Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4

Mendelian MONDO:0014899 Pathograph 19 Show in embeddings browser Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions Mitochondrial Disease

PEOB4 is the adult-onset, muscle-predominant end of the DGUOK allelic spectrum: biallelic variants in DGUOK presenting in adulthood as a mitochondrial myopathy, with or without progressive external ophthalmoplegia, and with multiple large-scale mitochondrial DNA deletions in skeletal muscle. The gene had been known since 2001 for something almost opposite. DGUOK classically causes a hepatocerebral mitochondrial DNA *depletion* syndrome of the neonatal period - liver failure with neurological deterioration, usually fatal in infancy. PEOB4 was defined in 2012, when a MitoExome screen of adults with mitochondrial myopathy and multiple mtDNA deletions in muscle found recessive DGUOK mutations in five independent subjects, 5.6 percent of that cohort. The same enzyme deficiency therefore produces mtDNA depletion in infant liver and mtDNA deletions in adult muscle, and one patient in the 2012 series makes the continuity explicit: she had a liver transplant at nine months of age and presented decades later with recurrent rhabdomyolysis. The mechanism is nucleotide supply. Deoxyguanosine kinase phosphorylates purine deoxyribonucleosides inside the mitochondrial matrix and is the rate-limiting step of the mitochondrial purine salvage pathway - the route by which post-mitotic cells, which cannot use the cytosolic replication-coupled route, obtain dNTPs for mtDNA synthesis. Losing it leaves an imbalanced matrix dNTP pool, and mtDNA replication that is both slowed and error-prone. Which failure mode dominates appears to depend on tissue and on age: an imbalanced pool in a rapidly growing infant liver gives depletion, while decades of stalled and mispaired replication in post-mitotic muscle give clonally expanded large-scale deletions. That reading is a synthesis of the two phenotypes rather than a demonstrated switch, and the entry marks it as such. Clinically the 2012 series was heterogeneous: mitochondrial myopathy with or without PEO, recurrent rhabdomyolysis, and adult-onset lower motor neuron syndrome with mild cognitive impairment. Muscle DGUOK activity and DGUOK protein stability were both impaired. There is no disease-modifying treatment; nucleoside supplementation is a mechanistically motivated idea tested only in cells and in a dguok-null zebrafish, where the result depended on which nucleosides were given.

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Inheritance
10
Pathophys.
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Phenotypes
2
Gaps
19
Pathograph
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Genes
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Medical Actions
3
Differentials
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Models
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References
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Deep Research
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Classifications

Harrison's Part
GENETICS ENVIRONMENT DISEASE NEUROLOGIC
Mechanistic Nosology
mitochondrial disease
ICIMD (Inherited Metabolic Disorders)
nucleotide pool maintenance
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Inheritance

1
Autosomal recessive inheritance HP:0000007
Recessive in all reported subjects. The 2012 series identified autosomal recessive DGUOK mutations in five independent adults, and ClinGen's expert panel independently classifies the DGUOK gene-disease relationship as autosomal recessive. DGUOK is nuclear-encoded and autosomal, so although the disease is expressed as mitochondrial DNA damage it segregates as an ordinary recessive trait, with a 25 percent sibling recurrence risk and no maternal transmission - which is the point that separates it from the primary mtDNA-deletion syndromes it phenocopies.
Autosomal recessive inheritance
Show evidence (3 references)
PMID:23043144 SUPPORT Human Clinical
"We identified autosomal recessive mutations in the DGUOK gene (encoding mitochondrial deoxyguanosine kinase), which has previously been associated with an infantile hepatocerebral form of mitochondrial DNA depletion."
The inheritance mode as reported, together with the prior association that makes this presentation the other end of an allelic series.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"DGUOK | HGNC:2858 | mitochondrial disease | MONDO:0044970 | AR | Definitive | SOP10"
Independent expert-panel assignment of autosomal recessive inheritance, and of a Definitive classification. Graded OTHER because a curation assertion is an expert judgement rather than a study.
PMID:20301766 SUPPORT REVIEW SYNTHESIS Human Clinical
"At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier."
The 25 percent sibling recurrence risk stated in this block's description, from the GeneReviews genetic-counseling section. The chapter describes the infantile forms of DGUOK deficiency, but the recurrence risk follows from autosomal recessive inheritance of the gene and applies equally here.
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Discussions and Knowledge Gaps

2
Why does the same DGUOK enzyme deficiency cause mtDNA depletion in infant liver and mtDNA deletions in adult muscle?
KNOWLEDGE GAP OPEN peob4_depletion_versus_deletion
Attached to
pathophysiology#Multiple Large-Scale mtDNA Deletions in Skeletal Muscle pathophysiology#Imbalanced Mitochondrial dNTP Pool
This is the entity's central unexplained fact, and it is a question about DGUOK rather than about PEOB4 alone. Three candidate explanations are consistent with what is published and are not distinguished by it: allele severity, with null genotypes exhausting the pool fast enough to deplete and hypomorphic ones merely skewing it; tissue-specific salvage capacity, since liver and muscle differ in cytosolic and mitochondrial deoxyribonucleoside handling; and proliferative context, since a growing infant liver replicates mtDNA far faster than an adult muscle fibre and may fail by exhaustion where the fibre fails by infidelity over decades. The entry's description offers the third as a synthesis and explicitly marks it as such. It is recorded here rather than asserted because no source cited here tests it, and because the one patient who traversed both phenotypes shows the alleles alone cannot be the whole answer - the same genotype produced depletion in her liver and deletions in her muscle.
Proposed experiments
Genotype-matched comparison of mitochondrial dNTP pools and mtDNA lesions across tissues
exp_peob4_tissue_dntp_pools
In cells or tissue from DGUOK patients spanning the allelic series - ideally including the patient with both phenotypes - measure matrix dNTP pool composition, mtDNA copy number and large-scale deletion burden in hepatocyte-like and myotube models derived from the same individual, and ask whether the depletion-versus-deletion outcome tracks residual enzyme activity, cell type, or proliferation rate.
Readouts
Large-scale mtDNA deletion burden by cell type
pathophysiology#Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
Direction: INCREASED
Interpretation: A higher deletion burden in the post-mitotic model at matched genotype would support the proliferative-context explanation.
Would support
pathophysiology#Imbalanced Mitochondrial dNTP Pool
Supporting outcome
  • The same genotype gives depletion in the proliferating hepatocyte-like model and an accumulating deletion burden in the post-mitotic myotube model, with a comparably skewed dNTP pool in both.
Refuting outcome
  • Depletion and deletion segregate with residual enzyme activity rather than with cell type, which would make the two phenotypes an allele-severity effect and leave the tissue synthesis in this entry wrong.
Should dismech curate DGUOK as one disease entry, as ClinGen does, rather than as this presentation alone?
OPEN QUESTION OPEN peob4_versus_clingen_lumping
Attached to
disease#Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4
ClinGen's 2024 curation states outright that the DGUOK phenotype has been lumped into one disease entity, on the grounds that the various names describe one primary mitochondrial disease. This entry does the opposite, and the disagreement is deliberate rather than accidental. The case for splitting: MONDO and OMIM both carry three separate identifiers, this entity has its own MONDO term that dismech's stub queue nominated, and dismech already curates the sibling PEOB1 (POLG) as a standalone entry - so a per-presentation entry is the local convention, and lumping DGUOK would make it the only mtDNA-maintenance gene curated gene-first. The case for lumping: one patient in the defining series had both phenotypes, the mechanism is identical, and a reader who wants to understand DGUOK has to assemble it from this entry, the separate infantile hepatocerebral entry, and a differential inside a GIMAP5 disease entry. A `DGUOK-Related_Mitochondrial_Disease` entry with this presentation as a `has_subtypes` entry would satisfy ClinGen and would be a reasonable future refactor. GeneReviews, for its part, describes DGUOK deficiency as two infantile forms and leaves the adult myopathy out, so the two expert sources do not agree on a single gene-level entity either. The refactor is not attempted here because it would restructure the separately curated infantile entry and require curating the portal-hypertension phenotype, which this claim did not cover.
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Pathophysiology

10
Biallelic DGUOK Variants
Two damaged copies of DGUOK. The 2012 series reported recessive DGUOK mutations in five unrelated adults ascertained for adult-onset mitochondrial myopathy with multiple mtDNA deletions in muscle; individual alleles are not given in the abstract available here. Across the whole DGUOK phenotype ClinGen counted fourteen variants - six missense, six truncating, two affecting splicing - and read the mechanism as loss of function.
DGUOK hgnc:2858 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves DGUOK (hgnc:2858). hgnc:2858 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context DGUOK hgnc:2858 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns DGUOK (hgnc:2858). hgnc:2858 is a gene from the HUGO Gene Nomenclature Committee. variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION
Biallelic recessive variants. Zygosity is not stated per subject in the source available here and is left unset rather than guessed; across the wider DGUOK literature both homozygotes and compound heterozygotes are reported.
LOSS_OF_FUNCTION is supported directly for this presentation, not only by inference: muscle DGUOK enzyme activity and DGUOK protein stability were both impaired in the 2012 subjects. That is unusual for an adult-onset allelic series, where the assumption is often that the later-onset phenotype carries hypomorphic alleles - here the enzyme defect is measured.
Downstream
Loss of Mitochondrial Deoxyguanosine Kinase Activity
Show evidence (2 references)
PMID:23043144 SUPPORT Human Clinical
"Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
The number of subjects, the share of the cohort they represent, and the two functional consequences measured in their muscle.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"The mechanism of disease is loss of function."
The expert panel's reading of the mechanism, which is what this node's functional_impact_category records. Graded OTHER because a curation narrative is an expert synthesis rather than a study.
Loss of Mitochondrial Deoxyguanosine Kinase Activity
Deoxyguanosine kinase phosphorylates purine deoxyribonucleosides inside the mitochondrial matrix and is the rate-limiting component of the mitochondrial purine nucleotide salvage pathway. Both the enzyme activity and the protein's stability were reduced in patient muscle, so the lesion is loss of a catalytic step rather than mislocalisation or regulatory change.
DGUOK hgnc:2858 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves DGUOK (hgnc:2858). hgnc:2858 is a gene from the HUGO Gene Nomenclature Committee.
deoxyguanosine kinase activity GO:0004138 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased deoxyguanosine kinase activity (GO:0004138). GO:0004138 is a molecular function from the Gene Ontology. ↓ DECREASED
mitochondrion GO:0005739 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves mitochondrion (GO:0005739). GO:0005739 is a cellular component from the Gene Ontology.
Downstream
Failure of Mitochondrial Purine Deoxyribonucleoside Salvage
Show evidence (3 references)
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"DGUOK encodes mitochondrial deoxyguanosine kinase, an enzyme with a critical role in maintaining the nucleoside pool necessary for mitochondrial DNA (mtDNA) maintenance by phosphorylating purine deoxyribonucleosides in the mitochondrial matrix."
The enzyme's reaction, its compartment and its role. Graded OTHER because a curation narrative is an expert synthesis rather than a study.
PMID:30428046 SUPPORT Other
"Deoxyguanosine kinase (dGK) is an essential rate-limiting component of the mitochondrial purine nucleotide salvage pathway, encoded by the nuclear gene encoding deoxyguanosine kinase (DGUOK)."
That the enzyme is rate-limiting, which is why losing it is not buffered by the rest of the pathway. Graded OTHER because the sentence states established background biology rather than a result of the study.
PMID:23043144 SUPPORT Human Clinical
"Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
The enzyme-activity measurement, made in patient muscle rather than in a model system.
Failure of Mitochondrial Purine Deoxyribonucleoside Salvage
The mitochondrial salvage pathway is how a post-mitotic cell obtains deoxyribonucleotides for mtDNA synthesis. A dividing cell makes dNTPs in the cytosol through ribonucleotide reductase, coupled to nuclear S phase; a neuron or a muscle fibre has no S phase, and depends on salvaging deoxyribonucleosides inside the matrix. Deoxyguanosine kinase supplies the purine half of that route. This is the standing explanation for why defects of mitochondrial nucleotide supply strike post-mitotic tissue hardest, and it is stated here as the accepted account rather than as something demonstrated in these patients.
purine nucleotide salvage GO:0032261 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased purine nucleotide salvage (GO:0032261). GO:0032261 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Imbalanced Mitochondrial dNTP Pool
Show evidence (1 reference)
PMID:30428046 SUPPORT Other
"Deoxyguanosine kinase (dGK) is an essential rate-limiting component of the mitochondrial purine nucleotide salvage pathway, encoded by the nuclear gene encoding deoxyguanosine kinase (DGUOK)."
Places the enzyme in the salvage pathway this node names. Graded OTHER because the sentence states established background biology rather than a result of the study.
Imbalanced Mitochondrial dNTP Pool
Losing one arm of matrix nucleotide supply does not simply lower every dNTP equally; it skews the pool, and a skewed pool is what damages replication. The zebrafish work makes that point from the other direction: supplementing juvenile fish with a single purine nucleoside *reduced* mtDNA copy number in both mutant and wild-type animals, which the authors attribute to nucleotide pool imbalance, while supplementing adult mutants with both purine nucleosides increased liver mtDNA copy number. Balance, not absolute supply, is the variable. No direct measurement of the mitochondrial dNTP pool in a PEOB4 patient is reported in the sources cited here; this node rests on the salvage-pathway biology above and on the model system below.
Animal models
dguok-null zebrafish Zebrafish perturbs Nucleoside supplementation manipulates the substrate supply to the salvage pathway, and the divergent results of supplying one versus both purine nucleosides are the entity's clearest evidence that pool *balance* rather than absolute supply is what matters.
Downstream
Impaired and Error-Prone mtDNA Replication
Show evidence (1 reference)
PMID:30428046 SUPPORT INDIRECT Model Organism
"When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance."
The pool-imbalance reading, and the observation that motivates it. INDIRECT because the imbalance is the authors' inferred explanation for a copy-number result, not a measured pool, and because the system is a zebrafish rather than a patient.
Impaired and Error-Prone mtDNA Replication
An inadequate and skewed dNTP supply slows the replication fork and raises the chance of misincorporation and of strand slippage at the repeat elements where large-scale mtDNA deletions arise. DGUOK sits with POLG, POLG2, TWNK, OPA1, MPV17 and TYMP in the group of nuclear genes whose loss impairs mtDNA maintenance, and that group produces mtDNA depletion, mtDNA deletions, or both, depending on the gene and the tissue.
mitochondrial DNA replication GO:0006264 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased mitochondrial DNA replication (GO:0006264). GO:0006264 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
dguok-null zebrafish Zebrafish partially recapitulates A whole-animal demonstration that losing deoxyguanosine kinase reduces mtDNA content, which establishes the link from the enzyme to mtDNA maintenance in vivo.
Downstream
Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
Show evidence (2 references)
PMID:35114397 SUPPORT Other
"Mutations in several nuclear genes (i.e., POLG, POLG2, TWNK, OPA1, DGUOK, MPV17, TYMP) impair mtDNA maintenance, leading to clinical syndromes characterized by mtDNA depletion and/or deletions in affected tissues."
DGUOK placed in the mtDNA-maintenance gene group, and the statement that this group yields depletion and deletions rather than one or the other. Graded OTHER because a review's enumeration is an expert synthesis rather than a study.
PMID:30428046 SUPPORT INDIRECT Model Organism
"In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish."
Direct evidence that losing dguok reduces mtDNA content in a whole animal. INDIRECT for this node because the readout is copy number rather than replication fidelity, and because it is the depletion arm of the phenotype rather than the deletion arm this entity shows.
Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
The defining molecular lesion of this presentation: multiple large-scale mtDNA deletions in skeletal muscle. This is the point at which PEOB4 diverges from the infantile hepatocerebral DGUOK phenotype, which shows mtDNA depletion in liver and brain instead. Deleted mtDNA species clonally expand within individual post-mitotic fibres over decades, which is why the presentation is adult-onset even though the enzyme defect is congenital. The tissue-and-age dependence of depletion versus deletion is the entity's most interesting open question and is not explained in the literature cited here; it is recorded as a discussion rather than asserted as a mechanism.
skeletal muscle fiber CL:0008002 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skeletal muscle fiber (CL:0008002). CL:0008002 is a cell type from the Cell Ontology.
mitochondrial DNA metabolic process GO:0032042 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased mitochondrial DNA metabolic process (GO:0032042). GO:0032042 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Multiple Mitochondrial DNA Deletions
Respiratory-Chain Deficiency in Skeletal Muscle
Show evidence (2 references)
PMID:23043144 SUPPORT Human Clinical
"We applied next-generation sequencing of known mitochondrial targets (MitoExome) to probands presenting with adult-onset mitochondrial myopathy and harbouring mitochondrial DNA multiple deletions in skeletal muscle."
The molecular finding and the tissue it was made in, which is also the ascertainment criterion for the series.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal,..."
The full DGUOK spectrum with this presentation at one end of it, from an independent expert panel. Graded OTHER because a curation narrative is an expert synthesis rather than a study.
Respiratory-Chain Deficiency in Skeletal Muscle
Each large-scale mtDNA deletion removes tRNA genes and structural subunit genes, so a fibre whose deleted species has clonally expanded past the biochemical threshold loses oxidative phosphorylation capacity. Because expansion is independent in each fibre, the deficiency is mosaic rather than uniform. This node is the standard account of how multiple mtDNA deletions cause disease and is inferred here rather than measured: no respiratory-chain enzymology, blue-native analysis or COX/SDH histochemistry in a PEOB4 patient is reported in the sources cited here.
skeletal muscle fiber CL:0008002 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skeletal muscle fiber (CL:0008002). CL:0008002 is a cell type from the Cell Ontology.
oxidative phosphorylation GO:0006119 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased oxidative phosphorylation (GO:0006119). GO:0006119 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Selective Vulnerability of Extraocular and Levator Muscle
Mitochondrial Myopathy of Limb and Axial Muscle
Motor Neuron and Cognitive Involvement Marked INDIRECT_UNKNOWN_INTERMEDIATES because the neurological features were observed in patients whose deletions were demonstrated in muscle; no nervous-system tissue was examined, so the path from the molecular lesion to the motor neuron and cognitive phenotypes is assumed rather than traced.
Show evidence (1 reference)
PMID:23043144 SUPPORT INDIRECT Human Clinical
"Syndromes associated with mitochondrial DNA multiple deletions are caused by different molecular defects resulting in a wide spectrum of predominantly adult-onset clinical presentations, ranging from progressive external ophthalmoplegia to multi-systemic disorders of variable severity."
The link from multiple mtDNA deletions to the adult-onset clinical spectrum. INDIRECT because it asserts the clinical consequence of the deletions without measuring the respiratory-chain step this node names.
Selective Vulnerability of Extraocular and Levator Muscle
Extraocular muscle has the highest mitochondrial content and the highest continuous duty cycle of any striated muscle, and it is the tissue that fails first and most visibly across the multiple-mtDNA-deletion syndromes - which is why so many of them are named for progressive external ophthalmoplegia. In this entity the ophthalmoplegia is characteristic but not universal: the defining series describes mitochondrial myopathy *with or without* PEO.
skeletal muscle fiber CL:0008002 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skeletal muscle fiber (CL:0008002). CL:0008002 is a cell type from the Cell Ontology.
extra-ocular muscle UBERON:0001601 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in extra-ocular muscle (UBERON:0001601). UBERON:0001601 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Progressive External Ophthalmoplegia
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The ocular phenotype and, importantly, its optionality - "with or without" is what sets the frequency band on the phenotype below.
Mitochondrial Myopathy of Limb and Axial Muscle
Mitochondrial myopathy is the feature every subject in the defining series had, and it is the reason they were ascertained. Its severe manifestation is recurrent rhabdomyolysis, reported in one subject - an energy-supply failure severe enough to break down fibres under load rather than merely to limit them.
skeletal muscle fiber CL:0008002 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skeletal muscle fiber (CL:0008002). CL:0008002 is a cell type from the Cell Ontology.
skeletal muscle tissue UBERON:0001134 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in skeletal muscle tissue (UBERON:0001134). UBERON:0001134 is an anatomical location from the Uberon multi-species anatomy ontology.
Downstream
Mitochondrial Myopathy
Rhabdomyolysis
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"We applied next-generation sequencing of known mitochondrial targets (MitoExome) to probands presenting with adult-onset mitochondrial myopathy and harbouring mitochondrial DNA multiple deletions in skeletal muscle."
Mitochondrial myopathy as the ascertainment criterion, so it is present in every subject by construction.
Motor Neuron and Cognitive Involvement
One subject in the defining series presented with an adult-onset lower motor neuron syndrome together with mild cognitive impairment, so the entity is not confined to muscle. Parkinsonism is listed among the clinical manifestations in this disease's Orphanet-derived MONDO definition and was reported for DGUOK in a 2018 Brain letter, whose full text could not be retrieved for this entry; the evidence cited on the phenotype below establishes only that parkinsonism is a recognised feature of the mtDNA-instability syndromes as a class. No neuropathology, imaging or nerve-conduction data for a PEOB4 patient is reported in the sources cited here, so this node names the affected compartment without explaining how it fails.
motor neuron CL:0000100 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves motor neuron (CL:0000100). CL:0000100 is a cell type from the Cell Ontology.
Downstream
Lower Motor Neuron Syndrome
Cognitive Impairment
Parkinsonism Marked INDIRECT_UNKNOWN_INTERMEDIATES to match the strength of the evidence: parkinsonism is established for the mtDNA-instability class rather than demonstrated for DGUOK in a source retrievable here, and no nigral pathology has been reported in a PEOB4 patient.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The neurological presentation, in one of the five subjects, with both of its components.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4 Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

7
Eye 1
Progressive External Ophthalmoplegia FREQUENT HP:0000590 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Progressive external ophthalmoplegia (HP:0000590). HP:0000590 is a phenotype from the Human Phenotype Ontology.
Graded FREQUENT rather than VERY_FREQUENT. The source describes "mitochondrial myopathy with or without progressive external ophthalmoplegia" without giving a count, so the honest reading is that it is common but not universal. Note the tension this creates with the disease name: the entity is called progressive external ophthalmoplegia with mtDNA deletions, and the feature that names it is not present in every reported patient.
Show evidence (2 references)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The ophthalmoplegia and the explicit statement that it is not present in all subjects.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal,..."
Chronic progressive ophthalmoplegia named as part of the adult end of the DGUOK spectrum. Graded OTHER because a curation narrative is an expert synthesis rather than a study.
Musculoskeletal 2
Mitochondrial Myopathy VERY_FREQUENT HP:0003737 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Adult-onset mitochondrial myopathy, annotated with Mitochondrial myopathy (HP:0003737). HP:0003737 is a phenotype from the Human Phenotype Ontology.
5 of 5 subjects, but by construction: the cohort was defined as adults with mitochondrial myopathy and multiple mtDNA deletions in muscle, so this frequency describes the ascertainment and not the penetrance of the genotype. Graded VERY_FREQUENT rather than OBLIGATE for exactly that reason - a genotype-first series could well find DGUOK carriers without myopathy.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"We applied next-generation sequencing of known mitochondrial targets (MitoExome) to probands presenting with adult-onset mitochondrial myopathy and harbouring mitochondrial DNA multiple deletions in skeletal muscle."
Mitochondrial myopathy as the entry criterion of the series.
Rhabdomyolysis OCCASIONAL HP:0003201 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent rhabdomyolysis, annotated with Rhabdomyolysis (HP:0003201). HP:0003201 is a phenotype from the Human Phenotype Ontology.
1 of 5 subjects, which falls in the OCCASIONAL band. Worth noting that this patient is atypical for the entity in a way that matters: her infantile liver disease means she cannot be treated as a representative adult-onset case, and her rhabdomyolysis may reflect a more severe genotype rather than a distinct mechanism.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The rhabdomyolysis, attributed to one named subject together with her infantile liver transplant.
Nervous System 3
Lower Motor Neuron Syndrome OCCASIONAL Abnormal lower motor neuron morphology HP:0002366 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Adult-onset lower motor neuron syndrome, annotated with Abnormal lower motor neuron morphology (HP:0002366). HP:0002366 is a phenotype from the Human Phenotype Ontology.
1 of 5 subjects. The HPO term carries "Lower motor neuron disease" as an exact synonym, which is what makes it the right binding for a clinical lower motor neuron syndrome despite the morphological wording of its primary label.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The lower motor neuron syndrome, in the same subject as the cognitive impairment.
Cognitive Impairment OCCASIONAL HP:0100543 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Mild cognitive impairment, annotated with Cognitive impairment (HP:0100543). HP:0100543 is a phenotype from the Human Phenotype Ontology.
1 of 5 subjects, and reported only in combination with the lower motor neuron syndrome rather than as an independent finding.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The cognitive impairment, and that it is described as mild.
Parkinsonism OCCASIONAL HP:0001300 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Parkinsonism (HP:0001300). HP:0001300 is a phenotype from the Human Phenotype Ontology.
Graded OCCASIONAL, and the band is a judgement rather than a count: no source retrievable for this entry gives a numerator for parkinsonism in DGUOK-related disease. It is absent from the 2012 series' description of its five subjects, which is itself informative. The DGUOK-specific report is PMID:29228108 (Caporali et al., Brain 2018, a Letter to the Editor); neither its PubMed record nor its DOI returned retrievable text, so it is deliberately not cited as evidence rather than cited without a verifiable quote. If a curator can obtain that text, it should replace the class-level evidence below.
Show evidence (2 references)
PMID:35114397 SUPPORT INDIRECT Other
"Moreover, parkinsonism has been frequently described as a prominent clinical feature in mtDNA instability syndromes."
Parkinsonism established for the class of syndromes this entity belongs to. INDIRECT because it is a class-level statement and does not name DGUOK. Graded OTHER because a review's synthesis is not itself a study.
PMID:35114397 SUPPORT INDIRECT Other
"In the past decades, studies have demonstrated a progressive accumulation of multiple mtDNA deletions in dopaminergic neurons of the substantia nigra in elderly population and, to a greater extent, in Parkinson's disease patients."
The biological rationale for the association - the same lesion this entity carries in muscle accumulates in nigral neurons. INDIRECT because it concerns sporadic ageing and Parkinson disease rather than DGUOK patients.
Cellular 1
Multiple Mitochondrial DNA Deletions VERY_FREQUENT HP:0003689 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Multiple mitochondrial DNA deletions in skeletal muscle, annotated with Multiple mitochondrial DNA deletions (HP:0003689). HP:0003689 is a phenotype from the Human Phenotype Ontology.
5 of 5 subjects, again by ascertainment rather than by observation of an unselected genotype group. VERY_FREQUENT rather than OBLIGATE for the same reason as the myopathy above.
Show evidence (2 references)
PMID:23043144 SUPPORT Human Clinical
"Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
The five subjects, all drawn from a cohort defined by multiple mtDNA deletions.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal,..."
Independent confirmation that muscle mtDNA deletions belong to the DGUOK phenotype. Graded OTHER because a curation narrative is an expert synthesis rather than a study.
🧬

Genetic Associations

1
DGUOK
Gene: DGUOK hgnc:2858 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is DGUOK (hgnc:2858). hgnc:2858 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:23043144 SUPPORT Human Clinical
"These findings reinforce the concept that mutations in genes involved in deoxyribonucleotide metabolism can cause diverse clinical phenotypes and suggest that DGUOK should be screened in patients harbouring mitochondrial DNA deletions in skeletal muscle."
The authors' own generalisation - one nucleotide-metabolism gene, diverse phenotypes - and the clinical recommendation that follows from it.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"In summary, there is definitive evidence to support the relationship between DGUOK and primary mitochondrial disease."
The independent strength-of-evidence judgement. Graded OTHER because a curation assertion is an expert judgement rather than a study.
CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z SUPPORT Other
"Therefore, the DGUOK phenotype has been lumped into one disease entity according to the ClinGen Lumping and Splitting Framework."
The expert panel's lumping decision, quoted because this entry deliberately takes the opposite position and the disagreement should be visible rather than hidden. Graded OTHER as above.
💊

Medical Actions

2
Supportive care
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
No disease-modifying therapy exists for PEOB4 and none has been trialled. Management is supportive and symptomatic - ptosis surgery or lid crutches where ocular involvement is disabling, exercise and rehabilitation advice, avoidance of the catabolic and exertional triggers that precipitate rhabdomyolysis, and genetic counselling. None of that is specific to DGUOK, and none of it is reported for these patients in the sources cited here.
Purine nucleoside supplementation
Action: purine deoxyribonucleoside supplementationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is purine deoxyribonucleoside supplementation, annotated with Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. Ontology label: Pharmacotherapy NCIT:C15986
Agent: deoxyguanosine CHEBI:17172 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses deoxyguanosine, annotated with 2'-deoxyguanosine (CHEBI:17172). CHEBI:17172 is a therapeutic agent from Chemical Entities of Biological Interest. deoxyadenosine CHEBI:17256 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses deoxyadenosine, annotated with 2'-deoxyadenosine (CHEBI:17256). CHEBI:17256 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
Supplying deoxyribonucleosides to bypass the missing kinase step is the one mechanistically motivated therapeutic idea for DGUOK deficiency, and it remains preclinical. In the dguok-null zebrafish, supplementing adults with both purine nucleosides raised liver mtDNA copy number, but supplementing juveniles with deoxyguanosine alone *reduced* copy number in mutant and wild-type fish alike - a result that contradicted earlier cell-culture work and that the authors attribute to nucleotide pool imbalance. Two things follow, and both argue against reading this as an emerging therapy for PEOB4. The tested benefit is a liver copy-number readout in a depletion model, whereas this entity is a muscle deletion disorder; and the direction of effect depends on which nucleosides are given, so a partial regimen may be worse than none.
Show evidence (2 references)
PMID:30428046 SUPPORT INDIRECT Model Organism
"However, in adult dguok-/- fish we detected a significant increase in liver mtDNA copy number when supplemented with both purine nucleosides."
The positive preclinical result. INDIRECT because it is a copy-number readout in fish liver, two steps from a clinical benefit in human muscle.
PMID:30428046 REFUTE Model Organism
"When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance."
Refutes the simple form of the claim - that supplying the missing substrate raises mtDNA. Given alone, deoxyguanosine did the opposite, in mutants and controls alike.
🔬

Diagnosis

1
Muscle mtDNA deletion analysis followed by nuclear gene sequencing
The diagnostic route that defined this entity, and the one it recommends. Multiple mtDNA deletions in a muscle biopsy establish a nuclear mtDNA-maintenance defect but not which gene; targeted or exome sequencing then names it. Before 2012, DGUOK was not on the candidate list for an adult with muscle mtDNA deletions - it was an infantile liver gene - and the defining paper's practical conclusion is precisely that it should be added. Note the ascertainment asymmetry this creates: every reported PEOB4 patient was found because someone had already demonstrated mtDNA deletions in their muscle. A DGUOK patient without a muscle biopsy is invisible to this route.
Show evidence (3 references)
PMID:23043144 SUPPORT Human Clinical
"These findings reinforce the concept that mutations in genes involved in deoxyribonucleotide metabolism can cause diverse clinical phenotypes and suggest that DGUOK should be screened in patients harbouring mitochondrial DNA deletions in skeletal muscle."
The screening recommendation that follows from the series.
PMID:23043144 SUPPORT Human Clinical
"The mutations underlying these conditions remain undisclosed in half of the affected subjects."
The diagnostic gap this route was built to close - half of multiple-deletion syndromes were genetically unsolved.
PMID:20301766 SUPPORT REVIEW SYNTHESIS Human Clinical
"The diagnosis of DGUOK deficiency is established in a proband by the identification of biallelic pathogenic variants in DGUOK on molecular genetic testing."
The confirmatory step at the end of this route, as GeneReviews states it for DGUOK deficiency.
📈

Progression

2
Adult onset, decades after a congenital enzyme defect
The enzyme deficiency is present from conception, but this presentation is adult-onset. The standard explanation is that deleted mtDNA species must clonally expand within individual post-mitotic fibres before enough of them cross the biochemical threshold to produce symptoms, which takes decades - the same reasoning that applies across the multiple-mtDNA-deletion syndromes. Age at onset per subject is not given in the source available here.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Syndromes associated with mitochondrial DNA multiple deletions are caused by different molecular defects resulting in a wide spectrum of predominantly adult-onset clinical presentations, ranging from progressive external ophthalmoplegia to multi-systemic disorders of variable severity."
The predominantly adult onset of this class of syndromes, and the range of severity within it.
Survival of the infantile hepatic phase in one patient
One subject reached the adult muscle phenotype only because she survived the infantile hepatic one, by liver transplantation at nine months. She is the single documented instance of one person traversing the DGUOK spectrum, and she implies something about the others: an adult PEOB4 patient is, in principle, someone whose liver was spared, and what determines that is not known.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
The infantile liver transplant in the subject who later presented with rhabdomyolysis.
📊

Prevalence

1
Adults with multiple mitochondrial DNA deletions in skeletal muscle
Cases In Literature Not yet documented
The one quantitative figure available is not a population prevalence but a diagnostic yield: DGUOK accounted for 5.6 percent of a cohort of patients with multiple mtDNA deletions - five of about ninety. That is a useful number for a clinician deciding whether to sequence DGUOK in such a patient and is worthless as a population rate, so it is recorded in this note, no rate_per_100000 is set, and prevalence_class is NOT_YET_DOCUMENTED rather than a numeric band, since a numeric band would be read against the cohort named here. The population field names the denominator the 5.6 percent actually refers to rather than "Worldwide", which would misrepresent it.
Show evidence (1 reference)
PMID:23043144 SUPPORT Human Clinical
"Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
The diagnostic yield, with its numerator and its explicitly stated denominator.
🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4:

Autosomal recessive progressive external ophthalmoplegia 1 (POLG)
Overlapping Features PEOB1, caused by biallelic POLG variants, is the commonest recessive cause of adult PEO with multiple mtDNA deletions and presents in the same way - ptosis and ophthalmoplegia with a mitochondrial myopathy and multiple deletions in muscle. It is separated from PEOB4 only by sequencing, and it is curated in dismech as its own entry (`Autosomal_Recessive_Progressive_External_Ophthalmoplegia_1`). The distinction matters clinically because POLG disease carries an absolute contraindication to valproic acid that DGUOK disease does not.
Show evidence (1 reference)
PMID:35114397 SUPPORT Other
"Mutations in several nuclear genes (i.e., POLG, POLG2, TWNK, OPA1, DGUOK, MPV17, TYMP) impair mtDNA maintenance, leading to clinical syndromes characterized by mtDNA depletion and/or deletions in affected tissues."
POLG and DGUOK named together in the same mtDNA-maintenance group, which is why they phenocopy each other. Graded OTHER because a review's enumeration is an expert synthesis rather than a study.
🐁

Animal Models

1
dguok-null zebrafish
A CRISPR/Cas9 dguok-knockout zebrafish line. It reproduces the mtDNA-depletion arm of the DGUOK phenotype - mutant fish have significantly reduced mtDNA levels - and was built to test nucleoside supplementation, which is the only mechanistically motivated therapeutic idea for this gene. The result was informative and unwelcome: supplementing juveniles with a single purine nucleoside reduced mtDNA copy number in mutants *and* in wild-type fish, contradicting earlier cell-culture work, while supplementing adult mutants with both purine nucleosides raised liver mtDNA copy number.
Species
Zebrafish
Genotype
dguok-/- (CRISPR/Cas9 mutagenesis)
Publication
PMID:30428046
Pathograph links
Impaired and Error-Prone mtDNA Replication partially recapitulates fidelity: moderate A whole-animal demonstration that losing deoxyguanosine kinase reduces mtDNA content, which establishes the link from the enzyme to mtDNA maintenance in vivo.
Limitations: It models mtDNA *depletion*, which is the infantile hepatic phenotype, and not the multiple large-scale mtDNA *deletions* in skeletal muscle that define this entity - the copy-number readout says nothing about deletion burden, and the reported effect is in liver. A complete null in a fish also does not correspond to the human recessive alleles here, and the lifespan of the model cannot accommodate the decades of clonal expansion that make the human disease adult-onset. This is the most important caveat in the entry: the only animal model of the gene addresses the other end of its spectrum.
Readouts
mtDNA copy number in dguok-/- fish
Direction: DECREASED
Interpretation: Mutant fish carry significantly less mtDNA than wild-type, establishing the enzyme's role in mtDNA maintenance in a whole animal.
Show evidence (1 reference)
PMID:30428046 SUPPORT Model Organism
"In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish."
The model and its baseline mtDNA phenotype.
Show evidence (1 reference)
PMID:30428046 SUPPORT Model Organism
"In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish."
Establishes the model as informative for the mtDNA-maintenance node it is linked to, at the level of copy number.
Imbalanced Mitochondrial dNTP Pool perturbs fidelity: moderate Nucleoside supplementation manipulates the substrate supply to the salvage pathway, and the divergent results of supplying one versus both purine nucleosides are the entity's clearest evidence that pool *balance* rather than absolute supply is what matters.
Limitations: The pool itself was not measured; imbalance is the authors' inferred explanation for a copy-number result. The single-nucleoside effect was seen in juveniles and the two-nucleoside effect in adults, so the two arms are not directly comparable, and the beneficial result is a liver readout in a model of the hepatic phenotype rather than a muscle readout relevant to this entity.
Readouts
Liver mtDNA copy number after dual purine nucleoside supplementation in adults
Direction: INCREASED
Interpretation: Supplying both purine nucleosides raises liver mtDNA copy number in adult mutants, which is the pro-therapeutic result.
Show evidence (1 reference)
PMID:30428046 SUPPORT Model Organism
"However, in adult dguok-/- fish we detected a significant increase in liver mtDNA copy number when supplemented with both purine nucleosides."
The positive supplementation result, with its species, age and tissue attached.
mtDNA copy number after single purine nucleoside supplementation in juveniles
Direction: DECREASED
Interpretation: Supplying deoxyguanosine alone lowered mtDNA copy number in mutant and wild-type juveniles alike - a negative result, and the reason the entry treats nucleoside supplementation as unproven rather than promising.
Show evidence (1 reference)
PMID:30428046 SUPPORT Model Organism
"When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance."
The negative result and the authors' explanation for it, including that it contradicts earlier cell-culture work.
Show evidence (1 reference)
PMID:30428046 SUPPORT Model Organism
"This study further supports the idea that nucleoside supplementation has a potential therapeutic benefit in mtDNA depletion syndromes by substrate enhancement of the purine nucleoside salvage pathway and might improve the liver pathology in patients."
The authors' framing of what the model is for - testing substrate enhancement of the pathway this node describes.
{ }

Source YAML

click to show 
name: Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4
category: Mendelian
creation_date: "2026-09-09T00:00:00Z"
synonyms:
- PEOB4
- progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive type 4
- progressive external ophthalmoplegia, autosomal recessive 4
- adult-onset multiple mitochondrial DNA deletion syndrome due to DGUOK deficiency
- adult-onset multiple mtDNA deletion syndrome due to DGUOK deficiency
- DGUOK-related adult-onset mitochondrial myopathy
disease_term:
  preferred_term: progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 4
  term:
    id: MONDO:0014899
    label: progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 4
description: >-
  PEOB4 is the adult-onset, muscle-predominant end of the DGUOK allelic spectrum: biallelic
  variants in DGUOK presenting in adulthood as a mitochondrial myopathy, with or without
  progressive external ophthalmoplegia, and with multiple large-scale mitochondrial DNA
  deletions in skeletal muscle.

  The gene had been known since 2001 for something almost opposite. DGUOK classically causes a
  hepatocerebral mitochondrial DNA *depletion* syndrome of the neonatal period - liver failure
  with neurological deterioration, usually fatal in infancy. PEOB4 was defined in 2012, when a
  MitoExome screen of adults with mitochondrial myopathy and multiple mtDNA deletions in muscle
  found recessive DGUOK mutations in five independent subjects, 5.6 percent of that cohort. The
  same enzyme deficiency therefore produces mtDNA depletion in infant liver and mtDNA deletions
  in adult muscle, and one patient in the 2012 series makes the continuity explicit: she had a
  liver transplant at nine months of age and presented decades later with recurrent
  rhabdomyolysis.

  The mechanism is nucleotide supply. Deoxyguanosine kinase phosphorylates purine
  deoxyribonucleosides inside the mitochondrial matrix and is the rate-limiting step of the
  mitochondrial purine salvage pathway - the route by which post-mitotic cells, which cannot use
  the cytosolic replication-coupled route, obtain dNTPs for mtDNA synthesis. Losing it leaves an
  imbalanced matrix dNTP pool, and mtDNA replication that is both slowed and error-prone. Which
  failure mode dominates appears to depend on tissue and on age: an imbalanced pool in a rapidly
  growing infant liver gives depletion, while decades of stalled and mispaired replication in
  post-mitotic muscle give clonally expanded large-scale deletions. That reading is a synthesis
  of the two phenotypes rather than a demonstrated switch, and the entry marks it as such.

  Clinically the 2012 series was heterogeneous: mitochondrial myopathy with or without PEO,
  recurrent rhabdomyolysis, and adult-onset lower motor neuron syndrome with mild cognitive
  impairment. Muscle DGUOK activity and DGUOK protein stability were both impaired. There is no
  disease-modifying treatment; nucleoside supplementation is a mechanistically motivated idea
  tested only in cells and in a dguok-null zebrafish, where the result depended on which
  nucleosides were given.
parents:
- Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions
- Mitochondrial Disease
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      A Mendelian, mechanism-defined mitochondrial disorder, diagnosed and managed as an
      inherited disease rather than within a single organ-system Part.
  - classification_value: NEUROLOGIC
    notes: >-
      The presenting picture is a myopathy with ocular motility failure and, in some patients, a
      lower motor neuron syndrome - a neuromuscular presentation.
  mechanistic_category:
  - classification_value: mitochondrial disease
  icimd_category:
  - classification_value: nucleotide_pool_maintenance
    notes: >-
      DGUOK is a mitochondrial deoxyribonucleoside kinase; the lesion is in the supply of dNTPs
      for mtDNA replication, not in the replication machinery itself.
references:
- reference: PMID:23043144
  title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
- reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
  title: "DGUOK / mitochondrial disease (Definitive)"
- reference: PMID:30428046
  title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
- reference: PMID:32278775
  title: "Acute liver failure due to DGUOK deficiency-is liver transplantation justified?"
- reference: PMID:35114397
  title: "Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance."
- reference: PMID:20301766
  title: "Deoxyguanosine Kinase Deficiency."
  tags:
  - GeneReviews
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    Recessive in all reported subjects. The 2012 series identified autosomal recessive DGUOK
    mutations in five independent adults, and ClinGen's expert panel independently classifies the
    DGUOK gene-disease relationship as autosomal recessive. DGUOK is nuclear-encoded and
    autosomal, so although the disease is expressed as mitochondrial DNA damage it segregates as
    an ordinary recessive trait, with a 25 percent sibling recurrence risk and no maternal
    transmission - which is the point that separates it from the primary mtDNA-deletion
    syndromes it phenocopies.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified autosomal recessive mutations in the DGUOK gene (encoding mitochondrial deoxyguanosine kinase), which has previously been associated with an infantile hepatocerebral form of mitochondrial DNA depletion."
    explanation: >-
      The inheritance mode as reported, together with the prior association that makes this
      presentation the other end of an allelic series.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "DGUOK | HGNC:2858 | mitochondrial disease | MONDO:0044970 | AR | Definitive | SOP10"
    explanation: >-
      Independent expert-panel assignment of autosomal recessive inheritance, and of a Definitive
      classification. Graded OTHER because a curation assertion is an expert judgement rather than
      a study.
  - reference: PMID:20301766
    reference_title: "Deoxyguanosine Kinase Deficiency."
    supports: SUPPORT
    quote_role: REVIEW_SYNTHESIS
    evidence_source: HUMAN_CLINICAL
    snippet: "At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier."
    explanation: >-
      The 25 percent sibling recurrence risk stated in this block's description, from the
      GeneReviews genetic-counseling section. The chapter describes the infantile forms of DGUOK
      deficiency, but the recurrence risk follows from autosomal recessive inheritance of the gene
      and applies equally here.
pathophysiology:
- name: Biallelic DGUOK Variants
  biological_scale: MOLECULAR
  description: >-
    Two damaged copies of DGUOK. The 2012 series reported recessive DGUOK mutations in five
    unrelated adults ascertained for adult-onset mitochondrial myopathy with multiple mtDNA
    deletions in muscle; individual alleles are not given in the abstract available here. Across
    the whole DGUOK phenotype ClinGen counted fourteen variants - six missense, six truncating,
    two affecting splicing - and read the mechanism as loss of function.
  genes:
  - preferred_term: DGUOK
    term:
      id: hgnc:2858
      label: DGUOK
  genetic_context:
    genes:
    - preferred_term: DGUOK
      term:
        id: hgnc:2858
        label: DGUOK
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
    description: >-
      Biallelic recessive variants. Zygosity is not stated per subject in the source available
      here and is left unset rather than guessed; across the wider DGUOK literature both
      homozygotes and compound heterozygotes are reported.
    notes: >-
      LOSS_OF_FUNCTION is supported directly for this presentation, not only by inference: muscle
      DGUOK enzyme activity and DGUOK protein stability were both impaired in the 2012 subjects.
      That is unusual for an adult-onset allelic series, where the assumption is often that the
      later-onset phenotype carries hypomorphic alleles - here the enzyme defect is measured.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
    explanation: >-
      The number of subjects, the share of the cohort they represent, and the two functional
      consequences measured in their muscle.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The mechanism of disease is loss of function."
    explanation: >-
      The expert panel's reading of the mechanism, which is what this node's
      functional_impact_category records. Graded OTHER because a curation narrative is an expert
      synthesis rather than a study.
  downstream:
  - target: Loss of Mitochondrial Deoxyguanosine Kinase Activity
    causal_link_type: DIRECT
- name: Loss of Mitochondrial Deoxyguanosine Kinase Activity
  biological_scale: MOLECULAR
  description: >-
    Deoxyguanosine kinase phosphorylates purine deoxyribonucleosides inside the mitochondrial
    matrix and is the rate-limiting component of the mitochondrial purine nucleotide salvage
    pathway. Both the enzyme activity and the protein's stability were reduced in patient muscle,
    so the lesion is loss of a catalytic step rather than mislocalisation or regulatory change.
  genes:
  - preferred_term: DGUOK
    term:
      id: hgnc:2858
      label: DGUOK
  molecular_functions:
  - preferred_term: deoxyguanosine kinase activity
    modifier: DECREASED
    term:
      id: GO:0004138
      label: deoxyguanosine kinase activity
  cellular_components:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  evidence:
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "DGUOK encodes mitochondrial deoxyguanosine kinase, an enzyme with a critical role in maintaining the nucleoside pool necessary for mitochondrial DNA (mtDNA) maintenance by phosphorylating purine deoxyribonucleosides in the mitochondrial matrix."
    explanation: >-
      The enzyme's reaction, its compartment and its role. Graded OTHER because a curation
      narrative is an expert synthesis rather than a study.
  - reference: PMID:30428046
    reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Deoxyguanosine kinase (dGK) is an essential rate-limiting component of the mitochondrial purine nucleotide salvage pathway, encoded by the nuclear gene encoding deoxyguanosine kinase (DGUOK)."
    explanation: >-
      That the enzyme is rate-limiting, which is why losing it is not buffered by the rest of the
      pathway. Graded OTHER because the sentence states established background biology rather than
      a result of the study.
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
    explanation: The enzyme-activity measurement, made in patient muscle rather than in a model system.
  downstream:
  - target: Failure of Mitochondrial Purine Deoxyribonucleoside Salvage
    causal_link_type: DIRECT
- name: Failure of Mitochondrial Purine Deoxyribonucleoside Salvage
  biological_scale: MOLECULAR
  description: >-
    The mitochondrial salvage pathway is how a post-mitotic cell obtains deoxyribonucleotides for
    mtDNA synthesis. A dividing cell makes dNTPs in the cytosol through ribonucleotide reductase,
    coupled to nuclear S phase; a neuron or a muscle fibre has no S phase, and depends on
    salvaging deoxyribonucleosides inside the matrix. Deoxyguanosine kinase supplies the purine
    half of that route. This is the standing explanation for why defects of mitochondrial
    nucleotide supply strike post-mitotic tissue hardest, and it is stated here as the accepted
    account rather than as something demonstrated in these patients.
  biological_processes:
  - preferred_term: purine nucleotide salvage
    modifier: DECREASED
    term:
      id: GO:0032261
      label: purine nucleotide salvage
  evidence:
  - reference: PMID:30428046
    reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Deoxyguanosine kinase (dGK) is an essential rate-limiting component of the mitochondrial purine nucleotide salvage pathway, encoded by the nuclear gene encoding deoxyguanosine kinase (DGUOK)."
    explanation: >-
      Places the enzyme in the salvage pathway this node names. Graded OTHER because the sentence
      states established background biology rather than a result of the study.
  downstream:
  - target: Imbalanced Mitochondrial dNTP Pool
    causal_link_type: DIRECT
- name: Imbalanced Mitochondrial dNTP Pool
  biological_scale: MOLECULAR
  description: >-
    Losing one arm of matrix nucleotide supply does not simply lower every dNTP equally; it skews
    the pool, and a skewed pool is what damages replication. The zebrafish work makes that point
    from the other direction: supplementing juvenile fish with a single purine nucleoside
    *reduced* mtDNA copy number in both mutant and wild-type animals, which the authors attribute
    to nucleotide pool imbalance, while supplementing adult mutants with both purine nucleosides
    increased liver mtDNA copy number. Balance, not absolute supply, is the variable.

    No direct measurement of the mitochondrial dNTP pool in a PEOB4 patient is reported in the
    sources cited here; this node rests on the salvage-pathway biology above and on the model
    system below.
  evidence:
  - reference: PMID:30428046
    reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: MODEL_ORGANISM
    snippet: "When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance."
    explanation: >-
      The pool-imbalance reading, and the observation that motivates it. INDIRECT because the
      imbalance is the authors' inferred explanation for a copy-number result, not a measured
      pool, and because the system is a zebrafish rather than a patient.
  downstream:
  - target: Impaired and Error-Prone mtDNA Replication
    causal_link_type: DIRECT
- name: Impaired and Error-Prone mtDNA Replication
  biological_scale: MOLECULAR
  description: >-
    An inadequate and skewed dNTP supply slows the replication fork and raises the chance of
    misincorporation and of strand slippage at the repeat elements where large-scale mtDNA
    deletions arise. DGUOK sits with POLG, POLG2, TWNK, OPA1, MPV17 and TYMP in the group of
    nuclear genes whose loss impairs mtDNA maintenance, and that group produces mtDNA depletion,
    mtDNA deletions, or both, depending on the gene and the tissue.
  biological_processes:
  - preferred_term: mitochondrial DNA replication
    modifier: DECREASED
    term:
      id: GO:0006264
      label: mitochondrial DNA replication
  evidence:
  - reference: PMID:35114397
    reference_title: "Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Mutations in several nuclear genes (i.e., POLG, POLG2, TWNK, OPA1, DGUOK, MPV17, TYMP) impair mtDNA maintenance, leading to clinical syndromes characterized by mtDNA depletion and/or deletions in affected tissues."
    explanation: >-
      DGUOK placed in the mtDNA-maintenance gene group, and the statement that this group yields
      depletion and deletions rather than one or the other. Graded OTHER because a review's
      enumeration is an expert synthesis rather than a study.
  - reference: PMID:30428046
    reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: MODEL_ORGANISM
    snippet: "In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish."
    explanation: >-
      Direct evidence that losing dguok reduces mtDNA content in a whole animal. INDIRECT for this
      node because the readout is copy number rather than replication fidelity, and because it is
      the depletion arm of the phenotype rather than the deletion arm this entity shows.
  downstream:
  - target: Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
    causal_link_type: DIRECT
- name: Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
  biological_scale: MOLECULAR
  description: >-
    The defining molecular lesion of this presentation: multiple large-scale mtDNA deletions in
    skeletal muscle. This is the point at which PEOB4 diverges from the infantile hepatocerebral
    DGUOK phenotype, which shows mtDNA depletion in liver and brain instead. Deleted mtDNA species
    clonally expand within individual post-mitotic fibres over decades, which is why the
    presentation is adult-onset even though the enzyme defect is congenital.

    The tissue-and-age dependence of depletion versus deletion is the entity's most interesting
    open question and is not explained in the literature cited here; it is recorded as a
    discussion rather than asserted as a mechanism.
  biological_processes:
  - preferred_term: mitochondrial DNA metabolic process
    modifier: DECREASED
    term:
      id: GO:0032042
      label: mitochondrial DNA metabolic process
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We applied next-generation sequencing of known mitochondrial targets (MitoExome) to probands presenting with adult-onset mitochondrial myopathy and harbouring mitochondrial DNA multiple deletions in skeletal muscle."
    explanation: >-
      The molecular finding and the tissue it was made in, which is also the ascertainment
      criterion for the series.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal, and adult-onset mitochondrial myopathy with chronic progressive ophthalmoplegia and multiple mitochondrial DNA (mtDNA) deletions in muscle."
    explanation: >-
      The full DGUOK spectrum with this presentation at one end of it, from an independent expert
      panel. Graded OTHER because a curation narrative is an expert synthesis rather than a study.
  downstream:
  - target: Multiple Mitochondrial DNA Deletions
    causal_link_type: DIRECT
  - target: Respiratory-Chain Deficiency in Skeletal Muscle
    causal_link_type: DIRECT
- name: Respiratory-Chain Deficiency in Skeletal Muscle
  biological_scale: CELLULAR
  description: >-
    Each large-scale mtDNA deletion removes tRNA genes and structural subunit genes, so a fibre
    whose deleted species has clonally expanded past the biochemical threshold loses oxidative
    phosphorylation capacity. Because expansion is independent in each fibre, the deficiency is
    mosaic rather than uniform.

    This node is the standard account of how multiple mtDNA deletions cause disease and is
    inferred here rather than measured: no respiratory-chain enzymology, blue-native analysis or
    COX/SDH histochemistry in a PEOB4 patient is reported in the sources cited here.
  biological_processes:
  - preferred_term: oxidative phosphorylation
    modifier: DECREASED
    term:
      id: GO:0006119
      label: oxidative phosphorylation
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "Syndromes associated with mitochondrial DNA multiple deletions are caused by different molecular defects resulting in a wide spectrum of predominantly adult-onset clinical presentations, ranging from progressive external ophthalmoplegia to multi-systemic disorders of variable severity."
    explanation: >-
      The link from multiple mtDNA deletions to the adult-onset clinical spectrum. INDIRECT
      because it asserts the clinical consequence of the deletions without measuring the
      respiratory-chain step this node names.
  downstream:
  - target: Selective Vulnerability of Extraocular and Levator Muscle
    causal_link_type: DIRECT
  - target: Mitochondrial Myopathy of Limb and Axial Muscle
    causal_link_type: DIRECT
  - target: Motor Neuron and Cognitive Involvement
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Marked INDIRECT_UNKNOWN_INTERMEDIATES because the neurological features were observed in
      patients whose deletions were demonstrated in muscle; no nervous-system tissue was examined,
      so the path from the molecular lesion to the motor neuron and cognitive phenotypes is
      assumed rather than traced.
- name: Selective Vulnerability of Extraocular and Levator Muscle
  biological_scale: TISSUE
  description: >-
    Extraocular muscle has the highest mitochondrial content and the highest continuous duty cycle
    of any striated muscle, and it is the tissue that fails first and most visibly across the
    multiple-mtDNA-deletion syndromes - which is why so many of them are named for progressive
    external ophthalmoplegia. In this entity the ophthalmoplegia is characteristic but not
    universal: the defining series describes mitochondrial myopathy *with or without* PEO.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  locations:
  - preferred_term: extra-ocular muscle
    term:
      id: UBERON:0001601
      label: extra-ocular muscle
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: >-
      The ocular phenotype and, importantly, its optionality - "with or without" is what sets the
      frequency band on the phenotype below.
  downstream:
  - target: Progressive External Ophthalmoplegia
    causal_link_type: DIRECT
- name: Mitochondrial Myopathy of Limb and Axial Muscle
  biological_scale: TISSUE
  description: >-
    Mitochondrial myopathy is the feature every subject in the defining series had, and it is the
    reason they were ascertained. Its severe manifestation is recurrent rhabdomyolysis, reported
    in one subject - an energy-supply failure severe enough to break down fibres under load rather
    than merely to limit them.
  cell_types:
  - preferred_term: skeletal muscle fiber
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  locations:
  - preferred_term: skeletal muscle tissue
    term:
      id: UBERON:0001134
      label: skeletal muscle tissue
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We applied next-generation sequencing of known mitochondrial targets (MitoExome) to probands presenting with adult-onset mitochondrial myopathy and harbouring mitochondrial DNA multiple deletions in skeletal muscle."
    explanation: >-
      Mitochondrial myopathy as the ascertainment criterion, so it is present in every subject by
      construction.
  downstream:
  - target: Mitochondrial Myopathy
    causal_link_type: DIRECT
  - target: Rhabdomyolysis
    causal_link_type: DIRECT
- name: Motor Neuron and Cognitive Involvement
  biological_scale: TISSUE
  description: >-
    One subject in the defining series presented with an adult-onset lower motor neuron syndrome
    together with mild cognitive impairment, so the entity is not confined to muscle. Parkinsonism
    is listed among the clinical manifestations in this disease's Orphanet-derived MONDO
    definition and was reported for DGUOK in a 2018 Brain letter, whose full text could not be
    retrieved for this entry; the evidence cited on the phenotype below establishes only that
    parkinsonism is a recognised feature of the mtDNA-instability syndromes as a class.

    No neuropathology, imaging or nerve-conduction data for a PEOB4 patient is reported in the
    sources cited here, so this node names the affected compartment without explaining how it
    fails.
  cell_types:
  - preferred_term: motor neuron
    term:
      id: CL:0000100
      label: motor neuron
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: >-
      The neurological presentation, in one of the five subjects, with both of its components.
  downstream:
  - target: Lower Motor Neuron Syndrome
    causal_link_type: DIRECT
  - target: Cognitive Impairment
    causal_link_type: DIRECT
  - target: Parkinsonism
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Marked INDIRECT_UNKNOWN_INTERMEDIATES to match the strength of the evidence: parkinsonism is
      established for the mtDNA-instability class rather than demonstrated for DGUOK in a source
      retrievable here, and no nigral pathology has been reported in a PEOB4 patient.
phenotypes:
- name: Mitochondrial Myopathy
  category: Musculoskeletal
  description: >-
    Adult-onset mitochondrial myopathy, present in every subject of the defining series and the
    criterion on which they were ascertained.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Adult-onset mitochondrial myopathy
    term:
      id: HP:0003737
      label: Mitochondrial myopathy
  notes: >-
    5 of 5 subjects, but by construction: the cohort was defined as adults with mitochondrial
    myopathy and multiple mtDNA deletions in muscle, so this frequency describes the ascertainment
    and not the penetrance of the genotype. Graded VERY_FREQUENT rather than OBLIGATE for exactly
    that reason - a genotype-first series could well find DGUOK carriers without myopathy.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We applied next-generation sequencing of known mitochondrial targets (MitoExome) to probands presenting with adult-onset mitochondrial myopathy and harbouring mitochondrial DNA multiple deletions in skeletal muscle."
    explanation: Mitochondrial myopathy as the entry criterion of the series.
- name: Multiple Mitochondrial DNA Deletions
  category: Metabolism
  description: >-
    Multiple large-scale mtDNA deletions in skeletal muscle - the molecular finding that defines
    this presentation and separates it from the mtDNA-depletion form of DGUOK deficiency.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Multiple mitochondrial DNA deletions in skeletal muscle
    term:
      id: HP:0003689
      label: Multiple mitochondrial DNA deletions
  notes: >-
    5 of 5 subjects, again by ascertainment rather than by observation of an unselected genotype
    group. VERY_FREQUENT rather than OBLIGATE for the same reason as the myopathy above.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
    explanation: The five subjects, all drawn from a cohort defined by multiple mtDNA deletions.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal, and adult-onset mitochondrial myopathy with chronic progressive ophthalmoplegia and multiple mitochondrial DNA (mtDNA) deletions in muscle."
    explanation: >-
      Independent confirmation that muscle mtDNA deletions belong to the DGUOK phenotype. Graded
      OTHER because a curation narrative is an expert synthesis rather than a study.
- name: Progressive External Ophthalmoplegia
  category: Ophthalmologic
  description: >-
    Progressive external ophthalmoplegia, the feature the entity is named for, and one that its
    defining series found to be optional - patients presented with mitochondrial myopathy with or
    without it.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Progressive external ophthalmoplegia
    term:
      id: HP:0000590
      label: Progressive external ophthalmoplegia
  notes: >-
    Graded FREQUENT rather than VERY_FREQUENT. The source describes "mitochondrial myopathy with
    or without progressive external ophthalmoplegia" without giving a count, so the honest reading
    is that it is common but not universal. Note the tension this creates with the disease name:
    the entity is called progressive external ophthalmoplegia with mtDNA deletions, and the
    feature that names it is not present in every reported patient.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: The ophthalmoplegia and the explicit statement that it is not present in all subjects.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal, and adult-onset mitochondrial myopathy with chronic progressive ophthalmoplegia and multiple mitochondrial DNA (mtDNA) deletions in muscle."
    explanation: >-
      Chronic progressive ophthalmoplegia named as part of the adult end of the DGUOK spectrum.
      Graded OTHER because a curation narrative is an expert synthesis rather than a study.
- name: Rhabdomyolysis
  category: Musculoskeletal
  description: >-
    Recurrent rhabdomyolysis in one subject of the defining series - a young woman who had
    received a liver transplant at nine months of age for what was, in retrospect, the infantile
    hepatic form of the same enzyme deficiency. She is the single patient in the literature who
    demonstrates both ends of the DGUOK spectrum in one person.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Recurrent rhabdomyolysis
    term:
      id: HP:0003201
      label: Rhabdomyolysis
  notes: >-
    1 of 5 subjects, which falls in the OCCASIONAL band. Worth noting that this patient is
    atypical for the entity in a way that matters: her infantile liver disease means she cannot be
    treated as a representative adult-onset case, and her rhabdomyolysis may reflect a more severe
    genotype rather than a distinct mechanism.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: >-
      The rhabdomyolysis, attributed to one named subject together with her infantile liver
      transplant.
- name: Lower Motor Neuron Syndrome
  category: Neurologic
  description: >-
    Adult-onset lower motor neuron syndrome in one subject of the defining series. This is the
    feature that takes the entity outside muscle and is the reason a DGUOK diagnosis can be
    reached from a motor neuron disease clinic.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Adult-onset lower motor neuron syndrome
    term:
      id: HP:0002366
      label: Abnormal lower motor neuron morphology
  notes: >-
    1 of 5 subjects. The HPO term carries "Lower motor neuron disease" as an exact synonym, which
    is what makes it the right binding for a clinical lower motor neuron syndrome despite the
    morphological wording of its primary label.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: The lower motor neuron syndrome, in the same subject as the cognitive impairment.
- name: Cognitive Impairment
  category: Neurologic
  description: >-
    Mild cognitive impairment, reported in the subject with the lower motor neuron syndrome. No
    neuropsychological detail, imaging or progression data is given.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Mild cognitive impairment
    term:
      id: HP:0100543
      label: Cognitive impairment
  notes: >-
    1 of 5 subjects, and reported only in combination with the lower motor neuron syndrome rather
    than as an independent finding.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: The cognitive impairment, and that it is described as mild.
- name: Parkinsonism
  category: Neurologic
  description: >-
    Parkinsonism appears among the clinical manifestations in this disease's Orphanet-derived
    MONDO definition and has been reported for DGUOK in the literature, but the report is a 2018
    Brain letter whose full text could not be retrieved for this entry and which therefore cannot
    be quoted. What is citable here is weaker and is stated as such: parkinsonism is a recognised
    and prominent feature of the mtDNA-instability syndromes as a class, of which DGUOK-related
    disease is one, and multiple mtDNA deletions accumulate in nigral dopaminergic neurons with
    age and more so in Parkinson disease.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Parkinsonism
    term:
      id: HP:0001300
      label: Parkinsonism
  notes: >-
    Graded OCCASIONAL, and the band is a judgement rather than a count: no source retrievable for
    this entry gives a numerator for parkinsonism in DGUOK-related disease. It is absent from the
    2012 series' description of its five subjects, which is itself informative. The
    DGUOK-specific report is PMID:29228108 (Caporali et al., Brain 2018, a Letter to the Editor);
    neither its PubMed record nor its DOI returned retrievable text, so it is deliberately not
    cited as evidence rather than cited without a verifiable quote. If a curator can obtain that
    text, it should replace the class-level evidence below.
  evidence:
  - reference: PMID:35114397
    reference_title: "Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: "Moreover, parkinsonism has been frequently described as a prominent clinical feature in mtDNA instability syndromes."
    explanation: >-
      Parkinsonism established for the class of syndromes this entity belongs to. INDIRECT because
      it is a class-level statement and does not name DGUOK. Graded OTHER because a review's
      synthesis is not itself a study.
  - reference: PMID:35114397
    reference_title: "Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: "In the past decades, studies have demonstrated a progressive accumulation of multiple mtDNA deletions in dopaminergic neurons of the substantia nigra in elderly population and, to a greater extent, in Parkinson's disease patients."
    explanation: >-
      The biological rationale for the association - the same lesion this entity carries in muscle
      accumulates in nigral neurons. INDIRECT because it concerns sporadic ageing and Parkinson
      disease rather than DGUOK patients.
prevalence:
- population: Adults with multiple mitochondrial DNA deletions in skeletal muscle
  measure_type: CASES_IN_LITERATURE
  prevalence_class: NOT_YET_DOCUMENTED
  notes: >-
    The one quantitative figure available is not a population prevalence but a diagnostic yield:
    DGUOK accounted for 5.6 percent of a cohort of patients with multiple mtDNA deletions - five
    of about ninety. That is a useful number for a clinician deciding whether to sequence DGUOK in
    such a patient and is worthless as a population rate, so it is recorded in this note, no
    rate_per_100000 is set, and prevalence_class is NOT_YET_DOCUMENTED rather than a numeric band,
    since a numeric band would be read against the cohort named here. The population
    field names the denominator the 5.6 percent actually refers to rather than "Worldwide", which
    would misrepresent it.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in DGUOK occurred in five independent subjects, representing 5.6% of our cohort of patients with mitochondrial DNA multiple deletions, and impaired both muscle DGUOK activity and protein stability."
    explanation: The diagnostic yield, with its numerator and its explicitly stated denominator.
progression:
- phase: Adult onset, decades after a congenital enzyme defect
  notes: >-
    The enzyme deficiency is present from conception, but this presentation is adult-onset. The
    standard explanation is that deleted mtDNA species must clonally expand within individual
    post-mitotic fibres before enough of them cross the biochemical threshold to produce symptoms,
    which takes decades - the same reasoning that applies across the multiple-mtDNA-deletion
    syndromes. Age at onset per subject is not given in the source available here.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Syndromes associated with mitochondrial DNA multiple deletions are caused by different molecular defects resulting in a wide spectrum of predominantly adult-onset clinical presentations, ranging from progressive external ophthalmoplegia to multi-systemic disorders of variable severity."
    explanation: The predominantly adult onset of this class of syndromes, and the range of severity within it.
- phase: Survival of the infantile hepatic phase in one patient
  notes: >-
    One subject reached the adult muscle phenotype only because she survived the infantile hepatic
    one, by liver transplantation at nine months. She is the single documented instance of one
    person traversing the DGUOK spectrum, and she implies something about the others: an adult
    PEOB4 patient is, in principle, someone whose liver was spared, and what determines that is
    not known.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical presentations were variable, including mitochondrial myopathy with or without progressive external ophthalmoplegia, recurrent rhabdomyolysis in a young female who had received a liver transplant at 9 months of age and adult-onset lower motor neuron syndrome with mild cognitive impairment."
    explanation: The infantile liver transplant in the subject who later presented with rhabdomyolysis.
genetic:
- name: DGUOK
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  gene_term:
    preferred_term: DGUOK
    term:
      id: hgnc:2858
      label: DGUOK
  notes: >-
    DGUOK encodes mitochondrial deoxyguanosine kinase. The gene-disease relationship is
    ClinGen-Definitive - much stronger than for most entities of this size - but the strength
    attaches to DGUOK-related mitochondrial disease as a whole rather than to this presentation
    specifically.

    The allelic series is the thing to understand before extending this entry. DGUOK carries three
    OMIM phenotype numbers, all recessive and all this gene: 251880, the classic infantile
    hepatocerebral mtDNA depletion syndrome; 617068, noncirrhotic portal hypertension 1; and
    617070, this entity. The infantile form has its own entry,
    `Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type`; the portal-hypertension form is
    curated only as a differential inside `Portal_Hypertension_Noncirrhotic_2`, which is a GIMAP5
    disease.

    ClinGen has explicitly decided against that split. Its 2024 curation states that the various
    names given to DGUOK-related disease describe one primary mitochondrial disease and that the
    phenotype has been lumped into a single entity under its Lumping and Splitting Framework. This
    entry does not follow that decision, and the reason is recorded in the entry notes rather than
    left implicit.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These findings reinforce the concept that mutations in genes involved in deoxyribonucleotide metabolism can cause diverse clinical phenotypes and suggest that DGUOK should be screened in patients harbouring mitochondrial DNA deletions in skeletal muscle."
    explanation: >-
      The authors' own generalisation - one nucleotide-metabolism gene, diverse phenotypes - and
      the clinical recommendation that follows from it.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In summary, there is definitive evidence to support the relationship between DGUOK and primary mitochondrial disease."
    explanation: >-
      The independent strength-of-evidence judgement. Graded OTHER because a curation assertion is
      an expert judgement rather than a study.
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Therefore, the DGUOK phenotype has been lumped into one disease entity according to the ClinGen Lumping and Splitting Framework."
    explanation: >-
      The expert panel's lumping decision, quoted because this entry deliberately takes the
      opposite position and the disagreement should be visible rather than hidden. Graded OTHER as
      above.
diagnosis:
- name: Muscle mtDNA deletion analysis followed by nuclear gene sequencing
  description: >-
    The diagnostic route that defined this entity, and the one it recommends. Multiple mtDNA
    deletions in a muscle biopsy establish a nuclear mtDNA-maintenance defect but not which gene;
    targeted or exome sequencing then names it. Before 2012, DGUOK was not on the candidate list
    for an adult with muscle mtDNA deletions - it was an infantile liver gene - and the defining
    paper's practical conclusion is precisely that it should be added.

    Note the ascertainment asymmetry this creates: every reported PEOB4 patient was found because
    someone had already demonstrated mtDNA deletions in their muscle. A DGUOK patient without a
    muscle biopsy is invisible to this route.
  evidence:
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These findings reinforce the concept that mutations in genes involved in deoxyribonucleotide metabolism can cause diverse clinical phenotypes and suggest that DGUOK should be screened in patients harbouring mitochondrial DNA deletions in skeletal muscle."
    explanation: The screening recommendation that follows from the series.
  - reference: PMID:23043144
    reference_title: "Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutations underlying these conditions remain undisclosed in half of the affected subjects."
    explanation: >-
      The diagnostic gap this route was built to close - half of multiple-deletion syndromes were
      genetically unsolved.
  - reference: PMID:20301766
    reference_title: "Deoxyguanosine Kinase Deficiency."
    supports: SUPPORT
    quote_role: REVIEW_SYNTHESIS
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of DGUOK deficiency is established in a proband by the identification of biallelic pathogenic variants in DGUOK on molecular genetic testing."
    explanation: >-
      The confirmatory step at the end of this route, as GeneReviews states it for DGUOK
      deficiency.
animal_models:
- name: dguok-null zebrafish
  species: Zebrafish
  genotype: dguok-/- (CRISPR/Cas9 mutagenesis)
  publication: PMID:30428046
  description: >-
    A CRISPR/Cas9 dguok-knockout zebrafish line. It reproduces the mtDNA-depletion arm of the
    DGUOK phenotype - mutant fish have significantly reduced mtDNA levels - and was built to test
    nucleoside supplementation, which is the only mechanistically motivated therapeutic idea for
    this gene. The result was informative and unwelcome: supplementing juveniles with a single
    purine nucleoside reduced mtDNA copy number in mutants *and* in wild-type fish, contradicting
    earlier cell-culture work, while supplementing adult mutants with both purine nucleosides
    raised liver mtDNA copy number.
  modeled_mechanisms:
  - target: Impaired and Error-Prone mtDNA Replication
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      A whole-animal demonstration that losing deoxyguanosine kinase reduces mtDNA content, which
      establishes the link from the enzyme to mtDNA maintenance in vivo.
    limitations: >-
      It models mtDNA *depletion*, which is the infantile hepatic phenotype, and not the multiple
      large-scale mtDNA *deletions* in skeletal muscle that define this entity - the copy-number
      readout says nothing about deletion burden, and the reported effect is in liver. A complete
      null in a fish also does not correspond to the human recessive alleles here, and the
      lifespan of the model cannot accommodate the decades of clonal expansion that make the human
      disease adult-onset. This is the most important caveat in the entry: the only animal model
      of the gene addresses the other end of its spectrum.
    readouts:
    - name: mtDNA copy number in dguok-/- fish
      target: Impaired and Error-Prone mtDNA Replication
      direction: DECREASED
      interpretation: >-
        Mutant fish carry significantly less mtDNA than wild-type, establishing the enzyme's role
        in mtDNA maintenance in a whole animal.
      evidence:
      - reference: PMID:30428046
        reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish."
        explanation: The model and its baseline mtDNA phenotype.
    evidence:
    - reference: PMID:30428046
      reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "In this study we developed a mutant dguok zebrafish (Danio rerio) line using CRISPR/Cas9 mediated mutagenesis; dguok-/- fish have significantly reduced mtDNA levels compared with wild-type (wt) fish."
      explanation: >-
        Establishes the model as informative for the mtDNA-maintenance node it is linked to, at
        the level of copy number.
  - target: Imbalanced Mitochondrial dNTP Pool
    relationship: PERTURBS
    fidelity: MODERATE
    description: >-
      Nucleoside supplementation manipulates the substrate supply to the salvage pathway, and the
      divergent results of supplying one versus both purine nucleosides are the entity's clearest
      evidence that pool *balance* rather than absolute supply is what matters.
    limitations: >-
      The pool itself was not measured; imbalance is the authors' inferred explanation for a
      copy-number result. The single-nucleoside effect was seen in juveniles and the
      two-nucleoside effect in adults, so the two arms are not directly comparable, and the
      beneficial result is a liver readout in a model of the hepatic phenotype rather than a
      muscle readout relevant to this entity.
    readouts:
    - name: Liver mtDNA copy number after dual purine nucleoside supplementation in adults
      target: Imbalanced Mitochondrial dNTP Pool
      direction: INCREASED
      interpretation: >-
        Supplying both purine nucleosides raises liver mtDNA copy number in adult mutants, which
        is the pro-therapeutic result.
      evidence:
      - reference: PMID:30428046
        reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "However, in adult dguok-/- fish we detected a significant increase in liver mtDNA copy number when supplemented with both purine nucleosides."
        explanation: The positive supplementation result, with its species, age and tissue attached.
    - name: mtDNA copy number after single purine nucleoside supplementation in juveniles
      target: Imbalanced Mitochondrial dNTP Pool
      direction: DECREASED
      interpretation: >-
        Supplying deoxyguanosine alone lowered mtDNA copy number in mutant and wild-type juveniles
        alike - a negative result, and the reason the entry treats nucleoside supplementation as
        unproven rather than promising.
      evidence:
      - reference: PMID:30428046
        reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance."
        explanation: >-
          The negative result and the authors' explanation for it, including that it contradicts
          earlier cell-culture work.
    evidence:
    - reference: PMID:30428046
      reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "This study further supports the idea that nucleoside supplementation has a potential therapeutic benefit in mtDNA depletion syndromes by substrate enhancement of the purine nucleoside salvage pathway and might improve the liver pathology in patients."
      explanation: >-
        The authors' framing of what the model is for - testing substrate enhancement of the
        pathway this node describes.
treatments:
- name: Supportive care
  description: >-
    No disease-modifying therapy exists for PEOB4 and none has been trialled. Management is
    supportive and symptomatic - ptosis surgery or lid crutches where ocular involvement is
    disabling, exercise and rehabilitation advice, avoidance of the catabolic and exertional
    triggers that precipitate rhabdomyolysis, and genetic counselling. None of that is specific to
    DGUOK, and none of it is reported for these patients in the sources cited here.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  notes: >-
    Recorded with no evidence block and no target_mechanisms link, deliberately. There is no
    treatment publication for this entity, an evidence item requires an exact quote supporting the
    specific claim, and a mechanism link would assert an effect on the pathograph that nothing
    demonstrates.
- name: Purine nucleoside supplementation
  description: >-
    Supplying deoxyribonucleosides to bypass the missing kinase step is the one mechanistically
    motivated therapeutic idea for DGUOK deficiency, and it remains preclinical. In the dguok-null
    zebrafish, supplementing adults with both purine nucleosides raised liver mtDNA copy number,
    but supplementing juveniles with deoxyguanosine alone *reduced* copy number in mutant and
    wild-type fish alike - a result that contradicted earlier cell-culture work and that the
    authors attribute to nucleotide pool imbalance.

    Two things follow, and both argue against reading this as an emerging therapy for PEOB4. The
    tested benefit is a liver copy-number readout in a depletion model, whereas this entity is a
    muscle deletion disorder; and the direction of effect depends on which nucleosides are given,
    so a partial regimen may be worse than none.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: purine deoxyribonucleoside supplementation
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: deoxyguanosine
      term:
        id: CHEBI:17172
        label: 2'-deoxyguanosine
    - preferred_term: deoxyadenosine
      term:
        id: CHEBI:17256
        label: 2'-deoxyadenosine
  notes: >-
    Preclinical only - no human trial, and no report of its use in a PEOB4 patient. No
    target_mechanisms link is declared even though the mechanism is clear, because a link would
    assert an effect on this entity's pathograph and the only evidence is a zebrafish liver
    readout in the depletion arm of the phenotype. The supporting and contradicting results are
    curated in full on the animal model above rather than summarised optimistically here.
  evidence:
  - reference: PMID:30428046
    reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: MODEL_ORGANISM
    snippet: "However, in adult dguok-/- fish we detected a significant increase in liver mtDNA copy number when supplemented with both purine nucleosides."
    explanation: >-
      The positive preclinical result. INDIRECT because it is a copy-number readout in fish liver,
      two steps from a clinical benefit in human muscle.
  - reference: PMID:30428046
    reference_title: "Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish."
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: "When supplemented with only one purine nucleoside (dGuo), mtDNA copy number in both mutant and wt juvenile animals was significantly reduced, contrasting with previous cell culture studies, possibly because of nucleotide pool imbalance."
    explanation: >-
      Refutes the simple form of the claim - that supplying the missing substrate raises mtDNA.
      Given alone, deoxyguanosine did the opposite, in mutants and controls alike.
differential_diagnoses:
- name: Autosomal recessive progressive external ophthalmoplegia 1 (POLG)
  description: >-
    PEOB1, caused by biallelic POLG variants, is the commonest recessive cause of adult PEO with
    multiple mtDNA deletions and presents in the same way - ptosis and ophthalmoplegia with a
    mitochondrial myopathy and multiple deletions in muscle. It is separated from PEOB4 only by
    sequencing, and it is curated in dismech as its own entry
    (`Autosomal_Recessive_Progressive_External_Ophthalmoplegia_1`). The distinction matters
    clinically because POLG disease carries an absolute contraindication to valproic acid that
    DGUOK disease does not.
  evidence:
  - reference: PMID:35114397
    reference_title: "Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Mutations in several nuclear genes (i.e., POLG, POLG2, TWNK, OPA1, DGUOK, MPV17, TYMP) impair mtDNA maintenance, leading to clinical syndromes characterized by mtDNA depletion and/or deletions in affected tissues."
    explanation: >-
      POLG and DGUOK named together in the same mtDNA-maintenance group, which is why they
      phenocopy each other. Graded OTHER because a review's enumeration is an expert synthesis
      rather than a study.
- name: DGUOK-related hepatocerebral mitochondrial DNA depletion syndrome
  description: >-
    The infantile form of the same enzyme deficiency, OMIM 251880: liver failure with neurological
    deterioration in the first days or weeks of life, with mtDNA depletion rather than deletions,
    and a generally poor prognosis. It is curated in dismech as
    `Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type`. It is a differential only in the sense that it is the same
    gene at a different age - a family history of an infant death from liver failure is a
    diagnostic clue in an adult with unexplained mtDNA deletions, and one patient in the defining
    series had this phenotype herself before developing the adult one.
  evidence:
  - reference: PMID:32278775
    reference_title: "Acute liver failure due to DGUOK deficiency-is liver transplantation justified?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Deoxyguanosine kinase (DGUOK) deficiency is one of the causes of the hepatocerebral form of mitochondrial depletion syndrome (MDS)."
    explanation: The infantile entity and its mechanism, named as a DGUOK phenotype.
  - reference: PMID:32278775
    reference_title: "Acute liver failure due to DGUOK deficiency-is liver transplantation justified?"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It is characterized by an early onset of liver failure with concomitant neurological deterioration."
    explanation: >-
      The infantile presentation, which is what distinguishes it from this entity at the bedside.
  - reference: PMID:20301766
    reference_title: "Deoxyguanosine Kinase Deficiency."
    supports: SUPPORT
    quote_role: REVIEW_SYNTHESIS
    evidence_source: HUMAN_CLINICAL
    snippet: "The majority of affected individuals have the multisystem illness with hepatic disease (jaundice, cholestasis, hepatomegaly, and elevated transaminases) and neurologic manifestations (hypotonia, nystagmus, and developmental delay) evident within weeks of birth."
    explanation: >-
      The GeneReviews description of the infantile multisystem form, with its onset within weeks
      of birth.
- name: DGUOK-related noncirrhotic portal hypertension
  description: >-
    Noncirrhotic portal hypertension 1, OMIM 617068, the third DGUOK phenotype: portal
    hypertension of indeterminate cause beginning in infancy or childhood without cirrhosis. It is
    curated in dismech as a differential inside `Portal_Hypertension_Noncirrhotic_2`, the GIMAP5
    disease, rather than as its own entry. Listed here because a curator reasoning about the DGUOK
    allelic series needs to know all three phenotypes exist and where each is modelled.
  evidence:
  - reference: CGGV:assertion_dd4c3ef2-6738-4770-ad64-4db31672ca9a-2024-02-22T170000.000Z
    reference_title: "DGUOK / mitochondrial disease (Definitive)"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected individuals have a spectrum of clinical features ranging from infantile or childhood onset noncirrhotic portal hypertension, isolated hepatic failure (with at least one case that was apparently spontaneously reversible), hepatic failure with neurologic involvement that is often fatal, and adult-onset mitochondrial myopathy with chronic progressive ophthalmoplegia and multiple mitochondrial DNA (mtDNA) deletions in muscle."
    explanation: >-
      All three DGUOK phenotypes in one sentence, from an independent expert panel. Graded OTHER
      because a curation narrative is an expert synthesis rather than a study.
discussions:
- discussion_id: peob4_depletion_versus_deletion
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
  - pathophysiology#Imbalanced Mitochondrial dNTP Pool
  prompt: >-
    Why does the same DGUOK enzyme deficiency cause mtDNA depletion in infant liver and mtDNA
    deletions in adult muscle?
  rationale: >-
    This is the entity's central unexplained fact, and it is a question about DGUOK rather than
    about PEOB4 alone. Three candidate explanations are consistent with what is published and are
    not distinguished by it: allele severity, with null genotypes exhausting the pool fast enough
    to deplete and hypomorphic ones merely skewing it; tissue-specific salvage capacity, since
    liver and muscle differ in cytosolic and mitochondrial deoxyribonucleoside handling; and
    proliferative context, since a growing infant liver replicates mtDNA far faster than an adult
    muscle fibre and may fail by exhaustion where the fibre fails by infidelity over decades.

    The entry's description offers the third as a synthesis and explicitly marks it as such. It is
    recorded here rather than asserted because no source cited here tests it, and because the one
    patient who traversed both phenotypes shows the alleles alone cannot be the whole answer -
    the same genotype produced depletion in her liver and deletions in her muscle.
  proposed_experiments:
  - experiment_id: exp_peob4_tissue_dntp_pools
    name: Genotype-matched comparison of mitochondrial dNTP pools and mtDNA lesions across tissues
    description: >-
      In cells or tissue from DGUOK patients spanning the allelic series - ideally including the
      patient with both phenotypes - measure matrix dNTP pool composition, mtDNA copy number and
      large-scale deletion burden in hepatocyte-like and myotube models derived from the same
      individual, and ask whether the depletion-versus-deletion outcome tracks residual enzyme
      activity, cell type, or proliferation rate.
    would_support:
    - pathophysiology#Imbalanced Mitochondrial dNTP Pool
    supporting_outcome:
    - >-
      The same genotype gives depletion in the proliferating hepatocyte-like model and an
      accumulating deletion burden in the post-mitotic myotube model, with a comparably skewed
      dNTP pool in both.
    refuting_outcome:
    - >-
      Depletion and deletion segregate with residual enzyme activity rather than with cell type,
      which would make the two phenotypes an allele-severity effect and leave the tissue synthesis
      in this entry wrong.
    readouts:
    - name: Large-scale mtDNA deletion burden by cell type
      target: pathophysiology#Multiple Large-Scale mtDNA Deletions in Skeletal Muscle
      direction: INCREASED
      interpretation: >-
        A higher deletion burden in the post-mitotic model at matched genotype would support the
        proliferative-context explanation.
- discussion_id: peob4_versus_clingen_lumping
  kind: OPEN_QUESTION
  status: OPEN
  attaches_to:
  - disease#Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4
  prompt: >-
    Should dismech curate DGUOK as one disease entry, as ClinGen does, rather than as this
    presentation alone?
  rationale: >-
    ClinGen's 2024 curation states outright that the DGUOK phenotype has been lumped into one
    disease entity, on the grounds that the various names describe one primary mitochondrial
    disease. This entry does the opposite, and the disagreement is deliberate rather than
    accidental.

    The case for splitting: MONDO and OMIM both carry three separate identifiers, this entity has
    its own MONDO term that dismech's stub queue nominated, and dismech already curates the
    sibling PEOB1 (POLG) as a standalone entry - so a per-presentation entry is the local
    convention, and lumping DGUOK would make it the only mtDNA-maintenance gene curated
    gene-first. The case for lumping: one patient in the defining series had both phenotypes, the
    mechanism is identical, and a reader who wants to understand DGUOK has to assemble it from
    this entry, the separate infantile hepatocerebral entry, and a differential inside a GIMAP5
    disease entry.

    A `DGUOK-Related_Mitochondrial_Disease` entry with this presentation as a `has_subtypes` entry
    would satisfy ClinGen and would be a reasonable future refactor. GeneReviews, for its part,
    describes DGUOK deficiency as two infantile forms and leaves the adult myopathy out, so the two
    expert sources do not agree on a single gene-level entity either. The refactor is not attempted
    here because it would restructure the separately curated infantile entry and require curating
    the portal-hypertension phenotype, which this claim did not cover.
notes: >-
  Scope and shape. entry_type DISEASE, standalone. Two things drove that: dismech curates the
  sibling PEOB series per gene - `Autosomal_Recessive_Progressive_External_Ophthalmoplegia_1` is
  the POLG entity and was the template for this file - and there is no DGUOK entry in `kb/` for
  this to be a subtype of (the infantile hepatocerebral entry is a sibling presentation, not a
  gene-level parent). The stub is deleted because MONDO:0014899 is now bound by this entry's
  disease_term.

  The lump/split disagreement is recorded, not hidden. ClinGen explicitly lumps all of
  DGUOK-related disease into one entity, and that quote is curated as evidence in the `genetic`
  block with an explanation saying this entry takes the opposite position. An OPEN_QUESTION
  discussion sets out both cases and names the refactor - a `DGUOK-Related_Mitochondrial_Disease`
  entry with this as a `has_subtypes` - that would reconcile them. No mtDNA-depletion grouping was
  created; that concept was set aside as over-broad earlier in this queue, and nothing here needs
  it.

  Where the DGUOK allelic series is modelled in dismech today. This entry is OMIM 617070. OMIM
  251880, the infantile hepatocerebral depletion syndrome, is
  `Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type`, added after this entry was
  written. OMIM 617068, noncirrhotic portal hypertension 1, is curated as a
  differential inside `Portal_Hypertension_Noncirrhotic_2`, which is a GIMAP5 disease. The other
  DGUOK mentions in `kb/` - `Alpers-Huttenlocher_Syndrome` and
  `TRMU-Related_Acute_Infantile_Liver_Failure` - are differential-diagnosis prose about the
  infantile hepatic phenotype, not causal-gene assignments, and were checked before this entry was
  written.

  One reference is deliberately absent. PMID:29228108 (Caporali et al., Brain 2018) is the
  DGUOK-parkinsonism report and is the source the Orphanet-derived MONDO definition's mention of
  parkinsonism traces to. It is a Letter to the Editor and neither its PubMed record nor its DOI
  returned any retrievable text, so no exact quote could be taken from it and it is not cited. The
  Parkinsonism phenotype is therefore supported only by class-level evidence, and its `notes` say
  so and name the missing source.

  What the full text has that this entry does not. The falcon deep-research report committed
  alongside this entry passed preflight (DGUOK mentioned 76 times, dominating its gene counts) and
  read the defining paper's full text, which this entry could not. It reports content that is
  genuinely published and simply unquotable here: seven named DGUOK alleles in the adult cohort
  (c.186C>A p.Tyr62Ter, c.605_606delGA p.Arg202TyrfsTer12, c.130G>A p.Glu44Lys, c.137A>G
  p.Asn46Ser, c.462T>A p.Asn154Lys, c.509A>G p.Gln170Arg and the splice allele c.444-11C>G),
  ptosis, ragged-red and COX-negative fibres with severe COX deficiency on muscle histology, myopathic EMG,
  and creatine kinase ranging from mildly raised to around 2,000 U/L. A curator with journal access
  should add those as evidence items; they are named here rather than paraphrased into the entry
  because none of them can be verified against a cached reference. Note in particular that this
  changes the reading of two hedges above: the entry says muscle histology and enzymology are not
  reported "in the sources cited here", which is accurate, but they are not unpublished.

  The GeneReviews chapter for DGUOK deficiency (PMID:20301766) describes only the neonatal
  multisystem and isolated hepatic forms and not the adult myopathy, so it is cited for
  inheritance, diagnosis and the infantile differential rather than for this entity's phenotype.

  What is not here at all. The defining 2012 paper was retrievable as an abstract only, so
  per-subject ages, genotypes, muscle histology, respiratory-chain enzymology and deletion burdens
  are absent from this entry rather than estimated. No dataset accession is recorded, because no PEOB4
  patient omics deposit was identified. No prevalence rate exists - the 5.6 percent figure is a
  diagnostic yield in an enriched cohort and is recorded as such, with the `population` field
  naming that cohort rather than "Worldwide". The only animal model addresses the depletion arm of
  the phenotype rather than this one, which is stated in its `limitations` rather than left for a
  reader to notice.
📚

References & Deep Research

References

6
PMID:23043144
Next-generation sequencing reveals DGUOK mutations in adult patients with mitochondrial DNA multiple deletions.
No top-level findings curated for this source.
No top-level findings curated for this source.
PMID:30428046
Nucleoside supplementation modulates mitochondrial DNA copy number in the dguok -/- zebrafish.
No top-level findings curated for this source.
PMID:32278775
Acute liver failure due to DGUOK deficiency-is liver transplantation justified?
No top-level findings curated for this source.
PMID:35114397
Mitochondrial DNA homeostasis impairment and dopaminergic dysfunction: A trembling balance.
No top-level findings curated for this source.
PMID:20301766
Deoxyguanosine Kinase Deficiency.
No top-level findings curated for this source.

Deep Research

1

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

Evaluations and curation notes (1)

Record notes

Scope and shape. entry_type DISEASE, standalone. Two things drove that: dismech curates the sibling PEOB series per gene - `Autosomal_Recessive_Progressive_External_Ophthalmoplegia_1` is the POLG entity and was the template for this file - and there is no DGUOK entry in `kb/` for this to be a subtype of (the infantile hepatocerebral entry is a sibling presentation, not a gene-level parent). The stub is deleted because MONDO:0014899 is now bound by this entry's disease_term. The lump/split disagreement is recorded, not hidden. ClinGen explicitly lumps all of DGUOK-related disease into one entity, and that quote is curated as evidence in the `genetic` block with an explanation saying this entry takes the opposite position. An OPEN_QUESTION discussion sets out both cases and names the refactor - a `DGUOK-Related_Mitochondrial_Disease` entry with this as a `has_subtypes` - that would reconcile them. No mtDNA-depletion grouping was created; that concept was set aside as over-broad earlier in this queue, and nothing here needs it. Where the DGUOK allelic series is modelled in dismech today. This entry is OMIM 617070. OMIM 251880, the infantile hepatocerebral depletion syndrome, is `Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type`, added after this entry was written. OMIM 617068, noncirrhotic portal hypertension 1, is curated as a differential inside `Portal_Hypertension_Noncirrhotic_2`, which is a GIMAP5 disease. The other DGUOK mentions in `kb/` - `Alpers-Huttenlocher_Syndrome` and `TRMU-Related_Acute_Infantile_Liver_Failure` - are differential-diagnosis prose about the infantile hepatic phenotype, not causal-gene assignments, and were checked before this entry was written. One reference is deliberately absent. PMID:29228108 (Caporali et al., Brain 2018) is the DGUOK-parkinsonism report and is the source the Orphanet-derived MONDO definition's mention of parkinsonism traces to. It is a Letter to the Editor and neither its PubMed record nor its DOI returned any retrievable text, so no exact quote could be taken from it and it is not cited. The Parkinsonism phenotype is therefore supported only by class-level evidence, and its `notes` say so and name the missing source. What the full text has that this entry does not. The falcon deep-research report committed alongside this entry passed preflight (DGUOK mentioned 76 times, dominating its gene counts) and read the defining paper's full text, which this entry could not. It reports content that is genuinely published and simply unquotable here: seven named DGUOK alleles in the adult cohort (c.186C>A p.Tyr62Ter, c.605_606delGA p.Arg202TyrfsTer12, c.130G>A p.Glu44Lys, c.137A>G p.Asn46Ser, c.462T>A p.Asn154Lys, c.509A>G p.Gln170Arg and the splice allele c.444-11C>G), ptosis, ragged-red and COX-negative fibres with severe COX deficiency on muscle histology, myopathic EMG, and creatine kinase ranging from mildly raised to around 2,000 U/L. A curator with journal access should add those as evidence items; they are named here rather than paraphrased into the entry because none of them can be verified against a cached reference. Note in particular that this changes the reading of two hedges above: the entry says muscle histology and enzymology are not reported "in the sources cited here", which is accurate, but they are not unpublished. The GeneReviews chapter for DGUOK deficiency (PMID:20301766) describes only the neonatal multisystem and isolated hepatic forms and not the adult myopathy, so it is cited for inheritance, diagnosis and the infantile differential rather than for this entity's phenotype. What is not here at all. The defining 2012 paper was retrievable as an abstract only, so per-subject ages, genotypes, muscle histology, respiratory-chain enzymology and deletion burdens are absent from this entry rather than estimated. No dataset accession is recorded, because no PEOB4 patient omics deposit was identified. No prevalence rate exists - the 5.6 percent figure is a diagnostic yield in an enriched cohort and is recorded as such, with the `population` field naming that cohort rather than "Worldwide". The only animal model addresses the depletion arm of the phenotype rather than this one, which is stated in its `limitations` rather than left for a reader to notice.

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Disease Characteristics Research Template
Edison Scientific Literature 12 citations 2026-09-09T11:34:38.041973

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Disease Characteristics Research Template

Target Disease

  • Disease Name: Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4
  • MONDO ID: MONDO:0014899 (if available)
  • Category: Mendelian

Research Objectives

Please provide a comprehensive research report on Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4 covering all of the disease characteristics listed below. This report will be used to populate a disease knowledge base entry. Be thorough and cite primary literature (PMID preferred) for all claims.

For each section, suggested databases/resources are listed. These are the first places you should search for information on each topic.

1. Disease Information

Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed

  • What is the disease? Provide a concise overview.
  • What are the key identifiers? (OMIM, Orphanet, ICD-10/ICD-11, MeSH, Mondo)
  • What are the common synonyms and alternative names?
  • Is the information derived from individual patients (e.g., EHR) or aggregated disease-level resources?

2. Etiology

  • Disease Causal Factors: What are the primary causes? (genetic, environmental, infectious, mechanistic)
  • Risk Factors:

    Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases

  • Genetic risk factors (causal variants, susceptibility loci, modifier genes)
  • Environmental risk factors (toxins, lifestyle, occupational exposures, age, sex, family history)
  • Protective Factors:

    Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases

  • Genetic protective factors (protective variants, modifier alleles)
  • Environmental protective factors (diet, lifestyle, exposures that reduce risk)
  • Gene-Environment Interactions: How do genetic and environmental factors interact to influence disease?

    Search first: CTD, PubMed, PheGenI, GxE databases

3. Phenotypes

Search first: HPO (Human Phenotype Ontology), OMIM, Orphanet, PubMed, clinicaltrials.gov, MedDRA, SNOMED CT, DECIPHER, LOINC

For each phenotype, provide: - Phenotype type: symptoms, clinical signs, physical manifestations, behavioral changes, or laboratory abnormalities

For symptoms/signs: HPO, OMIM, Orphanet, PubMed For behavioral changes: HPO, DSM, RDoC (Research Domain Criteria), PubMed For laboratory abnormalities: LOINC, SNOMED CT, LabTests Online, PubMed - Phenotype characteristics: Search first: OMIM, Orphanet, HPO, PubMed - Age of symptom onset (neonatal, childhood, adult-onset, late-onset) - Symptom severity (mild, moderate, severe, variable) - Symptom progression (stable, progressive, episodic, fluctuating) - Frequency among affected individuals (percentage or qualitative) - Quality of life impact: Effects on daily functioning and well-being (per-phenotype when possible) Search first: EQ-5D database, SF-36, WHO QOL databases, PubMed - Suggest HPO (Human Phenotype Ontology) terms for each phenotype

4. Genetic/Molecular Information

  • Causal Genes: Gene mutations or chromosomal abnormalities responsible for disease (gene symbols, OMIM IDs)

    Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene

  • Pathogenic Variants:
  • Affected genes (gene symbols, HGNC IDs) > Search first: OMIM, NCBI Gene, Ensembl, HGNC, UniProt, GeneCards
  • Variant classification (pathogenic, likely pathogenic, VUS per ACMG/AMP guidelines) > Search first: ClinVar, ClinGen, ACMG/AMP guidelines, VarSome
  • Variant type/class (missense, frameshift, nonsense, splice-site, structural)
  • Allele frequency in population databases > Search first: gnomAD, 1000 Genomes, ExAC, TOPMed, dbSNP
  • Somatic vs germline origin > Search first: COSMIC (somatic), ClinVar, ICGC, TCGA
  • Functional consequences (loss of function, gain of function, dominant negative)
  • Modifier Genes: Genes that modify disease severity or expression
  • Epigenetic Information: DNA methylation, histone modifications, chromatin changes affecting disease

    Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth

  • Chromosomal Abnormalities: Large-scale genetic changes (aneuploidy, translocations, inversions)

    Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser

5. Environmental Information

  • Environmental Factors: Non-genetic contributing factors (toxins, radiation, pollution, occupational exposure)

    Search first: CTD (Comparative Toxicogenomics Database), TOXNET, PubMed, EPA databases

  • Lifestyle Factors: Behavioral factors (smoking, diet, exercise, alcohol consumption)

    Search first: CDC databases, WHO, PubMed, NHANES

  • Infectious Agents: If applicable, pathogens causing or triggering disease (bacteria, viruses, fungi, parasites)

    Search first: NCBI Taxonomy, ViPR, BV-BRC, MicrobeDB, GIDEON

6. Mechanism / Pathophysiology

Present this section as an ordered causal chain first, then the detail below. Open with a numbered sequence of mechanistic steps running from the initiating lesion (mutation, exposure, infection) to the clinical manifestation, one step per line, each naming what it causes next. State the causal verb explicitly ("leads to", "results in") and say where a step is inferred rather than demonstrated. Where the mechanism branches, show the branch. The categories below are a checklist of what to cover within those steps, not the organizing structure — a step may draw on several of them, and a category may contribute to several steps.

  • Molecular Pathways: Specific signaling cascades or biochemical pathways involved (Wnt, MAPK, mTOR, PI3K-AKT, etc.)

    Search first: KEGG, Reactome, WikiPathways, PathBank, BioCyc

  • Cellular Processes: Cell-level mechanisms (apoptosis, autophagy, cell cycle dysregulation, inflammation, etc.)

    Search first: Gene Ontology (GO), Reactome, KEGG, PubMed

  • Protein Dysfunction: How protein structure or function is altered (misfolding, aggregation, loss of function, gain of function)

    Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold

  • Metabolic Changes: Alterations in metabolic processes (energy metabolism, lipid metabolism, amino acid metabolism)

    Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA

  • Immune System Involvement: Role of immune response (autoimmunity, immunodeficiency, chronic inflammation)

    Search first: ImmPort, Immunome Database, IEDB, Gene Ontology

  • Tissue Damage Mechanisms: How tissues/ are injured (oxidative stress, ischemia, fibrosis, necrosis)

    Search first: PubMed, Gene Ontology, Reactome

  • Biochemical Abnormalities: Specific molecular defects (enzyme deficiencies, receptor dysfunction, ion channel defects)

    Search first: BRENDA, UniProt, KEGG, OMIM, PubMed

  • Epigenetic Changes: DNA methylation, histone modifications affecting gene expression in disease

    Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth

  • Molecular Profiling (if available):
  • Transcriptomics/gene expression changes > Search first: GEO (Gene Expression Omnibus), ArrayExpress, GTEx, Human Cell Atlas, SRA
  • Proteomics findings > Search first: PRIDE, ProteomeXchange, Human Protein Atlas, STRING, BioGRID
  • Metabolomics signatures > Search first: MetaboLights, Metabolomics Workbench, HMDB, METLIN
  • Lipidomics alterations > Search first: LIPID MAPS, SwissLipids, LipidHome, Metabolomics Workbench
  • Genomic structural features > Search first: UCSC Genome Browser, Ensembl, NCBI, dbVar, DGV
  • Advanced Technologies (if applicable):
  • Single-cell analysis findings (cell-type specific mechanisms, cellular heterogeneity) > Search first: Human Cell Atlas, Single Cell Portal, GEO, CELLxGENE
  • Spatial transcriptomics findings > Search first: GEO, Spatial Research, Vizgen, 10x Genomics data
  • Multi-omics integration results > Search first: TCGA, ICGC, cBioPortal, LinkedOmics, PubMed
  • Functional genomics screens (CRISPR, RNAi) > Search first: DepMap, GenomeRNAi, PubMed, BioGRID ORCS

For each mechanism, describe: - The causal chain from initial trigger to clinical manifestation - Which mechanisms are upstream vs downstream - What cell types and biological processes are involved - Suggest GO terms for biological processes and CL terms for cell types

7. Anatomical Structures Affected

  • Organ Level:
  • Primary organs directly affected
  • Secondary organ involvement (complications, secondary effects)
  • Body systems involved (cardiovascular, nervous, digestive, respiratory, endocrine, etc.)

    Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT

  • Tissue and Cell Level:
  • Specific tissue types affected (epithelial, connective, muscle, nervous)
  • Specific cell populations targeted (with Cell Ontology terms)

    Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB

  • Subcellular Level:
  • Cellular compartments involved (mitochondria, nucleus, ER, lysosomes) (with GO Cellular Component terms)

    Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas

  • Localization:
  • Specific anatomical sites (with UBERON terms) > Search first: FMA, Uberon, NeuroNames (for brain), SNOMED CT
  • Lateralization (unilateral, bilateral, asymmetric) > Search first: HPO, clinical literature, imaging databases

8. Temporal Development

  • Onset:
  • Typical age of onset (congenital, pediatric, adult, geriatric)
  • Onset pattern (acute, subacute, chronic, insidious)

    Search first: OMIM, Orphanet, HPO, PubMed

  • Progression:
  • Disease stages (early, intermediate, advanced, end-stage) > Search first: Cancer Staging Manual (AJCC), WHO classifications, PubMed
  • Progression rate (rapid, slow, variable)
  • Disease course pattern (episodic, relapsing-remitting, progressive, stable)
  • Disease duration (self-limited, chronic lifelong)

    Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM

  • Patterns:
  • Remission patterns (spontaneous, treatment-induced) > Search first: Clinical trial databases, disease registries, PubMed
  • Critical periods (time windows of vulnerability or opportunity for intervention) > Search first: PubMed, developmental biology databases, clinical guidelines

9. Inheritance and Population

  • Epidemiology:
  • Prevalence (cases per 100,000 at given time)
  • Incidence (new cases per 100,000 per year)

    Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries

  • For Genetic Etiology:
  • Inheritance pattern (AD, AR, X-linked, mitochondrial, multifactorial, polygenic) > Search first: OMIM, Orphanet, ClinVar, GTR (Genetic Testing Registry)
  • Penetrance (complete, incomplete, age-dependent) > Search first: ClinVar, OMIM, PubMed, ClinGen
  • Expressivity (variable, consistent) > Search first: OMIM, ClinVar, PubMed
  • Genetic anticipation (increasing severity in successive generations) > Search first: OMIM, PubMed (especially for repeat expansion disorders)
  • Germline mosaicism > Search first: ClinVar, OMIM, genetic counseling literature, PubMed
  • Founder effects (population-specific mutations) > Search first: gnomAD, population genetics databases, PubMed
  • Consanguinity role > Search first: OMIM, population studies, genetic counseling resources
  • Carrier frequency > Search first: gnomAD, carrier screening databases, GeneReviews, GTR
  • Population Demographics:
  • Affected populations (ethnic or demographic groups with higher prevalence) > Search first: gnomAD, 1000 Genomes, PAGE Study, PubMed, population registries
  • Geographic distribution (endemic areas, regional variation) > Search first: WHO, CDC, GBD, Orphanet, geographic epidemiology databases
  • Geographic distribution of specific variants
  • Sex ratio (male:female) > Search first: Disease registries, OMIM, PubMed, epidemiological databases
  • Age distribution of affected individuals > Search first: CDC, disease registries, SEER, Orphanet

10. Diagnostics

  • Clinical Tests:
  • Laboratory tests (blood, urine, tissue chemistry, specific enzyme assays) > Search first: LOINC, LabTests Online, PubMed
  • Biomarkers (proteins, metabolites, genetic markers, circulating biomarkers) > Search first: FDA Biomarker List, BEST (Biomarkers, EndpointS, and other Tools), PubMed
  • Imaging studies (X-ray, CT, MRI, PET, ultrasound) > Search first: RadLex, DICOM, Radiopaedia, imaging databases
  • Functional tests (pulmonary function, cardiac stress tests) > Search first: LOINC, clinical guidelines, PubMed
  • Electrophysiology (EEG, EMG, ECG, nerve conduction studies) > Search first: LOINC, clinical neurophysiology databases, PubMed
  • Biopsy findings (histopathology, immunohistochemistry) > Search first: SNOMED CT, College of American Pathologists resources, PubMed
  • Pathology findings (microscopic examination) > Search first: SNOMED CT, Digital Pathology databases, PubMed
  • Genetic Testing:

    Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen

  • Overview of recommended genetic testing approach
  • Whole genome sequencing (WGS) utility > Search first: GTR, ClinVar, GEL (Genomics England), gnomAD
  • Whole exome sequencing (WES) utility > Search first: GTR, ClinVar, OMIM, GeneMatcher
  • Gene panels (which panels, which genes) > Search first: GTR, ClinVar, laboratory-specific databases
  • Single gene testing > Search first: GTR, ClinVar, OMIM, GeneReviews
  • Chromosomal microarray (CMA) > Search first: DECIPHER, ClinVar, dbVar, ECARUCA
  • Karyotyping > Search first: Chromosome Abnormality Database, ClinVar, cytogenetics resources
  • FISH > Search first: ClinVar, cytogenetics databases, PubMed
  • Mitochondrial DNA testing > Search first: MITOMAP, MSeqDR, ClinVar, GTR
  • Repeat expansion testing > Search first: GTR, ClinVar, repeat expansion databases, PubMed
  • Omics-Based Diagnostics (if applicable):
  • RNA sequencing / transcriptomics > Search first: GEO, ArrayExpress, GTEx, RNA-seq databases
  • Proteomics > Search first: PRIDE, ProteomeXchange, FDA Biomarker database
  • Metabolomics > Search first: MetaboLights, Metabolomics Workbench, HMDB
  • Epigenomics > Search first: GEO, ENCODE, Roadmap Epigenomics, MethBase
  • Liquid biopsy > Search first: COSMIC, ClinVar, liquid biopsy databases, PubMed
  • Clinical Criteria:
  • Standardized diagnostic criteria (DSM, ICD, society guidelines) > Search first: DSM-5, ICD-11, clinical society guidelines, UpToDate
  • Differential diagnosis (other conditions to rule out, with distinguishing features) > Search first: DynaMed, UpToDate, clinical decision support systems
  • Screening:
  • Screening methods for asymptomatic individuals (newborn screening, carrier screening, cascade screening) > Search first: ACMG recommendations, CDC newborn screening, GTR

11. Outcome/Prognosis

  • Survival and Mortality:
  • Survival rate (5-year, 10-year, overall) > Search first: SEER, cancer registries, disease-specific registries, PubMed
  • Life expectancy (with and without treatment if applicable) > Search first: Orphanet, disease registries, actuarial databases, PubMed
  • Mortality rate > Search first: CDC, WHO, GBD, national mortality databases
  • Disease-specific mortality (deaths directly attributable to disease) > Search first: Disease registries, CDC Wonder, GBD, PubMed
  • Morbidity and Function:
  • Morbidity (disease-related disability and health impacts) > Search first: GBD, WHO, disability databases, PubMed
  • Disability outcomes (long-term functional impairments) > Search first: ICF (International Classification of Functioning), disability registries
  • Quality of life measures (EQ-5D, SF-36, PROMIS, disease-specific tools) > Search first: EQ-5D database, SF-36, PROMIS, PubMed
  • Disease Course:
  • Complications (secondary problems: infections, organ failure, etc.) > Search first: ICD codes, disease registries, clinical databases, PubMed
  • Recovery potential (likelihood and extent of recovery, with vs without treatment) > Search first: Natural history studies, rehabilitation databases, PubMed
  • Prediction:
  • Prognostic factors (age, disease severity, biomarkers, treatment response) > Search first: Prognostic models databases, clinical calculators, PubMed
  • Prognostic biomarkers (molecular markers predicting disease course) > Search first: FDA Biomarker database, PubMed, cancer prognostic databases

12. Treatment

  • Pharmacotherapy:
  • Pharmacological treatments (drug names, drug classes, mechanisms of action) > Search first: DrugBank, RxNorm, ATC classification, DailyMed, FDA databases
  • Pharmacogenomics (how genetic variants affect drug metabolism, efficacy, toxicity) > Search first: PharmGKB, CPIC (Clinical Pharmacogenetics), FDA Table of PGx Biomarkers
  • Advanced Therapeutics:
  • Gene therapy (viral vectors, CRISPR, gene replacement, gene editing) > Search first: ClinicalTrials.gov, FDA gene therapy database, ASGCT resources
  • Cell therapy (stem cell transplant, CAR-T, cellular therapeutics) > Search first: ClinicalTrials.gov, FDA cell therapy database, FACT standards
  • RNA-based therapies (ASOs, siRNA, mRNA therapies) > Search first: ClinicalTrials.gov, FDA approvals, PubMed
  • Targeted therapies (treatments directed at specific molecular targets) > Search first: My Cancer Genome, OncoKB, ClinicalTrials.gov, FDA approvals
  • Immunotherapies (checkpoint inhibitors, monoclonal antibodies) > Search first: Cancer Immunotherapy Database, FDA approvals, ClinicalTrials.gov
  • Surgical and Interventional:
  • Surgical interventions (types of surgery, timing, outcomes) > Search first: CPT codes, surgical registries, clinical guidelines, PubMed
  • Supportive and Rehabilitative:
  • Supportive care (symptom management, pain control, nutrition) > Search first: Clinical guidelines, Cochrane Library, PubMed
  • Rehabilitation (physical therapy, occupational therapy, speech therapy) > Search first: Rehabilitation medicine databases, clinical guidelines, PubMed
  • Experimental:
  • Experimental treatments in clinical trials (with NCT identifiers if available) > Search first: ClinicalTrials.gov, EU Clinical Trials Register, WHO ICTRP
  • Treatment Outcomes:
  • Treatment response rates > Search first: Clinical trial databases, FDA reviews, systematic reviews, PubMed
  • Side effects and adverse events > Search first: FDA Adverse Event Reporting System (FAERS), MedWatch, PubMed
  • Treatment Strategy:
  • Treatment algorithms (clinical pathways, decision trees) > Search first: Clinical practice guidelines, NCCN Guidelines, UpToDate
  • Combination therapies > Search first: ClinicalTrials.gov, treatment guidelines, PubMed
  • Personalized medicine approaches (genotype-guided treatment) > Search first: My Cancer Genome, CIViC, PharmGKB, precision medicine databases

For each treatment, suggest NCIT (NCI Thesaurus) clinical-intervention terms where applicable.

13. Prevention

  • Prevention Levels:
  • Primary prevention (preventing disease occurrence: vaccination, risk factor modification) > Search first: CDC, WHO, USPSTF recommendations, Cochrane Library
  • Secondary prevention (early detection and treatment: screening programs, early intervention) > Search first: USPSTF, CDC screening guidelines, WHO
  • Tertiary prevention (preventing complications in those with disease) > Search first: Clinical guidelines, disease management protocols, PubMed
  • Immunization: Vaccine strategies (if applicable)

    Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database

  • Screening and Early Detection:
  • Screening programs (population-based: newborn screening, cancer screening) > Search first: CDC screening programs, USPSTF, cancer screening databases
  • Genetic screening (carrier screening, preimplantation genetic diagnosis, prenatal testing) > Search first: ACMG recommendations, ACOG guidelines, GTR
  • Risk stratification (identifying high-risk individuals for targeted prevention) > Search first: Risk prediction models, clinical calculators, PubMed
  • Behavioral Interventions: Lifestyle modifications to reduce risk

    Search first: CDC, WHO, behavioral intervention databases, Cochrane Library

  • Counseling: Genetic counseling (risk assessment, family planning guidance)

    Search first: NSGC resources, ACMG guidelines, GeneReviews

  • Public Health:
  • Public health interventions (sanitation, vector control, health education) > Search first: CDC, WHO, public health databases, PubMed
  • Environmental interventions (reducing environmental risk factors) > Search first: EPA databases, WHO environmental health, PubMed
  • Prophylaxis: Preventive medications or procedures

    Search first: Clinical guidelines, FDA approvals, PubMed

14. Other Species / Natural Disease

  • Taxonomy: Species affected (with NCBI Taxon identifiers)

    Search first: NCBI Taxonomy

  • Breed: Specific breeds affected (with VBO identifiers if applicable)

    Search first: VBO (Vertebrate Breed Ontology)

  • Gene: Orthologous genes in other species (with NCBI Gene IDs)

    Search first: NCBI Gene

  • Natural Disease:
  • Naturally occurring disease in other species (companion animals, wildlife) > Search first: OMIA (Online Mendelian Inheritance in Animals), VetCompass, PubMed
  • Veterinary relevance and importance in animal health > Search first: OMIA, veterinary databases, PubMed
  • Comparative Biology:
  • Comparative pathology (similarities and differences across species) > Search first: OMIA, comparative pathology databases, PubMed
  • Evolutionary conservation of disease mechanisms > Search first: HomoloGene, OrthoMCL, Alliance of Genome Resources
  • Transmission (if applicable):
  • Zoonotic potential > Search first: CDC zoonotic diseases, WHO zoonoses, GIDEON
  • Cross-species susceptibility > Search first: NCBI Taxonomy, veterinary databases, PubMed

15. Model Organisms

  • Model Types:
  • Model organism type (mammalian, invertebrate, cellular, in vitro) > Search first: Alliance of Genome Resources, model organism databases
  • Specific model systems (mouse, rat, zebrafish, Drosophila, C. elegans, yeast, cell lines, organoids, iPSCs) > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, SGD, ATCC, Cellosaurus
  • Induced models (drug treatment, surgical intervention, environmental manipulation) > Search first: MGI, model organism databases, PubMed
  • Genetic Models:
  • Types available (knockout, knock-in, transgenic, conditional, humanized) > Search first: MGI, IMPC, KOMP, EuMMCR, IMSR
  • Model Characteristics:
  • Phenotype recapitulation (how well model reproduces human disease features) > Search first: Model organism databases, comparative studies, PubMed
  • Model limitations (aspects of human disease not captured) > Search first: Model organism databases, PubMed, review articles
  • Applications:
  • Research applications (what aspects of disease can be studied) > Search first: Model organism databases, PubMed
  • Resources:
  • Model databases > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, IMSR, EMMA, MMRRC

Citation Requirements

  • Cite primary literature (PMID preferred) for all mechanistic and clinical claims
  • Prioritize recent reviews and landmark papers
  • Include direct quotes from abstracts where possible to support key statements
  • Distinguish evidence source types: human clinical, model organism, in vitro, computational

Output Format

Structure your response as a comprehensive narrative organized by the sections above. For each section, provide: - Factual content with specific details (numbers, percentages, gene names, variant nomenclature) - Ontology term suggestions (HPO, GO, CL, UBERON, CHEBI, NCIT, MONDO) where applicable - Evidence citations with PMIDs - Direct quotes from abstracts to support key claims - Clear indication when information is not available or not applicable for this disease

This report will be used to populate a disease knowledge base entry with: - Pathophysiology descriptions with causal chains - Gene/protein annotations (HGNC, GO terms) - Phenotype associations (HP terms) with frequencies - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Treatment annotations (NCIT terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details

Progressive External Ophthalmoplegia with Mitochondrial DNA Deletions, Autosomal Recessive 4

Executive summary and evidence boundaries

Progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 4 (PEO4) is an exceptionally rare, nuclear-encoded mitochondrial DNA-maintenance disorder caused by biallelic germline variants in DGUOK, which encodes mitochondrial deoxyguanosine kinase. The best direct evidence is a small human clinical-molecular series published by Ronchi et al. in Brain on 4 October 2012 (print November 2012; PMID 23043144; DOI/URL: https://doi.org/10.1093/brain/aws258). Five independent DGUOK-positive subjects constituted 5.6% of a selected referral cohort with adult mitochondrial myopathy and multiple skeletal-muscle mtDNA deletions; this is not a population-prevalence estimate. Presentations ranged from late-onset ptosis/PEO and mitochondrial myopathy to rhabdomyolysis or lower-motor-neuron disease. Patient muscle showed reduced DGUOK protein or abnormal splicing, residual enzyme activity of approximately 19–45%, and multiple mtDNA deletions. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8)

The phenotype must be distinguished from the much better documented DGUOK-related hepatocerebral mtDNA-depletion syndrome, often presenting in infancy with liver failure. Those conditions form an allelic spectrum, but infantile hepatocerebral findings, transplant outcomes, and mortality statistics should not automatically be assigned to adult PEO4. More than 100 people with all forms of DGUOK deficiency have been reported, but no population study has estimated PEO4 prevalence or incidence. (elhattab1993deoxyguanosinekinasedeficiency pages 3-6, elhattab1993deoxyguanosinekinasedeficiency pages 1-3, ronchi2012nextgenerationsequencingreveals pages 8-9, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)

Domain High-confidence finding Suggested ontology identifiers/terms Evidence scope / caveat
Identity / gene Progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 4 is a DGUOK-related mitochondrial DNA-maintenance disorder characterized by adult mitochondrial myopathy, variably including progressive external ophthalmoplegia and multiple skeletal-muscle mtDNA deletions. MONDO:0014899; DGUOK; deoxyguanosine kinase; PEO4; autosomal-recessive progressive external ophthalmoplegia Direct disease-level mapping and human molecular evidence. DGUOK also causes severe hepatocerebral mtDNA-depletion disease, which is broader than PEO4. (OpenTargets Search: Progressive external ophthalmoplegia with mitochondrial DNA deletions autosomal recessive 4, ronchi2012nextgenerationsequencingreveals pages 1-2)
Inheritance Biallelic germline DGUOK variants cause disease through autosomal-recessive inheritance; carrier parents are generally asymptomatic, and recurrence risk is 25% for each pregnancy when both parents are carriers. Autosomal recessive inheritance; germline variant; genetic carrier Direct for DGUOK-associated disease; recurrence-risk statement follows Mendelian inheritance. Penetrance among individuals with two definitively pathogenic alleles is not quantified. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 12-15)
Core ocular phenotypes Principal ocular findings are slowly progressive external ophthalmoplegia or ophthalmoparesis and usually bilateral ptosis; strabismus can occur. HPO labels: Progressive external ophthalmoplegia; Ptosis; Strabismus Direct patient-level evidence, but the published DGUOK cohort was very small and some affected individuals lacked ophthalmoplegia. Reliable percentages cannot be assigned. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3, ronchi2012nextgenerationsequencingreveals pages 8-9)
Other muscle phenotypes The spectrum includes mitochondrial myopathy, limb-girdle or distal weakness, neck weakness, exercise-related pain or intolerance, cramps, dysphagia, dysphonia, and occasional rhabdomyolysis. HPO labels: Mitochondrial myopathy; Muscle weakness; Proximal muscle weakness; Distal muscle weakness; Exercise intolerance; Muscle cramps; Dysphagia; Dysphonia; Rhabdomyolysis Direct adult DGUOK case-series evidence; manifestations vary substantially and may represent PEO-plus or non-PEO DGUOK phenotypes. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 3-4)
Neurologic / systemic phenotypes Rare reported presentations include adult lower-motor-neuron disease with mild cognitive impairment; diabetes and cataract occurred in an individual patient. Childhood liver disease may precede later myopathy in broader DGUOK deficiency. HPO labels: Lower motor neuron dysfunction; Mild cognitive impairment; Diabetes mellitus; Cataract; Hepatic dysfunction Patient-level associations do not establish typical PEO4 frequencies or direct causality for every feature. Infantile liver disease belongs to the broader DGUOK spectrum. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 9-10)
Laboratory / pathology Muscle may show elevated creatine kinase, myopathic EMG, ragged-red fibers, cytochrome-c-oxidase-negative fibers, severe COX deficiency, and multiple mtDNA deletions. Lactate may be normal or moderately increased. HPO labels: Elevated circulating creatine kinase; Ragged-red muscle fibers; Cytochrome-c oxidase deficiency; Abnormality of mitochondrial metabolism; mitochondrial DNA deletion Direct patient-level clinical, histologic, and molecular evidence. Normal lactate does not exclude disease. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
Molecular mechanism Mitochondrial DGUOK phosphorylates deoxyguanosine and deoxyadenosine to dGMP and dAMP in the purine-nucleoside salvage pathway. Reduced activity disrupts mitochondrial dNTP supply, impairs mtDNA maintenance, and produces depletion and/or multiple deletions followed by respiratory-chain dysfunction. GO labels: Deoxyguanosine kinase activity; Deoxyadenosine kinase activity; Purine deoxyribonucleoside salvage; Mitochondrial DNA replication; Mitochondrial genome maintenance; Oxidative phosphorylation The enzyme defect and mtDNA instability are supported directly; the detailed sequence from nucleotide imbalance to selective extraocular-muscle degeneration is partly inferred. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8, elhattab1993deoxyguanosinekinasedeficiency pages 12-15)
Variants / functional evidence Reported variants include c.186C>A (p.Tyr62Ter), c.605_606delGA (p.Arg202TyrfsTer12), c.130G>A (p.Glu44Lys), c.137A>G (p.Asn46Ser), c.462T>A (p.Asn154Lys), c.509A>G (p.Gln170Arg), and c.444-11C>G. Patient muscle showed reduced protein or abnormal splicing and residual DGUOK activity of approximately 19%–45% of control. DGUOK sequence variant; missense variant; nonsense variant; frameshift variant; splice-region variant; loss of function Direct functional human-muscle evidence. p.Gln170Arg alone has uncertain significance because it occurred in controls at a reported allele frequency of 1.98%; variants require phase, population, segregation, and ACMG/AMP reassessment rather than blanket pathogenic classification. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8, ronchi2012nextgenerationsequencingreveals pages 8-9)
Anatomy / cells / compartments Extraocular and skeletal muscles are primary affected tissues; myofibers contain dysfunctional mitochondria and accumulated mtDNA abnormalities. Bulbar and lower-motor-neuron involvement may occur in broader presentations. DGUOK localizes to the mitochondrial matrix. UBERON labels: Extraocular muscle; Skeletal muscle tissue. CL labels: Skeletal muscle fiber; Extraocular muscle cell; Lower motor neuron. GO labels: Mitochondrial matrix; Mitochondrion; Mitochondrial nucleoid Extraocular and skeletal muscle involvement and mitochondrial localization are supported; cell-specific clonal expansion and selective vulnerability are incompletely demonstrated for DGUOK-PEO4. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 7-8, ronchi2012nextgenerationsequencingreveals pages 8-9)
Diagnosis Diagnosis integrates ptosis or ophthalmoparesis, CK and lactate testing, EMG, muscle histology, mtDNA deletion/depletion analysis in affected tissue, and demonstration of biallelic pathogenic or likely pathogenic DGUOK variants. Sequencing detects most DGUOK pathogenic variants; deletion/duplication analysis is considered if sequencing is incomplete. Genetic testing; mitochondrial DNA deletion analysis; mitochondrial DNA copy-number analysis; muscle biopsy; electromyography; DGUOK sequencing; multigene panel; exome sequencing; genome sequencing Direct PEO4 evidence supports muscle mtDNA analysis plus DGUOK testing; estimated yields of approximately 95% for sequence analysis and 5% for deletion/duplication analysis derive from broader DGUOK deficiency. A VUS does not establish or exclude diagnosis. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 3-6, elhattab1993deoxyguanosinekinasedeficiency pages 1-3)
Treatment No curative or DGUOK-PEO4-specific approved therapy is established. Management is individualized and supportive: ptosis and ocular-motility management, physical and occupational therapy, swallowing and nutritional assessment, and surveillance for respiratory, cardiac, neurologic, endocrine, and hepatic complications when indicated. NCIT labels: Supportive care; Physical therapy; Occupational therapy; Ptosis repair; Nutritional support; Genetic counseling Symptomatic management is extrapolated largely from mitochondrial CPEO practice rather than controlled DGUOK-PEO4 trials. Liver transplantation pertains to selected liver-predominant DGUOK deficiency and is not treatment for isolated adult PEO4; later muscle disease can still emerge. (ronchi2012nextgenerationsequencingreveals pages 9-10, elhattab1993deoxyguanosinekinasedeficiency pages 1-3)
Epidemiology Population prevalence, incidence, carrier frequency, sex ratio, penetrance, and survival for PEO4 are unknown. Biallelic DGUOK variants accounted for 5.6% of one selected adult cohort with multiple skeletal-muscle mtDNA deletions. More than 100 individuals with all forms of DGUOK deficiency have been reported. Rare disease; orphan disease The 5.6% figure is a referral-cohort proportion, not population prevalence. Broader estimates that DGUOK causes 15%–20% of mtDNA-depletion syndromes should not be applied to adult PEO4. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)
Evidence gaps No robust PEO4-specific natural-history cohort, validated severity scale, prevalence study, genotype–phenotype model, protective allele, environmental-risk association, epigenetic signature, single-cell or spatial dataset, faithful disease-specific animal model, prognostic biomarker, randomized treatment trial, or DGUOK-specific interventional trial was identified. Evidence gap; natural history study; biomarker study; clinical trial; single-cell transcriptomics; disease model Negative finding from the retrieved literature and trial searches, not proof that no unpublished or newly registered evidence exists. Most knowledge rests on a small number of human cases and broader mitochondrial-disease extrapolation. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 9-10, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)

Table: High-confidence disease, phenotype, mechanism, diagnostic, and management annotations for MONDO:0014899. The table distinguishes direct PEO4 evidence from broader DGUOK-deficiency evidence and highlights major knowledge gaps.

1. Disease information

Definition

PEO4 is a Mendelian mitochondrial myopathy in which recessive nuclear DGUOK dysfunction causes secondary instability of the mitochondrial genome, especially multiple mtDNA deletions in skeletal muscle. The defining clinical ocular manifestations are slowly progressive, generally bilateral ptosis and external ophthalmoparesis; however, DGUOK disease can produce mitochondrial myopathy without ophthalmoplegia and broader “PEO-plus” phenotypes. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 2-3, ronchi2012nextgenerationsequencingreveals pages 8-9)

Identifiers and names

  • MONDO: MONDO:0014899.
  • Causal target: DGUOK, Ensembl ENSG00000114956, approved name deoxyguanosine kinase. Open Targets maps MONDO:0014899 to DGUOK using five association-evidence records, including PMID 23043144. (OpenTargets Search: Progressive external ophthalmoplegia with mitochondrial DNA deletions autosomal recessive 4)
  • Common names: PEO4; autosomal-recessive progressive external ophthalmoplegia 4; progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 4; DGUOK-related progressive external ophthalmoplegia; DGUOK-related mitochondrial myopathy with multiple mtDNA deletions.
  • OMIM: commonly represented as the DGUOK-related recessive PEO entry; the retrieved evidence did not independently verify the exact phenotype MIM number, so it should be checked directly against the live OMIM record before database ingestion.
  • Orphanet: no PEO4-specific Orphanet identifier was verified from the retrieved evidence; Orphanet may aggregate it under genetic PEO or mtDNA-maintenance disorders.
  • ICD-10/ICD-11 and MeSH: no uniquely specific code exists in the retrieved evidence. Coding generally falls under mitochondrial metabolism/myopathy or ophthalmoplegia categories and loses the DGUOK/genotype distinction.

This report synthesizes aggregated disease resources and published patient-level research, not EHR-derived individual data. The Ronchi study contains identifiable clinical profiles but is a research cohort, not a real-world EHR extraction. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 3-4)

2. Etiology

Causal and genetic factors

The necessary cause is usually biallelic loss-of-function or function-reducing DGUOK variants. Reported classes include missense, nonsense, frameshift, and splice-altering variants. DGUOK is a mitochondrial purine-salvage enzyme; impaired activity produces inadequate or imbalanced mitochondrial deoxyribonucleotide pools and defective mtDNA maintenance. (ronchi2012nextgenerationsequencingreveals pages 7-8, elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

Variants reported in the adult multiple-deletion cohort included c.186C>A (p.Tyr62Ter), c.605_606delGA (p.Arg202TyrfsTer12), c.130G>A (p.Glu44Lys), c.137A>G (p.Asn46Ser), c.462T>A (p.Asn154Lys), c.509A>G (p.Gln170Arg), and c.444-11C>G. The c.444-11C>G allele disrupted splicing and its mutant transcript was absent from muscle cDNA, consistent with degradation; several genotypes reduced protein abundance or enzyme activity. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8)

Variant-interpretation warning: p.Gln170Arg occurred in healthy Italian controls at a reported allele frequency of 1.98%, making it unsuitable for blanket classification as a fully penetrant pathogenic allele without phase, segregation, functional, and current population-database reassessment. A variant of uncertain significance neither confirms nor excludes diagnosis. (elhattab1993deoxyguanosinekinasedeficiency pages 1-3, ronchi2012nextgenerationsequencingreveals pages 8-9)

Environmental, infectious, and lifestyle risks

No toxin, infection, radiation exposure, occupation, diet, smoking behavior, alcohol exposure, or other environmental factor is established as a cause of PEO4. Physiologic stress, illness, fasting, or medications may aggravate symptoms in mitochondrial disease generally, but no DGUOK-PEO4-specific gene–environment interaction has been demonstrated. Infectious-agent and zoonotic categories are therefore not applicable etiologically.

Protective factors and modifiers

No validated protective DGUOK allele, nuclear modifier, mtDNA haplogroup modifier, diet, supplement, exercise program, or exposure has been shown to prevent PEO4. Residual DGUOK activity is biologically plausible as a severity modifier, but the original adult series found no simple relationship between residual muscle activity and age at myopathic onset. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 8-9)

3. Phenotypes

Because the direct cohort is very small, percentages would be misleading. Frequencies below are qualitative unless explicitly stated.

  • Progressive external ophthalmoplegia/ophthalmoparesis — clinical sign: adult or late-adult onset in documented patients; chronic, slowly progressive, generally bilateral. Suggested HPO: Progressive external ophthalmoplegia. One woman had an 11-year history at age 69; another older woman developed bilateral ptosis/PEO followed by limb-girdle weakness. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
  • Ptosis — sign: usually bilateral and progressive; may be mild or prominent. Suggested HPO: Ptosis, Bilateral ptosis. Ptosis can impair superior visual fields and reading and can cause compensatory frontalis activation or neck extension, although DGUOK-specific quality-of-life scores are unavailable. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
  • Strabismus — sign: reported in an adult with mild ptosis and wider neuromuscular disease. Suggested HPO: Strabismus. (ronchi2012nextgenerationsequencingreveals pages 3-4)
  • Mitochondrial myopathy and weakness — sign: variable limb-girdle, distal, neck, tongue, or generalized weakness; slowly progressive in adult cases. Suggested HPO: Mitochondrial myopathy, Proximal muscle weakness, Distal muscle weakness, Neck flexor weakness. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 3-4)
  • Exercise symptoms — symptom: exercise-induced pain, exercise intolerance, cramps, and CK elevation were documented. Suggested HPO: Exercise intolerance, Myalgia, Muscle cramps, Elevated circulating creatine kinase. These limit walking, work, and sustained activity, but no EQ-5D, SF-36, or PROMIS dataset exists for PEO4. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
  • Bulbar/laryngeal involvement — symptoms/signs: dysphagia, occasional liquid dysphagia, dysphonia, and tongue hypotrophy. Suggested HPO: Dysphagia, Dysphonia, Tongue atrophy. Aspiration and nutritional risk should be assessed clinically. (ronchi2012nextgenerationsequencingreveals pages 3-4)
  • Rhabdomyolysis — laboratory/clinical episode: recurrent episodes were described in a young woman with previous infantile DGUOK liver disease and transplantation; this represents the broader DGUOK spectrum rather than classic isolated PEO4. Suggested HPO: Rhabdomyolysis. (ronchi2012nextgenerationsequencingreveals pages 1-2)
  • Lower-motor-neuron syndrome and mild cognitive impairment: observed in siblings with multiple mtDNA deletions. Suggested HPO: Lower motor neuron dysfunction, Mild cognitive impairment. These are uncommon PEO-plus manifestations, not established core features. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 9-10)
  • Individual systemic observations: diabetes and cataract occurred in one elderly patient; causality and frequency cannot be determined. Suggested HPO: Diabetes mellitus, Cataract. (ronchi2012nextgenerationsequencingreveals pages 3-4)
  • Laboratory/pathology: CK ranged from mild elevation to approximately 2,000 U/L in one patient; lactate could be normal or moderately increased; EMG was myopathic; muscle contained ragged-red and COX-negative fibers, severe COX deficiency, and multiple mtDNA deletions. Suggested HPO: Elevated circulating creatine kinase, Ragged-red muscle fibers, Cytochrome-c oxidase deficiency, Lactic acidosis only when documented. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)

4. Genetic and molecular information

DGUOK encodes a nuclear-synthesized enzyme imported into the mitochondrial matrix. It phosphorylates deoxyguanosine and deoxyadenosine to dGMP and dAMP—the first step of mitochondrial purine-deoxyribonucleoside salvage. Suggested GO annotations include deoxyguanosine kinase activity, deoxyadenosine kinase activity, purine deoxyribonucleoside salvage, mitochondrial DNA replication, and mitochondrial genome maintenance. (ronchi2012nextgenerationsequencingreveals pages 7-8, ronchi2012nextgenerationsequencingreveals pages 8-9, elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

The adult study supplied unusually strong functional evidence: abnormal muscle splicing, reduced DGUOK protein on western blot, and residual enzyme activities of 19–45% of controls. This supports a predominantly loss-of-function mechanism. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8)

The variants are germline, not somatic cancer drivers. No recurrent PEO4-associated chromosomal aneuploidy, translocation, inversion, methylation defect, histone signature, or disease-specific chromatin abnormality is established. A Druze founder allele, c.255delA (p.Ala86ProfsTer13), is known in broader DGUOK deficiency, but is not specifically established as a PEO4 founder variant. (elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

Current ClinVar classifications and gnomAD allele frequencies should be retrieved variant by variant at the time of interpretation; the primary paper predates modern ACMG/AMP curation. Particularly, p.Gln170Arg should not be treated as pathogenic in isolation. (ronchi2012nextgenerationsequencingreveals pages 8-9)

5. Environmental information

PEO4 is a genetic mtDNA-maintenance disorder. There is no demonstrated causal role for pollution, occupational exposure, radiation, tobacco, alcohol, diet, or infection. Sensible mitochondrial-disease practice—avoiding prolonged fasting, dehydration, extreme unaccustomed exertion, and mitochondrial-toxic drugs when alternatives exist—is precautionary and individualized, not evidence-based primary prevention for DGUOK-PEO4. No PEO4-specific CTD interaction, exposure-response statistic, or infectious trigger was identified.

6. Mechanism and pathophysiology

Ordered causal chain

  1. Biallelic DGUOK function-reducing variants lead to impaired synthesis, stability, splicing, or catalytic activity of mitochondrial deoxyguanosine kinase. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8)
  2. Reduced DGUOK activity leads to inadequate phosphorylation of deoxyguanosine/deoxyadenosine to dGMP/dAMP and disturbed mitochondrial purine-dNTP supply. (ronchi2012nextgenerationsequencingreveals pages 7-8, elhattab1993deoxyguanosinekinasedeficiency pages 12-15)
  3. Disturbed dNTP homeostasis leads to defective mtDNA replication and maintenance; the exact biochemical transition from pool imbalance to deletion formation is partly inferred. (ronchi2012nextgenerationsequencingreveals pages 9-10, elhattab1993deoxyguanosinekinasedeficiency pages 12-15)
  4. Defective maintenance results in multiple mtDNA deletions in postmitotic skeletal muscle; profound alleles in other tissues can instead produce mtDNA depletion. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 1-3, ronchi2012nextgenerationsequencingreveals pages 7-8)
  5. Deleted/depleted mtDNA results in deficient synthesis of mtDNA-encoded respiratory-chain subunits and mosaic OXPHOS failure, demonstrated histologically by COX-negative and ragged-red fibers. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
  6. OXPHOS failure leads to impaired ATP production and compensatory mitochondrial proliferation in energy-demanding myofibers; oxidative-stress and cell-death contributions are plausible but not directly demonstrated in PEO4.
  7. Energetic failure in extraocular muscle fibers leads to bilateral ptosis and progressive ophthalmoparesis; involvement of limb, neck, bulbar, or respiratory muscles branches to weakness, exercise intolerance, dysphagia, and dysphonia. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
  8. In some genotypes/tissues, broader mtDNA instability leads to rhabdomyolysis, lower-motor-neuron disease, cognitive findings, or childhood hepatopathy; these branches are variable and not obligatory PEO4 features. (ronchi2012nextgenerationsequencingreveals pages 1-2, ronchi2012nextgenerationsequencingreveals pages 9-10)

Upstream: DGUOK genotype, transcript/protein stability, kinase activity, and mitochondrial dNTP supply. Intermediate: mtDNA replication/maintenance, deletion burden, depletion, and respiratory-chain assembly. Downstream: mosaic OXPHOS deficiency, myofiber dysfunction, and clinical weakness/ophthalmoplegia.

Suggested biological-process GO terms are purine deoxyribonucleoside salvage, mitochondrial DNA replication, mitochondrial genome maintenance, oxidative phosphorylation, ATP metabolic process, and muscle contraction. Suggested cell terms are skeletal muscle fiber, extraocular muscle cell, and—only for PEO-plus cases—lower motor neuron. No PEO4-specific immune, inflammatory, Wnt, MAPK, PI3K–AKT, mTOR, ferroptosis, autophagy, or apoptosis mechanism has been established.

No validated disease-specific transcriptomic, proteomic, metabolomic, lipidomic, single-cell, spatial-transcriptomic, or multi-omics signature was identified. Functional evidence currently rests mainly on human muscle mtDNA assays, histology, western blotting, RT-PCR, and kinase assays. (ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8)

7. Anatomical structures affected

  • Primary organ/tissue: bilateral extraocular muscles and other skeletal muscle. Suggested UBERON labels: extraocular muscle, skeletal muscle tissue.
  • Secondary sites: eyelid elevators; limb-girdle, distal, cervical, lingual, pharyngeal, and laryngeal musculature; lower motor neurons in rare PEO-plus disease. Liver and brain involvement belong mainly to broader DGUOK deficiency.
  • Cell level: skeletal and extraocular myofibers, with mosaic COX deficiency and ragged-red transformation. Suggested CL labels: skeletal muscle fiber, extraocular muscle cell, lower motor neuron where applicable.
  • Subcellular: mitochondrial matrix, mitochondrial nucleoid/mtDNA, respiratory-chain inner-membrane complexes. Suggested GO cellular components: mitochondrial matrix, mitochondrion, mitochondrial nucleoid, mitochondrial respiratory-chain complex.
  • Lateralization: ptosis and ophthalmoparesis are generally bilateral, although severity can be asymmetric. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3, ronchi2012nextgenerationsequencingreveals pages 8-9)

8. Temporal development

The documented PEO phenotype is usually adult-onset, insidious, chronic, and slowly progressive. Examples include onset around age 58 in a woman assessed at 69, onset at 69 in a man assessed at 80, and an eight-year progression in a 48-year-old woman. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)

A practical, nonvalidated staging description is: early ptosis/exercise symptoms; intermediate ophthalmoparesis and focal or limb-girdle weakness; advanced PEO-plus disease with bulbar, widespread muscle, respiratory, or neurologic involvement. No validated PEO4 staging system, progression-rate estimate, remission pattern, or intervention window exists. Spontaneous remission is not expected; surgery may improve ptosis but does not correct the mitochondrial defect.

The broader allelic spectrum ranges from neonatal liver failure to late-adult myopathy, implying strong genotype-, tissue-, and residual-function dependence, but reliable genotype–age prediction is unavailable. (elhattab1993deoxyguanosinekinasedeficiency pages 3-6, ronchi2012nextgenerationsequencingreveals pages 8-9)

9. Inheritance and population

Inheritance is autosomal recessive. If both parents carry a pathogenic allele, each pregnancy has a 25% affected, 50% carrier, and 25% noncarrier probability; heterozygous carriers are generally asymptomatic. Prenatal and preimplantation genetic testing are possible after familial variants are established. (elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

Penetrance for two definitively pathogenic alleles is probably high for some DGUOK phenotype but is not quantified for PEO4; expressivity is markedly variable and age dependent. There is no evidence of genetic anticipation. Germline mosaicism is theoretically possible but not documented as a material PEO4 contributor.

No PEO4-specific prevalence, incidence, carrier frequency, sex ratio, geographic concentration, or survival distribution is known. DGUOK variants represented 5.6% of one selected adult multiple-mtDNA-deletion cohort; all DGUOK phenotypes together account for an estimated 15–20% of mtDNA-depletion syndromes, but that figure must not be transferred to PEO4. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)

Consanguinity increases the probability of homozygosity for rare recessive alleles. The c.255delA founder allele occurs in Druze ancestry in broader DGUOK deficiency. No PEO4-specific founder population has been demonstrated. (elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

10. Diagnostics

Recommended workflow

  1. Clinical recognition: bilateral progressive ptosis/ophthalmoparesis with myopathy, exercise intolerance, dysphagia, or family structure compatible with recessive inheritance.
  2. Baseline assessment: CK, lactate and pyruvate, glucose, liver profile, ECG/echocardiography where indicated, respiratory function, hearing/vision examination, and neurologic assessment. Normal lactate does not exclude disease. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3)
  3. Electrophysiology: EMG may show a myopathic pattern; nerve-conduction testing is appropriate when neuropathy or motor-neuron disease is suspected.
  4. Genomic testing: a mitochondrial-myopathy/mtDNA-maintenance panel including DGUOK, POLG, TWNK, TK2, RRM2B, RNASEH1, MGME1, OPA1, MPV17, SUCLA2, SUCLG1, or exome/genome sequencing with copy-number analysis. Confirmation requires two pathogenic/likely pathogenic DGUOK alleles in trans. (elhattab1993deoxyguanosinekinasedeficiency pages 3-6, elhattab1993deoxyguanosinekinasedeficiency pages 1-3, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)
  5. Affected-tissue mtDNA analysis: long-range PCR or validated NGS/Southern-blot methods for multiple deletions and quantitative copy-number testing. Muscle can be more informative than blood for late-onset mtDNA-maintenance disease.
  6. Muscle biopsy if genetics is inconclusive: modified Gomori trichrome for ragged-red fibers, COX/SDH histochemistry, respiratory-chain enzymology, mtDNA deletion and depletion testing, and—research/functional setting—DGUOK activity, western blot, or RNA studies. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 9-10, ronchi2012nextgenerationsequencingreveals pages 7-8)

Across broader DGUOK deficiency, sequence analysis detects approximately 95% of identifiable pathogenic variants and deletion/duplication analysis about 5%; these are not PEO4-specific diagnostic yields. CMA, karyotyping, FISH, and repeat-expansion testing are not first-line unless another diagnosis is suspected. (elhattab1993deoxyguanosinekinasedeficiency pages 3-6)

Differential diagnosis

Important alternatives include mtDNA single large-scale deletion syndromes; POLG-, TWNK-, TK2-, RRM2B-, RNASEH1-, MGME1-, OPA1-, MPV17-, SUCLA2-, and SUCLG1-related disorders; primary mtDNA point variants; oculopharyngeal muscular dystrophy; myasthenia gravis; congenital myasthenic syndromes; thyroid eye disease; and structural orbital/brainstem disease. Distinguishing evidence for DGUOK-PEO4 is biallelic DGUOK variants plus multiple mtDNA deletions and mitochondrial pathology in muscle. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)

There is no population or newborn screening program for PEO4. Cascade testing of relatives and reproductive carrier testing are appropriate after a molecular diagnosis. (elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

11. Outcome and prognosis

No PEO4-specific five- or ten-year survival, mortality rate, or life-expectancy estimate exists. Adult PEO can progress slowly over decades, but morbidity includes restricted gaze, ptosis-related visual-field loss, exercise limitation, weakness, falls, dysphagia, aspiration risk, and reduced independence. Rare PEO-plus presentations may add lower-motor-neuron or cognitive disability. (ronchi2012nextgenerationsequencingreveals pages 3-4, ronchi2012nextgenerationsequencingreveals pages 2-3, ronchi2012nextgenerationsequencingreveals pages 9-10)

Recovery of lost extraocular motility or established mitochondrial myopathy is generally unlikely because current care does not correct DGUOK deficiency. Functional improvement can follow rehabilitation, assistive devices, swallowing intervention, or ptosis management. Prognostic biomarkers have not been validated; residual enzyme activity, deletion burden, age at onset, bulbar/respiratory involvement, and multisystem disease are plausible but unproven predictors.

Severe early mortality before age four and variable liver-transplant outcomes apply to neonatal hepatocerebral DGUOK deficiency, not automatically to PEO4. (elhattab1993deoxyguanosinekinasedeficiency pages 3-6, elhattab1993deoxyguanosinekinasedeficiency pages 6-8)

12. Treatment and real-world implementation

There is no approved disease-modifying treatment specifically for DGUOK-PEO4 and no DGUOK-specific randomized trial was identified. Management is multidisciplinary and phenotype directed:

  • Ptosis/ophthalmic care: lubrication for exposure, prisms where useful, and carefully selected ptosis repair or frontalis suspension, balancing visual benefit against exposure keratopathy. Suggested NCIT terms: Ptosis Repair, Supportive Care.
  • Rehabilitation: graded, supervised aerobic/resistance activity as tolerated, physical and occupational therapy, fall prevention, and mobility aids. Suggested NCIT: Physical Therapy, Occupational Therapy.
  • Bulbar/nutrition: speech-language and swallowing assessment, diet modification, aspiration prevention, and enteral support if necessary. Suggested NCIT: Nutritional Support, Speech Therapy.
  • Surveillance/treatment of complications: pulmonary function and sleep/ventilatory assessment when weak; ECG and cardiac evaluation; endocrine, hearing, neurologic, and hepatic assessment according to phenotype.
  • Supplements: coenzyme Q10, riboflavin, creatine, antioxidants, or commercial “mitochondrial cocktails” are sometimes used empirically, but no DGUOK-PEO4 response rate or controlled efficacy evidence exists.
  • Liver transplantation: relevant only to carefully selected liver-predominant DGUOK deficiency with minimal neurologic disease; it does not correct extrahepatic DGUOK deficiency, and later muscle disease has occurred after transplantation. It is not a treatment for isolated PEO4. (ronchi2012nextgenerationsequencingreveals pages 9-10, elhattab1993deoxyguanosinekinasedeficiency pages 1-3)

Broad mitochondrial trials involving elamipretide, nicotinamide riboside/niacin, or vitamin/cofactor approaches cannot presently be considered evidence for DGUOK-PEO4. No validated pharmacogenomic rule, gene replacement, CRISPR therapy, RNA therapy, or cell therapy is clinically available for this disorder.

13. Prevention

Primary prevention after conception is not currently possible. No vaccine, prophylactic medication, or behavioral intervention prevents biallelic DGUOK disease.

Genetic counseling is the principal preventive strategy: confirm phase and pathogenicity, test parents and at-risk relatives, offer cascade carrier testing, and discuss prenatal diagnosis or preimplantation genetic testing for monogenic disease. Each pregnancy of two confirmed carriers has a 25% recurrence risk. (elhattab1993deoxyguanosinekinasedeficiency pages 12-15)

Secondary prevention consists of earlier recognition and molecular diagnosis before avoidable procedures or prolonged misdiagnosis. Tertiary prevention includes fall prevention, aspiration and exposure-keratopathy prevention, maintenance of mobility, and surveillance for respiratory, cardiac, endocrine, neurologic, or hepatic complications.

14. Other species and natural disease

DGUOK and mitochondrial purine salvage are evolutionarily conserved, but no naturally occurring companion-animal, livestock, or wildlife disease was identified that faithfully corresponds to human recessive PEO4. There is no zoonotic potential or cross-species transmission because the disorder is inherited, not infectious. Exact ortholog NCBI Gene and taxonomy identifiers should be obtained directly from current NCBI/Alliance records before structured ingestion.

15. Model organisms and experimental systems

No validated animal model was identified that reproduces the full adult DGUOK-PEO4 combination of ptosis/ophthalmoplegia, skeletal-muscle multiple mtDNA deletions, and slow progression. Available disease biology relies most strongly on human muscle biopsy, patient-derived molecular assays, and broader DGUOK-deficiency cellular systems. Useful models would include DGUOK-knockout or patient-variant cell lines, myotubes, iPSC-derived skeletal/extraocular muscle, and hepatocyte-like cells for the broader depletion phenotype.

Such systems can test nucleotide-pool imbalance, mtDNA copy number/deletions, OXPHOS, ATP production, membrane potential, and rescue by wild-type DGUOK or nucleoside manipulation. Their principal limitation is that cultured proliferating cells may depend more on cytosolic de novo dNTP synthesis and may not reproduce decades-long deletion accumulation in postmitotic extraocular muscle.

Recent developments and expert assessment

The major 2023–2024 development is not a new DGUOK-specific therapy, but improved recognition of mitochondrial CPEO as a genetically heterogeneous syndrome and wider use of sequencing integrated with affected-tissue mtDNA analysis. For this ultra-rare subtype, the decisive evidence remains the 2012 primary series. The current expert interpretation is therefore conservative: diagnose PEO4 only when clinical and muscle-mtDNA findings align with two appropriately classified DGUOK variants; avoid extrapolating infantile liver-disease prognosis to adult PEO; and regard supplements or broad mitochondrial trials as unproven for this genotype. (ronchi2012nextgenerationsequencingreveals pages 1-2, elhattab1993deoxyguanosinekinasedeficiency pages 3-6, elhattab1993deoxyguanosinekinasedeficiency pages 1-3)

Key primary evidence quotation

The central paper’s conclusion can be summarized by its reported finding that recessive DGUOK mutations were identified in adults with “mitochondrial myopathy with or without progressive external ophthalmoplegia” and that these mutations impaired “muscle DGUOK activity and protein stability.” This is direct human clinical and functional evidence, whereas the finer chain from nucleotide imbalance to selective extraocular-muscle vulnerability remains partly inferred. (ronchi2012nextgenerationsequencingreveals pages 1-2)

Knowledge gaps

No robust PEO4-specific natural-history cohort, incidence/prevalence study, penetrance estimate, validated phenotype frequency, quality-of-life study, longitudinal biomarker, modifier-gene analysis, epigenetic signature, single-cell/spatial dataset, faithful animal model, genotype-guided treatment, or interventional trial was identified. Consequently, database entries should preserve evidence provenance and distinguish direct PEO4 evidence, broader DGUOK allelic-spectrum evidence, and general mitochondrial-disease extrapolation.

References

  1. (ronchi2012nextgenerationsequencingreveals pages 1-2): D. Ronchi, C. Garone, A. Bordoni, Purificación Gutiérrez Ríos, S. Calvo, M. Ripolone, M. Ranieri, M. Rizzuti, L. Villa, F. Magri, S. Corti, N. Bresolin, V. Mootha, M. Moggio, S. Dimauro, G. Comi, and M. Sciacco. Next-generation sequencing reveals dguok mutations in adult patients with mitochondrial dna multiple deletions. Brain : a journal of neurology, 135 Pt 11:3404-15, Nov 2012. URL: https://doi.org/10.1093/brain/aws258, doi:10.1093/brain/aws258. This article has 124 citations.

  2. (ronchi2012nextgenerationsequencingreveals pages 9-10): D. Ronchi, C. Garone, A. Bordoni, Purificación Gutiérrez Ríos, S. Calvo, M. Ripolone, M. Ranieri, M. Rizzuti, L. Villa, F. Magri, S. Corti, N. Bresolin, V. Mootha, M. Moggio, S. Dimauro, G. Comi, and M. Sciacco. Next-generation sequencing reveals dguok mutations in adult patients with mitochondrial dna multiple deletions. Brain : a journal of neurology, 135 Pt 11:3404-15, Nov 2012. URL: https://doi.org/10.1093/brain/aws258, doi:10.1093/brain/aws258. This article has 124 citations.

  3. (ronchi2012nextgenerationsequencingreveals pages 7-8): D. Ronchi, C. Garone, A. Bordoni, Purificación Gutiérrez Ríos, S. Calvo, M. Ripolone, M. Ranieri, M. Rizzuti, L. Villa, F. Magri, S. Corti, N. Bresolin, V. Mootha, M. Moggio, S. Dimauro, G. Comi, and M. Sciacco. Next-generation sequencing reveals dguok mutations in adult patients with mitochondrial dna multiple deletions. Brain : a journal of neurology, 135 Pt 11:3404-15, Nov 2012. URL: https://doi.org/10.1093/brain/aws258, doi:10.1093/brain/aws258. This article has 124 citations.

  4. (elhattab1993deoxyguanosinekinasedeficiency pages 3-6): AW El-Hattab and F Scaglia. Deoxyguanosine kinase deficiency. Unknown journal, 1993.

  5. (elhattab1993deoxyguanosinekinasedeficiency pages 1-3): AW El-Hattab and F Scaglia. Deoxyguanosine kinase deficiency. Unknown journal, 1993.

  6. (ronchi2012nextgenerationsequencingreveals pages 8-9): D. Ronchi, C. Garone, A. Bordoni, Purificación Gutiérrez Ríos, S. Calvo, M. Ripolone, M. Ranieri, M. Rizzuti, L. Villa, F. Magri, S. Corti, N. Bresolin, V. Mootha, M. Moggio, S. Dimauro, G. Comi, and M. Sciacco. Next-generation sequencing reveals dguok mutations in adult patients with mitochondrial dna multiple deletions. Brain : a journal of neurology, 135 Pt 11:3404-15, Nov 2012. URL: https://doi.org/10.1093/brain/aws258, doi:10.1093/brain/aws258. This article has 124 citations.

  7. (elhattab1993deoxyguanosinekinasedeficiency pages 6-8): AW El-Hattab and F Scaglia. Deoxyguanosine kinase deficiency. Unknown journal, 1993.

  8. (OpenTargets Search: Progressive external ophthalmoplegia with mitochondrial DNA deletions autosomal recessive 4): Open Targets Query (Progressive external ophthalmoplegia with mitochondrial DNA deletions autosomal recessive 4, 1 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.

  9. (elhattab1993deoxyguanosinekinasedeficiency pages 12-15): AW El-Hattab and F Scaglia. Deoxyguanosine kinase deficiency. Unknown journal, 1993.

  10. (ronchi2012nextgenerationsequencingreveals pages 3-4): D. Ronchi, C. Garone, A. Bordoni, Purificación Gutiérrez Ríos, S. Calvo, M. Ripolone, M. Ranieri, M. Rizzuti, L. Villa, F. Magri, S. Corti, N. Bresolin, V. Mootha, M. Moggio, S. Dimauro, G. Comi, and M. Sciacco. Next-generation sequencing reveals dguok mutations in adult patients with mitochondrial dna multiple deletions. Brain : a journal of neurology, 135 Pt 11:3404-15, Nov 2012. URL: https://doi.org/10.1093/brain/aws258, doi:10.1093/brain/aws258. This article has 124 citations.

  11. (ronchi2012nextgenerationsequencingreveals pages 2-3): D. Ronchi, C. Garone, A. Bordoni, Purificación Gutiérrez Ríos, S. Calvo, M. Ripolone, M. Ranieri, M. Rizzuti, L. Villa, F. Magri, S. Corti, N. Bresolin, V. Mootha, M. Moggio, S. Dimauro, G. Comi, and M. Sciacco. Next-generation sequencing reveals dguok mutations in adult patients with mitochondrial dna multiple deletions. Brain : a journal of neurology, 135 Pt 11:3404-15, Nov 2012. URL: https://doi.org/10.1093/brain/aws258, doi:10.1093/brain/aws258. This article has 124 citations.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 2
Resolved 2
Unresolved (possible confabulation) 0
Unverifiable 0
References weighed for topical relevance 2
On topic 1
Off topic 0

All extracted references resolved successfully.

Term Validation

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

Outcome Count
Terms checked 1
Resolved 1
Unresolved (possible confabulation) 0
Obsolete 0
Unverifiable 0
Terms whose name was checked 1
Terms named correctly 0
Terms named as a different term 1

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

  • MONDO:0014899 (5 mentions) - the report calls it "if available"; MONDO calls it progressive external ophthalmoplegia with mitochondrial DNA deletions, autosomal recessive 4