The gene editing system CRISPR-Cas9 is promising for its potential to treat diseases caused by gene irregularities, but fears of off-target effects threaten to delay its development. Scientists at the University of Texas at Austin are now proposing that replacing Cas9 with a different protein, Cas12a, may be the best way to address CRISPR’s shortcomings.
Cas9 is the scalpel of CRISPR, meaning it actually cuts the strands of DNA. But sometimes it edits the wrong part of the genome, disturbing healthy functions instead of fixing disease-causing genes. Scientists have long feared that this could inadvertently cause healthy cells to turn cancerous. Those fears were compounded just last month, when two studies suggested that even the cells that are targeted and successfully edited can be more susceptible to cancer-causing mutations.
So why Cas12a? The University of Texas team found that the difference between Cas9 and Cas12a is in their binding behavior. Cas9 binds to its target like super glue, whereas Cas12a acts more like Velcro, according to a statement.
As a result, Cas9 latches onto the first few letters in its genomic target, but then stops paying attention. If there’s a mismatch later in the DNA strand, the enzyme might overlook it and edit the wrong section of the genome. Cas12a, on the other hand, checks each base pair in its target before moving on, making it possible that mistakes can be corrected, the researchers demonstrated. They published their research in the journal Molecular Cell.
Safety worries with CRISPR have prompted scientists all over the world to propose various methods for improving the technology. In May, a team at Stanford University and the National Institute of Standards and Technology announced they had invented a new gene-editing system called MAGESTIC that’s designed to improve the process by which edited DNA repairs itself. Earlier in the year, scientists in Poland said they were perfecting a version of CRISPR that uses a variant of Cas9 to cut one strand of DNA instead of two.
Still, safety concerns continue to dog the burgeoning CRISPR industry. In July, research published in Nature Biotechnology suggested that Cas9 could cause undetectable genetic changes. The stock prices of well-known players in the CRISPR arena like CRISPR Therapeutics, Intellia Therapeutics and Sangamo all took hits. The prospect of unexpected gene edits could delay human trials of CRISPR, which were expected to start later this year.
Cas12a is not a new discovery—other research teams had suggested it might be more precise than Cas9—but previous evidence was inconclusive, the University of Texas team said. They believe that with further development of Cas12a, they may be able to make an error-proof gene-editing system.
"On the whole, Cas12a is better, but there were some areas where Cas12a was still surprisingly blind to some mispairing between its RNA and the genomic target," said co-author Ilya Finkelstein, an assistant professor of molecular biosciences at the university, in the statement. "So what our work does is show a clear path forward for improving Cas12a further."