Clinical characteristics.

TK2-related mitochondrial DNA (mtDNA) maintenance defect is a phenotypic continuum that ranges from severe to mild. To date, approximately 107 individuals with a molecularly confirmed diagnosis have been reported.

Three main subtypes of presentation have been described:

• Infantile-onset myopathy with neurologic involvement and rapid progression to early death. Affected individuals experience progressive muscle weakness leading to respiratory failure. Some individuals develop dysarthria, dysphagia, and/or hearing loss. Cognitive function is typically spared.
• 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. Some affected individuals develop respiratory insufficiency, chronic progressive external ophthalmoplegia, dysphagia, and dysarthria.

Diagnosis/testing.

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.

Management.

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.

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.

Genetic counseling.

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.

Suggestive Findings

TK2-related mtDNA maintenance defect should be suspected in individuals with the following clinical features (by age), supportive laboratory findings, electromyography results, skeletal muscle pathology, mtDNA content (copy number) analysis, and electron transport chain activity in skeletal muscle.

Establishing the Diagnosis

The diagnosis of TK2-related mitochondrial DNA 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; AND/OR
  • Biallelic pathogenic (or likely pathogenic) variants in TK2 identified by molecular genetic testing (see Table 1).
• 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; AND/OR
  • Biallelic pathogenic (or likely pathogenic) variants in TK2 identified by molecular genetic testing (see Table 1).

Note: (1) Per ACMG/AMP variant interpretation guidelines, the terms "pathogenic variants" and "likely pathogenic variants" are synonymous in a clinical setting, meaning that both are considered diagnostic and both can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this section is understood to include any likely pathogenic variants. (2) Identification of biallelic TK2 variants of uncertain significance (or of one known TK2 pathogenic variant and one TK2 variant of uncertain significance) does not establish or rule out a diagnosis.

Molecular genetic testing approaches can include a combination of gene-targeted testing (single-gene testing, concurrent or serial single-gene testing, multigene panel) and comprehensive genomic testing (chromosomal microarray analysis, exome sequencing, exome array, genome sequencing) depending on the phenotype.

Gene-targeted testing requires that the clinician determine which gene(s) are likely involved, whereas genomic testing does not. Because the phenotype of TK2-related mitochondrial DNA maintenance defect is broad, individuals with the distinctive findings described in Suggestive Findings are likely to be diagnosed using gene-targeted testing (see Option 1), whereas those with a phenotype indistinguishable from many other mitochondrial myopathies are more likely to be diagnosed using genomic testing (see Option 2).

Clinical Description

To date, approximately 107 individuals with molecularly confirmed TK2-related mtDNA maintenance defect have been reported [Pons et al 1996, Galbiati et al 2006, Oskoui et al 2006, Blakely et al 2008, Götz et al 2008, Collins et al 2009, Lesko et al 2010, Martí et al 2010, Zhang et al 2010, Béhin et al 2012, Garone et al 2018, Wang et al 2018].

The clinical presentation of TK2-related mtDNA maintenance defect is variable; as understanding of the disorder increases, the phenotype continues to broaden (Table 2).

Infantile onset (<2 years). The prenatal and perinatal histories are usually unremarkable; onset is typically in the first two years of life.

• Initial development is normal, followed by gradual onset of hypotonia:
  • A subset of affected individuals have early severe muscle weakness with encephalopathy and intractable epilepsy.
  • Some affected individuals have elevated serum concentrations of aminotransferases and CK in the first year of life.
    Note: The observed elevation in serum transaminases may reflect skeletal muscle involvement [Zhang et al 2010].
• Subsequently generalized fatigue, decreased physical stamina, proximal muscle weakness (manifest as difficulty getting to standing or walking), and feeding difficulties develop.
• Muscle atrophy becomes evident [Martí et al 2010].
• Some children develop bulbar weakness including dysarthria and dysphagia. Previously acquired motor skills are lost.
• Sensorineural hearing loss develops.
• Cognitive function is typically spared.

Muscle weakness rapidly progresses leading to respiratory failure and death within a few years after onset. Most children succumb to complications of respiratory muscle weakness; several are ventilator dependent before age six years. The most common cause of death is pulmonary infection.

Juvenile-/childhood-onset (ages 2-18 years) disease is characterized by distinct disease progression:

• Moderate-to-severe progression of generalized weakness
  • The severity of muscle weakness can vary widely among affected individuals.
  • In its mildest form, affected individuals may report myalgia and muscle weakness; in severe cases muscle weakness can progress to the point that ventilator assistance is required.
• Survival to at least age 13 years, with respiratory failure as the primary cause of death

Adult-onset (>18 years) disease is characterized by:

• Mild proximal limb muscle weakness due to progressive mitochondrial myopathy;
• Slow progression to respiratory failure in some affected individuals;
• Involvement of the facial and extraocular muscles with manifestations including chronic progressive external ophthalmoplegia, ptosis, dysphagia, and dysarthria.

Genotype-Phenotype Correlations

It is possible that the range of phenotypes observed may be explained by the variability in the amount of residual activity of mutated enzymes [Poulton et al 2009].

The small number of individuals reported to date precludes identification of genotype-phenotype correlations; however, the following have been observed:

• The pathogenic variant p.Arg130Trp appears to be associated with the most severe phenotype, with CNS involvement (i.e., seizures and loss of previously acquired motor skills) during the first months of life [Lesko et al 2010].
• Homozygosity for p.Arg183Trp is associated with myopathy and severe mtDNA depletion restricted to skeletal muscle [Götz et al 2008].
• Homozygosity for p.Lys202del has only been found in individuals with adult-onset TK2-related mtDNA maintenance defect [Wang et al 2018].
• When the p.Arg192Lys pathogenic variant is found in trans with other variants (p.Thr108Met and p.Lys202del), childhood onset of symptoms with prolonged survival is typical [Wang et al 2018].

Prevalence

The prevalence of TK2-related mtDNA maintenance defect is unknown; the disorder appears to be rare, with only approximately 107 affected individuals reported to date.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs of an individual diagnosed with TK2-related mitochondrial DNA maintenance defect, myopathic form, the following evaluations are recommended, if not completed as part of the diagnostic evaluation.

Treatment of Manifestations

Treatment is primarily supportive; management should involve a multidisciplinary team.

The following information represents typical management recommendations for individuals with developmental concerns in the United States; standard recommendations may vary from country to country.

Prevention of Secondary Complications

Chest physiotherapy can help reduce the risk of pulmonary infection (see Treatment of Manifestations).

Physical therapy can help maintain muscle function and prevent joint contractures (see Treatment of Manifestations).

Surveillance

No disease-specific clinical guidelines are available; treating physicians should consider the evaluations included in Table 6.

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions.

Mode of Inheritance

TK2-related mtDNA maintenance defect, myopathic form is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• The parents of an affected child are obligate heterozygotes (i.e., carriers of one TK2 pathogenic variant).
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Sibs of a proband

• At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
• Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband

• Most individuals with TK2-related mtDNA maintenance defect, myopathic form have early-onset severe disease and do not survive to reproduce.
• If individuals with less severe manifestations of aTK2-related mtDNA maintenance defect reproduce, their offspring are obligate heterozygotes for a TK2 pathogenic variant.

Other family members. Each sib of the proband's parents is at a 50% risk of being a carrier of a TK2 pathogenic variant.

Carrier Detection

Carrier testing for at-risk family members requires prior identification of the TK2 pathogenic variants in the family.

Related Genetic Counseling Issues

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are carriers or are at risk of being carriers.

DNA banking. Because it is likely that testing methodology and our understanding of genes, pathogenic mechanisms, and diseases will improve in the future, consideration should be given to banking DNA from probands in whom a molecular diagnosis has not been confirmed (i.e., the causative pathogenic mechanism is unknown). For more information, see Huang et al [2022].

Prenatal Testing and Preimplantation Genetic Testing

Once the TK2 pathogenic variants have been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Molecular Pathogenesis

Mitochondrial DNA (mtDNA) maintenance defect is characterized by a significant reduction in the number of copies of mtDNA in one or more tissues. TK2, encoding thymidine kinase 2 (which mediates the first and rate-limiting step in the phosphorylation of deoxypyrimidine nucleosides in the mitochondrial matrix), was the first gene to be associated with the myopathic form of mtDNA maintenance defect. To date, biallelic pathogenic variants in TK2 account for approximately 20% of myopathic mtDNA maintenance defect [Martí et al 2010].

Gene structure. TK2 comprises ten coding exons. Alternate splicing results in multiple transcript variants (see Table A, Gene). The longest transcript variant is NM_004614.4.

Pathogenic variants. To date, more than 30 different TK2 pathogenic variants have been reported in persons with myopathic mtDNA depletion (Table 7) [Pons et al 1996, Galbiati et al 2006, Oskoui et al 2006, Blakely et al 2008, Götz et al 2008, Collins et al 2009, Lesko et al 2010, Martí et al 2010, Zhang et al 2010, Béhin et al 2012].

About 70% of reported pathogenic variants are missense; the remainder include nonsense and splice site variants and small (1- to 4-nucleotide) deletions and insertions.

A gross deletion spanning 5.8 kb [Zhang et al 2010] and a complex rearrangement (Table 7) have also been reported.

All pathogenic variants are private except for the two variants p.Arg130Trp and p.Arg183Trp, observed in affected individuals from Finland (Table 7) – most likely founder variants in the Finnish population [Götz et al 2008].

Normal gene product. The TK2 isoform is 265 amino acids. TK2 encodes thymidine kinase 2, the first and rate-limiting step in phosphorylation of deoxypyrimidine nucleosides (salvage pathway) in the mitochondrial matrix.

Deoxynucloside triphosphate (dNTPs) can be synthesized via either the de novo pathway which is cell-cycle regulated or via the salvage pathway in which dNTPs are produced by utilizing preexisting deoxynucleosides to synthesize DNA precursors. Both pathways may be required for mtDNA maintenance in postmitotic tissues. Since mtDNA synthesis is continuous throughout the cell cycle, thymidine kinase 2 becomes indispensable for mtDNA maintenance.

Abnormal gene product. TK2 pathogenic variants result in dysfunction of the enzyme thymidine kinase 2 resulting in impaired synthesis of mtDNA precursors leading to mtDNA depletion.

Author History

Sirisak Chanprasert, MD; Baylor College of Medicine (2012-2018)
Ayman W El-Hattab, MD, FAAP, FACMG (2018-present)
Fernando Scaglia, MD, FACMG; Baylor College of Medicine (2012-2018)
Jing Wang, MD; Baylor College of Medicine (2012-2018)
Julia Wang, BS (2018-present)
Lee-Jun C Wong, PhD, FACMG (2012-present)

Revision History

• 26 July 2018 (ma) Comprehensive update posted live
• 6 December 2012 (me) Review posted live
• 23 August 2012 (fs) Original submission

Literature Cited

• Béhin A, Jardel C, Claeys KG, Fagart J, Louha M, Romero NB, Laforet P, Eymard B, Lombes A. Adult cases of mitochondrial DNA depletion due to TK2 defect: an expanding spectrum. Neurology. 2012;78:644–8. [PubMed: 22345218]
• Blakely E, He L, Gardner JL, Hudson G, Walter J, Hughes I, Turnbull DM, Taylor RW. Novel mutationin the TK2 gene associated with fatal mitochondrial DNA depletion myopathy. Neuromuscul Disord. 2008;18:557–60. [PubMed: 18508266]
• Collins J, Bove KE, Dimmock D, Morehart P, Wong LJ, Wong B. Progressive myofiber loss with extensive fibro-fatty replacement in a child with mitochondrial DNA depletion syndrome and novel thymidine kinase 2 gene mutations. Neuromuscul Disord. 2009;19:784–7. [PubMed: 19736010]
• Galbiati S, Bordoni A, Papadimitriou D, Toscano A, Rodolico C, Katsarou E, Sciacco M, Garufi A, Prelle A, Aguennouz M, Bonsignore M, Crimi M, Martinussi A, Bresolin N, Papadimitriou A, Comi GP. New mutation in TK2 gene associated with mitochondrial DNA depletion. Pediatr Neurol. 2006;34:177–85. [PubMed: 16504786]
• Garone C, Taylor RW, Nascimento A, Poulton J, Fratter C, Domínguez-González C, Evans JC, Loos M, Isohanni P, Suomalainen A, Ram D, Hughes MI, McFarland R, Barca E, Lopez Gomez C, Jayawant S, Thomas ND, Manzur AY, Kleinsteuber K, Martin MA, Kerr T, Gorman GS, Sommerville EW, Chinnery PF, Hofer M, Karch C, Ralph J, Cámara Y, Madruga-Garrido M, Domínguez-Carral J, Ortez C, Emperador S, Montoya J, Chakrapani A, Kriger JF, Schoenaker R, Levin B, Thompson JLP, Long Y, Rahman S, Donati MA, DiMauro S, Hirano M. Retrospective natural history of thymidine kinase 2 deficiency. J Med Genet. 2018;55:515–21. [PMC free article: PMC6073909] [PubMed: 29602790]
• Götz A, Isohanni P, Pihko H, Paetau A, Herva R, Saarenpää-Heikkilä O, Valanne L, Marjavaara S, Suomalainen A. Thymidine kinase 3 defects can cause multi-tissue mtDNA depletion syndrome. Brain. 2008;131:2841–50. [PubMed: 18819985]
• Huang SJ, Amendola LM, Sternen DL. Variation among DNA banking consent forms: points for clinicians to bank on. J Community Genet. 2022;13:389–97. [PMC free article: PMC9314484] [PubMed: 35834113]
• Leshinsky-Silver E, Michelson M, Cohen S, Ginsberg M, Sadeh M, Barash V, Lerman-Sagie T, Lev D. A defect in thymidine kinase 2 gene causing isolated mitochondrial myopathy without mtDNA depletion. Eur J Paediatr Neurol. 2008;12:309–13. [PubMed: 17951082]
• Lesko N, Naess K, Wibom R, Solaroli N, Nennesmo I, von Dobeln U, Karlsson A, Larsson NG. Two novel mutations in thymidine kinase-2 cause early onset fatal encephalomyopathy and severe mtDNA depletion. Neuromuscul Disord. 2010;20:198–203. [PubMed: 20083405]
• Martí R, Nascimento A, Colomer J, Lara MC, Lopez-gallardo E, Ruiz-Pesini E, Montaya J, Andreu AL, Briones P, Pineda M. Hearing loss in a patient with the myopathic form of mitochondrial DNA depletion syndrome and novel mutation in the TK2 gene. Pediatr Res. 2010;68:151–4. [PubMed: 20421844]
• Oskoui M, Davidzon G, Pascual J, Erazo R, Gurgel-Giannetti J, Krishna S, Bonilla E, De Vivo DC, Shanske S, Dimauro S. Clinical spectrum of mitochondrial DNA depletion due to mutation in the thymidine kinase 2 gene. Arch Neurol. 2006;63:1122–6. [PubMed: 16908738]
• Pons R, Andreetta F, Wang CH, Vu TH, Bonilla E, DiMauro S, De Vivo DC. Mitochondrial myopathy simulating spinal muscular atrophy. Pediatr Neurol. 1996;15:153–8. [PubMed: 8888051]
• Poulton J, Hirano M, Spinazzola A, Arenas Hernandez M, Jardel C, Lombès A, Czermin B, Horvath R, Taanman JW, Rotig A, Zeviani M, Fratter C. Collated mutations in mitochondrial DNA (mtDNA) depletion syndrome (excluding the mitochondrial gamma polymerase, POLG1). Biochim Biophys Acta. 2009;1792:1109–12. [PubMed: 19748572]
• Richards S, Aziz N, Bale S, Bick D, Das S, Gastier-Foster J, Grody WW, Hegde M, Lyon E, Spector E, Voelkerding K, Rehm HL, et al. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology. Genet Med. 2015;17:405–24. [PMC free article: PMC4544753] [PubMed: 25741868]
• Vilà MR, Segovia-Silvestre T, Gámez J, Marina A, Naini AB, Meseguer A, Lombès A, Bonilla E, DiMauro S, Hirano M, Andreu AL. Reversion of mtDNA depletion in a patient with TK2 deficiency. Neurology. 2003;60:1203–5. [PubMed: 12682338]
• Wang J, Kim E, Dai H, Stefans V, Vogel H, Al Jasmi F, Vergano SA, Castro D, Bernes S, Bhambhani V, Long C. Spectrum of clinical and molecular aspects of thymidine kinase 2-related mtDNA maintenance defect. Mol Genet Metab. 2018;124:124–30. [PubMed: 29735374]
• Zhang S, Li FY, Bass HN, Pursley A, Schmitt ES, Brown BL, Brundage EK, Mardach R, Wong LJ. Application of oligonucleotide array CGH to the simultaneous detection of a deletion in the nuclear TK2 gene and mtDNA depletion. Mol Genet Metab. 2010;99:53–7. [PubMed: 19815440]