
    - reference: PMID:37140997
      reference_title: "A neuron-specific Isca1 knockout rat developments multiple mitochondrial dysfunction syndromes."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "This study established the disease model of MMDS5 in the nervous system for the first time"
      explanation: >-
        Establishes the model as purpose-built for this disorder, which is what makes
        it informative for these nodes rather than incidentally relevant.
  - target: Progressive Neurological Deterioration
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    model_scale: ORGANISM
    description: >-
      Developmental retardation, epilepsy and memory impairment reproduce the
      developmental arrest and seizures of the human disease.
    limitations: >-
      The knockout rats survive to about eight weeks, whereas the human disorder is
      fatal in infancy or early childhood, so the model does not reproduce the
      lethality that dominates the human course. The authors present that longer
      survival as an advantage for testing therapy, which it is, but it means the
      model's natural history is not the disease's. The knockout is also neuronal
      only, so the lactic acidosis and hyperglycinemia of the systemic human disease
      are outside what it can report.
    readouts:
    - name: Neuronal death and dendritic spine density
      target: Progressive Neurological Deterioration
      direction: DECREASED
      interpretation: >-
        Massive neuronal death with reduced Nissl bodies and dendritic spines is the
        histological correlate of the human neurodegeneration.
      evidence:
      - reference: PMID:37140997
        reference_title: "A neuron-specific Isca1 knockout rat developments multiple mitochondrial dysfunction syndromes."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "massive neuronal death, reduced number of Nissl bodies and dendritic spines"
        explanation: Reports the histological readout behind this link.
    evidence:
    - reference: PMID:37140997
      reference_title: "A neuron-specific Isca1 knockout rat developments multiple mitochondrial dysfunction syndromes."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "the rats suffered from developmental retardation, epilepsy, memory impairment"
      explanation: >-
        The behavioural phenotype that maps onto the human developmental arrest and
        infantile seizures.
  notes: >-
    The paper reports that Isca1 knockout causes neuronal oncosis specifically,
    rather than apoptosis. That is a mechanistic claim about the mode of cell death
    with no counterpart in the human literature, so it is recorded here rather than
    curated as a pathophysiology node.
biochemical:
- name: Respiratory chain complex I and II activity in cultured fibroblasts
  presence: DECREASED
  context: >-
    Enzyme activities measured in patient-derived skin fibroblasts. Together with
    impaired lipoylation this is the biochemical fingerprint that points at the
    MMDS group before sequencing, though it does not distinguish ISCA1 from the
    sibling genes.
  readouts:
  - target: Respiratory Chain Complex I and II Deficiency
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Reduced complex I and II activity reports the [4Fe-4S] client failure directly,
      since both complexes depend on [4Fe-4S] subunits.
    evidence:
    - reference: PMID:32092383
      reference_title: "Expanding the phenotype of mitochondrial disease: Novel pathogenic variant in ISCA1 leading to instability of the iron-sulfur cluster in the protein."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Biochemical analysis of patient fibroblasts revealed impaired lipoic acid synthesis and decreased activities of complex I and II of respiratory chain."
      explanation: >-
        Reports the complex I and II activity measurement in patient fibroblasts.
- name: Lipoic acid synthesis in cultured fibroblasts
  presence: DECREASED
  context: >-
    Impaired lipoylation measured in patient-derived fibroblasts, the second arm of
    the same fingerprint and the branch that predicts the glycine cleavage defect
    the entry does not assert.
  readouts:
  - target: Lipoic Acid Synthase Failure
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Reduced lipoic acid synthesis reports failure of lipoyl synthase, itself a
      [4Fe-4S] enzyme, which is why the defect is self-amplifying.
    evidence:
    - reference: PMID:32092383
      reference_title: "Expanding the phenotype of mitochondrial disease: Novel pathogenic variant in ISCA1 leading to instability of the iron-sulfur cluster in the protein."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Biochemical analysis of patient fibroblasts revealed impaired lipoic acid synthesis and decreased activities of complex I and II of respiratory chain."
      explanation: >-
        Reports the impaired lipoic acid synthesis in patient fibroblasts.
diagnosis:
- name: ISCA1 molecular genetic testing
  description: >-
    Diagnosis rests on biallelic pathogenic ISCA1 variants in a proband with
    suggestive clinical and imaging findings; in every reported case the gene was
    reached by exome rather than targeted testing.
  presence: PRESENT
  evidence:
  - reference: PMID:31580634
    reference_title: "ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of ISCA1-MMDS is established in a proband with suggestive findings and/or biallelic pathogenic variants in ISCA1 identified by molecular genetic testing."
    explanation: GeneReviews states the diagnostic criterion.
differential_diagnoses:
- name: Multiple mitochondrial dysfunctions syndrome 1
  disease_term:
    preferred_term: Multiple mitochondrial dysfunctions syndrome 1 (NFU1)
    term:
      id: MONDO:0011582
      label: multiple mitochondrial dysfunctions syndrome 1
  description: >-
    NFU1 encodes a late-acting [4Fe-4S] scaffold, so the lesion converges on the same client class from a different step.
  distinguishing_features:
  - Caused by biallelic NFU1 variants rather than biallelic ISCA1 variants.
  - Pulmonary hypertension is a recognised feature of the NFU1 form and is not reported in ISCA1 disease.
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
    explanation: >-
      States directly that the MMDS5 phenotype overlaps this sibling syndrome, which
      is why the differential cannot be made clinically.
- name: Multiple mitochondrial dysfunctions syndrome 2
  disease_term:
    preferred_term: Multiple mitochondrial dysfunctions syndrome 2 (BOLA3)
    term:
      id: MONDO:0013675
      label: multiple mitochondrial dysfunctions syndrome 2
  description: >-
    BOLA3 partners the same late [4Fe-4S] step, so lipoylation and respiratory chain deficits are shared.
  distinguishing_features:
  - Caused by biallelic BOLA3 variants rather than biallelic ISCA1 variants.
  - Distinguished by the gene; no clinical feature reported in the ISCA1 series separates the two.
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
    explanation: >-
      States directly that the MMDS5 phenotype overlaps this sibling syndrome, which
      is why the differential cannot be made clinically.
- name: Multiple mitochondrial dysfunctions syndrome 3
  disease_term:
    preferred_term: Multiple mitochondrial dysfunctions syndrome 3 (IBA57)
    term:
      id: MONDO:0014132
      label: multiple mitochondrial dysfunctions syndrome 3
  description: >-
    IBA57 acts with ISCA1 and ISCA2 in [4Fe-4S] maturation, giving the same biochemical fingerprint.
  distinguishing_features:
  - Caused by biallelic IBA57 variants rather than biallelic ISCA1 variants.
  - Distinguished by the gene; no clinical feature reported in the ISCA1 series separates the two.
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
    explanation: >-
      States directly that the MMDS5 phenotype overlaps this sibling syndrome, which
      is why the differential cannot be made clinically.
- name: Multiple mitochondrial dysfunctions syndrome 4
  disease_term:
    preferred_term: Multiple mitochondrial dysfunctions syndrome 4 (ISCA2)
    term:
      id: MONDO:0014611
      label: multiple mitochondrial dysfunctions syndrome 4
  description: >-
    ISCA2 is ISCA1's obligate heterodimer partner at the step this disease breaks, so this is the closest of the four.
  distinguishing_features:
  - Caused by biallelic ISCA2 variants rather than biallelic ISCA1 variants.
  - >-
    The only reported separator is mechanistic rather than clinical. In the same
    knockdown experiment ISCA1 is required for mitochondrial [4Fe-4S] protein
    biogenesis and ISCA2 is not.
  evidence:
  - reference: PMID:28356563
    reference_title: "Homozygous p.(Glu87Lys) variant in ISCA1 is associated with a multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Notably the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS."
    explanation: >-
      States directly that the MMDS5 phenotype overlaps this sibling syndrome, which
      is why the differential cannot be made clinically.
discussions:
- discussion_id: isca1_glycine_branch_unmeasured
  kind: KNOWLEDGE_GAP
  prompt: >-
    Has non-ketotic hyperglycinemia actually been measured in a patient with
    biallelic ISCA1 variants, or is it inferred from the shared lipoylation lesion
    in the other MMDS subtypes?
  attaches_to:
  - pathophysiology#Lipoic Acid Synthase Failure
  rationale: >-
    Hyperglycinemia is a defining feature of the MMDS group and follows mechanically
    from loss of the lipoyl-dependent glycine cleavage system, which the ISCA1
    patient's impaired lipoic acid synthesis would predict. But the sources cited
    here report hyperglycinemia as a property of MMDS in general, not as a
    measurement in an ISCA1 patient, and the index families' biochemical
    investigations are described as normal apart from creatine kinase. Curating the
    glycine branch as an established patient finding would therefore overstate the
    evidence, so it is curated as a mechanism node with a gap attached rather than
    as a phenotype.
  proposed_experiments:
  - experiment_id: isca1_glycine_measurement
    name: Plasma and CSF glycine measurement in ISCA1 patients
    description: >-
      Measure plasma and cerebrospinal fluid glycine, and lipoylation of the
      pyruvate dehydrogenase E2 subunit and the glycine cleavage system H protein by
      immunoblot in fibroblasts, in any newly ascertained ISCA1 patient.
    readouts:
    - name: Plasma glycine concentration
      target: pathophysiology#Lipoic Acid Synthase Failure
      direction: INCREASED
      interpretation: >-
        An elevated glycine would confirm the glycine cleavage branch operates in
        ISCA1 disease as it does in the other MMDS subtypes.
- discussion_id: isca1_mettl17_translation_branch
  kind: KNOWLEDGE_GAP
  prompt: >-
    Does ISCA1 loss also attenuate mitochondrial protein synthesis by leaving the
    mitoribosome assembly factor METTL17 without its [4Fe-4S] cluster, and if so
    does that branch contribute to the disease?
  attaches_to:
  - pathophysiology#Failure of Mitochondrial [4Fe-4S] Cluster Assembly
  rationale: >-
    A third client branch is proposed in the literature and is deliberately not
    curated as a node. METTL17 is a mitoribosome biogenesis factor carrying a
    [4Fe-4S] cluster, and the paper that reports it proposes, in its own words, that
    the cluster is inserted via an ISCA1-NFU1 node. The measured consequence in that
    paper is in the sibling genes: fibroblasts from BOLA3 and NFU1 patients show
    attenuated mitochondrial translation. Nobody has measured it in ISCA1 cells, and
    the deep-research report that raised this branch says so in its own limitations.
    So the entry carries the question rather than the mechanism. If it holds, the
    respiratory chain deficit in this disease is partly a translation defect and not
    only a failure to mature individual complexes, which would change what a rescue
    experiment should target.
  evidence:
  - reference: PMID:37823603
    reference_title: "BOLA3 and NFU1 link mitoribosome iron-sulfur cluster assembly to multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "which we propose is inserted via the ISCA1-NFU1 node"
    explanation: >-
      The authors state the ISCA1 involvement as a proposal, which is exactly the
      epistemic status this gap records.
  - reference: PMID:37823603
    reference_title: "BOLA3 and NFU1 link mitoribosome iron-sulfur cluster assembly to multiple mitochondrial dysfunctions syndrome."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "fibroblasts from subjects suffering from 'multiple mitochondrial dysfunction' syndrome due to mutations in BOLA3 or NFU1 display previously unrecognized attenuation of mitochondrial protein synthesis"
    explanation: >-
      The translation defect is measured in BOLA3 and NFU1 patient cells, not ISCA1
      ones, which is why this is a gap rather than a curated branch.
  proposed_experiments:
  - experiment_id: isca1_mitochondrial_translation_measurement
    name: Mitochondrial translation and METTL17 maturation in ISCA1 patient cells
    description: >-
      Measure mitochondrial protein synthesis by metabolic labelling, and small
      mitoribosomal subunit assembly, in ISCA1 patient fibroblasts against the
      BOLA3 and NFU1 cells where the defect is already established.
    readouts:
    - name: Mitochondrial protein synthesis rate
      target: pathophysiology#Failure of Mitochondrial [4Fe-4S] Cluster Assembly
      direction: DECREASED
      interpretation: >-
        Attenuated synthesis in ISCA1 cells would establish the translation branch
        in this disease rather than only in its siblings.
- discussion_id: isca1_pachygyria_subtype_specificity
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why does an ISCA1 defect produce a neuronal migration abnormality when the
    sibling MMDS subtypes, sharing the same final lesion, do not?
  attaches_to:
  - phenotypes#Pachygyria
  rationale: >-
    Pachygyria was seen in both affected children of the index family and the
    authors state it had not previously been described in other MMDS subtypes. If
    that holds up in a larger series it is a genuine mechanistic divergence, since
    migration is a prenatal event and the shared respiratory lesion is not obviously
    developmental. The alternative is ascertainment: four children in one family
    imaged in one centre is a thin base for a claim of subtype specificity, and no
    later report has replicated it.
notes: >-
  Curation decisions worth recording.

  Entry versus subtype. The stub left entry_type UNDECIDED and flagged that the
  neonatal Fe-S-scaffold forms might belong together as one entry with per-gene
  subtypes. This is curated as its own DISEASE on two grounds: ISCA1 is a
  non-redundant node rather than a redundant paralogue of ISCA2 (the same knockdown
  experiment in PMID:28492233 finds ISCA1 required and ISCA2 not), and the reported
  phenotype carries a migration abnormality the authors say the other subtypes lack.
  The counter-argument is real and is recorded in the differential: the discoverers
  themselves say the phenotype resembles all four sibling syndromes. If the project
  later lumps the series, this entry is the ISCA1 subtype.

  The glycine branch is a mechanism node, not a phenotype. See the knowledge gap.
  No source cited here reports a glycine measurement in an ISCA1 patient.

  Frequency bands. With four children in two families plus one unrelated patient,
  band assignment is division over a handful of cases and is not a penetrance
  estimate. OBLIGATE is used where GeneReviews states the feature as definitional;
  OCCASIONAL where GeneReviews hedges ("risk for") or the feature was present in one
  of four.

  Early death is not curated as a phenotype. HP:0003819 (Death in childhood) sits
  outside the branch the PhenotypeTerm dynamic enum expands from, so there is no
  valid binding; the mortality claim and the reported ages at death are carried in
  the progression record instead.

  Deep research. An OpenScientist report is committed with the entry. It added the
  third allele (p.Tyr101Cys, PMID:32092383) and the neuron-specific Isca1 knockout
  rat (PMID:37140997), which is the only in vivo model of this disorder.

  Two of the report's identifiers were wrong and are not used. It gives HGNC:16947
  for ISCA1 in both its header and its genetics section; that code is ERLIN1, and
  ISCA1 is hgnc:28660. Nothing flagged it, because `just validate-research-terms`
  hardcodes --skip-prefix HGNC, so gene identifiers are the one class the report's
  term validation never examines (#9845). It also reports MMDS5 as OMIM #617613,
  which is correct, alongside ISCA1 as OMIM *611006, which belongs to a different
  gene; neither OMIM number is bound here.

  The report's own quote validation reports 7 of 20 quotes unsupported and sets
  needs_review: true. All seven are verbatim. Six fail on iron-sulfur notation
  ([4Fe-4S], [2Fe-2S]) and one on an HGVS designation, all stripped by the snippet
  matcher before comparison. That is #9364 and #10192, and it is why several
  snippets in this entry quote a bracket-free sentence where the bracketed one
  states the mechanism more directly.

  The sibling MMDS syndromes are four differential entries, not one. They were
  lumped as "MMDS1-4" in the first draft, which left the relationship out of reach
  of any query. Each now carries its own MONDO term. The distinguishing_features
  say the separator is the gene, because that is what the sources support: the
  discoverers describe the MMDS5 phenotype as similar to all four, and the only
  non-genetic separator anyone reports is the ISCA1/ISCA2 non-redundancy in
  PMID:28492233. MMDS4 (ISCA2) is now curated as its own entry on main
  (Multiple_Mitochondrial_Dysfunctions_Syndrome_4.yaml, MONDO:0014611), so the
  differential entry above is a live cross-reference rather than a forward link.

  Evidence source on patient-derived material. An assay run on cultured cells is
  IN_VITRO whatever the cells came from, so both the HeLa complementation
  (PMID:29767723) and the patient-fibroblast lipoylation and complex I/II
  measurements (PMID:32092383) are graded that way. HUMAN_CLINICAL is reserved for
  observations of the patient, such as the clinical description in PMID:29767723.

  Penetrance is deliberately not recorded. No reference cited here states one, and
  the five reported individuals were all ascertained through the phenotype, so
  "every biallelic carrier is affected" describes how they were found rather than
  how often the genotype manifests. The same reasoning governs the frequency bands
  above.

  Two name collisions in the flat pathograph namespace, resolved differently.
  "Leukodystrophy" names both a pathophysiology node (the tissue-scale imaging
  finding) and a phenotype, with identical spelling and case; the graph collapses
  them into one node, and that is treated here as one concept correctly rendered
  once rather than a defect to fix. "Lactic Acidosis"/"Lactic acidosis" differ
  only by case, so they do not collapse; that pair is treated as two concepts (the
  organism-scale biochemical node and the clinical/laboratory phenotype it
  produces) and is now joined by an explicit downstream edge instead.
📚

References & Deep Research

References

1
PMID:31580634
ISCA1-Related Multiple Mitochondrial Dysfunctions Syndrome.
No top-level findings curated for this source.

Deep Research

1

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

Evaluations and curation notes (2)

Record notes

Curation decisions worth recording. Entry versus subtype. The stub left entry_type UNDECIDED and flagged that the neonatal Fe-S-scaffold forms might belong together as one entry with per-gene subtypes. This is curated as its own DISEASE on two grounds: ISCA1 is a non-redundant node rather than a redundant paralogue of ISCA2 (the same knockdown experiment in PMID:28492233 finds ISCA1 required and ISCA2 not), and the reported phenotype carries a migration abnormality the authors say the other subtypes lack. The counter-argument is real and is recorded in the differential: the discoverers themselves say the phenotype resembles all four sibling syndromes. If the project later lumps the series, this entry is the ISCA1 subtype. The glycine branch is a mechanism node, not a phenotype. See the knowledge gap. No source cited here reports a glycine measurement in an ISCA1 patient. Frequency bands. With four children in two families plus one unrelated patient, band assignment is division over a handful of cases and is not a penetrance estimate. OBLIGATE is used where GeneReviews states the feature as definitional; OCCASIONAL where GeneReviews hedges ("risk for") or the feature was present in one of four. Early death is not curated as a phenotype. HP:0003819 (Death in childhood) sits outside the branch the PhenotypeTerm dynamic enum expands from, so there is no valid binding; the mortality claim and the reported ages at death are carried in the progression record instead. Deep research. An OpenScientist report is committed with the entry. It added the third allele (p.Tyr101Cys, PMID:32092383) and the neuron-specific Isca1 knockout rat (PMID:37140997), which is the only in vivo model of this disorder. Two of the report's identifiers were wrong and are not used. It gives HGNC:16947 for ISCA1 in both its header and its genetics section; that code is ERLIN1, and ISCA1 is hgnc:28660. Nothing flagged it, because `just validate-research-terms` hardcodes --skip-prefix HGNC, so gene identifiers are the one class the report's term validation never examines (#9845). It also reports MMDS5 as OMIM #617613, which is correct, alongside ISCA1 as OMIM *611006, which belongs to a different gene; neither OMIM number is bound here. The report's own quote validation reports 7 of 20 quotes unsupported and sets needs_review: true. All seven are verbatim. Six fail on iron-sulfur notation ([4Fe-4S], [2Fe-2S]) and one on an HGVS designation, all stripped by the snippet matcher before comparison. That is #9364 and #10192, and it is why several snippets in this entry quote a bracket-free sentence where the bracketed one states the mechanism more directly. The sibling MMDS syndromes are four differential entries, not one. They were lumped as "MMDS1-4" in the first draft, which left the relationship out of reach of any query. Each now carries its own MONDO term. The distinguishing_features say the separator is the gene, because that is what the sources support: the discoverers describe the MMDS5 phenotype as similar to all four, and the only non-genetic separator anyone reports is the ISCA1/ISCA2 non-redundancy in PMID:28492233. MMDS4 (ISCA2) is now curated as its own entry on main (Multiple_Mitochondrial_Dysfunctions_Syndrome_4.yaml, MONDO:0014611), so the differential entry above is a live cross-reference rather than a forward link. Evidence source on patient-derived material. An assay run on cultured cells is IN_VITRO whatever the cells came from, so both the HeLa complementation (PMID:29767723) and the patient-fibroblast lipoylation and complex I/II measurements (PMID:32092383) are graded that way. HUMAN_CLINICAL is reserved for observations of the patient, such as the clinical description in PMID:29767723. Penetrance is deliberately not recorded. No reference cited here states one, and the five reported individuals were all ascertained through the phenotype, so "every biallelic carrier is affected" describes how they were found rather than how often the genotype manifests. The same reasoning governs the frequency bands above. Two name collisions in the flat pathograph namespace, resolved differently. "Leukodystrophy" names both a pathophysiology node (the tissue-scale imaging finding) and a phenotype, with identical spelling and case; the graph collapses them into one node, and that is treated here as one concept correctly rendered once rather than a defect to fix. "Lactic Acidosis"/"Lactic acidosis" differ only by case, so they do not collapse; that pair is treated as two concepts (the organism-scale biochemical node and the clinical/laboratory phenotype it produces) and is now joined by an explicit downstream edge instead.

Create: Multiple Mitochondrial Dysfunctions Syndrome 5 (ISCA1) · 2026-09-05T19:36:48Z · View source

De-novo curation of MMDS5 (MONDO:0033282, ISCA1), claimed at #11155. entry_type resolved to DISEASE and the stub deleted. Sources: the ISCA1 GeneReviews chapter (PMID:31580634) as the mandatory phenotype baseline, plus Shukla 2017 (PMID:28356563), Shukla 2018 (PMID:30105122), Torraco 2018 (PMID:29767723), Beilschmidt 2017 (PMID:28492233), Suraci 2021 (PMID:33711344), Bogdan 2020 (PMID:32092383) and the Isca1 knockout rat (PMID:37140997). Deep research: one OpenScientist run, committed with the entry. It contributed the third allele p.(Tyr101Cys) and the neuron-specific Isca1 knockout rat, which is the only in vivo model of this disorder and is now linked to two pathograph nodes through modeled_mechanisms with readouts and per-link limitations. Two report identifiers were wrong and were not used. HGNC:16947, given for ISCA1 in both the report header and its genetics section, is ERLIN1; ISCA1 is hgnc:28660. Nothing flagged it because just validate-research-terms hardcodes --skip-prefix HGNC, so gene CURIEs are never checked in a report; reported on #9845. The report also pairs the correct disease OMIM (#617613) with an ISCA1 gene OMIM (*611006) that belongs to a different gene; no OMIM number is bound. An earlier draft of this entry independently bound hgnc:14059 for ISCA1, which is CELF6. just validate-terms caught it on the first run because the KB path does resolve HGNC through conf/oak_config.yaml. Two wrong gene identifiers for one gene in one session, one caught in seconds and one invisible; that asymmetry is what was reported on #9845. The report's quote validation reports 7 of 20 quotes unsupported and sets needs_review: true. All seven were checked against the cached text and are verbatim; six fail on iron-sulfur notation and one on an HGVS designation, both stripped by the snippet matcher. Reported on #9364. Several snippets in this entry therefore quote a bracket-free sentence where the bracketed one states the mechanism more directly. preflight-dr returns WARN rather than PASS, on ISCA2 being mentioned at 36% of ISCA1's rate. That is expected here and is not entity confusion: ISCA1 and ISCA2 are obligate heterodimer partners in the step this disease breaks, so a correct mechanistic report has to discuss both. No ISCA2-specific (MMDS4) clinical content was taken. Curation decisions. The glycine-cleavage branch is a mechanism node with a KNOWLEDGE_GAP attached, not a phenotype, because no cited source reports a glycine measurement in an ISCA1 patient. HP:0003819 (Death in childhood) is outside the PhenotypeTerm dynamic enum, so mortality is carried in progression instead, with the reason in notes. The case for a separate entry rather than a subtype of the MMDS series rests on ISCA1 being non-redundant with ISCA2 in the same knockdown experiment and on a migration abnormality the other subtypes are said to lack; the counter-argument is recorded in differential_diagnoses. Validation: just validate-disorders passes schema, terms and 41/41 snippets. check-entity-refs, check-causal-targets, check-duplicate-keys and check-enum-values are clean. Eight references fetched during research but not cited were pruned from references_cache rather than staged.

OpenScientist ▸
Key Findings
openscientist-autonomous 12 citations 2026-09-05T19:41:08.525907

Key Findings

Finding 1 — MMDS5 is caused by biallelic ISCA1 variants disrupting mitochondrial [4Fe-4S] cluster assembly

Multiple independent families with autosomal-recessive MMDS5 carry biallelic pathogenic variants in ISCA1 (chromosome 9; HGNC:16947; OMIM 611006). The reported allelic spectrum is small but consistent: a homozygous c.259G>A, p.(Glu87Lys) founder variant identified in two families and subsequently a third family; a homozygous c.29T>G, p.(Val10Gly) variant located in the mitochondrial presequence; and a p.(Tyr101Cys)* variant reported in the compound/heterozygous context. Functional studies across these reports converge on a loss-of-function mechanism: reduced ISCA1 protein level and stability, decreased [2Fe-2S]/[4Fe-4S] cluster stability, impaired lipoic-acid synthesis, and reduced respiratory Complex I and Complex II activities.

"revealed a homozygous c.259G>A [p.(Glu87Lys)] variant in ISCA1 and Mendelian segregation was confirmed in both families" — PMID: 28356563

"A homozygous missense mutation in ISCA1 (c.29T>G; p.V10G) identified by targeted MitoExome sequencing resulted in dramatic reduction of ISCA1 protein level" — PMID: 29767723

"a novel pathogenic variant p.(Tyr101Cys) in ISCA1 leading to MMDS type 5" — PMID: 32092383

"IBA57, along with ISCA1 and ISCA2, play a role in maturation of [4Fe-4S] clusters which are required for multiple mitochondrial enzymes including mitochondrial Complex I, Complex II, lipoic acid synthase, and aconitase" — PMID: 37903659

Finding 2 — MMDS5 presents as an infantile-onset severe neurodegenerative/leukodystrophy phenotype with early death

Reported MMDS5 patients show early-onset neurological deterioration: psychomotor regression with loss of previously acquired gait and language, seizures, a tetrapyramidal/spastic syndrome, extensive white-matter abnormalities (leukodystrophy), cortical migrational abnormalities, lactic acidosis, and early demise. Two independent clinical descriptions frame the disorder as, respectively, "early onset neurological deterioration" with "extensive white matter abnormalities" and a "severe early onset leukodystrophy." The neuron-specific Isca1 knockout rat reproduces this constellation, showing developmental retardation, epilepsy, memory impairment, and massive neuronal death.

"two affected children each with early onset neurological deterioration, seizures, extensive white matter abnormalities, cortical migrational abnormalities, lactic acidosis and early demise" — PMID: 28356563

"a patient with a severe early onset leukodystrophy, multiple defects of respiratory complexes and a severe impairment of lipoic acid synthesis" — PMID: 29767723

"the rats suffered from developmental retardation, epilepsy, memory impairment, massive neuronal death, reduced number of Nissl bodies and dendritic spines, mitochondrial fragmentation, cristae fracture, reduced content of respiratory chain complex protein, and reduced production of ATP" — PMID: 37140997

Suggested HPO terms: Developmental regression (HP:0002376), Seizure (HP:0001250), Spasticity (HP:0001257), Leukodystrophy (HP:0002415), Lactic acidosis (HP:0003128), Global developmental delay (HP:0001263), Abnormality of neuronal migration (HP:0002269), Hyperglycinemia (HP:0002154).

Finding 3 — Mechanistic causal chain: ISCA1 loss → failed [4Fe-4S] maturation → lipoylation & OXPHOS failure → energy deficit

ISCA1 operates in the late mitochondrial ISC machinery (with ISCA2 and IBA57), converting [2Fe-2S] to [4Fe-4S] clusters and inserting them into apoproteins. Its loss impairs lipoic acid synthase (LIAS), abolishing lipoylation of the pyruvate dehydrogenase complex, α-ketoglutarate dehydrogenase, and the glycine cleavage system—explaining hyperglycinemia and elevated organic acids—while simultaneously impairing Complex I, Complex II/SDH, and aconitase. ISCA1 also feeds an ISCA1–NFU1 node required to insert a [4Fe-4S] cluster into the mitoribosome assembly factor METTL17, linking the defect to attenuated mitochondrial translation. The net result is a combined respiratory-chain deficiency with reduced ATP, increased reactive oxygen species, lactic acidosis, and neuronal death (oncosis), as demonstrated in the Isca1-knockout rat.

"lipoic acid synthase, which supports the Fe-S-dependent process of lipoylation of components of multiple key enzyme complexes, including pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and the glycine cleavage complex" — PMID: 32776106

"the assembly of the small subunit depends on the mitoribosome biogenesis factor METTL17, recently reported containing a [4Fe-4S] cluster, which we propose is inserted via the ISCA1-NFU1 node" — PMID: 37823603

"This machinery ensures the correct assembly of both [2Fe-2S] and [4Fe-4S] clusters and their insertion in the mitochondrial target proteins" — PMID: 35883565

Finding 4 — ISCA1 is the central organizer of the late ISC machinery

In vitro NMR and biochemical studies show that ISCA1 and ISCA2 form a non-redundant heterodimer that receives two [2Fe-2S] clusters from GLRX5/GRX5 and assembles a [4Fe-4S] cluster. ISCA1 orchestrates maturation by binding both ISCA2 and NFU1—two proteins that do not interact with each other—forming a transient ternary complex that transfers the [4Fe-4S] cluster to NFU1-dependent apoproteins. ISCA2 additionally partners with IBA57. Disease-associated missense mutations across these accessory proteins map to protein/complex interfaces and destabilize cluster binding, which explains why loss of the ISCA1 hub produces a broad, multi-enzyme phenotype.

"two molecules of 2Fe-2S GRX5 donate their cluster to a heterodimeric ISCA1/ISCA2 complex. This complex acts as an 'assembler' of [4Fe-4S] clusters" — PMID: 25347204

"ISCA1 is the key player of the [4Fe-4S] protein maturation process because of its ability to interact with both NFU1 and ISCA2, which, instead do not interact each other" — PMID: 33711344

Finding 5 — No disease-specific therapy; supportive management with poor prognosis

No curative or FDA-approved disease-specific treatment exists for MMDS5 or any MMDS subtype. Care follows general primary mitochondrial disease practice: symptomatic and supportive management (seizure control, nutritional support, physiotherapy) plus empiric "mitochondrial cocktails" of antioxidants and cofactors (e.g., riboflavin, thiamine, coenzyme Q10, L-carnitine, N-acetylcysteine) whose efficacy is unproven. A Canadian physician survey documented that management relies on unstandardized empiric cofactor use. Prognosis is poor, with rapid neurological deterioration and early death. The neuron-specific Isca1-knockout rat survives to ~8 weeks—longer than human patients—offering a therapeutic testing window.

"Approximately half (49%) of participants would recommend 'mitochondrial cocktails' for all or most patients, but we identified variation in responses regarding specific vitamins and cofactors" — PMID: 31387656

"compared with human MMDS5, the rat model can survive up to 8 weeks of age, effectively extending the window of clinical treatment research" — PMID: 37140997

Finding 6 — Ultra-rare autosomal-recessive disorder with a South Indian founder variant and consanguinity association

MMDS5 (OMIM #617613) is inherited autosomal-recessively; all reported patients carry biallelic ISCA1 variants. Fewer than ~10 patients from a handful of families are described in the literature, so precise prevalence and incidence are unknown (ultra-rare). The recurrent homozygous c.259G>A p.(Glu87Lys) variant occurs on a shared region of homozygosity consistent with a founder effect in the Indian population, and affected families are typically consanguineous. No sex predilection is reported—both sexes are affected.

"The ISCA1 variant lies in the only shared region of homozygosity between the two families suggesting the possibility of a founder effect" — PMID: 28356563

"Report of the Third Family with Multiple Mitochondrial Dysfunctions Syndrome 5 Caused by the Founder Variant p.(Glu87Lys)" — PMID: 30105122

Finding 7 — Model systems recapitulate the [4Fe-4S] defect

Three complementary experimental models reproduce ISCA1 loss-of-function. (1) A neuron-specific Isca1 conditional knockout rat (Rattus norvegicus, NCBI Taxon 10116) shows developmental retardation, epilepsy, memory impairment, massive neuronal death (oncosis), reduced Nissl bodies and dendritic spines, mitochondrial fragmentation, cristae fracture, reduced respiratory-chain complex protein and ATP—recapitulating the human phenotype and surviving ~8 weeks. (2) ISCA1 RNAi knockdown in HeLa cells impairs biogenesis of mitochondrial [4Fe-4S] proteins and is rescued by wild-type but only partially by mutant ISCA1. (3) Patient-derived fibroblasts show impaired lipoic-acid synthesis and reduced Complex I/II activity. In vitro reconstituted human ISCA1/ISCA2 proteins model the underlying biochemistry.

"This study established the disease model of MMDS5 in the nervous system for the first time" — PMID: 37140997

"Down-regulation of ISCA1 in HeLa cells by RNAi impaired the biogenesis of mitochondrial [4Fe-4S] proteins, yet could be complemented by expression of wild-type ISCA1" — PMID: 29767723

Finding 8 — Phenotypically indistinguishable from other MMDS subtypes; diagnosed by combined enzyme + lipoylation defect plus ISCA1 genotyping

MMDS5 shares its core phenotype with MMDS1 (NFU1), MMDS2 (BOLA3), MMDS3 (IBA57), and MMDS4 (ISCA2). The shared biochemical signature—combined respiratory-chain (Complex I/II) deficiency plus impaired lipoic-acid synthesis with elevated glycine, lactate, and organic acids—flags the FeS-assembly disorder group, but definitive subtyping requires identifying biallelic ISCA1 variants by WES/WGS or a mitochondrial gene panel. A genotype–function correlation is evident: the presequence variant p.Val10Gly severely reduces import/stability and only partially complements in vitro, consistent with severe disease.

"the phenotype observed in all affected subjects with the ISCA1 pathogenic variant is similar to that previously described in all four types of MMDS" — PMID: 28356563

"the ISCA1 p.V10G mutant protein only partially complemented the defects, closely resembling the biochemical phenotypes observed for ISCA1 patient" — PMID: 29767723

Sectioned Disease Characterization

1. Disease Information

MMDS5 is a monogenic, autosomal-recessive mitochondrial disease characterized by combined deficiency of multiple mitochondrial [4Fe-4S] enzymes due to a defect in iron–sulfur cluster assembly. Key identifiers: MONDO:0033282, OMIM #617613 (phenotype), gene ISCA1 OMIM *611006 / HGNC:16947. There is no specific ICD-10 code beyond the mitochondrial disorder umbrella (e.g., ICD-10 E88.40; ICD-11 5C53.1 for mitochondrial disease). Synonyms/alternative names: "MMDS5," "Multiple mitochondrial dysfunctions syndrome 5," and descriptors used in the literature such as "ISCA1-related mitochondrial disease" and "ISCA1 infantile-onset leukodystrophy." Information is derived from aggregated disease-level resources and individual case reports (a small number of published families), not from large EHR cohorts.

2. Etiology

Causal factor: genetic—biallelic (homozygous or compound heterozygous) loss-of-function variants in ISCA1. Genetic risk factors: the disease-causing variants themselves (p.Glu87Lys founder allele, p.Val10Gly, p.Tyr101Cys); consanguinity strongly increases risk of homozygosity for recessive alleles. Environmental/protective/gene-environment factors: none established for this Mendelian disorder. Because MMDS5 is fully penetrant Mendelian disease, environmental modifiers, lifestyle factors, and protective alleles have not been described. Metabolic stressors (intercurrent illness, catabolic states) may precipitate acute decompensation, as in mitochondrial disease generally, though this is inferred rather than specifically documented for MMDS5.

3. Phenotypes

Core phenotypes (all clinical signs/symptoms and laboratory abnormalities): neurological deterioration/psychomotor regression (HP:0002376), seizures (HP:0001250), spasticity/tetrapyramidal syndrome (HP:0001257), leukodystrophy/white-matter abnormalities (HP:0002415), cortical/neuronal migration abnormalities (HP:0002269), global developmental delay (HP:0001263), lactic acidosis (HP:0003128), and hyperglycinemia (HP:0002154). Onset: neonatal to infantile. Severity: severe. Progression: rapidly progressive/neurodegenerative. Frequency: neurological features and lactic acidosis appear in essentially all reported patients (qualitative "very frequent"), though absolute percentages cannot be reliably derived from <10 cases. Quality-of-life impact: profound—affected children lose developmental milestones and become fully dependent, with early death.

4. Genetic/Molecular Information

Causal gene: ISCA1 (chromosome 9q21.33). Pathogenic variants: c.259G>A p.(Glu87Lys) (missense, founder); c.29T>G p.(Val10Gly) (missense, presequence/import); p.(Tyr101Cys) (missense). All are missense LOF variants; classified pathogenic/likely pathogenic under ACMG/AMP given functional and segregation evidence. Allele frequency: ultra-rare/absent-to-very-low in gnomAD (consistent with a recessive founder allele). Origin: germline. Functional consequence: loss of function—reduced protein level/stability and impaired [4Fe-4S] cluster assembly. Modifier/epigenetic/chromosomal: none established. This is a single-gene disorder without reported large structural rearrangements.

5. Environmental Information

Not applicable. MMDS5 is a purely genetic Mendelian disease; no environmental factors, lifestyle factors, or infectious agents are known to cause or trigger it. (Copper toxicity has been shown in unrelated work to inhibit ISCA1/ISCA2/ISCU-mediated Fe-S biogenesis biochemically, but this pertains to Wilson-disease pathophysiology, not to MMDS5 etiology.)

6. Mechanism / Pathophysiology

Ordered causal chain:

  1. Biallelic ISCA1 loss-of-function variants → reduced ISCA1 protein level and stability (demonstrated for p.Val10Gly; inferred for others).
  2. → Failure of the ISCA1–ISCA2 heterodimer to assemble [4Fe-4S] clusters from [2Fe-2S] precursors donated by GLRX5/GRX5 (demonstrated in vitro).
  3. → Failure of ISCA1 to bridge ISCA2 and NFU1, disrupting the transient ternary complex that hands [4Fe-4S] clusters to client apoproteins (demonstrated in vitro).
  4. Branch A → Loss of lipoic acid synthase (LIAS) activity → failed lipoylation of pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and the glycine cleavage system → hyperglycinemia, elevated organic acids, impaired pyruvate oxidation → lactic acidosis (demonstrated biochemically).
  5. Branch B → Loss of Complex I, Complex II/SDH, and aconitase activity → impaired oxidative phosphorylation and TCA-cycle flux → reduced ATP, increased ROS (demonstrated).
  6. Branch C → Failure of the ISCA1–NFU1 node to mature METTL17 → attenuated mitochondrial small-subunit ribosome assembly → reduced mitochondrial translation (proposed/inferred).
  7. → Bioenergetic failure in high-demand tissues (CNS) → mitochondrial fragmentation, cristae disruption, neuronal death (oncosis), demyelination/leukodystrophy → infantile neurodegeneration, seizures, spasticity, regression, early death (demonstrated in rat model; consistent with human).

Molecular pathways / processes: mitochondrial iron–sulfur cluster assembly (GO:0016226), [4Fe-4S] cluster assembly (GO:0051539), protein lipoylation (GO:0009249/GO:0036211), oxidative phosphorylation, TCA cycle, mitochondrial translation. Protein dysfunction: loss of function / reduced stability of ISCA1. Metabolic changes: impaired energy metabolism, lactic acidosis, glycine accumulation. Tissue damage: oxidative stress and energy deficit driving neuronal oncosis. Cell types: neurons (CL:0000540) and oligodendrocytes/white matter (CL:0000128). Subcellular compartment: mitochondrion / mitochondrial matrix (GO:0005739, GO:0005759). CHEBI entities: iron-sulfur cluster (CHEBI:30408 family), lipoic acid (CHEBI:16494), glycine (CHEBI:15428), L-lactate (CHEBI:16651).

7. Anatomical Structures Affected

Primary organ/system: central nervous system / brain (UBERON:0000955), especially cerebral white matter (UBERON:0002316) and cerebral cortex (UBERON:0000956). Body system: nervous system (UBERON:0001016). Secondary: systemic metabolic derangement (lactic acidosis) affecting multiple organs; skeletal muscle may show respiratory-chain deficiency. Tissue/cell: neurons and myelinating glia; subcellular: mitochondria. Lateralization: bilateral/diffuse (leukodystrophy is symmetric/diffuse rather than focal).

8. Temporal Development

Onset: congenital-to-infantile, insidious-to-subacute, often unmasked or accelerated by intercurrent illness. Course: progressive neurodegeneration with psychomotor regression; no remission. Duration: short—early death in infancy or early childhood in reported patients. Critical period: early infancy (the therapeutic window during which brain injury is accruing).

9. Inheritance and Population

Inheritance: autosomal recessive. Penetrance: complete (all reported biallelic carriers affected). Expressivity: severe and relatively consistent within the small reported cohort. Founder effect: the p.(Glu87Lys) allele on a shared haplotype in the Indian population. Consanguinity: commonly associated (homozygosity). Carrier frequency / prevalence / incidence: unknown; ultra-rare (<10 reported patients). Sex ratio: no predilection (both sexes affected). Anticipation / mosaicism: not reported/not applicable.

10. Diagnostics

Biochemical: elevated blood/CSF lactate; elevated plasma glycine (hyperglycinemia); elevated organic acids; reduced fibroblast/muscle Complex I and Complex II activities; reduced protein lipoylation on immunoblot. Imaging: brain MRI showing leukodystrophy/white-matter signal abnormalities and cortical migrational abnormalities. Genetic testing (definitive): whole-exome or whole-genome sequencing, or a mitochondrial/nuclear-mitochondrial gene panel including ISCA1; single-gene testing appropriate when the founder allele is suspected in an Indian consanguineous family. Differential diagnosis: other MMDS subtypes (MMDS1 NFU1, MMDS2 BOLA3, MMDS3 IBA57, MMDS4 ISCA2), other leukodystrophies, PDH deficiency, and glycine encephalopathy—distinguished by gene identification. Screening: carrier and cascade testing in affected families; prenatal testing where the familial variant is known.

11. Outcome/Prognosis

Prognosis is poor: rapid neurological deterioration and early death in infancy/early childhood in reported patients. Morbidity: profound neurodevelopmental disability, seizures, and loss of motor and language function. Complications: recurrent metabolic decompensation with lactic acidosis, feeding difficulties, and seizure-related morbidity. Recovery potential: none (progressive, no disease-modifying therapy). Prognostic factors: genotype severity (e.g., import-disrupting presequence variants) and degree of enzymatic/biochemical defect are plausible but not formally validated across the tiny cohort.

12. Treatment

No disease-specific or curative therapy. Supportive/pharmacologic: antiseizure medications; nutritional support; physiotherapy/occupational/speech therapy. Empiric "mitochondrial cocktail": riboflavin, thiamine, coenzyme Q10, L-carnitine, N-acetylcysteine—efficacy unproven. Experimental/preclinical: combinatorial glucose + nicotinic acid + N-acetylcysteine showed synergistic rescue in C. elegans and zebrafish complex I disease models (not MMDS5-specific); the Isca1-KO rat provides a platform for testing candidate therapies within its ~8-week survival window. NCIT-type intervention categories: supportive care, anticonvulsant therapy, dietary/nutritional supplementation. No gene, cell, or RNA therapies are approved or in trials for MMDS5.

13. Prevention

Primary prevention: genetic counseling for at-risk (especially consanguineous) families; carrier screening. Secondary/tertiary: cascade testing, prenatal diagnosis, and preimplantation genetic testing where the familial variant is known; anticipatory management of metabolic crises. No vaccine or public-health intervention is applicable (non-infectious, non-environmental Mendelian disorder).

14. Other Species / Natural Disease

Orthologs / model species: Rattus norvegicus (NCBI Taxon 10116; Isca1), Homo sapiens (ISCA1); the gene family is deeply conserved (bacterial IscA/SufA homologs). No naturally occurring companion-animal or wildlife MMDS5 disease is reported. Evolutionary conservation of the ISC assembly machinery underlies the utility of cross-species models.

15. Model Organisms

  • Neuron-specific Isca1 conditional knockout rat — first in vivo MMDS5 model; recapitulates developmental retardation, epilepsy, memory impairment, neuronal death, mitochondrial ultrastructural damage, reduced respiratory-chain protein and ATP; survives ~8 weeks (PMID: 37140997).
  • HeLa ISCA1 RNAi knockdown — impaired [4Fe-4S] protein biogenesis, rescued by WT (fully) and mutant (partially) ISCA1 (PMID: 29767723).
  • Patient-derived fibroblasts — impaired lipoic-acid synthesis, reduced Complex I/II activity (PMID: 32092383; PMID: 29767723).
  • In vitro reconstituted human ISCA1/ISCA2 proteins — biochemical model of [4Fe-4S] assembly (PMID: 25347204; PMID: 33711344).

Limitation of models: the rat's longer survival and the incomplete replication of the full human leukodystrophy spectrum mean therapeutic findings require careful translation.

Mechanistic Model / Interpretation

   Biallelic ISCA1 LOF variants (p.Glu87Lys / p.Val10Gly / p.Tyr101Cys)
              │  reduced ISCA1 protein & stability
              ▼
ISCA1–ISCA2 heterodimer fails to assemble [4Fe-4S] clusters
  (receives [2Fe-2S] from GLRX5; ISCA1 bridges ISCA2 & NFU1)
              │
┌─────────────────────┼──────────────────────────┐
▼                     ▼                           ▼
  LIAS not matured     Complex I, II/SDH,          METTL17 not matured
│              aconitase not matured        (ISCA1–NFU1 node)
▼                     ▼                           ▼
 Loss of lipoylation:   OXPHOS + TCA failure     Attenuated mito translation
 PDH, KGDH, GCS               │                     (proposed/inferred)
│                     ▼
▼             ↓ATP, ↑ROS, ↑lactate
 Hyperglycinemia,             │
 lactic acidosis,             ▼
 organic aciduria    Bioenergetic failure in neurons & glia
└───────────────┬─────┘
        ▼
   Neuronal death (oncosis), leukodystrophy, cortical migration defects
        ▼
   Infantile neurodegeneration, seizures, spasticity, regression, early death

The unifying concept is that ISCA1 is a non-redundant hub of the late ISC machinery. Because a single upstream lesion (failed [4Fe-4S] assembly) propagates to every downstream client enzyme, MMDS5 is fundamentally a combined enzyme-deficiency disorder rather than a single-enzyme defect. The lipoylation branch explains the distinctive biochemical fingerprint (hyperglycinemia + lactic acidosis + organic aciduria), while the OXPHOS branch explains the energy deficit; the two branches together account for the severe, brain-predominant, rapidly fatal clinical picture. This same convergence explains why the five MMDS subtypes (which affect different nodes of the same pathway) are clinically indistinguishable and require molecular genotyping to separate.

MMDS subtype Gene OMIM Pathway role
MMDS1 NFU1 #605711 [4Fe-4S] carrier / target insertion
MMDS2 BOLA3 #614299 Mitoribosome / Fe-S assembly cofactor
MMDS3 IBA57 #615330 Late ISC maturation (with ISCA2)
MMDS4 ISCA2 #616370 [4Fe-4S] assembler (ISCA1 partner)
MMDS5 ISCA1 #617613 [4Fe-4S] assembler / central organizer

Evidence Base

PMID Title (abbrev.) Evidence type Supports
28356563 Homozygous p.(Glu87Lys) in ISCA1 → MMDS Human clinical + genetics F1, F2, F6, F8 (founder variant; core phenotype; MMDS overlap)
29767723 ISCA1 mutation → infantile leukodystrophy, [4Fe-4S] defects Human + cellular (HeLa RNAi) F1, F2, F7, F8 (second variant; leukodystrophy; complementation)
32092383 Novel ISCA1 p.(Tyr101Cys), cluster instability Human + in vitro F1, F7 (third variant; fibroblast defect)
30105122 Third family with founder p.(Glu87Lys) Human clinical F6 (recurrence, ultra-rarity)
37140997 Neuron-specific Isca1 KO rat Model organism F2, F5, F7 (in vivo phenotype; treatment window)
32776106 NFU1 [4Fe-4S] assembly from ISCU2/ISCA1 In vitro biochemistry F3 (lipoylation link, PDH/KGDH/GCS)
37823603 BOLA3/NFU1 link mitoribosome ISC to MMDS In vitro / molecular F3 (METTL17 / mito translation branch)
35883565 Molecular basis of [4Fe-4S] maturation diseases Review F3, F4 (machinery, interface mutations)
25347204 [4Fe-4S] formation in mitochondrial ISC machinery In vitro biochemistry F4 (ISCA1/ISCA2 assembler)
33711344 ISCA1 orchestrates ISCA2 & NFU1 In vitro biochemistry F4 (ISCA1 as central organizer)
37903659 IBA57 MMDS case; client enzymes Human + review F1 (client enzyme definition)
31387656 Canadian survey of mitochondrial disease care Clinical survey F5 (empiric cocktail practice)
33640978 Glu+NA+NAC synergy in complex I models Model organism F5 (candidate therapy, non-specific)

All quoted snippets above were validated against the corresponding abstracts during the investigation.

Limitations and Knowledge Gaps

  • Extremely small evidence base: fewer than ~10 patients from a handful of families. Phenotype frequencies, penetrance nuances, and genotype–phenotype correlations cannot be quantified statistically; qualitative descriptors are the best available.
  • Allelic spectrum is narrow: only three missense variants are well described; the full spectrum (truncating, splice, structural) and their consequences are unknown.
  • Epidemiology unknown: no reliable prevalence, incidence, or carrier-frequency estimates; the founder allele's population frequency in India is not precisely quantified.
  • Mechanistic branches partly inferred: the METTL17/mitochondrial-translation branch is proposed rather than demonstrated specifically in MMDS5 patient tissue; the precise driver of selective CNS vulnerability is not resolved.
  • No natural-history study or validated biomarkers for progression or treatment response.
  • Model translation gap: the rat model survives longer and may not fully reproduce the human leukodystrophy; no MMDS5-specific therapeutic has been tested in vivo.

Proposed Follow-up Experiments / Actions

  1. Establish an international MMDS5 patient registry to aggregate cases, standardize phenotyping (HPO-coded), and estimate natural history and genotype–phenotype correlations.
  2. Expand the variant catalog via systematic ISCA1 screening in undiagnosed leukodystrophy/combined-OXPHOS cohorts; deposit and curate variants in ClinVar with functional evidence.
  3. Quantify the Indian founder allele frequency in population and carrier-screening datasets to inform prevalence and targeted carrier screening.
  4. Test candidate therapeutics in the neuron-specific Isca1-KO rat within its ~8-week window—e.g., the Glu+NA+NAC regimen, lipoic-acid/lipoylation-supporting strategies, and AAV-mediated ISCA1 gene replacement.
  5. Dissect selective neuronal vulnerability using patient iPSC-derived neurons/organoids and single-cell transcriptomics to map which [4Fe-4S] clients are most rate-limiting in CNS energetics.
  6. Validate the METTL17/mitochondrial-translation branch directly in MMDS5 patient-derived cells to confirm its contribution to pathology.
  7. Develop diagnostic biochemical panels (lipoylation immunoblot + glycine/lactate/organic-acid signatures) to accelerate recognition and prompt confirmatory ISCA1 genotyping.

Report compiled from an autonomous multi-iteration investigation (8 confirmed findings, 27 papers reviewed). Evidence types span human clinical/genetic reports, in vitro biochemistry, and model-organism studies, as annotated above.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 13
Resolved 13
Unresolved (possible confabulation) 0
Unverifiable 0
Quoted claims checked 20
Quoted claims found in source 13
Quoted claims not found in source 7
References weighed for topical relevance 13
On topic 11
Off topic 0

Quotes not found in the cited source

Searched the abstract, any retrieved full text, and the title. A quote drawn from a part of the paper that was not retrieved will appear here too, so check before treating one as invented:

4 of these was searched against an abstract alone, with no full text retrieved - marked abstract only below. Where full text can be fetched, re-running with it will settle them; where the source publishes only a summary to PubMed, as GeneReviews chapters do, it will not, and the quote has to be checked by hand against the chapter itself.

  • PMID:28356563: "revealed a homozygous c.259G>A [p.(Glu87Lys)] variant in ISCA1 and Mendelian segregation was confirmed in both families"
  • closest text in source: "revealed a homozygous c.259G>A [p.(Glu87Lys)] variant in ISCA1 and Mendelian segregation was confirmed in both families"
  • PMID:37903659 (abstract only): "IBA57, along with ISCA1 and ISCA2, play a role in maturation of [4Fe-4S] clusters which are required for multiple mitochondrial enzymes including mitochondrial Complex I, Complex II, lipoic acid synthase, and aconitase"
  • closest text in source: "IBA57, along with ISCA1 and ISCA2, play a role in maturation of [4Fe-4S] clusters which are required for multiple mitochondrial enzymes including mitochondrial Complex I, Complex II, lipoic acid synthase, and aconitase"
  • PMID:37823603: "the assembly of the small subunit depends on the mitoribosome biogenesis factor METTL17, recently reported containing a [4Fe-4S] cluster, which we propose is inserted via the ISCA1-NFU1 node"
  • closest text in source: "the assembly of the small subunit depends on the mitoribosome biogenesis factor METTL17, recently reported containing a [4Fe-4S] cluster, which we propose is inserted via the ISCA1-NFU1 node"
  • PMID:35883565 (abstract only): "This machinery ensures the correct assembly of both [2Fe-2S] and [4Fe-4S] clusters and their insertion in the mitochondrial target proteins"
  • closest text in source: "This machinery ensures the correct assembly of both [2Fe-2S] and [4Fe-4S] clusters and their insertion in the mitochondrial target proteins"
  • PMID:25347204 (abstract only): "two molecules of 2Fe-2S GRX5 donate their cluster to a heterodimeric ISCA1/ISCA2 complex. This complex acts as an 'assembler' of [4Fe-4S] clusters"
  • closest text in source: "We found that (i) ISCA2 binds either [2Fe-2S] or [4Fe-4S] cluster in a dimeric state, and (ii) two molecules of 2Fe-2S GRX5 donate their cluster to a heterodimeric ISCA1/ISCA2 complex"
  • PMID:33711344: "ISCA1 is the key player of the [4Fe-4S] protein maturation process because of its ability to interact with both NFU1 and ISCA2, which, instead do not interact each other"
  • closest text in source: "ISCA1 is the key player of the [4Fe-4S] protein maturation process because of its ability to interact with both NFU1 and ISCA2, which, instead do not interact each other"
  • PMID:29767723 (abstract only): "Down-regulation of ISCA1 in HeLa cells by RNAi impaired the biogenesis of mitochondrial [4Fe-4S] proteins, yet could be complemented by expression of wild-type ISCA1"
  • closest text in source: "Down-regulation of ISCA1 in HeLa cells by RNAi impaired the biogenesis of mitochondrial [4Fe-4S] proteins, yet could be complemented by expression of wild-type ISCA1"

Term Validation

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

Outcome Count
Terms checked 26
Resolved 25
Unresolved (possible confabulation) 0
Obsolete 0
Unverifiable 1

25 of 26 terms resolved to a current term; the rest could not be looked up either way.