If we can eliminate disease, why shouldn't we?

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Using what is called a CRISPR gene-editing technique, the scientists said, they repaired the embryos so that a heart condition would not manifest in later life.

While these embryos, which a team led by researchers at the Oregon Health and Science University edited using a novel gene-editing procedure known as CRISPR-Cas9, were destroyed rather than implanted into a womb, some said this type of genetic manipulation opens the door to other possibilities in human engineering.

The study, published today in Nature, involved taking eggs from 12 healthy female donors and sperm from a male donor who carries the MYBPC3 gene that causes hypertrophic cardiomyopathy, or HCM, which is a disease of the heart muscles that can cause sudden cardiac death.

In theory, the genes could be corrected, preventing generations of women from developing cancer caused by those mutations.

American scientists have accomplished a major first: For the first time on United States soil, a human embryo has been genetically modified.

In a world first, they used gene-editing to cut out DNA from a fertilised egg.

At present, South Korea's ethical guidelines ban experimenting on human embryos in the country.

Critics of the study were quick to allege that the research team - whose work can not be taken much further in the United States given legal limitations - had already pushed the boundaries too far.

The scientists utilized healthy donated human germ cells carrying the said gene mutation. Critics worry about attempts at "designer babies" instead of just preventing disease, and a few previous attempts at learning to edit embryos, in China, didn't work well and, more importantly, raised safety concerns. In the other 30% of CRISPR-treated embryos, a less-accurate mechanism introduced errors as it repaired the DNA cut. That suggests that the CRISPR process inadvertently triggered a powerful and unexpected form of natural DNA fix in human embryos, one not seen before in studies of mice or other creatures.

OHSU's team found that they were able to get around those problems by injecting the CRISPR-Cas9 molecular tool into a human egg at the same time that it was being fertilized by sperm. The Salk scientists contributed by developing the gene editing strategy, initially testing it in stem cells derived from the patient's skin cells.

Furthermore, to be ethical, any applications or experiments utilizing CRISPR or other gene editing technology can not use any other methods in its process which are themselves intrinsically immoral, Fr.Pacholczyk said.

Scientists contacted by KPBS saw these results as a small yet successful step toward the development of new strategies for fighting genetic diseases in future generations.

None of the embryos were permitted to develop beyond five days after conception. "It's the best way to treat the disease before the genetic mutation is actually transmitted to the embryo".

"The question that will be most debated is whether the same principle of modifying the genes of an embryo in vitro is acceptable", analyzed by an independent expert, professor Darren Griffin (University of Kent), quoted by the Science Media Centre.

It's now illegal in the US and many countries to implant a gene-edited embryo into a mother's womb to produce a baby. He and Kahn were part of a National Academy of Sciences report earlier this year that said if germline editing ever were allowed, it should be only for serious diseases with no good alternatives and done with strict oversight.

"Although it implies a future in which we could choose to have children without particular genetic diseases, or even design for particular traits, that is still likely a very long time from now", he wrote in an email to KPBS.