Abstract 159: Thymidine kinase 1 and cell cycle control in breast cancer
Authors: Bitter Eliza E. , Mortimer Toni , Morris Rachel , Barlow Kai , Schekall Abby , Townsend Michelle , Pickett Brett E. , O'Neill Kim L. 
Published: June 2022

Abstract
Recent advances in cancer immunotherapy have transformed biomarkers into diagnostic and prognostic indicators and immunotherapeutic targets in clinical oncology. To ensure higher treatment success, it is critical to identify unique biomarkers to mitigate “off-target” effects. Therefore, the identification of novel biomarkers and appropriate target selection has never been more important to improve cancer immunotherapy. The DNA salvage pathway protein thymidine kinase 1 (TK1) plays an important role in DNA synthesis and repair. Normally, it is highly expressed during S phase and provides recycled nucleotides by phosphorylating thymidine to make thymidine monophosphate. Past studies have suggested that TK1 may influence cell cycle control and, therefore, provides a platform for the understanding of disease progression and possible means of mitigation.
The purpose of our study was to verify elevated TK1 levels in breast cancer cells and further investigate TK1’s potential influence over cell cycle control. We hypothesized that TK1 may be important in regulating the cell cycle and other cell checkpoint proteins. Immunohistochemistry analysis was employed to quantify TK1 expression in ductal and lobular primary and matched metastatic breast cancer samples compared to normal tissue. Bioinformatics was then used to analyze RNA-seq data of cell cycle checkpoint factors and TK1 in BRCA patients from the Cancer Genome Atlas. To demonstrate TK1’s potential effect on the cell cycle checkpoint pathway, we utilized HCC1806 breast cancer cells and generated a CRISPR-Cas9 TK1 knockdown (L133) cell line for in vitro testing. These were validated by western blotting and qPCR. Propidium iodide staining and cell cycle analysis were performed via flow cytometry on both cell lines. Immunohistochemistry results indicated that TK1 levels increase as breast cancer progresses and, thus, may correlate with breast cancer aggressiveness. Bioinformatics analysis showed that strong positive correlations exist between cell cycle checkpoint factors and TK1 in BRCA patients, such as BRCA1 and CHEK2. Flow cytometry cell cycle analysis showed that mean differences of L133 cells in S phase were significantly higher than in G1 compared to HCC 1806 cells, suggesting that TK1 levels influence cell cycle arrest.
Our preliminary results suggest that TK1 may affect cell cycle checkpoint pathways, which are essential to maintaining homeostasis in normal cells and preventing cancer progression. Further investigation to understand TK1’s potential interactions with important checkpoint factors, such as p53 or Gas6, may help elucidate other therapeutic targets for preventing disease progression.
Citation Format: Eliza E. Bitter, Toni Mortimer, Rachel Morris, Kai Barlow, Abby Schekall, Michelle Townsend, Brett E. Pickett, Kim L. O'Neill. Thymidine kinase 1 and cell cycle control in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 159.