Abstract 344: A high-throughput functional screen reveals synthetic lethal interactions associated with replication stress in HORMAD1-expressing triple-negative breast cancers

Abstract

Abstract HORMAD1, expression of which is usually restricted to germline cells, is expressed in ~60% of triple-negative breast cancers (TNBCs) where it is associated with higher levels of allelic imbalance. In experimental systems, HORMAD1 also drives Homologous Recombination (HR) deficiency, PARP inhibitor and cisplatin sensitivity. To provide further insights into the role of HORMAD1 in TNBC, we generated a doxycycline-inducible HORMAD1 expression system in the copy number stable, HORMAD1-negative, TNBC cell line SUM159. Clonal, HORMAD1-positive, populations were established which displayed increased levels of genomic instability and reduced levels of homologous recombination. To identify synthetic lethal therapeutic targets associated with HORMAD1 expression in TNBC, we performed an RNAi library screen, in which the viability of HORMAD1-expressing SUM159 clonal cell populations was assessed following the targeted depletion of 1743 genes. Candidate synthetic lethal (SL) genes were validated in a secondary screen using 3 additional HORMAD1-expressing SUM159 clones. The primary siRNA screen identified 63 genes which were SL with elevated HORMAD1 expression, including XRCC1, TDP1, Pol η, BRIP1 and ATR - genes with a known function in mitigating replication stress. DNA polymerase η (POLH) is a Y-family DNA polymerase involved in translesion synthesis (TLS), a DNA damage tolerance pathway employed by proliferating cells to bypass replication fork stalling DNA lesions and to prevent replication fork collapse. We hypothesised that HORMAD1 expression leads to Pol η localisation to replication factories, to mediate lesion bypass. Consistent with this, we found an enrichment in Pol η and monoubiquitylated-PCNA in the chromatin fraction of HORMAD1-expressing SUM159 cells, suggesting that Pol η and TLS might buffer the effect of elevated HORMAD1 expression in TNBC. Our data suggest that HORMAD1 expression in mitotic cells leads to a dependency on pathways implicated in preventing or resolving replication stress, such as TLS, and suggests that vulnerabilities associated with specific DNA repair processes could be used to target HORMAD1-positive TNBCs. Citation Format: Dalia Tarantino, Callum Walker, Daniel Weekes, Helen Pemberton, Jessica Frankum, Rachel Brough, Christopher J. Lord, Anita Grigoriadis, Andrew Tutt. A high-throughput functional screen reveals synthetic lethal interactions associated with replication stress in HORMAD1-expressing triple-negative breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 344.