In May, a study claimed that the revolutionary CRISPR gene editing technique can cause thousands of unwanted and potentially dangerous mutations. The authors called for regulators to reassess the safety of the technique.
But doubts were raised about these claims from the very beginning, not least because it was a tiny study involving just three mice. Some critics have called for the paper to be withdrawn. Now a paper posted online on 5 July has proposed a simple and more plausible explanation for the controversial results.
Tsang and colleagues claimed that by sequencing the entire genome, they found off-target mutations missed by studies that only looked at sites resembling the target sequence. But there is a much simpler explanation, says the latest study: the two CRISPR-edited mice just happened to be more closely related and thus shared more mutations.
DNA mutations blamed on CRISPR more likely because lab mice were inbreeding relations
Tsang and colleagues assumed the three mice they studied were essentially genetically identical because their parents were very closely related, but the way the colony of mice was maintained means this was probably not the case, the team, which includes Luca Pinello of Harvard University.
Schaefer et al. recently advanced the provocative conclusion that CRISPR-Cas9 nuclease can induce off-target alterations at genomic loci that do not resemble the intended on-target site. Using high-coverage whole genome sequencing (WGS), these authors reported finding SNPs and indels in two CRISPR-Cas9-treated mice that were not present in a single untreated control mouse. On the basis of this association, Schaefer et al. concluded that these sequence variants were caused by CRISPR-Cas9. This new proposed CRISPR-Cas9 off-target activity runs contrary to previously published work and, if the authors are correct, could have profound implications for research and therapeutic applications. Here we demonstrate that the simplest interpretation of Schaefer et al.’s data is that the two CRISPR-Cas9-treated mice are actually more closely related genetically to each other than to the control mouse. This strongly suggests that the so-called “unexpected mutations” simply represent SNPs and indels shared in common by these mice prior to nuclease treatment. In addition, given the genomic and sequence distribution profiles of these variants, we show that it is challenging to explain how CRISPR-Cas9 might be expected to induce such changes. Finally, we argue that the lack of appropriate controls in Schaefer et al.’s experimental design precludes assignment of causality to CRISPR-Cas9. Given these substantial issues, we urge Schaefer et al. to revise or re-state the original conclusions of their published work so as to avoid leaving misleading and unsupported statements to persist in the literature.