Professor David Liu, professor of chemistry and chemical biology at Harvard University in Cambridge, Massachusetts, told the Los Angeles Times: 'Most human genetic variants that cause disease are point mutations – a single base-pair that, for whatever reason, has been changed to a different base-pair. So we set out to take a new approach to genome editing that would be especially good at correcting point mutations.'
Writing in Nature, the team described how they set out to improve the CRISPR/Cas9 technique, in which a whole section of the double-stranded DNA helix is cut out and replaced.
The whole genome consists of base-pairs – A (adenine) always pairs with T (thymidine), and C (cytosine) always pairs with G (guanine). Professor Liu and colleagues were able to nick just a single strand of the DNA helix, and then turn a single cytosine (C) base into a uracil (U) base, using an enzyme called APOBEC1. They call their technique 'base editing'.
Since U is very similar to T, following replication or DNA repair it pairs up with an (A) on the other DNA strand, effectively converting the G into an A. (The U is replaced with a T during this process.)
Thus, a C:G base-pair is permanently converted into an A:T base-pair.
However, the repair mechanism does not always work in the desired way. Sometimes the cell converts the U back into a C. 'When we first sketched this idea out on paper … I knew the cell was going to fight this change as hard as it could,' said Professor Liu. His team overcame this hurdle by fusing a small protein to the Cas9/APOBEC1 pair, preventing the U being converted back into a C.
Although the system only worked on two out of 12 occasions, the team is hopeful that it could be improved to the point where it could one day be used to correct disease-causing mutations. They are also researching ways to turn an A:T base-pair into a C:G base-pair.
Dr Lei Stanley Qi, assistant professor of bioengineering and chemical and systems biology at Stanford University, California, who was not involved in the study, told Nature News: 'This method could be much simpler than the old CRISPR method one, which requires inserting a DNA template that is prone to degradation and was sometimes even toxic to cells. This avoids a major barrier. It really expands the scope of CRISPR's applications.'