Safe gene therapy

The study showed that CRISPR does not cause unintentional mutations in monkeys

Vladimir Mirny, Naked Science

There has been a lot of noise and excitement around CRISPR lately. One of the most important questions about the technology has not yet been answered: will it lead only to the necessary results or will it provoke undesirable changes?

CRISPR is still a relatively new technology, and at the moment its potential side effects are unknown to science. What about unintentional mutations? To get to the heart of the problem, a team of researchers from China evaluated monkeys that had undergone CRISPR modification, looking for any unplanned changes in the genomes of animals.

The work has not yet received expert evaluation, but the bioRxiv preprint service has already published an early version of it (Luo et al., Trio deep-sequencing does not reveal unexpected mutations in Cas9-edited monkeys).

To conduct the study, scientists have developed a CRISPR-Cas9 system for deactivating the MCPH1 gene in rhesus monkeys. Due to mutations in this gene, the brain develops abnormally, leading to a disease such as microcephaly. Specialists have introduced their CRISPR-Cas9 system into fifteen rhesus macaque zygotes. They found out that MCPH1 was successfully deactivated in thirteen embryos, which indicated the effectiveness of their technique.

Armed with new data, scientists could begin searching for an answer to the question of unwanted mutations. The team introduced the same CRISPR-Cas9 system into thirty zygotes, twenty-four of which developed normally. These zygotes were then implanted into six surrogate female rhesus monkeys, two of whom became pregnant. One individual gave birth to twins – a male and a female, the first of which survived. Another gave birth to triplets by Caesarean section: one female and two males, all of them survived. And then it's time to search for unprogrammed mutations.

The scientists found that their CRISPR gene modification worked in all but one offspring. They focused on finding any de novo mutations (new ones) - those that are not inherited in monkeys. They sequenced the entire genomes of each individual and found that their CRISPR corrections did not produce a single de novo mutation (monkeys had some new mutations, but within the normal range for rhesus monkeys).

As a result, the researchers reviewed all published data on whole genome sequencing from another study that used CRISPR to replace genes in crab-eating macaques, often used in medical research. The information received confirmed their own research: the animals did not show de novo mutations more than is the norm for the species.

What conclusion did the scientists come to?

"CRISPR-Cas9 can be considered a relatively safe genomic editing system for primates," the report says.

Up to this point, most studies of the potential occurrence of unplanned mutations when using CRISPR have been carried out on rats or human cells. This study was the first to analyze the entire genomes of non-human primates that had undergone CRISPR editing. It was also an important step towards the future safe application of technology in humans, since genetically and physically our species are very similar.

And while de novo mutations aren't the only potential unwanted side effects of gene editing, they were the ones people were most worried about. Now there is some confidence that such changes do not pose a danger and CRISPR is getting closer to its original application.

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