Formation and Structure of ecDNA in Cancer Cells

ecDNA is crucial to understanding intratumoral heterogeneity, carcinogenesis, and therapeutic resistance in oncogenic cancer biology. They are often characterized by circular, acentric, and telomeric topology, allowing them to avoid typical chromosomal limits to host and amplify oncogenes, including MYC, EGFR, and CDK4, sometimes with enhancer regions that promote transcriptional hyperactivity. This chapter covers cancer cell ecDNA biogenesis, structure, and function. Genomic instability events like chromothripsis, breakage–fusion–bridge (BFB) cycles, and replication stress-induced fork collapse, facilitated by error-prone DNA repair pathways like NHEJ and MMEJ, form ecDNA. For structural reasons, ecDNAs have more active histone modifications (e.g., H3K27ac, H3K4me3), which make the chromatin more accessible and allow for transcriptional condensates and super-enhancer hijacking. Their non-Mendelian inheritance via random mitotic segregation promotes clonal diversity and adaptive evolution, underpinning phenotypic plasticity and therapeutic resistance. Furthermore, ecDNAs modulate immune evasion via the cGAS-STING pathway, and their presence in the cytoplasm or micronuclei contributes to pro-inflammatory signaling and immune checkpoint upregulation. Advanced detection methods—such as Circle-Seq, long-read sequencing, Hi-C, and live-cell imaging—offer unprecedented insights into the dynamics and topology of ecDNA. A noninvasive biomarker for disease burden, relapse prediction, and treatment stratification is circulating ecDNA. Promising treatments that address ecDNA replication and chromatin defects include ATR, BET, and CRISPR-mediated disruption. Thus, ecDNA drives malignancy rather than just being a product of a genomic disorder.