Abstract 3363: PUM3 is a triple-negative breast cancer dependency gene that functions in replication fork restart and repair

Abstract

Abstract Triple-Negative Breast Cancers (TNBC) are characterized by high levels of structural chromosome alterations and harbor mutational scars of defects in homologous recombination. Importantly, by acting either as drivers, or as tumor specific dependencies mutated genes in these regions represent potential therapeutic targets In search of such genes we identified that knockdown of the PUM3 gene, which is contained in a 9p24 recurrent ampliicon, impaired the growth of a subset of breast cancer but not mammary epithelial cell lines. Furthermore, the growth defects observed upon PUM3 knockdown in SUM149, a BRCA1 deficient cell line, were reversed by reactivation of BRCA1's function suggesting a BRCA1-deficiency associated dependency. PUM3 knockdown led to an increase in gH2Ax and 53BP1 nuclear foci, impaired restart of stalled replication forks, and decreased RAD51 mediated repair of collapsed replication forks. As a result PUM3 depleted cells have reduced DNA replication rate and slowed cell cycle progression. Knockdown of PUM3 also impaired replication stress induced PARylation and PAR foci formation and PUM3 and PARP1 knockdown showed phenotypic epistasis with respect to cell growth suggesting a potential mechanism for PUM3s replication fork related functions. PUM3 forms a complex with gH2Ax and the replication stalling agent hyroxyurea and PARP-inhibitors increases the nuclear concentration of PUM3. Furthermore PUM3 knockdown increased the sensitivity of breast cancer cells to these agents indicating a role for PUM3 in the repair of lesions generating by these DNA damaging agents. By studying PUM3 we have identified a novel player in replication fork restart and repair whose activity may be particularly relevant in homologous recombination deficient TNBCs. A greater understanding of PUM3's function and the pathways that regulate it may identify novel therapeutic targets for TNBCs. Importantly, direct or indirect modulation of PUM's functions may potentiate the effects of replication stress inducing therapies such as PARP-inhibitors. Citation Format: Daniel Weekes, Elodie Noel, Callum Walker, Nick Balan, Vandna Shah, Bhavna Sidhu, Anna Pardix, Stephen Pettitt, Christopher J. Lord, Anita Grigoriadis, Tutt N. Andrew. PUM3 is a triple-negative breast cancer dependency gene that functions in replication fork restart and repair [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 3363.