Concordance of DNA damage repair (DDR) gene mutations in paired primary and metastatic prostate cancer (PC) samples.

Abstract

5020 Background: Mutations in DDR genes represent actionable alterations that can be used to guide precision medicine strategies in men with advanced PC. However, acquisition of contemporary tissue samples for advanced molecular testing can be a barrier to deploying precision medicine approaches. We hypothesized that most DDR alterations represent early truncal events in PC and that archival primary tissue would faithfully reflect mutations found in cell-free circulating tumor (ctDNA) and/or metastatic tissue. Methods: Patients were included in this study if a DDR pathway mutation was detected in metastatic tissue or ctDNA and primary tissue sequencing was available for comparison. Sequencing data from three cohorts were analyzed: 1) FoundationOne, 2) University of Washington (UW-OncoPlex or SU2C/PCF International Dream Team sequencing pipelines) and 3) University of Washington rapid autopsy series. Only pathogenic somatic mutations were included and we required ≥30 days between primary tumor tissue and ctDNA/tumor tissue acquisition. Clonal hematopoiesis of indeterminant potential (CHIP) and germline events were adjudicated by an expert molecular pathologist and excluded. Variants detected only in plasma were considered likely to be CHIP or low subclones if the variant fraction was <1% and/or >5-fold less than the estimate tumor content in plasma. Results: Paired primary and ctDNA/metastatic samples were sequenced from 72 individuals with known DDR alterations. After excluding ctDNA cases where only CHIP (N=13) and/or germline events (N=7) were observed, 51 subjects remained and were included in the final analysis. The median time from acquisition of primary tissue to acquisition of ctDNA or tumor tissue was 52 mos (range: 1 – 193 mos). Concordance in DDR genes across samples was 86% (95% CI: 74-93%). Rates of concordance between metastatic-primary and ctDNA-primary pairs were similar when CHIP cases were excluded (87% and 85%, respectively). BRCA2 reversion mutations associated with resistance to PARP inhibitors and platinum chemotherapy were detected in ctDNA from two subjects. Conclusions: These data provide evidence that primary prostate tissue accurately reflect the mutational status of actionable DDR genes in men with metastatic PC, supporting the hypothesis that DDR alterations are early truncal events. After excluding likely CHIP events, ctDNA profiling accurately captured these truncal DDR mutations, while also detecting reversion alterations that may suggest potential resistance mechanisms.[Table: see text]