Thymidine Kinase 1

Thymidine Kinase 1 (TK1) is a key enzyme in DNA precursor synthesis. It is upregulated during the late G1 phase and early S phase of the cell cycle and its presence in cells is an indicator of active cell proliferation. Increased levels of TK1 in the blood can indicate active cell proliferation as a consequence of abnormal cell turnover and cell disruption triggered by for example therapeutic agents.

A unique proliferation biomarker

Proliferation biomarkers are proteins that are expressed in proliferating cells. Such biomarkers include, PCNA, Ki-67 and TK11,2. Ki-67 is commonly used and referred to in clinical guidelines as proliferation biomarker. Ki-67 is detected by using immunohistological techniques which have the drawbacks of being labor intensive, requiring tissue biopsy material and being somewhat subjective. Pathologists have become a scarce resource and can be a bottleneck in clinical routine laboratories. Furthermore, as thin needles are used to capture the tissue samples, only a small part of the total tissue mass is studied.

AroCell TK 210 ELISA is a standard in vitro diagnostic test that requires only a simple liquid biopsy to determine the level of TK1 and indicates the total rate of proliferation and cell disruption. As such it is more convenient, provides unique and valuable information, and may be a substitute for Ki-67 over time. In addition, the assay is easy to use, robust, reproducible, and can be scaled up using standard equipment for ELISA immunoassays commonly available in many clinical laboratories.

A key enzyme in DNA synthesis that measure cell proliferation and disruption

There are two major pathways involved in DNA precursor synthesis; the de novo pathway and the salvage pathway. In the de novo pathway, purine and pyrimidine nucleotides are synthesized from low molecular weight precursors, whereas the salvage pathway recycles deoxyribonucleosides (dNs) from degraded DNA. TK1 is a dNs kinase involved in the utilization of thymidine as a part of the salvage pathway. Thymidine Kinases are coupling a phosphate group from ATP to thymidine as part of the DNA synthesis and repair machinery in all living cells. There are two Thymidine Kinases in mammalian cells. TK1 is mainly found in the cytosol and TK2 is present in the mitochondria. TK1 is supporting the synthesis of nuclear DNA both during repair and replication of new DNA, while TK2 is responsible for the synthesis of building blocks for mitochondrial DNA synthesis, particularly in resting cells3.

 

Thymidine Kinase 1 is involved in the DNA synthesis via the metabolic salvage pathway.

TK1 exists in different forms depending on the source and purity. The basic intracellular form of TK1 is a homo dimer with a subunit size of 25 kDa. The monomer lacks enzyme activity, but in the presence of ATP at high concentrations, it polymerizes the dimers to tetramers with a molecular weight of approximately 100 kDa4. The intracellular TK1 dimers are rapidly converted to the active tetramer in the event of DNA damage.


TK1 complexes, immunoreactivity and enzyme activities in serum from a subject with myelodysplasia7.

TK1 is released from cells that die and disrupt during proliferation. Thus, the concentration of TK1 in extracellular fluid is a measure of disruption of dividing cells. Normal cells seldom disintegrate during proliferation and it usually only occurs during accelerated or unregulated proliferation, as in malignancies. Following its release, TK1 forms large complexes of different molecular weights and enzyme activities, a factor which is of key importance when developing assays to detect all forms of TK1.

References

  1. O’Neill KL, Buckwalter M, Murray BK. Thymidine Kinase: diagnostic and prognostic potential. Rev. Mol. Diagn. 1(4), (2001).
  2. Topolcan O, Holubec Jr L. The role of Thymidine Kinase in cancer diseases. Expert Opin. Med. Diagn. 2(2), (2008), 129–141.
  3. Chen Y-L, Eriksson S, Chang Z-F. Regulation and functional contribution of Thymidine Kinase 1 in repair of DNA damage. The J. Biol. 285(35) (2010), 27327–27335.
  4. Munch-Petersen B. Enzymatic regulation of cytosolic Thymidine Kinase 1 and mitochondrial Thymidine Kinase 2: a mini review. Nucleosides Nucleotides Nucleic Acids 29(4-6) (2010), 363–369.
  5. Sherley JL, Kelly TJ. Regulation of human Thymidine Kinase during the cell cycle. Biol. Chem. 263(17) (1988), 8350–8358.
  6. Chang ZF, Huang DY. The regulation of Thymidine Kinase in HL-60 human promyeloleukemia cells. J. Biol. Chem. 268(2) (1993), 1266–1271.
  7. Jagarlamudi K.K., Shaw, M.. Biomarkers in Medicine, Vol. 12: Issue 9, p:1035-1048, Published online 24 July 2018.