Breaking the vicious circle: Extrachromosomal circular DNA as an emerging player in tumour evolution.

Extrachromosomal circular DNA (ecDNA) or double minutes have gained renewed interest since its discovery more than five decades ago, emerging as potent drivers of tumour evolution. This has largely been motivated by recent discovery that the tumour-exclusive ecDNA are highly prevalent in almost all cancers unlike previously thought. EcDNAs contribute to elevated oncogene expression, intratumoural heterogeneity, tumour adaptation and therapy resistance independently of canonical chromosomal alterations. Importantly, ecDNAs play a critical role in patient survival as ecDNA-based oncogene amplification adversely affects clinical outcome to a significantly greater extent than intrachromosomal amplification. Chromothripsis, a major driver of ecDNA biogenesis and gene amplification, is a mutational process characterised by chromosomal shattering and localised complex genome rearrangement. Chemotherapeutic drugs can lead to chromothriptic rearrangements and therapy resistance. In this review, we examine how ecDNAs mediate oncogene overexpression, facilitate accelerated tumour malignancy and enhance rapid adaptation independently of linear chromosomes. We delve into discoveries pertaining to mechanisms of biogenesis, distinctive features of ecDNA, gene regulation and topological interactions with active chromatin. We also discuss the critical role of chromothripsis in engendering ecDNA amplification and evolution. One envisions that insights into ecDNA biology not only hold importance for the cancer genome and tumour evolutionary dynamics, but could also inform prognostication and clinical intervention, particularly for cancers characterised by high oncogene amplification.