Abstract PR01: Assessment of PARPi and cisplatin resistance in BRCA1 exon 11 mutant patient -derived xenografts

Abstract

Abstract Introduction: The BRCA1 protein contributes to homologous recombination (HR) repair of DNA breaks. BRCA1 mutations often result in dysfunction or absence of BRCA1 proteins. Consequently, BRCA1 mutations induce PARP inhibitor (PARPi) and cisplatin sensitivity. BRCA1 exon 11 is the largest exon and harbors a plethora of germline mutations. BRCA1 exon 11 is subject to alternative splicing and can generate the BRCA1-Δ11q isoform. This isoform is a hypomorphic variant that removes germline exon 11 mutations allowing for translation through to the end of the protein. In this study we examined PARPi and cisplatin sensitivity and resistance in a panel of BRCA1 exon 11 mutant PDX tumors. Experimental Procedures: Wild-type BRCA1 PDX036, exon 13 truncating BRCA1 mutant PDX1126, and BRCA1 exon 11 mutants PDX017, PDX056, and PDX124 were utilized. Using these models, we developed several isogenic sets of cisplatin and PARPi sensitive/resistant PDX using serial passaging and treatments. Additionally, we developed a method for lentiviral manipulation of PDX models to study resistance mechanisms. We assessed tumors for cisplatin and PARPi sensitivity and examined BRCA1 and 53BP1 expression by Western blotting and immunohistochemistry. Results: We observed variable responses to cisplatin and PARPi treatments among PDX models despite the presence of BRCA1 mutations that abrogated expression of full-length BRCA1. PDX196 expressed an abundance of BRCA1-Δ11q and exhibited resistance to both PARPi and cisplatin treatments. Interestingly, exon 11 mutant PDX017, 056, and 124 initially responded to both cisplatin and PARPi treatments. Serial passaging and treatments ultimately elicited resistance in each of these models and in some tumors coincided with an increase in BRCA1-Δ11q expression. Additionally, PARPi and cisplatin-resistant PDX017 tumors exhibited a simultaneous reduction of 53BP1 and increased BRCA1-Δ11q expression. In contrast to the BRCA1 exon 11 mutant PDX models, exon 13 mutant PDX1126 failed to develop resistance after multiple rounds of serial passaging with either cisplatin or PARPi treatments. However, when PDX1126 cells were transduced with lentivirus containing BRCA1-Δ11q cDNA, a rapid acquisition of cisplatin resistance occurred. Conclusions: Our data indicate BRCA1-Δ11q expression is variable in BRCA1 exon 11 mutant tumors, but nevertheless, when abundantly expressed, supports resistance to cisplatin and PARPi. This abstract is also being presented as Poster A46. Citation Format: John J Krais, Emma Clausen, Vladimir Khazak, Igor Astsaturov, Clare L Scott, Neil Johnson. Assessment of PARPi and cisplatin resistance in BRCA1 exon 11 mutant patient -derived xenografts [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr PR01.