Abstract 1878: CTPS1: An unexplored vulnerability in breast and ovarian cancer
Authors: Wang Xiyin , Emch Michael , Voll Lauren , Russell Rebecca , Rodman Esther , Pearson Nicole , Hou Xiaonan , Kaufmann Scott , Weroha Saravut , Beer Philip , Hawse John 
Published: March 2024

Abstract
Breast and ovarian cancer represent the most common cancer types in women worldwide and are among the top fatal cancers. Despite the use of cytotoxic chemotherapy and targeted agents, recurrence is frequent and survival rates for patients with advanced metastatic disease are dismal. These realities highlight the need to identify novel biomarkers and therapeutic vulnerabilities in such tumors and to develop alternative treatment strategies. We took an unbiased approach to tackle this problem in which we leveraged publically available (DepMap) and in-house generated genome-wide CRISPR screens conducted in over 1000 cancer cell lines. We initially focused on the identification of essential genes that were unique to triple-negative breast cancer (TNBC) cells and whose expression levels were associated with worse patient outcomes. This interrogation revealed six genes (CTPS1, RHOA, PRKRA, RAD9A, HUS1, and RAD1) meeting these initial criteria. The essentiality of these genes was validated using gene-specific siRNAs in a panel of TNBC cells and knockdown of CTPS1 was shown to elicit the most inhibitory effect. CTPS1 depletion resulted in a rapid S-phase cell cycle arrest followed by apoptosis, not only in TNBC cells but also in highly aggressive ovarian cancer cells which share many molecular features with TNBC. CTPS1 converts uridine triphosphate to cytidine triphosphate, forming an essential nucleic acid required for multiple cellular processes. CTPS1 mRNA expression was substantially elevated in tumor vs. normal tissue, for both breast and ovarian cancer. Using over 40 breast and ovarian cancer cell lines, RT-PCR analyses confirmed increased expression of CTPS1 in TNBC and most ovarian cancer cell lines compared to other disease sub-types and “normal” cell line/tissue controls. Interestingly, chemoresistant and PARP inhibitor-resistant models exhibited the highest levels of CTPS1 among all cells analyzed. We subsequently identified and acquired STP938, a first-in-class highly selective CTPS1 inhibitor being developed by Step-Pharma. STP938 was found to elicit potent anti-cancer effects at nM concentrations across many cell line and patient-derived models, in both 2D and 3D culture systems. RNAseq analysis in aggressive cell line models following STP938 treatment or CTPS1 knockdown revealed significant regulation of genes related to DNA replication and the cell cycle pathways suggesting that combinatorial/sequential treatments of STP938 with replication stress/DNA damage response related drugs may be synergistic. STP938 is currently being developed for the treatment of lymphoma, and our findings support the repurposing of STP938 for breast and ovarian cancer, an avenue that we are currently developing.
Citation Format: Xiyin Wang, Michael Emch, Lauren Voll, Rebecca Russell, Esther Rodman, Nicole Pearson, Xiaonan Hou, Scott Kaufmann, Saravut Weroha, Philip Beer, John Hawse. CTPS1: An unexplored vulnerability in breast and ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1878.