Abstract
Extrachromosomal circular DNA (eccDNA) of chromosomal origin is common in eukaryotic cells. Amplification of oncogenes on large eccDNA (ecDNA) can drive biological processes such as tumorigenesis, and identification of eccDNA by sequencing after removal of chromosomal DNA is therefore important for understanding their impact on the expressed phenotype. However, the circular mitochondrial DNA (mtDNA) might challenge the detection of eccDNA because the average somatic cell has hundreds of copies of mtDNA. Here we show that 61.2-99.5% of reads from eccDNA-enriched samples correspond to mtDNA in mouse tissues. We have developed a method to selectively remove mtDNA from total circular DNA by CRISPR/Cas9 guided cleavage of mtDNA with one single-guide RNA (sgRNA) or two sgRNAs followed by exonuclease degradation of the linearized mtDNA. Sequencing revealed that mtDNA reads were 85.9% ± 12.6% removed from eccDNA of 9 investigated mouse tissues. CRISPR/Cas9 cleavage also efficiently removed mtDNA from a human HeLa cell line and colorectal cancer samples. We identified up to 14 times more, and also larger eccDNA in CRISPR/Cas9 treated colorectal cancer samples than in untreated samples. We foresee that the method can be applied to effectively remove mtDNA from any eukaryotic species to obtain higher eccDNA yields.