Abstract P3-09-09: Serial circulating tumor DNA from patients with metastatic breast cancer with and without BRCA1/2 mutations
Authors: Collier Katharine A , Tallman David , Weber Zachary T. , Haynam Marcy , Adams Elizabeth J. , Jenison Janet , Asad Sarah , Lustberg Maryam , Cherian Mathew , Ramaswamy Bhuvaneswari , Sardesai Sagar , Williams Nicole , Wesolowski Robert , Vandeusen Jeffrey , Gatti-Mays Margaret E. , Pariser Ashley , Mortazavi Amir , Stover Daniel G. 
Published: March 2022

Background: Analysis of circulating tumor DNA (ctDNA) over time allows non-invasive evaluation of tumor genomic evolution. We characterize changes in tumor fraction (TFx), somatic copy number alterations (SCNAs), and somatic mutations over time in patients (pts) with and without BRCA1/2 mutations and metastatic breast cancer (mBC) who received a PARP inhibitor (PARPi) or platinum chemotherapy. Specifically, we seek to identify the frequency of BRCA1/2 reversion mutations. Methods: Pts with mBC and germline or somatic BRCA1/2 mutations were identified on a banking protocol of prospectively-collected serial samples of blood and plasma. Control pts without a BRCA1/2 mutation were matched 2:1 by age and hormone receptor (HR) status. Ultra-low-pass whole genome sequencing (ULPWGS) with 0.1x depth was performed on all plasma samples (n=103) and the ichorCNA algorithm was used to determine TFx and SCNAs. Targeted panel sequencing (TPS) of 402 cancer-related genes was performed at 10,000x depth on plasma samples, and one blood sample per pt. The panel includes BRCA1/2 and 38 other DNA damage repair (DDR) genes. Somatic mutations were identified by joint calling with Mutect2 across plasma timepoints with paired pt normal blood. Germline variant calling from TPS on blood with HaplotypeCaller was used to confirm germline mutations in BRCA1/2. Results: We identified 10 pts with mBC with a germline (n=7) or somatic (n=3) BRCA1 (n=2) or BRCA2 (n=8) mutation and banked blood and plasma samples at 2-9 timepoints at a median of 8 weeks apart (range 1-43). The control cohort of 20 pts with mBC and wildtype BRCA1/2 was well matched by age and HR status. All pts with BRCA1/2 mutations received a PARPi and/or platinum chemotherapy at some point during sample collection. Half of control pts received platinum chemotherapy. Germline BRCA1/2 mutations were confirmed in all 7 pts with known germline mutations. Somatic BRCA2 mutations were confirmed in ctDNA in 2 of 3 patients. Among all samples, median TFx was 0.05 (range 0-0.80) with 35% of samples having TFx &gt;0.10. There was no significant difference in TFx by age, receptor status, or active treatment with a PARPi or platinum. There was no significant change in the percent of genome with a SCNA over time. A reversion mutation of a germline BRCA2 mutation, restoring the open reading frame of BRCA2, was discovered at the last timepoint from 1 pt while receiving carboplatin. She had radiographic progression 4 weeks later. A germline BRCA1/2 reversion mutation in this cohort occurred in 2.3% of samples, 14.3% of pts. The somatic mutation landscape and clonal evolution of TPS using PyClone will be presented. Clonal evolution can show emerging and responding clusters of variants. For pts with available tissue specimens, somatic variants in ctDNA will be compared to somatic mutations detected in tissue with TPS. Conclusions: Evaluation of serial ctDNA samples for TFx, SCNAs, and somatic mutations from banked plasma and blood from pts with mBC is feasible. SCNAs were stable over time. The frequency of reversion mutations in BRCA1/2 was low, suggesting that either their incidence is low or ctDNA TPS is not sensitive enough to detect them.

Citation Format: Katharine A Collier, David Tallman, Zachary T. Weber, Marcy Haynam, Elizabeth J. Adams, Janet Jenison, Sarah Asad, Maryam Lustberg, Mathew Cherian, Bhuvaneswari Ramaswamy, Sagar Sardesai, Nicole Williams, Robert Wesolowski, Jeffrey Vandeusen, Margaret E. Gatti-Mays, Ashley Pariser, Amir Mortazavi, Daniel G. Stover. Serial circulating tumor DNA from patients with metastatic breast cancer with and without BRCA1/2 mutations [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-09-09.