Selection of Extended CRISPR RNAs with Enhanced Targeting and Specificity

As CRISPR effectors like Cas9 increasingly enter clinical trials for therapeutic gene editing, a future for personalized medicine will require efficient methods to protect individuals from the potential of off-target mutations that may also occur at specific sequences in their genomes that are similar to the therapeutic target. A Cas9 enzyme's ability to recognize their targets (and off-targets) are determined by the sequence of their RNA-cofactors (their guide RNAs or gRNAs). These are the sequencing results of a screen of hundreds of thousands of gRNA variants with short, randomized 5' nucleotide extensions near its DNA-targeting segment--a modification that can increase gene editing specificity by orders of magnitude--to identify extended gRNAs (x-gRNAs) that effectively block any activity at those off-target sites while still maintaining strong activity at their intended targets to make CRISPR therapies safer.