The recent approval of PARP inhibitor, olaparib, in HER2-negative breast cancer expands the therapeutic options for patients with germline BRCA1/2 alterations. The role of somatic BRCA alteration as a predictive biomarker in breast cancer is currently unclear. NCCN guidelines call for germline testing in all young patients (<46 yrs) and patients with triple negative breast cancer (TNBC) (< 61 yrs) with a personal history of breast cancer. Here we examined the landscape of BRCA mutations to assess whether additional populations may have potential benefit from PARP inhibitors.


Hybrid-capture based comprehensive genomic profiling of 395 cancer-related genes using the FoundationOne assay (Foundation Medicine, MA) was performed on 12,508 breast carcinomas. Somatic/germline/zygosity status for BRCA1/2 variants was analyzed as described in Sun, 2018 (PMID: 29415044). High genome-wide loss-of-heterozygosity (gLOH) was classified as ≥16% LOH. Subgroups were analyzed on histological {Invasive Ductal Carcinoma (IDC), Invasive Lobular Carcinoma (ILC)} and molecular subtypes [ER-positive (ER+), HER2-amplified (HER2+), TNBC], patient age [≤45, 46-60, 61+], and gender.


Consistent with previous reports, the frequency of BRCA1/2 alterations was highest in young patients (<46; 15%), TNBC (10%), and male breast cancer (14%); BRCA1/2 alterations were also identified in HER2+, ILC, and ER+ tumors (5.3%, 6.5%, 8.3%). Overall, BRCA1 was more frequently mutated in TNBC and young patients (67%), whereas BRCA2 was more frequently mutated in ER+, HER2+, ILC, and male breast cancer (63%, 57%, 79%, 100%).

The fraction of somatic BRCA mutations (sBRCA) was 38%, with the lowest fraction of sBRCA in TNBC and young patients (30%, 30% overall, 36%, 42% for BRCA1) and highest in HER2+, ILC, and older patients (46%, 52%, 51%); the absolute frequency of sBRCA is approximately 3.5%. In tumors with ESR1 mutations, we detected concurrent BRCA mutation in 6.4% (85/1319), with 48% predicted somatic.

Almost all tumors with BRCA mutations had biallelic inactivation, with LOH of the second allele irrespective of predicted germline status (90% of gBRCA and 86% of sBRCA under LOH). gLOH score was used as a phenotypic measure of homologous recombination deficiency (HRD): patients harboring biallelic BRCA alterations had elevated rates of gLOH with 89% of gBRCA and 84% of sBRCA tumors harboring a high gLOH score, vs 47% with heterozygous BRCA, 34% with no BRCA alteration, and 80% for patients with BRCA deletion.


Thirty-eight percent of deleterious BRCA alterations in breast are predicted somatic, including 36% of BRCA1 alterations in TNBC. Germline testing would miss these alterations even though they are frequently under LOH and are associated with high gLOH, a biomarker with predictive value in ovarian cancer. While patients with ER+ and HER2+ tumors have low rates of gBRCA alterations, the overall BRCA mutation rate, including somatic alterations, is appreciable at 8.3% and 5.3%. Our findings demonstrate that sBRCA alterations are associated with a comparable HRD phenotype to gBRCA altertions and suggests that PARP inhibitors may have potential value for a wider range of breast cancer patients.

Citation Format: Sokol ES, Frampton GM, Ross J, Ali S, Chung J, Oesterreich S. BRCA1/2 alterations are present at significant rates across breast cancer subtypes and are associated with a high genome-wide loss of heterozygosity signature [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-10-05.