Accurate detection of off-target activity in primary human cells is crucial for ensuring the safety of gene therapies, yet existing methods often lack sufficient sensitivity. To address this limitation, we develop Tracking-seq2, an advanced technology that integrates exogenous 5′ → 3′ exonuclease treatment and non-homologous end joining (NHEJ) pathway inhibitors with the original Tracking-seq. Tracking-seq2 exhibits enhanced sensitivity in profiling off-target sites of diverse genome editors—including Cas9, Cas12a, cytosine base editors (CBEs), adenine base editors (ABEs), and prime editors (PEs). Critically, Tracking-seq2 is directly applicable to clinically relevant primary human cell types, such as T cells and CD34
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hematopoietic stem and progenitor cells (HSPCs). Furthermore, our findings reveal that genomic variations drive distinct off-target heterogeneity across different individuals, highlighting the necessity for personalized safety assessment in clinical genome editing applications. Tracking-seq2 provides a robust platform for sensitive off-target detection in primary cells, with sensitivity comparable to or exceeding current state-of-the-art methods.
Base editors hold great promise in endogenous mutagenesis for genetic screening. However, the development of base editors that induce saturated multi-base conversions with diverse mutation spectrum is challenging. Here, we develop triple base editors (smACGs) that simultaneously mutagenize adenine, cytosine, and guanine within the same allele. Through screening and embedding engineered deaminase and alkyladenine DNA glycosylase variants in Cas9 structure, smACGmax is generated to catalyze robust triple-base conversion efficiencies of up to 41% across varied sequence contexts while maintaining low RNA off-target effects compared to previous dual-base editors. We apply smACGmax to enable high coverage (94%) of targeted HBEGF mutagenesis that identified diphtheria toxin-resistant mutations and to dissect SF3B1 variants with alternative splicing specificity via complex single, double, and triple base conversion screening. smACGmax expands base conversion capability from single and double substrates to trinucleotide level, which facilitates the generation of high-diversity and complex genetic variants, providing a useful platform for mutagenesis-based application. Broad-spectrum base mutagenesis at the same endogenous loci with base editors remains a challenge. Here, the authors developed smACGmax to catalyze efficient multi-base conversions across adenine, cytosine and guanine, and enable high-diversity functional screening in HBEGF and SF3B1 variants.