Concurrent RB1 loss andBRCA-deficiency predicts enhanced immunological response and long-term survival in tubo-ovarian high-grade serous carcinoma

Abstract

ABSTRACTBackgroundSomatic loss of the tumour suppressor RB1 is a common event in tubo-ovarian high-grade serous carcinoma (HGSC), which frequently co-occurs with alterations in homologous recombination DNA repair genes includingBRCA1andBRCA2(BRCA). We examined whether tumour expression of RB1 was associated with survival across ovarian cancer histotypes (HGSC, endometrioid (ENOC), clear cell (CCOC), mucinous (MOC), low-grade serous carcinoma (LGSC)), and how co-occurrence of germlineBRCApathogenic variants and RB1 loss influences long-term survival in a large series of HGSC.Patients and methodsRB1 protein expression patterns were classified by immunohistochemistry in epithelial ovarian carcinomas of 7436 patients from 20 studies participating in the Ovarian Tumor Tissue Analysis consortium and assessed for associations with overall survival (OS), accounting for patient age at diagnosis and FIGO stage. We examined RB1 expression and germlineBRCAstatus in a subset of 1134 HGSC, and related genotype to survival, tumour infiltrating CD8+ lymphocyte counts and transcriptomic subtypes. Using CRISPR-Cas9, we deletedRB1in HGSC cell lines with and withoutBRCA1mutations to model co-loss with treatment response. We also performed genomic analyses on 126 primary HGSC to explore the molecular characteristics of concurrent homologous recombination deficiency andRB1loss.ResultsRB1 protein loss was most frequent in HGSC (16.4%) and was highly correlated withRB1mRNA expression. RB1 loss was associated with longer OS in HGSC (hazard ratio [HR] 0.74, 95% confidence interval [CI] 0.66-0.83,P= 6.8 × 10-7), but with poorer prognosis in ENOC (HR 2.17, 95% CI 1.17-4.03,P= 0.0140). GermlineBRCAmutations and RB1 loss co-occurred in HGSC (P< 0.0001). Patients with both RB1 loss and germlineBRCAmutations had a superior OS (HR 0.38, 95% CI 0.25-0.58,P= 5.2 x10-6) compared to patients with either alteration alone, and their median OS was three times longer than non-carriers whose tumours retained RB1 expression (9.3 years vs. 3.1 years). Enhanced sensitivity to cisplatin (P< 0.01) and paclitaxel (P< 0.05) was seen inBRCA1mutated cell lines withRB1knockout. Among 126 patients with whole-genome and transcriptome sequence data, combinedRB1loss and genomic evidence of homologous recombination deficiency was correlated with transcriptional markers of enhanced interferon response, cell cycle deregulation, and reduced epithelial-mesenchymal transition in primary HGSC. CD8+ lymphocytes were most prevalent inBRCA-deficient HGSC with co-loss ofRB1.ConclusionsCo-occurrence of RB1 loss andBRCAmutation was associated with exceptionally long survival in patients with HGSC, potentially due to better treatment response and immune stimulation.