BRCA1 intronic Alu elements drive gene rearrangements and PARP inhibitor resistance

Resistència terapèutica contra el càncer; Càncer d'ovaris Resistencia terapéutica al cáncer; Cáncer de ovarios Cancer therapeutic resistance; Ovarian cancer BRCA1 mutant carcinomas are sensitive to PARP inhibitor (PARPi) therapy; however, resistance arises. BRCA1 BRCT domain mutant proteins do not fold correctly and are subject to proteasomal degradation, resulting in PARPi sensitivity. In this study, we show that cell lines and patient-derived tumors, with highly disruptive BRCT domain mutations, have readily detectable BRCA1 protein expression, and are able to proliferate in the presence of PARPi. Peptide analyses reveal that chemo-resistant cancers contain residues encoded by BRCA1 intron 15. Mechanistically, cancers with BRCT domain mutations harbor BRCA1 gene breakpoints within or adjacent to Alu elements in intron 15; producing partial gene duplications, inversions and translocations, and terminating transcription prior to the mutation-containing BRCT domain. BRCA1 BRCT domain-deficient protein isoforms avoid mutation-induced proteasomal degradation, support homology-dependent DNA repair, and promote PARPi resistance. Taken together, Alu-mediated BRCA1 gene rearrangements are responsible for generating hypomorphic proteins, and may represent a biomarker of PARPi resistance. This work was supported by the US National Institutes of Health (NIH) Grants P50 CA083638 and 5P30 CA006927, as well as R01CA214799, Susan Komen CCRCR17499048, and OC130212 Department of Defense to N.J., and a Pilot Award and Nested Teal Postdoctoral Scholar supported Y.W. (OC140040). We thank Drs. Judith Balmaña, Cristina Saura and Joaquin Arribas for providing materials. PDXs were established thanks to the GHD-Pink program, the FERO Foundation and the Catalan Agency AGAUR [2017 SGR 540]. V.S. is supported by the Instituto de Salud Carlos III (CP14/00228) and ALG by a PERIS fellowship from the Departament de Salut, Generalitat de Catalunya (SLT002/16/00477). Clovis Oncology supplied rucaparib. We are grateful to FCCC Genomics, Cell Culture and Cell Sorting facilities. We thank Hsin Yao Tang at the Wistar Proteomics facility for help with mass spectrometry.