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TK2 gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

TK2 gene

The TK2 gene encodes an enzyme necessary for maintaining mitochondrial DNA, and mutations are associated with a form of mitochondrial DNA depletion syndrome.

Normal Function

The TK2 gene provides instructions for making an enzyme called thymidine kinase 2 that functions within cell structures called mitochondria, which are found in all tissues. Mitochondria are involved in a wide variety of cellular activities, including energy production; chemical signaling; and regulation of cell growth, cell division, and cell death. Mitochondria contain their own genetic material, known as mitochondrial DNA (mtDNA), which is essential for the normal function of these structures. Thymidine kinase 2 is involved in the production and maintenance of mtDNA. Specifically, this enzyme plays a role in recycling mtDNA building blocks (nucleotides) so that errors in mtDNA sequencing can be repaired and new mtDNA molecules can be produced.

Health Conditions Related to Genetic Changes

TK2-related mitochondrial DNA depletion syndrome, myopathic form

More than 30 mutations in the TK2 gene have been found to cause TK2-related mitochondrial DNA depletion syndrome, myopathic form (TK2-MDS). TK2-MDS is an inherited condition that causes progressive muscle weakness (myopathy), typically beginning in early childhood. About two-thirds of the mutations that cause this condition change single protein building blocks (amino acids) in thymidine kinase 2. All TK2 gene mutations result in a decrease of enzyme activity, which impairs recycling of mtDNA nucleotides. A shortage of nucleotides available for the repair and production of mtDNA molecules leads to a reduction in the amount of mtDNA (known as mtDNA depletion) and impairs mitochondrial function.

Greater mtDNA depletion tends to cause more severe signs and symptoms. The muscle cells of people with TK2-MDS have very low amounts of mtDNA, ranging from 5 to 30 percent of normal. Other tissues can have 60 percent of normal to normal amounts of mtDNA. The cause for the variability in the amount of mtDNA lost among affected individuals, even those with the same mutations, is unknown. It is unclear why TK2 gene mutations typically affect only muscle tissue, but the high energy demands of muscle cells may make them the most susceptible to cell death when mtDNA is lost and less energy is produced in cells.

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Other Names for This Gene

  • KITM_HUMAN
  • mt-TK
  • MTDPS2
  • MTTK

Frequently Asked Questions

Diagnosis starts with clinical evaluation, blood tests showing elevated creatine kinase, and muscle MRI or biopsy showing mitochondrial abnormalities. Confirmatory testing requires a blood sample for DNA sequencing of the TK2 gene; no special fasting is required, but it helps to bring any previous lab reports and a detailed family history to the appointment.
While no cure exists, nucleoside supplementation (oral deoxythymidine and deoxycytidine) has shown benefit in clinical trials by bypassing the enzymatic block and improving muscle strength in some patients. Regular low‑impact exercise, physical therapy to maintain range of motion, and monitoring for respiratory complications are also recommended. Early involvement of a multidisciplinary team (neurology, genetics, pulmonology) can help address complications promptly.
The first clues often appear before age 5 and include progressive muscle weakness, difficulty climbing stairs or getting up from a seated position, delayed motor milestones, and sometimes exercise‑induced fatigue or muscle pain. Parents may also notice a limp, frequent falls, or trouble with fine motor tasks such as buttoning clothes.
TK2‑related mitochondrial DNA depletion syndrome follows an autosomal recessive pattern: both parents must carry one pathogenic copy of the gene. Each subsequent pregnancy has a 25 % chance of being affected, a 50 % chance of being a carrier, and a 25 % chance of being unaffected. Carrier testing for the parents and prenatal or pre‑implantation genetic diagnosis can be discussed with a genetic counselor.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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