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Classifications

Harrison's Chapter

Mechanistic Nosology

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Inheritance

1

Autosomal dominant inheritance HP:0000006

adPEO is inherited in an autosomal dominant manner at all six loci: a single heterozygous pathogenic variant in POLG, SLC25A4, TWNK, POLG2, RRM2B or DNA2 is sufficient. Each child of an affected individual has a 50% risk of inheriting the variant. Family history was positive in 22 of 25 patients in the genotyped TWNK cohort. Two caveats are important. First, germline mosaicism has been documented at the SLC25A4 locus in a parent who did not carry the variant in blood, so an apparently de novo case does not guarantee a negligible sibling recurrence risk. Second, both POLG and RRM2B also cause distinct autosomal RECESSIVE disorders, so the number of pathogenic alleles, not merely the gene, determines the inheritance mode and the counselling.

Autosomal dominant inheritance

Show evidence (4 references)

PMID:20301791 SUPPORT Human Clinical

"Late-onset PEO may be caused by a heterozygous POLG pathogenic variant and inherited in an autosomal dominant manner."

GeneReviews states the dominant single-allele mechanism at the POLG locus.

PMID:24741716 SUPPORT Human Clinical

"If a parent of the proband is affected and/or is known to have the RRM2B pathogenic variant identified in the proband, the risk to sibs of inheriting the pathogenic variant is 50%."

GeneReviews states the 50% transmission risk for the dominant RRM2B genotype.

PMID:35011763 SUPPORT Human Clinical

"Family history was positive in 22 patients."

Documents the dominant pedigree pattern in a genotyped adPEO cohort.

+ 1 more reference

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Subtypes

6

PEOA1 (POLG, polymerase gamma catalytic subunit) MONDO:0024528

POLG hgnc:9179

adPEO caused by a heterozygous pathogenic variant in POLG, the catalytic A subunit of mitochondrial DNA polymerase gamma. The dominant alleles cluster in the polymerase domain (classically p.Y955C in motif B, plus p.G923D, p.R943H and p.A957S) and reduce both catalytic efficiency and nucleotide selectivity. PEOA1 is the single commonest identified cause of familial PEO with multiple mtDNA deletions and carries the heaviest burden of multisystem extension, including parkinsonism and premature ovarian insufficiency. It must not be conflated with its recessive sibling PEOB1, which requires two POLG alleles.

Show evidence (2 references)

PMID:11431686 SUPPORT Human Clinical

"We mapped a new locus for dominant PEO at 15q22-q26 in a Belgian pedigree and identified a heterozygous mutation (Y955C) in the polymerase motif B of the mtDNA polymerase gamma (POLG)."

Founding identification of heterozygous POLG p.Y955C as the cause of dominant PEO, defining the PEOA1 subtype.

PMID:12210792 SUPPORT Human Clinical

"Our data show that mutations of POLG1 are the most frequent cause of familial progressive external ophthalmoplegia associated with accumulation of multiple mitochondrial DNA deletions, accounting for approximately 45% of our family cohort."

Establishes POLG as the most frequent single cause of familial PEO with multiple mtDNA deletions.

PEOA2 (SLC25A4/ANT1, adenine nucleotide translocator 1) MONDO:0012238

SLC25A4 hgnc:10990

adPEO caused by a heterozygous missense variant in SLC25A4, encoding the heart/skeletal-muscle isoform of the adenine nucleotide translocator (ANT1), the inner-membrane ADP/ATP carrier. ANT1 is not part of the replisome; the dominant variants are thought to perturb the mitochondrial nucleotide and energy homeostasis on which faithful mtDNA replication depends. PEOA2 was the first demonstration that a mitochondrial disease could be produced by a dominant mechanism. The phenotype is usually myopathic, and germline mosaicism has been documented in at least one pedigree.

Show evidence (1 reference)

PMID:10926541 SUPPORT Human Clinical

"We have identified two heterozygous missense mutations in the nuclear gene encoding the heart/skeletal muscle isoform of the adenine nucleotide translocator (ANT1) in five families and one sporadic patient."

Founding identification of heterozygous SLC25A4/ANT1 variants in adPEO families, defining the PEOA2 subtype.

PEOA3 (TWNK/C10orf2, Twinkle mtDNA helicase) MONDO:0012241

TWNK hgnc:1160

adPEO caused by a heterozygous variant in TWNK (formerly C10orf2/PEO1), encoding Twinkle, the hexameric replicative 5'-3' DNA helicase that unwinds mtDNA ahead of polymerase gamma and colocalises with mtDNA in mitochondrial nucleoids. The dominant variants cluster in the region mediating subunit interactions, consistent with poisoning of the helicase hexamer. PEOA3 is the most ocular-restricted of the dominant subtypes, with ptosis and ophthalmoparesis near-universal and comparatively little CNS involvement, though cardiac abnormalities occur in roughly a quarter of patients.

Show evidence (2 references)

PMID:11431692 SUPPORT Human Clinical

"Screening of the gene encoding Twinkle in individuals with autosomal dominant progressive external ophthalmoplegia (adPEO), associated with multiple mtDNA deletions, identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins."

Founding identification of TWNK (Twinkle) coding-region mutations segregating in twelve adPEO pedigrees, defining the PEOA3 subtype.

PMID:35011763 SUPPORT Human Clinical

"Autosomal dominant mutations in the TWNK gene, which encodes a mitochondrial DNA helicase, cause adult-onset progressive external ophthalmoplegia (PEO) and PEO-plus presentations."

Contemporary 25-patient cohort confirming TWNK as a cause of adult-onset adPEO and PEO-plus.

PEOA4 (POLG2, polymerase gamma p55 accessory subunit) MONDO:0012415

POLG2 hgnc:9180

adPEO caused by a heterozygous variant in POLG2, encoding the p55 accessory subunit that confers processivity on the polymerase gamma holoenzyme. The prototype allele p.G451E binds DNA normally but fails to stimulate the catalytic p140 subunit or to enhance the DNA-binding strength of the p140-p55 complex, so disease arises through haploinsufficiency or heterodimerisation of mutant with wild-type protein. PEOA4 is a rare subtype.

Show evidence (1 reference)

PMID:16685652 SUPPORT Human Clinical

"that causes progressive external ophthalmoplegia with multiple mtDNA deletions and cytochrome c oxidase (COX)-deficient muscle fibers"

Founding identification of the heterozygous dominant POLG2 p.G451E allele in PEO with multiple mtDNA deletions and COX-deficient fibres.

PEOA5 (RRM2B/p53R2, ribonucleotide reductase small subunit) MONDO:0013117

RRM2B hgnc:17296

adPEO caused by a heterozygous variant in RRM2B, encoding p53R2, the small subunit of the ribonucleotide reductase that supplies deoxyribonucleotides for mtDNA synthesis in non-dividing cells. The prototype allele is the C-terminal nonsense variant p.R327X, whose transcript escapes nonsense-mediated decay and yields a truncated protein that can no longer engage the R1 subunit, acting dominant-negatively. RRM2B is unusual in that biallelic variants in the same gene cause a much more severe, early-onset recessive encephalomyopathic mtDNA depletion syndrome, so the mode of inheritance must be established explicitly.

Show evidence (2 references)

PMID:19664747 SUPPORT Human Clinical

"Mutation screening of RRM2B revealed a heterozygous nonsense mutation in exon 9"

Founding identification of the heterozygous RRM2B p.R327X allele segregating in two adPEO families, defining the PEOA5 subtype.

PMID:24741716 SUPPORT Human Clinical

"Autosomal dominant progressive external ophthalmoplegia (adPEO), typically adult onset; other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness."

GeneReviews defines the adPEO phenotype within the RRM2B mtDNA maintenance defect spectrum.

PEOA6 (DNA2, helicase/nuclease; mtDNA deletion syndrome with progressive myopathy) MONDO:0014062

DNA2 hgnc:2939

adPEO caused by a heterozygous variant in DNA2, encoding a helicase/nuclease involved in mtDNA replication and in the long-patch base-excision repair pathway. Mutant proteins show severe impairment of nuclease, helicase and ATPase activity. The phenotype was originally delineated as an adult-onset progressive myopathy with muscle mtDNA instability and limb-girdle weakness, with PEO a frequent but not invariant component, which is why the MONDO entity carries the myopathy-first label with PEOA6 as a synonym.

Show evidence (1 reference)

PMID:23352259 SUPPORT Human Clinical

"We report the identification, by exome sequencing, of mutations in DNA2 in adult-onset individuals with a form of mitochondrial myopathy featuring instability of muscle mtDNA."

Founding identification of DNA2 mutations in adult-onset mitochondrial myopathy with mtDNA instability, defining the PEOA6 subtype.

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Discussions and Knowledge Gaps

4

Why is a single heterozygous variant sufficient to destabilise the mitochondrial genome at six mechanistically different nuclear loci, when the recessive counterparts at two of those same loci (POLG, RRM2B) require two alleles?

KNOWLEDGE GAP OPEN adpeo_dominance_mechanism_gap

Attached to

pathophysiology#Dominant-Negative Subunit Poisoning and Haploinsufficiency

Three distinct explanations are invoked in the literature and have never been discriminated head to head: dominant-negative poisoning of a multimer (Twinkle hexamer), haploinsufficiency or heterodimerisation within an obligate heterodimer (POLG2 p.G451E), and dominant-negative truncation of an interaction surface (RRM2B p.R327X). It is not established whether the SLC25A4/ANT1 alleles act by any of these routes or by a fourth, transport-based mechanism. This matters because the mechanism determines whether allele-specific silencing would be therapeutic (dominant-negative) or counterproductive (haploinsufficiency), and it is the axis on which any future gene-directed therapy would be designed.

Proposed experiments

Allelic titration of mutant versus wild-type replisome subunits

adpeo_allelic_titration_replisome

In an isogenic human cell system, titrate the mutant:wild-type ratio of Twinkle, POLG, POLG2 and RRM2B and measure mtDNA copy number, deletion burden and fork progression, to distinguish a haploinsufficiency dose-response (linear from 50%) from dominant-negative poisoning (worse than 50% function at 50% mutant).

Allele-specific knockdown of the mutant transcript in patient cells

adpeo_allele_specific_knockdown

Selectively silence the mutant allele in patient-derived myoblasts for each of the six genes and ask whether mtDNA deletion burden falls (supporting dominant-negative action) or worsens (supporting haploinsufficiency).

Show evidence (1 reference)

PMID:16685652 SUPPORT In Vitro

"In vivo, the disease most likely arises through haplotype insufficiency or heterodimerization of the mutated and wild-type proteins, which promote mtDNA deletions by stalling the DNA replication fork."

The authors themselves leave the choice between haploinsufficiency and heterodimerisation unresolved, which is exactly the open question.

Is the selective vulnerability of extraocular muscle in adPEO driven by a lower biochemical threshold, a faster rate of deletion generation, more permissive clonal expansion, or reduced mitochondrial quality control?

KNOWLEDGE GAP OPEN adpeo_extraocular_selectivity_gap

Attached to

pathophysiology#Extraocular Muscle Selective Vulnerability

Extraocular muscle carries three times the COX-deficient fibre burden of limb muscle in CPEO and crosses into COX deficiency at a lower mutational load, and the authors of that study explicitly attribute the pattern to unspecified tissue-specific mechanisms in clonal expansion and expression. The candidate explanations have not been discriminated. This matters because tissue selectivity, not enzymology, is what makes PEO a distinct clinical entity from the other phenotypes produced by the same alleles.

Proposed experiments

Single-fibre deletion load and threshold mapping across muscle allotypes

adpeo_eom_single_fibre_threshold

Measure single-fibre mtDNA deletion load and the heteroplasmy threshold for COX negativity in extraocular, levator palpebrae and limb muscle from the same adPEO donor, to separate a lower threshold from a faster accumulation rate.

Allotype-resolved mitophagy and mitochondrial dynamics measurement

adpeo_eom_mitophagy_flux

Compare mitophagy flux and fusion/fission dynamics between human extraocular and limb muscle to test the quality-control-capacity explanation for selective vulnerability.

Show evidence (1 reference)

PMID:20164463 SUPPORT Human Clinical

"The data also suggest that tissue-specific mechanisms are involved in the clonal expansion and expression of secondary mtDNA deletions in CPEO patients with nuclear genetic defects."

The source explicitly attributes the selectivity to unresolved tissue-specific mechanisms.

What determines whether an adPEO genotype produces ocular-restricted disease or a broad PEO-plus phenotype with neuropathy, ataxia, parkinsonism and premature ovarian insufficiency?

KNOWLEDGE GAP OPEN adpeo_genotype_phenotype_breadth_gap

Attached to

pathophysiology#Multisystem PEO-Plus Extension

The gene identity clearly matters - TWNK-related adPEO shows rare CNS involvement while POLG-related adPEO carries a heavy neurological burden - but gene identity alone does not predict the phenotype within a locus, and the reviewers of the largest mixed cohort concluded that phenotype-genotype correlations cannot be drawn in mitochondrial PEO. No modifier gene, deletion-burden threshold or epigenetic signature has been shown to explain the residual variance. This is the single largest obstacle to prognostic counselling at the time of molecular diagnosis.

Proposed experiments

Cross-locus deep-phenotyping cohort with matched tissue deletion burden

adpeo_multilocus_deep_phenotyping

Assemble a multi-centre adPEO cohort spanning all six loci with standardised neurological, cardiac, audiological and endocrine phenotyping plus quantified muscle deletion burden, and test whether burden, gene, allele class or age explains the breadth of extra-ocular involvement.

Modifier screen in genotype-matched discordant relatives

adpeo_modifier_gwas

Within families segregating a single adPEO allele, compare relatives who are ocular-restricted with those who have PEO-plus disease, to search for nuclear or mtDNA-haplogroup modifiers of phenotypic breadth.

Show evidence (2 references)

PMID:32161153 SUPPORT Human Clinical

"Phenotype-genotype correlations cannot be brought in mitochondrial PEO."

Explicit statement from a large mitochondrial PEO series that genotype does not predict phenotype, defining the gap.

PMID:20479361 SUPPORT Human Clinical

"Our data suggest a shared clinical phenotype with variable mild multiorgan involvement, and that the contribution of PEO1 mutations as a cause of adPEO may well be underestimated."

Documents variable multiorgan involvement within a single locus, showing that gene identity alone is insufficient to predict breadth.

Do the available mouse, cell and yeast models of adPEO genes actually reproduce the defining human disease features, given that no model organism has the human extraocular muscle allotype and that cultured cells cannot reproduce decades-long age-dependent clonal expansion?

HUMAN MODEL MISMATCH OPEN adpeo_model_fidelity_mismatch

Attached to

pathophysiology#Clonal Expansion of Deleted mtDNA in Post-Mitotic Cells pathophysiology#Extraocular Muscle Selective Vulnerability

Model evidence for adPEO is substantial and mechanistically informative - Twinkle and POLG catalytic mutants stall replication in human cells, and transgenic mice establish Twinkle as rate-limiting for mtDNA copy number - but the two features that define the human disease are precisely the ones the models cannot supply. The ocular phenotype depends on the extraocular muscle allotype, which no standard model recapitulates, and the adult onset depends on somatic clonal expansion over four decades, which no cell culture or short-lived rodent can span. Evidence therefore exists but its translational validity to the defining human features is the open question, which is why this is recorded as a human-model mismatch rather than a knowledge gap.

Proposed experiments

Long-duration ageing study in a knock-in adPEO model with ocular phenotyping

adpeo_long_lived_model_aging

Follow a heterozygous knock-in model carrying a human dominant allele across the full lifespan with serial extraocular and limb muscle single-fibre COX/SDH and deletion-burden quantification, to test whether age-dependent ocular-predominant pathology emerges at all.

Human extraocular-allotype myotube model of clonal expansion

adpeo_human_eom_organoid

Derive extraocular-allotype myotubes from patient iPSCs and apply an accelerated mtDNA turnover protocol, to test whether the lower COX threshold of extraocular muscle is cell-autonomous and reproducible outside the orbit.

Show evidence (2 references)

PMID:17452351 PARTIAL In Vitro

"Both groups of mutants reduced mitochondrial DNA copy number by severe replication stalling."

Cell-culture evidence establishes the fork-stalling mechanism but is measured over days and cannot address decades-long clonal expansion or ocular selectivity.

PMID:15509589 PARTIAL Model Organism

"we generated two transgenic mouse lines overexpressing wild-type Twinkle"

The available mouse evidence uses wild-type overexpression to establish copy number control, which is informative for Twinkle biology but is not a model of the heterozygous dominant human allele or of the ocular phenotype.

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Pathophysiology

12

Heterozygous Nuclear mtDNA-Maintenance Gene Variant

A single heterozygous germline variant in one of at least six nuclear genes initiates adPEO. The genes belong to two functional classes that converge on the same molecular lesion. The replisome class comprises POLG (catalytic polymerase gamma A subunit), POLG2 (p55 accessory subunit), TWNK (Twinkle replicative helicase) and DNA2 (helicase/nuclease of mtDNA replication and long-patch base-excision repair). The nucleotide-supply class comprises SLC25A4/ANT1 (the heart and skeletal-muscle ADP/ATP carrier) and RRM2B/p53R2 (the ribonucleotide reductase small subunit that provides dNTPs to non-dividing cells). Because mtDNA is replicated by a small, dedicated protein set with no redundant backup polymerase, loss of function in any one of these components is not buffered.

mitochondrial DNA metabolic process GO:0032042 ⚠ ABNORMAL

mitochondrion GO:0005739

Downstream

Dominant-Negative Subunit Poisoning and Haploinsufficiency In the replisome-class genes the mutant polypeptide is incorporated into a multimeric complex alongside wild-type protein, so one mutant allele degrades the function of the whole assembly.

Imbalanced Mitochondrial dNTP Supply In the nucleotide-supply-class genes (SLC25A4/ANT1 and RRM2B) the primary lesion is not in the replisome but in the provision of balanced deoxyribonucleotide precursors to the replicating mitochondrial genome.

Show evidence (4 references)

PMID:19664747 SUPPORT Human Clinical

"The disorder is heterogeneous, with five known nuclear disease genes that encode the proteins ANT1, Twinkle, POLG, POLG2, and OPA1."

States the locus heterogeneity of adPEO across nuclear mtDNA-maintenance genes.

PMID:23352259 SUPPORT Human Clinical

"Recently, causative mutations have been reported in several nuclear genes that encode proteins of the mtDNA replisome machinery (POLG, POLG2, and C10orf2) or that are involved in pathways for the synthesis of deoxyribonuclotides (ANT1 and RRM2B)."

Directly states the two functional classes - replisome machinery versus deoxyribonucleotide synthesis - into which the adPEO genes partition.

PMID:28324239 SUPPORT Human Clinical

"The genes involved encode proteins belonging to at least three pathways: mtDNA replication and maintenance, nucleotide supply and balance, and mitochondrial dynamics and quality control."

Review-level statement of the pathway classes underlying mtDNA maintenance defects, of which adPEO is the late-onset myopathic end.

+ 1 more reference

Dominant-Negative Subunit Poisoning and Haploinsufficiency

The defining feature that separates adPEO from its recessive counterparts is that heterozygosity suffices. Three mechanisms operate. First, dominant-negative poisoning of a multimer: Twinkle functions as a hexameric ring helicase and the dominant TWNK variants cluster precisely in the region mediating subunit interactions, so mutant monomers incorporate into and disable the ring. Second, haploinsufficiency or heterodimerisation within the polymerase gamma holoenzyme: the POLG2 p.G451E accessory subunit binds DNA normally but fails to stimulate the catalytic p140 subunit or to strengthen the DNA binding of the p140-p55 complex. Third, dominant-negative truncation: the RRM2B p.R327X transcript escapes nonsense-mediated decay and yields a protein lacking the 25 C-terminal residues required to engage the R1 subunit of ribonucleotide reductase. Dominant POLG alleles cluster in the polymerase domain and degrade both catalytic efficiency and nucleotide selectivity, and the magnitude of the biochemical defect tracks clinical severity.

DNA-directed DNA polymerase activity GO:0003887 ↓ DECREASED DNA helicase activity GO:0003678 ↓ DECREASED

mitochondrial nucleoid GO:0042645

Downstream

Replication Fork Stalling on mtDNA A poisoned or under-stimulated replisome cannot sustain processive synthesis around the 16.6 kb mitochondrial genome, so replication forks stall.

Show evidence (6 references)

PMID:11431692 SUPPORT Human Clinical

"The mutations cluster in a region of the protein proposed to be involved in subunit interactions."

The clustering of dominant TWNK variants at the subunit-interaction interface of a hexameric helicase is the structural basis for dominant-negative poisoning.

PMID:16685652 SUPPORT In Vitro

"Although G451E p55 retains a wild-type ability to bind DNA, it fails to enhance the DNA-binding strength of the p140-p55 complex."

Biochemical demonstration that the dominant POLG2 allele produces a subunit that assembles but does not function.

PMID:16685652 SUPPORT In Vitro

"In vivo, the disease most likely arises through haplotype insufficiency or heterodimerization of the mutated and wild-type proteins, which promote mtDNA deletions by stalling the DNA replication fork."

States the haploinsufficiency-or-heterodimerisation mechanism by which a single mutant allele stalls the fork.

+ 3 more references

Imbalanced Mitochondrial dNTP Supply

Post-mitotic cells cannot draw on the S-phase ribonucleotide reductase used by dividing cells and depend instead on the p53R2 (RRM2B) isoform for de novo dNTP synthesis, and on the adenine nucleotide translocator ANT1 for the ADP/ATP exchange that sustains matrix nucleotide and energy homeostasis. Variants in either gene distort the size and balance of the mitochondrial dNTP pool. Replication of mtDNA in the face of a skewed or insufficient precursor pool is slow and error-prone, producing the same downstream fork-stalling lesion that arises from a defective replisome. This is why two mechanistically unrelated protein classes yield an indistinguishable clinical and molecular phenotype.

2'-deoxyribonucleotide metabolic process GO:0009394 ⚠ ABNORMAL

ATP:ADP antiporter activity GO:0005471 ↓ DECREASED ribonucleoside-diphosphate reductase activity, thioredoxin disulfide as acceptor GO:0004748 ↓ DECREASED

mitochondrial inner membrane GO:0005743

Downstream

Replication Fork Stalling on mtDNA A skewed or insufficient mitochondrial dNTP pool slows and destabilises polymerase gamma synthesis, producing the same stalled-fork lesion as a primary replisome defect.

Show evidence (3 references)

PMID:10926541 SUPPORT Human Clinical

"These results indicate that ANT has a role in mtDNA maintenance and that a mitochondrial disease can be caused by a dominant mechanism."

Establishes that the ADP/ATP carrier, though not a replisome component, is required for mtDNA maintenance and acts dominantly.

PMID:23352259 SUPPORT Human Clinical

"or that are involved in pathways for the synthesis of deoxyribonuclotides (ANT1 and RRM2B)"

Assigns ANT1 and RRM2B specifically to the deoxyribonucleotide-supply arm of adPEO pathogenesis.

PMID:28324239 SUPPORT Human Clinical

"These errors of nuclear-mitochondrial intergenomic signaling may lead to mtDNA depletion, accumulation of mtDNA multiple deletions, or both, in critical tissues."

Frames both gene classes as errors of nuclear-mitochondrial intergenomic signalling converging on multiple deletions.

Replication Fork Stalling on mtDNA

The shared proximal lesion of all adPEO genotypes is arrest of the mtDNA replication fork. Expression of catalytically deficient Twinkle or polymerase gamma in human cells causes severe replication stalling and a fall in mtDNA copy number, and the two proteins produce distinguishable stalling signatures, confirming that each acts at the fork. In DNA2-related disease the same endpoint is reached through loss of the nuclease, helicase and ATPase activities needed for flap processing during mtDNA replication and long-patch base-excision repair.

mitochondrial DNA replication GO:0006264 ↓ DECREASED base-excision repair GO:0006284 ↓ DECREASED

DNA nuclease activity GO:0004536 ↓ DECREASED

mitochondrial nucleoid GO:0042645

Downstream

Multiple Large-Scale mtDNA Deletion Formation Stalled and abortive replication intermediates are resolved by illegitimate recombination or slipped-strand mispairing, generating heterogeneous large-scale deletions of the mitochondrial genome.

Show evidence (4 references)

PMID:17452351 SUPPORT In Vitro

"Both groups of mutants reduced mitochondrial DNA copy number by severe replication stalling."

Direct experimental demonstration that catalytically deficient Twinkle and polymerase gamma stall mtDNA replication.

PMID:19664747 SUPPORT Human Clinical

"Defects in these proteins affect mtDNA maintenance, probably leading to stalled replication forks, consequent mtDNA deletion formation, and progressive respiratory chain deficiency."

States the canonical stalled-fork model linking all adPEO gene defects to deletion formation and respiratory chain failure.

PMID:23352259 SUPPORT In Vitro

"In vitro biochemical analysis of purified mutant proteins revealed a severe impairment of nuclease, helicase, and ATPase activities."

Shows the enzymatic basis by which DNA2 variants compromise replication and repair at the fork.

+ 1 more reference

Multiple Large-Scale mtDNA Deletion Formation

The molecular hallmark of adPEO is the accumulation of MULTIPLE, heterogeneous large-scale mtDNA deletions in clinically affected tissue - as distinct from the single, clonal large-scale deletion of sporadic Kearns-Sayre syndrome and sporadic CPEO. The deletions are somatic and arise over a lifetime on the background of the germline nuclear variant, so they are a secondary rather than a primary genetic lesion. The founding pedigree studied by Zeviani and colleagues showed that the deletions in affected relatives all began within a 12-nucleotide stretch at the 5' end of the D-loop, establishing that a nuclear-coded protein defect can destroy mitochondrial genome integrity in a specific, heritable way.

mitochondrial DNA metabolic process GO:0032042 ⚠ ABNORMAL

Downstream

Clonal Expansion of Deleted mtDNA in Post-Mitotic Cells Individual deleted mtDNA molecules replicate within a single post-mitotic cell and progressively displace wild-type genomes over decades.

Show evidence (4 references)

PMID:2725645 SUPPORT Human Clinical

"We have studied four members of a family with autosomal dominant mitochondrial myopathy. Multiple deletions, involving the same portion of muscle mtDNA, were identified in all patients."

The founding clinical description of multiple mtDNA deletions segregating as an autosomal dominant trait.

PMID:2725645 SUPPORT Human Clinical

"The data indicate that a mutation of a nuclear-coded protein can destroy the integrity of the mitochondrial genome in a specific, heritable way."

States the central inference of the disease group: a nuclear lesion producing a mitochondrial genome lesion.

PMID:10926541 SUPPORT Human Clinical

"Autosomal dominant progressive external ophthalmoplegia is a rare human disease that shows a Mendelian inheritance pattern, but is characterized by large-scale mitochondrial DNA (mtDNA) deletions."

Captures the defining paradox of adPEO: Mendelian nuclear transmission with a mitochondrial genome lesion.

+ 1 more reference

Clonal Expansion of Deleted mtDNA in Post-Mitotic Cells

A newly formed deletion is initially a negligible fraction of the mtDNA in a cell and is phenotypically silent. Because post-mitotic cells such as myofibres and neurons turn mtDNA over continuously without dividing, a deleted genome that replicates even slightly faster than wild type expands clonally within a single cell or fibre segment across decades. This is the step that converts a lifelong constitutional nuclear variant into an adult-onset disease and explains the characteristic onset in the third to fifth decade. Clonal expansion is tissue-specific, which contributes to the selective vulnerability of extraocular muscle.

skeletal muscle fiber CL:0008002

Downstream

Threshold-Dependent Mosaic Respiratory Chain Deficiency Once the deleted mtDNA fraction within a cell exceeds roughly 60 to 90 percent, the supply of mtDNA-encoded respiratory chain subunits and tRNAs falls below what oxidative phosphorylation requires.

Show evidence (3 references)

PMID:20164463 SUPPORT Human Clinical

"The data also suggest that tissue-specific mechanisms are involved in the clonal expansion and expression of secondary mtDNA deletions in CPEO patients with nuclear genetic defects."

Directly implicates tissue-specific clonal expansion of secondary mtDNA deletions in patients whose primary defect is nuclear.

PMID:16685652 SUPPORT Human Clinical

"The progressive accumulation of mtDNA deletions causes COX deficiency in muscle fibers and results in the clinical phenotype."

Explicitly links progressive deletion accumulation to fibre-level COX deficiency and the clinical phenotype.

PMID:35011763 SUPPORT Human Clinical

"The mean ages of onset and diagnosis were 43 and 63 years, respectively."

The decades-long latency between a constitutional germline variant and clinical onset is the clinical signature of slow somatic clonal expansion.

Threshold-Dependent Mosaic Respiratory Chain Deficiency

Because mtDNA is multicopy, deleted genomes are functionally recessive until they cross a critical fraction of the cellular mtDNA population. Above this biochemical threshold the affected cell can no longer synthesise the thirteen mtDNA-encoded oxidative phosphorylation subunits and the tRNAs needed to translate them, and oxidative phosphorylation fails. Because expansion is independent in each cell, the resulting deficiency is mosaic: individual COX-negative fibre segments sit alongside normally respiring neighbours. Complex IV is the histochemical readout because three of its catalytic subunits are mtDNA-encoded, whereas complex II (SDH) is entirely nuclear-encoded and is preserved or increased, giving the COX-negative/SDH-hyperreactive signature that is specific for an mtDNA rather than a nuclear OXPHOS lesion.

skeletal muscle fiber CL:0008002

oxidative phosphorylation GO:0006119 ↓ DECREASED

cytochrome-c oxidase activity GO:0004129 ↓ DECREASED

Downstream

Extraocular Muscle Selective Vulnerability Extraocular muscle crosses the biochemical threshold earliest and most completely, so the ocular phenotype dominates the presentation.

Skeletal and Bulbar Mitochondrial Myopathy Limb, neck and bulbar muscle accumulate COX-negative fibre segments more slowly but by the same mechanism, producing proximal weakness and exercise intolerance.

Multisystem PEO-Plus Extension Other high-energy post-mitotic tissues - peripheral nerve, cerebellum, substantia nigra, cardiac conduction system, cochlea, lens and ovary - reach the same threshold in a genotype-dependent subset of patients.

Ragged-red muscle fibers Compensatory proliferation of mitochondria in respiratory-chain-deficient fibre segments produces the subsarcolemmal masses seen as ragged-red fibres.

Cytochrome C oxidase-negative muscle fibers Loss of mtDNA-encoded complex IV subunits above threshold abolishes COX histochemical reactivity in the affected fibre segment.

Multiple mitochondrial DNA deletions The deletions demonstrable in muscle by long-range PCR or Southern blot are the direct molecular readout of this process.

Show evidence (3 references)

PMID:16987890 SUPPORT Human Clinical

"The secondary mtDNA defects cause cell and tissue-specific deficiencies of mitochondrial oxidative phosphorylation, leading to organ dysfunction and human disease."

States the causal step from secondary mtDNA defect to cell- and tissue-specific OXPHOS deficiency.

PMID:20479361 SUPPORT Human Clinical

"Mutations in the Twinkle (PEO1) gene are a recognized cause of autosomal dominant progressive external ophthalmoplegia (adPEO), resulting in the accumulation of multiple mitochondrial DNA (mtDNA) deletions and cytochrome c oxidase (COX)-deficient fibers in skeletal muscle secondary to a disorder..."

Links the mtDNA maintenance defect to accumulation of deletions and COX-deficient fibres in a genotyped adPEO cohort.

PMID:20164463 SUPPORT Human Clinical

"single-fiber analysis revealed a lower mutational threshold for COX deficiency in EOM"

Demonstrates the threshold nature of the biochemical defect at single-fibre resolution.

Extraocular Muscle Selective Vulnerability

Extraocular muscle and the levator palpebrae superioris are affected earliest and most severely in every mtDNA-deletion disorder. The explanation is quantitative: in CPEO patients with mtDNA deletions, 41.6% of extraocular muscle fibres are COX-deficient compared with 13.7% of limb (quadriceps) fibres, and single-fibre analysis shows that extraocular muscle crosses into COX deficiency at a LOWER mutational load than skeletal muscle. Extraocular muscle is a distinct allotype with exceptionally high mitochondrial content, tonic firing rates and continuous activity, which plausibly underlies both its higher deletion burden and its lower threshold. Because the levator and the extraocular muscles fail together and symmetrically, patients develop bilateral ptosis and symmetric ophthalmoparesis and often do NOT report diplopia despite marked ductional limitation.

skeletal muscle fiber CL:0008002

extra-ocular muscle UBERON:0001601 levator palpebrae superioris UBERON:0001604

Downstream

Ptosis Failure of the levator palpebrae superioris, an extraocular-allotype muscle, produces insidious bilateral eyelid droop, usually the presenting sign.

Progressive external ophthalmoplegia Symmetric respiratory-chain failure across the recti and obliques produces slowly progressive limitation of eye movement in all directions.

Diplopia Asymmetric involvement of individual extraocular muscles produces ocular misalignment and double vision in a minority of patients.

Show evidence (2 references)

PMID:20164463 SUPPORT Human Clinical

"CPEO patients with mtDNA deletions had more COX-deficient fibers in EOM (41.6%) than in skeletal muscle (13.7%, P > 0.0001), and single-fiber analysis revealed a lower mutational threshold for COX deficiency in EOM."

Quantifies both the greater deletion burden and the lower biochemical threshold of extraocular muscle, the mechanistic basis of the ocular phenotype.

PMID:20164463 SUPPORT Human Clinical

"The more pronounced mitochondrial biochemical defect and lower mutational threshold in EOM compared with skeletal muscle fibers provide an explanation of the selective muscle involvement in CPEO."

States the authors' mechanistic conclusion for selective extraocular involvement.

Skeletal and Bulbar Mitochondrial Myopathy

Limb, neck, facial and bulbar muscles accumulate COX-negative fibre segments by the same threshold mechanism but more slowly than extraocular muscle, producing the "PEO-plus" myopathic phenotype. In the largest genotyped adPEO cohort (25 TWNK patients), weakness affecting proximal limbs, neck and bulbar muscles was present in 48% and exercise intolerance in 28%. Deficient oxidative phosphorylation limits sustained aerobic work, so fatigue and exercise intolerance often precede fixed weakness. Bulbar involvement produces dysphagia and dysphonia and is the main driver of aspiration risk.

skeletal muscle fiber CL:0008002

oxidative phosphorylation GO:0006119 ↓ DECREASED

muscle organ UBERON:0001630

Downstream

Proximal muscle weakness Accumulating respiratory-chain-deficient fibre segments in limb-girdle and neck muscle produce proximal weakness.

Exercise intolerance Impaired oxidative ATP regeneration limits sustained aerobic muscle work.

Fatigue Reduced oxidative capacity across skeletal muscle produces pervasive fatigue, often the earliest systemic symptom.

Dysphagia Bulbar muscle involvement impairs swallowing and raises aspiration risk.

Elevated circulating creatine kinase concentration Ongoing myofibre stress produces mild hyperCKemia in a minority of patients.

Dysarthria Bulbar muscle involvement degrades articulation, typically alongside dysphagia.

Dysphonia Laryngeal muscle involvement alters voice quality in bulbar-predominant families.

Respiratory insufficiency due to muscle weakness Chest-wall and diaphragmatic myopathy produces restrictive ventilatory failure.

Mitochondrial myopathy The aggregate skeletal-muscle phenotype is a mitochondrial myopathy.

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles. Exercise intolerance was present in 28%."

Quantifies the myopathic and bulbar extension of adPEO in a genotyped cohort.

PMID:20479361 SUPPORT Human Clinical

"Ptosis and ophthalmoparesis were almost universal clinical features among this cohort, with 52% (17/33) reporting fatigue and 33% (11/33) having mild proximal myopathy."

Independent genotyped cohort quantifying fatigue and proximal myopathy alongside the near-universal ocular features.

Multisystem PEO-Plus Extension

Because the nuclear variant is constitutional, every post-mitotic tissue accumulates mtDNA deletions; whether a given tissue becomes clinically involved depends on its energetic demand, its mitotic status and the specific genotype. The breadth of extension is strongly genotype-dependent. TWNK-related adPEO is comparatively ocular-restricted, with CNS features rarely described but cardiac abnormalities in around a quarter of patients, whereas POLG-related adPEO carries the greatest burden of neurological extension. Reported extensions include peripheral neuropathy, ataxia, sensorineural hearing loss, cataract, cardiac conduction disease and cardiomyopathy, respiratory muscle involvement, gastrointestinal dysmotility, depression and other psychiatric features.

Downstream

Nigrostriatal Dopaminergic Degeneration In POLG-related adPEO, deletion-driven respiratory chain failure in substantia nigra neurons produces dopaminergic cell loss.

Ovarian Follicular Depletion Oocyte and granulosa-cell mitochondrial dysfunction accelerates follicular attrition in POLG-related adPEO.

Peripheral axonal neuropathy Length-dependent failure of oxidative ATP supply in long sensory and motor axons produces an axonal neuropathy.

Ataxia Cerebellar and sensory-afferent involvement produces gait and limb ataxia in a minority of patients.

Sensorineural hearing impairment Cochlear hair cells and spiral ganglion neurons are high-energy post-mitotic cells vulnerable to the same threshold mechanism.

Cataract Lens epithelial oxidative stress and impaired energy supply contribute to early-onset lens opacification in some patients.

Arrhythmia Respiratory chain deficiency in the cardiac conduction system and myocardium produces conduction disease and rhythm disturbance.

Depression Psychiatric features including depression and, in one ANT1 pedigree, a schizoaffective disorder have been reported in adPEO families.

Show evidence (4 references)

PMID:35011763 SUPPORT Human Clinical

"Less frequent manifestations were cardiac (24%) and respiratory (4%) involvement, neuropathy (8%), ataxia (4%), and parkinsonism (4%)."

Quantifies the multisystem extension of adPEO in a genotyped TWNK cohort.

PMID:20479361 SUPPORT Human Clinical

"Features consistent with CNS involvement were rarely described; however, in 24% (8/33) of the patients, cardiac abnormalities were reported."

Establishes the genotype-specific pattern in TWNK disease: little CNS involvement but appreciable cardiac involvement.

PMID:20301791 SUPPORT Human Clinical

"Late-onset disease (after age 40 years): Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."

GeneReviews defines the late-onset POLG phenotype, which is the dominant (PEOA1) end of the POLG spectrum, as PEO plus neuropathy and ataxia.

+ 1 more reference

Nigrostriatal Dopaminergic Degeneration

In POLG-related adPEO, mtDNA deletions accumulate in substantia nigra pars compacta dopaminergic neurons, which are exceptionally vulnerable because of their long unmyelinated axons, high basal energy demand and pacemaking calcium load. Parkinsonism cosegregated highly significantly with POLG mutations across seven adPEO families, PET showed dopaminergic neuron loss, and post-mortem examination in two individuals showed loss of pigmented neurons and pigment phagocytosis in the substantia nigra WITHOUT Lewy bodies. The absence of Lewy pathology is mechanistically informative: this is a primary bioenergetic nigrostriatal degeneration rather than an alpha-synucleinopathy, which is why conformance is declared at the nigrostriatal-neurodegeneration node of the parkinsonism module and NOT at its alpha-synuclein aggregation node.

dopaminergic neuron CL:0000700

dopamine biosynthetic process GO:0042416 ↓ DECREASED

substantia nigra UBERON:0002038

Downstream

Parkinsonism Loss of nigrostriatal dopaminergic projection produces striatal dopamine deficiency and clinical parkinsonism.

Show evidence (3 references)

PMID:15351195 SUPPORT Human Clinical

"Clinical assessment showed significant cosegregation of parkinsonism with POLG mutations (p<0.0001), and PET findings were consistent with dopaminergic neuron loss."

Establishes genetic cosegregation of parkinsonism with POLG mutations and confirms dopaminergic neuron loss in vivo by PET.

PMID:15351195 SUPPORT Human Clinical

"Post-mortem examination in two individuals showed loss of pigmented neurons and pigment phagocytosis in substantia nigra without Lewy bodies."

Neuropathological confirmation of nigral degeneration and the key negative finding that distinguishes this from an alpha-synucleinopathy.

PMID:15351195 SUPPORT Human Clinical

"The POLG gene defect resulted in secondary accumulation of mtDNA deletions in patients' tissues."

Confirms that the nigral phenotype sits downstream of the same secondary mtDNA deletion lesion as the ocular myopathy.

Ovarian Follicular Depletion

Oocytes carry the largest mtDNA complement of any human cell and depend on mitochondrial competence for meiotic and developmental viability. In POLG-related adPEO families most affected women reached menopause before age 35, a striking departure from the usual mitochondrial disease phenotype and a reproductive-counselling consideration independent of transmission risk. The presumed mechanism is accelerated attrition of the ovarian follicular reserve on a background of accumulating mtDNA deletions, though the intermediate steps have not been directly demonstrated in human ovary.

ovary UBERON:0000992

Downstream

Premature ovarian insufficiency Accelerated depletion of the follicular reserve produces menopause before age 40, and in these families typically before age 35.

Show evidence (2 references)

PMID:15351195 SUPPORT Human Clinical

"Furthermore, most women with progressive external ophthalmoplegia had early menopause-before age 35 years."

Direct clinical observation of premature menopause in POLG-mutation PEO families.

PMID:15351195 SUPPORT Human Clinical

"Dysfunction of mitochondrial POLG causes a severe progressive multisystem disorder including parkinsonism and premature menopause, which are not typical of mitochondrial disease."

States the authors' conclusion that premature menopause is a genuine and atypical component of the POLG multisystem phenotype.

✶

Histopathology

2

Mitochondrial Change on Skeletal Muscle Biopsy

Muscle biopsy in adPEO shows the composite mitochondrial signature: ragged-red fibres on modified Gomori trichrome, subsarcolemmal mitochondrial accumulation, and mosaic COX-negative fibre segments on sequential COX/SDH histochemistry. Mitochondrial change was present in all 19 available biopsies in the genotyped TWNK adPEO cohort and in 95% of a mixed-aetiology 89-case mitochondrial PEO series. The burden of change varies remarkably between patients, so a sparse or equivocal biopsy does not exclude the diagnosis.

Show evidence (3 references)

PMID:35011763 SUPPORT Human Clinical

"All 19 available muscle biopsies showed signs of mitochondrial dysfunction."

Establishes the near-universal presence of mitochondrial histological change in genetically confirmed adPEO.

PMID:32161153 SUPPORT Human Clinical

"Muscle biopsy was the most accurate test, showing mitochondrial changes in 95%."

Quantifies the diagnostic yield of muscle biopsy in mitochondrial PEO.

PMID:20479361 SUPPORT Human Clinical

"Mitochondrial histochemical changes observed in muscle showed remarkable variability"

Documents the between-patient variability that limits the negative predictive value of biopsy.

Multiple Large-Scale mtDNA Rearrangements in Muscle

Molecular analysis of the biopsy by long-range PCR, Southern blot or deep sequencing demonstrates multiple heterogeneous large-scale mtDNA deletions. This is the finding that redirects the diagnostic workup from the mitochondrial to the nuclear genome and is the discriminator against the single clonal deletion of sporadic CPEO and Kearns-Sayre syndrome. In the 89-case mitochondrial PEO series, multiple mtDNA deletions accounted for 26% of cases and were attributable to TWNK, POLG, TK2 or RRM2B.

Show evidence (2 references)

PMID:32161153 SUPPORT Human Clinical

"Single large-scale mitochondrial DNA (mtDNA) deletion was the most frequent finding (63%), followed by multiple mtDNA deletions (26%) due to mutations in TWNK (n=8), POLG (n=7), TK2 (n=6) or RRM2B (n=2) genes, and point mtDNA mutations (7%)."

Quantifies the molecular architecture of mitochondrial PEO and the nuclear genes responsible for the multiple-deletion subset.

PMID:2725645 SUPPORT Human Clinical

"Multiple deletions, involving the same portion of muscle mtDNA, were identified in all patients."

Founding demonstration of multiple mtDNA deletions in muscle in a dominantly inherited pedigree.

⬡

Pathograph

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

●

Phenotypes

23

Cardiovascular 1

Arrhythmia OCCASIONAL Arrhythmia HP:0011675

Show evidence (2 references)

PMID:20479361 SUPPORT Human Clinical

"however, in 24% (8/33) of the patients, cardiac abnormalities were reported"

Quantifies cardiac abnormality at 24% in a genotyped adPEO cohort.

PMID:35011763 SUPPORT Human Clinical

"Less frequent manifestations were cardiac (24%) and respiratory (4%) involvement"

Independent genotyped cohort reporting the same 24% cardiac involvement rate.

Digestive 1

Dysphagia OCCASIONAL Dysphagia HP:0002015

Course: PROGRESSIVE

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles."

Documents bulbar muscle involvement in the genotyped adPEO cohort; bulbar weakness is the substrate for dysphagia.

PMID:24741716 SUPPORT Human Clinical

"other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness"

GeneReviews lists bulbar dysfunction among the manifestations of RRM2B-related adPEO.

Ear 1

Sensorineural hearing impairment Sensorineural hearing impairment HP:0000407

Course: PROGRESSIVE

No adPEO-specific frequency for hearing loss was found in a quotable abstract, so frequency is deliberately omitted rather than estimated.

Show evidence (1 reference)

PMID:24741716 PARTIAL Human Clinical

"Subsequent assessments are likely to reveal multisystem involvement including sensorineural hearing loss, renal tubulopathy, and respiratory failure."

GeneReviews documents sensorineural hearing loss within the RRM2B mtDNA maintenance defect spectrum; cited as PARTIAL because the sentence describes the severe encephalomyopathic phenotype rather than adPEO specifically.

Eye 3

Ptosis VERY_FREQUENT Ptosis HP:0000508

Course: PROGRESSIVE

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"Ptosis and PEO (92% and 80%) were the most common findings."

Quantifies ptosis at 92% in a genotyped adPEO cohort, supporting the VERY_FREQUENT band.

PMID:20479361 SUPPORT Human Clinical

"Ptosis and ophthalmoparesis were almost universal clinical features among this cohort"

Independent genotyped cohort describing ptosis as almost universal.

Diplopia OCCASIONAL Diplopia HP:0000651

No adPEO-specific quantitative frequency for diplopia was found in a quotable abstract; the OCCASIONAL band reflects the consistent clinical observation that diplopia is disproportionately uncommon relative to the ductional deficit, and should be treated as a clinical estimate rather than a measured figure.

Show evidence (1 reference)

PMID:38391710 PARTIAL Human Clinical

"Chronic progressive external ophthalmoplegia (CPEO) is a rare disorder that can be at the forefront of several mitochondrial diseases."

Review context for the ocular motility phenotype; cited as PARTIAL because the abstract does not itself quantify diplopia frequency.

Cataract VERY_RARE Cataract HP:0000518

The specific cataract count (2 patients, ages 48 and 50) comes from the body of the Bermejo-Guerrero cohort paper via deep research and could NOT be verified against the cached abstract; only the cohort framing is quotable. The VERY_RARE band reflects 2/25 patients.

Show evidence (1 reference)

PMID:35011763 PARTIAL Human Clinical

"Autosomal dominant mutations in the TWNK gene, which encodes a mitochondrial DNA helicase, cause adult-onset progressive external ophthalmoplegia (PEO) and PEO-plus presentations."

Establishes the cohort as genotyped adPEO; the cataract observation itself (two related subjects, ages 48 and 50) is reported in the full text rather than the abstract, so this evidence item is marked PARTIAL.

Genitourinary 1

Premature ovarian insufficiency Premature ovarian insufficiency HP:0008209

Frequency is omitted because the source reports "most women" within seven selected POLG families rather than a cohort-wide percentage; the finding is genotype-specific to POLG-related adPEO and has not been established for the other five loci.

Show evidence (1 reference)

PMID:15351195 SUPPORT Human Clinical

"Furthermore, most women with progressive external ophthalmoplegia had early menopause-before age 35 years."

Direct clinical observation of premature menopause in POLG-mutation PEO families.

Metabolism 1

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

Severity: MILD

Show evidence (1 reference)

PMID:35011763 SUPPORT Human Clinical

"Only 28% had mild hyperCKemia."

Quantifies mild hyperCKemia at 28%, supporting the OCCASIONAL band and the clinical point that a normal CK does not exclude the diagnosis.

Musculoskeletal 2

Proximal muscle weakness FREQUENT Proximal muscle weakness HP:0003701

Course: PROGRESSIVE

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles."

Quantifies proximal, neck and bulbar weakness at 48%, supporting the FREQUENT band.

PMID:20479361 SUPPORT Human Clinical

"33% (11/33) having mild proximal myopathy"

Independent genotyped cohort quantifying mild proximal myopathy at 33%.

Respiratory insufficiency due to muscle weakness Respiratory insufficiency due to muscle weakness HP:0002747

Show evidence (1 reference)

PMID:20880070 SUPPORT Human Clinical

"Respiratory insufficiency caused by chest wall weakness was reported in other families with different Twinkle gene mutations"

Directly reports respiratory insufficiency from chest-wall weakness in TWNK adPEO families.

Nervous System 4

Ataxia VERY_RARE Ataxia HP:0001251

Course: PROGRESSIVE

The VERY_RARE band is genotype-specific to TWNK-related adPEO. In POLG-related adPEO (PEOA1) ataxia is considerably more common, and no single frequency band applies across all six loci.

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"neuropathy (8%), ataxia (4%), and parkinsonism (4%)"

Quantifies ataxia at 4% in TWNK-related adPEO, supporting the VERY_RARE band for that genotype.

PMID:20301791 PARTIAL Human Clinical

"Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."

GeneReviews lists ataxia among late-onset POLG features; cited as PARTIAL because the frequency band recorded here is derived from the TWNK cohort and understates POLG-related disease.

Parkinsonism VERY_RARE Parkinsonism HP:0001300

Course: PROGRESSIVE

The VERY_RARE band is taken from the TWNK cohort. In POLG-related adPEO parkinsonism is substantially more frequent; the Luoma study found significant cosegregation across seven families but did not report a single population percentage that could be mapped to an enum band.

Show evidence (2 references)

PMID:15351195 SUPPORT Human Clinical

"Clinical assessment showed significant cosegregation of parkinsonism with POLG mutations (p<0.0001), and PET findings were consistent with dopaminergic neuron loss."

Establishes parkinsonism as a genuine, genetically cosegregating feature of POLG-related PEO with imaging confirmation.

PMID:35011763 SUPPORT Human Clinical

"neuropathy (8%), ataxia (4%), and parkinsonism (4%)"

Quantifies parkinsonism at 4% in TWNK-related adPEO.

Depression Depression HP:0000716

Frequency is omitted deliberately. The only quotable adPEO psychiatric evidence describes a single sibling with schizoaffective disorder, which does not support a frequency band for depression.

Show evidence (1 reference)

PMID:15792871 PARTIAL Human Clinical

"The mutation was identified in three siblings with PEO, one of them additionally suffered from schizoaffective disorder."

Documents psychiatric comorbidity in a molecularly confirmed adPEO family; cited as PARTIAL because the reported diagnosis is schizoaffective disorder rather than depression specifically.

Dysarthria Dysarthria HP:0001260

Show evidence (1 reference)

PMID:20880070 SUPPORT Human Clinical

"Symptoms reported in association with this mutation were myopathy, peripheral neuropathy, dysarthria and/or dysphagia, respiratory insufficiency and parkinsonism."

Lists dysarthria among the features of the recurrent TWNK p.R374Q adPEO allele.

Constitutional 2

Exercise intolerance OCCASIONAL Exercise intolerance HP:0003546

Show evidence (1 reference)

PMID:35011763 SUPPORT Human Clinical

"Exercise intolerance was present in 28%."

Quantifies exercise intolerance at 28%, supporting the OCCASIONAL band.

Fatigue FREQUENT Fatigue HP:0012378

Temporal: CHRONIC

Show evidence (1 reference)

PMID:20479361 SUPPORT Human Clinical

"with 52% (17/33) reporting fatigue"

Quantifies fatigue at 52% in a genotyped adPEO cohort, supporting the FREQUENT band.

Other 7

Progressive external ophthalmoplegia VERY_FREQUENT Progressive external ophthalmoplegia HP:0000590

Course: PROGRESSIVE

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"Ptosis and PEO (92% and 80%) were the most common findings."

Quantifies PEO at 80% in a genotyped adPEO cohort, supporting the VERY_FREQUENT band.

PMID:32161153 PARTIAL Human Clinical

"'pure PEO' (42%), consisting of isolated palpebral ptosis with ophthalmoparesis"

Defines the pure-PEO phenotype and its share in a mixed-aetiology mitochondrial PEO series; cited as PARTIAL because the cohort includes single-deletion and recessive causes as well as adPEO.

Mitochondrial myopathy VERY_FREQUENT Mitochondrial myopathy HP:0003737

Show evidence (1 reference)

PMID:35011763 SUPPORT Human Clinical

"All 19 available muscle biopsies showed signs of mitochondrial dysfunction."

Establishes that the mitochondrial myopathy is essentially universal in genetically confirmed adPEO.

Ragged-red muscle fibers FREQUENT Ragged-red muscle fibers HP:0003200

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"All 19 available muscle biopsies showed signs of mitochondrial dysfunction."

Documents the universal presence of mitochondrial histological change on adPEO muscle biopsy, of which ragged-red fibres are the canonical form.

PMID:20479361 PARTIAL Human Clinical

"Mitochondrial histochemical changes observed in muscle showed remarkable variability"

Important qualifier: the histochemical burden varies widely between patients, so a sparse biopsy does not exclude the diagnosis.

Cytochrome C oxidase-negative muscle fibers FREQUENT Cytochrome C oxidase-negative muscle fibers HP:0003688

Show evidence (2 references)

PMID:20479361 SUPPORT Human Clinical

"resulting in the accumulation of multiple mitochondrial DNA (mtDNA) deletions and cytochrome c oxidase (COX)-deficient fibers in skeletal muscle secondary to a disorder of mtDNA maintenance"

Directly documents COX-deficient fibres as the histochemical consequence of the mtDNA maintenance defect in genotyped adPEO.

PMID:16685652 SUPPORT Human Clinical

"The progressive accumulation of mtDNA deletions causes COX deficiency in muscle fibers and results in the clinical phenotype."

Independent genotype (POLG2) demonstrating the same COX-deficient fibre readout.

Multiple mitochondrial DNA deletions OBLIGATE Multiple mitochondrial DNA deletions HP:0003689

Show evidence (2 references)

PMID:19664747 SUPPORT Human Clinical

"Autosomal-dominant progressive external ophthalmoplegia (adPEO) is a mitochondrial disorder that is characterized by accumulation of multiple mitochondrial DNA (mtDNA) deletions in postmitotic tissues."

Defines multiple mtDNA deletions in post-mitotic tissue as the characteristic feature of adPEO, supporting the OBLIGATE band.

PMID:20479361 SUPPORT Human Clinical

"as did the secondary mtDNA deletions, which in some patients were only detected by PCR-based assays and not Southern blotting"

Important methodological caveat on detection sensitivity for the defining molecular phenotype.

Peripheral axonal neuropathy OCCASIONAL Peripheral axonal neuropathy HP:0003477

Course: PROGRESSIVE

Show evidence (2 references)

PMID:35011763 SUPPORT Human Clinical

"neuropathy (8%), ataxia (4%), and parkinsonism (4%)"

Quantifies neuropathy at 8% in TWNK-related adPEO.

PMID:20301791 SUPPORT Human Clinical

"Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."

GeneReviews places peripheral neuropathy among the core features of late-onset POLG disease, the genotype in which the neurological extension is greatest.

Dysphonia Dysphonia HP:0001618

Show evidence (1 reference)

PMID:26838077 SUPPORT Human Clinical

"We have studied a large Iranian family with myopathy, dysphonia, dysphagia, and behavior change in addition to PEO in affected members."

Documents dysphonia alongside dysphagia in a large TWNK adPEO family.

🧬

Genetic Associations

6

POLG (A single heterozygous pathogenic variant in POLG, encoding the catalytic A subunit of mitochondrial DNA polymerase gamma, causes PEOA1. The dominant alleles cluster in the polymerase domain (p.G923D, p.R943H, p.Y955C, p.A957S), where they reduce catalytic efficiency and nucleotide selectivity; p.Y955C in motif B is the prototype. POLG is the most frequent single cause of familial PEO with multiple mtDNA deletions, accounting for approximately 45% of one family cohort. The same gene causes a wide autosomal RECESSIVE spectrum (PEOB1, Alpers-Huttenlocher, MIRAS, SANDO), so the number of pathogenic alleles must be established explicitly.)

Gene: POLG hgnc:9179 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (4 references)

PMID:11431686 SUPPORT Human Clinical

"We mapped a new locus for dominant PEO at 15q22-q26 in a Belgian pedigree and identified a heterozygous mutation (Y955C) in the polymerase motif B of the mtDNA polymerase gamma (POLG)."

Founding identification of the dominant POLG allele.

PMID:12210792 SUPPORT Human Clinical

"We screened the POLG1 gene in several PEO families and identified five different heterozygous missense mutations of POLG1 in 10 autosomal dominant families."

Establishes heterozygous POLG missense variants in ten dominant PEO families.

PMID:15258572 SUPPORT In Vitro

"Four autosomal dominant mutations that cause PEO encode the amino acid substitutions G923D, R943H, Y955C and A957S in the polymerase domain of pol gamma."

Enumerates the four canonical dominant POLG alleles and localises them to the polymerase domain.

+ 1 more reference

SLC25A4 (A single heterozygous missense variant in SLC25A4, encoding the heart and skeletal-muscle isoform of the adenine nucleotide translocator (ANT1), causes PEOA2. The prototype familial allele substitutes proline for a highly conserved alanine at position 114; the analogous substitution in yeast produces a respiratory defect. A second reported allele converts a conserved alanine at codon 90 to aspartic acid. Germline mosaicism has been documented, which has direct implications for recurrence-risk counselling in apparently sporadic cases.)

Gene: SLC25A4 hgnc:10990 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:10926541 SUPPORT Human Clinical

"The familial mutation substitutes a proline for a highly conserved alanine at position 114 in the ANT1 protein. The analogous mutation in yeast caused a respiratory defect."

Identifies the prototype ANT1 allele and provides cross-species functional support.

PMID:15792871 SUPPORT Human Clinical

"Microsatellite analysis showed that the mutation was dominant and inherited from the mother who did not carry the mutation in blood, indicating germ-line mosaicism."

Documents germline mosaicism at the SLC25A4 locus, a recurrence-risk consideration in apparently de novo adPEO.

TWNK (A single heterozygous variant in TWNK (formerly C10orf2/PEO1), encoding the Twinkle replicative mtDNA helicase, causes PEOA3. Eleven different coding-region mutations co-segregated with disease in twelve adPEO pedigrees in the founding report, clustering in the region mediating subunit interactions of the hexameric helicase. In a contemporary 25-patient Spanish cohort, ten different TWNK variants were found, with c.1361T>G (p.Val454Gly) and c.1070G>C (p.Arg357Pro) the most common. TWNK is a leading cause of adPEO when the family history is positive.)

Gene: TWNK hgnc:1160 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (3 references)

PMID:11431692 SUPPORT Human Clinical

"identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins"

Founding co-segregation evidence for TWNK in adPEO.

PMID:35011763 SUPPORT Human Clinical

"Ten different TWNK mutations were identified, with c.1361T>G (p.Val454Gly) and c.1070G>C (p.Arg357Pro) being the most common."

Contemporary allelic spectrum of TWNK-related adPEO.

PMID:35011763 SUPPORT Human Clinical

"TWNK is an important cause when positive family history is present"

Positions TWNK as a high-priority gene when the pedigree is dominant.

POLG2 (A single heterozygous variant in POLG2, encoding the p55 accessory subunit of polymerase gamma, causes PEOA4. The prototype allele c.1352G>A (p.G451E) retains DNA binding but fails to stimulate the catalytic p140 subunit or to strengthen p140-p55 complex DNA binding, so disease arises through haploinsufficiency or heterodimerisation with wild-type protein. PEOA4 is rare.)

Gene: POLG2 hgnc:9180 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (1 reference)

PMID:16685652 SUPPORT Human Clinical

"Biochemical characterization of purified, recombinant G451E-substituted p55 protein in vitro revealed incomplete stimulation of the catalytic subunit due to compromised subunit interaction."

Biochemical characterisation of the prototype dominant POLG2 allele.

RRM2B (A single heterozygous variant in RRM2B, encoding p53R2, the small subunit of the ribonucleotide reductase supplying deoxyribonucleotides to non-dividing cells, causes PEOA5. The prototype allele is the C-terminal nonsense variant p.R327X, whose transcript escapes nonsense-mediated decay and produces a dominant-negative truncated protein. Biallelic RRM2B variants cause a mechanistically related but far more severe recessive encephalomyopathic mtDNA depletion syndrome, so the inheritance mode must be established explicitly for each family.)

Gene: RRM2B hgnc:17296 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:19664747 SUPPORT Human Clinical

"We conclude that dominant-negative or gain-of-function mutations in RRM2B are a cause of multiple mtDNA deletions and adPEO."

States the dominant mechanism at the RRM2B locus.

PMID:24741716 SUPPORT Human Clinical

"The diagnosis of an RRM2B-MDMD is established in a proband with suggestive findings and either biallelic RRM2B pathogenic variants or a heterozygous RRM2B pathogenic variant identified by molecular genetic testing."

GeneReviews establishes that both monoallelic (dominant) and biallelic (recessive) RRM2B genotypes cause disease.

DNA2 (A single heterozygous variant in DNA2, encoding a helicase/nuclease acting in mtDNA replication and in the long-patch base-excision repair pathway, causes PEOA6. Mutant proteins show severe impairment of nuclease, helicase and ATPase activity. The phenotype was delineated as adult-onset progressive myopathy with muscle mtDNA instability and limb-girdle weakness, with PEO a frequent component.)

Gene: DNA2 hgnc:2939 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:23352259 SUPPORT Human Clinical

"DNA2 encodes a helicase/nuclease family member that is most likely involved in mtDNA replication, as well as in the long-patch base-excision repair (LP-BER) pathway."

Defines the molecular function of DNA2 in mtDNA maintenance.

PMID:23352259 SUPPORT In Vitro

"In vitro biochemical analysis of purified mutant proteins revealed a severe impairment of nuclease, helicase, and ATPase activities."

Functional confirmation that the PEOA6 alleles are loss-of-function for all three catalytic activities.

💊

Medical Actions

6

Supportive and Multidisciplinary Care

Action: Supportive Care NCIT:C15747

There is no approved disease-modifying therapy for adPEO. Management is supportive and multidisciplinary: occupational, physical and speech therapy for the myopathy and bulbar involvement; nutritional and swallowing support; respiratory support where needed; hearing and low-vision support; and periodic cardiac and respiratory surveillance. Coenzyme Q10, riboflavin and other "mitochondrial cocktail" supplements are widely used empirically, but no adPEO-specific efficacy evidence was identified.

Show evidence (3 references)

PMID:38391710 SUPPORT Human Clinical

"No definitive treatment option is available for mitochondrial diseases, and management is mainly focused on lifestyle risk modification and supplementation to reduce mitochondrial load and symptomatic relief, such as ptosis repair in the case of CPEO."

Contemporary review stating the absence of definitive therapy and the supportive, symptom-directed model of care for mitochondrial CPEO.

PMID:24741716 SUPPORT Human Clinical

"To date, there are no known cures and few effective treatments for any forms of mitochondrial disease, including the RRM2B-MDMDs."

GeneReviews confirms the absence of curative or disease-modifying therapy for the mtDNA maintenance defects.

PMID:20301791 SUPPORT Human Clinical

"Clinical management is largely supportive and involves standard approaches for associated complications including occupational, physical, and speech therapy; nutritional support; respiratory support"

GeneReviews specifies the components of supportive multidisciplinary management.

Ptosis Surgery (Levator Advancement or Frontalis Suspension)

Action: Ophthalmologic Surgical Procedure NCIT:C15331

Surgical correction is the mainstay of symptomatic treatment for the ptosis of adPEO, chosen according to levator excursion: levator advancement or resection where levator function is preserved, frontalis suspension (sling) where it is not. Myogenic ptosis is a particularly difficult surgical target because the ptosis continues to worsen and because the same mitochondrial myopathy impairs protective corneal mechanisms (Bell phenomenon, orbicularis closure). Deliberate under-correction is therefore standard, at the cost of a high recurrence rate.

Target Phenotypes: Ptosis HP:0000508

Show evidence (3 references)

PMID:36178005 SUPPORT Human Clinical

"Surgical techniques predominantly involved levator advancement, levator resection, frontalis sling, blepharoplasty, and Fasanella-Servat."

Enumerates the surgical options used for progressive myogenic ptosis, of which CPEO is an explicitly analysed subgroup.

PMID:36178005 SUPPORT Human Clinical

"However, myogenic ptosis is especially challenging as it is characterized by worsening ptosis and the loss of protective corneal mechanisms."

Explains why mitochondrial myogenic ptosis carries a higher surgical risk than other ptosis aetiologies.

PMID:36178005 SUPPORT Human Clinical

"The goals of care with myogenic ptosis involves repairing ptosis just sufficiently to alleviate visual obstruction while avoiding adverse post-operative complications."

Establishes the deliberate-under-correction principle governing ptosis surgery in mitochondrial ocular myopathy.

Avoidance of Valproic Acid (Contraindicated)

Action: Pharmacotherapy NCIT:C15986

Agent: valproic acid CHEBI:39867

Valproic acid and sodium divalproate must be avoided in POLG-related disease, including PEOA1, because they can precipitate or accelerate liver disease and fatal hepatic failure; POLG genotype prospectively identifies individuals at high risk. GeneReviews extends an equivalent caution to RRM2B-related disease, where valproate should be used only in exceptional circumstances. Because epilepsy can occur in POLG-related disease and valproate is a common anticonvulsant choice, this is the single most actionable prescribing fact in adPEO. This record is curated with negative valence: the intervention is withholding the drug, not administering it. SCOPE LIMIT: the caution is GENOTYPE-DIRECTED, not adPEO-wide. It is established for POLG (PEOA1) and extended by GeneReviews to RRM2B (PEOA5); no equivalent evidence exists for the SLC25A4 (PEOA2), TWNK (PEOA3), POLG2 (PEOA4) or DNA2 (PEOA6) subtypes, and this entry does not assert one for them. Note also that the underlying hepatotoxicity literature is dominated by BIALLELIC POLG disease, so its transfer to heterozygous dominant adPEO is an extrapolation of prescribing caution rather than a directly demonstrated dominant-genotype risk.

Show evidence (3 references)

PMID:20301791 SUPPORT Human Clinical

"because of the risk of precipitating and/or accelerating liver disease"

GeneReviews lists valproic acid and sodium divalproate under Agents/circumstances to avoid for POLG-related disorders.

PMID:24741716 SUPPORT Human Clinical

"Valproic acid should be used only in exceptional circumstances."

GeneReviews extends the valproate caution to RRM2B mtDNA maintenance defects, which include the adPEO phenotype.

PMID:21038416 SUPPORT Human Clinical

"These findings implicate impaired liver regeneration in VPA toxicity and show that prospective genetic testing of POLG will identify individuals at high risk of this potentially fatal consequence of treatment."

Demonstrates that POLG genotype prospectively identifies valproate hepatotoxicity risk, making avoidance an actionable, genotype-driven decision.

Genetic Counseling and Cascade Testing

Action: Genetic Counseling NCIT:C15240

Because adPEO is dominantly transmitted, each child of an affected individual has a 50% risk. Counselling should cover the transmission risk, the variable expressivity and late onset that make the phenotype easy to miss in relatives, the availability of prenatal and preimplantation genetic testing once the familial variant is known, and the germline-mosaicism caveat that an apparently de novo case does not guarantee negligible sibling recurrence risk. For POLG and RRM2B, counselling must establish whether the family carries a dominant (monoallelic) or a recessive (biallelic) genotype, since the recurrence risks differ fundamentally.

Show evidence (2 references)

PMID:24741716 SUPPORT Human Clinical

"Once the RRM2B pathogenic variant(s) have been identified in an affected family member, prenatal and preimplantation genetic testing for RRM2B-MDMD are possible."

GeneReviews confirms the availability of prenatal and preimplantation testing once the familial variant is known.

PMID:35011763 SUPPORT Human Clinical

"Accurate differential diagnosis and early confirmation with appropriately chosen complementary studies allow genetic counseling and the avoidance of unnecessary treatments."

States the dual value of early molecular confirmation: enabling counselling and preventing futile treatment.

Physical Therapy and Paced Exercise

Action: Physical Therapy NCIT:C15302

Individualised submaximal aerobic and resistance training, physiotherapy, activity pacing and fall prevention are offered for the myopathic component. Exercise is supportive rather than disease-modifying and should be prescribed at submaximal intensity given the limited oxidative reserve.

Target Phenotypes: Proximal muscle weakness HP:0003701

Show evidence (1 reference)

PMID:20301791 SUPPORT Human Clinical

"including occupational, physical, and speech therapy"

GeneReviews includes physical therapy among the standard supportive measures.

Elamipretide (Investigational, Not Approved)

Action: Pharmacotherapy NCIT:C15986

Agent: elamipretide NCIT:C171874

Elamipretide is a mitochondria-targeted tetrapeptide investigated for primary mitochondrial myopathy. It is NOT approved for adPEO or for any mitochondrial myopathy indication, and the Phase 3 MMPOWER-3 trial was NEGATIVE in its genotypically diverse primary mitochondrial myopathy population. A POST HOC genotype-stratified analysis found that participants carrying mtDNA replisome variants who also had CPEO - broadly the adPEO population - improved on the six-minute walk test by 37.3 +/- 9.5 m versus -8.0 +/- 10.7 m on placebo at week 24 (p=0.0024), with the replisome cohort as a whole only trending toward benefit (p=0.06). This is a hypothesis-generating exploratory subgroup finding from a trial that failed its primary endpoints; it is NOT evidence of efficacy. It motivated the dedicated Phase 3 NuPOWER trial (NCT05162768) in nuclear-DNA primary mitochondrial disease, and no efficacy conclusion can be drawn until those results are published. This treatment is curated so that the negative parent-trial result is on the record alongside the subgroup signal.

Target Phenotypes: Exercise intolerance HP:0003546

Show evidence (3 references)

PMID:39574155 REFUTE Human Clinical

"As previously published, the MMPOWER-3 clinical trial did not demonstrate a significant benefit of elamipretide treatment in a genotypically diverse population of adults with primary mitochondrial myopathy (PMM)."

The negative primary result of the parent Phase 3 trial, recorded explicitly and first so that the post-hoc subgroup signal below cannot be read as established efficacy.

PMID:39574155 PARTIAL Human Clinical

"The 6MWT results at week 24 in subjects with replisome variants showed a significant change in the elamipretide group subjects who had chronic progressive external ophthalmoplegia (CPEO) (37.3 ± 9.5 m versus - 8.0 ± 10.7 m for the placebo group; p = 0.0024)."

The quantitative result for the replisome-plus-CPEO subgroup, which is the subgroup corresponding to adPEO. Cited as PARTIAL because it is an exploratory post-hoc subgroup analysis of a trial that failed its primary endpoints, not confirmatory efficacy evidence.

PMID:39574155 PARTIAL Human Clinical

"These data serve as the foundation for a follow-up Phase 3 clinical trial (NuPOWER) which has been designed as described in this paper to determine the efficacy of elamipretide in patients with mtDNA maintenance-related disorders."

Confirms that efficacy in exactly the adPEO population remains an open question to be resolved by a dedicated confirmatory trial.

🔬

Biochemical Markers

2

Serum Creatine Kinase (VARIABLE)

Show evidence (1 reference)

PMID:35011763 SUPPORT Human Clinical

"Only 28% had mild hyperCKemia."

Only a minority of genotyped adPEO patients have raised creatine kinase, and the elevation is mild.

Serum Lactate (VARIABLE)

Show evidence (1 reference)

PMID:28695364 PARTIAL Human Clinical

"Increased lactate was associated with central neurological involvement."

In a large genetically diagnosed mitochondrial cohort, raised lactate tracked with CNS involvement rather than with the ocular myopathy itself.

🔀

Differential Diagnoses

6

Conditions with similar clinical presentations that must be differentiated from Autosomal Dominant Progressive External Ophthalmoplegia:

Myasthenia Gravis MONDO:0009688

Overlapping Features The commonest misdiagnosis. Both cause ptosis and ophthalmoparesis, but myasthenic ptosis fluctuates, fatigues on sustained upgaze, and is typically accompanied by diplopia, whereas adPEO ptosis is fixed, slowly progressive over years and disproportionately diplopia-free. In the genotyped TWNK adPEO cohort, 36% of patients had previously been misdiagnosed as myasthenia. Acetylcholine receptor antibodies, repetitive nerve stimulation and single-fibre EMG separate the two.

Distinguishing Features

Fluctuating and fatigable weakness in myasthenia versus fixed slow progression in adPEO
Diplopia usual in myasthenia, disproportionately uncommon in adPEO
Positive acetylcholine receptor antibodies and decrement on repetitive nerve stimulation in myasthenia
Response to anticholinesterase in myasthenia

Show evidence (1 reference)

PMID:35011763 SUPPORT Human Clinical

"Before definitive genetic confirmation, 56% of patients were misdiagnosed (36% with myasthenia, 20% with oculopharyngeal muscle dystrophy)."

Quantifies myasthenia gravis as the leading misdiagnosis of adPEO, making it the most important differential.

Oculopharyngeal Muscular Dystrophy Not Yet Curated MONDO:0008116

Overlapping Features An autosomal dominant late-onset myopathy caused by a short GCN trinucleotide repeat expansion in PABPN1, presenting with ptosis and dysphagia. It shares the dominant pedigree, the late onset and the ptosis-plus-bulbar phenotype with adPEO and accounted for 20% of prior misdiagnoses in the genotyped TWNK cohort. Extraocular motility is relatively spared in OPMD relative to the severity of the ptosis, and muscle biopsy shows rimmed vacuoles and intranuclear filamentous inclusions rather than ragged-red and COX-negative fibres.

Distinguishing Features

PABPN1 GCN repeat expansion rather than a nuclear mtDNA maintenance gene variant
Rimmed vacuoles and intranuclear inclusions rather than ragged-red and COX-negative fibres
Ophthalmoparesis relatively mild for the degree of ptosis
No multiple mtDNA deletions in muscle

Show evidence (1 reference)

PMID:35011763 SUPPORT Human Clinical

"36% with myasthenia, 20% with oculopharyngeal muscle dystrophy"

Quantifies oculopharyngeal muscular dystrophy as the second commonest misdiagnosis of adPEO.

Single Large-Scale mtDNA Deletion Syndromes (Sporadic CPEO, Kearns-Sayre) MONDO:0010787

Overlapping Features Sporadic CPEO and Kearns-Sayre syndrome are caused by a SINGLE clonal large-scale mtDNA deletion arising in the germline or early embryogenesis, with no nuclear gene lesion and negligible recurrence risk. They are clinically almost indistinguishable from adPEO at the bedside, and are in fact the majority of mitochondrial PEO seen in clinic (63% of an 89-case series versus 26% with multiple deletions). Distinguishing single from multiple deletions on muscle mtDNA analysis is the decisive step and completely changes the genetic counselling.

Distinguishing Features

Single clonal large-scale mtDNA deletion versus multiple heterogeneous deletions
Sporadic occurrence with negligible recurrence risk versus 50% dominant transmission
Kearns-Sayre adds retinopathy, cardiac conduction block and onset before age 20
No causative nuclear gene variant identified

Show evidence (1 reference)

PMID:32161153 SUPPORT Human Clinical

"Single large-scale mitochondrial DNA (mtDNA) deletion was the most frequent finding (63%), followed by multiple mtDNA deletions (26%)"

Quantifies the relative frequency of the single-deletion and multiple-deletion classes, showing that the single-deletion syndromes are the larger group and the primary differential.

Autosomal Recessive Progressive External Ophthalmoplegia (PEOB1 and Related) MONDO:0009783

Overlapping Features The recessive counterparts at the same loci: biallelic POLG variants cause PEOB1, and biallelic RRM2B variants cause a severe encephalomyopathic mtDNA depletion syndrome and an arPEO phenotype. The molecular endpoint (multiple mtDNA deletions) and much of the clinical picture are shared, so the distinction rests entirely on the NUMBER of pathogenic alleles and the pedigree, not on the gene or the phenotype. Recessive disease tends to earlier onset and heavier multisystem involvement.

Distinguishing Features

Biallelic rather than monoallelic pathogenic variants
Typically earlier onset and heavier neurological extension
25% sibling recurrence risk rather than 50% offspring transmission
Common European POLG founder alleles p.A467T and p.W748S in the recessive form

Show evidence (2 references)

PMID:20301791 SUPPORT Human Clinical

"Establishing the diagnosis of a POLG-related disorder relies on clinical findings and the identification of biallelic POLG pathogenic variants on molecular genetic testing for all phenotypes except autosomal dominant progressive external ophthalmoplegia (adPEO), for which identification of a..."

GeneReviews states the allele-count discriminator between the dominant and recessive POLG phenotypes.

PMID:24741716 SUPPORT Human Clinical

"RRM2B autosomal recessive progressive external ophthalmoplegia (arPEO), a typically childhood-onset predominantly myopathic phenotype of PEO, ptosis, proximal muscle weakness, and bulbar dysfunction."

Defines the recessive RRM2B PEO phenotype that must be distinguished from dominant PEOA5.

OPA1-Related Dominant Optic Atrophy Plus MONDO:0014720

Overlapping Features Heterozygous OPA1 variants cause a dominant syndrome that can include CPEO and multiple mtDNA deletions in muscle, so it overlaps adPEO both molecularly and clinically and is listed among the adPEO genes by some authors. It is excluded from the subtype list of this entry and modelled here as a differential instead, because its cardinal feature is bilateral progressive optic atrophy with central and colour vision loss, and because it acts through mitochondrial inner-membrane fusion and cristae architecture rather than through the mtDNA replisome or the nucleotide pool. dismech curates it separately as Autosomal_Dominant_Optic_Atrophy_Plus.

Distinguishing Features

Bilateral optic atrophy with central and colour vision loss as the cardinal feature
Mechanism is mitochondrial inner-membrane fusion and cristae architecture, not the mtDNA replisome or dNTP supply
Distinct MONDO identity (MONDO:0014720) and a separate dismech entry

Show evidence (1 reference)

PMID:19664747 PARTIAL Human Clinical

"The disorder is heterogeneous, with five known nuclear disease genes that encode the proteins ANT1, Twinkle, POLG, POLG2, and OPA1."

Confirms that OPA1 is counted among the dominant nuclear genes producing an adPEO-overlapping phenotype, which is why it must be actively differentiated rather than ignored.

Thyroid Eye Disease and Other Orbital Disease

Overlapping Features Restrictive orbital myopathy from thyroid eye disease, orbital inflammatory disease or an orbital mass can mimic ophthalmoparesis. These are distinguished by proptosis, lid retraction rather than ptosis, orbital pain, forced-duction restriction and characteristic orbital imaging, and by the absence of a dominant family history or muscle mitochondrial pathology.

Distinguishing Features

Proptosis and lid retraction rather than ptosis
Abnormal orbital imaging with extraocular muscle enlargement
Positive forced duction testing indicating a restrictive rather than myogenic process
Thyroid function and thyroid autoantibody abnormalities

Show evidence (1 reference)

PMID:38391710 PARTIAL Human Clinical

"Understanding the varying presentations and genetic aspects of mitochondrial CPEO is crucial for accurate diagnosis and management."

Review emphasising the diagnostic-accuracy problem in CPEO; cited as PARTIAL because the abstract does not itself enumerate the orbital differentials.

🔬

Clinical Trials

1

NCT05162768 PHASE_III COMPLETED

NuPOWER (SPIMD-301): a 48-week randomised, double-blind, parallel-group, placebo-controlled Phase 3 trial of daily subcutaneous elamipretide in subjects with primary mitochondrial myopathy caused by nuclear DNA mutations - the population that includes adPEO. Designed as the confirmatory follow-up to the post-hoc replisome/CPEO signal from MMPOWER-3. Listed as completed, but results were not available in the registry record at the time of curation, so no efficacy conclusion can be drawn from completion status alone.

Target Phenotypes: Exercise intolerance HP:0003546 Proximal muscle weakness HP:0003701

Show evidence (1 reference)

clinicaltrials:NCT05162768 SUPPORT Human Clinical

"SPIMD-301 is a 48-week, randomized, double-blind, parallel-group, placebo-controlled trial to assess efficacy and safety of single daily subcutaneous (SC) administration of elamipretide as a treatment for subjects with primary mitochondrial myopathy associated with nuclear DNA mutations (nPMD)."

The registry description of the confirmatory Phase 3 trial targeting the nuclear-DNA mitochondrial myopathy population to which adPEO belongs.

{ }

Source YAML

click to show

name: Autosomal Dominant Progressive External Ophthalmoplegia
creation_date: "2026-07-31T00:00:00Z"
category: Mendelian
synonyms:
- adPEO
- adCPEO
- PEOA
- progressive external ophthalmoplegia, autosomal dominant
- progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal
  dominant
description: >
  Autosomal dominant progressive external ophthalmoplegia (adPEO) is a genetically
  heterogeneous, nuclear-encoded disorder of mitochondrial DNA (mtDNA) maintenance.
  A single heterozygous pathogenic variant in one of at least six nuclear genes is
  sufficient to destabilise the mitochondrial genome, and the shared molecular
  signature of the group is the accumulation of MULTIPLE large-scale mtDNA deletions
  in post-mitotic tissue. The causative genes fall into two functional classes that
  converge on the same lesion: components of the mtDNA replisome itself (POLG, the
  catalytic polymerase gamma subunit, in PEOA1; POLG2, its p55 accessory subunit, in
  PEOA4; TWNK/C10orf2, the replicative Twinkle helicase, in PEOA3; and DNA2, a
  helicase/nuclease of mtDNA replication and long-patch base-excision repair, in
  PEOA6), and proteins that govern the mitochondrial deoxyribonucleotide (dNTP)
  supply (SLC25A4/ANT1, the heart and skeletal-muscle adenine nucleotide
  translocator, in PEOA2; and RRM2B/p53R2, the small subunit of ribonucleotide
  reductase that supplies dNTPs to non-dividing cells, in PEOA5).

  Whichever gene is affected, the mutant allele acts through dominant-negative
  subunit poisoning or haploinsufficiency within a multimeric replisome, so a single
  mutant copy is enough to stall the mtDNA replication fork. Stalled and abortive
  replication generates deleted mtDNA species that, over decades, clonally expand
  within individual post-mitotic cells until they cross the biochemical threshold at
  which oxidative phosphorylation fails. The result is a mosaic respiratory-chain
  defect seen on muscle biopsy as ragged-red and cytochrome c oxidase (COX)-negative
  fibres. Extraocular muscle and the levator palpebrae superioris are the earliest
  and most severely affected tissues because they have a lower mutational threshold
  for COX deficiency than limb muscle, which explains the cardinal presentation of
  slowly progressive bilateral ptosis and symmetric ophthalmoparesis, typically with
  adult onset in the third to fifth decade.

  Beyond the eyes, adPEO is frequently a "PEO-plus" disorder with proximal and
  bulbar myopathy, exercise intolerance and fatigue, cardiac involvement, peripheral
  neuropathy, ataxia, sensorineural hearing loss, cataract, parkinsonism and
  premature ovarian insufficiency. The breadth of extra-ocular involvement is
  strongly genotype-dependent: TWNK-related disease is comparatively restricted to
  the eyes, while POLG-related adPEO carries the highest burden of multisystem
  neurological extension. There is no disease-modifying therapy; management is
  supportive, centred on ptosis surgery, prism or strabismus correction,
  multidisciplinary rehabilitation, cardiac and respiratory surveillance, and
  genetic counselling for the 50% transmission risk. Valproic acid should be
  avoided.
disease_term:
  preferred_term: autosomal dominant progressive external ophthalmoplegia
  term:
    id: MONDO:0008003
    label: autosomal dominant progressive external ophthalmoplegia
classifications:
  harrisons_chapter:
  - classification_value: NEUROLOGIC
  mechanistic_category:
  - classification_value: mitochondrial disease
  icimd_category:
  - classification_value: mtdna_replication_and_maintenance
parents:
- Progressive External Ophthalmoplegia with Multiple mtDNA Deletions
- Mitochondrial DNA Maintenance Disorder
- Mitochondrial Disease
references:
- reference: PMID:20301791
  title: "POLG-Related Disorders."
  tags:
  - GeneReviews
- reference: PMID:24741716
  title: "RRM2B Mitochondrial DNA Maintenance Defects."
  tags:
  - GeneReviews
has_subtypes:
- name: PEOA1
  display_name: PEOA1 (POLG, polymerase gamma catalytic subunit)
  subtype_term:
    preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions,
      autosomal dominant 1
    term:
      id: MONDO:0024528
      label: progressive external ophthalmoplegia with mitochondrial DNA deletions,
        autosomal dominant 1
  description: >-
    adPEO caused by a heterozygous pathogenic variant in POLG, the catalytic A
    subunit of mitochondrial DNA polymerase gamma. The dominant alleles cluster in
    the polymerase domain (classically p.Y955C in motif B, plus p.G923D, p.R943H and
    p.A957S) and reduce both catalytic efficiency and nucleotide selectivity. PEOA1
    is the single commonest identified cause of familial PEO with multiple mtDNA
    deletions and carries the heaviest burden of multisystem extension, including
    parkinsonism and premature ovarian insufficiency. It must not be conflated with
    its recessive sibling PEOB1, which requires two POLG alleles.
  genes:
  - preferred_term: POLG
    term:
      id: hgnc:9179
      label: POLG
  evidence:
  - reference: PMID:11431686
    reference_title: "Mutation of POLG is associated with progressive external ophthalmoplegia characterized by mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We mapped a new locus for dominant PEO at 15q22-q26 in a Belgian pedigree and identified a heterozygous mutation (Y955C) in the polymerase motif B of the mtDNA polymerase gamma (POLG)."
    explanation: >-
      Founding identification of heterozygous POLG p.Y955C as the cause of dominant
      PEO, defining the PEOA1 subtype.
  - reference: PMID:12210792
    reference_title: "Mutations of mitochondrial DNA polymerase gammaA are a frequent cause of autosomal dominant or recessive progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our data show that mutations of POLG1 are the most frequent cause of familial progressive external ophthalmoplegia associated with accumulation of multiple mitochondrial DNA deletions, accounting for approximately 45% of our family cohort."
    explanation: >-
      Establishes POLG as the most frequent single cause of familial PEO with
      multiple mtDNA deletions.
- name: PEOA2
  display_name: PEOA2 (SLC25A4/ANT1, adenine nucleotide translocator 1)
  subtype_term:
    preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions,
      autosomal dominant 2
    term:
      id: MONDO:0012238
      label: progressive external ophthalmoplegia with mitochondrial DNA deletions,
        autosomal dominant 2
  description: >-
    adPEO caused by a heterozygous missense variant in SLC25A4, encoding the
    heart/skeletal-muscle isoform of the adenine nucleotide translocator (ANT1), the
    inner-membrane ADP/ATP carrier. ANT1 is not part of the replisome; the dominant
    variants are thought to perturb the mitochondrial nucleotide and energy
    homeostasis on which faithful mtDNA replication depends. PEOA2 was the first
    demonstration that a mitochondrial disease could be produced by a dominant
    mechanism. The phenotype is usually myopathic, and germline mosaicism has been
    documented in at least one pedigree.
  genes:
  - preferred_term: SLC25A4
    term:
      id: hgnc:10990
      label: SLC25A4
  evidence:
  - reference: PMID:10926541
    reference_title: "Role of adenine nucleotide translocator 1 in mtDNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have identified two heterozygous missense mutations in the nuclear gene encoding the heart/skeletal muscle isoform of the adenine nucleotide translocator (ANT1) in five families and one sporadic patient."
    explanation: >-
      Founding identification of heterozygous SLC25A4/ANT1 variants in adPEO
      families, defining the PEOA2 subtype.
- name: PEOA3
  display_name: PEOA3 (TWNK/C10orf2, Twinkle mtDNA helicase)
  subtype_term:
    preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions,
      autosomal dominant 3
    term:
      id: MONDO:0012241
      label: progressive external ophthalmoplegia with mitochondrial DNA deletions,
        autosomal dominant 3
  description: >-
    adPEO caused by a heterozygous variant in TWNK (formerly C10orf2/PEO1), encoding
    Twinkle, the hexameric replicative 5'-3' DNA helicase that unwinds mtDNA ahead of
    polymerase gamma and colocalises with mtDNA in mitochondrial nucleoids. The
    dominant variants cluster in the region mediating subunit interactions,
    consistent with poisoning of the helicase hexamer. PEOA3 is the most
    ocular-restricted of the dominant subtypes, with ptosis and ophthalmoparesis
    near-universal and comparatively little CNS involvement, though cardiac
    abnormalities occur in roughly a quarter of patients.
  genes:
  - preferred_term: TWNK
    term:
      id: hgnc:1160
      label: TWNK
  evidence:
  - reference: PMID:11431692
    reference_title: "Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Screening of the gene encoding Twinkle in individuals with autosomal dominant progressive external ophthalmoplegia (adPEO), associated with multiple mtDNA deletions, identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins."
    explanation: >-
      Founding identification of TWNK (Twinkle) coding-region mutations segregating
      in twelve adPEO pedigrees, defining the PEOA3 subtype.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autosomal dominant mutations in the TWNK gene, which encodes a mitochondrial DNA helicase, cause adult-onset progressive external ophthalmoplegia (PEO) and PEO-plus presentations."
    explanation: >-
      Contemporary 25-patient cohort confirming TWNK as a cause of adult-onset adPEO
      and PEO-plus.
- name: PEOA4
  display_name: PEOA4 (POLG2, polymerase gamma p55 accessory subunit)
  subtype_term:
    preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions,
      autosomal dominant 4
    term:
      id: MONDO:0012415
      label: progressive external ophthalmoplegia with mitochondrial DNA deletions,
        autosomal dominant 4
  description: >-
    adPEO caused by a heterozygous variant in POLG2, encoding the p55 accessory
    subunit that confers processivity on the polymerase gamma holoenzyme. The
    prototype allele p.G451E binds DNA normally but fails to stimulate the catalytic
    p140 subunit or to enhance the DNA-binding strength of the p140-p55 complex, so
    disease arises through haploinsufficiency or heterodimerisation of mutant with
    wild-type protein. PEOA4 is a rare subtype.
  genes:
  - preferred_term: POLG2
    term:
      id: hgnc:9180
      label: POLG2
  evidence:
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "that causes progressive external ophthalmoplegia with multiple mtDNA deletions and cytochrome c oxidase (COX)-deficient muscle fibers"
    explanation: >-
      Founding identification of the heterozygous dominant POLG2 p.G451E allele in
      PEO with multiple mtDNA deletions and COX-deficient fibres.
- name: PEOA5
  display_name: PEOA5 (RRM2B/p53R2, ribonucleotide reductase small subunit)
  subtype_term:
    preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions,
      autosomal dominant 5
    term:
      id: MONDO:0013117
      label: progressive external ophthalmoplegia with mitochondrial DNA deletions,
        autosomal dominant 5
  description: >-
    adPEO caused by a heterozygous variant in RRM2B, encoding p53R2, the small
    subunit of the ribonucleotide reductase that supplies deoxyribonucleotides for
    mtDNA synthesis in non-dividing cells. The prototype allele is the C-terminal
    nonsense variant p.R327X, whose transcript escapes nonsense-mediated decay and
    yields a truncated protein that can no longer engage the R1 subunit, acting
    dominant-negatively. RRM2B is unusual in that biallelic variants in the same gene
    cause a much more severe, early-onset recessive encephalomyopathic mtDNA
    depletion syndrome, so the mode of inheritance must be established explicitly.
  genes:
  - preferred_term: RRM2B
    term:
      id: hgnc:17296
      label: RRM2B
  evidence:
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutation screening of RRM2B revealed a heterozygous nonsense mutation in exon 9"
    explanation: >-
      Founding identification of the heterozygous RRM2B p.R327X allele segregating in
      two adPEO families, defining the PEOA5 subtype.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autosomal dominant progressive external ophthalmoplegia (adPEO), typically adult onset; other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness."
    explanation: >-
      GeneReviews defines the adPEO phenotype within the RRM2B mtDNA maintenance
      defect spectrum.
- name: PEOA6
  display_name: PEOA6 (DNA2, helicase/nuclease; mtDNA deletion syndrome with progressive
    myopathy)
  subtype_term:
    preferred_term: mitochondrial DNA deletion syndrome with progressive myopathy
    term:
      id: MONDO:0014062
      label: mitochondrial DNA deletion syndrome with progressive myopathy
  description: >-
    adPEO caused by a heterozygous variant in DNA2, encoding a helicase/nuclease
    involved in mtDNA replication and in the long-patch base-excision repair
    pathway. Mutant proteins show severe impairment of nuclease, helicase and ATPase
    activity. The phenotype was originally delineated as an adult-onset progressive
    myopathy with muscle mtDNA instability and limb-girdle weakness, with PEO a
    frequent but not invariant component, which is why the MONDO entity carries the
    myopathy-first label with PEOA6 as a synonym.
  genes:
  - preferred_term: DNA2
    term:
      id: hgnc:2939
      label: DNA2
  evidence:
  - reference: PMID:23352259
    reference_title: "Mutations in DNA2 link progressive myopathy to mitochondrial DNA instability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report the identification, by exome sequencing, of mutations in DNA2 in adult-onset individuals with a form of mitochondrial myopathy featuring instability of muscle mtDNA."
    explanation: >-
      Founding identification of DNA2 mutations in adult-onset mitochondrial myopathy
      with mtDNA instability, defining the PEOA6 subtype.
pathophysiology:
- name: Heterozygous Nuclear mtDNA-Maintenance Gene Variant
  role: trigger
  biological_scale: MOLECULAR
  description: >
    A single heterozygous germline variant in one of at least six nuclear genes
    initiates adPEO. The genes belong to two functional classes that converge on the
    same molecular lesion. The replisome class comprises POLG (catalytic polymerase
    gamma A subunit), POLG2 (p55 accessory subunit), TWNK (Twinkle replicative
    helicase) and DNA2 (helicase/nuclease of mtDNA replication and long-patch
    base-excision repair). The nucleotide-supply class comprises SLC25A4/ANT1 (the
    heart and skeletal-muscle ADP/ATP carrier) and RRM2B/p53R2 (the ribonucleotide
    reductase small subunit that provides dNTPs to non-dividing cells). Because
    mtDNA is replicated by a small, dedicated protein set with no redundant backup
    polymerase, loss of function in any one of these components is not buffered.
  locations:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  biological_processes:
  - preferred_term: mitochondrial DNA metabolic process
    term:
      id: GO:0032042
      label: mitochondrial DNA metabolic process
    modifier: ABNORMAL
  evidence:
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder is heterogeneous, with five known nuclear disease genes that encode the proteins ANT1, Twinkle, POLG, POLG2, and OPA1."
    explanation: >-
      States the locus heterogeneity of adPEO across nuclear mtDNA-maintenance genes.
  - reference: PMID:23352259
    reference_title: "Mutations in DNA2 link progressive myopathy to mitochondrial DNA instability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Recently, causative mutations have been reported in several nuclear genes that encode proteins of the mtDNA replisome machinery (POLG, POLG2, and C10orf2) or that are involved in pathways for the synthesis of deoxyribonuclotides (ANT1 and RRM2B)."
    explanation: >-
      Directly states the two functional classes - replisome machinery versus
      deoxyribonucleotide synthesis - into which the adPEO genes partition.
  - reference: PMID:28324239
    reference_title: "MtDNA-maintenance defects: syndromes and genes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The genes involved encode proteins belonging to at least three pathways: mtDNA replication and maintenance, nucleotide supply and balance, and mitochondrial dynamics and quality control."
    explanation: >-
      Review-level statement of the pathway classes underlying mtDNA maintenance
      defects, of which adPEO is the late-onset myopathic end.
  - reference: PMID:11431686
    reference_title: "Mutation of POLG is associated with progressive external ophthalmoplegia characterized by mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "POLG is the only DNA polymerase responsible for mtDNA replication."
    explanation: >-
      Establishes the absence of a redundant backup polymerase, which is why a
      partial loss of polymerase gamma function is not buffered.
  downstream:
  - target: Dominant-Negative Subunit Poisoning and Haploinsufficiency
    description: >-
      In the replisome-class genes the mutant polypeptide is incorporated into a
      multimeric complex alongside wild-type protein, so one mutant allele degrades
      the function of the whole assembly.
    causal_link_type: DIRECT
  - target: Imbalanced Mitochondrial dNTP Supply
    description: >-
      In the nucleotide-supply-class genes (SLC25A4/ANT1 and RRM2B) the primary
      lesion is not in the replisome but in the provision of balanced
      deoxyribonucleotide precursors to the replicating mitochondrial genome.
    causal_link_type: DIRECT
- name: Dominant-Negative Subunit Poisoning and Haploinsufficiency
  role: effector
  biological_scale: MOLECULAR
  description: >
    The defining feature that separates adPEO from its recessive counterparts is
    that heterozygosity suffices. Three mechanisms operate. First, dominant-negative
    poisoning of a multimer: Twinkle functions as a hexameric ring helicase and the
    dominant TWNK variants cluster precisely in the region mediating subunit
    interactions, so mutant monomers incorporate into and disable the ring. Second,
    haploinsufficiency or heterodimerisation within the polymerase gamma holoenzyme:
    the POLG2 p.G451E accessory subunit binds DNA normally but fails to stimulate the
    catalytic p140 subunit or to strengthen the DNA binding of the p140-p55 complex.
    Third, dominant-negative truncation: the RRM2B p.R327X transcript escapes
    nonsense-mediated decay and yields a protein lacking the 25 C-terminal residues
    required to engage the R1 subunit of ribonucleotide reductase. Dominant POLG
    alleles cluster in the polymerase domain and degrade both catalytic efficiency
    and nucleotide selectivity, and the magnitude of the biochemical defect tracks
    clinical severity.
  molecular_functions:
  - preferred_term: DNA-directed DNA polymerase activity
    term:
      id: GO:0003887
      label: DNA-directed DNA polymerase activity
    modifier: DECREASED
  - preferred_term: DNA helicase activity
    term:
      id: GO:0003678
      label: DNA helicase activity
    modifier: DECREASED
  locations:
  - preferred_term: mitochondrial nucleoid
    term:
      id: GO:0042645
      label: mitochondrial nucleoid
  evidence:
  - reference: PMID:11431692
    reference_title: "Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutations cluster in a region of the protein proposed to be involved in subunit interactions."
    explanation: >-
      The clustering of dominant TWNK variants at the subunit-interaction interface
      of a hexameric helicase is the structural basis for dominant-negative poisoning.
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Although G451E p55 retains a wild-type ability to bind DNA, it fails to enhance the DNA-binding strength of the p140-p55 complex."
    explanation: >-
      Biochemical demonstration that the dominant POLG2 allele produces a subunit
      that assembles but does not function.
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vivo, the disease most likely arises through haplotype insufficiency or heterodimerization of the mutated and wild-type proteins, which promote mtDNA deletions by stalling the DNA replication fork."
    explanation: >-
      States the haploinsufficiency-or-heterodimerisation mechanism by which a single
      mutant allele stalls the fork.
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutant mRNA escaped nonsense-mediated decay and resulted in a protein with truncation of 25 highly conserved C-terminal amino acids essential for the interaction with the ribonucleotide reductase subunit R1."
    explanation: >-
      Shows how a nonsense allele produces a dominant-negative rather than a null
      effect, explaining dominance at the RRM2B locus.
  - reference: PMID:15258572
    reference_title: "Structure-function defects of human mitochondrial DNA polymerase in autosomal dominant progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Polymerase mutants exhibit 0.03-30% wild-type polymerase activity and a 2- to 35-fold decrease in nucleotide selectivity in vitro."
    explanation: >-
      Quantifies the catalytic and fidelity defect of the dominant POLG alleles.
  - reference: PMID:15258572
    reference_title: "Structure-function defects of human mitochondrial DNA polymerase in autosomal dominant progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The reduced selectivity and catalytic efficiency of the autosomal dominant PEO mutants predict in vivo dysfunction, and the extent of biochemical defects correlates with the clinical severity of the disease."
    explanation: >-
      Links the magnitude of the biochemical lesion to clinical severity, supporting
      a dose-of-dysfunction model of dominance.
  downstream:
  - target: Replication Fork Stalling on mtDNA
    description: >-
      A poisoned or under-stimulated replisome cannot sustain processive synthesis
      around the 16.6 kb mitochondrial genome, so replication forks stall.
    causal_link_type: DIRECT
- name: Imbalanced Mitochondrial dNTP Supply
  role: effector
  biological_scale: MOLECULAR
  description: >
    Post-mitotic cells cannot draw on the S-phase ribonucleotide reductase used by
    dividing cells and depend instead on the p53R2 (RRM2B) isoform for de novo dNTP
    synthesis, and on the adenine nucleotide translocator ANT1 for the ADP/ATP
    exchange that sustains matrix nucleotide and energy homeostasis. Variants in
    either gene distort the size and balance of the mitochondrial dNTP pool.
    Replication of mtDNA in the face of a skewed or insufficient precursor pool is
    slow and error-prone, producing the same downstream fork-stalling lesion that
    arises from a defective replisome. This is why two mechanistically unrelated
    protein classes yield an indistinguishable clinical and molecular phenotype.
  locations:
  - preferred_term: mitochondrial inner membrane
    term:
      id: GO:0005743
      label: mitochondrial inner membrane
  molecular_functions:
  - preferred_term: ATP:ADP antiporter activity
    term:
      id: GO:0005471
      label: ATP:ADP antiporter activity
    modifier: DECREASED
  - preferred_term: ribonucleoside-diphosphate reductase activity, thioredoxin disulfide
      as acceptor
    term:
      id: GO:0004748
      label: ribonucleoside-diphosphate reductase activity, thioredoxin disulfide as
        acceptor
    modifier: DECREASED
  biological_processes:
  - preferred_term: 2'-deoxyribonucleotide metabolic process
    term:
      id: GO:0009394
      label: 2'-deoxyribonucleotide metabolic process
    modifier: ABNORMAL
  evidence:
  - reference: PMID:10926541
    reference_title: "Role of adenine nucleotide translocator 1 in mtDNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These results indicate that ANT has a role in mtDNA maintenance and that a mitochondrial disease can be caused by a dominant mechanism."
    explanation: >-
      Establishes that the ADP/ATP carrier, though not a replisome component, is
      required for mtDNA maintenance and acts dominantly.
  - reference: PMID:23352259
    reference_title: "Mutations in DNA2 link progressive myopathy to mitochondrial DNA instability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "or that are involved in pathways for the synthesis of deoxyribonuclotides (ANT1 and RRM2B)"
    explanation: >-
      Assigns ANT1 and RRM2B specifically to the deoxyribonucleotide-supply arm of
      adPEO pathogenesis.
  - reference: PMID:28324239
    reference_title: "MtDNA-maintenance defects: syndromes and genes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These errors of nuclear-mitochondrial intergenomic signaling may lead to mtDNA depletion, accumulation of mtDNA multiple deletions, or both, in critical tissues."
    explanation: >-
      Frames both gene classes as errors of nuclear-mitochondrial intergenomic
      signalling converging on multiple deletions.
  downstream:
  - target: Replication Fork Stalling on mtDNA
    description: >-
      A skewed or insufficient mitochondrial dNTP pool slows and destabilises
      polymerase gamma synthesis, producing the same stalled-fork lesion as a
      primary replisome defect.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
- name: Replication Fork Stalling on mtDNA
  role: central_effector
  biological_scale: MOLECULAR
  description: >
    The shared proximal lesion of all adPEO genotypes is arrest of the mtDNA
    replication fork. Expression of catalytically deficient Twinkle or polymerase
    gamma in human cells causes severe replication stalling and a fall in mtDNA copy
    number, and the two proteins produce distinguishable stalling signatures,
    confirming that each acts at the fork. In DNA2-related disease the same endpoint
    is reached through loss of the nuclease, helicase and ATPase activities needed
    for flap processing during mtDNA replication and long-patch base-excision repair.
  locations:
  - preferred_term: mitochondrial nucleoid
    term:
      id: GO:0042645
      label: mitochondrial nucleoid
  biological_processes:
  - preferred_term: mitochondrial DNA replication
    term:
      id: GO:0006264
      label: mitochondrial DNA replication
    modifier: DECREASED
  - preferred_term: base-excision repair
    term:
      id: GO:0006284
      label: base-excision repair
    modifier: DECREASED
  molecular_functions:
  - preferred_term: DNA nuclease activity
    term:
      id: GO:0004536
      label: DNA nuclease activity
    modifier: DECREASED
  evidence:
  - reference: PMID:17452351
    reference_title: "Expression of catalytic mutants of the mtDNA helicase Twinkle and polymerase POLG causes distinct replication stalling phenotypes."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Both groups of mutants reduced mitochondrial DNA copy number by severe replication stalling."
    explanation: >-
      Direct experimental demonstration that catalytically deficient Twinkle and
      polymerase gamma stall mtDNA replication.
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Defects in these proteins affect mtDNA maintenance, probably leading to stalled replication forks, consequent mtDNA deletion formation, and progressive respiratory chain deficiency."
    explanation: >-
      States the canonical stalled-fork model linking all adPEO gene defects to
      deletion formation and respiratory chain failure.
  - reference: PMID:23352259
    reference_title: "Mutations in DNA2 link progressive myopathy to mitochondrial DNA instability."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vitro biochemical analysis of purified mutant proteins revealed a severe impairment of nuclease, helicase, and ATPase activities."
    explanation: >-
      Shows the enzymatic basis by which DNA2 variants compromise replication and
      repair at the fork.
  - reference: PMID:15509589
    reference_title: "Twinkle helicase is essential for mtDNA maintenance and regulates mtDNA copy number."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "These data demonstrate that Twinkle helicase is essential for mtDNA maintenance, and that it may be a key regulator of mtDNA copy number in mammals."
    explanation: >-
      Mouse transgenic and RNAi data establishing Twinkle as rate-limiting for mtDNA
      maintenance and copy number.
  downstream:
  - target: Multiple Large-Scale mtDNA Deletion Formation
    description: >-
      Stalled and abortive replication intermediates are resolved by illegitimate
      recombination or slipped-strand mispairing, generating heterogeneous
      large-scale deletions of the mitochondrial genome.
    causal_link_type: DIRECT
- name: Multiple Large-Scale mtDNA Deletion Formation
  role: central_effector
  biological_scale: MOLECULAR
  description: >
    The molecular hallmark of adPEO is the accumulation of MULTIPLE, heterogeneous
    large-scale mtDNA deletions in clinically affected tissue - as distinct from the
    single, clonal large-scale deletion of sporadic Kearns-Sayre syndrome and
    sporadic CPEO. The deletions are somatic and arise over a lifetime on the
    background of the germline nuclear variant, so they are a secondary rather than
    a primary genetic lesion. The founding pedigree studied by Zeviani and colleagues
    showed that the deletions in affected relatives all began within a
    12-nucleotide stretch at the 5' end of the D-loop, establishing that a
    nuclear-coded protein defect can destroy mitochondrial genome integrity in a
    specific, heritable way.
  biological_processes:
  - preferred_term: mitochondrial DNA metabolic process
    term:
      id: GO:0032042
      label: mitochondrial DNA metabolic process
    modifier: ABNORMAL
  evidence:
  - reference: PMID:2725645
    reference_title: "An autosomal dominant disorder with multiple deletions of mitochondrial DNA starting at the D-loop region."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have studied four members of a family with autosomal dominant mitochondrial myopathy. Multiple deletions, involving the same portion of muscle mtDNA, were identified in all patients."
    explanation: >-
      The founding clinical description of multiple mtDNA deletions segregating as an
      autosomal dominant trait.
  - reference: PMID:2725645
    reference_title: "An autosomal dominant disorder with multiple deletions of mitochondrial DNA starting at the D-loop region."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The data indicate that a mutation of a nuclear-coded protein can destroy the integrity of the mitochondrial genome in a specific, heritable way."
    explanation: >-
      States the central inference of the disease group: a nuclear lesion producing a
      mitochondrial genome lesion.
  - reference: PMID:10926541
    reference_title: "Role of adenine nucleotide translocator 1 in mtDNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autosomal dominant progressive external ophthalmoplegia is a rare human disease that shows a Mendelian inheritance pattern, but is characterized by large-scale mitochondrial DNA (mtDNA) deletions."
    explanation: >-
      Captures the defining paradox of adPEO: Mendelian nuclear transmission with a
      mitochondrial genome lesion.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All 19 available muscle biopsies showed signs of mitochondrial dysfunction."
    explanation: >-
      Contemporary confirmation that the muscle mitochondrial lesion is essentially
      universal in genetically confirmed adPEO.
  downstream:
  - target: Clonal Expansion of Deleted mtDNA in Post-Mitotic Cells
    description: >-
      Individual deleted mtDNA molecules replicate within a single post-mitotic cell
      and progressively displace wild-type genomes over decades.
    causal_link_type: DIRECT
- name: Clonal Expansion of Deleted mtDNA in Post-Mitotic Cells
  role: amplifier
  biological_scale: CELLULAR
  description: >
    A newly formed deletion is initially a negligible fraction of the mtDNA in a
    cell and is phenotypically silent. Because post-mitotic cells such as myofibres
    and neurons turn mtDNA over continuously without dividing, a deleted genome that
    replicates even slightly faster than wild type expands clonally within a single
    cell or fibre segment across decades. This is the step that converts a lifelong
    constitutional nuclear variant into an adult-onset disease and explains the
    characteristic onset in the third to fifth decade. Clonal expansion is
    tissue-specific, which contributes to the selective vulnerability of extraocular
    muscle.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:20164463
    reference_title: "Mitochondrial DNA defects and selective extraocular muscle involvement in CPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The data also suggest that tissue-specific mechanisms are involved in the clonal expansion and expression of secondary mtDNA deletions in CPEO patients with nuclear genetic defects."
    explanation: >-
      Directly implicates tissue-specific clonal expansion of secondary mtDNA
      deletions in patients whose primary defect is nuclear.
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The progressive accumulation of mtDNA deletions causes COX deficiency in muscle fibers and results in the clinical phenotype."
    explanation: >-
      Explicitly links progressive deletion accumulation to fibre-level COX
      deficiency and the clinical phenotype.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mean ages of onset and diagnosis were 43 and 63 years, respectively."
    explanation: >-
      The decades-long latency between a constitutional germline variant and clinical
      onset is the clinical signature of slow somatic clonal expansion.
  downstream:
  - target: Threshold-Dependent Mosaic Respiratory Chain Deficiency
    description: >-
      Once the deleted mtDNA fraction within a cell exceeds roughly 60 to 90 percent,
      the supply of mtDNA-encoded respiratory chain subunits and tRNAs falls below
      what oxidative phosphorylation requires.
    causal_link_type: DIRECT
- name: Threshold-Dependent Mosaic Respiratory Chain Deficiency
  role: central_effector
  biological_scale: CELLULAR
  description: >
    Because mtDNA is multicopy, deleted genomes are functionally recessive until
    they cross a critical fraction of the cellular mtDNA population. Above this
    biochemical threshold the affected cell can no longer synthesise the thirteen
    mtDNA-encoded oxidative phosphorylation subunits and the tRNAs needed to
    translate them, and oxidative phosphorylation fails. Because expansion is
    independent in each cell, the resulting deficiency is mosaic: individual
    COX-negative fibre segments sit alongside normally respiring neighbours. Complex
    IV is the histochemical readout because three of its catalytic subunits are
    mtDNA-encoded, whereas complex II (SDH) is entirely nuclear-encoded and is
    preserved or increased, giving the COX-negative/SDH-hyperreactive signature that
    is specific for an mtDNA rather than a nuclear OXPHOS lesion.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  biological_processes:
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  molecular_functions:
  - preferred_term: cytochrome-c oxidase activity
    term:
      id: GO:0004129
      label: cytochrome-c oxidase activity
    modifier: DECREASED
  evidence:
  - reference: PMID:16987890
    reference_title: "Mitochondrial DNA polymerase-gamma and human disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The secondary mtDNA defects cause cell and tissue-specific deficiencies of mitochondrial oxidative phosphorylation, leading to organ dysfunction and human disease."
    explanation: >-
      States the causal step from secondary mtDNA defect to cell- and tissue-specific
      OXPHOS deficiency.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in the Twinkle (PEO1) gene are a recognized cause of autosomal dominant progressive external ophthalmoplegia (adPEO), resulting in the accumulation of multiple mitochondrial DNA (mtDNA) deletions and cytochrome c oxidase (COX)-deficient fibers in skeletal muscle secondary to a disorder of mtDNA maintenance."
    explanation: >-
      Links the mtDNA maintenance defect to accumulation of deletions and
      COX-deficient fibres in a genotyped adPEO cohort.
  - reference: PMID:20164463
    reference_title: "Mitochondrial DNA defects and selective extraocular muscle involvement in CPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "single-fiber analysis revealed a lower mutational threshold for COX deficiency in EOM"
    explanation: >-
      Demonstrates the threshold nature of the biochemical defect at single-fibre
      resolution.
  downstream:
  - target: Extraocular Muscle Selective Vulnerability
    description: >-
      Extraocular muscle crosses the biochemical threshold earliest and most
      completely, so the ocular phenotype dominates the presentation.
    causal_link_type: DIRECT
  - target: Skeletal and Bulbar Mitochondrial Myopathy
    description: >-
      Limb, neck and bulbar muscle accumulate COX-negative fibre segments more slowly
      but by the same mechanism, producing proximal weakness and exercise intolerance.
    causal_link_type: DIRECT
  - target: Multisystem PEO-Plus Extension
    description: >-
      Other high-energy post-mitotic tissues - peripheral nerve, cerebellum,
      substantia nigra, cardiac conduction system, cochlea, lens and ovary - reach the
      same threshold in a genotype-dependent subset of patients.
    causal_link_type: DIRECT
  - target: Ragged-red muscle fibers
    description: >-
      Compensatory proliferation of mitochondria in respiratory-chain-deficient fibre
      segments produces the subsarcolemmal masses seen as ragged-red fibres.
    causal_link_type: DIRECT
  - target: Cytochrome C oxidase-negative muscle fibers
    description: >-
      Loss of mtDNA-encoded complex IV subunits above threshold abolishes COX
      histochemical reactivity in the affected fibre segment.
    causal_link_type: DIRECT
  - target: Multiple mitochondrial DNA deletions
    description: >-
      The deletions demonstrable in muscle by long-range PCR or Southern blot are the
      direct molecular readout of this process.
    causal_link_type: DIRECT
- name: Extraocular Muscle Selective Vulnerability
  role: consequence
  biological_scale: TISSUE
  description: >
    Extraocular muscle and the levator palpebrae superioris are affected earliest and
    most severely in every mtDNA-deletion disorder. The explanation is quantitative:
    in CPEO patients with mtDNA deletions, 41.6% of extraocular muscle fibres are
    COX-deficient compared with 13.7% of limb (quadriceps) fibres, and single-fibre
    analysis shows that extraocular muscle crosses into COX deficiency at a LOWER
    mutational load than skeletal muscle. Extraocular muscle is a distinct allotype
    with exceptionally high mitochondrial content, tonic firing rates and continuous
    activity, which plausibly underlies both its higher deletion burden and its lower
    threshold. Because the levator and the extraocular muscles fail together and
    symmetrically, patients develop bilateral ptosis and symmetric ophthalmoparesis
    and often do NOT report diplopia despite marked ductional limitation.
  locations:
  - preferred_term: extra-ocular muscle
    term:
      id: UBERON:0001601
      label: extra-ocular muscle
  - preferred_term: levator palpebrae superioris
    term:
      id: UBERON:0001604
      label: levator palpebrae superioris
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:20164463
    reference_title: "Mitochondrial DNA defects and selective extraocular muscle involvement in CPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CPEO patients with mtDNA deletions had more COX-deficient fibers in EOM (41.6%) than in skeletal muscle (13.7%, P > 0.0001), and single-fiber analysis revealed a lower mutational threshold for COX deficiency in EOM."
    explanation: >-
      Quantifies both the greater deletion burden and the lower biochemical threshold
      of extraocular muscle, the mechanistic basis of the ocular phenotype.
  - reference: PMID:20164463
    reference_title: "Mitochondrial DNA defects and selective extraocular muscle involvement in CPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The more pronounced mitochondrial biochemical defect and lower mutational threshold in EOM compared with skeletal muscle fibers provide an explanation of the selective muscle involvement in CPEO."
    explanation: >-
      States the authors' mechanistic conclusion for selective extraocular
      involvement.
  downstream:
  - target: Ptosis
    description: >-
      Failure of the levator palpebrae superioris, an extraocular-allotype muscle,
      produces insidious bilateral eyelid droop, usually the presenting sign.
    causal_link_type: DIRECT
  - target: Progressive external ophthalmoplegia
    description: >-
      Symmetric respiratory-chain failure across the recti and obliques produces
      slowly progressive limitation of eye movement in all directions.
    causal_link_type: DIRECT
  - target: Diplopia
    description: >-
      Asymmetric involvement of individual extraocular muscles produces
      ocular misalignment and double vision in a minority of patients.
    causal_link_type: DIRECT
- name: Skeletal and Bulbar Mitochondrial Myopathy
  role: consequence
  biological_scale: TISSUE
  description: >
    Limb, neck, facial and bulbar muscles accumulate COX-negative fibre segments by
    the same threshold mechanism but more slowly than extraocular muscle, producing
    the "PEO-plus" myopathic phenotype. In the largest genotyped adPEO cohort
    (25 TWNK patients), weakness affecting proximal limbs, neck and bulbar muscles
    was present in 48% and exercise intolerance in 28%. Deficient oxidative
    phosphorylation limits sustained aerobic work, so fatigue and exercise
    intolerance often precede fixed weakness. Bulbar involvement produces dysphagia
    and dysphonia and is the main driver of aspiration risk.
  locations:
  - preferred_term: muscle organ
    term:
      id: UBERON:0001630
      label: muscle organ
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  biological_processes:
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles. Exercise intolerance was present in 28%."
    explanation: >-
      Quantifies the myopathic and bulbar extension of adPEO in a genotyped cohort.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ptosis and ophthalmoparesis were almost universal clinical features among this cohort, with 52% (17/33) reporting fatigue and 33% (11/33) having mild proximal myopathy."
    explanation: >-
      Independent genotyped cohort quantifying fatigue and proximal myopathy
      alongside the near-universal ocular features.
  downstream:
  - target: Proximal muscle weakness
    description: >-
      Accumulating respiratory-chain-deficient fibre segments in limb-girdle and neck
      muscle produce proximal weakness.
    causal_link_type: DIRECT
  - target: Exercise intolerance
    description: >-
      Impaired oxidative ATP regeneration limits sustained aerobic muscle work.
    causal_link_type: DIRECT
  - target: Fatigue
    description: >-
      Reduced oxidative capacity across skeletal muscle produces pervasive fatigue,
      often the earliest systemic symptom.
    causal_link_type: DIRECT
  - target: Dysphagia
    description: >-
      Bulbar muscle involvement impairs swallowing and raises aspiration risk.
    causal_link_type: DIRECT
  - target: Elevated circulating creatine kinase concentration
    description: >-
      Ongoing myofibre stress produces mild hyperCKemia in a minority of patients.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  - target: Dysarthria
    description: >-
      Bulbar muscle involvement degrades articulation, typically alongside dysphagia.
    causal_link_type: DIRECT
  - target: Dysphonia
    description: >-
      Laryngeal muscle involvement alters voice quality in bulbar-predominant families.
    causal_link_type: DIRECT
  - target: Respiratory insufficiency due to muscle weakness
    description: >-
      Chest-wall and diaphragmatic myopathy produces restrictive ventilatory failure.
    causal_link_type: DIRECT
  - target: Mitochondrial myopathy
    description: >-
      The aggregate skeletal-muscle phenotype is a mitochondrial myopathy.
    causal_link_type: DIRECT
- name: Multisystem PEO-Plus Extension
  role: consequence
  biological_scale: ORGANISM
  description: >
    Because the nuclear variant is constitutional, every post-mitotic tissue
    accumulates mtDNA deletions; whether a given tissue becomes clinically involved
    depends on its energetic demand, its mitotic status and the specific genotype.
    The breadth of extension is strongly genotype-dependent. TWNK-related adPEO is
    comparatively ocular-restricted, with CNS features rarely described but cardiac
    abnormalities in around a quarter of patients, whereas POLG-related adPEO carries
    the greatest burden of neurological extension. Reported extensions include
    peripheral neuropathy, ataxia, sensorineural hearing loss, cataract, cardiac
    conduction disease and cardiomyopathy, respiratory muscle involvement,
    gastrointestinal dysmotility, depression and other psychiatric features.
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Less frequent manifestations were cardiac (24%) and respiratory (4%) involvement, neuropathy (8%), ataxia (4%), and parkinsonism (4%)."
    explanation: >-
      Quantifies the multisystem extension of adPEO in a genotyped TWNK cohort.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Features consistent with CNS involvement were rarely described; however, in 24% (8/33) of the patients, cardiac abnormalities were reported."
    explanation: >-
      Establishes the genotype-specific pattern in TWNK disease: little CNS
      involvement but appreciable cardiac involvement.
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Late-onset disease (after age 40 years): Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."
    explanation: >-
      GeneReviews defines the late-onset POLG phenotype, which is the dominant
      (PEOA1) end of the POLG spectrum, as PEO plus neuropathy and ataxia.
  - reference: PMID:16987890
    reference_title: "Mitochondrial DNA polymerase-gamma and human disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinically, POLG mutations can present from early neonatal life to late middle age, with a spectrum of phenotypes that includes common neurological disorders such as migraine, epilepsy and Parkinsonism."
    explanation: >-
      Establishes the breadth of neurological extension available to POLG-related
      disease.
  downstream:
  - target: Nigrostriatal Dopaminergic Degeneration
    description: >-
      In POLG-related adPEO, deletion-driven respiratory chain failure in substantia
      nigra neurons produces dopaminergic cell loss.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  - target: Ovarian Follicular Depletion
    description: >-
      Oocyte and granulosa-cell mitochondrial dysfunction accelerates follicular
      attrition in POLG-related adPEO.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Peripheral axonal neuropathy
    description: >-
      Length-dependent failure of oxidative ATP supply in long sensory and motor
      axons produces an axonal neuropathy.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  - target: Ataxia
    description: >-
      Cerebellar and sensory-afferent involvement produces gait and limb ataxia in a
      minority of patients.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  - target: Sensorineural hearing impairment
    description: >-
      Cochlear hair cells and spiral ganglion neurons are high-energy post-mitotic
      cells vulnerable to the same threshold mechanism.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  - target: Cataract
    description: >-
      Lens epithelial oxidative stress and impaired energy supply contribute to
      early-onset lens opacification in some patients.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Arrhythmia
    description: >-
      Respiratory chain deficiency in the cardiac conduction system and myocardium
      produces conduction disease and rhythm disturbance.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  - target: Depression
    description: >-
      Psychiatric features including depression and, in one ANT1 pedigree, a
      schizoaffective disorder have been reported in adPEO families.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- name: Nigrostriatal Dopaminergic Degeneration
  role: outcome
  biological_scale: TISSUE
  conforms_to: "parkinsonism_dopaminergic_degeneration#Nigrostriatal Dopaminergic Neurodegeneration"
  description: >
    In POLG-related adPEO, mtDNA deletions accumulate in substantia nigra pars
    compacta dopaminergic neurons, which are exceptionally vulnerable because of
    their long unmyelinated axons, high basal energy demand and pacemaking calcium
    load. Parkinsonism cosegregated highly significantly with POLG mutations across
    seven adPEO families, PET showed dopaminergic neuron loss, and post-mortem
    examination in two individuals showed loss of pigmented neurons and pigment
    phagocytosis in the substantia nigra WITHOUT Lewy bodies. The absence of Lewy
    pathology is mechanistically informative: this is a primary bioenergetic
    nigrostriatal degeneration rather than an alpha-synucleinopathy, which is why
    conformance is declared at the nigrostriatal-neurodegeneration node of the
    parkinsonism module and NOT at its alpha-synuclein aggregation node.
  locations:
  - preferred_term: substantia nigra
    term:
      id: UBERON:0002038
      label: substantia nigra
  cell_types:
  - preferred_term: dopaminergic neuron
    term:
      id: CL:0000700
      label: dopaminergic neuron
  biological_processes:
  - preferred_term: dopamine biosynthetic process
    term:
      id: GO:0042416
      label: dopamine biosynthetic process
    modifier: DECREASED
  evidence:
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical assessment showed significant cosegregation of parkinsonism with POLG mutations (p<0.0001), and PET findings were consistent with dopaminergic neuron loss."
    explanation: >-
      Establishes genetic cosegregation of parkinsonism with POLG mutations and
      confirms dopaminergic neuron loss in vivo by PET.
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Post-mortem examination in two individuals showed loss of pigmented neurons and pigment phagocytosis in substantia nigra without Lewy bodies."
    explanation: >-
      Neuropathological confirmation of nigral degeneration and the key negative
      finding that distinguishes this from an alpha-synucleinopathy.
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The POLG gene defect resulted in secondary accumulation of mtDNA deletions in patients' tissues."
    explanation: >-
      Confirms that the nigral phenotype sits downstream of the same secondary mtDNA
      deletion lesion as the ocular myopathy.
  downstream:
  - target: Parkinsonism
    description: >-
      Loss of nigrostriatal dopaminergic projection produces striatal dopamine
      deficiency and clinical parkinsonism.
    causal_link_type: DIRECT
- name: Ovarian Follicular Depletion
  role: outcome
  biological_scale: TISSUE
  description: >
    Oocytes carry the largest mtDNA complement of any human cell and depend on
    mitochondrial competence for meiotic and developmental viability. In
    POLG-related adPEO families most affected women reached menopause before age 35,
    a striking departure from the usual mitochondrial disease phenotype and a
    reproductive-counselling consideration independent of transmission risk. The
    presumed mechanism is accelerated attrition of the ovarian follicular reserve on
    a background of accumulating mtDNA deletions, though the intermediate steps have
    not been directly demonstrated in human ovary.
  locations:
  - preferred_term: ovary
    term:
      id: UBERON:0000992
      label: ovary
  evidence:
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Furthermore, most women with progressive external ophthalmoplegia had early menopause-before age 35 years."
    explanation: >-
      Direct clinical observation of premature menopause in POLG-mutation PEO
      families.
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Dysfunction of mitochondrial POLG causes a severe progressive multisystem disorder including parkinsonism and premature menopause, which are not typical of mitochondrial disease."
    explanation: >-
      States the authors' conclusion that premature menopause is a genuine and
      atypical component of the POLG multisystem phenotype.
  downstream:
  - target: Premature ovarian insufficiency
    description: >-
      Accelerated depletion of the follicular reserve produces menopause before age
      40, and in these families typically before age 35.
    causal_link_type: DIRECT
phenotypes:
- category: Ocular
  name: Ptosis
  description: >
    Insidious, slowly progressive, usually bilateral and largely symmetric drooping
    of the eyelids from respiratory chain failure in the levator palpebrae superioris.
    Ptosis is the commonest presenting sign of adPEO, present in 92% of a genotyped
    25-patient TWNK cohort, and in advanced disease patients adopt a chin-up head
    posture and use frontalis contraction to clear the visual axis.
  phenotype_term:
    preferred_term: Ptosis
    term:
      id: HP:0000508
      label: Ptosis
    clinical_course: PROGRESSIVE
  frequency: VERY_FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ptosis and PEO (92% and 80%) were the most common findings."
    explanation: >-
      Quantifies ptosis at 92% in a genotyped adPEO cohort, supporting the
      VERY_FREQUENT band.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ptosis and ophthalmoparesis were almost universal clinical features among this cohort"
    explanation: >-
      Independent genotyped cohort describing ptosis as almost universal.
- category: Ocular
  name: Progressive external ophthalmoplegia
  description: >
    Slowly progressive, usually symmetric limitation of eye movement in all
    directions of gaze, the eponymous feature of the disorder. Present in 80% of the
    genotyped TWNK adPEO cohort. Because involvement is typically symmetric, patients
    frequently do not complain of diplopia despite marked ductional restriction, and
    the deficit is often first noticed by an examiner rather than the patient.
  phenotype_term:
    preferred_term: Progressive external ophthalmoplegia
    term:
      id: HP:0000590
      label: Progressive external ophthalmoplegia
    clinical_course: PROGRESSIVE
  frequency: VERY_FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ptosis and PEO (92% and 80%) were the most common findings."
    explanation: >-
      Quantifies PEO at 80% in a genotyped adPEO cohort, supporting the VERY_FREQUENT
      band.
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "'pure PEO' (42%), consisting of isolated palpebral ptosis with ophthalmoparesis"
    explanation: >-
      Defines the pure-PEO phenotype and its share in a mixed-aetiology mitochondrial
      PEO series; cited as PARTIAL because the cohort includes single-deletion and
      recessive causes as well as adPEO.
- category: Ocular
  name: Diplopia
  description: >
    Double vision arising when involvement of individual extraocular muscles is
    asymmetric enough to produce ocular misalignment. Diplopia is notably LESS common
    than the degree of ophthalmoparesis would predict, because the symmetric,
    concurrent failure of yoked muscles preserves alignment in many patients.
  phenotype_term:
    preferred_term: Diplopia
    term:
      id: HP:0000651
      label: Diplopia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38391710
    reference_title: "Mitochondrial Chronic Progressive External Ophthalmoplegia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Chronic progressive external ophthalmoplegia (CPEO) is a rare disorder that can be at the forefront of several mitochondrial diseases."
    explanation: >-
      Review context for the ocular motility phenotype; cited as PARTIAL because the
      abstract does not itself quantify diplopia frequency.
  notes: >-
    No adPEO-specific quantitative frequency for diplopia was found in a quotable
    abstract; the OCCASIONAL band reflects the consistent clinical observation that
    diplopia is disproportionately uncommon relative to the ductional deficit, and
    should be treated as a clinical estimate rather than a measured figure.
- category: Muscular
  name: Proximal muscle weakness
  description: >
    Weakness affecting proximal limb, neck, facial and bulbar muscles, present in 48%
    of the genotyped TWNK adPEO cohort and 33% of the PEO1 cohort. This is the core
    of the "PEO-plus" myopathic phenotype and is the feature most often mistaken for
    oculopharyngeal muscular dystrophy or myasthenia gravis.
  phenotype_term:
    preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
    clinical_course: PROGRESSIVE
  frequency: FREQUENT
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles."
    explanation: >-
      Quantifies proximal, neck and bulbar weakness at 48%, supporting the FREQUENT
      band.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "33% (11/33) having mild proximal myopathy"
    explanation: >-
      Independent genotyped cohort quantifying mild proximal myopathy at 33%.
- category: Muscular
  name: Exercise intolerance
  description: >
    Reduced tolerance of sustained aerobic activity, with premature muscle fatigue,
    breathlessness and myalgia on exertion, reflecting the inability of
    respiratory-chain-deficient fibres to regenerate ATP oxidatively. Present in 28%
    of the genotyped TWNK adPEO cohort.
  phenotype_term:
    preferred_term: Exercise intolerance
    term:
      id: HP:0003546
      label: Exercise intolerance
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Exercise intolerance was present in 28%."
    explanation: >-
      Quantifies exercise intolerance at 28%, supporting the OCCASIONAL band.
- category: Constitutional
  name: Fatigue
  description: >
    Pervasive fatigue, reported by 52% of the PEO1 (Twinkle) adPEO cohort, and often
    the earliest and most disabling systemic symptom, preceding fixed weakness by
    years.
  phenotype_term:
    preferred_term: Fatigue
    term:
      id: HP:0012378
      label: Fatigue
    temporality: CHRONIC
  frequency: FREQUENT
  evidence:
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "with 52% (17/33) reporting fatigue"
    explanation: >-
      Quantifies fatigue at 52% in a genotyped adPEO cohort, supporting the FREQUENT
      band.
- category: Muscular
  name: Mitochondrial myopathy
  description: >
    The aggregate skeletal-muscle phenotype of adPEO is a mitochondrial myopathy,
    defined by mosaic respiratory-chain-deficient fibres on biopsy together with
    clinical weakness, exercise intolerance and fatigue. All 19 available muscle
    biopsies in the genotyped TWNK cohort showed signs of mitochondrial dysfunction.
  phenotype_term:
    preferred_term: Mitochondrial myopathy
    term:
      id: HP:0003737
      label: Mitochondrial myopathy
  frequency: VERY_FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All 19 available muscle biopsies showed signs of mitochondrial dysfunction."
    explanation: >-
      Establishes that the mitochondrial myopathy is essentially universal in
      genetically confirmed adPEO.
- category: Muscular
  name: Ragged-red muscle fibers
  description: >
    Scattered muscle fibres with irregular subsarcolemmal red-staining masses of
    proliferated mitochondria on modified Gomori trichrome, the classic compensatory
    response of a respiratory-chain-deficient fibre segment.
  phenotype_term:
    preferred_term: Ragged-red muscle fibers
    term:
      id: HP:0003200
      label: Ragged-red muscle fibers
  frequency: FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All 19 available muscle biopsies showed signs of mitochondrial dysfunction."
    explanation: >-
      Documents the universal presence of mitochondrial histological change on adPEO
      muscle biopsy, of which ragged-red fibres are the canonical form.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Mitochondrial histochemical changes observed in muscle showed remarkable variability"
    explanation: >-
      Important qualifier: the histochemical burden varies widely between patients, so
      a sparse biopsy does not exclude the diagnosis.
- category: Muscular
  name: Cytochrome C oxidase-negative muscle fibers
  description: >
    Mosaic loss of cytochrome c oxidase (complex IV) histochemical reactivity in
    scattered fibre segments, with preserved or increased succinate dehydrogenase
    (complex II) reactivity. Because three complex IV catalytic subunits are
    mtDNA-encoded while complex II is entirely nuclear-encoded, this
    COX-negative/SDH-positive pattern is specific for an mtDNA rather than a nuclear
    OXPHOS lesion.
  phenotype_term:
    preferred_term: Cytochrome C oxidase-negative muscle fibers
    term:
      id: HP:0003688
      label: Cytochrome C oxidase-negative muscle fibers
  frequency: FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "resulting in the accumulation of multiple mitochondrial DNA (mtDNA) deletions and cytochrome c oxidase (COX)-deficient fibers in skeletal muscle secondary to a disorder of mtDNA maintenance"
    explanation: >-
      Directly documents COX-deficient fibres as the histochemical consequence of the
      mtDNA maintenance defect in genotyped adPEO.
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The progressive accumulation of mtDNA deletions causes COX deficiency in muscle fibers and results in the clinical phenotype."
    explanation: >-
      Independent genotype (POLG2) demonstrating the same COX-deficient fibre
      readout.
- category: Molecular
  name: Multiple mitochondrial DNA deletions
  description: >
    Multiple heterogeneous large-scale deletions of the mitochondrial genome
    demonstrable in clinically affected tissue (usually skeletal muscle) by
    long-range PCR, Southern blot or deep sequencing. This is the defining molecular
    phenotype of the disease group and the finding that redirects the diagnostic
    workup from the mitochondrial to the nuclear genome. Sensitivity depends on
    method: in some patients the deletions are detectable only by PCR-based assays
    and not by Southern blotting.
  phenotype_term:
    preferred_term: Multiple mitochondrial DNA deletions
    term:
      id: HP:0003689
      label: Multiple mitochondrial DNA deletions
  frequency: OBLIGATE
  diagnostic: true
  evidence:
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autosomal-dominant progressive external ophthalmoplegia (adPEO) is a mitochondrial disorder that is characterized by accumulation of multiple mitochondrial DNA (mtDNA) deletions in postmitotic tissues."
    explanation: >-
      Defines multiple mtDNA deletions in post-mitotic tissue as the characteristic
      feature of adPEO, supporting the OBLIGATE band.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "as did the secondary mtDNA deletions, which in some patients were only detected by PCR-based assays and not Southern blotting"
    explanation: >-
      Important methodological caveat on detection sensitivity for the defining
      molecular phenotype.
- category: Muscular
  name: Elevated circulating creatine kinase concentration
  description: >
    Mild hyperCKemia, present in only 28% of the genotyped TWNK adPEO cohort. A
    normal creatine kinase does NOT exclude adPEO and CK is not a useful screening
    test.
  phenotype_term:
    preferred_term: Elevated circulating creatine kinase concentration
    term:
      id: HP:0003236
      label: Elevated circulating creatine kinase concentration
    severity: MILD
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Only 28% had mild hyperCKemia."
    explanation: >-
      Quantifies mild hyperCKemia at 28%, supporting the OCCASIONAL band and the
      clinical point that a normal CK does not exclude the diagnosis.
- category: Gastrointestinal
  name: Dysphagia
  description: >
    Swallowing difficulty from bulbar muscle involvement, part of the 48% weakness
    burden in the genotyped TWNK cohort, which also encompassed neck and bulbar
    muscles. Dysphagia is the principal driver of aspiration risk and warrants formal
    speech and swallow assessment.
  phenotype_term:
    preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
    clinical_course: PROGRESSIVE
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Weakness was present in 48%, affecting proximal limbs, neck, and bulbar muscles."
    explanation: >-
      Documents bulbar muscle involvement in the genotyped adPEO cohort; bulbar
      weakness is the substrate for dysphagia.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness"
    explanation: >-
      GeneReviews lists bulbar dysfunction among the manifestations of RRM2B-related
      adPEO.
- category: Cardiovascular
  name: Arrhythmia
  description: >
    Cardiac abnormality, chiefly conduction disease and rhythm disturbance, was
    reported in 24% of both large genotyped adPEO cohorts. This is a clinically
    important and potentially fatal extension that justifies baseline and periodic
    ECG and echocardiographic surveillance even in apparently ocular-restricted
    disease.
  phenotype_term:
    preferred_term: Arrhythmia
    term:
      id: HP:0011675
      label: Arrhythmia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "however, in 24% (8/33) of the patients, cardiac abnormalities were reported"
    explanation: >-
      Quantifies cardiac abnormality at 24% in a genotyped adPEO cohort.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Less frequent manifestations were cardiac (24%) and respiratory (4%) involvement"
    explanation: >-
      Independent genotyped cohort reporting the same 24% cardiac involvement rate.
- category: Neurologic
  name: Peripheral axonal neuropathy
  description: >
    Length-dependent sensory and sensorimotor axonal neuropathy, present in 8% of the
    genotyped TWNK cohort but substantially more common in POLG-related adPEO, where
    GeneReviews lists peripheral neuropathy among the defining features of late-onset
    disease.
  phenotype_term:
    preferred_term: Peripheral axonal neuropathy
    term:
      id: HP:0003477
      label: Peripheral axonal neuropathy
    clinical_course: PROGRESSIVE
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neuropathy (8%), ataxia (4%), and parkinsonism (4%)"
    explanation: >-
      Quantifies neuropathy at 8% in TWNK-related adPEO.
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."
    explanation: >-
      GeneReviews places peripheral neuropathy among the core features of late-onset
      POLG disease, the genotype in which the neurological extension is greatest.
- category: Neurologic
  name: Ataxia
  description: >
    Gait and limb ataxia, of mixed cerebellar and sensory-afferent origin, present in
    4% of the genotyped TWNK cohort and considerably more frequent in POLG-related
    adPEO. Where ataxia dominates, the phenotype merges with the POLG ataxia and
    neuropathy spectrum.
  phenotype_term:
    preferred_term: Ataxia
    term:
      id: HP:0001251
      label: Ataxia
    clinical_course: PROGRESSIVE
  frequency: VERY_RARE
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neuropathy (8%), ataxia (4%), and parkinsonism (4%)"
    explanation: >-
      Quantifies ataxia at 4% in TWNK-related adPEO, supporting the VERY_RARE band for
      that genotype.
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness."
    explanation: >-
      GeneReviews lists ataxia among late-onset POLG features; cited as PARTIAL
      because the frequency band recorded here is derived from the TWNK cohort and
      understates POLG-related disease.
  notes: >-
    The VERY_RARE band is genotype-specific to TWNK-related adPEO. In POLG-related
    adPEO (PEOA1) ataxia is considerably more common, and no single frequency band
    applies across all six loci.
- category: Neurologic
  name: Parkinsonism
  description: >
    Levodopa-responsive parkinsonism from nigrostriatal dopaminergic degeneration,
    strongly associated with POLG-related adPEO, where parkinsonism cosegregated with
    POLG mutations across seven families and PET confirmed dopaminergic neuron loss.
    Neuropathology shows loss of pigmented nigral neurons WITHOUT Lewy bodies,
    distinguishing this from idiopathic Parkinson disease. It occurs in 4% of the
    TWNK cohort, so it is not exclusive to POLG.
  phenotype_term:
    preferred_term: Parkinsonism
    term:
      id: HP:0001300
      label: Parkinsonism
    clinical_course: PROGRESSIVE
  frequency: VERY_RARE
  evidence:
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical assessment showed significant cosegregation of parkinsonism with POLG mutations (p<0.0001), and PET findings were consistent with dopaminergic neuron loss."
    explanation: >-
      Establishes parkinsonism as a genuine, genetically cosegregating feature of
      POLG-related PEO with imaging confirmation.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "neuropathy (8%), ataxia (4%), and parkinsonism (4%)"
    explanation: >-
      Quantifies parkinsonism at 4% in TWNK-related adPEO.
  notes: >-
    The VERY_RARE band is taken from the TWNK cohort. In POLG-related adPEO
    parkinsonism is substantially more frequent; the Luoma study found significant
    cosegregation across seven families but did not report a single population
    percentage that could be mapped to an enum band.
- category: Neurologic
  name: Sensorineural hearing impairment
  description: >
    Cochlear hair cells and spiral ganglion neurons are high-energy post-mitotic
    cells that accumulate the same mtDNA deletion burden, and sensorineural hearing
    loss is a recognised extension of the mtDNA maintenance defect spectrum,
    particularly in RRM2B-related disease.
  phenotype_term:
    preferred_term: Sensorineural hearing impairment
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Subsequent assessments are likely to reveal multisystem involvement including sensorineural hearing loss, renal tubulopathy, and respiratory failure."
    explanation: >-
      GeneReviews documents sensorineural hearing loss within the RRM2B mtDNA
      maintenance defect spectrum; cited as PARTIAL because the sentence describes the
      severe encephalomyopathic phenotype rather than adPEO specifically.
  notes: >-
    No adPEO-specific frequency for hearing loss was found in a quotable abstract, so
    frequency is deliberately omitted rather than estimated.
- category: Ocular
  name: Cataract
  description: >
    Early-onset lens opacification, reported in two related subjects aged 48 and 50
    in the genotyped TWNK adPEO cohort. Cataract is a recognised but uncommon ocular
    extension of mtDNA maintenance disease.
  phenotype_term:
    preferred_term: Cataract
    term:
      id: HP:0000518
      label: Cataract
  frequency: VERY_RARE
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Autosomal dominant mutations in the TWNK gene, which encodes a mitochondrial DNA helicase, cause adult-onset progressive external ophthalmoplegia (PEO) and PEO-plus presentations."
    explanation: >-
      Establishes the cohort as genotyped adPEO; the cataract observation itself
      (two related subjects, ages 48 and 50) is reported in the full text rather than
      the abstract, so this evidence item is marked PARTIAL.
  notes: >-
    The specific cataract count (2 patients, ages 48 and 50) comes from the body of
    the Bermejo-Guerrero cohort paper via deep research and could NOT be verified
    against the cached abstract; only the cohort framing is quotable. The VERY_RARE
    band reflects 2/25 patients.
- category: Psychiatric
  name: Depression
  description: >
    Psychiatric involvement is a recognised extension of adPEO. In a German ANT1
    (SLC25A4) pedigree one of three affected siblings had a schizoaffective disorder,
    and depression is commonly reported across mtDNA maintenance disorders.
  phenotype_term:
    preferred_term: Depression
    term:
      id: HP:0000716
      label: Depression
  evidence:
  - reference: PMID:15792871
    reference_title: "A novel ANT1 gene mutation with probable germline mosaicism in autosomal dominant progressive external ophthalmoplegia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutation was identified in three siblings with PEO, one of them additionally suffered from schizoaffective disorder."
    explanation: >-
      Documents psychiatric comorbidity in a molecularly confirmed adPEO family;
      cited as PARTIAL because the reported diagnosis is schizoaffective disorder
      rather than depression specifically.
  notes: >-
    Frequency is omitted deliberately. The only quotable adPEO psychiatric evidence
    describes a single sibling with schizoaffective disorder, which does not support a
    frequency band for depression.
- category: Endocrine
  name: Premature ovarian insufficiency
  description: >
    Most affected women in the POLG-related adPEO families studied by Luoma and
    colleagues reached menopause before age 35 years, a reproductive-counselling
    consideration independent of the 50% transmission risk.
  phenotype_term:
    preferred_term: Premature ovarian insufficiency
    term:
      id: HP:0008209
      label: Premature ovarian insufficiency
  evidence:
  - reference: PMID:15351195
    reference_title: "Parkinsonism, premature menopause, and mitochondrial DNA polymerase gamma mutations: clinical and molecular genetic study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Furthermore, most women with progressive external ophthalmoplegia had early menopause-before age 35 years."
    explanation: >-
      Direct clinical observation of premature menopause in POLG-mutation PEO
      families.
  notes: >-
    Frequency is omitted because the source reports "most women" within seven
    selected POLG families rather than a cohort-wide percentage; the finding is
    genotype-specific to POLG-related adPEO and has not been established for the
    other five loci.
- category: Neurologic
  name: Dysarthria
  description: >
    Slurred speech from bulbar muscle involvement, frequently accompanying dysphagia
    in TWNK-related adPEO.
  phenotype_term:
    preferred_term: Dysarthria
    term:
      id: HP:0001260
      label: Dysarthria
  evidence:
  - reference: PMID:20880070
    reference_title: "TWINKLE gene mutation: report of a French family with an autosomal dominant progressive external ophthalmoplegia and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Symptoms reported in association with this mutation were myopathy, peripheral neuropathy, dysarthria and/or dysphagia, respiratory insufficiency and parkinsonism."
    explanation: >-
      Lists dysarthria among the features of the recurrent TWNK p.R374Q adPEO allele.
- category: Neurologic
  name: Dysphonia
  description: >
    Voice change from laryngeal muscle involvement, reported alongside dysphagia and
    myopathy in a large TWNK-related adPEO family.
  phenotype_term:
    preferred_term: Dysphonia
    term:
      id: HP:0001618
      label: Dysphonia
  evidence:
  - reference: PMID:26838077
    reference_title: "Mutation in TWINKLE in a Large Iranian Family with Progressive External Ophthalmoplegia, Myopathy, Dysphagia and Dysphonia, and Behavior Change."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have studied a large Iranian family with myopathy, dysphonia, dysphagia, and behavior change in addition to PEO in affected members."
    explanation: >-
      Documents dysphonia alongside dysphagia in a large TWNK adPEO family.
- category: Respiratory
  name: Respiratory insufficiency due to muscle weakness
  description: >
    Restrictive ventilatory failure from chest-wall and diaphragmatic weakness,
    reported in several TWNK adPEO families. It is a major determinant of morbidity
    when present and warrants pulmonary function testing and consideration of
    non-invasive ventilation.
  phenotype_term:
    preferred_term: Respiratory insufficiency due to muscle weakness
    term:
      id: HP:0002747
      label: Respiratory insufficiency due to muscle weakness
  evidence:
  - reference: PMID:20880070
    reference_title: "TWINKLE gene mutation: report of a French family with an autosomal dominant progressive external ophthalmoplegia and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Respiratory insufficiency caused by chest wall weakness was reported in other families with different Twinkle gene mutations"
    explanation: >-
      Directly reports respiratory insufficiency from chest-wall weakness in TWNK adPEO
      families.
histopathology:
- name: Mitochondrial Change on Skeletal Muscle Biopsy
  description: >
    Muscle biopsy in adPEO shows the composite mitochondrial signature: ragged-red
    fibres on modified Gomori trichrome, subsarcolemmal mitochondrial accumulation,
    and mosaic COX-negative fibre segments on sequential COX/SDH histochemistry.
    Mitochondrial change was present in all 19 available biopsies in the genotyped
    TWNK adPEO cohort and in 95% of a mixed-aetiology 89-case mitochondrial PEO
    series. The burden of change varies remarkably between patients, so a sparse or
    equivocal biopsy does not exclude the diagnosis.
  diagnostic: true
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All 19 available muscle biopsies showed signs of mitochondrial dysfunction."
    explanation: >-
      Establishes the near-universal presence of mitochondrial histological change in
      genetically confirmed adPEO.
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsy was the most accurate test, showing mitochondrial changes in 95%."
    explanation: >-
      Quantifies the diagnostic yield of muscle biopsy in mitochondrial PEO.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mitochondrial histochemical changes observed in muscle showed remarkable variability"
    explanation: >-
      Documents the between-patient variability that limits the negative predictive
      value of biopsy.
- name: Multiple Large-Scale mtDNA Rearrangements in Muscle
  description: >
    Molecular analysis of the biopsy by long-range PCR, Southern blot or deep
    sequencing demonstrates multiple heterogeneous large-scale mtDNA deletions. This
    is the finding that redirects the diagnostic workup from the mitochondrial to the
    nuclear genome and is the discriminator against the single clonal deletion of
    sporadic CPEO and Kearns-Sayre syndrome. In the 89-case mitochondrial PEO series,
    multiple mtDNA deletions accounted for 26% of cases and were attributable to
    TWNK, POLG, TK2 or RRM2B.
  diagnostic: true
  evidence:
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Single large-scale mitochondrial DNA (mtDNA) deletion was the most frequent finding (63%), followed by multiple mtDNA deletions (26%) due to mutations in TWNK (n=8), POLG (n=7), TK2 (n=6) or RRM2B (n=2) genes, and point mtDNA mutations (7%)."
    explanation: >-
      Quantifies the molecular architecture of mitochondrial PEO and the nuclear genes
      responsible for the multiple-deletion subset.
  - reference: PMID:2725645
    reference_title: "An autosomal dominant disorder with multiple deletions of mitochondrial DNA starting at the D-loop region."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multiple deletions, involving the same portion of muscle mtDNA, were identified in all patients."
    explanation: >-
      Founding demonstration of multiple mtDNA deletions in muscle in a dominantly
      inherited pedigree.
biochemical:
- name: Serum Creatine Kinase
  biomarker_term:
    preferred_term: Elevated circulating creatine kinase concentration
    term:
      id: HP:0003236
      label: Elevated circulating creatine kinase concentration
  presence: VARIABLE
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Only 28% had mild hyperCKemia."
    explanation: >-
      Only a minority of genotyped adPEO patients have raised creatine kinase, and the
      elevation is mild.
  notes: >-
    Creatine kinase is normal in nearly three-quarters of adPEO patients and, when
    raised, is only mildly so. It is not a useful screening or exclusion test.
- name: Serum Lactate
  biomarker_term:
    preferred_term: Increased circulating lactate concentration
    term:
      id: HP:0002151
      label: Increased circulating lactate concentration
  presence: VARIABLE
  evidence:
  - reference: PMID:28695364
    reference_title: "Revisiting mitochondrial ocular myopathies: a study from the Italian Network."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Increased lactate was associated with central neurological involvement."
    explanation: >-
      In a large genetically diagnosed mitochondrial cohort, raised lactate tracked
      with CNS involvement rather than with the ocular myopathy itself.
  notes: >-
    Resting serum lactate is frequently normal in ocular myopathy and is not a
    sensitive screening test. An elevated lactate should prompt a search for
    encephalopathic or other central features rather than being taken as
    confirmation of the ocular phenotype.
genetic:
- name: POLG
  gene_term:
    preferred_term: POLG
    term:
      id: hgnc:9179
      label: POLG
  relationship_type: CAUSATIVE
  subtype: PEOA1
  variant_origin: GERMLINE
  association: >-
    A single heterozygous pathogenic variant in POLG, encoding the catalytic A
    subunit of mitochondrial DNA polymerase gamma, causes PEOA1. The dominant alleles
    cluster in the polymerase domain (p.G923D, p.R943H, p.Y955C, p.A957S), where they
    reduce catalytic efficiency and nucleotide selectivity; p.Y955C in motif B is the
    prototype. POLG is the most frequent single cause of familial PEO with multiple
    mtDNA deletions, accounting for approximately 45% of one family cohort. The same
    gene causes a wide autosomal RECESSIVE spectrum (PEOB1, Alpers-Huttenlocher,
    MIRAS, SANDO), so the number of pathogenic alleles must be established explicitly.
  evidence:
  - reference: PMID:11431686
    reference_title: "Mutation of POLG is associated with progressive external ophthalmoplegia characterized by mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We mapped a new locus for dominant PEO at 15q22-q26 in a Belgian pedigree and identified a heterozygous mutation (Y955C) in the polymerase motif B of the mtDNA polymerase gamma (POLG)."
    explanation: >-
      Founding identification of the dominant POLG allele.
  - reference: PMID:12210792
    reference_title: "Mutations of mitochondrial DNA polymerase gammaA are a frequent cause of autosomal dominant or recessive progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We screened the POLG1 gene in several PEO families and identified five different heterozygous missense mutations of POLG1 in 10 autosomal dominant families."
    explanation: >-
      Establishes heterozygous POLG missense variants in ten dominant PEO families.
  - reference: PMID:15258572
    reference_title: "Structure-function defects of human mitochondrial DNA polymerase in autosomal dominant progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Four autosomal dominant mutations that cause PEO encode the amino acid substitutions G923D, R943H, Y955C and A957S in the polymerase domain of pol gamma."
    explanation: >-
      Enumerates the four canonical dominant POLG alleles and localises them to the
      polymerase domain.
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Late-onset PEO may be caused by a heterozygous POLG pathogenic variant and inherited in an autosomal dominant manner."
    explanation: >-
      GeneReviews confirms the dominant, single-allele mechanism for late-onset POLG
      PEO.
- name: SLC25A4
  gene_term:
    preferred_term: SLC25A4
    term:
      id: hgnc:10990
      label: SLC25A4
  relationship_type: CAUSATIVE
  subtype: PEOA2
  variant_origin: GERMLINE
  association: >-
    A single heterozygous missense variant in SLC25A4, encoding the heart and
    skeletal-muscle isoform of the adenine nucleotide translocator (ANT1), causes
    PEOA2. The prototype familial allele substitutes proline for a highly conserved
    alanine at position 114; the analogous substitution in yeast produces a
    respiratory defect. A second reported allele converts a conserved alanine at
    codon 90 to aspartic acid. Germline mosaicism has been documented, which has
    direct implications for recurrence-risk counselling in apparently sporadic cases.
  evidence:
  - reference: PMID:10926541
    reference_title: "Role of adenine nucleotide translocator 1 in mtDNA maintenance."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The familial mutation substitutes a proline for a highly conserved alanine at position 114 in the ANT1 protein. The analogous mutation in yeast caused a respiratory defect."
    explanation: >-
      Identifies the prototype ANT1 allele and provides cross-species functional
      support.
  - reference: PMID:15792871
    reference_title: "A novel ANT1 gene mutation with probable germline mosaicism in autosomal dominant progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microsatellite analysis showed that the mutation was dominant and inherited from the mother who did not carry the mutation in blood, indicating germ-line mosaicism."
    explanation: >-
      Documents germline mosaicism at the SLC25A4 locus, a recurrence-risk
      consideration in apparently de novo adPEO.
- name: TWNK
  gene_term:
    preferred_term: TWNK
    term:
      id: hgnc:1160
      label: TWNK
  relationship_type: CAUSATIVE
  subtype: PEOA3
  variant_origin: GERMLINE
  association: >-
    A single heterozygous variant in TWNK (formerly C10orf2/PEO1), encoding the
    Twinkle replicative mtDNA helicase, causes PEOA3. Eleven different
    coding-region mutations co-segregated with disease in twelve adPEO pedigrees in
    the founding report, clustering in the region mediating subunit interactions of
    the hexameric helicase. In a contemporary 25-patient Spanish cohort, ten
    different TWNK variants were found, with c.1361T>G (p.Val454Gly) and c.1070G>C
    (p.Arg357Pro) the most common. TWNK is a leading cause of adPEO when the family
    history is positive.
  evidence:
  - reference: PMID:11431692
    reference_title: "Human mitochondrial DNA deletions associated with mutations in the gene encoding Twinkle, a phage T7 gene 4-like protein localized in mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "identified 11 different coding-region mutations co-segregating with the disorder in 12 adPEO pedigrees of various ethnic origins"
    explanation: >-
      Founding co-segregation evidence for TWNK in adPEO.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ten different TWNK mutations were identified, with c.1361T>G (p.Val454Gly) and c.1070G>C (p.Arg357Pro) being the most common."
    explanation: >-
      Contemporary allelic spectrum of TWNK-related adPEO.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TWNK is an important cause when positive family history is present"
    explanation: >-
      Positions TWNK as a high-priority gene when the pedigree is dominant.
- name: POLG2
  gene_term:
    preferred_term: POLG2
    term:
      id: hgnc:9180
      label: POLG2
  relationship_type: CAUSATIVE
  subtype: PEOA4
  variant_origin: GERMLINE
  association: >-
    A single heterozygous variant in POLG2, encoding the p55 accessory subunit of
    polymerase gamma, causes PEOA4. The prototype allele c.1352G>A (p.G451E) retains
    DNA binding but fails to stimulate the catalytic p140 subunit or to strengthen
    p140-p55 complex DNA binding, so disease arises through haploinsufficiency or
    heterodimerisation with wild-type protein. PEOA4 is rare.
  evidence:
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Biochemical characterization of purified, recombinant G451E-substituted p55 protein in vitro revealed incomplete stimulation of the catalytic subunit due to compromised subunit interaction."
    explanation: >-
      Biochemical characterisation of the prototype dominant POLG2 allele.
- name: RRM2B
  gene_term:
    preferred_term: RRM2B
    term:
      id: hgnc:17296
      label: RRM2B
  relationship_type: CAUSATIVE
  subtype: PEOA5
  variant_origin: GERMLINE
  association: >-
    A single heterozygous variant in RRM2B, encoding p53R2, the small subunit of the
    ribonucleotide reductase supplying deoxyribonucleotides to non-dividing cells,
    causes PEOA5. The prototype allele is the C-terminal nonsense variant p.R327X,
    whose transcript escapes nonsense-mediated decay and produces a dominant-negative
    truncated protein. Biallelic RRM2B variants cause a mechanistically related but
    far more severe recessive encephalomyopathic mtDNA depletion syndrome, so the
    inheritance mode must be established explicitly for each family.
  evidence:
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We conclude that dominant-negative or gain-of-function mutations in RRM2B are a cause of multiple mtDNA deletions and adPEO."
    explanation: >-
      States the dominant mechanism at the RRM2B locus.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of an RRM2B-MDMD is established in a proband with suggestive findings and either biallelic RRM2B pathogenic variants or a heterozygous RRM2B pathogenic variant identified by molecular genetic testing."
    explanation: >-
      GeneReviews establishes that both monoallelic (dominant) and biallelic
      (recessive) RRM2B genotypes cause disease.
- name: DNA2
  gene_term:
    preferred_term: DNA2
    term:
      id: hgnc:2939
      label: DNA2
  relationship_type: CAUSATIVE
  subtype: PEOA6
  variant_origin: GERMLINE
  association: >-
    A single heterozygous variant in DNA2, encoding a helicase/nuclease acting in
    mtDNA replication and in the long-patch base-excision repair pathway, causes
    PEOA6. Mutant proteins show severe impairment of nuclease, helicase and ATPase
    activity. The phenotype was delineated as adult-onset progressive myopathy with
    muscle mtDNA instability and limb-girdle weakness, with PEO a frequent component.
  evidence:
  - reference: PMID:23352259
    reference_title: "Mutations in DNA2 link progressive myopathy to mitochondrial DNA instability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "DNA2 encodes a helicase/nuclease family member that is most likely involved in mtDNA replication, as well as in the long-patch base-excision repair (LP-BER) pathway."
    explanation: >-
      Defines the molecular function of DNA2 in mtDNA maintenance.
  - reference: PMID:23352259
    reference_title: "Mutations in DNA2 link progressive myopathy to mitochondrial DNA instability."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vitro biochemical analysis of purified mutant proteins revealed a severe impairment of nuclease, helicase, and ATPase activities."
    explanation: >-
      Functional confirmation that the PEOA6 alleles are loss-of-function for all
      three catalytic activities.
inheritance:
- name: Autosomal dominant inheritance
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  description: >-
    adPEO is inherited in an autosomal dominant manner at all six loci: a single
    heterozygous pathogenic variant in POLG, SLC25A4, TWNK, POLG2, RRM2B or DNA2 is
    sufficient. Each child of an affected individual has a 50% risk of inheriting the
    variant. Family history was positive in 22 of 25 patients in the genotyped TWNK
    cohort. Two caveats are important. First, germline mosaicism has been documented
    at the SLC25A4 locus in a parent who did not carry the variant in blood, so an
    apparently de novo case does not guarantee a negligible sibling recurrence risk.
    Second, both POLG and RRM2B also cause distinct autosomal RECESSIVE disorders, so
    the number of pathogenic alleles, not merely the gene, determines the inheritance
    mode and the counselling.
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Late-onset PEO may be caused by a heterozygous POLG pathogenic variant and inherited in an autosomal dominant manner."
    explanation: >-
      GeneReviews states the dominant single-allele mechanism at the POLG locus.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "If a parent of the proband is affected and/or is known to have the RRM2B pathogenic variant identified in the proband, the risk to sibs of inheriting the pathogenic variant is 50%."
    explanation: >-
      GeneReviews states the 50% transmission risk for the dominant RRM2B genotype.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Family history was positive in 22 patients."
    explanation: >-
      Documents the dominant pedigree pattern in a genotyped adPEO cohort.
  - reference: PMID:15792871
    reference_title: "A novel ANT1 gene mutation with probable germline mosaicism in autosomal dominant progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microsatellite analysis showed that the mutation was dominant and inherited from the mother who did not carry the mutation in blood, indicating germ-line mosaicism."
    explanation: >-
      Documents the germline mosaicism caveat that modifies recurrence-risk
      counselling.
prevalence:
- population: North East England adults (all nuclear-gene adult mitochondrial disease)
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 2.9
  notes: >-
    Upper-bound proxy. This is the population prevalence of ALL clinically overt adult
    mitochondrial disease caused by nuclear-gene mutations, the class to which adPEO
    belongs; adPEO specifically is a fraction of this figure. No adPEO-specific
    population prevalence estimate has been published.
  evidence:
  - reference: PMID:25652200
    reference_title: "Prevalence of nuclear and mitochondrial DNA mutations related to adult mitochondrial disease."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "In this population, nuclear mutations were responsible for clinically overt adult mitochondrial disease in 2.9 per 100,000 adults."
    explanation: >-
      Population-based minimum prevalence of nuclear-gene adult mitochondrial disease.
      Cited as PARTIAL because it is a class-level rather than an adPEO-specific
      figure.
- population: Mitochondrial PEO diagnostic cohort (Spain, 89 cases)
  measure_type: PERIOD_PREVALENCE
  prevalence_class: UNKNOWN
  notes: >-
    Not a population rate but a diagnostic-yield composition figure: among 89
    consecutive mitochondrial PEO cases, 26% had multiple mtDNA deletions (the adPEO
    molecular class) versus 63% with a single large-scale deletion and 7% with mtDNA
    point mutations. This is the best available estimate of how large a share of the
    PEO clinic adPEO represents.
  evidence:
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Single large-scale mitochondrial DNA (mtDNA) deletion was the most frequent finding (63%), followed by multiple mtDNA deletions (26%)"
    explanation: >-
      Gives the share of a mitochondrial PEO diagnostic cohort attributable to the
      multiple-deletion (nuclear-gene) class. PARTIAL because it is a clinic-referral
      composition, not a population rate, and includes recessive as well as dominant
      nuclear causes.
treatments:
- name: Supportive and Multidisciplinary Care
  description: >
    There is no approved disease-modifying therapy for adPEO. Management is
    supportive and multidisciplinary: occupational, physical and speech therapy for
    the myopathy and bulbar involvement; nutritional and swallowing support;
    respiratory support where needed; hearing and low-vision support; and periodic
    cardiac and respiratory surveillance. Coenzyme Q10, riboflavin and other
    "mitochondrial cocktail" supplements are widely used empirically, but no
    adPEO-specific efficacy evidence was identified.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:38391710
    reference_title: "Mitochondrial Chronic Progressive External Ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No definitive treatment option is available for mitochondrial diseases, and management is mainly focused on lifestyle risk modification and supplementation to reduce mitochondrial load and symptomatic relief, such as ptosis repair in the case of CPEO."
    explanation: >-
      Contemporary review stating the absence of definitive therapy and the
      supportive, symptom-directed model of care for mitochondrial CPEO.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To date, there are no known cures and few effective treatments for any forms of mitochondrial disease, including the RRM2B-MDMDs."
    explanation: >-
      GeneReviews confirms the absence of curative or disease-modifying therapy for
      the mtDNA maintenance defects.
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical management is largely supportive and involves standard approaches for associated complications including occupational, physical, and speech therapy; nutritional support; respiratory support"
    explanation: >-
      GeneReviews specifies the components of supportive multidisciplinary management.
- name: Ptosis Surgery (Levator Advancement or Frontalis Suspension)
  description: >
    Surgical correction is the mainstay of symptomatic treatment for the ptosis of
    adPEO, chosen according to levator excursion: levator advancement or resection
    where levator function is preserved, frontalis suspension (sling) where it is
    not. Myogenic ptosis is a particularly difficult surgical target because the
    ptosis continues to worsen and because the same mitochondrial myopathy impairs
    protective corneal mechanisms (Bell phenomenon, orbicularis closure). Deliberate
    under-correction is therefore standard, at the cost of a high recurrence rate.
  treatment_term:
    preferred_term: Ophthalmologic Surgical Procedure
    term:
      id: NCIT:C15331
      label: Ophthalmologic Surgical Procedure
  therapeutic_modality: SURGERY
  target_phenotypes:
  - preferred_term: Ptosis
    term:
      id: HP:0000508
      label: Ptosis
  evidence:
  - reference: PMID:36178005
    reference_title: "A review of surgical management of progressive myogenic ptosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Surgical techniques predominantly involved levator advancement, levator resection, frontalis sling, blepharoplasty, and Fasanella-Servat."
    explanation: >-
      Enumerates the surgical options used for progressive myogenic ptosis, of which
      CPEO is an explicitly analysed subgroup.
  - reference: PMID:36178005
    reference_title: "A review of surgical management of progressive myogenic ptosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, myogenic ptosis is especially challenging as it is characterized by worsening ptosis and the loss of protective corneal mechanisms."
    explanation: >-
      Explains why mitochondrial myogenic ptosis carries a higher surgical risk than
      other ptosis aetiologies.
  - reference: PMID:36178005
    reference_title: "A review of surgical management of progressive myogenic ptosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The goals of care with myogenic ptosis involves repairing ptosis just sufficiently to alleviate visual obstruction while avoiding adverse post-operative complications."
    explanation: >-
      Establishes the deliberate-under-correction principle governing ptosis surgery
      in mitochondrial ocular myopathy.
- name: Avoidance of Valproic Acid (Contraindicated)
  description: >
    Valproic acid and sodium divalproate must be avoided in POLG-related disease,
    including PEOA1, because they can precipitate or accelerate liver disease and
    fatal hepatic failure; POLG genotype prospectively identifies individuals at
    high risk. GeneReviews extends an equivalent caution to RRM2B-related disease,
    where valproate should be used only in exceptional circumstances. Because
    epilepsy can occur in POLG-related disease and valproate is a common
    anticonvulsant choice, this is the single most actionable prescribing fact in
    adPEO. This record is curated with negative valence: the intervention is
    withholding the drug, not administering it. SCOPE LIMIT: the caution is
    GENOTYPE-DIRECTED, not adPEO-wide. It is established for POLG (PEOA1) and extended
    by GeneReviews to RRM2B (PEOA5); no equivalent evidence exists for the SLC25A4
    (PEOA2), TWNK (PEOA3), POLG2 (PEOA4) or DNA2 (PEOA6) subtypes, and this entry does
    not assert one for them. Note also that the underlying hepatotoxicity literature is
    dominated by BIALLELIC POLG disease, so its transfer to heterozygous dominant adPEO
    is an extrapolation of prescribing caution rather than a directly demonstrated
    dominant-genotype risk.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: valproic acid
      term:
        id: CHEBI:39867
        label: valproic acid
  therapeutic_modality: SMALL_MOLECULE
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "because of the risk of precipitating and/or accelerating liver disease"
    explanation: >-
      GeneReviews lists valproic acid and sodium divalproate under
      Agents/circumstances to avoid for POLG-related disorders.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Valproic acid should be used only in exceptional circumstances."
    explanation: >-
      GeneReviews extends the valproate caution to RRM2B mtDNA maintenance defects,
      which include the adPEO phenotype.
  - reference: PMID:21038416
    reference_title: "Polymerase gamma gene POLG determines the risk of sodium valproate-induced liver toxicity."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These findings implicate impaired liver regeneration in VPA toxicity and show that prospective genetic testing of POLG will identify individuals at high risk of this potentially fatal consequence of treatment."
    explanation: >-
      Demonstrates that POLG genotype prospectively identifies valproate
      hepatotoxicity risk, making avoidance an actionable, genotype-driven decision.
- name: Genetic Counseling and Cascade Testing
  description: >
    Because adPEO is dominantly transmitted, each child of an affected individual
    has a 50% risk. Counselling should cover the transmission risk, the variable
    expressivity and late onset that make the phenotype easy to miss in relatives,
    the availability of prenatal and preimplantation genetic testing once the
    familial variant is known, and the germline-mosaicism caveat that an apparently
    de novo case does not guarantee negligible sibling recurrence risk. For POLG and
    RRM2B, counselling must establish whether the family carries a dominant
    (monoallelic) or a recessive (biallelic) genotype, since the recurrence risks
    differ fundamentally.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Once the RRM2B pathogenic variant(s) have been identified in an affected family member, prenatal and preimplantation genetic testing for RRM2B-MDMD are possible."
    explanation: >-
      GeneReviews confirms the availability of prenatal and preimplantation testing
      once the familial variant is known.
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Accurate differential diagnosis and early confirmation with appropriately chosen complementary studies allow genetic counseling and the avoidance of unnecessary treatments."
    explanation: >-
      States the dual value of early molecular confirmation: enabling counselling and
      preventing futile treatment.
- name: Physical Therapy and Paced Exercise
  description: >
    Individualised submaximal aerobic and resistance training, physiotherapy, activity
    pacing and fall prevention are offered for the myopathic component. Exercise is
    supportive rather than disease-modifying and should be prescribed at submaximal
    intensity given the limited oxidative reserve.
  treatment_term:
    preferred_term: Physical Therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  therapeutic_modality: BEHAVIORAL
  target_phenotypes:
  - preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "including occupational, physical, and speech therapy"
    explanation: >-
      GeneReviews includes physical therapy among the standard supportive measures.
- name: Elamipretide (Investigational, Not Approved)
  description: >
    Elamipretide is a mitochondria-targeted tetrapeptide investigated for primary
    mitochondrial myopathy. It is NOT approved for adPEO or for any mitochondrial
    myopathy indication, and the Phase 3 MMPOWER-3 trial was NEGATIVE in its
    genotypically diverse primary mitochondrial myopathy population. A POST HOC
    genotype-stratified analysis found that participants carrying mtDNA replisome
    variants who also had CPEO - broadly the adPEO population - improved on the
    six-minute walk test by 37.3 +/- 9.5 m versus -8.0 +/- 10.7 m on placebo at week
    24 (p=0.0024), with the replisome cohort as a whole only trending toward benefit
    (p=0.06). This is a hypothesis-generating exploratory subgroup finding from a
    trial that failed its primary endpoints; it is NOT evidence of efficacy. It
    motivated the dedicated Phase 3 NuPOWER trial (NCT05162768) in nuclear-DNA
    primary mitochondrial disease, and no efficacy conclusion can be drawn until
    those results are published. This treatment is curated so that the negative
    parent-trial result is on the record alongside the subgroup signal.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: elamipretide
      term:
        id: NCIT:C171874
        label: Elamipretide
  therapeutic_modality: PEPTIDE
  target_phenotypes:
  - preferred_term: Exercise intolerance
    term:
      id: HP:0003546
      label: Exercise intolerance
  evidence:
  - reference: PMID:39574155
    reference_title: "Genotype-specific effects of elamipretide in patients with primary mitochondrial myopathy: a post hoc analysis of the MMPOWER-3 trial."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "As previously published, the MMPOWER-3 clinical trial did not demonstrate a significant benefit of elamipretide treatment in a genotypically diverse population of adults with primary mitochondrial myopathy (PMM)."
    explanation: >-
      The negative primary result of the parent Phase 3 trial, recorded explicitly and
      first so that the post-hoc subgroup signal below cannot be read as established
      efficacy.
  - reference: PMID:39574155
    reference_title: "Genotype-specific effects of elamipretide in patients with primary mitochondrial myopathy: a post hoc analysis of the MMPOWER-3 trial."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "The 6MWT results at week 24 in subjects with replisome variants showed a significant change in the elamipretide group subjects who had chronic progressive external ophthalmoplegia (CPEO) (37.3 ± 9.5 m versus - 8.0 ± 10.7 m for the placebo group; p = 0.0024)."
    explanation: >-
      The quantitative result for the replisome-plus-CPEO subgroup, which is the
      subgroup corresponding to adPEO. Cited as PARTIAL because it is an exploratory
      post-hoc subgroup analysis of a trial that failed its primary endpoints, not
      confirmatory efficacy evidence.
  - reference: PMID:39574155
    reference_title: "Genotype-specific effects of elamipretide in patients with primary mitochondrial myopathy: a post hoc analysis of the MMPOWER-3 trial."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "These data serve as the foundation for a follow-up Phase 3 clinical trial (NuPOWER) which has been designed as described in this paper to determine the efficacy of elamipretide in patients with mtDNA maintenance-related disorders."
    explanation: >-
      Confirms that efficacy in exactly the adPEO population remains an open question
      to be resolved by a dedicated confirmatory trial.
  notes: >
    Not approved for adPEO or any mitochondrial myopathy indication at the time of
    curation. Do not present the post-hoc 6MWT figure as a treatment effect estimate.
diagnosis:
- name: Molecular Diagnosis by a Single Heterozygous Nuclear Variant
  description: >
    Diagnosis rests on the clinical picture of adult-onset progressive ptosis and
    external ophthalmoplegia, usually with a dominant family history, plus
    identification of a SINGLE heterozygous pathogenic variant in POLG, SLC25A4,
    TWNK, POLG2, RRM2B or DNA2. The monoallelic requirement is what distinguishes
    adPEO from its recessive counterparts at the shared POLG and RRM2B loci, and
    GeneReviews states it explicitly for POLG. Practice has shifted toward
    panel or exome-first testing: in TWNK-related adPEO, direct sequencing is
    recommended before muscle biopsy. Where genetics is inconclusive, muscle biopsy
    with COX/SDH histochemistry and mtDNA rearrangement analysis remains the most
    accurate single test, and analysis of a clinically affected tissue is required
    because the secondary deletions can be tissue-restricted.
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "for which identification of a heterozygous POLG pathogenic variant on molecular genetic testing is diagnostic"
    explanation: >-
      GeneReviews states that a single heterozygous POLG variant is diagnostic for
      adPEO, in explicit contrast to the biallelic requirement for the recessive
      phenotypes.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Direct sequencing of the PEO1 gene should be considered in adPEO patients prior to muscle biopsy."
    explanation: >-
      Supports a genetics-first diagnostic pathway in adPEO.
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsy was the most accurate test, showing mitochondrial changes in 95%. Genetic diagnosis was achieved in 96% of the patients."
    explanation: >-
      Quantifies the yield of biopsy and of molecular testing in mitochondrial PEO.
- name: Demonstration of Multiple (Not Single) mtDNA Deletions in Muscle
  description: >
    The decisive molecular discriminator between adPEO and sporadic CPEO or
    Kearns-Sayre syndrome is whether muscle carries MULTIPLE heterogeneous deletions
    (nuclear-gene disease, dominant recurrence risk) or a SINGLE clonal large-scale
    deletion (sporadic, negligible recurrence risk). In an 89-case mitochondrial PEO
    series, a single deletion accounted for 63% and multiple deletions for 26%.
    Detection method matters: in some patients the multiple deletions are visible only
    on PCR-based assays and not on Southern blotting.
  evidence:
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "If no mtDNA deletions are identified, whole mtDNA sequencing should be performed."
    explanation: >-
      States the escalation step in the diagnostic algorithm for mitochondrial PEO.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "which in some patients were only detected by PCR-based assays and not Southern blotting"
    explanation: >-
      Important methodological caveat: Southern blotting alone can miss the defining
      molecular finding.
- name: Serum FGF21 and GDF15 as Non-Invasive Adjuncts
  description: >
    FGF21 and GDF15 are circulating markers of the mitochondrial integrated stress
    response and outperform muscle histology as a screening test in mitochondrial
    disease: in a 194-sample study only 39% of genetically verified patients had
    mitochondrial pathology on muscle histology, whereas biomarkers were elevated in
    62%. In a myopathic patient with at least one elevated biomarker, an
    mtDNA-expression disorder - the class that includes adPEO - was the cause with 94%
    probability. They are best positioned as non-invasive first-line tests preceding
    genetic sequencing. Normal values do not exclude disease, and neither marker is
    specific for the dominant nuclear form or for adPEO in particular.
  evidence:
  - reference: PMID:32857451
    reference_title: "Diagnostic value of serum biomarkers FGF21 and GDF15 compared to muscle sample in mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Only 39% of patients with genetically verified mitochondrial disease had mitochondrial pathology in their muscle histology. In contrast, biomarkers were elevated in 62% of patients with genetically verified mitochondrial disease."
    explanation: >-
      Quantifies the diagnostic advantage of the serum biomarkers over muscle histology
      in a 194-sample cohort.
  - reference: PMID:32857451
    reference_title: "Diagnostic value of serum biomarkers FGF21 and GDF15 compared to muscle sample in mitochondrial disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "If at least one of the biomarkers was induced and the patient had a myopathic disease, a mitochondrial DNA expression disease was the cause with 94% probability."
    explanation: >-
      Gives the positive predictive value in exactly the clinical situation adPEO
      presents in - a myopathic patient with an mtDNA-expression defect.
  - reference: PMID:38391710
    reference_title: "Mitochondrial Chronic Progressive External Ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Genetic sequencing is the gold standard for diagnosing mitochondrial encephalomyopathies, preceded by non-invasive tests such as fibroblast growth factor-21 and growth differentiation factor-15."
    explanation: >-
      A CPEO-specific review positioning FGF21 and GDF15 ahead of sequencing in the
      diagnostic pathway.
clinical_trials:
- name: NCT05162768
  phase: PHASE_III
  status: COMPLETED
  description: >
    NuPOWER (SPIMD-301): a 48-week randomised, double-blind, parallel-group,
    placebo-controlled Phase 3 trial of daily subcutaneous elamipretide in subjects
    with primary mitochondrial myopathy caused by nuclear DNA mutations - the
    population that includes adPEO. Designed as the confirmatory follow-up to the
    post-hoc replisome/CPEO signal from MMPOWER-3. Listed as completed, but results
    were not available in the registry record at the time of curation, so no efficacy
    conclusion can be drawn from completion status alone.
  target_phenotypes:
  - preferred_term: Exercise intolerance
    term:
      id: HP:0003546
      label: Exercise intolerance
  - preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
  evidence:
  - reference: clinicaltrials:NCT05162768
    reference_title: >-
      A Phase 3 Randomized, Double-Blind, Parallel-Group, Placebo-Controlled Trial to
      Evaluate the Efficacy and Safety of Daily Subcutaneous Injections of Elamipretide
      in Subjects With Primary Mitochondrial Disease Resulting From Pathogenic Nuclear
      DNA Mutations (nPMD) NuPower
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SPIMD-301 is a 48-week, randomized, double-blind, parallel-group, placebo-controlled trial to assess efficacy and safety of single daily subcutaneous (SC) administration of elamipretide as a treatment for subjects with primary mitochondrial myopathy associated with nuclear DNA mutations (nPMD)."
    explanation: >-
      The registry description of the confirmatory Phase 3 trial targeting the
      nuclear-DNA mitochondrial myopathy population to which adPEO belongs.
differential_diagnoses:
- name: Myasthenia Gravis
  disease_term:
    preferred_term: myasthenia gravis
    term:
      id: MONDO:0009688
      label: myasthenia gravis
  description: >
    The commonest misdiagnosis. Both cause ptosis and ophthalmoparesis, but
    myasthenic ptosis fluctuates, fatigues on sustained upgaze, and is typically
    accompanied by diplopia, whereas adPEO ptosis is fixed, slowly progressive over
    years and disproportionately diplopia-free. In the genotyped TWNK adPEO cohort,
    36% of patients had previously been misdiagnosed as myasthenia. Acetylcholine
    receptor antibodies, repetitive nerve stimulation and single-fibre EMG separate
    the two.
  distinguishing_features:
  - Fluctuating and fatigable weakness in myasthenia versus fixed slow progression in
    adPEO
  - Diplopia usual in myasthenia, disproportionately uncommon in adPEO
  - Positive acetylcholine receptor antibodies and decrement on repetitive nerve
    stimulation in myasthenia
  - Response to anticholinesterase in myasthenia
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Before definitive genetic confirmation, 56% of patients were misdiagnosed (36% with myasthenia, 20% with oculopharyngeal muscle dystrophy)."
    explanation: >-
      Quantifies myasthenia gravis as the leading misdiagnosis of adPEO, making it the
      most important differential.
- name: Oculopharyngeal Muscular Dystrophy
  disease_term:
    preferred_term: oculopharyngeal muscular dystrophy
    term:
      id: MONDO:0008116
      label: oculopharyngeal muscular dystrophy
  description: >
    An autosomal dominant late-onset myopathy caused by a short GCN trinucleotide
    repeat expansion in PABPN1, presenting with ptosis and dysphagia. It shares the
    dominant pedigree, the late onset and the ptosis-plus-bulbar phenotype with adPEO
    and accounted for 20% of prior misdiagnoses in the genotyped TWNK cohort. Extraocular
    motility is relatively spared in OPMD relative to the severity of the ptosis, and
    muscle biopsy shows rimmed vacuoles and intranuclear filamentous inclusions rather
    than ragged-red and COX-negative fibres.
  distinguishing_features:
  - PABPN1 GCN repeat expansion rather than a nuclear mtDNA maintenance gene variant
  - Rimmed vacuoles and intranuclear inclusions rather than ragged-red and
    COX-negative fibres
  - Ophthalmoparesis relatively mild for the degree of ptosis
  - No multiple mtDNA deletions in muscle
  evidence:
  - reference: PMID:35011763
    reference_title: "Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "36% with myasthenia, 20% with oculopharyngeal muscle dystrophy"
    explanation: >-
      Quantifies oculopharyngeal muscular dystrophy as the second commonest
      misdiagnosis of adPEO.
- name: Single Large-Scale mtDNA Deletion Syndromes (Sporadic CPEO, Kearns-Sayre)
  disease_term:
    preferred_term: Kearns-Sayre syndrome
    term:
      id: MONDO:0010787
      label: Kearns-Sayre syndrome
  description: >
    Sporadic CPEO and Kearns-Sayre syndrome are caused by a SINGLE clonal large-scale
    mtDNA deletion arising in the germline or early embryogenesis, with no nuclear
    gene lesion and negligible recurrence risk. They are clinically almost
    indistinguishable from adPEO at the bedside, and are in fact the majority of
    mitochondrial PEO seen in clinic (63% of an 89-case series versus 26% with
    multiple deletions). Distinguishing single from multiple deletions on muscle mtDNA
    analysis is the decisive step and completely changes the genetic counselling.
  distinguishing_features:
  - Single clonal large-scale mtDNA deletion versus multiple heterogeneous deletions
  - Sporadic occurrence with negligible recurrence risk versus 50% dominant
    transmission
  - Kearns-Sayre adds retinopathy, cardiac conduction block and onset before age 20
  - No causative nuclear gene variant identified
  evidence:
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Single large-scale mitochondrial DNA (mtDNA) deletion was the most frequent finding (63%), followed by multiple mtDNA deletions (26%)"
    explanation: >-
      Quantifies the relative frequency of the single-deletion and multiple-deletion
      classes, showing that the single-deletion syndromes are the larger group and the
      primary differential.
- name: Autosomal Recessive Progressive External Ophthalmoplegia (PEOB1 and Related)
  disease_term:
    preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 1
    term:
      id: MONDO:0009783
      label: progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 1
  description: >
    The recessive counterparts at the same loci: biallelic POLG variants cause PEOB1,
    and biallelic RRM2B variants cause a severe encephalomyopathic mtDNA depletion
    syndrome and an arPEO phenotype. The molecular endpoint (multiple mtDNA deletions)
    and much of the clinical picture are shared, so the distinction rests entirely on
    the NUMBER of pathogenic alleles and the pedigree, not on the gene or the
    phenotype. Recessive disease tends to earlier onset and heavier multisystem
    involvement.
  distinguishing_features:
  - Biallelic rather than monoallelic pathogenic variants
  - Typically earlier onset and heavier neurological extension
  - 25% sibling recurrence risk rather than 50% offspring transmission
  - Common European POLG founder alleles p.A467T and p.W748S in the recessive form
  evidence:
  - reference: PMID:20301791
    reference_title: "POLG-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Establishing the diagnosis of a POLG-related disorder relies on clinical findings and the identification of biallelic POLG pathogenic variants on molecular genetic testing for all phenotypes except autosomal dominant progressive external ophthalmoplegia (adPEO), for which identification of a heterozygous POLG pathogenic variant on molecular genetic testing is diagnostic."
    explanation: >-
      GeneReviews states the allele-count discriminator between the dominant and
      recessive POLG phenotypes.
  - reference: PMID:24741716
    reference_title: "RRM2B Mitochondrial DNA Maintenance Defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "RRM2B autosomal recessive progressive external ophthalmoplegia (arPEO), a typically childhood-onset predominantly myopathic phenotype of PEO, ptosis, proximal muscle weakness, and bulbar dysfunction."
    explanation: >-
      Defines the recessive RRM2B PEO phenotype that must be distinguished from
      dominant PEOA5.
- name: OPA1-Related Dominant Optic Atrophy Plus
  disease_term:
    preferred_term: autosomal dominant optic atrophy plus syndrome
    term:
      id: MONDO:0014720
      label: autosomal dominant optic atrophy plus syndrome
  description: >
    Heterozygous OPA1 variants cause a dominant syndrome that can include CPEO and
    multiple mtDNA deletions in muscle, so it overlaps adPEO both molecularly and
    clinically and is listed among the adPEO genes by some authors. It is excluded
    from the subtype list of this entry and modelled here as a differential instead,
    because its cardinal feature is bilateral progressive optic atrophy with central
    and colour vision loss, and because it acts through mitochondrial inner-membrane
    fusion and cristae architecture rather than through the mtDNA replisome or the
    nucleotide pool. dismech curates it separately as
    Autosomal_Dominant_Optic_Atrophy_Plus.
  distinguishing_features:
  - Bilateral optic atrophy with central and colour vision loss as the cardinal feature
  - Mechanism is mitochondrial inner-membrane fusion and cristae architecture, not the
    mtDNA replisome or dNTP supply
  - Distinct MONDO identity (MONDO:0014720) and a separate dismech entry
  evidence:
  - reference: PMID:19664747
    reference_title: "A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder is heterogeneous, with five known nuclear disease genes that encode the proteins ANT1, Twinkle, POLG, POLG2, and OPA1."
    explanation: >-
      Confirms that OPA1 is counted among the dominant nuclear genes producing an
      adPEO-overlapping phenotype, which is why it must be actively differentiated
      rather than ignored.
- name: Thyroid Eye Disease and Other Orbital Disease
  description: >
    Restrictive orbital myopathy from thyroid eye disease, orbital inflammatory
    disease or an orbital mass can mimic ophthalmoparesis. These are distinguished by
    proptosis, lid retraction rather than ptosis, orbital pain, forced-duction
    restriction and characteristic orbital imaging, and by the absence of a dominant
    family history or muscle mitochondrial pathology.
  distinguishing_features:
  - Proptosis and lid retraction rather than ptosis
  - Abnormal orbital imaging with extraocular muscle enlargement
  - Positive forced duction testing indicating a restrictive rather than myogenic
    process
  - Thyroid function and thyroid autoantibody abnormalities
  evidence:
  - reference: PMID:38391710
    reference_title: "Mitochondrial Chronic Progressive External Ophthalmoplegia."
    supports: PARTIAL
    evidence_source: HUMAN_CLINICAL
    snippet: "Understanding the varying presentations and genetic aspects of mitochondrial CPEO is crucial for accurate diagnosis and management."
    explanation: >-
      Review emphasising the diagnostic-accuracy problem in CPEO; cited as PARTIAL
      because the abstract does not itself enumerate the orbital differentials.
discussions:
- discussion_id: adpeo_dominance_mechanism_gap
  prompt: >-
    Why is a single heterozygous variant sufficient to destabilise the mitochondrial
    genome at six mechanistically different nuclear loci, when the recessive
    counterparts at two of those same loci (POLG, RRM2B) require two alleles?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Dominant-Negative Subunit Poisoning and Haploinsufficiency
  rationale: >-
    Three distinct explanations are invoked in the literature and have never been
    discriminated head to head: dominant-negative poisoning of a multimer (Twinkle
    hexamer), haploinsufficiency or heterodimerisation within an obligate heterodimer
    (POLG2 p.G451E), and dominant-negative truncation of an interaction surface
    (RRM2B p.R327X). It is not established whether the SLC25A4/ANT1 alleles act by any
    of these routes or by a fourth, transport-based mechanism. This matters because
    the mechanism determines whether allele-specific silencing would be therapeutic
    (dominant-negative) or counterproductive (haploinsufficiency), and it is the axis
    on which any future gene-directed therapy would be designed.
  proposed_experiments:
  - experiment_id: adpeo_allelic_titration_replisome
    name: Allelic titration of mutant versus wild-type replisome subunits
    description: >-
      In an isogenic human cell system, titrate the mutant:wild-type ratio of Twinkle,
      POLG, POLG2 and RRM2B and measure mtDNA copy number, deletion burden and fork
      progression, to distinguish a haploinsufficiency dose-response (linear from 50%)
      from dominant-negative poisoning (worse than 50% function at 50% mutant).
  - experiment_id: adpeo_allele_specific_knockdown
    name: Allele-specific knockdown of the mutant transcript in patient cells
    description: >-
      Selectively silence the mutant allele in patient-derived myoblasts for each of
      the six genes and ask whether mtDNA deletion burden falls (supporting
      dominant-negative action) or worsens (supporting haploinsufficiency).
  evidence:
  - reference: PMID:16685652
    reference_title: "Mutant POLG2 disrupts DNA polymerase gamma subunits and causes progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vivo, the disease most likely arises through haplotype insufficiency or heterodimerization of the mutated and wild-type proteins, which promote mtDNA deletions by stalling the DNA replication fork."
    explanation: >-
      The authors themselves leave the choice between haploinsufficiency and
      heterodimerisation unresolved, which is exactly the open question.
- discussion_id: adpeo_extraocular_selectivity_gap
  prompt: >-
    Is the selective vulnerability of extraocular muscle in adPEO driven by a lower
    biochemical threshold, a faster rate of deletion generation, more permissive clonal
    expansion, or reduced mitochondrial quality control?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Extraocular Muscle Selective Vulnerability
  rationale: >-
    Extraocular muscle carries three times the COX-deficient fibre burden of limb
    muscle in CPEO and crosses into COX deficiency at a lower mutational load, and the
    authors of that study explicitly attribute the pattern to unspecified
    tissue-specific mechanisms in clonal expansion and expression. The candidate
    explanations have not been discriminated. This matters because tissue selectivity,
    not enzymology, is what makes PEO a distinct clinical entity from the other
    phenotypes produced by the same alleles.
  proposed_experiments:
  - experiment_id: adpeo_eom_single_fibre_threshold
    name: Single-fibre deletion load and threshold mapping across muscle allotypes
    description: >-
      Measure single-fibre mtDNA deletion load and the heteroplasmy threshold for COX
      negativity in extraocular, levator palpebrae and limb muscle from the same
      adPEO donor, to separate a lower threshold from a faster accumulation rate.
  - experiment_id: adpeo_eom_mitophagy_flux
    name: Allotype-resolved mitophagy and mitochondrial dynamics measurement
    description: >-
      Compare mitophagy flux and fusion/fission dynamics between human extraocular and
      limb muscle to test the quality-control-capacity explanation for selective
      vulnerability.
  evidence:
  - reference: PMID:20164463
    reference_title: "Mitochondrial DNA defects and selective extraocular muscle involvement in CPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The data also suggest that tissue-specific mechanisms are involved in the clonal expansion and expression of secondary mtDNA deletions in CPEO patients with nuclear genetic defects."
    explanation: >-
      The source explicitly attributes the selectivity to unresolved tissue-specific
      mechanisms.
- discussion_id: adpeo_genotype_phenotype_breadth_gap
  prompt: >-
    What determines whether an adPEO genotype produces ocular-restricted disease or a
    broad PEO-plus phenotype with neuropathy, ataxia, parkinsonism and premature
    ovarian insufficiency?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Multisystem PEO-Plus Extension
  rationale: >-
    The gene identity clearly matters - TWNK-related adPEO shows rare CNS involvement
    while POLG-related adPEO carries a heavy neurological burden - but gene identity
    alone does not predict the phenotype within a locus, and the reviewers of the
    largest mixed cohort concluded that phenotype-genotype correlations cannot be
    drawn in mitochondrial PEO. No modifier gene, deletion-burden threshold or
    epigenetic signature has been shown to explain the residual variance. This is the
    single largest obstacle to prognostic counselling at the time of molecular
    diagnosis.
  proposed_experiments:
  - experiment_id: adpeo_multilocus_deep_phenotyping
    name: Cross-locus deep-phenotyping cohort with matched tissue deletion burden
    description: >-
      Assemble a multi-centre adPEO cohort spanning all six loci with standardised
      neurological, cardiac, audiological and endocrine phenotyping plus quantified
      muscle deletion burden, and test whether burden, gene, allele class or age
      explains the breadth of extra-ocular involvement.
  - experiment_id: adpeo_modifier_gwas
    name: Modifier screen in genotype-matched discordant relatives
    description: >-
      Within families segregating a single adPEO allele, compare relatives who are
      ocular-restricted with those who have PEO-plus disease, to search for nuclear or
      mtDNA-haplogroup modifiers of phenotypic breadth.
  evidence:
  - reference: PMID:32161153
    reference_title: "Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Phenotype-genotype correlations cannot be brought in mitochondrial PEO."
    explanation: >-
      Explicit statement from a large mitochondrial PEO series that genotype does not
      predict phenotype, defining the gap.
  - reference: PMID:20479361
    reference_title: "The clinical, histochemical, and molecular spectrum of PEO1 (Twinkle)-linked adPEO."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our data suggest a shared clinical phenotype with variable mild multiorgan involvement, and that the contribution of PEO1 mutations as a cause of adPEO may well be underestimated."
    explanation: >-
      Documents variable multiorgan involvement within a single locus, showing that
      gene identity alone is insufficient to predict breadth.
- discussion_id: adpeo_model_fidelity_mismatch
  prompt: >-
    Do the available mouse, cell and yeast models of adPEO genes actually reproduce
    the defining human disease features, given that no model organism has the human
    extraocular muscle allotype and that cultured cells cannot reproduce
    decades-long age-dependent clonal expansion?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Clonal Expansion of Deleted mtDNA in Post-Mitotic Cells
  - pathophysiology#Extraocular Muscle Selective Vulnerability
  rationale: >-
    Model evidence for adPEO is substantial and mechanistically informative - Twinkle
    and POLG catalytic mutants stall replication in human cells, and transgenic mice
    establish Twinkle as rate-limiting for mtDNA copy number - but the two features
    that define the human disease are precisely the ones the models cannot supply. The
    ocular phenotype depends on the extraocular muscle allotype, which no standard
    model recapitulates, and the adult onset depends on somatic clonal expansion over
    four decades, which no cell culture or short-lived rodent can span. Evidence
    therefore exists but its translational validity to the defining human features is
    the open question, which is why this is recorded as a human-model mismatch rather
    than a knowledge gap.
  proposed_experiments:
  - experiment_id: adpeo_long_lived_model_aging
    name: Long-duration ageing study in a knock-in adPEO model with ocular phenotyping
    description: >-
      Follow a heterozygous knock-in model carrying a human dominant allele across the
      full lifespan with serial extraocular and limb muscle single-fibre COX/SDH and
      deletion-burden quantification, to test whether age-dependent ocular-predominant
      pathology emerges at all.
  - experiment_id: adpeo_human_eom_organoid
    name: Human extraocular-allotype myotube model of clonal expansion
    description: >-
      Derive extraocular-allotype myotubes from patient iPSCs and apply an accelerated
      mtDNA turnover protocol, to test whether the lower COX threshold of extraocular
      muscle is cell-autonomous and reproducible outside the orbit.
  evidence:
  - reference: PMID:17452351
    reference_title: "Expression of catalytic mutants of the mtDNA helicase Twinkle and polymerase POLG causes distinct replication stalling phenotypes."
    supports: PARTIAL
    evidence_source: IN_VITRO
    snippet: "Both groups of mutants reduced mitochondrial DNA copy number by severe replication stalling."
    explanation: >-
      Cell-culture evidence establishes the fork-stalling mechanism but is measured
      over days and cannot address decades-long clonal expansion or ocular
      selectivity.
  - reference: PMID:15509589
    reference_title: "Twinkle helicase is essential for mtDNA maintenance and regulates mtDNA copy number."
    supports: PARTIAL
    evidence_source: MODEL_ORGANISM
    snippet: "we generated two transgenic mouse lines overexpressing wild-type Twinkle"
    explanation: >-
      The available mouse evidence uses wild-type overexpression to establish copy
      number control, which is informative for Twinkle biology but is not a model of
      the heterozygous dominant human allele or of the ocular phenotype.
notes: >
  SUBTYPE-GENE MAPPING - READ BEFORE EDITING. The PEOA1-PEOA6 numbering used here was
  verified directly against MONDO and OMIM with OAK rather than taken from any
  secondary source, because this series is a known named-entity-confusion hazard. The
  authoritative mapping is PEOA1=POLG (OMIM 157640, MONDO:0024528), PEOA2=SLC25A4/ANT1
  (609283, MONDO:0012238), PEOA3=TWNK/C10orf2 (609286, MONDO:0012241), PEOA4=POLG2
  (610131, MONDO:0012415), PEOA5=RRM2B (613077, MONDO:0013117), PEOA6=DNA2 (615156,
  MONDO:0014062, curated by MONDO under the label "mitochondrial DNA deletion syndrome
  with progressive myopathy"). THE TRAP: the historical GENE symbol "PEO1" denotes
  TWNK/C10orf2 - the first adPEO locus mapped - whereas the DISEASE designation PEOA1
  denotes POLG. These are one apart and are frequently conflated; GitHub issue #7469,
  from which this entry was curated, originally listed the series as PEOA1=TWNK,
  PEOA2=POLG, PEOA3=SLC25A4, which is shifted by one and is wrong. Any source that
  equates PEO1 with PEOA1 must not be trusted for this numbering. Re-verify with OAK
  against MONDO/OMIM before changing any subtype-gene assignment in this entry.

  Scope and boundaries. This entry is the umbrella adPEO entity (MONDO:0008003,
  Orphanet:254892), curated as a single disease with six locus-specific subtypes
  (PEOA1-PEOA6) rather than as six separate entries, because the six genotypes share
  a single mechanism (heterozygous nuclear defect leading to multiple mtDNA
  deletions leading to threshold-dependent mosaic COX deficiency), a single cardinal
  phenotype, and a single management pathway, and because they are routinely tested
  as one gene panel. Note an ontology-level ambiguity worth flagging: MONDO:0008003
  carries "PEOA1" as a RELATED synonym even though PEOA1 has its own term
  (MONDO:0024528, defined by POLG). The label of MONDO:0008003 ("autosomal dominant
  progressive external ophthalmoplegia") and its Orphanet:254892 xref support its use
  as the umbrella term here, and each subtype is separately bound to its own
  gene-specific MONDO term.

  OPA1 is deliberately excluded. Tyynismaa and colleagues list OPA1 among the adPEO
  genes, and dominant OPA1 variants can produce PEO with multiple mtDNA deletions
  within a "DOA-plus" phenotype. It is excluded from the subtype list here because
  OPA1 disease is primarily an optic atrophy syndrome with a distinct MONDO identity,
  its mechanism is inner-membrane fusion and cristae organisation rather than
  replisome or nucleotide-supply failure, and dismech already carries a separate
  Autosomal Dominant Optic Atrophy Plus entry. It is modelled here as an explicit
  differential_diagnoses entry bound to MONDO:0014720 rather than being silently
  omitted. TK2 likewise causes multiple mtDNA
  deletions and PEO but is recessive and is out of scope for a dominant entry.

  Frequency bands are genotype-limited. Almost all quantitative frequencies in this
  entry come from two genotyped cohorts, both TWNK-predominant: Bermejo-Guerrero 2021
  (25 TWNK patients) and Fratter 2010 (33 PEO1/Twinkle patients). These are the best
  available adPEO-specific figures, but they systematically UNDERSTATE the
  neurological extension of POLG-related adPEO, where ataxia, neuropathy and
  parkinsonism are considerably more frequent than the 4-8% recorded here. Per-locus
  frequency bands could not be assigned because no comparable genotyped cohort exists
  for POLG-, POLG2-, SLC25A4-, RRM2B- or DNA2-related adPEO. Where no quotable
  frequency was found (hearing loss, depression, premature ovarian insufficiency),
  frequency has been omitted rather than estimated.

  Deep research provenance. This entry was curated with an Edison Scientific (falcon)
  deep-research report as a LEAD source only. The named entity confusion preflight was
  run against MONDO:0008003 and all six subtype terms with OAK before any deep-research
  content was used, and the report's primary disease identity, gene set and OMIM/Orphanet
  anchors matched. Every PMID cited here was independently fetched with
  just fetch-reference and every snippet verified as an exact substring of the cached
  abstract. Two claims surfaced by deep research could NOT be verified against a
  quotable abstract and are recorded as such rather than asserted: the cataract count
  in the TWNK cohort (two related subjects aged 48 and 50, reported in the paper body,
  not the abstract) and the clinical-trial records NCT02161848 and NCT04678115, which
  the report itself notes do not establish genotype-specific efficacy in adPEO and
  which are therefore not curated as clinical_trials entries. NCT05162768 (NuPOWER) IS
  curated, but only as an unreported confirmatory trial: it is recorded together with
  an explicit supports: REFUTE evidence item for the NEGATIVE primary result of the
  parent MMPOWER-3 Phase 3 trial (PMID:39574155), so that the post-hoc
  replisome/CPEO six-minute-walk subgroup signal cannot be misread as demonstrated
  efficacy. Elamipretide is not approved for adPEO or for any mitochondrial myopathy
  indication.

📚

References & Deep Research

References

2

PMID:20301791

POLG-Related Disorders.

No top-level findings curated for this source.

PMID:24741716

RRM2B Mitochondrial DNA Maintenance Defects.

No top-level findings curated for this source.

Deep Research

1

Falcon ▸

Autosomal Dominant Progressive External Ophthalmoplegia: Disease-Characteristics Report

Edison Scientific Literature 21 citations 2026-07-31T17:05:04.304211

Autosomal Dominant Progressive External Ophthalmoplegia: Disease-Characteristics Report

Scope and evidence note

Autosomal dominant progressive external ophthalmoplegia (adPEO; often adCPEO) is a genetically heterogeneous nuclear-genome mitochondrial DNA (mtDNA)-maintenance disorder. It must be distinguished from the broader phenotype “mitochondrial CPEO,” which also includes sporadic single large-scale mtDNA deletions, mtDNA point variants, and recessive nuclear disorders. Statistics from mixed CPEO cohorts are therefore labeled as such rather than attributed directly to adPEO.

The strongest directly applicable human evidence retrieved was a 25-patient TWNK-adPEO cohort published online in December 2021, together with mitochondrial PEO cohorts and 2023–2024 clinical reviews. The following table summarizes the principal knowledge-base fields.

domain	established finding	quantitative evidence	ontology suggestions	evidence scope/caveat
Disease definition	Autosomal dominant progressive external ophthalmoplegia (adPEO) is an adult-onset mitochondrial DNA maintenance disorder caused by heterozygous nuclear-gene defects, typically with secondary multiple mtDNA deletions in muscle and progressive ptosis/ophthalmoparesis (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, bermejoguerrero2021clinicalhistologicaland pages 1-2)	Typical onset reported at 20–40 years in review literature; TWNK cohort mean onset 43 years, mean diagnosis 63 years (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, bermejoguerrero2021clinicalhistologicaland pages 1-2)	HP:0000508 Ptosis; HP:0000602 Ophthalmoplegia; GO:0006260 DNA replication; GO:0005739 mitochondrion	Distinct from all-cause mitochondrial CPEO, which also includes mtDNA deletions/point mutations and recessive nuclear etiologies (ali2024mitochondrialchronicprogressive pages 1-3, chen2023mitochondriaandthe pages 1-2)
Core causal genes	Established adPEO genes include TWNK, POLG, POLG2, SLC25A4, DNA2, and RRM2B; OPA1 can produce a syndromic dominant phenotype with external ophthalmoplegia plus optic atrophy/deafness/ataxia and multiple mtDNA deletions (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, kierdaszuk2020progressiveexternalophthalmoplegia pages 11-12)	No single gene frequency is established for all adPEO; in one TWNK-specific cohort, 25/25 had heterozygous TWNK variants (bermejoguerrero2021clinicalhistologicaland pages 1-2, bermejoguerrero2021clinicalhistologicaland pages 2-4)	HGNC gene symbols as listed; GO:0003678 DNA helicase activity (TWNK); GO:0003887 DNA-directed DNA polymerase activity (POLG); GO:0140355 ADP/ATP transmembrane transporter activity (SLC25A4)	Gene list is disease-level and heterogeneous; OPA1 should be annotated as syndromic/overlap rather than core isolated adPEO in all cases (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)
TWNK-specific variant data	TWNK-related adPEO is caused by heterozygous missense variants affecting mtDNA helicase function and replication (bermejoguerrero2021clinicalhistologicaland pages 1-2)	10 different TWNK mutations in 25 patients; most frequent c.1361T>G (p.Val454Gly) in 7 patients/6 families and c.1070G>C (p.Arg357Pro) in 7 patients/5 families; c.1121G>A (p.Arg374Gln) in 3 patients; c.1411T>G (p.Tyr471Asp) in 2 siblings (bermejoguerrero2021clinicalhistologicaland pages 7-9)	Sequence Ontology: missense_variant; GO:0004386 helicase activity; GO:0006268 DNA unwinding involved in DNA replication	These frequencies apply to one Madrid laboratory cohort, not all adPEO populations (bermejoguerrero2021clinicalhistologicaland pages 2-4, bermejoguerrero2021clinicalhistologicaland pages 7-9)
Inheritance/family history	Inheritance is autosomal dominant with variable expressivity and adult presentation (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, kierdaszuk2020progressiveexternalophthalmoplegia pages 11-12)	In TWNK cohort, 22/25 (88%) had first-degree relatives with ptosis and/or ophthalmoplegia; 64% female (16/25) (bermejoguerrero2021clinicalhistologicaland pages 2-4)	HP:0000006 Autosomal dominant inheritance	Penetrance was not quantified in the retrieved evidence; sex distribution from one cohort should not be generalized (bermejoguerrero2021clinicalhistologicaland pages 2-4)
Core phenotype	Hallmark phenotype is bilateral progressive ptosis with progressive external ophthalmoplegia; weakness and exercise intolerance are common extrasocular features (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, ali2024mitochondrialchronicprogressive pages 13-14, bermejoguerrero2021clinicalhistologicaland pages 1-2)	TWNK cohort: ptosis 92%, PEO 80%, weakness 48%, exercise intolerance 28% (bermejoguerrero2021clinicalhistologicaland pages 1-2)	HP:0000508 Ptosis; HP:0000602 Ophthalmoplegia; HP:0001324 Muscle weakness; HP:0003546 Exercise intolerance	Frequencies are strongest for TWNK-related adPEO; broader adPEO due to other genes may differ (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, bermejoguerrero2021clinicalhistologicaland pages 1-2)
Additional/systemic phenotypes	adPEO may include bulbar dysfunction, hearing loss, ataxia, neuropathy, parkinsonism, cataracts, cardiac and respiratory involvement (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, ali2024mitochondrialchronicprogressive pages 13-14, bermejoguerrero2021clinicalhistologicaland pages 5-6)	TWNK cohort: bulbar involvement 24%, cardiac 24%, respiratory 4%, neuropathy 8%, ataxia 4%, parkinsonism 4%; cataracts reported in 2 related patients (bermejoguerrero2021clinicalhistologicaland pages 5-6, bermejoguerrero2021clinicalhistologicaland pages 1-2)	HP:0002015 Dysphagia; HP:0000407 Sensorineural hearing impairment; HP:0001251 Ataxia; HP:0002355 Difficulty walking; HP:0000518 Cataract; HP:0011675 Arrhythmia/cardiac conduction abnormality	Systemic manifestations are variable and may be gene- and family-specific (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)
Pathology/biopsy	Skeletal muscle typically shows mitochondrial myopathy with ragged-red fibers, COX-negative fibers, and multiple mtDNA deletions (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, bermejoguerrero2021clinicalhistologicaland pages 6-7, bermejoguerrero2021clinicalhistologicaland pages 7-9)	In mitochondrial PEO series, biopsy showed mitochondrial changes in 95%; in TWNK cohort, all 19 available biopsies showed mitochondrial dysfunction and all 17 tested muscle samples had multiple mtDNA deletions (ali2024mitochondrialchronicprogressive pages 1-3, bermejoguerrero2021clinicalhistologicaland pages 7-9)	HP:0003200 Ragged-red muscle fibers; HP:0003688 Abnormal muscle mitochondria; GO:0006119 oxidative phosphorylation	95% biopsy yield was for mixed mitochondrial PEO, not adPEO alone; deletion testing is tissue-sensitive (ali2024mitochondrialchronicprogressive pages 1-3)
Mechanism/pathophysiology	Upstream defects fall into mtDNA replication/repair, nucleotide supply/balance, and mitochondrial dynamics/quality control; downstream consequence is multiple mtDNA deletions causing respiratory-chain deficiency and energy failure in high-demand muscle (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)	Respiratory complex activities can range from normal to ~50% of control means in affected muscle (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)	GO:0006260 DNA replication; GO:0007005 mitochondrion organization; GO:0006091 generation of precursor metabolites and energy; CL:0000187 skeletal muscle cell	Mechanistic framework is established for mtDNA-maintenance disorders broadly; adPEO-specific omics and cell-type-resolved datasets were not identified (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)
Electrophysiology/labs	EMG may show myopathic or mixed changes; CK and lactate can be normal or mildly elevated; GDF-15 may be elevated (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, bermejoguerrero2021clinicalhistologicaland pages 5-6, bermejoguerrero2021clinicalhistologicaland pages 6-7)	TWNK cohort: hyperCKemia 28%; mild hyperlactacidemia in 3/6 tested; GDF-15 elevated in 2/3 tested (1454–2727 pg/mL); EMG myopathic in 60%, neurogenic in 13%, normal in 27% of 15 tested (bermejoguerrero2021clinicalhistologicaland pages 5-6, bermejoguerrero2021clinicalhistologicaland pages 6-7)	LOINC/biomarkers: CK, lactate, GDF15; HP:0003236 Elevated serum creatine phosphokinase	Biomarker evidence is limited and from small tested subsets (bermejoguerrero2021clinicalhistologicaland pages 6-7)
Diagnosis	Current understanding supports sequencing-based diagnosis, complemented by muscle biopsy and mtDNA rearrangement analysis when suspicion remains high; in one mitochondrial PEO algorithm, biopsy was proposed as first step because it enables mtDNA rearrangement studies (ali2024mitochondrialchronicprogressive pages 1-3, bermejoguerrero2021clinicalhistologicaland pages 1-2)	Mixed mitochondrial PEO series achieved genetic diagnosis in 96%; biopsy informative in 95% (ali2024mitochondrialchronicprogressive pages 1-3)	NCIT: Genetic Testing; HP terms above; GO:0003723? not needed	These performance metrics are from all-cause mitochondrial PEO rather than strictly adPEO; practice has shifted toward broader NGS-first workflows in recent reviews (ali2024mitochondrialchronicprogressive pages 1-3, chen2023mitochondriaandthe pages 1-2)
Differential diagnosis	adPEO is commonly misdiagnosed as seronegative myasthenia gravis or oculopharyngeal muscular dystrophy (bermejoguerrero2021clinicalhistologicaland pages 1-2)	In TWNK cohort, 56% were misdiagnosed before genetic confirmation: 36% myasthenia, 20% oculopharyngeal muscular dystrophy (bermejoguerrero2021clinicalhistologicaland pages 1-2)	Differential ontology suggestions: HP:0000602 Ophthalmoplegia; disease comparators include myasthenia gravis and OPMD	Differential data derive from TWNK-related cases and specialist-center experience (bermejoguerrero2021clinicalhistologicaland pages 1-2)
Management	No definitive disease-modifying therapy is established; management is supportive, includes lifestyle/risk modification, supplements used empirically in mitochondrial care, ptosis repair, and monitoring for multisystem complications (ali2024mitochondrialchronicprogressive pages 1-3, ali2024mitochondrialchronicprogressive pages 18-19)	No adPEO-specific response rates were identified in retrieved evidence (ali2024mitochondrialchronicprogressive pages 18-19)	NCIT: Supportive Care; NCIT: Blepharoplasty/Ptosis Repair; NCIT: Physical Therapy	Do not extrapolate LHON gene-therapy efficacy to adPEO; retrieved therapeutic advances largely concern other mitochondrial phenotypes (ali2024mitochondrialchronicprogressive pages 18-19)
Real-world implementation / trials	Real-world care currently relies on diagnostic genomics and symptom-directed interventions; relevant mitochondrial/PEO trials exist but are not adPEO genotype-specific efficacy trials (clinical-trial contexts summarized in prior retrieval)	Examples retrieved: NCT02161848 observational MRI study in CPEO, enrollment 133, completed; NCT04678115 severe blepharoptosis non-surgical treatment trial, enrollment 16, completed; NCT05162768 phase 3 elamipretide in primary mitochondrial disease from nuclear DNA mutations, enrollment 102, completed	NCIT: Magnetic Resonance Imaging; NCIT: Elamipretide; NCIT: Blepharoptosis intervention	Trial records were not specific to autosomal dominant PEO and do not establish standard-of-care efficacy for adPEO
Epidemiology	Robust adPEO-specific prevalence/incidence estimates were not identified in the retrieved evidence	Only broader CPEO estimates were found: incidence 1–2 per 100,000 and prevalence ~1 in 30,000 or 3.4 per 100,000 in review sources (ali2024mitochondrialchronicprogressive pages 1-3, chen2023mitochondriaandthe pages 1-2)	MONDO/Orphanet term suggestion should await authoritative identifier confirmation	These figures apply to mitochondrial CPEO broadly, not specifically to autosomal dominant PEO; epidemiology remains a knowledge gap for the AD subtype (ali2024mitochondrialchronicprogressive pages 1-3, chen2023mitochondriaandthe pages 1-2)

Table: This table summarizes established, citable knowledge for autosomal dominant progressive external ophthalmoplegia, emphasizing what is supported directly for the AD subtype versus broader mitochondrial CPEO. It is useful for populating structured disease fields while preserving important scope caveats and evidence gaps.

1. Disease information

Definition and nomenclature

adPEO is usually a slowly progressive, adult-onset mitochondrial myopathy characterized by bilateral ptosis and limitation of extraocular movements, with secondary accumulation of multiple mtDNA deletions in post-mitotic tissues. “PEO-plus” denotes ophthalmoplegia accompanied by systemic manifestations such as proximal, neck, or bulbar weakness; exercise intolerance; neuropathy; ataxia; hearing loss; cataract; cardiac disease; or parkinsonism. The typical review-level onset range is approximately 20–40 years, although onset and severity vary considerably. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)

Common names are autosomal dominant progressive external ophthalmoplegia, autosomal dominant chronic progressive external ophthalmoplegia, adPEO, adCPEO, dominant PEO, and, where systemic disease is present, adPEO-plus/CPEO-plus.

Identifiers

OMIM phenotype: the literature explicitly cites autosomal dominant PEO as OMIM 157640; individual gene-related phenotypes can also have separate OMIM entries.
MONDO: a stable adPEO-specific MONDO identifier was not confirmed in the retrieved evidence; it should not be populated without direct MONDO verification. A broader progressive external ophthalmoplegia concept may be available.
Orphanet: CPEO and autosomal-dominant PEO are represented in Orphanet’s mitochondrial-disease hierarchy, but an exact ORPHA number was not confirmed here.
ICD: there is no highly specific, universally used ICD-10 code for genetically confirmed adPEO. Cases are generally coded under mitochondrial metabolism/myopathy or ophthalmoplegia categories. ICD-11 provides more granular mitochondrial-disease concepts, but local coding should be verified.
MeSH: relevant concepts include Ophthalmoplegia, Chronic Progressive External and Mitochondrial Myopathies.

The report describes aggregated disease-level evidence, not an individual EHR. The TWNK statistics derive from a specialist laboratory/clinical cohort of individual patients subsequently aggregated for publication. (bermejoguerrero2021clinicalhistologicaland pages 2-4)

2. Etiology, risk, protection, and environment

Primary cause

adPEO is caused by a heterozygous germline pathogenic variant in a nuclear gene required for mtDNA replication, repair, nucleotide balance, or mitochondrial dynamics. Established genes include TWNK, POLG, POLG2, SLC25A4, DNA2, and RRM2B. Dominant OPA1 variants can produce an overlapping syndromic disorder with optic atrophy, PEO, deafness, ataxia, and multiple mtDNA deletions. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, kierdaszuk2020progressiveexternalophthalmoplegia pages 11-12)

The causal hierarchy is:

heterozygous nuclear variant;
defective mtDNA replication/maintenance or nucleotide homeostasis;
progressive formation and clonal expansion of multiple mtDNA deletions;
mosaic respiratory-chain deficiency;
ATP failure in high-demand, long-lived cells;
extraocular and skeletal-muscle dysfunction, with variable multisystem disease.

Risk factors

Genetic: an affected parent, a pathogenic heterozygous allele, and increasing age are the major established risk determinants. In the TWNK cohort, 22/25 patients (88%) had a first-degree relative with ptosis and/or ophthalmoplegia. (bermejoguerrero2021clinicalhistologicaland pages 2-4)
Age: deletion burden and clinical manifestations generally increase with age; presymptomatic carriers may have subclinical biochemical abnormalities. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)
Sex: no established biological sex predisposition exists. The TWNK cohort was 64% female, but this small referral cohort cannot establish a sex ratio. (bermejoguerrero2021clinicalhistologicaland pages 2-4)
Environmental/infectious: no toxin, infection, occupational exposure, diet, smoking behavior, or lifestyle factor is known to cause Mendelian adPEO.

Protective factors and gene–environment interaction

No validated protective variant, diet, drug, or behavioral intervention prevents adPEO. Avoidance of mitochondrial stressors is prudent clinical practice but is not demonstrated primary prevention. Potentially mitochondrion-toxic drugs and prolonged metabolic stress may aggravate mitochondrial symptoms generally, but a quantified adPEO-specific gene–environment interaction has not been established. No infectious trigger is implicated.

3. Phenotypes

Core and systemic manifestations

Phenotype	Type/course	Direct TWNK-adPEO frequency	Suggested HPO term
Ptosis, generally bilateral	Sign; insidious, progressive; mild to severe	92%	HP:0000508
External ophthalmoplegia/ophthalmoparesis	Sign; usually symmetric and progressive	80%	HP:0000602
Skeletal-muscle weakness	Sign; proximal, neck, facial, or bulbar; progressive/variable	48%	HP:0001324
Exercise intolerance/fatigue	Symptom; chronic	28%	HP:0003546
Bulbar weakness/dysphagia/dysphonia	Symptom/sign; variable	24%	HP:0002015, HP:0001618
Cardiac abnormality	Sign; variable, clinically important	24%	HP:0011675 or finding-specific term
Peripheral neuropathy	Sign; variable	8%	HP:0009830
Respiratory involvement	Sign; uncommon in this cohort	4%	HP:0002795
Ataxia	Sign	4%	HP:0001251
Parkinsonism	Sign	4%	HP:0001300
Cataract	Sign; two related subjects, ages 48 and 50	2 patients	HP:0000518
Mild CK elevation	Laboratory abnormality	28%	HP:0003236

These percentages are from 25 individuals with TWNK variants and should not be treated as universal frequencies for POLG-, POLG2-, SLC25A4-, DNA2-, RRM2B-, or OPA1-related disease. (bermejoguerrero2021clinicalhistologicaland pages 5-6, bermejoguerrero2021clinicalhistologicaland pages 1-2)

Additional reported adPEO-spectrum manifestations include sensorineural hearing loss, facial weakness, hypogonadism, psychiatric abnormalities, gastrointestinal dysmotility, and cataracts. OPA1-related “DOA-plus” particularly suggests optic atrophy and hearing loss. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, ali2024mitochondrialchronicprogressive pages 13-14)

Quality of life

Ptosis can obstruct the visual axis, fixed ophthalmoplegia restricts gaze, and weakness, dysphagia, fatigue, neuropathy, or hearing loss can limit mobility, work, communication, and participation. Robust adPEO-specific EQ-5D, SF-36, PROMIS, survival, and disability-transition estimates were not identified. Accordingly, quality-of-life impact is clinically credible but insufficiently quantified by genotype.

4. Genetic and molecular information

Genes and functional classes

TWNK encodes the mitochondrial replicative helicase. Dominant missense variants impair helicase function, stall replication, and produce multiple mtDNA deletions.
POLG encodes the catalytic polymerase-γ subunit; dominant variants can alter polymerase, proofreading, or replisome function.
POLG2 encodes the accessory polymerase-γ subunit.
SLC25A4/ANT1 encodes the muscle-predominant mitochondrial ADP/ATP carrier and influences nucleotide/energy homeostasis.
DNA2 participates in mtDNA replication/repair and flap processing.
RRM2B supports deoxyribonucleotide production in non-dividing cells.
OPA1 controls inner-membrane fusion/cristae organization and, in dominant syndromic disease, indirectly compromises mtDNA maintenance.

These proteins converge on mtDNA replication/maintenance despite performing different proximal functions. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)

Variant evidence

In the TWNK cohort, all variants were heterozygous missense substitutions in exons 1–2. Ten variants were found: c.1361T>G (p.Val454Gly) occurred in seven patients from six families; c.1070G>C (p.Arg357Pro) in seven patients from five families; c.1121G>A (p.Arg374Gln) in three patients; and c.1411T>G (p.Tyr471Asp) in two siblings. All 17 tested muscle samples contained multiple mtDNA deletions. (bermejoguerrero2021clinicalhistologicaland pages 7-9)

Variants should be classified individually under ACMG/AMP criteria using ClinVar/ClinGen evidence, segregation, functional data, phenotype specificity, and population frequency. Disease association alone does not make every rare variant pathogenic. Causal alleles are germline; secondary mtDNA deletions arise somatically and clonally in affected tissues. Pathogenic dominant alleles are generally absent or extremely rare in population databases, but no single allele-frequency threshold is valid for every gene or variant.

Modifiers, epigenetics, and chromosome abnormalities

No reproducible modifier gene, protective allele, disease-specific methylation signature, histone alteration, or recurrent chromosomal rearrangement is established for adPEO. Conventional aneuploidy or translocation is not the characteristic lesion. The clinically relevant structural abnormalities are multiple mtDNA deletions, not a nuclear karyotypic defect.

Suggested annotations include SO:0001583 missense_variant, GO:0006260 DNA replication, GO:0006281 DNA repair, GO:0007005 mitochondrion organization, and GO:0006119 oxidative phosphorylation.

5. Environmental information

adPEO is not infectious, contagious, toxic, or environmentally acquired. No causal association with pollution, radiation, smoking, alcohol, occupation, or a specific diet has been demonstrated. Lifestyle measures—adequate nutrition, avoidance of prolonged fasting/dehydration, appropriately paced aerobic activity, and avoidance of unnecessary mitochondrial-toxic exposures—are supportive rather than curative or preventive. Vaccination has no disease-specific etiologic role, although routine immunization is appropriate.

6. Mechanism and pathophysiology

Upstream pathways

The principal upstream modules are mtDNA replication/repair (TWNK, POLG, POLG2, DNA2), nucleotide supply/balance (RRM2B and indirectly SLC25A4), and mitochondrial dynamics/quality control (OPA1). A major review summarizes mtDNA-maintenance disorders as involving “mtDNA replication and maintenance, nucleotide supply and balance, and mitochondrial dynamics and quality control.” (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)

Downstream cellular injury

Multiple deletions remove mtDNA genes encoding respiratory-chain subunits and RNAs. Deletion burden varies among fibers, producing a mosaic of cytochrome-c-oxidase-negative cells. Respiratory-chain activities may range from normal to approximately 50% of control means, explaining why blood biomarkers can be normal despite muscle disease. Energy failure and compensatory mitochondrial proliferation produce ragged-red fibers; extraocular muscles are particularly vulnerable because of sustained activity and high mitochondrial demand. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)

Suggested terms:

GO biological process: GO:0006260 mitochondrial-DNA replication; GO:0006119 oxidative phosphorylation; GO:0007005 mitochondrion organization; GO:0006091 generation of precursor metabolites and energy.
GO cellular component: GO:0005739 mitochondrion, GO:0005743 mitochondrial inner membrane, GO:0005759 mitochondrial matrix, GO:0005758 mitochondrial intermembrane space.
Cell Ontology: CL:0000187 skeletal muscle cell; extraocular myofiber-specific annotations may require anatomy plus skeletal-muscle-cell terms.
Metabolic consequence: impaired ATP production, variable lactate elevation, and compensatory mitochondrial biogenesis.

No primary autoimmune mechanism is established. Oxidative stress may be downstream of respiratory dysfunction, but chronic inflammation is not considered the initiating pathology.

Molecular profiling and advanced technologies

Routine pathology and bulk biochemical/genomic testing dominate the evidence base. No validated adPEO-specific transcriptomic, proteomic, metabolomic, lipidomic, single-cell, spatial-transcriptomic, or integrated multi-omics diagnostic signature was identified. GDF-15 is promising as a general mitochondrial biomarker but not specific to adPEO: it was elevated in 2/3 tested TWNK patients at 1,454–2,727 pg/mL. (bermejoguerrero2021clinicalhistologicaland pages 6-7)

7. Anatomical structures affected

The primary structures are the extraocular muscles and levator palpebrae superioris, generally bilaterally and relatively symmetrically. Other skeletal muscles—proximal limb, cervical, facial, pharyngeal, and respiratory muscles—can be involved. Secondary sites vary by genotype and can include peripheral nerves, cochlea/auditory pathways, optic nerve, basal ganglia/cerebellar pathways, gastrointestinal smooth muscle/enteric nervous system, and heart. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)

Suggested anatomy terms include UBERON:0001135 skeletal muscle tissue, UBERON extraocular-muscle and eyelid terms after database verification, and organ-specific terms for heart, peripheral nerve, optic nerve, and cochlea. At the subcellular level, mitochondria—especially matrix replisomes and inner-membrane OXPHOS machinery—are central.

8. Temporal development

adPEO is typically insidious and chronic. Review literature places usual onset around 20–40 years; broader mitochondrial CPEO has a reported mean onset around 29 years. In the TWNK series, 80% began at age ≥18, mean onset was 43 years, and mean molecular diagnosis was 63 years—a roughly 20-year average diagnostic gap. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, chen2023mitochondriaandthe pages 1-2, bermejoguerrero2021clinicalhistologicaland pages 1-2)

A practical trajectory is:

subtle asymmetric or bilateral ptosis;
gradually restricted ocular motility, often with limited diplopia because progression is symmetric;
neck/proximal weakness or exercise intolerance;
variable PEO-plus manifestations later in life.

There are no validated formal stages, predictable annual progression rate, spontaneous remission pattern, or defined end-stage. Disease is lifelong and generally progressive. Critical opportunities are early genetic diagnosis, surveillance before cardiac/respiratory complications, and ptosis management before visual-axis obstruction or compensatory head posture becomes disabling.

9. Inheritance and population

Inheritance is autosomal dominant: an affected heterozygous individual generally has a 50% transmission probability per pregnancy, independent of sex. Expressivity is markedly variable and penetrance is likely age-dependent, but robust gene- and variant-specific penetrance estimates are unavailable. Anticipation is not established. Germline mosaicism is biologically possible but not a defining feature; consanguinity is not relevant to dominant transmission. (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4, kierdaszuk2020progressiveexternalophthalmoplegia pages 11-12)

adPEO-specific incidence, prevalence, carrier frequency, ethnic enrichment, geographic distribution, and sex ratio remain inadequately characterized. Broader CPEO estimates—1–2 per 100,000 incidence, approximately 1 in 30,000 prevalence, or 3.4 per 100,000—must not be interpreted as adPEO-specific. (ali2024mitochondrialchronicprogressive pages 1-3, chen2023mitochondriaandthe pages 1-2)

No universal founder allele was established. Recurrent TWNK variants in a Spanish cohort may reflect referral or ancestry patterns; their cohort frequencies are not population frequencies. (bermejoguerrero2021clinicalhistologicaland pages 7-9)

10. Diagnostics

Clinical and laboratory approach

Clinical suspicion is raised by slowly progressive bilateral ptosis and ophthalmoparesis, especially with an autosomal-dominant pedigree or proximal/bulbar weakness. Examination should document eyelid position, Bell phenomenon, corneal exposure, ocular ductions, diplopia, visual acuity, fundus/optic nerve, hearing, bulbar function, strength, gait, neuropathy, and systemic involvement.

CK and lactate may be normal or mildly elevated. In the TWNK cohort, mild hyperCKemia occurred in 28%; mild hyperlactacidemia in 3/6 tested; EMG was myopathic in 9/15 (60%), neurogenic in 2/15 (13%), and normal in 4/15 (27%). Brain and muscle imaging were often normal or nonspecific. (bermejoguerrero2021clinicalhistologicaland pages 5-6, bermejoguerrero2021clinicalhistologicaland pages 6-7)

Genetic testing strategy

Use a mitochondrial disease/PEO nuclear-gene panel including at least TWNK, POLG, POLG2, SLC25A4, DNA2, RRM2B, OPA1 and relevant recessive genes.
Concurrently or reflexively assess mtDNA for point variants, single deletions, and multiple deletions.
If blood is nondiagnostic, test a clinically affected tissue—usually skeletal muscle—because secondary deletions can be tissue-restricted.
Use WES/WGS when panel testing is negative; WGS may improve detection of nuclear structural/noncoding variants and mtDNA abnormalities, but tissue and analytic validation remain important.
Perform segregation testing in relatives and interpret variants under ACMG/AMP standards.

CMA, karyotyping, FISH, and repeat-expansion testing have low first-line yield unless another diagnosis is suspected. RNA sequencing may clarify splice variants but is not routine. Prenatal and preimplantation testing require a known familial pathogenic nuclear variant.

Biopsy and pathology

Muscle shows ragged-red fibers, subsarcolemmal mitochondrial accumulation, COX-negative fibers, and sometimes neurogenic changes. All 19 biopsied patients in the TWNK cohort had mitochondrial abnormalities; all 17 tested had multiple mtDNA deletions. In a mixed 89-patient mitochondrial PEO series, biopsy showed mitochondrial changes in 95% and molecular diagnosis was reached in 96%. Those mixed-cohort performance estimates should not be assigned specifically to adPEO. (bermejoguerrero2021clinicalhistologicaland pages 7-9, ali2024mitochondrialchronicprogressive pages 1-3)

Differential diagnosis

Important alternatives are myasthenia gravis, oculopharyngeal muscular dystrophy, congenital myasthenic syndromes, thyroid eye disease, orbital disease, cranial neuropathies, myotonic dystrophy, and other mitochondrial PEO etiologies. In the TWNK cohort, 56% were initially misdiagnosed—36% as myasthenia and 20% as oculopharyngeal muscular dystrophy—showing the real-world value of early molecular testing. (bermejoguerrero2021clinicalhistologicaland pages 1-2)

11. Outcome and prognosis

adPEO commonly causes chronic visual-functional and neuromuscular morbidity but isolated ocular disease is not usually directly fatal. Prognosis depends on PEO-plus involvement, particularly cardiac conduction disease/cardiomyopathy, respiratory weakness, dysphagia/aspiration, severe neuropathy, and central neurologic disease. No reliable adPEO-specific 5- or 10-year survival, mortality rate, or life-expectancy estimate was found.

Recovery of lost extraocular movement is not expected with current care. Ptosis and diplopia can be palliated, and rehabilitation may preserve function, but treatment does not remove the causal nuclear variant or accumulated mtDNA deletions. No validated molecular prognostic biomarker exists; genotype, age, deletion burden in muscle, respiratory-chain deficiency, cardiac involvement, dysphagia, and respiratory function are plausible clinical prognostic variables.

12. Treatment and current implementation

There is no approved disease-modifying therapy specifically for adPEO. The 2024 review states that “No definitive treatment option is available for mitochondrial diseases,” with management focused on lifestyle measures, supplements, and symptomatic relief such as ptosis repair. (ali2024mitochondrialchronicprogressive pages 1-3)

Symptomatic and supportive care

Ptosis: lubricants and exposure prevention; ptosis crutch or eyelid tape in selected patients; cautious levator advancement or frontalis suspension. Surgery must account for poor Bell phenomenon and risk of exposure keratopathy. Suggested NCIT concepts: Supportive Care, Blepharoplasty/Ptosis Repair.
Diplopia/ophthalmoplegia: prisms when deviation is suitable; selected strabismus surgery, with counseling that progression can alter alignment.
Myopathy/fatigue: individualized submaximal aerobic/resistance exercise, physiotherapy, pacing, and fall prevention. NCIT: Physical Therapy, Exercise Therapy.
Dysphagia: speech/swallow assessment, dietary modification, and aspiration prevention; enteral feeding if severe. In the TWNK cohort, no patient required enteral nutrition despite bulbar involvement. (bermejoguerrero2021clinicalhistologicaland pages 5-6)
Hearing/vision: hearing aids or cochlear assessment; low-vision support where optic or retinal disease occurs.
Cardiac/respiratory: ECG, echocardiographic and rhythm surveillance; pulmonary-function and sleep assessment when indicated; standard specialist treatment.

Coenzyme Q10, riboflavin, antioxidants, and other “mitochondrial cocktails” are often used empirically, but no adPEO-specific response rate or high-quality efficacy evidence was identified. Pharmacogenomic dosing guidance is not established.

Trials and advanced therapy

Relevant but nonspecific studies include NCT02161848, an observational MRI study in CPEO (completed; 133 participants); NCT04678115, a completed 16-participant trial of nonsurgical approaches for severe blepharoptosis; and NCT05162768, a completed phase III study of elamipretide in nuclear-DNA primary mitochondrial disease (102 participants). These records do not establish genotype-specific efficacy or standard-of-care use in adPEO.

Gene replacement/editing is conceptually more tractable for nuclear adPEO than for primary mtDNA mutations, but no approved TWNK-, POLG-, POLG2-, SLC25A4-, DNA2-, or RRM2B-directed gene therapy exists. LHON ND4 gene-therapy results and mitochondrial augmentation studies concern other disorders and should not be extrapolated to adPEO. (ali2024mitochondrialchronicprogressive pages 18-19)

13. Prevention

Primary prevention by lifestyle or medication is not currently possible. The principal preventive intervention is genetic counseling: molecular confirmation, cascade testing of adult relatives, and discussion of the 50% transmission risk. If a familial pathogenic variant is known, prenatal diagnosis and PGT-M are technically possible; reproductive decisions require nondirective counseling because penetrance and severity can be variable.

Secondary prevention consists of identifying presymptomatic carriers and detecting cardiac, respiratory, hearing, swallowing, visual, or neurologic complications early. Tertiary prevention includes fall and aspiration prevention, corneal protection, paced exercise, hearing support, and timely cardiac/respiratory care. Population newborn screening, vaccination, antimicrobial prophylaxis, and public-health environmental control are not disease-specific interventions.

14. Other species and natural disease

No well-established naturally occurring veterinary disorder directly equivalent to human adPEO was identified. Thus, breed prevalence, VBO identifiers, animal incidence, and veterinary management cannot currently be populated reliably. The disease is noninfectious and has no zoonotic or cross-species transmission potential.

The implicated genes are evolutionarily conserved across mammals and many eukaryotes, particularly POLG, TWNK, OPA1, and adenine nucleotide translocators. Conservation permits functional modeling, but orthologous-gene presence does not imply naturally occurring clinical adPEO.

15. Model organisms

Experimental systems include engineered mouse models affecting Twinkle, Polg, Slc25a4/Ant1, or Opa1; patient fibroblasts/myoblasts; transmitochondrial or cybrid systems; and Saccharomyces cerevisiae models of conserved mitochondrial maintenance genes. Twinkle mutants can reproduce replication stalling and progressive multiple-deletion/respiratory-chain pathology; polymerase-γ models interrogate replication fidelity and age-related mtDNA mutation accumulation.

Yeast is useful for rapid functional validation and phenotype-based drug screening because its mitochondrial biogenesis and metabolism are experimentally tractable. A 2023 review emphasizes that yeast has been used both “to validate alleged pathogenic variants” and to identify potentially beneficial molecules, although yeast lacks extraocular muscles and cannot reproduce the complete human phenotype.

Model limitations include species-specific eye-muscle physiology, differences in mtDNA replication and lifespan, severe phenotypes in some knockouts that do not match heterozygous adult disease, and failure of cultured cells to reproduce age-dependent clonal deletion expansion. Relevant resources include MGI, IMPC, IMSR, SGD, Cellosaurus, and the Alliance of Genome Resources.

Key recent and primary sources

Ali A, Esmaeil A, Behbehani R. “Mitochondrial Chronic Progressive External Ophthalmoplegia.” Brain Sciences. Published January 2024. DOI: https://doi.org/10.3390/brainsci14020135. Abstract: “Genetic sequencing is the gold standard for diagnosing mitochondrial encephalomyopathies…” (ali2024mitochondrialchronicprogressive pages 1-3)
Chen BS et al. “Mitochondria and the eye—manifestations of mitochondrial diseases and their management.” Eye. Published April 2023. DOI: https://doi.org/10.1038/s41433-023-02523-x. The abstract notes that genotype–phenotype correlations “can be imprecise.” (chen2023mitochondriaandthe pages 1-2)
Bermejo-Guerrero L et al. “Clinical, Histological, and Genetic Features of 25 Patients with Autosomal Dominant Progressive External Ophthalmoplegia (ad-PEO)/PEO-Plus Due to TWNK Mutations.” Journal of Clinical Medicine. Published December 2021. DOI: https://doi.org/10.3390/jcm11010022. The abstract reports mean onset at 43 years, diagnosis at 63 years, ptosis in 92%, PEO in 80%, and prior misdiagnosis in 56%. (bermejoguerrero2021clinicalhistologicaland pages 1-2)
Rodríguez-López C et al. “Clinical, pathological and genetic spectrum in 89 cases of mitochondrial progressive external ophthalmoplegia.” Journal of Medical Genetics. Published March 2020. DOI: https://doi.org/10.1136/jmedgenet-2019-106649. The abstract reports “pure PEO” in 42%, mitochondrial biopsy changes in 95%, and genetic diagnosis in 96%; this was a mixed-etiology PEO cohort. (ali2024mitochondrialchronicprogressive pages 1-3)
Viscomi C, Zeviani M. “MtDNA-maintenance defects: syndromes and genes.” Journal of Inherited Metabolic Disease. Published March 2017. DOI: https://doi.org/10.1007/s10545-017-0027-5. Its abstract states that these disorders can cause “mtDNA depletion, accumulation of mtDNA multiple deletions, or both.” (viscomi2017mtdnamaintenancedefectssyndromes pages 2-4)

Principal knowledge gaps

The most important unresolved fields are adPEO-specific prevalence and incidence, gene- and variant-specific penetrance, longitudinal progression rates, standardized patient-reported outcomes, validated prognostic biomarkers, environmental modifiers, cell-resolved multi-omics, and controlled genotype-specific treatment trials. Broader CPEO or mitochondrial-disease findings should remain separately annotated until directly validated in autosomal-dominant mtDNA-maintenance PEO.

References

(viscomi2017mtdnamaintenancedefectssyndromes pages 2-4): Carlo Viscomi and Massimo Zeviani. Mtdna-maintenance defects: syndromes and genes. Journal of Inherited Metabolic Disease, 40:587-599, Mar 2017. URL: https://doi.org/10.1007/s10545-017-0027-5, doi:10.1007/s10545-017-0027-5. This article has 226 citations and is from a peer-reviewed journal.
(bermejoguerrero2021clinicalhistologicaland pages 1-2): Laura Bermejo-Guerrero, Carlos Pablo de Fuenmayor-Fernández de la Hoz, Pablo Serrano-Lorenzo, Alberto Blázquez-Encinar, Gerardo Gutiérrez-Gutiérrez, Laura Martínez-Vicente, Lucía Galán-Dávila, Jorge García-García, Joaquín Arenas, Nuria Muelas, Aurelio Hernández-Laín, Cristina Domínguez-González, and Miguel A. Martín. Clinical, histological, and genetic features of 25 patients with autosomal dominant progressive external ophthalmoplegia (ad-peo)/peo-plus due to twnk mutations. Journal of Clinical Medicine, 11:22, Dec 2021. URL: https://doi.org/10.3390/jcm11010022, doi:10.3390/jcm11010022. This article has 12 citations.
(ali2024mitochondrialchronicprogressive pages 1-3): Ali Ali, Ali Esmaeil, and Raed Behbehani. Mitochondrial chronic progressive external ophthalmoplegia. Brain Sciences, 14:135, Jan 2024. URL: https://doi.org/10.3390/brainsci14020135, doi:10.3390/brainsci14020135. This article has 25 citations.
(chen2023mitochondriaandthe pages 1-2): Benson S. Chen, Joshua P. Harvey, Michael J. Gilhooley, Neringa Jurkute, and Patrick Yu-Wai-Man. Mitochondria and the eye—manifestations of mitochondrial diseases and their management. Eye, 37:2416-2425, Apr 2023. URL: https://doi.org/10.1038/s41433-023-02523-x, doi:10.1038/s41433-023-02523-x. This article has 44 citations and is from a peer-reviewed journal.
(kierdaszuk2020progressiveexternalophthalmoplegia pages 11-12): Biruta Kierdaszuk, Magdalena Kaliszewska, Joanna Rusecka, Joanna Kosińska, Ewa Bartnik, Katarzyna Tońska, Anna M. Kamińska, and Anna Kostera-Pruszczyk. Progressive external ophthalmoplegia in polish patients—from clinical evaluation to genetic confirmation. Genes, 12:54, Dec 2020. URL: https://doi.org/10.3390/genes12010054, doi:10.3390/genes12010054. This article has 6 citations.
(bermejoguerrero2021clinicalhistologicaland pages 2-4): Laura Bermejo-Guerrero, Carlos Pablo de Fuenmayor-Fernández de la Hoz, Pablo Serrano-Lorenzo, Alberto Blázquez-Encinar, Gerardo Gutiérrez-Gutiérrez, Laura Martínez-Vicente, Lucía Galán-Dávila, Jorge García-García, Joaquín Arenas, Nuria Muelas, Aurelio Hernández-Laín, Cristina Domínguez-González, and Miguel A. Martín. Clinical, histological, and genetic features of 25 patients with autosomal dominant progressive external ophthalmoplegia (ad-peo)/peo-plus due to twnk mutations. Journal of Clinical Medicine, 11:22, Dec 2021. URL: https://doi.org/10.3390/jcm11010022, doi:10.3390/jcm11010022. This article has 12 citations.
(bermejoguerrero2021clinicalhistologicaland pages 7-9): Laura Bermejo-Guerrero, Carlos Pablo de Fuenmayor-Fernández de la Hoz, Pablo Serrano-Lorenzo, Alberto Blázquez-Encinar, Gerardo Gutiérrez-Gutiérrez, Laura Martínez-Vicente, Lucía Galán-Dávila, Jorge García-García, Joaquín Arenas, Nuria Muelas, Aurelio Hernández-Laín, Cristina Domínguez-González, and Miguel A. Martín. Clinical, histological, and genetic features of 25 patients with autosomal dominant progressive external ophthalmoplegia (ad-peo)/peo-plus due to twnk mutations. Journal of Clinical Medicine, 11:22, Dec 2021. URL: https://doi.org/10.3390/jcm11010022, doi:10.3390/jcm11010022. This article has 12 citations.
(ali2024mitochondrialchronicprogressive pages 13-14): Ali Ali, Ali Esmaeil, and Raed Behbehani. Mitochondrial chronic progressive external ophthalmoplegia. Brain Sciences, 14:135, Jan 2024. URL: https://doi.org/10.3390/brainsci14020135, doi:10.3390/brainsci14020135. This article has 25 citations.
(bermejoguerrero2021clinicalhistologicaland pages 5-6): Laura Bermejo-Guerrero, Carlos Pablo de Fuenmayor-Fernández de la Hoz, Pablo Serrano-Lorenzo, Alberto Blázquez-Encinar, Gerardo Gutiérrez-Gutiérrez, Laura Martínez-Vicente, Lucía Galán-Dávila, Jorge García-García, Joaquín Arenas, Nuria Muelas, Aurelio Hernández-Laín, Cristina Domínguez-González, and Miguel A. Martín. Clinical, histological, and genetic features of 25 patients with autosomal dominant progressive external ophthalmoplegia (ad-peo)/peo-plus due to twnk mutations. Journal of Clinical Medicine, 11:22, Dec 2021. URL: https://doi.org/10.3390/jcm11010022, doi:10.3390/jcm11010022. This article has 12 citations.
(bermejoguerrero2021clinicalhistologicaland pages 6-7): Laura Bermejo-Guerrero, Carlos Pablo de Fuenmayor-Fernández de la Hoz, Pablo Serrano-Lorenzo, Alberto Blázquez-Encinar, Gerardo Gutiérrez-Gutiérrez, Laura Martínez-Vicente, Lucía Galán-Dávila, Jorge García-García, Joaquín Arenas, Nuria Muelas, Aurelio Hernández-Laín, Cristina Domínguez-González, and Miguel A. Martín. Clinical, histological, and genetic features of 25 patients with autosomal dominant progressive external ophthalmoplegia (ad-peo)/peo-plus due to twnk mutations. Journal of Clinical Medicine, 11:22, Dec 2021. URL: https://doi.org/10.3390/jcm11010022, doi:10.3390/jcm11010022. This article has 12 citations.
(ali2024mitochondrialchronicprogressive pages 18-19): Ali Ali, Ali Esmaeil, and Raed Behbehani. Mitochondrial chronic progressive external ophthalmoplegia. Brain Sciences, 14:135, Jan 2024. URL: https://doi.org/10.3390/brainsci14020135, doi:10.3390/brainsci14020135. This article has 25 citations.

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Discussions and Knowledge Gaps

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Genetic Associations

Medical Actions

Biochemical Markers

Differential Diagnoses

Clinical Trials

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Autosomal Dominant Progressive External Ophthalmoplegia: Disease-Characteristics Report

Scope and evidence note

1. Disease information

Definition and nomenclature

Identifiers

2. Etiology, risk, protection, and environment

Primary cause

Risk factors

Protective factors and gene–environment interaction

3. Phenotypes

Core and systemic manifestations

Quality of life

4. Genetic and molecular information

Genes and functional classes

Variant evidence

Modifiers, epigenetics, and chromosome abnormalities

5. Environmental information

6. Mechanism and pathophysiology

Upstream pathways

Downstream cellular injury

Molecular profiling and advanced technologies

7. Anatomical structures affected

8. Temporal development

9. Inheritance and population

10. Diagnostics

Clinical and laboratory approach

Genetic testing strategy

Biopsy and pathology

Differential diagnosis

11. Outcome and prognosis

12. Treatment and current implementation

Symptomatic and supportive care

Trials and advanced therapy

13. Prevention

14. Other species and natural disease

15. Model organisms

Key recent and primary sources

Principal knowledge gaps

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