Off-target genomic DNA site (Off-target site)

Partially complementary genomic DNA sites with compatible PAMs are sequences within the genome that resemble the intended target of a CRISPR-Cas system but contain one or more mismatches. These sites are characterized by the presence of a Protospacer Adjacent Motif (PAM) that allows the Cas nuclease to initiate binding, followed by sufficient base-pairing with the guide RNA (gRNA) to trigger DNA cleavage or modification (Hsu et al., 2013, Nature Biotechnology). In the context of therapeutic gene editing, these locations are considered off-target sites and represent a major safety hurdle rather than a desired therapeutic target. Unintended activity at these sites can result in permanent genetic alterations, including deletions, insertions, or translocations, which may disrupt tumor suppressor genes or activate oncogenes (Fu et al., 2013, Nature Biotechnology). These sites are often identified through bioinformatic algorithms that score the likelihood of Cas9 binding based on the number and position of mismatches (Zhang et al., 2015, Genome Biology). Experimental methods like GUIDE-seq or CIRCLE-seq are used to empirically detect these sites in a cell-type-specific manner (Tsai et al., 2015, Nature Biotechnology). Minimizing the frequency of interactions at these sites is critical for the clinical translation of CRISPR-based therapies. Strategies to reduce off-target effects include the use of high-fidelity Cas variants and optimized guide RNA designs.

Other names

Off-target sequenceMismatched DNA siteNon-specific binding siteCRISPR off-targetPartially complementary DNA site

Unintended RNA-guided DNA cleavage or modification at sequences with partial homology to the guide RNA.