An NGS-based approach for precise and footprint-free CRISPR-based gene editing in human stem cells.

Precise gene editing with conventional CRISPR/Cas9 is often constrained by low knock-in (KI) efficiencies (≈ 2-20 %) in human induced pluripotent stem cells (hiPSCs) and human embryonic stem cells (hESCs). This limitation typically necessitates labour-intensive manual isolation and genotyping of hundreds of colonies to identify correctly edited cells. Fluorescence- or antibiotic-based enrichment methods facilitate the identification process but can compromise cell viability and genomic integrity. Here, we present a footprint-free editing strategy that combines low-density seeding with next-generation sequencing (NGS) to rapidly identify cell populations containing precisely modified clones. By optimising the transfection workflow and adhering to CRISPR/Cas9 KI design principles, we achieved high average editing efficiencies of 64 % in hiPSCs (introducing a Brugada syndrome-associated variant) and 51 % in hESCs (introducing a neurodevelopmental disorder (NDD)-associated variant). Furthermore, under suboptimal CRISPR design conditions, this approach successfully identified hESC clones carrying a second NDD-associated variant, despite average KI efficiencies below 1 %. Importantly, genomic integrity was preserved throughout subcloning rounds, as confirmed by Sanger sequencing and single nucleotide polymorphism (SNP) array analysis. Hence, this NGS-based enrichment strategy reliably identifies desired KI clones under both optimal and challenging conditions, reducing the need for extensive colony screening and offering an effective alternative to fluorescence- and antibiotic-based selection methods.