Abstract A058: Mesenchymal stem cells act as a selective pressure for High Grade Serous Ovarian Cancer initiation

Abstract High grade serous ovarian cancer (HGSOC) is the most common and lethal subtype of ovarian cancer accounting for 80-90% of diagnoses. Despite its high incidence and mortality, the mechanisms of cancer initiation are unclear. The majority of HGSOC originates in the fallopian tube epithelium (FTE) with support of underlying mesenchymal stromal/stem cells (MSCs). MSCs are stromal progenitor cells that play important roles in wound healing and, in the context of cancer, tumor progression. We showed that MSCs within the tumor microenvironment, termed cancer associated MSCs (CA-MSCs) harbor a unique, age-associated epigenetic signature that is essential for tumor growth and metastasis. Surprisingly, we identified rare subsets of MSCs from benign tubes that exhibit similar epigenetic changes to cancer educated CA-MSCs suggesting that the pro-oncogenic MSC phenotype exists prior to cancer initiation. These “high risk” MSCs (hrMSCs) increase with age, are enriched in BRCA mutation carrier patients, promote cancer cell proliferation, and most notably, elicit tumor formation in primary, non-cancerous FTE. To elucidate the mechanism of hrMSC-induced malignant transformation of FTE, we characterized the effect that hrMSCs have on primary FTE. HrMSCs promoted increased growth of FTE relative to normal/low risk MSCs (lrMSCs) derived from the same patient. These MSCs lack the epigenetic signature/transcriptome of hrMSCs/CA-MSCs. Intriguingly, we co-cultured FTE with hrMSCs at a 1: 1 ratio and discovered robust DNA damage in the form of DNA double strand breaks (53BP1 foci) in FTE at 24 hours. Furthermore, conditioned media from hrMSCs supported the recovery of FTE following damage with other DNA damaging agents. We hypothesize that the DNA damage function of hrMSCs serves as a selective pressure for mutation in FTE (1st hit). Additionally, we hypothesize that hrMSCs simultaneously provide growth signals to FTE allowing for evasion of DNA damage-induced cell death (2nd hit). Lastly, we hypothesize that both hrMSC functions are required for genesis of HGSOC. Citation Format: Geyon L. Garcia, Huda I. Atiya, Lan G. Coffman. Mesenchymal stem cells act as a selective pressure for High Grade Serous Ovarian Cancer initiation abstract. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84 (5 Suppl₂): Abstract nr A058.