Abstract A22: Homologous recombination deficiency negatively predicts for immune infiltration and antitumor immune activity in breast tumors with BRCA1/2 alterations

Tumors with germline or somatic BRCA1 or BRCA2 alterations are sensitive to DNA-damaging agents but often develop therapeutic resistance. Breast cancers associated with BRCA1/2 alterations have a relatively high mutational load, defined as the number of nonsynonymous single nucleotide mutations, relative to BRCA1/2 wild-type tumors, leading to suggestions that patients with BRCA1/2 mutations are possible candidates for immune checkpoint blockade. However, immune infiltration can range widely in tumors with BRCA1/2 alterations, and molecular determinants that account for this variability remain unclear. Our goal was to delineate the molecular features associated with immunogenicity in breast cancers associated with germline or somatic BRCA1/2 alterations through genetic/genomic and histopathologic analyses in 115 tumors from the TCGA (n=89) and Penn (n=26). In the TCGA dataset, we found that the level of homologous recombination deficiency (HRD), as measured by the copy number-based HRD score (high vs. low, dichotomized by median), correlated positively with neoantigen load in BRCA1/2 tumors (p=0.02), but unexpectedly inversely with immunogenicity as measured by cytolytic index (p=0.04) (geometric mean of PRF1 and GZMA expression) and immune ESTIMATE (p=0.002), an RNA-seq-based measure of immune cell infiltration. In prior studies, we had found that a surprisingly high percentage of BRCA1 and BRCA2 germline mutation-associated breast cancers lacked allele-specific loss of heterozygosity (LOH), 10% and 46%, respectively. We thus evaluated whether allele-specific LOH of germline BRCA2 mutations and somatic BRCA1/2 mutation clonality were drivers of the negative association between HRD level and cytolytic index/immune ESTIMATE. We found that allele-specific LOH was associated with higher HRD score (p=0.015) but lower enrichment of the Type II IFN signaling immune metagene (p=0.01), lower cytolytic index (p=0.04), and lower immune ESTIMATE score (p=0.01); tumors with clonal somatic BRCA1/2 mutations had significantly higher HRD score (p=2.4E-07) and mutational burden (p=0.05) but lower cytolytic index (p=0.003) and immune ESTIMATE scores (p=5E-05) than tumors with subclonal somatic BRCA1/2 mutations. Stratifying by both hormone receptor expression and median HRD score, we found that triple-negative breast cancers with low HRD were the most immunogenic and hormone receptor-positive tumors with high HRD the least (p=0.001). We confirmed these TCGA findings by histopathology in tumors associated with germline mutations in BRCA1/2 from Penn. We found that CD45+ leukocyte (p=0.03) and CD8+ cytotoxic T-cell infiltration (p=0.03) as well as expression of the immune effector PRF1 (p=0.048) were significantly lower with high HRD. Further, tumors with allele-specific LOH of the germline BRCA1/2 mutation had significantly lower PRF1 staining (p=0.004), lower membrane levels of the immune checkpoint protein PDL1 (p=0.05), and lower CD8+ (p=0.001), FoxP3+ (p=0.03), and CD20+ (p=0.04) immune cells, suggesting an immunosuppressive role for allele-specific LOH leading to high HRD. These data are consistent with a prior pan-TCGA analysis demonstrating that increased tumor chromosome/arm level copy number alterations suppress immunogenicity. We have found, for the first time, that genomic instability in BRCA1/2 tumors is associated with lower antitumor immune activity, and that the immunogenicity of BRCA1/2 mutation-associated breast cancers is directly related to their HRD level (driven by allele-specific LOH and clonality) and receptor status, both clinically evaluable biomarkers. These findings have immediate implications for the stratification of patients with BRCA1/2 mutation-associated breast cancer for checkpoint blockade therapy. Citation Format: Adam A. Kraya, Kara N. Maxwell, Bradley Wubbenhorst, Brandon M. Wenz, John Pluta, Andrew J. Rech, Nicole Lunceford, Amanda Barrett, Nandita Mitra, Jennifer J.D. Morrissette, Anupma Nayak, Michael Feldman, Susan M. Domchek, Robert H. Vonderheide, Katherine L. Nathanson. Homologous recombination deficiency negatively predicts for immune infiltration and antitumor immune activity in breast tumors with BRCA1/2 alterations [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr A22.