MSTO1-Related Mitochondrial Myopathy

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Classifications

Harrison's Chapter

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Inheritance

1

Autosomal recessive inheritance HP:0000007

Disease requires two damaging MSTO1 alleles, in the great majority of reported families as compound heterozygous missense, truncating or splice-site variants, and in at least one Ashkenazi Jewish family as a homozygous missense variant. Heterozygous parents are unaffected, and the largest cohort makes that point explicitly as an argument about mechanism: haploinsufficiency for MSTO1 does not produce disease. Penetrance in bi-allelic individuals appears complete on present evidence and is recorded as such, with the caveat that the total published experience is around 30 patients and no unaffected bi-allelic individual has been sought systematically in population databases. The historically competing dominant mode is deliberately not asserted. It rested on a single family whose variant was later shown to lie in the MSTO2P pseudogene; see the entry-level notes and the `msto1-dominant-claim-retracted` discussion. Reviews published between 2018 and 2019 that describe "both autosomal dominant and recessive modes" predate that correction, and the 2018 statement is quoted below as evidence of the state of the field at the time rather than as a current claim.

Autosomal recessive inheritance Penetrance: COMPLETE

Show evidence (6 references)

PMID:28544275 SUPPORT Human Clinical

"We report here the first families carrying recessive variants in the MSTO1 gene: compound heterozygous mutations were identified in two sisters and in an unrelated singleton case, who presented a multisystem complex phenotype mainly characterized by myopathy and cerebellar ataxia."

The first demonstration of recessive transmission at this locus, in three probands from two families.

PMID:31463572 SUPPORT Human Clinical

"We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants"

Fifteen patients from twelve independent families, all bi-allelic, is the strongest single body of evidence for recessive inheritance.

PMID:31463572 SUPPORT Human Clinical

"Since haploinsufficiency, as is present in some of the heterozygous parents in our cohort, does not cause a clinical phenotype"

Establishes that a single damaged MSTO1 allele is tolerated, which is the genetic argument against a simple loss-of-function dominant mechanism and the reason carrier parents need no surveillance.

+ 3 more references

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Mechanistic Hypotheses

3

Fusion failure is the primary lesion and mtDNA depletion is downstream of it

msto1_fusion_first CANONICAL

Evidence balance 2 support

The standard model, held since the first recessive families were described in 2017 and restated by every clinical group that has reported the disorder since. Loss of cytosolic MSTO1 impairs mitochondrial fusion; the fragmented, poorly distributed network then fails to distribute the mtDNA replication machinery and to segregate nucleoids, and mtDNA content falls. The strongest direct argument for the ordering comes from rescue kinetics in patient fibroblasts: re-expressing wild-type MSTO1 fully restores mitochondrial and lysosomal morphology by 48 hours but only partially restores mtDNA, which is what one expects if the genome defect is the later, downstream event. The model is also supported by analogy, since the same fusion-to-depletion relationship is established for MFN2 and OPA1. Its weakness is that the connecting steps have never been demonstrated in MSTO1 cells, and that a cytosolic protein with no demonstrated enzymatic activity is an odd fusion factor.

Pathograph links

Loss of Cytosolic MSTO1 Protein -> Impaired Mitochondrial Fusion and Network Fragmentation DIRECT The canonical route. Without MSTO1 at the cytosol-outer-membrane interface, fusion events fall and the network fragments; this is the oldest and most reproduced observation in the field.

Impaired Mitochondrial Fusion and Network Fragmentation -> mtDNA Depletion and Nucleoid Reorganisation INDIRECT KNOWN INTERMEDIATES Reduced fusion is proposed to deplete mtDNA by failing to distribute the mtDNA replication machinery and by leaving nucleoids unsegregated, the same route established for the fusion GTPases MFN2 and OPA1.

Show evidence (2 references)

PMID:31463572 SUPPORT In Vitro

"further suggests that impairments in mitochondrial dynamics are upstream of mtDNA depletion in MSTO1 patient fibroblasts"

The authors' own statement of the causal ordering, based on the differential recovery kinetics after rescue.

PMID:28554942 SUPPORT In Vitro

"MSTO1 likely has a physiologically relevant role in mitochondrial morphogenesis by supporting mitochondrial fusion."

The canonical functional assignment for the protein.

MSTO1 is a TRiC/CCT chaperonin assembly factor and the mitochondrial phenotype is secondary to a cytosolic proteostasis defect

msto1_tric_proteostasis EMERGING

Evidence balance 2 support

Proposed in a 2026 preprint that has not been peer reviewed. Rapid, auxin-induced degradation of MSTO1 lowers TRiC within two days while mitochondrial fragmentation does not appear until day six; early TRiC assembly subcomplexes accumulate in the absence of MSTO1; MSTO1 co-immunoprecipitates with TRiC; TRiC is reduced in patient fibroblasts; and depleting TRiC alone is sufficient to fragment mitochondria. If confirmed, this reverses the direction of the canonical model and would make MSTO1-related disease a cytosolic chaperonopathy with a mitochondrial readout. It would also predict phenotypic overlap with CCT-subunit disorders that has not yet been looked for clinically. It is recorded as EMERGING rather than ALTERNATIVE because it currently rests on one unreviewed preprint from one laboratory.

Pathograph links

Loss of Cytosolic MSTO1 Protein -> Failure of TRiC Chaperonin Assembly DIRECT The 2026 proteostasis route. On this model the proximate consequence of losing MSTO1 is not a mitochondrial event at all but failure to assemble the cytosolic chaperonin TRiC, with the mitochondrial phenotype following later.

Failure of TRiC Chaperonin Assembly -> Impaired Mitochondrial Fusion and Network Fragmentation INDIRECT UNKNOWN INTERMEDIATES Depleting TRiC alone fragments mitochondria, and does so independently of changes in tubulin or actin, so the relevant TRiC substrate linking proteostasis to the organelle is not yet identified.

Show evidence (2 references)

PMID:42539026 SUPPORT In Vitro

"Together, our findings identify MSTO1 as a TRiC assembly factor and connect mitochondrial defects caused by MSTO1-depletion to the loss of TRiC."

The authors' summary statement of the hypothesis.

PMID:42539026 SUPPORT In Vitro

"MSTO1 patient-derived fibroblasts have fragmented mitochondria and a striking loss of mtDNA."

Confirms that the model is being built against the same cellular phenotype that defines the human disease, rather than against an unrelated cell system.

MSTO1 restrains RAD51 at the mitochondrial genome and its loss depletes mtDNA directly

msto1_rad51_mtdna EMERGING

Evidence balance 1 support

Proposed in 2026 from cancer cell models. MSTO1 binds RAD51 through a conserved FxxA motif and restrains its mitochondrial, but not its nuclear, activity; losing MSTO1 increases RAD51 occupancy on mtDNA, disrupts mtDNA replication and depletes the genome. This would make the mtDNA phenotype a direct consequence of MSTO1 loss rather than a sequela of fragmentation, and it is not mutually exclusive with the fusion model - both could contribute. The principal limitation is that the work is entirely in cancer and cultured cell systems, with no patient material, and that the accompanying cGAS-STING inflammatory arm has never been sought in a patient.

Pathograph links

Loss of Cytosolic MSTO1 Protein -> Unrestrained RAD51 Binding to Mitochondrial DNA DIRECT The 2026 genome-maintenance route. On this model MSTO1 restrains RAD51 at the mitochondrial genome, and losing it disrupts mtDNA replication directly rather than through the fusion machinery.

Unrestrained RAD51 Binding to Mitochondrial DNA -> mtDNA Depletion and Nucleoid Reorganisation DIRECT On this model the depletion is a direct consequence of disordered mtDNA replication at the nucleoid, not a distribution problem inherited from the fusion defect.

Show evidence (1 reference)

PMID:42098462 SUPPORT In Vitro

"Loss of MSTO1 impairs mitochondrial membrane potential, reduces mtDNA content, and increases susceptibility to oxidative stress."

Shows that MSTO1 loss reproduces the patient mtDNA phenotype in a system where the proposed direct mechanism was mapped.

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Discussions and Knowledge Gaps

5

Is autosomal dominant inheritance a real mode for MSTO1 disease, given that it was reported in 2017, repeated in reviews through 2019, and still appears in the HPO annotation set for OMIM:617675?

CONTROVERSY RESOLVED msto1-dominant-claim-retracted

Attached to

genetic#MSTO1 pathophysiology#Bi-allelic Loss-of-Function Variants in MSTO1

This entry answers no, and the reasoning should be auditable rather than implicit. The claim originated with a single Hungarian family in which a heterozygous c.22G>A (p.Val8Met) change segregated with a multisystem phenotype dominated by psychiatric and endocrine features; the report was titled as revealing the first dominant MSTO1 mutation, and the authors also proposed a possible dominant-negative mechanism. In 2023 Gerber et al challenged the variant assignment, and in their published reply the original authors confirmed that the mutation lies in the MSTO2P pseudogene rather than in MSTO1; EMBO Molecular Medicine issued an erratum to the 2017 paper. Independently, the largest cohort had already observed that heterozygous parents carrying MSTO1 loss-of-function alleles are unaffected, which removes haploinsufficiency as a plausible dominant route. Every other reported family is bi-allelic. The residual open question, and the reason the discussion is retained rather than deleted, is what the MSTO2P variant was doing in that family: the original authors maintain that their patients genuinely had reduced MSTO1 mRNA and protein, and whether the pseudogene variant contributed to that reduction is explicitly unresolved.

Proposed experiments

Test whether MSTO2P variants modulate MSTO1 expression

msto1-msto2p-expression-test

Introduce the MSTO2P c.22G>A-equivalent change into cells on an otherwise isogenic background and measure MSTO1 mRNA and protein, to determine whether a pseudogene variant can account for the reduced MSTO1 levels reported in the original family or whether that observation had another cause.

Audit of MSTO1 variant calls for pseudogene mis-mapping

msto1-pseudogene-alignment-audit

Re-examine reported heterozygous MSTO1 variants in clinical databases with alignment methods that discriminate MSTO1 from MSTO2P, to establish how often the misassignment recurs in routine diagnostics.

Resolution: Resolved for the purpose of this entry: only autosomal recessive inheritance is asserted, and the phenotypes unique to the MSTO2P family are excluded. The subsidiary question of whether an MSTO2P variant can lower MSTO1 expression remains open and is recorded in the rationale.

Show evidence (3 references)

PMID:37431816 SUPPORT Human Clinical

"Gerber et al report 2 autosomal recessive pathogenic Misato homolog 1 (MSTO1) variants causing hereditary optic atrophy and raise concerns about a previously identified dominant variant of MSTO1 by Gal et al (2017)."

The published challenge to the dominant variant.

PMID:37431815 SUPPORT Human Clinical

"but also confirm finding of Gerber et al that the mutation is in MSTO2p pseudogene"

The original authors' confirmation, which is what makes this resolved rather than merely disputed.

PMID:37431815 PARTIAL Human Clinical

"Whether MSTO2p variant contributes to the observed decrease in MSTO1 levels in patients remains unclear."

The explicitly unresolved residue, which is why the discussion carries proposed experiments rather than being closed outright.

What fraction of patients with bi-allelic MSTO1 variants actually have a pigmentary retinopathy, given that the feature names the disease term but was absent from the largest published cohort?

KNOWLEDGE GAP OPEN msto1-retinopathy-denominator

Attached to

phenotypes#Pigmentary retinopathy pathophysiology#Retinal Degeneration with Pigmentary Change

The MONDO and Orphanet labels for this disorder include pigmentary retinopathy, and the feature is unambiguously real: both probands of the report that named it, the index patient of the first recessive series and two Chinese sisters followed for seventeen years all have it, and the 2025 ophthalmological study calls retinal dystrophy a key characteristic. Yet none of the fifteen patients in the largest cohort had it recorded. The authors of that cohort state that formal ophthalmological examination was not pursued in all patients, so the conflict may be entirely one of ascertainment - a rod-predominant dystrophy with concentric field loss and preserved acuity is asymptomatic for years and will not be found unless it is looked for. The alternative is a genuine genotype- or ancestry-dependent difference in retinal involvement. The distinction matters for practice, because if the retinopathy is common but silent then every patient needs baseline retinal imaging, whereas if it is genuinely rare then targeted examination suffices. It also matters for nosology: a disease named after a feature that most patients lack is a naming problem that MONDO may eventually need to address.

Proposed experiments

Protocolised retinal imaging in every ascertained MSTO1 patient

msto1-protocolised-retinal-imaging

Apply a common protocol - wide-field fundus autofluorescence, optical coherence tomography, visual fields and full-field electroretinography - to all newly ascertained and, where possible, previously reported patients, to convert an inconsistent set of case observations into a real numerator and denominator.

Genotype-retinal phenotype correlation across the published cohort

msto1-genotype-retina-correlation

Test whether retinal involvement tracks with allele class or residual protein level by collating genotype alongside protocolised retinal phenotyping, which would distinguish an ascertainment artefact from a genotype effect.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"This finding was not reported in any of the patients in our cohort; however, formal ophthalmologic examination was not pursued in all patients."

States both halves of the gap - the absence of the finding, and the incomplete ascertainment that prevents interpreting the absence.

PMID:40542878 SUPPORT Human Clinical

"Increased awareness and comprehensive ophthalmic examination are crucial for obtaining an early and accurate diagnosis."

The ophthalmological group's own conclusion that systematic examination is what is missing, which is the substance of the proposed experiments.

Is mtDNA depletion present in the tissues that are actually diseased - skeletal muscle and cerebellum - or only in the cultured fibroblasts where it has almost always been measured?

KNOWLEDGE GAP OPEN msto1-mtdna-depletion-tissue-gap

Attached to

pathophysiology#mtDNA Depletion and Nucleoid Reorganisation pathophysiology#Dystrophic Degeneration of Skeletal Muscle Fibres

mtDNA depletion is the observation that reclassified this disorder as an mtDNA depletion syndrome, and it is consistent wherever it has been looked for. But the evidence base is narrower than the claim. Seven of seven fibroblast lines in the largest cohort showed 30-70% depletion, and the same paper states that muscle tissue was not available for further mtDNA content studies; the only muscle measurement in the literature is 27% of control in a single patient in a different series. No cerebellar tissue from an MSTO1 patient has ever been examined. Fibroblasts divide and muscle fibres and Purkinje cells do not, and mtDNA copy number regulation differs substantially between proliferating and postmitotic cells, so the extrapolation is not safe. Until depletion is measured in muscle in a series rather than in one patient, the causal edge from mtDNA depletion to muscle and cerebellar pathology in this entry rests on an inference.

Proposed experiments

Quantitative mtDNA content in MSTO1 patient muscle across a series

msto1-muscle-mtdna-series

Measure mtDNA relative to nuclear DNA in stored muscle biopsy material from as many reported patients as can be recovered, with single-fibre analysis where possible, to establish whether the fibroblast depletion is reproduced in the diseased tissue and whether it is uniform or mosaic across fibres.

Patient iPSC-derived myotubes and cerebellar organoids

msto1-ipsc-neurons-and-myotubes

Differentiate patient iPSC into myotubes and cerebellar organoids and measure mtDNA content, nucleoid distribution and network morphology in postmitotic, disease-relevant cell types rather than in dividing fibroblasts.

Show evidence (2 references)

PMID:31463572 SUPPORT In Vitro

"all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids"

The strength of the fibroblast evidence, which is exactly what makes the absence of tissue evidence conspicuous.

PMID:28544275 PARTIAL Human Clinical

"mtDNA amount was reduced to 27% of the mean control value"

The entire muscle evidence base for this disorder is this one measurement, in one patient. Marked PARTIAL for that reason.

Is impaired mitochondrial fusion the primary consequence of losing MSTO1, or is it downstream of a cytosolic proteostasis defect, or of a direct effect on mtDNA replication through RAD51?

EMERGING HYPOTHESIS OPEN msto1-mechanism-ordering

Attached to

pathophysiology#Loss of Cytosolic MSTO1 Protein pathophysiology#Failure of TRiC Chaperonin Assembly pathophysiology#Unrestrained RAD51 Binding to Mitochondrial DNA

Three accounts of MSTO1's molecular function are now on the table and they make different predictions. The canonical fusion-first model, in place since 2017, is supported by rescue kinetics in patient fibroblasts and by analogy with MFN2 and OPA1, but has never demonstrated the connecting steps in MSTO1 cells and leaves the protein without a defined molecular activity. The 2026 TRiC model gives it one - chaperonin assembly factor - and its temporal-ordering experiment directly contradicts the canonical ordering, with TRiC falling at day two and mitochondria fragmenting only at day six; but it is an unreviewed preprint from one laboratory, and the TRiC substrate that connects proteostasis to the organelle is unidentified since tubulin and actin were excluded. The 2026 RAD51 model would explain mtDNA depletion without a fusion intermediate, but it is entirely in cancer cell systems. The three are not mutually exclusive and the entry models all three as separate hypothesis groups rather than picking one. Which is correct determines what a therapeutic target would even be: the fusion machinery, the chaperonin system, or mtDNA replication.

Proposed experiments

Independent replication of the TRiC assembly result in patient fibroblasts

msto1-tric-replication-in-patient-cells

Measure TRiC/CCT subunit levels and assembly intermediates in the established panel of MSTO1 patient fibroblast lines in a second laboratory, with peer review, before the proteostasis model is treated as more than a lead.

Identify the TRiC substrate that links proteostasis to mitochondrial morphology

msto1-tric-substrate-screen

Screen TRiC substrates for one whose depletion reproduces mitochondrial fragmentation, given that tubulin and actin were explicitly excluded, to supply the missing intermediate in the proteostasis model.

Test the MSTO1-RAD51 interaction in patient rather than cancer cells

msto1-rad51-in-patient-fibroblasts

Assay mitochondrial RAD51 occupancy on mtDNA in MSTO1 patient fibroblasts and ask whether inhibiting mitochondrial RAD51 rescues mtDNA content, which would establish whether the RAD51 route operates in the human disease.

Show evidence (2 references)

PMID:42539026 SUPPORT In Vitro

"Using rapid depletion of MSTO1, we found that mitochondrial fragmentation appears only after six days."

The kinetic observation that puts the canonical ordering in question.

PMID:31463572 SUPPORT In Vitro

"further suggests that impairments in mitochondrial dynamics are upstream of mtDNA depletion in MSTO1 patient fibroblasts"

The canonical ordering, stated by its proponents on the basis of rescue kinetics.

Why has no vertebrate animal model of MSTO1 deficiency been reported, and what does its absence prevent?

KNOWLEDGE GAP OPEN msto1-no-animal-model

Attached to

pathophysiology#Dystrophic Degeneration of Skeletal Muscle Fibres pathophysiology#Cerebellar Hypoplasia and Cortical Atrophy pathophysiology#Retinal Degeneration with Pigmentary Change

A literature search on 2026-08-01 across all 32 PubMed records for MSTO1 found no mouse, zebrafish or other vertebrate model of MSTO1 loss. The non-vertebrate precedents are informative but distant: null mutations of misato in Drosophila cause irregular chromosome segregation, and disruption of DML1 in Saccharomyces cerevisiae impairs mtDNA inheritance and mitochondrial segregation - and it is notable that the yeast phenotype is about mtDNA, which prefigures the human depletion finding by nearly two decades. But neither organism has a cerebellum, skeletal muscle of the mammalian type, or a neurosensory retina, so the three affected tissues in this disorder are unmodelled. The consequence is concrete: every tissue-level causal edge in this entry - from mtDNA depletion to muscle degeneration, to cerebellar hypoplasia, to retinal degeneration - is an inference from fibroblast biology plus clinical observation, with no in vivo corroboration in any species. There is also no preclinical platform on which any therapeutic idea could be tested.

Proposed experiments

Constitutive and conditional Msto1 knockout mouse

msto1-mouse-knockout

Generate a germline null and, if it is embryonic lethal, muscle-specific and Purkinje-cell-specific conditional knockouts, and phenotype muscle histology, cerebellar volume, retinal structure and mtDNA content in each tissue - the experiment that would test the tissue-level causal edges this entry currently infers.

Stable germline msto1 zebrafish mutant

msto1-zebrafish-mutant

Create a CRISPR germline mutant and assess mitochondrial network morphology in muscle, motor behaviour, cerebellar development and retinal structure, providing a tractable vertebrate system for modifier and compound screens.

The Drosophila misato and yeast DML1 results referred to in the rationale are described in the introductions of PMID:28554942 and PMID:31604776 as citations to primary work from 1997 and 2002 that is not itself cached here. They are therefore recorded in this rationale rather than quoted as evidence snippets, and no `animal_models:` block is asserted for this entry.

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Pathophysiology

14

Bi-allelic Loss-of-Function Variants in MSTO1

The primary lesion is bi-allelic damage to MSTO1 at 1q22, reported against transcript NM_018116.3. The allelic spectrum is broad and includes missense substitutions across the length of the protein, frameshift and nonsense alleles, a start-loss allele, splice-acceptor variants and at least one multi-exon intragenic deletion, and there is no recurrent founder allele. The functional consequence converges regardless of allele class: in every patient fibroblast line assayed to date the MSTO1 protein is absent or strongly reduced, so the disease behaves as a loss of protein rather than as a set of allele-specific effects. Heterozygous parents are unaffected, so one intact allele suffices.

MSTO1 hgnc:29678

Downstream

Loss of Cytosolic MSTO1 Protein Truncating, frameshift, start-loss and deletion alleles remove the coding capacity outright; the missense alleles tested behave as protein-destabilising rather than as separation-of-function changes, so both classes converge on absence of the protein.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants"

Establishes both the bi-allelic requirement and the breadth of the allelic spectrum.

PMID:31604776 SUPPORT Human Clinical

"deletion encompassing exons 7-14 of MSTO1"

Documents that one of the two alleles can be an intragenic copy-number deletion rather than a sequence variant, which is the diagnostic reason a negative exome does not exclude the disorder.

Loss of Cytosolic MSTO1 Protein

MSTO1 is a soluble cytosolic protein, not a mitochondrial matrix or inner-membrane resident, and this is the feature that makes the disorder mechanistically unusual among mitochondrial diseases. Its localisation was initially reported as the mitochondrial outer membrane, but two later groups working independently on patient material and on transfected cells placed it predominantly in the cytosol, interacting with the outer-membrane fusion machinery from the cytosolic face rather than residing in the organelle. The protein carries a region homologous to the tubulin/FtsZ GTPase superfamily, the same fold family used for chromosome segregation in eukaryotes and for organelle division in bacteria and algae, but no GTPase activity has been demonstrated for the human protein and its molecular activity remained undefined until 2026. In patient fibroblasts the protein is undetectable by western blot in every line examined in the largest series, and strongly reduced in the others, so this node is the convergence point of the whole allelic spectrum.

Fusion failure is the primary lesion and mtDNA depletion is downstream of it MSTO1 is a TRiC/CCT chaperonin assembly factor and the mitochondrial phenotype is secondary to a cytosolic proteostasis defect MSTO1 restrains RAD51 at the mitochondrial genome and its loss depletes mtDNA directly

patient-derived dermal fibroblast CL:0000057

MSTO1 hgnc:29678

cytosol GO:0005829 mitochondrial outer membrane GO:0005741

Downstream

Impaired Mitochondrial Fusion and Network Fragmentation The canonical route. Without MSTO1 at the cytosol-outer-membrane interface, fusion events fall and the network fragments; this is the oldest and most reproduced observation in the field.

Failure of TRiC Chaperonin Assembly The 2026 proteostasis route. On this model the proximate consequence of losing MSTO1 is not a mitochondrial event at all but failure to assemble the cytosolic chaperonin TRiC, with the mitochondrial phenotype following later.

Unrestrained RAD51 Binding to Mitochondrial DNA The 2026 genome-maintenance route. On this model MSTO1 restrains RAD51 at the mitochondrial genome, and losing it disrupts mtDNA replication directly rather than through the fusion machinery.

Show evidence (5 references)

PMID:31463572 SUPPORT In Vitro

"MSTO1 protein was not detectable in the cultured fibroblasts of all seven patients evaluated, suggesting that pathogenic variants result in a loss of protein expression and/or affect protein stability."

Demonstrates that the disease-causing genotypes converge on absence of the protein, in seven independent patient lines.

PMID:28544275 SUPPORT In Vitro

"We also found that MSTO1 is mainly a cytosolic protein."

Independent assignment of the protein to the cytosol rather than to the organelle.

PMID:28554942 SUPPORT In Vitro

"At variance with a previous report, we find MSTO1 to be localized in the cytoplasmic area with limited colocalization with mitochondria."

The explicit contradiction of the earlier outer-membrane assignment. Cited for its cell-biology result only; the patient-genotype claims of this paper were later corrected and are not used anywhere in this entry.

+ 2 more references

Impaired Mitochondrial Fusion and Network Fragmentation

Mitochondria are maintained as a dynamic, partly continuous reticulum by balanced fusion and fission. Fusion allows content mixing - complementation of damaged mtDNA and proteins between organelles - and its failure leaves a population of small, isolated, functionally isolated fragments. Every patient fibroblast line examined across four independent groups shows a fragmented mitochondrial network, and in one series the fragments coexist with abnormally enlarged structures. The effect is MSTO1-dependent in both directions: acute knockdown of MSTO1 in HeLa cells reproduces the fragmentation, over-expression of the protein drives the opposite phenotype of perinuclear mitochondrial aggregation, and re-expression of wild-type MSTO1 in patient cells restores the network. Loss of network continuity also degrades organelle distribution - the property for which the gene is named - which is expected to matter most in the long, highly polarised cells of muscle and the cerebellum.

Fusion failure is the primary lesion and mtDNA depletion is downstream of it

patient-derived dermal fibroblast CL:0000057

mitochondrial fusion GO:0008053 ↓ DECREASED mitochondrion distribution within the cell GO:0048312 ⚠ ABNORMAL

mitochondrion GO:0005739

Downstream

mtDNA Depletion and Nucleoid Reorganisation Reduced fusion is proposed to deplete mtDNA by failing to distribute the mtDNA replication machinery and by leaving nucleoids unsegregated, the same route established for the fusion GTPases MFN2 and OPA1.

Enlarged Lysosomal Vacuoles Patient fibroblasts show markedly enlarged lysosomal vacuoles alongside the fragmented network. The connecting steps are not established, hence the unknown-intermediates typing.

Show evidence (6 references)

PMID:31463572 SUPPORT In Vitro

"Consistent with impaired mitochondrial fusion, mitochondrial networks in fibroblasts were found to be fragmented."

Fragmentation confirmed across the seven patient lines of the largest cohort.

PMID:28544275 SUPPORT In Vitro

"Studies in patients' fibroblasts revealed that MSTO1 protein levels were strongly reduced, the mitochondrial network was fragmented, and the fusion events among mitochondria were decreased"

Couples the morphological readout to a direct measurement of fusion events, which is what makes this a fusion defect rather than merely an altered shape.

PMID:33612823 SUPPORT In Vitro

"we found that MSTO1 protein level was reduced and that mitochondrial network was fragmented or formed enlarged structures"

Independent replication in a homozygous genotype, with the additional observation of enlarged as well as fragmented structures.

+ 3 more references

Failure of TRiC Chaperonin Assembly

An alternative account of what MSTO1 actually does, published as a preprint in July 2026 and not yet peer reviewed. Using auxin-inducible degradation to strip MSTO1 from cells within hours, the authors found that TRiC (the chaperonin-containing T-complex, CCT), the essential cytosolic ATP-dependent chaperonin that folds actin, tubulin and a large set of other substrates, falls within two days, whereas mitochondrial fragmentation does not appear until day six. Early TRiC assembly subcomplexes accumulate when MSTO1 is absent, MSTO1 co-immunoprecipitates with TRiC, and TRiC is also reduced in MSTO1 patient fibroblasts. Critically, knocking down TRiC on its own is sufficient to fragment mitochondria, and does so without measurable changes in tubulin or actin. If this holds, the ordering of the whole pathograph changes: the mitochondrial phenotype becomes a downstream consequence of a cytosolic proteostasis defect rather than the primary lesion, and MSTO1 becomes a chaperonin assembly factor that happens to have been discovered through a mitochondrial phenotype.

MSTO1 is a TRiC/CCT chaperonin assembly factor and the mitochondrial phenotype is secondary to a cytosolic proteostasis defect

MSTO1 hgnc:29678

TRiC/CCT chaperonin GO:0005832

cytosol GO:0005829

Downstream

Impaired Mitochondrial Fusion and Network Fragmentation Depleting TRiC alone fragments mitochondria, and does so independently of changes in tubulin or actin, so the relevant TRiC substrate linking proteostasis to the organelle is not yet identified.

Show evidence (3 references)

PMID:42539026 SUPPORT In Vitro

"Surprisingly, prior to any changes in mitochondria, we show that MSTO1-depleted cells have a significant decrease in TRiC levels, an essential cytosolic ATP-dependent chaperone required to fold diverse substrates, including actin and tubulin."

The temporal-ordering result on which the whole alternative model rests, and the reason it is a challenge to the fusion-first account.

PMID:42539026 SUPPORT In Vitro

"We also observe accumulation of early TRiC assembly subcomplexes in the absence of MSTO1 suggesting that MSTO1 facilitates assembly of TRiC."

Assigns MSTO1 a specific molecular activity, chaperonin assembly, for the first time.

PMID:42539026 SUPPORT In Vitro

"Using rapid depletion of MSTO1, we found that mitochondrial fragmentation appears only after six days."

The kinetic gap between TRiC loss at two days and mitochondrial fragmentation at six days is the specific observation that makes the mitochondrial event look secondary.

Unrestrained RAD51 Binding to Mitochondrial DNA

A second 2026 account, this one placing MSTO1 directly at the mitochondrial genome. In cancer cell models MSTO1 interacts with the recombinase RAD51 through a conserved FxxA motif and restrains its activity specifically within mitochondria, without affecting RAD51's nuclear functions. When MSTO1 is lost, RAD51 binding to mtDNA increases, mtDNA replication is disrupted and mtDNA damage accumulates, membrane potential falls and mtDNA content drops. The same work reports that mtDNA then leaks to the cytosol through a BAX/BAK-dependent route and activates cGAS-STING signalling, which would give MSTO1 deficiency an innate-immune arm. This route would explain mtDNA depletion without requiring the fusion defect as an intermediate, and it is therefore modelled as a competing hypothesis rather than as an addition to the canonical chain.

MSTO1 restrains RAD51 at the mitochondrial genome and its loss depletes mtDNA directly

MSTO1 hgnc:29678

mitochondrial DNA replication GO:0006264 ↓ DECREASED

mitochondrial nucleoid GO:0042645

Downstream

mtDNA Depletion and Nucleoid Reorganisation On this model the depletion is a direct consequence of disordered mtDNA replication at the nucleoid, not a distribution problem inherited from the fusion defect.

Show evidence (3 references)

PMID:42098462 SUPPORT In Vitro

"This study identifies MSTO1 as a crucial regulator of mitochondrial genome integrity through direct interaction with RAD51."

States the proposed direct molecular link between MSTO1 and mitochondrial genome maintenance.

PMID:42098462 SUPPORT In Vitro

"MSTO1 deficiency enhances RAD51 binding to mitochondrial DNA (mtDNA), disrupting mtDNA replication and causing mtDNA damage."

The specific mechanism by which loss of MSTO1 would deplete mtDNA without passing through a fusion defect.

PMID:42098462 PARTIAL In Vitro

"Furthermore, MSTO1 deficiency triggers BAX/BAK-dependent mtDNA leakage, activating the cGAS-STING pathway and driving inflammatory responses."

A potentially important innate-immune consequence. Marked PARTIAL because it is shown only in cancer cell models and no inflammatory phenotype has been reported in any patient with MSTO1-related disease.

mtDNA Depletion and Nucleoid Reorganisation

The finding that reclassified this disorder. Mitochondrial DNA content is reduced in patient cells to roughly a third to two thirds of control, and the surviving genomes are redistributed: nucleoids become fewer and larger, and some mitochondrial fragments contain no genome at all. The depletion is reproducible across independent laboratories, appears in muscle as well as in fibroblasts, and is quantitatively in the same range as that seen in fibroblasts from patients with OPA1 or MFN2 disease and in the established mtDNA depletion syndromes, which is the basis for the proposal that MSTO1 deficiency be classified as an mtDNA depletion syndrome.

patient-derived dermal fibroblast CL:0000057 skeletal muscle fibre CL:0008002

mitochondrial DNA metabolic process GO:0032042 ↓ DECREASED

mitochondrial nucleoid GO:0042645

Downstream

Dystrophic Degeneration of Skeletal Muscle Fibres Skeletal muscle is the most affected tissue. The steps between a 30-70% fall in mtDNA and fibre degeneration have not been demonstrated in MSTO1 patient muscle, so the link is typed as having unknown intermediates.

Cerebellar Hypoplasia and Cortical Atrophy The cerebellar lesion is present from early childhood and often before age two, so at least part of it is developmental rather than degenerative; how mtDNA depletion produces a hypoplastic cerebellum is not known.

Corticospinal Tract Dysfunction Long descending motor axons are among the most energetically demanding structures in the nervous system and are a recurrent target in mitochondrial dynamics disorders, but no pathological material from an MSTO1 patient's spinal cord exists.

Retinal Degeneration with Pigmentary Change Photoreceptors and retinal pigment epithelium have among the highest oxidative demands of any tissue. The link is inconstant across cohorts, which is itself curated as a knowledge gap.

Dissociation Between mtDNA Depletion and Respiratory Chain Function The depletion is real but, in most patients studied, is not accompanied by the respiratory chain deficiency that mtDNA depletion syndromes usually produce. That dissociation is modelled explicitly rather than ignored.

Show evidence (8 references)

PMID:31463572 SUPPORT In Vitro

"all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids"

The quantitative anchor for the depletion claim, in seven independent patient lines.

PMID:31463572 SUPPORT In Vitro

"contained fewer nucleoids, which were larger in size compared to control lines"

Establishes that the genomes are not merely fewer but abnormally packaged and distributed.

PMID:31463572 SUPPORT In Vitro

"fragments were devoid of mitochondrial genomes in patient cells"

The distribution failure made concrete: some organelle fragments have no genome at all, which is the expected consequence of losing fusion-mediated content mixing.

+ 5 more references

Dissociation Between mtDNA Depletion and Respiratory Chain Function

A patient with MSTO1 disease usually does not look biochemically like a classical mitochondrial patient. Lactate is normal, respiratory chain complex activities in muscle homogenate are normal or near normal, and fibroblast respiration measured by micro-oxygraphy is normal, even though mtDNA is reduced and the network is fragmented. What is abnormal is citrate synthase activity, which is markedly low and indexes mitochondrial mass rather than mitochondrial function, so the coherent reading is that these cells have fewer, poorly distributed mitochondria carrying less mtDNA rather than mitochondria that respire badly. This matters diagnostically: a normal mitochondrial biochemical screen does not exclude the diagnosis, and it is a large part of why the disorder was reached by exome sequencing rather than by metabolic workup. The dissociation is not absolute - the 2026 adult-onset case did show reduced membrane potential and respiratory capacity - and that exception is carried here rather than hidden.

patient-derived dermal fibroblast CL:0000057 skeletal muscle fibre CL:0008002

Show evidence (4 references)

PMID:28544275 SUPPORT Human Clinical

"Our biochemical analyses were not indicative of a mitochondrial disorder: patients showed normal lactate levels, and hardly any defect of MRC complex activities or cell respiration"

The clearest statement that the standard mitochondrial biochemical screen is normal in this disorder.

PMID:28544275 SUPPORT Human Clinical

"However, we detected markedly low activity of citrate synthase and reduced mtDNA amount, suggesting a reduction in mitochondrial mass."

Reframes the abnormality as reduced mitochondrial mass rather than reduced per-mitochondrion function, which is the interpretation this node encodes.

PMID:28554942 SUPPORT In Vitro

"Short-term silencing of MSTO1 in HeLa cells reproduced the impairment of mitochondrial morphology and dynamics observed in the fibroblasts without damaging bioenergetics."

Acute MSTO1 depletion fragments the network without a bioenergetic penalty, which is the experimental counterpart of the clinical dissociation.

+ 1 more reference

Enlarged Lysosomal Vacuoles

Markedly enlarged lysosomal vacuoles were found across all MSTO1 patient fibroblast lines in the largest series, a finding not previously reported in the context of MSTO1 dysfunction. Mitochondrial and lysosomal compartments are functionally coupled, and lysosomal enlargement is a recurring accompaniment of mitochondrial dysfunction, but for MSTO1 the connecting steps are undetermined and the finding has not yet been replicated by another group. It is curated because it recovered on re-expression of wild-type MSTO1 on the same timescale as mitochondrial morphology, which argues it is a genuine consequence of losing the protein rather than a culture artefact.

patient-derived dermal fibroblast CL:0000057

lysosome GO:0005764

Show evidence (1 reference)

PMID:31463572 SUPPORT In Vitro

"Enlarged lysosomal vacuoles in MSTO1 patient fibroblasts"

The figure legend stating the finding directly; the result is described in the same paper as not previously reported for MSTO1.

Dystrophic Degeneration of Skeletal Muscle Fibres

Muscle is the tissue in which MSTO1 loss produces the most consistent structural damage. Biopsies show a dystrophic rather than a purely myopathic process: variation in fibre size, internalised nuclei, whorled fibres, and a mild degree of necrosis and regeneration, with electron microscopy in earlier reports showing increased subsarcolemmal and intermyofibrillar mitochondria with lipid and glycogen accumulation. Notably the classical oxidative histochemical stigmata of mitochondrial myopathy - ragged red fibres, cytochrome c oxidase negative fibres - were absent in the cases where they were sought, consistent with the biochemical dissociation node above. Ongoing fibre necrosis and regeneration is what produces the raised creatine kinase, which is the single most useful screening abnormality in this disorder.

skeletal muscle fibre CL:0008002

skeletal muscle tissue UBERON:0001134

Downstream

Proximal Weakness with Restrictive Respiratory Compromise Progressive loss of functional muscle fibres produces the clinical weakness and the restrictive ventilatory defect.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Muscle biopsies were performed in ten patients and were consistent with a dystrophic process"

Ten biopsies in one cohort establish the dystrophic character of the muscle pathology.

PMID:31463572 SUPPORT Human Clinical

"Serum creatine kinase (CK) levels were significantly increased in all patients, except for P15, ranging from 300 to 5000 U/L."

Quantifies the leak of muscle enzyme that accompanies the dystrophic process, in 14 of 15 patients.

Cerebellar Hypoplasia and Cortical Atrophy

The cerebellar lesion is the second constant of the disorder and is developmental at least in part. Brain MRI showed moderate to severe cerebellar atrophy or hypoplasia involving the vermis and both hemispheres in all twelve patients imaged in the largest cohort, four of whom were imaged before the age of two, and repeat imaging in four patients confirmed no progression. That pattern - present early, stable thereafter - is unusual among mitochondrial cerebellar syndromes and argues for a defect in cerebellar development rather than for ongoing neurodegeneration. It is not universal: an additional case with clinical and radiological evidence of progressive cerebellar involvement has been reported, and a 2026 patient presented with adult-onset progressive ataxia, so this entry records both courses. Purkinje cells are the obvious candidate substrate given their energetic demands and their sensitivity in other mitochondrial disorders, but no neuropathological material from an MSTO1 patient has been published.

cerebellum UBERON:0002037 cerebellar vermis UBERON:0004720

Downstream

Ataxia, Dysmetria and Dysarthria Loss of cerebellar cortical output produces the appendicular and truncal cerebellar signs and the cerebellar speech disturbance.

Show evidence (3 references)

PMID:31463572 SUPPORT Human Clinical

"involving the vermis and both hemispheres in all 12"

Establishes that the cerebellar lesion involves vermis and hemispheres and was present in every patient imaged.

PMID:31463572 SUPPORT Human Clinical

"Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."

States the early-onset, non-progressive character of the cerebellar atrophy for the cohort as a whole.

PMID:31130378 PARTIAL Human Clinical

"This case confirms a consistent MSTO1-related neuromuscular phenotype and in addition suggests a progressive neurological component at least in some patients, in keeping with the mitochondrial role of the defective protein."

The counter-observation to non-progression. Marked PARTIAL because it is a single case and its authors themselves frame it as applying to "some patients".

Corticospinal Tract Dysfunction

An upper motor neuron component sits alongside the myopathy and is easy to miss because weakness is usually attributed to the muscle disease. Increased tone, a spastic catch, clonus and brisk deep tendon reflexes were recorded in eight of fifteen patients in the largest cohort, and upper motor neuron signs are singled out in an independent report as part of the typical picture. Its presence is diagnostically useful: brisk reflexes in a child with a dystrophic myopathy and a high creatine kinase point away from a pure muscular dystrophy and towards a central component.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."

Names corticospinal tract dysfunction as one of the three core components of the phenotype.

PMID:33612823 SUPPORT Human Clinical

"often with upper motor neuron signs and varied cognitive impairment"

Independent confirmation that upper motor neuron signs are a recurrent feature rather than a single-cohort observation.

Retinal Degeneration with Pigmentary Change

The retinal component is the feature that names the MONDO and Orphanet terms and simultaneously the least consistent part of the phenotype. Where it is present it is a genuine retinal dystrophy: widespread salt-and-pepper pigmentation, macular atrophy, mottled hypoautofluorescence and loss of the ellipsoid zone on multimodal imaging in a Chinese sibling pair followed for seventeen years, and pigmentary retinopathy with pallor of the disc and concentric visual field constriction, but preserved visual acuity, in the index patient of the first recessive series (PMID:28544275). Both probands of the report that gave the syndrome its MONDO name (PMID:29339779) had pigmentary retinopathy. Yet it was reported in none of the fifteen patients of the largest cohort, where formal ophthalmological examination was not performed in every patient - so the true denominator is unknown and the discrepancy may be ascertainment rather than biology.

retinal pigment epithelial cell CL:0002586 photoreceptor cell CL:0000210

retina UBERON:0000966

Show evidence (4 references)

PMID:29339779 SUPPORT Human Clinical

"We performed whole-exome sequencing in two unrelated patients showing cerebellar atrophy, intellectual disability, and pigmentary retinopathy."

The report that established pigmentary retinopathy as part of the MSTO1 phenotype and gave the MONDO term its name.

PMID:40542878 SUPPORT Human Clinical

"Ophthalmic multimodal imaging examination revealed typical characteristics of retinal dystrophy in both patients, including binocular widespread salt-and-pepper pigmentation, macular atrophy, a mottled pattern of hypoautofluorescence, and degeneration of the ellipsoid zone."

The most detailed retinal characterisation available, and the source of the cell-layer inference recorded in this node's notes.

PMID:28544275 SUPPORT Human Clinical

"Ophthalmological examination revealed the presence of pigmentary retinopathy with papillary pallor, associated with concentric restriction of the visual field but no abnormality of visual acuity."

Shows the functional signature - field constriction with preserved acuity - that distinguishes this rod-predominant retinopathy from a maculopathy or an optic neuropathy.

+ 1 more reference

Proximal Weakness with Restrictive Respiratory Compromise

The organism-level expression of the muscle disease. Presentation is with hypotonia and delayed motor milestones between birth and three years, followed by independent but waddling, wide-based ambulation and predominantly proximal weakness that then remains relatively stable. Loss of respiratory muscle strength shows up as a reduced forced vital capacity before it produces symptoms, which is the argument for scheduled pulmonary function testing rather than symptom-triggered testing. A later-onset arm of the spectrum exists: a 2026 series reported diagnosis between 13 and 30 years, and a separate 2026 case had adult-onset disease.

Show evidence (3 references)

PMID:31463572 SUPPORT Human Clinical

"Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."

Establishes the age window and the presenting features of the muscle disease.

PMID:31463572 SUPPORT Human Clinical

"All 15 patients reported either relatively slow progression or no progression of their muscle weakness."

The natural-history claim that distinguishes this from the rapidly progressive congenital muscular dystrophies.

PMID:31463572 SUPPORT Human Clinical

"revealed reduced forced vital capacity (FVC) measurements, ranging from 60 to 79% predicted"

Quantifies subclinical restrictive lung physiology in nine tested patients, which is the basis for routine spirometry.

Ataxia, Dysmetria and Dysarthria

The organism-level expression of the cerebellar lesion. In the largest cohort dysmetria was recorded in 11 of 15, gait ataxia in 9 of 15 and abnormal speech in 12 of 15, the last comprising speech delay in five and dysarthria in eight. Because the cerebellar atrophy is usually stable, these signs typically do not worsen substantially over time, and the functional trajectory of most patients is dominated by the muscle disease rather than by the cerebellar disease.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."

The per-sign denominators for the cerebellar phenotype in the only cohort large enough to provide them.

✶

Histopathology

1

Dystrophic muscle with increased variability in fibre diameter

Muscle biopsies in ten patients of the largest cohort were consistent with a dystrophic process, with variation in fibre type size, mild necrosis and regeneration, internalised nuclei and whorled fibres. The oxidative histochemical hallmarks of mitochondrial myopathy - ragged red fibres and cytochrome c oxidase negative fibres - were not present where they were sought, and the earlier 2017 recessive series explicitly recorded no oxidative histochemical defect in COX or SDH. A pathologist reading this biopsy without genetic information would report a congenital muscular dystrophy, not a mitochondrial myopathy.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"Muscle biopsies were performed in ten patients and were consistent with a dystrophic process"

The histological conclusion from ten biopsies.

⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

●

Phenotypes

19

Digestive 1

Dysphagia Dysphagia HP:0002015

Reported in a single patient. No frequency band is asserted, and the feature is retained because its clinical consequence (aspiration risk) is high even at low frequency.

Show evidence (1 reference)

PMID:31604776 SUPPORT Human Clinical

"Additionally, our patient presents with dysphagia and restrictive lung disease, not previously reported for MSTO1-associated disorders."

The single report of dysphagia in this disorder, with the authors' own note that it was novel.

Eye 1

Pigmentary retinopathy Pigmentary retinopathy HP:0000580

No frequency band is asserted and the reason is a genuine conflict rather than missing data. Present in both probands of PMID:29339779, in patient A1 of PMID:28544275 and in both sisters of PMID:40542878; absent in 0 of 15 of PMID:31463572, where the authors note that formal ophthalmological examination was not performed in every patient. Pooling those numerators across cohorts that used different ophthalmological protocols would manufacture a precision the literature does not support. See the `msto1-retinopathy-denominator` discussion.

Show evidence (3 references)

PMID:29339779 SUPPORT Human Clinical

"We performed whole-exome sequencing in two unrelated patients showing cerebellar atrophy, intellectual disability, and pigmentary retinopathy."

Two unrelated probands with pigmentary retinopathy, the observation that put the feature into the disease name.

PMID:40542878 SUPPORT Human Clinical

"Our study provides a comprehensive ophthalmic characterization of MMYAT, indicating that retinal dystrophy is a key characteristic of this disease."

The strongest positive statement in the literature, based on 17 years of multimodal imaging follow-up in two sisters.

PMID:31463572 REFUTE Human Clinical

"major cognitive involvement, seizures, retinopathy, optic atrophy or hearing loss were not seen"

Explicit absence of retinopathy in the 15-patient cohort. Retained as REFUTE evidence against a universal claim; it does not refute the existence of the feature in other patients.

Metabolism 1

Elevated creatine kinase VERY_FREQUENT Elevated circulating creatine kinase concentration HP:0003236

Frequency band derived directly from the quoted count of 14 of 15 patients (93%) in PMID:31463572, which falls in the VERY_FREQUENT (80-99%) band.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Serum creatine kinase (CK) levels were significantly increased in all patients, except for P15, ranging from 300 to 5000 U/L."

Both the numerator/denominator for the frequency band and the reported concentration range.

PMID:31604776 SUPPORT Human Clinical

"Testing revealed elevated plasma creatine kinase, and electromyogram results were consistent with longstanding generalized myopathy."

Independent confirmation in a separately reported adult patient.

Musculoskeletal 5

Muscle weakness VERY_FREQUENT Proximal muscle weakness HP:0003701

Course: STABLE

Frequency band derived from an explicit count: at last examination all 15 patients in PMID:31463572 had predominantly proximal weakness, and all four patients in PMID:41653646 exhibited myopathy of variable severity. VERY_FREQUENT (80-99%) rather than OBLIGATE because the total published denominator is around 30 and no systematic ascertainment has been done.

Show evidence (3 references)

PMID:31463572 SUPPORT Human Clinical

"Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."

Documents the presenting features and the age window.

PMID:31463572 SUPPORT Human Clinical

"All 15 patients reported either relatively slow progression or no progression of their muscle weakness."

Supports both the near-universal frequency and the STABLE clinical course qualifier.

PMID:41653646 SUPPORT Human Clinical

"All patients exhibited myopathy of variable severity."

Independent 2026 replication of the near-universal myopathy in a separate four-patient series.

Muscular dystrophy Muscular dystrophy HP:0003560

No frequency band is asserted. Muscle biopsy was performed in 10 of 15 patients in PMID:31463572 and all were dystrophic, but a 10/15 denominator reflects who was biopsied rather than who has the pathology, and biopsy in the era of first-line genome sequencing is increasingly selective.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Muscle biopsies were performed in ten patients and were consistent with a dystrophic process"

Establishes the dystrophic character of the muscle pathology in the largest series.

PMID:31130378 SUPPORT Human Clinical

"Recessive mutations in the MSTO1 gene, encoding for a mitochondrial distribution and morphology regulator, have been recently described in a very limited number of patients with multisystem involvement, mostly characterized by myopathy or dystrophy, cerebellar ataxia, pigmentary retinopathy and..."

Summarises the histological spectrum from myopathic to frankly dystrophic across reported patients.

Generalized hypotonia Generalized hypotonia HP:0001290

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."

Hypotonia named as a typical presenting feature.

PMID:41653646 SUPPORT Human Clinical

"Two had congenital hypotonia and global developmental delay, while the remaining two developed muscle weakness at ages 2 and 5."

Documents the split between congenital and later-onset presentations within a single small series.

Scoliosis Scoliosis HP:0002650

Show evidence (3 references)

PMID:41653646 SUPPORT Human Clinical

"The most common non-neurologic abnormality noted in our cases was skeletal abnormalities."

Identifies skeletal abnormality as the leading extra-neurological finding in the most recent series.

PMID:31604776 SUPPORT Human Clinical

"His phenotype included muscle weakness, hypotonia, early motor developmental delay, pectus excavatum, and scoliosis."

Scoliosis with pectus excavatum in an independently reported adult patient.

PMID:28544275 SUPPORT Human Clinical

"The patient showed severe asymmetry of the chest, with enlargement of the right hemithorax, pectus excavatum, and marked scoliosis"

The most severe reported thoracic deformity, showing how far the axial phenotype can go.

Pectus excavatum Pectus excavatum HP:0000767

Show evidence (2 references)

PMID:31604776 SUPPORT Human Clinical

"His phenotype included muscle weakness, hypotonia, early motor developmental delay, pectus excavatum, and scoliosis."

Direct report of pectus excavatum in an MSTO1 patient.

PMID:28544275 SUPPORT Human Clinical

"The patient showed severe asymmetry of the chest, with enlargement of the right hemithorax, pectus excavatum, and marked scoliosis"

Independent replication with a description of the associated chest asymmetry.

Nervous System 9

Cerebellar atrophy VERY_FREQUENT Cerebellar atrophy HP:0001272

Frequency band derived from an explicit count: 12 of 12 patients imaged in PMID:31463572. VERY_FREQUENT rather than OBLIGATE because only 12 of the 15 patients in that cohort had imaging available, and because PMID:41653646 reported MRI evidence of cerebellar atrophy in only one of four patients - a discrepancy that is itself notable and that argues against treating the cerebellar lesion as obligate.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"involving the vermis and both hemispheres in all 12"

The 12/12 denominator and the anatomical distribution.

PMID:41653646 PARTIAL Human Clinical

"Magnetic resonance imaging evidence of cerebellar atrophy was noted in Patient 3."

Marked PARTIAL because in this four-patient 2026 series cerebellar atrophy was noted in only one patient, which is the counterweight to the 12/12 figure and the reason the band is not OBLIGATE.

Ataxia FREQUENT Gait ataxia HP:0002066

Frequency band derived from the explicit count of gait ataxia in 9 of 15 patients (60%) in PMID:31463572, which falls in the FREQUENT (30-79%) band. Dysmetria at 11 of 15 is curated as a separate phenotype below.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."

Provides the numerator and denominator for the frequency band.

PMID:36468072 SUPPORT Human Clinical

"Here, we reported two siblings with progressive cerebellar atrophy and ataxia in a Chinese family."

Independent replication, and one of the reports describing a progressive rather than a static cerebellar course.

Dysarthria FREQUENT Dysarthria HP:0001260

Frequency band derived from the explicit count of 8 of 15 (53%) in PMID:31463572, in the FREQUENT (30-79%) band. The broader category of any abnormal speech (12 of 15) is not used for the band because it aggregates dysarthria with developmental speech delay, which is a different phenotype and is curated separately below.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."

The count of 8 of 15 for dysarthria specifically.

Delayed speech and language development FREQUENT Delayed speech and language development HP:0000750

Frequency band derived from the explicit count of 5 of 15 (33%) in PMID:31463572, which just enters the FREQUENT (30-79%) band. This is a borderline assignment and a single reclassified patient would move it to OCCASIONAL.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."

The count of 5 of 15 for speech delay.

PMID:36035138 SUPPORT Human Clinical

"The clinical features are infancy-onset mental and motor retardation, language disorder, dysarthria, scoliosis, cerebellar atrophy, tremor, lower-extremity muscle weakness, elevated muscle enzymes, extensive myopathy with chronic atrophy, hyperventilation lungs, and previously unreported hairy..."

Independent case documenting language disorder alongside dysarthria in the same patient.

Motor delay FREQUENT Motor delay HP:0001270

Frequency band derived from the statement that all but five of 15 patients (10 of 15, 67%) had delayed motor milestones in PMID:31463572, which falls in the FREQUENT (30-79%) band.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."

Establishes delayed motor milestones as the typical presentation and gives the age window.

PMID:31604776 SUPPORT Human Clinical

"His phenotype included muscle weakness, hypotonia, early motor developmental delay, pectus excavatum, and scoliosis."

Independent replication of hypotonia with early motor delay in a separately reported patient.

Hyperreflexia FREQUENT Hyperreflexia HP:0001347

Frequency band derived from the count of corticospinal tract manifestations in 8 of 15 patients (53%) in PMID:31463572, in the FREQUENT (30-79%) band. The source counts increased tone, spastic catch, clonus and brisk reflexes together, so the band applies to the corticospinal syndrome as a whole rather than to hyperreflexia in isolation.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."

Corticospinal tract dysfunction named as a core component of the phenotype.

PMID:33612823 SUPPORT Human Clinical

"often with upper motor neuron signs and varied cognitive impairment"

Independent statement that upper motor neuron signs are a recurring feature.

Mild intellectual disability FREQUENT Mild intellectual disability HP:0001256

Frequency band derived from the count of learning difficulties in 9 of 15 patients (60%) in PMID:31463572, in the FREQUENT (30-79%) band. The severity qualifier "mild" comes from the same source's statement that no patient had major cognitive involvement, which is quoted below.

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"major cognitive involvement, seizures, retinopathy, optic atrophy or hearing loss were not seen"

Sets the ceiling on cognitive impairment in this disorder, which is what justifies the "mild" HPO term rather than a generic intellectual disability term.

PMID:30684668 SUPPORT Human Clinical

"Here we have performed a whole-exome sequencing in a family which includes two brothers showing cerebellar atrophy and ataxia, intellectual disability, and myopathy."

Independent replication of intellectual disability as part of the triad in two affected brothers.

Tremor Tremor HP:0001337

No frequency band is asserted: tremor is reported in individual cases and is not among the signs counted in PMID:31463572, so there is no denominator to band.

Show evidence (1 reference)

PMID:36035138 SUPPORT Human Clinical

"The clinical features are infancy-onset mental and motor retardation, language disorder, dysarthria, scoliosis, cerebellar atrophy, tremor, lower-extremity muscle weakness, elevated muscle enzymes, extensive myopathy with chronic atrophy, hyperventilation lungs, and previously unreported hairy..."

Tremor listed as part of the clinical picture in an independently reported patient.

Absence of seizures, cataract, deafness and cardiac involvement Seizure HP:0001250

∅ ABSENT

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"None of the patients had a history of seizures, cataracts, hearing or cardiac involvement."

The explicit negative-findings statement on which this entry rests.

Respiratory 1

Restrictive ventilatory defect Restrictive ventilatory defect HP:0002091

Show evidence (2 references)

PMID:31463572 SUPPORT Human Clinical

"revealed reduced forced vital capacity (FVC) measurements, ranging from 60 to 79% predicted"

Quantifies the restrictive defect in the nine patients who underwent pulmonary function testing.

PMID:31604776 SUPPORT Human Clinical

"Additionally, our patient presents with dysphagia and restrictive lung disease, not previously reported for MSTO1-associated disorders."

Independent confirmation of restrictive lung disease, reported as novel at the time.

Other 1

Dysmetria FREQUENT Dysmetria HP:0001310

Frequency band derived from the explicit count of 11 of 15 (73%) in PMID:31463572, in the FREQUENT (30-79%) band.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."

The count of 11 of 15 for dysmetria.

🧬

Genetic Associations

1

MSTO1 (Bi-allelic loss-of-function variants; no recurrent founder allele; a single reported dominant family was subsequently shown to carry its variant in the MSTO2P pseudogene)

Gene: MSTO1 hgnc:29678 relationship_type: CAUSATIVE

Show evidence (4 references)

PMID:31463572 SUPPORT Human Clinical

"We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants"

The largest single body of gene-disease evidence - twelve independent families with bi-allelic variants and matching functional data.

PMID:37431815 SUPPORT Human Clinical

"but also confirm finding of Gerber et al that the mutation is in MSTO2p pseudogene"

The original authors' own confirmation that the variant underlying the reported dominant family lies in MSTO2P and not in MSTO1. This is the single most important genetic correction for this disease and the reason no dominant mode is asserted.

PMID:31604776 SUPPORT Human Clinical

"The majority of patients with disease-associated variants in MSTO1 present with biallelic variants suggesting autosomal recessive inheritance; however, one family has been reported with a single variant and presumed autosomal dominant inheritance."

Documents the pre-2023 balance of evidence, in which a single family stood against all others, and identifies precisely which claim was later overturned.

+ 1 more reference

💊

Medical Actions

5

Multidisciplinary supportive care

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

No disease-modifying therapy exists and none is in trial. Management follows the components of the phenotype: physiotherapy and mobility aids for the proximal weakness and ataxia, scheduled spirometry with non-invasive ventilation if vital capacity falls, orthopaedic surveillance and intervention for scoliosis and chest deformity, speech and language therapy for dysarthria and speech delay, educational support for learning difficulty, and low-vision services where retinopathy is present. Routine cardiac surveillance has no established indication in this disorder, which distinguishes management from that of most muscular dystrophies.

Physiotherapy and mobility support

Action: physical therapy Ontology label: Physical Therapy NCIT:C15302

Directed at the proximal weakness, the waddling wide-based gait and the cerebellar incoordination. Because weakness is slowly progressive or static in most patients, maintenance of function over decades is a realistic goal rather than a holding measure.

Show evidence (1 reference)

PMID:31463572 PARTIAL Human Clinical

"All 15 patients reported either relatively slow progression or no progression of their muscle weakness."

Marked PARTIAL: this supports the rationale for long-term functional maintenance but is not evidence that physiotherapy alters outcome in this disorder, which has never been tested.

Respiratory surveillance and non-invasive ventilation

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

Forced vital capacity was reduced to 60-79% of predicted in every patient tested in the largest cohort, and these measurements came from screening rather than from symptoms. Scheduled spirometry, with sleep studies and non-invasive ventilation if values decline, is therefore the rational approach; chest wall deformity from scoliosis and pectus excavatum compounds the diaphragmatic weakness.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"revealed reduced forced vital capacity (FVC) measurements, ranging from 60 to 79% predicted"

Establishes that clinically silent restrictive physiology is present, which is the justification for scheduled rather than symptom-triggered testing.

Orthopaedic management of scoliosis and chest deformity

Action: orthopedic surgical procedure Ontology label: Orthopedic Surgical Procedure NCIT:C16186

Spinal deformity is the most common non-neurological problem and can be severe, including marked scoliosis with hemithorax asymmetry. Management follows standard neuromuscular scoliosis practice, with bracing and surgical correction considered on curve progression and respiratory grounds.

Show evidence (1 reference)

PMID:28544275 SUPPORT Human Clinical

"The patient showed severe asymmetry of the chest, with enlargement of the right hemithorax, pectus excavatum, and marked scoliosis"

Documents the severity that the orthopaedic component of management has to address.

Genetic counselling

Action: genetic counseling Ontology label: Genetic Counseling NCIT:C15240

Counselling is straightforward once the genotype is established - autosomal recessive, with a 25% recurrence risk for siblings and unaffected carrier parents - but it depends on the two variants having been confirmed in trans, and on the reported variants not being pseudogene artefacts. The recessive model should be stated without the historical dominant caveat, which no longer applies.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"Since haploinsufficiency, as is present in some of the heterozygous parents in our cohort, does not cause a clinical phenotype"

The evidence that carrier parents are unaffected, which is the substance of the counselling message.

🔬

Biochemical Markers

3

Normal blood lactate and normal respiratory chain enzyme activities (Normal)

Show evidence (2 references)

PMID:28544275 SUPPORT Human Clinical

"Our biochemical analyses were not indicative of a mitochondrial disorder: patients showed normal lactate levels, and hardly any defect of MRC complex activities or cell respiration"

The direct statement that the mitochondrial biochemical workup is uninformative in this disorder.

PMID:28544275 SUPPORT Human Clinical

"As for other mutations affecting genes involved in mitochondrial dynamics, no biochemical defects typical of mitochondrial disorders were reported."

Places the negative biochemistry in context as a property of mitochondrial-dynamics disorders generally, not an anomaly of this cohort.

Reduced citrate synthase activity in muscle (Decreased)

Show evidence (1 reference)

PMID:28544275 SUPPORT Human Clinical

"However, we detected markedly low activity of citrate synthase and reduced mtDNA amount, suggesting a reduction in mitochondrial mass."

The finding and the authors' interpretation of it as reduced mitochondrial mass.

Reduced mitochondrial DNA copy number (Decreased)

Show evidence (2 references)

PMID:31463572 SUPPORT In Vitro

"We observed a significant reduction in mtDNA copy number across all fibroblast lines, ranging from 30 to 70% depletion"

The quantitative fibroblast result across all seven patient lines.

PMID:28544275 SUPPORT Human Clinical

"mtDNA amount was reduced to 27% of the mean control value"

The only muscle measurement, in the index patient of the 2017 recessive series.

🔀

Differential Diagnoses

5

Conditions with similar clinical presentations that must be differentiated from MSTO1-Related Mitochondrial Myopathy:

Marinesco-Sjogren syndrome Not Yet Curated MONDO:0009567

Overlapping Features The closest clinical mimic and the one most likely to be considered first. Marinesco-Sjogren syndrome, caused by bi-allelic SIL1 variants, pairs a myopathy with raised creatine kinase and cerebellar atrophy with ataxia, which is precisely the MSTO1 core. The index patient of the first recessive MSTO1 family had SIL1 sequenced and excluded before the MSTO1 diagnosis was reached, which is direct documentation that this differential is worked through in practice.

Distinguishing Features

Early-onset bilateral cataract is a cardinal feature of Marinesco-Sjogren syndrome and was absent in all 15 patients of the largest MSTO1 cohort.
Hypergonadotropic hypogonadism and short stature are typical of Marinesco-Sjogren syndrome and are not established features of MSTO1 disease.
Marinesco-Sjogren muscle shows rimmed autophagic vacuoles with abnormal membranous whorls, whereas MSTO1 muscle shows a non-specific dystrophic pattern.
Only the causal gene distinguishes the two definitively; SIL1 was verified as HGNC:24624 and Marinesco-Sjogren syndrome as MONDO:0009567 with OMIM:248800 using OAK and the HGNC REST API on 2026-08-01.

Show evidence (2 references)

PMID:28544275 SUPPORT Human Clinical

"The following genes were previously excluded by Sanger sequencing: SPG7, FKTN, POMT1, POMT2, LARGE, POMGNT1, and SIL1."

Documents that SIL1 - and the dystroglycanopathy genes - were actively excluded in an MSTO1 patient, which is what makes this a real rather than a theoretical differential.

PMID:31463572 SUPPORT Human Clinical

"None of the patients had a history of seizures, cataracts, hearing or cardiac involvement."

The absence of cataract in every MSTO1 patient is the single most useful clinical discriminator from Marinesco-Sjogren syndrome.

Dystroglycanopathy-type congenital muscular dystrophy

Overlapping Features A child with congenital or early-childhood dystrophic weakness, a raised creatine kinase and cerebellar abnormality on MRI fits the dystroglycanopathy phenotype closely, and these genes are on every congenital muscular dystrophy panel while MSTO1 historically was not. The first recessive MSTO1 family had FKTN, POMT1, POMT2, LARGE and POMGNT1 sequenced and excluded before MSTO1 was found, so this is the differential that actually delays diagnosis.

Distinguishing Features

Dystroglycanopathies typically show cobblestone lissencephaly, brainstem and cerebellar dysplasia with cysts, and ocular malformation; MSTO1 disease shows isolated cerebellar volume loss with a structurally normal cortex.
Immunohistochemistry for glycosylated alpha-dystroglycan is reduced in dystroglycanopathies and is not reported as abnormal in MSTO1 muscle.
Corticospinal tract signs are a recurrent feature of MSTO1 disease and are not characteristic of the dystroglycanopathies.
No MONDO term is bound for this differential because it names a genetically heterogeneous group rather than a single ontology entity; the genes are named instead, quoted from the source below.

Show evidence (1 reference)

PMID:28544275 SUPPORT Human Clinical

"The following genes were previously excluded by Sanger sequencing: SPG7, FKTN, POMT1, POMT2, LARGE, POMGNT1, and SIL1."

The dystroglycanopathy genes named as having been excluded in a patient who turned out to have MSTO1 disease.

Mitochondrial DNA depletion syndrome, myopathic form Not Yet Curated MONDO:0012301

Overlapping Features The nosological neighbour. TK2-related myopathic mtDNA depletion syndrome is the disorder MSTO1 disease most resembles at the level of laboratory mechanism - both present with a myopathy and reduced mtDNA copy number - and the largest MSTO1 cohort argued explicitly that MSTO1 deficiency should be classified as an mtDNA depletion syndrome. The distinction is therefore genetic and clinical rather than biochemical.

Distinguishing Features

TK2 disease is a defect of the mitochondrial nucleoside salvage pathway (TK2 verified as HGNC:11831 via the HGNC REST API on 2026-08-01), whereas MSTO1 encodes a cytosolic protein acting on organelle dynamics.
TK2 myopathy is typically relentlessly progressive with respiratory failure, whereas MSTO1 weakness is slowly progressive or static in every patient of the largest cohort.
Cerebellar atrophy with ataxia and corticospinal signs is core to MSTO1 disease and is not a feature of the myopathic form of TK2 disease.
Ragged red and COX-negative fibres with respiratory chain deficiency are usual in TK2 muscle; MSTO1 muscle shows a dystrophic pattern with normal oxidative histochemistry and normal respiratory chain activities.
MONDO:0012301 was verified with OAK on 2026-08-01, returning OMIM:609560 and RO:0004003 HGNC:11831 TK2.

Show evidence (1 reference)

PMID:31463572 SUPPORT In Vitro

"this novel link to mtDNA depletion in patient fibroblasts suggests that MSTO1-deficiency should also be considered a mtDNA depletion syndrome"

Establishes that the two disorders belong to the same mechanistic class, which is what makes the clinical distinctions above necessary.

Kearns-Sayre syndrome MONDO:0010787

Overlapping Features The differential that the pigmentary retinopathy generates. A patient with pigmentary retinopathy, ataxia and a myopathy will be investigated for Kearns-Sayre syndrome, and the single-large-scale mtDNA deletion syndromes are a standard part of that workup. The two are distinguished by the remainder of the Kearns-Sayre triad and by the genetic mechanism.

Distinguishing Features

Kearns-Sayre syndrome requires chronic progressive external ophthalmoplegia with onset before age 20; ophthalmoplegia and ptosis are not features of MSTO1 disease.
Cardiac conduction block is a defining and life-limiting component of Kearns-Sayre syndrome; cardiac involvement was absent in all 11 MSTO1 patients who had echocardiography.
Kearns-Sayre syndrome is caused by a single large-scale mtDNA deletion, usually sporadic, whereas MSTO1 disease is nuclear and autosomal recessive with an inheritance risk to siblings.
Ragged red fibres and COX-negative fibres are typical in Kearns-Sayre muscle and absent in MSTO1 muscle.
MONDO:0010787 was verified with OAK on 2026-08-01 (OMIM:530000) and matches the disease_term already bound in kb/disorders/Kearns-Sayre_Syndrome.yaml.

Show evidence (1 reference)

PMID:31463572 SUPPORT Human Clinical

"None of the patients had a history of seizures, cataracts, hearing or cardiac involvement."

Absence of cardiac and audiological involvement in MSTO1 disease is the practical discriminator from Kearns-Sayre syndrome.

Charcot-Marie-Tooth disease type 2A2 Not Yet Curated MONDO:0012231

Overlapping Features Included as a mechanistic rather than a clinical differential. CMT2A2 is caused by variants in MFN2, the outer-membrane mitofusin that executes the fusion reaction MSTO1 is thought to support, and MFN2 disease is one of the two fusion disorders in which mtDNA depletion was first demonstrated. The pairing establishes that lesions at adjacent points of one fusion pathway both produce neurological disease with mtDNA depletion, which is the argument for treating fusion-related mtDNA depletion as a coherent group.

Distinguishing Features

CMT2A2 is a length-dependent axonal neuropathy with distal weakness, sensory loss, areflexia and foot deformity; MSTO1 disease is proximal, myopathic and often hyperreflexic.
CMT2A2 is usually autosomal dominant, whereas MSTO1 disease is recessive with unaffected carrier parents.
Creatine kinase is normal or minimally raised in CMT2A2 and substantially raised in most MSTO1 patients.
MFN2 acts at the mitochondrial outer membrane as a GTPase; MSTO1 is cytosolic and no GTPase activity has been demonstrated for it.
MONDO:0012231 was verified with OAK on 2026-08-01, returning OMIM:609260 and RO:0004003 HGNC:16877 MFN2. Note that MONDO:0007308 (CMT2A1) is a distinct term associated with KIF1B and is deliberately NOT the term bound here.

{ }

Source YAML

click to show

name: MSTO1-Related Mitochondrial Myopathy
creation_date: "2026-08-01T00:00:00Z"
category: Mendelian
disease_term:
  preferred_term: mitochondrial myopathy-cerebellar ataxia-pigmentary retinopathy syndrome
  term:
    id: MONDO:0044714
    label: mitochondrial myopathy-cerebellar ataxia-pigmentary retinopathy syndrome
description: >-
  Mitochondrial myopathy and ataxia (MMYAT) is an ultra-rare disorder caused by
  bi-allelic loss-of-function variants in MSTO1, the gene encoding misato
  homolog 1. Unusually for a mitochondrial disease gene, MSTO1 is not a
  mitochondrial resident enzyme: it is a soluble cytosolic protein, distantly
  related to the tubulin/FtsZ GTPase superfamily, that acts at the interface
  between the cytosol and the mitochondrial outer membrane to support
  mitochondrial fusion, network continuity and organelle distribution. Loss of
  the protein produces a stereotyped cellular triad in patient fibroblasts -
  absent or strongly reduced MSTO1, a fragmented mitochondrial network, and
  depletion of mitochondrial DNA with reorganisation of mtDNA nucleoids - which
  is why MSTO1 deficiency has been proposed as a mitochondrial DNA depletion
  syndrome even though the disease was first described as a disorder of
  mitochondrial dynamics.

  Clinically the core is neuromuscular rather than encephalopathic: early-onset,
  largely non-progressive proximal weakness with dystrophic muscle histology and
  a raised creatine kinase, cerebellar atrophy or hypoplasia that is present
  from early childhood and manifests as dysmetria, gait ataxia and dysarthria,
  and corticospinal tract signs. Skeletal features (scoliosis, pectus
  excavatum), restrictive lung physiology and learning difficulties without
  major cognitive impairment are common. The pigmentary retinopathy that names
  the MONDO and Orphanet terms is a genuine but inconstant feature: it was
  central to the two probands whose report gave the syndrome its name,
  and a Chinese sibling pair followed for seventeen years shows a well
  characterised retinal dystrophy, yet it was reported in none of the fifteen
  patients of the largest published cohort, in which formal ophthalmological
  examination was not universally performed. This entry therefore treats
  retinopathy as an important but non-obligate component rather than as a
  defining criterion.

  Two things about this entity are contested and are curated as such rather than
  smoothed over. First, inheritance: the single family that established a
  dominant mode was subsequently shown to carry its variant in the MSTO2P
  pseudogene rather than in MSTO1, and every other reported family is
  bi-allelic, so autosomal recessive is the only mode this entry asserts.
  Second, mechanism: whether mtDNA depletion is downstream of failed fusion,
  downstream of a direct MSTO1-RAD51 interaction at the mitochondrial genome, or
  downstream of a cytosolic proteostasis defect (MSTO1 as an assembly factor for
  the TRiC/CCT chaperonin) is unresolved, and all three routes are modelled
  explicitly as competing hypothesis groups.
parents:
- Mitochondrial Disease
synonyms:
- MMYAT
- mitochondrial myopathy and ataxia
- mitochondrial myopathy-cerebellar atrophy-pigmentary retinopathy syndrome
- MSTO1-related mitochondrial myopathy and ataxia syndrome
- MSTO1-related myopathy
- MSTO1-RD
- misato homolog 1 deficiency
classifications:
  harrisons_chapter:
  - classification_value: NEUROLOGIC
    notes: >-
      The morbidity of this disorder is almost entirely neuromuscular and
      neurological: a dystrophic myopathy with proximal weakness, congenital or
      early-childhood cerebellar atrophy with ataxia and dysarthria, and
      corticospinal tract signs. The skeletal deformities (scoliosis, pectus
      excavatum) and the restrictive respiratory physiology are secondary to the
      myopathy rather than independent organ disease, and the retinopathy is a
      lesion of the neurosensory retina. Cardiac, hepatic, renal and endocrine
      involvement have not been established (echocardiography was normal in all
      11 patients screened in the largest cohort), so a single NEUROLOGIC
      chapter assignment is correct. Endocrine and psychiatric features that
      appear in the OMIM/HPO annotation set for this disease derive from the
      family whose variant was later localised to the MSTO2P pseudogene and are
      deliberately not carried here; see the entry-level notes.
    evidence:
    - reference: PMID:31463572
      reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."
      explanation: Defines the phenotype of the largest cohort as a muscular dystrophy plus two central-nervous-system components, which is the basis for the NEUROLOGIC assignment.
notes: >-
  Entity verification. Before any content was written, the target term was
  confirmed with OAK on 2026-08-01: `runoak -i sqlite:obo:mondo info
  MONDO:0044714 -O obo` gives name "mitochondrial myopathy-cerebellar
  ataxia-pigmentary retinopathy syndrome", xrefs OMIM:617675, Orphanet:502423,
  MEDGEN:1620960, UMLS:C4540096, GARD:0017934, synonym "MMYAT", and
  `is_a MONDO:0009637` (inborn mitochondrial myopathy). `runoak -i
  sqlite:obo:mondo relationships -p RO:0004003 MONDO:0044714` returns exactly one
  gene, HGNC:29678 MSTO1. HGNC:29678 was independently confirmed against the HGNC
  REST API as symbol MSTO1, name "misato mitochondrial distribution and
  morphology regulator 1", locus 1q22, Entrez 55154, Ensembl ENSG00000125459,
  UniProt Q9BUK6, alias symbols FLJ10504, LST005, MST and misato (retrieved
  2026-08-01). The alias list matters for reading the literature: the founding
  cell-biology paper (PMID:17349998) calls the protein "Misato" throughout and
  never uses the string MSTO1.

  NEC preflight. Every one of the 32 PubMed records returned for the query
  `MSTO1` on 2026-08-01 was inspected. MSTO1 is the dominant and usually the only
  disease gene named in the clinical papers used here; there is no competing gene
  in this literature. Four records were audited and rejected as not bearing on
  this disorder: PMID:42388255 and PMID:41503457 (a glucocorticoid-induced ocular
  hypertension GWAS and its preprint, in which MSTO1 appears only as a locus
  annotation), PMID:40646730 (a mud-crab weight-trait GWAS), PMID:23047795 (a
  prostate-radiotherapy fatigue expression study), and PMID:37565517, which is an
  FDX2 rhabdomyolysis case report that mentions MSTO1 only in differential
  discussion. PMID:39129082 (renal-cell-carcinoma prognostic signature) was
  likewise excluded. No named-entity confusion risk was identified: the disorder
  has no eponym, and its acronym MMYAT is unique in MONDO.

  The dominant-inheritance claim, and why it is not asserted here. This is the
  single most important correction in the entry. Gal et al 2017 (PMID:28554942)
  reported a Hungarian family with a heterozygous c.22G>A (p.Val8Met) change and
  titled their conclusion "the first dominant mutation in MSTO1". Gerber et al
  (PMID:37431816) subsequently raised concerns about that variant, and in their
  reply (PMID:37431815) the original authors confirmed that the mutation lies in
  the MSTO2P pseudogene (HGNC:26329, verified against the HGNC REST API on
  2026-08-01) rather than in MSTO1. EMBO Molecular Medicine published a
  corresponding erratum to the 2017 paper (PMID:37431817). The practical
  consequences are followed through consistently in this entry: (i) only
  autosomal recessive inheritance is asserted; (ii) the clinical features unique
  to that family - schizophrenia, depression, anxiety, hyperthyroidism, raised
  prolactin, primary amenorrhoea, multiple lipomas, mandibular prognathia,
  hearing impairment - are NOT curated as phenotypes of this disorder even though
  they remain in the HPO annotation set for OMIM:617675; and (iii) the
  cell-biology results of PMID:28554942 that do not depend on the patients'
  genotype (siRNA silencing of MSTO1 in HeLa cells, subcellular localisation,
  interaction with the fusion machinery) are still cited, tagged IN_VITRO, while
  its patient-fibroblast results are not used to support any claim about this
  disease.

  Contaminated annotation source, flagged not propagated. The HPO annotation set
  retrieved from https://ontology.jax.org/api/network/annotation/OMIM:617675 on
  2026-08-01 lists both HP:0000007 (Autosomal recessive inheritance) and
  HP:0000006 (Autosomal dominant inheritance), together with HP:0100753
  Schizophrenia, HP:0000716 Depression, HP:0000739 Anxiety, HP:0000836
  Hyperthyroidism, HP:0000870 Increased circulating prolactin concentration,
  HP:0000786 Primary amenorrhea and HP:0001012 Multiple lipomas. Those
  annotations trace to the Hungarian family described above. They are recorded
  here as an upstream data-quality observation and are not reproduced as
  phenotypes or as an inheritance mode. No file outside this entry was edited.

  Structured-source evidence was unavailable. `data/orphadata/` and
  `data/clingen/` in this checkout contain only `MANIFEST.yaml` with no
  downloaded payload, and no `references_cache/ORPHA_502423.md` or MSTO1-bearing
  `CGGV_*` cache file exists, so neither an Orphanet prevalence class nor a
  ClinGen gene-disease-validity classification could be consulted or quoted. This
  is recorded as a gap, not as an assertion that no such classification exists.
  Per the scope rules the manifests were not touched.

  No GeneReviews chapter was found. A PubMed search on 2026-08-01 for
  `MSTO1 AND GeneReviews[All Fields]` returned no records, so no GeneReviews
  phenotype baseline was available to this curation and the phenotype list is
  assembled from the primary cohorts instead. This records the result of that
  search, not an assertion that no such chapter exists.

  Frequency bands. `frequency:` is asserted only where a published denominator
  exists in a cached source - chiefly the 15-patient cohort of PMID:31463572,
  which reports explicit counts. Where the only support is an aggregate statement
  across the literature, the band is omitted and the count is recorded in the
  phenotype `notes:` instead. Total published experience is small enough (31-33
  cases as of 2025-2026) that a single reclassified patient moves most features
  across a band boundary.

  Cohort arithmetic. Two recent reviews give slightly different literature
  totals: PMID:40542878 (2025) counts 33 confirmed cases across 11 articles up to
  2025, while PMID:42051546 (2026) states 31 cases reported globally. Both are
  quoted verbatim in `prevalence:` rather than reconciled into a single number,
  because the difference reflects different inclusion decisions (most obviously
  whether the MSTO2P family is counted) that neither paper spells out in its
  abstract. PMID:41653646 adds four further patients from three families in 2026.
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    Disease requires two damaging MSTO1 alleles, in the great majority of
    reported families as compound heterozygous missense, truncating or
    splice-site variants, and in at least one Ashkenazi Jewish family as a
    homozygous missense variant. Heterozygous parents are unaffected, and the
    largest cohort makes that point explicitly as an argument about mechanism:
    haploinsufficiency for MSTO1 does not produce disease. Penetrance in
    bi-allelic individuals appears complete on present evidence and is recorded
    as such, with the caveat that the total published experience is around 30
    patients and no unaffected bi-allelic individual has been sought
    systematically in population databases.

    The historically competing dominant mode is deliberately not asserted. It
    rested on a single family whose variant was later shown to lie in the MSTO2P
    pseudogene; see the entry-level notes and the `msto1-dominant-claim-retracted`
    discussion. Reviews published between 2018 and 2019 that describe "both
    autosomal dominant and recessive modes" predate that correction, and the 2018
    statement is quoted below as evidence of the state of the field at the time
    rather than as a current claim.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  penetrance: COMPLETE
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report here the first families carrying recessive variants in the MSTO1 gene: compound heterozygous mutations were identified in two sisters and in an unrelated singleton case, who presented a multisystem complex phenotype mainly characterized by myopathy and cerebellar ataxia."
    explanation: The first demonstration of recessive transmission at this locus, in three probands from two families.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants"
    explanation: Fifteen patients from twelve independent families, all bi-allelic, is the strongest single body of evidence for recessive inheritance.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since haploinsufficiency, as is present in some of the heterozygous parents in our cohort, does not cause a clinical phenotype"
    explanation: >-
      Establishes that a single damaged MSTO1 allele is tolerated, which is the
      genetic argument against a simple loss-of-function dominant mechanism and
      the reason carrier parents need no surveillance.
  - reference: PMID:33612823
    reference_title: "A novel homozygous MSTO1 mutation in Ashkenazi Jewish siblings with ataxia and myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our report confirms that MSTO1 mutations are typically recessive, and associated with clinical phenotypes characterized by early-onset muscle impairment and ataxia, often with upper motor neuron signs and varied cognitive impairment."
    explanation: An independent group's explicit statement of the recessive conclusion, based on a homozygous family.
  - reference: PMID:41653646
    reference_title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report clinical and genetic features in four patients from 3 independent families with an ultra-rare autosomal recessive myopathy associated with biallelic pathogenic or likely pathogenic variants in MSTO1."
    explanation: The most recent case series, in 2026, still describes the disorder as autosomal recessive and bi-allelic.
  - reference: PMID:29339779
    reference_title: "Novel recessive mutations in MSTO1 cause cerebellar atrophy with pigmentary retinopathy."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Both autosomal dominant and recessive modes of inheritance have been suggested."
    explanation: >-
      Marked PARTIAL because it documents the state of the field in 2018, before
      the dominant family's variant was localised to the MSTO2P pseudogene. It
      is quoted to show that the ambiguity was real and contemporaneous, not to
      support a dominant mode.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: NOT_YET_DOCUMENTED
  notes: >-
    No prevalence or incidence estimate of any kind has been published for this
    disorder, and no Orphanet epidemiology record was reachable from this
    checkout, so the class is NOT_YET_DOCUMENTED rather than a numeric or
    qualitative band and `rate_per_100000` is deliberately left empty. What is
    available is a case count. A 2025 ophthalmological review counted 33
    confirmed cases across 11 published articles; a 2026 report states 31 cases
    reported globally; a 2022 case report put the figure at 31 patients. A 2026
    Mayo Clinic series adds four more patients from three families. The
    discrepancy between the counts is not resolved in any of the abstracts and
    is left visible here. Most reports state no ancestry for their patients at
    all, so no geographic distribution should be read off the author
    affiliations. Where ancestry is stated it is heterogeneous - the ethnicity
    row of the largest cohort lists Caucasian, African, Chinese,
    Caucasian/Hispanic and Native American across its fifteen patients, one
    patient of the 2017 recessive series is recorded as of British Caucasian
    descent, and separate reports describe Chinese families and one Ashkenazi
    Jewish sibship - which is consistent with ascertainment through
    neuromuscular and exome-sequencing referral networks rather than with any
    population restriction. No founder allele has been established.
  evidence:
  - reference: PMID:40542878
    reference_title: "Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A literature review was conducted, analysing the data from 11 published articles encompassing 33 confirmed cases of MMYAT up to 2025."
    explanation: The highest published case count, from a 2025 systematic literature review of the disorder.
  - reference: PMID:42051546
    reference_title: "Mild-cerebellar ataxia due to impaired mitochondrial function caused by the MSTO1 variations."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in MSTO1 are linked to a rare mitochondrial disorder characterized by early-onset myopathy and cerebellar ataxia, with 31 cases reported globally to date, which underscores its exceptional rarity."
    explanation: >-
      A second, slightly lower literature count published a year later. Marked
      PARTIAL because it disagrees with the 2025 review's total of 33 and neither
      source explains the difference.
pathophysiology:
- name: Bi-allelic Loss-of-Function Variants in MSTO1
  biological_scale: MOLECULAR
  description: >-
    The primary lesion is bi-allelic damage to MSTO1 at 1q22, reported against
    transcript NM_018116.3. The allelic spectrum is broad and includes missense
    substitutions across the length of the protein, frameshift and nonsense
    alleles, a start-loss allele, splice-acceptor variants and at least one
    multi-exon intragenic deletion, and there is no recurrent founder allele.
    The functional consequence converges regardless of allele class: in every
    patient fibroblast line assayed to date the MSTO1 protein is absent or
    strongly reduced, so the disease behaves as a loss of protein rather than as
    a set of allele-specific effects. Heterozygous parents are unaffected, so
    one intact allele suffices.
  genes:
  - preferred_term: MSTO1
    term:
      id: hgnc:29678
      label: MSTO1
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants"
    explanation: Establishes both the bi-allelic requirement and the breadth of the allelic spectrum.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "deletion encompassing exons 7-14 of MSTO1"
    explanation: >-
      Documents that one of the two alleles can be an intragenic copy-number
      deletion rather than a sequence variant, which is the diagnostic reason a
      negative exome does not exclude the disorder.
  downstream:
  - target: Loss of Cytosolic MSTO1 Protein
    causal_link_type: DIRECT
    description: >-
      Truncating, frameshift, start-loss and deletion alleles remove the coding
      capacity outright; the missense alleles tested behave as
      protein-destabilising rather than as separation-of-function changes, so
      both classes converge on absence of the protein.
- name: Loss of Cytosolic MSTO1 Protein
  biological_scale: MOLECULAR
  description: >-
    MSTO1 is a soluble cytosolic protein, not a mitochondrial matrix or
    inner-membrane resident, and this is the feature that makes the disorder
    mechanistically unusual among mitochondrial diseases. Its localisation was
    initially reported as the mitochondrial outer membrane, but two later groups
    working independently on patient material and on transfected cells placed it
    predominantly in the cytosol, interacting with the outer-membrane fusion
    machinery from the cytosolic face rather than residing in the organelle. The
    protein carries a region homologous to the tubulin/FtsZ GTPase superfamily,
    the same fold family used for chromosome segregation in eukaryotes and for
    organelle division in bacteria and algae, but no GTPase activity has been
    demonstrated for the human protein and its molecular activity remained
    undefined until 2026. In patient fibroblasts the protein is undetectable by
    western blot in every line examined in the largest series, and strongly
    reduced in the others, so this node is the convergence point of the whole
    allelic spectrum.
  genes:
  - preferred_term: MSTO1
    term:
      id: hgnc:29678
      label: MSTO1
  cellular_components:
  - preferred_term: cytosol
    term:
      id: GO:0005829
      label: cytosol
  - preferred_term: mitochondrial outer membrane
    modifier: ABNORMAL
    term:
      id: GO:0005741
      label: mitochondrial outer membrane
  cell_types:
  - preferred_term: patient-derived dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  notes: >-
    Localisation is a genuine, unresolved disagreement in the primary
    literature and is presented as such rather than harmonised. Kimura and Okano
    (PMID:17349998) fractionated mitochondria and assigned Misato to the outer
    membrane. Gal et al (PMID:28554942) explicitly contradicted that result, and
    Nasca et al (PMID:28544275) independently found the protein to be mainly
    cytosolic. The weight of evidence favours a cytosolic protein that acts at
    the outer-membrane interface, which is how this entry models it, but the two
    outer-membrane and cytosolic evidence items below are both retained so the
    disagreement stays visible.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "MSTO1 protein was not detectable in the cultured fibroblasts of all seven patients evaluated, suggesting that pathogenic variants result in a loss of protein expression and/or affect protein stability."
    explanation: Demonstrates that the disease-causing genotypes converge on absence of the protein, in seven independent patient lines.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We also found that MSTO1 is mainly a cytosolic protein."
    explanation: Independent assignment of the protein to the cytosol rather than to the organelle.
  - reference: PMID:28554942
    reference_title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "At variance with a previous report, we find MSTO1 to be localized in the cytoplasmic area with limited colocalization with mitochondria."
    explanation: >-
      The explicit contradiction of the earlier outer-membrane assignment. Cited
      for its cell-biology result only; the patient-genotype claims of this paper
      were later corrected and are not used anywhere in this entry.
  - reference: PMID:28554942
    reference_title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "MSTO1 interacts with the fusion machinery as a soluble factor at the cytoplasm-mitochondrial outer membrane interface."
    explanation: Places the protein's site of action at the cytosolic face of the outer membrane, which is the topology this entry models.
  - reference: PMID:17349998
    reference_title: "Human Misato regulates mitochondrial distribution and morphology."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Further experiments of fractionating mitochondria revealed that Misato was localized to the outer membrane."
    explanation: >-
      The original outer-membrane assignment, retained as PARTIAL because two
      subsequent groups could not reproduce it and reported a predominantly
      cytosolic distribution.
  downstream:
  - target: Impaired Mitochondrial Fusion and Network Fragmentation
    causal_link_type: DIRECT
    hypothesis_groups:
    - msto1_fusion_first
    description: >-
      The canonical route. Without MSTO1 at the cytosol-outer-membrane
      interface, fusion events fall and the network fragments; this is the
      oldest and most reproduced observation in the field.
  - target: Failure of TRiC Chaperonin Assembly
    causal_link_type: DIRECT
    hypothesis_groups:
    - msto1_tric_proteostasis
    description: >-
      The 2026 proteostasis route. On this model the proximate consequence of
      losing MSTO1 is not a mitochondrial event at all but failure to assemble
      the cytosolic chaperonin TRiC, with the mitochondrial phenotype following
      later.
  - target: Unrestrained RAD51 Binding to Mitochondrial DNA
    causal_link_type: DIRECT
    hypothesis_groups:
    - msto1_rad51_mtdna
    description: >-
      The 2026 genome-maintenance route. On this model MSTO1 restrains RAD51 at
      the mitochondrial genome, and losing it disrupts mtDNA replication
      directly rather than through the fusion machinery.
- name: Impaired Mitochondrial Fusion and Network Fragmentation
  biological_scale: CELLULAR
  description: >-
    Mitochondria are maintained as a dynamic, partly continuous reticulum by
    balanced fusion and fission. Fusion allows content mixing - complementation
    of damaged mtDNA and proteins between organelles - and its failure leaves a
    population of small, isolated, functionally isolated fragments. Every
    patient fibroblast line examined across four independent groups shows a
    fragmented mitochondrial network, and in one series the fragments coexist
    with abnormally enlarged structures. The effect is MSTO1-dependent in both
    directions: acute knockdown of MSTO1 in HeLa cells reproduces the
    fragmentation, over-expression of the protein drives the opposite phenotype
    of perinuclear mitochondrial aggregation, and re-expression of wild-type
    MSTO1 in patient cells restores the network. Loss of network continuity also
    degrades organelle distribution - the property for which the gene is named -
    which is expected to matter most in the long, highly polarised cells of
    muscle and the cerebellum.
  cell_types:
  - preferred_term: patient-derived dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: mitochondrial fusion
    modifier: DECREASED
    term:
      id: GO:0008053
      label: mitochondrial fusion
  - preferred_term: mitochondrion distribution within the cell
    modifier: ABNORMAL
    term:
      id: GO:0048312
      label: intracellular distribution of mitochondria
  cellular_components:
  - preferred_term: mitochondrion
    modifier: ABNORMAL
    term:
      id: GO:0005739
      label: mitochondrion
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Consistent with impaired mitochondrial fusion, mitochondrial networks in fibroblasts were found to be fragmented."
    explanation: Fragmentation confirmed across the seven patient lines of the largest cohort.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Studies in patients' fibroblasts revealed that MSTO1 protein levels were strongly reduced, the mitochondrial network was fragmented, and the fusion events among mitochondria were decreased"
    explanation: >-
      Couples the morphological readout to a direct measurement of fusion
      events, which is what makes this a fusion defect rather than merely an
      altered shape.
  - reference: PMID:33612823
    reference_title: "A novel homozygous MSTO1 mutation in Ashkenazi Jewish siblings with ataxia and myopathy."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we found that MSTO1 protein level was reduced and that mitochondrial network was fragmented or formed enlarged structures"
    explanation: Independent replication in a homozygous genotype, with the additional observation of enlarged as well as fragmented structures.
  - reference: PMID:28554942
    reference_title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Short-term silencing of MSTO1 in HeLa cells reproduced the impairment of mitochondrial morphology and dynamics observed in the fibroblasts without damaging bioenergetics."
    explanation: >-
      Acute knockdown in a clean cell line reproduces the phenotype, so
      fragmentation follows loss of MSTO1 rather than being a secondary property
      of patient cells. The clause about preserved bioenergetics is itself
      informative and is carried into the dissociation node below.
  - reference: PMID:17349998
    reference_title: "Human Misato regulates mitochondrial distribution and morphology."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "After silencing of Misato, the filamentous mitochondrial network disappeared and fragmented mitochondria were observed, indicating human Misato has a role in mitochondrial fusion."
    explanation: The original demonstration, a decade before any patient was described, that depleting this protein collapses the mitochondrial reticulum.
  - reference: PMID:17349998
    reference_title: "Human Misato regulates mitochondrial distribution and morphology."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Its overexpression resulted in the formation of perinuclear aggregations of mitochondria in these cells."
    explanation: >-
      The reciprocal gain-of-dosage phenotype, which establishes that MSTO1
      levels set network morphology bidirectionally rather than merely
      permitting it.
  downstream:
  - target: mtDNA Depletion and Nucleoid Reorganisation
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    hypothesis_groups:
    - msto1_fusion_first
    description: >-
      Reduced fusion is proposed to deplete mtDNA by failing to distribute the
      mtDNA replication machinery and by leaving nucleoids unsegregated, the
      same route established for the fusion GTPases MFN2 and OPA1.
  - target: Enlarged Lysosomal Vacuoles
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Patient fibroblasts show markedly enlarged lysosomal vacuoles alongside
      the fragmented network. The connecting steps are not established, hence
      the unknown-intermediates typing.
- name: Failure of TRiC Chaperonin Assembly
  biological_scale: MOLECULAR
  description: >-
    An alternative account of what MSTO1 actually does, published as a preprint
    in July 2026 and not yet peer reviewed. Using auxin-inducible degradation to
    strip MSTO1 from cells within hours, the authors found that TRiC (the
    chaperonin-containing T-complex, CCT), the essential cytosolic ATP-dependent
    chaperonin that folds actin, tubulin and a large set of other substrates,
    falls within two days, whereas mitochondrial fragmentation does not appear
    until day six. Early TRiC assembly subcomplexes accumulate when MSTO1 is
    absent, MSTO1 co-immunoprecipitates with TRiC, and TRiC is also reduced in
    MSTO1 patient fibroblasts. Critically, knocking down TRiC on its own is
    sufficient to fragment mitochondria, and does so without measurable changes
    in tubulin or actin. If this holds, the ordering of the whole pathograph
    changes: the mitochondrial phenotype becomes a downstream consequence of a
    cytosolic proteostasis defect rather than the primary lesion, and MSTO1
    becomes a chaperonin assembly factor that happens to have been discovered
    through a mitochondrial phenotype.
  genes:
  - preferred_term: MSTO1
    term:
      id: hgnc:29678
      label: MSTO1
  protein_complexes:
  - preferred_term: TRiC/CCT chaperonin
    modifier: DECREASED
    term:
      id: GO:0005832
      label: chaperonin-containing T-complex
  cellular_components:
  - preferred_term: cytosol
    term:
      id: GO:0005829
      label: cytosol
  notes: >-
    Evidence status matters here and is stated rather than implied.
    PMID:42539026 is a bioRxiv preprint (posted 2026-07-23) and has not been
    peer reviewed; it is the sole source for this node. It is included because
    it is the only work that proposes a concrete molecular activity for MSTO1
    and because its temporal-ordering experiment is a direct test of the
    canonical model, but no clinical or diagnostic claim in this entry depends
    on it, and the node is grouped under an EMERGING hypothesis rather than
    placed on the canonical causal path.
  evidence:
  - reference: PMID:42539026
    reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Surprisingly, prior to any changes in mitochondria, we show that MSTO1-depleted cells have a significant decrease in TRiC levels, an essential cytosolic ATP-dependent chaperone required to fold diverse substrates, including actin and tubulin."
    explanation: The temporal-ordering result on which the whole alternative model rests, and the reason it is a challenge to the fusion-first account.
  - reference: PMID:42539026
    reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We also observe accumulation of early TRiC assembly subcomplexes in the absence of MSTO1 suggesting that MSTO1 facilitates assembly of TRiC."
    explanation: Assigns MSTO1 a specific molecular activity, chaperonin assembly, for the first time.
  - reference: PMID:42539026
    reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Using rapid depletion of MSTO1, we found that mitochondrial fragmentation appears only after six days."
    explanation: >-
      The kinetic gap between TRiC loss at two days and mitochondrial
      fragmentation at six days is the specific observation that makes the
      mitochondrial event look secondary.
  downstream:
  - target: Impaired Mitochondrial Fusion and Network Fragmentation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    hypothesis_groups:
    - msto1_tric_proteostasis
    description: >-
      Depleting TRiC alone fragments mitochondria, and does so independently of
      changes in tubulin or actin, so the relevant TRiC substrate linking
      proteostasis to the organelle is not yet identified.
    evidence:
    - reference: PMID:42539026
      reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We further demonstrate that knockdown of TRiC leads to a decrease in MSTO1 protein levels and remarkably, was sufficient to induce a fragmented mitochondrial phenotype, independent of changes in tubulin or actin."
      explanation: Sufficiency of TRiC loss for the mitochondrial phenotype, plus a reciprocal dependence of MSTO1 levels on TRiC.
- name: Unrestrained RAD51 Binding to Mitochondrial DNA
  biological_scale: MOLECULAR
  description: >-
    A second 2026 account, this one placing MSTO1 directly at the mitochondrial
    genome. In cancer cell models MSTO1 interacts with the recombinase RAD51
    through a conserved FxxA motif and restrains its activity specifically
    within mitochondria, without affecting RAD51's nuclear functions. When MSTO1
    is lost, RAD51 binding to mtDNA increases, mtDNA replication is disrupted
    and mtDNA damage accumulates, membrane potential falls and mtDNA content
    drops. The same work reports that mtDNA then leaks to the cytosol through a
    BAX/BAK-dependent route and activates cGAS-STING signalling, which would
    give MSTO1 deficiency an innate-immune arm. This route would explain mtDNA
    depletion without requiring the fusion defect as an intermediate, and it is
    therefore modelled as a competing hypothesis rather than as an addition to
    the canonical chain.
  genes:
  - preferred_term: MSTO1
    term:
      id: hgnc:29678
      label: MSTO1
  cellular_components:
  - preferred_term: mitochondrial nucleoid
    modifier: ABNORMAL
    term:
      id: GO:0042645
      label: mitochondrial nucleoid
  biological_processes:
  - preferred_term: mitochondrial DNA replication
    modifier: DECREASED
    term:
      id: GO:0006264
      label: mitochondrial DNA replication
  notes: >-
    Scope caveat. PMID:42098462 is a cancer-biology paper: the mechanistic work
    is in tumour and cultured cell models and the clinical bioinformatics arm
    uses TCGA. It contains no patient with an MSTO1-related myopathy. It is
    curated here because it is direct experimental evidence about the molecular
    activity of the disease protein at the mitochondrial genome, but the
    cGAS-STING inflammatory arm in particular has never been looked for in a
    patient with this disorder and is not asserted as part of the human disease.
  evidence:
  - reference: PMID:42098462
    reference_title: "MSTO1 modulates RAD51 activity to safeguard mitochondrial DNA integrity and control immune responses."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "This study identifies MSTO1 as a crucial regulator of mitochondrial genome integrity through direct interaction with RAD51."
    explanation: States the proposed direct molecular link between MSTO1 and mitochondrial genome maintenance.
  - reference: PMID:42098462
    reference_title: "MSTO1 modulates RAD51 activity to safeguard mitochondrial DNA integrity and control immune responses."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "MSTO1 deficiency enhances RAD51 binding to mitochondrial DNA (mtDNA), disrupting mtDNA replication and causing mtDNA damage."
    explanation: The specific mechanism by which loss of MSTO1 would deplete mtDNA without passing through a fusion defect.
  - reference: PMID:42098462
    reference_title: "MSTO1 modulates RAD51 activity to safeguard mitochondrial DNA integrity and control immune responses."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Furthermore, MSTO1 deficiency triggers BAX/BAK-dependent mtDNA leakage, activating the cGAS-STING pathway and driving inflammatory responses."
    explanation: >-
      A potentially important innate-immune consequence. Marked PARTIAL because
      it is shown only in cancer cell models and no inflammatory phenotype has
      been reported in any patient with MSTO1-related disease.
  downstream:
  - target: mtDNA Depletion and Nucleoid Reorganisation
    causal_link_type: DIRECT
    hypothesis_groups:
    - msto1_rad51_mtdna
    description: >-
      On this model the depletion is a direct consequence of disordered mtDNA
      replication at the nucleoid, not a distribution problem inherited from the
      fusion defect.
- name: mtDNA Depletion and Nucleoid Reorganisation
  biological_scale: MOLECULAR
  description: >-
    The finding that reclassified this disorder. Mitochondrial DNA content is
    reduced in patient cells to roughly a third to two thirds of control, and
    the surviving genomes are redistributed: nucleoids become fewer and larger,
    and some mitochondrial fragments contain no genome at all. The depletion is
    reproducible across independent laboratories, appears in muscle as well as
    in fibroblasts, and is quantitatively in the same range as that seen in
    fibroblasts from patients with OPA1 or MFN2 disease and in the established
    mtDNA depletion syndromes, which is the basis for the proposal that MSTO1
    deficiency be classified as an mtDNA depletion syndrome.
  cell_types:
  - preferred_term: patient-derived dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  - preferred_term: skeletal muscle fibre
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  cellular_components:
  - preferred_term: mitochondrial nucleoid
    modifier: ABNORMAL
    term:
      id: GO:0042645
      label: mitochondrial nucleoid
  biological_processes:
  - preferred_term: mitochondrial DNA metabolic process
    modifier: DECREASED
    term:
      id: GO:0032042
      label: mitochondrial DNA metabolic process
  notes: >-
    Consistency assessment, which was the specific question this curation was
    asked to settle. mtDNA depletion is CONSISTENT wherever it has been
    measured, but it has been measured in a small number of laboratories and
    almost entirely in cultured fibroblasts. Positive: 7 of 7 fibroblast lines
    in PMID:31463572 (30-70% depletion); muscle from patient A1 at 27% of
    control and fibroblasts from A1 and A2 at 39% and 68% in PMID:28544275; the
    homozygous proband of PMID:33612823; the adult-onset patient of
    PMID:42051546. No published report of a normal mtDNA content in an
    MSTO1 patient was found. Two important limits on that consistency: the
    largest cohort could not obtain muscle tissue for mtDNA studies at all
    ("Unfortunately, muscle tissue was not available for further mtDNA content
    studies"), so the muscle evidence rests on a single patient in a different
    series; and depletion in a dividing fibroblast may not predict depletion in
    postmitotic muscle or cerebellum, the tissues that are actually diseased.
    The claim curated here is therefore "consistently found in patient
    fibroblasts, with single-patient support in muscle", not "consistently found
    in patient tissue".
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids"
    explanation: The quantitative anchor for the depletion claim, in seven independent patient lines.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "contained fewer nucleoids, which were larger in size compared to control lines"
    explanation: Establishes that the genomes are not merely fewer but abnormally packaged and distributed.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "fragments were devoid of mitochondrial genomes in patient cells"
    explanation: >-
      The distribution failure made concrete: some organelle fragments have no
      genome at all, which is the expected consequence of losing fusion-mediated
      content mixing.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "this novel link to mtDNA depletion in patient fibroblasts suggests that MSTO1-deficiency should also be considered a mtDNA depletion syndrome"
    explanation: The nosological consequence drawn by the authors of the largest cohort - that MSTO1 deficiency belongs among the mtDNA depletion syndromes.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "mtDNA amount was reduced to 27% of the mean control value"
    explanation: >-
      The only published measurement of mtDNA content in MSTO1 patient muscle,
      predating and independent of the fibroblast series.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "the amount of mtDNA in total DNA extracted from fibroblasts, assessed by quantitative PCR, was also lower in the patients than in controls"
    explanation: Confirms the muscle finding in cultured cells from the same two patients.
  - reference: PMID:42051546
    reference_title: "Mild-cerebellar ataxia due to impaired mitochondrial function caused by the MSTO1 variations."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Functional studies revealed significantly reduced MSTO1 protein expression, mtDNA depletion, and impaired mitochondrial function, as reflected by decreased mitochondrial membrane potential and respiratory capacity, suggesting a pathogenic role for these variants."
    explanation: >-
      Replicates depletion in an adult-onset patient and, unlike the earlier
      series, also finds impaired membrane potential and respiration - the
      observation that complicates the dissociation node below.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "recent work suggesting that reduced fusion causes mtDNA depletion due to insufficient distribution of the mtDNA replication machinery"
    explanation: >-
      The authors' proposed mechanism for the fusion-to-depletion link. Marked
      PARTIAL because it is an inference from work on other fusion genes rather
      than a demonstration in MSTO1 cells.
  downstream:
  - target: Dystrophic Degeneration of Skeletal Muscle Fibres
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Skeletal muscle is the most affected tissue. The steps between a
      30-70% fall in mtDNA and fibre degeneration have not been demonstrated in
      MSTO1 patient muscle, so the link is typed as having unknown
      intermediates.
  - target: Cerebellar Hypoplasia and Cortical Atrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The cerebellar lesion is present from early childhood and often before age
      two, so at least part of it is developmental rather than degenerative; how
      mtDNA depletion produces a hypoplastic cerebellum is not known.
  - target: Corticospinal Tract Dysfunction
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Long descending motor axons are among the most energetically demanding
      structures in the nervous system and are a recurrent target in
      mitochondrial dynamics disorders, but no pathological material from an
      MSTO1 patient's spinal cord exists.
  - target: Retinal Degeneration with Pigmentary Change
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Photoreceptors and retinal pigment epithelium have among the highest
      oxidative demands of any tissue. The link is inconstant across cohorts,
      which is itself curated as a knowledge gap.
  - target: Dissociation Between mtDNA Depletion and Respiratory Chain Function
    causal_link_type: DIRECT
    description: >-
      The depletion is real but, in most patients studied, is not accompanied by
      the respiratory chain deficiency that mtDNA depletion syndromes usually
      produce. That dissociation is modelled explicitly rather than ignored.
- name: Dissociation Between mtDNA Depletion and Respiratory Chain Function
  biological_scale: CELLULAR
  description: >-
    A patient with MSTO1 disease usually does not look biochemically like a
    classical mitochondrial patient. Lactate is normal, respiratory chain
    complex activities in muscle homogenate are normal or near normal, and
    fibroblast respiration measured by micro-oxygraphy is normal, even though
    mtDNA is reduced and the network is fragmented. What is abnormal is citrate
    synthase activity, which is markedly low and indexes mitochondrial mass
    rather than mitochondrial function, so the coherent reading is that these
    cells have fewer, poorly distributed mitochondria carrying less mtDNA rather
    than mitochondria that respire badly. This matters diagnostically: a normal
    mitochondrial biochemical screen does not exclude the diagnosis, and it is a
    large part of why the disorder was reached by exome sequencing rather than
    by metabolic workup. The dissociation is not absolute - the 2026 adult-onset
    case did show reduced membrane potential and respiratory capacity - and that
    exception is carried here rather than hidden.
  cell_types:
  - preferred_term: patient-derived dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  - preferred_term: skeletal muscle fibre
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our biochemical analyses were not indicative of a mitochondrial disorder: patients showed normal lactate levels, and hardly any defect of MRC complex activities or cell respiration"
    explanation: The clearest statement that the standard mitochondrial biochemical screen is normal in this disorder.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, we detected markedly low activity of citrate synthase and reduced mtDNA amount, suggesting a reduction in mitochondrial mass."
    explanation: >-
      Reframes the abnormality as reduced mitochondrial mass rather than reduced
      per-mitochondrion function, which is the interpretation this node encodes.
  - reference: PMID:28554942
    reference_title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Short-term silencing of MSTO1 in HeLa cells reproduced the impairment of mitochondrial morphology and dynamics observed in the fibroblasts without damaging bioenergetics."
    explanation: Acute MSTO1 depletion fragments the network without a bioenergetic penalty, which is the experimental counterpart of the clinical dissociation.
  - reference: PMID:42051546
    reference_title: "Mild-cerebellar ataxia due to impaired mitochondrial function caused by the MSTO1 variations."
    supports: REFUTE
    evidence_source: IN_VITRO
    snippet: "Functional studies revealed significantly reduced MSTO1 protein expression, mtDNA depletion, and impaired mitochondrial function, as reflected by decreased mitochondrial membrane potential and respiratory capacity, suggesting a pathogenic role for these variants."
    explanation: >-
      The counter-example. In this 2026 adult-onset patient membrane potential
      and respiratory capacity were both reduced, so the dissociation between
      mtDNA depletion and bioenergetic failure is not universal. Tagged REFUTE
      against the node's general claim rather than removed, because a single
      contradicting line is exactly the sort of finding that should stay visible.
- name: Enlarged Lysosomal Vacuoles
  biological_scale: CELLULAR
  description: >-
    Markedly enlarged lysosomal vacuoles were found across all MSTO1 patient
    fibroblast lines in the largest series, a finding not previously reported in
    the context of MSTO1 dysfunction. Mitochondrial and lysosomal compartments
    are functionally coupled, and lysosomal enlargement is a recurring accompaniment
    of mitochondrial dysfunction, but for MSTO1 the connecting steps are
    undetermined and the finding has not yet been replicated by another group.
    It is curated because it recovered on re-expression of wild-type MSTO1 on the
    same timescale as mitochondrial morphology, which argues it is a genuine
    consequence of losing the protein rather than a culture artefact.
  cell_types:
  - preferred_term: patient-derived dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  cellular_components:
  - preferred_term: lysosome
    modifier: ABNORMAL
    term:
      id: GO:0005764
      label: lysosome
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Enlarged lysosomal vacuoles in MSTO1 patient fibroblasts"
    explanation: The figure legend stating the finding directly; the result is described in the same paper as not previously reported for MSTO1.
- name: Dystrophic Degeneration of Skeletal Muscle Fibres
  biological_scale: TISSUE
  description: >-
    Muscle is the tissue in which MSTO1 loss produces the most consistent
    structural damage. Biopsies show a dystrophic rather than a purely myopathic
    process: variation in fibre size, internalised nuclei, whorled fibres, and a
    mild degree of necrosis and regeneration, with electron microscopy in
    earlier reports showing increased subsarcolemmal and intermyofibrillar
    mitochondria with lipid and glycogen accumulation. Notably the classical
    oxidative histochemical stigmata of mitochondrial myopathy - ragged red
    fibres, cytochrome c oxidase negative fibres - were absent in the cases
    where they were sought, consistent with the biochemical dissociation node
    above. Ongoing fibre necrosis and regeneration is what produces the raised
    creatine kinase, which is the single most useful screening abnormality in
    this disorder.
  cell_types:
  - preferred_term: skeletal muscle fibre
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  locations:
  - preferred_term: skeletal muscle tissue
    term:
      id: UBERON:0001134
      label: skeletal muscle tissue
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsies were performed in ten patients and were consistent with a dystrophic process"
    explanation: Ten biopsies in one cohort establish the dystrophic character of the muscle pathology.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serum creatine kinase (CK) levels were significantly increased in all patients, except for P15, ranging from 300 to 5000 U/L."
    explanation: Quantifies the leak of muscle enzyme that accompanies the dystrophic process, in 14 of 15 patients.
  downstream:
  - target: Proximal Weakness with Restrictive Respiratory Compromise
    causal_link_type: DIRECT
    description: Progressive loss of functional muscle fibres produces the clinical weakness and the restrictive ventilatory defect.
- name: Cerebellar Hypoplasia and Cortical Atrophy
  biological_scale: TISSUE
  description: >-
    The cerebellar lesion is the second constant of the disorder and is
    developmental at least in part. Brain MRI showed moderate to severe
    cerebellar atrophy or hypoplasia involving the vermis and both hemispheres
    in all twelve patients imaged in the largest cohort, four of whom were
    imaged before the age of two, and repeat imaging in four patients confirmed
    no progression. That pattern - present early, stable thereafter - is
    unusual among mitochondrial cerebellar syndromes and argues for a defect in
    cerebellar development rather than for ongoing neurodegeneration. It is not
    universal: an additional case with clinical and radiological evidence of
    progressive cerebellar involvement has been reported, and a 2026 patient
    presented with adult-onset progressive ataxia, so this entry records both
    courses. Purkinje cells are the obvious candidate substrate given their
    energetic demands and their sensitivity in other mitochondrial disorders,
    but no neuropathological material from an MSTO1 patient has been published.
  locations:
  - preferred_term: cerebellum
    term:
      id: UBERON:0002037
      label: cerebellum
  - preferred_term: cerebellar vermis
    term:
      id: UBERON:0004720
      label: cerebellar vermis
  notes: >-
    The Purkinje cell is deliberately NOT bound as a cell type on this node. No
    autopsy, biopsy or imaging-based cell-type-resolved evidence exists for
    Purkinje cell loss in MSTO1 disease; asserting it would import a mechanism
    from other cerebellar ataxias rather than record one demonstrated here.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "involving the vermis and both hemispheres in all 12"
    explanation: Establishes that the cerebellar lesion involves vermis and hemispheres and was present in every patient imaged.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."
    explanation: States the early-onset, non-progressive character of the cerebellar atrophy for the cohort as a whole.
  - reference: PMID:31130378
    reference_title: "A novel case of MSTO1 gene related congenital muscular dystrophy with progressive neurological involvement."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "This case confirms a consistent MSTO1-related neuromuscular phenotype and in addition suggests a progressive neurological component at least in some patients, in keeping with the mitochondrial role of the defective protein."
    explanation: >-
      The counter-observation to non-progression. Marked PARTIAL because it is a
      single case and its authors themselves frame it as applying to "some
      patients".
  downstream:
  - target: Ataxia, Dysmetria and Dysarthria
    causal_link_type: DIRECT
    description: Loss of cerebellar cortical output produces the appendicular and truncal cerebellar signs and the cerebellar speech disturbance.
- name: Corticospinal Tract Dysfunction
  biological_scale: TISSUE
  description: >-
    An upper motor neuron component sits alongside the myopathy and is easy to
    miss because weakness is usually attributed to the muscle disease. Increased
    tone, a spastic catch, clonus and brisk deep tendon reflexes were recorded
    in eight of fifteen patients in the largest cohort, and upper motor neuron
    signs are singled out in an independent report as part of the typical
    picture. Its presence is diagnostically useful: brisk reflexes in a child
    with a dystrophic myopathy and a high creatine kinase point away from a
    pure muscular dystrophy and towards a central component.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."
    explanation: Names corticospinal tract dysfunction as one of the three core components of the phenotype.
  - reference: PMID:33612823
    reference_title: "A novel homozygous MSTO1 mutation in Ashkenazi Jewish siblings with ataxia and myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "often with upper motor neuron signs and varied cognitive impairment"
    explanation: Independent confirmation that upper motor neuron signs are a recurrent feature rather than a single-cohort observation.
- name: Retinal Degeneration with Pigmentary Change
  biological_scale: TISSUE
  description: >-
    The retinal component is the feature that names the MONDO and Orphanet terms
    and simultaneously the least consistent part of the phenotype. Where it is
    present it is a genuine retinal dystrophy: widespread salt-and-pepper
    pigmentation, macular atrophy, mottled hypoautofluorescence and loss of the
    ellipsoid zone on multimodal imaging in a Chinese sibling pair followed for
    seventeen years, and pigmentary retinopathy with pallor of the disc and
    concentric visual field constriction, but preserved visual acuity, in the
    index patient of the first recessive series (PMID:28544275). Both probands
    of the report that gave the syndrome its MONDO name (PMID:29339779) had
    pigmentary retinopathy. Yet it was reported in none of
    the fifteen patients of the largest cohort, where formal ophthalmological
    examination was not performed in every patient - so the true denominator is
    unknown and the discrepancy may be ascertainment rather than biology.
  locations:
  - preferred_term: retina
    term:
      id: UBERON:0000966
      label: retina
  cell_types:
  - preferred_term: retinal pigment epithelial cell
    term:
      id: CL:0002586
      label: retinal pigment epithelial cell
  - preferred_term: photoreceptor cell
    term:
      id: CL:0000210
      label: photoreceptor cell
  notes: >-
    The two cell types bound here are the cells whose degeneration produces the
    imaging phenotype described in PMID:40542878 (ellipsoid-zone loss is a
    photoreceptor readout; mottled hypoautofluorescence and salt-and-pepper
    pigmentation are retinal pigment epithelium readouts). No histopathology of
    an MSTO1 patient retina exists, so the cell-type assignment is inferred from
    the imaging modality rather than demonstrated.
  evidence:
  - reference: PMID:29339779
    reference_title: "Novel recessive mutations in MSTO1 cause cerebellar atrophy with pigmentary retinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We performed whole-exome sequencing in two unrelated patients showing cerebellar atrophy, intellectual disability, and pigmentary retinopathy."
    explanation: The report that established pigmentary retinopathy as part of the MSTO1 phenotype and gave the MONDO term its name.
  - reference: PMID:40542878
    reference_title: "Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ophthalmic multimodal imaging examination revealed typical characteristics of retinal dystrophy in both patients, including binocular widespread salt-and-pepper pigmentation, macular atrophy, a mottled pattern of hypoautofluorescence, and degeneration of the ellipsoid zone."
    explanation: The most detailed retinal characterisation available, and the source of the cell-layer inference recorded in this node's notes.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ophthalmological examination revealed the presence of pigmentary retinopathy with papillary pallor, associated with concentric restriction of the visual field but no abnormality of visual acuity."
    explanation: >-
      Shows the functional signature - field constriction with preserved acuity
      - that distinguishes this rod-predominant retinopathy from a maculopathy
      or an optic neuropathy.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "This finding was not reported in any of the patients in our cohort; however, formal ophthalmologic examination was not pursued in all patients."
    explanation: >-
      The direct counter-evidence: zero of fifteen patients in the largest
      cohort, with the authors' own caveat that ophthalmological assessment was
      incomplete. This is why retinopathy is curated as inconstant rather than
      as a defining feature.
- name: Proximal Weakness with Restrictive Respiratory Compromise
  biological_scale: ORGANISM
  description: >-
    The organism-level expression of the muscle disease. Presentation is with
    hypotonia and delayed motor milestones between birth and three years,
    followed by independent but waddling, wide-based ambulation and
    predominantly proximal weakness that then remains relatively stable. Loss of
    respiratory muscle strength shows up as a reduced forced vital capacity
    before it produces symptoms, which is the argument for scheduled pulmonary
    function testing rather than symptom-triggered testing. A later-onset arm of
    the spectrum exists: a 2026 series reported diagnosis between 13 and 30
    years, and a separate 2026 case had adult-onset disease.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."
    explanation: Establishes the age window and the presenting features of the muscle disease.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All 15 patients reported either relatively slow progression or no progression of their muscle weakness."
    explanation: The natural-history claim that distinguishes this from the rapidly progressive congenital muscular dystrophies.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "revealed reduced forced vital capacity (FVC) measurements, ranging from 60 to 79% predicted"
    explanation: Quantifies subclinical restrictive lung physiology in nine tested patients, which is the basis for routine spirometry.
- name: Ataxia, Dysmetria and Dysarthria
  biological_scale: ORGANISM
  description: >-
    The organism-level expression of the cerebellar lesion. In the largest
    cohort dysmetria was recorded in 11 of 15, gait ataxia in 9 of 15 and
    abnormal speech in 12 of 15, the last comprising speech delay in five and
    dysarthria in eight. Because the cerebellar atrophy is usually stable, these
    signs typically do not worsen substantially over time, and the functional
    trajectory of most patients is dominated by the muscle disease rather than
    by the cerebellar disease.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."
    explanation: The per-sign denominators for the cerebellar phenotype in the only cohort large enough to provide them.
mechanistic_hypotheses:
- hypothesis_group_id: msto1_fusion_first
  hypothesis_label: Fusion failure is the primary lesion and mtDNA depletion is downstream of it
  status: CANONICAL
  description: >-
    The standard model, held since the first recessive families were described
    in 2017 and restated by every clinical group that has reported the disorder
    since. Loss of cytosolic
    MSTO1 impairs mitochondrial fusion; the fragmented, poorly distributed
    network then fails to distribute the mtDNA replication machinery and to
    segregate nucleoids, and mtDNA content falls. The strongest direct argument
    for the ordering comes from rescue kinetics in patient fibroblasts:
    re-expressing wild-type MSTO1 fully restores mitochondrial and lysosomal
    morphology by 48 hours but only partially restores mtDNA, which is what one
    expects if the genome defect is the later, downstream event. The model is
    also supported by analogy, since the same fusion-to-depletion relationship
    is established for MFN2 and OPA1. Its weakness is that the connecting steps
    have never been demonstrated in MSTO1 cells, and that a cytosolic protein
    with no demonstrated enzymatic activity is an odd fusion factor.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "further suggests that impairments in mitochondrial dynamics are upstream of mtDNA depletion in MSTO1 patient fibroblasts"
    explanation: The authors' own statement of the causal ordering, based on the differential recovery kinetics after rescue.
  - reference: PMID:28554942
    reference_title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "MSTO1 likely has a physiologically relevant role in mitochondrial morphogenesis by supporting mitochondrial fusion."
    explanation: The canonical functional assignment for the protein.
- hypothesis_group_id: msto1_tric_proteostasis
  hypothesis_label: MSTO1 is a TRiC/CCT chaperonin assembly factor and the mitochondrial phenotype is secondary to a cytosolic proteostasis defect
  status: EMERGING
  description: >-
    Proposed in a 2026 preprint that has not been peer reviewed. Rapid,
    auxin-induced degradation of MSTO1 lowers TRiC within two days while
    mitochondrial fragmentation does not appear until day six; early TRiC
    assembly subcomplexes accumulate in the absence of MSTO1; MSTO1
    co-immunoprecipitates with TRiC; TRiC is reduced in patient fibroblasts; and
    depleting TRiC alone is sufficient to fragment mitochondria. If confirmed,
    this reverses the direction of the canonical model and would make
    MSTO1-related disease a cytosolic chaperonopathy with a mitochondrial
    readout. It would also predict phenotypic overlap with CCT-subunit disorders
    that has not yet been looked for clinically. It is recorded as EMERGING
    rather than ALTERNATIVE because it currently rests on one unreviewed
    preprint from one laboratory.
  evidence:
  - reference: PMID:42539026
    reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Together, our findings identify MSTO1 as a TRiC assembly factor and connect mitochondrial defects caused by MSTO1-depletion to the loss of TRiC."
    explanation: The authors' summary statement of the hypothesis.
  - reference: PMID:42539026
    reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "MSTO1 patient-derived fibroblasts have fragmented mitochondria and a striking loss of mtDNA."
    explanation: >-
      Confirms that the model is being built against the same cellular phenotype
      that defines the human disease, rather than against an unrelated cell
      system.
- hypothesis_group_id: msto1_rad51_mtdna
  hypothesis_label: MSTO1 restrains RAD51 at the mitochondrial genome and its loss depletes mtDNA directly
  status: EMERGING
  description: >-
    Proposed in 2026 from cancer cell models. MSTO1 binds RAD51 through a
    conserved FxxA motif and restrains its mitochondrial, but not its nuclear,
    activity; losing MSTO1 increases RAD51 occupancy on mtDNA, disrupts mtDNA
    replication and depletes the genome. This would make the mtDNA phenotype a
    direct consequence of MSTO1 loss rather than a sequela of fragmentation, and
    it is not mutually exclusive with the fusion model - both could contribute.
    The principal limitation is that the work is entirely in cancer and cultured
    cell systems, with no patient material, and that the accompanying
    cGAS-STING inflammatory arm has never been sought in a patient.
  evidence:
  - reference: PMID:42098462
    reference_title: "MSTO1 modulates RAD51 activity to safeguard mitochondrial DNA integrity and control immune responses."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Loss of MSTO1 impairs mitochondrial membrane potential, reduces mtDNA content, and increases susceptibility to oxidative stress."
    explanation: Shows that MSTO1 loss reproduces the patient mtDNA phenotype in a system where the proposed direct mechanism was mapped.
genetic:
- name: MSTO1
  association: Bi-allelic loss-of-function variants; no recurrent founder allele; a single reported dominant family was subsequently shown to carry its variant in the MSTO2P pseudogene
  relationship_type: CAUSATIVE
  gene_term:
    preferred_term: MSTO1
    term:
      id: hgnc:29678
      label: MSTO1
  notes: >-
    Nomenclature and locus. HGNC:29678 is symbol MSTO1, "misato mitochondrial
    distribution and morphology regulator 1", at 1q22, Entrez 55154, Ensembl
    ENSG00000125459, UniProt Q9BUK6, with alias symbols FLJ10504, LST005, MST and
    misato (HGNC REST API, retrieved 2026-08-01). The reference transcript used
    consistently across the clinical literature is NM_018116.3.

    The MSTO2P pseudogene problem, which is a practical laboratory issue and not
    only a historical curiosity. MSTO2P (HGNC:26329, verified against the HGNC
    REST API on 2026-08-01, which returns symbol MSTO2P, name "misato family
    member 2, pseudogene", locus_type "pseudogene", locus 1q22) is not a
    processed pseudogene: PMID:31463572 places it in a roughly 240 kb tandem
    segmental duplication at 1q22 arising from a duplication event in the human
    lineage, with 99.5% exonic nucleotide identity to MSTO1. That identity
    means short-read alignments can place pseudogene reads on the parent gene and
    vice versa. This is exactly what happened to the family reported as the first
    dominant MSTO1 case: the c.22G>A (p.Val8Met) variant was later confirmed by
    the original authors to lie in MSTO2P. Any laboratory reporting a
    heterozygous MSTO1 variant, and particularly a variant near the 5' end of the
    gene, should confirm that the read alignment is not pseudogene-derived before
    issuing a report.

    Allelic architecture. Reported alleles span the protein and include missense
    (for example p.Arg256Gln, p.Arg279His, p.Asp236His, p.Asp236Gly, p.Phe217Leu,
    p.Thr324Ile, p.Arg191Trp, p.Arg256Trp, p.Pro479Ser, p.Ala243Pro,
    p.Leu468Gln, p.Glu447Lys), frameshift (p.Gly420ValfsTer2), nonsense
    (p.Gln27*), splice-acceptor (c.1099-1G>A) and start-loss (c.1A>G, p.M1?)
    changes, together with at least one multi-exon intragenic deletion covering
    exons 7 to 14. Variant names in this note are transcribed from the abstracts
    of the cached references and are recorded here rather than as evidence
    snippets because each comes from a different paper. Because a copy-number
    allele can be one of the two hits, an exome that finds a single heterozygous
    MSTO1 variant in a compatible phenotype should be followed by
    deletion/duplication analysis rather than closed.

    Not asserted, deliberately. gnomAD constraint metrics (pLI, o/e LoF, missense
    Z) are not recorded anywhere in this entry because no cached source consulted
    here reports them and they must not be written from memory. No ClinGen
    gene-disease validity classification could be retrieved (see the entry-level
    notes), so the gene-disease relationship is typed CAUSATIVE on the strength
    of the published families - at least 20 independent families across the
    cited reports, with segregation, patient-cell functional data and rescue -
    rather than on a curated external assertion. No modifier locus, methylation
    episignature, founder allele or somatic mechanism has been established.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We present extensive phenotypic and genetic data from 12 independent families, including 15 new patients harbouring a broad array of bi-allelic MSTO1 pathogenic variants"
    explanation: >-
      The largest single body of gene-disease evidence - twelve independent
      families with bi-allelic variants and matching functional data.
  - reference: PMID:37431815
    reference_title: "Answer to Gerber et al. \"Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy\"."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "but also confirm finding of Gerber et al that the mutation is in MSTO2p pseudogene"
    explanation: >-
      The original authors' own confirmation that the variant underlying the
      reported dominant family lies in MSTO2P and not in MSTO1. This is the
      single most important genetic correction for this disease and the reason
      no dominant mode is asserted.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The majority of patients with disease-associated variants in MSTO1 present with biallelic variants suggesting autosomal recessive inheritance; however, one family has been reported with a single variant and presumed autosomal dominant inheritance."
    explanation: >-
      Documents the pre-2023 balance of evidence, in which a single family stood
      against all others, and identifies precisely which claim was later
      overturned.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "deletion encompassing exons 7-14 of MSTO1"
    explanation: Establishes that an intragenic copy-number deletion can constitute the second allele, which drives the diagnostic recommendation above.
phenotypes:
- name: Muscle weakness
  category: Musculoskeletal
  diagnostic: true
  description: >-
    Predominantly proximal weakness is the most constant clinical feature and
    usually the presenting one. Onset is between birth and three years in most
    patients, with hypotonia and delayed motor milestones; independent
    ambulation is generally achieved but with a waddling, wide-based gait.
    Progression is slow or absent, so this behaves as a static or very slowly
    progressive myopathy rather than as a relentlessly progressive dystrophy.
  phenotype_term:
    preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
    clinical_course: STABLE
  frequency: VERY_FREQUENT
  notes: >-
    Frequency band derived from an explicit count: at last examination all 15
    patients in PMID:31463572 had predominantly proximal weakness, and all four
    patients in PMID:41653646 exhibited myopathy of variable severity.
    VERY_FREQUENT (80-99%) rather than OBLIGATE because the total published
    denominator is around 30 and no systematic ascertainment has been done.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."
    explanation: Documents the presenting features and the age window.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All 15 patients reported either relatively slow progression or no progression of their muscle weakness."
    explanation: Supports both the near-universal frequency and the STABLE clinical course qualifier.
  - reference: PMID:41653646
    reference_title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients exhibited myopathy of variable severity."
    explanation: Independent 2026 replication of the near-universal myopathy in a separate four-patient series.
- name: Muscular dystrophy
  category: Musculoskeletal
  diagnostic: true
  description: >-
    Muscle histology in this disorder is dystrophic rather than merely
    myopathic, which is diagnostically important because it sends the workup
    towards the congenital muscular dystrophy panel rather than towards a
    mitochondrial panel. Biopsies show variation in fibre size, internalised
    nuclei, whorled fibres and mild necrosis with regeneration. Importantly the
    conventional oxidative histochemical markers of mitochondrial myopathy were
    absent where they were sought, so a normal COX and SDH stain does not
    exclude the diagnosis.
  phenotype_term:
    preferred_term: Muscular dystrophy
    term:
      id: HP:0003560
      label: Muscular dystrophy
  notes: >-
    No frequency band is asserted. Muscle biopsy was performed in 10 of 15
    patients in PMID:31463572 and all were dystrophic, but a 10/15 denominator
    reflects who was biopsied rather than who has the pathology, and biopsy in
    the era of first-line genome sequencing is increasingly selective.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsies were performed in ten patients and were consistent with a dystrophic process"
    explanation: Establishes the dystrophic character of the muscle pathology in the largest series.
  - reference: PMID:31130378
    reference_title: "A novel case of MSTO1 gene related congenital muscular dystrophy with progressive neurological involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Recessive mutations in the MSTO1 gene, encoding for a mitochondrial distribution and morphology regulator, have been recently described in a very limited number of patients with multisystem involvement, mostly characterized by myopathy or dystrophy, cerebellar ataxia, pigmentary retinopathy and raised creatine kinase levels."
    explanation: Summarises the histological spectrum from myopathic to frankly dystrophic across reported patients.
- name: Elevated creatine kinase
  category: Metabolic
  diagnostic: true
  description: >-
    A raised serum creatine kinase is the most useful single screening
    abnormality. Levels in the largest cohort ranged from 300 to 5000 U/L and
    were increased in 14 of 15 patients, so a normal creatine kinase makes the
    diagnosis less likely but does not exclude it. Combined with brisk reflexes
    and a wide-based gait in a child with proximal weakness, a raised creatine
    kinase is the pattern that should prompt sequencing.
  phenotype_term:
    preferred_term: Elevated circulating creatine kinase concentration
    term:
      id: HP:0003236
      label: Elevated circulating creatine kinase concentration
  frequency: VERY_FREQUENT
  notes: >-
    Frequency band derived directly from the quoted count of 14 of 15 patients
    (93%) in PMID:31463572, which falls in the VERY_FREQUENT (80-99%) band.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serum creatine kinase (CK) levels were significantly increased in all patients, except for P15, ranging from 300 to 5000 U/L."
    explanation: Both the numerator/denominator for the frequency band and the reported concentration range.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Testing revealed elevated plasma creatine kinase, and electromyogram results were consistent with longstanding generalized myopathy."
    explanation: Independent confirmation in a separately reported adult patient.
- name: Cerebellar atrophy
  category: Neurologic
  diagnostic: true
  description: >-
    Cerebellar atrophy or hypoplasia involving the vermis and both hemispheres
    is the imaging hallmark. It is present early - four patients in the largest
    cohort were imaged before age two and already showed significantly reduced
    cerebellar volume - and in most patients it does not progress on repeat
    imaging, which distinguishes it from most acquired and many genetic
    cerebellar degenerations. A minority of reports describe progression.
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
  frequency: VERY_FREQUENT
  notes: >-
    Frequency band derived from an explicit count: 12 of 12 patients imaged in
    PMID:31463572. VERY_FREQUENT rather than OBLIGATE because only 12 of the 15
    patients in that cohort had imaging available, and because PMID:41653646
    reported MRI evidence of cerebellar atrophy in only one of four patients -
    a discrepancy that is itself notable and that argues against treating the
    cerebellar lesion as obligate.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "involving the vermis and both hemispheres in all 12"
    explanation: The 12/12 denominator and the anatomical distribution.
  - reference: PMID:41653646
    reference_title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Magnetic resonance imaging evidence of cerebellar atrophy was noted in Patient 3."
    explanation: >-
      Marked PARTIAL because in this four-patient 2026 series cerebellar atrophy
      was noted in only one patient, which is the counterweight to the 12/12
      figure and the reason the band is not OBLIGATE.
- name: Ataxia
  category: Neurologic
  diagnostic: true
  description: >-
    Cerebellar signs are the second clinical pillar of the syndrome. Gait ataxia
    was present in 9 of 15 patients in the largest cohort and dysmetria in 11 of
    15. Because the underlying cerebellar atrophy is usually stable, the ataxia
    is typically non-progressive; the exceptions are the reported cases with
    progressive cerebellar involvement and the adult-onset progressive case.
  phenotype_term:
    preferred_term: Gait ataxia
    term:
      id: HP:0002066
      label: Gait ataxia
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the explicit count of gait ataxia in 9 of 15
    patients (60%) in PMID:31463572, which falls in the FREQUENT (30-79%) band.
    Dysmetria at 11 of 15 is curated as a separate phenotype below.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."
    explanation: Provides the numerator and denominator for the frequency band.
  - reference: PMID:36468072
    reference_title: "Indentification of novel MSTO1 compound heterozygous mutations in a Chinese family with recessive cerebellar atrophy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we reported two siblings with progressive cerebellar atrophy and ataxia in a Chinese family."
    explanation: Independent replication, and one of the reports describing a progressive rather than a static cerebellar course.
- name: Dysmetria
  category: Neurologic
  description: >-
    Appendicular cerebellar dysfunction, recorded in 11 of 15 patients in the
    largest cohort and the most frequently detected cerebellar sign in that
    series - more common than gait ataxia, which means examination of limb
    coordination is a higher-yield test than observing gait in this disorder.
  phenotype_term:
    preferred_term: Dysmetria
    term:
      id: HP:0001310
      label: Dysmetria
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the explicit count of 11 of 15 (73%) in
    PMID:31463572, in the FREQUENT (30-79%) band.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."
    explanation: The count of 11 of 15 for dysmetria.
- name: Dysarthria
  category: Neurologic
  description: >-
    Cerebellar speech disturbance, present in 8 of 15 patients in the largest
    cohort. Speech abnormality of some kind - dysarthria or a history of speech
    delay - was the single most frequent cerebellar manifestation, affecting 12
    of 15.
  phenotype_term:
    preferred_term: Dysarthria
    term:
      id: HP:0001260
      label: Dysarthria
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the explicit count of 8 of 15 (53%) in
    PMID:31463572, in the FREQUENT (30-79%) band. The broader category of any
    abnormal speech (12 of 15) is not used for the band because it aggregates
    dysarthria with developmental speech delay, which is a different phenotype
    and is curated separately below.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."
    explanation: The count of 8 of 15 for dysarthria specifically.
- name: Delayed speech and language development
  category: Neurologic
  description: >-
    A history of speech delay was recorded in 5 of 15 patients in the largest
    cohort, and language disorder features in independent case reports. It sits
    alongside, rather than within, the cerebellar dysarthria: the two are
    counted separately in the source series.
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the explicit count of 5 of 15 (33%) in
    PMID:31463572, which just enters the FREQUENT (30-79%) band. This is a
    borderline assignment and a single reclassified patient would move it to
    OCCASIONAL.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cerebellar symptoms manifested as dysmetria in 11, gait ataxia in nine and abnormal speech in 12, which included a history of speech delay in five and dysarthria in eight patients, respectively."
    explanation: The count of 5 of 15 for speech delay.
  - reference: PMID:36035138
    reference_title: "Case Report: Evidences of myasthenia and cerebellar atrophy in a chinese patient with novel compound heterozygous MSTO1 variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical features are infancy-onset mental and motor retardation, language disorder, dysarthria, scoliosis, cerebellar atrophy, tremor, lower-extremity muscle weakness, elevated muscle enzymes, extensive myopathy with chronic atrophy, hyperventilation lungs, and previously unreported hairy back and enlarged gastrocnemius."
    explanation: Independent case documenting language disorder alongside dysarthria in the same patient.
- name: Motor delay
  category: Neurologic
  diagnostic: true
  description: >-
    Delayed motor milestones are the usual presentation, recognised between
    birth and three years. All but five patients in the largest cohort had
    delayed milestones and subsequently achieved independent ambulation. The
    delay is attributable both to the muscle disease and to the cerebellar
    lesion, and the two cannot be separated clinically.
  phenotype_term:
    preferred_term: Motor delay
    term:
      id: HP:0001270
      label: Motor delay
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the statement that all but five of 15 patients
    (10 of 15, 67%) had delayed motor milestones in PMID:31463572, which falls
    in the FREQUENT (30-79%) band.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."
    explanation: Establishes delayed motor milestones as the typical presentation and gives the age window.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "His phenotype included muscle weakness, hypotonia, early motor developmental delay, pectus excavatum, and scoliosis."
    explanation: Independent replication of hypotonia with early motor delay in a separately reported patient.
- name: Generalized hypotonia
  category: Neurologic
  description: >-
    Hypotonia is part of the usual presentation and in a minority is congenital.
    In the 2026 Mayo series two of four patients had congenital hypotonia with
    global developmental delay while the other two developed weakness at ages
    two and five, illustrating that the disorder has both a congenital-onset and
    a later-childhood-onset presentation.
  phenotype_term:
    preferred_term: Generalized hypotonia
    term:
      id: HP:0001290
      label: Generalized hypotonia
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients typically presented with hypotonia and delayed motor milestones with first symptoms recognized between birth and three years of age."
    explanation: Hypotonia named as a typical presenting feature.
  - reference: PMID:41653646
    reference_title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two had congenital hypotonia and global developmental delay, while the remaining two developed muscle weakness at ages 2 and 5."
    explanation: Documents the split between congenital and later-onset presentations within a single small series.
- name: Pigmentary retinopathy
  category: Ophthalmologic
  description: >-
    A rod-predominant pigmentary retinal dystrophy. Where it has been imaged in
    detail it shows widespread salt-and-pepper pigmentation, macular atrophy,
    mottled hypoautofluorescence and ellipsoid-zone degeneration; functionally
    it produces concentric visual field constriction with initially preserved
    acuity. It names the MONDO term, but it is not present in every patient and
    was absent from the largest reported cohort, so it should be understood as a
    feature to look for rather than a criterion to require.
  phenotype_term:
    preferred_term: Pigmentary retinopathy
    term:
      id: HP:0000580
      label: Pigmentary retinopathy
  notes: >-
    No frequency band is asserted and the reason is a genuine conflict rather
    than missing data. Present in both probands of PMID:29339779, in patient A1
    of PMID:28544275 and in both sisters of PMID:40542878; absent in 0 of 15 of
    PMID:31463572, where the authors note that formal ophthalmological
    examination was not performed in every patient. Pooling those numerators
    across cohorts that used different ophthalmological protocols would
    manufacture a precision the literature does not support. See the
    `msto1-retinopathy-denominator` discussion.
  evidence:
  - reference: PMID:29339779
    reference_title: "Novel recessive mutations in MSTO1 cause cerebellar atrophy with pigmentary retinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We performed whole-exome sequencing in two unrelated patients showing cerebellar atrophy, intellectual disability, and pigmentary retinopathy."
    explanation: Two unrelated probands with pigmentary retinopathy, the observation that put the feature into the disease name.
  - reference: PMID:40542878
    reference_title: "Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our study provides a comprehensive ophthalmic characterization of MMYAT, indicating that retinal dystrophy is a key characteristic of this disease."
    explanation: The strongest positive statement in the literature, based on 17 years of multimodal imaging follow-up in two sisters.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "major cognitive involvement, seizures, retinopathy, optic atrophy or hearing loss were not seen"
    explanation: >-
      Explicit absence of retinopathy in the 15-patient cohort. Retained as
      REFUTE evidence against a universal claim; it does not refute the
      existence of the feature in other patients.
- name: Scoliosis
  category: Musculoskeletal
  description: >-
    Spinal deformity is the most frequently reported non-neurological
    abnormality and is a consequence of longstanding axial muscle weakness
    rather than a primary skeletal dysplasia. It recurs across independent
    cohorts and case reports and is one of the features that brings these
    patients to orthopaedic attention before the diagnosis is made.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  evidence:
  - reference: PMID:41653646
    reference_title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most common non-neurologic abnormality noted in our cases was skeletal abnormalities."
    explanation: Identifies skeletal abnormality as the leading extra-neurological finding in the most recent series.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "His phenotype included muscle weakness, hypotonia, early motor developmental delay, pectus excavatum, and scoliosis."
    explanation: Scoliosis with pectus excavatum in an independently reported adult patient.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient showed severe asymmetry of the chest, with enlargement of the right hemithorax, pectus excavatum, and marked scoliosis"
    explanation: The most severe reported thoracic deformity, showing how far the axial phenotype can go.
- name: Pectus excavatum
  category: Musculoskeletal
  description: >-
    Anterior chest wall deformity accompanies the scoliosis and, like it,
    reflects chronic weakness of the muscles that shape the growing thorax. It
    is clinically consequential because it compounds the restrictive
    respiratory physiology.
  phenotype_term:
    preferred_term: Pectus excavatum
    term:
      id: HP:0000767
      label: Pectus excavatum
  evidence:
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "His phenotype included muscle weakness, hypotonia, early motor developmental delay, pectus excavatum, and scoliosis."
    explanation: Direct report of pectus excavatum in an MSTO1 patient.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient showed severe asymmetry of the chest, with enlargement of the right hemithorax, pectus excavatum, and marked scoliosis"
    explanation: Independent replication with a description of the associated chest asymmetry.
- name: Restrictive ventilatory defect
  category: Respiratory
  description: >-
    Reduced forced vital capacity was found in the nine patients tested in the
    largest cohort, ranging from 60 to 79% of predicted, and restrictive lung
    disease has been reported independently. Because these values were found on
    screening rather than because of symptoms, and because chest wall deformity
    compounds the diaphragmatic weakness, pulmonary function testing should be
    scheduled rather than reactive.
  phenotype_term:
    preferred_term: Restrictive ventilatory defect
    term:
      id: HP:0002091
      label: Restrictive ventilatory defect
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "revealed reduced forced vital capacity (FVC) measurements, ranging from 60 to 79% predicted"
    explanation: Quantifies the restrictive defect in the nine patients who underwent pulmonary function testing.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additionally, our patient presents with dysphagia and restrictive lung disease, not previously reported for MSTO1-associated disorders."
    explanation: Independent confirmation of restrictive lung disease, reported as novel at the time.
- name: Hyperreflexia
  category: Neurologic
  description: >-
    Brisk deep tendon reflexes, increased tone, a spastic catch and clonus were
    recorded in 8 of 15 patients in the largest cohort. Hyperreflexia in a child
    whose weakness is otherwise myopathic is the clinical signature that
    something central is also involved, and it is one of the more useful
    discriminating signs at the bedside.
  phenotype_term:
    preferred_term: Hyperreflexia
    term:
      id: HP:0001347
      label: Hyperreflexia
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the count of corticospinal tract manifestations
    in 8 of 15 patients (53%) in PMID:31463572, in the FREQUENT (30-79%) band.
    The source counts increased tone, spastic catch, clonus and brisk reflexes
    together, so the band applies to the corticospinal syndrome as a whole
    rather than to hyperreflexia in isolation.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bi-allelic loss-of-function variants in MSTO1 manifest clinically with a remarkably consistent phenotype of childhood-onset muscular dystrophy, corticospinal tract dysfunction and early-onset non-progressive cerebellar atrophy."
    explanation: Corticospinal tract dysfunction named as a core component of the phenotype.
  - reference: PMID:33612823
    reference_title: "A novel homozygous MSTO1 mutation in Ashkenazi Jewish siblings with ataxia and myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "often with upper motor neuron signs and varied cognitive impairment"
    explanation: Independent statement that upper motor neuron signs are a recurring feature.
- name: Mild intellectual disability
  category: Neurologic
  description: >-
    Cognition is affected but only mildly. Learning difficulties were reported
    in 9 of 15 patients in the largest cohort while none had major cognitive
    involvement, and independent reports describe intellectual disability
    alongside the motor phenotype. The consistent picture is of learning
    difficulty rather than of the profound impairment seen in encephalopathic
    mitochondrial disease, and this is one of the features that separates MSTO1
    disease from the classical mtDNA depletion syndromes.
  phenotype_term:
    preferred_term: Mild intellectual disability
    term:
      id: HP:0001256
      label: Mild intellectual disability
  frequency: FREQUENT
  notes: >-
    Frequency band derived from the count of learning difficulties in 9 of 15
    patients (60%) in PMID:31463572, in the FREQUENT (30-79%) band. The
    severity qualifier "mild" comes from the same source's statement that no
    patient had major cognitive involvement, which is quoted below.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "major cognitive involvement, seizures, retinopathy, optic atrophy or hearing loss were not seen"
    explanation: Sets the ceiling on cognitive impairment in this disorder, which is what justifies the "mild" HPO term rather than a generic intellectual disability term.
  - reference: PMID:30684668
    reference_title: "Whole-exome sequencing identifies rare compound heterozygous mutations in the MSTO1 gene associated with cerebellar ataxia and myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we have performed a whole-exome sequencing in a family which includes two brothers showing cerebellar atrophy and ataxia, intellectual disability, and myopathy."
    explanation: Independent replication of intellectual disability as part of the triad in two affected brothers.
- name: Tremor
  category: Neurologic
  description: >-
    Tremor is reported in individual patients, including the index patient of
    the first recessive series in whom fine tremors were noted from age five,
    and in a Chinese case
    report. It is presumed cerebellar in origin. It is not counted in the
    largest cohort, so its frequency is unknown.
  phenotype_term:
    preferred_term: Tremor
    term:
      id: HP:0001337
      label: Tremor
  notes: >-
    No frequency band is asserted: tremor is reported in individual cases and
    is not among the signs counted in PMID:31463572, so there is no denominator
    to band.
  evidence:
  - reference: PMID:36035138
    reference_title: "Case Report: Evidences of myasthenia and cerebellar atrophy in a chinese patient with novel compound heterozygous MSTO1 variants."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical features are infancy-onset mental and motor retardation, language disorder, dysarthria, scoliosis, cerebellar atrophy, tremor, lower-extremity muscle weakness, elevated muscle enzymes, extensive myopathy with chronic atrophy, hyperventilation lungs, and previously unreported hairy back and enlarged gastrocnemius."
    explanation: Tremor listed as part of the clinical picture in an independently reported patient.
- name: Dysphagia
  category: Gastrointestinal
  description: >-
    Swallowing difficulty has been reported in an adult patient and, like the
    restrictive lung disease in the same report, was described as not
    previously recognised in MSTO1 disease. It is mechanistically consistent
    with bulbar and pharyngeal involvement in a generalised dystrophic myopathy
    and matters clinically because of aspiration risk.
  phenotype_term:
    preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
  notes: >-
    Reported in a single patient. No frequency band is asserted, and the
    feature is retained because its clinical consequence (aspiration risk) is
    high even at low frequency.
  evidence:
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additionally, our patient presents with dysphagia and restrictive lung disease, not previously reported for MSTO1-associated disorders."
    explanation: The single report of dysphagia in this disorder, with the authors' own note that it was novel.
- name: Absence of seizures, cataract, deafness and cardiac involvement
  category: Neurologic
  description: >-
    A deliberately negative phenotype entry, because what this disorder does NOT
    do is diagnostically load-bearing. None of the 15 patients in the largest
    cohort had seizures, cataracts, hearing loss or cardiac involvement, and
    echocardiography was normal in all 11 who underwent it. This separates MSTO1
    disease from Marinesco-Sjogren syndrome (cataract), from Kearns-Sayre
    syndrome and other mtDNA disorders (cardiac conduction disease,
    sensorineural deafness), and from the encephalopathic mtDNA depletion
    syndromes (seizures). It also means routine cardiac surveillance, which is
    standard in most muscular dystrophies, has no established indication here.
  phenotype_term:
    preferred_term: Seizure
    modifier: ABSENT
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "None of the patients had a history of seizures, cataracts, hearing or cardiac involvement."
    explanation: The explicit negative-findings statement on which this entry rests.
histopathology:
- name: Dystrophic muscle with increased variability in fibre diameter
  description: >-
    Muscle biopsies in ten patients of the largest cohort were consistent with a
    dystrophic process, with variation in fibre type size, mild necrosis and
    regeneration, internalised nuclei and whorled fibres. The oxidative
    histochemical hallmarks of mitochondrial myopathy - ragged red fibres and
    cytochrome c oxidase negative fibres - were not present where they were
    sought, and the earlier 2017 recessive series explicitly recorded no oxidative
    histochemical defect in COX or SDH. A pathologist reading this biopsy
    without genetic information would report a congenital muscular dystrophy,
    not a mitochondrial myopathy.
  finding_term:
    preferred_term: Dystrophic muscle with increased variability in fibre diameter, internalised nuclei, whorled fibres and mild necrosis with regeneration
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsies were performed in ten patients and were consistent with a dystrophic process"
    explanation: The histological conclusion from ten biopsies.
imaging_findings:
- name: Cerebellar atrophy and hypoplasia of vermis and hemispheres on brain MRI
  modality: MRI
  imaging_finding_term:
    preferred_term: Cerebellar atrophy or hypoplasia of vermis and both hemispheres
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
  located_in:
    preferred_term: cerebellum
    term:
      id: UBERON:0002037
      label: cerebellum
  laterality: BILATERAL
  description: >-
    Brain MRI is the single most informative imaging study. Moderate to severe
    cerebellar atrophy or hypoplasia involving the vermis and both hemispheres
    was present in all twelve patients imaged in the largest cohort, four of
    whom were imaged before age two. Repeat imaging in four patients confirmed
    no progression, with only one patient showing mild progression between ages
    one and six. Supratentorial white matter signal change has been reported in
    individual patients but is not a consistent feature.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "involving the vermis and both hemispheres in all 12"
    explanation: Establishes the distribution and the 12/12 imaging denominator.
- name: Selective pattern of muscle involvement on lower-limb muscle MRI
  modality: MRI
  imaging_finding_term:
    preferred_term: Selective fatty replacement of thigh muscles with relative sparing of semimembranosus and biceps femoris
  located_in:
    preferred_term: skeletal muscle tissue
    term:
      id: UBERON:0001134
      label: skeletal muscle tissue
  description: >-
    Muscle MRI shows a distinct and potentially recognisable pattern. In the
    largest cohort, imaging in four patients ranged from severe fatty
    replacement of all upper-leg muscles except semimembranosus and biceps
    femoris to mild fatty infiltration of upper and lower leg. An independent
    report describes a distinct pattern of selective involvement in
    MSTO1-related disease. With only a handful of imaged patients the pattern is
    a lead for pattern-recognition diagnosis rather than an established
    diagnostic sign.
  evidence:
  - reference: PMID:31130378
    reference_title: "A novel case of MSTO1 gene related congenital muscular dystrophy with progressive neurological involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We also report a distinct pattern of selective involvement on muscle MRI in MSTO1-RD."
    explanation: The claim that muscle MRI shows a disorder-specific pattern, from an independent group.
biochemical:
- name: Normal blood lactate and normal respiratory chain enzyme activities
  presence: Normal
  notes: >-
    The standard mitochondrial biochemical screen is normal in most patients.
    Lactate is not elevated, respiratory chain complex activities in muscle
    homogenate are normal or near normal, and fibroblast respiration is normal.
    This is the opposite of what a clinician expects in a disease that has been
    proposed to belong among the mtDNA depletion syndromes, and it is the main
    reason these
    patients were historically not identified through metabolic pathways. The
    2026 adult-onset case is the exception, with reduced membrane potential and
    respiratory capacity.

    Curated as a normal-result biomarker because the negative result carries
    diagnostic information - it must not be used to exclude the diagnosis. No
    LOINC-coded reference range is attached because the sources report
    qualitative normality rather than an interval.
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our biochemical analyses were not indicative of a mitochondrial disorder: patients showed normal lactate levels, and hardly any defect of MRC complex activities or cell respiration"
    explanation: The direct statement that the mitochondrial biochemical workup is uninformative in this disorder.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As for other mutations affecting genes involved in mitochondrial dynamics, no biochemical defects typical of mitochondrial disorders were reported."
    explanation: Places the negative biochemistry in context as a property of mitochondrial-dynamics disorders generally, not an anomaly of this cohort.
- name: Reduced citrate synthase activity in muscle
  presence: Decreased
  notes: >-
    Citrate synthase activity, the conventional index of mitochondrial mass, was
    markedly reduced in patient muscle in the 2017 recessive series even though
    respiratory chain complex activities normalised to citrate synthase were
    unremarkable. Read together with the reduced mtDNA content, this supports a
    model of fewer mitochondria rather than defective ones, and it is the
    biochemical counterpart of the dissociation node in the pathograph.

    Reported in a single family (patients A1 and A2 of PMID:28544275). It has
    not been measured systematically in later cohorts, so its sensitivity as a
    biomarker is unknown and it should not be used as a diagnostic test.
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, we detected markedly low activity of citrate synthase and reduced mtDNA amount, suggesting a reduction in mitochondrial mass."
    explanation: The finding and the authors' interpretation of it as reduced mitochondrial mass.
- name: Reduced mitochondrial DNA copy number
  presence: Decreased
  notes: >-
    Quantitative measurement of mtDNA relative to nuclear DNA is the one
    laboratory assay that is reliably abnormal. Patient fibroblasts show 30-70%
    depletion, and the single patient in whom muscle was assayed had 27% of
    control. Because the assay is available in most mitochondrial diagnostic
    laboratories, it is a reasonable supporting test once an MSTO1 genotype is
    suspected - though not a screening test, since mtDNA depletion is shared
    with many other genetic causes.
  cell_types:
  - preferred_term: patient-derived dermal fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We observed a significant reduction in mtDNA copy number across all fibroblast lines, ranging from 30 to 70% depletion"
    explanation: The quantitative fibroblast result across all seven patient lines.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "mtDNA amount was reduced to 27% of the mean control value"
    explanation: The only muscle measurement, in the index patient of the 2017 recessive series.
diagnosis:
- name: Exome or genome sequencing with bi-allelic MSTO1 variant interpretation
  description: >-
    Molecular diagnosis rests on identifying bi-allelic MSTO1 variants against
    transcript NM_018116.3, and in practice the diagnosis has almost always been
    made by broad sequencing after targeted neuromuscular panels returned
    negative. Three interpretive points follow from the published experience.
    First, both parents should be tested to establish that the two variants are
    in trans, since heterozygotes are unaffected. Second, if only one
    heterozygous sequence variant is found in a compatible phenotype, proceed to
    deletion/duplication analysis, because at least one reported patient carried
    a multi-exon intragenic deletion as the second allele. Third, and specific
    to this gene, confirm that a called variant is not a mis-mapped read from
    the adjacent MSTO2P pseudogene - the misassignment that produced the
    literature's only reported dominant family.
  evidence:
  - reference: PMID:41653646
    reference_title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Exome or genome sequencing was used to identify genetic variants in patients with suspected hereditary myopathy who had negative results on targeted genetic panels."
    explanation: Describes the diagnostic route actually used, and the reason panel-negative myopathy should prompt broad sequencing.
  - reference: PMID:31604776
    reference_title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "deletion encompassing exons 7-14 of MSTO1"
    explanation: Supports the specific recommendation to pursue copy-number analysis when only one sequence variant is found.
  - reference: PMID:37431815
    reference_title: "Answer to Gerber et al. \"Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy\"."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "but also confirm finding of Gerber et al that the mutation is in MSTO2p pseudogene"
    explanation: The documented instance of pseudogene misassignment that justifies the third interpretive caution.
- name: Serum creatine kinase and neurological examination
  description: >-
    The pre-test clinical pattern that should trigger sequencing is a child with
    early-onset, slowly progressive or static proximal weakness, a raised
    creatine kinase, a wide-based waddling gait, cerebellar signs on limb
    coordination testing, and brisk reflexes. The combination of a myopathic
    weakness pattern with upper motor neuron signs is unusual and is the
    strongest bedside pointer. Creatine kinase is elevated in the large majority
    but not all patients.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serum creatine kinase (CK) levels were significantly increased in all patients, except for P15, ranging from 300 to 5000 U/L."
    explanation: Establishes both the diagnostic value and the limits of creatine kinase as a screening test.
- name: Brain MRI
  description: >-
    Brain MRI is indicated in any child with an unexplained dystrophic myopathy
    and should be read specifically for cerebellar volume. Cerebellar atrophy or
    hypoplasia of the vermis and both hemispheres was present in every patient
    imaged in the largest cohort, and finding it in a child whose muscle biopsy
    reads as congenital muscular dystrophy substantially narrows the
    differential.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "involving the vermis and both hemispheres in all 12"
    explanation: The imaging finding and its consistency across the cohort.
- name: Ophthalmological assessment with multimodal retinal imaging
  description: >-
    Formal ophthalmological review with fundus autofluorescence and optical
    coherence tomography is recommended even in patients without visual symptoms,
    for two reasons. First, the retinopathy causes concentric field constriction
    with initially preserved acuity, so it is asymptomatic until late. Second,
    the largest cohort reported no retinopathy but did not examine every patient,
    so the true frequency is unknown and can only be established by
    protocolised examination.
  evidence:
  - reference: PMID:40542878
    reference_title: "Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multimodal imaging of the retina is beneficial for diagnosing MMYAT-associated retinal dystrophy."
    explanation: The authors' explicit recommendation for the imaging modality, from the only detailed ophthalmological study of this disorder.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This finding was not reported in any of the patients in our cohort; however, formal ophthalmologic examination was not pursued in all patients."
    explanation: The incomplete-ascertainment admission that makes protocolised examination necessary.
- name: Mitochondrial DNA copy number quantification
  description: >-
    Quantitative mtDNA content in fibroblasts or muscle is a supportive rather
    than a diagnostic test. It is consistently reduced in MSTO1 patients, which
    can help resolve a variant of uncertain significance, but depletion is
    shared with many other nuclear-gene mtDNA depletion syndromes and so has no
    specificity on its own. Conversely, a normal respiratory chain assay must
    not be used to exclude the diagnosis.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We observed a significant reduction in mtDNA copy number across all fibroblast lines, ranging from 30 to 70% depletion"
    explanation: Establishes the consistency of the finding that makes it useful as supporting evidence.
differential_diagnoses:
- name: Marinesco-Sjogren syndrome
  disease_term:
    preferred_term: Marinesco-Sjogren syndrome
    term:
      id: MONDO:0009567
      label: Marinesco-Sjogren syndrome
  description: >-
    The closest clinical mimic and the one most likely to be considered first.
    Marinesco-Sjogren syndrome, caused by bi-allelic SIL1 variants, pairs a
    myopathy with raised creatine kinase and cerebellar atrophy with ataxia,
    which is precisely the MSTO1 core. The index patient of the first recessive
    MSTO1 family had SIL1 sequenced and excluded before the MSTO1 diagnosis was
    reached, which is direct documentation that this differential is worked
    through in practice.
  distinguishing_features:
  - Early-onset bilateral cataract is a cardinal feature of Marinesco-Sjogren syndrome and was absent in all 15 patients of the largest MSTO1 cohort.
  - Hypergonadotropic hypogonadism and short stature are typical of Marinesco-Sjogren syndrome and are not established features of MSTO1 disease.
  - Marinesco-Sjogren muscle shows rimmed autophagic vacuoles with abnormal membranous whorls, whereas MSTO1 muscle shows a non-specific dystrophic pattern.
  - Only the causal gene distinguishes the two definitively; SIL1 was verified as HGNC:24624 and Marinesco-Sjogren syndrome as MONDO:0009567 with OMIM:248800 using OAK and the HGNC REST API on 2026-08-01.
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The following genes were previously excluded by Sanger sequencing: SPG7, FKTN, POMT1, POMT2, LARGE, POMGNT1, and SIL1."
    explanation: >-
      Documents that SIL1 - and the dystroglycanopathy genes - were actively
      excluded in an MSTO1 patient, which is what makes this a real rather than
      a theoretical differential.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "None of the patients had a history of seizures, cataracts, hearing or cardiac involvement."
    explanation: The absence of cataract in every MSTO1 patient is the single most useful clinical discriminator from Marinesco-Sjogren syndrome.
- name: Dystroglycanopathy-type congenital muscular dystrophy
  description: >-
    A child with congenital or early-childhood dystrophic weakness, a raised
    creatine kinase and cerebellar abnormality on MRI fits the
    dystroglycanopathy phenotype closely, and these genes are on every
    congenital muscular dystrophy panel while MSTO1 historically was not. The
    first recessive MSTO1 family had FKTN, POMT1, POMT2, LARGE and POMGNT1
    sequenced and excluded before MSTO1 was found, so this is the differential
    that actually delays diagnosis.
  distinguishing_features:
  - Dystroglycanopathies typically show cobblestone lissencephaly, brainstem and cerebellar dysplasia with cysts, and ocular malformation; MSTO1 disease shows isolated cerebellar volume loss with a structurally normal cortex.
  - Immunohistochemistry for glycosylated alpha-dystroglycan is reduced in dystroglycanopathies and is not reported as abnormal in MSTO1 muscle.
  - Corticospinal tract signs are a recurrent feature of MSTO1 disease and are not characteristic of the dystroglycanopathies.
  - No MONDO term is bound for this differential because it names a genetically heterogeneous group rather than a single ontology entity; the genes are named instead, quoted from the source below.
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The following genes were previously excluded by Sanger sequencing: SPG7, FKTN, POMT1, POMT2, LARGE, POMGNT1, and SIL1."
    explanation: The dystroglycanopathy genes named as having been excluded in a patient who turned out to have MSTO1 disease.
- name: Mitochondrial DNA depletion syndrome, myopathic form
  disease_term:
    preferred_term: mitochondrial DNA depletion syndrome, myopathic form
    term:
      id: MONDO:0012301
      label: mitochondrial DNA depletion syndrome, myopathic form
  description: >-
    The nosological neighbour. TK2-related myopathic mtDNA depletion syndrome is
    the disorder MSTO1 disease most resembles at the level of laboratory
    mechanism - both present with a myopathy and reduced mtDNA copy number - and
    the largest MSTO1 cohort argued explicitly that MSTO1 deficiency should be
    classified as an mtDNA depletion syndrome. The distinction is therefore
    genetic and clinical rather than biochemical.
  distinguishing_features:
  - TK2 disease is a defect of the mitochondrial nucleoside salvage pathway (TK2 verified as HGNC:11831 via the HGNC REST API on 2026-08-01), whereas MSTO1 encodes a cytosolic protein acting on organelle dynamics.
  - TK2 myopathy is typically relentlessly progressive with respiratory failure, whereas MSTO1 weakness is slowly progressive or static in every patient of the largest cohort.
  - Cerebellar atrophy with ataxia and corticospinal signs is core to MSTO1 disease and is not a feature of the myopathic form of TK2 disease.
  - Ragged red and COX-negative fibres with respiratory chain deficiency are usual in TK2 muscle; MSTO1 muscle shows a dystrophic pattern with normal oxidative histochemistry and normal respiratory chain activities.
  - MONDO:0012301 was verified with OAK on 2026-08-01, returning OMIM:609560 and RO:0004003 HGNC:11831 TK2.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "this novel link to mtDNA depletion in patient fibroblasts suggests that MSTO1-deficiency should also be considered a mtDNA depletion syndrome"
    explanation: Establishes that the two disorders belong to the same mechanistic class, which is what makes the clinical distinctions above necessary.
- name: Kearns-Sayre syndrome
  disease_term:
    preferred_term: Kearns-Sayre syndrome
    term:
      id: MONDO:0010787
      label: Kearns-Sayre syndrome
  description: >-
    The differential that the pigmentary retinopathy generates. A patient with
    pigmentary retinopathy, ataxia and a myopathy will be investigated for
    Kearns-Sayre syndrome, and the single-large-scale mtDNA deletion syndromes
    are a standard part of that workup. The two are distinguished by the
    remainder of the Kearns-Sayre triad and by the genetic mechanism.
  distinguishing_features:
  - Kearns-Sayre syndrome requires chronic progressive external ophthalmoplegia with onset before age 20; ophthalmoplegia and ptosis are not features of MSTO1 disease.
  - Cardiac conduction block is a defining and life-limiting component of Kearns-Sayre syndrome; cardiac involvement was absent in all 11 MSTO1 patients who had echocardiography.
  - Kearns-Sayre syndrome is caused by a single large-scale mtDNA deletion, usually sporadic, whereas MSTO1 disease is nuclear and autosomal recessive with an inheritance risk to siblings.
  - Ragged red fibres and COX-negative fibres are typical in Kearns-Sayre muscle and absent in MSTO1 muscle.
  - MONDO:0010787 was verified with OAK on 2026-08-01 (OMIM:530000) and matches the disease_term already bound in kb/disorders/Kearns-Sayre_Syndrome.yaml.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "None of the patients had a history of seizures, cataracts, hearing or cardiac involvement."
    explanation: Absence of cardiac and audiological involvement in MSTO1 disease is the practical discriminator from Kearns-Sayre syndrome.
- name: Charcot-Marie-Tooth disease type 2A2
  disease_term:
    preferred_term: Charcot-Marie-Tooth disease type 2A2
    term:
      id: MONDO:0012231
      label: Charcot-Marie-Tooth disease type 2A2
  description: >-
    Included as a mechanistic rather than a clinical differential. CMT2A2 is
    caused by variants in MFN2, the outer-membrane mitofusin that executes the
    fusion reaction MSTO1 is thought to support, and MFN2 disease is one of the
    two fusion disorders in which mtDNA depletion was first demonstrated. The
    pairing establishes that lesions at adjacent points of one fusion pathway
    both produce neurological disease with mtDNA depletion, which is the
    argument for treating fusion-related mtDNA depletion as a coherent group.
  distinguishing_features:
  - CMT2A2 is a length-dependent axonal neuropathy with distal weakness, sensory loss, areflexia and foot deformity; MSTO1 disease is proximal, myopathic and often hyperreflexic.
  - CMT2A2 is usually autosomal dominant, whereas MSTO1 disease is recessive with unaffected carrier parents.
  - Creatine kinase is normal or minimally raised in CMT2A2 and substantially raised in most MSTO1 patients.
  - MFN2 acts at the mitochondrial outer membrane as a GTPase; MSTO1 is cytosolic and no GTPase activity has been demonstrated for it.
  - MONDO:0012231 was verified with OAK on 2026-08-01, returning OMIM:609260 and RO:0004003 HGNC:16877 MFN2. Note that MONDO:0007308 (CMT2A1) is a distinct term associated with KIF1B and is deliberately NOT the term bound here.
  notes: >-
    The MONDO identifier for this differential was checked rather than recalled,
    and the check mattered: the obvious search hit for "Charcot-Marie-Tooth
    disease type 2A" is MONDO:0007308, which OAK reports as CMT2A1 with causal
    gene KIF1B, not MFN2. Binding that term would have attached the wrong gene
    to the mechanistic argument this differential makes. No evidence item is
    attached because the comparison is a reasoned cross-reference rather than a
    quoted finding.
treatments:
- name: Multidisciplinary supportive care
  description: >-
    No disease-modifying therapy exists and none is in trial. Management follows
    the components of the phenotype: physiotherapy and mobility aids for the
    proximal weakness and ataxia, scheduled spirometry with non-invasive
    ventilation if vital capacity falls, orthopaedic surveillance and
    intervention for scoliosis and chest deformity, speech and language therapy
    for dysarthria and speech delay, educational support for learning
    difficulty, and low-vision services where retinopathy is present. Routine
    cardiac surveillance has no established indication in this disorder, which
    distinguishes management from that of most muscular dystrophies.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  notes: >-
    No treatment claim in this entry is supported by an interventional study in
    MSTO1 disease, because none exists. The management components above are
    inferred from the documented phenotype rather than from trial evidence, and
    are recorded here rather than as evidenced treatment entries for that
    reason.
- name: Physiotherapy and mobility support
  description: >-
    Directed at the proximal weakness, the waddling wide-based gait and the
    cerebellar incoordination. Because weakness is slowly progressive or static
    in most patients, maintenance of function over decades is a realistic goal
    rather than a holding measure.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "All 15 patients reported either relatively slow progression or no progression of their muscle weakness."
    explanation: >-
      Marked PARTIAL: this supports the rationale for long-term functional
      maintenance but is not evidence that physiotherapy alters outcome in this
      disorder, which has never been tested.
- name: Respiratory surveillance and non-invasive ventilation
  description: >-
    Forced vital capacity was reduced to 60-79% of predicted in every patient
    tested in the largest cohort, and these measurements came from screening
    rather than from symptoms. Scheduled spirometry, with sleep studies and
    non-invasive ventilation if values decline, is therefore the rational
    approach; chest wall deformity from scoliosis and pectus excavatum
    compounds the diaphragmatic weakness.
  therapeutic_modality: DEVICE
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "revealed reduced forced vital capacity (FVC) measurements, ranging from 60 to 79% predicted"
    explanation: Establishes that clinically silent restrictive physiology is present, which is the justification for scheduled rather than symptom-triggered testing.
- name: Orthopaedic management of scoliosis and chest deformity
  description: >-
    Spinal deformity is the most common non-neurological problem and can be
    severe, including marked scoliosis with hemithorax asymmetry. Management
    follows standard neuromuscular scoliosis practice, with bracing and surgical
    correction considered on curve progression and respiratory grounds.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: orthopedic surgical procedure
    term:
      id: NCIT:C16186
      label: Orthopedic Surgical Procedure
  evidence:
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient showed severe asymmetry of the chest, with enlargement of the right hemithorax, pectus excavatum, and marked scoliosis"
    explanation: Documents the severity that the orthopaedic component of management has to address.
- name: Genetic counselling
  description: >-
    Counselling is straightforward once the genotype is established -
    autosomal recessive, with a 25% recurrence risk for siblings and unaffected
    carrier parents - but it depends on the two variants having been confirmed
    in trans, and on the reported variants not being pseudogene artefacts. The
    recessive model should be stated without the historical dominant caveat,
    which no longer applies.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since haploinsufficiency, as is present in some of the heterozygous parents in our cohort, does not cause a clinical phenotype"
    explanation: The evidence that carrier parents are unaffected, which is the substance of the counselling message.
discussions:
- discussion_id: msto1-dominant-claim-retracted
  kind: CONTROVERSY
  status: RESOLVED
  prompt: >-
    Is autosomal dominant inheritance a real mode for MSTO1 disease, given that
    it was reported in 2017, repeated in reviews through 2019, and still appears
    in the HPO annotation set for OMIM:617675?
  rationale: >-
    This entry answers no, and the reasoning should be auditable rather than
    implicit. The claim originated with a single Hungarian family in which a
    heterozygous c.22G>A (p.Val8Met) change segregated with a multisystem
    phenotype dominated by psychiatric and endocrine features; the report was
    titled as revealing the first dominant MSTO1 mutation, and the authors also
    proposed a possible dominant-negative mechanism. In 2023 Gerber et al
    challenged the variant assignment, and in their published reply the original
    authors confirmed that the mutation lies in the MSTO2P pseudogene rather
    than in MSTO1; EMBO Molecular Medicine issued an erratum to the 2017 paper.
    Independently, the largest cohort had already observed that heterozygous
    parents carrying MSTO1 loss-of-function alleles are unaffected, which
    removes haploinsufficiency as a plausible dominant route. Every other
    reported family is bi-allelic. The residual open question, and the reason
    the discussion is retained rather than deleted, is what the MSTO2P variant
    was doing in that family: the original authors maintain that their patients
    genuinely had reduced MSTO1 mRNA and protein, and whether the pseudogene
    variant contributed to that reduction is explicitly unresolved.
  attaches_to:
  - "genetic#MSTO1"
  - "pathophysiology#Bi-allelic Loss-of-Function Variants in MSTO1"
  resolution_note: >-
    Resolved for the purpose of this entry: only autosomal recessive inheritance
    is asserted, and the phenotypes unique to the MSTO2P family are excluded.
    The subsidiary question of whether an MSTO2P variant can lower MSTO1
    expression remains open and is recorded in the rationale.
  proposed_experiments:
  - experiment_id: msto1-msto2p-expression-test
    name: Test whether MSTO2P variants modulate MSTO1 expression
    description: >-
      Introduce the MSTO2P c.22G>A-equivalent change into cells on an otherwise
      isogenic background and measure MSTO1 mRNA and protein, to determine
      whether a pseudogene variant can account for the reduced MSTO1 levels
      reported in the original family or whether that observation had another
      cause.
  - experiment_id: msto1-pseudogene-alignment-audit
    name: Audit of MSTO1 variant calls for pseudogene mis-mapping
    description: >-
      Re-examine reported heterozygous MSTO1 variants in clinical databases with
      alignment methods that discriminate MSTO1 from MSTO2P, to establish how
      often the misassignment recurs in routine diagnostics.
  evidence:
  - reference: PMID:37431816
    reference_title: "Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Gerber et al report 2 autosomal recessive pathogenic Misato homolog 1 (MSTO1) variants causing hereditary optic atrophy and raise concerns about a previously identified dominant variant of MSTO1 by Gal et al (2017)."
    explanation: The published challenge to the dominant variant.
  - reference: PMID:37431815
    reference_title: "Answer to Gerber et al. \"Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy\"."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "but also confirm finding of Gerber et al that the mutation is in MSTO2p pseudogene"
    explanation: The original authors' confirmation, which is what makes this resolved rather than merely disputed.
  - reference: PMID:37431815
    reference_title: "Answer to Gerber et al. \"Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy\"."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Whether MSTO2p variant contributes to the observed decrease in MSTO1 levels in patients remains unclear."
    explanation: The explicitly unresolved residue, which is why the discussion carries proposed experiments rather than being closed outright.
- discussion_id: msto1-retinopathy-denominator
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What fraction of patients with bi-allelic MSTO1 variants actually have a
    pigmentary retinopathy, given that the feature names the disease term but
    was absent from the largest published cohort?
  rationale: >-
    The MONDO and Orphanet labels for this disorder include pigmentary
    retinopathy, and the feature is unambiguously real: both probands of the
    report that named it, the index patient of the first recessive series and
    two Chinese sisters followed for seventeen
    years all have it, and the 2025 ophthalmological study calls retinal
    dystrophy a key characteristic. Yet none of the fifteen patients in the
    largest cohort had it recorded. The authors of that cohort state that formal
    ophthalmological examination was not pursued in all patients, so the
    conflict may be entirely one of ascertainment - a rod-predominant dystrophy
    with concentric field loss and preserved acuity is asymptomatic for years
    and will not be found unless it is looked for. The alternative is a genuine
    genotype- or ancestry-dependent difference in retinal involvement. The
    distinction matters for practice, because if the retinopathy is common but
    silent then every patient needs baseline retinal imaging, whereas if it is
    genuinely rare then targeted examination suffices. It also matters for
    nosology: a disease named after a feature that most patients lack is a
    naming problem that MONDO may eventually need to address.
  attaches_to:
  - "phenotypes#Pigmentary retinopathy"
  - "pathophysiology#Retinal Degeneration with Pigmentary Change"
  proposed_experiments:
  - experiment_id: msto1-protocolised-retinal-imaging
    name: Protocolised retinal imaging in every ascertained MSTO1 patient
    description: >-
      Apply a common protocol - wide-field fundus autofluorescence, optical
      coherence tomography, visual fields and full-field electroretinography -
      to all newly ascertained and, where possible, previously reported patients,
      to convert an inconsistent set of case observations into a real numerator
      and denominator.
  - experiment_id: msto1-genotype-retina-correlation
    name: Genotype-retinal phenotype correlation across the published cohort
    description: >-
      Test whether retinal involvement tracks with allele class or residual
      protein level by collating genotype alongside protocolised retinal
      phenotyping, which would distinguish an ascertainment artefact from a
      genotype effect.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This finding was not reported in any of the patients in our cohort; however, formal ophthalmologic examination was not pursued in all patients."
    explanation: >-
      States both halves of the gap - the absence of the finding, and the
      incomplete ascertainment that prevents interpreting the absence.
  - reference: PMID:40542878
    reference_title: "Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Increased awareness and comprehensive ophthalmic examination are crucial for obtaining an early and accurate diagnosis."
    explanation: The ophthalmological group's own conclusion that systematic examination is what is missing, which is the substance of the proposed experiments.
- discussion_id: msto1-mtdna-depletion-tissue-gap
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Is mtDNA depletion present in the tissues that are actually diseased -
    skeletal muscle and cerebellum - or only in the cultured fibroblasts where
    it has almost always been measured?
  rationale: >-
    mtDNA depletion is the observation that reclassified this disorder as an
    mtDNA depletion syndrome, and it is consistent wherever it has been looked
    for. But the evidence base is narrower than the claim. Seven of seven
    fibroblast lines in the largest cohort showed 30-70% depletion, and the
    same paper states that muscle tissue was not available for further mtDNA
    content studies; the only muscle measurement in the literature is 27% of
    control in a single patient in a different series. No cerebellar tissue from
    an MSTO1 patient has ever been examined. Fibroblasts divide and muscle
    fibres and Purkinje cells do not, and mtDNA copy number regulation differs
    substantially between proliferating and postmitotic cells, so the
    extrapolation is not safe. Until depletion is measured in muscle in a series
    rather than in one patient, the causal edge from mtDNA depletion to muscle
    and cerebellar pathology in this entry rests on an inference.
  attaches_to:
  - "pathophysiology#mtDNA Depletion and Nucleoid Reorganisation"
  - "pathophysiology#Dystrophic Degeneration of Skeletal Muscle Fibres"
  proposed_experiments:
  - experiment_id: msto1-muscle-mtdna-series
    name: Quantitative mtDNA content in MSTO1 patient muscle across a series
    description: >-
      Measure mtDNA relative to nuclear DNA in stored muscle biopsy material
      from as many reported patients as can be recovered, with single-fibre
      analysis where possible, to establish whether the fibroblast depletion is
      reproduced in the diseased tissue and whether it is uniform or mosaic
      across fibres.
  - experiment_id: msto1-ipsc-neurons-and-myotubes
    name: Patient iPSC-derived myotubes and cerebellar organoids
    description: >-
      Differentiate patient iPSC into myotubes and cerebellar organoids and
      measure mtDNA content, nucleoid distribution and network morphology in
      postmitotic, disease-relevant cell types rather than in dividing
      fibroblasts.
  evidence:
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "all fibroblasts were found to have depletion of mtDNA ranging from 30 to 70% along with alterations to mtDNA nucleoids"
    explanation: The strength of the fibroblast evidence, which is exactly what makes the absence of tissue evidence conspicuous.
  - reference: PMID:28544275
    reference_title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "mtDNA amount was reduced to 27% of the mean control value"
    explanation: >-
      The entire muscle evidence base for this disorder is this one
      measurement, in one patient. Marked PARTIAL for that reason.
- discussion_id: msto1-mechanism-ordering
  kind: EMERGING_HYPOTHESIS
  status: OPEN
  prompt: >-
    Is impaired mitochondrial fusion the primary consequence of losing MSTO1, or
    is it downstream of a cytosolic proteostasis defect, or of a direct effect on
    mtDNA replication through RAD51?
  rationale: >-
    Three accounts of MSTO1's molecular function are now on the table and they
    make different predictions. The canonical fusion-first model, in place since
    2017, is supported by rescue kinetics in patient fibroblasts and by analogy
    with MFN2 and OPA1, but has never demonstrated the connecting steps in MSTO1
    cells and leaves the protein without a defined molecular activity. The 2026
    TRiC model gives it one - chaperonin assembly factor - and its
    temporal-ordering experiment directly contradicts the canonical ordering,
    with TRiC falling at day two and mitochondria fragmenting only at day six;
    but it is an unreviewed preprint from one laboratory, and the TRiC substrate
    that connects proteostasis to the organelle is unidentified since tubulin
    and actin were excluded. The 2026 RAD51 model would explain mtDNA depletion
    without a fusion intermediate, but it is entirely in cancer cell systems. The
    three are not mutually exclusive and the entry models all three as separate
    hypothesis groups rather than picking one. Which is correct determines what a
    therapeutic target would even be: the fusion machinery, the chaperonin
    system, or mtDNA replication.
  attaches_to:
  - "pathophysiology#Loss of Cytosolic MSTO1 Protein"
  - "pathophysiology#Failure of TRiC Chaperonin Assembly"
  - "pathophysiology#Unrestrained RAD51 Binding to Mitochondrial DNA"
  proposed_experiments:
  - experiment_id: msto1-tric-replication-in-patient-cells
    name: Independent replication of the TRiC assembly result in patient fibroblasts
    description: >-
      Measure TRiC/CCT subunit levels and assembly intermediates in the
      established panel of MSTO1 patient fibroblast lines in a second
      laboratory, with peer review, before the proteostasis model is treated as
      more than a lead.
  - experiment_id: msto1-tric-substrate-screen
    name: Identify the TRiC substrate that links proteostasis to mitochondrial morphology
    description: >-
      Screen TRiC substrates for one whose depletion reproduces mitochondrial
      fragmentation, given that tubulin and actin were explicitly excluded, to
      supply the missing intermediate in the proteostasis model.
  - experiment_id: msto1-rad51-in-patient-fibroblasts
    name: Test the MSTO1-RAD51 interaction in patient rather than cancer cells
    description: >-
      Assay mitochondrial RAD51 occupancy on mtDNA in MSTO1 patient fibroblasts
      and ask whether inhibiting mitochondrial RAD51 rescues mtDNA content, which
      would establish whether the RAD51 route operates in the human disease.
  evidence:
  - reference: PMID:42539026
    reference_title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Using rapid depletion of MSTO1, we found that mitochondrial fragmentation appears only after six days."
    explanation: The kinetic observation that puts the canonical ordering in question.
  - reference: PMID:31463572
    reference_title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "further suggests that impairments in mitochondrial dynamics are upstream of mtDNA depletion in MSTO1 patient fibroblasts"
    explanation: The canonical ordering, stated by its proponents on the basis of rescue kinetics.
- discussion_id: msto1-no-animal-model
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Why has no vertebrate animal model of MSTO1 deficiency been reported, and
    what does its absence prevent?
  rationale: >-
    A literature search on 2026-08-01 across all 32 PubMed records for MSTO1
    found no mouse, zebrafish or other vertebrate model of MSTO1 loss. The
    non-vertebrate precedents are informative but distant: null mutations of
    misato in Drosophila cause irregular chromosome segregation, and disruption
    of DML1 in Saccharomyces cerevisiae impairs mtDNA inheritance and
    mitochondrial segregation - and it is notable that the yeast phenotype is
    about mtDNA, which prefigures the human depletion finding by nearly two
    decades. But neither organism has a cerebellum, skeletal muscle of the
    mammalian type, or a neurosensory retina, so the three affected tissues in
    this disorder are unmodelled. The consequence is concrete: every
    tissue-level causal edge in this entry - from mtDNA depletion to muscle
    degeneration, to cerebellar hypoplasia, to retinal degeneration - is an
    inference from fibroblast biology plus clinical observation, with no in vivo
    corroboration in any species. There is also no preclinical platform on which
    any therapeutic idea could be tested.
  attaches_to:
  - "pathophysiology#Dystrophic Degeneration of Skeletal Muscle Fibres"
  - "pathophysiology#Cerebellar Hypoplasia and Cortical Atrophy"
  - "pathophysiology#Retinal Degeneration with Pigmentary Change"
  proposed_experiments:
  - experiment_id: msto1-mouse-knockout
    name: Constitutive and conditional Msto1 knockout mouse
    description: >-
      Generate a germline null and, if it is embryonic lethal, muscle-specific
      and Purkinje-cell-specific conditional knockouts, and phenotype muscle
      histology, cerebellar volume, retinal structure and mtDNA content in each
      tissue - the experiment that would test the tissue-level causal edges this
      entry currently infers.
  - experiment_id: msto1-zebrafish-mutant
    name: Stable germline msto1 zebrafish mutant
    description: >-
      Create a CRISPR germline mutant and assess mitochondrial network
      morphology in muscle, motor behaviour, cerebellar development and retinal
      structure, providing a tractable vertebrate system for modifier and
      compound screens.
  notes: >-
    The Drosophila misato and yeast DML1 results referred to in the rationale
    are described in the introductions of PMID:28554942 and PMID:31604776 as
    citations to primary work from 1997 and 2002 that is not itself cached here.
    They are therefore recorded in this rationale rather than quoted as evidence
    snippets, and no `animal_models:` block is asserted for this entry.
references:
- reference: PMID:31463572
  title: "MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement."
- reference: PMID:28544275
  title: "Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia."
- reference: PMID:28554942
  title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
- reference: PMID:29339779
  title: "Novel recessive mutations in MSTO1 cause cerebellar atrophy with pigmentary retinopathy."
- reference: PMID:17349998
  title: "Human Misato regulates mitochondrial distribution and morphology."
- reference: PMID:37431816
  title: "Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy."
- reference: PMID:37431815
  title: "Answer to Gerber et al. \"Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy\"."
- reference: PMID:37431817
  title: "MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans."
- reference: PMID:31604776
  title: "Novel biallelic variants in MSTO1 associated with mitochondrial myopathy."
- reference: PMID:31130378
  title: "A novel case of MSTO1 gene related congenital muscular dystrophy with progressive neurological involvement."
- reference: PMID:33612823
  title: "A novel homozygous MSTO1 mutation in Ashkenazi Jewish siblings with ataxia and myopathy."
- reference: PMID:30684668
  title: "Whole-exome sequencing identifies rare compound heterozygous mutations in the MSTO1 gene associated with cerebellar ataxia and myopathy."
- reference: PMID:36035138
  title: "Case Report: Evidences of myasthenia and cerebellar atrophy in a chinese patient with novel compound heterozygous MSTO1 variants."
- reference: PMID:36468072
  title: "Indentification of novel MSTO1 compound heterozygous mutations in a Chinese family with recessive cerebellar atrophy and ataxia."
- reference: PMID:40542878
  title: "Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review."
- reference: PMID:41653646
  title: "MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review."
- reference: PMID:42051546
  title: "Mild-cerebellar ataxia due to impaired mitochondrial function caused by the MSTO1 variations."
- reference: PMID:42098462
  title: "MSTO1 modulates RAD51 activity to safeguard mitochondrial DNA integrity and control immune responses."
- reference: PMID:42539026
  title: "MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function."

📚

References & Deep Research

References

19

PMID:31463572

MSTO1 mutations cause mtDNA depletion, manifesting as muscular dystrophy with cerebellar involvement.

No top-level findings curated for this source.

PMID:28544275

Recessive mutations in MSTO1 cause mitochondrial dynamics impairment, leading to myopathy and ataxia.

No top-level findings curated for this source.

PMID:28554942

MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans.

No top-level findings curated for this source.

PMID:29339779

Novel recessive mutations in MSTO1 cause cerebellar atrophy with pigmentary retinopathy.

No top-level findings curated for this source.

PMID:17349998

Human Misato regulates mitochondrial distribution and morphology.

No top-level findings curated for this source.

PMID:37431816

Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy.

No top-level findings curated for this source.

PMID:37431815

Answer to Gerber et al. "Autosomal recessive pathogenic MSTO1 variants in hereditary optic atrophy".

No top-level findings curated for this source.

PMID:37431817

MSTO1 is a cytoplasmic pro-mitochondrial fusion protein, whose mutation induces myopathy and ataxia in humans.

No top-level findings curated for this source.

PMID:31604776

Novel biallelic variants in MSTO1 associated with mitochondrial myopathy.

No top-level findings curated for this source.

PMID:31130378

A novel case of MSTO1 gene related congenital muscular dystrophy with progressive neurological involvement.

No top-level findings curated for this source.

PMID:33612823

A novel homozygous MSTO1 mutation in Ashkenazi Jewish siblings with ataxia and myopathy.

No top-level findings curated for this source.

PMID:30684668

Whole-exome sequencing identifies rare compound heterozygous mutations in the MSTO1 gene associated with cerebellar ataxia and myopathy.

No top-level findings curated for this source.

PMID:36035138

Case Report: Evidences of myasthenia and cerebellar atrophy in a chinese patient with novel compound heterozygous MSTO1 variants.

No top-level findings curated for this source.

PMID:36468072

Indentification of novel MSTO1 compound heterozygous mutations in a Chinese family with recessive cerebellar atrophy and ataxia.

No top-level findings curated for this source.

PMID:40542878

Seventeen-year follow-up of mitochondrial myopathy and ataxia in a Chinese family: case reports and literature review.

No top-level findings curated for this source.

PMID:41653646

MSTO1-related mitochondrial myopathy and ataxia syndrome: Case series and literature review.

No top-level findings curated for this source.

PMID:42051546

Mild-cerebellar ataxia due to impaired mitochondrial function caused by the MSTO1 variations.

No top-level findings curated for this source.

PMID:42098462

MSTO1 modulates RAD51 activity to safeguard mitochondrial DNA integrity and control immune responses.

No top-level findings curated for this source.

PMID:42539026

MSTO1 functions as a TRiC assembly factor linking cytosolic proteostasis to mitochondrial function.

No top-level findings curated for this source.

MSTO1-Related Mitochondrial Myopathy

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