Abstract 25: Reversion patterns in homologous recombination repair genes in metastatic cancer patients profiled by cell-free DNA
Authors: Fu Yu , Yen Jennifer , Aartyomenko Alexander , Gnerre Sante , Weiport Caroline , Helman Elena , Yablonovitch Arielle , Pettitt Stephen , Llord Christopher , Tutt Andrew , Nagy Rebecca , Odegaard Justin , Chudova Darya , Talasaz AmirAli 
Published: July 2021

Abstract
Background: Inactivating mutations in homologous recombination repair (HRR) genes are associated with a response to PARP inhibitors and platinum-based therapies. However, secondary reversion mutations in HRR genes remain a major mechanism of resistance. We sought to characterize the prevalence and spectrum of these reversions in advanced cancer patients in BRCA1/2 and other HRR genes, such as ATM and CDK12.
Methods: We analyzed cfDNA from &gt;75,000 late-stage cancer patients previously tested as part of routine clinical care using Guardant360®, a 73- to 74-gene panel. This assay detects single-nucleotide variants (SNVs), indels, fusions, and copy-number alterations in cfDNA with a reportable range of ≥0.04%, 0.02%, 0.04%, and ≥2.12 copies, respectively. Secondary reversion mutations were identified by re-analysis using a bespoke HRR deficiency (HRD) Module, which annotates SNV/indels and calls multi-exon deletions and copy-number loss with a 95% limit of detection of ≥ 0.1% and 10 to 25% tumor fraction, respectively.
Results: Among &gt;20,000 breast, ovarian, prostate and pancreatic cancer patients, deleterious germline or somatic mutations were detected in BRCA1 (2.7%), BRCA2 (4.8%), ATM (2.9%) and CDK12 (1.7%). The majority of reversions occurred in BRCA1/2-mut (8.7%), compared to 0.07% of CDK12-mut and none in ATM. Reversions showed selective gene and allele representation. In breast cancer, reversions in BRCA2-mut patients were twice as frequent as those in BRCA1-mut (12.9% vs 5.5%, p-value=0.0046), while the near opposite was true for ovarian (7.6% vs 12.8%, p-value=0.73). In pancreatic cancer (&gt;7,500 patients), reversions presented exclusively in BRCA2-mut (8.4%, n=131 vs 0 in BRCA1-mut, n=63). CDK12 reversions were relatively rare: 1 detected in &gt;5,000 prostate cancer patients. Across all genes, the majority of reversions were small indels while 18% of reversions removed the exon entirely. Pathogenic hotspots BRCA2 codons A938 and S1982 - accounted for ≥10% of reversions and were more likely to revert compared to other deleterious sites (p-value&lt;1.0×10-6). Overall, 16.0% of BRCA1/2 mutant breast, ovarian, pancreatic, prostate patients reverted, consistent with previous reports (Lin et al. 2018).
Conclusions: cfDNA has been previously shown to detect BRCA1/2 germline, somatic and polyclonal reversion mutations acquired over PARP and platinum-based therapy. We demonstrate that the Guardant HRD Module can robustly identify these reversions in the cfDNA of metastatic patients. We emphasize that reversions were frequent among BRCA1/2-mut breast, ovarian, prostate and pancreatic cancers, with preferential enrichment based on gene and tumor type. Understanding resistance patterns may aid in the treatment selection for progressing patients. Further work is needed to define these ideal treatment options.
Citation Format: Yu Fu, Jennifer Yen, Alexander Aartyomenko, Sante Gnerre, Caroline Weiport, Elena Helman, Arielle Yablonovitch, Stephen Pettitt, Christopher Llord, Andrew Tutt, Rebecca Nagy, Justin Odegaard, Darya Chudova, AmirAli Talasaz. Reversion patterns in homologous recombination repair genes in metastatic cancer patients profiled by cell-free DNA [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 25.