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.
The technology’s main strength is its easy programmability, meaning that any gene can be pinpointed by a custom-made guide. To understand how the process works, just imagine a molecular Swiss army knife, provided with a positioning compass, a vise to grip the target and scissors to cut it. Researchers at the Broad Institute in Boston have modified a bit the essential equipment. Instead of the traditional DNA-cutting scissors (Cas9), Zhang and colleagues introduced an RNA-cutting enzyme (Cas13). They tested about fifteen variants belonging to different microorganisms, then selected the best from Leptotrichia wadei. Therefore they used this tiny molecular machine to reduce the expression of three genes associated with cancer. Zhang’s team has also developed a new biotech tool intended for RNA visual tracking. By disabling the molecular scissors and by equipping CRISPR with a fluorescent tag, they could follow the translocation of selected RNA inside the cell.
Another group at the UC San Diego is already repurposing CRISPR to destroy toxic RNA building up in diseases like amyotrophic lateral sclerosis, Huntington and some types of muscular dystrophy. A therapy targeting RNA would probably require repeat treatments over the years, while a one-time procedure could suffice with DNA editing. But Gene Yao, who presented promising results in Cell in August, thinks the RNA approach could be safer, and its temporary effect may be hopefully extended long enough with the right tricks. According to the MIT Technology Review, he estimates that more than 20 genetic diseases caused by toxic RNA repeats could potentially be treated this way.