• Chromosome instability (CIN) is prevalent and dynamic in high-grade serous ovarian cancer (HGSOC). • Quantitative imaging microscopy reveals 91% of ascites (26 samples) and 100% of solid tumours (36 samples) exhibit CIN. • CIN increases with disease progression and decreases following chemotherapy treatment. • Higher levels of CIN occur in solid tumour samples relative to patient-matched ascites samples. • Data suggest that cells with lower levels of CIN may be more amendable to metastatic spread.

High-grade serous ovarian cancer (HGSOC) is the most aggressive epithelial ovarian cancer subtype, and >75% of patients experience tumor recurrence following initial treatment, often with drug-resistant disease. Unfortunately, the aberrant events driving tumor progression, recurrence, and multidrug resistance in HGSOC remain largely unknown. Chromosome instability (CIN) is defined as an increased rate at which whole chromosomes (or large fragments) are gained or lost, and in many cancer types CIN is associated with tumor initiation, disease recurrence, multidrug resistance, and poor patient prognosis. Conceptually, the ongoing chromosome changes associated with CIN promote the production of genetically distinct daughter cells and drive cell-to-cell heterogeneity. As CIN remains largely unstudied in HGSOC, we now seek to characterize the prevalence and dynamics of CIN in chemonaïve, chemosensitive, and chemoresistant HGSOC patient samples, including samples isolated from ascites (metastatic cells) and solid tumors (primary tumor). Using single-cell quantitative imaging techniques, we evaluated CIN by assessing CIN phenotypes, including changes in nuclear areas and chromosome numbers. Changes in nuclear areas and increases in cell-to-cell heterogeneity are suggestive of CIN. Next, we employed fluorescence in situ hybridization and chromosome enumeration probes (CEPs) recognizing pericentric regions of chromosomes 8, 11, and 17 to quantify changes in chromosome numbers. Deviations from the expected number of two CEP foci/chromosome/nucleus (i.e., diploid state) are indicative of CIN. CEP specificity was validated within diploid and aneuploid fallopian tube (FT) secretory epithelial cell lines, a cell type of origin for HGSOC. The diploid FT cell lines show two CEPs/chromosome/nucleus and the aneuploid FT cell lines show an aberrant number of CEPs/chromosome/nucleus. CEPs are now being employed in HGSOC ascites samples and solid tumors within a HGSOC tissue microarray. Preliminary data show that nuclear areas and CEP numbers are heterogeneous and dynamic over time. More specifically, our data show CIN is highly prevalent in HGSOC cells isolated from ascites, increases with disease progression and drug resistance, and decreases in response to treatment. Additionally, we show that solid tumor samples (primary tumor) typically contain more frequent gains and/or losses in CEP foci (i.e., higher level of CIN) compared to the ascites samples (metastatic cells). Importantly, characterizing the prevalence of CIN in HGSOC will help researchers understand the association between CIN and tumor development, progression, response, and acquisition of drug resistance. Citation Format: Claire Morden, Ally Farrell, Mirka Sliwowski, Zelda Lichtensztejn, Mark Nachtigal, Kirk McManus. Characterizing chromosome instability in chemonaïve, chemosensitive, and chemoresistant high-grade serous ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr A20.