Howard Hughes Medical Institute (HHMI) Investigator David Liu and colleagues have created a new enzyme that can directly change a DNA base pair from an adenine thymine pair (A●T) to a guanine cystine pair (G●C). The work was published in yesterday's Nature.
The new system is a "really exciting addition to the genome engineering toolbox," says Feng Zhang, an HHMI-Simons Faculty Scholar and molecular biologist at the Broad Institute of MIT and Harvard, who was not involved in the study. "It's a great example of how we can harness natural enzymes and processes to accelerate scientific research." The team has created a tool called a base editor to add to that genome engineering toolbox. These base editors have the capability to change A●T to G●C.
When starting the project, Liu and his team knew it was going to be tough because they had to create an enzyme that didn't exist. Nicole Gaudelli, a postdoctoral researcher in the group, started with an enzyme called TadA that was able to convert adenine to a molecule called inosine (which cells treat as guanine), but in transfer RNA rather than in DNA. She then created libraries of TadA mutants in bacteria and required them to convert A to inosine in antibiotic-resistant genes so that they would survive against the presence of antibiotics. Those bacteria that survived had the TadA mutations that were capable of performing the adenine-to-inosine conversion on DNA. The researchers saw some bacterial colonies were able to fix their own mutations with chemical surgery and survive the antibiotic challenge.
Then the researchers decided to tweak the enzyme more and attach a Cas9 nickase. That add-on would allow the base editor to find it's target and snip the opposing strand of DNA, thus prompting the cell to complete the swap of A●T to G●C.
Currently, the team's most efficient enzyme is called ABE7.10, which is capable of performing the swap in both human and bacterial genomes. It operates with more than 50 percent efficiency and few undesired byproducts if any.
Mutations where G●C mutates into an A●T account for a large number of single point mutation associated diseases. This study provides promise to addressing those and there are still many other questions that need to be addressed before it can be used in human therapeutics.
Image: A newly created DNA base editor contains an atom-rearranging enzyme (red) that can change adenine into inosine (read and copied as guanine), guide RNA (green) which directs the molecule to the right spot, and Cas9 nickase (blue), which snips the opposing strand of DNA and tricks the cell into swapping the complementary base. Image courtesy of Gaudelli et al./ Nature 2017,