Chromosomal Instability as a Driver of cGAS-STING Dysfunction in High Grade Serous Ovarian Cancer

ABSTRACT

Chromosomal instability (CIN) is defined as the continual gain or loss of chromosome fragments or whole chromosomes and is a feature of high grade serous ovarian cancer (HGSOC). Numerous drivers of CIN in HGSOC have been identified including homologous recombination repair deficiency (HRD). One consequence of ongoing CIN is the accumulation of cytoplasmic self-DNA which promotes the activation of DNA sensing pathways such as cGAS-STING. Acute cGAS-STING activation by DNA damaging agents has been associated with immune activation and synergy with immunotherapy. Recently, several publications have demonstrated a pro-tumorigenic role for chronic cGAS-STING signalling in CINhigh models of various cancers. The consequences of chronic cGAS-STING activation in homologous recombination repair deficient (HRD) HGSOC remains unexplored. Therefore, we investigated the differences in cGAS-STING functionality in CINlow and CINhigh HGSOC-representative murine cell lines, in addition to evaluating the prevalence of cGAS-STING dysfunction in panels of human HGSOC cell lines. CINhigh HSGOC murine cells demonstrated reduced basal STING mRNA and protein expression in comparison to CINlow cells. As a consequence, CINhigh cells failed to induce PD-L1 and cytokines in response to STING agonism. This phenotype was mimicked in CINlow cell lines through repetitive STING agonism. We hypothesised that STING downregulation would be reversed upon alleviation of basal cGAS activation in CINhigh cells and indeed, evidence of this was observed upon cGAS knockout. However, this phenotype was not consistent across knockout clones, which suggests heterogeneity within the parental population with regard to how STING is being downregulated. Previously published CIN-induced cGAS-STING phenotypes, including metastasis-related and NFκB-related signatures, were not recapitulated in the CINhigh murine model. In human HGSOC cell lines, extensive cGAS-STING dysfunction was observed irrespective of CIN rate, which suggests that CIN is not the sole contributing factor to cGAS-STING dysfunction in HGSOC cell line models. Overall, it was determined that cGAS-STING dysfunction is apparent in CINhigh mouse and human cell line models of HGSOC, though important differences from previous studies using different mechanistic drivers of CIN were noted. Future work aims to explore the mechanisms mediating STING suppression in the CINhigh models, and to produce further CINhigh models to evaluate if cGAS-STING dysfunction becomes apparent over time as CIN increases.