He Jiankui, the Chinese scientist who created the first gene-edited twin children last year, could have unknowingly shortened their lives by more than 1.9 years. A study into the DNA and death records of 400,000 volunteers in the UK Biobank found the genetic mutations to gene CCR5 were “of quite strong effect.”

This finding validates what many critics warned of when the news of Jiankui’s experiment broke – that as genes often have more than one role, unintentional and potentially dangerous side effects could well occur.

Variations in CCR5 have also been linked with memory recovery post-stroke, suggesting they could change brain functions too. There is also evidence such mutations could make individuals more susceptible to other diseases like West Nile virus or influenza.

Rasmus Nielsen of the University of California, who undertook the study alongside colleague Xinzhu Wei and published in Nature Medicine, said that there were significantly fewer Biobank volunteers with the double mutation than would be expected by chance:  “That tells us there is a process that removes individuals with two copies, and that process is probably natural selection. People die.”

When volunteers’ DNA was compared with death records, it was further found that individuals with two non-working CCR5 genes died 20% faster than other volunteers. 

This study should go a long way to silence proponents of Jiankui’s work, as he performed it: while intentions were positive, to heedlessly alter genes is reckless at best and damaging at worst. Evidence around the outcomes of these changes must be studied in depth, with mortality rates examined closely to ensure that side effects of the editing are both understood and accepted.