An article published in the journal Nature Biotechnology this Monday reported that base editing procedure can now be more effective and efficient with the help of a newer version of CRISPR–Cas9 system. The CRISPR genome-editing tool is now better than before.
The scientists working on the project explained that with this better technique, the accuracy of the system has increased and now DNA can be precisely targeted without the addition of unwanted mutations.
Clustered regularly interspaced short palindromic repeats or CRISPR are DNA sequences found in genomes of prokaryotes. These DNA sequences are from the bacteriophages that have previously infected the prokaryotic organism. They help detect and defeat subsequent infections from similar bacteriophages. Cas9 (or “CRISPR-associated protein 9”) is an enzyme that uses CRISPR sequences as guides and cuts the DNA strands that are complementary to that sequence.
So, the CRISPR-Cas9 system can be used to edit genes within organisms. This technique has different practical applications like biological research, treatment of diseases and development of new biological products.
Introduced in 2016, the base editing technique offers greater control than the conventional CRISPR–Cas9 editing. In base editing, one nucleotide (DNA component) is changed into another without cutting the DNA. Base editing hence is an attractive option to treat disorders like sickle-cell anaemia, which is caused by single base changes of DNA.
There are two types of base editors: adenine base editor which converts an adenosine (A) to a guanosine (G) and cytidine base editor which converts a cytosine (C) nucleotide to a thymine (T). Both were created at Broad Institute of MIT & Harvard in David Liu’s Lab.
Previously, red flags were raised as the the CRISPR-Cas9 system was reportedly causing off target mutations. Off-target mutations are caused when the system cuts DNA at a location it was not supposed to touch. David Liu has previously explained that human gene editing is in its infancy and everyone feels the responsibility of making it safer, ‘especially as these agents begin to enter clinical trials’.
But finding these mutations can be tough as even the different cells of the same organism has different mutations from each other, arising over time. Unless the edited and unedited cell had identical genomes originally, it is hard to surmise if the mutations are naturally occurring or CRISPR caused.
The only way to identify these mutations is to sequence the full edited genome which can be lengthy and expensive process. Liu and his team had to come with a new system to identify these off-target mutations. In one of the methods the base editors were inserted into bacteria and then tested for resistance to an antibiotic drug. The higher the frequency with which bacterial cells became resistant, the more active the base editor was in mutating the DNA in resistance genes.
Several enzymes were tested and a group of enzymes was identified which could convert C to T without causing as many mutations. Since new enzymes are less likely to cause these mutations, this would allow scientists to work on safer and effective gene therapies.
Other works of the team include the introduction of a new CRISPR technique called prime editing, which is a more controlled process of introducing changes to genome. The team has also developed Cas9 enzymes that can reach previously inaccessible regions of DNA. Over the years, Liu has distributed 8,000 base-editing tools to numerous laboratories. This year another company Beam Therapeutics (also co-founded by Liu) went public in an effort to commercialize this technology and raised 180 million US dollars in funding.