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A team at Sun Yat-sen University has used a technique known as base editing ("chemical surgery") to correct an error out of the genetic code's three billion "letters." The procedure saw the team altering lab-made embryos to extract the disease beta-thalassemia, an inherited blood disorder that affects one in 100,000 births.Beta-thalassemia is generated by a single error to a single base, and scientists would call it a "single point mutation." These single letter errors are responsible for two-thirds of inherited genetic diseases.The team, which was the pioneer of CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) gene editing, stated that base editing could treat a variety of inherited diseases one day.
Base editing amends DNA's basic building blocks, which are adenine, cytosine, guanine, and thymine, the four bases.The blocks are represented by their respective first letters, A, C, G, and T.The four bases would contain all the instructions for creating and facilitating the human body, but the combinations would be obstructed.In base editing, the team scanned the DNA for the error and convert a G to an A to correct the fault.
Junjiu Huang, who is among the researchers, expressed his delight over the fact that their team was "the first to demonstrate the feasibility of curing genetic disease in human embryos by base editor system." He added that the study has provided new methods for treating patients and preventing babies to be born with beta-thalassemia, "and even other inherited diseases."
As a refresher, CRISPR utilizes an enzyme (Cas9 nuclease) to extract a slab of undesired DNA and would rely on the cell's reaction to the double-stranded slice, by initiating changes in genomic DNA at the target site.However, base editing is a different story as David Liu, the inventor of the said technique, would explain:
"[Base editing] does not make a double-stranded cut in the target DNA. Instead, base editors directly perform chemical surgery on the target DNA base to convert one base pair to a different base pair (that is, to make a point mutation).Direct chemical surgery is an appropriate analogy because base editors actually catalyze the rearrangement of atoms on the target DNA base to transform that DNA base from a C to something that looks like a T (for example)."
Liu described the new paper as an "interesting work," since it demonstrates base editing's ability to "directly correct a pathogenic beta-thalassemia mutation in the promoter of the beta-thalassemia gene in cells from beta-thalassemia patients, and in cloned human embryos derived from these patient [skin] cells." He observed that the researchers exercised "efficient corrections" of the mutation behind the blood disorder.
Liu understood that the study is the first to use base editing to correct a pathogenic mutation in human embryos, although there were previously published papers that present base editing's effectiveness.Harvard Medical School geneticist Seth Shipman clarified that base editing was not engineered to rival CRISPR-Cas9.Rather, it is a modified version equipped with various attributes.He mentioned: "Because only certain base changes are possible with this editor, one challenge has been to find cases, where the changes that can be made would potentially fix a disease.Puping Liang and colleagues found one and showed it in this paper."
Seth Shipman, a geneticist at Harvard Medical School, also believed that base editing can be harnessed to treat the living person's faulty cells."Their final experiments are in these nuclear transfer embryos as a way of showing whether one might be able to make a germline modification to correct this mutation," he said. "In the case of beta-thalassemia though, if they were able to edit the...stem cells instead, they may be able to treat or even cure the disease in patients who are currently suffering from it, rather than potentially modify their offspring.That could be a more relevant line of research—and one they hint at in their discussion."
University of Kent geneticist Darren Griffin commented about base editing's prospects: "For many years, we have been saying that direct gene editing in embryos is some way into the future.Now the future is here and there is much to consider." Griffin added that the breakthrough will pave way to an array of ethical inquiries and safety concerns over manipulating genes in embryos, yet he deemed that the "chemical surgery" is critical advancement, since the Chinese researchers used "precise and targeted “base editor” techniques to zero in on one mutant gene among billions of genetic components, and then selectively fix that gene."
Griffin, in the end, emphasized the importance of not getting carried away about its widespread utility, should base editing become a clinical practice. "An embryo would still need to be diagnosed as abnormal, then the base editor applied, then re-diagnosed to make sure that it had worked.This would be an involved procedure that would be very expensive," he explained.
Francis Crick Institute's Robin Lovell mentioned that the recent experiment "looks very promising," yet advised that modified embryos still have their share of problems, He stated that humans possess a pair of copies in every gene (alleles) and a sizeable portion of these genes are needed to be normal in order to avoid disease.Lovell-Badge observed that the new method would fix one mutation in the two pairs and create "mosaic" embryos that are both healthy and mutated cells.