Targeted profiling of human extrachromosomal DNA by CRISPR-CATCH

Abstract

Extrachromosomal DNA (ecDNA) is a common mode of oncogene amplification but is challenging to analyze. Here, we present a method for targeted purification of megabase-sized ecDNA by combining in-vitro CRISPR-Cas9 treatment and pulsed field gel electrophoresis of agarose-entrapped genomic DNA (CRISPR-CATCH). We demonstrate strong enrichment of ecDNA molecules containing EGFR, FGFR2 and MYC from human cancer cells. Targeted purification of ecDNA versus chromosomal DNA enabled phasing of genetic variants and provided definitive proof of an EGFRvIII mutation on ecDNA and wild-type EGFR on chromosomal DNA in a glioblastoma neurosphere model. CRISPR-CATCH followed by nanopore sequencing enabled single-molecule ecDNA methylation profiling and revealed hypomethylation of the EGFR promoter on ecDNA compared to the native chromosomal locus in the same cells. Finally, separation of ecDNA species by size and sequencing allowed accurate reconstruction of megabase- sized ecDNA structures with base-pair resolution. CRISPR-CATCH is a new addition to the toolkit for studying focal amplifications in cancer and will accelerate studies aiming to explore the genetic and epigenetic landscapes of ecDNA.