CRISPR Accuracy and Precision

A recent paper by Chakrabarti et al notes that:

• The outcome of CRISPR-mediated editing can be predicted
• Not all target sites are edited in a predictable manner
• The precision of DNA editing is mainly determined by the fourth nucleotide upstream of the PAM site
• Chromatin states affect editing of imprecise, but not precise, target sites

In the corresponding Eureka article they note:

Guided by the RNA molecule, the Cas-9 enzyme scans along the genome until it finds the region of interest. When the RNA guide matches the correct DNA sequence, it sticks like Velcro and Cas9 cuts through the DNA. The DNA is broken three letters from the end of the target sequence, and bits of genetic code are then inserted or deleted, seemingly haphazardly, when the cell attempts to repair the break. In this study, the researchers found that the outcome of a particular gene edit depends on the fourth letter from the end of the RNA guide, adjacent to the cutting site. The team discovered that if this letter is an A or a T, there will be a very precise genetic insertion; a C will lead to a relatively precise deletion and a G will lead to many imprecise deletions. Thus, simply avoiding sites containing a G makes genome editing much more predictable.

This means that precision and accuracy may be attained. It has been known that CRISPRs seek certain defined sites on the DNA but there may be multiple sites or sites blocked by closed chromatin. This appears to remedy the issue.