CRISPR for gene therapy: surmountable difficulties
When investigating the capabilities of the CRISPR genomic editor, it was found that our immune system can interfere with the use of a promising method on a person
Marina Astvatsaturyan, Echo of Moscow
An article with the results of a new study (Charlesworth et al., Identification of Pre-Existing Adaptive Immunity to Cas9 Proteins in Humans), published on the bioRxiv preprint server, but not yet reviewed, raises questions about the potential of the popular molecular genetic method CRISPR to correct mutations that cause human diseases.
The presented data were evaluated by the editors of the journal Nature. The CRISPR-Cas9 system itself is a primitive immune system that exists in many microbes. Its action is based on the ability of the Cas9 enzyme to cut DNA at a location determined by a short sequence of special RNA. Scientists can change the sequence of this RNA "guide" in order to target Cas9 to a specific segment of DNA, and then there is a high probability that the system will correct an undesirable genetic mutation that is localized on this segment.
But the fact is that, being foreign, the Cas9 protein can provoke a long-term immune response, and both versions of the Cas9 enzyme, most appreciated by molecular experimental biologists, originate from common bacteria that can exist in the human body, staphylococci and streptococci, therefore, it is quite possible that many people have already formed immunity against these proteins.
In a study conducted by a group of hematologists led by Matthew Porteus and Kenneth Weinberg from Stanford University in California, blood samples from 22 children and 12 healthy adults were analyzed for the presence of an immune response against the two most widely used variants of the Cas 9 enzyme by experimenters.
79 percent of the study participants had antibodies against Cas9 from Staphlicoccus aureus, and in 65 percent – on an enzyme from Streptococcus pyogenes. In a companion experiment, half of the 13 adult participants produced immune cells – T lymphocytes, which were targeted at Cas9 from S.aureus. Thus, the detected immune responses can sabotage gene therapy: antibodies against Cas9 can bind the enzyme in the bloodstream before it starts acting in the right place, and T cells targeting this protein can destroy the cells in which it should be produced.
"Can we assume that these data put an end to gene therapy with the CRISPR system?", asks Nature, and answers: "most likely not," given the huge interest in the approach and a rich selection of alternative enzymes for cutting DNA, not to mention that Cas9 can be slightly modified, and then it will avoid already the existing immune response in the body.
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