BRCA2 abrogation triggers innate immune responses potentiated by treatment with PARP inhibitors

Heterozygous germline mutations in BRCA2 predispose to breast and ovarian cancer. Contrary to non-cancerous cells, where BRCA2 deletion causes cell cycle arrest or cell death, BRCA2 inactivation in tumors is associated with uncontrolled cell proliferation. We set out to investigate this conundrum by exploring modalities of cell adaptation to loss of BRCA2 and focused on genome-wide transcriptome alterations. Human cells in which BRCA2 expression was inhibited using a doxycycline (DOX)-inducible shRNA for 4 or 28 days were subjected to RNA-seq analyses. Gene sets differentially expressed in BRCA2-deficient versus -proficient cells revealed a biphasic response to BRCA2 abrogation. The early, acute response consisted of downregulation of genes involved in cell cycle progression, DNA replication and repair and was associated with cell cycle arrest in G1. Surprisingly, the late, chronic response consisted predominantly of upregulation of innate immune response genes controlled by interferon. Activation of the cGAS-STING pathway detected in these cells further substantiated the concept that long-term BRCA2 abrogation triggers cell-intrinsic immune signaling. Importantly, we found that treatment with PARP inhibitors stimulated the interferon response in cells and tumors lacking BRCA2. We propose that PARP inhibitors may suppress growth of BRCA2-deficient cells and tumors, in part, by activating interferon signaling.