In the studies, published in Nature Medicine, scientists found that cells whose genomes are successfully edited by CRISPR-Cas9 have the potential to seed tumors inside a patient. That could make some CRISPR'd cells ticking time bombs, according to researchers from Sweden's Karolinska Institute and, separately, Novartis.
CRISPR has already dodged two potentially fatal bullets - a 2017 claim that it causes sky-high numbers of off-target effects was retracted in March, and a report of human immunity to Cas9 was largely shrugged off as solvable. But experts are taking the cancer-risk finding seriously.
The CEO of CRISPR Therapeutics, Sam Kulkarni, told STAT the results are "plausible." Although they likely apply to only one of the ways that CRISPR edits genomes (replacing disease-causing DNA with healthy versions) and not the other (just excising DNA), he said, "it's something we need to pay attention to, especially as CRISPR expands to more diseases. We need to do the work and make sure edited cells returned to patients don't become cancerous."
Another leading CRISPR scientist, who asked not to be named because of involvement with genome-editing companies, called the new data "pretty striking," and raised concerns that a potential fatal flaw in some uses of CRISPR had "been missed."
On the other hand, the Novartis paper has been available in preliminary form since last summer, and CRISPR experts "haven't freaked out," said Erik Sontheimer of the University of Massachusetts Medical School, whose CRISPR research centers on novel enzymes and off-target effects. "This is something that bears paying attention to, but I don't think it's a deal-breaker" for CRISPR therapies.
The Karolinska and Novartis groups tested CRISPR on different kinds of human cells - retinal cells and pluripotent stem cells, respectively. But they found essentially the same phenomenon. Standard CRISPR-Cas9 works by cutting both strands of the DNA double helix. That injury causes a cell to activate a biochemical first-aid kit orchestrated by a gene called p53, which either mends the DNA break or makes the cell self-destruct.
Whichever action p53 takes, the consequence is the same: CRISPR doesn't work, either because the genome edit is stitched up or the cell is dead. (The Novartis team calculated that p53 reduces CRISPR efficiency in pluripotent stem cells seventeenfold.) That might explain something found over and over: CRISPR is woefully inefficient, with only a small minority of cells into which CRISPR is introduced, usually by a virus, actually having their genomes edited as intended.
"We found that cutting the genome with CRISPR-Cas9 induced the activation of ... p53," said Emma Haapaniemi, the lead author of the Karolinska study. That "makes editing much more difficult."
The flip side of p53 repairing CRISPR edits, or killing cells that accept the edits, is that cells that survive with the edits do so precisely because they have a dysfunctional p53 and therefore lack this fix-it-or-kill-it mechanism.
Comment: Additional information about the concerns of CRISPR tech: