Abstract A12: Analysis of tumor immunogenicity in germline BRCA1/2 mutation associated breast and ovarian cancers

Germline mutations in BRCA1 or BRCA 2 can lead to increased genomic instability and mutational burden in mutation-associated breast and ovarian cancer. Given the association of tumor neoepitope load and clinical response to immune checkpoint therapy, we used whole exome sequencing (WES) to predict missense mutational burden and potential neoepitope expression in 34 breast and 71 ovarian tumors with germline BRCA1/2 mutations (combining Penn and TCGA cohorts). Analysis of WES data found that tumors from patients with BRCA1/2 mutations had significantly higher mutational burden than non-BRCA1/2 tumors (p<0.01). Based on a novel bioinformatics pipeline for human leukocyte antigen (HLA) genotyping and MHC class I neoantigen prediction, we found that neoantigen load (predicted IC50 <500 nM) varied across patients (breast: mean - 147, std. dev. – 127; ovarian: mean – 126, std. dev. – 96). While higher neoantigen load promotes cytotoxic CD8+ T-cell responses in a number of cancers, we found among the BRCA1/2 subset that neoantigen load did not correlate with cytolytic signature, an RNA-based metric of anti-tumor immune response. From TCGA RNA-seq data, immune pathways (q<0.01) including PD-1 signaling, reflective of counter-regulatory responses often observed in response to immune pressure, positively correlated with cytolytic signature. Neoantigen load, however, correlated only with GPCR signaling and extracellular matrix (ECM) proteins (q<0.01). By contrast, the expression of genes involved in ECM and angiogenesis, which are involved in tumor immune escape and suppression of immune cell infiltration, negatively correlated with cytolytic signature (q<0.01). Increased PDL1 and PDL2 (p<0.05) expression associated with copy number loss in PTEN. These data were confirmed by tissue microarray analysis, which found that levels of T cell infiltration did not associate with neoantigen load, whereas PTEN loss was associated with decreased T cell infiltration (p<0.05). Our work demonstrates that neoantigen load alone is not sufficient to induce T-cell infiltration and cytotoxicity in tumors from patients with BRCA1/2 mutations, but rather points to tumor intrinsic pathways involving PTEN copy alterations and a dense extracellular matrix as determinants of tumor immunity, both of which may lead to novel opportunities for combination immune therapy in these cancers.

Citation Format: Adam Kraya, Kara N. Maxwell, Brandon M. Wenz, Bradley Wubbenhorst, Daniel De Sloover, Nicole Lunceford, Amanda Barrett, Jennifer D. Morrissette, Michael Feldman, Robert H. Vonderheide, Susan M. Domcheck, Katherine L. Nathanson. Analysis of tumor immunogenicity in germline BRCA1/2 mutation associated breast and ovarian cancers. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr A12.

Abstract 2990: Evidence for diverse mechanisms of tumorigenesis in breast and ovarian tumors of BRCA1/2 carriers

Germline mutations in the tumor suppressors BRCA1 and BRCA2 lead to increased risks of breast and ovarian cancers at least in part due to their roles in homologous recombination based double stranded DNA repair. Genetic alterations including loss of the wild-type BRCA1/2 allele, PTEN loss and TP53 mutations are thought to contribute to genomic instability in susceptible tissues and therefore tumor formation. In order to further investigate the mechanisms of tumorigenesis in BRCA1/2 carriers, we performed whole exome sequencing of 39 breast and ovarian tumors and matched blood germline DNA. Sequencing data were analyzed using GATK and MuTECT for variant calls and ngCGH and Sequenza for copy number analysis. Using cellularity corrected tumor versus germline allele frequency calculations, we found no evidence of loss of heterozygosity (LOH) in 17% of BRCA1 tumors (n = 4 of 24) and 40% of BRCA2 tumors (n = 6 of 15). Genomic instability as measured by percentage of genome in the diploid state showed that BRCA1/2 tumors with no evidence of LOH had decreased genomic instability (average% genome diploid 76+27% vs 57+16%, p = 0.01) with no difference in the overall somatic mutation rate (average 50+71 vs 83+156 single nucleotide variants per megabase, p = NS). While there were also no significant differences in the average number of predicted deleterious somatic mutations in BRCA1/2 tumors with no LOH versus tumors with LOH (n = 13+15 vs 13+6, p = NS), tumors without LOH were significantly less likely to have mutations or loss in TP53 or loss of PTEN (50% versus 93%, p = 0.007). Of the ten tumors with no LOH, five were from patients who had received prior cytotoxic chemotherapy. Other tumors with no LOH were from patients with atypical presentations including a mixed histology epithelial ovarian tumor in a BRCA1 carrier, and a triple negative breast cancer in a 70 year old BRCA2 carrier. Of the 29 tumors with evidence of LOH, no patient had received cytotoxic chemotherapy for a prior malignancy or in the neoadjuvant setting. No recurrent driver mutations other than TP53 mutations were identified in the BRCA1/2 tumors with or without LOH. Twenty-six pathogenic likely driver mutations were identified in 25 cancer genes outside of TP53, including NRAS, PIK3CA, and ABL2. Our results indicate that approximately 75% of tumors in BRCA1/2 germline mutation carriers arise via a classic pathway involving LOH of the wildtype allele, loss of function of TP53 and/or PTEN and significant genomic instability. However, approximately 25% of tumors may arise via an alternative pathway, possibly related to prior chemotherapy in some cases. Further studies are needed to determine the molecular and clinical factors associated with this proposed classic versus atypical pathway of tumorigenesis in BRCA1/2 carriers and whether these characteristics are associated with outcomes such as survival and platinum and/or PARP inhibitor sensitivity.

Citation Format: Kara N. Maxwell, Daniel De Sloover, Bradley Wubbenhorst, Brandon Wenz, Nicole Lunceford, Lyndsey Emery, Kurt D'Andrea, Robert D. Daber, Michael D. Feldman, Susan M. Domchek, Katherine L. Nathanson. Evidence for diverse mechanisms of tumorigenesis in breast and ovarian tumors of BRCA1/2 carriers. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2990. doi:10.1158/1538-7445.AM2015-2990

The mutational spectrum of breast and ovarian tumors from BRCA1 and BRCA2 mutation carriers

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Background: Individuals who carry one mutated copy of the BRCA1 or BRCA2 genes have elevated lifetime risks of breast and ovarian cancer. A number of studies have investigated the somatic mutational spectra of breast and ovarian tumors; however, BRCA1/2mutated tumors are underrepresented. Methods: Sixty-eight formalin-fixed paraffin embedded samples from BRCA1/2patients have been identified. Massively parallel sequencing using 48 gene capture is in process, whole exome sequencing of tumor and matched germline DNA is planned. Data are analyzed using a custom bioinformatics pipeline. Results: In analysis of data from the first 26 breast (4 BRCA1, 6 BRCA2) and ovarian (8 BRCA1, 8 BRCA2) tumors, the majority (23/26, 88%) had 0-2 variants in 48 cancer genes. Known deleterious TP53 mutations were the only variants identified in 2/4 BRCA1 and 2/6 BRCA2 breast tumors. Of those remaining, 2 BRCA1 and 1 BRCA2 breast tumors had no identified deleterious mutations. Two BRCA2 breast tumors with no TP53 mutations had known deleterious mutations in a single gene each - FGFR2 and PI3KCA. One BRCA2 breast tumor with no TP53 mutation had a variant of uncertain significance in FLT3. Finally, one BRCA2 breast tumor had a very high mutational rate, with one deleterious TP53 mutation and 7 other small deletion and single nucleotide variants. For the ovarian tumors, 15/16 BRCA1 and BRCA2 tumors had known deleterious TP53 mutations; the ovarian tumor with no TP53 mutation had no other variants. TP53 mutations were the sole identified mutations in 8 ovarian tumors. One ovarian tumor carried a known JAK3 activating mutation and 4 ovarian tumors carried one variant of uncertain significance in a single gene - SMO, PDGFRA, GNA11 and NRAS. Finally, two ovarian tumors were found to have high mutational rates. Conclusions: Using a targeted resequencing panel, we confirmed the high rate of TP53 mutations in BRCA1/2 breast tumors and observed a higher than expected rate in BRCA1/2 ovarian tumors. Importantly, we have identified mutations in other known driver genes using FFPE samples, allowing generalizability to other sites. These analyses may uncover novel mutations that could be exploited in the development of targeted therapeutic agents for BRCA1/2 carriers.