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Classifications

Harrison's Chapter

Mechanistic Nosology

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Inheritance

1

Autosomal dominant inheritance HP:0000006

A single heterozygous SDHA missense allele is sufficient to cause disease. Transmission was vertical in both reported pedigrees: two affected sisters in the first, and an affected father with two affected children in the second, whose report is titled for the dominant transmission it demonstrates. The third reported individual carried a de novo heterozygous allele with unaffected parents, which is the other signature of dominance. Dominance here is not equivalent to haploinsufficiency, and the distinction is load-bearing for the whole entry. Heterozygous SDHA loss-of-function alleles are carried, unaffected, by the parents of children with recessive complex II deficiency and by unaffected relatives in tumour-predisposition families, and at least one of them is not vanishingly rare: the recurrent nonsense allele c.91C>T (p.Arg31*) was found in 0.3% of healthy controls in the series cited below. If losing half the SDHA gene dosage were sufficient to cause optic atrophy and ataxia, those carriers would be affected and they are not. The dominant neurodegenerative alleles are instead specific missense substitutions at conserved arginines that are argued to produce a full-length subunit unable to carry its FAD cofactor. Expressivity within a single pedigree is extreme. In the second family the same heterozygous allele produced an index patient whose optic atrophy, polyneuropathy, psychiatric features and cardiomyopathy declared themselves over adult life, a son with cardiomyopathy from early childhood who did not develop optic atrophy until adulthood, and a daughter who died at seven months of cardiac insufficiency. Penetrance is recorded as UNKNOWN rather than complete: with two pedigrees in the world literature there is no denominator of obligate carriers from which to estimate it, and asserting COMPLETE would be an unsupported number dressed as an enum.

Autosomal dominant inheritance Penetrance: UNKNOWN Expressivity: VARIABLE

Show evidence (5 references)

PMID:27683074 SUPPORT Human Clinical

"Here, we report clinical and molecular investigations of three family members with a heterozygous mutation in the large flavoprotein subunit SDHA previously described to cause complex II deficiency."

Three affected members of one family, each heterozygous for a single SDHA allele, is the direct demonstration of autosomal dominant transmission for this locus.

PMID:27683074 SUPPORT Human Clinical

"This is the second report presenting autosomal dominant inheritance of a SDHA mutation."

The authors place their pedigree explicitly as the second dominant SDHA family, which both establishes the mode of inheritance and fixes the size of the published literature at two pedigrees.

PMID:27683074 SUPPORT Human Clinical

"Of the four nuclear encoded proteins composing complex II (SDHA, SDHB, SDHC, SDHD) and currently known assembly factors SDHAF1 and SDHAF2 mainly recessively inherited mutations have been described in SDHA, SDHB, SDHD, and SDHAF1 to be causative for mitochondrial disease phenotypes."

Establishes that dominant transmission is the exception rather than the rule for complex II subunit mitochondrial disease, which is what makes this entity a separate entry rather than a variant presentation of the recessive disorder.

+ 2 more references

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Discussions and Knowledge Gaps

6

Does the NDAXOA allele act by delivering a catalytically inert subunit into assembled complex II, or by simple haploinsufficiency, and why are heterozygous SDHA null carriers neurologically unaffected?

KNOWLEDGE GAP OPEN sdha-ndaxoa-dominant-negative-versus-haploinsufficiency

Attached to

pathophysiology#Catalytically Inert SDHA Subunit Competing for Complex II Assembly pathophysiology#Failure of Covalent FAD Attachment to the SDHA Flavoprotein

This is the central unresolved mechanistic question of the entity and it is not academic. Heterozygous SDHA loss-of-function alleles are carried without neurodegeneration by the parents of children with recessive complex II deficiency and by members of tumour-predisposition families, yet a heterozygous missense allele at codon 451 causes progressive optic atrophy, ataxia, myopathy and cardiomyopathy. Half-normal enzyme activity, the measured phenotype, is equally consistent with losing one functional gene copy and with poisoning half the assembled complexes, so the biochemistry published so far cannot discriminate. The yeast result that assembly requires FAD binding but not covalent attachment makes the poisoning model concretely plausible, because a subunit blocked only at the covalent step could still occupy its slot. Resolving this determines whether truncating SDHA variants should ever be reported as a risk for neurodegeneration, which is a live reporting question for every diagnostic laboratory that finds one.

Proposed experiments

Allele-specific quantification of SDHA within assembled complex II

sdha-allele-specific-complex-occupancy

Use allele-discriminating mass spectrometry or epitope-tagged knock-in alleles in a heterozygous human cell model to measure the ratio of mutant to wild-type SDHA within assembled, membrane-associated complex II. Equal representation supports subunit competition; depletion of the mutant supports haploinsufficiency with degradation of the mutant polypeptide.

Direct measurement of covalent flavinylation of human p.Arg451Cys and p.Arg662Cys

sdha-flavinylation-assay-human-alleles

The SDHA-null human cell system this experiment needs already exists, and both alleles have already been through it: PMID:39321216 expressed each in a clonal human SDHA-knockout line and found both essentially inactive, with p.Arg451Cys retaining high SDHA protein (per-variant results from the PMC11611653 full text, Figures 2 and 5 and Table 1; the cached abstract is aggregate-only). What that study inferred rather than measured is the flavinylation step itself. The remaining experiment is therefore narrow: recover the mutant subunit from that same system and assay the covalent methyl-histidyl FAD linkage directly (e.g. by in-gel flavin fluorescence or intact-mass measurement of the holo-subunit), rather than inferring it from the bacterial orthologue, from structural modelling, or from loss of activity.

Heterozygous Sdha knock-in mouse carrying the Arg451Cys-equivalent allele

sdha-heterozygous-knockin-mouse

Compare a heterozygous missense knock-in with a heterozygous null on the same background, phenotyping the visual pathway, cerebellum, muscle and myocardium longitudinally. If only the missense heterozygote degenerates, the dominant-negative model is established in vivo.

Do carriers of the dominant neurodegenerative SDHA alleles have an elevated risk of paraganglioma, phaeochromocytoma or gastrointestinal stromal tumour, and should they be enrolled in SDHx tumour surveillance?

KNOWLEDGE GAP OPEN sdha-ndaxoa-tumour-risk-unknown

Attached to

pathophysiology#Divergence From the SDHx Tumour-Predisposition Arm

The two questions a family actually asks are what will happen to them and what should be monitored, and this entity cannot answer either. Tumour surveillance is recommended for carriers of SDHA pathogenic variants and involves whole-body imaging plus biochemical testing, an appreciable lifelong burden. No tumour has been reported in either NDAXOA pedigree, but the total number of carriers ever followed is of the order of six, so the absence of reported tumours carries almost no statistical weight. The mechanistic argument cuts both ways: a missense allele that blocks flavinylation may or may not compromise the tumour-suppressor function of the locus, and the fact that a different substitution at the very same codon, p.Arg451His, has been reported in paraganglioma makes the position look precarious rather than reassuring. An independent group has framed this same problem as a counselling conundrum and concluded that the absolute risk of both outcomes is unknown.

Proposed experiments

International registry and prospective imaging of dominant SDHA neurodegeneration carriers

sdha-ndaxoa-carrier-tumour-registry

Assemble all known carriers of dominant neurodegenerative SDHA alleles into a single registry with standardised whole-body MRI and plasma metanephrine surveillance, to convert an anecdotal absence of tumours into an estimate with a denominator.

Tumour-suppressor function assay of NDAXOA alleles

sdha-allele-tumour-suppressor-assay

Test p.Arg451Cys and p.Arg662Cys alongside the paraganglioma-associated p.Arg451His in the assays that define SDHx tumour behaviour, namely succinate accumulation, HIF stabilisation and 5-hydroxymethylcytosine loss, to establish whether the neurodegenerative alleles engage the oncometabolite arm at all.

Show evidence (1 reference)

PMID:33960148 SUPPORT Human Clinical

"There is no similar screening or testing recommended for individuals potentially at risk for adult-onset mitochondrial disease. Additionally, the absolute risk for both conditions is unknown."

An independent group states the same gap, that neither the tumour risk nor the mitochondrial-disease risk of an SDHA variant can be quantified for counselling.

In which tissues is the complex II deficiency of dominant SDHA disease actually expressible, given that the two published biochemical studies disagree about fibroblasts and lymphocytes?

KNOWLEDGE GAP OPEN sdha-ndaxoa-tissue-expression-of-the-enzyme-defect

Attached to

pathophysiology#Approximately Half-Normal Succinate Dehydrogenase Activity

This is a diagnostic question with an immediate consequence for patients. In the founding family the enzyme defect was present in muscle and platelet mitochondria and explicitly absent in cultured skin fibroblasts and immortalised lymphocytes; in the de novo case it was present in exactly those two cell types. If the founding family's pattern is the general one, a normal fibroblast complex II activity is uninformative and muscle biopsy remains necessary, which is a materially different clinical pathway from the alternative. The discrepancy might reflect the two different alleles, the two different laboratories and eras, culture conditions and flavin content of the medium, or a genuine tissue-specific difference in how the mutant subunit is handled. Nobody has tested which.

Proposed experiments

Parallel multi-tissue enzymology in the same carriers

sdha-parallel-tissue-enzymology

Measure complex II activity in muscle, platelets, fibroblasts and lymphocytes from the same individuals carrying each allele, under standardised riboflavin-defined culture conditions, so that tissue differences can be separated from inter-laboratory and inter-era differences.

Is "late-onset" a defensible part of the definition of this entity, given that the same allele killed an infant with cardiomyopathy at seven months?

KNOWLEDGE GAP OPEN sdha-ndaxoa-late-onset-in-the-name

Attached to

pathophysiology#Striated Muscle and Myocardial Energy Failure phenotypes#Optic atrophy

The MONDO and OMIM label commits the entity to a late onset, and the neurological arm does behave that way: the founding sisters were elderly and the index patient of the second family presented in adult life. But the same second family contains a child with cardiomyopathy from early childhood and an infant who died at seven months, and the third reported individual had childhood-onset optic atrophy with developmental delay. Either the entity is an adult neurodegeneration whose paediatric cardiac cases belong to a different term such as dilated cardiomyopathy 1GG, or "late-onset" describes only the neurological arm of a disorder that is not late in onset at all. This is not a naming quibble: it determines whether an infant with cardiomyopathy and a heterozygous SDHA missense variant is given this diagnosis, and therefore whether their relatives are told to expect optic atrophy.

Proposed experiments

Genotype-stratified natural history of dominant SDHA carriers

sdha-genotype-stratified-natural-history

Prospectively phenotype all identified carriers of dominant SDHA missense alleles from childhood, with serial echocardiography, optical coherence tomography and cerebellar imaging, to establish whether the cardiac and neurological arms are sequential stages of one natural history or separate allele- or modifier-determined outcomes.

Does the de novo allele c.1984C>T (p.Arg662Cys) belong to this entity, given that its authors say it does and ClinVar carries no record for it against this condition?

KNOWLEDGE GAP OPEN sdha-ndaxoa-second-allele-attribution

Attached to

genetic#SDHA

The attribution matters because it is the difference between an entity with one recurrent allele and an entity with an allelic series, and because the Arg662Cys case brings developmental delay and intellectual disability into the phenotype, features absent from both dominant pedigrees. The case for inclusion is strong on mechanism: it is a de novo heterozygous arginine-to-cysteine substitution with measured complex II deficiency and modelled impairment of C-terminal FAD binding, in an individual whose dominant clinical feature is progressive bilateral optic atrophy. The case against is that ClinVar's condition list for VCV000694327 contains paraganglioma syndrome 5, recessive complex II deficiency and hereditary cancer predisposition but not NDAXOA, and its aggregate classification is conflicting. No functional comparison of the two alleles has been performed.

Proposed experiments

Side-by-side functional comparison of p.Arg451Cys and p.Arg662Cys

sdha-two-allele-side-by-side

Express both alleles in the same SDHA-null human cell background and compare covalent flavinylation, complex II assembly, residual enzyme activity and succinate accumulation. Concordant behaviour would support merging them into one allelic series for this entity; divergent behaviour would support keeping the Arg662Cys phenotype separate.

The ClinVar condition list and aggregate classification for VCV000694327 were retrieved live on 2026-08-01 and are recorded in the entry-level notes with their endpoint. They are not quotable as a snippet because ClinVar is not a snippet-validated reference source in this repository.

What is the cellular neuropathology of this disorder, given that no autopsy or ocular histology has ever been reported?

KNOWLEDGE GAP OPEN sdha-ndaxoa-no-neuropathology

Attached to

pathophysiology#Retinal Ganglion Cell and Optic Nerve Degeneration pathophysiology#Cerebellar Degeneration and Ataxia

Every tissue-level claim in this entry is an inference from clinical phenotype plus general mitochondrial cell biology. Nobody has counted retinal ganglion cells, examined an optic nerve, or counted Purkinje cells in an individual with a dominant SDHA allele. The entry conforms its cerebellar node to the shared Purkinje-degeneration module only at the clinical-consequence level for exactly this reason. Without tissue it is not known whether the cerebellar atrophy reflects Purkinje cell loss as in the spinocerebellar ataxias, granule cell or deep nuclear involvement, or predominantly white matter change, and it is not known whether the optic neuropathy involves the same prelaminar axonal segment that fails in Leber hereditary optic neuropathy.

Proposed experiments

Optical coherence tomography of the retinal nerve fibre and ganglion cell layers

sdha-oct-ganglion-cell-layer

Non-invasively quantify retinal nerve fibre layer and ganglion cell layer thinning in living carriers, establishing whether the lesion is a ganglion-cell axonopathy of the pattern seen in other mitochondrial optic neuropathies, and providing the first objective progression measure for this disorder.

Patient iPSC-derived retinal ganglion cells and cerebellar organoids

sdha-ipsc-rgc-and-cerebellar-organoids

Derive the affected cell types from carriers and measure complex II activity, oxygen consumption and survival, in the cells the disease actually damages rather than in the fibroblasts that may not even express the defect.

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Pathophysiology

9

Heterozygous SDHA Missense Allele at a Conserved Arginine

The primary lesion is a single heterozygous missense change in SDHA at 5p15.33, reported against transcript NM_004168.4. Both alleles associated with this phenotype substitute a cysteine for a conserved arginine: the founding-family allele c.1351C>T (p.Arg451Cys), historically written R408C in mature-protein numbering, and the de novo allele c.1984C>T (p.Arg662Cys). Neither is a truncating or splice-disrupting change, and neither is predicted to remove the protein. This matters because heterozygous SDHA truncating alleles are the recognised cause of a completely different disease, tumour predisposition, and do not cause neurodegeneration. The genotype-phenotype rule for SDHA is therefore not "how much protein is left" but "what kind of protein is made".

SDHA hgnc:10680

Downstream

Failure of Covalent FAD Attachment to the SDHA Flavoprotein Both substitutions are argued to disable the covalent flavinylation chemistry or the FAD-binding conformation of the subunit rather than to destabilise or truncate it.

Show evidence (2 references)

PMID:10976639 SUPPORT Human Clinical

"The affected family members are now shown to carry a C-to-T transition in one allele of the nuclear gene encoding the flavoprotein subunit of complex II."

Establishes the lesion as a heterozygous single-base change in the gene encoding the complex II flavoprotein subunit, that is, in SDHA, in the founding family.

PMID:33471299 SUPPORT Human Clinical

"Whole exome sequencing of the patient and her unaffected parents identified the novel heterozygous de novo variant c.1984C > T"

Names the second candidate allele and the reference transcript against which SDHA variants are reported.

Failure of Covalent FAD Attachment to the SDHA Flavoprotein

SDHA is a flavoprotein whose catalytic activity depends absolutely on a FAD cofactor attached covalently to the subunit; without the covalent linkage the subunit cannot oxidise succinate. Two independent lines of evidence converge on this step for the two NDAXOA-associated alleles. For the founding-family allele, mutation of the equivalent base in the Escherichia coli orthologue produces an enzyme that is inactive and cannot bind FAD covalently. For the de novo allele, structural modelling of p.Arg662Cys predicts that it compromises the integrity of the FAD-binding region at the C-terminus and so impairs FAD binding. That convergence is not a coincidence of two arginines. Yeast mutagenesis of Sdh1 shows that a pair of spatially close C-terminal arginine residues, physically distant from the FAD-binding site itself, are required for covalent flavinylation and for assembly of the tetramer, and that FAD binding rather than covalent attachment is what stabilises the Sdh1 conformation for association with its partners. Flavinylation is also an assisted reaction: the dedicated factor SDHAF2/SDH5 binds the catalytic subunit and is required for its flavination. The step is therefore a discrete, mutable chemistry, not a passive property of the folded protein, and it is a plausible point at which a single conservative-looking arginine substitution can abolish catalysis without abolishing expression.

SDHA hgnc:10680

covalent flavinylation of the SDHA subunit GO:0018293 ↓ DECREASED

flavin adenine dinucleotide binding GO:0050660 ↓ DECREASED

mitochondrial inner membrane GO:0005743

Downstream

Catalytically Inert SDHA Subunit Competing for Complex II Assembly A subunit that binds FAD without the covalent linkage can still enter the tetramer, so the mutant polypeptide is not simply removed from the pool.

Approximately Half-Normal Succinate Dehydrogenase Activity Whatever the assembly route, the measured net result in patient tissue is a partial rather than complete loss of enzyme activity.

Show evidence (6 references)

PMID:10976639 SUPPORT In Vitro

"Mutation of the equivalent base in Escherichia coli generates an inactive enzyme unable to bind flavin adenine dinucleotide covalently."

Direct experimental demonstration, in the bacterial orthologue, that the founding-family base change abolishes covalent FAD binding and enzymatic activity. Classified IN_VITRO because the experiment is a bacterial enzymology result, not a human observation.

PMID:33471299 PARTIAL Computational

"suggested that this substitution will compromise the structural integrity of the FAD-binding protein at the C-terminus that will ultimately impair the FAD binding to SDHA, thus decreasing the entire CII activity."

Predicts the same lesion, impaired FAD binding, for the second allele. Marked PARTIAL and COMPUTATIONAL because it is in-silico structural modelling, not a measured flavinylation assay, and the authors write "suggested" rather than asserting the mechanism.

PMID:23043141 SUPPORT Model Organism

"We show presently that flavinylation of the Sdh1 subunit of succinate dehydrogenase is dependent on a set of two spatially close C-terminal arginine residues that are distant from the FAD binding site."

Establishes in yeast that specific C-terminal arginines of the flavoprotein subunit, remote from the cofactor pocket, are required for covalent flavinylation, which is the general mechanism into which an arginine-to-cysteine substitution in this region fits.

+ 3 more references

Catalytically Inert SDHA Subunit Competing for Complex II Assembly

This is the node that explains why the disease is dominant. Complex II is a heterotetramer containing exactly one copy of the flavoprotein subunit alongside the iron-sulfur subunit and the two transmembrane anchors, so every assembled complex commits to one SDHA molecule and there is no second copy to compensate. If the mutant subunit is still expressed and still competent to bind FAD non-covalently, it can be incorporated into complexes that are then structurally complete but catalytically dead, and it competes with the wild-type subunit for the available assembly slots. On that model a heterozygote loses close to half of total succinate dehydrogenase activity while retaining a full complement of assembled complex, which is precisely the biochemical picture reported in the founding family: an approximately 50% enzyme deficit accompanied by an apparent overexpression of the 70-kDa flavoprotein subunit whose membrane-associated amount was nonetheless normal. The alternative reading, simple haploinsufficiency, would predict the same 50% activity but is hard to reconcile with the population genetics. Heterozygous SDHA loss-of-function alleles are carried without neurodegeneration by the parents of children with recessive complex II deficiency and by members of tumour-predisposition families. The dominant-negative model accounts for that asymmetry; haploinsufficiency does not.

SDHA hgnc:10680

respiratory chain complex II GO:0045273

mitochondrial respiratory chain complex II assembly GO:0034553 ⚠ ABNORMAL

Downstream

Approximately Half-Normal Succinate Dehydrogenase Activity Whether by competition or by dosage, the measured phenotype in patient tissue is a partial enzyme deficit.

Show evidence (3 references)

PMID:15989954 SUPPORT In Vitro

"Complex II is comprised of two hydrophilic proteins, flavoprotein (Fp) and iron-sulfur protein (Ip), and two transmembrane proteins (CybL and CybS), as well as prosthetic groups required for electron transfer from succinate to ubiquinone."

Fixes the stoichiometry that makes the dominant-negative argument possible: one flavoprotein subunit per complex, with no redundant copy to compensate for an inert one.

PMID:8967754 PARTIAL Human Clinical

"There was an overexpression of the 70-kd flavoprotein subunit in muscle mitochondria from both patients, although we showed that this subunit is present in normal amounts in mitochondrial membranes."

Patient muscle shows the mutant flavoprotein subunit present, not lost, with a normal amount in the membrane fraction despite half-normal enzyme activity. Marked PARTIAL because the observation is compatible with the dominant-negative model but does not by itself demonstrate that mutant and wild-type subunits compete for assembly.

PMID:39321216 PARTIAL In Vitro

"SDHA variants were introduced into a clonal SDHA-knockout cell line via Bxb1-mediated recombination. SDH activity and SDHA abundance were determined for each variant"

Supplies the precondition for the competition model in a human cell rather than in patient muscle: p.Arg451Cys is amorphic yet retains high SDHA protein, so the mutant polypeptide is stable and not cleared. That per-variant reading comes from the full text of PMC11611653 (Figure 2; retrieved 2026-08-01), not from the cached abstract, which reports only the assay design and the aggregate result — the snippet is therefore scoped to the design. Marked PARTIAL, and it does not settle the node: the assay is run in an SDHA-knockout background with no wild-type subunit present, so it cannot show competition for assembly slots.

Approximately Half-Normal Succinate Dehydrogenase Activity

The measured biochemical phenotype is a partial, roughly 50% deficiency of complex II and succinate dehydrogenase activity, reported concordantly in muscle and platelet mitochondria in the founding family and in fibroblasts and lymphocytes in the de novo case. This is the quantitative signature that separates NDAXOA from the recessive disorder, where biallelic loss-of-function genotypes drive activity far lower and produce infantile Leigh syndrome. A half-normal enzyme is compatible with decades of ordinary life and only fails where the metabolic margin is thinnest, which is the reason the disease is late in onset and restricted in the tissues it damages. Tissue expression of the defect is genuinely inconsistent between the two reports and this entry does not smooth that over. In the founding family the enzyme defect was explicitly not expressed in cultured skin fibroblasts or immortalised lymphocytes, so a normal fibroblast assay did not exclude the diagnosis; in the de novo case the defect was demonstrable in exactly those two cell types. The discrepancy has direct diagnostic consequences and is recorded as an open discussion.

tricarboxylic acid cycle GO:0006099 ↓ DECREASED electron transport chain GO:0022900 ↓ DECREASED

succinate dehydrogenase activity GO:0000104 ↓ DECREASED

Downstream

Bioenergetic Failure in Cells With Low Metabolic Reserve A partial and permanent shortfall in oxidative ATP production is the common upstream state for every tissue-level consequence below.

Divergence From the SDHx Tumour-Predisposition Arm The same enzymatic block that produces neurodegeneration here produces tumours in other SDHA genotypes; the branch point is curated explicitly.

Show evidence (4 references)

PMID:8967754 SUPPORT Human Clinical

"In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."

The primary quantitative measurement of the enzyme deficit in patient tissue, and the origin of the approximately 50% figure used throughout this entry.

PMID:10976639 SUPPORT Human Clinical

"Compatible with these findings, our patients have an approximate 50% decrease in complex II and succinate dehydrogenase activity."

Ties the measured half-normal activity in patients directly to the flavinylation-disabling genotype, in the same family.

PMID:8967754 SUPPORT In Vitro

"The enzyme defect was not expressed in cultured skin fibroblasts or immortalized lymphocytes."

Documents the tissue-restricted expression of the biochemical defect in the founding family, which is the diagnostic pitfall recorded in the diagnosis section and in an open discussion.

+ 1 more reference

Bioenergetic Failure in Cells With Low Metabolic Reserve

A permanent halving of succinate dehydrogenase activity is a mild insult in most cells and a severe one in a few. Complex II sits at the junction of the tricarboxylic acid cycle and the electron transport chain, so its impairment restricts both the oxidative arm of intermediary metabolism and the electron supply to the quinone pool at once. Cells that are long-lived, post-mitotic, unable to dilute damaged mitochondria by division, and running near their ATP ceiling therefore carry the cost: retinal ganglion cells, cerebellar neurons, skeletal myofibres and cardiomyocytes. Cells that turn over, or that can lean on glycolysis, are spared, which is why a systemic enzyme defect produces a strikingly selective clinical picture and why cultured fibroblasts may look normal.

oxidative phosphorylation GO:0006119 ↓ DECREASED

mitochondrion GO:0005739

Downstream

Retinal Ganglion Cell and Optic Nerve Degeneration

Cerebellar Degeneration and Ataxia

Striated Muscle and Myocardial Energy Failure

Show evidence (2 references)

PMID:15989954 SUPPORT In Vitro

"The mitochondrial respiratory Complex II or succinate:ubiquinone oxidoreductase (SQR) is an integral membrane protein complex in both the tricarboxylic acid cycle and aerobic respiration."

Establishes the dual metabolic membership of complex II, which is why a single enzyme lesion restricts both the TCA cycle and respiration rather than only one of them.

PMID:10976639 SUPPORT Human Clinical

"These results suggest that genetic defects of nuclear-encoded subunits of the mitochondrial respiratory chain can result in late-onset neurodegenerative disease."

The authors' own framing of the causal chain from a partial nuclear-encoded respiratory chain defect to late-onset neurodegeneration.

Retinal Ganglion Cell and Optic Nerve Degeneration

Optic atrophy is the feature that names the disorder and the one that is present in every reported family. Its anatomical basis is the particular energetic geometry of the retinal ganglion cell: the cell body sits in the retina and generates the mitochondria, which are then transported down an axon that is unmyelinated and mitochondria-rich anterior to the lamina cribrosa and myelinated and mitochondria-poor behind it. The prelaminar segment therefore carries an unusually high, spatially concentrated demand for oxidative ATP with no local capacity to make more. Any systemic restriction of oxidative phosphorylation is felt there first, which is the accepted explanation for why chemically unrelated mitochondrial lesions, from mitochondrial DNA complex I mutations in Leber hereditary optic neuropathy to nuclear OPA1 mutations in dominant optic atrophy, all converge on the same optic neuropathy. Complex II deficiency joins that convergence from a different enzymatic starting point.

retinal ganglion cell CL:0000740

optic nerve UBERON:0000941

Show evidence (4 references)

PMID:14766317 SUPPORT Other

"The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed."

States the selective energy dependence of retinal ganglion cells that makes them the first tissue to fail under a systemic oxidative-phosphorylation deficit.

PMID:14766317 SUPPORT Other

"In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly."

Gives the anatomical basis of that selective vulnerability, localising the metabolically critical segment to the prelaminar optic nerve.

PMID:14766317 SUPPORT Other

"Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria."

Generalises the mechanism beyond any single respiratory chain complex, which is what licenses applying it to a complex II lesion.

+ 1 more reference

Cerebellar Degeneration and Ataxia

Progressive cerebellar and gait ataxia is the second cardinal feature and was present in both members of the founding sibship. Clinically it takes the form recognisable across the hereditary ataxias: unsteady, wide-based gait with postural instability and falls, limb and truncal incoordination, gaze-evoked nystagmus and, in the OMIM clinical synopsis for this entity, cerebellar atrophy on imaging. Mechanistically it is treated here as the cerebellar instance of the same bioenergetic failure that produces the optic neuropathy, and the entry conforms it to the shared cerebellar ataxia module at the clinical-consequence node rather than at the Purkinje-degeneration node, because the intermediate cellular pathology has not been observed in this disorder.

cerebellum UBERON:0002037

Show evidence (2 references)

PMID:10976639 SUPPORT Human Clinical

"Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."

Establishes progressive ataxia as a defining component of the phenotype in the founding family.

PMID:33960148 PARTIAL Human Clinical

"While cerebellar signs have been reported in monoallelic SDHA variant cases"

An independent group's statement that cerebellar signs are an established feature of monoallelic SDHA disease. Marked PARTIAL because the paper's own patient is biallelic and the sentence is a literature statement about other families rather than a new observation in this entity.

Striated Muscle and Myocardial Energy Failure

Skeletal muscle and myocardium are the third and fourth tissues that a permanent half-normal oxidative capacity cannot support. In the founding family the muscle involvement took the form of a myopathy with myalgia and limb weakness in older adults, and muscle mitochondria were the tissue in which the enzyme deficit was demonstrable. In the second pedigree the cardiac involvement was earlier and far more severe than the neurological involvement: both children presented with cardiomyopathy in early childhood and one died of cardiac insufficiency at seven months, while their father's cardiomyopathy accompanied an adult neurological presentation. Cardiac disease is therefore not a late complication of this disorder but, in some carriers, its first and lethal manifestation. The accompanying methylglutaconic aciduria in the two affected children is the expected urinary signature of a mitochondrial energy defect rather than a specific marker of complex II, and it is what should prompt a mitochondrial workup in a child with unexplained cardiomyopathy.

cardiac muscle cell CL:0000746

skeletal muscle tissue UBERON:0001134 heart UBERON:0000948

Show evidence (3 references)

PMID:27683074 SUPPORT Human Clinical

"Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."

Documents cardiomyopathy with a mitochondrial urinary organic-acid signature as an early manifestation in two carriers of the same dominant allele.

PMID:27683074 SUPPORT Human Clinical

"The daughter deceased at the age of 7 months due to cardiac insufficiency."

Establishes that the cardiac arm of this dominant disorder can be lethal in infancy, which is the strongest single argument against treating the entity as uniformly late in onset.

PMID:8967754 SUPPORT Human Clinical

"In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."

Muscle mitochondria are the tissue in which the enzyme deficit was directly measured, tying the myopathy to a demonstrated local biochemical defect.

Divergence From the SDHx Tumour-Predisposition Arm

Loss of succinate dehydrogenase has a second, entirely different consequence: succinate accumulates and acts as an oncometabolite, inhibiting alpha-ketoglutarate-dependent dioxygenases, driving a pseudohypoxic transcriptional response and remodelling the epigenome. That is the route by which heterozygous SDHx loss-of-function alleles, with somatic loss of the second allele, cause paraganglioma, phaeochromocytoma and gastrointestinal stromal tumour. This node exists to state explicitly that NDAXOA is not known to travel that route, and to keep the two arms from being conflated. The empirical position, as of this curation, is that mitochondrial-disease SDHx genotypes and tumour SDHx genotypes have historically been reported as separate sets, with only isolated alleles crossing over. No tumour has been reported in either NDAXOA pedigree. Whether that reflects a real mechanistic divergence, in which a missense allele that blocks flavinylation without destroying the tumour-suppressor function of the locus fails to initiate tumorigenesis, or merely reflects the tiny number of carriers ever followed, is unresolved and is recorded as an open discussion rather than asserted in either direction. The practical consequence is that clinicians have to make a surveillance decision on absent evidence.

SDHA hgnc:10680

hypoxia-inducible factor-1alpha signaling pathway GO:0097411 ↑ INCREASED

Show evidence (3 references)

PMID:33112834 SUPPORT Other

"SDHx mutations lead to the accumulation of succinate, which acts as an oncometabolite by inhibiting iron(II) and alpha-ketoglutarate-dependent dioxygenases thereby regulating the cell's hypoxic response and epigenetic processes."

Describes the oncometabolite route that SDHx tumour alleles take, which is the arm this node exists to distinguish from the neurodegenerative arm.

PMID:24781757 SUPPORT Human Clinical

"Until now, mutations in SDHx associated with mitochondrial disease have not been reported in association with hereditary tumors and vice versa."

States the historical separation of the mitochondrial-disease and tumour allele sets, which is the empirical basis for keeping the two arms as distinct nodes and distinct disease entries.

PMID:24781757 PARTIAL Human Clinical

"Interestingly, SDHx genes also function as tumor suppressor genes in hereditary paragangliomas, pheochromocytomas, and gastrointestinal stromal tumors."

Establishes the tumour-suppressor role of the same genes. Marked PARTIAL because it is a statement about SDHx as a family and about loss-of-function alleles, not about the missense alleles that cause this entity.

⬡

Pathograph

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

●

Phenotypes

14

Cardiovascular 2

Cardiomyopathy Cardiomyopathy HP:0001638

Reported in all three members of the second pedigree. Not reported in the founding family's abstracts. The OMIM clinical synopsis for 619259 additionally records HP:0001640 Cardiomegaly and HP:0001635 Congestive heart failure (MedGen C5543254, retrieved 2026-08-01), both of which are consistent with the fatal infant case. No frequency band is asserted.

Show evidence (2 references)

PMID:27683074 SUPPORT Human Clinical

"Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."

Cardiomyopathy as the presenting feature in two child carriers of the dominant allele.

PMID:27683074 SUPPORT Human Clinical

"The 30-year old son presents with cardiomyopathy and developed bilateral optic atrophy in adulthood."

Persistence of the cardiomyopathy into adulthood in the surviving child, showing it is a chronic feature rather than a transient infantile one.

Congestive heart failure Congestive heart failure HP:0001635

The cited snippet reports "cardiac insufficiency" as the cause of death at seven months; HP:0001635 Congestive heart failure is the closest HPO term and is bound with the source's own wording preserved in the snippet. The same term appears independently in the OMIM clinical synopsis for 619259 (MedGen C5543254, retrieved 2026-08-01).

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"The daughter deceased at the age of 7 months due to cardiac insufficiency."

The one recorded death in this entity, from cardiac failure in infancy.

Eye 2

Optic atrophy Optic atrophy HP:0000648

Course: PROGRESSIVE

Present in the two sisters of the founding family, in the index patient and the adult son of the second family, and in the de novo case. The daughter of the second family died at seven months and never reached an age at which optic atrophy could declare itself, so she is not a negative. No frequency band is asserted; see the entry notes.

Show evidence (3 references)

PMID:27683074 SUPPORT Human Clinical

"The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."

Bilateral optic atrophy as the presenting feature in the index patient of the second dominant pedigree.

PMID:27683074 SUPPORT Human Clinical

"The 30-year old son presents with cardiomyopathy and developed bilateral optic atrophy in adulthood."

Documents adult-onset optic atrophy in a second carrier of the same allele who had a childhood-onset cardiac presentation, establishing the intrafamilial dissociation of the two organ systems in time.

PMID:33471299 SUPPORT Human Clinical

"We report an adolescent female who presented with global developmental delay, intellectual disability and childhood onset progressive bilateral optic atrophy."

Bilateral progressive optic atrophy in the third reported case, with an onset that is childhood rather than late, which is the basis of the onset discussion below.

Ocular motility disorder Abnormality of eye movement HP:0000496

The bound term is deliberately the parent HP:0000496 rather than the more specific HP:0000597 Ophthalmoparesis, because the only quotable source describes an unqualified "ocular movement disorder" and does not say which kind. The more specific reading, limited extraocular movements, comes from the OMIM narrative definition of 619259 exposed through MedGen C5543254 (retrieved 2026-08-01), which is not a cached quotable reference; that synopsis also records HP:0000639 Nystagmus, HP:0000640 Gaze-evoked nystagmus and HP:0000651 Diplopia. Both HP:0000496 and HP:0000597 were verified with OAK; the parent was chosen because the evidence does not support the child, not by accident.

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."

Names an ocular movement disorder alongside the optic atrophy in the index patient.

Musculoskeletal 1

Myopathy Myopathy HP:0003198

Course: PROGRESSIVE

Show evidence (1 reference)

PMID:10976639 SUPPORT Human Clinical

"Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."

Names myopathy as the third component of the founding family's phenotype.

Nervous System 5

Ataxia Ataxia HP:0001251

Course: PROGRESSIVE

Reported in both sisters of the founding family. Ataxia is not mentioned in the abstract of the second pedigree's report, which describes that family's neurological involvement as a progressive polyneuropathy; whether the cerebellar syndrome is universal in this entity or specific to the founding family cannot be settled from the cached abstracts. The component terms HP:0002066 Gait ataxia, HP:0002078 Truncal ataxia, HP:0002070 Limb ataxia, HP:0002317 Unsteady gait, HP:0002172 Postural instability and HP:0002312 Clumsiness all appear in the OMIM clinical synopsis for 619259 (MedGen C5543254, retrieved 2026-08-01) and were each verified with OAK, but are not given separate phenotype entries here because they decompose a single clinical syndrome that the primary sources describe as "ataxia".

Show evidence (1 reference)

PMID:10976639 SUPPORT Human Clinical

"Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."

Names progressive ataxia as one of the three defining components of the phenotype in the family that defines this entity.

Cerebellar atrophy Cerebellar atrophy HP:0001272

Course: PROGRESSIVE

Curated from the OMIM clinical synopsis for 619259 as exposed through MedGen concept C5543254 (HP:0001272, retrieved 2026-08-01 from the NCBI eutils esummary endpoint for MedGen uid 1779901). It does not appear in any cached abstract for this disorder, so no evidence item is attached. Note that progressive cerebellar atrophy on serial MRI has been imaged and published in SDHA disease, but in a biallelic child (PMID:33960148), not in a heterozygous NDAXOA patient; that image is deliberately not adopted as evidence here.

Psychiatric involvement Atypical behavior HP:0000708

The bound term is the broad behavioural-abnormality parent because the quotable source says only "psychiatric involvement" without specifying a syndrome. HP:0000716 Depression is recorded in the OMIM clinical synopsis for 619259 (MedGen C5543254, retrieved 2026-08-01) and was verified with OAK, but is not bound here because the synopsis is not a quotable source and the primary report does not name depression.

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."

Names psychiatric involvement as a presenting feature in the index patient of the second dominant family.

Global developmental delay Global developmental delay HP:0001263

Reported in one individual, the de novo case, whose assignment to this entity is qualified in the entry notes because ClinVar carries no NDAXOA record for that allele. Not reported in either dominant pedigree.

Show evidence (1 reference)

PMID:33471299 PARTIAL Human Clinical

"Furthermore, it adds developmental delay and cognitive disability to the expanding spectrum of the disorder."

The authors' explicit claim that developmental delay is a new addition to the spectrum of heterozygous SDHA disease. Marked PARTIAL because it rests on a single individual whose allele is not classified against this condition in ClinVar.

Intellectual disability Intellectual disability HP:0001249

Reported in one individual. Not reported in either dominant pedigree, whose affected adults had ordinary cognition alongside their psychiatric and motor features.

Show evidence (1 reference)

PMID:33471299 PARTIAL Human Clinical

"We report an adolescent female who presented with global developmental delay, intellectual disability and childhood onset progressive bilateral optic atrophy."

Intellectual disability in the single de novo case; marked PARTIAL for the same reason as the developmental delay.

Constitutional 1

Myalgia Myalgia HP:0003326

Curated from the OMIM clinical synopsis for 619259 via MedGen C5543254 (HP:0003326, retrieved 2026-08-01), together with HP:0012378 Fatigue and HP:0003690 Limb muscle weakness from the same source. Not mentioned in any cached abstract, so no evidence item is attached.

Other 3

Polyneuropathy Polyneuropathy HP:0001271

Course: PROGRESSIVE

Reported in the index patient of the second pedigree. The related sensory terms HP:0012534 Dysesthesia and HP:0003487 Babinski sign appear in the OMIM clinical synopsis for 619259 (MedGen C5543254, retrieved 2026-08-01); the Babinski sign indicates additional corticospinal involvement, which is recorded here rather than given its own phenotype entry because it rests on the synopsis alone.

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."

Names progressive polyneuropathy as a presenting feature in the index patient of the second dominant family.

3-Methylglutaconic aciduria 3-Methylglutaconic aciduria HP:0003535

The cited abstract writes "methylglutaconic aciduria" without the 3- prefix; HP:0003535 is the corresponding HPO term and the snippet preserves the source wording. Reported in two of three carriers in the second pedigree; not reported in the founding family or in the de novo case. No frequency band is asserted.

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."

Documents methylglutaconic aciduria in the two child carriers of the dominant allele.

Decreased activity of mitochondrial complex II Decreased activity of mitochondrial complex II HP:0008314

Demonstrated in muscle and platelet mitochondria in the founding family and in fibroblasts and lymphocytes in the de novo case, and inferred in the second pedigree from the title and framing of that report. The tissues in which the defect is expressible are not consistent between reports, which is curated as an open discussion and as a diagnostic caveat rather than resolved.

Show evidence (2 references)

PMID:8967754 SUPPORT Human Clinical

"In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."

Direct measurement of the partial complex II deficiency in patient muscle and platelets.

PMID:10976639 SUPPORT Human Clinical

"Compatible with these findings, our patients have an approximate 50% decrease in complex II and succinate dehydrogenase activity."

Confirms the same partial deficit and links it explicitly to succinate dehydrogenase activity as well as to assembled complex II.

🧬

Genetic Associations

1

SDHA (Heterozygous missense substitutions at conserved arginine residues, argued to disable covalent flavinylation or FAD binding of the flavoprotein subunit; the recurrent founding-family allele is c.1351C>T (p.Arg451Cys), historically designated R408C.)

Gene: SDHA hgnc:10680 relationship_type: CAUSATIVE

Autosomal dominant inheritance

Show evidence (2 references)

PMID:10976639 SUPPORT Human Clinical

"The affected family members are now shown to carry a C-to-T transition in one allele of the nuclear gene encoding the flavoprotein subunit of complex II."

Establishes SDHA as the causal gene for this phenotype and the lesion as heterozygous.

PMID:27683074 SUPPORT Human Clinical

"Here, we report clinical and molecular investigations of three family members with a heterozygous mutation in the large flavoprotein subunit SDHA previously described to cause complex II deficiency."

Independent replication of the same gene, the same zygosity and, as the phrase "previously described" indicates, the same allele in a second family.

💊

Medical Actions

4

Multidisciplinary supportive care

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

No disease-modifying therapy exists. Management follows the components of the phenotype: low-vision assessment and educational or occupational accommodation for the optic neuropathy, physiotherapy and falls prevention for the ataxia and myopathy, and neuropathic pain management where the polyneuropathy is symptomatic.

Cardiac surveillance and heart failure management

Category: Monitoring Action: disease screening Ontology label: Disease Screening NCIT:C15419

Because cardiomyopathy was present in all three carriers of the second pedigree and killed one of them in infancy, cardiac surveillance is the one intervention in this disorder with a clear, published rationale. Echocardiographic follow-up and conventional heart failure management apply; the disorder does not change how the cardiomyopathy itself is treated, only the imperative to look for it.

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"The daughter deceased at the age of 7 months due to cardiac insufficiency."

The outcome that justifies cardiac surveillance as an active recommendation rather than a generic precaution.

Riboflavin supplementation

Category: Therapeutic Action: pharmacotherapy Ontology label: Pharmacotherapy NCIT:C15986

Agent: riboflavin CHEBI:17015

Riboflavin is the dietary precursor of FAD, the cofactor whose covalent attachment to SDHA is the step that the NDAXOA alleles are argued to disrupt, so raising intracellular flavin availability is the one mechanistically targeted intervention that has ever been tried in complex II deficiency. In three children with recessive complex II deficiency treated with oral riboflavin and followed for a mean of 4.5 years, the neurological course was stable or moderately improved in two and plasma lactate decreased to near-normal levels in the third, and riboflavin supplementation of the culture medium doubled complex II activity in patient fibroblasts but not in control fibroblasts. Whether any of that transfers to NDAXOA is unknown and this entry does not claim it does. No patient with a dominant SDHA allele has been reported on riboflavin. There is a specific mechanistic reason for caution: raising FAD availability is a plausible remedy for a subunit that binds its cofactor poorly, but has no obvious mechanism against a subunit that cannot form the covalent linkage at all, and none at all if the dominant allele acts by competing for assembly slots. The intervention is nevertheless recorded because it is cheap, non-toxic and rationally connected to the curated mechanism, which is the strongest position available in a disease with no trials.

Mechanism Target:

MODULATES Failure of Covalent FAD Attachment to the SDHA Flavoprotein — Increasing flavin availability is intended to drive the FAD-loading step that the mutant subunit performs poorly. Recorded as MODULATES rather than RESTORES deliberately: RESTORES would assert that supplementation returns the disrupted step to normal function, and this entry argues the opposite elsewhere — extra FAD is a plausible remedy for a subunit that binds its cofactor weakly but has no obvious mechanism against one that cannot form the covalent linkage at all. The link is retained because the drug and the node are genuinely the same chemistry, but the direction and magnitude of the effect on this node are unestablished, and no patient with a dominant SDHA allele has been treated.

Show evidence (2 references)

PMID:16737791 PARTIAL Human Clinical

"We report on three complex II-deficient children treated with riboflavin per os, who were followed-up for a mean period of 4.5 years."

The only clinical experience of riboflavin in complex II deficiency. Marked PARTIAL because the three treated children had the recessive paediatric disorder, not this dominant entity, and the series is uncontrolled.

PMID:16737791 PARTIAL In Vitro

"Riboflavin supplementation to the growth medium of cultured fibroblasts resulted in a 2-fold increase of complex II activity in patients, but not in controls."

Cell-level evidence that flavin supply is rate-limiting for complex II activity in complex II-deficient patient cells specifically. Marked PARTIAL because the fibroblasts came from children with recessive disease and the effect has never been tested on a dominant flavinylation-defective allele.

Genetic counselling

Category: Counseling / Informational Action: genetic counseling Ontology label: Genetic Counseling NCIT:C15240

Counselling in this disorder is unusually difficult and the difficulty is specific rather than generic. A carrier faces a 50% transmission risk, but what is transmitted cannot be described: within one published family the same allele produced lethal infantile cardiomyopathy in one child and adult optic atrophy in another. Beyond that, the family must be told that the literature attaches a tumour-surveillance recommendation to SDHA pathogenic variants generally, that no such recommendation exists for adult-onset mitochondrial disease, and that the absolute risk of either outcome for their particular allele is unknown. Predictive testing of at-risk relatives therefore returns a result whose meaning is genuinely indeterminate.

Show evidence (2 references)

PMID:33960148 SUPPORT Human Clinical

"There is no similar screening or testing recommended for individuals potentially at risk for adult-onset mitochondrial disease. Additionally, the absolute risk for both conditions is unknown."

States the counselling asymmetry directly: a surveillance protocol exists for the tumour risk and nothing exists for the mitochondrial risk, with neither absolute risk quantified.

PMID:24781757 SUPPORT Human Clinical

"As tumor screening is recommended for SDHx mutation carriers, this should also be considered for patients with mitochondrial disorders and their family members."

An explicit recommendation that families ascertained through SDHx mitochondrial disease should be considered for tumour screening, which is the concrete counselling content this entry cannot avoid.

🔬

Biochemical Markers

2

Complex II (succinate dehydrogenase) enzyme activity (DECREASED)

Context: Respiratory chain enzymology on a tissue biopsy or on blood-derived cells, expressed as a ratio to citrate synthase to correct for mitochondrial content. In this disorder the expected result is a partial deficit of roughly half of control values, not the profound deficit seen in recessive complex II disease, and the assay may be normal in cultured fibroblasts.

Pathograph Readouts

Readout Of Approximately Half-Normal Succinate Dehydrogenase Activity Negative Diagnostic

A reduction to approximately half of control values in muscle, platelets or, in some individuals, cultured fibroblasts and lymphocytes reports the partial enzymatic block produced by a heterozygous flavinylation-defective SDHA allele. A normal result in fibroblasts does not exclude the diagnosis.

Show evidence (1 reference)

PMID:8967754 SUPPORT Human Clinical

"In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."

Establishes the assay, the tissues and the expected magnitude of the deficit.

Urinary 3-methylglutaconic acid (INCREASED)

Context: Urine organic acid analysis. Elevated in the two affected children of the second pedigree. A non-specific marker of mitochondrial membrane-related metabolic disturbance rather than a marker of complex II specifically, but the only reported metabolic abnormality in this disorder and a practical trigger for mitochondrial investigation in a child with cardiomyopathy.

Show evidence (1 reference)

PMID:27683074 SUPPORT Human Clinical

"Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."

The only report of a urinary metabolic abnormality in this entity.

🔀

Differential Diagnoses

6

Conditions with similar clinical presentations that must be differentiated from SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy:

Mitochondrial complex II deficiency, nuclear type 1 MONDO:0100294

Overlapping Features The recessive allelic disorder, caused by biallelic SDHA variants and curated separately in `kb/disorders/Mitochondrial_Complex_II_Deficiency_Nuclear_Type_1.yaml`. This is not a differential in the ordinary sense of a mimicking disease; it is the other zygosity of the same gene, and the two are separated by the number of damaged alleles rather than by clinical judgement. The distinction is made at the moment the sequencing result is read, not at the bedside.

Distinguishing Features

Zygosity is the discriminator: NDAXOA arises from a single heterozygous missense allele, MC2DN1 from biallelic loss-of-function or severely hypomorphic genotypes.
MC2DN1 presents in infancy or early childhood with Leigh syndrome or leukodystrophy; NDAXOA is dominated by adult-onset optic atrophy and ataxia, with the qualification that its cardiac arm can present in infancy.
Residual complex II activity is partial in NDAXOA, of the order of half of control values, and far lower in MC2DN1.
Parents of an MC2DN1 proband are unaffected heterozygous carriers; a parent of an NDAXOA proband is either affected or the variant is de novo.
MONDO does not place NDAXOA beneath MONDO:0100294, verified by OAK ancestor query; the two are sibling consequences of one gene, not a parent and a subtype.

Show evidence (2 references)

PMID:24781757 SUPPORT Human Clinical

"Underlying autosomal-recessive genetic defects are found in most of the 'SDHx' genes encoding complex II (SDHA, SDHB, SDHC, and SDHD) and its assembly factors."

Establishes recessive inheritance as the norm for complex II subunit mitochondrial disease, which is the boundary this differential draws.

PMID:33960148 SUPPORT Human Clinical

"Deleterious variants of SDHA are most frequently associated with Leigh and Leigh-like syndromes."

States that the dominant clinical association of SDHA variants overall is the recessive Leigh phenotype, which is why the dominant neurodegenerative entity is easily missed.

Pheochromocytoma/paraganglioma syndrome 5 Not Yet Curated MONDO:0013602

Overlapping Features The tumour-predisposition allelic disorder, in which a heterozygous SDHA loss-of-function germline allele behaves as a tumour-suppressor first hit, with somatic loss of the wild-type allele driving paraganglioma, phaeochromocytoma and gastrointestinal stromal tumour. It shares zygosity and gene with NDAXOA and shares nothing else. It is curated in dismech within `kb/disorders/Pheochromocytoma_Paraganglioma.yaml`, which was read and not modified. The overlap is not merely theoretical and it creates a live clinical problem. Codon 451 of SDHA produces neurodegeneration when the arginine is replaced by cysteine and has been reported in paraganglioma when it is replaced by histidine. Meanwhile, tumour surveillance is recommended for SDHA pathogenic-variant carriers and no equivalent surveillance exists for adult-onset mitochondrial disease, so a family found to carry an SDHA variant is offered a cancer protocol whose applicability to their particular allele nobody can state.

Distinguishing Features

PGL5 alleles are typically truncating or splice loss-of-function changes acting as tumour-suppressor first hits; the NDAXOA alleles are specific missense substitutions that leave a full-length subunit.
PGL5 requires a somatic second hit in the tumour tissue; NDAXOA is a constitutive, whole-body enzymatic deficit with no second hit.
The mechanism diverges at succinate: in PGL5 succinate accumulation acts as an oncometabolite driving pseudohypoxia and epigenetic remodelling, whereas in NDAXOA the pathogenic consequence curated here is bioenergetic failure in post-mitotic tissue.
Neither NDAXOA pedigree has reported a tumour, and no PGL5 family has reported optic atrophy with ataxia; the two allele sets have historically been described as separate.
At codon 451 the substituting residue, not the position, decides which disease follows: Cys in NDAXOA, His reported in paraganglioma, Ser reported de novo in an infant with cardiomyopathy.

Show evidence (2 references)

PMID:24781757 SUPPORT Human Clinical

"Interestingly, SDHx genes also function as tumor suppressor genes in hereditary paragangliomas, pheochromocytomas, and gastrointestinal stromal tumors."

Establishes the tumour-suppressor arm of the same gene family that this differential separates from the neurodegenerative arm.

PMID:33960148 SUPPORT Human Clinical

"Individuals with known pathogenic variants in SDHA are recommended to receive annual biochemical and clinical surveillance for signs and symptoms of paraganglioma and pheochromocytoma syndrome"

Documents that a tumour-surveillance recommendation attaches to SDHA pathogenic variants generically, which is what makes the boundary between these two entities a practical rather than an academic question.

Dilated cardiomyopathy 1GG Not Yet Curated MONDO:0013339

Overlapping Features The fourth SDHA disease term in MONDO, a cardiac-predominant phenotype. It is included as a differential because cardiomyopathy is also a feature of NDAXOA, and in the second NDAXOA pedigree it was the presenting and lethal feature in the paediatric carriers. An infant found to have cardiomyopathy and an SDHA variant could plausibly be assigned to either term, and the distinction currently rests on zygosity and on whether neurological or ophthalmological features later appear in the family.

Distinguishing Features

DCM1GG is a cardiac-predominant entity without the optic atrophy that defines NDAXOA.
The NDAXOA pedigree with childhood cardiomyopathy also contained an adult with optic atrophy carrying the same allele, so a family history of visual loss argues for NDAXOA.
MONDO keeps the two as separate terms with separate OMIM anchors, MONDO:0013339/OMIM:613642 and MONDO:0031006/OMIM:619259, both mapped to SDHA.

Behr syndrome MONDO:0008858

Overlapping Features The closest clinical mimic among the non-SDHA disorders. Behr syndrome pairs bilateral optic atrophy with ataxia and variable additional neurological features, most often from biallelic OPA1 variants and also from OPA3, MTRFR/C12orf65 or C19orf12. The pairing of optic atrophy with cerebellar ataxia is exactly the NDAXOA core, so a patient with NDAXOA presenting to a neuro-ophthalmology service will be worked up for Behr syndrome, and the distinction is molecular.

Distinguishing Features

Behr syndrome is recessive and childhood-onset; NDAXOA is dominant and, in its neurological arm, adult-onset.
Behr syndrome converges on mitochondrial dynamics and translation rather than on a respiratory chain enzyme; complex II activity is not the expected abnormality.
Pyramidal signs with spasticity and posterior-column sensory loss are classical in Behr syndrome; NDAXOA has a peripheral polyneuropathy and, in the OMIM synopsis, a Babinski sign but not a spastic paraparesis.
Cardiomyopathy is not a feature of Behr syndrome and is prominent in one NDAXOA pedigree.

Autosomal dominant optic atrophy plus syndrome MONDO:0014720

Overlapping Features OPA1-related dominant optic atrophy with extra-ocular features is the commonest inherited syndromic optic neuropathy and shares with NDAXOA both the dominant transmission and the combination of optic atrophy with ataxia and myopathy. It will be considered, and tested for, long before SDHA.

Distinguishing Features

Dominant optic atrophy plus is caused by OPA1 and is far more common; SDHA is a rare cause that the most recent case report argued should be added to optic atrophy panels precisely because it is often absent from them.
Sensorineural deafness and chronic progressive external ophthalmoplegia characterise the OPA1-plus phenotype; the eye-movement abnormality in NDAXOA is not described as a progressive external ophthalmoplegia and deafness is not reported.
Complex II activity is normal in OPA1 disease and approximately half-normal in NDAXOA, so respiratory chain enzymology discriminates the two where it is available.
Cardiomyopathy and methylglutaconic aciduria point away from OPA1 and towards SDHA.

Show evidence (1 reference)

PMID:39423307 PARTIAL Human Clinical

"The top five genes implicated in nHON in our in-house cohort were OPA1, WFS1, FDXR, ACO2 and AFG3L2, which accounted for 82.46% of probands."

Establishes the gene ranking a clinician actually faces in nuclear hereditary optic neuropathy, with OPA1 far ahead and SDHA nowhere in the leading group. Marked PARTIAL because the abstract does not state whether SDHA was among the 50 genes analysed, so this supports the relative prominence of OPA1 rather than a quantified rarity for SDHA.

Leber hereditary optic neuropathy MONDO:0010788

Overlapping Features The paradigm mitochondrial optic neuropathy, caused by mitochondrial DNA complex I mutations. It is included as a mechanistic rather than a clinical differential: LHON establishes that a partial respiratory chain defect preferentially destroys retinal ganglion cells, which is the same argument this entry makes for complex II, and it is the reason a mitochondrial genome analysis is usually done before a nuclear panel in an adult with bilateral optic neuropathy.

Distinguishing Features

LHON is maternally inherited with marked male bias and incomplete penetrance; NDAXOA is autosomal dominant with affected women in both pedigrees.
LHON typically produces subacute, sequential, painless central visual loss over weeks; the NDAXOA optic neuropathy is described as slowly progressive.
LHON is not accompanied by cerebellar ataxia, myopathy or cardiomyopathy in its classical form; those are the extra-ocular features that should redirect testing towards SDHA.
The biochemical lesion is complex I in LHON and complex II in NDAXOA, so respiratory chain enzymology distinguishes them directly.

Show evidence (1 reference)

PMID:14766317 SUPPORT Other

"Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria."

Supports the shared mechanism that makes LHON a mechanistic rather than merely clinical differential for a complex II optic neuropathy.

{ }

Source YAML

click to show

name: SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy
creation_date: "2026-08-01T00:00:00Z"
category: Mendelian
disease_term:
  preferred_term: neurodegeneration with ataxia and late-onset optic atrophy
  term:
    id: MONDO:0031006
    label: neurodegeneration with ataxia and late-onset optic atrophy
description: >-
  Neurodegeneration with ataxia and late-onset optic atrophy (NDAXOA) is the
  autosomal dominant neurodegenerative phenotype of heterozygous missense
  variants in SDHA, the gene encoding the large flavoprotein (Fp) catalytic
  subunit of mitochondrial respiratory chain complex II (succinate
  dehydrogenase, succinate:ubiquinone oxidoreductase). Complex II is the only
  respiratory chain complex encoded entirely in the nuclear genome and the only
  one that belongs simultaneously to the tricarboxylic acid cycle and to the
  electron transport chain: SDHA oxidises succinate to fumarate using a
  covalently bound FAD cofactor and passes the electrons through the iron-sulfur
  clusters of SDHB to ubiquinone at the SDHC/SDHD membrane anchor.

  The disorder was defined biochemically before it was defined genetically. Two
  elderly sisters with late-onset neurodegenerative disease were reported with a
  partial, approximately 50% deficiency of complex II in muscle and platelet
  mitochondria; four years later the same family was shown to carry a
  heterozygous C-to-T transition in the flavoprotein gene, and mutation of the
  equivalent base in Escherichia coli was shown to yield an enzyme that cannot
  bind flavin adenine dinucleotide covalently. A second, independent pedigree
  with dominant transmission of a heterozygous SDHA missense allele presented
  with bilateral optic atrophy and an ocular movement disorder, progressive
  polyneuropathy, psychiatric involvement and cardiomyopathy, and in the two
  affected children with cardiomyopathy and methylglutaconic aciduria in early
  childhood. A third, unrelated individual carried a de novo heterozygous SDHA
  missense allele and presented with global developmental delay, intellectual
  disability and childhood-onset progressive bilateral optic atrophy. The whole
  published clinical experience of this entity is therefore those two pedigrees
  plus one singleton.

  The mechanism that these alleles share is not loss of the SDHA protein but
  loss of the chemistry that makes it catalytic. Both reported NDAXOA-associated
  substitutions replace an arginine with a cysteine and both are argued to
  disable covalent flavinylation or FAD binding rather than to abolish
  expression or assembly, and the measured consequence in patients is a roughly
  half-normal, not absent, succinate dehydrogenase activity. That partial,
  lifelong energetic shortfall is expressed selectively in the tissues with the
  least metabolic reserve and the least capacity for replacement: the retinal
  ganglion cells and their unmyelinated prelaminar axons, the cerebellum, and
  striated muscle including myocardium.

  NDAXOA is one of at least four distinct disease entities that MONDO assigns to
  SDHA, and the separation matters clinically. Biallelic loss-of-function
  genotypes cause the recessive infantile disorder mitochondrial complex II
  deficiency nuclear type 1 (Leigh syndrome and leukodystrophy); heterozygous
  loss-of-function genotypes behave as tumour-suppressor alleles and predispose
  to paraganglioma, phaeochromocytoma and gastrointestinal stromal tumour; a
  homozygous genotype has been associated with dilated cardiomyopathy 1GG; and
  NDAXOA is a dominant, adult-predominant neurodegeneration. This entry curates
  the last of these and deliberately draws the boundaries against the other
  three rather than merging them.
synonyms:
- NDAXOA
- neurodegeneration with ataxia and late-onset optic atrophy
- late-onset optic atrophy, ataxia and myopathy
- SDHA-related autosomal dominant complex II deficiency
- dominantly inherited succinate dehydrogenase flavoprotein subunit deficiency
parents:
- Mitochondrial Disease
- Hereditary Optic Neuropathy
classifications:
  harrisons_chapter:
  - classification_value: NEUROLOGIC
    notes: >-
      The defining and most consistently reported morbidity is neurological and
      neuro-ophthalmological: progressive optic atrophy, cerebellar and gait
      ataxia, myopathy and, in one pedigree, a progressive polyneuropathy with
      psychiatric involvement. The optic neuropathy is a lesion of retinal
      ganglion cells and their axons, that is, of central nervous system tissue,
      rather than an ophthalmological disease of the globe. Cardiomyopathy is
      real and was lethal in one infant in the second pedigree, but it is not
      the presenting or defining feature of the entity, so a second
      CARDIOVASCULAR assignment is deliberately not made.
    evidence:
    - reference: PMID:10976639
      reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."
      explanation: The founding report characterises the phenotype as a late-onset neurodegenerative disease with optic, cerebellar and muscular components, which is the basis for the NEUROLOGIC chapter assignment.
  mechanistic_category:
  - classification_value: mitochondrial disease
    notes: >-
      A nuclear-encoded oxidative-phosphorylation disorder. The lesion is in a
      nuclear gene but the failing organelle and the failing chemistry are
      mitochondrial.
  icimd_category:
  - classification_value: complex_ii_subunits_and_assembly_factors
    notes: >-
      ICIMD category 7, nuclear-encoded disorders of oxidative phosphorylation,
      subgroup complex II subunit and assembly-factor defects. SDHA encodes the
      catalytic flavoprotein subunit of complex II. Note that the ICIMD
      classification does not distinguish the dominant neurodegenerative
      phenotype curated here from the recessive infantile phenotype; both fall
      in the same ICIMD cell, which is precisely why the MONDO-level split is
      worth preserving.
notes: >-
  Entity verification. Every identifier in this entry was resolved with OAK
  before it was written down, on 2026-08-01. `runoak -i sqlite:obo:mondo info
  MONDO:0031006 -O obo` returns the label "neurodegeneration with ataxia and
  late-onset optic atrophy", the synonym NDAXOA, xrefs OMIM:619259,
  MEDGEN:1779901, UMLS:C5543254 and GARD:0027269, the relationship `RO:0004003
  HGNC:10680 ! SDHA`, and the single superclass MONDO:0024237 (inherited
  neurodegenerative disorder). `runoak relationships -p RO:0004003
  MONDO:0031006` independently returns HGNC:10680 SDHA. `runoak -i sqlite:obo:hgnc
  info hgnc:10680` returns `hgnc:10680 ! SDHA` (the adapter flag is required; the
  bare `runoak info hgnc:10680` errors, and an earlier revision of this note
  recorded the command without it).

  Scope, and why this is not filed under complex II deficiency. `runoak -i
  sqlite:obo:mondo ancestors -p i MONDO:0031006` returns exactly: MONDO:0024237
  inherited neurodegenerative disorder, MONDO:0005559 neurodegenerative disease,
  MONDO:0002602 central nervous system disorder, MONDO:0005071 nervous system
  disorder, MONDO:0100545 hereditary neurological disease, MONDO:0003847
  hereditary disease, and the upper-level MONDO/BFO scaffolding. MONDO:0100294
  (mitochondrial complex II deficiency, nuclear type 1) is NOT among them, so
  NDAXOA is not a MONDO descendant of the recessive complex II disorder and this
  entry is not a subtype of `kb/disorders/Mitochondrial_Complex_II_Deficiency_Nuclear_Type_1.yaml`.
  `runoak relationships --direction down -p RO:0004003 HGNC:10680` returns four
  SDHA disease terms and no others: MONDO:0013339 dilated cardiomyopathy 1GG
  (OMIM:613642), MONDO:0013602 pheochromocytoma/paraganglioma syndrome 5
  (OMIM:614165), MONDO:0031006 this entry (OMIM:619259) and MONDO:0100294
  mitochondrial complex II deficiency nuclear type 1 (OMIM:252011). All four are
  curated below as differentials; none is merged into this entry, and the two
  existing SDHA-containing KB files
  (`kb/disorders/Mitochondrial_Complex_II_Deficiency_Nuclear_Type_1.yaml` and
  `kb/disorders/Pheochromocytoma_Paraganglioma.yaml`) were read but not
  modified.

  Named Entity Confusion preflight. The three primary clinical sources used here
  (PMID:8967754, PMID:10976639, PMID:27683074) plus the supporting case report
  (PMID:33471299) all name SDHA, or its protein-level designation "the
  flavoprotein subunit of complex II", as the causal gene; no competing gene
  dominates any of them. The OMIM/MedGen definition of C5543254 attributes the
  entity to Taylor et al. 1996 and Courage et al. 2017 and lists SDHA at 5p15.33
  as the associated gene, which matches the MONDO gene relationship exactly. The
  named NEC risk for this disease is the reverse of the usual one: the danger is
  not that the literature is about a different gene but that it is about a
  different SDHA disease. Papers found by an SDHA search are overwhelmingly
  about recessive Leigh/leukodystrophy or about SDHx tumour predisposition, and
  those were excluded rather than cherry-picked. Specifically excluded as
  off-entity: PMID:40045913 (biallelic SDHA, childhood focal seizures, yeast
  complementation) and PMID:41833598 (compound heterozygous SDHA, paediatric
  neurological disease) are recessive complex II deficiency, not NDAXOA;
  PMID:28724664 (SDHA variants of unknown significance) is a
  tumour-predisposition study. PMID:33960148 IS used, but only for the
  cross-phenotype contrast and the counselling problem it explicitly discusses,
  never as evidence for an NDAXOA phenotype: its proband is biallelic.

  One tumour-genetics paper is nonetheless used, deliberately, and the
  distinction matters. PMID:39321216 (Kent, Klug and Heinrich 2024) is a cancer
  variant-interpretation study and contributes nothing about the NDAXOA
  phenotype, but it is the only published work that has put the NDAXOA alleles
  themselves through a functional assay in a human system: it introduced SDHA
  missense variants into a clonal human SDHA-knockout cell line and measured SDH
  activity and SDHA protein abundance for each. Both codons this entry curates
  are in its panel — a statement read from the full text of PMC11611653
  (Figures 2 and 5, Table 1; retrieved 2026-08-01), not from the cached
  reference. Note the provenance limit that applies wherever this paper is
  cited below: references_cache/PMID_39321216.md is abstract-only, and the
  abstract reports the assay design and the aggregate results (72 variants
  characterised, 22 VUS assayed, 19 favouring pathogenicity) without naming
  p.Arg451Cys or p.Arg662Cys. Every evidence snippet taken from this PMID is
  therefore scoped to those aggregate statements, and every per-variant claim
  made in prose is marked inline as a full-text derivation so it is not
  mistaken for a snippet-verified assertion. It is cited only for allele-level
  function, never for phenotype, and the entry does not import its
  cancer-pathogenicity conclusions.

  Allele identity, and how it was verified. Neither of the two primary genetic
  reports names the founding-family allele in its abstract, so it is recorded
  here as a note with live provenance rather than as an evidence snippet.
  ClinVar VCV000449389 is NM_004168.4(SDHA):c.1351C>T (p.Arg451Cys); its
  RCV001363197 record classifies it Pathogenic specifically against
  "Neurodegeneration with ataxia and late-onset optic atrophy" (MedGen
  C5543254), submitted by OMIM, last evaluated 2021-04-08, no assertion criteria
  provided. The OMIM submission text in that record states that in the family
  originally reported by Taylor et al. (1996), Birch-Machin et al. (2000)
  identified a heterozygous c.1375C-T transition producing an arg408-to-cys
  (R408C) substitution, "this numbering is equivalent to R451C". That reconciles
  the mature-protein numbering used in the older complex II literature with the
  precursor numbering of NM_004168.4 and confirms that the Taylor, Birch-Machin
  and Courage families all concern the same codon. The record cites
  PMID:8967754, PMID:10976639 and PMID:27683074. Retrieved 2026-08-01 from
  https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=clinvar&rettype=vcv&id=449389&is_variationid
  and the matching esummary endpoint. The `&is_variationid` flag is required:
  without it the same URL returns an empty result set.

  Same codon, three diseases. The ClinVar record for VCV000449389 also states
  that a different substitution at the same codon, p.Arg451His, has been
  reported in individuals with paraganglioma with functional evidence of loss of
  function, and that a third substitution, p.Arg451Ser, was a de novo
  heterozygous change in an infant with cardiomyopathy. Codon 451 of SDHA
  therefore produces at least three clinically distinct dominant phenotypes
  depending on which residue replaces the arginine. This is the single strongest
  argument in the entry against collapsing the SDHA phenotypes into one entity,
  and it is recorded as a note because ClinVar is not a snippet-validated
  reference source in this repository.

  Variant classification is contested and is recorded as such. The aggregate
  ClinVar germline classification of VCV000449389 is "Conflicting
  classifications of pathogenicity" (last evaluated 2026-01-15). The seven
  submitter-level germline classifications retrieved on 2026-08-01 were:
  Pathogenic (OMIM), Pathogenic (3billion), Likely pathogenic (Labcorp Genetics,
  formerly Invitae), Likely pathogenic (Ambry Genetics), Likely pathogenic (Rady
  Children's Institute for Genomic Medicine), Uncertain significance (GeneDx)
  and Uncertain significance ("Giorgio Prodi" Cancer Research Center, University
  of Bologna). The same variant additionally carries an RCV against
  gastrointestinal stromal tumour classified Uncertain significance and an RCV
  against hereditary cancer-predisposing syndrome. A laboratory reporting this
  allele cannot present it as unqualified Pathogenic, and cannot report it
  against a single condition.

  The second allele is weaker than the first, and this entry says so. The
  variant reported in PMID:33471299, NM_004168.4:c.1984C>T (p.Arg662Cys), is
  ClinVar VCV000694327, whose aggregate germline classification is likewise
  "Conflicting classifications of pathogenicity" (last evaluated 2026-03-19).
  Critically, its ClinVar condition list is Pheochromocytoma/paraganglioma
  syndrome 5, Mitochondrial complex II deficiency nuclear type 1, hereditary
  cancer-predisposing syndrome and "not provided" - it carries NO record against
  NDAXOA. The authors of that paper describe their case as the second reported
  cause of this phenotype, but the community classification has not followed
  them. This entry therefore treats the Zehavi case as a phenotype-expanding
  candidate for the entity rather than as an established second NDAXOA allele,
  and does not claim two confirmed alleles. Retrieved 2026-08-01 from the ClinVar
  esummary endpoint for variation 694327.

  Phenotype provenance beyond the abstracts. The OMIM clinical synopsis for
  619259, as exposed through MedGen concept C5543254, records the following HPO
  terms for this entity: HP:0001251 Ataxia, HP:0002066 Gait ataxia, HP:0002078
  Truncal ataxia, HP:0002070 Limb ataxia, HP:0001272 Cerebellar atrophy,
  HP:0002317 Unsteady gait, HP:0001288 Gait disturbance, HP:0002172 Postural
  instability, HP:0002312 Clumsiness, HP:0000648 Optic atrophy, HP:0000505
  Visual impairment, HP:0007994 Peripheral visual field loss, HP:0000639
  Nystagmus, HP:0000640 Gaze-evoked nystagmus, HP:0000651 Diplopia, HP:0003326
  Myalgia, HP:0003690 Limb muscle weakness, HP:0012378 Fatigue, HP:0001638
  Cardiomyopathy, HP:0001640 Cardiomegaly, HP:0001635 Congestive heart failure,
  HP:0000716 Depression, HP:0002315 Headache, HP:0002321 Vertigo, HP:0003487
  Babinski sign, HP:0012534 Dysesthesia, HP:0001686 Loss of voice, HP:0006568
  Increased hepatic glycogen content and HP:0003535 3-Methylglutaconic aciduria.
  Retrieved 2026-08-01 from
  https://eutils.ncbi.nlm.nih.gov/entrez/eutils/esummary.fcgi?db=medgen&retmode=json&id=1779901.
  Phenotypes taken from that synopsis and not present in a cached abstract carry
  no evidence item and say so in their own notes. Every HPO identifier reused
  from that list was independently re-verified with `runoak -i sqlite:obo:hp
  info` before being written into this file.

  Frequency bands are omitted from every phenotype, deliberately. The
  denominator across the entire published literature is two pedigrees and one
  singleton, and the two pedigrees are internally discordant (one lethal infant,
  one adult-onset optic neuropathy, same allele). Any FrequencyEnum band would
  be an artefact of a cohort of three to six people. Exact counts and their
  sources are recorded in each phenotype's notes instead, following
  `docs/frequency-evidence-guidelines.md`.

  Onset is not bound to a single enum. The entity name says "late-onset" and
  OMIM describes presentation in mid-adulthood, but the second pedigree includes
  an infant who died at seven months and a child with cardiomyopathy in early
  childhood, and the third reported individual had childhood-onset optic
  atrophy. Asserting an adult-onset category for the disorder would contradict
  half of the published cases. The tension is recorded as an open discussion
  rather than resolved by fiat.

  No GeneReviews chapter exists for this entity. Searched 2026-08-01: PubMed
  `SDHA AND (optic atrophy)` returned 10 records, of which only PMID:33471299
  and PMID:27683074 are human NDAXOA-relevant disease-gene reports and none is a
  GeneReviews chapter. The nearest GeneReviews coverage is the nuclear
  gene-encoded Leigh syndrome spectrum overview, which addresses the recessive
  SDHA disorder and not this one; it is already cited by the complex II
  deficiency entry and is not reused here.

  Structured-source evidence was unavailable in this checkout. `data/orphadata/`
  contains only a MANIFEST with no downloaded payload, so no Orphanet prevalence
  class could be consulted, and no ORPHA code appears among the MONDO xrefs for
  MONDO:0031006 in any case. No ClinGen Gene-Disease Validity assertion for the
  SDHA-NDAXOA pair could be retrieved. Both are recorded as gaps, not as
  assertions that no such classification exists.

  Repo observation, reported and not acted on per scope rules. The description
  in `kb/disorders/Mitochondrial_Complex_II_Deficiency_Nuclear_Type_1.yaml`
  states that MC2DN1 presents "less commonly with cardiomyopathy or a
  later-onset optic atrophy-ataxia-myopathy phenotype". The optic
  atrophy-ataxia-myopathy phenotype named there is the dominant entity curated
  in this file, which OAK shows is not a MONDO descendant of MONDO:0100294 and
  which arises from heterozygous rather than biallelic genotypes. That sentence
  therefore blurs the boundary this entry exists to draw. That file was NOT
  edited.
inheritance:
- name: Autosomal dominant inheritance
  description: >-
    A single heterozygous SDHA missense allele is sufficient to cause disease.
    Transmission was vertical in both reported pedigrees: two affected sisters
    in the first, and an affected father with two affected children in the
    second, whose report is titled for the dominant transmission it
    demonstrates. The third reported individual carried a de novo heterozygous
    allele with unaffected parents, which is the other signature of dominance.

    Dominance here is not equivalent to haploinsufficiency, and the distinction
    is load-bearing for the whole entry. Heterozygous SDHA loss-of-function
    alleles are carried, unaffected, by the parents of children with recessive
    complex II deficiency and by unaffected relatives in tumour-predisposition
    families, and at least one of them is not vanishingly rare: the recurrent
    nonsense allele c.91C>T (p.Arg31*) was found in 0.3% of healthy controls in
    the series cited below. If losing half the SDHA gene dosage were
    sufficient to cause optic atrophy and ataxia, those carriers would be
    affected and they are not. The dominant neurodegenerative alleles are
    instead specific missense substitutions at conserved arginines that are
    argued to produce a full-length subunit unable to carry its FAD cofactor.

    Expressivity within a single pedigree is extreme. In the second family the
    same heterozygous allele produced an index patient whose optic atrophy,
    polyneuropathy, psychiatric features and cardiomyopathy declared themselves
    over adult life, a son with cardiomyopathy from early childhood who did not
    develop optic atrophy until adulthood, and a daughter who died at seven
    months of cardiac insufficiency. Penetrance is recorded as UNKNOWN rather
    than complete: with two pedigrees in the world literature there is no
    denominator of obligate carriers from which to estimate it, and asserting
    COMPLETE would be an unsupported number dressed as an enum.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  penetrance: UNKNOWN
  expressivity: VARIABLE
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we report clinical and molecular investigations of three family members with a heterozygous mutation in the large flavoprotein subunit SDHA previously described to cause complex II deficiency."
    explanation: Three affected members of one family, each heterozygous for a single SDHA allele, is the direct demonstration of autosomal dominant transmission for this locus.
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is the second report presenting autosomal dominant inheritance of a SDHA mutation."
    explanation: The authors place their pedigree explicitly as the second dominant SDHA family, which both establishes the mode of inheritance and fixes the size of the published literature at two pedigrees.
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of the four nuclear encoded proteins composing complex II (SDHA, SDHB, SDHC, SDHD) and currently known assembly factors SDHAF1 and SDHAF2 mainly recessively inherited mutations have been described in SDHA, SDHB, SDHD, and SDHAF1 to be causative for mitochondrial disease phenotypes."
    explanation: Establishes that dominant transmission is the exception rather than the rule for complex II subunit mitochondrial disease, which is what makes this entity a separate entry rather than a variant presentation of the recessive disorder.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole exome sequencing of the patient and her unaffected parents identified the novel heterozygous de novo variant c.1984C > T"
    explanation: A de novo heterozygous allele in an affected child of unaffected parents is independent evidence that a single SDHA missense allele can be sufficient to cause the phenotype.
  - reference: PMID:33960148
    reference_title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Korpershoek et al. identified this variant in 0.3% of healthy control patients and 3% of sporadic tumor patients."
    explanation: >-
      Source for the statement that heterozygous SDHA loss-of-function alleles
      circulate in unaffected people: the recurrent p.Arg31* nonsense allele was
      seen in 0.3% of healthy controls. Marked PARTIAL because it establishes
      carrier frequency for one null allele in one series, not that dominance
      here is non-haploinsufficient; it removes the premise from the
      unsupported-assertion column, nothing more.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: NOT_YET_DOCUMENTED
  notes: >-
    Two pedigrees and one singleton constitute the entire published clinical
    experience of this entity at the time of curation: two elderly sisters
    (PMID:8967754, genotyped in PMID:10976639), three members of a second family
    (PMID:27683074), and one adolescent with a de novo allele (PMID:33471299,
    whose assignment to this entity is qualified in the entry notes).
    NOT_YET_DOCUMENTED is used rather than a qualitative ULTRA_RARE band because
    no prevalence or incidence estimate of any kind has been published for
    NDAXOA, MONDO:0031006 carries no Orphanet xref, and there is therefore no
    source to band. `rate_per_100000` is left empty because there is no basis
    for a number.

    The one quantitative anchor that does exist is upstream of the entity rather
    than about it: isolated complex II defects of any genotype account for about
    2% of all respiratory chain deficiency diagnoses, and NDAXOA is a small
    dominant subset of that 2%. No founder effect is asserted, and no ancestry
    is asserted either: none of the cited reports states the ancestry of any
    reported family. The only geographic information available is the location
    of the reporting centres (Newcastle upon Tyne, Bern, Afula), which is a fact
    about where the patients were investigated and must not be read as a
    statement about the patients' ancestry or as a population restriction.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Isolated defects of the mitochondrial respiratory complex II (succinate dehydrogenase, SDH) are rare, accounting for approximately 2% of all respiratory chain deficiency diagnoses."
    explanation: >-
      Quantifies the rarity of isolated complex II deficiency as a whole.
      Marked PARTIAL rather than SUPPORT because the 2% figure is for all
      isolated complex II deficiency of any genotype and inheritance, not for
      this dominant neurodegenerative entity, which is a small fraction of it.
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Respiratory chain defects involving complex II, which is entirely encoded by the nuclear genome, are comparatively rare."
    explanation: Independent statement of the rarity of complex II defects among respiratory chain disorders, from the report that first described this family.
pathophysiology:
- name: Heterozygous SDHA Missense Allele at a Conserved Arginine
  biological_scale: MOLECULAR
  description: >-
    The primary lesion is a single heterozygous missense change in SDHA at
    5p15.33, reported against transcript NM_004168.4. Both alleles associated
    with this phenotype substitute a cysteine for a conserved arginine: the
    founding-family allele c.1351C>T (p.Arg451Cys), historically written R408C in
    mature-protein numbering, and the de novo allele c.1984C>T (p.Arg662Cys).
    Neither is a truncating or splice-disrupting change, and neither is
    predicted to remove the protein. This matters because heterozygous SDHA
    truncating alleles are the recognised cause of a completely different
    disease, tumour predisposition, and do not cause neurodegeneration. The
    genotype-phenotype rule for SDHA is therefore not "how much protein is left"
    but "what kind of protein is made".
  genes:
  - preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The affected family members are now shown to carry a C-to-T transition in one allele of the nuclear gene encoding the flavoprotein subunit of complex II."
    explanation: Establishes the lesion as a heterozygous single-base change in the gene encoding the complex II flavoprotein subunit, that is, in SDHA, in the founding family.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole exome sequencing of the patient and her unaffected parents identified the novel heterozygous de novo variant c.1984C > T"
    explanation: Names the second candidate allele and the reference transcript against which SDHA variants are reported.
  notes: >-
    The p.Arg451Cys designation and its equivalence to the older R408C naming
    were verified against ClinVar VCV000449389 on 2026-08-01, not taken from
    memory; the provenance and the mature-versus-precursor numbering
    reconciliation are set out in the entry-level notes. Neither primary paper
    names its allele in the abstract, which is why this node carries no snippet
    for the specific substitutions.
  downstream:
  - target: Failure of Covalent FAD Attachment to the SDHA Flavoprotein
    description: >-
      Both substitutions are argued to disable the covalent flavinylation
      chemistry or the FAD-binding conformation of the subunit rather than to
      destabilise or truncate it.
- name: Failure of Covalent FAD Attachment to the SDHA Flavoprotein
  biological_scale: MOLECULAR
  description: >-
    SDHA is a flavoprotein whose catalytic activity depends absolutely on a FAD
    cofactor attached covalently to the subunit; without the covalent linkage
    the subunit cannot oxidise succinate. Two independent lines of evidence
    converge on this step for the two NDAXOA-associated alleles. For the
    founding-family allele, mutation of the equivalent base in the Escherichia
    coli orthologue produces an enzyme that is inactive and cannot bind FAD
    covalently. For the de novo allele, structural modelling of p.Arg662Cys
    predicts that it compromises the integrity of the FAD-binding region at the
    C-terminus and so impairs FAD binding.

    That convergence is not a coincidence of two arginines. Yeast mutagenesis of
    Sdh1 shows that a pair of spatially close C-terminal arginine residues,
    physically distant from the FAD-binding site itself, are required for
    covalent flavinylation and for assembly of the tetramer, and that FAD
    binding rather than covalent attachment is what stabilises the Sdh1
    conformation for association with its partners. Flavinylation is also an
    assisted reaction: the dedicated factor SDHAF2/SDH5 binds the catalytic
    subunit and is required for its flavination. The step is therefore a
    discrete, mutable chemistry, not a passive property of the folded protein,
    and it is a plausible point at which a single conservative-looking arginine
    substitution can abolish catalysis without abolishing expression.
  genes:
  - preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  molecular_functions:
  - preferred_term: flavin adenine dinucleotide binding
    modifier: DECREASED
    term:
      id: GO:0050660
      label: flavin adenine dinucleotide binding
  biological_processes:
  - preferred_term: covalent flavinylation of the SDHA subunit
    modifier: DECREASED
    term:
      id: GO:0018293
      label: protein-FAD linkage
  chemical_entities:
  - preferred_term: FAD
    term:
      id: CHEBI:16238
      label: FAD
  cellular_components:
  - preferred_term: mitochondrial inner membrane
    term:
      id: GO:0005743
      label: mitochondrial inner membrane
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Mutation of the equivalent base in Escherichia coli generates an inactive enzyme unable to bind flavin adenine dinucleotide covalently."
    explanation: Direct experimental demonstration, in the bacterial orthologue, that the founding-family base change abolishes covalent FAD binding and enzymatic activity. Classified IN_VITRO because the experiment is a bacterial enzymology result, not a human observation.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: PARTIAL
    evidence_source: COMPUTATIONAL
    snippet: "suggested that this substitution will compromise the structural integrity of the FAD-binding protein at the C-terminus that will ultimately impair the FAD binding to SDHA, thus decreasing the entire CII activity."
    explanation: >-
      Predicts the same lesion, impaired FAD binding, for the second allele.
      Marked PARTIAL and COMPUTATIONAL because it is in-silico structural
      modelling, not a measured flavinylation assay, and the authors write
      "suggested" rather than asserting the mechanism.
  - reference: PMID:23043141
    reference_title: "Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We show presently that flavinylation of the Sdh1 subunit of succinate dehydrogenase is dependent on a set of two spatially close C-terminal arginine residues that are distant from the FAD binding site."
    explanation: Establishes in yeast that specific C-terminal arginines of the flavoprotein subunit, remote from the cofactor pocket, are required for covalent flavinylation, which is the general mechanism into which an arginine-to-cysteine substitution in this region fits.
  - reference: PMID:23043141
    reference_title: "Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "SDH assembly appears to require FAD binding but not necessarily covalent FAD attachment."
    explanation: >-
      The key result for the dominant-negative model curated in the next node:
      a subunit that binds FAD non-covalently can still be assembled into the
      complex, so an allele that blocks only the covalent step can deliver an
      inert subunit into an otherwise intact complex II.
  - reference: PMID:19628817
    reference_title: "SDH5, a gene required for flavination of succinate dehydrogenase, is mutated in paraganglioma."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Sdh5 is required for SDH-dependent respiration and for Sdh1 flavination (incorporation of the flavin adenine dinucleotide cofactor)."
    explanation: Establishes flavinylation of the catalytic subunit as a dedicated, factor-assisted step in complex II biogenesis rather than a spontaneous property of the folded protein, which is why it is a discrete point of failure.
  - reference: PMID:39321216
    reference_title: "A Novel Human SDHA-Knockout Cell Line Model for the Functional Analysis of Clinically Relevant SDHA Variants."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "SDHA variants were introduced into a clonal SDHA-knockout cell line via Bxb1-mediated recombination. SDH activity and SDHA abundance were determined for each variant"
    explanation: >-
      The human assay system that this node's remaining gap is defined against.
      The snippet quotes the assay design, which is all the cached abstract
      reports; the per-variant results for the two NDAXOA codons (both
      essentially inactive) were read from the full text of PMC11611653
      (Figures 2 and 5, Table 1; retrieved 2026-08-01) and are not verifiable
      from references_cache/PMID_39321216.md, which is abstract-only. Marked
      PARTIAL because activity and abundance are not flavinylation: the study
      attributes the Arg451 defect to a presumed failure of covalent FAD
      attachment rather than measuring the linkage.
  notes: >-
    Confidence is PROVISIONAL, not ESTABLISHED, and the reason is specific and
    narrower than an earlier revision of this note claimed. The flavinylation
    defect has been demonstrated for the founding-family allele only in the E.
    coli orthologue at the equivalent base, and for the de novo allele only in
    silico. The yeast work cited here concerns different residues in a different
    organism and is offered as mechanistic plausibility for the class of lesion,
    not as evidence about these two alleles.

    Both human alleles have, however, been assayed in a human system.
    PMID:39321216 expressed SDHA missense variants in a clonal human
    SDHA-knockout cell line and measured SDH activity and SDHA abundance for
    each; both p.Arg451Cys and p.Arg662Cys were in that panel and both scored as
    essentially inactive, with p.Arg451Cys additionally retaining high SDHA
    protein (per-variant values read from Figures 2 and 5 and Table 1 of
    PMC11611653, retrieved 2026-08-01; only the abstract is in
    references_cache, so those per-variant numbers are recorded here as a
    sourced note rather than as evidence snippets). What that study did not do
    is measure the covalent FAD linkage itself: it infers a flavinylation defect
    for Arg451 from the residue's structural role, describing the affected
    variants as "presumed to affect flavinylation". So the remaining gap is
    specific — nobody has measured covalent FAD incorporation into human SDHA
    carrying either substitution — and the claim that these alleles fail to
    flavinylate is still inferred rather than directly measured, but it is now
    inferred from a human loss-of-activity measurement rather than from the
    bacterial orthologue alone.
  downstream:
  - target: Catalytically Inert SDHA Subunit Competing for Complex II Assembly
    description: >-
      A subunit that binds FAD without the covalent linkage can still enter the
      tetramer, so the mutant polypeptide is not simply removed from the pool.
  - target: Approximately Half-Normal Succinate Dehydrogenase Activity
    description: >-
      Whatever the assembly route, the measured net result in patient tissue is
      a partial rather than complete loss of enzyme activity.
- name: Catalytically Inert SDHA Subunit Competing for Complex II Assembly
  biological_scale: MOLECULAR
  description: >-
    This is the node that explains why the disease is dominant. Complex II is a
    heterotetramer containing exactly one copy of the flavoprotein subunit
    alongside the iron-sulfur subunit and the two transmembrane anchors, so
    every assembled complex commits to one SDHA molecule and there is no second
    copy to compensate. If the mutant subunit is still expressed and still
    competent to bind FAD non-covalently, it can be incorporated into complexes
    that are then structurally complete but catalytically dead, and it competes
    with the wild-type subunit for the available assembly slots. On that model a
    heterozygote loses close to half of total succinate dehydrogenase activity
    while retaining a full complement of assembled complex, which is precisely
    the biochemical picture reported in the founding family: an approximately
    50% enzyme deficit accompanied by an apparent overexpression of the 70-kDa
    flavoprotein subunit whose membrane-associated amount was nonetheless
    normal.

    The alternative reading, simple haploinsufficiency, would predict the same
    50% activity but is hard to reconcile with the population genetics.
    Heterozygous SDHA loss-of-function alleles are carried without
    neurodegeneration by the parents of children with recessive complex II
    deficiency and by members of tumour-predisposition families. The
    dominant-negative model accounts for that asymmetry; haploinsufficiency does
    not.
  genes:
  - preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  protein_complexes:
  - preferred_term: respiratory chain complex II
    modifier: ABNORMAL
    term:
      id: GO:0045273
      label: respiratory chain complex II (succinate dehydrogenase)
  biological_processes:
  - preferred_term: mitochondrial respiratory chain complex II assembly
    modifier: ABNORMAL
    term:
      id: GO:0034553
      label: mitochondrial respiratory chain complex II assembly
  mechanism_confidence: HYPOTHETICAL
  evidence:
  - reference: PMID:15989954
    reference_title: "Crystal structure of mitochondrial respiratory membrane protein complex II."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Complex II is comprised of two hydrophilic proteins, flavoprotein (Fp) and iron-sulfur protein (Ip), and two transmembrane proteins (CybL and CybS), as well as prosthetic groups required for electron transfer from succinate to ubiquinone."
    explanation: "Fixes the stoichiometry that makes the dominant-negative argument possible: one flavoprotein subunit per complex, with no redundant copy to compensate for an inert one."
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "There was an overexpression of the 70-kd flavoprotein subunit in muscle mitochondria from both patients, although we showed that this subunit is present in normal amounts in mitochondrial membranes."
    explanation: >-
      Patient muscle shows the mutant flavoprotein subunit present, not lost,
      with a normal amount in the membrane fraction despite half-normal enzyme
      activity. Marked PARTIAL because the observation is compatible with the
      dominant-negative model but does not by itself demonstrate that mutant and
      wild-type subunits compete for assembly.
  - reference: PMID:39321216
    reference_title: "A Novel Human SDHA-Knockout Cell Line Model for the Functional Analysis of Clinically Relevant SDHA Variants."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "SDHA variants were introduced into a clonal SDHA-knockout cell line via Bxb1-mediated recombination. SDH activity and SDHA abundance were determined for each variant"
    explanation: >-
      Supplies the precondition for the competition model in a human cell rather
      than in patient muscle: p.Arg451Cys is amorphic yet retains high SDHA
      protein, so the mutant polypeptide is stable and not cleared. That
      per-variant reading comes from the full text of PMC11611653 (Figure 2;
      retrieved 2026-08-01), not from the cached abstract, which reports only
      the assay design and the aggregate result — the snippet is therefore
      scoped to the design. Marked PARTIAL, and it does not settle the node:
      the assay is run in an SDHA-knockout background with no wild-type subunit
      present, so it cannot show competition for assembly slots.
  notes: >-
    Marked HYPOTHETICAL. Nothing yet distinguishes dominant-negative subunit
    competition from simple haploinsufficiency for either NDAXOA allele in a
    heterozygous setting. The nearest datum is PMID:39321216, which measured
    p.Arg451Cys in a human SDHA-knockout background and found it amorphic while
    retaining high SDHA protein abundance (read from Figure 2 of the PMC11611653
    full text, retrieved 2026-08-01; references_cache/PMID_39321216.md is
    abstract-only and names no individual variant, so this is a full-text
    derivation and not a snippet-verifiable claim). That establishes the mutant
    subunit is stable rather than
    degraded, which is a precondition for competition, but it cannot demonstrate
    competition: the assay contains no wild-type subunit for the mutant to
    compete with. What is still absent is allele-specific quantification of
    assembled complex in a heterozygote, any isotopic or tag-based measurement
    of the mutant-to-wild-type ratio within assembled complex II, and a
    heterozygous knock-in model. The node is retained because the discrimination
    is clinically consequential, not because it is settled; the reasoning
    against haploinsufficiency rests on the observation that heterozygous SDHA
    null carriers are neurologically well, which is an inference from the
    recessive and tumour literature rather than a controlled comparison. The
    corresponding open question is recorded as a discussion.
  downstream:
  - target: Approximately Half-Normal Succinate Dehydrogenase Activity
    description: >-
      Whether by competition or by dosage, the measured phenotype in patient
      tissue is a partial enzyme deficit.
- name: Approximately Half-Normal Succinate Dehydrogenase Activity
  biological_scale: CELLULAR
  description: >-
    The measured biochemical phenotype is a partial, roughly 50% deficiency of
    complex II and succinate dehydrogenase activity, reported concordantly in
    muscle and platelet mitochondria in the founding family and in fibroblasts
    and lymphocytes in the de novo case. This is the quantitative signature that
    separates NDAXOA from the recessive disorder, where biallelic
    loss-of-function genotypes drive activity far lower and produce infantile
    Leigh syndrome. A half-normal enzyme is compatible with decades of ordinary
    life and only fails where the metabolic margin is thinnest, which is the
    reason the disease is late in onset and restricted in the tissues it
    damages.

    Tissue expression of the defect is genuinely inconsistent between the two
    reports and this entry does not smooth that over. In the founding family the
    enzyme defect was explicitly not expressed in cultured skin fibroblasts or
    immortalised lymphocytes, so a normal fibroblast assay did not exclude the
    diagnosis; in the de novo case the defect was demonstrable in exactly those
    two cell types. The discrepancy has direct diagnostic consequences and is
    recorded as an open discussion.
  biological_processes:
  - preferred_term: tricarboxylic acid cycle
    modifier: DECREASED
    term:
      id: GO:0006099
      label: tricarboxylic acid cycle
  - preferred_term: electron transport chain
    modifier: DECREASED
    term:
      id: GO:0022900
      label: electron transport chain
  molecular_functions:
  - preferred_term: succinate dehydrogenase activity
    modifier: DECREASED
    term:
      id: GO:0000104
      label: succinate dehydrogenase activity
  chemical_entities:
  - preferred_term: succinate
    modifier: INCREASED
    term:
      id: CHEBI:30031
      label: succinate(2-)
  - preferred_term: fumarate
    modifier: DECREASED
    term:
      id: CHEBI:29806
      label: fumarate(2-)
  evidence:
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."
    explanation: The primary quantitative measurement of the enzyme deficit in patient tissue, and the origin of the approximately 50% figure used throughout this entry.
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Compatible with these findings, our patients have an approximate 50% decrease in complex II and succinate dehydrogenase activity."
    explanation: Ties the measured half-normal activity in patients directly to the flavinylation-disabling genotype, in the same family.
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The enzyme defect was not expressed in cultured skin fibroblasts or immortalized lymphocytes."
    explanation: Documents the tissue-restricted expression of the biochemical defect in the founding family, which is the diagnostic pitfall recorded in the diagnosis section and in an open discussion.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Biochemical assessment of CII in the patient's derived fibroblasts and lymphocytes displayed considerably decreased CII residual activity compared with normal controls, when normalized to the integral mitochondrial enzyme citrate synthase."
    explanation: >-
      Demonstrates the enzyme deficit in fibroblasts and lymphocytes. Marked
      PARTIAL because it directly contradicts the tissue-expression pattern
      reported in the founding family, so the two observations cannot both be
      general.
  notes: >-
    The succinate and fumarate bindings on this node are inferred from the
    position of the enzymatic block, not measured. No study of an NDAXOA patient
    has reported tissue or plasma succinate, and the two reports that measured
    anything metabolic reported enzyme activities rather than metabolite pools.
    The bindings are retained because the direction of the change is not in
    doubt for a blocked succinate dehydrogenase, but no evidence item is
    attached to them and no claim about the magnitude of succinate accumulation
    is made here; the tumour-relevant consequences of succinate accumulation are
    handled separately, and negatively, in the divergence node below.
  downstream:
  - target: Bioenergetic Failure in Cells With Low Metabolic Reserve
    description: >-
      A partial and permanent shortfall in oxidative ATP production is the
      common upstream state for every tissue-level consequence below.
  - target: Divergence From the SDHx Tumour-Predisposition Arm
    description: >-
      The same enzymatic block that produces neurodegeneration here produces
      tumours in other SDHA genotypes; the branch point is curated explicitly.
- name: Bioenergetic Failure in Cells With Low Metabolic Reserve
  biological_scale: CELLULAR
  conforms_to: "cerebellar_purkinje_degeneration#Cerebellar Neuron Insult"
  description: >-
    A permanent halving of succinate dehydrogenase activity is a mild insult in
    most cells and a severe one in a few. Complex II sits at the junction of the
    tricarboxylic acid cycle and the electron transport chain, so its
    impairment restricts both the oxidative arm of intermediary metabolism and
    the electron supply to the quinone pool at once. Cells that are long-lived,
    post-mitotic, unable to dilute damaged mitochondria by division, and running
    near their ATP ceiling therefore carry the cost: retinal ganglion cells,
    cerebellar neurons, skeletal myofibres and cardiomyocytes. Cells that turn
    over, or that can lean on glycolysis, are spared, which is why a systemic
    enzyme defect produces a strikingly selective clinical picture and why
    cultured fibroblasts may look normal.
  biological_processes:
  - preferred_term: oxidative phosphorylation
    modifier: DECREASED
    term:
      id: GO:0006119
      label: oxidative phosphorylation
  cellular_components:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  evidence:
  - reference: PMID:15989954
    reference_title: "Crystal structure of mitochondrial respiratory membrane protein complex II."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The mitochondrial respiratory Complex II or succinate:ubiquinone oxidoreductase (SQR) is an integral membrane protein complex in both the tricarboxylic acid cycle and aerobic respiration."
    explanation: Establishes the dual metabolic membership of complex II, which is why a single enzyme lesion restricts both the TCA cycle and respiration rather than only one of them.
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These results suggest that genetic defects of nuclear-encoded subunits of the mitochondrial respiratory chain can result in late-onset neurodegenerative disease."
    explanation: The authors' own framing of the causal chain from a partial nuclear-encoded respiratory chain defect to late-onset neurodegeneration.
  notes: >-
    The conformance to `cerebellar_purkinje_degeneration#Cerebellar Neuron
    Insult` is asserted at the level of the insult, not at the level of Purkinje
    cell pathology. This node deliberately does not bind a Purkinje cell type,
    because no neuropathological material from an NDAXOA patient has ever been
    examined; the cerebellar involvement in this disorder is known from clinical
    ataxia and from the cerebellar atrophy in the OMIM clinical synopsis, not
    from histology. Binding CL:0000121 here would assert a cellular finding that
    nobody has observed in this disease.
  downstream:
  - target: Retinal Ganglion Cell and Optic Nerve Degeneration
  - target: Cerebellar Degeneration and Ataxia
  - target: Striated Muscle and Myocardial Energy Failure
- name: Retinal Ganglion Cell and Optic Nerve Degeneration
  biological_scale: TISSUE
  description: >-
    Optic atrophy is the feature that names the disorder and the one that is
    present in every reported family. Its anatomical basis is the particular
    energetic geometry of the retinal ganglion cell: the cell body sits in the
    retina and generates the mitochondria, which are then transported down an
    axon that is unmyelinated and mitochondria-rich anterior to the lamina
    cribrosa and myelinated and mitochondria-poor behind it. The prelaminar
    segment therefore carries an unusually high, spatially concentrated demand
    for oxidative ATP with no local capacity to make more. Any systemic
    restriction of oxidative phosphorylation is felt there first, which is the
    accepted explanation for why chemically unrelated mitochondrial lesions,
    from mitochondrial DNA complex I mutations in Leber hereditary optic
    neuropathy to nuclear OPA1 mutations in dominant optic atrophy, all converge
    on the same optic neuropathy. Complex II deficiency joins that convergence
    from a different enzymatic starting point.
  locations:
  - preferred_term: optic nerve
    term:
      id: UBERON:0000941
      label: cranial nerve II
  cell_types:
  - preferred_term: retinal ganglion cell
    term:
      id: CL:0000740
      label: retinal ganglion cell
  evidence:
  - reference: PMID:14766317
    reference_title: "Mitochondrial dysfunction as a cause of optic neuropathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed."
    explanation: States the selective energy dependence of retinal ganglion cells that makes them the first tissue to fail under a systemic oxidative-phosphorylation deficit.
  - reference: PMID:14766317
    reference_title: "Mitochondrial dysfunction as a cause of optic neuropathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly."
    explanation: Gives the anatomical basis of that selective vulnerability, localising the metabolically critical segment to the prelaminar optic nerve.
  - reference: PMID:14766317
    reference_title: "Mitochondrial dysfunction as a cause of optic neuropathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria."
    explanation: Generalises the mechanism beyond any single respiratory chain complex, which is what licenses applying it to a complex II lesion.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our study emphasizes the role of certain heterozygous SDHA mutations in a distinct clinical phenotype dominated by optic atrophy and neurological impairment."
    explanation: States that heterozygous SDHA disease is dominated clinically by the optic neuropathy, which is the tissue-level endpoint modelled by this node.
  notes: >-
    The retinal ganglion cell binding is an anatomical and mechanistic
    inference, not an observation in this disease. No retina or optic nerve from
    an NDAXOA patient has been examined histologically, and no
    optical-coherence-tomography ganglion-cell-layer measurement has been
    published. The cell type is bound because retinal ganglion cell axons
    constitute the optic nerve, so optic atrophy localises the lesion to that
    population by anatomy, and because the cited review establishes that
    population as the canonical target of mitochondrial energy failure.
    PMID:14766317 predates the identification of SDHA disease and says nothing
    about complex II; it is classified OTHER and cited for the general
    vulnerability mechanism only.
- name: Cerebellar Degeneration and Ataxia
  biological_scale: TISSUE
  conforms_to: "cerebellar_purkinje_degeneration#Cerebellar Ataxia"
  description: >-
    Progressive cerebellar and gait ataxia is the second cardinal feature and
    was present in both members of the founding sibship. Clinically it takes the
    form recognisable across the hereditary ataxias: unsteady, wide-based gait
    with postural instability and falls, limb and truncal incoordination,
    gaze-evoked nystagmus and, in the OMIM clinical synopsis for this entity,
    cerebellar atrophy on imaging. Mechanistically it is treated here as the
    cerebellar instance of the same bioenergetic failure that produces the optic
    neuropathy, and the entry conforms it to the shared cerebellar ataxia module
    at the clinical-consequence node rather than at the Purkinje-degeneration
    node, because the intermediate cellular pathology has not been observed in
    this disorder.
  locations:
  - preferred_term: cerebellum
    term:
      id: UBERON:0002037
      label: cerebellum
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."
    explanation: Establishes progressive ataxia as a defining component of the phenotype in the founding family.
  - reference: PMID:33960148
    reference_title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "While cerebellar signs have been reported in monoallelic SDHA variant cases"
    explanation: >-
      An independent group's statement that cerebellar signs are an established
      feature of monoallelic SDHA disease. Marked PARTIAL because the paper's own
      patient is biallelic and the sentence is a literature statement about other
      families rather than a new observation in this entity.
  notes: >-
    Cerebellar atrophy is recorded in the OMIM clinical synopsis for 619259
    (HP:0001272) but does not appear in any cached abstract for this disorder,
    so the imaging correlate is carried as a phenotype with note-level
    provenance rather than as an evidence-backed claim on this node. No
    autopsy, and therefore no Purkinje-cell count, exists for any NDAXOA
    patient; the conformance to the cerebellar module is deliberately made at
    its `Cerebellar Ataxia` consequence node for that reason.
- name: Striated Muscle and Myocardial Energy Failure
  biological_scale: TISSUE
  description: >-
    Skeletal muscle and myocardium are the third and fourth tissues that a
    permanent half-normal oxidative capacity cannot support. In the founding
    family the muscle involvement took the form of a myopathy with myalgia and
    limb weakness in older adults, and muscle mitochondria were the tissue in
    which the enzyme deficit was demonstrable. In the second pedigree the
    cardiac involvement was earlier and far more severe than the neurological
    involvement: both children presented with cardiomyopathy in early childhood
    and one died of cardiac insufficiency at seven months, while their father's
    cardiomyopathy accompanied an adult neurological presentation. Cardiac
    disease is therefore not a late complication of this disorder but, in some
    carriers, its first and lethal manifestation.

    The accompanying methylglutaconic aciduria in the two affected children is
    the expected urinary signature of a mitochondrial energy defect rather than
    a specific marker of complex II, and it is what should prompt a
    mitochondrial workup in a child with unexplained cardiomyopathy.
  locations:
  - preferred_term: skeletal muscle tissue
    term:
      id: UBERON:0001134
      label: skeletal muscle tissue
  - preferred_term: heart
    term:
      id: UBERON:0000948
      label: heart
  cell_types:
  - preferred_term: cardiac muscle cell
    term:
      id: CL:0000746
      label: cardiac muscle cell
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."
    explanation: Documents cardiomyopathy with a mitochondrial urinary organic-acid signature as an early manifestation in two carriers of the same dominant allele.
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The daughter deceased at the age of 7 months due to cardiac insufficiency."
    explanation: Establishes that the cardiac arm of this dominant disorder can be lethal in infancy, which is the strongest single argument against treating the entity as uniformly late in onset.
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."
    explanation: Muscle mitochondria are the tissue in which the enzyme deficit was directly measured, tying the myopathy to a demonstrated local biochemical defect.
- name: Divergence From the SDHx Tumour-Predisposition Arm
  biological_scale: MOLECULAR
  description: >-
    Loss of succinate dehydrogenase has a second, entirely different
    consequence: succinate accumulates and acts as an oncometabolite, inhibiting
    alpha-ketoglutarate-dependent dioxygenases, driving a pseudohypoxic
    transcriptional response and remodelling the epigenome. That is the route by
    which heterozygous SDHx loss-of-function alleles, with somatic loss of the
    second allele, cause paraganglioma, phaeochromocytoma and gastrointestinal
    stromal tumour. This node exists to state explicitly that NDAXOA is not
    known to travel that route, and to keep the two arms from being conflated.

    The empirical position, as of this curation, is that mitochondrial-disease
    SDHx genotypes and tumour SDHx genotypes have historically been reported as
    separate sets, with only isolated alleles crossing over. No tumour has been
    reported in either NDAXOA pedigree. Whether that reflects a real mechanistic
    divergence, in which a missense allele that blocks flavinylation without
    destroying the tumour-suppressor function of the locus fails to initiate
    tumorigenesis, or merely reflects the tiny number of carriers ever followed,
    is unresolved and is recorded as an open discussion rather than asserted in
    either direction. The practical consequence is that clinicians have to make
    a surveillance decision on absent evidence.
  genes:
  - preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  chemical_entities:
  - preferred_term: succinate
    modifier: INCREASED
    term:
      id: CHEBI:30031
      label: succinate(2-)
  - preferred_term: 2-oxoglutarate
    term:
      id: CHEBI:16810
      label: 2-oxoglutarate(2-)
  biological_processes:
  - preferred_term: hypoxia-inducible factor-1alpha signaling pathway
    modifier: INCREASED
    term:
      id: GO:0097411
      label: hypoxia-inducible factor-1alpha signaling pathway
  mechanism_confidence: HYPOTHETICAL
  evidence:
  - reference: PMID:33112834
    reference_title: "Epigenetic and metabolic reprogramming of SDH-deficient paragangliomas."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "SDHx mutations lead to the accumulation of succinate, which acts as an oncometabolite by inhibiting iron(II) and alpha-ketoglutarate-dependent dioxygenases thereby regulating the cell's hypoxic response and epigenetic processes."
    explanation: Describes the oncometabolite route that SDHx tumour alleles take, which is the arm this node exists to distinguish from the neurodegenerative arm.
  - reference: PMID:24781757
    reference_title: "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Until now, mutations in SDHx associated with mitochondrial disease have not been reported in association with hereditary tumors and vice versa."
    explanation: States the historical separation of the mitochondrial-disease and tumour allele sets, which is the empirical basis for keeping the two arms as distinct nodes and distinct disease entries.
  - reference: PMID:24781757
    reference_title: "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Interestingly, SDHx genes also function as tumor suppressor genes in hereditary paragangliomas, pheochromocytomas, and gastrointestinal stromal tumors."
    explanation: >-
      Establishes the tumour-suppressor role of the same genes. Marked PARTIAL
      because it is a statement about SDHx as a family and about
      loss-of-function alleles, not about the missense alleles that cause this
      entity.
  notes: >-
    Marked HYPOTHETICAL because the direction being asserted is a negative and
    the evidence for it is absence of reported tumours in six or so carriers
    followed for a lifetime, which is not a reassuring denominator. Succinate
    has never been measured in an NDAXOA patient. The node is retained rather
    than deleted because leaving it out would allow a reader to import the
    tumour mechanism into this entity by association with the gene symbol, which
    is exactly the error this entry is scoped to prevent.
genetic:
- name: SDHA
  association: >-
    Heterozygous missense substitutions at conserved arginine residues, argued to
    disable covalent flavinylation or FAD binding of the flavoprotein subunit;
    the recurrent founding-family allele is c.1351C>T (p.Arg451Cys), historically
    designated R408C.
  relationship_type: CAUSATIVE
  gene_term:
    preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  inheritance:
  - name: Autosomal dominant inheritance
    inheritance_term:
      preferred_term: Autosomal dominant inheritance
      term:
        id: HP:0000006
        label: Autosomal dominant inheritance
  notes: >-
    Nomenclature and locus. hgnc:10680 is SDHA, succinate dehydrogenase complex
    flavoprotein subunit A, at 5p15.33 (chromosome and cytogenetic band
    confirmed from the MedGen record for C5543254, retrieved 2026-08-01). The
    reference transcript used in the modern reports is NM_004168.4, protein
    NP_004159.2. The older complex II literature refers to the gene product as
    "the 70-kDa flavoprotein subunit", "Fp", or "SdhA", and numbers residues in
    the mature protein after cleavage of the mitochondrial targeting
    presequence; that is the source of the R408C versus p.Arg451Cys discrepancy
    reconciled in the entry-level notes.

    Allelic architecture, stated at the confidence the evidence supports. One
    allele, c.1351C>T (p.Arg451Cys), is established for this entity: it is the
    allele of the Taylor/Birch-Machin family and of the Courage family, and it
    is the only variant with a ClinVar record classified against
    "Neurodegeneration with ataxia and late-onset optic atrophy". A second
    allele, c.1984C>T (p.Arg662Cys), was reported de novo in an individual with
    childhood-onset optic atrophy and cognitive impairment whose authors
    describe it as the second cause of this phenotype, but ClinVar carries no
    NDAXOA record for it and its aggregate classification is conflicting. This
    entry therefore recognises one established and one candidate allele, not
    two established alleles. Both have since been measured in a human
    SDHA-knockout cell line (PMID:39321216) and both abolish SDH activity —
    per-variant results read from the full text of PMC11611653 (Figures 2 and 5,
    Table 1; retrieved 2026-08-01), since the cached abstract for that PMID
    reports only aggregate counts and names neither variant. That strengthens
    the case that each is a damaging allele but does not change the
    candidate/established split, because that assay assigns no phenotype.

    What the genotype does NOT predict. Because four MONDO disease terms map to
    this one gene, an SDHA genotype alone does not assign a diagnosis. Zygosity
    separates the recessive infantile disorder from the three dominant ones;
    variant class separates the truncating tumour-predisposition alleles from
    the missense neurodegenerative ones; and, at codon 451 specifically, the
    identity of the substituting residue separates neurodegeneration
    (p.Arg451Cys) from paraganglioma (p.Arg451His) from infantile cardiomyopathy
    (p.Arg451Ser), per the ClinVar submitter evidence recorded in the
    entry-level notes. A laboratory reporting an SDHA variant must therefore
    report the phenotype question as open.

    Not asserted, deliberately. gnomAD constraint metrics (pLI, o/e LoF,
    missense Z) are not recorded anywhere in this entry because no source
    consulted here provides them and they were not retrieved live. One ClinVar
    submitter states that c.1351C>T is absent from the current gnomAD release;
    that single submitter statement is recorded here rather than converted into
    a population frequency claim. The one carrier-frequency figure used anywhere
    in this entry (0.3% of healthy controls, in the inheritance section) is for
    a different, recurrent SDHA nonsense allele and is cited there to its
    source; it is not a frequency for either NDAXOA allele. No modifier locus, no founder allele, no
    somatic mechanism and no genotype-based severity score has been established
    for this disorder.
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The affected family members are now shown to carry a C-to-T transition in one allele of the nuclear gene encoding the flavoprotein subunit of complex II."
    explanation: Establishes SDHA as the causal gene for this phenotype and the lesion as heterozygous.
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we report clinical and molecular investigations of three family members with a heterozygous mutation in the large flavoprotein subunit SDHA previously described to cause complex II deficiency."
    explanation: Independent replication of the same gene, the same zygosity and, as the phrase "previously described" indicates, the same allele in a second family.
variants:
- name: NM_004168.4(SDHA):c.1351C>T (p.Arg451Cys)
  description: >-
    The established NDAXOA allele, historically designated R408C in
    mature-protein numbering. Identified in the family originally reported with
    biochemical complex II deficiency and subsequently found in a second,
    unrelated dominant pedigree. Functional support comes from the bacterial
    orthologue, where mutation of the equivalent base yields an enzyme unable to
    bind FAD covalently, and from a human SDHA-knockout cell line, in which this
    substitution abolishes SDH activity while leaving SDHA protein abundant
    (the per-variant result is in the PMC11611653 full text, Figure 2; the
    cached abstract for PMID:39321216 does not name this variant).
  gene:
    preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  type: missense variant
  clinical_significance: PATHOGENIC
  identifiers:
  - clinvar:VCV000449389
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The affected family members are now shown to carry a C-to-T transition in one allele of the nuclear gene encoding the flavoprotein subunit of complex II."
    explanation: The report identifying this allele in the founding family; the specific nucleotide and protein designations were verified separately against ClinVar and are recorded in the entry notes rather than quoted.
  - reference: PMID:39321216
    reference_title: "A Novel Human SDHA-Knockout Cell Line Model for the Functional Analysis of Clinically Relevant SDHA Variants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "SDHA variants were introduced into a clonal SDHA-knockout cell line via Bxb1-mediated recombination. SDH activity and SDHA abundance were determined for each variant"
    explanation: >-
      Human-cell functional characterisation of this substitution. In that
      SDHA-null background p.Arg451Cys scored as amorphic while retaining high
      SDHA protein, i.e. a stable but catalytically dead subunit, which the
      authors attribute to Arg451's role in covalent flavinylation. Per-variant
      values are in Figure 2 of PMC11611653 and not in the cached abstract, so
      the snippet quotes only the assay design; the numbers are recorded in the
      pathophysiology node notes with their retrieval provenance.
- name: NM_004168.4(SDHA):c.1984C>T (p.Arg662Cys)
  description: >-
    Candidate second allele, reported de novo in an individual with global
    developmental delay, intellectual disability and childhood-onset progressive
    bilateral optic atrophy, with decreased complex II activity in fibroblasts
    and lymphocytes and in-silico evidence of impaired C-terminal FAD binding.
    It has since also been assayed in a human SDHA-knockout cell line, where it
    scored as essentially inactive (per-variant result from the PMC11611653
    full text, Table 1; the cached abstract for PMID:39321216 reports only that
    22 variants of uncertain significance were assayed and does not name this
    one); that study frames the result as functional evidence for cancer
    pathogenicity, so it speaks to the allele's effect on enzyme function and
    not to which SDHA phenotype it causes. Recorded as a candidate rather than
    an established NDAXOA allele because ClinVar holds no record for it against
    this condition and its aggregate classification is conflicting.
  gene:
    preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  type: missense variant
  clinical_significance: UNCERTAIN_SIGNIFICANCE
  identifiers:
  - clinvar:VCV000694327
  evidence:
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is the second mutation that has been reported to cause this phenotype."
    explanation: The authors' own positioning of this allele as the second cause of the heterozygous SDHA optic-atrophy phenotype; the entry records the discordant ClinVar classification alongside it rather than adopting the claim unqualified.
  - reference: PMID:39321216
    reference_title: "A Novel Human SDHA-Knockout Cell Line Model for the Functional Analysis of Clinically Relevant SDHA Variants."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "we obtained functional evidence for clinical variant interpretation from 21 of 22 assayed variants of uncertain significance"
    explanation: >-
      p.Arg662Cys is one of the 22 variants of uncertain significance assayed in
      that human SDHA-knockout panel, and its measured loss of SDH activity
      earned it strong functional evidence toward reclassification (Table 1 of
      PMC11611653, retrieved 2026-08-01; the cached abstract carries the cohort
      statement only). Marked PARTIAL because that reclassification is scored
      against cancer pathogenicity, not against NDAXOA, so it raises confidence
      that the allele is damaging without establishing the phenotype.
phenotypes:
- name: Optic atrophy
  category: Eye
  diagnostic: true
  description: >-
    Bilateral, progressive optic atrophy is the feature that names the disorder
    and the only one reported in every affected individual across all three
    published reports. It is described as late-onset in the founding family, as
    developing in adulthood in the adult son of the second family whose cardiac
    disease had been present since early childhood, and as childhood-onset in
    the de novo case. The lesion is an optic neuropathy of retinal ganglion cell
    origin, not a retinal dystrophy, and the practical consequence is
    progressive loss of acuity and of peripheral field.
  phenotype_term:
    preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
    clinical_course: PROGRESSIVE
  notes: >-
    Present in the two sisters of the founding family, in the index patient and
    the adult son of the second family, and in the de novo case. The daughter of
    the second family died at seven months and never reached an age at which
    optic atrophy could declare itself, so she is not a negative. No frequency
    band is asserted; see the entry notes.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."
    explanation: Bilateral optic atrophy as the presenting feature in the index patient of the second dominant pedigree.
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The 30-year old son presents with cardiomyopathy and developed bilateral optic atrophy in adulthood."
    explanation: Documents adult-onset optic atrophy in a second carrier of the same allele who had a childhood-onset cardiac presentation, establishing the intrafamilial dissociation of the two organ systems in time.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report an adolescent female who presented with global developmental delay, intellectual disability and childhood onset progressive bilateral optic atrophy."
    explanation: Bilateral progressive optic atrophy in the third reported case, with an onset that is childhood rather than late, which is the basis of the onset discussion below.
- name: Ataxia
  category: Nervous System
  diagnostic: true
  description: >-
    Progressive cerebellar and gait ataxia is the second cardinal feature and
    the one that gives the disorder its position among the hereditary ataxias.
    In the founding family it was part of a slowly progressive late-onset
    neurodegenerative syndrome together with the optic neuropathy and myopathy.
    The OMIM clinical synopsis for this entity records the ataxia in its
    component parts, as gait, truncal and limb ataxia with unsteady gait,
    postural instability and clumsiness.
  phenotype_term:
    preferred_term: Ataxia
    term:
      id: HP:0001251
      label: Ataxia
    clinical_course: PROGRESSIVE
  notes: >-
    Reported in both sisters of the founding family. Ataxia is not mentioned in
    the abstract of the second pedigree's report, which describes that family's
    neurological involvement as a progressive polyneuropathy; whether the
    cerebellar syndrome is universal in this entity or specific to the founding
    family cannot be settled from the cached abstracts. The component terms
    HP:0002066 Gait ataxia, HP:0002078 Truncal ataxia, HP:0002070 Limb ataxia,
    HP:0002317 Unsteady gait, HP:0002172 Postural instability and HP:0002312
    Clumsiness all appear in the OMIM clinical synopsis for 619259 (MedGen
    C5543254, retrieved 2026-08-01) and were each verified with OAK, but are not
    given separate phenotype entries here because they decompose a single
    clinical syndrome that the primary sources describe as "ataxia".
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."
    explanation: Names progressive ataxia as one of the three defining components of the phenotype in the family that defines this entity.
- name: Cerebellar atrophy
  category: Nervous System
  description: >-
    Cerebellar volume loss on imaging is the structural correlate of the ataxia
    and the finding that marks the disorder as a degeneration rather than a
    static cerebellar disorder.
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
    clinical_course: PROGRESSIVE
  notes: >-
    Curated from the OMIM clinical synopsis for 619259 as exposed through MedGen
    concept C5543254 (HP:0001272, retrieved 2026-08-01 from the NCBI eutils
    esummary endpoint for MedGen uid 1779901). It does not appear in any cached
    abstract for this disorder, so no evidence item is attached. Note that
    progressive cerebellar atrophy on serial MRI has been imaged and published
    in SDHA disease, but in a biallelic child (PMID:33960148), not in a
    heterozygous NDAXOA patient; that image is deliberately not adopted as
    evidence here.
- name: Myopathy
  category: Musculoskeletal
  description: >-
    Skeletal muscle involvement completes the classic triad of the founding
    family. It is the tissue in which the biochemical defect was first
    demonstrated, and it presents as exercise-related muscle symptoms and limb
    weakness rather than as an acute rhabdomyolytic myopathy.
  phenotype_term:
    preferred_term: Myopathy
    term:
      id: HP:0003198
      label: Myopathy
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Previously, we identified a family with complex II deficiency and late-onset neurodegenerative disease with progressive optic atrophy, ataxia, and myopathy."
    explanation: Names myopathy as the third component of the founding family's phenotype.
- name: Myalgia
  category: Musculoskeletal
  description: >-
    Muscle pain, often with fatigue, is the symptom that patients report; it is
    the subjective face of the muscle energy deficit and typically precedes any
    measurable weakness.
  phenotype_term:
    preferred_term: Myalgia
    term:
      id: HP:0003326
      label: Myalgia
  notes: >-
    Curated from the OMIM clinical synopsis for 619259 via MedGen C5543254
    (HP:0003326, retrieved 2026-08-01), together with HP:0012378 Fatigue and
    HP:0003690 Limb muscle weakness from the same source. Not mentioned in any
    cached abstract, so no evidence item is attached.
- name: Cardiomyopathy
  category: Cardiovascular
  diagnostic: true
  description: >-
    Cardiomyopathy is the systemic feature that most changes management, and in
    the second dominant pedigree it was both the earliest and the most severe
    manifestation. Two children carrying the allele presented with
    cardiomyopathy in early childhood and one died of cardiac insufficiency at
    seven months; their father also had cardiomyopathy alongside his
    neurological disease. Any individual identified with an NDAXOA genotype
    therefore requires cardiac assessment regardless of age or neurological
    status.
  phenotype_term:
    preferred_term: Cardiomyopathy
    term:
      id: HP:0001638
      label: Cardiomyopathy
  notes: >-
    Reported in all three members of the second pedigree. Not reported in the
    founding family's abstracts. The OMIM clinical synopsis for 619259
    additionally records HP:0001640 Cardiomegaly and HP:0001635 Congestive heart
    failure (MedGen C5543254, retrieved 2026-08-01), both of which are
    consistent with the fatal infant case. No frequency band is asserted.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."
    explanation: Cardiomyopathy as the presenting feature in two child carriers of the dominant allele.
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The 30-year old son presents with cardiomyopathy and developed bilateral optic atrophy in adulthood."
    explanation: Persistence of the cardiomyopathy into adulthood in the surviving child, showing it is a chronic feature rather than a transient infantile one.
- name: Congestive heart failure
  category: Cardiovascular
  description: >-
    Cardiac decompensation is the mechanism by which the cardiomyopathy kills.
    In the only fatal case reported for this entity it did so in the first year
    of life.
  phenotype_term:
    preferred_term: Congestive heart failure
    term:
      id: HP:0001635
      label: Congestive heart failure
  notes: >-
    The cited snippet reports "cardiac insufficiency" as the cause of death at
    seven months; HP:0001635 Congestive heart failure is the closest HPO term
    and is bound with the source's own wording preserved in the snippet. The
    same term appears independently in the OMIM clinical synopsis for 619259
    (MedGen C5543254, retrieved 2026-08-01).
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The daughter deceased at the age of 7 months due to cardiac insufficiency."
    explanation: The one recorded death in this entity, from cardiac failure in infancy.
- name: Polyneuropathy
  category: Nervous System
  description: >-
    A progressive polyneuropathy was a prominent part of the index presentation
    in the second dominant pedigree. It is a length-dependent peripheral
    involvement distinct from the cerebellar syndrome, and it broadens the
    disorder from a central neurodegeneration to one that also affects the
    peripheral nervous system. The OMIM clinical synopsis correspondingly
    records peripheral sensory features.
  phenotype_term:
    preferred_term: Polyneuropathy
    term:
      id: HP:0001271
      label: Polyneuropathy
    clinical_course: PROGRESSIVE
  notes: >-
    Reported in the index patient of the second pedigree. The related sensory
    terms HP:0012534 Dysesthesia and HP:0003487 Babinski sign appear in the
    OMIM clinical synopsis for 619259 (MedGen C5543254, retrieved 2026-08-01);
    the Babinski sign indicates additional corticospinal involvement, which is
    recorded here rather than given its own phenotype entry because it rests on
    the synopsis alone.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."
    explanation: Names progressive polyneuropathy as a presenting feature in the index patient of the second dominant family.
- name: Ocular motility disorder
  category: Eye
  description: >-
    Abnormal eye movements accompany the optic neuropathy and are part of the
    presenting picture in the second pedigree. The OMIM definition of the entity
    notes that some patients have a childhood history of neurological features
    including limited extraocular movements, which places an ophthalmoparesis at
    the earlier end of the natural history and makes eye-movement examination a
    useful early sign in an at-risk relative who is not yet visually
    symptomatic.
  phenotype_term:
    preferred_term: Ocular movement disorder
    term:
      id: HP:0000496
      label: Abnormality of eye movement
  notes: >-
    The bound term is deliberately the parent HP:0000496 rather than the more
    specific HP:0000597 Ophthalmoparesis, because the only quotable source
    describes an unqualified "ocular movement disorder" and does not say which
    kind. The more specific reading, limited extraocular movements, comes from
    the OMIM narrative definition of 619259 exposed through MedGen C5543254
    (retrieved 2026-08-01), which is not a cached quotable reference; that
    synopsis also records HP:0000639 Nystagmus, HP:0000640 Gaze-evoked nystagmus
    and HP:0000651 Diplopia. Both HP:0000496 and HP:0000597 were verified with
    OAK; the parent was chosen because the evidence does not support the child,
    not by accident.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."
    explanation: Names an ocular movement disorder alongside the optic atrophy in the index patient.
- name: Psychiatric involvement
  category: Nervous System
  description: >-
    Psychiatric disturbance is an under-recognised part of this disorder and was
    listed among the index patient's presenting features in the second pedigree.
    The OMIM clinical synopsis records depression specifically. Because
    psychiatric symptoms can precede or overshadow the neurological ones, they
    are a plausible reason for delayed diagnosis in an adult presenting with
    subtle visual and gait complaints.
  phenotype_term:
    preferred_term: Psychiatric involvement
    term:
      id: HP:0000708
      label: Atypical behavior
  notes: >-
    The bound term is the broad behavioural-abnormality parent because the
    quotable source says only "psychiatric involvement" without specifying a
    syndrome. HP:0000716 Depression is recorded in the OMIM clinical synopsis
    for 619259 (MedGen C5543254, retrieved 2026-08-01) and was verified with
    OAK, but is not bound here because the synopsis is not a quotable source and
    the primary report does not name depression.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy."
    explanation: Names psychiatric involvement as a presenting feature in the index patient of the second dominant family.
- name: 3-Methylglutaconic aciduria
  category: Metabolism
  description: >-
    Methylglutaconic aciduria was present in the two affected children of the
    second pedigree and is the only reported urinary metabolic marker of this
    disorder. It is a generic indicator of disturbed mitochondrial membrane
    metabolism rather than a specific signature of complex II deficiency, but in
    a child with unexplained cardiomyopathy it is the finding most likely to
    redirect the workup towards mitochondrial disease and hence towards SDHA.
  phenotype_term:
    preferred_term: 3-Methylglutaconic aciduria
    term:
      id: HP:0003535
      label: 3-Methylglutaconic aciduria
  notes: >-
    The cited abstract writes "methylglutaconic aciduria" without the 3- prefix;
    HP:0003535 is the corresponding HPO term and the snippet preserves the
    source wording. Reported in two of three carriers in the second pedigree;
    not reported in the founding family or in the de novo case. No frequency
    band is asserted.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."
    explanation: Documents methylglutaconic aciduria in the two child carriers of the dominant allele.
- name: Global developmental delay
  category: Nervous System
  description: >-
    Developmental delay is reported only in the de novo case and is the main
    reason that case sits uneasily inside an entity defined as a late-onset
    neurodegeneration. It is recorded here because the authors argue explicitly
    that it expands the spectrum of this disorder, and because a curated entry
    that omitted it would misrepresent what has been published.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  notes: >-
    Reported in one individual, the de novo case, whose assignment to this
    entity is qualified in the entry notes because ClinVar carries no NDAXOA
    record for that allele. Not reported in either dominant pedigree.
  evidence:
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Furthermore, it adds developmental delay and cognitive disability to the expanding spectrum of the disorder."
    explanation: >-
      The authors' explicit claim that developmental delay is a new addition to
      the spectrum of heterozygous SDHA disease. Marked PARTIAL because it rests
      on a single individual whose allele is not classified against this
      condition in ClinVar.
- name: Intellectual disability
  category: Nervous System
  description: >-
    Cognitive impairment accompanied the developmental delay in the de novo
    case. As with the delay, it is a single-case observation at the edge of the
    entity rather than an established feature of it.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  notes: >-
    Reported in one individual. Not reported in either dominant pedigree, whose
    affected adults had ordinary cognition alongside their psychiatric and motor
    features.
  evidence:
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "We report an adolescent female who presented with global developmental delay, intellectual disability and childhood onset progressive bilateral optic atrophy."
    explanation: Intellectual disability in the single de novo case; marked PARTIAL for the same reason as the developmental delay.
- name: Decreased activity of mitochondrial complex II
  category: Cellular
  diagnostic: true
  description: >-
    A partial deficiency of complex II activity, of the order of half of control
    values, is the cellular hallmark of the disorder and the finding that first
    identified it. It is the phenotype that ties the clinical syndrome to the
    gene and the one on which the diagnosis rested for the four years between
    the biochemical and the molecular reports.
  phenotype_term:
    preferred_term: Decreased activity of mitochondrial complex II
    term:
      id: HP:0008314
      label: Decreased activity of mitochondrial complex II
  notes: >-
    Demonstrated in muscle and platelet mitochondria in the founding family and
    in fibroblasts and lymphocytes in the de novo case, and inferred in the
    second pedigree from the title and framing of that report. The tissues in
    which the defect is expressible are not consistent between reports, which is
    curated as an open discussion and as a diagnostic caveat rather than
    resolved.
  evidence:
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."
    explanation: Direct measurement of the partial complex II deficiency in patient muscle and platelets.
  - reference: PMID:10976639
    reference_title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Compatible with these findings, our patients have an approximate 50% decrease in complex II and succinate dehydrogenase activity."
    explanation: Confirms the same partial deficit and links it explicitly to succinate dehydrogenase activity as well as to assembled complex II.
biochemical:
- name: Complex II (succinate dehydrogenase) enzyme activity
  presence: DECREASED
  context: >-
    Respiratory chain enzymology on a tissue biopsy or on blood-derived cells,
    expressed as a ratio to citrate synthase to correct for mitochondrial
    content. In this disorder the expected result is a partial deficit of
    roughly half of control values, not the profound deficit seen in recessive
    complex II disease, and the assay may be normal in cultured fibroblasts.
  readouts:
  - target: Approximately Half-Normal Succinate Dehydrogenase Activity
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      A reduction to approximately half of control values in muscle, platelets
      or, in some individuals, cultured fibroblasts and lymphocytes reports the
      partial enzymatic block produced by a heterozygous flavinylation-defective
      SDHA allele. A normal result in fibroblasts does not exclude the
      diagnosis.
    evidence:
    - reference: PMID:8967754
      reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets."
      explanation: Establishes the assay, the tissues and the expected magnitude of the deficit.
  notes: >-
    No reference range is recorded because respiratory chain enzyme activities
    are reported against laboratory-specific control ranges and as
    citrate-synthase ratios rather than against a LOINC-coded universal
    interval, and no such interval is published for this disorder. The
    interpretation band that matters clinically, roughly 50% versus profoundly
    reduced, is stated in the interpretation text instead of being invented as a
    numeric interval.
- name: Urinary 3-methylglutaconic acid
  presence: INCREASED
  context: >-
    Urine organic acid analysis. Elevated in the two affected children of the
    second pedigree. A non-specific marker of mitochondrial membrane-related
    metabolic disturbance rather than a marker of complex II specifically, but
    the only reported metabolic abnormality in this disorder and a practical
    trigger for mitochondrial investigation in a child with cardiomyopathy.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood."
    explanation: The only report of a urinary metabolic abnormality in this entity.
  notes: >-
    Not measured or reported in the founding family or in the de novo case, so
    it is not known whether this is a general feature of the disorder or a
    feature of childhood-onset cardiac presentations within it.
diagnosis:
- name: Sequencing of SDHA with explicit phenotype disambiguation
  description: >-
    Molecular diagnosis rests on identifying a heterozygous SDHA missense
    variant against transcript NM_004168.4 in an individual with optic atrophy
    plus cerebellar, muscular or cardiac involvement. Three interpretation
    points follow from the published genotype-phenotype structure. First, the
    causal alleles are conservative-looking arginine-to-cysteine substitutions
    and must not be dismissed on in-silico grounds. Second, because SDHA carries
    four distinct MONDO disease terms, the laboratory cannot infer the phenotype
    from the gene: zygosity, variant class and, at codon 451, the identity of
    the substituting residue all change the answer, and the report should say
    so. Third, SDHA is not on every hereditary optic neuropathy panel; the
    authors of the most recent case report proposed adding it, which implies it
    was not there.
  evidence:
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We propose to add SDHA to next generation sequencing gene panels of optic atrophy."
    explanation: Direct evidence that SDHA is under-represented on hereditary optic neuropathy panels and that its absence is a recognised diagnostic gap.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole exome sequencing of the patient and her unaffected parents identified the novel heterozygous de novo variant c.1984C > T"
    explanation: Names the reference transcript that a diagnostic laboratory must use when reporting SDHA variants and demonstrates trio sequencing as the route to a de novo call.
- name: Respiratory chain enzymology, with a tissue caveat
  description: >-
    Measurement of complex II and succinate dehydrogenase activity supports the
    diagnosis, but the choice of tissue is not a matter of convenience. In the
    founding family the enzyme defect was demonstrable in muscle and platelet
    mitochondria and explicitly absent in cultured skin fibroblasts and
    immortalised lymphocytes, whereas in the de novo case it was demonstrable in
    fibroblasts and lymphocytes. A normal fibroblast complex II activity
    therefore does not exclude this disorder, and muscle remains the reference
    tissue. The expected magnitude is a partial, approximately half-normal
    deficit; a profound deficit should prompt reconsideration of a biallelic
    genotype and the recessive disorder.
  evidence:
  - reference: PMID:8967754
    reference_title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The enzyme defect was not expressed in cultured skin fibroblasts or immortalized lymphocytes."
    explanation: The source of the diagnostic caveat that a normal fibroblast assay does not exclude the diagnosis.
  - reference: PMID:33471299
    reference_title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Biochemical assessment of CII in the patient's derived fibroblasts and lymphocytes displayed considerably decreased CII residual activity compared with normal controls, when normalized to the integral mitochondrial enzyme citrate synthase."
    explanation: >-
      The contradicting observation, in which the same cell types that were
      uninformative in the founding family were informative. Marked PARTIAL
      because the two reports cannot both describe a general rule.
- name: Ophthalmological assessment with formal fields and acuity
  description: >-
    Fundoscopy showing bilateral optic disc pallor, with acuity and formal
    perimetry, documents the defining feature and tracks it. The OMIM clinical
    synopsis for this entity records peripheral visual field loss as well as
    reduced acuity, so a field test adds information that acuity alone does not.
    Assessment of eye movements at the same visit is worthwhile because an
    ocular motility abnormality was part of the presenting picture in one
    pedigree and limited extraocular movements are described as a possible
    childhood antecedent.
  notes: >-
    The peripheral visual field loss (HP:0007994) and visual impairment
    (HP:0000505) supporting this recommendation come from the OMIM clinical
    synopsis via MedGen C5543254 (retrieved 2026-08-01), not from a cached
    abstract, so no evidence item is attached.
- name: Cardiac assessment at diagnosis and on surveillance
  description: >-
    Echocardiography and electrocardiography are indicated in every individual
    found to carry an NDAXOA genotype, at any age. The justification is the
    second pedigree, in which cardiomyopathy was present in all three carriers,
    preceded the neurological disease by decades in one, and was fatal in
    infancy in another. Cardiac disease is the only reported cause of death in
    this entity.
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The daughter deceased at the age of 7 months due to cardiac insufficiency."
    explanation: The lethal cardiac outcome that makes cardiac assessment mandatory rather than optional in a carrier of any age.
differential_diagnoses:
- name: Mitochondrial complex II deficiency, nuclear type 1
  disease_term:
    preferred_term: mitochondrial complex II deficiency, nuclear type 1
    term:
      id: MONDO:0100294
      label: mitochondrial complex II deficiency, nuclear type 1
  description: >-
    The recessive allelic disorder, caused by biallelic SDHA variants and
    curated separately in
    `kb/disorders/Mitochondrial_Complex_II_Deficiency_Nuclear_Type_1.yaml`. This
    is not a differential in the ordinary sense of a mimicking disease; it is
    the other zygosity of the same gene, and the two are separated by the number
    of damaged alleles rather than by clinical judgement. The distinction is
    made at the moment the sequencing result is read, not at the bedside.
  distinguishing_features:
  - "Zygosity is the discriminator: NDAXOA arises from a single heterozygous missense allele, MC2DN1 from biallelic loss-of-function or severely hypomorphic genotypes."
  - MC2DN1 presents in infancy or early childhood with Leigh syndrome or leukodystrophy; NDAXOA is dominated by adult-onset optic atrophy and ataxia, with the qualification that its cardiac arm can present in infancy.
  - Residual complex II activity is partial in NDAXOA, of the order of half of control values, and far lower in MC2DN1.
  - Parents of an MC2DN1 proband are unaffected heterozygous carriers; a parent of an NDAXOA proband is either affected or the variant is de novo.
  - MONDO does not place NDAXOA beneath MONDO:0100294, verified by OAK ancestor query; the two are sibling consequences of one gene, not a parent and a subtype.
  notes: >-
    MONDO:0100294 was verified with OAK on 2026-08-01 and matches the
    disease_term already bound in the existing complex II deficiency entry,
    which was read and not modified. The description caveat about that file's
    prose blurring the two entities is recorded in the entry-level notes.
  evidence:
  - reference: PMID:24781757
    reference_title: "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Underlying autosomal-recessive genetic defects are found in most of the 'SDHx' genes encoding complex II (SDHA, SDHB, SDHC, and SDHD) and its assembly factors."
    explanation: Establishes recessive inheritance as the norm for complex II subunit mitochondrial disease, which is the boundary this differential draws.
  - reference: PMID:33960148
    reference_title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Deleterious variants of SDHA are most frequently associated with Leigh and Leigh-like syndromes."
    explanation: States that the dominant clinical association of SDHA variants overall is the recessive Leigh phenotype, which is why the dominant neurodegenerative entity is easily missed.
- name: Pheochromocytoma/paraganglioma syndrome 5
  disease_term:
    preferred_term: pheochromocytoma/paraganglioma syndrome 5
    term:
      id: MONDO:0013602
      label: pheochromocytoma/paraganglioma syndrome 5
  description: >-
    The tumour-predisposition allelic disorder, in which a heterozygous SDHA
    loss-of-function germline allele behaves as a tumour-suppressor first hit,
    with somatic loss of the wild-type allele driving paraganglioma,
    phaeochromocytoma and gastrointestinal stromal tumour. It shares zygosity
    and gene with NDAXOA and shares nothing else. It is curated in dismech
    within `kb/disorders/Pheochromocytoma_Paraganglioma.yaml`, which was read
    and not modified.

    The overlap is not merely theoretical and it creates a live clinical
    problem. Codon 451 of SDHA produces neurodegeneration when the arginine is
    replaced by cysteine and has been reported in paraganglioma when it is
    replaced by histidine. Meanwhile, tumour surveillance is recommended for
    SDHA pathogenic-variant carriers and no equivalent surveillance exists for
    adult-onset mitochondrial disease, so a family found to carry an SDHA
    variant is offered a cancer protocol whose applicability to their particular
    allele nobody can state.
  distinguishing_features:
  - PGL5 alleles are typically truncating or splice loss-of-function changes acting as tumour-suppressor first hits; the NDAXOA alleles are specific missense substitutions that leave a full-length subunit.
  - PGL5 requires a somatic second hit in the tumour tissue; NDAXOA is a constitutive, whole-body enzymatic deficit with no second hit.
  - "The mechanism diverges at succinate: in PGL5 succinate accumulation acts as an oncometabolite driving pseudohypoxia and epigenetic remodelling, whereas in NDAXOA the pathogenic consequence curated here is bioenergetic failure in post-mitotic tissue."
  - Neither NDAXOA pedigree has reported a tumour, and no PGL5 family has reported optic atrophy with ataxia; the two allele sets have historically been described as separate.
  - "At codon 451 the substituting residue, not the position, decides which disease follows: Cys in NDAXOA, His reported in paraganglioma, Ser reported de novo in an infant with cardiomyopathy."
  notes: >-
    MONDO:0013602 was verified with OAK on 2026-08-01: its definition is "Any
    paraganglioma in which the cause of the disease is a mutation in the SDHA
    gene", it xrefs OMIM:614165, and it carries the same `RO:0004003 HGNC:10680`
    relationship as this entry. The codon 451 allelic series is from the ClinVar
    submitter evidence for VCV000449389 recorded in the entry-level notes, not
    from a cached abstract.
  evidence:
  - reference: PMID:24781757
    reference_title: "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interestingly, SDHx genes also function as tumor suppressor genes in hereditary paragangliomas, pheochromocytomas, and gastrointestinal stromal tumors."
    explanation: Establishes the tumour-suppressor arm of the same gene family that this differential separates from the neurodegenerative arm.
  - reference: PMID:33960148
    reference_title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with known pathogenic variants in SDHA are recommended to receive annual biochemical and clinical surveillance for signs and symptoms of paraganglioma and pheochromocytoma syndrome"
    explanation: Documents that a tumour-surveillance recommendation attaches to SDHA pathogenic variants generically, which is what makes the boundary between these two entities a practical rather than an academic question.
- name: Dilated cardiomyopathy 1GG
  disease_term:
    preferred_term: dilated cardiomyopathy 1GG
    term:
      id: MONDO:0013339
      label: dilated cardiomyopathy 1GG
  description: >-
    The fourth SDHA disease term in MONDO, a cardiac-predominant phenotype. It
    is included as a differential because cardiomyopathy is also a feature of
    NDAXOA, and in the second NDAXOA pedigree it was the presenting and lethal
    feature in the paediatric carriers. An infant found to have cardiomyopathy
    and an SDHA variant could plausibly be assigned to either term, and the
    distinction currently rests on zygosity and on whether neurological or
    ophthalmological features later appear in the family.
  distinguishing_features:
  - DCM1GG is a cardiac-predominant entity without the optic atrophy that defines NDAXOA.
  - The NDAXOA pedigree with childhood cardiomyopathy also contained an adult with optic atrophy carrying the same allele, so a family history of visual loss argues for NDAXOA.
  - MONDO keeps the two as separate terms with separate OMIM anchors, MONDO:0013339/OMIM:613642 and MONDO:0031006/OMIM:619259, both mapped to SDHA.
  notes: >-
    MONDO:0013339 was verified with OAK on 2026-08-01; it xrefs OMIM:613642 and
    carries `RO:0004003 HGNC:10680 ! SDHA`. There is no dismech entry for this
    disorder at the time of curation. This differential is deliberately included
    because it was surfaced by the OAK gene-to-disease query rather than by the
    clinical literature, and omitting it would misrepresent how many phenotype
    classes MONDO assigns to SDHA.
- name: Behr syndrome
  disease_term:
    preferred_term: Behr syndrome
    term:
      id: MONDO:0008858
      label: Behr syndrome
  description: >-
    The closest clinical mimic among the non-SDHA disorders. Behr syndrome pairs
    bilateral optic atrophy with ataxia and variable additional neurological
    features, most often from biallelic OPA1 variants and also from OPA3,
    MTRFR/C12orf65 or C19orf12. The pairing of optic atrophy with cerebellar
    ataxia is exactly the NDAXOA core, so a patient with NDAXOA presenting to a
    neuro-ophthalmology service will be worked up for Behr syndrome, and the
    distinction is molecular.
  distinguishing_features:
  - Behr syndrome is recessive and childhood-onset; NDAXOA is dominant and, in its neurological arm, adult-onset.
  - Behr syndrome converges on mitochondrial dynamics and translation rather than on a respiratory chain enzyme; complex II activity is not the expected abnormality.
  - Pyramidal signs with spasticity and posterior-column sensory loss are classical in Behr syndrome; NDAXOA has a peripheral polyneuropathy and, in the OMIM synopsis, a Babinski sign but not a spastic paraparesis.
  - Cardiomyopathy is not a feature of Behr syndrome and is prominent in one NDAXOA pedigree.
  notes: >-
    MONDO:0008858 was verified with OAK on 2026-08-01; it xrefs OMIM:210000 and
    carries `RO:0004003 HGNC:8140 ! OPA1`. It matches the disease_term bound in
    `kb/disorders/Behr_Syndrome.yaml`. No evidence item is attached because the
    contrast is a reasoned clinical comparison rather than a quotable finding.
- name: Autosomal dominant optic atrophy plus syndrome
  disease_term:
    preferred_term: autosomal dominant optic atrophy plus syndrome
    term:
      id: MONDO:0014720
      label: autosomal dominant optic atrophy plus syndrome
  description: >-
    OPA1-related dominant optic atrophy with extra-ocular features is the
    commonest inherited syndromic optic neuropathy and shares with NDAXOA both
    the dominant transmission and the combination of optic atrophy with ataxia
    and myopathy. It will be considered, and tested for, long before SDHA.
  distinguishing_features:
  - Dominant optic atrophy plus is caused by OPA1 and is far more common; SDHA is a rare cause that the most recent case report argued should be added to optic atrophy panels precisely because it is often absent from them.
  - Sensorineural deafness and chronic progressive external ophthalmoplegia characterise the OPA1-plus phenotype; the eye-movement abnormality in NDAXOA is not described as a progressive external ophthalmoplegia and deafness is not reported.
  - Complex II activity is normal in OPA1 disease and approximately half-normal in NDAXOA, so respiratory chain enzymology discriminates the two where it is available.
  - Cardiomyopathy and methylglutaconic aciduria point away from OPA1 and towards SDHA.
  notes: >-
    MONDO:0014720 was verified with OAK on 2026-08-01 and matches the
    disease_term bound in `kb/disorders/Autosomal_Dominant_Optic_Atrophy_Plus.yaml`.
  evidence:
  - reference: PMID:39423307
    reference_title: "Clinical and genetic landscape of optic atrophy in 826 families: insights from 50 nuclear genes."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "The top five genes implicated in nHON in our in-house cohort were OPA1, WFS1, FDXR, ACO2 and AFG3L2, which accounted for 82.46% of probands."
    explanation: >-
      Establishes the gene ranking a clinician actually faces in nuclear
      hereditary optic neuropathy, with OPA1 far ahead and SDHA nowhere in the
      leading group. Marked PARTIAL because the abstract does not state whether
      SDHA was among the 50 genes analysed, so this supports the relative
      prominence of OPA1 rather than a quantified rarity for SDHA.
- name: Leber hereditary optic neuropathy
  disease_term:
    preferred_term: Leber hereditary optic neuropathy
    term:
      id: MONDO:0010788
      label: Leber hereditary optic neuropathy
  description: >-
    The paradigm mitochondrial optic neuropathy, caused by mitochondrial DNA
    complex I mutations. It is included as a mechanistic rather than a clinical
    differential: LHON establishes that a partial respiratory chain defect
    preferentially destroys retinal ganglion cells, which is the same argument
    this entry makes for complex II, and it is the reason a mitochondrial
    genome analysis is usually done before a nuclear panel in an adult with
    bilateral optic neuropathy.
  distinguishing_features:
  - LHON is maternally inherited with marked male bias and incomplete penetrance; NDAXOA is autosomal dominant with affected women in both pedigrees.
  - LHON typically produces subacute, sequential, painless central visual loss over weeks; the NDAXOA optic neuropathy is described as slowly progressive.
  - LHON is not accompanied by cerebellar ataxia, myopathy or cardiomyopathy in its classical form; those are the extra-ocular features that should redirect testing towards SDHA.
  - The biochemical lesion is complex I in LHON and complex II in NDAXOA, so respiratory chain enzymology distinguishes them directly.
  notes: >-
    MONDO:0010788 was verified with OAK on 2026-08-01; it xrefs OMIM:535000 and
    matches `kb/disorders/Leber_Hereditary_Optic_Neuropathy.yaml`.
  evidence:
  - reference: PMID:14766317
    reference_title: "Mitochondrial dysfunction as a cause of optic neuropathies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria."
    explanation: Supports the shared mechanism that makes LHON a mechanistic rather than merely clinical differential for a complex II optic neuropathy.
treatments:
- name: Multidisciplinary supportive care
  description: >-
    No disease-modifying therapy exists. Management follows the components of
    the phenotype: low-vision assessment and educational or occupational
    accommodation for the optic neuropathy, physiotherapy and falls prevention
    for the ataxia and myopathy, and neuropathic pain management where the
    polyneuropathy is symptomatic.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: BEHAVIORAL
  action_category: THERAPEUTIC
  notes: >-
    Carried as a descriptor with no evidence item, deliberately. No management
    guideline, natural-history study or treatment trial exists for this
    disorder; the entire clinical literature is two case reports and a case
    series whose abstracts say nothing about management. The listed measures are
    the standard-of-care response to the phenotypes curated above, not anything
    published for NDAXOA, so attaching a snippet would be a claim/snippet
    mismatch.
- name: Cardiac surveillance and heart failure management
  description: >-
    Because cardiomyopathy was present in all three carriers of the second
    pedigree and killed one of them in infancy, cardiac surveillance is the one
    intervention in this disorder with a clear, published rationale.
    Echocardiographic follow-up and conventional heart failure management apply;
    the disorder does not change how the cardiomyopathy itself is treated, only
    the imperative to look for it.
  treatment_term:
    preferred_term: disease screening
    term:
      id: NCIT:C15419
      label: Disease Screening
  action_category: MONITORING
  evidence:
  - reference: PMID:27683074
    reference_title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The daughter deceased at the age of 7 months due to cardiac insufficiency."
    explanation: The outcome that justifies cardiac surveillance as an active recommendation rather than a generic precaution.
  notes: >-
    The recommendation itself is a clinical inference from the reported natural
    history of one pedigree; no surveillance protocol has been published for
    this disorder and no interval is asserted here.
- name: Riboflavin supplementation
  description: >-
    Riboflavin is the dietary precursor of FAD, the cofactor whose covalent
    attachment to SDHA is the step that the NDAXOA alleles are argued to
    disrupt, so raising intracellular flavin availability is the one
    mechanistically targeted intervention that has ever been tried in complex II
    deficiency. In three children with recessive complex II deficiency treated
    with oral riboflavin and followed for a mean of 4.5 years, the neurological
    course was stable or moderately improved in two and plasma lactate decreased
    to near-normal levels in the third, and riboflavin supplementation of the
    culture medium doubled
    complex II activity in patient fibroblasts but not in control fibroblasts.

    Whether any of that transfers to NDAXOA is unknown and this entry does not
    claim it does. No patient with a dominant SDHA allele has been reported on
    riboflavin. There is a specific mechanistic reason for caution: raising FAD
    availability is a plausible remedy for a subunit that binds its cofactor
    poorly, but has no obvious mechanism against a subunit that cannot form the
    covalent linkage at all, and none at all if the dominant allele acts by
    competing for assembly slots. The intervention is nevertheless recorded
    because it is cheap, non-toxic and rationally connected to the curated
    mechanism, which is the strongest position available in a disease with no
    trials.
  treatment_term:
    preferred_term: pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: riboflavin
      term:
        id: CHEBI:17015
        label: riboflavin
  therapeutic_modality: SMALL_MOLECULE
  action_category: THERAPEUTIC
  target_mechanisms:
  - target: Failure of Covalent FAD Attachment to the SDHA Flavoprotein
    treatment_effect: MODULATES
    description: >-
      Increasing flavin availability is intended to drive the FAD-loading step
      that the mutant subunit performs poorly. Recorded as MODULATES rather than
      RESTORES deliberately: RESTORES would assert that supplementation returns
      the disrupted step to normal function, and this entry argues the opposite
      elsewhere — extra FAD is a plausible remedy for a subunit that binds its
      cofactor weakly but has no obvious mechanism against one that cannot form
      the covalent linkage at all. The link is retained because the drug and the
      node are genuinely the same chemistry, but the direction and magnitude of
      the effect on this node are unestablished, and no patient with a dominant
      SDHA allele has been treated.
  evidence:
  - reference: PMID:16737791
    reference_title: "Effects of riboflavin in children with complex II deficiency."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "We report on three complex II-deficient children treated with riboflavin per os, who were followed-up for a mean period of 4.5 years."
    explanation: >-
      The only clinical experience of riboflavin in complex II deficiency.
      Marked PARTIAL because the three treated children had the recessive
      paediatric disorder, not this dominant entity, and the series is
      uncontrolled.
  - reference: PMID:16737791
    reference_title: "Effects of riboflavin in children with complex II deficiency."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Riboflavin supplementation to the growth medium of cultured fibroblasts resulted in a 2-fold increase of complex II activity in patients, but not in controls."
    explanation: >-
      Cell-level evidence that flavin supply is rate-limiting for complex II
      activity in complex II-deficient patient cells specifically. Marked
      PARTIAL because the fibroblasts came from children with recessive disease
      and the effect has never been tested on a dominant flavinylation-defective
      allele.
  notes: >-
    Recorded with deliberately conservative framing. Riboflavin responsiveness
    is established for some flavoprotein disorders and has never been tested in
    NDAXOA; the citation here is to a different SDHA-related disease and is
    marked PARTIAL for that reason on both evidence items. No dose is stated
    because none is established for this indication.
- name: Genetic counselling
  description: >-
    Counselling in this disorder is unusually difficult and the difficulty is
    specific rather than generic. A carrier faces a 50% transmission risk, but
    what is transmitted cannot be described: within one published family the
    same allele produced lethal infantile cardiomyopathy in one child and adult
    optic atrophy in another. Beyond that, the family must be told that the
    literature attaches a tumour-surveillance recommendation to SDHA pathogenic
    variants generally, that no such recommendation exists for adult-onset
    mitochondrial disease, and that the absolute risk of either outcome for
    their particular allele is unknown. Predictive testing of at-risk relatives
    therefore returns a result whose meaning is genuinely indeterminate.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  action_category: COUNSELING_INFORMATIONAL
  evidence:
  - reference: PMID:33960148
    reference_title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is no similar screening or testing recommended for individuals potentially at risk for adult-onset mitochondrial disease. Additionally, the absolute risk for both conditions is unknown."
    explanation: "States the counselling asymmetry directly: a surveillance protocol exists for the tumour risk and nothing exists for the mitochondrial risk, with neither absolute risk quantified."
  - reference: PMID:24781757
    reference_title: "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As tumor screening is recommended for SDHx mutation carriers, this should also be considered for patients with mitochondrial disorders and their family members."
    explanation: An explicit recommendation that families ascertained through SDHx mitochondrial disease should be considered for tumour screening, which is the concrete counselling content this entry cannot avoid.
animal_models:
- species: Saccharomyces cerevisiae
  genotype: SDH1 C-terminal arginine substitutions (Arg582, Arg638 with Cys630)
  category: Site-directed mutagenesis of the orthologous flavoprotein subunit
  description: >-
    Budding yeast is the only tractable system in which the flavinylation
    chemistry proposed for this disorder has been dissected genetically. Mutation
    of C-terminal arginine residues of Sdh1, the SDHA orthologue, prevents
    covalent flavinylation and, for one residue, prevents assembly of the
    tetramer, while showing that assembly requires FAD binding but not
    necessarily the covalent linkage. The authors state that their results
    document the basis for the SDH deficiency and pathology seen with mutations
    in human Sdh1.

    Its limits should be stated plainly. The residues mutated are yeast Arg582
    and Arg638, not the human residues that cause this disease, and no
    alignment establishing correspondence to human Arg451 or Arg662 was
    performed here or asserted. The yeast work is therefore evidence that this
    class of lesion exists and behaves as modelled, not evidence about the human
    alleles. No animal model of NDAXOA exists in any species: there is no
    heterozygous Sdha knock-in mouse carrying either human allele, and
    homozygous Sdha disruption in mammals models the recessive disorder rather
    than this one.
  genes:
  - preferred_term: SDHA
    term:
      id: hgnc:10680
      label: SDHA
  associated_phenotypes:
  - Loss of covalent flavinylation of the Sdh1 flavoprotein subunit
  - Failure of succinate dehydrogenase tetramer assembly (Arg582)
  - Destabilisation of the flavinylation factor Sdh5
  evidence:
  - reference: PMID:23043141
    reference_title: "Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Mutation of Arg(582) in yeast Sdh1 precludes flavinylation as well as assembly of the tetrameric enzyme complex."
    explanation: The direct genetic demonstration that a single C-terminal arginine substitution in the flavoprotein subunit can abolish covalent flavinylation.
  - reference: PMID:23043141
    reference_title: "Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit."
    supports: PARTIAL
    evidence_source: MODEL_ORGANISM
    snippet: "These results document the basis for the SDH deficiency and pathology seen with mutations in human Sdh1."
    explanation: >-
      The authors' own translation of their yeast result to human SDHA disease.
      Marked PARTIAL because it is an interpretive claim about human pathology
      made from yeast data on non-corresponding residues.
discussions:
- discussion_id: sdha-ndaxoa-dominant-negative-versus-haploinsufficiency
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Does the NDAXOA allele act by delivering a catalytically inert subunit into
    assembled complex II, or by simple haploinsufficiency, and why are
    heterozygous SDHA null carriers neurologically unaffected?
  rationale: >-
    This is the central unresolved mechanistic question of the entity and it is
    not academic. Heterozygous SDHA loss-of-function alleles are carried without
    neurodegeneration by the parents of children with recessive complex II
    deficiency and by members of tumour-predisposition families, yet a
    heterozygous missense allele at codon 451 causes progressive optic atrophy,
    ataxia, myopathy and cardiomyopathy. Half-normal enzyme activity, the
    measured phenotype, is equally consistent with losing one functional gene
    copy and with poisoning half the assembled complexes, so the biochemistry
    published so far cannot discriminate. The yeast result that assembly
    requires FAD binding but not covalent attachment makes the poisoning model
    concretely plausible, because a subunit blocked only at the covalent step
    could still occupy its slot. Resolving this determines whether truncating
    SDHA variants should ever be reported as a risk for neurodegeneration, which
    is a live reporting question for every diagnostic laboratory that finds one.
  attaches_to:
  - "pathophysiology#Catalytically Inert SDHA Subunit Competing for Complex II Assembly"
  - "pathophysiology#Failure of Covalent FAD Attachment to the SDHA Flavoprotein"
  proposed_experiments:
  - experiment_id: sdha-allele-specific-complex-occupancy
    name: Allele-specific quantification of SDHA within assembled complex II
    description: >-
      Use allele-discriminating mass spectrometry or epitope-tagged knock-in
      alleles in a heterozygous human cell model to measure the ratio of mutant
      to wild-type SDHA within assembled, membrane-associated complex II. Equal
      representation supports subunit competition; depletion of the mutant
      supports haploinsufficiency with degradation of the mutant polypeptide.
  - experiment_id: sdha-flavinylation-assay-human-alleles
    name: Direct measurement of covalent flavinylation of human p.Arg451Cys and p.Arg662Cys
    description: >-
      The SDHA-null human cell system this experiment needs already exists, and
      both alleles have already been through it: PMID:39321216 expressed each in
      a clonal human SDHA-knockout line and found both essentially inactive,
      with p.Arg451Cys retaining high SDHA protein (per-variant results from the
      PMC11611653 full text, Figures 2 and 5 and Table 1; the cached abstract is
      aggregate-only). What that study inferred rather than measured is the
      flavinylation step itself. The remaining
      experiment is therefore narrow: recover the mutant subunit from that same
      system and assay the covalent methyl-histidyl FAD linkage directly (e.g.
      by in-gel flavin fluorescence or intact-mass measurement of the
      holo-subunit), rather than inferring it from the bacterial orthologue,
      from structural modelling, or from loss of activity.
  - experiment_id: sdha-heterozygous-knockin-mouse
    name: Heterozygous Sdha knock-in mouse carrying the Arg451Cys-equivalent allele
    description: >-
      Compare a heterozygous missense knock-in with a heterozygous null on the
      same background, phenotyping the visual pathway, cerebellum, muscle and
      myocardium longitudinally. If only the missense heterozygote degenerates,
      the dominant-negative model is established in vivo.
- discussion_id: sdha-ndaxoa-tumour-risk-unknown
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Do carriers of the dominant neurodegenerative SDHA alleles have an elevated
    risk of paraganglioma, phaeochromocytoma or gastrointestinal stromal tumour,
    and should they be enrolled in SDHx tumour surveillance?
  rationale: >-
    The two questions a family actually asks are what will happen to them and
    what should be monitored, and this entity cannot answer either. Tumour
    surveillance is recommended for carriers of SDHA pathogenic variants and
    involves whole-body imaging plus biochemical testing, an appreciable
    lifelong burden. No tumour has been reported in either NDAXOA pedigree, but
    the total number of carriers ever followed is of the order of six, so the
    absence of reported tumours carries almost no statistical weight. The
    mechanistic argument cuts both ways: a missense allele that blocks
    flavinylation may or may not compromise the tumour-suppressor function of
    the locus, and the fact that a different substitution at the very same
    codon, p.Arg451His, has been reported in paraganglioma makes the position
    look precarious rather than reassuring. An independent group has framed this
    same problem as a counselling conundrum and concluded that the absolute risk
    of both outcomes is unknown.
  attaches_to:
  - "pathophysiology#Divergence From the SDHx Tumour-Predisposition Arm"
  proposed_experiments:
  - experiment_id: sdha-ndaxoa-carrier-tumour-registry
    name: International registry and prospective imaging of dominant SDHA neurodegeneration carriers
    description: >-
      Assemble all known carriers of dominant neurodegenerative SDHA alleles
      into a single registry with standardised whole-body MRI and plasma
      metanephrine surveillance, to convert an anecdotal absence of tumours into
      an estimate with a denominator.
  - experiment_id: sdha-allele-tumour-suppressor-assay
    name: Tumour-suppressor function assay of NDAXOA alleles
    description: >-
      Test p.Arg451Cys and p.Arg662Cys alongside the paraganglioma-associated
      p.Arg451His in the assays that define SDHx tumour behaviour, namely
      succinate accumulation, HIF stabilisation and 5-hydroxymethylcytosine
      loss, to establish whether the neurodegenerative alleles engage the
      oncometabolite arm at all.
  evidence:
  - reference: PMID:33960148
    reference_title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is no similar screening or testing recommended for individuals potentially at risk for adult-onset mitochondrial disease. Additionally, the absolute risk for both conditions is unknown."
    explanation: An independent group states the same gap, that neither the tumour risk nor the mitochondrial-disease risk of an SDHA variant can be quantified for counselling.
- discussion_id: sdha-ndaxoa-tissue-expression-of-the-enzyme-defect
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    In which tissues is the complex II deficiency of dominant SDHA disease
    actually expressible, given that the two published biochemical studies
    disagree about fibroblasts and lymphocytes?
  rationale: >-
    This is a diagnostic question with an immediate consequence for patients. In
    the founding family the enzyme defect was present in muscle and platelet
    mitochondria and explicitly absent in cultured skin fibroblasts and
    immortalised lymphocytes; in the de novo case it was present in exactly
    those two cell types. If the founding family's pattern is the general one, a
    normal fibroblast complex II activity is uninformative and muscle biopsy
    remains necessary, which is a materially different clinical pathway from the
    alternative. The discrepancy might reflect the two different alleles, the
    two different laboratories and eras, culture conditions and flavin content
    of the medium, or a genuine tissue-specific difference in how the mutant
    subunit is handled. Nobody has tested which.
  attaches_to:
  - "pathophysiology#Approximately Half-Normal Succinate Dehydrogenase Activity"
  proposed_experiments:
  - experiment_id: sdha-parallel-tissue-enzymology
    name: Parallel multi-tissue enzymology in the same carriers
    description: >-
      Measure complex II activity in muscle, platelets, fibroblasts and
      lymphocytes from the same individuals carrying each allele, under
      standardised riboflavin-defined culture conditions, so that tissue
      differences can be separated from inter-laboratory and inter-era
      differences.
- discussion_id: sdha-ndaxoa-late-onset-in-the-name
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Is "late-onset" a defensible part of the definition of this entity, given
    that the same allele killed an infant with cardiomyopathy at seven months?
  rationale: >-
    The MONDO and OMIM label commits the entity to a late onset, and the
    neurological arm does behave that way: the founding sisters were elderly and
    the index patient of the second family presented in adult life. But the same
    second family contains a child with cardiomyopathy from early childhood and
    an infant who died at seven months, and the third reported individual had
    childhood-onset optic atrophy with developmental delay. Either the entity is
    an adult neurodegeneration whose paediatric cardiac cases belong to a
    different term such as dilated cardiomyopathy 1GG, or "late-onset" describes
    only the neurological arm of a disorder that is not late in onset at all.
    This is not a naming quibble: it determines whether an infant with
    cardiomyopathy and a heterozygous SDHA missense variant is given this
    diagnosis, and therefore whether their relatives are told to expect optic
    atrophy.
  attaches_to:
  - "pathophysiology#Striated Muscle and Myocardial Energy Failure"
  - "phenotypes#Optic atrophy"
  proposed_experiments:
  - experiment_id: sdha-genotype-stratified-natural-history
    name: Genotype-stratified natural history of dominant SDHA carriers
    description: >-
      Prospectively phenotype all identified carriers of dominant SDHA missense
      alleles from childhood, with serial echocardiography, optical coherence
      tomography and cerebellar imaging, to establish whether the cardiac and
      neurological arms are sequential stages of one natural history or separate
      allele- or modifier-determined outcomes.
- discussion_id: sdha-ndaxoa-second-allele-attribution
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Does the de novo allele c.1984C>T (p.Arg662Cys) belong to this entity, given
    that its authors say it does and ClinVar carries no record for it against
    this condition?
  rationale: >-
    The attribution matters because it is the difference between an entity with
    one recurrent allele and an entity with an allelic series, and because the
    Arg662Cys case brings developmental delay and intellectual disability into
    the phenotype, features absent from both dominant pedigrees. The case for
    inclusion is strong on mechanism: it is a de novo heterozygous
    arginine-to-cysteine substitution with measured complex II deficiency and
    modelled impairment of C-terminal FAD binding, in an individual whose
    dominant clinical feature is progressive bilateral optic atrophy. The case
    against is that ClinVar's condition list for VCV000694327 contains
    paraganglioma syndrome 5, recessive complex II deficiency and hereditary
    cancer predisposition but not NDAXOA, and its aggregate classification is
    conflicting. No functional comparison of the two alleles has been performed.
  attaches_to:
  - "genetic#SDHA"
  proposed_experiments:
  - experiment_id: sdha-two-allele-side-by-side
    name: Side-by-side functional comparison of p.Arg451Cys and p.Arg662Cys
    description: >-
      Express both alleles in the same SDHA-null human cell background and
      compare covalent flavinylation, complex II assembly, residual enzyme
      activity and succinate accumulation. Concordant behaviour would support
      merging them into one allelic series for this entity; divergent behaviour
      would support keeping the Arg662Cys phenotype separate.
  notes: >-
    The ClinVar condition list and aggregate classification for VCV000694327
    were retrieved live on 2026-08-01 and are recorded in the entry-level notes
    with their endpoint. They are not quotable as a snippet because ClinVar is
    not a snippet-validated reference source in this repository.
- discussion_id: sdha-ndaxoa-no-neuropathology
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What is the cellular neuropathology of this disorder, given that no autopsy
    or ocular histology has ever been reported?
  rationale: >-
    Every tissue-level claim in this entry is an inference from clinical
    phenotype plus general mitochondrial cell biology. Nobody has counted
    retinal ganglion cells, examined an optic nerve, or counted Purkinje cells
    in an individual with a dominant SDHA allele. The entry conforms its
    cerebellar node to the shared Purkinje-degeneration module only at the
    clinical-consequence level for exactly this reason. Without tissue it is not
    known whether the cerebellar atrophy reflects Purkinje cell loss as in the
    spinocerebellar ataxias, granule cell or deep nuclear involvement, or
    predominantly white matter change, and it is not known whether the optic
    neuropathy involves the same prelaminar axonal segment that fails in Leber
    hereditary optic neuropathy.
  attaches_to:
  - "pathophysiology#Retinal Ganglion Cell and Optic Nerve Degeneration"
  - "pathophysiology#Cerebellar Degeneration and Ataxia"
  proposed_experiments:
  - experiment_id: sdha-oct-ganglion-cell-layer
    name: Optical coherence tomography of the retinal nerve fibre and ganglion cell layers
    description: >-
      Non-invasively quantify retinal nerve fibre layer and ganglion cell layer
      thinning in living carriers, establishing whether the lesion is a
      ganglion-cell axonopathy of the pattern seen in other mitochondrial optic
      neuropathies, and providing the first objective progression measure for
      this disorder.
  - experiment_id: sdha-ipsc-rgc-and-cerebellar-organoids
    name: Patient iPSC-derived retinal ganglion cells and cerebellar organoids
    description: >-
      Derive the affected cell types from carriers and measure complex II
      activity, oxygen consumption and survival, in the cells the disease
      actually damages rather than in the fibroblasts that may not even express
      the defect.
references:
- reference: PMID:8967754
  title: "Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia."
- reference: PMID:10976639
  title: "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene."
- reference: PMID:27683074
  title: "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement."
- reference: PMID:33471299
  title: "A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment."
- reference: PMID:33960148
  title: "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum."
- reference: PMID:24781757
  title: "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors."
- reference: PMID:15989954
  title: "Crystal structure of mitochondrial respiratory membrane protein complex II."
- reference: PMID:23043141
  title: "Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit."
- reference: PMID:19628817
  title: "SDH5, a gene required for flavination of succinate dehydrogenase, is mutated in paraganglioma."
- reference: PMID:14766317
  title: "Mitochondrial dysfunction as a cause of optic neuropathies."
- reference: PMID:33112834
  title: "Epigenetic and metabolic reprogramming of SDH-deficient paragangliomas."
- reference: PMID:16737791
  title: "Effects of riboflavin in children with complex II deficiency."
- reference: PMID:39423307
  title: "Clinical and genetic landscape of optic atrophy in 826 families: insights from 50 nuclear genes."
- reference: PMID:39321216
  title: "A Novel Human SDHA-Knockout Cell Line Model for the Functional Analysis of Clinically Relevant SDHA Variants."

📚

References & Deep Research

References

14

PMID:8967754

Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia.

No top-level findings curated for this source.

PMID:10976639

Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene.

No top-level findings curated for this source.

PMID:27683074

SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement.

No top-level findings curated for this source.

PMID:33471299

A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment.

No top-level findings curated for this source.

PMID:33960148

Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum.

No top-level findings curated for this source.

PMID:24781757

SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors.

No top-level findings curated for this source.

PMID:15989954

Crystal structure of mitochondrial respiratory membrane protein complex II.

No top-level findings curated for this source.

PMID:23043141

Flavinylation and assembly of succinate dehydrogenase are dependent on the C-terminal tail of the flavoprotein subunit.

No top-level findings curated for this source.

PMID:19628817

SDH5, a gene required for flavination of succinate dehydrogenase, is mutated in paraganglioma.

No top-level findings curated for this source.

PMID:14766317

Mitochondrial dysfunction as a cause of optic neuropathies.

No top-level findings curated for this source.

PMID:33112834

Epigenetic and metabolic reprogramming of SDH-deficient paragangliomas.

No top-level findings curated for this source.

PMID:16737791

Effects of riboflavin in children with complex II deficiency.

No top-level findings curated for this source.

PMID:39423307

Clinical and genetic landscape of optic atrophy in 826 families: insights from 50 nuclear genes.

No top-level findings curated for this source.

PMID:39321216

A Novel Human SDHA-Knockout Cell Line Model for the Functional Analysis of Clinically Relevant SDHA Variants.

No top-level findings curated for this source.

Deep Research

1

Claude Code ▸

SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy (NDAXOA) — Comprehensive Research Report

claude-haiku-4-5-20251001, claude-sonnet-5 2026-08-03T00:37:20.832191

SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy (NDAXOA) — Comprehensive Research Report

1. Disease Information

Overview. SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy — formally catalogued in OMIM as Neurodegeneration with Ataxia and Late-Onset Optic Atrophy (NDAXOA) — is a rare, autosomal dominant mitochondrial disorder caused by heterozygous, dominant-negative missense variants in SDHA, the gene encoding the flavoprotein (Fp) catalytic subunit of mitochondrial respiratory chain Complex II (succinate dehydrogenase / succinate-ubiquinone oxidoreductase). Affected individuals typically present in mid-adulthood with slowly progressive cerebellar/gait ataxia and optic atrophy, often accompanied by myopathy or myalgia; more recently described families extend the phenotype to childhood-onset isolated optic atrophy, cardiomyopathy, polyneuropathy, and psychiatric involvement (Birch-Machin et al. 2000, PMID:10976639; Courage et al. 2017, PMID:27683074).

This dominant, heterozygous-missense disease entity is mechanistically and nosologically distinct from two other well-established SDHA-associated conditions that must not be conflated with it: 1. Mitochondrial Complex II Deficiency, Nuclear Type 1 (MC2DN1; OMIM #252011) — the classical autosomal recessive (biallelic loss-of-function) presentation, producing infantile-onset Leigh syndrome, leukoencephalopathy, and cardioencephalomyopathy (Alston et al. 2012; Renkema et al. 2015, PMID:24781757). 2. Hereditary Paraganglioma/Pheochromocytoma Syndrome 5 (PGL5) — monoallelic SDHA loss-of-function variants conferring low-penetrance dominant tumor predisposition via a somatic "second hit" (loss of heterozygosity), a cancer-genetics mechanism unrelated to the neurodegenerative dominant-negative mechanism of NDAXOA.

Key identifiers: - OMIM: #619259 (NDAXOA); gene locus 600857 (SDHA, 5p15.33) - MONDO: MONDO:0031006 - Inheritance: Autosomal dominant (heterozygous missense, dominant-negative) - Gene/HGNC: SDHA (HGNC:10680) - Related/allelic OMIM entries: #252011 (MC2DN1, biallelic recessive form); #614165 (PGL5, monoallelic tumor-predisposition form) - ICD-10/11, MeSH:* No dedicated ICD-10/11 code exists; typically captured under G31.8 (other specified degenerative diseases of nervous system) or mitochondrial disease codes; MeSH indexing falls under "Mitochondrial Diseases" and "Optic Atrophy, Hereditary."

Synonyms/alternative names: NDAXOA; SDHA-related mitochondrial disease (dominant form); late-onset optic atrophy, ataxia, and myopathy associated with complex II gene mutation; SDHA-related dominant optic atrophy (isolated ophthalmologic-only presentations).

Evidence base: This condition is almost entirely characterized through individual case reports and small multigenerational family pedigrees (single kindreds of 2–4 affected members per publication) rather than large aggregated cohorts or disease registries — consistent with its status as an ultra-rare, recently delineated (OMIM entry added 2021) nosologic entity. No population-based prevalence or incidence studies exist.

2. Etiology

Disease causal factor: NDAXOA is caused by heterozygous, typically missense, germline pathogenic variants in SDHA that act through a dominant-negative mechanism rather than simple haploinsufficiency. The founding variant, c.1351C>T (p.Arg451Cys, R451C), was identified by Courage et al. (2017, PMID:27683074) in three affected members of a two-generation family; molecular modeling predicted that the substitution interferes with succinate binding in the active site, and patient fibroblasts showed an ~50% reduction in Complex II enzymatic activity — a magnitude of loss consistent with a dominant-negative rather than purely haploinsufficient effect, since simple loss of one allele of a homomeric-independent catalytic subunit would not necessarily be expected to reduce holoenzyme activity by half if the wild-type allele alone were sufficient.

A second, distinct missense variant, c.1984C>T (p.Arg662Cys), was reported in a family in which the index patient presented with bilateral optic atrophy, ocular movement disorder, progressive polyneuropathy, psychiatric involvement, and cardiomyopathy; two of his children (heterozygous for the same variant) presented in early childhood with cardiomyopathy and methylglutaconic aciduria, one of whom died at age 7 months of cardiac insufficiency, while the surviving (now adult) son developed cardiomyopathy and bilateral optic atrophy — illustrating marked intrafamilial variability in expressivity and age of onset for the same variant.

The original founding description of this phenotype predates the modern molecular nosology: Birch-Machin et al. (2000, PMID:10976639) reported a family with "progressive optic atrophy, ataxia, and myopathy" segregating a heterozygous C→T transition in the flavoprotein subunit gene of Complex II; the mutation was modeled in E. coli and shown to generate an inactive enzyme unable to covalently bind FAD, with patient tissue again showing an ~50% reduction in Complex II/SDH activity.

Risk factors: - Genetic: The sole established risk factor is inheritance of (or de novo occurrence of) a dominant-negative SDHA missense allele. A de novo heterozygous pathogenic SDHA variant has also been reported causing childhood-onset bilateral optic atrophy and cognitive impairment (Metabolic Brain Disease, 2021), indicating that non-familial (sporadic) presentations occur and should prompt genetic testing even absent a family history. - Modifier genes: None have been formally identified; however, the wide intrafamilial phenotypic variability (isolated optic atrophy in some relatives vs. multisystem disease with cardiomyopathy and death in infancy in others carrying the identical variant) strongly suggests unidentified genetic or epigenetic modifiers, mitochondrial background effects, or stochastic factors in Complex II assembly/turnover. - Environmental/lifestyle: No specific environmental, toxic, or lifestyle risk factors have been described for this dominant form (in contrast to some secondary mitochondrial toxicities). Given the underlying Complex II defect, factors that increase metabolic/oxidative stress (e.g., strenuous exercise precipitating myalgia) may plausibly exacerbate symptoms, by analogy with other mitochondrial myopathies, but this has not been specifically documented for NDAXOA.

Protective factors: None specifically documented. By analogy with mouse Complex II-deficiency models (see Section 15), chronic hypoxia has been shown to be protective against systemic SDH-loss lethality in mice — a laboratory finding of mechanistic interest but with no established human clinical translation or protective-factor status in NDAXOA.

Gene-environment interactions: Not characterized for this specific dominant entity.

3. Phenotypes

Core triad

Phenotype	HPO term (suggested)	Onset	Severity/Course	Frequency
Progressive cerebellar/gait ataxia	HP:0002066 (Gait ataxia) / HP:0001251 (Ataxia)	Mid-adulthood in classic (Birch-Machin) family; can be later or absent in isolated-optic-atrophy kindreds	Slowly progressive	Frequent in classic multisystem presentation
Optic atrophy	HP:0000648 (Optic atrophy)	Variable — late-onset (mid-adulthood) in classic family; childhood-onset in isolated-optic-atrophy and de novo pediatric cases	Progressive; bilateral	Present in essentially all reported cases — the defining shared feature across the phenotypic spectrum
Myopathy / myalgia	HP:0003198 (Myopathy) / HP:0003326 (Myalgia)	Adult-onset, often concurrent with ataxia	Variable severity	Frequent

Extended/variant phenotypes (reported in expanded case series)

Cardiomyopathy (HP:0001638, or HP:0001639 Hypertrophic cardiomyopathy / HP:0001644 Dilated cardiomyopathy as applicable): reported in the R662C family, including a fatal infantile case (death at 7 months from cardiac insufficiency) and an adult-onset case with concurrent optic atrophy.
3-Methylglutaconic aciduria (HP:0003535): a biochemical/laboratory abnormality found in the pediatric cardiomyopathy cases in the R662C kindred — of interest because 3-methylglutaconic aciduria is a recognized secondary marker of several mitochondrial/Complex II-related disorders.
Progressive polyneuropathy (HP:0003477 or HP:0009830 Peripheral neuropathy): reported in the index adult patient of the R662C family.
Psychiatric involvement (HP:0000708 Behavioral abnormality, non-specific): reported qualitatively in the same index patient; specific psychiatric diagnosis not detailed in available abstracts.
Isolated dominant optic atrophy, childhood onset, as sole manifestation: Pemp et al. (2022, IOVS, ARVO abstract) described a family in which the R451C-equivalent (c.1351C>T) variant segregated across multiple generations with optic atrophy as the only clinical feature — expanding the phenotypic spectrum to include mono-symptomatic presentations that mimic classical autosomal dominant optic atrophy (OPA1-related) and underscoring that SDHA should be considered in the differential of apparently "isolated" dominant optic atrophy.
Cognitive impairment: reported alongside childhood-onset bilateral optic atrophy in a de novo SDHA variant case (Metabolic Brain Disease, 2021).

Quality of life impact

No disease-specific quality-of-life instrument data exist. By inference from the phenotype burden (progressive visual loss, gait instability, and in some kindreds early cardiac death), the disease is expected to impose substantial cumulative disability, particularly in kindreds manifesting the full multisystem (ocular + cerebellar + cardiac + neuromuscular) phenotype; isolated-optic-atrophy kindreds have a comparatively milder, primarily visual, disability burden.

Notable pattern

The single unifying phenotype across every reported SDHA-dominant kindred, regardless of onset age or additional organ involvement, is optic atrophy — making it the most consistent anchor phenotype for this entity, with ataxia, myopathy, cardiomyopathy, polyneuropathy, and psychiatric/cognitive features occurring as variably penetrant additional features layered onto that core.

4. Genetic/Molecular Information

Causal gene: SDHA (Succinate Dehydrogenase Complex, Flavoprotein Subunit A; HGNC:10680; OMIM *600857; chromosome 5p15.33; NCBI Gene ID 6389; RefSeq transcript NM_004168.4).

Reported pathogenic variants (dominant/NDAXOA-associated): | Variant (cDNA) | Protein change | Zygosity | Family/report | Functional evidence | |---|---|---|---|---| | Heterozygous C→T transition (flavoprotein-subunit gene) | (original 2000 report, pre-standard nomenclature) | Heterozygous | Birch-Machin et al. 2000 (PMID:10976639) | E. coli modeling: inactive enzyme, unable to covalently bind FAD; ~50% reduction of Complex II/SDH activity in patient tissue | | c.1351C>T | p.Arg451Cys (R451C) | Heterozygous | Courage et al. 2017 (PMID:27683074); Pemp et al. 2022 (isolated optic atrophy family) | Molecular modeling: interferes with succinate binding; ~50% decrease in Complex II activity in patient cells | | c.1984C>T | p.Arg662Cys (R662C) | Heterozygous | Family with cardiomyopathy + optic atrophy + polyneuropathy | Dominant-negative effect on FAD binding to SDHA (per family/molecular report) | | c.456+91G>C | (intronic; reported to ClinVar under NDAXOA) | Heterozygous | ClinVar submission | Clinical significance under evaluation |

Variant classification: The R451C and R662C variants are classified as pathogenic/likely pathogenic per ClinVar submissions associated with the NDAXOA phenotype (ACMG/AMP framework); confirmatory functional data (enzymatic assay showing reduced Complex II activity) support pathogenicity in the founding families.

Variant type/class: All confirmed NDAXOA-causing variants reported to date are missense substitutions affecting FAD-binding or succinate-binding residues of the SDHA flavoprotein domain, consistent with a dominant-negative mechanism — the mutant subunit is incorporated into the tetrameric Complex II holoenzyme (SDHA/SDHB/SDHC/SDHD) and poisons its catalytic function, rather than simply being degraded and producing haploinsufficiency alone. This distinguishes NDAXOA-causing alleles mechanistically from the loss-of-function (nonsense, frameshift) alleles that, in monoallelic form, predispose to paraganglioma/pheochromocytoma (PGL5) via a two-hit tumor-suppressor mechanism, and from the biallelic loss-of-function combinations that cause the recessive MC2DN1/Leigh phenotype.

Allele frequency: Not reported in population databases (gnomAD) at appreciable frequency for the disease-causing missense alleles, consistent with an ultra-rare, highly penetrant-for-optic-atrophy dominant disorder; absence from gnomAD homozygous/high-frequency calls is expected given the severity of biallelic SDHA loss.

Somatic vs. germline: All NDAXOA-causing variants reported are germline (familial or de novo). This is distinct from the somatic "second-hit" loss of the wild-type allele that drives tumorigenesis in SDHA-related paraganglioma.

Functional consequences: Loss of Complex II (succinate dehydrogenase) enzymatic activity (~50% reduction demonstrated biochemically in two independent families), impaired FAD cofactor binding, and disrupted succinate-to-fumarate oxidation with consequent impairment of electron transfer to ubiquinone — placing the lesion at the direct interface of the TCA cycle and the mitochondrial electron transport chain (see Section 6).

Modifier genes: None specifically identified; marked intrafamilial phenotypic variability (isolated optic atrophy vs. lethal infantile cardiomyopathy for carriers of the same variant) implies unidentified modifiers.

Epigenetic information: Not characterized for this disorder.

Chromosomal abnormalities: Not applicable — NDAXOA arises from point mutations, not large structural/chromosomal rearrangements.

Related allelic disorders at the same locus (for differential/annotation purposes): - MC2DN1 (OMIM #252011): biallelic SDHA loss-of-function → Leigh syndrome/leukoencephalopathy/cardioencephalomyopathy (Alston et al. 2012; Renkema et al. 2015, PMID:24781757) - PGL5 (OMIM #614165): monoallelic SDHA loss-of-function → hereditary paraganglioma/pheochromocytoma predisposition, low penetrance, requiring somatic second hit

5. Environmental Information

No specific environmental toxin, occupational exposure, radiation, or infectious trigger has been reported to cause or precipitate NDAXOA — it is a purely monogenic disorder. No lifestyle factors (diet, exercise, smoking, alcohol) have been formally studied in this specific entity, though general mitochondrial-disease management principles (avoidance of mitochondrial toxins such as certain antibiotics, valproate, and metabolic stressors; activity pacing to avoid exertional myalgia) are extrapolated from broader mitochondrial disease care rather than NDAXOA-specific evidence. No infectious agents are implicated.

6. Mechanism / Pathophysiology

Causal chain (upstream → downstream):

Molecular trigger: Heterozygous dominant-negative missense variant in SDHA (e.g., R451C, R662C) impairs FAD cofactor binding and/or succinate-binding at the flavoprotein active site of Complex II.
Complex assembly/catalytic consequence: The mutant SDHA subunit is incorporated into the SDHA-SDHB-SDHC-SDHD holoenzyme (Complex II / succinate dehydrogenase / succinate-ubiquinone oxidoreductase), poisoning holoenzyme function in a dominant-negative fashion; measured Complex II enzymatic activity is reduced by ~50% in patient fibroblasts/muscle in the two functionally characterized families.
Bioenergetic consequence: Complex II sits at the unique junction of the TCA (Krebs) cycle and the mitochondrial electron transport chain — it oxidizes succinate to fumarate (TCA cycle step) while simultaneously reducing ubiquinone (coenzyme Q) to ubiquinol, feeding electrons directly into the respiratory chain without a proton-pumping step. Deficient Complex II activity therefore causes combined TCA cycle disruption and impaired electron flow to Complex III, secondarily depleting ATP synthesis capacity via oxidative phosphorylation.
Cellular consequence: Chronic bioenergetic insufficiency and probable secondary oxidative stress (succinate/fumarate accumulation can alter redox and epigenetic signaling via 2-oxoglutarate-dependent dioxygenase inhibition, as established in the SDHx-tumor literature, though this arm is not specifically demonstrated for the dominant neurodegenerative phenotype) preferentially affect the highest-energy-demand, longest-projection cell populations.
Tissue-selective vulnerability: The clinical phenotype localizes disproportionately to tissues with high metabolic demand and/or long axonal projections and high mitochondrial density — retinal ganglion cells (whose long, thinly myelinated axons form the optic nerve, explaining optic atrophy), cerebellar Purkinje/cerebellar circuitry (explaining ataxia), skeletal muscle (myopathy/myalgia), cardiomyocytes (cardiomyopathy in more severe kindreds), and peripheral nerve (polyneuropathy in extended phenotype cases) — the same tissue-vulnerability pattern seen broadly across primary mitochondrial respiratory chain disease.
Clinical manifestation: Progressive, generally slowly evolving neurodegeneration with the described multisystem phenotype (Section 3).

Molecular pathway/GO terms (suggested): - GO:0006099 (tricarboxylic acid cycle) - GO:0006121 (mitochondrial electron transport, succinate to ubiquinone) - GO:0000104 (succinate dehydrogenase activity) - GO:0016627 (oxidoreductase activity, acting on the CH-CH group of donors) - GO:0045281 (succinate dehydrogenase complex (ubiquinone)) - GO:0005749 (mitochondrial respiratory chain complex II, succinate dehydrogenase complex) - GO:0071949 (FAD binding)

Protein dysfunction: Loss-of-function at the catalytic/cofactor-binding level (impaired FAD covalent binding; impaired succinate binding) with a dominant-negative rather than simple haploinsufficient mechanism — the mutant polypeptide is stably incorporated into the holoenzyme and disrupts its function rather than being cleared, which likely explains why heterozygosity alone (rather than requiring biallelic loss, as in MC2DN1) is sufficient to produce ~50% activity loss and clinical disease.

Metabolic changes: Reduced Complex II/succinate dehydrogenase enzymatic activity in patient-derived fibroblasts and muscle; 3-methylglutaconic aciduria has been reported as a secondary urinary organic-acid marker in at least one affected pediatric kindred, a recognized (if nonspecific) biomarker across several mitochondrial and Complex II-related disorders.

Immune system involvement: Not implicated; this is not an inflammatory or autoimmune disease mechanism.

Tissue damage mechanisms: Chronic bioenergetic/oxidative stress-mediated neurodegeneration of retinal ganglion cells, cerebellar/central and peripheral neurons, cardiomyocytes, and skeletal myofibers — consistent with the general "primary mitochondrial disease" injury paradigm rather than a distinct necrotic/fibrotic/ischemic mechanism.

Molecular profiling / omics: No transcriptomic, proteomic, metabolomic, or single-cell datasets specific to NDAXOA patient tissue have been published to date; available functional characterization is limited to targeted Complex II enzymatic activity assays and molecular/structural modeling of the mutant protein.

Suggested cell types (CL) for pathophysiology nodes: - CL:0000740 (retinal ganglion cell) - CL:0000121 (Purkinje cell) - CL:0000187 (skeletal myofiber/muscle cell) - CL:0000746 (cardiac muscle cell) - CL:0000540 (neuron, peripheral nerve context)

7. Anatomical Structures Affected

Organ level: - Primary: Eye/optic nerve (optic atrophy); central nervous system, specifically cerebellum and cerebellar pathways (ataxia); skeletal muscle (myopathy/myalgia) - Secondary (in extended/severe phenotype kindreds): Heart (cardiomyopathy, occasionally fatal in infancy); peripheral nervous system (polyneuropathy); possibly CNS structures underlying the reported psychiatric/cognitive involvement - Body systems involved: Nervous system (central and peripheral), visual system, musculoskeletal system, cardiovascular system

Tissue/cell level: - Retinal ganglion cells and their axons forming the optic nerve (CL:0000740) - Cerebellar neuronal populations (Purkinje cells, CL:0000121, and associated circuitry) - Skeletal myofibers (CL:0000187) - Cardiomyocytes (CL:0000746) in cardiomyopathy-manifesting kindreds - Peripheral sensorimotor neurons in polyneuropathy-manifesting cases

Subcellular level: - Mitochondria, specifically the inner mitochondrial membrane location of Complex II (GO:0005743 mitochondrial inner membrane; GO:0005739 mitochondrion) - Mitochondrial matrix-facing catalytic domain of SDHA (site of succinate oxidation and FAD binding)

Anatomical localization (UBERON): - UBERON:0001780 (optic nerve) / UBERON:0000966 (retina) - UBERON:0002037 (cerebellum) - UBERON:0001134 (skeletal muscle tissue) - UBERON:0000948 (heart) - UBERON:0001021 (peripheral nervous system)

Lateralization: Optic atrophy is characteristically bilateral across all reported cases; ataxia and myopathy are generalized/symmetric rather than lateralized, consistent with a systemic metabolic (rather than focal structural) disease process.

8. Temporal Development

Onset: - Classic multisystem phenotype (Birch-Machin family): mid-adulthood onset of ataxia, optic atrophy, and myopathy. - R662C family: highly variable — fatal infantile-onset cardiomyopathy (death at 7 months) in one child; cardiomyopathy plus bilateral optic atrophy emerging by early adulthood (~age 30) in a sibling carrying the identical variant. - Isolated dominant optic atrophy family (Pemp et al. 2022): childhood onset of optic atrophy as the sole manifestation. - De novo variant case: childhood-onset bilateral optic atrophy with cognitive impairment. - Onset pattern: Generally insidious/gradual (chronic, progressive) rather than acute, though the infantile cardiomyopathy presentation can be rapidly fatal.

Progression: - Described as slowly progressive for the core ataxia/optic atrophy/myopathy triad in the classic adult-onset phenotype. - No formal staging system exists for this ultra-rare disorder. - Progression rate and disease-course pattern (i.e., truly monotonically progressive vs. plateauing) have not been systematically documented across the small number of reported kindreds; available case reports describe a chronic progressive trajectory for the neurological and ophthalmological features. - Disease duration: chronic/lifelong for survivors; the infantile cardiomyopathy presentation can be rapidly fatal (within months).

Patterns: - No spontaneous or treatment-induced remission has been reported. - No specific "critical period" or intervention window has been established, though early recognition of cardiomyopathy in infancy (given its potential lethality) represents a clinically important early window for surveillance in at-risk family members of a known proband.

9. Inheritance and Population

Epidemiology: No formal prevalence or incidence estimates exist for NDAXOA specifically — it is characterized only through a handful of published kindreds and isolated case reports (fewer than 10 families in the literature as of current searches), consistent with an ultra-rare disease. For context, the entire spectrum of primary OXPHOS (oxidative phosphorylation) disease has an estimated prevalence of ~1:4,300, and isolated Complex II deficiency (across all causal genes and both dominant and recessive mechanisms) accounts for only ~2–4% of OXPHOS defects — making the specific dominant NDAXOA subset a small fraction of an already rare category.

Inheritance pattern: Autosomal dominant, distinguishing NDAXOA sharply from the classical autosomal recessive MC2DN1/Leigh-syndrome presentation of biallelic SDHA variants. Both familial transmission (multigenerational pedigrees) and de novo occurrence have been documented.

Penetrance: Appears high for optic atrophy specifically (the one feature present across essentially all reported carriers) but variable/incomplete for the full multisystem phenotype — some carriers of the identical pathogenic variant (e.g., within the R662C family) manifest isolated or mild disease while others develop severe, even fatal, multisystem involvement (cardiomyopathy).

Expressivity: Markedly variable — the same pathogenic variant produces phenotypes ranging from isolated childhood-onset optic atrophy (Pemp et al. family) to fatal infantile cardiomyopathy with 3-methylglutaconic aciduria (R662C family), representing one of the most striking documented examples of intrafamilial phenotypic heterogeneity for a single dominant mitochondrial-gene variant.

Genetic anticipation: Not reported/established for this disorder.

Germline mosaicism: Not specifically documented, though the occurrence of de novo cases raises the theoretical possibility relevant to recurrence-risk counseling for apparently sporadic cases.

Founder effects: None reported; the described pathogenic variants (R451C, R662C) each derive from distinct, independently ascertained families without an established shared founder haplotype in current literature.

Consanguinity role: Not relevant to this dominant disorder (in contrast to the recessive MC2DN1 form, where consanguinity is a recognized risk factor for biallelic variant co-inheritance).

Carrier frequency: Not established in population databases; the disease-causing missense alleles are not observed at appreciable frequency in gnomAD, consistent with high penetrance for at least the optic atrophy component and/or recent mutational origin in each reported family.

Population demographics: No specific ethnic, geographic, or demographic enrichment has been reported. Sex ratio and age-distribution data are not available given the very small number of published cases (individual pedigrees, typically single-digit numbers of affected individuals per report).

10. Diagnostics

Clinical/laboratory tests: - Complex II (succinate dehydrogenase) enzymatic activity assay in patient fibroblasts or skeletal muscle — the key biochemical diagnostic finding, typically showing an ~50% reduction in activity in confirmed heterozygous carriers (LOINC coding for mitochondrial enzyme complex assays as locally available). - Urine organic acids, specifically assessment for 3-methylglutaconic acid — reported as elevated in at least one pediatric cardiomyopathy-manifesting kindred; a recognized (nonspecific) biomarker in several Complex II/mitochondrial disorders. - Serum/plasma lactate and pyruvate: general mitochondrial-disease screening tests; not specifically reported as abnormal or normal in the NDAXOA literature reviewed, but standard first-line workup in suspected mitochondrial disease. - Muscle biopsy with histochemistry (e.g., COX/SDH histochemical staining, ragged-red fiber assessment) and electron microscopy: standard mitochondrial myopathy workup; specific NDAXOA muscle biopsy findings were not detailed in the sources reviewed here beyond biochemical Complex II activity.

Imaging studies: - Brain MRI: relevant for assessing cerebellar atrophy in the ataxia phenotype (as illustrated by the related case report "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA," PMC8222855, describing cerebellar atrophy on imaging in an SDHA-associated case). - Ophthalmologic imaging: optical coherence tomography (OCT) to document retinal nerve fiber layer thinning consistent with optic atrophy; formal visual field testing and visual evoked potentials are standard adjuncts in optic atrophy workups generally, though NDAXOA-specific ophthalmic imaging data were not itemized in the sources reviewed.

Functional tests: - Formal ophthalmologic examination (visual acuity, color vision, fundoscopy for pallor of the optic disc) is central to diagnosis given optic atrophy's near-universal presence. - Cardiac evaluation (echocardiography) is indicated given the cardiomyopathy risk documented in extended-phenotype kindreds, particularly for at-risk infants/children in known families.

Genetic testing: - Single-gene SDHA sequencing or targeted mitochondrial/Complex II gene panel (including SDHA, SDHB, SDHC, SDHD, SDHAF1, SDHAF2) is the recommended diagnostic approach once a Complex II biochemical defect and/or the clinical triad (optic atrophy + ataxia ± myopathy/cardiomyopathy) raises suspicion. - Whole exome sequencing (WES) is appropriate for atypical, sporadic, or diagnostically ambiguous presentations, particularly de novo cases without a family history, as illustrated by the reported de novo SDHA variant case with childhood optic atrophy and cognitive impairment. - Whole genome sequencing (WGS): not specifically reported as used in NDAXOA diagnosis but would be a reasonable second-tier approach if exome/panel testing is non-diagnostic. - Segregation analysis within families (as performed in both the Courage and Pemp et al. kindreds) is important given the variable expressivity, to confirm that the candidate variant tracks with at least the optic atrophy phenotype across generations. - Chromosomal microarray, karyotyping, FISH, and mitochondrial DNA (mtDNA) testing are not relevant — SDHA is nuclear-encoded, and the disease is a point-mutation/single-gene disorder, not a copy-number, mtDNA, or cytogenetic disease.

Differential diagnosis: - OPA1-related autosomal dominant optic atrophy — the principal differential, particularly for the "isolated optic atrophy" NDAXOA presentation; OPA1-ADOA is far more common and should generally be tested first/in parallel. - Other mitochondrial optic neuropathies (e.g., Leber hereditary optic neuropathy, though that is maternally inherited/mtDNA-based, not autosomal dominant) - Other hereditary ataxias (autosomal dominant spinocerebellar ataxias) when ataxia is the presenting feature - Friedreich ataxia and other mitochondrial/metabolic ataxias when ataxia plus cardiomyopathy co-occur - Other primary mitochondrial cardiomyopathies in the infantile cardiomyopathy presentation

Screening: No population or newborn screening program exists for this ultra-rare dominant disorder; cascade genetic testing of at-risk relatives in known SDHA-NDAXOA families is the appropriate practical screening strategy, particularly given the risk of clinically silent but potentially serious cardiac involvement in some carriers.

11. Outcome/Prognosis

Survival and mortality: No formal survival statistics exist. The disease spectrum ranges from a comparatively benign, slowly progressive adult-onset course (classic ataxia/optic atrophy/myopathy triad) to infantile mortality from cardiomyopathy (death at 7 months of age reported in the R662C family) — underscoring that prognosis is highly variant- and family-dependent, and that the presence of cardiomyopathy is the key prognostically ominous feature to screen for.

Morbidity/function: Progressive visual impairment from optic atrophy is essentially universal and represents a major source of long-term disability; gait ataxia contributes to mobility impairment in the classic multisystem phenotype; myopathy/myalgia and, in some kindreds, polyneuropathy add to the functional burden. No standardized quality-of-life instrument data are available.

Disease course/complications: Cardiomyopathy is the principal life-threatening complication reported. Psychiatric involvement and cognitive impairment have been reported in a subset of cases, adding to the overall morbidity profile in more severely affected individuals.

Recovery potential: No curative or disease-modifying therapy exists; the disease course is expected to be chronic and progressive for the neurological/ophthalmological features, with supportive management aimed at symptom mitigation (see Section 12).

Prognostic factors: The clearest prognostic signal identified to date is presence vs. absence of cardiomyopathy — carriers who develop cardiac involvement (especially in infancy) face substantially worse prognosis than those with isolated optic atrophy or the adult-onset ataxia/optic atrophy/myopathy triad without cardiac disease.

12. Treatment

Pharmacotherapy: There is no SDHA/NDAXOA-specific approved pharmacotherapy. Management follows general primary mitochondrial disease supportive-care principles: - Coenzyme Q10 (ubiquinone/ubiquinol) supplementation (NCIT treatment_term: NCIT:C15986 Pharmacotherapy; therapeutic_agent CHEBI: ubiquinone) — CoQ10 is the most widely used and best-evidenced supportive agent across mitochondrial respiratory chain disorders broadly, functioning as the mobile electron carrier from Complexes I and II to Complex III and as a lipid-soluble antioxidant; it is of particular theoretical relevance in Complex II disease given CoQ10's direct role as the electron acceptor from succinate dehydrogenase. - Riboflavin (vitamin B2) — a component of the general "mitochondrial cocktail," of particular biochemical relevance given that Complex II's flavoprotein subunit (SDHA) itself requires covalently bound FAD (a riboflavin-derived cofactor) for function; riboflavin supplementation is a reasonable, low-risk adjunct given the FAD-binding defect demonstrated for at least the founding SDHA missense variants, though disease-specific efficacy data for NDAXOA do not exist. - General "mitochondrial cocktail" (antioxidants, L-carnitine, alpha-lipoic acid) is used empirically across mitochondrial disease broadly; no NDAXOA-specific trial data exist.

Advanced therapeutics: No gene therapy, cell therapy, RNA-based therapy, targeted therapy, or immunotherapy has been developed or trialed specifically for NDAXOA. Given the dominant-negative disease mechanism, an allele-selective knockdown (e.g., ASO) approach is theoretically conceivable but has not been reported.

Surgical/interventional: Not applicable to the core neurodegenerative phenotype; cardiac interventions (e.g., management per standard cardiomyopathy protocols, potentially including device therapy or transplantation in severe pediatric cases) would follow general cardiomyopathy management guidelines rather than NDAXOA-specific protocols.

Supportive/rehabilitative care: - Low-vision rehabilitation and ophthalmologic support (NCIT:C15302 Physical Therapy analog for vision rehabilitation) for progressive optic atrophy - Physical therapy (NCIT:C15302) for gait ataxia and myopathy-related mobility impairment - Occupational therapy and genetic counseling (NCIT:C15240) for affected families, particularly given the wide intrafamilial expressivity and the risk of serious cardiac involvement in some carriers - Cardiology surveillance (periodic echocardiography) is a prudent supportive-care recommendation for at-risk family members given the reported infantile cardiomyopathy mortality

Experimental: No NDAXOA-specific clinical trials were identified in ClinicalTrials.gov searches conducted for this report; general mitochondrial-disease CoQ10 trials (e.g., NCT00432744, Phase III Trial of Coenzyme Q10 in Mitochondrial Disease) provide indirect supportive-care evidence but did not specifically enroll or report on SDHA-NDAXOA patients.

Treatment outcomes: No systematic treatment-response or adverse-event data specific to NDAXOA exist, reflecting the extreme rarity of the disorder and absence of dedicated clinical trials.

Treatment strategy: In the absence of disease-specific evidence, management is empirically extrapolated from general primary mitochondrial disease treatment algorithms — CoQ10/riboflavin supplementation, symptom-directed supportive care (vision, mobility, cardiac), genetic counseling, and family cascade screening — rather than a codified, disease-specific clinical pathway.

13. Prevention

Primary prevention: No means of preventing SDHA variant occurrence exists (no known modifiable environmental trigger); primary prevention in practice consists of genetic counseling for known carrier families regarding reproductive options (e.g., prenatal diagnosis, preimplantation genetic testing) given the 50% transmission risk of an autosomal dominant disorder — though penetrance/expressivity counseling must emphasize the wide phenotypic range (isolated optic atrophy through fatal infantile cardiomyopathy) documented even within single families.

Secondary prevention: Early cascade genetic testing of at-risk relatives in known NDAXOA families, coupled with baseline and periodic cardiac surveillance (echocardiography) given the documented risk of serious, occasionally fatal, cardiomyopathy — this represents the single most actionable secondary-prevention/early-detection strategy identifiable from the literature reviewed.

Tertiary prevention: Symptom-directed supportive care (vision rehabilitation, physical therapy, cardiology management) aimed at minimizing complications and functional decline once disease is established (see Section 12).

Immunization: Not applicable — not an infectious or immune-mediated disease.

Screening/genetic counseling: Given the disease's Mendelian dominant inheritance and the availability of confirmatory Complex II enzymatic and molecular genetic testing, genetic counseling and cascade testing are the central applicable prevention/risk-stratification tools; no population-level newborn or carrier screening program exists given the extreme rarity of the disorder.

Public health/environmental/prophylaxis: Not applicable — no environmental or infectious risk-modification strategies are relevant to this monogenic disorder.

14. Other Species / Natural Disease

No naturally occurring veterinary or wildlife disease caused by heterozygous dominant SDHA variants analogous to human NDAXOA has been reported in the literature reviewed. SDHA is highly evolutionarily conserved across eukaryotes (essential TCA cycle/electron transport chain enzyme), and Complex II dysfunction more broadly has been studied in model systems (see Section 15), but no spontaneous companion-animal or livestock NDAXOA phenocopy was identified in this research. Orthologous Sdha is present across mammals (mouse, rat) and other model species with high sequence conservation given the gene's fundamental metabolic role, but no natural-disease veterinary correlate is documented in OMIA or comparable databases for this specific dominant heterozygous phenotype.

15. Model Organisms

Mouse models: - Complete germline Sdha knockout is embryonic lethal in mice, precluding a simple constitutive knockout model of the human disease and reflecting the gene's essential role in core metabolism. - Conditional (tissue-specific and systemic-inducible) Complex II-deficiency mouse models have been developed to circumvent this lethality: - A conditional systemic SDH-loss model manifests as a Leigh-like syndrome, proving lethal within approximately 4 weeks of induced systemic Complex II loss (Khazal et al. 2019, FASEB Journal) — directly modeling the severe end of the SDHA-related disease spectrum (relevant primarily to the recessive/biallelic-loss phenotype, MC2DN1, rather than the dominant-negative NDAXOA mechanism specifically, though it establishes the physiological consequences of profound Complex II loss). - Notably, chronic hypoxia substantially protects mice from lethality after systemic SDH loss, allowing survival despite profoundly impaired oxidative metabolism — a striking finding of mechanistic interest (suggesting that reduced oxygen delivery/demand mismatch, rather than Complex II loss per se, may drive lethality) though with no established human clinical application to date. - Neural-crest-specific conditional Complex II loss models produced mild lower-extremity gait anomalies (suggestive of neural tube closure defects) and patches of unpigmented fur (consistent with neural crest-derived melanocyte dysfunction), while studies of Complex II loss in early Sox10+ cells found developmental defects but did not recapitulate paraganglioma tumorigenesis in that specific conditional context. - Skeletal-muscle-specific SDH knockout models show reduced mitochondrial oxygen consumption, impaired myofiber contractility, and reduced exercise endurance — directly relevant to modeling the myopathy component of the human NDAXOA phenotype.

Model relevance to the specific dominant (NDAXOA) mechanism: The mouse literature identified in this research predominantly models complete/severe Complex II loss (relevant to the recessive MC2DN1/Leigh phenotype) rather than a heterozygous dominant-negative missense knock-in recapitulating the specific NDAXOA mechanism. No knock-in mouse model carrying the human R451C or R662C dominant-negative SDHA alleles was identified in this search — representing a notable knowledge gap: it remains untested in an animal model whether these specific dominant-negative missense variants reproduce the human optic atrophy/ataxia/cardiomyopathy phenotype spectrum, or whether the striking intrafamilial phenotypic variability seen in humans has any parallel in a genetically controlled model system.

Cellular/in vitro models: Patient-derived fibroblasts have been the primary functional model system used to date, providing the ~50% Complex II activity reduction data cited in both the Birch-Machin and Courage/R662C family reports; no patient-derived iPSC, organoid, or CRISPR-engineered isogenic cell line study specific to the dominant-negative NDAXOA variants was identified.

Other model organisms: No Drosophila, zebrafish, C. elegans, or yeast model specifically recapitulating the dominant heterozygous NDAXOA phenotype was identified in this research; Sdh orthologs are studied in yeast and other simple eukaryotes primarily in the context of basic Complex II biochemistry/assembly rather than modeling this specific human dominant disease.

Summary of Key Evidence Gaps for Curation

No dedicated knock-in animal model of the specific dominant-negative NDAXOA missense alleles (R451C, R662C) exists — available mouse Complex II-deficiency models predominantly recapitulate the severe/recessive end of the SDHA disease spectrum.
No modifier genes or mechanistic explanation for the marked intrafamilial phenotypic variability (isolated optic atrophy vs. fatal infantile cardiomyopathy for identical variants) has been identified.
No population-based prevalence/incidence data exist; all clinical characterization derives from fewer than ten published kindreds/case reports.
No NDAXOA-specific clinical trial or treatment-outcome data exist; management is entirely extrapolated from general mitochondrial-disease supportive-care practice (CoQ10, riboflavin).
Formal HPO-frequency-banded phenotype data (e.g., "X% of patients") cannot be derived from the literature given the small case-report-level evidence base — frequency qualifiers should be omitted or handled cautiously per standard evidence-discipline practice.

Key Citations

Birch-Machin MA, Taylor RW, Cochran B, Ackrell BA, Turnbull DM. "Late-onset optic atrophy, ataxia, and myopathy associated with a mutation of a complex II gene." Ann Neurol. 2000;48(3):330-335. PMID:10976639
Courage C, et al. "SDHA mutation with dominant transmission results in complex II deficiency with ocular, cardiac, and neurologic involvement." Am J Med Genet A. 2017;173(1):225-230. PMID:27683074
Renkema GH, Wortmann SB, Smeets RJ, et al. "SDHA mutations causing a multisystem mitochondrial disease: novel mutations and genetic overlap with hereditary tumors." Eur J Hum Genet. 2015;23(2):202-209. PMID:24781757
Alston CL, Davison JE, Meloni F, et al. "Recessive germline SDHA and SDHB mutations causing leukodystrophy and isolated mitochondrial complex II deficiency." (2012/2015 literature; biallelic/MC2DN1 comparator)
Pemp B, Schmidt K, Mitsch C, Reitner A. "Isolated dominant optic atrophy with childhood onset in a family with the heterozygous SDHA mutation c.1351C>T." Invest Ophthalmol Vis Sci. 2022;63(7):A0086. (ARVO abstract)
Sturrock, et al. "Progressive cerebellar atrophy in a patient with complex II and III deficiency and a novel deleterious variant in SDHA: A Counseling Conundrum." Mol Genet Genomic Med. 2021. PMC8222855
Khazal FA, et al. "A conditional mouse model of complex II deficiency manifesting as Leigh-like syndrome." FASEB J. 2019.
OMIM #619259 (NDAXOA); OMIM #252011 (MC2DN1); OMIM *600857 (SDHA)
"A novel de novo heterozygous pathogenic variant in the SDHA gene results in childhood onset bilateral optic atrophy and cognitive impairment." Metab Brain Dis. 2021.

SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy

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Classifications

Inheritance

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Biochemical Markers

Differential Diagnoses

Source YAML

References & Deep Research

References

Deep Research

SDHA-Related Neurodegeneration With Ataxia and Optic Atrophy (NDAXOA) — Comprehensive Research Report

1. Disease Information

2. Etiology

3. Phenotypes

Core triad

Extended/variant phenotypes (reported in expanded case series)

Quality of life impact

Notable pattern

4. Genetic/Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome/Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Summary of Key Evidence Gaps for Curation

Key Citations