CRISPR-Cas9 had captured the imagination of the world when it first became public—scientists and health professional were suddenly wondering aloud if this mean the end of genetic diseases. So, when Shoukhrat Mitalipov and his team of researchers at the Oregon Health Sciences Center published a report on how they were successfully able to eliminate a disease-coding mutation in ova and create viable zygotes with CRISPR-Cas9, the genetic disease-free future suddenly seemed very near. Then, another research questioned Mitalipov’s findings, and explained how its action, even if the end result of the editing is the same, could be imprecise. Now, US scientists have again fuelled optimism over gene editing with two new tools. David Liu of Harvard University and the Broad Institute of MIT and Harvard has developed a ultra-precise point-mutation editing tool—point mutations or single-letter mutations occur when the wrong pair of DNA bases, adenine (A)-thymine (T) or cytosine (C)-guanine (G), is present at a locus on a codon—that can correct the error.
Point mutations cause diseases like sickle-cell anaemia, colour blindness, cystic fibrosis and are believed to be responsible for some cancers as well. The second technique, developed by Feng Zhang of the Broad Institute, edits the RNA—the genetic messenger for coding proteins—without altering the DNA. Given how the human genome consists of 6 billion DNA bases (A,T,C & G), the precision these tools bring is mind-boggling. And this is also where they score over CRISPR Cas9, though they are built off it. CRISPR edits out entire genes by cutting them out, enabling their replacement by a healthy gene. But the two new tools reach a level much finer than this.
Liu rightly says CRISPR is a scissor while base editors are fine-tipped pencils. Liu’s base editor uses an enzyme that can convert an A-T pairing to a C-G pairing, something that hadn’t been done in a lab before. Zhang’s REPAIR, on the other hand, works on the mRNA strand to repair base errors. But given mRNA are an intermediary in transcription—they later become one strand of the double-stranded DNA—the fixing of the error is unlikely to be replicated as the cell divides. These tools have considerable potential in to help eradicate some of the most dreaded diseases, but both researchers are not ready for human trials just yet.