# Characterization of Extrachromosomal Circular DNA in Primary and Cisplatin-Resistant High-Grade Serous Ovarian Cancer

## TL;DR

This study explores how extrachromosomal circular DNA contributes to cisplatin resistance in ovarian cancer, identifying a key role for HIF1A-carrying ecDNA.

## Contribution

The study identifies a specific HIF1A-carrying ecDNA linked to cisplatin resistance in high-grade serous ovarian cancer.

## Key findings

- A large ecDNA carrying HIF1A was found in the SKOV3 cisR cell line, contributing to cisplatin resistance.
- Most eccDNAs were less than 1000 bp and located in non-coding repetitive elements.
- CisR cell lines showed significant differences in eccDNA numbers on chromosome 21 and enriched pathways related to drug resistance.

## Abstract

Background: Cisplatin resistance is a major cause of tumor recurrence and mortality in high-grade serous ovarian cancer (HGSOC). Extrachromosomal circular DNA (eccDNA) has emerged as a critical factor in tumor evolution and drug resistance. However, the specific contribution of eccDNA to cisplatin resistance in HGSOC remains unclear. Methods: We performed whole-genome sequencing, Circle-Seq, and RNA-Seq in four pairs of primary and cisplatin-resistant (cisR) HGSOC cell lines to characterize genome-wide eccDNA distribution and features. Functional enrichment analyses were subsequently conducted on differentially expressed eccDNA-related genes. Results: In the SKOV3 cisR cell line, we identified a large extrachromosomal circular DNA (ecDNA) carrying the HIF1A gene, which regulates DNA repair, drug efflux, and epithelial–mesenchymal transition, contributing to cisplatin resistance. Using Circle-Seq, we detected a total of 161,062 eccDNAs, most of which were less than 1000 bp and distributed across all chromosomes. Notably, the number of eccDNAs on chromosome 21 differed significantly between the primary and cisR cell lines. Additionally, eccDNAs were predominantly located in non-coding repetitive elements. Functional analysis of eccDNA-related differentially expressed genes revealed that, compared to primary cell lines, cisR cell lines were associated with mitotic spindle assembly, regulation of vascular permeability, and cell differentiation. eccDNA-related genes involved in these pathways include MISP, WIPF1, RHOD, KRT80, and PLVAP. Conclusions: Our findings suggest that eccDNAs, particularly ecDNA amplifications like HIF1A, contribute significantly to cisplatin resistance mechanisms in HGSOC. These insights highlight eccDNA as a potential target for overcoming therapeutic resistance and improving treatment outcomes in ovarian cancer.