TK1 in Cancer Progression: elucidating its influence on cellular invasion and survival

1. Background: The salvage pathway enzyme thymidine kinase 1 (TK1) is elevated in the serum of several different cancer types and higher expression is associated with more aggressive tumor grade. As a result, it has potential as a biomarker for diagnosis and prognosis. Recent studies indicate that TK1 may be involved in cancer pathogenesis; however, its direct involvement has not been identified. We propose to evaluate the effects of TK1 on cancer progression in vitro through measuring cellular invasion and survival of breast cancer cells.2.Methods: Breast cancer cells MDA-MB-231, HCC 1806, and MCF7 were cultured according to standard techniques. We employed the use of TK1 target siRNA and a CRISPR-Cas9 TK1 knockout plasmid to compare transfected cell lines to wild type cell lines. Protein factors in survival and invasive pathways were also tested for correlations to TK1 in BRCA RNA-seq patient data (n=1095) using the TIMER program. Cellular invasion was quantified in cell index (factor of impedance) over a 24-hour period. Cell survival was measured by apoptosis under metabolic and DNA stress using flow cytometry. All results were statistically assessed using an ANOVA or t-test in GraphPad PRISM®.3.Results: Cellular invasion assays assessing wild type and TK1 knockdown/knockout (TK1-/-) cell types showed TK1-/- cell lines had increased invasion potential (p= 0.0001). Bioinformatically, we saw a strong overall negative correlation between apoptotic factors and TK1 (p ≤ 0.05). When testing TK1 effects on cell survival we saw a protective affect under DNA stress (p ≤ 0.05), but not under metabolic stress (p= 0.0001).4.Conclusion From cell cycle analysis, we observed a shift towards S phase in TK1-/- cells. This shift to S phase would promote growth and account for the increased cellular invasion and decrease in metabolic induced stress in TK1-/- cells. We propose that cancer cells still may elicit a cancer progressive phenotype based on effects of TK1, but that a system which isolates TK1 is not effective to understand the effects. Instead, identifying protein networks inclusive of TK1 will help to elucidate its effects on cancer progression.