Characterization of gRNA-dependent and gRNA-independent off-target binding sites of PspCas13b and RfxCas13d in mammalian cells.

CRISPR-Cas13 systems, harnessed for RNA-guided transcriptome editing, hold significant promise for clinical and in vivo therapeutic applications. However, understanding their in vivo target specificity and recognition rules remains a challenge. In this study, we employed the uSpyCLIP method, which enhances sensitivity and specificity for identifying RNA-binding protein (RBP) binding sites, to map the transcriptome-wide binding sites of catalytically inactive PspCas13b (dPspCas13b) and RfxCas13d (dRfxCas13d) in HEK293T cells, using a variety of single guide RNAs (gRNAs). Surprisingly, we identified both gRNA-dependent and gRNA-independent off-target binding sites for both dCas13 complexes. These gRNA-independent off-target sites exhibited distinct RNA structural and sequence signatures: dPspCas13b's gRNA-independent binding was associated with specific RNA structural features, while dRfxCas13d's was linked to unique sequence motifs. Analysis of gRNA-dependent off-target sites revealed the crucial role of the DR-distal and middle regions of the gRNA in determining binding specificity. Further analysis demonstrated that some off-target binding events led to changes in gene expression at the messenger RNA and/or protein level. Collectively, our findings provide important insights into the characteristics of gRNA-dependent and gRNA-independent off-target binding for PspCas13b and RfxCas13d, offering valuable guidance for optimizing Cas13 and gRNA design in future applications.