Adult-Onset Ataxia and Polyneuropathy

Mendelian MONDO:0010781 Pathograph 12 Show in embeddings browser hereditary disease mitochondrial disease

Adult-onset ataxia and polyneuropathy (OMIM 500010) is a maternally inherited mitochondrial disorder defined by a kindred carrying the heteroplasmic m.8993T>C variant of MT-ATP6, in which slowly progressive cerebellar ataxia and axonal sensorimotor polyneuropathy begin in adult life. The same nucleotide-8993 substitution more commonly causes infantile or childhood disease (maternally inherited Leigh syndrome, NARP), so this entity marks the mild, late-onset end of a heteroplasmy- and modifier-dependent MT-ATP6 spectrum rather than a distinct molecular lesion. Because standard adult mitochondrial screening (cytochrome c oxidase muscle histochemistry, respiratory-chain enzymology) is typically normal in MT-ATP6 disease, affected adults are readily misclassified as undiagnosed spinocerebellar ataxia or Charcot-Marie-Tooth disease unless mtDNA is sequenced. Other MT-ATP6 variants (m.9035T>C, m.9185T>C) produce clinically indistinguishable adult-onset ataxia with variable pyramidal and peripheral-nerve involvement.

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1
Inheritance
7
Pathophys.
9
Phenotypes
3
Gaps
12
Pathograph
1
Genes
4
Medical Actions
5
Differentials
1
References
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Deep Research
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Inheritance

1
Maternal mitochondrial inheritance HP:0001427
The disorder is caused by heteroplasmic MT-ATP6 variants (m.8993T>C, m.9035T>C, m.9185T>C) carried on mitochondrial DNA and is transmitted maternally. Adult onset in these kindreds reflects a heteroplasmy load that stays below the threshold producing infantile NARP or Leigh syndrome.
Mitochondrial inheritance
Show evidence (1 reference)
PMID:35159298 SUPPORT Human Clinical
"Pathogenic variants in MT-ATP6 are a well-known cause of maternally-inherited mitochondrial disorders"
Establishes maternal transmission as the inheritance mode for MT-ATP6 disorders, the gene carrying this entry's causal variants.
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Discussions and Knowledge Gaps

3
Which cerebellar cell population is lost in adult-onset MT-ATP6 ataxia, and is the Purkinje neuron the primary target as it is in most hereditary ataxias?
KNOWLEDGE GAP OPEN purkinje_neuropathology_unproven
Attached to
pathophysiology#Cerebellar Degeneration
Cerebellar atrophy in m.8993T>C carriers is documented only by MRI. No autopsy or quantitative neuropathological series exists for the adult-onset MT-ATP6 phenotype, so the assignment of the degeneration to Purkinje neurons is inference from the wider ataxia literature rather than a finding in this entity. This matters mechanistically because MT-ATP6 disease elsewhere (Leigh/NARP) preferentially injures brainstem and basal ganglia rather than cerebellar cortex, and the m.8993T>C ROS-weighted lesion may select a different population than the calcium-mediated Purkinje degeneration of the dominant spinocerebellar ataxias.
Proposed experiments
Post-mortem cerebellar neuropathology in genetically confirmed adult-onset MT-ATP6 ataxia
cerebellar_neuropathology_mtatp6
Quantitative stereology of Purkinje, granule and dentate neurons with per-cell heteroplasmy measurement, compared against age-matched controls and against NARP/Leigh cerebellum.
Show evidence (1 reference)
PMID:20953793 SUPPORT Human Clinical
"High-resolution retinal and brain imaging in NARP syndrome revealed analogous patterns of tissue injury characterized by heterogeneous areas of neuronal loss."
The deepest tissue-level characterisation available for m.8993T>C carriers is imaging, which resolves regions rather than cell types. It is cited here to document the ceiling of current evidence, which is exactly what this knowledge gap asserts.
Why do some MT-ATP6 pedigrees with very high mutant load present in infancy with Leigh syndrome while others present only in adulthood with ataxia and neuropathy?
OPEN QUESTION OPEN heteroplasmy_does_not_explain_onset
Attached to
pathophysiology#Heteroplasmy Load Above the Phenotypic Threshold
Mutant load is necessary but demonstrably not sufficient to predict age of onset or severity: patients with childhood-onset and adult-onset MT-ATP6 ataxia have both been reported at 90-100% heteroplasmy, and within a single m.8993T>C family, phenotype differed markedly between members with comparably high ectodermal and mesodermal loads. Candidate modifiers include mtDNA haplogroup, nuclear background, mtDNA copy number, and differences in antioxidant scavenging capacity, none of which has been tested for this entity.
Show evidence (1 reference)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"There were marked phenotypic differences between family members, even between individuals with the greatest degrees of ectodermal and mesodermal heteroplasmy."
Demonstrates that measured heteroplasmy fails to explain within-family phenotypic variation in an m.8993T>C kindred. Graded INDIRECT because that kindred was diagnosed as NARP syndrome (MONDO:0010794); the open question about load-versus-onset is nonetheless shared across the MT-ATP6 spectrum.
Does the rescue of MT-ATP6 mutant cell death by CoQ10 and vitamin E in transmitochondrial cybrids translate into any clinical benefit in adult-onset MT-ATP6 ataxia?
HUMAN MODEL MISMATCH OPEN antioxidant_rescue_in_cybrids_only
Attached to
pathophysiology#Neuronal Energy Deficit and Oxidative Stress
Cybrids carrying an MT-ATP6 ataxia variant died by apoptosis or necrosis under metabolic stress, and both outcomes were prevented by CoQ10 and vitamin E, prompting the authors to propose antioxidant therapy. This is an in-vitro result in an immortalised cell background carrying a different MT-ATP6 variant (m.9035T>C) from the m.8993T>C that defines this entity, and the stressors used (glucose deprivation, tert-butyl hydroperoxide) are not models of the decades-long neurodegeneration seen in patients. No controlled trial of antioxidants exists for this disorder, and the entry therefore records no antioxidant treatment.
Show evidence (1 reference)
PMID:19626676 SUPPORT In Vitro
"Both of these outcomes were prevented by the antioxidants CoQ(10) and vitamin E, suggesting that the abnormally high levels of ROS were the triggers of cell death."
The cybrid rescue result whose translational validity this discussion questions.
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Pathophysiology

7
MT-ATP6 m.8993T>C Variant
A heteroplasmic thymine-to-cytosine transition at mtDNA nucleotide 8993 substitutes the highly conserved leucine at codon 156 of the ATP6 subunit (subunit a) of mitochondrial ATP synthase. The same nucleotide is the site of the more severe m.8993T>G (Leu156Arg) NARP/MILS variant.
MT-ATP6 hgnc:7414 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MT-ATP6 (hgnc:7414). hgnc:7414 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context MT-ATP6 hgnc:7414 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns MT-ATP6 (hgnc:7414). hgnc:7414 is a gene from the HUGO Gene Nomenclature Committee. allele_type: SNV variant_origin: GERMLINE functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
Heteroplasmic mitochondrial missense substitution (Leu156Pro) inherited through the maternal line.
proton-transporting ATP synthase complex GO:0045259 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves proton-transporting ATP synthase complex (GO:0045259). GO:0045259 is a cellular component from the Gene Ontology.
Downstream
Impaired ATP Synthase Function Substitution of the conserved Leu-156 residue in the membrane sector of subunit a degrades proton-driven ATP synthesis.
Heteroplasmy Load Above the Phenotypic Threshold Because the variant is heteroplasmic, disease expression depends on the proportion of mutant mtDNA molecules reached in a given tissue.
Show evidence (2 references)
PMID:16049925 SUPPORT Human Clinical
"We report a kindred with adult-onset slowly progressive ataxia and polyneuropathy and with the heteroplasmic 8993T-->C mutation."
This is the defining report for OMIM 500010 and directly ties the heteroplasmic m.8993T>C MT-ATP6 variant to the adult-onset ataxia-plus-polyneuropathy phenotype.
PMID:17568559 SUPPORT In Vitro
"Two point mutations (T>G and T>C) at the same 8993 nucleotide of mitochondrial DNA (at comparable mutant load), affecting the ATPase 6 subunit of the F1F0-ATPase, result in neurological phenotypes of variable severity in humans."
Locates the lesion in the ATPase 6 subunit of F1F0-ATP synthase and records that the two nucleotide-8993 substitutions differ in severity.
Impaired ATP Synthase Function
Loss of the conserved subunit-a leucine impairs the proton-translocating membrane sector of complex V, so the proton-motive force is used inefficiently for ATP synthesis. Whether the primary lesion is catalytic or one of assembly and stability is not settled: complex V subcomplexes are detectable on native gels in patient tissue, which points toward an assembly or stability problem, and the authors who reported them said further work is needed to decide between the two readings.
proton motive force-driven ATP synthesis GO:0015986 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased proton motive force-driven ATP synthesis (GO:0015986). GO:0015986 is a biological process from the Gene Ontology. ↓ DECREASED
proton-transporting ATP synthase activity, rotational mechanism GO:0046933 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased proton-transporting ATP synthase activity, rotational mechanism (GO:0046933). GO:0046933 is a molecular function from the Gene Ontology. ↓ DECREASED
proton-transporting ATP synthase complex GO:0045259 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves proton-transporting ATP synthase complex (GO:0045259). GO:0045259 is a cellular component from the Gene Ontology.
Downstream
Neuronal Energy Deficit and Oxidative Stress Inefficient oxidative phosphorylation lowers cellular ATP and raises reactive oxygen species output from the respiratory chain.
Show evidence (3 references)
PMID:19626676 SUPPORT INDIRECT In Vitro
"We confirmed that the 4452T>C transition in tRNA(Met) represented a polymorphism; however, 9035T>C conversion in the ATP6 gene was responsible for a defective F(0)-ATPase."
Cybrid evidence that an ataxia-associated MT-ATP6 missense variant produces a defective F0-ATPase. Graded INDIRECT because this cybrid work used m.9035T>C, a different MT-ATP6 variant in the same adult-onset ataxia spectrum, not the m.8993T>C variant that defines this entry.
PMID:35159298 SUPPORT INDIRECT Human Clinical
"Native gels found presence of subcomplexes of complex V in fibroblast and/or skeletal muscle."
Patient-tissue evidence that an MT-ATP6 ataxia variant perturbs complex V. Graded INDIRECT because the patients carried m.9035T>C rather than m.8993T>C.
PMID:35159298 SUPPORT INDIRECT Human Clinical
"The occurrence of subcomplexes suggest that variant NC_012920.1(MT-ATP6):m.9035T>C could cause problems with complex V assembly or stability. However, further research is warranted to further evaluate these hypotheses."
The authors' own reading of the subcomplex finding, and their statement that the assembly-versus-catalysis question is unresolved. Cited here so the node description does not over-read the native-gel result. Graded INDIRECT because the variant is m.9035T>C rather than m.8993T>C.
Heteroplasmy Load Above the Phenotypic Threshold
Mitochondrial heteroplasmy is the gate between carrier state and disease. MT-ATP6 ataxia variants require a very high mutant load before symptoms appear, and mutant load segregates unpredictably between tissues and between generations through the germline bottleneck. This is the most plausible explanation for why the same nucleotide-8993 change causes infantile Leigh syndrome in some pedigrees and only adult-onset ataxia in others, although load alone does not fully predict severity.
Downstream
Neuronal Energy Deficit and Oxidative Stress Only tissues whose mutant load exceeds the threshold express a bioenergetic defect, which is why the phenotype is regionally restricted and appears late.
Show evidence (3 references)
PMID:35159298 SUPPORT INDIRECT Human Clinical
"The above mentioned findings suggest there is a very high heteroplasmy threshold required for NC_012920.1(MT-ATP6):m.9035T>C to trigger phenotypic manifestations, but once this threshold is exceeded, this can result in different clinical manifestations."
States the high-threshold, poorly-predictive relationship between MT-ATP6 mutant load and phenotype. Graded INDIRECT because it was established for m.9035T>C rather than m.8993T>C.
PMID:20953793 SUPPORT INDIRECT Human Clinical
"There were marked phenotypic differences between family members, even between individuals with the greatest degrees of ectodermal and mesodermal heteroplasmy."
In an m.8993T>C family, measured heteroplasmy did not account for the within-family phenotypic spread, supporting the modulator role and the limits of load as a predictor. Graded INDIRECT because the cohort was diagnosed as NARP syndrome (MONDO:0010794), an entity this entry keeps distinct; the observation carries across because the modulator claim is about m.8993T>C heteroplasmy behaviour rather than about either phenotype.
PMID:20301352 SUPPORT INDIRECT Other
"Sibs may inherit the pathogenic variant at varying heteroplasmy levels due to the bottleneck effect and variant-specific segregation patterns."
GeneReviews statement of the germline-bottleneck mechanism that generates the variable mutant loads this node depends on. Graded INDIRECT because it is a general statement about mtDNA disorders rather than about this entity.
Neuronal Energy Deficit and Oxidative Stress
Cells above the heteroplasmy threshold sustain a chronic ATP shortfall and an increased reactive-oxygen-species burden, and compensate by shifting toward glycolysis. For the m.8993T>C substitution specifically the balance is tilted toward ROS overproduction rather than the more severe energy failure seen with m.8993T>G, which fits the milder, later-onset phenotype. Long projection neurons and cerebellar neurons, which depend on distributed mitochondrial ATP supply, are the tissues in which this shortfall becomes clinically apparent.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
oxidative phosphorylation GO:0006119 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased oxidative phosphorylation (GO:0006119). GO:0006119 is a biological process from the Gene Ontology. ↓ DECREASED reactive oxygen species metabolic process GO:0072593 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased reactive oxygen species metabolic process (GO:0072593). GO:0072593 is a biological process from the Gene Ontology. ↑ INCREASED response to oxidative stress GO:0006979 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased response to oxidative stress (GO:0006979). GO:0006979 is a biological process from the Gene Ontology. ↑ INCREASED mitochondrion organization GO:0007005 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal mitochondrion organization (GO:0007005). GO:0007005 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Distal Axonal Degeneration of Peripheral Nerves The longest peripheral axons, which must maintain mitochondria far from the cell body, are the first to fail.
Cerebellar Degeneration Chronic bioenergetic stress in cerebellar cortex produces progressive cerebellar volume loss.
Show evidence (3 references)
PMID:17568559 SUPPORT In Vitro
"The 8993T>G change mainly induces an energy deficiency, whereas the 8993T>C favours an increased ROS production."
Establishes the ROS-weighted balance of the bioenergetic lesion for the m.8993T>C variant that defines this entry.
PMID:19626676 SUPPORT INDIRECT In Vitro
"They had less than half of the steady-state content of ATP and nearly an 8-fold higher basal level of reactive oxygen species (ROS)."
Quantifies the combined ATP shortfall and ROS burden in cybrids carrying an MT-ATP6 ataxia variant. Graded INDIRECT because the cybrids carried m.9035T>C.
PMID:35159298 SUPPORT INDIRECT Human Clinical
"Bioenergetic measurements in fibroblasts from both patients detected reduced spare respiratory capacities and altered extracellular acidification rates, revealing a switch from mitochondrial respiration to glycolysis to uphold ATP production."
Patient-derived fibroblast evidence of the compensatory glycolytic shift described in this node. Graded INDIRECT because the variant was m.9035T>C.
Distal Axonal Degeneration of Peripheral Nerves
A length-dependent, predominantly axonal sensorimotor polyneuropathy develops, producing distal sensory loss, depressed or absent ankle reflexes, and distal weakness. Nerve conduction studies show axonal rather than demyelinating features, which is why these patients are commonly referred with a working diagnosis of Charcot-Marie-Tooth disease.
sensory neuron CL:0000101 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves sensory neuron (CL:0000101). CL:0000101 is a cell type from the Cell Ontology. neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
axonal transport GO:0098930 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased axonal transport (GO:0098930). GO:0098930 is a biological process from the Gene Ontology. ↓ DECREASED transmission of nerve impulse GO:0019226 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased transmission of nerve impulse (GO:0019226). GO:0019226 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Adult-Onset Ataxia with Axonal Polyneuropathy Peripheral sensory loss compounds cerebellar incoordination, worsening gait instability.
Show evidence (2 references)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal polyneuropathy."
Electrophysiological demonstration of length-dependent axonal sensorimotor polyneuropathy in an m.8993T>C family. Graded INDIRECT because that family was diagnosed as NARP syndrome (MONDO:0010794), which this entry curates as a separate entity and a differential.
PMID:16049925 SUPPORT Human Clinical
"Our findings suggest that the 8993T-->C mtDNA mutation should be considered in the differential diagnosis of nondominant adult-onset ataxia and axonal neuropathy."
The defining report characterises the neuropathy of this entity as axonal.
Cerebellar Degeneration
Progressive cerebellar volume loss underlies the ataxia. Cerebellar atrophy is demonstrable on brain MRI in m.8993T>C carriers. The cell-level neuropathology has not been established for this adult-onset entity: no autopsy series localises the loss to Purkinje neurons specifically, so the Purkinje cell type is recorded here as the presumed but unproven substrate (see the KNOWLEDGE_GAP discussion).
Purkinje cell CL:0000121 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves Purkinje cell (CL:0000121). CL:0000121 is a cell type from the Cell Ontology.
nervous system process GO:0050877 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal nervous system process (GO:0050877). GO:0050877 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Adult-Onset Ataxia with Axonal Polyneuropathy Loss of cerebellar cortical output produces gait and limb ataxia, dysarthria, and abnormal eye movements.
Show evidence (1 reference)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"3-T MRI revealed cerebellar atrophy and cystic and cavitary T2 hyperintensities in the basal ganglia."
Imaging evidence of cerebellar atrophy in an m.8993T>C family, supporting cerebellar degeneration as the anatomical substrate of the ataxia. Graded INDIRECT because that family was diagnosed as NARP syndrome (MONDO:0010794), which this entry curates as a separate entity and a differential.
Adult-Onset Ataxia with Axonal Polyneuropathy
The combined clinical syndrome: slowly progressive cerebellar ataxia beginning in adulthood together with an axonal sensorimotor polyneuropathy, variably accompanied by pyramidal signs. It is clinically indistinguishable from other adult spinocerebellar ataxias, and the routine adult mitochondrial screening tests are normal, so mtDNA sequencing is the only route to diagnosis.
Show evidence (1 reference)
PMID:22577227 SUPPORT Human Clinical
"The authors report two families with onset of ataxia in adulthood (with pyramidal dysfunction and/or peripheral neuropathy variably present), who are clinically indistinguishable from other spinocerebellar ataxia patients."
Characterises the adult-onset MT-ATP6 syndrome as an ataxia with variable neuropathy and pyramidal involvement that mimics common spinocerebellar ataxias.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Adult-Onset Ataxia and Polyneuropathy Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

9
Nervous System 5
Progressive Cerebellar Ataxia OBLIGATE HP:0002073 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Progressive cerebellar ataxia (HP:0002073), qualified as course progressive. HP:0002073 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:16049925 SUPPORT Human Clinical
"We report a kindred with adult-onset slowly progressive ataxia and polyneuropathy and with the heteroplasmic 8993T-->C mutation."
The defining kindred is described as having adult-onset slowly progressive ataxia, which is by definition present in every member of this entity.
Gait Ataxia HP:0002066 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Gait ataxia (HP:0002066). HP:0002066 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:35159298 SUPPORT INDIRECT Human Clinical
"Furthermore, the patient had an atactic gait pattern, a decreased sense of balance and could not walk without support."
An examination finding of ataxic gait in MT-ATP6 ataxia. Graded INDIRECT because the patient carried m.9035T>C and presented in adolescence rather than adulthood.
PMID:22577227 SUPPORT INDIRECT Human Clinical
"The authors report two families with onset of ataxia in adulthood (with pyramidal dysfunction and/or peripheral neuropathy variably present), who are clinically indistinguishable from other spinocerebellar ataxia patients."
Graded INDIRECT because the quote establishes adult-onset ataxia clinically indistinguishable from spinocerebellar ataxia, but does not name gait, so it supports the cerebellar syndrome rather than this specific HPO term.
Cerebellar Atrophy HP:0001272 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebellar atrophy (HP:0001272). HP:0001272 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"3-T MRI revealed cerebellar atrophy and cystic and cavitary T2 hyperintensities in the basal ganglia."
High-field MRI in an m.8993T>C family documents cerebellar atrophy. Graded INDIRECT because that family was diagnosed as NARP syndrome (MONDO:0010794) rather than as this adult-onset entity.
Episodic Ataxia VERY_RARE HP:0002131 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Episodic ataxia (HP:0002131). HP:0002131 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:18055910 SUPPORT Human Clinical
"One subject had episodic ataxia (EA) and transient hemipareses, broadening the phenotype."
A single m.8993T>C carrier with episodic ataxia, reported explicitly as a broadening of the phenotype, which is why the frequency band is VERY_RARE.
Transient Hemiparesis VERY_RARE HP:0001269 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hemiparesis (HP:0001269), qualified as temporality transient. HP:0001269 is a phenotype from the Human Phenotype Ontology.
Temporal: TRANSIENT
Show evidence (1 reference)
PMID:18055910 SUPPORT Human Clinical
"One subject had episodic ataxia (EA) and transient hemipareses, broadening the phenotype."
Directly reports transient hemiparesis in an m.8993T>C carrier.
Other 4
Peripheral Axonal Neuropathy OBLIGATE HP:0003477 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Peripheral axonal neuropathy (HP:0003477). HP:0003477 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:16049925 SUPPORT Human Clinical
"Our findings suggest that the 8993T-->C mtDNA mutation should be considered in the differential diagnosis of nondominant adult-onset ataxia and axonal neuropathy."
The defining report identifies the neuropathy of this entity as axonal.
PMID:20953793 SUPPORT INDIRECT Human Clinical
"EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal polyneuropathy."
Electrophysiological confirmation of length-dependent axonal sensorimotor polyneuropathy in an m.8993T>C family. Graded INDIRECT because that family was diagnosed as NARP syndrome (MONDO:0010794), a separate entity in this knowledge base; the neuropathy phenotype is shared, the diagnosis is not.
Sensorimotor Neuropathy HP:0007141 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sensorimotor neuropathy (HP:0007141). HP:0007141 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal polyneuropathy."
Documents the sensorimotor character of the neuropathy. Graded INDIRECT because the cohort was diagnosed as NARP syndrome (MONDO:0010794) rather than as this adult-onset entity.
Pyramidal Dysfunction OCCASIONAL Babinski sign HP:0003487 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Babinski sign (HP:0003487). HP:0003487 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22577227 SUPPORT INDIRECT Human Clinical
"The authors report two families with onset of ataxia in adulthood (with pyramidal dysfunction and/or peripheral neuropathy variably present), who are clinically indistinguishable from other spinocerebellar ataxia patients."
Records pyramidal dysfunction as a variable feature. Graded INDIRECT because the abstract names pyramidal dysfunction generically rather than the Babinski sign used here as the closest HPO anchor.
Ophthalmoplegia OCCASIONAL HP:0000602 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Ophthalmoplegia (HP:0000602). HP:0000602 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:35159298 SUPPORT INDIRECT Human Clinical
"Patient 2 presented in adolescence with ataxia and ophthalmoplegia without cognitive or motor impairment."
Documents ophthalmoplegia accompanying later-onset MT-ATP6 ataxia. Graded INDIRECT because the patient carried m.9035T>C and presented in adolescence rather than adulthood.
🧬

Genetic Associations

1
MT-ATP6 (Heteroplasmic m.8993T>C (Leu156Pro) is the defining variant of OMIM 500010. Other MT-ATP6 missense variants (m.9035T>C, m.9185T>C) cause clinically indistinguishable adult-onset ataxia syndromes.)
Gene: MT-ATP6 hgnc:7414 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is MT-ATP6 (hgnc:7414). hgnc:7414 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Mitochondrial inheritance
Show evidence (2 references)
PMID:16049925 SUPPORT Human Clinical
"We report a kindred with adult-onset slowly progressive ataxia and polyneuropathy and with the heteroplasmic 8993T-->C mutation."
Establishes MT-ATP6 m.8993T>C as the causal variant for this entity.
PMID:22577227 SUPPORT Human Clinical
"These families had the m.9185T>C and m.9035T>C mutations, respectively, which have not previously been associated with adult-onset cerebellar syndromes."
Extends the causal gene beyond m.8993T>C to other MT-ATP6 missense variants producing adult-onset cerebellar syndromes.
💊

Medical Actions

4
Supportive and Symptomatic Management
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
No disease-modifying therapy exists. Care is supportive and symptom-directed across the mtDNA disorders, with regular neurologic review for progression of ataxia and neuropathy.
Target Phenotypes: Progressive cerebellar ataxia HP:0002073 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Progressive cerebellar ataxia (HP:0002073). HP:0002073 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20301352 SUPPORT INDIRECT Other
"MANAGEMENT: Treatment of manifestations: Treatment is supportive."
GeneReviews states that management of mtDNA disease is supportive. Graded INDIRECT because the chapter addresses the Leigh syndrome spectrum rather than this adult-onset entity specifically.
Physical and Occupational Therapy
Action: physical therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is physical therapy (NCIT:C15302). NCIT:C15302 is a clinical intervention from the NCI Thesaurus. Ontology label: Physical Therapy NCIT:C15302
Gait, balance and hand-function rehabilitation for progressive cerebellar ataxia and distal weakness.
Target Phenotypes: Gait ataxia HP:0002066 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Gait ataxia (HP:0002066). HP:0002066 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:20301352 SUPPORT INDIRECT Other
"physical therapy and occupational therapy; standard treatment of eye movement disorders"
GeneReviews lists physical and occupational therapy in the supportive management of mtDNA disease. Graded INDIRECT because the recommendation is made for the Leigh syndrome spectrum rather than this entity.
Avoidance of Mitochondrial Toxins
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Sodium valproate, drugs that provoke acidosis, and dichloroacetate should be avoided or used with caution in mtDNA disease, and anaesthesia requires careful planning. This is an agents-to-avoid caution carried over from the mtDNA disorder literature, not a therapy.
Show evidence (1 reference)
PMID:20301352 SUPPORT INDIRECT Other
"Sodium valproate, medications that cause acidosis, and dichloroacetate should be avoided or used with caution; administration of anesthesia requires careful consideration to avoid aggravation of respiratory symptoms and precipitation of respiratory failure."
GeneReviews Agents/Circumstances to Avoid for mtDNA disease. Graded INDIRECT because it is stated for the Leigh syndrome spectrum; the pharmacologic hazard is shared by MT-ATP6 disease but has not been studied in this adult-onset entity.
Genetic Counseling and Reproductive Options
Action: genetic counselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is genetic counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. Ontology label: Genetic Counseling NCIT:C15240
Counselling covers maternal transmission, the unpredictability of heteroplasmy segregation, and reproductive options including prenatal testing, preimplantation genetic testing, and oocyte donation.
Show evidence (1 reference)
PMID:20301352 SUPPORT INDIRECT Other
"Reproductive options for the family members of a proband with an mtDNA pathogenic variant may include prenatal testing, preimplantation genetic testing, and oocyte donation."
GeneReviews enumerates the reproductive options relevant to any maternally inherited mtDNA variant. Graded INDIRECT because the chapter addresses the Leigh syndrome spectrum.
🔬

Biochemical Markers

2
Cytochrome c oxidase muscle histochemistry (Normal)
Show evidence (1 reference)
PMID:18055910 SUPPORT Human Clinical
"Cytochrome c oxidase muscle histochemistry, which is the standard clinical investigation for mitochondrial disease in adults, is usually normal in patients with MTATP6 mutations."
States directly that the standard adult mitochondrial screening test is normal in MT-ATP6 disease.
Respiratory chain enzyme activities (Normal)
Show evidence (1 reference)
PMID:22577227 SUPPORT Human Clinical
"Other investigations including muscle biopsy and respiratory chain enzyme activity were non-specific or normal."
Directly reports normal or non-specific respiratory-chain enzymology in adult-onset MT-ATP6 ataxia families.
🔬

Diagnosis

3
Mitochondrial DNA sequencing
Sequencing of MT-ATP6 with heteroplasmy quantification is the only reliable route to diagnosis, and should be pursued in undiagnosed adult ataxia even when muscle biopsy and respiratory-chain studies are unremarkable. Because heteroplasmy differs between tissues, a negative or low-level blood result does not exclude the diagnosis.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: Identification of a pathogenic heteroplasmic MT-ATP6 variant (m.8993T>C, or another MT-ATP6 missense variant such as m.9035T>C or m.9185T>C) establishes the diagnosis.
Show evidence (2 references)
PMID:22577227 SUPPORT Human Clinical
"MTATP6 sequencing should be considered in the workup of undiagnosed ataxia, even if other investigations do not suggest a mitochondrial DNA disorder."
Direct recommendation that MT-ATP6 sequencing be part of the undiagnosed adult ataxia workup regardless of other test results.
PMID:20953793 SUPPORT INDIRECT Human Clinical
"The degree of mutant heteroplasmy for the m.8993T>C mutation was evaluated by real-time allele refractory mutation system quantitative PCR of mtDNA from hair bulbs (ectoderm) and blood leukocytes (mesoderm)."
Illustrates the multi-tissue heteroplasmy quantification this diagnostic step calls for. Graded INDIRECT because the study quantified heteroplasmy in an m.8993T>C family diagnosed as NARP syndrome (MONDO:0010794); the method transfers, the cohort is a different entity.
Nerve conduction studies and electromyography
Electrophysiology characterises the neuropathy as axonal and length-dependent, separating it from demyelinating hereditary neuropathies.
nerve conduction velocity test NCIT:C88502 NCI Thesaurus (NCIT)
Results: Length-dependent sensorimotor axonal polyneuropathy of variable severity.
Show evidence (1 reference)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal polyneuropathy."
Reports the EMG/NCS findings this diagnostic study is expected to yield. Graded INDIRECT because the findings come from an m.8993T>C family diagnosed as NARP syndrome (MONDO:0010794) rather than from this adult-onset entity.
Brain magnetic resonance imaging
MRI documents cerebellar atrophy and may show basal ganglia signal change, supporting a mitochondrial rather than an acquired cause.
magnetic resonance imaging NCIT:C16809 NCI Thesaurus (NCIT)
Results: Cerebellar atrophy, with variable basal ganglia T2 hyperintensity.
Show evidence (1 reference)
PMID:20953793 SUPPORT INDIRECT Human Clinical
"3-T MRI revealed cerebellar atrophy and cystic and cavitary T2 hyperintensities in the basal ganglia."
Reports the MRI findings expected from this investigation. Graded INDIRECT because they come from an m.8993T>C family diagnosed as NARP syndrome (MONDO:0010794) rather than from this adult-onset entity.
📊

Prevalence

1
Adults with unexplained ataxia or suspected Charcot-Marie-Tooth disease (United Kingdom referral cohorts)
Cases In Literature Ultra Rare
Screening 308 patients with unexplained ataxia and 96 with suspected Charcot-Marie-Tooth disease identified a single m.8993T>C family; a further 191 episodic-ataxia patients yielded none. Only a handful of kindreds with adult-onset MT-ATP6 ataxia have been reported worldwide.
Show evidence (2 references)
PMID:18055910 SUPPORT Human Clinical
"We therefore studied 308 patients with unexplained ataxia and 96 patients with suspected Charcot-Marie-Tooth disease to determine whether the m.8993T-->C MTATP6 mutation is common in unexplained inherited ataxia and/or polyneuropathy."
Defines the screened cohorts from which the rarity estimate is drawn.
PMID:18055910 SUPPORT Human Clinical
"In conclusion, m.8993T-->C MTATP6 should be considered in patients with unexplained ataxia, CMT or EA, but cases are uncommon."
The authors' own summary that cases are uncommon, supporting the ULTRA_RARE band.
🔀

Differential Diagnoses

5

Conditions with similar clinical presentations that must be differentiated from Adult-Onset Ataxia and Polyneuropathy:

NARP syndrome MONDO:0010794
Overlapping Features NARP is caused by the same MT-ATP6 nucleotide-8993 substitutions but presents in childhood with the triad of neuropathy, ataxia and retinitis pigmentosa. Retinal involvement and earlier onset separate it from this adult entity, which is defined by ataxia plus axonal neuropathy without a required pigmentary retinopathy.
Distinguishing Features
  • Childhood onset
  • Retinitis pigmentosa
  • Generally higher mutant load
Show evidence (1 reference)
PMID:16049925 SUPPORT Human Clinical
"The 8993T-->C mutation in mitochondrial DNA (mtDNA) has been described previously to be associated with infantile- or childhood-onset phenotypes, ranging from Leigh's syndrome to neurogenic weakness, ataxia, and retinitis pigmentosa syndrome."
The defining report explicitly contrasts this adult entity with the previously described childhood NARP and Leigh phenotypes of the same variant.
Leigh syndrome MONDO:0009723
Overlapping Features Maternally inherited Leigh syndrome is the severe infantile end of the same MT-ATP6 spectrum, with subacute necrotising encephalopathy, developmental regression and high early mortality.
Distinguishing Features
  • Onset typically between three and 12 months
  • Decompensation with elevated lactate
  • Bilateral symmetric basal ganglia and brainstem lesions
Show evidence (1 reference)
PMID:20301352 SUPPORT Other
"Mitochondrial DNA-LSS is characterized by onset of manifestations typically between ages three and 12 months, often following an intercurrent illness (usually viral) or metabolic challenge (vaccinations, surgery, prolonged fasting)."
GeneReviews establishes the infantile onset that distinguishes Leigh syndrome spectrum from this adult-onset entity.
Autosomal recessive and dominant spinocerebellar ataxias Not Yet Curated MONDO:0000437
Overlapping Features Adult-onset MT-ATP6 ataxia is clinically indistinguishable from the common hereditary spinocerebellar ataxias, and is only separated from them by mtDNA sequencing.
Distinguishing Features
  • Non-dominant (maternal) pedigree pattern
  • Normal cytochrome c oxidase histochemistry does not exclude the diagnosis
  • Requires mitochondrial DNA sequencing
Show evidence (1 reference)
PMID:22577227 SUPPORT Human Clinical
"The authors report two families with onset of ataxia in adulthood (with pyramidal dysfunction and/or peripheral neuropathy variably present), who are clinically indistinguishable from other spinocerebellar ataxia patients."
States the clinical indistinguishability that makes spinocerebellar ataxia the primary differential.
CANVAS / RFC1 spectrum disorder MONDO:0044720
Overlapping Features The single most important differential for the descriptive phrase "adult-onset ataxia and polyneuropathy". Biallelic intronic AAGGG repeat expansion in RFC1 is a common, recessively inherited cause of late-onset ataxia with sensory neuronopathy, and is far more prevalent than MT-ATP6 disease. It is a mechanistically unrelated entity and must not be conflated with this MONDO/OMIM concept, which is defined by a maternally inherited MT-ATP6 variant.
Distinguishing Features
  • Bilateral vestibular areflexia
  • Sensory neuronopathy (ganglionopathy) rather than length-dependent axonal polyneuropathy
  • Chronic cough often preceding ataxia by years
  • Autosomal recessive biallelic RFC1 repeat expansion, not maternal inheritance
Show evidence (1 reference)
PMID:30926972 SUPPORT Human Clinical
"We used non-parametric linkage analysis and genome sequencing to identify a biallelic intronic AAGGG repeat expansion in the replication factor C subunit 1 (RFC1) gene as the cause of familial CANVAS and a frequent cause of late-onset ataxia, particularly if sensory neuronopathy and bilateral..."
Establishes biallelic RFC1 repeat expansion as a frequent cause of late-onset ataxia with neuropathy, making it the leading differential for an adult presenting with ataxia plus polyneuropathy.
Charcot-Marie-Tooth disease MONDO:0015626
Overlapping Features The axonal sensorimotor neuropathy can dominate the presentation and lead to referral as suspected hereditary motor and sensory neuropathy.
Distinguishing Features
  • Accompanying cerebellar ataxia
  • Maternal inheritance pattern
  • MT-ATP6 variant on mitochondrial DNA sequencing
Show evidence (1 reference)
PMID:18055910 SUPPORT Human Clinical
"In conclusion, m.8993T-->C MTATP6 should be considered in patients with unexplained ataxia, CMT or EA, but cases are uncommon."
Names suspected Charcot-Marie-Tooth disease as one of the clinical contexts in which this variant should be sought.
{ }

Source YAML

click to show 
name: Adult-Onset Ataxia and Polyneuropathy
creation_date: '2026-08-19T00:00:00Z'
description: >-
  Adult-onset ataxia and polyneuropathy (OMIM 500010) is a maternally inherited
  mitochondrial disorder defined by a kindred carrying the heteroplasmic
  m.8993T>C variant of MT-ATP6, in which slowly progressive cerebellar ataxia
  and axonal sensorimotor polyneuropathy begin in adult life. The same
  nucleotide-8993 substitution more commonly causes infantile or childhood
  disease (maternally inherited Leigh syndrome, NARP), so this entity marks the
  mild, late-onset end of a heteroplasmy- and modifier-dependent MT-ATP6
  spectrum rather than a distinct molecular lesion. Because standard adult
  mitochondrial screening (cytochrome c oxidase muscle histochemistry,
  respiratory-chain enzymology) is typically normal in MT-ATP6 disease, affected
  adults are readily misclassified as undiagnosed spinocerebellar ataxia or
  Charcot-Marie-Tooth disease unless mtDNA is sequenced. Other MT-ATP6 variants
  (m.9035T>C, m.9185T>C) produce clinically indistinguishable adult-onset
  ataxia with variable pyramidal and peripheral-nerve involvement.
category: Mendelian
parents:
- hereditary disease
- mitochondrial disease
synonyms:
- Ataxia and polyneuropathy, adult-onset
- Adult-onset MT-ATP6 ataxia and axonal neuropathy
disease_term:
  preferred_term: adult-onset ataxia and polyneuropathy
  term:
    id: MONDO:0010781
    label: ataxia and polyneuropathy, adult-onset

inheritance:
- name: Maternal mitochondrial inheritance
  description: >-
    The disorder is caused by heteroplasmic MT-ATP6 variants (m.8993T>C,
    m.9035T>C, m.9185T>C) carried on mitochondrial DNA and is transmitted
    maternally. Adult onset in these kindreds reflects a heteroplasmy load that
    stays below the threshold producing infantile NARP or Leigh syndrome.
  inheritance_term:
    preferred_term: Mitochondrial inheritance
    term:
      id: HP:0001427
      label: Mitochondrial inheritance
  evidence:
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Pathogenic variants in MT-ATP6 are a well-known cause of
      maternally-inherited mitochondrial disorders
    explanation: >-
      Establishes maternal transmission as the inheritance mode for MT-ATP6
      disorders, the gene carrying this entry's causal variants.
pathophysiology:
- name: MT-ATP6 m.8993T>C Variant
  role: trigger
  biological_scale: MOLECULAR
  description: >-
    A heteroplasmic thymine-to-cytosine transition at mtDNA nucleotide 8993
    substitutes the highly conserved leucine at codon 156 of the ATP6 subunit
    (subunit a) of mitochondrial ATP synthase. The same nucleotide is the site
    of the more severe m.8993T>G (Leu156Arg) NARP/MILS variant.
  genes:
  - preferred_term: MT-ATP6
    term:
      id: hgnc:7414
      label: MT-ATP6
  cellular_components:
  - preferred_term: proton-transporting ATP synthase complex
    term:
      id: GO:0045259
      label: proton-transporting ATP synthase complex
  genetic_context:
    gene:
      preferred_term: MT-ATP6
      term:
        id: hgnc:7414
        label: MT-ATP6
    allele_type: SNV
    variant_origin: GERMLINE
    functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
    description: >-
      Heteroplasmic mitochondrial missense substitution (Leu156Pro) inherited
      through the maternal line.
  evidence:
  - reference: PMID:16049925
    reference_title: Adult-onset ataxia and polyneuropathy caused by mitochondrial 8993T-->C mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report a kindred with adult-onset slowly progressive ataxia and
      polyneuropathy and with the heteroplasmic 8993T-->C mutation.
    explanation: >-
      This is the defining report for OMIM 500010 and directly ties the
      heteroplasmic m.8993T>C MT-ATP6 variant to the adult-onset
      ataxia-plus-polyneuropathy phenotype.
  - reference: PMID:17568559
    reference_title: Biochemical phenotypes associated with the mitochondrial ATP6 gene mutations at nt8993.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Two point mutations (T>G and T>C) at the same 8993 nucleotide of
      mitochondrial DNA (at comparable mutant load), affecting the ATPase 6
      subunit of the F1F0-ATPase, result in neurological phenotypes of variable
      severity in humans.
    explanation: >-
      Locates the lesion in the ATPase 6 subunit of F1F0-ATP synthase and
      records that the two nucleotide-8993 substitutions differ in severity.
  downstream:
  - target: Impaired ATP Synthase Function
    causal_link_type: DIRECT
    description: >-
      Substitution of the conserved Leu-156 residue in the membrane sector of
      subunit a degrades proton-driven ATP synthesis.
    evidence:
    - reference: PMID:17568559
      reference_title: Biochemical phenotypes associated with the mitochondrial ATP6 gene mutations at nt8993.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        These results possibly highlight the different pathogenic mechanism
        generated by the two mutations at position 8993 and provide useful
        information to better characterize the biochemical role of the highly
        conserved Leu-156 in ATPase 6 subunit of the mitochondrial ATP synthase
        complex.
      explanation: >-
        Directly attributes the biochemical defect to the conserved Leu-156
        residue of the ATP synthase ATPase 6 subunit.
  - target: Heteroplasmy Load Above the Phenotypic Threshold
    causal_link_type: DIRECT
    description: >-
      Because the variant is heteroplasmic, disease expression depends on the
      proportion of mutant mtDNA molecules reached in a given tissue.

- name: Impaired ATP Synthase Function
  role: amplifier
  biological_scale: MOLECULAR
  description: >-
    Loss of the conserved subunit-a leucine impairs the proton-translocating
    membrane sector of complex V, so the proton-motive force is used
    inefficiently for ATP synthesis. Whether the primary lesion is catalytic or
    one of assembly and stability is not settled: complex V subcomplexes are
    detectable on native gels in patient tissue, which points toward an assembly
    or stability problem, and the authors who reported them said further work is
    needed to decide between the two readings.
  molecular_functions:
  - preferred_term: proton-transporting ATP synthase activity, rotational mechanism
    modifier: DECREASED
    term:
      id: GO:0046933
      label: proton-transporting ATP synthase activity, rotational mechanism
  biological_processes:
  - preferred_term: proton motive force-driven ATP synthesis
    modifier: DECREASED
    term:
      id: GO:0015986
      label: proton motive force-driven ATP synthesis
  cellular_components:
  - preferred_term: proton-transporting ATP synthase complex
    term:
      id: GO:0045259
      label: proton-transporting ATP synthase complex
  evidence:
  - reference: PMID:19626676
    reference_title: Identification of ataxia-associated mtDNA mutations (m.4052T>C and m.9035T>C) and evaluation of their pathogenicity in transmitochondrial cybrids.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: >-
      We confirmed that the 4452T>C transition in tRNA(Met) represented a
      polymorphism; however, 9035T>C conversion in the ATP6 gene was
      responsible for a defective F(0)-ATPase.
    explanation: >-
      Cybrid evidence that an ataxia-associated MT-ATP6 missense variant produces a
      defective F0-ATPase. Graded INDIRECT because this cybrid work used m.9035T>C, a
      different MT-ATP6 variant in the same adult-onset ataxia spectrum, not the
      m.8993T>C variant that defines this entry.
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Native gels found presence of subcomplexes of complex V in fibroblast
      and/or skeletal muscle.
    explanation: >-
      Patient-tissue evidence that an MT-ATP6 ataxia variant perturbs complex V. Graded
      INDIRECT because the patients carried m.9035T>C rather than m.8993T>C.
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The occurrence of subcomplexes suggest that variant
      NC_012920.1(MT-ATP6):m.9035T>C could cause problems with complex V
      assembly or stability. However, further research is warranted to further
      evaluate these hypotheses.
    explanation: >-
      The authors' own reading of the subcomplex finding, and their statement that the
      assembly-versus-catalysis question is unresolved. Cited here so the node
      description does not over-read the native-gel result. Graded INDIRECT because the
      variant is m.9035T>C rather than m.8993T>C.
  downstream:
  - target: Neuronal Energy Deficit and Oxidative Stress
    causal_link_type: DIRECT
    description: >-
      Inefficient oxidative phosphorylation lowers cellular ATP and raises
      reactive oxygen species output from the respiratory chain.
    evidence:
    - reference: PMID:17568559
      reference_title: Biochemical phenotypes associated with the mitochondrial ATP6 gene mutations at nt8993.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Both 8993T>G and 8993T>C mutations led to energy deprivation and ROS
        overproduction.
      explanation: >-
        Patient lymphocytes carrying m.8993T>C show both of the downstream
        consequences asserted by this edge.

- name: Heteroplasmy Load Above the Phenotypic Threshold
  role: modulator
  biological_scale: CELLULAR
  description: >-
    Mitochondrial heteroplasmy is the gate between carrier state and disease.
    MT-ATP6 ataxia variants require a very high mutant load before symptoms
    appear, and mutant load segregates unpredictably between tissues and between
    generations through the germline bottleneck. This is the most plausible
    explanation for why the same nucleotide-8993 change causes infantile Leigh
    syndrome in some pedigrees and only adult-onset ataxia in others, although
    load alone does not fully predict severity.
  evidence:
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The above mentioned findings suggest there is a very high heteroplasmy
      threshold required for NC_012920.1(MT-ATP6):m.9035T>C to trigger
      phenotypic manifestations, but once this threshold is exceeded, this can
      result in different clinical manifestations.
    explanation: >-
      States the high-threshold, poorly-predictive relationship between MT-ATP6 mutant
      load and phenotype. Graded INDIRECT because it was established for m.9035T>C
      rather than m.8993T>C.
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      There were marked phenotypic differences between family members, even
      between individuals with the greatest degrees of ectodermal and mesodermal
      heteroplasmy.
    explanation: >-
      In an m.8993T>C family, measured heteroplasmy did not account for the
      within-family phenotypic spread, supporting the modulator role and the limits of
      load as a predictor. Graded INDIRECT because the cohort was diagnosed as NARP
      syndrome (MONDO:0010794), an entity this entry keeps distinct; the observation
      carries across because the modulator claim is about m.8993T>C heteroplasmy
      behaviour rather than about either phenotype.
  - reference: PMID:20301352
    reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: >-
      Sibs may inherit the pathogenic variant at varying heteroplasmy levels due
      to the bottleneck effect and variant-specific segregation patterns.
    explanation: >-
      GeneReviews statement of the germline-bottleneck mechanism that generates the
      variable mutant loads this node depends on. Graded INDIRECT because it is a
      general statement about mtDNA disorders rather than about this entity.
  downstream:
  - target: Neuronal Energy Deficit and Oxidative Stress
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Only tissues whose mutant load exceeds the threshold express a
      bioenergetic defect, which is why the phenotype is regionally restricted
      and appears late.

- name: Neuronal Energy Deficit and Oxidative Stress
  role: central_effector
  biological_scale: CELLULAR
  conforms_to: peripheral_axonal_degeneration#Axonal Transport Impairment and Mitochondrial Dysfunction
  description: >-
    Cells above the heteroplasmy threshold sustain a chronic ATP shortfall and
    an increased reactive-oxygen-species burden, and compensate by shifting
    toward glycolysis. For the m.8993T>C substitution specifically the balance
    is tilted toward ROS overproduction rather than the more severe energy
    failure seen with m.8993T>G, which fits the milder, later-onset phenotype.
    Long projection neurons and cerebellar neurons, which depend on distributed
    mitochondrial ATP supply, are the tissues in which this shortfall becomes
    clinically apparent.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: oxidative phosphorylation
    modifier: DECREASED
    term:
      id: GO:0006119
      label: oxidative phosphorylation
  - preferred_term: reactive oxygen species metabolic process
    modifier: INCREASED
    term:
      id: GO:0072593
      label: reactive oxygen species metabolic process
  - preferred_term: response to oxidative stress
    modifier: INCREASED
    term:
      id: GO:0006979
      label: response to oxidative stress
  - preferred_term: mitochondrion organization
    modifier: ABNORMAL
    term:
      id: GO:0007005
      label: mitochondrion organization
  chemical_entities:
  - preferred_term: ATP
    modifier: DECREASED
    term:
      id: CHEBI:15422
      label: ATP
  - preferred_term: reactive oxygen species
    modifier: INCREASED
    term:
      id: CHEBI:26523
      label: reactive oxygen species
  evidence:
  - reference: PMID:17568559
    reference_title: Biochemical phenotypes associated with the mitochondrial ATP6 gene mutations at nt8993.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The 8993T>G change mainly induces an energy deficiency, whereas the
      8993T>C favours an increased ROS production.
    explanation: >-
      Establishes the ROS-weighted balance of the bioenergetic lesion for the
      m.8993T>C variant that defines this entry.
  - reference: PMID:19626676
    reference_title: Identification of ataxia-associated mtDNA mutations (m.4052T>C and m.9035T>C) and evaluation of their pathogenicity in transmitochondrial cybrids.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: >-
      They had less than half of the steady-state content of ATP and nearly an
      8-fold higher basal level of reactive oxygen species (ROS).
    explanation: >-
      Quantifies the combined ATP shortfall and ROS burden in cybrids carrying an
      MT-ATP6 ataxia variant. Graded INDIRECT because the cybrids carried m.9035T>C.
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Bioenergetic measurements in fibroblasts from both patients detected
      reduced spare respiratory capacities and altered extracellular
      acidification rates, revealing a switch from mitochondrial respiration to
      glycolysis to uphold ATP production.
    explanation: >-
      Patient-derived fibroblast evidence of the compensatory glycolytic shift described
      in this node. Graded INDIRECT because the variant was m.9035T>C.
  downstream:
  - target: Distal Axonal Degeneration of Peripheral Nerves
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The longest peripheral axons, which must maintain mitochondria far from
      the cell body, are the first to fail.
  - target: Cerebellar Degeneration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Chronic bioenergetic stress in cerebellar cortex produces progressive
      cerebellar volume loss.

- name: Distal Axonal Degeneration of Peripheral Nerves
  role: effector
  biological_scale: TISSUE
  conforms_to: peripheral_axonal_degeneration#Length-Dependent Nerve Fiber Dysfunction
  description: >-
    A length-dependent, predominantly axonal sensorimotor polyneuropathy
    develops, producing distal sensory loss, depressed or absent ankle reflexes,
    and distal weakness. Nerve conduction studies show axonal rather than
    demyelinating features, which is why these patients are commonly referred
    with a working diagnosis of Charcot-Marie-Tooth disease.
  cell_types:
  - preferred_term: sensory neuron
    term:
      id: CL:0000101
      label: sensory neuron
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: axonal transport
    modifier: DECREASED
    term:
      id: GO:0098930
      label: axonal transport
  - preferred_term: transmission of nerve impulse
    modifier: DECREASED
    term:
      id: GO:0019226
      label: transmission of nerve impulse
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal
      polyneuropathy.
    explanation: >-
      Electrophysiological demonstration of length-dependent axonal sensorimotor
      polyneuropathy in an m.8993T>C family. Graded INDIRECT because that family was
      diagnosed as NARP syndrome (MONDO:0010794), which this entry curates as a separate
      entity and a differential.
  - reference: PMID:16049925
    reference_title: Adult-onset ataxia and polyneuropathy caused by mitochondrial 8993T-->C mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Our findings suggest that the 8993T-->C mtDNA mutation should be
      considered in the differential diagnosis of nondominant adult-onset ataxia
      and axonal neuropathy.
    explanation: >-
      The defining report characterises the neuropathy of this entity as
      axonal.
  downstream:
  - target: Adult-Onset Ataxia with Axonal Polyneuropathy
    causal_link_type: DIRECT
    description: >-
      Peripheral sensory loss compounds cerebellar incoordination, worsening
      gait instability.

- name: Cerebellar Degeneration
  role: effector
  biological_scale: TISSUE
  mechanism_confidence: PROVISIONAL
  description: >-
    Progressive cerebellar volume loss underlies the ataxia. Cerebellar atrophy
    is demonstrable on brain MRI in m.8993T>C carriers. The cell-level
    neuropathology has not been established for this adult-onset entity: no
    autopsy series localises the loss to Purkinje neurons specifically, so the
    Purkinje cell type is recorded here as the presumed but unproven substrate
    (see the KNOWLEDGE_GAP discussion).
  cell_types:
  - preferred_term: Purkinje cell
    term:
      id: CL:0000121
      label: Purkinje cell
  biological_processes:
  - preferred_term: nervous system process
    modifier: ABNORMAL
    term:
      id: GO:0050877
      label: nervous system process
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      3-T MRI revealed cerebellar atrophy and cystic and cavitary T2
      hyperintensities in the basal ganglia.
    explanation: >-
      Imaging evidence of cerebellar atrophy in an m.8993T>C family, supporting
      cerebellar degeneration as the anatomical substrate of the ataxia. Graded INDIRECT
      because that family was diagnosed as NARP syndrome (MONDO:0010794), which this
      entry curates as a separate entity and a differential.
  downstream:
  - target: Adult-Onset Ataxia with Axonal Polyneuropathy
    causal_link_type: DIRECT
    description: >-
      Loss of cerebellar cortical output produces gait and limb ataxia,
      dysarthria, and abnormal eye movements.

- name: Adult-Onset Ataxia with Axonal Polyneuropathy
  role: consequence
  biological_scale: ORGANISM
  conforms_to: cerebellar_purkinje_degeneration#Cerebellar Ataxia
  description: >-
    The combined clinical syndrome: slowly progressive cerebellar ataxia
    beginning in adulthood together with an axonal sensorimotor polyneuropathy,
    variably accompanied by pyramidal signs. It is clinically indistinguishable
    from other adult spinocerebellar ataxias, and the routine adult
    mitochondrial screening tests are normal, so mtDNA sequencing is the only
    route to diagnosis.
  evidence:
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The authors report two families with onset of ataxia in adulthood (with
      pyramidal dysfunction and/or peripheral neuropathy variably present), who
      are clinically indistinguishable from other spinocerebellar ataxia
      patients.
    explanation: >-
      Characterises the adult-onset MT-ATP6 syndrome as an ataxia with variable
      neuropathy and pyramidal involvement that mimics common spinocerebellar
      ataxias.

phenotypes:
- category: Neurological
  name: Progressive Cerebellar Ataxia
  description: >-
    Slowly progressive gait and limb ataxia with adult onset, the defining
    manifestation of this entity.
  phenotype_term:
    preferred_term: Progressive cerebellar ataxia
    term:
      id: HP:0002073
      label: Progressive cerebellar ataxia
    clinical_course: PROGRESSIVE
  frequency: OBLIGATE
  diagnostic: true
  evidence:
  - reference: PMID:16049925
    reference_title: Adult-onset ataxia and polyneuropathy caused by mitochondrial 8993T-->C mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report a kindred with adult-onset slowly progressive ataxia and
      polyneuropathy and with the heteroplasmic 8993T-->C mutation.
    explanation: >-
      The defining kindred is described as having adult-onset slowly
      progressive ataxia, which is by definition present in every member of
      this entity.

- category: Neurological
  name: Gait Ataxia
  description: Unsteady, wide-based gait is typically the presenting complaint.
  phenotype_term:
    preferred_term: Gait ataxia
    term:
      id: HP:0002066
      label: Gait ataxia
  evidence:
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Furthermore, the patient had an atactic gait pattern, a decreased sense of
      balance and could not walk without support.
    explanation: >-
      An examination finding of ataxic gait in MT-ATP6 ataxia. Graded INDIRECT because
      the patient carried m.9035T>C and presented in adolescence rather than adulthood.
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The authors report two families with onset of ataxia in adulthood (with
      pyramidal dysfunction and/or peripheral neuropathy variably present), who
      are clinically indistinguishable from other spinocerebellar ataxia
      patients.
    explanation: >-
      Graded INDIRECT because the quote establishes adult-onset ataxia clinically
      indistinguishable from spinocerebellar ataxia, but does not name gait, so it
      supports the cerebellar syndrome rather than this specific HPO term.
- category: Neurological
  name: Peripheral Axonal Neuropathy
  description: >-
    Length-dependent sensorimotor polyneuropathy with axonal features on nerve
    conduction studies, present alongside the ataxia.
  phenotype_term:
    preferred_term: Peripheral axonal neuropathy
    term:
      id: HP:0003477
      label: Peripheral axonal neuropathy
  frequency: OBLIGATE
  diagnostic: true
  evidence:
  - reference: PMID:16049925
    reference_title: Adult-onset ataxia and polyneuropathy caused by mitochondrial 8993T-->C mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Our findings suggest that the 8993T-->C mtDNA mutation should be
      considered in the differential diagnosis of nondominant adult-onset ataxia
      and axonal neuropathy.
    explanation: >-
      The defining report identifies the neuropathy of this entity as axonal.
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal
      polyneuropathy.
    explanation: >-
      Electrophysiological confirmation of length-dependent axonal sensorimotor
      polyneuropathy in an m.8993T>C family. Graded INDIRECT because that family was
      diagnosed as NARP syndrome (MONDO:0010794), a separate entity in this knowledge
      base; the neuropathy phenotype is shared, the diagnosis is not.
- category: Neurological
  name: Sensorimotor Neuropathy
  description: >-
    Both sensory and motor fibres are affected, producing distal sensory loss
    together with distal weakness.
  phenotype_term:
    preferred_term: Sensorimotor neuropathy
    term:
      id: HP:0007141
      label: Sensorimotor neuropathy
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal
      polyneuropathy.
    explanation: >-
      Documents the sensorimotor character of the neuropathy. Graded INDIRECT because
      the cohort was diagnosed as NARP syndrome (MONDO:0010794) rather than as this
      adult-onset entity.
- category: Neuroimaging
  name: Cerebellar Atrophy
  description: Brain MRI shows cerebellar volume loss.
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      3-T MRI revealed cerebellar atrophy and cystic and cavitary T2
      hyperintensities in the basal ganglia.
    explanation: >-
      High-field MRI in an m.8993T>C family documents cerebellar atrophy. Graded
      INDIRECT because that family was diagnosed as NARP syndrome (MONDO:0010794) rather
      than as this adult-onset entity.
- category: Neurological
  name: Pyramidal Dysfunction
  description: >-
    Corticospinal tract signs (spasticity, brisk reflexes, extensor plantar
    responses) are variably present in adult-onset MT-ATP6 ataxia.
  phenotype_term:
    preferred_term: Babinski sign
    term:
      id: HP:0003487
      label: Babinski sign
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The authors report two families with onset of ataxia in adulthood (with
      pyramidal dysfunction and/or peripheral neuropathy variably present), who
      are clinically indistinguishable from other spinocerebellar ataxia
      patients.
    explanation: >-
      Records pyramidal dysfunction as a variable feature. Graded INDIRECT because the
      abstract names pyramidal dysfunction generically rather than the Babinski sign
      used here as the closest HPO anchor.
- category: Neurological
  name: Episodic Ataxia
  description: >-
    A phenotypic extension observed in one m.8993T>C carrier, who had discrete
    attacks of ataxia rather than (or in addition to) steady progression.
  phenotype_term:
    preferred_term: Episodic ataxia
    term:
      id: HP:0002131
      label: Episodic ataxia
  frequency: VERY_RARE
  evidence:
  - reference: PMID:18055910
    reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      One subject had episodic ataxia (EA) and transient hemipareses,
      broadening the phenotype.
    explanation: >-
      A single m.8993T>C carrier with episodic ataxia, reported explicitly as a
      broadening of the phenotype, which is why the frequency band is VERY_RARE.

- category: Neurological
  name: Transient Hemiparesis
  description: >-
    Reversible unilateral weakness reported in the same m.8993T>C carrier who
    had episodic ataxia.
  phenotype_term:
    preferred_term: Hemiparesis
    term:
      id: HP:0001269
      label: Hemiparesis
    temporality: TRANSIENT
  frequency: VERY_RARE
  evidence:
  - reference: PMID:18055910
    reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      One subject had episodic ataxia (EA) and transient hemipareses,
      broadening the phenotype.
    explanation: >-
      Directly reports transient hemiparesis in an m.8993T>C carrier.

- category: Neurological
  name: Ophthalmoplegia
  description: >-
    Impaired eye movements have been reported in adult-onset MT-ATP6 ataxia,
    though not in the defining m.8993T>C kindred.
  phenotype_term:
    preferred_term: Ophthalmoplegia
    term:
      id: HP:0000602
      label: Ophthalmoplegia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:35159298
    reference_title: "Clinical Heterogeneity in MT-ATP6 Pathogenic Variants: Same Genotype-Different Onset."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patient 2 presented in adolescence with ataxia and ophthalmoplegia
      without cognitive or motor impairment.
    explanation: >-
      Documents ophthalmoplegia accompanying later-onset MT-ATP6 ataxia. Graded INDIRECT
      because the patient carried m.9035T>C and presented in adolescence rather than
      adulthood.
biochemical:
- name: Cytochrome c oxidase muscle histochemistry
  notes: >-
    Unlike most adult mitochondrial disease, COX histochemistry on muscle is
    typically normal in MT-ATP6 disorders. This normal result is diagnostically
    important: it is the reason these patients are missed by the standard adult
    mitochondrial workup.
  presence: Normal
  evidence:
  - reference: PMID:18055910
    reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Cytochrome c oxidase muscle histochemistry, which is the standard
      clinical investigation for mitochondrial disease in adults, is usually
      normal in patients with MTATP6 mutations.
    explanation: >-
      States directly that the standard adult mitochondrial screening test is
      normal in MT-ATP6 disease.

- name: Respiratory chain enzyme activities
  notes: >-
    Respiratory-chain enzymology and muscle biopsy are non-specific or normal in
    adult-onset MT-ATP6 ataxia. The commonly used spectrophotometric complex V
    assay measures ATP hydrolysis, the reverse reaction, which does not require
    proton pumping and so under-detects this class of defect.
  presence: Normal
  evidence:
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Other investigations including muscle biopsy and respiratory chain enzyme
      activity were non-specific or normal.
    explanation: >-
      Directly reports normal or non-specific respiratory-chain enzymology in
      adult-onset MT-ATP6 ataxia families.

genetic:
- name: MT-ATP6
  gene_term:
    preferred_term: MT-ATP6
    term:
      id: hgnc:7414
      label: MT-ATP6
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  association: >-
    Heteroplasmic m.8993T>C (Leu156Pro) is the defining variant of OMIM 500010.
    Other MT-ATP6 missense variants (m.9035T>C, m.9185T>C) cause clinically
    indistinguishable adult-onset ataxia syndromes.
  inheritance:
  - name: Mitochondrial inheritance
    inheritance_term:
      preferred_term: Mitochondrial inheritance
      term:
        id: HP:0001427
        label: Mitochondrial inheritance
    description: >-
      Maternal transmission of a heteroplasmic mtDNA variant; mutant load
      segregates through the germline bottleneck, so sibs and offspring inherit
      varying loads and clinical expression is incompletely predictable.
    evidence:
    - reference: PMID:20301352
      reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: OTHER
      snippet: >-
        Sibs may inherit the pathogenic variant at varying heteroplasmy levels
        due to the bottleneck effect and variant-specific segregation patterns.
      explanation: >-
        GeneReviews statement of maternal transmission with bottleneck-driven
        heteroplasmy variation. Graded INDIRECT because the chapter covers mtDNA-Leigh
        syndrome spectrum generally rather than this adult-onset entity.
    - reference: PMID:18055910
      reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We identified a three-generation family with the m.8993T-->C mutation
        of mtDNA.
      explanation: >-
        A three-generation pedigree carrying the variant is direct evidence of
        vertical (maternal) transmission of m.8993T>C.
  evidence:
  - reference: PMID:16049925
    reference_title: Adult-onset ataxia and polyneuropathy caused by mitochondrial 8993T-->C mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report a kindred with adult-onset slowly progressive ataxia and
      polyneuropathy and with the heteroplasmic 8993T-->C mutation.
    explanation: >-
      Establishes MT-ATP6 m.8993T>C as the causal variant for this entity.
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These families had the m.9185T>C and m.9035T>C mutations, respectively,
      which have not previously been associated with adult-onset cerebellar
      syndromes.
    explanation: >-
      Extends the causal gene beyond m.8993T>C to other MT-ATP6 missense
      variants producing adult-onset cerebellar syndromes.

diagnosis:
- name: Mitochondrial DNA sequencing
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Sequencing of MT-ATP6 with heteroplasmy quantification is the only reliable
    route to diagnosis, and should be pursued in undiagnosed adult ataxia even
    when muscle biopsy and respiratory-chain studies are unremarkable. Because
    heteroplasmy differs between tissues, a negative or low-level blood result
    does not exclude the diagnosis.
  results: >-
    Identification of a pathogenic heteroplasmic MT-ATP6 variant (m.8993T>C, or
    another MT-ATP6 missense variant such as m.9035T>C or m.9185T>C) establishes
    the diagnosis.
  evidence:
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      MTATP6 sequencing should be considered in the workup of undiagnosed
      ataxia, even if other investigations do not suggest a mitochondrial DNA
      disorder.
    explanation: >-
      Direct recommendation that MT-ATP6 sequencing be part of the undiagnosed
      adult ataxia workup regardless of other test results.
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The degree of mutant heteroplasmy for the m.8993T>C mutation was evaluated
      by real-time allele refractory mutation system quantitative PCR of mtDNA
      from hair bulbs (ectoderm) and blood leukocytes (mesoderm).
    explanation: >-
      Illustrates the multi-tissue heteroplasmy quantification this diagnostic step
      calls for. Graded INDIRECT because the study quantified heteroplasmy in an
      m.8993T>C family diagnosed as NARP syndrome (MONDO:0010794); the method transfers,
      the cohort is a different entity.
- name: Nerve conduction studies and electromyography
  diagnosis_term:
    preferred_term: nerve conduction velocity test
    term:
      id: NCIT:C88502
      label: Nerve Conduction Velocity Test
  description: >-
    Electrophysiology characterises the neuropathy as axonal and
    length-dependent, separating it from demyelinating hereditary neuropathies.
  results: >-
    Length-dependent sensorimotor axonal polyneuropathy of variable severity.
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      EMG-NCS revealed varying degrees of length-dependent sensorimotor axonal
      polyneuropathy.
    explanation: >-
      Reports the EMG/NCS findings this diagnostic study is expected to yield. Graded
      INDIRECT because the findings come from an m.8993T>C family diagnosed as NARP
      syndrome (MONDO:0010794) rather than from this adult-onset entity.
- name: Brain magnetic resonance imaging
  diagnosis_term:
    preferred_term: magnetic resonance imaging
    term:
      id: NCIT:C16809
      label: Magnetic Resonance Imaging
  description: >-
    MRI documents cerebellar atrophy and may show basal ganglia signal change,
    supporting a mitochondrial rather than an acquired cause.
  results: Cerebellar atrophy, with variable basal ganglia T2 hyperintensity.
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      3-T MRI revealed cerebellar atrophy and cystic and cavitary T2
      hyperintensities in the basal ganglia.
    explanation: >-
      Reports the MRI findings expected from this investigation. Graded INDIRECT because
      they come from an m.8993T>C family diagnosed as NARP syndrome (MONDO:0010794)
      rather than from this adult-onset entity.
differential_diagnoses:
- name: NARP syndrome
  disease_term:
    preferred_term: NARP syndrome
    term:
      id: MONDO:0010794
      label: NARP syndrome
  description: >-
    NARP is caused by the same MT-ATP6 nucleotide-8993 substitutions but
    presents in childhood with the triad of neuropathy, ataxia and retinitis
    pigmentosa. Retinal involvement and earlier onset separate it from this
    adult entity, which is defined by ataxia plus axonal neuropathy without a
    required pigmentary retinopathy.
  distinguishing_features:
  - Childhood onset
  - Retinitis pigmentosa
  - Generally higher mutant load
  evidence:
  - reference: PMID:16049925
    reference_title: Adult-onset ataxia and polyneuropathy caused by mitochondrial 8993T-->C mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The 8993T-->C mutation in mitochondrial DNA (mtDNA) has been described
      previously to be associated with infantile- or childhood-onset
      phenotypes, ranging from Leigh's syndrome to neurogenic weakness, ataxia,
      and retinitis pigmentosa syndrome.
    explanation: >-
      The defining report explicitly contrasts this adult entity with the
      previously described childhood NARP and Leigh phenotypes of the same
      variant.

- name: Leigh syndrome
  disease_term:
    preferred_term: Leigh syndrome
    term:
      id: MONDO:0009723
      label: Leigh syndrome
  description: >-
    Maternally inherited Leigh syndrome is the severe infantile end of the same
    MT-ATP6 spectrum, with subacute necrotising encephalopathy, developmental
    regression and high early mortality.
  distinguishing_features:
  - Onset typically between three and 12 months
  - Decompensation with elevated lactate
  - Bilateral symmetric basal ganglia and brainstem lesions
  evidence:
  - reference: PMID:20301352
    reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Mitochondrial DNA-LSS is characterized by onset of manifestations
      typically between ages three and 12 months, often following an
      intercurrent illness (usually viral) or metabolic challenge
      (vaccinations, surgery, prolonged fasting).
    explanation: >-
      GeneReviews establishes the infantile onset that distinguishes Leigh
      syndrome spectrum from this adult-onset entity.

- name: Autosomal recessive and dominant spinocerebellar ataxias
  disease_term:
    preferred_term: cerebellar ataxia
    term:
      id: MONDO:0000437
      label: cerebellar ataxia
  description: >-
    Adult-onset MT-ATP6 ataxia is clinically indistinguishable from the common
    hereditary spinocerebellar ataxias, and is only separated from them by mtDNA
    sequencing.
  distinguishing_features:
  - Non-dominant (maternal) pedigree pattern
  - Normal cytochrome c oxidase histochemistry does not exclude the diagnosis
  - Requires mitochondrial DNA sequencing
  evidence:
  - reference: PMID:22577227
    reference_title: Adult-onset spinocerebellar ataxia syndromes due to MTATP6 mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The authors report two families with onset of ataxia in adulthood (with
      pyramidal dysfunction and/or peripheral neuropathy variably present), who
      are clinically indistinguishable from other spinocerebellar ataxia
      patients.
    explanation: >-
      States the clinical indistinguishability that makes spinocerebellar ataxia
      the primary differential.

- name: CANVAS / RFC1 spectrum disorder
  disease_term:
    preferred_term: cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome
    term:
      id: MONDO:0044720
      label: cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome
  description: >-
    The single most important differential for the descriptive phrase
    "adult-onset ataxia and polyneuropathy". Biallelic intronic AAGGG repeat
    expansion in RFC1 is a common, recessively inherited cause of late-onset
    ataxia with sensory neuronopathy, and is far more prevalent than MT-ATP6
    disease. It is a mechanistically unrelated entity and must not be conflated
    with this MONDO/OMIM concept, which is defined by a maternally inherited
    MT-ATP6 variant.
  distinguishing_features:
  - Bilateral vestibular areflexia
  - Sensory neuronopathy (ganglionopathy) rather than length-dependent axonal polyneuropathy
  - Chronic cough often preceding ataxia by years
  - Autosomal recessive biallelic RFC1 repeat expansion, not maternal inheritance
  evidence:
  - reference: PMID:30926972
    reference_title: Biallelic expansion of an intronic repeat in RFC1 is a common cause of late-onset ataxia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We used non-parametric linkage analysis and genome sequencing to identify
      a biallelic intronic AAGGG repeat expansion in the replication factor C
      subunit 1 (RFC1) gene as the cause of familial CANVAS and a frequent cause
      of late-onset ataxia, particularly if sensory neuronopathy and bilateral
      vestibular areflexia coexist.
    explanation: >-
      Establishes biallelic RFC1 repeat expansion as a frequent cause of
      late-onset ataxia with neuropathy, making it the leading differential for
      an adult presenting with ataxia plus polyneuropathy.

- name: Charcot-Marie-Tooth disease
  disease_term:
    preferred_term: Charcot-Marie-Tooth disease
    term:
      id: MONDO:0015626
      label: Charcot-Marie-Tooth disease
  description: >-
    The axonal sensorimotor neuropathy can dominate the presentation and lead to
    referral as suspected hereditary motor and sensory neuropathy.
  distinguishing_features:
  - Accompanying cerebellar ataxia
  - Maternal inheritance pattern
  - MT-ATP6 variant on mitochondrial DNA sequencing
  evidence:
  - reference: PMID:18055910
    reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In conclusion, m.8993T-->C MTATP6 should be considered in patients with
      unexplained ataxia, CMT or EA, but cases are uncommon.
    explanation: >-
      Names suspected Charcot-Marie-Tooth disease as one of the clinical
      contexts in which this variant should be sought.

treatments:
- name: Supportive and Symptomatic Management
  description: >-
    No disease-modifying therapy exists. Care is supportive and symptom-directed
    across the mtDNA disorders, with regular neurologic review for progression
    of ataxia and neuropathy.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: OTHER
  target_phenotypes:
  - preferred_term: Progressive cerebellar ataxia
    term:
      id: HP:0002073
      label: Progressive cerebellar ataxia
  evidence:
  - reference: PMID:20301352
    reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: "MANAGEMENT: Treatment of manifestations: Treatment is supportive."
    explanation: >-
      GeneReviews states that management of mtDNA disease is supportive. Graded INDIRECT
      because the chapter addresses the Leigh syndrome spectrum rather than this
      adult-onset entity specifically.
- name: Physical and Occupational Therapy
  description: >-
    Gait, balance and hand-function rehabilitation for progressive cerebellar
    ataxia and distal weakness.
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  therapeutic_modality: BEHAVIORAL
  target_phenotypes:
  - preferred_term: Gait ataxia
    term:
      id: HP:0002066
      label: Gait ataxia
  evidence:
  - reference: PMID:20301352
    reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: >-
      physical therapy and occupational therapy; standard treatment of eye
      movement disorders
    explanation: >-
      GeneReviews lists physical and occupational therapy in the supportive management
      of mtDNA disease. Graded INDIRECT because the recommendation is made for the Leigh
      syndrome spectrum rather than this entity.
- name: Avoidance of Mitochondrial Toxins
  description: >-
    Sodium valproate, drugs that provoke acidosis, and dichloroacetate should be
    avoided or used with caution in mtDNA disease, and anaesthesia requires
    careful planning. This is an agents-to-avoid caution carried over from the
    mtDNA disorder literature, not a therapy.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: OTHER
  evidence:
  - reference: PMID:20301352
    reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: >-
      Sodium valproate, medications that cause acidosis, and dichloroacetate
      should be avoided or used with caution; administration of anesthesia
      requires careful consideration to avoid aggravation of respiratory
      symptoms and precipitation of respiratory failure.
    explanation: >-
      GeneReviews Agents/Circumstances to Avoid for mtDNA disease. Graded INDIRECT
      because it is stated for the Leigh syndrome spectrum; the pharmacologic hazard is
      shared by MT-ATP6 disease but has not been studied in this adult-onset entity.
- name: Genetic Counseling and Reproductive Options
  description: >-
    Counselling covers maternal transmission, the unpredictability of
    heteroplasmy segregation, and reproductive options including prenatal
    testing, preimplantation genetic testing, and oocyte donation.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  therapeutic_modality: OTHER
  evidence:
  - reference: PMID:20301352
    reference_title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: >-
      Reproductive options for the family members of a proband with an mtDNA
      pathogenic variant may include prenatal testing, preimplantation genetic
      testing, and oocyte donation.
    explanation: >-
      GeneReviews enumerates the reproductive options relevant to any maternally
      inherited mtDNA variant. Graded INDIRECT because the chapter addresses the Leigh
      syndrome spectrum.
prevalence:
- population: Adults with unexplained ataxia or suspected Charcot-Marie-Tooth disease (United Kingdom referral cohorts)
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Screening 308 patients with unexplained ataxia and 96 with suspected
    Charcot-Marie-Tooth disease identified a single m.8993T>C family; a further
    191 episodic-ataxia patients yielded none. Only a handful of kindreds with
    adult-onset MT-ATP6 ataxia have been reported worldwide.
  evidence:
  - reference: PMID:18055910
    reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We therefore studied 308 patients with unexplained ataxia and 96 patients
      with suspected Charcot-Marie-Tooth disease to determine whether the
      m.8993T-->C MTATP6 mutation is common in unexplained inherited ataxia
      and/or polyneuropathy.
    explanation: >-
      Defines the screened cohorts from which the rarity estimate is drawn.
  - reference: PMID:18055910
    reference_title: Episodic ataxia and hemiplegia caused by the 8993T->C mitochondrial DNA mutation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In conclusion, m.8993T-->C MTATP6 should be considered in patients with
      unexplained ataxia, CMT or EA, but cases are uncommon.
    explanation: >-
      The authors' own summary that cases are uncommon, supporting the
      ULTRA_RARE band.

discussions:
- discussion_id: purkinje_neuropathology_unproven
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Which cerebellar cell population is lost in adult-onset MT-ATP6 ataxia, and
    is the Purkinje neuron the primary target as it is in most hereditary
    ataxias?
  attaches_to:
  - pathophysiology#Cerebellar Degeneration
  rationale: >-
    Cerebellar atrophy in m.8993T>C carriers is documented only by MRI. No
    autopsy or quantitative neuropathological series exists for the adult-onset
    MT-ATP6 phenotype, so the assignment of the degeneration to Purkinje neurons
    is inference from the wider ataxia literature rather than a finding in this
    entity. This matters mechanistically because MT-ATP6 disease elsewhere
    (Leigh/NARP) preferentially injures brainstem and basal ganglia rather than
    cerebellar cortex, and the m.8993T>C ROS-weighted lesion may select a
    different population than the calcium-mediated Purkinje degeneration of the
    dominant spinocerebellar ataxias.
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      High-resolution retinal and brain imaging in NARP syndrome revealed
      analogous patterns of tissue injury characterized by heterogeneous areas
      of neuronal loss.
    explanation: >-
      The deepest tissue-level characterisation available for m.8993T>C carriers is
      imaging, which resolves regions rather than cell types. It is cited here to
      document the ceiling of current evidence, which is exactly what this knowledge gap
      asserts.
  proposed_experiments:
  - experiment_id: cerebellar_neuropathology_mtatp6
    name: Post-mortem cerebellar neuropathology in genetically confirmed adult-onset MT-ATP6 ataxia
    description: >-
      Quantitative stereology of Purkinje, granule and dentate neurons with
      per-cell heteroplasmy measurement, compared against age-matched controls
      and against NARP/Leigh cerebellum.

- discussion_id: heteroplasmy_does_not_explain_onset
  kind: OPEN_QUESTION
  status: OPEN
  prompt: >-
    Why do some MT-ATP6 pedigrees with very high mutant load present in infancy
    with Leigh syndrome while others present only in adulthood with ataxia and
    neuropathy?
  attaches_to:
  - pathophysiology#Heteroplasmy Load Above the Phenotypic Threshold
  rationale: >-
    Mutant load is necessary but demonstrably not sufficient to predict age of
    onset or severity: patients with childhood-onset and adult-onset MT-ATP6
    ataxia have both been reported at 90-100% heteroplasmy, and within a single
    m.8993T>C family, phenotype differed markedly between members with
    comparably high ectodermal and mesodermal loads. Candidate modifiers include
    mtDNA haplogroup, nuclear background, mtDNA copy number, and differences in
    antioxidant scavenging capacity, none of which has been tested for this
    entity.
  evidence:
  - reference: PMID:20953793
    reference_title: Heterogeneous patterns of tissue injury in NARP syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      There were marked phenotypic differences between family members, even
      between individuals with the greatest degrees of ectodermal and mesodermal
      heteroplasmy.
    explanation: >-
      Demonstrates that measured heteroplasmy fails to explain within-family phenotypic
      variation in an m.8993T>C kindred. Graded INDIRECT because that kindred was
      diagnosed as NARP syndrome (MONDO:0010794); the open question about
      load-versus-onset is nonetheless shared across the MT-ATP6 spectrum.
- discussion_id: antioxidant_rescue_in_cybrids_only
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Does the rescue of MT-ATP6 mutant cell death by CoQ10 and vitamin E in
    transmitochondrial cybrids translate into any clinical benefit in adult-onset
    MT-ATP6 ataxia?
  attaches_to:
  - pathophysiology#Neuronal Energy Deficit and Oxidative Stress
  rationale: >-
    Cybrids carrying an MT-ATP6 ataxia variant died by apoptosis or necrosis
    under metabolic stress, and both outcomes were prevented by CoQ10 and
    vitamin E, prompting the authors to propose antioxidant therapy. This is an
    in-vitro result in an immortalised cell background carrying a different
    MT-ATP6 variant (m.9035T>C) from the m.8993T>C that defines this entity, and
    the stressors used (glucose deprivation, tert-butyl hydroperoxide) are not
    models of the decades-long neurodegeneration seen in patients. No controlled
    trial of antioxidants exists for this disorder, and the entry therefore
    records no antioxidant treatment.
  evidence:
  - reference: PMID:19626676
    reference_title: Identification of ataxia-associated mtDNA mutations (m.4052T>C and m.9035T>C) and evaluation of their pathogenicity in transmitochondrial cybrids.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Both of these outcomes were prevented by the antioxidants CoQ(10) and
      vitamin E, suggesting that the abnormally high levels of ROS were the
      triggers of cell death.
    explanation: >-
      The cybrid rescue result whose translational validity this discussion
      questions.

notes: >-
  Scope. This entry is anchored on OMIM 500010 / MONDO:0010781, whose defining
  report is the Rantamaki 2005 Finnish kindred with heteroplasmic MT-ATP6
  m.8993T>C. It is deliberately kept distinct from NARP syndrome
  (MONDO:0010794), which shares the nucleotide-8993 substitutions but is defined
  by childhood onset with retinitis pigmentosa, and from Leigh syndrome
  (MONDO:0009723). Where the only available mechanistic or clinical evidence
  comes from a different MT-ATP6 variant in the same adult-onset ataxia spectrum
  (m.9035T>C, m.9185T>C), the evidence item carries directness: INDIRECT and the
  variant is named in its explanation.

  The same discipline is applied on the entity axis. PMID:20953793 phenotyped an
  m.8993T>C family diagnosed as NARP syndrome, and is the best available source
  for this entity's electrophysiology, cerebellar imaging, and heteroplasmy
  behaviour: the variant is the defining one, but the published diagnosis is the
  entity curated separately at MONDO:0010794. Every item drawn from it
  therefore carries directness: INDIRECT with the NARP cohort named, so that the
  file does not simultaneously assert that NARP is a distinct disease and treat
  NARP data as unqualified evidence for this one.

  Deep-research provenance and a Named Entity Confusion event. A falcon
  (Edison) deep-research run was performed for this entry and was DISCARDED in
  full. `just preflight-dr` returned SKIP (MONDO records no causal gene for
  MONDO:0010781), so the manual OMIM/gene preflight was applied and the report
  failed it: its dominant gene was RFC1 (61 mentions) and it contained zero
  mentions of MT-ATP6 or nucleotide 8993, having resolved the descriptive label
  "adult-onset ataxia and polyneuropathy" to RFC1 spectrum disorder / CANVAS
  (MONDO:0044720) instead of to OMIM 500010. No content, citation, or ontology
  term in this entry comes from that report; every reference here was found and
  cached independently from PubMed. CANVAS is nevertheless a real and important
  differential, and is curated as one below from an independently retrieved
  primary source.

  MONDO classification. MONDO:0010781 is currently placed under MONDO:0021190
  (DNA repair disease). That parent looks incorrect for a maternally inherited
  MT-ATP6 oxidative-phosphorylation defect - MedGen and MeSH both map this
  concept to mitochondrial disease and to Mitochondrial Proton-Translocating
  ATPases deficiency. The `parents` slot here records mitochondrial disease
  instead; the MONDO placement is worth reporting upstream.

references:
- reference: PMID:20301352
  title: Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
  tags:
  - GeneReviews
📚

References & Deep Research

References

1
PMID:20301352
Mitochondrial DNA-Associated Leigh Syndrome Spectrum.
No top-level findings curated for this source.

Deep Research

1
Falcon ▸
Adult-Onset Ataxia and Polyneuropathy: Comprehensive Disease Report
Edison Scientific Literature 27 citations 2026-08-19T21:11:49.054617

Adult-Onset Ataxia and Polyneuropathy: Comprehensive Disease Report

Scope and disease mapping

The requested label is not a consistently maintained standalone disease name. In a Mendelian context, it maps most closely to RFC1 spectrum disorder, especially cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome (CANVAS). Open Targets identifies this entity as MONDO:0044720 and links it specifically to RFC1 (ENSG00000035928), supported by human genetic evidence including PMID 30926972, PMID 31230722, and ClinGen/ClinVar records. “Adult-onset ataxia and polyneuropathy” can also describe genetically heterogeneous disorders; therefore, a knowledge-base entry should state explicitly that this report concerns RFC1-related disease, not every disorder producing that phenotype. (OpenTargets Search: CANVAS)

The evidence summarized below is aggregated disease-level evidence from cohorts, case series, molecular studies, and ClinicalTrials.gov—not individual EHR data.

Domain Core finding Quantitative details Ontology suggestions Evidence
Disease identity Adult-onset ataxia and polyneuropathy in this Mendelian context maps best to RFC1 spectrum disorder / CANVAS = cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome; MONDO:0044720 Open Targets links MONDO:0044720 to RFC1 with literature support MONDO:0044720; HPO: HP:0001251 Ataxia, HP:0009830 Peripheral neuropathy, HP:0007646 Bilateral vestibular hypofunction (OpenTargets Search: CANVAS, traschutz2021naturalhistoryphenotypic pages 1-2)
Inheritance / causal gene Usually autosomal recessive; primary cause is biallelic intronic RFC1 pentanucleotide repeat expansion in intron 2 Typical pathogenic motif AAGGG; reported pathogenic spectrum also includes population-specific ACAGG and Māori-associated configurations; compound heterozygosity with truncating RFC1 variants also reported Gene: RFC1; HPO: HP:0000007 Autosomal recessive inheritance (cortese2019biallelicexpansionof pages 4-6, ronco2023truncatingvariantsin pages 1-2, scriba2023rfc1inan pages 3-4)
Variant / molecular lesion Pathogenic alleles are large nonreference intronic repeat expansions; truncating coding variants can act in trans with one expansion in rare cases Expansion size about ~400–2,000 repeats (median ~1,000) in early discovery work; 2024 data show repeat size modifies onset/severity SO: tandem repeat expansion; intron variant (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12, curro2024roleofthe pages 15-17)
Typical onset / course Adult onset, usually insidious and slowly progressive multisystem neurodegeneration Mean onset 54±9 y (range 35–73) in 2019 series; median onset 54 y (range 25–80) in 2024 cohort; progression about 1.3 SARA points/year HPO: HP:0003581 Adult onset, HP:0003676 Progressive neurologic deterioration (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12, traschutz2021naturalhistoryphenotypic pages 1-2)
Core phenotype: sensory neuropathy / neuronopathy Hallmark feature; often presents as sensory ataxic neuropathy and may precede full CANVAS All tested/examined cases had sensory neuropathy in major cohorts; in 2024 cohort 24% had isolated sensory neuropathy and 38% complex neuropathy HPO: HP:0009830 Peripheral neuropathy, HP:0002355 Difficulty walking; UBERON: dorsal root ganglion (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12, curro2024roleofthe pages 4-7)
Core phenotype: cerebellar ataxia Cerebellar dysfunction is common but not universal early; often evolves over time Cerebellar involvement 80% in 2019 series; cerebellar signs 72% at first assessment and 84% at follow-up in 2024 cohort HPO: HP:0001251 Ataxia, HP:0002060 Cerebellar atrophy; UBERON: cerebellum (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12)
Core phenotype: vestibular dysfunction Bilateral vestibular areflexia/hypofunction is a defining axis of CANVAS but may be incomplete early Bilateral vestibular areflexia 54% in 2019 series; 75% in 2024 cohort HPO: HP:0007993 Vestibular dysfunction, HP:0007646 Bilateral vestibular hypofunction; UBERON: vestibular labyrinth (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12, gisatulin2020clinicalspectrumof pages 5-9)
Core phenotype: chronic cough Highly discriminative associated symptom and may precede neurologic signs 37% in 2019 series; 75% overall and initial symptom in 50% in 2024 cohort; ACC phenotype can be highly suggestive HPO: HP:0012735 Chronic cough (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12, traschutz2021naturalhistoryphenotypic pages 1-2)
Core phenotype: dysautonomia / small-fiber involvement Autonomic and small-fiber involvement are common in multisystem disease Autonomic involvement 23% in 2019 series; dysautonomia 62% in 2021 natural history study; 2023 Portuguese cohort: multisystem features beyond CANVAS in 82%, dysautonomia 43%, severe epidermal denervation in all biopsied patients HPO: HP:0000789 Autonomic dysfunction; CL: sensory neuron; UBERON: skin, peripheral nerve (cortese2019biallelicexpansionof pages 4-6, traschutz2021naturalhistoryphenotypic pages 1-2)
Other multisystem phenotypes Can overlap with parkinsonism/MSA-C, movement disorder, cranial or motor neuron features Bradykinesia 28%, postural instability 49%, slow vertical saccades 17%, chorea/dystonia 11% in 2021 study; motor neuron/motor neuropathy 18% in 2023 Portuguese cohort HPO: HP:0002067 Bradykinesia, HP:0002136 Dysphagia, HP:0001260 Dysarthria (traschutz2021naturalhistoryphenotypic pages 1-2)
Diagnostics Diagnosis relies on targeted RFC1 repeat testing plus phenotype-specific neurologic workup Methods reported: flanking PCR, repeat-primed PCR, duplex PCR, Southern blot, fragment analysis, Sanger sequencing, short-read screening, targeted long-read sequencing; MRI/CT showed cerebellar atrophy in 83% of 42 cases HPO: HP:0003401 Cerebellar MRI abnormality; UBERON: cerebellar vermis (cortese2019biallelicexpansionof pages 4-6, gisatulin2020clinicalspectrumof pages 1-5, scriba2023rfc1inan pages 3-4)
Diagnostic yield / testing strategy RFC1 testing should be prioritized in adult-onset ataxia with sensory neuropathy, vestibular failure, or chronic cough Prevalence in enriched cohorts: 67% in suspected cohort, 68% in clinical CANVAS, 100% in ataxia with chronic cough, 14% in unselected late-onset ataxia; Australasian cohort found pathogenic expansions in 15.3% (37/242) HPO-guided testing: ataxia + neuropathy + cough + vestibular dysfunction (traschutz2021naturalhistoryphenotypic pages 1-2, scriba2023rfc1inan pages 3-4)
Key 2024 mechanism Patient iPSC-derived neurons support repeat-dependent, RFC1-protein-independent synaptic dysfunction 2024 Science Advances study found normal RFC1 splicing/expression and intact DNA repair, no consistent RNA foci/peptide toxicity in iNeurons, but reduced neuronal development/synaptic connectivity; CRISPR deletion of one expanded allele rescued deficits, RFC1 knockdown did not phenocopy, and RFC1 re-expression did not rescue GO: synapse organization, chemical synaptic transmission; CL: glutamatergic neuron; UBERON: cerebellum/peripheral nervous system (maltby2024aagggrepeatexpansions pages 1-2, maltby2024aagggrepeatexpansions pages 13-14, maltby2024aagggrepeatexpansions pages 11-13)
Genotype–phenotype modifier (2024) Repeat size is a prognostic modifier Larger smaller allele HR 2.06 and larger allele HR 1.53 for earlier neurologic onset; loss of independent walking HR 2.78 (smaller allele) and 1.60 (larger allele); dysarthria/dysphagia HR 3.40 and 1.71; larger expansions associated with more severe vermian atrophy HPO: HP:0001260 Dysarthria, HP:0002015 Dysphagia; UBERON: cerebellar vermis (curro2024roleofthe pages 10-12, curro2024roleofthe pages 15-17)
Prognosis Chronic progressive disability; some premature mortality in advanced disease In 2024 cohort 54% required walking aids after median 10 y, 17% wheelchairs after 14 y; mortality 8%, with disease-related deaths including aspiration pneumonia/immobility complications HPO: HP:0002829 Wheelchair dependence, HP:0040296 Aspiration pneumonia (curro2024roleofthe pages 10-12, traschutz2021naturalhistoryphenotypic pages 1-2)
Current treatment / trial status No established disease-modifying therapy; care is supportive and multidisciplinary; RFC1-specific biomarker/natural-history trial is recruiting Trial planning estimate: 330 total patients for 1-year or 132 for 2-year study to detect 50% slowing; NCT07156214 recruiting from 2024-10-14, primary completion 2026-06-30, evaluating scales, NfL/oxidative stress biomarkers, imaging, and patient-derived cells NCIT: supportive care; physical therapy/rehabilitation; biomarker study (traschutz2021naturalhistoryphenotypic pages 1-2, NCT07156214 chunk 1)

Table: This table summarizes the core knowledge-base fields for RFC1 spectrum disorder/CANVAS, the main Mendelian interpretation of adult-onset ataxia and polyneuropathy. It highlights identifiers, causal genetics, phenotype frequencies, diagnostics, 2024 mechanistic insights, prognosis, and current trial status with ontology suggestions and evidence IDs.

1. Disease information

RFC1 disease is an autosomal-recessive, usually late-onset neurodegenerative disorder whose manifestations form a continuum from isolated sensory neuronopathy/neuropathy to combined sensory, cerebellar, and bilateral vestibular dysfunction. The complete triad is termed CANVAS. Other common labels are RFC1 spectrum disorder, RFC1-related disorder, RFC1-associated disease, CANVAS syndrome, and ataxia with chronic cough.

Identifiers and classifications

  • MONDO: MONDO:0044720, cerebellar ataxia with neuropathy and bilateral vestibular areflexia syndrome.
  • OMIM: CANVAS is commonly indexed as 614575; RFC1 is 102579. Database versions should be checked before ingestion.
  • Gene: RFC1, replication factor C subunit 1; Ensembl ENSG00000035928.
  • MeSH: no uniquely specific RFC1-CANVAS descriptor was established in the retrieved evidence; broader terms include cerebellar ataxia and peripheral nervous system disease.
  • ICD-10/ICD-11: no uniquely disease-specific code was established. Coding generally combines hereditary/degenerative ataxia, polyneuropathy, and vestibular dysfunction. A knowledge base should not assign a single exact ICD code without jurisdictional validation.

The landmark discovery study described a mean onset of 54±9 years (range 35–73) and mean observed disease duration of 11±7 years. Its defining molecular lesion was a biallelic, large intronic RFC1 repeat expansion. (cortese2019biallelicexpansionof pages 4-6)

2. Etiology, risk, protection, and gene–environment interaction

Causal factor

The principal cause is a germline biallelic nonreference pentanucleotide-repeat expansion in intron 2 of RFC1, usually (AAGGG)n. Typical pathogenic alleles contain approximately 400–2,000 repeats, often near 800–1,000, whereas the reference motif is (AAAAG)n. The expansion lies in the poly-A tail of an AluSx3 element. (cortese2019biallelicexpansionof pages 4-6, gisatulin2020clinicalspectrumof pages 5-9, gisatulin2020clinicalspectrumof pages 1-5)

Pathogenic configurations are population dependent. Besides AAGGG, disease-associated ACAGG and complex Māori-associated (AAAGG)exp(AAGGG)exp alleles have been reported. Consequently, an AAGGG-only assay can miss disease in non-European populations. (curro2024roleofthe pages 4-7, scriba2023rfc1inan pages 3-4)

Rare patients have one expanded allele and a truncating RFC1 variant in trans. Reported variants include c.1267C>T (p.Arg423Ter), c.1739_1740del (p.Lys580SerfsTer9), c.2191del (p.Gly731GlufsTer6), and c.2876del (p.Pro959GlnfsTer24). Fibroblasts carrying two tested truncating alleles showed nonsense-mediated decay and reduced RFC1 RNA and protein. Full RFC1 sequencing is therefore appropriate when classic CANVAS accompanies only one detected expansion. (ronco2023truncatingvariantsin pages 1-2)

Risk and modifying factors

  • Genetic risk: two pathogenic RFC1 alleles are causal; heterozygous carriers are generally unaffected.
  • Repeat size: the strongest established modifier. In 553 expansion carriers, with size measured in 392, larger alleles predicted earlier onset and more severe disease. Earlier neurologic onset had HR 2.06 for the smaller allele and 1.53 for the larger allele; earlier dysarthria/dysphagia had HR 3.40 and 1.71; earlier loss of independent walking had HR 2.78 and 1.60, respectively. Larger repeats were also associated with complex neuropathy, complete CANVAS, and greater vermian atrophy. Repeat size explained only about 6% of onset variability, implying additional modifiers. (curro2024roleofthe pages 10-12, curro2024roleofthe pages 15-17)
  • Age: manifestations are age dependent, explaining apparently sporadic presentations and unaffected younger siblings.
  • Family history: its absence does not argue strongly against disease because inheritance is recessive and onset is late.

No reproducible sex, lifestyle, occupational, dietary, infectious, or toxic risk factor has been established for genetically confirmed RFC1 disease. No validated protective allele, diet, drug, or environmental exposure is known. Environmental or epigenetic modifiers are plausible because repeat size explains little phenotypic variance, but specific gene–environment interactions remain unproven. The 2024 repeat study found no major meiotic or tissue-specific somatic instability, arguing against marked anticipation driven by unstable expansion growth. (curro2024roleofthe pages 15-17)

3. Phenotypes

The disease is chronic and progressive, but component systems emerge at different times. Sensory neuropathy often precedes cerebellar and vestibular signs; chronic spasmodic cough can precede neurologic disability by years.

  • Sensory neuronopathy/neuropathy: hallmark phenotype, generally non-length-dependent, affecting large fibers and proprioception; motor conduction is often preserved. Suggested terms: HP:0009830 peripheral neuropathy, HP:0000763 sensory neuropathy, HP:0003477 impaired vibration sense, HP:0010871 sensory ataxia. It was found in all examined patients in major cohorts. (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12)
  • Cerebellar ataxia: gait and limb ataxia, dysarthria, impaired pursuit, and cerebellar atrophy; HP:0001251, HP:0002072 chorea only where present, HP:0001260 dysarthria, HP:0002060 cerebellar atrophy. Cerebellar involvement was 80% in the discovery cohort and rose from 72% at first assessment to 84% at follow-up in a later large cohort. (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12)
  • Bilateral vestibulopathy: oscillopsia, impaired vestibulo-ocular reflex, and imbalance worsened in darkness; HP:0007993 vestibular dysfunction, HP:0007646 bilateral vestibular hypofunction, HP:0010544 oscillopsia. Reported prevalence was 54–75%, depending on cohort and disease stage. (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12, gisatulin2020clinicalspectrumof pages 5-9)
  • Chronic cough: dry, spasmodic cough, frequently prodromal; HP:0012735. Frequencies ranged from 37% in the initial series to 75% in a 2024 cohort; in the latter it was the initial symptom in 50%. (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12)
  • Dysautonomia and small-fiber disease: urinary, bowel, cardiovascular, cardiovagal, and sudomotor dysfunction; HP:0000789 autonomic dysfunction, HP:0000020 urinary incontinence where applicable. Dysautonomia occurred in 23% of the discovery series and 62% in a deeply phenotyped natural-history cohort. (cortese2019biallelicexpansionof pages 4-6, traschutz2021naturalhistoryphenotypic pages 1-2)
  • Other manifestations: bradykinesia 28%, postural instability 49%, slow vertical saccades 17%, and chorea/dystonia 11% in one cohort. Motor-neuron/motor-neuropathy phenotypes, sleep disorders, cranial neuropathy, and cognitive impairment have been reported but are less securely quantified and may represent spectrum extremes. (traschutz2021naturalhistoryphenotypic pages 1-2)
  • Advanced disability: dysarthria/dysphagia increased from 6% to 51% during follow-up. Suggested terms include HP:0002015 dysphagia, HP:0002829 wheelchair dependence, and HP:0040296 aspiration pneumonia. (curro2024roleofthe pages 10-12)

Quality-of-life effects include impaired walking in darkness or on uneven surfaces, falls, oscillopsia, chronic cough, pain or sensory discomfort, loss of driving and employment, dysphagia, and eventual dependence for mobility. RFC1-specific EQ-5D, SF-36, or PROMIS population estimates were not found.

4. Genetic and molecular information

RFC1 encodes the large subunit of replication factor C, a clamp-loader complex involved in DNA replication and repair. The conventional disease genotype is homozygous or compound-heterozygous pathogenic repeat expansion. Variants are germline, not somatic cancer alterations.

Population screening in the discovery study found expanded AAGGG alleles on approximately 0.7% of chromosomes; subsequent estimates range from 0.7% to 4%, depending on ancestry, motif definition, and ascertainment. These figures are allele frequencies, not disease prevalence. (cortese2019biallelicexpansionof pages 4-6, traschutz2021naturalhistoryphenotypic pages 1-2)

Large repeat expansions are not represented reliably by standard SNV/indel nomenclature, and their frequencies are poorly captured by conventional gnomAD short-read variant tables. For knowledge-base purposes, pathogenicity should be assigned at the repeat-configuration level, with motif, size, zygosity, and assay recorded. Benign or uncertain expansions, including AAAAG/AAAGG or interrupted configurations, must not be automatically classified as pathogenic. (gisatulin2020clinicalspectrumof pages 1-5, scriba2023rfc1inan pages 3-4)

No validated modifier gene, disease-specific methylation signature, recurrent chromosomal rearrangement, or pathogenic epigenetic lesion has been established. Rare RFC1 deletions and truncating alleles broaden the allelic spectrum but do not define a recurrent chromosomal syndrome. (ronco2023truncatingvariantsin pages 1-2)

5. Environmental and lifestyle information

RFC1 disease is not known to be caused by infection, radiation, pollution, alcohol, smoking, diet, or occupational exposure. Such factors remain relevant in the differential diagnosis of adult-onset ataxia/polyneuropathy—particularly alcohol, neurotoxic medications, vitamin deficiencies, immune disease, and paraneoplastic processes—but are not demonstrated causal cofactors for biallelic RFC1 disease. No vaccine or pathogen-specific intervention applies.

6. Mechanism and pathophysiology

Causal chain

Biallelic pathogenic RFC1 repeat configuration → repeat-dependent neuronal dysfunction → impaired synaptic development/signaling and selective vulnerability of sensory ganglia, cerebellar circuits, and vestibular pathways → sensory ataxia, cerebellar ataxia, and vestibular areflexia. Downstream degeneration includes Purkinje-cell loss, dorsal-root-ganglionopathy, peripheral small/large-fiber loss, and cerebellar atrophy. Severe Purkinje-cell depletion with Bergmann gliosis has been documented neuropathologically. (cortese2019biallelicexpansionof pages 4-6)

Earlier studies found unchanged RFC1/WDR19 expression and no RFC1 intron retention, leaving mechanism uncertain. (gisatulin2020clinicalspectrumof pages 1-5)

The key 2024 mechanistic development used patient-derived iPSCs and glutamatergic iNeurons. RFC1 expression, splicing, DNA-repair function, DNA-damage accumulation, and UV-repair recovery were not materially abnormal. Reporter constructs permitted pentapeptide-repeat translation, but repeat peptides and convincing RNA-foci toxicity were not consistently demonstrated in patient iNeurons. Conversely, patient neurons had abnormal neuronal-development and synaptic-gene programs, reduced burst rate, burst strength, and firing correlation, and impaired connectivity. Deleting one expanded allele with CRISPR rescued molecular and functional defects; RFC1 knockdown did not reproduce them, and RFC1 replacement did not rescue them. The authors’ abstract conclusion was: “These findings support a repeat-dependent but RFC1 protein–independent cause of neuronal dysfunction in CANVAS.” (maltby2024aagggrepeatexpansions pages 1-2, maltby2024aagggrepeatexpansions pages 13-14, maltby2024aagggrepeatexpansions pages 11-13)

This creates tension with truncating-variant evidence supporting partial RFC1 loss of function. The most defensible current interpretation is that more than one mechanism may operate: repeat-sequence/configuration toxicity is strongly supported in neurons, while RFC1 haploinsufficiency or reduced dosage can contribute in rare compound genotypes. (ronco2023truncatingvariantsin pages 1-2, maltby2024aagggrepeatexpansions pages 1-2)

Suggested annotations: GO:0050808 synapse organization; GO:0007268 chemical synaptic transmission; GO:0006281 DNA repair, with a qualifier that canonical repair dysfunction was not demonstrated; GO:0006950 response to stress; CL:0000540 neuron, CL:0000121 Purkinje cell, CL:0000101 sensory neuron, CL:0000709 retinal?—the latter should not be used without phenotype evidence. Relevant compartments include nucleus, cytoplasm, synapse, and axon.

No validated disease-level epigenomic, proteomic, metabolomic, lipidomic, single-cell, or spatial-transcriptomic signature is yet available. INSIDE-CANVAS is investigating oxidative stress, neurofilament light, mitochondrial function, and patient-derived cells. (NCT07156214 chunk 1)

7. Anatomical structures affected

Primary systems are the peripheral sensory nervous system, cerebellum, and vestibular system.

  • Dorsal-root ganglia and sensory peripheral nerves: neuronopathy with large- and small-fiber loss; UBERON:0000044 dorsal root ganglion, UBERON:0001021 nerve, CL:0000101 sensory neuron.
  • Cerebellum: Purkinje cells, vermis, and hemispheric circuits; UBERON:0002037 cerebellum, UBERON:0004729 cerebellar vermis, CL:0000121 Purkinje cell.
  • Vestibular apparatus/pathways: bilateral vestibular hypofunction; UBERON:0001843 vestibular labyrinth and vestibular nerve/pathway terms.
  • Spinal proprioceptive pathways and brainstem oculomotor networks may contribute secondarily.
  • Autonomic and cutaneous small fibers are involved in a subset.

MRI showed cerebellar atrophy in 83% of 42 imaged cases in the discovery series. Larger repeats, especially the smaller expanded allele, correlated with greater atrophy of vermian lobules I–V and VI–VII. Disease is generally bilateral/systemic rather than lateralized. (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12)

8. Temporal development

Onset is usually insidious in the fifth or sixth decade, but observed onset spans approximately 25–80 years. Cough or sensory symptoms may be the first manifestation; cerebellar and vestibular components accumulate later. (cortese2019biallelicexpansionof pages 4-6, curro2024roleofthe pages 10-12)

A practical staging model is:

  1. Prodromal/early: chronic cough, distal or patchy sensory symptoms, imbalance in darkness.
  2. Intermediate: sensory ataxia plus cerebellar or vestibular dysfunction; falls and oscillopsia.
  3. Established CANVAS: all three systems affected, with dysarthria and reduced independent mobility.
  4. Advanced: walking aids or wheelchair use, dysphagia, aspiration risk, and multisystem/autonomic complications.

Mean ataxia progression was approximately 1.3 SARA points/year, although nonlinear MSA-C-like phases of 2.5–5.5 points/year occurred. No spontaneous remission pattern is recognized. (traschutz2021naturalhistoryphenotypic pages 1-2)

9. Inheritance and population

Inheritance is autosomal recessive, with variable expressivity and strongly age-dependent penetrance. Precise lifetime penetrance for each repeat motif and size is not established. Repeat-size correlation supports dose-dependent expressivity. Unlike many repeat diseases, marked meiotic/somatic instability and classic genetic anticipation were not observed. (curro2024roleofthe pages 10-12, curro2024roleofthe pages 15-17)

RFC1 expansions were found in 14% of unselected late-onset ataxia, 67% of an enriched suspected cohort, 68% of clinical CANVAS, and 100% of ataxia-with-chronic-cough cases in one study. These are diagnostic yields, not population prevalence. (traschutz2021naturalhistoryphenotypic pages 1-2)

An Australasian study found pathogenic biallelic expansions in 37/242 (15.3%) neurologic cases, including AAGGG, ACAGG, and Māori-associated configurations. A Japanese hereditary sensory/autonomic neuropathy cohort found biallelic RFC1 expansions in 20/79 (25.3%) cases. These findings demonstrate ancestry-specific motif distributions. (scriba2023rfc1inan pages 3-4)

Population incidence, point prevalence, sex ratio, carrier frequency by ancestry, and survival-adjusted prevalence remain inadequately measured. Consanguinity can increase recessive disease probability but is not required, given relatively common carrier alleles.

10. Diagnostics

Clinical and physiologic evaluation

Evaluate cerebellar signs, sensory modalities, reflexes, gait in darkness, eye movements, autonomic symptoms, chronic cough, and family history. Recommended investigations include:

  • Nerve-conduction studies: typically absent/reduced sensory responses with relatively preserved motor conduction.
  • Vestibular testing: video head-impulse testing, calorics, rotational chair, vestibular-evoked myogenic potentials, and visually enhanced vestibulo-ocular reflex where available.
  • Brain MRI: cerebellar and vermian atrophy; spinal imaging when alternative diagnoses are suspected.
  • SARA or ICARS for ataxia, vestibular disability scales, swallowing assessment, and autonomic testing.
  • Laboratory exclusion of acquired ataxia/neuropathy: vitamin E/B12/copper, thyroid, diabetes, paraprotein, immune/paraneoplastic studies, toxins, infections, and medication review as clinically indicated.

Genetic testing algorithm

  1. Order RFC1 flanking PCR plus motif-specific repeat-primed PCR in adult-onset sensory neuronopathy, CANVAS, bilateral vestibulopathy with neuropathy, or ataxia with chronic cough.
  2. Confirm and size large alleles with Southern blot, optical genome mapping, or validated long-read sequencing.
  3. If the phenotype is compelling but only one expansion is found, sequence the complete RFC1 coding region and assess deletion/CNV alleles. (ronco2023truncatingvariantsin pages 1-2)
  4. If PCR patterns are atypical or ancestry suggests non-AAGGG motifs, use targeted long-read sequencing to resolve motif, interruptions, orientation, and size. The 2023 Australasian study recommended a multistep short-/long-read workflow. (scriba2023rfc1inan pages 3-4)
  5. If negative, test other repeat disorders and conventional ataxia/neuropathy genes with a repeat-expansion panel plus WES/WGS. Standard WES alone can miss RFC1 expansions.

The 2024 adult-onset ataxia review summarized the diagnostic challenge: targeted repeat testing remains necessary even when exome or short-read genome sequencing is performed, while long-read sequencing may eventually unify detection. (scriba2023rfc1inan pages 3-4)

Important differentials include Friedreich ataxia, SCA3 and other dominant SCAs, FGF14-GAA/SCA27B, FXTAS, POLG and other mitochondrial disease, spinocerebellar ataxia with axonal neuropathy, hereditary sensory/autonomic neuropathies, immune/paraneoplastic ataxia, superficial siderosis, multiple-system atrophy–cerebellar type, alcohol/toxin-associated disease, and vitamin deficiencies.

Cascade testing is appropriate after molecular confirmation. Population or newborn screening is not recommended; predictive testing of adult relatives requires genetic counseling.

11. Outcome and prognosis

Disease is lifelong and progressive; recovery is not expected, although rehabilitation can improve function and safety. In a large cohort, 54% required a walking aid after a median of 10 years, and 17% required a wheelchair after 14 years. Mortality was 8%; attributed deaths included aspiration pneumonia and complications of immobility. (curro2024roleofthe pages 10-12)

Repeat size is the best current molecular prognostic marker, but it is not deterministic. Baseline multisystem involvement, dysphagia, falls, and rapid SARA progression likely indicate poorer functional prognosis. No validated individual survival calculator exists.

12. Treatment and current applications

There is no approved disease-modifying therapy. Current real-world care is genotype-informed but symptomatic:

  • physiotherapy, balance and gait training, vestibular rehabilitation, fall prevention, and mobility aids;
  • occupational therapy and home adaptation;
  • speech therapy, swallow evaluation, texture modification, and aspiration prevention;
  • treatment of neuropathic pain using standard agents when present;
  • management of orthostatic, urinary, bowel, and sudomotor symptoms;
  • cough assessment and symptomatic management after excluding pulmonary causes;
  • nutritional, respiratory, psychological, and social support;
  • hearing/vision compensation and driving-safety review;
  • genetic counseling and cascade testing.

Suggested NCIT concepts include Physical Therapy, Occupational Therapy, Speech Therapy, Vestibular Rehabilitation, Supportive Care, and Genetic Counseling. No RFC1-specific pharmacogenomic guideline, surgery, cell therapy, ASO, siRNA, or gene-replacement therapy is established.

The 2024 iNeuron result argues that simply restoring RFC1 protein may be insufficient; excision, silencing, or sequence-specific targeting of the expanded repeat is a more mechanistically supported direction, although CRISPR rescue remains preclinical. (maltby2024aagggrepeatexpansions pages 1-2, maltby2024aagggrepeatexpansions pages 11-13)

Clinical research: INSIDE-CANVAS (NCT07156214) began October 14, 2024 and is recruiting. It targets 25 RFC1 patients plus matched controls, with one-year clinical scales, neurophysiology, vestibular testing, brain/spinal MRI, NfL and oxidative-stress biomarkers, fibroblasts, and iPSCs; estimated primary completion is June 30, 2026. It is a biomarker/pathogenesis study, not a proven therapeutic trial. (NCT07156214 chunk 1)

Natural-history modeling estimated that a trial detecting 50% slowing would require approximately 330 participants for one year or 132 for two years. (traschutz2021naturalhistoryphenotypic pages 1-2)

13. Prevention

Primary prevention through lifestyle modification is not available. For families with a known genotype, options include genetic counseling, adult cascade testing, prenatal diagnosis, and preimplantation genetic testing, subject to local ethics and regulation. Because penetrance is age dependent and genotype–phenotype prediction is incomplete, predictive counseling must address uncertainty.

Secondary prevention consists of early molecular diagnosis and surveillance for falls, vestibular failure, autonomic dysfunction, dysphagia, and aspiration. Tertiary prevention includes rehabilitation, mobility aids, home-safety modification, vaccination and respiratory care according to general standards, and prompt treatment of aspiration or immobility complications. No RFC1-specific vaccine or chemoprophylaxis applies.

14. Other species and natural disease

No well-established naturally occurring veterinary counterpart caused by orthologous RFC1 intronic pentanucleotide expansions was identified. RFC1 is evolutionarily conserved, but the human disease depends on a specific repeat architecture that may not be conserved across species. Zoonotic transmission is not applicable. A knowledge base should mark affected nonhuman species, breed associations, and natural transmission as not established rather than infer them from RFC1 conservation.

15. Model organisms and experimental systems

The most informative current model is the human patient-derived iPSC/iNeuron system. The 2024 study used four patient and three control lines and an isogenic CRISPR-corrected line. It reproduced neuronal-development, transcriptomic, calcium-signaling, and synaptic-connectivity abnormalities and demonstrated rescue after deletion of one expanded allele. (maltby2024aagggrepeatexpansions pages 13-14, maltby2024aagggrepeatexpansions pages 2-3)

Limitations are important: the neurons were predominantly glutamatergic rather than dorsal-root-ganglion sensory neurons, vestibular neurons, or mature Purkinje cells; the number of lines and isogenic controls was small; and early developmental phenotypes may not reproduce decades-long human degeneration. Pentapeptide products were detected in three of four postmortem cerebella but not convincingly in iNeurons, illustrating model dependence. (maltby2024aagggrepeatexpansions pages 13-14)

No validated mouse, rat, zebrafish, Drosophila, or C. elegans model was established in the retrieved evidence as recapitulating the complete human triad. Priority models include repeat knock-in animals, sensory-neuron and Purkinje-cell differentiation, cerebellar organoids, and isogenic long-repeat systems.

Key recent expert conclusions and exact abstract quotations

  • Currò et al., Brain 2024, DOI 10.1093/brain/awad436: “RFC1 repeat size, particularly of the smaller allele, is one of the determinants of variability in RFC1 disease and represents a key prognostic factor to predict disease onset, phenotype, and severity.” (curro2024roleofthe pages 10-12)
  • Maltby et al., Science Advances, September 2024, DOI 10.1126/sciadv.adn2321: “These findings support a repeat-dependent but RFC1 protein–independent cause of neuronal dysfunction in CANVAS, with implications for therapeutic development in this currently untreatable condition.” (maltby2024aagggrepeatexpansions pages 1-2)
  • Scriba et al., Brain Communications, July 2023, DOI 10.1093/braincomms/fcad208: the authors found pathogenic expansions in 15.3% of their neurological cohort and concluded that RFC1 expansions make a substantial contribution to Australasian neurological disease, supporting combined short- and long-read diagnostic workflows. (scriba2023rfc1inan pages 3-4)
  • Ronco et al., Neurology, January 2023, DOI 10.1212/WNL.0000000000201486, PMID 36478048: “Full RFC1 sequencing is recommended in cases affected by typical CANVAS and carrying monoallelic (AAGGG)n expansions.” (ronco2023truncatingvariantsin pages 1-2)

Evidence gaps

Reliable population incidence and prevalence, motif-specific penetrance, sex ratios, validated quality-of-life norms, environmental modifiers, fluid biomarkers, single-cell/spatial atlases, natural animal disease, robust whole-animal models, and disease-modifying treatment data remain unavailable. Frequencies vary substantially with ascertainment and disease stage; cohort percentages should therefore not be treated as universal penetrance estimates.

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