Sunday, October 16, 2016

How Good is CRISPR?

We have been following CRISPR technology for the past three years. Logically it makes sense. The CRISPR guide RNA targets a sequence and the Cas 9 or equivalent cuts it. There are few issues however. One is it creates a double stranded break which often creates a worse situation than before. Second, the RNA target may not find the sport we sought but some other identical but wrong sequence.

As noted in STAT:

As always, what worked in mice might not in patients. A constant concern with CRISPR is that it edits genes it isn’t supposed to, because the guide RNA mistakes a healthy region of DNA for the mutation. Testing the most likely of these “off-target” sites, the scientists found that the one that was mistakenly CRISPR’d the most often wasn’t a gene at all, or even near any genes. Other off-target sites were CRISPR’d in fewer than 0.10 percent of cells. But even that low level of error might be dangerous, perhaps triggering a cancer-causing gene, so Corn and his team are running more animal studies of whether their CRISPR approach will be safe.

 In a recent Science Translational Medicine they note in applying this to blood disorders:

Genetic diseases of blood cells are prime candidates for treatment through ex vivo gene editing of CD34+ hematopoietic stem/progenitor cells (HSPCs), and a variety of technologies have been proposed to treat these disorders. Sickle cell disease (SCD) is a recessive genetic disorder caused by a single-nucleotide polymorphism in the β-globin gene (HBB). Sickle hemoglobin damages erythrocytes, causing vasoocclusion, severe pain, progressive organ damage, and premature death. We optimize design and delivery parameters of a ribonucleoprotein (RNP) complex comprising Cas9 protein and unmodified single guide RNA, together with a single-stranded DNA oligonucleotide donor (ssODN), to enable efficient replacement of the SCD mutation in human HSPCs. Corrected HSPCs from SCD patients produced less sickle hemoglobin RNA and protein and correspondingly increased wild-type hemoglobin when differentiated into erythroblasts. When engrafted into immunocompromised mice, ex vivo treated human HSPCs maintain SCD gene edits throughout 16 weeks at a level likely to have clinical benefit. These results demonstrate that an accessible approach combining Cas9 RNP with an ssODN can mediate efficient HSPC genome editing, enables investigator-led exploration of gene editing reagents in primary hematopoietic stem cells, and suggests a path toward the development of new gene editing treatments for SCD and other hematopoietic diseases.

 Thus there is potential but also a concomitant risk.