Abstract 6094: Longitudinal analysis of PARP inhibitor and platinum resistance in BRCA1/2m breast cancer using liquid biopsy

Abstract Background: Assessing how clinical resistance to PARP inhibitors develops has been challenging, due to the lack of coverage of potential resistance genes in sequencing panels and biopsies being subject to spatial heterogeneity. We studied the development of PARPi and/or platinum resistant disease using both tissue and a novel liquid biopsy assay, in patients treated for BRCA1/2-mutated metastatic breast cancer (BRCA1/2m mBC). Approach: A cohort of 35 mBC patients with germline or somatic BRCA1/2 mutations were identified as having developed PARPi or platinum resistant disease. Tumour biopsies were analysed by exome and transcriptome sequencing whereas ctDNA isolated from plasma sampled across the pre-treatment, response and eventual progression journey were analysed using the Guardant INFINITY platform equipped to detect mutations in >800 genes, and genome wide methylation, including promoter methylation in 398 cancer-related genes. Somatic mutations or regions of methylation that were associated with resistance were identified, including BRCA1/2 reversion mutations. Somatic mutations with the potential to cause PARPi resistance were annotated using CRISPR screen data and other functional analyses describing genes that alter PARPi synthetic lethality. Results: The most common resistance mechanism was BRCA1/2 reversion mutation (51%; n=8 BRCA1m patients, n=10 BRCA2m patients). Most reversions (77%) occurred via deletions (BRCA1, 70%; BRCA2, 79%) and exhibited microhomology use at junctions (> 80% for both BRCA1 and BRCA2). In 14 patients, multiple concurrent reversion mutations were detected with VAFs ranging from 0.1-40% for BRCA1 and 0.05-18% for BRCA2. Changes in VAF over time indicated that different reversions in the same patient may impart different fitness advantages in the face of treatment. Loss-of-function mutations in the 53BP1-Shieldin pathway were identified in two BRCA1m patients, and VUS mutations in the pathway were seen in two further patients. In some patients, reversion mutations co-occurred with either 53BP1-Shieldin pathway mutations or with replication fork stability mutations (PAXIP1 in BRCA2m patients) - indicating concurrent but mechanistically distinct forms of resistance develop in the same patient. We isolated a PDX model from one patient that exhibits resistance via both BRCA1 reversion and loss of 53BP1, allowing this phenomenon to be modelled in more detail. We also detected BRCA1 methylation in ctDNA in a patient with a somatic rearrangement in BRCA1. Conclusions: Liquid biopsy profiling of PARPi-resistance breast cancer patients indicates that many patients develop multiple reversion mutations, and that alterations in the 53BP1-Shieldin pathway are present but less frequent. The co-occurrence of 53BP1-Shieldin pathway mutations and reversion mutations suggests parallel mechanisms of resistance can operate in the same patient. Citation Format: Elizabeth Harvey-Jones, Maya Raghunandan, Luisa Robbez-Masson, Alaguthurai Thanussuyah, Roberta Liccardo, Arielle Yablonovitch, Mingyang Cai, Leylah Drusbosky, Michael Dorschner, Lorena Magraner Pardo, Rebecca Marlow, Asha Konde, Jennifer Trendell, John Alexander, Syed Haider, Chris Starling, Ioannis Roxanis, Jennifer Yen, Stephen J. Pettitt, Christopher J. Lord, Andrew N. Tutt. Longitudinal analysis of PARP inhibitor and platinum resistance in BRCA1/2m breast cancer using liquid biopsy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6094.