CRISPR gene-editing technology brought science fiction to life with its ability to cut and paste DNA fragments, potentially eliminating serious inherited diseases. CRISPR-Cas9, in particular, has gotten scientists excited because,1 by modifying an enzyme called Cas9, the gene-editing capabilities are significantly improved. That's not to say they're perfect, however, as evidenced by a recent study that showed CRISPR may have significant unintended consequences to your DNA, including large deletions and complex rearrangements.2
Many of the concerns to date regarding CRISPR, or Clustered Regularly Interspaced Short Palindromic Repeat, technology have centered on off-target mutations. The featured study, published in Nature Biotechnology, looked at on-target mutations at the site of the "cuts," revealing potentially dangerous changes that could increase the risk of chronic diseases like cancer.
Is CRISPR Scrambling DNA?
Researchers at the U.K.'s Wellcome Sanger Institute systematically studied mutations from CRISPR-Cas9 in mouse and human cells, focusing on the gene-editing target site. Large genetic rearrangements were observed, including DNA deletions and insertions, that were spotted near the target site.
They were far enough away, however, that standard tests looking for CRISPR-related DNA damage would miss them. The DNA deletions could end up activating genes that should stay "off," such as cancer-causing genes, as well as silencing those that should be "on." One of the study's authors, professor Allan Bradley, said in a statement:3
"This is the first systematic assessment of unexpected events resulting from CRISPR/Cas9 editing in therapeutically relevant cells, and we found that changes in the DNA have been seriously underestimated before now. It is important that anyone thinking of using this technology for gene therapy proceeds with caution, and looks very carefully to check for possible harmful effects."
The deletions detected were at a scale of "thousands of bases," which is more than previously thought and enough to affect adjacent genes. For instance, deletions equivalent to thousands of DNA letters were revealed. "In one case, genomes in about two-thirds of the CRISPR'd cells showed the expected small-scale inadvertent havoc, but 21 percent had DNA deletions of more than 250 bases and up to 6,000 bases long," Scientific American reported.4
The cells targeted by CRISPR try to "stitch things back together," according to Bradley, "But it doesn't really know what bits of DNA lie adjacent to each other." As a result, the DNA may be rearranged, previously distant DNA sequences may become attached, or unrelated sections could be incorporated into the chromosome.5
Cas9, a bacteria enzyme that acts as the "scissors" in CRISPR, actually remains in the body for a period of hours to weeks. Even after the initial DNA segment had been cut out and a new section "pasted" into the gap to repair it, Cas9 continued to make cuts into the DNA. "[T]he scissors continued to cut the DNA over and over again. They found significant areas near the cut site where DNA had been removed, rearranged or inverted," The Conversation reported.6
Does This Mean CRISPR Isn't Safe?
It's too soon to say what the long-term effects of gene-editing technology will be, and there are many variables to the safety equation. The findings likely only apply to CRISPR-Cas9, which cuts through the DNA's double strand. Other CRISPR technologies exist that may alter only a single strand or not involve cutting at all, instead swapping DNA letters.
There are also CRISPR systems that target RNA instead of DNA and those that could potentially involve only cells isolated from the body, such as white blood cells, which could then be analyzed for potential mutations before being put back into the body.7
The Nature study did make waves in the industry, though, such that within the first 20 minutes of the results being made public three CRISPR companies lost more than $300 million in value.8
Some companies using CRISPR have said they're already on the lookout for large and small DNA deletions (including one company using the technology to make pig organs that could be transplanted into humans). One company also claims it hasn't found large deletions in their work on cells that do not divide often (the Nature study used actively dividing cells).9
The researchers are standing by their findings, however, which the journal took one year to publish. During that time, Bradley says, he was asked to conduct additional experiments and "the results all held up."10 Past studies have also found unexpected mutations, including one based on a study that used CRISPR-Cas9 to restore sight in blind mice by correcting a genetic mutation.
The researchers sequenced the entire genome of the CRISPR-edited mice to search for mutations. In addition to the intended genetic edit, they found more than 100 additional deletions and insertions along with more than 1,500 single-nucleotide mutations.11 The study was later retracted, however, due to insufficient data and a need for more research to confirm the results.12