AsianScientist (Aug. 8, 2017) – CRISPR-Cas9 gene editing technology has been used to correct disease-causing mutations in human embryos. This research, published in Nature, was conducted by an international collaboration including the Center for Genome Engineering within the Institute for Basic Science (IBS) in South Korea, the Oregon Health and Science University, the Salk Institute for Biological Studies, BGI-Qingdao and Shenzhen Engineering Laboratory for Innovative Molecular Diagnostics in China.
The scientists used CRISPR-Cas9 to repair a fragment of DNA that causes a common genetic heart disease known as hypertrophic cardiomyopathy. Hypertrophic cardiomyopathy is one of more than 10,000 inheritable diseases caused by an error in a single gene. This genetic disease manifests only in adulthood and affects an estimated 1 in 500 people. It can lead to heart failure and sudden death of apparently healthy people. Current treatments rely mainly on symptomatic relief.
Forty percent of all familial hypertrophic cardiomyopathy is caused by a mutation of the MYBPC3 gene on the 11th chromosome. In this study, the researchers dealt with a mutation characterized by four missing base pairs in the MYBPC3 gene. Reintroducing these four base pairs with CRISPR-Cas9 in the embryo prevents this mutation from appearing in future generations.
The experiments on the embryos were conducted in the US in compliance with ethical guidelines. Researchers worked with healthy egg cells and sperm of a man affected by hypertrophic cardiomyopathy. IBS researchers provided the CRISPR-Cas9 enzyme and analyzed the DNA of the embryos to make sure that the procedure worked correctly.
Previous studies have injected CRISPR-Cas9 into embryos after in vitro fertilization (IVF), but faced the problem of mosaicism, characterized by embryos having a mixture of cells with and without the repaired mutation. Mosaicism would lead to organisms with some tissues or organs that bear the mutations and some that do not. In this study, the researchers injected sperm and CRISPR-Cas9 into the egg at the same time to improve the accuracy of the gene correction.
CRISPR-Cas9 cut the DNA at the correct position in all tested embryos (100 percent) and 42 out of the 58 embryos (72.4 percent) did not carry the hypertrophic cardiomyopathy mutation. Moreover, the scientists also discovered that human embryos have an alternative DNA repair system, where the Cas9-induced cuts in the DNA coming from the sperm are repaired using the healthy egg’s DNA as a template. In the remaining 27.6 percent of the embryos, the cellular cut-repairing mechanism introduced some unwanted insertions or deletions near the cut.
Having confirmed that the disease-causing mutation is repaired correctly in human embryos, IBS researchers performed further analysis to make sure that the gene scissors did not cut any other sites of the human genome. The IBS team has previously developed a technique known as Digenome-seq to assess the accuracy of the gene scissors, as off-target cuts and editing mistakes could be a major problem and bring unwanted consequences. Sequencing the whole genome of the embryo did not find any off-target changes. The result was also confirmed with another DNA sequencing technique by researchers working at BGI-Qingdao and Shenzhen Engineering Laboratory for Innovative Molecular Diagnostics.
CRISPR-Cas9 combined with IVF and preimplantation genetic diagnosis (PGD) could be helpful for many other genetic diseases. However, the scientists stated that “genome editing approaches must be further optimized” before moving to clinical trials.
“We have succeeded in correcting the mutated gene which causes hypertrophic cardiomyopathy in human embryos with high efficiency and specificity,” said Professor Kim Jin-Soo, Director of the Center for Genome Engineering. “Research on human embryos has been a very sensitive subject. The application of this technology to clinical practice in the future requires not only additional research, but also social consensus.”
The article can be found at: Ma et al. (2017) Correction of a Pathogenic Gene Mutation in Human Embryos.
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Source: Institute for Basic Science.
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