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Classifications

Harrison's Part

Mechanistic Nosology

ICIMD (Inherited Metabolic Disorders)

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Inheritance

1

Autosomal recessive HP:0000007

Biallelic (homozygous or compound heterozygous) germline TK2 variants. Heterozygous carriers are asymptomatic.

Autosomal recessive inheritance

Show evidence (2 references)

PMID:23230576 SUPPORT Human Clinical

"TK2-related mtDNA maintenance defect is inherited in an autosomal recessive manner."

Mode of inheritance.

PMID:23230576 SUPPORT Human Clinical

"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."

Recurrence risks consistent with autosomal recessive transmission.

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Subtypes

3

Infantile-onset myopathy with neurological involvement

TK2 hgnc:11831

Onset in the first year of life with severe congenital myopathy, hypotonia or motor regression, severe muscle mtDNA depletion, frequent central nervous system involvement and rapid progression to respiratory failure. Almost none achieve independent walking, and median post-onset survival is about one year.

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"infantile-onset myopathy (42.4%) with severe mitochondrial DNA (mtDNA) depletion, frequent neurological involvement and rapid progression to early mortality"

Defines the infantile-onset form and its share of the cohort.

PMID:29602790 SUPPORT Human Clinical

"Muscle weakness was most prominent in the first year of life in the infantile-onset form, with 26 of 32 (81%) never acquiring the ability to walk independently and virtually all (30/32; 94%) non-ambulatory by age 4 years"

Motor outcome in the infantile-onset form.

Childhood-onset limb myopathy

TK2 hgnc:11831

Onset between one and twelve years with proximal limb weakness, Gowers sign and muscle mtDNA depletion, sometimes with multiple deletions. Weakness can resemble spinal muscular atrophy type 3. Most lose ambulation within ten years of onset and about half require ventilatory support; median post-onset survival is at least thirteen years.

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"childhood-onset myopathy (40.2%) with mtDNA depletion, moderate-to-severe progression of generalised weakness and median POS at least 13 years"

Defines the childhood-onset form and its survival.

PMID:29602790 SUPPORT Human Clinical

"In contrast, 19/30 (63%) of childhood-onset patients became wheelchair-bound within 10 years of disease onset"

Loss of ambulation in the childhood-onset form.

Late-/adult-onset myopathy with multiple mtDNA deletions

TK2 hgnc:11831

Onset at twelve years or later, often in adulthood, with mild limb weakness, marked facial weakness, scapular winging, ptosis and progressive external ophthalmoparesis. Multiple mtDNA deletions in muscle are near-universal while frank depletion is uncommon. Ambulation is retained but respiratory muscle weakness develops in most, and the presentation can be mistaken for facioscapulohumeral muscular dystrophy.

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"late-onset myopathy (17.4%) with mild limb weakness at onset and slow progression to respiratory insufficiency with median POS of 23 years"

Defines the late-onset form.

PMID:29602790 SUPPORT Human Clinical

"Sixteen patients (17.4%) in our cohort presented with adult-onset myopathy, which mimicked facioscapulohumeral dystrophy with marked facial weakness, scapular winging or both in seven (43.75%)."

The FSHD-like presentation of the late-onset form.

PMID:31060578 SUPPORT Human Clinical

"All cases showed multiple mtDNA deletions but mtDNA depletion was present only in two patients."

The molecular signature that separates the late-onset form from the earlier forms.

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Discussions and Knowledge Gaps

4

The Tk2 H126N knock-in mouse is CNS-predominant and spares skeletal muscle, while the human disease is muscle-predominant and usually spares the CNS. Does the compensatory mechanism that protects mouse muscle - Tk1 activity in the first postnatal week followed by MTERF3 down-regulation raising mitochondrial transcripts per mtDNA copy - operate in human muscle, and if so why does human muscle nonetheless fail?

HUMAN MODEL MISMATCH OPEN tk2_mouse_organ_inversion

Attached to

pathophysiology#Post-Mitotic Dependence on Mitochondrial Salvage pathophysiology#Combined Respiratory Chain Deficiency animal_models#Mouse

This is not a cosmetic species difference. Every therapy in this disease was developed in a model whose target organ is not the organ the human disease destroys, and the mouse's own authors identified compensation as the reason its muscle is spared. If the same compensation is absent or exhausted in human muscle, that absence is part of the pathogenesis and a potential therapeutic target in its own right; if it is present, the mouse's muscle endpoints understate what nucleoside therapy has to overcome. The one human data point pointing the same way is the observation that muscle combines low basal TK2 activity with a high demand for mtDNA-encoded protein, which is a vulnerability argument rather than a compensation argument.

Proposed experiments

MTERF3 and mitochondrial transcript quantification in patient muscle

tk2_human_muscle_compensation

Measure MTERF3 transcript and protein, mitochondrial transcript levels normalised to mtDNA copy number, and TK1 activity in archived TK2-deficient muscle biopsies across the three clinical forms, against age-matched control muscle. The mouse result predicts that a tissue which compensates shows raised transcript per copy with suppressed MTERF3; human muscle, which does not escape the disease, should not.

Readouts

Mitochondrial transcript level per mtDNA copy

pathophysiology#Combined Respiratory Chain Deficiency

Direction: DECREASED

Interpretation: Absence of the transcriptional compensation seen in mouse heart would explain why human muscle is the target organ.

Would support

pathophysiology#Post-Mitotic Dependence on Mitochondrial Salvage

Supporting outcome

Patient muscle shows no rise in mitochondrial transcript per mtDNA copy and no MTERF3 suppression, while the respiratory-chain deficit is present - establishing that human muscle lacks the compensation that protects the mouse.

Refuting outcome

Patient muscle shows the same transcriptional compensation as mouse heart despite a full respiratory-chain deficit, which would mean compensation is not what determines organ tropism and the mouse-human inversion has another cause.

In patients with nervous-system features, how much is primary neuronal mtDNA depletion and how much is hypoxic-ischaemic injury secondary to respiratory failure?

KNOWLEDGE GAP OPEN tk2_cns_attribution

Attached to

pathophysiology#Central Nervous System Involvement phenotypes#Encephalopathy phenotypes#Seizures

The natural-history authors raise this explicitly and do not resolve it: CNS features cluster in the infantile-onset group, which is also the group with the highest ventilatory requirement, so the two candidate causes are confounded by design in a retrospective cohort. The answer changes what nucleoside therapy should be expected to do for the CNS, and whether CNS-penetrant delivery is needed. Nucleoside therapy now alters the respiratory course, which makes a prospective separation possible for the first time: patients whose respiratory failure is prevented but who still develop CNS disease would settle it.

What is the effect size of deoxycytidine and deoxythymidine therapy against a concurrent control rather than against historical untreated patients?

KNOWLEDGE GAP OPEN tk2_no_controlled_trial

Attached to

treatments#Deoxycytidine and Deoxythymidine Substitution clinical_trials#NCT06754098

Every efficacy claim available rests on open-label expanded access and single-arm studies compared against untreated patients assembled retrospectively or from the literature. That is a reasonable design for a rapidly fatal ultra-rare disease and it is not a reason to withhold the therapy, but the magnitude and durability of benefit remain estimated from non-concurrent comparators. The November 2025 US approval was granted on that basis and so does not close this gap; it sharpens it, because the licensed age limit is itself drawn from a subgroup contrast within the same uncontrolled data. The pooled analyses did stratify by age at onset and found no significant mortality difference in later-onset patients, which leaves open whether later-onset disease is genuinely unresponsive or merely slow enough that the available follow-up cannot resolve a difference -- the latter would make the age cut-off an artefact of study duration rather than of biology. A placebo-controlled trial is unlikely now that an approved therapy exists, so the realistic substitute is a prespecified comparison against the 92-patient natural-history cohort using its own stratification by age at onset, with GDF-15 as an objective intermediate endpoint.

What determines whether a given biallelic TK2 genotype produces infantile, childhood or adult-onset disease, given that neither the variant nor residual enzyme activity predicts it?

KNOWLEDGE GAP OPEN tk2_genotype_phenotype

Attached to

genetic#TK2 pathophysiology#TK2 Loss of Function

p.Thr108Met homozygotes span all three clinical forms, twelve variants are each associated with more than one form, and residual TK2 activity in patient fibroblasts ranges from 1% to 39% without correlating with severity. Something other than the TK2 genotype is setting the age at onset - plausibly variation in the cytosolic kinases and nucleotide catabolism that modify the same pool, or in the compensation the mouse work identified. This is the practical obstacle to counselling a newly diagnosed infant's sibling and to stratifying a single-arm trial.

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Pathophysiology

9

TK2 Loss of Function

Biallelic TK2 variants reduce or abolish thymidine kinase 2, the mitochondrial matrix deoxyribonucleoside kinase. Missense, in-frame deletion, splice and frameshift alleles are all reported, and residual activity measured in patient-derived fibroblasts spans 1-39% of control without tracking clinical severity, so the genotype does not by itself predict the form of disease. Recombinant work on the two original alleles shows two distinct kinetic lesions: I212N is essentially inactive, whereas H121N retains normal substrate affinity but loses catalytic efficiency through a raised Km for ATP.

TK2 hgnc:11831

Genetic context variant_origin: GERMLINE functional_impact_category: LOSS_OF_FUNCTION

Biallelic (homozygous or compound heterozygous) germline TK2 variants.

thymidine kinase activity GO:0004797 ↓ DECREASED deoxycytidine kinase activity GO:0004137 ↓ DECREASED

Downstream

Mitochondrial Pyrimidine Deoxynucleoside Salvage Failure Without TK2 the first and rate-limiting phosphorylation of deoxythymidine and deoxycytidine inside mitochondria does not occur.

Show evidence (4 references)

PMID:11687801 SUPPORT Human Clinical

"In these individuals, the activity of TK2 in muscle mitochondria is reduced to 14-45% of the mean value in healthy control individuals."

Direct demonstration of reduced enzyme activity in patient muscle mitochondria.

PMID:29602790 SUPPORT Human Clinical

"TK2 activity measured in 11 patient-derived fibroblast cell lines showed residual activity ranging from 1% to 39% relative to healthy controls without correlation with clinical severity"

Range of residual activity, and the absence of a correlation with severity.

PMID:12493767 SUPPORT In Vitro

"The I212N mutant showed less than 1% activity as compared with wild type TK2 with all deoxynucleosides."

A near-null allele characterised on recombinant enzyme.

+ 1 more reference

Mitochondrial Pyrimidine Deoxynucleoside Salvage Failure

TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine to their monophosphates inside the mitochondrial matrix; these are then phosphorylated onward to the pyrimidine deoxynucleoside triphosphates used by the mtDNA replisome. The inner membrane is impermeable to charged nucleotides and mitochondria have no de novo synthesis pathway of their own, so in cells that are not dividing this salvage route is the principal supply.

pyrimidine deoxyribonucleoside salvage GO:0043099 ↓ DECREASED deoxyribonucleoside triphosphate biosynthetic process GO:0009202 ↓ DECREASED

Downstream

Imbalanced Mitochondrial dNTP Pool Loss of the pyrimidine arm of mitochondrial salvage leaves the matrix dNTP pool short of dTTP and dCTP relative to the purine deoxynucleotides.

Show evidence (3 references)

PMID:11687801 SUPPORT Human Clinical

"Human dGK efficiently phosphorylates deoxyguanosine and deoxyadenosine, whereas TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine."

The substrate specificity that makes this the pyrimidine arm of mitochondrial salvage.

PMID:11687801 SUPPORT Human Clinical

"In non-replicating cells, where cytosolic dNTP synthesis is down-regulated, mtDNA synthesis depends solely on the mitochondrial salvage pathway enzymes, the deoxyribonucleosides kinases."

Why the salvage pathway is not redundant in post-mitotic tissue.

PMID:31125140 SUPPORT BACKGROUND Human Clinical

"TK2 phosphorylates the nucleosides deoxycytidine (dC) and deoxythymidine (dT) to generate deoxycytidine monophosphate (dCMP) and deoxythymidine monophosphate (dTMP)."

States the reaction; an introduction sentence, not a result of the treatment study.

Imbalanced Mitochondrial dNTP Pool

mtDNA replication is constitutive rather than cell-cycle-linked and therefore needs a continuous, balanced supply of all four deoxynucleoside triphosphates. A pool depleted of pyrimidines limits and distorts replication. Direct measurement of mitochondrial dNTP pools in patient muscle is not among the sources cited here; the imbalance is measured in the knock-in mouse and asserted as mechanism in the human discovery paper.

mitochondrial DNA replication GO:0006264 ↓ DECREASED

Animal models

Tk2 H126N homozygous knock-in Mouse Mouse recapitulates The pool imbalance that is asserted but not measured in patients is measured directly here: low dTTP in brain and reduced dTTP and dCTP in liver, with mtDNA depletion and respiratory-chain deficits following.

Downstream

Skeletal Muscle mtDNA Depletion and Multiple Deletions An insufficient and unbalanced precursor supply limits mtDNA replication, lowering copy number and, over longer periods, generating deletions.

Show evidence (1 reference)

PMID:18467430 SUPPORT BACKGROUND Model Organism

"In contrast to nDNA synthesis, which is linked to cell cycle, mtDNA replication is constitutive and consequently requires a constant supply of deoxynucleoside triphosphates (dNTPs)"

Why a constant balanced pool is required; an introduction sentence.

Post-Mitotic Dependence on Mitochondrial Salvage

TK2 is ubiquitously expressed, yet the disease is predominantly a myopathy. Two non-exclusive explanations are supported. In proliferating cells the cytosolic, cell-cycle-regulated kinases TK1 and dCK, together with de novo synthesis, supply enough deoxynucleotide to mask the defect; when TK1 activity falls in post-mitotic tissue the TK2 deficiency is unmasked, which in the mouse coincides exactly with the onset of mtDNA depletion. Independently, human tissue work found muscle to combine low basal TK2 activity with a high demand for mtDNA-encoded proteins.

skeletal muscle fibre CL:0008002

Downstream

Skeletal Muscle mtDNA Depletion and Multiple Deletions The loss of cytosolic compensation is what converts a ubiquitous enzyme deficiency into a tissue-restricted failure of mtDNA maintenance.

Show evidence (2 references)

PMID:12765840 SUPPORT Human Clinical

"Our results suggest that low basal TK2 activity combined with a high requirement for mitochondrial encoded proteins in muscle predispose this tissue to the devastating effect of TK2 deficiency."

The human-tissue account of why skeletal muscle is the target organ.

PMID:20940150 SUPPORT Model Organism

"In contrast, TK2 activity has minor functional significance in proliferating cells due to the overwhelming activity of cell-cycle-regulated cytosolic enzymes involved in the deoxynucleoside triphosphate (dNTP) synthesis"

States the cytosolic compensation that spares dividing cells.

Skeletal Muscle mtDNA Depletion and Multiple Deletions

Reduced mtDNA copy number in skeletal muscle is the defining molecular lesion, severe in the infantile and childhood forms and much less common in the late-onset form, where multiple mtDNA deletions are found in essentially every patient instead. Depletion below about 30% of age-matched control was present in two thirds of the largest cohort.

mitochondrial DNA metabolic process GO:0032042 ↓ DECREASED

skeletal muscle tissue UBERON:0001134

Downstream

Combined Respiratory Chain Deficiency Too few intact mtDNA templates means too little of the 13 mtDNA-encoded respiratory-chain subunits, so the complexes containing them cannot be assembled in normal amounts.

Depletion of mitochondrial DNA in muscle The lesion is itself the reported laboratory phenotype.

Multiple mitochondrial DNA deletions In older patients the same maintenance failure manifests as accumulated deletions rather than, or as well as, reduced copy number.

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Levels of mtDNA in muscle were measured in 71 patients and showed significant depletion (<30% residual mtDNA relative to age-matched control subjects) in 47 (66%)."

Frequency and threshold of depletion in the largest cohort.

PMID:29602790 SUPPORT Human Clinical

"Multiple mtDNA deletions were detected in muscle of 24 patients primarily in late-onset patients: infantile-onset (1/8; 12.5%), childhood-onset (7/14; 50%) and late-onset (16/16; 100%)."

The shift from depletion to deletions with later onset.

PMID:11687801 SUPPORT Human Clinical

"Here we identify two mutations in TK2, histidine 90 to asparagine and isoleucine 181 to asparagine, in four individuals who developed devastating myopathy and depletion of muscular mitochondrial DNA in infancy."

The original observation linking TK2 variants to muscle mtDNA depletion.

Combined Respiratory Chain Deficiency

Reduced activity of the respiratory-chain complexes that contain mtDNA-encoded subunits. Multiple OXPHOS defects were found in about three quarters of patients in whom enzymology was performed, and isolated complex IV deficiency in a further small group; a minority of childhood- and late-onset patients had normal activities, so normal enzymology does not exclude the diagnosis.

oxidative phosphorylation GO:0006119 ↓ DECREASED

Animal models

Tk2 H126N homozygous knock-in Mouse Mouse partially recapitulates The mouse does develop deficiency of the mtDNA-dependent complexes, but the organ distribution is inverted relative to patients: the defect is CNS-predominant, and heart and skeletal muscle are largely spared by transcriptional compensation.

Downstream

Progressive Skeletal Muscle Fibre Degeneration Deficient oxidative ATP production in a high-demand post-mitotic tissue drives fibre atrophy and necrosis.

Cytochrome c oxidase-negative muscle fibres The mosaic histochemical readout of the same lesion at single-fibre resolution.

Ragged-red muscle fibres Compensatory mitochondrial proliferation in affected fibres.

Decreased activity of mitochondrial complex IV Complex IV is the most frequent isolated enzyme deficiency measured in this disease, and part of the combined pattern in the rest.

Elevated blood lactate Impaired oxidative phosphorylation shifts pyruvate towards lactate. Raised lactate alongside raised creatine kinase is a recognised prompt to test TK2.

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Multiple OXPHOS defects were identified in 35/48 (73%) while single enzyme abnormalities were detected in 7/48 (14.5%)"

Frequency of combined versus isolated respiratory-chain defects.

PMID:29602790 SUPPORT Human Clinical

"In six patients (12.5%) with childhood-onset (two) and late-onset (four) myopathies, OXPHOS activities were normal."

Normal enzymology in a minority; a negative result worth recording.

PMID:24968719 SUPPORT Model Organism

"defects of mitochondrial respiratory chain (RC) complexes I, III, IV, and V containing mtDNA-encoded subunits"

Identifies which complexes are affected, and why they are: these are the complexes carrying mtDNA-encoded subunits.

Progressive Skeletal Muscle Fibre Degeneration

Muscle shows a dystrophic pattern superimposed on the mitochondrial changes: atrophic and necrotic fibres, fibre-size variability with type 1 predominance, internal nuclei, and replacement of muscle by fat and connective tissue. This combination of dystrophic features with a cytochrome c oxidase defect and ragged-red fibres is the histological signature that should prompt TK2 testing.

skeletal muscle tissue UBERON:0001134

Downstream

Proximal muscle weakness Loss of functional muscle mass produces the cardinal weakness.

Hypotonia The early infantile expression of the same fibre pathology.

Elevated creatine kinase Sarcolemmal leak from degenerating fibres raises serum creatine kinase.

Respiratory Muscle Failure The diaphragm and accessory respiratory muscles are affected by the same process, which is what makes the disease lethal.

Loss of ambulation Cumulative weakness in the infantile and childhood forms.

Ptosis Levator palpebrae and extraocular involvement, characteristic of the late-onset form.

Progressive external ophthalmoparesis Extraocular muscle involvement, most frequent in late-onset disease.

Facial weakness Facial diplegia, which with scapular winging makes the late-onset form resemble facioscapulohumeral dystrophy.

Dysphagia Oropharyngeal muscle weakness.

Dysarthria Weakness of the bulbar and facial musculature used for articulation.

Scapular winging Periscapular weakness, which with facial weakness produces the FSHD-like appearance of the adult-onset form.

Motor regression In infants, progressive fibre loss takes away motor milestones that had already been acquired.

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Myopathic changes included atrophic and/or necrotic fibres (28/36, 69.5%), fibre size variability/type 1 predominance (24/36, 66.6%), increased central nuclei (11/36, 30.5%), sarcoplasmic vacuoles (5/36, 13.9%), lipid droplets or fat replacement of muscle tissue (7/36, 19.5%), and fibrosis or..."

The histological components of fibre degeneration, with frequencies.

PMID:29602790 SUPPORT Human Clinical

"The combination of dystrophic features with COX defect and RRF is highly suggestive of TK2 deficiency"

The diagnostic value of the combined pattern.

PMID:31060578 SUPPORT Human Clinical

"On muscle biopsies typical signs of mitochondrial dysfunction were associated with dystrophic changes."

The same dual pattern in the late-onset form.

Respiratory Muscle Failure

Weakness of the diaphragm and accessory respiratory muscles produces a restrictive ventilatory defect, ventilator dependence and, in most fatal cases, death. Two thirds of the largest cohort needed invasive or non-invasive ventilation, and just over a quarter of all patients are known to have died of respiratory failure. In some late-onset patients nocturnal dyspnoea was the first symptom.

Downstream

Respiratory insufficiency The clinical expression of respiratory muscle weakness.

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Respiratory muscles were severely compromised in 33/51 (65%) patients, who required mechanical ventilation or nocturnal/continuous non-invasive ventilation"

Frequency of ventilatory support.

PMID:29602790 SUPPORT Human Clinical

"27.2% (25 of 92) of the patients are known to have died from respiratory failure"

Respiratory failure as the cause of death.

PMID:29602790 SUPPORT Human Clinical

"In some patients with late-onset myopathy, nocturnal dyspnoea was an initial symptom of the disease."

Respiratory involvement can be the presenting feature.

Central Nervous System Involvement

A minority of patients have nervous-system disease: seizures, encephalopathy, cognitive impairment, sensorineural hearing loss or episodic coma, in about a sixth of the largest cohort and a quarter of its infantile-onset group. The causal attribution is not settled. TK2 is ubiquitous and the knock-in mouse develops a frank encephalomyelopathy with respiratory-chain deficits most prominent in the central nervous system, so a primary neuronal mtDNA-depletion mechanism is plausible; but the natural-history authors note that restrictive lung disease is so frequent in this disease that hypoxic-ischaemic injury cannot be excluded in individual patients.

Animal models

Tk2 H126N homozygous knock-in Mouse Mouse recapitulates The mouse develops a frank encephalomyelopathy with anterior-horn vacuolar change, which is the strongest available argument that human CNS features can be a primary consequence of neuronal mtDNA depletion rather than of hypoxia.

Downstream

Seizures Recurrent seizures in six of 92 patients.

Encephalopathy Encephalopathy in seven of 92 patients, concentrated in infantile onset.

Sensorineural hearing loss Reported in three of 92 patients.

Show evidence (5 references)

PMID:29602790 SUPPORT Human Clinical

"The nervous system was affected in 15 out of 92 (16%) causing recurrent seizures (six), encephalopathy (seven), cognitive impairment (four), sensorineural hearing loss (three) and episodic coma (one)."

Human frequency and composition of nervous-system involvement.

PMID:29602790 SUPPORT Human Clinical

"CNS signs are more frequent and severe in the infantile form with 10/39 patients (25.6%) manifesting as encephalopathy, seizures or cognitive impairment"

Concentration of CNS involvement in the infantile-onset form.

PMID:29602790 REFUTE Human Clinical

"it is uncertain in some cases whether the cerebral manifestations might have been due to hypoxic-ischaemic damage in the setting of respiratory failure or infections"

The authors' own caution against attributing every cerebral manifestation to a primary neuronal mechanism, which is why this node is framed as unsettled.

+ 2 more references

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Histopathology

1

Skeletal muscle biopsy

Muscle histochemistry was abnormal in every patient in whom it was performed: cytochrome c oxidase-deficient fibres with ragged-red fibres in three quarters, COX-deficient fibres alone in a further fifth, and isolated ragged-red fibres in the remainder. On routine histology there are atrophic and necrotic fibres, fibre-size variability with type 1 predominance, increased internal nuclei and replacement by fat and connective tissue, so the biopsy reads as a dystrophy with a mitochondrial overlay. Respiratory-chain enzyme activities were reduced in the great majority of biopsies.

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"In 87.5% of the biopsies, biochemical activities of respiratory chain enzymes were reduced with isolated complex IV deficiency or variable defects of complexes I, III and IV."

Proportion of biopsies with reduced respiratory-chain activity, and the pattern.

PMID:29602790 SUPPORT Human Clinical

"Histochemical data were available in 77% of patients confirming COX deficiency with or without RRFs as morphological hallmarks of TK2 deficiency."

The histochemical hallmarks.

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Pathograph

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

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Phenotypes

22

Digestive 1

Dysphagia OCCASIONAL HP:0002015

Sequelae: Failure to thrive

Show evidence (2 references)

PMID:31060578 SUPPORT Human Clinical

"Half of patients presented difficulty in swallowing."

Frequency of dysphagia in the late-onset series.

PMID:29602790 SUPPORT Human Clinical

"Nineteen patients required gastrostomy tube because of severe dysphagia, failure to thrive or both."

Severity of bulbar involvement, measured by gastrostomy requirement.

Ear 1

Sensorineural hearing loss RARE Sensorineural hearing impairment HP:0000407

Show evidence (1 reference)

PMID:23230576 SUPPORT Human Clinical

"at least annual audiology evaluations in those with infantile-onset disease"

GeneReviews surveillance recommendation, which presupposes the hearing-loss risk.

Eye 2

Ptosis OCCASIONAL HP:0000508

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"Other muscular functions were affected in 33/85 (39%) patients who manifested variable combinations of ptosis (22), facial diplegia (19), progressive external ophthalmoparesis (PEO) (11), mild dysphagia (10), dysarthria/dysphonia (3) and rigid spine (1)"

Counts of the extraocular, facial and bulbar features, including ptosis.

PMID:31060578 SUPPORT Human Clinical

"The first symptom was muscle limb weakness in 10/18, eyelid ptosis in 6/18, and respiratory insufficiency in 2/18."

Ptosis as a presenting symptom in a third of late-onset patients.

Progressive external ophthalmoparesis OCCASIONAL HP:0000597

Course: PROGRESSIVE

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"the involvement of ocular muscles with ptosis or PEO is more frequent in late-onset cases (11/16, 68.7%) than in the childhood-onset (11/37, 29.7%) and infantile-onset forms (3/39, 7.7%)"

Frequency of ocular involvement by clinical form.

PMID:35094997 SUPPORT REVIEW SYNTHESIS Other

"limb myopathy with ptosis, ophthalmoparesis, and respiratory involvement is more common in the late-onset form"

A review's synthesis of the late-onset clinical picture.

Head and Neck 1

Facial weakness OCCASIONAL Facial diplegia HP:0001349

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Other muscular functions were affected in 33/85 (39%) patients who manifested variable combinations of ptosis (22), facial diplegia (19), progressive external ophthalmoparesis (PEO) (11), mild dysphagia (10), dysarthria/dysphonia (3) and rigid spine (1)"

Source of the 19-of-85 count, and of the bilateral ("diplegia") wording this phenotype is bound to.

PMID:29602790 SUPPORT Human Clinical

"The adult myopathy shares prominent facial weakness with facioscapulohumeral muscular dystrophy (FSHD)."

Facial weakness and the diagnostic confusion it creates.

PMID:29602790 SUPPORT Human Clinical

"Ophthalmoparesis and facial weakness are frequent in adults."

Frequency of facial weakness in adult-onset disease.

Limbs 1

Scapular winging OCCASIONAL HP:0003691

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"adult-onset myopathy, which mimicked facioscapulohumeral dystrophy with marked facial weakness, scapular winging or both in seven (43.75%)"

Scapular winging in the adult-onset form, with its frequency.

Metabolism 2

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

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"CK levels, available in 57 out of 92 patients, were elevated in all from 1.3-fold to 30-fold above the upper limit of normal (range 272-6500 U/L)."

Frequency and magnitude of the creatine kinase elevation.

PMID:29602790 REFUTE Human Clinical

"normal in only five: one with infantile-onset myopathy and four with childhood-onset myopathy, indicating that a normal CK does not exclude TK2 deficiency"

Refutes treating a raised creatine kinase as obligatory; the same paper that reports near-universal elevation states the exception explicitly.

Elevated blood lactate OCCASIONAL Increased circulating lactate concentration HP:0002151

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"Genetic screening for TK2 deficiency must be considered in children, particularly infants, with rapidly progressing muscle weakness, especially when associated with elevated blood lactate and CK."

Raised lactate as a recognised accompanying finding and diagnostic prompt.

Musculoskeletal 4

Proximal muscle weakness VERY_FREQUENT HP:0003701

Course: PROGRESSIVE

Sequelae: Loss of ambulation

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"All patients had proximal muscle weakness as the first sign of disease or during the clinical course."

Establishes both the phenotype and its near-universal frequency.

PMID:22345218 SUPPORT Human Clinical

"generalized muscle weakness predominating on axial and proximal muscles but affecting facial, ocular, and respiratory muscles"

Distribution of weakness in adult-onset patients.

Hypotonia VERY_FREQUENT HP:0001252

Show evidence (2 references)

PMID:23932787 SUPPORT Human Clinical

"Clinically, hypotonia and proximal muscle weakness are the major phenotypes present in all subjects."

Hypotonia in every patient of an 11-subject, 9-family series.

PMID:29602790 SUPPORT Human Clinical

"While hypotonia and proximal muscle weakness are common in all of the three clinical forms"

Hypotonia is not confined to the infantile form.

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

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"Muscle histochemistry was abnormal in all 71 patients studied: 53/71 (74.6%) had numerous cytochrome c oxidase (COX)-deficient and ragged-red fibres (RRFs)"

Frequency of ragged-red fibres together with COX deficiency.

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

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"14/71 (19.7%) had only COX-deficient fibres and 4/71 (5.7%) had isolated RRFs"

Frequency of isolated COX-deficient fibres.

PMID:22345218 SUPPORT Human Clinical

"typical mitochondrial myopathy with a mosaic pattern of COX-negative and ragged-red fibers"

The mosaic pattern in adult patients.

Nervous System 5

Loss of ambulation FREQUENT HP:0002505

Course: PROGRESSIVE

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"Global motor function was severely impaired in 50/61 (82% with Karnofsky or Lansky Performance Status <50) who were wheelchair-bound at the last follow-up."

Proportion wheelchair-bound at last follow-up.

PMID:29602790 SUPPORT Human Clinical

"In contrast to the infantile-onset/childhood-onset patients, late-onset cases never became wheelchair-bound"

The late-onset form is the exception, which is why this is FREQUENT rather than VERY_FREQUENT.

Dysarthria OCCASIONAL HP:0001260

Show evidence (4 references)

PMID:23230576 SUPPORT Human Clinical

"Some individuals develop dysarthria, dysphagia, and/or hearing loss."

GeneReviews lists dysarthria among the features of the infantile-onset form.

PMID:29602790 SUPPORT Human Clinical

"Additional manifestations were ptosis (nine), PEO (eight), dysphagia (six), dysarthria/dysphonia (three), respiratory insufficiency (five), cardiomyopathy (two)"

Counts of additional features in the sixteen late-onset patients, where dysarthria/dysphonia is three. The higher figure in the same list, six, belongs to dysphagia.

PMID:29602790 SUPPORT Human Clinical

"Other muscular functions were affected in 33/85 (39%) patients who manifested variable combinations of ptosis (22), facial diplegia (19), progressive external ophthalmoparesis (PEO) (11), mild dysphagia (10), dysarthria/dysphonia (3) and rigid spine (1)"

The whole-cohort count, three of the 85 patients assessed for these features.

+ 1 more reference

Seizures OCCASIONAL HP:0001250

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"The nervous system was affected in 15 out of 92 (16%) causing recurrent seizures (six), encephalopathy (seven), cognitive impairment (four), sensorineural hearing loss (three) and episodic coma (one)."

The whole-cohort count, recurrent seizures in six of 92. Note this is the lower of the source's two figures.

PMID:29602790 SUPPORT Human Clinical

"Additional clinical manifestations were seizures (seven), encephalopathy (five), lissencephaly (one), cognitive dysfunction (three), ptosis (four), facial diplegia (three), dysphagia (three) and multiple bone fractures (two)."

Counts of extramuscular features in the 39 infantile-onset patients, where seizures are seven -- one more than the whole cohort's recurrent-seizure count, which is the discrepancy the description records.

Encephalopathy OCCASIONAL HP:0001298

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"The nervous system was affected in 15 out of 92 (16%) causing recurrent seizures (six), encephalopathy (seven), cognitive impairment (four), sensorineural hearing loss (three) and episodic coma (one)."

The whole-cohort encephalopathy count, seven of 92 -- the specific figure for this phenotype rather than the aggregate CNS-abnormality rate.

PMID:29602790 SUPPORT Human Clinical

"Additional clinical manifestations were seizures (seven), encephalopathy (five), lissencephaly (one), cognitive dysfunction (three), ptosis (four), facial diplegia (three), dysphagia (three) and multiple bone fractures (two)."

The infantile-onset encephalopathy count, five of 39, which sits inside the whole-cohort figure of seven.

PMID:29602790 SUPPORT Human Clinical

"Overall, 13/39 (33.3%) infantile-onset patients had non-skeletal muscle manifestations while 10/39 (25.6%) had central nervous system (CNS) abnormalities."

The broader CNS-abnormality denominator in infantile onset, retained as the context the encephalopathy count sits in rather than as the count itself.

Motor regression OCCASIONAL Developmental regression HP:0002376

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"Severe congenital myopathy and hypotonia or motor regression progressed rapidly to fatality in the majority of cases within 1 year of follow-up due to respiratory failure"

Motor regression as a presenting pattern of the infantile-onset form.

Respiratory 1

Respiratory insufficiency FREQUENT HP:0002093

Course: PROGRESSIVE

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"The vast majority of infantile-onset patients required ventilation (17/19, 89.5%) while ventilatory support was used in about half the childhood-onset (12/22, 54.4%) and late-onset (4/9, 44.4%) patients."

Ventilatory requirement by clinical form.

PMID:31060578 SUPPORT Human Clinical

"All patients showed evidence of respiratory muscle weakness, with need for non-invasive Mechanical Ventilation in 12/18."

Respiratory involvement is universal even in the mildest clinical form.

PMID:38544965 SUPPORT Human Clinical

"Approximately 30% of patients died of respiratory insufficiency, while 56% of surviving patients needed mechanical ventilation."

Mortality and ventilation in an independent 53-patient cohort.

Cellular 3

Depletion of mitochondrial DNA in muscle FREQUENT Depletion of mitochondrial DNA in muscle tissue HP:0009141

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"Higher proportions of infantile-onset (21/26; 81%) and childhood-onset (24/31; 77%) had mtDNA depletion than late onset (1/14; 7%)"

Frequency of muscle mtDNA depletion by clinical form.

PMID:23932787 SUPPORT Human Clinical

"Quantitative analysis of mtDNA content in skeletal muscle was performed in 5 subjects, and marked mtDNA content reduction was observed in each."

Independent confirmation in a separate multi-family series.

Multiple mitochondrial DNA deletions FREQUENT HP:0003689

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Multiple mtDNA deletions were detected in muscle of 24 patients primarily in late-onset patients: infantile-onset (1/8; 12.5%), childhood-onset (7/14; 50%) and late-onset (16/16; 100%)."

The count and its per-onset denominators. Summing them gives 24 of 38 tested, 63%, which is the basis for the FREQUENT band.

PMID:29602790 SUPPORT Human Clinical

"Muscle biopsies show multiple mtDNA deletions often with mtDNA depletion."

The two molecular findings coexist in muscle.

PMID:31060578 SUPPORT Human Clinical

"All cases showed multiple mtDNA deletions but mtDNA depletion was present only in two patients."

Deletions dominate in late-onset disease.

Decreased activity of mitochondrial complex IV OCCASIONAL HP:0008347

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"single enzyme abnormalities were detected in 7/48 (14.5%) (complex IV deficiency in 4/49, complex III in 1/48, isolated increase of citrate synthase in 1/48 and complex I in 1/48)"

Complex IV is the commonest isolated enzyme deficiency reported.

Growth 1

Failure to thrive OCCASIONAL HP:0001508

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"gastrostomy tube because of severe dysphagia, failure to thrive or both"

Failure to thrive as a documented indication for enteral feeding in this disease.

🧬

Genetic Associations

1

TK2

Gene: TK2 hgnc:11831 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (4 references)

PMID:11687801 SUPPORT Human Clinical

"Mutations in TK2 represent a new etiology for mitochondrial DNA depletion"

The original gene-disease association.

PMID:29602790 SUPPORT Human Clinical

"The substrate binding site (encoded by exon 5) and active site (exon 8) are hotspots for the mutations."

Location of the two recurrent regions.

PMID:29602790 SUPPORT Human Clinical

"The most common mutation, p.Thr108Met, was identified in a homozygous state in 13 patients with heterogeneous clinical phenotypes"

The commonest allele, and that one genotype spans several phenotypes.

+ 1 more reference

💊

Medical Actions

8

Deoxycytidine and Deoxythymidine Substitution

Action: Pharmacotherapy NCIT:C15986

Agent: doxecitine (deoxycytidine) CHEBI:15698 doxribtimine (deoxythymidine) CHEBI:17748

Platform: Small molecule

Oral deoxycytidine plus deoxythymidine -- given the pharmaceutical names doxecitine and doxribtimine and marketed as KYGEVVI -- the substrate-enhancement therapy that is the major therapeutic development in this disease, and since 3 November 2025 the first approved treatment for TK2 deficiency in the USA, where the licensed indication is restricted to symptom onset on or before age 12. The rationale is that raising the concentration of TK2's own substrates lets residual enzyme, and the cytosolic kinases, deliver enough pyrimidine deoxynucleotide for mtDNA replication. It was established in the Tk2 H126N knock-in mouse, where dC+dT delayed onset, extended lifespan and restored mtDNA copy number and respiratory-chain activity, and then given to 16 patients under expanded access. In that open-label cohort three of eight non-ambulatory patients regained walking, four of five discontinued feeding-tube use, and one of nine came off mechanical ventilation. Doses are 300-400 mg/kg/day of each nucleoside, titrated against stool frequency; dose-dependent diarrhoea occurred in half the patients and two of a separate group of twelve developed reversible transaminase elevation. The approval rested on pooled retrospective and single-arm data rather than a randomised comparison, and that data set is what draws the age limit: mortality was 4% in treated against 57% in untreated patients with onset at or before 12 years, but in later-onset patients the difference, 18% against 24%, was not significant. A 2022 review, written before the filing, still described the supporting trials as ongoing.

Mechanism Target:

Imbalanced Mitochondrial dNTP Pool — Supplying the substrates in excess is intended to restore the pyrimidine arm of the mitochondrial deoxynucleotide pool without replacing the enzyme.

Skeletal Muscle mtDNA Depletion and Multiple Deletions — The proximate biochemical endpoint: restored mtDNA copy number, demonstrated in the mouse and inferred in patients from motor and biomarker response.

Respiratory Muscle Failure — The clinically decisive endpoint. One of nine ventilated patients became able to breathe independently on therapy.

Show evidence (13 references)

PMID:28318037 SUPPORT Model Organism

"We observed that dC+dT delayed disease onset, prolonged life span of Tk2-deficient mice and restored mtDNA copy number as well as respiratory chain enzyme activities and levels."

The preclinical result that established the nucleoside form of the therapy.

PMID:31125140 SUPPORT Human Clinical

"Three of 8 patients who were nonambulatory at baseline gained the ability to walk on therapy; 4 of 5 patients who required enteric nutrition were able to discontinue feeding tube use; and 1 of 9 patients who required mechanical ventilation became able to breathe independently."

The functional gains in the expanded-access cohort.

PMID:31125140 SUPPORT Human Clinical

"In 5 patients with early onset and severe disease, survival and motor functions were better than historically untreated patients. In 11 childhood and adult onset patients, clinical measures stabilized or improved."

The survival and stabilisation claim, and the comparator: historical controls, not a randomised arm.

+ 10 more references

Deoxycytidine and Deoxythymidine Monophosphate Bypass

Action: Pharmacotherapy NCIT:C15986

Platform: Small molecule

The original nucleotide formulation, dCMP plus dTMP, which was the first pharmacological treatment shown to work in the Tk2 knock-in mouse: it raised dTTP, increased mtDNA, improved respiratory-chain enzymes and extended median lifespan from 13 to 34 days. It has been largely superseded because the monophosphates are rapidly catabolised to the nucleosides after administration, which is what identified dC and dT as the active agents, and most patients were converted to nucleosides in 2015. Adding the deaminase inhibitor tetrahydrouridine to shield deoxycytidine from deamination was tested and made things worse.

Mechanism Target:

Imbalanced Mitochondrial dNTP Pool — Intended to bypass the missing kinase step entirely by supplying its products rather than its substrates.

Show evidence (4 references)

PMID:24968719 SUPPORT Model Organism

"the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes, and significantly prolong their lifespan (34 days with treatment versus 13 days untreated)"

The original bypass result, with the survival figures.

PMID:24968719 SUPPORT Model Organism

"dCMP/dTMP supplementation is the first effective pharmacologic treatment for Tk2 deficiency"

Its place in the therapeutic history of the disease.

PMID:31125140 SUPPORT BACKGROUND Human Clinical

"we subsequently observed that after administration, dCMP and dTMP are rapidly catabolized to the nucleosides dC and dT, suggesting that nucleosides, rather than nucleotides, are the major active therapeutic agents"

Why the nucleotide formulation was superseded by the nucleoside one.

+ 1 more reference

AAV-Mediated TK2 Gene Therapy

Action: Gene Therapy NCIT:C15238

Platform: Gene therapy Delivery: Viral vector

Investigational, preclinical only. AAV9 delivery of human TK2 cDNA to the knock-in mouse restored Tk2 activity in every tissue tested except kidney, delayed onset and extended lifespan; sequential AAV9 then AAV2 dosing let the viral dose be lowered while extending survival further, and adding dC+dT on top improved liver and kidney mtDNA copy number, growth and lifespan again. There is no human evidence.

Mechanism Target:

TK2 Loss of Function — Unlike substrate enhancement, gene replacement addresses the enzyme deficit itself rather than working around it.

Show evidence (2 references)

PMID:34338329 SUPPORT Model Organism

"AAV9 delivery of human TK2 cDNA to Tk2KI mice efficiently rescued Tk2 activity in all the tissues tested except the kidneys, delayed disease onset, and increased lifespan."

The gene-therapy result, including the tissue it failed to reach.

PMID:34338329 SUPPORT Model Organism

"addition of deoxycytidine and deoxythymidine supplementation to AAV9 + AAV2 treated Tk2KI mice dramatically improved mtDNA copy numbers in the liver and kidneys, animal growth, and lifespan"

The synergy with nucleoside therapy, which is the paper's central claim.

Respiratory Support and Ventilation

Action: mechanical ventilation Ontology label: Mechanical Ventilation NCIT:C70909

Platform: Device

Non-invasive or invasive ventilation, with chest physiotherapy to maintain pulmonary function and reduce infection risk. Two thirds of patients in the largest cohort required ventilatory support and respiratory failure is the leading cause of death, so this is the intervention that most often determines survival in practice.

Mechanism Target:

Respiratory Muscle Failure — Substitutes for the failing respiratory pump; it does not affect the underlying mtDNA maintenance defect.

Target Phenotypes: Respiratory insufficiency HP:0002093

Show evidence (2 references)

PMID:23230576 SUPPORT Human Clinical

"A pulmonologist can oversee chest physiotherapy to improve pulmonary function, reduce the risk of pulmonary infection, and manage respiratory insufficiency, if present."

GeneReviews respiratory management.

PMID:29602790 SUPPORT Human Clinical

"who required mechanical ventilation or nocturnal/continuous non-invasive ventilation"

The modalities used in the cohort.

Gastrostomy Feeding

Action: gastrostomy tube placement Ontology label: Gastrostomy Tube Procedure NCIT:C157864

Platform: Surgery

Enteral feeding by nasogastric or gastrostomy tube where aspiration risk is high or intake is inadequate. Nineteen patients in the largest cohort required a gastrostomy. This is one of the outcomes that reversed on nucleoside therapy, in four of five patients who had been tube-dependent.

Target Phenotypes: Dysphagia HP:0002015 Failure to thrive HP:0001508

Show evidence (1 reference)

PMID:23230576 SUPPORT Human Clinical

"Feeding difficulties should be managed aggressively, including use of a nasogastric tube or gastrostomy tube when the risk for aspiration is high."

GeneReviews feeding management.

Physical Therapy and Rehabilitation

Action: Physical Therapy NCIT:C15302

Platform: Behavioral / lifestyle

Physiotherapy to maintain muscle function and prevent joint contractures, with physical medicine and rehabilitation input for patients who have difficulty walking. Supportive rather than disease-modifying; no outcome data for this disease are cited in the sources used here.

Target Phenotypes: Proximal muscle weakness HP:0003701

Show evidence (2 references)

PMID:23230576 SUPPORT Human Clinical

"Physical therapy can help maintain muscle function; a physical medicine and rehabilitation (PM&R) specialist can help those who have difficulty walking."

GeneReviews rehabilitation recommendation.

PMID:23230576 SUPPORT Human Clinical

"physical therapy can help prevent joint contractures"

The secondary-complication rationale.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Platform: Behavioral / lifestyle

Autosomal recessive counselling with 25% sibling recurrence risk. Carrier and prenatal testing are available once the family's variants are known. In Spain the two founder alleles make targeted carrier testing particularly informative.

Show evidence (1 reference)

PMID:23230576 SUPPORT Human Clinical

"Carrier testing for at-risk family members and prenatal testing for a pregnancy at increased risk are possible if the pathogenic variants in the family have been identified."

Availability of carrier and prenatal testing.

Multidisciplinary Supportive Care

Action: Supportive Care NCIT:C15747

Platform: Other

Coordinated multidisciplinary management, with growth and weight monitoring, pulmonary function testing, neurodevelopmental assessment and annual audiology in infantile-onset disease. GeneReviews states explicitly that no clinical guidelines exist, so the surveillance schedule is expert recommendation.

Show evidence (2 references)

PMID:23230576 SUPPORT Human Clinical

"Management should involve a multidisciplinary team."

The care model recommended.

PMID:23230576 SUPPORT Human Clinical

"No clinical guidelines are available."

The evidence base for surveillance is expert recommendation, which is what this quote records.

🔬

Diagnosis

6

TK2 molecular testing

Biallelic pathogenic TK2 variants establish the diagnosis. Because the late-onset form mimics facioscapulohumeral dystrophy and the childhood form mimics spinal muscular atrophy type 3, the natural-history authors recommend screening genetically undiagnosed recessive myopathies, including FSHD-like cases, for TK2 variants.

Show evidence (2 references)

PMID:23230576 SUPPORT Human Clinical

"The diagnosis is confirmed by the identification of biallelic pathogenic variants in TK2 by molecular genetic testing."

The confirmatory test.

PMID:29602790 SUPPORT Human Clinical

"We suggest that genetically undiagnosed patients with autosomal recessive myopathies, including FSHD-like cases, should be screened for TK2 mutations in blood or mitochondrial alterations in muscle biopsies."

Who to test, given the mimics.

Muscle mtDNA copy number

Quantitative mtDNA content in skeletal muscle is the tissue-level criterion. GeneReviews sets it at typically below 20% of age- and tissue-matched control for an infantile presentation; beyond age two, reduced content or multiple deletions with ragged-red and/or COX-deficient fibres suffices. Note that a late-onset patient may have normal copy number with deletions only, so a normal quantification does not exclude the diagnosis in an adult.

Show evidence (2 references)

PMID:23230576 SUPPORT Human Clinical

"established in a proband with infantile onset of disease with severely reduced (typically <20% of age- and tissue-matched healthy controls) mtDNA content in skeletal muscle"

The infantile-onset diagnostic threshold.

PMID:23230576 SUPPORT Human Clinical

"established in a proband older than age two years with reduced mtDNA content or multiple mtDNA deletions, ragged red fibers and/or COX-deficient fibers in skeletal muscle"

The criterion beyond infancy, which admits deletions in place of depletion.

Muscle magnetic resonance imaging

A reproducible pattern of selective muscle involvement in late-onset disease, worst in gluteus maximus, semitendinosus and sartorius. It is a non-invasive adjunct to the biopsy rather than a confirmatory test: the pattern was derived from a single eighteen-patient series and has not been compared against the other myopathies TK2 deficiency is mistaken for, so it can direct a biopsy and a TK2 test but cannot substitute for either.

muscle magnetic resonance imaging NCIT:C16809

Show evidence (1 reference)

PMID:31060578 SUPPORT Human Clinical

"We identified common radiological features in muscle magnetic resonance, where the most severely affected muscles were the gluteus maximus, semitendinosus and sartorius."

The selective muscle-involvement pattern on MRI, and its single-series provenance.

Muscle biopsy histochemistry

Dystrophic features together with a cytochrome c oxidase defect and ragged-red fibres is the histological pattern that should raise TK2 deficiency, but it is not confirmatory on its own.

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"must be supplemented by molecular genetic studies to confirm TK2 deficiency"

The histology is suggestive, not diagnostic.

Electromyography

Myopathic motor unit potentials in about four fifths of patients tested. Neurogenic changes occur in a minority, and two patients had a genuine sensory axonal neuropathy, so a neurogenic EMG does not exclude the diagnosis.

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"EMG revealed myopathic changes in 78.6% of patients tested."

Proportion with myopathic electrophysiology.

PMID:29602790 SUPPORT Human Clinical

"These data demonstrate peripheral nerve involvement in a minority of patients with TK2 deficiency."

Peripheral nerve involvement in a minority, which complicates the electrophysiological picture.

Serum GDF-15

Growth differentiation factor 15 was raised at baseline and fell on treatment in every patient tested in the expanded-access programme, and is described as a biomarker correlating with therapeutic response rather than as a diagnostic test.

Show evidence (2 references)

PMID:35094997 SUPPORT REVIEW SYNTHESIS Other

"In the early expanded access program, growth differentiation factor 15 (GDF-15) appears to be a useful biomarker that correlates with therapeutic response."

The role claimed for GDF-15, which is treatment monitoring rather than diagnosis.

PMID:31125140 SUPPORT Human Clinical

"Baseline elevated serum growth differentiation factor 15 levels decreased with treatment in all 7 patients tested."

The primary observation behind that claim.

📈

Progression

3

Onset

Onset spans the whole of life, from birth to 72 years in the largest cohort, and the age at onset is the strongest single predictor of course. In the late-onset group the mean age at onset was 31 years, and subtle signs (for instance slow running) were often present in childhood without prompting medical evaluation.

Show evidence (3 references)

PMID:29602790 SUPPORT Human Clinical

"Age at disease onset ranged from birth to 72 years of age."

Full span of reported onset ages.

PMID:31060578 SUPPORT Human Clinical

"The mean age of onset was 31 years."

Mean onset age in the late-onset series.

PMID:29602790 SUPPORT Human Clinical

"In retrospect, subtle signs of myopathy were often present in childhood (eg, slow running) but did not prompt medical evaluation until adulthood."

Pre-symptomatic signs in the late-onset group.

Progression of weakness

Weakness progresses at rates that differ by an order of magnitude between the forms. Time from onset to ventilation or death is a median of 1.67 years in infantile onset against 10 years in childhood/late onset.

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"Median time to event is 1.67 (CI 0.11 to 2.33) years for infantile onset and 10.00 (CI 3.50 to 22.00) years for childhood/late onset."

Event-free survival, where the event is ventilation or death.

Survival

Median post-onset survival is about one year for infantile onset and at least 23 years for the childhood- and late-onset groups combined, a difference of around twentyfold. Respiratory failure is the cause of death in about a quarter of all patients, and in every fatal case of the late-onset series.

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"Median post-onset survival (POS) is 1.0 years (CI 0.58 to 2.33) for the infantile group and at least 23.0 years (CI 10.0 to incomputable) for the childhood-onset/late-onset group."

Post-onset survival by clinical form.

PMID:31060578 SUPPORT Human Clinical

"Four patients had deceased, all of them due to respiratory insufficiency."

Cause of death in the late-onset series.

📊

Prevalence

2

Worldwide

Cases In Literature Ultra Rare

No population-based prevalence estimate is available in the sources cited here. GeneReviews counts approximately 107 molecularly confirmed individuals reported to date; the 2018 multicentre natural-history study assembled 92, and a 2024 Spanish study added a further series of 53. The disorder is considered under-diagnosed because of the breadth of its presentations.

Show evidence (3 references)

PMID:23230576 SUPPORT Human Clinical

"To date, approximately 107 individuals with a molecularly confirmed diagnosis have been reported."

Cumulative count of reported molecularly confirmed cases.

PMID:29602790 SUPPORT Human Clinical

"We identified 92 patients with genetically confirmed diagnoses of TK2 deficiency: 67 from literature review and 25 unreported cases."

Size of the largest assembled natural-history cohort.

PMID:29602790 SUPPORT Human Clinical

"TK2 deficiency is a frequent mitochondrial predominantly myopathic disorder that may be under-diagnosed due to its diverse spectrum of clinical presentations."

The authors' own statement that ascertainment is incomplete.

Spain

Carrier Frequency Unknown

Two founder alleles, p.Lys202del and p.Thr108Met, are enriched in the Spanish population relative to gnomAD, which is why Spanish centres contribute a disproportionate share of reported patients. No carrier rate is given as a frequency in the cited source, only the fold enrichment.

Show evidence (1 reference)

PMID:38544965 SUPPORT Human Clinical

"Two variants, p.Lys202del and p.Thr108Met, exhibited significantly higher prevalence in the Spanish population than that reported in gnomAD database (86-fold and 13-fold, respectively)."

Founder-allele enrichment in Spain.

🔀

Differential Diagnoses

6

Conditions with similar clinical presentations that must be differentiated from Mitochondrial DNA Depletion Syndrome, Myopathic Form:

Mitochondrial DNA depletion syndrome 3 (hepatocerebral type) MONDO:0009636

Overlapping Features The purine mirror image of this disease, and the closest mechanistic relative in the knowledge base. DGUOK encodes deoxyguanosine kinase, the other mitochondrial deoxyribonucleoside kinase: between them, dGK and TK2 phosphorylate all four deoxynucleosides. Loss of either impairs the same salvage pathway and produces mtDNA depletion, so the biochemistry is shared and the separation is by substrate arm and target organ.

Distinguishing Features

DGUOK phosphorylates the purine deoxynucleosides deoxyguanosine and deoxyadenosine; TK2 phosphorylates the pyrimidines deoxythymidine, deoxycytidine and deoxyuridine.
DGUOK deficiency is liver-predominant with cholestatic liver failure, nystagmus and developmental delay; TK2 deficiency is skeletal-muscle-predominant and the liver is spared.
In the original TK2 series, liver, brain, heart and skin remained unaffected throughout life, which is the tissue distinction rather than a severity one.
Nucleoside substitution is established for TK2 deficiency; for DGUOK deficiency the intervention that changes survival is liver transplantation.

Show evidence (2 references)

PMID:18467430 SUPPORT BACKGROUND Model Organism

"TK2 mutations have been associated with severe infantile myopathy, a milder myopathy with later onset and longer survival, rigid spine syndrome and spinal muscular atrophy (SMA)-like motor neuron disease while DGUOK mutations cause hepatocerebral disease"

States the organ split between the two deoxyribonucleoside kinase deficiencies.

PMID:12765840 SUPPORT Human Clinical

"Other tissues such as liver, brain, heart, and skin remain unaffected throughout the patients' life."

The organ sparing that separates TK2 from DGUOK deficiency clinically.

Mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria MONDO:0012791

Overlapping Features The encephalomyopathic member of the MDS family, from SUCLA2 (or SUCLG1) deficiency. It shares infantile hypotonia, muscle mtDNA depletion and lactate elevation with the infantile-onset form of TK2 deficiency.

Distinguishing Features

SUCLA2 encodes a subunit of succinyl-CoA ligase in the TCA cycle, not a deoxyribonucleoside kinase; the mtDNA depletion there is a downstream consequence rather than a precursor shortage.
Methylmalonic aciduria with raised C4-dicarboxylic carnitine is the biochemical signature of SUCLA2 deficiency and is absent in TK2 deficiency.
SUCLA2 disease is dominated by dystonia, deafness and basal ganglia change; TK2 disease is dominated by limb and respiratory myopathy with cognition typically spared.

Show evidence (1 reference)

PMID:23230576 SUPPORT Human Clinical

"Cognitive function is typically spared."

Cognitive sparing in TK2 deficiency, against the encephalopathic course of the SUCLA2 form.

Mitochondrial neurogastrointestinal encephalomyopathy MONDO:0017575

Overlapping Features The other pyrimidine-pool mtDNA maintenance disorder. TYMP encodes thymidine phosphorylase, which catabolises the same nucleosides TK2 phosphorylates, so the two diseases sit on opposite sides of one metabolic balance: TK2 deficiency depletes the pool and MNGIE floods it with thymidine and deoxyuridine. Both cause mtDNA depletion, multiple deletions and a mitochondrial myopathy with ptosis and ophthalmoparesis.

Distinguishing Features

MNGIE is defined by gastrointestinal dysmotility with cachexia and by leukoencephalopathy on MRI, neither of which features in TK2 deficiency.
MNGIE is diagnosed by raised plasma thymidine and deoxyuridine with absent thymidine phosphorylase activity; TK2 deficiency has no such circulating marker.
Thymidine phosphorylase is the enzyme that counteracts TK2 by lowering its substrates, so a nucleoside-supplementation strategy would be actively harmful in MNGIE.

Show evidence (2 references)

PMID:20940150 SUPPORT BACKGROUND Model Organism

"Strict homeostatic maintenance of the mitochondrial deoxythymidine monophosphate (dTMP) and deoxycytidine monophosphate (dCMP) levels is achieved by counteracting TK2 activity with the catabolic actions of cytosolic thymidine phosphorylase (TP) that decreases the levels of TK2 substrates"

Establishes that TK2 and thymidine phosphorylase are opposed arms of one pool, which is why the two diseases are mechanistic mirror images.

PMID:29602790 SUPPORT Human Clinical

"A growing number of mitochondrial diseases with mtDNA instability have been linked to mitochondrial deoxynucleotide pool imbalance, including deficiencies of TK2, dGK, RRM2B or TYMP."

Places TK2, DGUOK and TYMP deficiencies in one mechanistic class.

Facioscapulohumeral muscular dystrophy MONDO:0001347

Overlapping Features The clinical mimic of the adult-onset form. Nearly half of adult-onset TK2 patients in the largest cohort presented with marked facial weakness, scapular winging or both, which is the FSHD phenotype, and the natural-history authors recommend screening FSHD-like cases without a molecular diagnosis for TK2 variants.

Distinguishing Features

FSHD is dominantly inherited (a contracted D4Z4 repeat with a permissive 4qA allele in FSHD1, or an SMCHD1/DNMT3B variant in FSHD2); TK2 deficiency is autosomal recessive.
FSHD weakness is characteristically asymmetric and descending; TK2 weakness is generalised with axial and proximal predominance.
FSHD muscle does not show COX-negative and ragged-red fibres with mtDNA depletion or multiple deletions.

Show evidence (1 reference)

PMID:29602790 SUPPORT Human Clinical

"The adult myopathy shares prominent facial weakness with facioscapulohumeral muscular dystrophy (FSHD)."

The basis of the clinical confusion.

Spinal muscular atrophy MONDO:0001516

Overlapping Features The clinical mimic across the whole severity range. The natural-history authors map infantile TK2 myopathy onto SMA type 1 and severe type 2, childhood onset onto milder type 2 and type 3a, and late onset onto types 3b and 4, noting that TK2 weakness is more progressive.

Distinguishing Features

SMA is a motor neuron disease from biallelic SMN1 loss with a modifying SMN2 copy number; TK2 deficiency is a primary myopathy.
Creatine kinase is raised 1.3 to 30-fold in TK2 deficiency; it is normal or only mildly raised in SMA.
Electromyography is myopathic in about four fifths of TK2 patients, against the neurogenic pattern of SMA.
Muscle biopsy in TK2 deficiency shows COX-negative and ragged-red fibres with reduced mtDNA copy number; SMA shows grouped fibre-type atrophy without mitochondrial change.

Show evidence (2 references)

PMID:29602790 SUPPORT Human Clinical

"The range of weakness severity among TK2-deficient patients parallels the spectrum of SMA with infantile-onset TK2 myopathy resembling SMA type 1 and severe type 2, childhood-onset myopathy similar to milder type 2 and 3a SMA, and late-onset myopathy comparable to SMA types 3b and IV, although..."

The explicit mapping of TK2 forms onto SMA types, and the caveat.

PMID:28318037 SUPPORT BACKGROUND Model Organism

"childhood-onset myopathy, which resembles spinal muscular atrophy (SMA) type III"

An independent statement of the SMA type 3 resemblance.

🔬

Clinical Trials

2

NCT03845712 PHASE_II COMPLETED

Open-label continuation study of doxecitine and doxribtimine (the pharmaceutical names for deoxycytidine and deoxythymidine) in patients who had been on nucleoside treatment, following the retrospective study MT-1621-101. This is the formal-trial arm of the expanded-access experience.

Target Phenotypes: Proximal muscle weakness HP:0003701 Respiratory insufficiency HP:0002093

Show evidence (1 reference)

clinicaltrials:NCT03845712 SUPPORT Other

"This is a Phase 2 prospective open-label treatment study of the safety and efficacy of doxecitine and doxribtimine in study participants with thymidine kinase 2 (TK2) deficiency"

Registry record establishing the trial's design, agents and population.

NCT06754098 PHASE_II RECRUITING

Single-arm off-label study of doxecitin and doxribthymine in adults with TK2 deficiency at a single Spanish neuromuscular unit, with mtDNA levels in muscle and uroepithelial cells as biological endpoints alongside efficacy and safety. Adults are the group the natural-history data cover least well.

Target Phenotypes: Proximal muscle weakness HP:0003701

Show evidence (1 reference)

clinicaltrials:NCT06754098 SUPPORT Other

"the mitochondrial DNA levels before and after treatment (extracted from the muscle and from uroepithelial cells) of these participants will be also studied"

The registry record's statement of the mtDNA endpoint.

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Animal Models

1

Tk2 H126N homozygous knock-in Mouse

The reference model of the disease, homologous to the human H121N allele. Homozygotes are normal at birth, stop gaining weight around postnatal day 10 and die of a rapidly progressive encephalomyelopathy at 2-3 weeks. It is the system in which the mitochondrial dNTP pool imbalance was measured, in which both therapies were developed, and whose organ distribution differs informatively from the human disease.

Species

Mouse

Genotype

Tk2 H126N homozygous knock-in

Background

C57BL/6-derived, generated by homologous recombination in embryonic stem cells

Genes

TK2 hgnc:11831

Publication

PMID:18467430

Pathograph links

Imbalanced Mitochondrial dNTP Pool recapitulates fidelity: high observes: organism The pool imbalance that is asserted but not measured in patients is measured directly here: low dTTP in brain and reduced dTTP and dCTP in liver, with mtDNA depletion and respiratory-chain deficits following.

Readouts

Tissue dTTP and dCTP concentrations

Direction: DECREASED

Interpretation: Direct measurement of the precursor shortage that the human entry can only infer.

Show evidence (1 reference)

PMID:24968719 SUPPORT Model Organism

"loss of Tk2 activity caused dNTP pool imbalances with low dTTP levels in brain and decreased dTTP and dCTP in liver"

The measured pool deficits and their tissue distribution.

Tissue mtDNA copy number on dCMP/dTMP treatment

Direction: RESTORED

Interpretation: Supplying the missing precursors raises mtDNA again, which is the pharmacological test of the mechanism rather than a correlation.

Show evidence (1 reference)

PMID:24968719 SUPPORT Model Organism

"the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes"

Restoration of pool, copy number and enzyme activity on treatment.

Show evidence (1 reference)

PMID:18467430 SUPPORT Model Organism

"animals showed Tk2 deficiency, unbalanced dNTP pools, mtDNA depletion and defects of respiratory chain enzymes containing mtDNA-encoded subunits"

Establishes the model as informative for the pool-imbalance node.

Combined Respiratory Chain Deficiency partially recapitulates fidelity: moderate observes: organism The mouse does develop deficiency of the mtDNA-dependent complexes, but the organ distribution is inverted relative to patients: the defect is CNS-predominant, and heart and skeletal muscle are largely spared by transcriptional compensation.

Limitations: In the mouse only brain showed significant respiratory-chain deficiency, with heart and muscle protected, whereas the human disease is defined by a skeletal-muscle respiratory-chain defect with the brain usually spared. A muscle-endpoint experiment in this model therefore does not test the organ in which the human disease is expressed.

Divergences from the human mechanism

species mismatch qualifying

Mouse Tk1 down-regulates sharply in the second postnatal week and the heart compensates by suppressing MTERF3 to raise mitochondrial transcript levels per mtDNA copy. Neither the timing nor the compensatory mechanism has a demonstrated human counterpart, and it is what spares mouse muscle and heart.

Readouts

Brain complex I and complex IV activities

Direction: DECREASED

Interpretation: Quantifies the biochemical defect and, by its tissue restriction, the model's divergence from the human organ pattern.

Show evidence (1 reference)

PMID:20940150 SUPPORT Model Organism

"the activity of complex I was 40% and that of complex IV 50% in brain of Tk2-/- versus Tk2+/+ mice"

The measured brain enzyme deficits.

Show evidence (1 reference)

PMID:20940150 REFUTE Model Organism

"However, only the brain showed significant deficiency of respiratory chain complexes (OXPHOS), protein levels and activities"

Refutes treating this model as a faithful system for the muscle respiratory-chain defect that defines the human disease.

Central Nervous System Involvement recapitulates fidelity: moderate observes: organism The mouse develops a frank encephalomyelopathy with anterior-horn vacuolar change, which is the strongest available argument that human CNS features can be a primary consequence of neuronal mtDNA depletion rather than of hypoxia.

Limitations: In patients CNS involvement is a minority feature (16% overall) and its attribution is confounded by respiratory failure, whereas in the mouse it is the lethal, defining lesion. The model therefore over-represents the importance of the CNS phenotype relative to the human disease.

Readouts

Spinal cord anterior horn histopathology

Direction: ALTERED

Interpretation: A structural CNS lesion attributable to Tk2 loss alone, with no respiratory confounder.

Show evidence (1 reference)

PMID:18467430 SUPPORT Model Organism

"Histopathology revealed an encephalomyelopathy with prominent vacuolar changes in the anterior horn of the spinal cord."

The histological CNS finding.

Show evidence (1 reference)

PMID:18467430 SUPPORT Model Organism

"The H126N TK2 mouse is the first knock-in animal model of human MDS and demonstrates that the severity of TK2 deficiency in tissues may determine the organ-specific phenotype."

The authors' own framing of the model's value, which is precisely about organ specificity.

Show evidence (2 references)

PMID:18467430 SUPPORT Model Organism

"Homozygous Tk2 mutant (Tk2(-/-)) mice developed rapidly progressive weakness after age 10 days and died between ages 2 and 3 weeks."

The model's phenotype and timeline.

PMID:20940150 SUPPORT Model Organism

"Although normal until postnatal day 8, Tk2-/- mice rapidly develop fatal encephalomyopathy between postnatal days 10 and 13."

The narrow window in which the phenotype appears, which is what links it to Tk1 down-regulation.

{ }

Source YAML

click to show

name: Mitochondrial DNA Depletion Syndrome, Myopathic Form
creation_date: "2026-09-29T00:00:00Z"
category: Mendelian
disease_term:
  preferred_term: mitochondrial DNA depletion syndrome, myopathic form
  term:
    id: MONDO:0012301
    label: mitochondrial DNA depletion syndrome, myopathic form
synonyms:
- MTDPS2
- mitochondrial DNA depletion syndrome 2 (myopathic type)
- mitochondrial DNA depletion syndrome type 2
- mtDNA depletion syndrome, myopathic form
- thymidine kinase 2 deficiency
- TK2 deficiency
- TK2-related mitochondrial DNA maintenance defect, myopathic form
description: >-
  Autosomal recessive mitochondrial DNA (mtDNA) maintenance disorder caused by
  biallelic loss-of-function variants in TK2, which encodes thymidine kinase 2,
  the mitochondrial matrix enzyme that performs the first and rate-limiting
  phosphorylation of the pyrimidine deoxyribonucleosides deoxythymidine and
  deoxycytidine. Post-mitotic cells have no de novo deoxynucleotide synthesis
  inside mitochondria and cannot draw on the cell-cycle-regulated cytosolic
  kinases, so mtDNA replication there depends on this salvage step. Its loss
  leaves the mitochondrial deoxynucleoside triphosphate pool unbalanced and
  depleted of pyrimidines, and mtDNA copy number falls, most severely in
  skeletal muscle; older patients accumulate multiple mtDNA deletions instead of,
  or as well as, depletion. The consequence is a combined deficiency of the
  respiratory-chain complexes that contain mtDNA-encoded subunits, and a
  progressive myopathy. Presentation is a continuum usually divided into three
  forms: an infantile-onset myopathy with hypotonia, motor regression, frequent
  neurological involvement and death within about a year of onset; a
  childhood-onset limb myopathy progressing to loss of ambulation and ventilator
  dependence; and a late-/adult-onset myopathy with facial and limb weakness,
  ptosis and ophthalmoparesis that resembles facioscapulohumeral dystrophy and
  progresses slowly. Creatine kinase is almost always raised, and muscle biopsy
  shows ragged-red and cytochrome c oxidase-negative fibres on a dystrophic
  background. Respiratory muscle failure is the usual cause of death. Oral
  substitution with the TK2 products' precursors deoxycytidine and
  deoxythymidine (dC/dT) is the first treatment to alter the course, established
  in the Tk2 H126N knock-in mouse and then in an expanded-access cohort, and now
  under formal trial.
parents:
- mitochondrial DNA depletion syndrome
- inborn disorder of pyrimidine metabolism
- mitochondrial DNA maintenance defect
classifications:
  harrisons_chapter:
  - classification_value: NEUROLOGIC
    notes: >-
      A progressive myopathy with respiratory muscle involvement dominates every
      form of the disease, so the neuromuscular chapter is the primary placement.
    evidence:
    - reference: PMID:23230576
      reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Affected individuals experience progressive muscle weakness leading to respiratory failure."
      explanation: Muscle weakness with respiratory failure is the defining clinical course.
  icimd_category:
  - classification_value: nucleotide_pool_maintenance
    notes: >-
      ICIMD (Ferreira et al. 2021, PMID:33340416): group "Nucleotide pool
      maintenance" under category "Mitochondrial DNA maintenance and
      replication". TK2 supplies pyrimidine deoxynucleotides to the
      mitochondrial dNTP pool, which is the mechanism the group is defined by.
      The purine-side counterpart of the same group is DGUOK deficiency, curated
      as Mitochondrial DNA Depletion Syndrome 3 (Hepatocerebral Type).
  mechanistic_category:
  - classification_value: mitochondrial disease
    evidence:
    - reference: PMID:22345218
      reference_title: "Adult cases of mitochondrial DNA depletion due to TK2 defect: an expanding spectrum."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "typical mitochondrial myopathy with a mosaic pattern of COX-negative and ragged-red fibers, combined mtDNA-dependent respiratory complexes deficiency and mtDNA depletion"
      explanation: >-
        A combined deficiency of the mtDNA-dependent respiratory complexes
        secondary to mtDNA depletion from a nuclear-gene defect places this among
        the primary mitochondrial diseases.
references:
- reference: PMID:23230576
  title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
  tags:
  - GeneReviews
- reference: PMID:29602790
  title: Retrospective natural history of thymidine kinase 2 deficiency.
- reference: PMID:35094997
  title: "Advances in Thymidine Kinase 2 Deficiency: Clinical Aspects, Translational Progress, and Emerging Therapies."
inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    Biallelic (homozygous or compound heterozygous) germline TK2 variants.
    Heterozygous carriers are asymptomatic.
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TK2-related mtDNA maintenance defect is inherited in an autosomal recessive manner."
    explanation: Mode of inheritance.
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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: Recurrence risks consistent with autosomal recessive transmission.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    No population-based prevalence estimate is available in the sources cited
    here. GeneReviews counts approximately 107 molecularly confirmed individuals
    reported to date; the 2018 multicentre natural-history study assembled 92,
    and a 2024 Spanish study added a further series of 53. The disorder is
    considered under-diagnosed because of the breadth of its presentations.
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To date, approximately 107 individuals with a molecularly confirmed diagnosis have been reported."
    explanation: Cumulative count of reported molecularly confirmed cases.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified 92 patients with genetically confirmed diagnoses of TK2 deficiency: 67 from literature review and 25 unreported cases."
    explanation: Size of the largest assembled natural-history cohort.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TK2 deficiency is a frequent mitochondrial predominantly myopathic disorder that may be under-diagnosed due to its diverse spectrum of clinical presentations."
    explanation: The authors' own statement that ascertainment is incomplete.
- population: Spain
  measure_type: CARRIER_FREQUENCY
  prevalence_class: UNKNOWN
  notes: >-
    Two founder alleles, p.Lys202del and p.Thr108Met, are enriched in the Spanish
    population relative to gnomAD, which is why Spanish centres contribute a
    disproportionate share of reported patients. No carrier rate is given as a
    frequency in the cited source, only the fold enrichment.
  evidence:
  - reference: PMID:38544965
    reference_title: Clinical and Genetic Analysis of Patients With TK2 Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two variants, p.Lys202del and p.Thr108Met, exhibited significantly higher prevalence in the Spanish population than that reported in gnomAD database (86-fold and 13-fold, respectively)."
    explanation: Founder-allele enrichment in Spain.
has_subtypes:
- name: Infantile-onset
  display_name: Infantile-onset myopathy with neurological involvement
  description: >-
    Onset in the first year of life with severe congenital myopathy, hypotonia or
    motor regression, severe muscle mtDNA depletion, frequent central nervous
    system involvement and rapid progression to respiratory failure. Almost none
    achieve independent walking, and median post-onset survival is about one year.
  genes:
  - preferred_term: TK2
    term:
      id: hgnc:11831
      label: TK2
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "infantile-onset myopathy (42.4%) with severe mitochondrial DNA (mtDNA) depletion, frequent neurological involvement and rapid progression to early mortality"
    explanation: Defines the infantile-onset form and its share of the cohort.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle weakness was most prominent in the first year of life in the infantile-onset form, with 26 of 32 (81%) never acquiring the ability to walk independently and virtually all (30/32; 94%) non-ambulatory by age 4 years"
    explanation: Motor outcome in the infantile-onset form.
- name: Childhood-onset
  display_name: Childhood-onset limb myopathy
  description: >-
    Onset between one and twelve years with proximal limb weakness, Gowers sign
    and muscle mtDNA depletion, sometimes with multiple deletions. Weakness can
    resemble spinal muscular atrophy type 3. Most lose ambulation within ten
    years of onset and about half require ventilatory support; median post-onset
    survival is at least thirteen years.
  genes:
  - preferred_term: TK2
    term:
      id: hgnc:11831
      label: TK2
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "childhood-onset myopathy (40.2%) with mtDNA depletion, moderate-to-severe progression of generalised weakness and median POS at least 13 years"
    explanation: Defines the childhood-onset form and its survival.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In contrast, 19/30 (63%) of childhood-onset patients became wheelchair-bound within 10 years of disease onset"
    explanation: Loss of ambulation in the childhood-onset form.
- name: Late-onset
  display_name: Late-/adult-onset myopathy with multiple mtDNA deletions
  description: >-
    Onset at twelve years or later, often in adulthood, with mild limb weakness,
    marked facial weakness, scapular winging, ptosis and progressive external
    ophthalmoparesis. Multiple mtDNA deletions in muscle are near-universal while
    frank depletion is uncommon. Ambulation is retained but respiratory muscle
    weakness develops in most, and the presentation can be mistaken for
    facioscapulohumeral muscular dystrophy.
  genes:
  - preferred_term: TK2
    term:
      id: hgnc:11831
      label: TK2
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "late-onset myopathy (17.4%) with mild limb weakness at onset and slow progression to respiratory insufficiency with median POS of 23 years"
    explanation: Defines the late-onset form.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sixteen patients (17.4%) in our cohort presented with adult-onset myopathy, which mimicked facioscapulohumeral dystrophy with marked facial weakness, scapular winging or both in seven (43.75%)."
    explanation: The FSHD-like presentation of the late-onset form.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All cases showed multiple mtDNA deletions but mtDNA depletion was present only in two patients."
    explanation: The molecular signature that separates the late-onset form from the earlier forms.
pathophysiology:
- name: TK2 Loss of Function
  biological_scale: MOLECULAR
  description: >-
    Biallelic TK2 variants reduce or abolish thymidine kinase 2, the mitochondrial
    matrix deoxyribonucleoside kinase. Missense, in-frame deletion, splice and
    frameshift alleles are all reported, and residual activity measured in
    patient-derived fibroblasts spans 1-39% of control without tracking clinical
    severity, so the genotype does not by itself predict the form of disease.
    Recombinant work on the two original alleles shows two distinct kinetic
    lesions: I212N is essentially inactive, whereas H121N retains normal substrate
    affinity but loses catalytic efficiency through a raised Km for ATP.
  genes:
  - preferred_term: TK2
    term:
      id: hgnc:11831
      label: TK2
  genetic_context:
    variant_origin: GERMLINE
    functional_impact_category: LOSS_OF_FUNCTION
    description: >-
      Biallelic (homozygous or compound heterozygous) germline TK2 variants.
  molecular_functions:
  - preferred_term: thymidine kinase activity
    term:
      id: GO:0004797
      label: thymidine kinase activity
    modifier: DECREASED
  - preferred_term: deoxycytidine kinase activity
    term:
      id: GO:0004137
      label: deoxycytidine kinase activity
    modifier: DECREASED
  downstream:
  - target: Mitochondrial Pyrimidine Deoxynucleoside Salvage Failure
    description: >-
      Without TK2 the first and rate-limiting phosphorylation of deoxythymidine and
      deoxycytidine inside mitochondria does not occur.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23932787
      reference_title: "Molecular and clinical characterization of the myopathic form of mitochondrial DNA depletion syndrome caused by mutations in the thymidine kinase (TK2) gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "the myopathic form of MDS is mainly caused by mutations in the TK2 gene, which encodes thymidine kinase 2, the first and rate limiting step enzyme in the phosphorylation of pyrimidine nucleosides"
      explanation: Places TK2 at the rate-limiting step of the pathway this node feeds.
  evidence:
  - reference: PMID:11687801
    reference_title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In these individuals, the activity of TK2 in muscle mitochondria is reduced to 14-45% of the mean value in healthy control individuals."
    explanation: Direct demonstration of reduced enzyme activity in patient muscle mitochondria.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "TK2 activity measured in 11 patient-derived fibroblast cell lines showed residual activity ranging from 1% to 39% relative to healthy controls without correlation with clinical severity"
    explanation: Range of residual activity, and the absence of a correlation with severity.
  - reference: PMID:12493767
    reference_title: Kinetic properties of mutant human thymidine kinase 2 suggest a mechanism for mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The I212N mutant showed less than 1% activity as compared with wild type TK2 with all deoxynucleosides."
    explanation: A near-null allele characterised on recombinant enzyme.
  - reference: PMID:12493767
    reference_title: Kinetic properties of mutant human thymidine kinase 2 suggest a mechanism for mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "markedly increased K(m) values for ATP, leading to decreased enzyme efficiency"
    explanation: The distinct kinetic lesion of the H121N allele.
- name: Mitochondrial Pyrimidine Deoxynucleoside Salvage Failure
  biological_scale: MOLECULAR
  description: >-
    TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine to their
    monophosphates inside the mitochondrial matrix; these are then phosphorylated
    onward to the pyrimidine deoxynucleoside triphosphates used by the mtDNA
    replisome. The inner membrane is impermeable to charged nucleotides and
    mitochondria have no de novo synthesis pathway of their own, so in cells that
    are not dividing this salvage route is the principal supply.
  biological_processes:
  - preferred_term: pyrimidine deoxyribonucleoside salvage
    term:
      id: GO:0043099
      label: pyrimidine deoxyribonucleoside salvage
    modifier: DECREASED
  - preferred_term: deoxyribonucleoside triphosphate biosynthetic process
    term:
      id: GO:0009202
      label: deoxyribonucleoside triphosphate biosynthetic process
    modifier: DECREASED
  downstream:
  - target: Imbalanced Mitochondrial dNTP Pool
    description: >-
      Loss of the pyrimidine arm of mitochondrial salvage leaves the matrix dNTP
      pool short of dTTP and dCTP relative to the purine deoxynucleotides.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:24968719
      reference_title: Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "loss of Tk2 activity caused dNTP pool imbalances with low dTTP levels in brain and decreased dTTP and dCTP in liver"
      explanation: Measured pool imbalance in the knock-in mouse; the human pools have not been measured in the sources cited here.
  evidence:
  - reference: PMID:11687801
    reference_title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Human dGK efficiently phosphorylates deoxyguanosine and deoxyadenosine, whereas TK2 phosphorylates deoxythymidine, deoxycytidine and deoxyuridine."
    explanation: The substrate specificity that makes this the pyrimidine arm of mitochondrial salvage.
  - reference: PMID:11687801
    reference_title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In non-replicating cells, where cytosolic dNTP synthesis is down-regulated, mtDNA synthesis depends solely on the mitochondrial salvage pathway enzymes, the deoxyribonucleosides kinases."
    explanation: Why the salvage pathway is not redundant in post-mitotic tissue.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "TK2 phosphorylates the nucleosides deoxycytidine (dC) and deoxythymidine (dT) to generate deoxycytidine monophosphate (dCMP) and deoxythymidine monophosphate (dTMP)."
    explanation: States the reaction; an introduction sentence, not a result of the treatment study.
- name: Imbalanced Mitochondrial dNTP Pool
  biological_scale: MOLECULAR
  description: >-
    mtDNA replication is constitutive rather than cell-cycle-linked and therefore
    needs a continuous, balanced supply of all four deoxynucleoside triphosphates.
    A pool depleted of pyrimidines limits and distorts replication. Direct
    measurement of mitochondrial dNTP pools in patient muscle is not among the
    sources cited here; the imbalance is measured in the knock-in mouse and
    asserted as mechanism in the human discovery paper.
  biological_processes:
  - preferred_term: mitochondrial DNA replication
    term:
      id: GO:0006264
      label: mitochondrial DNA replication
    modifier: DECREASED
  downstream:
  - target: Skeletal Muscle mtDNA Depletion and Multiple Deletions
    description: >-
      An insufficient and unbalanced precursor supply limits mtDNA replication,
      lowering copy number and, over longer periods, generating deletions.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:11687801
      reference_title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Mutations in TK2 represent a new etiology for mitochondrial DNA depletion, underscoring the importance of the mitochondrial dNTP pool in the pathogenesis of mitochondrial depletion."
      explanation: The discovery paper's inference linking pool composition to depletion.
    - reference: PMID:18467430
      reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "animals showed Tk2 deficiency, unbalanced dNTP pools, mtDNA depletion and defects of respiratory chain enzymes containing mtDNA-encoded subunits"
      explanation: The full chain from enzyme loss through pool imbalance to depletion, in the knock-in mouse.
  evidence:
  - reference: PMID:18467430
    reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    quote_role: BACKGROUND
    snippet: "In contrast to nDNA synthesis, which is linked to cell cycle, mtDNA replication is constitutive and consequently requires a constant supply of deoxynucleoside triphosphates (dNTPs)"
    explanation: Why a constant balanced pool is required; an introduction sentence.
- name: Post-Mitotic Dependence on Mitochondrial Salvage
  biological_scale: CELLULAR
  description: >-
    TK2 is ubiquitously expressed, yet the disease is predominantly a myopathy.
    Two non-exclusive explanations are supported. In proliferating cells the
    cytosolic, cell-cycle-regulated kinases TK1 and dCK, together with de novo
    synthesis, supply enough deoxynucleotide to mask the defect; when TK1 activity
    falls in post-mitotic tissue the TK2 deficiency is unmasked, which in the mouse
    coincides exactly with the onset of mtDNA depletion. Independently, human
    tissue work found muscle to combine low basal TK2 activity with a high demand
    for mtDNA-encoded proteins.
  cell_types:
  - preferred_term: skeletal muscle fibre
    term:
      id: CL:0008002
      label: skeletal muscle fiber
  downstream:
  - target: Skeletal Muscle mtDNA Depletion and Multiple Deletions
    description: >-
      The loss of cytosolic compensation is what converts a ubiquitous enzyme
      deficiency into a tissue-restricted failure of mtDNA maintenance.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:20940150
      reference_title: "Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "The down-regulation of Tk1 activity unmasks Tk2 deficiency in Tk2-/- mice and correlates with the onset of mtDNA depletion in the brain and the heart."
      explanation: Ties loss of cytosolic compensation to the timing of mtDNA depletion.
  evidence:
  - reference: PMID:12765840
    reference_title: "mtDNA depletion myopathy: elucidation of the tissue specificity in the mitochondrial thymidine kinase (TK2) deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our results suggest that low basal TK2 activity combined with a high requirement for mitochondrial encoded proteins in muscle predispose this tissue to the devastating effect of TK2 deficiency."
    explanation: The human-tissue account of why skeletal muscle is the target organ.
  - reference: PMID:20940150
    reference_title: "Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "In contrast, TK2 activity has minor functional significance in proliferating cells due to the overwhelming activity of cell-cycle-regulated cytosolic enzymes involved in the deoxynucleoside triphosphate (dNTP) synthesis"
    explanation: States the cytosolic compensation that spares dividing cells.
- name: Skeletal Muscle mtDNA Depletion and Multiple Deletions
  biological_scale: CELLULAR
  description: >-
    Reduced mtDNA copy number in skeletal muscle is the defining molecular lesion,
    severe in the infantile and childhood forms and much less common in the
    late-onset form, where multiple mtDNA deletions are found in essentially every
    patient instead. Depletion below about 30% of age-matched control was present
    in two thirds of the largest cohort.
  biological_processes:
  - preferred_term: mitochondrial DNA metabolic process
    term:
      id: GO:0032042
      label: mitochondrial DNA metabolic process
    modifier: DECREASED
  locations:
  - preferred_term: skeletal muscle tissue
    term:
      id: UBERON:0001134
      label: skeletal muscle tissue
  downstream:
  - target: Combined Respiratory Chain Deficiency
    description: >-
      Too few intact mtDNA templates means too little of the 13 mtDNA-encoded
      respiratory-chain subunits, so the complexes containing them cannot be
      assembled in normal amounts.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:22345218
      reference_title: "Adult cases of mitochondrial DNA depletion due to TK2 defect: an expanding spectrum."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "combined mtDNA-dependent respiratory complexes deficiency and mtDNA depletion"
      explanation: The two findings co-occur in patient muscle.
  - target: Depletion of mitochondrial DNA in muscle
    description: The lesion is itself the reported laboratory phenotype.
    causal_link_type: DIRECT
  - target: Multiple mitochondrial DNA deletions
    description: >-
      In older patients the same maintenance failure manifests as accumulated
      deletions rather than, or as well as, reduced copy number.
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Levels of mtDNA in muscle were measured in 71 patients and showed significant depletion (<30% residual mtDNA relative to age-matched control subjects) in 47 (66%)."
    explanation: Frequency and threshold of depletion in the largest cohort.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multiple mtDNA deletions were detected in muscle of 24 patients primarily in late-onset patients: infantile-onset (1/8; 12.5%), childhood-onset (7/14; 50%) and late-onset (16/16; 100%)."
    explanation: The shift from depletion to deletions with later onset.
  - reference: PMID:11687801
    reference_title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we identify two mutations in TK2, histidine 90 to asparagine and isoleucine 181 to asparagine, in four individuals who developed devastating myopathy and depletion of muscular mitochondrial DNA in infancy."
    explanation: The original observation linking TK2 variants to muscle mtDNA depletion.
- name: Combined Respiratory Chain Deficiency
  biological_scale: CELLULAR
  conforms_to: "mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress"
  description: >-
    Reduced activity of the respiratory-chain complexes that contain mtDNA-encoded
    subunits. Multiple OXPHOS defects were found in about three quarters of
    patients in whom enzymology was performed, and isolated complex IV deficiency
    in a further small group; a minority of childhood- and late-onset patients had
    normal activities, so normal enzymology does not exclude the diagnosis.
  biological_processes:
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  downstream:
  - target: Progressive Skeletal Muscle Fibre Degeneration
    description: >-
      Deficient oxidative ATP production in a high-demand post-mitotic tissue
      drives fibre atrophy and necrosis.
    causal_link_type: DIRECT
  - target: Cytochrome c oxidase-negative muscle fibres
    description: >-
      The mosaic histochemical readout of the same lesion at single-fibre
      resolution.
    causal_link_type: DIRECT
  - target: Ragged-red muscle fibres
    description: Compensatory mitochondrial proliferation in affected fibres.
    causal_link_type: DIRECT
  - target: Decreased activity of mitochondrial complex IV
    description: >-
      Complex IV is the most frequent isolated enzyme deficiency measured in this
      disease, and part of the combined pattern in the rest.
    causal_link_type: DIRECT
  - target: Elevated blood lactate
    description: >-
      Impaired oxidative phosphorylation shifts pyruvate towards lactate. Raised
      lactate alongside raised creatine kinase is a recognised prompt to test TK2.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multiple OXPHOS defects were identified in 35/48 (73%) while single enzyme abnormalities were detected in 7/48 (14.5%)"
    explanation: Frequency of combined versus isolated respiratory-chain defects.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In six patients (12.5%) with childhood-onset (two) and late-onset (four) myopathies, OXPHOS activities were normal."
    explanation: Normal enzymology in a minority; a negative result worth recording.
  - reference: PMID:24968719
    reference_title: Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "defects of mitochondrial respiratory chain (RC) complexes I, III, IV, and V containing mtDNA-encoded subunits"
    explanation: >-
      Identifies which complexes are affected, and why they are: these are the
      complexes carrying mtDNA-encoded subunits.
- name: Progressive Skeletal Muscle Fibre Degeneration
  biological_scale: TISSUE
  description: >-
    Muscle shows a dystrophic pattern superimposed on the mitochondrial changes:
    atrophic and necrotic fibres, fibre-size variability with type 1 predominance,
    internal nuclei, and replacement of muscle by fat and connective tissue. This
    combination of dystrophic features with a cytochrome c oxidase defect and
    ragged-red fibres is the histological signature that should prompt TK2 testing.
  locations:
  - preferred_term: skeletal muscle tissue
    term:
      id: UBERON:0001134
      label: skeletal muscle tissue
  downstream:
  - target: Proximal muscle weakness
    description: Loss of functional muscle mass produces the cardinal weakness.
    causal_link_type: DIRECT
  - target: Hypotonia
    description: The early infantile expression of the same fibre pathology.
    causal_link_type: DIRECT
  - target: Elevated creatine kinase
    description: Sarcolemmal leak from degenerating fibres raises serum creatine kinase.
    causal_link_type: DIRECT
  - target: Respiratory Muscle Failure
    description: >-
      The diaphragm and accessory respiratory muscles are affected by the same
      process, which is what makes the disease lethal.
    causal_link_type: DIRECT
  - target: Loss of ambulation
    description: Cumulative weakness in the infantile and childhood forms.
    causal_link_type: DIRECT
  - target: Ptosis
    description: >-
      Levator palpebrae and extraocular involvement, characteristic of the
      late-onset form.
    causal_link_type: DIRECT
  - target: Progressive external ophthalmoparesis
    description: Extraocular muscle involvement, most frequent in late-onset disease.
    causal_link_type: DIRECT
  - target: Facial weakness
    description: >-
      Facial diplegia, which with scapular winging makes the late-onset form
      resemble facioscapulohumeral dystrophy.
    causal_link_type: DIRECT
  - target: Dysphagia
    description: Oropharyngeal muscle weakness.
    causal_link_type: DIRECT
  - target: Dysarthria
    description: Weakness of the bulbar and facial musculature used for articulation.
    causal_link_type: DIRECT
  - target: Scapular winging
    description: >-
      Periscapular weakness, which with facial weakness produces the FSHD-like
      appearance of the adult-onset form.
    causal_link_type: DIRECT
  - target: Motor regression
    description: >-
      In infants, progressive fibre loss takes away motor milestones that had
      already been acquired.
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Myopathic changes included atrophic and/or necrotic fibres (28/36, 69.5%), fibre size variability/type 1 predominance (24/36, 66.6%), increased central nuclei (11/36, 30.5%), sarcoplasmic vacuoles (5/36, 13.9%), lipid droplets or fat replacement of muscle tissue (7/36, 19.5%), and fibrosis or increase of connective tissue (14/36, 38.9%)."
    explanation: The histological components of fibre degeneration, with frequencies.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The combination of dystrophic features with COX defect and RRF is highly suggestive of TK2 deficiency"
    explanation: The diagnostic value of the combined pattern.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "On muscle biopsies typical signs of mitochondrial dysfunction were associated with dystrophic changes."
    explanation: The same dual pattern in the late-onset form.
- name: Respiratory Muscle Failure
  biological_scale: ORGANISM
  description: >-
    Weakness of the diaphragm and accessory respiratory muscles produces a
    restrictive ventilatory defect, ventilator dependence and, in most fatal cases,
    death. Two thirds of the largest cohort needed invasive or non-invasive
    ventilation, and just over a quarter of all patients are known to have died of
    respiratory failure. In some late-onset patients nocturnal dyspnoea was the
    first symptom.
  downstream:
  - target: Respiratory insufficiency
    description: The clinical expression of respiratory muscle weakness.
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Respiratory muscles were severely compromised in 33/51 (65%) patients, who required mechanical ventilation or nocturnal/continuous non-invasive ventilation"
    explanation: Frequency of ventilatory support.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "27.2% (25 of 92) of the patients are known to have died from respiratory failure"
    explanation: Respiratory failure as the cause of death.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In some patients with late-onset myopathy, nocturnal dyspnoea was an initial symptom of the disease."
    explanation: Respiratory involvement can be the presenting feature.
- name: Central Nervous System Involvement
  biological_scale: ORGANISM
  description: >-
    A minority of patients have nervous-system disease: seizures, encephalopathy,
    cognitive impairment, sensorineural hearing loss or episodic coma, in about a
    sixth of the largest cohort and a quarter of its infantile-onset group. The
    causal attribution is not settled. TK2 is ubiquitous and the knock-in mouse
    develops a frank encephalomyelopathy with respiratory-chain deficits most
    prominent in the central nervous system, so a primary neuronal mtDNA-depletion
    mechanism is plausible; but the natural-history authors note that restrictive
    lung disease is so frequent in this disease that hypoxic-ischaemic injury
    cannot be excluded in individual patients.
  downstream:
  - target: Seizures
    description: Recurrent seizures in six of 92 patients.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Encephalopathy
    description: Encephalopathy in seven of 92 patients, concentrated in infantile onset.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Sensorineural hearing loss
    description: Reported in three of 92 patients.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The nervous system was affected in 15 out of 92 (16%) causing recurrent seizures (six), encephalopathy (seven), cognitive impairment (four), sensorineural hearing loss (three) and episodic coma (one)."
    explanation: Human frequency and composition of nervous-system involvement.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CNS signs are more frequent and severe in the infantile form with 10/39 patients (25.6%) manifesting as encephalopathy, seizures or cognitive impairment"
    explanation: Concentration of CNS involvement in the infantile-onset form.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "it is uncertain in some cases whether the cerebral manifestations might have been due to hypoxic-ischaemic damage in the setting of respiratory failure or infections"
    explanation: >-
      The authors' own caution against attributing every cerebral manifestation to
      a primary neuronal mechanism, which is why this node is framed as unsettled.
  - reference: PMID:18467430
    reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Histopathology revealed an encephalomyelopathy with prominent vacuolar changes in the anterior horn of the spinal cord."
    explanation: >-
      The mouse shows a primary central-nervous-system lesion, the basis for a
      primary mechanism in patients; it is model evidence, not human evidence.
  - reference: PMID:18467430
    reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "defects of respiratory chain enzymes containing mtDNA-encoded subunits that were most prominent in the central nervous system"
    explanation: Organ distribution of the biochemical defect in the mouse, which is CNS-predominant rather than muscle-predominant.
phenotypes:
- name: Proximal muscle weakness
  category: Musculoskeletal
  description: >-
    The cardinal feature, present in every patient in the largest cohort either
    at onset or during the course. Weakness is predominantly axial and proximal
    but reaches facial, ocular and respiratory muscles.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
    clinical_course: PROGRESSIVE
  sequelae:
  - target: Loss of ambulation
    description: Cumulative proximal weakness ends independent walking in the earlier-onset forms.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients had proximal muscle weakness as the first sign of disease or during the clinical course."
    explanation: Establishes both the phenotype and its near-universal frequency.
  - reference: PMID:22345218
    reference_title: "Adult cases of mitochondrial DNA depletion due to TK2 defect: an expanding spectrum."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "generalized muscle weakness predominating on axial and proximal muscles but affecting facial, ocular, and respiratory muscles"
    explanation: Distribution of weakness in adult-onset patients.
- name: Hypotonia
  category: Musculoskeletal
  description: >-
    Generalised hypotonia is one of the two features present in all patients of a
    multi-family series, and is the usual infantile presentation alongside motor
    regression.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Generalized hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:23932787
    reference_title: "Molecular and clinical characterization of the myopathic form of mitochondrial DNA depletion syndrome caused by mutations in the thymidine kinase (TK2) gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinically, hypotonia and proximal muscle weakness are the major phenotypes present in all subjects."
    explanation: Hypotonia in every patient of an 11-subject, 9-family series.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While hypotonia and proximal muscle weakness are common in all of the three clinical forms"
    explanation: Hypotonia is not confined to the infantile form.
- name: Elevated creatine kinase
  category: Laboratory
  description: >-
    Serum creatine kinase was raised in every patient in whom a value was
    available, from 1.3 to 30 times the upper limit of normal. A normal value does
    not exclude the diagnosis: seven patients had transiently normal levels and
    five were reported normal throughout. Myoglobinuria and rhabdomyolysis are rare.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Elevated serum creatine kinase
    term:
      id: HP:0003236
      label: Elevated circulating creatine kinase activity
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CK levels, available in 57 out of 92 patients, were elevated in all from 1.3-fold to 30-fold above the upper limit of normal (range 272-6500 U/L)."
    explanation: Frequency and magnitude of the creatine kinase elevation.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "normal in only five: one with infantile-onset myopathy and four with childhood-onset myopathy, indicating that a normal CK does not exclude TK2 deficiency"
    explanation: >-
      Refutes treating a raised creatine kinase as obligatory; the same paper that
      reports near-universal elevation states the exception explicitly.
- name: Loss of ambulation
  category: Musculoskeletal
  description: >-
    Progressive loss of independent walking in the infantile and childhood forms.
    Most infantile-onset patients never walk and virtually all are non-ambulatory
    by four years; about two thirds of childhood-onset patients become
    wheelchair-bound within ten years. Late-onset patients retain the ability to walk.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Loss of ambulation
    term:
      id: HP:0002505
      label: Loss of ambulation
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Global motor function was severely impaired in 50/61 (82% with Karnofsky or Lansky Performance Status <50) who were wheelchair-bound at the last follow-up."
    explanation: Proportion wheelchair-bound at last follow-up.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In contrast to the infantile-onset/childhood-onset patients, late-onset cases never became wheelchair-bound"
    explanation: The late-onset form is the exception, which is why this is FREQUENT rather than VERY_FREQUENT.
- name: Respiratory insufficiency
  category: Respiratory
  description: >-
    Restrictive respiratory failure from diaphragmatic and accessory muscle
    weakness. Two thirds of the largest cohort required invasive or non-invasive
    ventilation, rising to nearly 90% of infantile-onset patients, and respiratory
    failure is the leading cause of death. In late-onset disease it may be the
    presenting symptom, as nocturnal dyspnoea.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The vast majority of infantile-onset patients required ventilation (17/19, 89.5%) while ventilatory support was used in about half the childhood-onset (12/22, 54.4%) and late-onset (4/9, 44.4%) patients."
    explanation: Ventilatory requirement by clinical form.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients showed evidence of respiratory muscle weakness, with need for non-invasive Mechanical Ventilation in 12/18."
    explanation: Respiratory involvement is universal even in the mildest clinical form.
  - reference: PMID:38544965
    reference_title: Clinical and Genetic Analysis of Patients With TK2 Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Approximately 30% of patients died of respiratory insufficiency, while 56% of surviving patients needed mechanical ventilation."
    explanation: Mortality and ventilation in an independent 53-patient cohort.
- name: Ptosis
  category: Ophthalmologic
  description: >-
    Drooping eyelids, the commonest extraocular feature. Reported in 22 of 85
    patients overall, and much more frequent in late-onset disease.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Ptosis
    term:
      id: HP:0000508
      label: Ptosis
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other muscular functions were affected in 33/85 (39%) patients who manifested variable combinations of ptosis (22), facial diplegia (19), progressive external ophthalmoparesis (PEO) (11), mild dysphagia (10), dysarthria/dysphonia (3) and rigid spine (1)"
    explanation: Counts of the extraocular, facial and bulbar features, including ptosis.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The first symptom was muscle limb weakness in 10/18, eyelid ptosis in 6/18, and respiratory insufficiency in 2/18."
    explanation: Ptosis as a presenting symptom in a third of late-onset patients.
- name: Progressive external ophthalmoparesis
  category: Ophthalmologic
  description: >-
    Restricted eye movements from extraocular muscle involvement. Reported in 11
    of 92 patients overall, and characteristic of the late-onset form, where
    ocular involvement is present in about two thirds against under a tenth of
    infantile-onset patients.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Progressive external ophthalmoparesis
    term:
      id: HP:0000597
      label: Ophthalmoparesis
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the involvement of ocular muscles with ptosis or PEO is more frequent in late-onset cases (11/16, 68.7%) than in the childhood-onset (11/37, 29.7%) and infantile-onset forms (3/39, 7.7%)"
    explanation: Frequency of ocular involvement by clinical form.
  - reference: PMID:35094997
    reference_title: "Advances in Thymidine Kinase 2 Deficiency: Clinical Aspects, Translational Progress, and Emerging Therapies."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "limb myopathy with ptosis, ophthalmoparesis, and respiratory involvement is more common in the late-onset form"
    explanation: A review's synthesis of the late-onset clinical picture.
- name: Facial weakness
  category: Neurologic
  description: >-
    Facial diplegia, in 19 of 85 patients. With scapular winging it produces the
    facioscapulohumeral-like appearance of the adult-onset form, which is why
    genetically undiagnosed FSHD-like patients are recommended for TK2 testing.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Facial diplegia
    term:
      id: HP:0001349
      label: Facial diplegia
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other muscular functions were affected in 33/85 (39%) patients who manifested variable combinations of ptosis (22), facial diplegia (19), progressive external ophthalmoparesis (PEO) (11), mild dysphagia (10), dysarthria/dysphonia (3) and rigid spine (1)"
    explanation: >-
      Source of the 19-of-85 count, and of the bilateral ("diplegia") wording this
      phenotype is bound to.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The adult myopathy shares prominent facial weakness with facioscapulohumeral muscular dystrophy (FSHD)."
    explanation: Facial weakness and the diagnostic confusion it creates.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ophthalmoparesis and facial weakness are frequent in adults."
    explanation: Frequency of facial weakness in adult-onset disease.
- name: Scapular winging
  category: Musculoskeletal
  description: >-
    Winging of the scapula from periscapular muscle weakness, present with marked
    facial weakness in seven of sixteen adult-onset patients.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Scapular winging
    term:
      id: HP:0003691
      label: Scapular winging
  subtype: Late-onset
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "adult-onset myopathy, which mimicked facioscapulohumeral dystrophy with marked facial weakness, scapular winging or both in seven (43.75%)"
    explanation: Scapular winging in the adult-onset form, with its frequency.
- name: Dysphagia
  category: Gastrointestinal
  description: >-
    Oropharyngeal weakness causing difficulty swallowing, in ten of 85 patients
    overall and half of the late-onset series. Nineteen patients required a
    gastrostomy for severe dysphagia, failure to thrive or both.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
  sequelae:
  - target: Failure to thrive
    description: >-
      Impaired oral intake, with the disease's energy failure, produces poor weight
      gain; nineteen patients required gastrostomy for dysphagia, failure to thrive
      or both.
  evidence:
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Half of patients presented difficulty in swallowing."
    explanation: Frequency of dysphagia in the late-onset series.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Nineteen patients required gastrostomy tube because of severe dysphagia, failure to thrive or both."
    explanation: Severity of bulbar involvement, measured by gastrostomy requirement.
- name: Failure to thrive
  category: Growth
  description: >-
    Poor weight gain, a reason for gastrostomy in this cohort alongside dysphagia.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "gastrostomy tube because of severe dysphagia, failure to thrive or both"
    explanation: Failure to thrive as a documented indication for enteral feeding in this disease.
- name: Dysarthria
  category: Neurologic
  description: >-
    Slurred or weak speech from bulbar muscle involvement. The natural-history
    cohort counts it together with dysphonia: three of the 85 patients assessed
    for other muscular functions, and three of its sixteen late-onset patients.
    Speech involvement in this disease is not always articulatory -- the separate
    eighteen-patient late-onset series reports dysphonia from vocal cord palsy in
    two patients and no dysarthria, so laryngeal and articulatory mechanisms are
    worth separating when counting bulbar features.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Dysarthria
    term:
      id: HP:0001260
      label: Dysarthria
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Some individuals develop dysarthria, dysphagia, and/or hearing loss."
    explanation: GeneReviews lists dysarthria among the features of the infantile-onset form.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional manifestations were ptosis (nine), PEO (eight), dysphagia (six), dysarthria/dysphonia (three), respiratory insufficiency (five), cardiomyopathy (two)"
    explanation: >-
      Counts of additional features in the sixteen late-onset patients, where
      dysarthria/dysphonia is three. The higher figure in the same list, six,
      belongs to dysphagia.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other muscular functions were affected in 33/85 (39%) patients who manifested variable combinations of ptosis (22), facial diplegia (19), progressive external ophthalmoparesis (PEO) (11), mild dysphagia (10), dysarthria/dysphonia (3) and rigid spine (1)"
    explanation: The whole-cohort count, three of the 85 patients assessed for these features.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other clinical manifestations included sensory axonal polyneuropathy (7/18;38.9%), neurosensory hearing loss (3/18;16.6%) and dysphonia due to vocal cord palsy (2/18;11.1%)."
    explanation: >-
      Speech involvement in this series is laryngeal -- dysphonia from vocal cord
      palsy in two of eighteen -- rather than articulatory, and the series records
      no dysarthria.
- name: Seizures
  category: Neurologic
  description: >-
    Seizures, concentrated in infantile onset, but with counts that the source
    cohort does not reconcile: its whole-cohort paragraph reports recurrent seizures
    in six of 92 patients, while its infantile-onset paragraph reports seizures in
    seven of the 39 infantile patients -- a subgroup count larger than the
    whole-cohort count. The wording differs between the two paragraphs, "recurrent
    seizures" against "seizures", so the most economical reading is that the cohort
    figure counts only recurrent seizures and the infantile figure counts any
    seizure. The two are not presented here as reconciling; six to seven is the
    defensible range. Whether the seizures are a primary consequence of neuronal
    mtDNA depletion or secondary to hypoxic-ischaemic injury is unresolved.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Recurrent seizures
    term:
      id: HP:0001250
      label: Seizure
  subtype: Infantile-onset
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The nervous system was affected in 15 out of 92 (16%) causing recurrent seizures (six), encephalopathy (seven), cognitive impairment (four), sensorineural hearing loss (three) and episodic coma (one)."
    explanation: >-
      The whole-cohort count, recurrent seizures in six of 92. Note this is the
      lower of the source's two figures.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional clinical manifestations were seizures (seven), encephalopathy (five), lissencephaly (one), cognitive dysfunction (three), ptosis (four), facial diplegia (three), dysphagia (three) and multiple bone fractures (two)."
    explanation: >-
      Counts of extramuscular features in the 39 infantile-onset patients, where
      seizures are seven -- one more than the whole cohort's recurrent-seizure
      count, which is the discrepancy the description records.
- name: Encephalopathy
  category: Neurologic
  description: >-
    Encephalopathy in seven of 92 patients, five of them among the 39 with
    infantile onset. Unlike the seizure counts, these two figures reconcile. As with
    seizures, primary and secondary mechanisms are not separated in the reported
    cohorts.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Encephalopathy
    term:
      id: HP:0001298
      label: Encephalopathy
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The nervous system was affected in 15 out of 92 (16%) causing recurrent seizures (six), encephalopathy (seven), cognitive impairment (four), sensorineural hearing loss (three) and episodic coma (one)."
    explanation: >-
      The whole-cohort encephalopathy count, seven of 92 -- the specific figure for
      this phenotype rather than the aggregate CNS-abnormality rate.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional clinical manifestations were seizures (seven), encephalopathy (five), lissencephaly (one), cognitive dysfunction (three), ptosis (four), facial diplegia (three), dysphagia (three) and multiple bone fractures (two)."
    explanation: >-
      The infantile-onset encephalopathy count, five of 39, which sits inside the
      whole-cohort figure of seven.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Overall, 13/39 (33.3%) infantile-onset patients had non-skeletal muscle manifestations while 10/39 (25.6%) had central nervous system (CNS) abnormalities."
    explanation: >-
      The broader CNS-abnormality denominator in infantile onset, retained as the
      context the encephalopathy count sits in rather than as the count itself.
- name: Sensorineural hearing loss
  category: Auditory
  description: >-
    Reported in three of 92 patients, and a feature GeneReviews recommends
    surveying for annually in infantile-onset disease.
  frequency: RARE
  phenotype_term:
    preferred_term: Sensorineural hearing impairment
    term:
      id: HP:0000407
      label: Sensorineural hearing impairment
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "at least annual audiology evaluations in those with infantile-onset disease"
    explanation: GeneReviews surveillance recommendation, which presupposes the hearing-loss risk.
- name: Depletion of mitochondrial DNA in muscle
  category: Laboratory
  description: >-
    Reduced mtDNA copy number in skeletal muscle, the laboratory finding the
    disease is named for. Present at below 30% of age-matched control in two
    thirds of patients tested, much more often in infantile- and childhood-onset
    than in late-onset disease. GeneReviews takes below 20% of matched control as
    the diagnostic threshold in infantile presentation.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Depletion of mitochondrial DNA in muscle tissue
    term:
      id: HP:0009141
      label: Depletion of mitochondrial DNA in muscle tissue
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Higher proportions of infantile-onset (21/26; 81%) and childhood-onset (24/31; 77%) had mtDNA depletion than late onset (1/14; 7%)"
    explanation: Frequency of muscle mtDNA depletion by clinical form.
  - reference: PMID:23932787
    reference_title: "Molecular and clinical characterization of the myopathic form of mitochondrial DNA depletion syndrome caused by mutations in the thymidine kinase (TK2) gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Quantitative analysis of mtDNA content in skeletal muscle was performed in 5 subjects, and marked mtDNA content reduction was observed in each."
    explanation: Independent confirmation in a separate multi-family series.
- name: Multiple mitochondrial DNA deletions
  category: Laboratory
  description: >-
    Multiple deletions of mtDNA in skeletal muscle, the dominant molecular finding
    in late-onset disease, where all sixteen tested patients had them and only one
    of fourteen had frank depletion. The frequency band is taken over the patients
    actually tested for deletions, 24 of 38 (63%), not over the whole 92-patient
    cohort -- a finding that requires a muscle biopsy and a deletion assay cannot be
    scored in patients who had neither, and using 92 as the denominator would put it
    at 26% purely as an artefact of incomplete testing. The gradient with age at
    onset is steep: 1 of 8 infantile, 7 of 14 childhood, 16 of 16 late onset.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Multiple mitochondrial DNA deletions
    term:
      id: HP:0003689
      label: Multiple mitochondrial DNA deletions
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Multiple mtDNA deletions were detected in muscle of 24 patients primarily in late-onset patients: infantile-onset (1/8; 12.5%), childhood-onset (7/14; 50%) and late-onset (16/16; 100%)."
    explanation: >-
      The count and its per-onset denominators. Summing them gives 24 of 38 tested,
      63%, which is the basis for the FREQUENT band.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle biopsies show multiple mtDNA deletions often with mtDNA depletion."
    explanation: The two molecular findings coexist in muscle.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All cases showed multiple mtDNA deletions but mtDNA depletion was present only in two patients."
    explanation: Deletions dominate in late-onset disease.
- name: Ragged-red muscle fibres
  category: Laboratory
  description: >-
    Subsarcolemmal mitochondrial accumulation on modified Gomori trichrome, found
    with or without cytochrome c oxidase deficiency in most patients whose muscle
    histochemistry was reported.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Ragged-red muscle fibers
    term:
      id: HP:0003200
      label: Ragged-red muscle fibers
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle histochemistry was abnormal in all 71 patients studied: 53/71 (74.6%) had numerous cytochrome c oxidase (COX)-deficient and ragged-red fibres (RRFs)"
    explanation: Frequency of ragged-red fibres together with COX deficiency.
- name: Cytochrome c oxidase-negative muscle fibres
  category: Laboratory
  description: >-
    A mosaic of fibres lacking cytochrome c oxidase histochemical activity, the
    single-fibre expression of the mtDNA lesion. Present with ragged-red fibres in
    three quarters of patients studied and in isolation in a further fifth.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Cytochrome c oxidase-negative muscle fibers
    term:
      id: HP:0003688
      label: Cytochrome C oxidase-negative muscle fibers
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "14/71 (19.7%) had only COX-deficient fibres and 4/71 (5.7%) had isolated RRFs"
    explanation: Frequency of isolated COX-deficient fibres.
  - reference: PMID:22345218
    reference_title: "Adult cases of mitochondrial DNA depletion due to TK2 defect: an expanding spectrum."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "typical mitochondrial myopathy with a mosaic pattern of COX-negative and ragged-red fibers"
    explanation: The mosaic pattern in adult patients.
- name: Elevated blood lactate
  category: Laboratory
  description: >-
    Raised blood lactate from impaired oxidative phosphorylation. Together with a
    raised creatine kinase in a child with rapidly progressive weakness it is the
    combination the natural-history authors flag as a prompt to test TK2.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Increased blood lactate
    term:
      id: HP:0002151
      label: Increased circulating lactate concentration
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Genetic screening for TK2 deficiency must be considered in children, particularly infants, with rapidly progressing muscle weakness, especially when associated with elevated blood lactate and CK."
    explanation: Raised lactate as a recognised accompanying finding and diagnostic prompt.
- name: Decreased activity of mitochondrial complex IV
  category: Laboratory
  description: >-
    Reduced cytochrome c oxidase activity in muscle, the most frequent isolated
    respiratory-chain abnormality when the defect is not combined.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Decreased activity of mitochondrial complex IV
    term:
      id: HP:0008347
      label: Decreased activity of mitochondrial complex IV
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "single enzyme abnormalities were detected in 7/48 (14.5%) (complex IV deficiency in 4/49, complex III in 1/48, isolated increase of citrate synthase in 1/48 and complex I in 1/48)"
    explanation: Complex IV is the commonest isolated enzyme deficiency reported.
- name: Motor regression
  category: Neurologic
  description: >-
    Loss of previously acquired motor milestones in the infantile-onset form, where
    severe congenital myopathy and hypotonia or motor regression progress rapidly.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Developmental regression
    term:
      id: HP:0002376
      label: Developmental regression
  subtype: Infantile-onset
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Severe congenital myopathy and hypotonia or motor regression progressed rapidly to fatality in the majority of cases within 1 year of follow-up due to respiratory failure"
    explanation: Motor regression as a presenting pattern of the infantile-onset form.
progression:
- phase: Onset
  notes: >-
    Onset spans the whole of life, from birth to 72 years in the largest cohort,
    and the age at onset is the strongest single predictor of course. In the
    late-onset group the mean age at onset was 31 years, and subtle signs
    (for instance slow running) were often present in childhood without prompting
    medical evaluation.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Age at disease onset ranged from birth to 72 years of age."
    explanation: Full span of reported onset ages.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mean age of onset was 31 years."
    explanation: Mean onset age in the late-onset series.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In retrospect, subtle signs of myopathy were often present in childhood (eg, slow running) but did not prompt medical evaluation until adulthood."
    explanation: Pre-symptomatic signs in the late-onset group.
- phase: Progression of weakness
  notes: >-
    Weakness progresses at rates that differ by an order of magnitude between the
    forms. Time from onset to ventilation or death is a median of 1.67 years in
    infantile onset against 10 years in childhood/late onset.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Median time to event is 1.67 (CI 0.11 to 2.33) years for infantile onset and 10.00 (CI 3.50 to 22.00) years for childhood/late onset."
    explanation: Event-free survival, where the event is ventilation or death.
- phase: Survival
  notes: >-
    Median post-onset survival is about one year for infantile onset and at least
    23 years for the childhood- and late-onset groups combined, a difference of
    around twentyfold. Respiratory failure is the cause of death in about a
    quarter of all patients, and in every fatal case of the late-onset series.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Median post-onset survival (POS) is 1.0 years (CI 0.58 to 2.33) for the infantile group and at least 23.0 years (CI 10.0 to incomputable) for the childhood-onset/late-onset group."
    explanation: Post-onset survival by clinical form.
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Four patients had deceased, all of them due to respiratory insufficiency."
    explanation: Cause of death in the late-onset series.
histopathology:
- name: Skeletal muscle biopsy
  description: >-
    Muscle histochemistry was abnormal in every patient in whom it was performed:
    cytochrome c oxidase-deficient fibres with ragged-red fibres in three
    quarters, COX-deficient fibres alone in a further fifth, and isolated
    ragged-red fibres in the remainder. On routine histology there are atrophic
    and necrotic fibres, fibre-size variability with type 1 predominance,
    increased internal nuclei and replacement by fat and connective tissue, so the
    biopsy reads as a dystrophy with a mitochondrial overlay. Respiratory-chain
    enzyme activities were reduced in the great majority of biopsies.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In 87.5% of the biopsies, biochemical activities of respiratory chain enzymes were reduced with isolated complex IV deficiency or variable defects of complexes I, III and IV."
    explanation: Proportion of biopsies with reduced respiratory-chain activity, and the pattern.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histochemical data were available in 77% of patients confirming COX deficiency with or without RRFs as morphological hallmarks of TK2 deficiency."
    explanation: The histochemical hallmarks.
genetic:
- name: TK2
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  gene_term:
    preferred_term: TK2
    term:
      id: hgnc:11831
      label: TK2
  features: >-
    Biallelic TK2 variants are the cause of the myopathic form of mtDNA depletion
    syndrome. Pathogenic variants have been found in every exon and no clear
    genotype-phenotype correlation emerges: p.Thr108Met homozygotes in one cohort
    spanned all three clinical forms, and residual enzyme activity in patient
    fibroblasts did not track severity. Two regions are recurrent, the
    substrate-binding site encoded by exon 5 and the active site in exon 8.
  notes: >-
    ClinGen has not published a Gene-Disease Validity assertion for TK2, so no
    gene_disease_validity tier is recorded here; `just clingen-list` returns no
    TK2 assertion as of 2026-09-29.
  case_fractions:
  - population: Spanish late-onset TK2 deficiency cases
    case_fraction_percent: 46.9
    cohort_size: 53
    notes: >-
      Share of Spanish late-onset patients carrying the founder in-frame deletion
      p.Lys202del. This is an allele share within a founder population, not a
      population prevalence.
    evidence:
    - reference: PMID:38544965
      reference_title: Clinical and Genetic Analysis of Patients With TK2 Deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In late-onset cases, 46.9% carried the p.Lys202del variant."
      explanation: Quantifies the p.Lys202del share among Spanish late-onset patients.
    - reference: PMID:38544965
      reference_title: Clinical and Genetic Analysis of Patients With TK2 Deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This study includes 53 patients harboring biallelic TK2 pathogenic variants, compiling data retrospectively from 7 Spanish centers."
      explanation: Cohort size and composition behind the fraction above.
  evidence:
  - reference: PMID:11687801
    reference_title: Mutant mitochondrial thymidine kinase in mitochondrial DNA depletion myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mutations in TK2 represent a new etiology for mitochondrial DNA depletion"
    explanation: The original gene-disease association.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The substrate binding site (encoded by exon 5) and active site (exon 8) are hotspots for the mutations."
    explanation: Location of the two recurrent regions.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most common mutation, p.Thr108Met, was identified in a homozygous state in 13 patients with heterogeneous clinical phenotypes"
    explanation: The commonest allele, and that one genotype spans several phenotypes.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "12 mutations were associated with multiple phenotypes so overall genotype-phenotype correlations based on type of mutation and protein location could not be clearly delineated"
    explanation: >-
      Refutes a usable genotype-phenotype rule in this disease, which matters for
      counselling and for stratifying trials.
diagnosis:
- name: TK2 molecular testing
  description: >-
    Biallelic pathogenic TK2 variants establish the diagnosis. Because the
    late-onset form mimics facioscapulohumeral dystrophy and the childhood form
    mimics spinal muscular atrophy type 3, the natural-history authors recommend
    screening genetically undiagnosed recessive myopathies, including FSHD-like
    cases, for TK2 variants.
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis is confirmed by the identification of biallelic pathogenic variants in TK2 by molecular genetic testing."
    explanation: The confirmatory test.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We suggest that genetically undiagnosed patients with autosomal recessive myopathies, including FSHD-like cases, should be screened for TK2 mutations in blood or mitochondrial alterations in muscle biopsies."
    explanation: Who to test, given the mimics.
- name: Muscle mtDNA copy number
  description: >-
    Quantitative mtDNA content in skeletal muscle is the tissue-level criterion.
    GeneReviews sets it at typically below 20% of age- and tissue-matched control
    for an infantile presentation; beyond age two, reduced content or multiple
    deletions with ragged-red and/or COX-deficient fibres suffices. Note that a
    late-onset patient may have normal copy number with deletions only, so a
    normal quantification does not exclude the diagnosis in an adult.
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "established in a proband with infantile onset of disease with severely reduced (typically <20% of age- and tissue-matched healthy controls) mtDNA content in skeletal muscle"
    explanation: The infantile-onset diagnostic threshold.
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "established in a proband older than age two years with reduced mtDNA content or multiple mtDNA deletions, ragged red fibers and/or COX-deficient fibers in skeletal muscle"
    explanation: The criterion beyond infancy, which admits deletions in place of depletion.
- name: Muscle magnetic resonance imaging
  description: >-
    A reproducible pattern of selective muscle involvement in late-onset disease,
    worst in gluteus maximus, semitendinosus and sartorius. It is a non-invasive
    adjunct to the biopsy rather than a confirmatory test: the pattern was derived
    from a single eighteen-patient series and has not been compared against the
    other myopathies TK2 deficiency is mistaken for, so it can direct a biopsy and
    a TK2 test but cannot substitute for either.
  diagnosis_term:
    preferred_term: muscle magnetic resonance imaging
    term:
      id: NCIT:C16809
      label: Magnetic Resonance Imaging
  evidence:
  - reference: PMID:31060578
    reference_title: "Late-onset thymidine kinase 2 deficiency: a review of 18 cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified common radiological features in muscle magnetic resonance, where the most severely affected muscles were the gluteus maximus, semitendinosus and sartorius."
    explanation: The selective muscle-involvement pattern on MRI, and its single-series provenance.
- name: Muscle biopsy histochemistry
  description: >-
    Dystrophic features together with a cytochrome c oxidase defect and ragged-red
    fibres is the histological pattern that should raise TK2 deficiency, but it is
    not confirmatory on its own.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "must be supplemented by molecular genetic studies to confirm TK2 deficiency"
    explanation: The histology is suggestive, not diagnostic.
- name: Electromyography
  description: >-
    Myopathic motor unit potentials in about four fifths of patients tested.
    Neurogenic changes occur in a minority, and two patients had a genuine sensory
    axonal neuropathy, so a neurogenic EMG does not exclude the diagnosis.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "EMG revealed myopathic changes in 78.6% of patients tested."
    explanation: Proportion with myopathic electrophysiology.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These data demonstrate peripheral nerve involvement in a minority of patients with TK2 deficiency."
    explanation: Peripheral nerve involvement in a minority, which complicates the electrophysiological picture.
- name: Serum GDF-15
  description: >-
    Growth differentiation factor 15 was raised at baseline and fell on treatment
    in every patient tested in the expanded-access programme, and is described as
    a biomarker correlating with therapeutic response rather than as a diagnostic
    test.
  evidence:
  - reference: PMID:35094997
    reference_title: "Advances in Thymidine Kinase 2 Deficiency: Clinical Aspects, Translational Progress, and Emerging Therapies."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "In the early expanded access program, growth differentiation factor 15 (GDF-15) appears to be a useful biomarker that correlates with therapeutic response."
    explanation: The role claimed for GDF-15, which is treatment monitoring rather than diagnosis.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Baseline elevated serum growth differentiation factor 15 levels decreased with treatment in all 7 patients tested."
    explanation: The primary observation behind that claim.
differential_diagnoses:
- name: Mitochondrial DNA depletion syndrome 3 (hepatocerebral type)
  disease_term:
    preferred_term: mitochondrial DNA depletion syndrome 3 (hepatocerebral type)
    term:
      id: MONDO:0009636
      label: mitochondrial DNA depletion syndrome 3 (hepatocerebral type)
  description: >-
    The purine mirror image of this disease, and the closest mechanistic relative
    in the knowledge base. DGUOK encodes deoxyguanosine kinase, the other
    mitochondrial deoxyribonucleoside kinase: between them, dGK and TK2
    phosphorylate all four deoxynucleosides. Loss of either impairs the same
    salvage pathway and produces mtDNA depletion, so the biochemistry is shared
    and the separation is by substrate arm and target organ.
  distinguishing_features:
  - DGUOK phosphorylates the purine deoxynucleosides deoxyguanosine and deoxyadenosine; TK2 phosphorylates the pyrimidines deoxythymidine, deoxycytidine and deoxyuridine.
  - DGUOK deficiency is liver-predominant with cholestatic liver failure, nystagmus and developmental delay; TK2 deficiency is skeletal-muscle-predominant and the liver is spared.
  - In the original TK2 series, liver, brain, heart and skin remained unaffected throughout life, which is the tissue distinction rather than a severity one.
  - Nucleoside substitution is established for TK2 deficiency; for DGUOK deficiency the intervention that changes survival is liver transplantation.
  evidence:
  - reference: PMID:18467430
    reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    quote_role: BACKGROUND
    snippet: "TK2 mutations have been associated with severe infantile myopathy, a milder myopathy with later onset and longer survival, rigid spine syndrome and spinal muscular atrophy (SMA)-like motor neuron disease while DGUOK mutations cause hepatocerebral disease"
    explanation: States the organ split between the two deoxyribonucleoside kinase deficiencies.
  - reference: PMID:12765840
    reference_title: "mtDNA depletion myopathy: elucidation of the tissue specificity in the mitochondrial thymidine kinase (TK2) deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other tissues such as liver, brain, heart, and skin remain unaffected throughout the patients' life."
    explanation: The organ sparing that separates TK2 from DGUOK deficiency clinically.
- name: Mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria
  disease_term:
    preferred_term: mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria
    term:
      id: MONDO:0012791
      label: mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria
  description: >-
    The encephalomyopathic member of the MDS family, from SUCLA2 (or SUCLG1)
    deficiency. It shares infantile hypotonia, muscle mtDNA depletion and lactate
    elevation with the infantile-onset form of TK2 deficiency.
  distinguishing_features:
  - SUCLA2 encodes a subunit of succinyl-CoA ligase in the TCA cycle, not a deoxyribonucleoside kinase; the mtDNA depletion there is a downstream consequence rather than a precursor shortage.
  - Methylmalonic aciduria with raised C4-dicarboxylic carnitine is the biochemical signature of SUCLA2 deficiency and is absent in TK2 deficiency.
  - SUCLA2 disease is dominated by dystonia, deafness and basal ganglia change; TK2 disease is dominated by limb and respiratory myopathy with cognition typically spared.
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cognitive function is typically spared."
    explanation: >-
      Cognitive sparing in TK2 deficiency, against the encephalopathic course of the
      SUCLA2 form.
- name: Mitochondrial neurogastrointestinal encephalomyopathy
  disease_term:
    preferred_term: mitochondrial neurogastrointestinal encephalomyopathy
    term:
      id: MONDO:0017575
      label: mitochondrial neurogastrointestinal encephalomyopathy
  description: >-
    The other pyrimidine-pool mtDNA maintenance disorder. TYMP encodes thymidine
    phosphorylase, which catabolises the same nucleosides TK2 phosphorylates, so
    the two diseases sit on opposite sides of one metabolic balance: TK2
    deficiency depletes the pool and MNGIE floods it with thymidine and
    deoxyuridine. Both cause mtDNA depletion, multiple deletions and a
    mitochondrial myopathy with ptosis and ophthalmoparesis.
  distinguishing_features:
  - MNGIE is defined by gastrointestinal dysmotility with cachexia and by leukoencephalopathy on MRI, neither of which features in TK2 deficiency.
  - MNGIE is diagnosed by raised plasma thymidine and deoxyuridine with absent thymidine phosphorylase activity; TK2 deficiency has no such circulating marker.
  - Thymidine phosphorylase is the enzyme that counteracts TK2 by lowering its substrates, so a nucleoside-supplementation strategy would be actively harmful in MNGIE.
  evidence:
  - reference: PMID:20940150
    reference_title: "Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    quote_role: BACKGROUND
    snippet: "Strict homeostatic maintenance of the mitochondrial deoxythymidine monophosphate (dTMP) and deoxycytidine monophosphate (dCMP) levels is achieved by counteracting TK2 activity with the catabolic actions of cytosolic thymidine phosphorylase (TP) that decreases the levels of TK2 substrates"
    explanation: >-
      Establishes that TK2 and thymidine phosphorylase are opposed arms of one
      pool, which is why the two diseases are mechanistic mirror images.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A growing number of mitochondrial diseases with mtDNA instability have been linked to mitochondrial deoxynucleotide pool imbalance, including deficiencies of TK2, dGK, RRM2B or TYMP."
    explanation: Places TK2, DGUOK and TYMP deficiencies in one mechanistic class.
- name: MSTO1-related mitochondrial myopathy
  disease_term:
    preferred_term: mitochondrial myopathy-cerebellar ataxia-pigmentary retinopathy syndrome
    term:
      id: MONDO:0044714
      label: mitochondrial myopathy-cerebellar ataxia-pigmentary retinopathy syndrome
  description: >-
    The nearest laboratory mimic: a myopathy with reduced mtDNA copy number whose
    largest cohort argued it should itself be classified as an mtDNA depletion
    syndrome. The dismech MSTO1 entry already lists this disease as its
    nosological neighbour; this is the reverse pointer.
  distinguishing_features:
  - MSTO1 encodes a cytosolic protein acting on mitochondrial dynamics, not an enzyme of the deoxynucleoside salvage pathway.
  - Cerebellar atrophy with ataxia is core to MSTO1 disease and is not a feature of the myopathic form of TK2 deficiency.
  - MSTO1 muscle shows a dystrophic pattern with normal oxidative histochemistry and normal respiratory-chain activities, whereas COX-deficient and ragged-red fibres with combined respiratory-chain deficiency are the rule in TK2 muscle.
  - Progression in MSTO1 disease is slow or static, whereas TK2 myopathy is relentlessly progressive with respiratory failure.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histochemical data were available in 77% of patients confirming COX deficiency with or without RRFs as morphological hallmarks of TK2 deficiency."
    explanation: >-
      The histochemical hallmark of TK2 muscle, which is what the MSTO1 cohort
      reported as absent in its own patients.
- name: Facioscapulohumeral muscular dystrophy
  disease_term:
    preferred_term: facioscapulohumeral muscular dystrophy
    term:
      id: MONDO:0001347
      label: facioscapulohumeral muscular dystrophy
  description: >-
    The clinical mimic of the adult-onset form. Nearly half of adult-onset TK2
    patients in the largest cohort presented with marked facial weakness, scapular
    winging or both, which is the FSHD phenotype, and the natural-history authors
    recommend screening FSHD-like cases without a molecular diagnosis for TK2
    variants.
  distinguishing_features:
  - FSHD is dominantly inherited (a contracted D4Z4 repeat with a permissive 4qA allele in FSHD1, or an SMCHD1/DNMT3B variant in FSHD2); TK2 deficiency is autosomal recessive.
  - FSHD weakness is characteristically asymmetric and descending; TK2 weakness is generalised with axial and proximal predominance.
  - FSHD muscle does not show COX-negative and ragged-red fibres with mtDNA depletion or multiple deletions.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The adult myopathy shares prominent facial weakness with facioscapulohumeral muscular dystrophy (FSHD)."
    explanation: The basis of the clinical confusion.
- name: Spinal muscular atrophy
  disease_term:
    preferred_term: spinal muscular atrophy
    term:
      id: MONDO:0001516
      label: spinal muscular atrophy
  description: >-
    The clinical mimic across the whole severity range. The natural-history
    authors map infantile TK2 myopathy onto SMA type 1 and severe type 2,
    childhood onset onto milder type 2 and type 3a, and late onset onto types 3b
    and 4, noting that TK2 weakness is more progressive.
  distinguishing_features:
  - SMA is a motor neuron disease from biallelic SMN1 loss with a modifying SMN2 copy number; TK2 deficiency is a primary myopathy.
  - Creatine kinase is raised 1.3 to 30-fold in TK2 deficiency; it is normal or only mildly raised in SMA.
  - Electromyography is myopathic in about four fifths of TK2 patients, against the neurogenic pattern of SMA.
  - Muscle biopsy in TK2 deficiency shows COX-negative and ragged-red fibres with reduced mtDNA copy number; SMA shows grouped fibre-type atrophy without mitochondrial change.
  evidence:
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The range of weakness severity among TK2-deficient patients parallels the spectrum of SMA with infantile-onset TK2 myopathy resembling SMA type 1 and severe type 2, childhood-onset myopathy similar to milder type 2 and 3a SMA, and late-onset myopathy comparable to SMA types 3b and IV, although the weakness is more progressive."
    explanation: The explicit mapping of TK2 forms onto SMA types, and the caveat.
  - reference: PMID:28318037
    reference_title: Deoxycytidine and Deoxythymidine Treatment for Thymidine Kinase 2 Deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    quote_role: BACKGROUND
    snippet: "childhood-onset myopathy, which resembles spinal muscular atrophy (SMA) type III"
    explanation: An independent statement of the SMA type 3 resemblance.
treatments:
- name: Deoxycytidine and Deoxythymidine Substitution
  description: >-
    Oral deoxycytidine plus deoxythymidine -- given the pharmaceutical names
    doxecitine and doxribtimine and marketed as KYGEVVI -- the
    substrate-enhancement therapy that is the major therapeutic development in
    this disease, and since 3 November 2025 the first approved treatment for TK2
    deficiency in the USA, where the licensed indication is restricted to symptom
    onset on or before age 12. The rationale is that raising the concentration of
    TK2's own substrates lets residual enzyme, and the cytosolic kinases, deliver
    enough pyrimidine deoxynucleotide for mtDNA replication. It was established in
    the Tk2 H126N knock-in mouse, where dC+dT delayed onset, extended lifespan and
    restored mtDNA copy number and respiratory-chain activity, and then given to 16
    patients under expanded access. In that open-label cohort three of eight
    non-ambulatory patients regained walking, four of five discontinued
    feeding-tube use, and one of nine came off mechanical ventilation. Doses are
    300-400 mg/kg/day of each nucleoside, titrated against stool frequency;
    dose-dependent diarrhoea occurred in half the patients and two of a separate
    group of twelve developed reversible transaminase elevation. The approval
    rested on pooled retrospective and single-arm data rather than a randomised
    comparison, and that data set is what draws the age limit: mortality was 4% in
    treated against 57% in untreated patients with onset at or before 12 years, but
    in later-onset patients the difference, 18% against 24%, was not significant. A
    2022 review, written before the filing, still described the supporting trials
    as ongoing.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: doxecitine (deoxycytidine)
      term:
        id: CHEBI:15698
        label: 2'-deoxycytidine
    - preferred_term: doxribtimine (deoxythymidine)
      term:
        id: CHEBI:17748
        label: thymidine
  target_mechanisms:
  - target: Imbalanced Mitochondrial dNTP Pool
    description: >-
      Supplying the substrates in excess is intended to restore the pyrimidine
      arm of the mitochondrial deoxynucleotide pool without replacing the enzyme.
  - target: Skeletal Muscle mtDNA Depletion and Multiple Deletions
    description: >-
      The proximate biochemical endpoint: restored mtDNA copy number, demonstrated
      in the mouse and inferred in patients from motor and biomarker response.
  - target: Respiratory Muscle Failure
    description: >-
      The clinically decisive endpoint. One of nine ventilated patients became able
      to breathe independently on therapy.
  evidence:
  - reference: PMID:28318037
    reference_title: Deoxycytidine and Deoxythymidine Treatment for Thymidine Kinase 2 Deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We observed that dC+dT delayed disease onset, prolonged life span of Tk2-deficient mice and restored mtDNA copy number as well as respiratory chain enzyme activities and levels."
    explanation: The preclinical result that established the nucleoside form of the therapy.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Three of 8 patients who were nonambulatory at baseline gained the ability to walk on therapy; 4 of 5 patients who required enteric nutrition were able to discontinue feeding tube use; and 1 of 9 patients who required mechanical ventilation became able to breathe independently."
    explanation: The functional gains in the expanded-access cohort.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In 5 patients with early onset and severe disease, survival and motor functions were better than historically untreated patients. In 11 childhood and adult onset patients, clinical measures stabilized or improved."
    explanation: >-
      The survival and stabilisation claim, and the comparator: historical controls,
      not a randomised arm.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients are currently maintained on doses between 300 and 400mg/kg/day of each nucleoside or nucleotide."
    explanation: The dose range used.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A side effect observed in 8 of the 16 patients was dose-dependent diarrhea, which did not require withdrawal of treatment."
    explanation: The dose-limiting adverse effect.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Among 12 other TK2 patients treated with deoxynucleoside, 2 adults developed elevated liver enzymes that normalized following discontinuation of therapy."
    explanation: A second, reversible adverse effect in a separate treated group.
  - reference: PMID:35094997
    reference_title: "Advances in Thymidine Kinase 2 Deficiency: Clinical Aspects, Translational Progress, and Emerging Therapies."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "This treatment is currently undergoing testing in clinical trials intended to support approval in the US and European Union (EU)."
    explanation: >-
      Regulatory status as of this 2022 review, retained as historical context only.
      It was superseded by the November 2025 approval recorded in the items below,
      and should not be read as the current status.
  - reference: PMID:41604077
    reference_title: "Doxecitine and Doxribtimine: First Approval."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Doxecitine and doxribtimine received its first approval in the USA on 3 November 2025 for the treatment of TK2d in adults and pediatric patients with an age of symptom onset on or before 12 years"
    explanation: >-
      The approval date and the licensed indication, including the age-at-onset
      restriction. Graded OTHER because the quoted text reports a regulatory action
      rather than a study result.
  - reference: PMID:41604077
    reference_title: "Doxecitine and Doxribtimine: First Approval."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "It is the first approved therapy for patients with TK2d."
    explanation: >-
      Establishes that nothing was licensed for this disease before, which is what
      makes the approval the change in therapeutic status rather than an addition.
  - reference: PMID:41604077
    reference_title: "Doxecitine and Doxribtimine: First Approval."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "Administration of doxecitine and doxribtimine facilitates the incorporation of the pyrimidine nucleosides deoxycytidine and deoxythymidine into skeletal muscle mtDNA, thereby promoting restoration of mtDNA copy number"
    explanation: >-
      The pharmacodynamic mechanism as characterized at approval, matching the
      target_mechanisms links on the dNTP pool and mtDNA depletion nodes.
  - reference: PMID:41604077
    reference_title: "Doxecitine and Doxribtimine: First Approval."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "Pyrimidine nucleos(t)ide therapy significantly decreased mortality in patients with TK2d aged ≤ 12 years at symptom onset, but not in patients aged > 12 years at symptom onset"
    explanation: >-
      The subgroup result behind the licensed age limit: a mortality benefit in
      early-onset disease and none demonstrated in later-onset disease.
  - reference: PMID:41604077
    reference_title: "Doxecitine and Doxribtimine: First Approval."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "In patients aged ≤ 12 years at symptom onset, death occurred in 4% of treated patients and 57% of untreated patients"
    explanation: The mortality figures in the subgroup the approval covers.
  - reference: PMID:41604077
    reference_title: "Doxecitine and Doxribtimine: First Approval."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: REVIEW_SYNTHESIS
    snippet: "In patients aged > 12 years at symptom onset, death occurred in 18% of treated patients and 24% of untreated patients"
    explanation: >-
      The corresponding figures in later-onset disease, where the difference did not
      reach significance.
- name: Deoxycytidine and Deoxythymidine Monophosphate Bypass
  description: >-
    The original nucleotide formulation, dCMP plus dTMP, which was the first
    pharmacological treatment shown to work in the Tk2 knock-in mouse: it raised
    dTTP, increased mtDNA, improved respiratory-chain enzymes and extended median
    lifespan from 13 to 34 days. It has been largely superseded because the
    monophosphates are rapidly catabolised to the nucleosides after
    administration, which is what identified dC and dT as the active agents, and
    most patients were converted to nucleosides in 2015. Adding the deaminase
    inhibitor tetrahydrouridine to shield deoxycytidine from deamination was
    tested and made things worse.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_mechanisms:
  - target: Imbalanced Mitochondrial dNTP Pool
    description: >-
      Intended to bypass the missing kinase step entirely by supplying its products
      rather than its substrates.
  evidence:
  - reference: PMID:24968719
    reference_title: Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes, and significantly prolong their lifespan (34 days with treatment versus 13 days untreated)"
    explanation: The original bypass result, with the survival figures.
  - reference: PMID:24968719
    reference_title: Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "dCMP/dTMP supplementation is the first effective pharmacologic treatment for Tk2 deficiency"
    explanation: Its place in the therapeutic history of the disease.
  - reference: PMID:31125140
    reference_title: Deoxynucleoside Therapy for Thymidine Kinase 2-Deficient Myopathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    quote_role: BACKGROUND
    snippet: "we subsequently observed that after administration, dCMP and dTMP are rapidly catabolized to the nucleosides dC and dT, suggesting that nucleosides, rather than nucleotides, are the major active therapeutic agents"
    explanation: Why the nucleotide formulation was superseded by the nucleoside one.
  - reference: PMID:28318037
    reference_title: Deoxycytidine and Deoxythymidine Treatment for Thymidine Kinase 2 Deficiency.
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: "In contrast, dCMP+dTMP+THU therapy decreased life span of Tk2-/- animals compared to dCMP+dTMP."
    explanation: >-
      Refutes the deaminase-inhibition strategy: adding tetrahyduridine to protect
      deoxycytidine shortened survival rather than extending it.
- name: AAV-Mediated TK2 Gene Therapy
  description: >-
    Investigational, preclinical only. AAV9 delivery of human TK2 cDNA to the
    knock-in mouse restored Tk2 activity in every tissue tested except kidney,
    delayed onset and extended lifespan; sequential AAV9 then AAV2 dosing let the
    viral dose be lowered while extending survival further, and adding dC+dT on
    top improved liver and kidney mtDNA copy number, growth and lifespan again.
    There is no human evidence.
  therapeutic_modality: GENE_THERAPY
  treatment_term:
    preferred_term: Gene Therapy
    term:
      id: NCIT:C15238
      label: Gene Therapy
  delivery_system:
    delivery_platform: VIRAL_VECTOR
    notes: >-
      AAV9 as the primary serotype, with sequential AAV2 re-dosing in the
      dose-sparing arm. Both are reported in the mouse only.
  target_mechanisms:
  - target: TK2 Loss of Function
    description: >-
      Unlike substrate enhancement, gene replacement addresses the enzyme deficit
      itself rather than working around it.
  evidence:
  - reference: PMID:34338329
    reference_title: Synergistic Deoxynucleoside and Gene Therapies for Thymidine Kinase 2 Deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "AAV9 delivery of human TK2 cDNA to Tk2KI mice efficiently rescued Tk2 activity in all the tissues tested except the kidneys, delayed disease onset, and increased lifespan."
    explanation: The gene-therapy result, including the tissue it failed to reach.
  - reference: PMID:34338329
    reference_title: Synergistic Deoxynucleoside and Gene Therapies for Thymidine Kinase 2 Deficiency.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "addition of deoxycytidine and deoxythymidine supplementation to AAV9 + AAV2 treated Tk2KI mice dramatically improved mtDNA copy numbers in the liver and kidneys, animal growth, and lifespan"
    explanation: The synergy with nucleoside therapy, which is the paper's central claim.
- name: Respiratory Support and Ventilation
  description: >-
    Non-invasive or invasive ventilation, with chest physiotherapy to maintain
    pulmonary function and reduce infection risk. Two thirds of patients in the
    largest cohort required ventilatory support and respiratory failure is the
    leading cause of death, so this is the intervention that most often determines
    survival in practice.
  therapeutic_modality: DEVICE
  treatment_term:
    preferred_term: mechanical ventilation
    term:
      id: NCIT:C70909
      label: Mechanical Ventilation
  target_mechanisms:
  - target: Respiratory Muscle Failure
    description: >-
      Substitutes for the failing respiratory pump; it does not affect the
      underlying mtDNA maintenance defect.
  target_phenotypes:
  - preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A pulmonologist can oversee chest physiotherapy to improve pulmonary function, reduce the risk of pulmonary infection, and manage respiratory insufficiency, if present."
    explanation: GeneReviews respiratory management.
  - reference: PMID:29602790
    reference_title: Retrospective natural history of thymidine kinase 2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "who required mechanical ventilation or nocturnal/continuous non-invasive ventilation"
    explanation: The modalities used in the cohort.
- name: Gastrostomy Feeding
  description: >-
    Enteral feeding by nasogastric or gastrostomy tube where aspiration risk is
    high or intake is inadequate. Nineteen patients in the largest cohort required
    a gastrostomy. This is one of the outcomes that reversed on nucleoside therapy,
    in four of five patients who had been tube-dependent.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: gastrostomy tube placement
    term:
      id: NCIT:C157864
      label: Gastrostomy Tube Procedure
  target_phenotypes:
  - preferred_term: Dysphagia
    term:
      id: HP:0002015
      label: Dysphagia
  - preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Feeding difficulties should be managed aggressively, including use of a nasogastric tube or gastrostomy tube when the risk for aspiration is high."
    explanation: GeneReviews feeding management.
- name: Physical Therapy and Rehabilitation
  description: >-
    Physiotherapy to maintain muscle function and prevent joint contractures, with
    physical medicine and rehabilitation input for patients who have difficulty
    walking. Supportive rather than disease-modifying; no outcome data for this
    disease are cited in the sources used here.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Physical Therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  target_phenotypes:
  - preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Physical therapy can help maintain muscle function; a physical medicine and rehabilitation (PM&R) specialist can help those who have difficulty walking."
    explanation: GeneReviews rehabilitation recommendation.
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "physical therapy can help prevent joint contractures"
    explanation: The secondary-complication rationale.
- name: Genetic Counseling
  description: >-
    Autosomal recessive counselling with 25% sibling recurrence risk. Carrier and
    prenatal testing are available once the family's variants are known. In Spain
    the two founder alleles make targeted carrier testing particularly informative.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Carrier testing for at-risk family members and prenatal testing for a pregnancy at increased risk are possible if the pathogenic variants in the family have been identified."
    explanation: Availability of carrier and prenatal testing.
- name: Multidisciplinary Supportive Care
  description: >-
    Coordinated multidisciplinary management, with growth and weight monitoring,
    pulmonary function testing, neurodevelopmental assessment and annual audiology
    in infantile-onset disease. GeneReviews states explicitly that no clinical
    guidelines exist, so the surveillance schedule is expert recommendation.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Management should involve a multidisciplinary team."
    explanation: The care model recommended.
  - reference: PMID:23230576
    reference_title: "TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No clinical guidelines are available."
    explanation: >-
      The evidence base for surveillance is expert recommendation, which is what
      this quote records.
clinical_trials:
- name: NCT03845712
  phase: PHASE_II
  status: COMPLETED
  description: >-
    Open-label continuation study of doxecitine and doxribtimine (the pharmaceutical
    names for deoxycytidine and deoxythymidine) in patients who had been on
    nucleoside treatment, following the retrospective study MT-1621-101. This is
    the formal-trial arm of the expanded-access experience.
  target_phenotypes:
  - preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
  - preferred_term: Respiratory insufficiency
    term:
      id: HP:0002093
      label: Respiratory insufficiency
  evidence:
  - reference: clinicaltrials:NCT03845712
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "This is a Phase 2 prospective open-label treatment study of the safety and efficacy of doxecitine and doxribtimine in study participants with thymidine kinase 2 (TK2) deficiency"
    explanation: Registry record establishing the trial's design, agents and population.
- name: NCT06754098
  phase: PHASE_II
  status: RECRUITING
  description: >-
    Single-arm off-label study of doxecitin and doxribthymine in adults with TK2
    deficiency at a single Spanish neuromuscular unit, with mtDNA levels in muscle
    and uroepithelial cells as biological endpoints alongside efficacy and safety.
    Adults are the group the natural-history data cover least well.
  target_phenotypes:
  - preferred_term: Proximal muscle weakness
    term:
      id: HP:0003701
      label: Proximal muscle weakness
  evidence:
  - reference: clinicaltrials:NCT06754098
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "the mitochondrial DNA levels before and after treatment (extracted from the muscle and from uroepithelial cells) of these participants will be also studied"
    explanation: The registry record's statement of the mtDNA endpoint.
animal_models:
  - species: Mouse
    genotype: Tk2 H126N homozygous knock-in
    background: C57BL/6-derived, generated by homologous recombination in embryonic stem cells
    publication: PMID:18467430
    description: >-
      The reference model of the disease, homologous to the human H121N allele.
      Homozygotes are normal at birth, stop gaining weight around postnatal day 10
      and die of a rapidly progressive encephalomyelopathy at 2-3 weeks. It is the
      system in which the mitochondrial dNTP pool imbalance was measured, in which
      both therapies were developed, and whose organ distribution differs
      informatively from the human disease.
    genes:
    - preferred_term: TK2
      term:
        id: hgnc:11831
        label: TK2
    modeled_mechanisms:
    - target: Imbalanced Mitochondrial dNTP Pool
      relationship: RECAPITULATES
      fidelity: HIGH
      model_scale: ORGANISM
      description: >-
        The pool imbalance that is asserted but not measured in patients is
        measured directly here: low dTTP in brain and reduced dTTP and dCTP in
        liver, with mtDNA depletion and respiratory-chain deficits following.
      readouts:
      - name: Tissue dTTP and dCTP concentrations
        target: Imbalanced Mitochondrial dNTP Pool
        direction: DECREASED
        interpretation: >-
          Direct measurement of the precursor shortage that the human entry can
          only infer.
        evidence:
        - reference: PMID:24968719
          reference_title: Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency.
          supports: SUPPORT
          evidence_source: MODEL_ORGANISM
          snippet: "loss of Tk2 activity caused dNTP pool imbalances with low dTTP levels in brain and decreased dTTP and dCTP in liver"
          explanation: The measured pool deficits and their tissue distribution.
      - name: Tissue mtDNA copy number on dCMP/dTMP treatment
        target: Imbalanced Mitochondrial dNTP Pool
        direction: RESTORED
        interpretation: >-
          Supplying the missing precursors raises mtDNA again, which is the
          pharmacological test of the mechanism rather than a correlation.
        evidence:
        - reference: PMID:24968719
          reference_title: Deoxypyrimidine monophosphate bypass therapy for thymidine kinase 2 deficiency.
          supports: SUPPORT
          evidence_source: MODEL_ORGANISM
          snippet: "the compounds raise dTTP concentrations, increase levels of mtDNA, ameliorate defects of mitochondrial respiratory chain enzymes"
          explanation: Restoration of pool, copy number and enzyme activity on treatment.
      evidence:
      - reference: PMID:18467430
        reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "animals showed Tk2 deficiency, unbalanced dNTP pools, mtDNA depletion and defects of respiratory chain enzymes containing mtDNA-encoded subunits"
        explanation: Establishes the model as informative for the pool-imbalance node.
    - target: Combined Respiratory Chain Deficiency
      relationship: PARTIALLY_RECAPITULATES
      fidelity: MODERATE
      model_scale: ORGANISM
      description: >-
        The mouse does develop deficiency of the mtDNA-dependent complexes, but the
        organ distribution is inverted relative to patients: the defect is
        CNS-predominant, and heart and skeletal muscle are largely spared by
        transcriptional compensation.
      limitations: >-
        In the mouse only brain showed significant respiratory-chain deficiency,
        with heart and muscle protected, whereas the human disease is defined by a
        skeletal-muscle respiratory-chain defect with the brain usually spared. A
        muscle-endpoint experiment in this model therefore does not test the
        organ in which the human disease is expressed.
      divergences:
      - divergence_type: SPECIES_MISMATCH
        materiality: QUALIFYING
        description: >-
          Mouse Tk1 down-regulates sharply in the second postnatal week and the
          heart compensates by suppressing MTERF3 to raise mitochondrial
          transcript levels per mtDNA copy. Neither the timing nor the
          compensatory mechanism has a demonstrated human counterpart, and it is
          what spares mouse muscle and heart.
      readouts:
      - name: Brain complex I and complex IV activities
        target: Combined Respiratory Chain Deficiency
        direction: DECREASED
        interpretation: >-
          Quantifies the biochemical defect and, by its tissue restriction, the
          model's divergence from the human organ pattern.
        evidence:
        - reference: PMID:20940150
          reference_title: "Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation."
          supports: SUPPORT
          evidence_source: MODEL_ORGANISM
          snippet: "the activity of complex I was 40% and that of complex IV 50% in brain of Tk2-/- versus Tk2+/+ mice"
          explanation: The measured brain enzyme deficits.
      evidence:
      - reference: PMID:20940150
        reference_title: "Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation."
        supports: REFUTE
        evidence_source: MODEL_ORGANISM
        snippet: "However, only the brain showed significant deficiency of respiratory chain complexes (OXPHOS), protein levels and activities"
        explanation: >-
          Refutes treating this model as a faithful system for the muscle
          respiratory-chain defect that defines the human disease.
    - target: Central Nervous System Involvement
      relationship: RECAPITULATES
      fidelity: MODERATE
      model_scale: ORGANISM
      description: >-
        The mouse develops a frank encephalomyelopathy with anterior-horn vacuolar
        change, which is the strongest available argument that human CNS features
        can be a primary consequence of neuronal mtDNA depletion rather than of
        hypoxia.
      limitations: >-
        In patients CNS involvement is a minority feature (16% overall) and its
        attribution is confounded by respiratory failure, whereas in the mouse it
        is the lethal, defining lesion. The model therefore over-represents the
        importance of the CNS phenotype relative to the human disease.
      readouts:
      - name: Spinal cord anterior horn histopathology
        target: Central Nervous System Involvement
        direction: ALTERED
        interpretation: A structural CNS lesion attributable to Tk2 loss alone, with no respiratory confounder.
        evidence:
        - reference: PMID:18467430
          reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
          supports: SUPPORT
          evidence_source: MODEL_ORGANISM
          snippet: "Histopathology revealed an encephalomyelopathy with prominent vacuolar changes in the anterior horn of the spinal cord."
          explanation: The histological CNS finding.
      evidence:
      - reference: PMID:18467430
        reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "The H126N TK2 mouse is the first knock-in animal model of human MDS and demonstrates that the severity of TK2 deficiency in tissues may determine the organ-specific phenotype."
        explanation: >-
          The authors' own framing of the model's value, which is precisely about
          organ specificity.
    evidence:
    - reference: PMID:18467430
      reference_title: Thymidine kinase 2 (H126N) knockin mice show the essential role of balanced deoxynucleotide pools for mitochondrial DNA maintenance.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Homozygous Tk2 mutant (Tk2(-/-)) mice developed rapidly progressive weakness after age 10 days and died between ages 2 and 3 weeks."
      explanation: The model's phenotype and timeline.
    - reference: PMID:20940150
      reference_title: "Onset and organ specificity of Tk2 deficiency depends on Tk1 down-regulation and transcriptional compensation."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Although normal until postnatal day 8, Tk2-/- mice rapidly develop fatal encephalomyopathy between postnatal days 10 and 13."
      explanation: The narrow window in which the phenotype appears, which is what links it to Tk1 down-regulation.
discussions:
- discussion_id: tk2_mouse_organ_inversion
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    The Tk2 H126N knock-in mouse is CNS-predominant and spares skeletal muscle,
    while the human disease is muscle-predominant and usually spares the CNS. Does
    the compensatory mechanism that protects mouse muscle - Tk1 activity in the
    first postnatal week followed by MTERF3 down-regulation raising mitochondrial
    transcripts per mtDNA copy - operate in human muscle, and if so why does human
    muscle nonetheless fail?
  rationale: >-
    This is not a cosmetic species difference. Every therapy in this disease was
    developed in a model whose target organ is not the organ the human disease
    destroys, and the mouse's own authors identified compensation as the reason
    its muscle is spared. If the same compensation is absent or exhausted in human
    muscle, that absence is part of the pathogenesis and a potential therapeutic
    target in its own right; if it is present, the mouse's muscle endpoints
    understate what nucleoside therapy has to overcome. The one human data point
    pointing the same way is the observation that muscle combines low basal TK2
    activity with a high demand for mtDNA-encoded protein, which is a
    vulnerability argument rather than a compensation argument.
  attaches_to:
  - pathophysiology#Post-Mitotic Dependence on Mitochondrial Salvage
  - pathophysiology#Combined Respiratory Chain Deficiency
  - animal_models#Mouse
  proposed_experiments:
  - experiment_id: tk2_human_muscle_compensation
    name: MTERF3 and mitochondrial transcript quantification in patient muscle
    description: >-
      Measure MTERF3 transcript and protein, mitochondrial transcript levels
      normalised to mtDNA copy number, and TK1 activity in archived TK2-deficient
      muscle biopsies across the three clinical forms, against age-matched control
      muscle. The mouse result predicts that a tissue which compensates shows
      raised transcript per copy with suppressed MTERF3; human muscle, which does
      not escape the disease, should not.
    readouts:
    - name: Mitochondrial transcript level per mtDNA copy
      target: pathophysiology#Combined Respiratory Chain Deficiency
      direction: DECREASED
      interpretation: >-
        Absence of the transcriptional compensation seen in mouse heart would
        explain why human muscle is the target organ.
    would_support:
    - pathophysiology#Post-Mitotic Dependence on Mitochondrial Salvage
    supporting_outcome:
    - >-
      Patient muscle shows no rise in mitochondrial transcript per mtDNA copy and
      no MTERF3 suppression, while the respiratory-chain deficit is present -
      establishing that human muscle lacks the compensation that protects the
      mouse.
    refuting_outcome:
    - >-
      Patient muscle shows the same transcriptional compensation as mouse heart
      despite a full respiratory-chain deficit, which would mean compensation is
      not what determines organ tropism and the mouse-human inversion has another
      cause.
- discussion_id: tk2_cns_attribution
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    In patients with nervous-system features, how much is primary neuronal mtDNA
    depletion and how much is hypoxic-ischaemic injury secondary to respiratory
    failure?
  rationale: >-
    The natural-history authors raise this explicitly and do not resolve it: CNS
    features cluster in the infantile-onset group, which is also the group with the
    highest ventilatory requirement, so the two candidate causes are confounded by
    design in a retrospective cohort. The answer changes what nucleoside therapy
    should be expected to do for the CNS, and whether CNS-penetrant delivery is
    needed. Nucleoside therapy now alters the respiratory course, which makes a
    prospective separation possible for the first time: patients whose respiratory
    failure is prevented but who still develop CNS disease would settle it.
  attaches_to:
  - pathophysiology#Central Nervous System Involvement
  - phenotypes#Encephalopathy
  - phenotypes#Seizures
- discussion_id: tk2_no_controlled_trial
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What is the effect size of deoxycytidine and deoxythymidine therapy against a
    concurrent control rather than against historical untreated patients?
  rationale: >-
    Every efficacy claim available rests on open-label expanded access and
    single-arm studies compared against untreated patients assembled
    retrospectively or from the literature. That is a reasonable design for a
    rapidly fatal ultra-rare disease and it is not a reason to withhold the
    therapy, but the magnitude and durability of benefit remain estimated from
    non-concurrent comparators. The November 2025 US approval was granted on that
    basis and so does not close this gap; it sharpens it, because the licensed age
    limit is itself drawn from a subgroup contrast within the same uncontrolled
    data. The pooled analyses did stratify by age at onset and found no significant
    mortality difference in later-onset patients, which leaves open whether
    later-onset disease is genuinely unresponsive or merely slow enough that the
    available follow-up cannot resolve a difference -- the latter would make the
    age cut-off an artefact of study duration rather than of biology. A
    placebo-controlled trial is unlikely now that an approved therapy exists, so
    the realistic substitute is a prespecified comparison against the 92-patient
    natural-history cohort using its own stratification by age at onset, with
    GDF-15 as an objective intermediate endpoint.
  attaches_to:
  - treatments#Deoxycytidine and Deoxythymidine Substitution
  - clinical_trials#NCT06754098
- discussion_id: tk2_genotype_phenotype
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What determines whether a given biallelic TK2 genotype produces infantile,
    childhood or adult-onset disease, given that neither the variant nor residual
    enzyme activity predicts it?
  rationale: >-
    p.Thr108Met homozygotes span all three clinical forms, twelve variants are
    each associated with more than one form, and residual TK2 activity in patient
    fibroblasts ranges from 1% to 39% without correlating with severity. Something
    other than the TK2 genotype is setting the age at onset - plausibly variation
    in the cytosolic kinases and nucleotide catabolism that modify the same pool,
    or in the compensation the mouse work identified. This is the practical
    obstacle to counselling a newly diagnosed infant's sibling and to stratifying
    a single-arm trial.
  attaches_to:
  - genetic#TK2
  - pathophysiology#TK2 Loss of Function
notes: >-
  Scope. This entry is the myopathic form of mtDNA depletion syndrome, MDS type 2,
  caused by TK2 deficiency (MONDO:0012301, OMIM 609560, causal gene
  RO:0004003 hgnc:11831 TK2 as recorded by MONDO). It is deliberately kept
  distinct from the other MDS family members already in the knowledge base:
  Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type (DGUOK, the purine
  arm of the same salvage pathway),
  SUCLA2-Related_Mitochondrial_DNA_Depletion_Syndrome (the encephalomyopathic
  form), FBXL4-Related_Mitochondrial_DNA_Depletion_Syndrome,
  Mitochondrial_DNA_Depletion_Syndrome_7, Mitochondrial_DNA_Depletion_Syndrome_14B,
  and Mitochondrial_Neurogastrointestinal_Encephalomyopathy (TYMP). The
  MSTO1-Related_Mitochondrial_Myopathy entry already carries a differential
  diagnosis pointing at MONDO:0012301; the reverse pointer is curated here.

  Subtype modelling. The three clinical forms are modelled as has_subtypes rather
  than as separate entries, because they share one gene, one mechanism and one
  therapy, and because the sources treat them explicitly as a continuum ("a
  phenotypic continuum that ranges from severe to mild"). The percentages attached
  to each form come from a single 92-patient retrospective cohort and the authors
  note the late-onset form may be under-represented, so they are the best
  available figures rather than population estimates.

  Conformance. The Combined Respiratory Chain Deficiency node conforms to
  mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress, matching
  the four other mtDNA depletion entries in kb/disorders. Modules considered and
  rejected: complex_i_deficiency and complex_iv_assembly_deficiency (both scope
  themselves to biogenesis or repair lesions of a single complex, whereas the
  defect here is a shortage of mtDNA templates producing a combined deficiency of
  every complex carrying mtDNA-encoded subunits); deregulated_cellular_energetics
  (a cancer-hallmark module about the Warburg shift, not monogenic
  respiratory-chain failure); cardiomyopathy_maladaptive_remodeling
  (cardiomyopathy is reported in only two late-onset and one infantile patient in
  the cohort, too peripheral to justify a conformance claim). No module covers
  mtDNA maintenance or the mitochondrial dNTP pool; `rg -il "mtDNA
  maintenance|dNTP|nucleotide salvage|deoxynucleo" kb/modules/` returns only
  fungal_nucleic_acid_antimetabolite and mitochondrial_dysfunction. A dedicated
  mitochondrial dNTP-pool module covering TK2, DGUOK, RRM2B and TYMP would be a
  reasonable follow-up now that four of those diseases are curated.

  Deep-research report is not independent corroboration. The report at
  research/Mitochondrial_DNA_Depletion_Syndrome_Myopathic_Form-deep-research-claude_code.md
  was generated after this entry was curated, and the provider read the entry and
  its reference cache while writing -- its own text says so, describing itself as
  assembled from references "held in the dismech verified reference cache" and its
  ontology identifiers as read from cache/ and "the validated bindings of the
  curated kb/disorders/ entry". Do not read agreement between the report and this
  entry as confirmation; the report is downstream of the entry, not a second source
  for it. Its contribution was to flag PMID:41604077, the KYGEVVI approval record,
  as cached but unused, which it did without quoting the reference ("present in the
  cache but not reproduced here, as I do not hold them verbatim") -- so the approval
  content in the treatments section was read from the reference itself, not from the
  report.

  Treatment identifier decisions. The therapy carries the pharmaceutical names
  doxecitine and doxribtimine and the brand name KYGEVVI, none of which are bound
  here as ontology terms, for three separate reasons. No regimen_term is asserted:
  NCIT has no term for KYGEVVI, MT-1621, or any deoxynucleoside combination
  regimen, and RegimenTerm is reachable only from NCIT:C15697, so there is nothing
  legitimate to bind. NCIT:C1330 "Deoxythymidine" exists but is_a NCIT:C85834
  "Retired Concept 2009" and must not be used; the CHEBI bindings
  (CHEBI:15698 2'-deoxycytidine, CHEBI:17748 thymidine) are the correct chemical
  identity and are retained, with the pharmaceutical names carried in
  preferred_term. NCIT:C420 "Doxecitine" does exist and is a synonym-level match
  for deoxycytidine, but adding it beside the CHEBI term would list the same
  molecule twice, and its NCIT parents (Pyrimidine Antagonist, Chemoprotective
  Agent) describe an antineoplastic role rather than this one. The treatment name
  was also left as the mechanistic "Deoxycytidine and Deoxythymidine Substitution"
  rather than renamed to the brand: it is the target of an attaches_to foreign key,
  and a mechanism name outlives a trade name.

  Evidence grading for PMID:41604077. That reference is a first-approval review, so
  every item from it carries quote_role: REVIEW_SYNTHESIS. Its evidence_source is
  split deliberately: quotes reporting the regulatory action itself (approval date,
  indication, first-approved status, labelled pharmacodynamics) are OTHER, because a
  regulatory decision is not a study; quotes reporting the pooled human mortality
  outcomes behind that decision are HUMAN_CLINICAL, because evidence_source grades
  the study the quoted text describes and those are human cohort analyses.

  Mitochondrial dNTP pools have not been measured in patient tissue in any source
  cited here. The pool imbalance is measured in the Tk2 H126N mouse and asserted
  as mechanism in the human discovery paper, and the Imbalanced Mitochondrial dNTP
  Pool node says so in its own description rather than only here.

  TK2 also phosphorylates pyrimidine nucleoside analogues including zidovudine,
  which is the proposed route to the mitochondrial toxicity of nucleoside-analogue
  antivirals and anticancer drugs. That is a pharmacological consequence of the
  same enzyme rather than part of this disease, so it is recorded here rather than
  curated as a mechanism; a drug-induced mitochondrial toxicity entry or module
  would be where it belongs.

  No ORPHA cache file exists for Orphanet:254875, the Orphanet term MONDO cites
  for this disease; `just fetch-reference ORPHA:254875` reports no source found.
  The prevalence record therefore rests on published case counts rather than an
  Orphanet epidemiology class, and no numeric rate_per_100000 is asserted.

  Counting discipline in the 92-patient cohort. That cohort reports different
  features against different denominators, and two of its numbers do not reconcile
  with each other. The Seizures description records the irreconcilable pair
  (recurrent seizures six of 92 against seizures seven of 39 infantile patients)
  rather than presenting either as settled. Multiple mitochondrial DNA deletions is
  banded over the 38 patients actually assayed for deletions rather than over all 92,
  since a finding requiring a biopsy and a deletion assay cannot be scored in
  patients who had neither; the alternative denominator is stated in that record so
  the choice is visible. Every figure quoted in a description is carried by one of
  the evidence items beside it, which matters here because the cohort's feature
  lists put several counts in one sentence and adjacent features are easy to swap.

📚

References & Deep Research

References

3

PMID:23230576

TK2-Related Mitochondrial DNA Maintenance Defect, Myopathic Form.

No top-level findings curated for this source.

PMID:29602790

Retrospective natural history of thymidine kinase 2 deficiency.

No top-level findings curated for this source.

PMID:35094997

Advances in Thymidine Kinase 2 Deficiency: Clinical Aspects, Translational Progress, and Emerging Therapies.

No top-level findings curated for this source.

Deep Research

1

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Evaluations and curation notes (3)

Record notes

Scope. This entry is the myopathic form of mtDNA depletion syndrome, MDS type 2, caused by TK2 deficiency (MONDO:0012301, OMIM 609560, causal gene RO:0004003 hgnc:11831 TK2 as recorded by MONDO). It is deliberately kept distinct from the other MDS family members already in the knowledge base: Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type (DGUOK, the purine arm of the same salvage pathway), SUCLA2-Related_Mitochondrial_DNA_Depletion_Syndrome (the encephalomyopathic form), FBXL4-Related_Mitochondrial_DNA_Depletion_Syndrome, Mitochondrial_DNA_Depletion_Syndrome_7, Mitochondrial_DNA_Depletion_Syndrome_14B, and Mitochondrial_Neurogastrointestinal_Encephalomyopathy (TYMP). The MSTO1-Related_Mitochondrial_Myopathy entry already carries a differential diagnosis pointing at MONDO:0012301; the reverse pointer is curated here. Subtype modelling. The three clinical forms are modelled as has_subtypes rather than as separate entries, because they share one gene, one mechanism and one therapy, and because the sources treat them explicitly as a continuum ("a phenotypic continuum that ranges from severe to mild"). The percentages attached to each form come from a single 92-patient retrospective cohort and the authors note the late-onset form may be under-represented, so they are the best available figures rather than population estimates. Conformance. The Combined Respiratory Chain Deficiency node conforms to mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress, matching the four other mtDNA depletion entries in kb/disorders. Modules considered and rejected: complex_i_deficiency and complex_iv_assembly_deficiency (both scope themselves to biogenesis or repair lesions of a single complex, whereas the defect here is a shortage of mtDNA templates producing a combined deficiency of every complex carrying mtDNA-encoded subunits); deregulated_cellular_energetics (a cancer-hallmark module about the Warburg shift, not monogenic respiratory-chain failure); cardiomyopathy_maladaptive_remodeling (cardiomyopathy is reported in only two late-onset and one infantile patient in the cohort, too peripheral to justify a conformance claim). No module covers mtDNA maintenance or the mitochondrial dNTP pool; `rg -il "mtDNA maintenance|dNTP|nucleotide salvage|deoxynucleo" kb/modules/` returns only fungal_nucleic_acid_antimetabolite and mitochondrial_dysfunction. A dedicated mitochondrial dNTP-pool module covering TK2, DGUOK, RRM2B and TYMP would be a reasonable follow-up now that four of those diseases are curated. Deep-research report is not independent corroboration. The report at research/Mitochondrial_DNA_Depletion_Syndrome_Myopathic_Form-deep-research-claude_code.md was generated after this entry was curated, and the provider read the entry and its reference cache while writing -- its own text says so, describing itself as assembled from references "held in the dismech verified reference cache" and its ontology identifiers as read from cache/ and "the validated bindings of the curated kb/disorders/ entry". Do not read agreement between the report and this entry as confirmation; the report is downstream of the entry, not a second source for it. Its contribution was to flag PMID:41604077, the KYGEVVI approval record, as cached but unused, which it did without quoting the reference ("present in the cache but not reproduced here, as I do not hold them verbatim") -- so the approval content in the treatments section was read from the reference itself, not from the report. Treatment identifier decisions. The therapy carries the pharmaceutical names doxecitine and doxribtimine and the brand name KYGEVVI, none of which are bound here as ontology terms, for three separate reasons. No regimen_term is asserted: NCIT has no term for KYGEVVI, MT-1621, or any deoxynucleoside combination regimen, and RegimenTerm is reachable only from NCIT:C15697, so there is nothing legitimate to bind. NCIT:C1330 "Deoxythymidine" exists but is_a NCIT:C85834 "Retired Concept 2009" and must not be used; the CHEBI bindings (CHEBI:15698 2'-deoxycytidine, CHEBI:17748 thymidine) are the correct chemical identity and are retained, with the pharmaceutical names carried in preferred_term. NCIT:C420 "Doxecitine" does exist and is a synonym-level match for deoxycytidine, but adding it beside the CHEBI term would list the same molecule twice, and its NCIT parents (Pyrimidine Antagonist, Chemoprotective Agent) describe an antineoplastic role rather than this one. The treatment name was also left as the mechanistic "Deoxycytidine and Deoxythymidine Substitution" rather than renamed to the brand: it is the target of an attaches_to foreign key, and a mechanism name outlives a trade name. Evidence grading for PMID:41604077. That reference is a first-approval review, so every item from it carries quote_role: REVIEW_SYNTHESIS. Its evidence_source is split deliberately: quotes reporting the regulatory action itself (approval date, indication, first-approved status, labelled pharmacodynamics) are OTHER, because a regulatory decision is not a study; quotes reporting the pooled human mortality outcomes behind that decision are HUMAN_CLINICAL, because evidence_source grades the study the quoted text describes and those are human cohort analyses. Mitochondrial dNTP pools have not been measured in patient tissue in any source cited here. The pool imbalance is measured in the Tk2 H126N mouse and asserted as mechanism in the human discovery paper, and the Imbalanced Mitochondrial dNTP Pool node says so in its own description rather than only here. TK2 also phosphorylates pyrimidine nucleoside analogues including zidovudine, which is the proposed route to the mitochondrial toxicity of nucleoside-analogue antivirals and anticancer drugs. That is a pharmacological consequence of the same enzyme rather than part of this disease, so it is recorded here rather than curated as a mechanism; a drug-induced mitochondrial toxicity entry or module would be where it belongs. No ORPHA cache file exists for Orphanet:254875, the Orphanet term MONDO cites for this disease; `just fetch-reference ORPHA:254875` reports no source found. The prevalence record therefore rests on published case counts rather than an Orphanet epidemiology class, and no numeric rate_per_100000 is asserted. Counting discipline in the 92-patient cohort. That cohort reports different features against different denominators, and two of its numbers do not reconcile with each other. The Seizures description records the irreconcilable pair (recurrent seizures six of 92 against seizures seven of 39 infantile patients) rather than presenting either as settled. Multiple mitochondrial DNA deletions is banded over the 38 patients actually assayed for deletions rather than over all 92, since a finding requiring a biopsy and a deletion assay cannot be scored in patients who had neither; the alternative denominator is stated in that record so the choice is visible. Every figure quoted in a description is carried by one of the evidence items beside it, which matters here because the cohort's feature lists put several counts in one sentence and adjacent features are easy to swap.

Address review on #13196: dysarthria figure, facial diplegia term, KYGEVVI approval, DR artifact · 2026-09-30T03:25:20Z · View source

Second round on PR #13196, addressing a CHANGES_REQUESTED review. Three blocking findings and five suggestions were worked; all were actioned. BLOCKING 1 -- unsupported figure in the Dysarthria description. The description claimed "six of eighteen late-onset patients". Both cache bodies were grepped directly. PMID:31060578 (the eighteen-case late-onset series) never mentions dysarthria: its only speech finding is "dysphonia due to vocal cord palsy (2/18;11.1%)", and its "6" refers to swallowing difficulty ("difficulty in swallowing, which resulted in severe weight loss and/or detriment to the safety of oral feeding in 6 cases"). The reviewer's diagnosis was correct: a dysphagia count had been read as a dysarthria count. The supported figures are both in PMID:29602790 -- "dysarthria/dysphonia (3)" of the 85 patients assessed for other muscular functions, and "dysarthria/dysphonia (three)" of its sixteen late-onset patients. The description now gives those, and additionally records the laryngeal-versus-articulatory distinction, with the eighteen-case series' vocal cord palsy sentence cited for that point rather than for a dysarthria count. BLOCKING 2 -- Facial diplegia rebound to the exact term. Was HP:0010628 "Facial palsy". HP:0001349 was verified by OAK (`runoak -i sqlite:obo:hp info HP:0001349 -O obo`): canonical label "Facial diplegia", def "bilateral facial palsy", is_a HP:0010628. Present in cache/hp/terms.csv and in the phenotypeterm enum membership cache, so it validates offline. Rebound; `just validate-terms` passes. While in that block, the description's "19 of 85" figure had no cited quote containing it, so the whole-cohort sentence carrying "facial diplegia (19)" was added as evidence. BLOCKING 3 -- treatment status brought up to date. PMID:41604077 (Blair HA, "Doxecitine and Doxribtimine: First Approval", Paediatr Drugs 2026) was already fetched into the worktree and was read in full. KYGEVVI was approved in the USA on 3 November 2025 for TK2 deficiency with symptom onset on or before 12 years. The treatment description now states the approval as fact and attributes the 2022 "trials ongoing" sentence to that older review as historical context, with its explanation rewritten to say it is superseded and must not be read as current status. Six new evidence items were added from PMID:41604077, all with quote_role: REVIEW_SYNTHESIS. Their evidence_source is split deliberately: regulatory statements (approval date and indication, first-approved status, labelled pharmacodynamics) are OTHER, because a regulatory action is not a study; the pooled human mortality outcomes are HUMAN_CLINICAL, because evidence_source grades the study the quoted text describes. That reasoning is recorded in the entry's notes so a later reader does not have to reconstruct it. The approval record also supplied genuinely new clinical content: the pooled analyses show a mortality benefit only in onset at or before 12 years (4% treated against 57% untreated) and no significant difference in later onset (18% against 24%), which is where the licensed age limit comes from. That was added to the description and cited, and the tk2_no_controlled_trial knowledge gap was rewritten: the approval does not close it, because the age limit is itself drawn from a subgroup contrast inside the same uncontrolled data, and it is unresolved whether later-onset disease is unresponsive or merely slower than the available follow-up can resolve. Identifier decisions on the approved product, all recorded in notes. No regimen_term was added: NCIT has no term for KYGEVVI, MT-1621, or any deoxynucleoside combination regimen (searched `KYGEVVI`, `MT-1621`, `MT 1621`, `l~Nucleoside Regimen`, `l~Deoxynucleoside`, `l~Nucleoside Therapy`), and RegimenTerm is reachable only from NCIT:C15697. NCIT:C1330 "Deoxythymidine" exists but is_a NCIT:C85834 "Retired Concept 2009" and was not used. NCIT:C420 "Doxecitine" does exist, but adding it beside CHEBI:15698 would list the same molecule twice and its NCIT parents (Pyrimidine Antagonist, Chemoprotective Agent) describe an antineoplastic role. The existing CHEBI bindings were kept and the pharmaceutical names carried in preferred_term instead. The treatment name was left as the mechanistic "Deoxycytidine and Deoxythymidine Substitution": it is the target of an attaches_to foreign key and a mechanism name outlives a trade name. BLOCKING 4 -- deep-research artifact committed, and what it actually is. The report at research/Mitochondrial_DNA_Depletion_Syndrome_Myopathic_Form-deep-research-claude_code.md is committed with this round, together with the six DOI reference-cache files its own reference validation rests on. It carries genuine provider frontmatter (template_sha 1e7ea4ee81, 24/24 references verified, 0 confabulation, 72/72 terms verified) and `just preflight-dr ... MONDO:0012301` returns PASS with TK2 mentioned 51 times and matching OMIM 609560. It is NOT independent corroboration of this entry and must not be described as such. It was generated after the entry was curated, on 2026-09-29, and the provider read the entry and its reference cache while writing. The report says so itself: it describes being "assembled from 18 references held in the dismech verified reference cache", states that its ontology identifiers were "read from the repository's authority-backed term caches ... and from the validated bindings of the curated kb/disorders/ entry", and at one point enumerates what "the curated dismech entry binds". Agreement between the report and the entry is therefore circular. Its actual contribution was narrower than "found a new reference": it flagged PMID:41604077 as present in the cache but unused, explicitly declining to quote it ("present in the cache but not reproduced here, as I do not hold them verbatim -- read them from the source"). All approval content curated here was read from the reference, not from the report. SUGGESTIONS, all taken. Seizures -- the two source counts do not reconcile and are no longer presented as if they did. The whole-cohort paragraph gives "recurrent seizures (six)" of 92; the infantile paragraph gives "seizures (seven)" of 39, a subgroup count exceeding the cohort count. The wording differs between them, so the description offers the most economical reading (the cohort figure counts only recurrent seizures) while stating that six to seven is the defensible range rather than adopting either number. Both sentences are now cited. Encephalopathy -- the aggregate "10/39 ... CNS abnormalities" snippet was the only evidence. The specific counts were verified and added: "encephalopathy (seven)" of 92 and "encephalopathy (five)" of 39. Unlike the seizure figures these reconcile, and the description now says so. The aggregate quote was kept as denominator context rather than removed. Multiple mitochondrial DNA deletions -- frequency OCCASIONAL to FREQUENT. Summing the source's per-onset denominators (1/8, 7/14, 16/16) gives 24 of 38 tested, 63%, which is FREQUENT (30-79%); 24/92 would be 26% only as an artefact of incomplete testing, since a finding requiring a biopsy and a deletion assay cannot be scored in patients who had neither. The denominator choice and the rejected alternative are both stated in the record, and the quote carrying the breakdown was added as frequency evidence. Incidentally verified that the description's "one of fourteen had frank depletion" is correct -- it is the late-onset depletion figure (1/14; 7%). Evidence-provenance paragraph moved out of notes. The paragraph narrating the curation session ("found by independent PubMed search (E-utilities), fetched with `just fetch-reference` ... Sixteen PMIDs ...") was removed from notes; that history is in the CREATE record for this entry and in this one. It was also factually stale, since it asserted that no deep-research report exists. The "no ORPHA cache for Orphanet:254875" and "no module covers mtDNA maintenance" paragraphs were kept, as they record searches and their results rather than session narration. New notes paragraphs record the identifier decisions, the evidence_source split, the counting-denominator discipline, and the deep-research report's non-independence. Muscle MRI moved from histopathology to diagnosis. Its own description had conceded it was "not histology". Moved, that preamble dropped, and a diagnosis_term added: NCIT:C16809 "Magnetic Resonance Imaging", verified by OAK and present in cache/ncit/terms.csv. This follows the existing KB convention -- the Muscular_Dystrophy_Congenital_Hearing_Loss_And_Ovarian_Insufficiency_Syndrome entry files the same record under diagnosis with the same term. The description was also strengthened to say what the record is good for: the pattern comes from a single eighteen-patient series and has not been compared against the myopathies TK2 deficiency is mistaken for, so it can direct a biopsy but not replace one. NOT CHANGED, deliberately. The two sibling files on this branch (Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type, SUCLA2-Related_Mitochondrial_DNA_Depletion_Syndrome) carry only additive reciprocal differentials and were left untouched; they were included in the validation sweep only. Two unrelated reference-cache files (PMID_33231368, PMID_34026460, both SUCLA2 references) were rewritten by the validator's own fix-references-cache step with trailing-whitespace normalization; that churn is out of scope for this PR and was restored rather than swept in. It is deterministic and will reappear for whoever runs the validator next. VALIDATION. Each gate run separately, never chained. just validate: 162 snippets, 163 titles, 0 issues. just count-verified-snippets: 162/162 verified. just validate-terms: passed. just validate-disorders over all three files: 420/420 snippets, 0 issues. check-duplicate-keys, check-entity-refs, check-qualifier-terms (0 qualifier terms), check-enum-values: OK. check-causal-targets: OK, no new broken targets. check-genereviews: GeneReviews TAGGED=1, StatPearls NO_CHAPTER=1. check-snippet-grading, check-title-snippets, check-snippet-length, check-reference-titles, check-folded-hyphens, check-environmental-evidence: OK, no new entries against any baseline, and no baseline file was edited. check-reference-cache-frontmatter: OK, and none of the seven newly added cache files is among the advisory set. just list-disconnected-phenotypes: this entry does not appear, so every phenotype is causally connected. just validate-history-all: passed. Follow-up in the same round. The re-review approved and raised one optional point: the closing sentence of the new "Counting discipline" notes paragraph described this review round rather than the disease. That is the same defect as the provenance paragraph moved out above, and CLAUDE.md's rule that notes: is not a session log covers it, so it was taken rather than deferred. The sentence was replaced with the durable invariant it was gesturing at -- every figure in a description is carried by an evidence item beside it, which matters because the cohort's feature lists put several counts in one sentence. Re-ran just validate (162 snippets, 0 issues), check-duplicate-keys and check-folded-hyphens. This costs a review cycle under the one-push rule; it was judged worth it because the PR is inside the three-day auto-merge age window either way.

Create: mtDNA depletion syndrome, myopathic form (TK2) · 2026-09-29T23:47:58Z · View source

New entry for mitochondrial DNA depletion syndrome, myopathic form (MONDO:0012301; MDS type 2; TK2 deficiency), closing issue #11702 and retiring stubs/Mitochondrial_DNA_Depletion_Syndrome_Myopathic_Form.yaml. References were derived entirely by independent literature search, because no deep-research report exists for this disease. Candidate PMIDs came from NCBI E-utilities searches on TK2 deficiency, thymidine kinase 2 mutations myopathy, mtDNA depletion muscle, the mitochondrial dNTP salvage pathway, the Tk2 H126N knock-in mouse, and deoxynucleoside/deoxynucleoside- monophosphate therapy. Sixteen PMIDs and two ClinicalTrials.gov records were fetched with `just fetch-reference` and read from the resulting cache files before any snippet was written; nothing was cited that was not fetched. No research report was created or committed. Curated: disease_term, synonyms, description, parents, classifications (NEUROLOGIC / ICIMD nucleotide_pool_maintenance / mitochondrial disease), three has_subtypes (infantile-, childhood- and late-onset), inheritance, two prevalence records, nine pathophysiology nodes, 22 phenotypes, three progression phases, two histopathology records, one genetic record with a Spanish founder-allele case fraction, five diagnosis records, six differential diagnoses, eight treatments, two clinical trials, the Tk2 H126N knock-in mouse with three modeled_mechanisms links, four discussions, and entry notes. Mechanism chain: TK2 loss of function -> failure of mitochondrial pyrimidine deoxynucleoside salvage -> imbalanced mitochondrial dNTP pool -> skeletal muscle mtDNA depletion and multiple deletions -> combined respiratory chain deficiency -> progressive muscle fibre degeneration -> respiratory muscle failure, with a separate node for post-mitotic dependence on salvage (the tissue-specificity gate) and one for the unsettled CNS involvement. All 22 phenotypes are causally connected (`just list-disconnected-phenotypes` reports 22/22, 100%). Terms. TK2 was resolved in both directions before binding: `runoak -i sqlite:obo:hgnc search "l^TK2"` returned hgnc:11831, and `runoak -i sqlite:obo:hgnc info hgnc:11831` returned TK2. MONDO:0012301 was inspected with `runoak -i sqlite:obo:mondo info MONDO:0012301 -O obo`, confirming OMIM:609560 and RO:0004003 HGNC:11831. GO:0000002 (mitochondrial genome maintenance) was rejected as obsolete and GO:0006264 used instead. Every HP, GO, CL, UBERON, NCIT, CHEBI and MONDO CURIE was resolved with OAK in the same step it was written. CHEBI:15698 and CHEBI:17748 were checked against their definitions to confirm they are the 2'-deoxyribonucleosides rather than the nucleotides. HP:0003236's canonical label was corrected from "concentration" to "activity" after the validator reported the mismatch. Conformance: Combined Respiratory Chain Deficiency conforms to mitochondrial_dysfunction#Bioenergetic Decline and Oxidative Stress, consistent with the four other mtDNA depletion entries. complex_i_deficiency, complex_iv_assembly_deficiency, deregulated_cellular_energetics and cardiomyopathy_maladaptive_remodeling were considered and rejected, with reasons recorded in the entry notes. No module covers mtDNA maintenance or the mitochondrial dNTP pool. Sibling consistency: MSTO1-Related_Mitochondrial_Myopathy already carried a differential pointing at MONDO:0012301, so the reverse pointer was added here. Reverse differentials naming MONDO:0012301 were also added to Mitochondrial_DNA_Depletion_Syndrome_3_Hepatocerebral_Type (the purine arm of the same salvage pathway) and SUCLA2-Related_Mitochondrial_DNA_Depletion_Syndrome (the encephalomyopathic form), each with its own history record. No sibling entry's existing content was changed. Negative and limiting results were curated rather than dropped: a normal creatine kinase does not exclude the diagnosis (REFUTE), no usable genotype-phenotype correlation exists (REFUTE), adding tetrahydrouridine to the nucleotide therapy shortened mouse survival (REFUTE), the mouse's respiratory-chain defect is CNS-predominant with muscle spared so it is not a faithful muscle model (REFUTE, with a SPECIES_MISMATCH divergence), and CNS features in patients may be hypoxic-ischaemic rather than primary (REFUTE). Mouse evidence is graded MODEL_ORGANISM throughout and is never the sole support for a human phenotype. Not curated: no ClinGen Gene-Disease Validity assertion exists for TK2 (`just clingen-list` returns none), so gene_disease_validity is absent and the reason is recorded in the genetic record's notes. No ORPHA cache file exists for Orphanet:254875, so prevalence rests on published case counts with no rate_per_100000 asserted. Mitochondrial dNTP pools have not been measured in patient tissue in any cited source; the node says so. Validation, each run separately and read: `just validate` (149 snippets checked, 0 issues, 150 titles checked), `just count-verified-snippets`, `just validate-terms`, `just validate-disorders` over all three changed disorder files, `just check-duplicate-keys`, `just check-entity-refs`, `just check-causal-targets`, `just check-qualifier-terms` (0 qualifier terms; none used), `just check-enum-values`, `just check-coarse-phenotypes`, `just check-delivery-system`, `just check-genereviews` (GeneReviews chapter PMID:23230576 tagged, EXACT title match), and `just validate-history-all`.

Claude Code ▸

Disease Characteristics Research Report

claude-haiku-4-5-20251001, claude-opus-5 2026-09-29T20:55:28.335169

Disease Characteristics Research Report

Mitochondrial DNA Depletion Syndrome, Myopathic Form (TK2 Deficiency)

MONDO:0012301 · TK2-related mitochondrial DNA maintenance defect, myopathic form · MTDPS2

Report date: 2026-09-29 · Inheritance: Autosomal recessive (Mendelian)

Provenance and method note

This report was assembled from 18 references held in the dismech verified reference cache, of which 13 were read in full text or full abstract for this report. Eleven carry full PubMed Central text; the remainder are abstract-only records. Where a claim rests on a reference I did not read directly, I cite it as a pointer and attribute the substance to the secondary source I did read (chiefly Berardo et al. 2022, PMID:35094997), rather than presenting a quotation I could not verify.

Ontology identifiers in this report were read from the repository's authority-backed term caches (cache/<prefix>/terms.csv) and from the validated bindings of the curated kb/disorders/ entry, in the same step they were written. The Ontology Lookup Service MCP tool was unavailable in this session (permission not granted), so no identifier here was resolved by live OLS query, and none was written from recall. Concepts for which I could not produce a cache- or entry-backed identifier are named in prose and flagged as unresolved in §16.

Evidence-source typing follows the dismech convention and is marked per claim: [HUMAN CLINICAL], [MODEL ORGANISM], [IN VITRO], [COMPUTATIONAL], [REVIEW/SYNTHESIS].

1. Disease Identity and Identifiers

TK2 deficiency (TK2d) is an autosomal recessive disorder of mitochondrial DNA (mtDNA) maintenance caused by biallelic pathogenic variants in TK2, encoding mitochondrial thymidine kinase 2. Its cardinal presentation is a progressive myopathy with early and disproportionate involvement of respiratory, cervical, facial and bulbar musculature, accompanied in muscle by quantitative mtDNA depletion, qualitative multiple mtDNA deletions, or both.

Field	Value	Source
Primary ontology term	`MONDO:0012301` mitochondrial DNA depletion syndrome, myopathic form	curated entry binding
OMIM phenotype	MIM #609560	Ceballos et al. 2024, PMID:38544965
Gene	TK2, `hgnc:11831`	curated entry binding
Gene locus	chromosome 16q21	PMID:35094997
Reference transcript	NM_004614.5	PMID:35094997
Alternative designations	MTDPS2; mtDNA depletion syndrome 2 (myopathic type); TK2-related mtDNA maintenance defect, myopathic form	PMID:23230576

The gene's structure is given precisely by Berardo and colleagues: [REVIEW/SYNTHESIS]

"The TK2 gene is located at chromosome 16q21 and spans 42,410 base pairs with 10 exons encoding a 265 amino acid protein. The TK2 mRNA length is 5114 nucleotides with a 3.8 kilobase long 3'-untranslanted [sic] region (NM_004614.5)." — Berardo A, Domínguez-González C, Engelstad K, Hirano M. J Neuromuscul Dis. 2022. PMID:35094997, DOI 10.3233/JND-210786

The causal gene was identified in 2001 in patients with myopathic mtDNA depletion (Saada et al., Nat Genet; PMID:11687801) [HUMAN CLINICAL] [IN VITRO].

Nomenclature caution that matters for curation. GeneReviews titles the entity a "TK2-related mitochondrial DNA (mtDNA) maintenance defect" rather than a depletion syndrome, and this is not cosmetic: late-onset disease characteristically shows multiple mtDNA deletions with normal or near-normal mtDNA copy number, so "depletion syndrome" misdescribes a substantial minority of molecularly confirmed patients. See §8 and §9.

Identifiers I could not resolve in this session: Orphanet ORPHAcode, ICD-10-CM and ICD-11 codes, MeSH descriptor, UMLS CUI, and the TK2 MIM gene number (as distinct from the phenotype MIM). These require lookups that were not available; they are flagged in §16 rather than supplied from memory.

2. Classification and Nosology

TK2d sits at the intersection of three classification axes, and its position differs on each.

By mechanism — mtDNA maintenance defect. TK2d belongs to the group of nuclear-gene disorders that impair replication or nucleotide supply for the mitochondrial genome, rather than disorders of the mitochondrial genome itself. Lopez-Gomez et al. give the canonical genotype-to-phenotype partition of the mtDNA depletion syndromes: [REVIEW/SYNTHESIS]

Myopathic — TK2
Hepatocerebral — DGUOK, POLG, C10orf72 (TWNK), MPV17
Myopathy with renal tubulopathy — RRM2B
Encephalomyopathic — SUCLA2, SUCLG1
Severe infantile encephalomyopathy — OPA1, MFN2

— Lopez-Gomez C, et al. Ann Neurol. 2017. PMID:28318037, DOI 10.1002/ana.24922

Garone et al. name the eight genes then established for infantile mtDNA depletion syndrome: TK2, DGUOK, POLG, MPV17, RRM2B, SUCLA2, SUCLG1 and C10orf2 (PMID:24968719) [REVIEW/SYNTHESIS].

By biochemical pathway — deoxyribonucleoside salvage. Within the maintenance defects, TK2d is specifically a defect of the mitochondrial pyrimidine deoxyribonucleoside salvage pathway, grouping it with DGUOK (purine salvage) and functionally, though not mechanistically, with TYMP/MNGIE and RRM2B as disorders of nucleotide pool balance.

By clinical phenotype — a mitochondrial myopathy. Clinically the disorder presents as a myopathy and enters the differential of limb-girdle muscular dystrophy, spinal muscular atrophy, facioscapulohumeral dystrophy and congenital myopathy (§14) far more often than it enters the differential of a classical mitochondrial encephalopathy.

Phenotypic continuum, not discrete types. GeneReviews is explicit that the entity is graded rather than partitioned: [REVIEW/SYNTHESIS]

"TK2-related mitochondrial DNA (mtDNA) maintenance defect is a phenotypic continuum that ranges from severe to mild. ... Three main subtypes of presentation have been described: Infantile-onset myopathy with neurologic involvement and rapid progression to early death. ... Juvenile/childhood onset with generalized proximal weakness and survival to at least 13 years. Late-/adult-onset myopathy with facial and limb weakness and mtDNA deletions." — GeneReviews, PMID:23230576

These three bands — infantile (<12 months to ~2 years), childhood/juvenile, and late/adult — are used consistently throughout this report because every major series stratifies on them, but they are convenience bands on a continuum, and the Spanish cohort's onset distribution (§3) shows no natural breakpoints.

A placement within the International Classification of Inherited Metabolic Disorders (ICIMD) framework is available at PMID:33340416; I did not read that record in full and so do not quote its category assignment here.

3. Epidemiology

No direct prevalence measurement of TK2d exists. Every figure in the literature is an indirect estimate built by multiplying a mitochondrial-disease prevalence by the TK2 share of a referred mtDNA-depletion cohort. This is important to state plainly, because the resulting numbers are often quoted as though they were measured.

Berardo et al. give the derivation explicitly: [REVIEW/SYNTHESIS] a background mitochondrial disease prevalence of 11.5 per 100,000, and a TK2 fraction of 10%, 12.5% and 18% across three separate MDDS screening series, yielding

"a minimum prevalence of TK2d at 600 patients and a maximum prevalence of 2700 TK2-deficient patients in the United States." — PMID:35094997

Against that estimated denominator, the count of reported patients remains small. GeneReviews records that [REVIEW/SYNTHESIS]

"approximately 107 individuals with a molecularly confirmed diagnosis have been reported." — PMID:23230576

The single largest cohort published to date is Spanish: 53 molecularly confirmed patients (Ceballos et al., Neurol Genet 2024, PMID:38544965) [HUMAN CLINICAL], which by itself is roughly half the previously reported world literature — a strong indication that TK2d is substantially under-diagnosed, and that the "107 reported" figure reflects ascertainment rather than incidence.

Age-at-onset distribution (n = 53, Spanish cohort) [HUMAN CLINICAL]:

Onset band	n	%
< 1 year	4	7.5%
12–36 months	11	20.8%
3–12 years	6	11.3%
> 12 years	32	60.3%
— of which > 40 years	14	26.4%

— PMID:38544965

This distribution is itself a finding: the majority of molecularly confirmed patients in an unselected national cohort have adult-onset disease, inverting the older literature's impression of TK2d as a fatal infantile myopathy. The inversion is almost certainly an ascertainment correction — infantile cases died before molecular diagnosis was available, and adult cases were mis-labelled as limb-girdle dystrophy.

Founder effects and population genetics. The Spanish cohort provides the only population-genetic characterisation of TK2 alleles [HUMAN CLINICAL] [COMPUTATIONAL]:

Variant	Spanish allele freq.	gnomAD allele freq.	Enrichment
p.Lys202del (c.604_606del)	0.034%	0.0004%	86-fold
p.Thr108Met (c.323C>T)	0.062%	0.0046%	13-fold

Regional enrichment is sharper still: p.Thr108Met reaches 0.176% in Galicia, a 38-fold excess over gnomAD. gnomAD's Latino subpopulation carries p.Lys202del at 0.0029%, consistent with transmission of the Iberian allele to the Americas.

Haplotype dating placed the most recent common ancestor at 425 years (95% CI 125–1,550) for p.Thr108Met and 2,375 years (1,212–3,532) for p.Lys202del, corresponding to roughly 16.8 and 95.2 generations. Methods included runs-of-homozygosity analysis with FIS/FST/FROH inbreeding coefficients, PLINK, and the Axiom Spain Biobank Array. — PMID:38544965

The practical consequence: in Iberian and Iberian-descended populations, TK2d prior probability is materially higher than a global estimate implies, and p.Thr108Met / p.Lys202del warrant targeted testing.

Not established: incidence, sex ratio (no series reports a sex skew, but none powered to exclude one), prevalence outside Spain and referral-based US/European series, and carrier frequency in any non-Iberian population.

4. Clinical Presentation

4.1 The core syndrome

The presenting and dominant feature across all onset bands is proximal and axial muscle weakness (HP:0003701 Proximal muscle weakness) with a distribution that is the disorder's clinical signature: cervical extensor, facial and bulbar muscles are involved early and disproportionately relative to limb strength.

The Spanish cohort quantifies this: 80% of patients had cervical and facial weakness regardless of age of onset (PMID:38544965) [HUMAN CLINICAL]. This axial-and-facial predominance, in a patient with elevated creatine kinase, is what should prompt TK2 testing rather than a further dystrophy panel.

Respiratory involvement is the prognostically decisive feature. Weakness of the diaphragm (UBERON:0001103) and accessory respiratory muscles produces respiratory insufficiency (HP:0002093) that drives both ventilatory dependence and mortality (§11). GeneReviews: "Affected individuals experience progressive muscle weakness leading to respiratory failure." (PMID:23230576).

4.2 Presentation by onset band

Infantile onset (<2 years) [HUMAN CLINICAL] [REVIEW/SYNTHESIS] Rapidly progressive myopathy with hypotonia (HP:0001252), motor delay or developmental regression (HP:0002376), failure to thrive (HP:0001508), and progression to respiratory failure and early death. GeneReviews records that in this band "Some individuals develop dysarthria, dysphagia, and/or hearing loss. Cognitive function is typically spared." (PMID:23230576) — the sparing of cognition despite "neurologic involvement" is a clinically important and frequently misstated point. Seizures (HP:0001250), encephalopathy (HP:0001298) and sensorineural hearing impairment (HP:0000407) occur but are not universal.

Childhood/juvenile onset [REVIEW/SYNTHESIS] Generalised proximal weakness with survival to at least the teens. Berardo et al. report in this band: wheelchair dependence 60%, ventilator dependence 55%, CNS involvement 10%, and extraskeletal (non-muscle) involvement under 20% (PMID:35094997).

Late/adult onset (>12 years, and a large >40-year group) [HUMAN CLINICAL] [REVIEW/SYNTHESIS] Mean onset 31 years. Facial and limb weakness, with non-invasive ventilation in 12 of 18 (67%) and gastrostomy in approximately 30%, at a mean 19.6 years after onset (PMID:35094997). Chronic progressive external ophthalmoplegia (HP:0000590), ptosis (HP:0000508) and ophthalmoparesis (HP:0000597) are characteristic of this band and largely absent from the infantile one.

4.3 The ptosis/CPEO age gradient

The Spanish cohort documents an age-dependent ocular gradient that is among the most useful discriminating observations in the disorder [HUMAN CLINICAL]:

Onset band	Ptosis and CPEO	Ptosis only	Neither
< 12 years	18.8%	18.8%	62.5%
12–40 years	40%	46.7%	13.3%
> 40 years	13/13 (100%) ptosis	—	11/13 (85%) had both

— PMID:38544965

Read this as a rule of thumb: in adult-onset TK2d, ptosis is effectively obligatory; in infantile-onset TK2d, its absence is the norm. An adult with axial/facial weakness, high CK and no ptosis is a less likely TK2d candidate than the same patient with ptosis.

4.4 Additional and atypical features

Dysphagia (HP:0002015) in 40% of the Spanish cohort; dysarthria (HP:0001260) also recorded.
Scapular winging (HP:0003691), contributing to the FSHD and SMA mimicry described in §14.
Facial palsy / facial weakness (HP:0010628).
Loss of ambulation (HP:0002505) — but note this is less common than the older literature implies: only 16.8% (7/43) of the Spanish cohort were non-ambulant (PMID:38544965). Ventilatory failure outpaces loss of walking.
Isolated exercise intolerance with exertional rhabdomyolysis in 3 patients (6.8%) (HP:0003546, HP:0003201) — a genuinely oligosymptomatic presentation that would not normally prompt mitochondrial testing (PMID:38544965).
Hypertrophic cardiomyopathy reported in association with the p.Ala139Val and p.Phe70Ser variants (PMID:35094997) — cardiac involvement is otherwise rare.

4.5 Laboratory presentation

Creatine kinase (HP:0003236 Elevated circulating creatine kinase activity) is elevated in the great majority, spanning 272–6,500 U/L across 57 patients — but Berardo et al. record 6 patients with transient normal values, and state the consequence directly: [REVIEW/SYNTHESIS]

"indicating that a normal CK does not exclude TK2d." — PMID:35094997

Lactate (HP:0002151 Increased circulating lactate concentration) may be raised but is neither sensitive nor specific.

5. Natural History and Progression

The trajectory differs qualitatively, not merely in tempo, across onset bands.

Infantile onset follows a compressed course: onset in the first two years, rapid progression of weakness, respiratory failure, and death typically within the first years of life. The untreated mortality in this band is stark — the Spanish cohort found 86% of untreated patients with onset before 12 years had died by the September 2023 census (PMID:38544965) [HUMAN CLINICAL].

Childhood/juvenile onset progresses over years to a decade-plus, accumulating wheelchair dependence (≈60%) and ventilatory support (≈55%) but with survival into the teens and beyond (PMID:35094997).

Adult onset is the slowest course and the one in which the sequence of milestones is best described: weakness beginning at a mean of 31 years, ptosis and CPEO accruing, non-invasive ventilation in about two-thirds, and gastrostomy at a mean of 19.6 years after onset in the ~30% who require it (PMID:35094997) [REVIEW/SYNTHESIS].

Genotype predicts the timing of ventilatory failure. This is one of the clearest genotype–course correlations in TK2d, and it is statistically supported [HUMAN CLINICAL]:

All 6 p.Lys202del homozygotes began mechanical ventilation after age 40. 78% of 9 p.Thr108Met homozygotes began ventilation between ages 12 and 40. Association with variant, p < 0.01. — PMID:38544965

Dedicated natural-history analyses are available at PMID:29602790 (subtype frequencies and post-onset survival) and PMID:31060578 (late-onset series with respiratory, MRI and biomarker profiling); both are in the cache but I did not read them in full for this report and therefore quote no figures from them.

The single most useful framing for prognosis is that respiratory function, not ambulation, is the clock. Thirty percent of the Spanish cohort had died, all of respiratory failure (§11), while five in six patients remained ambulant.

6. Pathophysiology and Mechanism

6.1 Ordered causal chain

Biallelic pathogenic variants in TK2 (hgnc:11831) reduce or abolish the catalytic efficiency of mitochondrial thymidine kinase 2 (GO:0004797 thymidine kinase activity; GO:0004137 deoxycytidine kinase activity). Demonstrated in vitro by enzyme kinetics on recombinant mutant protein (PMID:12493767).
Loss of TK2 activity results in failure of the first and rate-limiting phosphorylation step of the mitochondrial pyrimidine deoxyribonucleoside salvage pathway (GO:0043099 pyrimidine deoxyribonucleoside salvage) — deoxythymidine → dTMP and deoxycytidine → dCMP. Demonstrated (PMID:20940150).
In proliferating cells, cytosolic thymidine kinase 1 (TK1) supplies deoxypyrimidine nucleotides independently, which masks the TK2 defect. Demonstrated in mouse tissue (PMID:20940150).
Post-mitotic differentiation leads to cell-cycle-dependent down-regulation of TK1, which unmasks the TK2 deficiency in that tissue — the step that sets both tissue selectivity and age of onset. Demonstrated in the Tk2⁻/⁻ mouse (PMID:20940150).
Unmasked TK2 deficiency results in depletion of the mitochondrial dTTP pool (CHEBI:18077), with the dCTP pool relatively preserved because de novo ribonucleotide reductase (R1–p53R2) can supply it. Demonstrated by tissue nucleotide measurement (PMID:20940150).
The resulting imbalanced mitochondrial dNTP pool (GO:0009202 deoxyribonucleoside triphosphate biosynthetic process) leads to failure of faithful mtDNA replication (GO:0006264 mitochondrial DNA replication; GO:0032042 mitochondrial DNA metabolic process). Demonstrated (PMID:20940150, PMID:24968719).
The mechanism branches here, and the branch is the disorder's central clinical puzzle:
7a — Quantitative branch: replication failure results in reduced mtDNA copy number — mtDNA depletion (HP:0009141 Depletion of mitochondrial DNA in muscle tissue). Dominant in early-onset disease. Demonstrated in human muscle and mouse tissue.
7b — Qualitative branch: replication error results in multiple mtDNA deletions (HP:0003689) with normal or near-normal copy number. Dominant in late-onset disease. Demonstrated in human muscle; the mechanistic route from dNTP imbalance to deletion formation specifically — as opposed to depletion — is inferred, not demonstrated.
A compensatory branch intervenes before OXPHOS fails. Reduced mtDNA copy number leads to down-regulation of the mitochondrial transcription terminator MTERF3, which results in increased mitochondrial transcript abundance per mtDNA molecule (GO:0006390 mitochondrial transcription) — approximately 3-fold in heart — thereby sparing OXPHOS despite substantial copy-number loss. Demonstrated in the Tk2⁻/⁻ mouse (PMID:20940150).
Where mtDNA loss exceeds the capacity of that transcriptional compensation — the "mtDNA depletion threshold" — it results in deficiency of mtDNA-encoded respiratory chain subunits and consequent oxidative phosphorylation deficiency (GO:0006119 oxidative phosphorylation; GO:0022904 respiratory electron transport chain), measurable as complex IV deficiency (HP:0008347) and cytochrome c oxidase-negative fibres (HP:0003688). Demonstrated (PMID:20940150).
OXPHOS deficiency in the skeletal muscle fibre (CL:0008002) of skeletal muscle tissue (UBERON:0001134) leads to fibre degeneration, ragged-red fibre formation (HP:0003200), necrosis and dystrophic remodelling. Demonstrated histologically in human muscle.
The degree of residual TK2 activity determines which tissues cross the threshold, and therefore the phenotype: 14–45% residual activity leads to myopathy; under 10% leads to encephalomyopathy. Demonstrated by correlating patient enzyme assays with phenotype (PMID:20940150).
Progressive loss of muscle fibres results in proximal, cervical, facial and bulbar weakness (HP:0003701, HP:0000467, HP:0010628), which leads to diaphragmatic and accessory respiratory muscle failure (UBERON:0001103; HP:0002093 Respiratory insufficiency).
Respiratory muscle failure results in ventilatory dependence and, in untreated disease, death — in 100% of the fatalities in the largest cohort (PMID:38544965). Demonstrated [HUMAN CLINICAL].

Therapeutic interruption point. Steps 2–5 are bypassable: exogenous deoxycytidine and deoxythymidine (CHEBI:15698, CHEBI:17748) enter mitochondria and are phosphorylated by pathways that do not require TK2, restoring dNTP pools downstream of the lesion. This is the entire rationale for nucleoside substrate-enhancement therapy (§12), and it is why the therapy works without correcting the genetic defect.

6.2 Detail

The rate-limiting-step argument. Dorado et al. state TK2's position in the pathway precisely: [MODEL ORGANISM] [REVIEW/SYNTHESIS]

"the first and rate-limiting step for the salvage pathway synthesis of deoxypyrimidine nucleoside triphosphates required for mitochondrial DNA (mtDNA) replication and maintenance in post-mitotic cells" — Dorado B, Area E, Akman HO, Hirano M. Hum Mol Genet. 2011. PMID:20940150, DOI 10.1093/hmg/ddq453

Because post-mitotic cells cannot rely on cell-cycle-coupled de novo synthesis, and because the mitochondrial pyrimidine supply has no redundant route, TK2 loss is uniquely consequential in differentiated tissue.

The TK1 hand-off is the tissue-selectivity mechanism. The same paper demonstrates the unmasking directly: [MODEL ORGANISM]

"The down-regulation of Tk1 activity unmasks Tk2 deficiency in Tk2-/- mice and correlates with the onset of mtDNA depletion in the brain and the heart" — PMID:20940150

The measured time course in Tk2⁻/⁻ mouse brain shows the sequence: dTTP fell from 57 ± 17% of normal at day 8 to 11 ± 8% at day 13, while dCTP remained preserved (attributed to de novo ribonucleotide reductase R1–p53R2 activity). Brain mtDNA fell from 45 ± 10% to 35 ± 8%; heart mtDNA fell from normal to 58 ± 27%.

Crucially, mtDNA loss begins before symptoms. Garone et al. measured a pre-symptomatic postnatal day 4 baseline in Tk2⁻/⁻ mice and already found cerebrum 38 ± 13%, cerebellum 54 ± 1%, muscle 28 ± 12% and kidney 62 ± 11% of normal mtDNA, with heart and liver still normal (PMID:24968719) [MODEL ORGANISM]. Any therapy that waits for symptoms is starting behind.

MTERF3 and the depletion threshold. The compensation mechanism in step 8 is specifically transcriptional and specifically MTERF3-mediated. MTERF3 fell to 43 ± 22% in brain and 63 ± 21% in heart. Dorado et al. explicitly excluded the canonical biogenesis regulators — PGC-1α, NRF-1, NRF-2, TFAM, TFB1M and TFB2M were not responsible. They define the concept: [MODEL ORGANISM]

"we can define the mtDNA depletion threshold as the reduction in mtDNA that overwhelms transcriptional compensation" — PMID:20940150

This resolves an otherwise paradoxical observation — that tissues can lose 40% of their mtDNA with preserved respiratory function — and it identifies MTERF3 inhibitors as a rational therapeutic class, proposed in that paper and, so far as I can establish, never developed.

Why muscle. Two independent and complementary explanations exist. The first, from Saada et al., is a supply-and-demand argument: [HUMAN CLINICAL] [IN VITRO]

"Our results suggest that low basal TK2 activity combined with a high requirement for mitochondrial encoded proteins in muscle predispose this tissue to the devastating effect of TK2 deficiency." — Saada A, Shaag A, Elpeleg O. Mol Genet Metab. 2003. PMID:12765840, DOI 10.1016/s1096-7192(03)00063-5

The same paper records the striking selectivity: "Other tissues such as liver, brain, heart, and skin remain unaffected throughout the patients' life." The second explanation is the TK1 timing argument above — muscle exits the cell cycle early and durably. These are not competing; they compound.

Mutant enzyme kinetics establish the genotype–activity–phenotype ladder. Wang, Saada and Eriksson characterised recombinant mutants [IN VITRO]:

"The I212N mutant showed less than 1% activity as compared with wild type TK2 with all deoxynucleosides." — Wang L, Saada A, Eriksson S. J Biol Chem. 2003. PMID:12493767, DOI 10.1074/jbc.M206143200

The p.His121Asn mutant behaves quite differently and instructively: normal Km for deoxythymidine (6 µM) and deoxycytidine (11 µM), but "2- and 3-fold lower V(max) values as compared with wild type TK2 and markedly increased K(m) values for ATP, leading to decreased enzyme efficiency." The lesion is in phosphate donor handling, not substrate recognition — consistent with the structural observation that His126 (mouse) / His121 (human) sits at the C-terminal end of helix α4 facing the ERS (Glu138–Arg139–Ser140) phosphate-transferring domain (PMID:20940150).

Substrate-specific kinetics matter for dosing. Lopez-Gomez et al. found TK2 follows Michaelis–Menten kinetics for deoxycytidine but shows negative cooperativity for deoxythymidine (PMID:28318037) [IN VITRO]. The two substrates are not interchangeable, and a therapy must supply both.

6.3 Ontology terms for the mechanism

Concept	Term	Modifier
Thymidine kinase activity	`GO:0004797` thymidine kinase activity	DECREASED / LOSS_OF_FUNCTION
Deoxycytidine kinase activity	`GO:0004137` deoxycytidine kinase activity	DECREASED
Pyrimidine salvage	`GO:0043099` pyrimidine deoxyribonucleoside salvage	DECREASED
dNTP synthesis	`GO:0009202` deoxyribonucleoside triphosphate biosynthetic process	DECREASED
mtDNA replication	`GO:0006264` mitochondrial DNA replication	DECREASED
mtDNA metabolism	`GO:0032042` mitochondrial DNA metabolic process	DECREASED
Mitochondrial transcription (MTERF3 compensation)	`GO:0006390` mitochondrial transcription	INCREASED (per mtDNA copy)
Mitochondrion organization	`GO:0007005` mitochondrion organization	—
OXPHOS	`GO:0006119` oxidative phosphorylation	DECREASED
Respiratory chain	`GO:0022904` respiratory electron transport chain	DECREASED
Affected cell type	`CL:0008002` skeletal muscle fiber	—
Affected tissue	`UBERON:0001134` skeletal muscle tissue	—
Respiratory failure site	`UBERON:0001103` diaphragm	—
Substrate (thymidine)	`CHEBI:17748` thymidine	—
Substrate (deoxycytidine)	`CHEBI:15698` 2'-deoxycytidine	—
Depleted pool	`CHEBI:18077` dTTP	DECREASED

Related cell types available for a fuller pathograph: CL:0002372 myotube, CL:0000594 skeletal muscle satellite cell, CL:0000189 slow muscle cell, CL:0000190 fast muscle cell, CL:0000540 neuron (encephalomyopathic band), CL:0000182 hepatocyte (spared — useful as a contrast with DGUOK/MPV17 hepatocerebral disease).

7. Genetics

Gene and inheritance. TK2 (hgnc:11831), chromosome 16q21, 10 exons, 265 amino acids, NM_004614.5. Inheritance is autosomal recessive (HP:0000007).

Variant spectrum. Berardo et al. record more than 30 pathogenic variants, with the frequently encountered ones being p.Thr108Met, p.Asn58Ser, p.Arg130Trp, p.Lys202del, p.His121Asn and p.Arg183Trp (PMID:35094997) [REVIEW/SYNTHESIS]. The Spanish cohort found 16 distinct variants across 53 patients (PMID:38544965) [HUMAN CLINICAL].

Allelic frequency in the largest cohort (PMID:38544965):

Variant	cDNA	Patients	Alleles
p.Lys202del	c.604_606del	21	35
p.Thr108Met	c.323C>T	18	32
p.Tyr208Cys	—	—	11
p.Ala139Thr	—	—	6

Two variants therefore account for the large majority of Spanish alleles — a founder architecture (§3), not a mutational hotspot.

A novel variant was reported in that cohort: c.503del (p.Ile168ThrfsTer2), classified on PVS1 + PM2 + observed in trans.

Variant distribution within the gene. Pathogenic variants were found in all exons except 3 and 4, with missense variants clustering at residues 100–141 — the region containing the Mg²⁺ and ATP binding determinants (PMID:38544965). This is mechanistically coherent with the p.His121Asn kinetics above (§6.2): the disease-dense region is the phosphate-transfer machinery, not the nucleoside-binding pocket.

Genotype–phenotype correlations. These are real but partial:

Variant	Association	Source
p.Arg130Trp	Most severe phenotype	PMID:35094997
p.Ile212Asn	<1% residual activity in vitro	PMID:12493767
p.Arg183Trp (homozygous)	Muscle-restricted mtDNA depletion	PMID:35094997
p.Lys202del	Adult-only onset; MV always after age 40	PMID:35094997, PMID:38544965
p.Thr108Met (homozygous)	MV between ages 12 and 40 in 78%	PMID:38544965
p.Ala139Val, p.Phe70Ser	Hypertrophic cardiomyopathy	PMID:35094997
p.His121Asn	Reduced Vmax, raised ATP Km	PMID:12493767

The unifying quantitative model is residual enzyme activity, not variant identity: 14–45% of normal TK2 activity produces myopathy; below 10% produces encephalomyopathy (PMID:20940150). Variant-specific correlations are best read as proxies for where a genotype falls on that scale.

Modifiers, penetrance and expressivity. No modifier loci are established. Penetrance appears complete for biallelic pathogenic genotypes, but expressivity is wide even within a genotype — p.Thr108Met homozygotes span onset bands — and no explanation for that intra-genotypic variability has been identified. This is a genuine open question, not an under-researched one.

8. Diagnosis

GeneReviews gives the diagnostic criteria, and they are age-stratified, which is the point most often missed: [REVIEW/SYNTHESIS]

"The diagnosis of TK2-related mtDNA maintenance defect is established in a proband with infantile onset of disease with severely reduced (typically <20% of age- and tissue-matched healthy controls) mtDNA content in skeletal muscle. The diagnosis of TK2-related mtDNA maintenance defect is established in a proband older than age two years with reduced mtDNA content or multiple mtDNA deletions, ragged red fibers and/or COX-deficient fibers in skeletal muscle. The diagnosis is confirmed by the identification of biallelic pathogenic variants in TK2 by molecular genetic testing." — PMID:23230576

Two consequences follow.

First, mtDNA quantification alone will miss late-onset disease. Berardo et al. report that muscle mtDNA averaged 14% of normal across 55 cases — but 15 cases had normal mtDNA content, and in those the mean age of onset was 19 years (PMID:35094997) [REVIEW/SYNTHESIS]. In adult-onset patients the abnormality is qualitative (multiple deletions), not quantitative. A normal muscle mtDNA copy number in an adult does not exclude TK2d.

Second, respiratory chain enzymology is an unreliable screen. Berardo et al. state it plainly:

"only 20% to 74% of confirmed TK2d patients have had deficiencies in OXPHOS activities." — PMID:35094997

Combined with the CK observation ("a normal CK does not exclude TK2d", §4.5), the diagnostic implication is that no single conventional screening test is sensitive, and molecular testing should be reached for early rather than after a normal biochemical workup.

Practical diagnostic pathway:

Clinical suspicion — proximal + cervical + facial/bulbar weakness; ptosis in an adult; elevated CK; respiratory involvement out of proportion to limb weakness.
Creatine kinase measurement (NCIT:C64489) — supportive, not exclusionary.
Muscle biopsy (NCIT:C51895) — histology, histochemistry (COX/SDH), respiratory chain enzymology, and mtDNA copy number plus deletion analysis (§9).
Molecular confirmation — biallelic TK2 variants by exome (NCIT:C101295 Whole Exome Sequencing) or targeted panel. In Iberian or Iberian-descended patients, targeted p.Thr108Met / p.Lys202del testing is high-yield.
Biomarkers — GDF-15 and FGF-21 (§9) as supportive, non-diagnostic adjuncts.

Biopsy yield in the Spanish cohort was essentially total: 50 of 53 patients (94.3%) underwent muscle biopsy and all were pathologic (PMID:38544965) — so while biopsy is not required for diagnosis once molecular testing is available, it is highly informative when performed.

9. Histopathology and Biomarkers

9.1 Muscle histopathology

The biopsy shows mitochondrial and dystrophic features together, which is itself diagnostically suggestive.

Mitochondrial features: - Ragged-red fibres (HP:0003200): 2–18% of fibres (PMID:38544965) - COX-negative fibres (HP:0003688): 4–40% of fibres (PMID:38544965) - Decreased complex IV activity (HP:0008347)

Dystrophic and neurogenic-mimicking features (Spanish cohort, PMID:38544965) [HUMAN CLINICAL]: - Myopathic change in 93% - Fibre-size variability in 93% - Endomysial fibrosis in 52% - Nuclear internalization in 50% - Fibre necrosis in 48%

Berardo et al. add a feature that directly explains a recurrent misdiagnosis: SMA-like fibre grouping with type 1 fibre predominance (HP:0003803 Type 1 muscle fiber predominance) (PMID:35094997). A biopsy read as neurogenic grouping in an infant is a classic route to a wrong SMA diagnosis (§14).

9.2 Molecular pathology of muscle

mtDNA depletion (HP:0009141): mean 14% of normal across 55 cases, but with 15 normal-content cases skewing toward late onset (PMID:35094997). Individual Spanish-cohort values span <10% to 66% (PMID:38544965).
Multiple mtDNA deletions (HP:0003689): characteristic of late-onset disease, and the abnormality present when copy number is normal.

The two findings are best understood as the two arms of the §6.1 step-7 branch rather than as alternative severities of one finding.

9.3 Circulating biomarkers

Berardo et al. give both analytes with onset-band-dependent magnitudes and reference ranges [REVIEW/SYNTHESIS]:

Biomarker	Early onset	Childhood/late onset	Reference range
GDF-15	> 10,000 pg/mL	> 1,000 pg/mL	≤ 750 pg/mL
FGF-21	> 1,000 pg/mL	100–1,000 pg/mL	< 350 pg/mL

— PMID:35094997

GDF-15 is the more informative of the two, with both a larger dynamic range and a clearer separation from reference. Both track disease severity and onset band rather than being simply present-or-absent, which makes them plausible — though not validated — treatment-response measures. Neither is specific to TK2d; both are general mitochondrial-disease biomarkers.

I could not resolve ontology identifiers for GDF-15, FGF-21, or a creatine-kinase analyte term (as distinct from the NCIT:C64489 measurement procedure) from the available caches; see §16.

10. Management and Surveillance

Management is multidisciplinary and, apart from nucleoside therapy (§12), entirely supportive. GeneReviews provides the only structured guidance [REVIEW/SYNTHESIS]:

"Treatment of manifestations: Management should involve a multidisciplinary team. Feeding difficulties should be managed aggressively, including use of a nasogastric tube or gastrostomy tube when the risk for aspiration is high. Physical therapy can help maintain muscle function; a physical medicine and rehabilitation (PM&R) specialist can help those who have difficulty walking. A pulmonologist can oversee chest physiotherapy to improve pulmonary function, reduce the risk of pulmonary infection, and manage respiratory insufficiency, if present. Hearing loss and seizures are managed in a standard manner. Prevention of secondary complications: Chest physiotherapy can help reduce the risk of pulmonary infection; physical therapy can help prevent joint contractures." — PMID:23230576

Surveillance recommendations, and the important caveat. GeneReviews is explicit that no guideline exists:

"Surveillance: No clinical guidelines are available. Treating physicians should consider: routine evaluation of growth and weight, pulmonary function tests with consideration of blood gases, neurodevelopmental assessments at each visit, and at least annual audiology evaluations in those with infantile-onset disease." — PMID:23230576

These are expert suggestions, not evidence-based intervals. Given that every death in the largest cohort was respiratory (§11), the case for prioritising serial pulmonary function testing — including supine FVC and nocturnal hypoventilation assessment — over other surveillance is strong on mechanistic grounds, though no trial supports a specific schedule.

Interventions and their ontology terms:

Intervention	Term	Notes
Mechanical / non-invasive ventilation	`NCIT:C70909` Mechanical Ventilation	Required by 55–67% depending on band
Gastrostomy	`NCIT:C157864` Gastrostomy Tube Procedure	~30% of adult-onset, mean 19.6 y post-onset
Physical therapy	`NCIT:C15302` Physical Therapy	Maintain function, prevent contracture
Occupational therapy	`NCIT:C121351` Occupational Therapy	—
Nutritional support	`NCIT:C15433` Nutritional Support	Failure to thrive, dysphagia
Supportive care	`NCIT:C15747` Supportive Care	Umbrella
Genetic counselling	`NCIT:C15240` Genetic Counseling	§13
Pharmacotherapy	`NCIT:C15986` Pharmacotherapy	Nucleoside therapy, §12
Gene therapy	`NCIT:C15238` Gene Therapy	Investigational, §12

Targeted phenotypes for these interventions: HP:0002093 Respiratory insufficiency, HP:0002015 Dysphagia, HP:0001508 Failure to thrive, HP:0003701 Proximal muscle weakness.

Not established: anaesthetic risk protocols, exercise prescription (versus avoidance), immunisation or infection-prophylaxis policy beyond general neuromuscular practice, pregnancy management, and whether any conventional "mitochondrial cocktail" (coenzyme Q10, riboflavin, carnitine) alters course — no trial addresses this in TK2d.

11. Prognosis

The cause of death is singular. The Spanish cohort's mortality data are the most informative prognostic dataset available [HUMAN CLINICAL]:

"Approximately 30% of patients died of respiratory insufficiency, while 56% of surviving patients needed mechanical ventilation." — PMID:38544965

By the September 2023 census, 16 of 53 patients (30%) had died, all of respiratory failure. No death in the cohort was attributed to cardiac, hepatic, renal or neurological cause. Respiratory muscle function is therefore the prognosis, and this single fact should organise both surveillance (§10) and trial endpoint selection.

Onset band is the dominant prognostic variable. 86% of untreated patients with onset before age 12 had died (PMID:38544965). Infantile onset carries a very poor untreated prognosis; adult onset is compatible with decades of survival with ventilatory support.

Treatment changes the picture, on observational evidence. In the same cohort, all 23 treated patients but one were alive at census (PMID:38544965). This is an uncontrolled comparison in a cohort where treatment allocation correlated with era and with survival to the point of being treatable, so it cannot be read as an effect size. It is nonetheless the strongest available human survival signal for nucleoside therapy, and it is concordant with the mouse survival data (§12) and the pooled clinical data underlying the 2025 regulatory approval.

Genotype refines timing rather than outcome. The p.Lys202del / p.Thr108Met ventilation-onset difference (§5, p < 0.01) is a timing predictor of real counselling value.

Favourable prognostic indicators: later onset; p.Lys202del genotype; residual TK2 activity in the 14–45% range; preserved ambulation (the majority — only 16.8% non-ambulant); receipt of nucleoside therapy.

Unfavourable: onset before 12 years; p.Arg130Trp genotype; residual activity below 10%; CNS involvement; early ventilatory requirement.

12. Treatment, Including Investigational

TK2d is now one of the few mitochondrial disorders with a mechanism-based, approved pharmacotherapy. The development path is unusually clean and worth following in order, because each step tested and discarded a specific hypothesis.

12.1 Step 1 — deoxynucleoside monophosphates (2014)

Garone et al. treated Tk2⁻/⁻ mice with dCMP + dTMP [MODEL ORGANISM]:

Dose	Survival	p
Untreated	13.2 ± 2.5 days	—
200 mg/kg/day	34.6 ± 3.2 days	0.0028 (n = 7)
400 mg/kg/day	44.3 ± 9.1 days	0.0071

"dCMP/dTMP supplementation is the first effective pharmacologic treatment for Tk2 deficiency." — Garone C, et al. EMBO Mol Med. 2014. PMID:24968719, DOI 10.15252/emmm.201404092

Two mechanistic observations from that paper shaped everything after it. First, the monophosphates were undetectable after gavage while deoxythymidine and deoxyuridine rose instead — implying the nucleosides, not the monophosphates, are the active species, which directly motivated step 2. Second, intestinal thymidine phosphorylase activity rises by postnatal day 29 and catabolises the drug, capping efficacy. (Curiously, treatment also raised Tk1 activity.)

12.2 Step 2 — deoxynucleosides, and the refutation of THU (2017)

Lopez-Gomez et al. tested dC + dT directly, and tested tetrahydrouridine (THU) as a catabolism inhibitor [MODEL ORGANISM] [IN VITRO]:

Regimen	Survival
dC+dT 260 mg/kg/day	P31 ± 4
dC+dT 520 mg/kg/day	P43 ± 10
dC+dT + THU	30 ± 1 (vs 44 ± 7 without THU, p = 0.007)

"double the molar amounts of nucleoside relative to deoxynucleoside monophosphates were required in order to achieve the same prolongation of lifespan."

"our results reveal a novel nucleoside therapy and refute the hypothesized therapeutic effect of THU for TK2 deficiency." — Lopez-Gomez C, et al. Ann Neurol. 2017. PMID:28318037

THU shortened survival. This is a clear negative result and worth recording as such: a plausible pharmacological adjuvant made the outcome worse.

The same paper reattributed the persistent failure to rescue brain to catabolism rather than blood–brain-barrier closure, noting that ENT1 and ENT2 do transport dT and dC. It also found the intestine uniquely maintained mtDNA at 82–86% of normal — consistent with continued proliferation there keeping TK1 active, i.e. a natural experiment confirming the §6.1 step-3/4 masking mechanism.

12.3 Step 3 — compassionate use in humans

Berardo et al. summarise the human compassionate-use experience [HUMAN CLINICAL] [REVIEW/SYNTHESIS]: 16 patients, doses up to 400 mg/kg/day, with diarrhoea and transaminase elevation the only notable adverse events. Functional outcomes: ventilation discontinued in 2 of 4, gastrostomy discontinued in 2 of 3, and two patients regained the ability to walk (PMID:35094997). The primary report is PMID:31125140 (Domínguez-González et al., Ann Neurol 2019), which I did not read in full for this report.

Regaining ambulation in a progressive mitochondrial myopathy is a substantial claim and, if it holds, indicates the disorder has a reversible component — presumably restoration of dNTP supply to surviving fibres rather than regeneration of lost ones.

12.4 Step 4 — trial and approval

Compassionate use was followed by formal development as MT1621 (Modis Therapeutics / Zogenix), trial NCT03845712 (PMID:35094997). This culminated in the November 2025 regulatory approval of doxecitine (deoxycytidine) and doxribtimine (deoxythymidine) for TK2 deficiency — the first approved therapy for the disorder, and, notably, an approval for a substrate-enhancement strategy rather than gene or enzyme replacement (PMID:41604077) [HUMAN CLINICAL].

That record contains dosing, pharmacokinetic and pooled survival data. I read it in the prior session but do not hold the specific dose, PK parameter and survival figures in a form I can reproduce verbatim here, and so I do not state them — they should be read from the source before use. One cache caveat for anyone quoting it: its properties table contains a typographical inconsistency, reading ">1% of unchanged drug excreted in urine" where the body text says "Less than 1%". The body text is correct; the table string must not be quoted.

Treatment curation pattern for the approved therapy:

treatment_term:
  preferred_term: Pharmacotherapy
  term:
    id: NCIT:C15986
    label: Pharmacotherapy
  therapeutic_agent:
  - preferred_term: deoxycytidine
    term:
      id: CHEBI:15698
      label: 2'-deoxycytidine
  - preferred_term: thymidine
    term:
      id: CHEBI:17748
      label: thymidine

12.5 Gene therapy (investigational)

AAV-mediated TK2 gene transfer has been tested in the mouse [MODEL ORGANISM]. Berardo et al. report that AAV9-TK2 rescued Tk2 activity in all tissues except kidney, and that AAV9 + AAV2 combined with dC/dT was synergistic (PMID:35094997); the primary report is PMID:34338329, which I did not read in full. Term: NCIT:C15238 Gene Therapy.

The synergy observation is mechanistically sensible — gene therapy restores the enzyme where transduction reaches, and nucleoside supplementation covers tissues it does not.

12.6 Untested rational target

Dorado et al. proposed MTERF3 inhibitors as a therapeutic class, on the logic that enhancing the endogenous transcriptional compensation (§6.1 step 8) could raise the mtDNA depletion threshold without touching nucleotide supply (PMID:20940150). I find no evidence this has been pursued. It remains the most clearly articulated untested therapeutic hypothesis in the disorder.

13. Genetic Counselling

GeneReviews gives the counselling content [REVIEW/SYNTHESIS]:

"TK2-related mtDNA maintenance defect is inherited in an autosomal recessive manner. 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. Carrier testing for at-risk family members and prenatal testing for a pregnancy at increased risk are possible if the pathogenic variants in the family have been identified." — PMID:23230576

Inheritance term: HP:0000007 Autosomal recessive inheritance.

Counselling points specific to TK2d, beyond generic autosomal recessive counselling:

Expressivity within a genotype is wide. A sibling inheriting the same biallelic genotype may have a materially different onset age. The p.Thr108Met homozygote group in the Spanish cohort spans onset bands. Counselling should not promise a sibling's course from the index case's.
Genotype does predict ventilatory timing. For the two founder alleles, the ventilation-onset association (p < 0.01) supports concrete anticipatory guidance: p.Lys202del homozygotes were ventilated only after 40; p.Thr108Met homozygotes mostly between 12 and 40 (PMID:38544965).
Founder-population carrier risk is elevated. In Iberian and Iberian-descended families, carrier frequencies for p.Thr108Met (0.062% Spanish, 0.176% Galicia) and p.Lys202del (0.034%) are 13- to 86-fold above gnomAD global. Reproductive counselling and partner carrier testing have a higher yield in these populations than global figures suggest (PMID:38544965).
Treatment availability changes the conversation. With an approved therapy (§12.4) and the observational survival signal (all but one of 23 treated Spanish patients alive), the prognostic counselling given before 2019 is no longer accurate. Early molecular diagnosis is now actionable, which strengthens the case for prenatal and newborn-relevant testing where a familial genotype is known.
Pre-symptomatic mtDNA loss in the mouse (§6.2) argues that a molecularly diagnosed but asymptomatic sibling is not necessarily unaffected at the tissue level — a consideration for surveillance rather than a basis for treating.

Not established: whether pre-symptomatic nucleoside therapy in a molecularly diagnosed asymptomatic individual is beneficial; newborn screening feasibility or candidacy; and any formal reproductive-option guidance (PGT-M, donor gametes) specific to TK2d.

14. Differential Diagnosis

Berardo et al. provide an age-banded differential grid (PMID:35094997) [REVIEW/SYNTHESIS]; the curated dismech entry binds the following differentials, all with validated MONDO terms:

Differential	MONDO term	Discriminating features
Spinal muscular atrophy	`MONDO:0001516` spinal muscular atrophy	The key infantile mimic. TK2d biopsy shows SMA-like fibre grouping with type 1 predominance, so histology can mislead; SMN1 testing and mtDNA quantification separate them
Facioscapulohumeral muscular dystrophy	`MONDO:0001347` facioscapulohumeral muscular dystrophy	Facial weakness and scapular winging are shared; FSHD lacks CK elevation of TK2d magnitude, ragged-red/COX-negative fibres and mtDNA depletion
mtDNA depletion syndrome 3 (hepatocerebral)	`MONDO:0009636`	DGUOK; hepatic failure dominant, muscle spared
mtDNA depletion, encephalomyopathic with methylmalonic aciduria	`MONDO:0012791`	SUCLA2/SUCLG1; methylmalonic aciduria is the discriminator
MNGIE	`MONDO:0017575` mitochondrial neurogastrointestinal encephalomyopathy	TYMP; also a nucleoside-pool disorder, but with GI dysmotility, leukoencephalopathy and raised plasma thymidine
Mitochondrial myopathy–cerebellar ataxia–pigmentary retinopathy	`MONDO:0044714`	Ataxia and retinopathy absent in TK2d

Additional differentials to consider (concepts named; MONDO identifiers not resolved in this session — see §16): limb-girdle muscular dystrophies (the commonest adult misdiagnosis, given proximal weakness and high CK); congenital myopathies; Pompe disease (proximal weakness with early diaphragmatic failure — a particularly close mimic of the TK2d respiratory pattern); other POLG-, TWNK- and RRM2B-related mtDNA maintenance disorders; single-large-deletion CPEO and Kearns–Sayre syndrome (for the adult ptosis/CPEO presentation); myasthenia gravis and congenital myasthenic syndromes (ptosis and bulbar weakness, but fatigable and with a decrement on repetitive stimulation).

The discriminating triad that should trigger TK2 testing: axial/cervical and facial weakness out of proportion to limb weakness, respiratory involvement out of proportion to ambulatory loss, and in adults, ptosis. Add elevated CK with mitochondrial and dystrophic changes on biopsy.

15. Model Systems

15.1 Mouse models

Two mouse models were published in 2008 and both are informative and both are limited.

Tk2 H126N knockin — the mouse equivalent of human p.His121Asn, a kinetically characterised partial-function allele (Akman et al. 2008, PMID:18467430). I did not read this record in full.
Tk2 knockout (Tk2⁻/⁻) — the model used for the mechanistic work in PMID:20940150 and the therapeutic work in PMID:24968719 and PMID:28318037.

Berardo et al. record the critical limitation directly [REVIEW/SYNTHESIS]:

Both models died at 14–16 days, none survived past 30 days, and "both mouse models did not reproduce the human myopathic phenotype." — PMID:35094997

This is a species/phenotype mismatch of the kind dismech records as FAILS_TO_RECAPITULATE or PARTIALLY_RECAPITULATES with explicit limitations, and it must be stated whenever mouse data are cited for a human claim. The mice die of an encephalopathic course in under three weeks; human myopathic TK2d is a myopathy that in the majority of patients begins after age 12. Any mouse survival endpoint is therefore an upward extrapolation from a model observing a different phenotype.

What the models do recapitulate, and what they legitimately support: - The TK1/TK2 developmental hand-off and its tissue-timing consequences (PMID:20940150) — this is a cell-biological mechanism, well served by the model. - The dTTP-selective nucleotide pool defect with dCTP sparing (PMID:20940150). - MTERF3-mediated transcriptional compensation and the depletion threshold (PMID:20940150). - Dose-responsive rescue by deoxynucleoside supplementation (PMID:24968719, PMID:28318037) — the model's most valuable contribution, and one whose translation to humans has since been confirmed clinically, which retrospectively validates its use for this specific purpose. - The negative THU result (PMID:28318037). - AAV9/AAV2 gene-therapy proof of concept, and its kidney gap (PMID:34338329, via PMID:35094997).

What they do not support: any claim about human age of onset, the myopathic distribution of weakness, ptosis/CPEO, or the qualitative (multiple-deletion) arm of the mechanism.

Model-link curation pattern for dismech: the Tk2⁻/⁻ mouse linked to a Mitochondrial dNTP Pool Imbalance or mtDNA Depletion node would be RECAPITULATES with fidelity: MODERATE and model_scale: ORGANISM; the same model linked to a clinical myopathy node would be FAILS_TO_RECAPITULATE with limitations naming the 14–16-day encephalopathic death and the absent myopathic phenotype, and a divergences entry of type INCOMPLETE_PHENOTYPE or SPECIES_MISMATCH.

15.2 In vitro systems

Recombinant TK2 enzymology — the workhorse for genotype-to-activity mapping. Wang, Saada and Eriksson's mutant kinetics (PMID:12493767) is the reference dataset; the substrate-specific kinetics (Michaelis–Menten for dC, negative cooperativity for dT; PMID:28318037) came from the same approach.
Patient fibroblasts and myoblasts — Saada et al.'s tissue-activity comparisons (PMID:12765840) underpin the muscle-selectivity argument. Note the mechanistic trap: proliferating patient cells express TK1 and therefore mask the defect, so a fibroblast assay can be misleadingly normal. Differentiated myotubes (CL:0002372) are the appropriate cellular model, and this is a direct corollary of §6.1 step 3.

15.3 Gaps in the model landscape

No published model, to my knowledge from these sources, reproduces the late-onset, multiple-deletion arm of the disorder — which is the majority phenotype in an unselected cohort (60.3% onset >12 years). Neither is there a model of the ptosis/CPEO presentation. A hypomorphic allelic series with survival into adulthood, or a muscle-restricted conditional knockout with delayed induction, would address a real gap. Patient-derived iPSC myotube systems are an obvious candidate and I found no record of one in these sources.

16. Explicit Statements of Unavailable Information

Per the template's requirement, the following were sought and not obtained in this session:

Identifiers not resolved (and deliberately not supplied from memory). The Ontology Lookup Service tool was unavailable (permission not granted), so the following have no identifier in this report: Orphanet ORPHAcode for MONDO:0012301; ICD-10-CM and ICD-11 codes; MeSH descriptor; UMLS CUI; TK2 MIM gene number; MONDO terms for limb-girdle muscular dystrophy, Pompe disease, congenital myopathy, Kearns–Sayre syndrome, CPEO, myasthenia gravis and the POLG/TWNK/RRM2B disorders; HPO terms for cervical extensor weakness specifically, nocturnal hypoventilation, and exertional myalgia; CHEBI terms for dCTP, dTMP and dCMP (only CHEBI:18077 dTTP, CHEBI:17748 thymidine and CHEBI:15698 2'-deoxycytidine were cache-resolvable); analyte terms for GDF-15 and FGF-21; NCIT terms for non-invasive positive-pressure ventilation specifically, mtDNA copy-number quantification, and the approved agents doxecitine and doxribtimine; and MGI, IMPC or OMIA accessions for the mouse models. Every concept above is named in prose so it can be bound once a lookup is available.

Epidemiology. No measured prevalence or incidence exists anywhere in the literature — every figure is derived. No sex ratio. No prevalence estimate outside Spain and referral-based series. No carrier frequency in any non-Iberian population.

Clinical. No validated outcome measure or severity score for TK2d. No anaesthetic risk protocol. No exercise prescription evidence. No pregnancy management data. No data on whether conventional mitochondrial supplement cocktails alter course.

Surveillance. GeneReviews states directly that "No clinical guidelines are available"; the listed items are expert suggestion, with no evidence-based intervals.

Treatment. Specific dose, pharmacokinetic and pooled-survival figures from the 2025 approval record (PMID:41604077) are present in the cache but not reproduced here, as I do not hold them verbatim — read them from the source. No controlled trial data on nucleoside therapy were located (the human evidence is compassionate-use and single-arm). No data on pre-symptomatic treatment. MTERF3 inhibition remains an untested proposal. Long-term (>5 year) safety data on nucleoside therapy were not located.

Mechanism. The route from dNTP imbalance specifically to multiple deletions rather than depletion is inferred, not demonstrated. The cause of intra-genotypic variability in onset age is unknown. Why kidney resists AAV9-TK2 rescue is unexplained.

Models. No model of late-onset, multiple-deletion TK2d. No iPSC-derived myotube model located. Both mouse models are explicitly stated not to reproduce the human myopathic phenotype.

References in the cache that I did not read in full for this report, and from which I therefore quoted nothing: PMID:18467430 (Akman, H126N knockin mouse), PMID:34338329 (AAV9 gene therapy), PMID:23932787 (Chanprasert), PMID:22345218 (Béhin, adult cases), PMID:33340416 (ICIMD nosology).

Appendix A — Consolidated Ontology Term Set

All identifiers below were read from cache/<prefix>/terms.csv or from the validated bindings of kb/disorders/Mitochondrial_DNA_Depletion_Syndrome_Myopathic_Form.yaml in the same step they were written. None was supplied from recall.

Disease (MONDO) MONDO:0012301 mitochondrial DNA depletion syndrome, myopathic form (primary) MONDO:0009636 mitochondrial DNA depletion syndrome 3 (hepatocerebral type) MONDO:0012791 mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria MONDO:0017575 mitochondrial neurogastrointestinal encephalomyopathy MONDO:0044714 mitochondrial myopathy-cerebellar ataxia-pigmentary retinopathy syndrome MONDO:0001347 facioscapulohumeral muscular dystrophy MONDO:0001516 spinal muscular atrophy

Gene (HGNC) hgnc:11831 TK2

Inheritance (HPO) HP:0000007 Autosomal recessive inheritance

Phenotype — muscle (HPO) HP:0003701 Proximal muscle weakness · HP:0003324 Generalized muscle weakness · HP:0000467 Neck muscle weakness · HP:0010628 Facial palsy · HP:0003691 Scapular winging · HP:0001252 Hypotonia · HP:0002505 Loss of ambulation · HP:0003198 Myopathy · HP:0003546 Exercise intolerance · HP:0003201 Rhabdomyolysis · HP:0001270 Motor delay

Phenotype — ocular / bulbar / respiratory (HPO) HP:0000508 Ptosis · HP:0000597 Ophthalmoparesis · HP:0000590 Progressive external ophthalmoplegia · HP:0002015 Dysphagia · HP:0001260 Dysarthria · HP:0002093 Respiratory insufficiency

Phenotype — CNS / systemic (HPO) HP:0001250 Seizure · HP:0001298 Encephalopathy · HP:0002376 Developmental regression · HP:0000407 Sensorineural hearing impairment · HP:0001508 Failure to thrive

Phenotype — laboratory / histopathology (HPO) HP:0003236 Elevated circulating creatine kinase activity · HP:0002151 Increased circulating lactate concentration · HP:0009141 Depletion of mitochondrial DNA in muscle tissue · HP:0003689 Multiple mitochondrial DNA deletions · HP:0003200 Ragged-red muscle fibers · HP:0003688 Cytochrome C oxidase-negative muscle fibers · HP:0008347 Decreased activity of mitochondrial complex IV · HP:0003803 Type 1 muscle fiber predominance

Biological process / molecular function (GO) GO:0004797 thymidine kinase activity · GO:0004137 deoxycytidine kinase activity · GO:0043099 pyrimidine deoxyribonucleoside salvage · GO:0009202 deoxyribonucleoside triphosphate biosynthetic process · GO:0006264 mitochondrial DNA replication · GO:0032042 mitochondrial DNA metabolic process · GO:0006390 mitochondrial transcription · GO:0007005 mitochondrion organization · GO:0006119 oxidative phosphorylation · GO:0022904 respiratory electron transport chain

Cell type (CL) CL:0008002 skeletal muscle fiber · CL:0002372 myotube · CL:0000594 skeletal muscle satellite cell · CL:0000189 slow muscle cell · CL:0000190 fast muscle cell · CL:0000540 neuron · CL:0000182 hepatocyte (spared — contrast term)

Anatomy (UBERON) UBERON:0001134 skeletal muscle tissue · UBERON:0001103 diaphragm

Chemical entity (CHEBI) CHEBI:17748 thymidine · CHEBI:15698 2'-deoxycytidine · CHEBI:18077 dTTP

Clinical intervention / procedure (NCIT) NCIT:C15986 Pharmacotherapy · NCIT:C15238 Gene Therapy · NCIT:C70909 Mechanical Ventilation · NCIT:C157864 Gastrostomy Tube Procedure · NCIT:C15302 Physical Therapy · NCIT:C121351 Occupational Therapy · NCIT:C15433 Nutritional Support · NCIT:C15240 Genetic Counseling · NCIT:C15747 Supportive Care · NCIT:C51895 Muscle Biopsy · NCIT:C101295 Whole Exome Sequencing · NCIT:C64489 Creatine Kinase Measurement

Appendix B — Reference List with Evidence Typing

PMID	Citation	DOI	Evidence type	Read in full
11687801	Saada A, et al. Nat Genet. 2001. Gene discovery	—	HUMAN CLINICAL / IN VITRO	Yes (prior session)
12493767	Wang L, Saada A, Eriksson S. J Biol Chem. 2003. Mutant enzyme kinetics	10.1074/jbc.M206143200	IN VITRO	Yes (abstract-only record)
12765840	Saada A, Shaag A, Elpeleg O. Mol Genet Metab. 2003. Tissue specificity	10.1016/s1096-7192(03)00063-5	HUMAN CLINICAL / IN VITRO	Yes (abstract-only record)
18467430	Akman HO, et al. 2008. Tk2 H126N knockin mouse	—	MODEL ORGANISM	No
20940150	Dorado B, Area E, Akman HO, Hirano M. Hum Mol Genet. 2011. TK1 unmasking, MTERF3, depletion threshold	10.1093/hmg/ddq453	MODEL ORGANISM	Yes (full PMC)
22345218	Béhin A, et al. Adult cases	—	HUMAN CLINICAL	No
23230576	GeneReviews. TK2-related mtDNA maintenance defect, myopathic form	—	REVIEW/SYNTHESIS	Yes (abstract-only record)
23932787	Chanprasert S, et al. Molecular/clinical	—	HUMAN CLINICAL	No
24968719	Garone C, et al. EMBO Mol Med. 2014. dCMP/dTMP therapy	10.15252/emmm.201404092	MODEL ORGANISM	Yes (full PMC)
28318037	Lopez-Gomez C, et al. Ann Neurol. 2017. dC/dT therapy; THU refuted	10.1002/ana.24922	MODEL ORGANISM / IN VITRO	Yes (full PMC)
29602790	Garone C, et al. 2018. Natural history	—	HUMAN CLINICAL	Yes (prior session)
31060578	Domínguez-González C, et al. 2019. Late-onset series	—	HUMAN CLINICAL	Yes (prior session)
31125140	Domínguez-González C, et al. Ann Neurol. 2019. Compassionate use	—	HUMAN CLINICAL	Yes (prior session)
33340416	ICIMD nosology	—	REVIEW/SYNTHESIS	No
34338329	AAV9 TK2 gene therapy + deoxynucleoside synergy	—	MODEL ORGANISM	No
35094997	Berardo A, Domínguez-González C, Engelstad K, Hirano M. J Neuromuscul Dis. 2022. Comprehensive review	10.3233/JND-210786	REVIEW/SYNTHESIS	Yes (full PMC + Table 1)
38544965	Ceballos A (Ceballos-Ceballos), et al. Neurol Genet. 2024. 53-patient Spanish cohort	10.1212/NXG.0000000000200138	HUMAN CLINICAL / COMPUTATIONAL	Yes (full PMC + Tables 1–2)
41604077	2025. Doxecitine / doxribtimine regulatory approval	—	HUMAN CLINICAL	Yes (prior session)

Recency note. The template asked for prioritisation of 2023–2024+ sources. The two most recent references are the 2024 Spanish cohort (PMID:38544965) and the 2025 approval record (PMID:41604077), and both are load-bearing here — the former for §3, §4, §5, §7, §9 and §11, the latter for §12.4. The 2022 Berardo review (PMID:35094997) is the most recent comprehensive synthesis. The mechanistic core (§6) rests on 2003–2017 work, because that is when it was established and no later source supersedes it; the MTERF3 compensation mechanism in particular has, so far as these sources show, neither been replicated nor extended since 2011, which is itself worth noting as a gap.

Reference Validation

Checked with linkml-reference-validator 0.3.0rc3.

Outcome	Count
References checked	24
Resolved	24
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	24
On topic	19
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	72
Resolved	72
Unresolved (possible confabulation)	0
Obsolete	0
Unverifiable	0
Terms whose name was checked	3
Terms named correctly	0
Terms named as a different term	0
Terms whose name is worth a second look	3

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

MONDO:0009636 (2 mentions) - the report calls it "mtDNA depletion syndrome 3 (hepatocerebral)"; MONDO calls it mitochondrial DNA depletion syndrome 3 (hepatocerebral type)
MONDO:0012791 (2 mentions) - the report calls it "mtDNA depletion, encephalomyopathic with methylmalonic aciduria"; MONDO calls it mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria, and lists "mtDNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria" among its other names
MONDO:0044714 (2 mentions) - the report calls it "Mitochondrial myopathy–cerebellar ataxia–pigmentary retinopathy"; MONDO calls it mitochondrial myopathy-cerebellar ataxia-pigmentary retinopathy syndrome

Every term resolved, and every label the report gave matched.

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Discussions and Knowledge Gaps

Pathophysiology

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Differential Diagnoses

Clinical Trials

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References & Deep Research

References

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Disease Characteristics Research Report

Mitochondrial DNA Depletion Syndrome, Myopathic Form (TK2 Deficiency)

Provenance and method note

1. Disease Identity and Identifiers

2. Classification and Nosology

3. Epidemiology

4. Clinical Presentation

4.1 The core syndrome

4.2 Presentation by onset band

4.3 The ptosis/CPEO age gradient

4.4 Additional and atypical features

4.5 Laboratory presentation

5. Natural History and Progression

6. Pathophysiology and Mechanism

6.1 Ordered causal chain

6.2 Detail

6.3 Ontology terms for the mechanism

7. Genetics

8. Diagnosis

9. Histopathology and Biomarkers

9.1 Muscle histopathology

9.2 Molecular pathology of muscle

9.3 Circulating biomarkers

10. Management and Surveillance

11. Prognosis

12. Treatment, Including Investigational

12.1 Step 1 — deoxynucleoside monophosphates (2014)

12.2 Step 2 — deoxynucleosides, and the refutation of THU (2017)

12.3 Step 3 — compassionate use in humans

12.4 Step 4 — trial and approval

12.5 Gene therapy (investigational)

12.6 Untested rational target

13. Genetic Counselling

14. Differential Diagnosis

15. Model Systems

15.1 Mouse models

15.2 In vitro systems

15.3 Gaps in the model landscape

16. Explicit Statements of Unavailable Information

Appendix A — Consolidated Ontology Term Set

Appendix B — Reference List with Evidence Typing

Reference Validation

Term Validation

Terms whose name is worth a second look