Until now, 'off-target effects' of the CRISPR/Cas9 system were thought to be minimal, with improvements to the technique reducing errors to nearly undetectable levels. However, now a new study suggests previous methods for detecting off-target mutations may have underestimated the true scale of unwanted effects.
'We feel it's critical that the scientific community consider the potential hazards of all off-target mutations caused by CRISPR,' said co-author Dr Stephen Tsang, from Columbia University Medical Centre, New York. 'Researchers who aren't using whole genome sequencing to find off-target effects may be missing potentially important mutations. Even a single nucleotide change can have a huge impact.'
The precise nature of CRISPR has contributed to its growing popularity in recent years. Yet until now, researchers have been using computer algorithms to check the accuracy of the technique, by identifying and scanning regions of the genome most likely to be affected by genome editing.
'These predictive algorithms seem to do a good job when CRISPR is performed in cells or tissues in a dish, but whole genome sequencing has not been employed to look for all off-target effects in living animals,' said co-author Dr Alexander Bassuk at the University of Iowa.
In a previous study, the team had used CRISPR in two mice to correct a faulty gene that causes blindness. In their new study, published in Nature Methods, the researchers compared the genomes of the edited mice to a single non-edited control mouse.
They found over 1500 single-nucleotide mutations and over 100 larger deletions or insertions in the two CRISPR mice. In spite of this, the mice appeared healthy and did not show any obvious side effects due to the off-target mutations.
Some experts in the field are sceptical of the findings, raising concerns over the experimental design of the study. Dr Gaétan Burgio from the Australian National University, Canberra, whose research group has a keen interest in CRISPR genome-editing technologies, stated to Medium: 'I would predict very few if not close to none of these variants are CRISPR related.' Instead he believes a number of issues with the methodology, as well as the small sample size, can explain the 'abnormal number of mutations'. Further investigation by other research groups will be needed to validate these results and determine the future implications for CRISPR methods.
The first CRISPR clinical trial in patients already underway in China and another is expected to start in the US in the next year.
The researchers say they hope other groups will use their methods to assess off-target mutations and improve the accuracy of CRISPR.
'We're still upbeat about CRISPR,' said co-author Dr Vinit Mahajan at Stanford University. 'We're physicians, and we know that every new therapy has some potential side effects - but we need to be aware of what they are.'