CRISPR screens identify PRMT7 as a therapeutic target to enhance T cell-mediated killing in breast cancer

Genome-wide association studies (GWAS) have identified more than 220 loci associated with breast cancer susceptibility, yet identifying effector genes, their modes of action and prioritising therapeutic targets remains a significant challenge. To address this, we performed pooled CRISPR knockout and inhibition screens to identify genes at risk loci that influence cytotoxic T lymphocyte (CTL) killing of MCF7 breast cancer cells in co-culture. These screens uncovered 33 candidate modulating genes, of which we validated six by single gene editing in two cell lines. Deletion of IRF1, ATF7IP, and CASP8 conferred resistance to CTL killing, while disruption of CFLAR, CREBBP and PRMT7 enhanced sensitivity. Analysis of clinical data showed that PRMT7 expression is negatively correlated with CD8+ infiltration and survival in breast cancer patient cohorts. Pharmacological inhibition of PRMT7 sensitized breast cells to CTL killing in vitro, and Prmt7-deficient tumors exhibited reduced growth and increased CD8+ T cell infiltration in immunocompetent mice. Enhanced Prmt7-dependent tumor growth was not observed in immunodeficient mice, implicating Prmt7 in immune evasion. This study underscores the utility of CRISPR screens for high-throughput functional follow-up of GWAS findings and identifies PRMT7 inhibition as a promising therapeutic strategy.