With the first UK and US human trials using the potentially revolutionary CRISPR gene editing technique about to commence, several questions still hover over the ultimate safety of this system. A team of Chinese scientists has now conducted the first full assessment of the effects of CRISPR-Cas9 on non-human primates to ascertain whether it results in off-target mutations.

A controversial study published last year suggested CRISPR could introduce unintended mutations, but after several scientists criticized the veracity of the data it was ultimately retracted.

Despite the earlier flawed research being retracted, there are still major concerns that gene-editing using the CRISPR technique can potentially result in unintended mutations. The Chinese research is the first to perform a genome-wide evaluation on the connection between the CRISPR process and de novo mutations – new mutations that spontaneously arise in a fertilized egg during the initial stages of embryogenesis – in rhesus macaque monkeys.

The study began by using the CRISPR system on macaque zygotes, or fertilized eggs. The researchers were testing the effectiveness of the technique's targeting specificity by focusing on the deletion of a gene called MCPH1, known to cause birth defects in humans. In the embryos initially treated, the process successfully knocked the MCPH1 gene out in 13 out of 15 samples.

Five monkeys were ultimately gestated, with four surviving delivery. All the offspring monkeys displayed successful CRISPR modifications, but the next steps were to undertake comprehensive whole genome sequencing to better understand if the gene-editing process had resulted in unintended mutations.

Several techniques revealed that there were in fact no detectable off-target effects, but some de novo mutations were ultimately identified. A certain volume of these spontaneous de novo mutations are to be generally expected in any new generation of primates, and a close study of the particular mutations detected in the monkeys led the researchers to safely conclude that these de novo mutations were regular spontaneous mutations and not related to the CRISPR process.

The study has not been officially published or peer-reviewed at this stage so it shouldn't be declared as entirely reliable, but it does offer promise that the CRISPR-Cas9 process is relatively accurate.

Of course, off-target mutations aren't the only potential problem the CRISPR process is currently facing. Two recent studies raised the new and concerning issue that the process could trigger an increased risk of cancer in modified cells. It will be years before we can get a full picture of the broader side-effects of this new technique, but there are sure to be many scientists watching these upcoming human clinical trials very closely in the hopes that this breakthrough technique is safe and successful.

The new study was published on the preprint server bioRxiv.