The results of the study are yet to be published, but the work – led by Professor Shoukhrat Mitalipov at the Oregon Health and Science University, Portland – is believed to include the largest number of successfully modified embryos produced using CRISPR technology.
Professor Mitalipov declined to comment on the study, but stated the results are pending publication next month. However, Dr Jun Wu, a collaborator at the Salk Institute, California told MIT Technology Review, 'So far as I know this will be the first study reported in the US.'
Three published attempts of genome editing in human embryos have been carried out in China to date, the first in 2015 (see BioNews 799). Professor Shirley Hodgson, a specialist in genetics at St George's, University of London, said the 2015 study had shown CRISPR to be 'a technique with significant errors in human embryos with regard to accuracy, resulting in "off-target" mutations in other genes, and mosaicism in the developing embryo, since usually the "repair" has only occurred in a proportion of embryonic cells.'
However, sources claim that the US study overcomes these two hurdles. Professor Mitalipov's group are believed to have injected eggs with CRISPR at the same time as they were fertilised with donor sperm, as opposed to after. The donor sperm contained inherited disease-related mutations, though it is not known which disease genes these were. It is reported that the majority of cells in the resulting embryos were successfully edited so that the disease genes were replaced with healthy genes, with few errors.
The successfully edited embryos of 'clinical quality' were discarded after a few days. The team had no intention of implanting them.
A scientist familiar with the project told MIT Technology Review, 'It is proof of principle that it can work. They significantly reduced mosaicism. I don't think it's the start of clinical trials yet, but it does take it further than anyone has before.'
Scientists are reluctant to comment on the results until they have been confirmed in a published scientific paper, yet some caution that there is still a long way to go until it would be possible to implant any edited embryos.
'While it might be tempting to consider a technology that potentially offers the prospect of curing serious genetic diseases at such an early stage of development, there are major risks, both of technical failure and unanticipated adverse consequences, which could affect generations to come,' said Professor Frances Flinter, a clinical geneticist at Guy's & St Thomas' NHS Foundation Trust, London, who was not involved in the study.
Besides the safety issues, the study also raises important ethical questions about how the technology could or should be used. Research involving human embryo editing is barred from federal funding in the US. Careful regulation is in place in the UK, with scientists granted permission from the Human Fertilisation and Embryology Authority (see BioNews 837) to genetically modify human embryos for research use only.
Dr Simon Waddington of University College London, who was not involved in the study, concluded, 'If what has been reported in the media is backed up by a paper published in a peer-reviewed journal, it would be a valuable increment but we still have a long way to go.'