The Quest to Make CRISPR Even More Precise
By contrast, CRISPR, the youngest technique on the block, is cheaper, more versatile, and more precise than its predecessors. And scientists are racing to improve it even further, developing new versions that are even more efficient, that can subtly change the emphasis of genetic words rather than deleting them outright, and that make fewer mistakes.
CRISPR consists of two components: An enzyme called Cas9—a pair of molecular scissors that grabs DNA and cuts it into two; and a guide RNA—a molecule that tells Cas9 exactly what to cut.
They also raise serious questions about proposed (speculative) uses for the technique, such as editing the genomes of embryos to prevent inherited disorders. If you’re going to try that, you’d better make sure that you’re not inadvertently activating a cancer gene or disrupting an essential one. Indeed, when a Chinese team recently (and controversially) used CRISPR to edit a disease gene in (inviable) human embryos, they found surprising and worrying levels of off-target cuts.
That’s why the many position statements about the ethics of CRISPR have universally highlighted the risks of off-target cuts, and the need to identify, understand, and avoid them. And it’s why several groups of scientists have been trying to develop ways of making CRISPR more specific, so that they make fewer off-target cuts.
CRISPR pioneers Feng Zhang and George Church are trying to change the Cas9 editor itself. First, they created half-hearted versions. The usual enzyme checks a DNA sequence against its guide RNA and then cuts both strands, but the mutant versions cut just one strand. So you need two of them, checking their own guide RNAs, to fully sever a stretch of DNA. It’s unlikely that both enzymes will get things wrong, so together, they become more specific.
Today, Zhang has unveiled yet another strategy for engineering Cas9. First, his team searched for mutations that will make Cas9 more discerning in its attacks, so that it only cuts DNA that perfectly matches its guide RNA template. They found five, which make the enzyme more specific but no less efficient. They then tested these mutations in combinations, until they settled on one particularly judicious version of Cas9, which they called esCas9 (“enhanced specificity Cas9”). This upgraded Cas9 cuts its targets just as well as Cas9 Classic, but never veers off target—at least, not that the team could detect. It’s a precision weapon that inflicts no collateral damage.
Meanwhile, Keith Joung noted that many teams, including his own, have developed ways of measuring off-target cuts. What’s missing is a way of comparing these methods against each other to see which performs best; without that, there’s no yardstick with which to gauge how common these problems are and how well the solutions are actually dealing with it. Partly, that’s because the field is moving very quickly and it would take time for each group to master the others’ methods. “I’m hoping that a company will compare these methods, so they can go to regulators,” he says.
http://www.theatlantic.com/science/archive/2015/12/who-edits-the-gene-editors/418209/