Abstract P5-12-03: Intratumor heterogeneity and intrinsic immune activation are associated with response to chemotherapy in BRCA-related breast cancers

Abstract Background Breast cancers in women with a germline BRCA1/2 mutation (gBRCAm) have homologous recombination deficiency (HRD) and are sensitive to therapies causing double-strand DNA breaks. In TBCRC 031 (INFORM), both neoadjuvant cisplatin and doxorubicin/cyclophosphamide (“AC”) in gBRCAm carriers resulted in a complete pathologic response in 18% and 26% respectively in patients with newly diagnosed HER2–negative breast cancer. Herein, we describe molecular features from tumor whole exome (WES) and transcriptome sequencing (RNAseq) associated with response. Methods TBCRC 031 (the INFORM Trial - NCT01670500) was a randomized phase II neoadjuvant trial comparing the efficacy of cisplatin versus AC in gBRCAm carriers with stage I-III HER2–negative breast cancer. Of 118 patients enrolled, 92 patients provided fresh frozen research biopsies, collected prior to chemotherapy initiation, which were subjected to WES and RNAseq. Variants were called using GATK best practices and intratumoral heterogeneity was inferred from mutations and variant allele frequencies using Mutant Allele Tumor Heterogeneity (MATH) scores. Mutational Signature 3 (Sig3) and Genomic Instability Score (GIS) were calculated with SigMA and scarHRD, respectively. RNAseq data were utilized to perform differential gene expression and functional analyses, while cellular deconvolution was performed with CIBERSORTx trained against breast cancer single cell data. Patients were grouped according to their residual cancer burden (RCB) as responders (RCB-0 or 1), or non-responders (RCB-2 or 3). P-values ≤ 0.05 were considered statistically significant and when appropriate, adjustment for multiple testing was performed using a false discovery rate ≤ 5%. Results Of the 92 patients with gBRCAm, 59 (64%) had triple-negative and 33 (36%) were hormone-receptor positive HER2-negative breast cancer, 40 (43%) were classified as responders and 52 (57%) as non-responders. WES analysis revealed that indices of HRD were high (Sig3 exposure predominant and GIS ≥ 42) across most samples irrespective of receptor status and not significantly different among responders and non-responders. In contrast, responders exhibited lower levels of intratumor heterogeneity than non-responders (median MATH 42.9 vs. 33.5, p = 0.01). 223 genes were differentially expressed between responders and non-responders following control for tumor hormone receptor status, BRCA1/2 mutation, and menopausal status. Pathways identified as significantly enriched in upregulated genes were indicative of intrinsic immune activation in responders (e.g., T-cell activation, immune response-signaling, and regulation of leukocyte activation). Cellular deconvolution of the tumor microenvironment confirmed that responders presented a higher proportion of T-cells (p = 0.025) and myeloid cells (p = 0.003) in the tumor samples, while perivascular-like cells were enriched in non-responders (p = 0.034). Conclusion In this analysis of the largest cohort of treatment-naïve gBRCAm breast cancer to date, we found that response to cytotoxic chemotherapy is associated with a transcriptional program of intrinsic immune activation and an increased population of intratumoral T-cells and myeloid cells. Lower levels of intratumor heterogeneity and higher immune activation were associated with response to chemotherapy gBRCAm carriers while HRD scores were not. Citation Format: Felipe Batalini, Doga C. Gulhan, Xanthi Lida Katopodi, Nikolas Kalavros, Antuan Tran, Dimitra Karagkouni, Emily Stern-Gatof, Stuart Schnitt, Judy E. Garber, Gerburg M. Wulf, Peter J. Park, Ioannis Vlachos, Nadine Tung. Intratumor heterogeneity and intrinsic immune activation are associated with response to chemotherapy in BRCA-related breast cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-12-03.