The gene-editing tool CRISPR may one day change the way humans approach medicine — or at least that’s how it’s been portrayed so far. But for all the talk of using CRISPR to eliminate disease, the method was never very good at doing one important thing: altering single letters of DNA. (DNA is made of four chemical units, represented by the letters A, T, G, and C.) Now, scientists at Harvard University say they've modified the CRISPR method so it can be used to effectively reverse mutations involving changes in one letter of the genetic code. That’s important because two-thirds of genetic illness in humans involve mutations where there’s a change in a single letter.
Ref: Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature (20 April 2016) | DOI: 10.1038/nature17946
Current genome-editing technologies introduce double-stranded (ds) DNA breaks at a target locus as the first step to gene correction. Although most genetic diseases arise from point mutations, current approaches to point mutation correction are inefficient and typically induce an abundance of random insertions and deletions (indels) at the target locus resulting from the cellular response to dsDNA breaks. Here we report the development of ‘base editing’, a new approach to genome editing that enables the direct, irreversible conversion of one target DNA base into another in a programmable manner, without requiring dsDNA backbone cleavage or a donor template. We engineered fusions of CRISPR/Cas9 and a cytidine deaminase enzyme that retain the ability to be programmed with a guide RNA, do not induce dsDNA breaks, and mediate the direct conversion of cytidine to uridine, thereby effecting a C→T (or G→A) substitution. The resulting ‘base editors’ convert cytidines within a window of approximately five nucleotides, and can efficiently correct a variety of point mutations relevant to human disease. In four transformed human and murine cell lines, second- and third-generation base editors that fuse uracil glycosylase inhibitor, and that use a Cas9 nickase targeting the non-edited strand, manipulate the cellular DNA repair response to favour desired base-editing outcomes, resulting in permanent correction of ~15–75% of total cellular DNA with minimal (typically ≤1%) indel formation. Base editing expands the scope and efficiency of genome editing of point mutations.