Kinetic basis for Cas12a off-target discrimination
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CRISPR-Cas12a proteins are RNA-guided endonucleases classified as type V-A effectors that function similarly to Cas9, but possess distinct biochemical features. Previous studies have reported that compared to Cas9, Cas12a exhibits reduced off-target activity, yet the mechanistic origin of this high specificity remains unclear. In this study, we used singlemolecule fluorescence assays to investigate the kinetic basis for the reduced off-target effects of Cas12a. Introducing double mismatches at various positions within the target DNA enabled systematic analysis of the off-target effects on individual reaction steps in the Cas12a-mediated DNA cleavage reaction: seeding, stable R–loop formation, and DNA cleavage. Our results show that mismatches within a 17 bp PAM–proximal seed region significantly impair stable R–loop formation and subsequent cleavage, whereas mismatches in the PAM–distal region exert minimal or negligible effects. These results suggest that the low off-target tolerance of Cas12a and the resulting high on-target selectivity arise from the high sensitivity of the R–loop formation rate to DNA mismatches in the PAM–proximal region, which strongly correlates with cleavage efficiency. This work establishes R–loop formation as a conformational checkpoint for specific target cleavage, and provides a mechanistic framework to improve the fidelity of genome editing.
Abstract
CRISPR-Cas12a proteins are RNA-guided endonucleases classified as type V-A effectors that function similarly to Cas9, but possess distinct biochemical features. Previous studies have reported that compared to Cas9, Cas12a exhibits reduced off-target activity, yet the mechanistic origin of this high specificity remains unclear. In this study, we used singlemolecule fluorescence assays to investigate the kinetic basis for the reduced off-target effects of Cas12a. Introducing double mismatches at various positions within the target DNA enabled systematic analysis of the off-target effects on individual reaction steps in the Cas12a-mediated DNA cleavage reaction: seeding, stable R–loop formation, and DNA cleavage. Our results show that mismatches within a 17 bp PAM–proximal seed region significantly impair stable R–loop formation and subsequent cleavage, whereas mismatches in the PAM–distal region exert minimal or negligible effects. These results suggest that the low off-target tolerance of Cas12a and the resulting high on-target selectivity arise from the high sensitivity of the R–loop formation rate to DNA mismatches in the PAM–proximal region, which strongly correlates with cleavage efficiency. This work establishes R–loop formation as a conformational checkpoint for specific target cleavage, and provides a mechanistic framework to improve the fidelity of genome editing.