CRaTER enrichment for on-target gene-editing enables generation of variant libraries in hiPSCs

Generation of transgenic cell lines is limited by inefficient gene editing requiring genotypic screening of hundreds to thousands of colonies to isolate correctly gene-edited cells. Here, we describe a novel method called CRISPRa On-Target Editing Retrieval (CRaTER) that enriches for cells with on-target knock-in of a promoterless cDNA-fluorescent reporter transgene by transiently overexpressing the targeted endogenous genetic locus and sorting for fluorescent cells. We use CRaTER to enrich for rare cells with heterozygous, biallelic-editing of the endogenous, transcriptionally-inactive MYH7 locus in human induced pluripotent stem cells (hiPSC), resulting in a 9-fold enrichment compared to antibiotics selection alone. We leveraged CRaTER to enrich for heterozygous knock-in of a library of single nucleotide variants (SNV) in MYH7, a gene encoding for sarcomeric MHC-ß wherein autosomal dominant missense mutations cause cardiac and skeletal myopathies. CRaTER enabled 90% enrichment of heterozygous, biallelically-edited hiPSCs – a 38.6-fold enrichment compared to antibiotics selection alone – to generate 113 SNVs comprising 78 missense variants in MYH7. hiPSCs that have undergone CRaTER enrichment can differentiate to cardiomyocytes and exhibit expected localization and expression of MHC-ß fusion proteins. Together, CRaTER substantially reduces the screening required for isolation of gene-edited cells, enabling the generation of transgenic cell lines at unprecedented scale. Overall design: pDNA, hiPSC, and cardiomyocyte MYH7 variant libraries