The genetic mutation given to Chinese twins last year rendering them immune to the HIV virus may significantly reduce life expectancy, scientists said Monday in a fresh warning against human gene-editing.

Chinese researcher He Jiankui last year provoked widespread outrage among doctors by unveiling the results of an experiment he conducted to alter the DNA of twin girls, prompting authorities in Beijing to announce a moratorium on the practice.

He used a gene-editing tool known as Crispr to insert a mutated variant of a CCR5 gene -- known as Delta32 -- into the girls' chromosome at the embryo stage meaning they are now immune to the AIDS-causing HIV virus.

But a new wide-ranging study of genetic make-up and death registry information suggests individuals carrying the D32 mutation face a 20-percent higher risk of early death compared with the global population.

Researchers from the University of Berkeley California examined the health data of 409,000 people of British ancestry and looked at whether or not they possessed the mutation, which occurs naturally in around one percent of the population, and how and when they died.

After correcting for the ages of those involved, they found those with the mutation were 21 percent more likely to die before the age of 76 than those who did not.

They found that D32 possessors were significantly more likely to die from diseases that are far more common than HIV, notably influenza.

"The cost of resistance to HIV may be increased susceptibility to other, and perhaps more common, diseases," the study's authors wrote.

- 'We don't know enough' -

The research, published in the journal Nature Medicine, doesn't explain why the mutation increases mortality risk, but the authors said there was a clear statistical trend that should discourage repeats of He's experiment.

"Introduction of new or derived mutations in humans using Crispr technology... comes with considerable risk even if the mutations provide a perceived advantage," they said.

China in November it said had ordered people involved in the twin experiment to halt their activities, ruling that it "seriously violates" national law and medical ethics.

David Curtis, honorary Professor at University College London's Genetics Institute, said the new study provided a clear look at the possible unintended consequences of gene-editing in humans.

"There are many other examples in medicine where an intervention intended to treat one condition inadvertently causes major unexpected problems elsewhere," said Curtis, who was not involved in the study.

"This sends us a warning that we should be extremely cautious around the introduction of therapies involving modifying the genetic code."

Robin Lovell-Badge, group leader at The Francis Crick Institute in London, added: "All this shows once more that He Jiankui was foolish to choose CCR5 to mutate in his attempts at germline genome editing. We simply do not yet know enough about the gene."

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