Identification and validation of drug-resistant mutations can provide important insights into the mechanism of action of a compound. Here we demonstrate the feasibility of such an approach in mammalian cells using next-generation sequencing of drug-resistant clones and CRISPR-Cas9-mediated gene editing on two drug-target pairs, 6-thioguanine-HPRT1 and triptolide-ERCC3. We showed that disrupting functional HPRT1 allele or introducing ERCC3 point mutations by gene editing can confer drug resistance in cells.

The development of custom-designed nucleases (CDNs), including zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), has made it possible to perform precise genetic engineering in many cell types and species. More recently, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system has been successfully employed for genome editing. These RNA-guided DNA endonucleases are shown to be more efficient and flexible than CDNs and hold great potential for applications in both biological studies and medicine. Here, we review the progress that has been made for all three genome editing technologies in modifying both cells and model organisms, compare important aspects of each approach, and summarize the applications of these tools in disease modeling and gene therapy. In the end, we discuss future prospects of the field.