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David Liu’s lab created the first base editor in 2016 (Komor et al., 2016) and since then has been trying to expand their precision editing capabilities. Base editors make specific DNA base changes and consist of a catalytically impaired Cas protein (dCas or Cas nickase) fused to a DNA-modifying enzyme, in this case a deaminase. Base changes from C•G-to-T•A are mediated by cytosine base editors (CBEs) and base changes from A•T-to-G•C are mediated by adenine base editors (ABEs). How does this work? Through molecular biology teamwork. The guide RNA (gRNA) specifies the editing target site on the DNA, the Cas domain directs the modifying enzyme to the target site, and the deaminase induces the DNA base change without a DNA double-strand break. But base editors aren’t perfect. They may be slow, can only target certain sites, or make only a subset of base substitutions. 

There are over 75,000 pathogenic genetic variants that have been identified in humans and catalogued in the ClinVar database. Previously developed genome editing methods using nucleases and base editors have the potential to correct only a minority of those variants in most cell types. A new technique from David Liu’s lab at the Broad Institute could add more precision and flexibility to the CRISPR editing world.

This post was contributed by Kutubuddin Molla, a Fulbright Visiting Scholar at the Pennsylvania State University.

Imagine you are dealing with a defective gene, Xm, the sequence of which is identical to the correct gene, Xw, except for a single base. If you heard about CRISPR, one question probably comes to mind: can CRISPR be applied to fix the defective base precisely?

This post was updated on Nov 27, 2017.

When we talk about CRISPR applications, one negative always comes up: the low editing efficiency of homology-directed repair (HDR). Compared to non-homologous end joining, HDR occurs at a relatively low frequency, and in nondividing cells, this pathway is further downregulated. Rather than try to improve HDR, Addgene depositor David Liu’s lab created new Cas9 fusion proteins that act as base editors. These fusions contain dCas9 or Cas9 nickase and a cytidine deaminase, and they can convert cytosine to uracil without cutting DNA. Uracil is then subsequently converted to thymine through DNA replication or repair. Later work has improved base editor targeting flexibility and specificity. New adenine base editors also allow you to change adenine to inosine, which is then converted to guanine.