xCas9: CRISPR gets easy-going

pam sequenceCRISPR needs to anchor itself near a short sequence called PAM to do its job. In the book “Modern Prometheus” (Cambridge University Press) James Kozubek says a PAM is like a shoehorn, where the Cas9 nuclease begins to clasp down to recognize the right site and cut. In order to fit every gene, a super-adjustable shoehorn would be needed. Think of it as the equivalent of a bump key that can open any door. A Broad Institute group led by David Liu has almost reached the goal with xCas9, the new super-adjustable Cas9 variant described in Nature this week. Continue reading

Adding the RNA string to the CRISPR bow


So far we have learned that CRISPR may turn a faulty gene off by cutting and mutating its sequence. But what if we want to proceed more cautiously and avoid permanent changes to the genome? We could leave the target gene intact but ineffective, by intercepting and destroying the RNA messages with which it gives the wrong orders to the diseased cells. In this way it would be easier to go back if necessary. The good news is that CRISPR is a jack-of-all-trades, well-suited for the task, and the new approach (call it RNA targeting with CRISPR) is going to help to study human biology and diseases. One of the technique pioneer, Feng Zhang, has demonstrated in Nature last week that it can efficiently target RNA in mammalian cells (and also plants), equalizing and even surpassing the performance of the current tool of choice for RNA knockdown (RNA interference). In short, besides advancing its career as DNA editor, CRISPR has also found a second job in the RNA business. Continue reading