Tracking plasma DNA mutation dynamics in estrogen receptor positive metastatic breast cancer with dPCR-SEQ

Serial monitoring of plasma DNA mutations in estrogen receptor positive metastatic breast cancer (ER + MBC) holds promise as an early predictor of therapeutic response. Here, we developed dPCR-SEQ, a customized assay that utilizes digital PCR-based target enrichment followed by next-generation sequencing to analyze plasma DNA mutations in ESR1 , PIK3CA , and TP53 . We validated dPCR-SEQ in a prospective cohort of 58 patients with ER + MBC and demonstrate excellent concordance with hotspot ESR1 mutation abundance measured by conventional digital PCR. The dPCR-SEQ assay revealed ESR1 , PIK3CA , and TP53 plasma ctDNA mutations in 55%, 32%, and 32% of the study patients, respectively. We also observed dynamic changes in ESR1 , PIK3CA , and TP53 ctDNA mutant allele fraction (MAF) that were frequently discordant between the different genes. Thus, monitoring plasma DNA mutation dynamics using a dPCR-SEQ assay is feasible, accurate, and may be investigated as a biomarker of therapeutic response in ER + MBC.