ecDNA replication is disorganized and vulnerable to replication stress.

Extrachromosomal DNA (ecDNA) is a critical driver of cancer progression, contributing to tumour growth, evolution, and therapeutic resistance through oncogene amplification. Despite its significance, the replication of ecDNA remains poorly understood. In this study, we investigated the replication dynamics of ecDNA using high-resolution replication timing analysis (Repli-seq) and DNAscent, a method for measuring origin firing and replication fork movement, that we applied to both bulk DNA and to ecDNA isolated with FINE (Fluorescence-activated cell sorting-based Isolation of Native ecDNA), a new method for isolating, chromatinized ecDNA without DNA or protein digestion. We demonstrate that ecDNA in the COLO 320DM colorectal cancer cell line exhibits largely asynchronous replication throughout the S phase, contrasting with the conserved replication timing of the corresponding chromosomal DNA in RPE-1 cells and the chromosomally reintegrated ecDNA in COLO 320HSR. Replication origins on ecDNA are redistributed, and replication forks exhibit reduced velocity and increased stalling. Under replication stress induced by hydroxyurea treatment, ecDNA replication is further compromised, leading to altered origin activation, reduced fork velocity and eventual ecDNA depletion from cells. Our findings reveal fundamental differences in the replication dynamics of ecDNA, providing insights that could inform the development of therapies targeting ecDNA-associated oncogene amplification in cancer.