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An global team of scientists has successfully edited of the genes of a viable human embryo, correcting a mutation that can cause the heart condition hypertrophic cardiomyopathy. Later, the injected the CRISPR-Cas9 editing tool into the eggs and found that about 72 per cent of embryos were free from the errant gene.

The US and the South Korean team allowed the embryos to develop for five days before stopping the experiment. In the medium term it holds out the prospect - if the law changes - of eliminating some single-gene defects from entire families, since embryos treated in this way will no longer transmit the defective form of the gene.

The breakthrough, announced Wednesday in the journal Nature, corrected the mutation for a heart condition at the earliest stage of embryonic development so that the defect would not be passed on to future generations.

The adjustment itself is heritable, meaning that the children and grandchildren of a person born with edited DNA will be safe from the same genetic disease. The latest research team reports it achieved "efficiency, accuracy and safety" with the approach. The Committee on Human Gene Editing recommended a series of general values: transparency, wellbeing of people, fairness, and responsible science, in an age of DNA manipulation.

"Genetic diseases that are heritable can be treated this way as early as possible", he says. For example, a case in which both copies of the gene were mutated rather than just one, which was the case in this experiment.

The researchers used donated sperm from an anonymous OHSU patient with the condition to fertilize eggs donated by 12 healthy women who were recruited in the Portland area through print and online ads. Work with skin cells reprogrammed to mimic embryos had suggested the mutation would be repaired in fewer than 30 percent of cells.

Nor were there unintended mutations in other parts of the genome. But, with other countries also carrying out similar experiments, as China first did previous year, the tide is turning.

Those teams had problems with "mosaicism", which occurs when some cells in an embryo are corrected, and some not.

Aside from helping the USA catch up to China in the genetic arms race, the study breaks some important new ground.

"We definitely want to replicate this study with other mutations and other donors", said Amato.

CRISPR is the acronym for Clustered Regularly Interspaced Short Palindromic Repeats and by using an enzyme called Cas9, it can snip specific target sequences on a mutant gene.

"We need more debate", Wu said. But the altered embryos created in the study were quickly destroyed and never meant to be implanted in a woman - a step that would be illegal under current regulations in the United States and many other countries. The treatment replaces faulty inherited DNA in the mitochondria, tiny rod-like bodies in cells that supply energy, to prevent devastating diseases. Sometimes eggs had already copied DNA, and a mutant gene escaped editing (top, middle).

Unlike research groups before them - which worked on embryos that were not capable of ever becoming a baby - this study involved the creation of healthy human embryos specifically for research purposes.

"What it does is, it searches out the one error in the 3 billion letters of the DNA code and it chops it out, and the DNA corrects itself", CBS News medical contributor Dr. David Agus explained on "CBS This Morning".

"I think that in a world in which some people are regarded as genetically superior to others because they have been genetically enhanced is a radically more unequal and oppressive world than what we have now", King says.


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