Thymidine Kinase (TK) is an enzyme involved in the synthesis of DNA, and it plays a crucial role in cell proliferation and DNA replication. It has two isoforms – Thymidine Kinase 1 (TK1) and Thymidine Kinase 2 (TK2). In breast cancer patients, sustained cell proliferation is a hallmark of the disease, and elevated TK1 levels have been associated with tumour growth. This has led to the development of the use of TK activity tests to indicate cell proliferation rate and cell turnover, which is used to provide insights into disease development.

In this article, we will review the use of Thymidine Kinase (TK) as a breast cancer biomarker. We will also explore how TK activity is measured in serum or plasma, some practical applications of measurement, and existing alternatives to cancer monitoring.

Thymidine Kinase (TK) as a Breast Cancer Biomarker

TK can be used in disease monitoring for breast cancer patients. There are two isoforms of the enzyme – TK1 and TK2. While they share the same general function of catalysing the phosphorylation of thymidine, they differ in cellular localisation, expression patterns, and physiological roles. In other words, they have different implications within various diseases.

In breast cancer monitoring, the isoform used as a biomarker is TK1. TK1 activity is closely associated with cell proliferation rates, and elevated TK1 levels can be used to evaluate characteristics related to disease progression, such as tumour growth and patient response to treatment.

On the other hand, TK2 is primarily associated with mitochondrial function and the maintenance of mitochondrial DNA (mtDNA). TK2 gene mutations can lead to certain rare genetic disorders, such as mitochondrial DNA depletion syndromes (MDS). In this case, TK2 activity tests are used to monitor and diagnose the condition.

How Thymidine Kinase 1 (TK1) activity is measured

In the context of breast cancer patients, TK1 activity can be measured using various lab techniques. The most common method is through the use of an enzyme-linked immunosorbent assay (ELISA). ELISA can be performed on blood-based samples, such as serum or plasma.

To measure TK1 activity in serum, blood samples are collected from breast cancer patients. ELISA is set up by coating the wells of a microplate with antibodies specific to TK1, and the plate is washed to remove any unbound molecules. When the patient’s blood sample is added to the wells and incubated, any TK1 present in the sample binds to the immobilised antibodies.

Detection antibodies for TK1 are added to the wells, and a sandwich complex is formed (with the target molecule captured between two antibodies). Following this, a secondary detection reagent (such as an enzyme-linked antibody) is added to produce a detectable signal through colorimetric or fluorometric reactions. TK1 presence is then indicated.

Finally, the detectable signal is measured using a plate reader or spectrophotometer, and TK1 activity in the sample can be determined. The level of activity can provide insights into the proliferation rate of cancer cells and tumour burden, which can offer researchers and medical practitioners guidance into treatment approach.

Applications of TK1 activity analysis in disease monitoring

Monitoring TK1 activity levels in patients allows medical practitioners to adapt breast cancer treatment approaches. This can lead to prolonging a patient’s life, reducing the severity of the side effects of their treatment, or both. Below are three main applications of TK1 activity testing:

Treatment Response Assessment

Monitoring TK1 activity during breast cancer treatment provides valuable insights into treatment response. Decreasing TK1 activity levels over the course of treatment may indicate a positive response to therapy, while increasing or persistently high levels may suggest resistance or disease progression. TK1 activity analysis can thus guide treatment decisions, and medical practitioners can facilitate adjustments to optimise patient outcomes.

Predicting Treatment Toxicity

TK1 activity analysis has also shown potential in predicting treatment-related toxicities. Higher TK1 activity level in breast cancer patients before treatment initiation have been associated with increased risk of chemotherapy-induced toxicities, such as haematological toxicity. When patients with high risk are identified, medical practitioners can tailor treatment regimens or adjust dosages to minimise severe side effects.

Minimal Residual Disease Monitoring

Breast cancer patients that have completed primary treatment can have their TK1 activity levels in their blood measured. Analysis can be used to identify the presence of minimal residual disease (MRD) – which refers to small amounts of cancer cells that may remain after treatment. MRD can potentially lead to disease recurrence, and identifying early signs can guide treatment strategies.

Alternatives to TK1 activity testing

In addition to TK1 activity testing, there are other blood-based biomarkers that can be used to monitor breast cancer development, such as CA15-3, CA27.29, and CTCs.

• CA15-3 is a carbohydrate antigen commonly used as a biomarker for breast cancer, measured through taking a blood sample of the patient. Elevated CA15-3 levels can indicate disease progression and response to treatment. However, CA15-3 is not specific to breast cancer and can also be elevated in other conditions.
• CA27.29 is another carbohydrate antigen commonly used for breast cancer monitoring, particularly for advanced cancer stages. However, it is not specific to breast cancer either and can be elevated in other non-cancerous conditions, making analysis less clear-cut.
• Circulating Tumour Cells (CTCs) are cancer cells that have detached from the primary tumour and are circulating in the bloodstream. By detecting CTCs and analysing them, valuable information on tumour biology and treatment response can be obtained as well.

Final words

Thymidine Kinase – and Thymidine Kinase 1 in particular – holds good promise as a valuable biomarker in breast cancer monitoring. Through a minimally invasive sample collection procedure, these blood-based biomarkers provide insight into disease progression, allowing medical practitioners to tailor treatment strategies to improve patient outcomes.