Loss of nuclear integrity at persistent DNA bridges ties PARP inhibitors to cGAS/STING signaling

BRCA mutant tumors, deficient in homologous recombination DNA repair (HRR), exhibit some of the highest levels of tumor cell-intrinsic interferon (IFN)-stimulated gene (ISG) expression among The Cancer Genome Atlas primary tumors, consistent with preclinical studies tying DNA repair deficiency to increased ISG expression through an as yet poorly understood mechanism. The goal of our ongoing studies is to examine the contribution of mitotic errors to IFN signaling in BRCA-deficient cancer. We find that DNA bridges that persist beyond the abscission checkpoint into the following interphase occur more frequently with BRCA1 deficiency. Treatment with PARP inhibitors (PARPi) including olaparib, saruparib, and AZD9574, which cause targeted tumor cell death of BRCA-deficient cells, further increases the frequency of persistent bridges. Similarly, PARPi induce an elevated number of persistent bridges in BRCA2-deficient prostate tumor xenografts but not in those that are competent for HRR. Our findings show that the majority of persistent bridges recruit the cytosolic DNA sensor cGAS in vitro , suggesting a loss of nuclear integrity that exposes the bridging DNA to the cytoplasm. Indeed, performing the first correlative light and electron microscopy (CLEM) of persistent DNA bridges, we find nuclear envelope (NE) ruptures at PARPi-induced bridges. cGAS is maximally activated by non-nucleosomal DNA, and we find the bridging DNA to be highly accessible as assessed by Tn5 transposition (ATAC-see). Surprisingly, our CLEM data reveals efficient NE repair at persistent bridges, which we tie to the robust recruitment of the NE repair factors BAF, LEM2 and CHMP7. This NE repair network may influence cGAS activation by the bridging DNA thereby influencing ISG expression. Consistent with this, knockdown of the most upstream NE repair factor, BAF, increases ISG expression and synergizes with PARPi. Taken together, our results demonstrate that PARPi treatment increases the frequency of persistent DNA bridges that are over-stretched, non-nucleosomal, and defective in nuclear integrity, leading to activation of cGAS-dependent IFN signaling. We are additionally investigating the prevalence of persistent bridges in patient samples and xenograft models to explore their potential use as a biomarker for PARPi sensitivity. Further elucidating the mechanisms contributing to ISG expression in BRCA-deficient cancers will provide novel insights into strategies for maximizing the therapeutic potential of PARPi treatment.