Starting around 2012, a genetic technique called CRISPR/Cas9 rose to prominence in the fields of genetic research and biotechnology. It was seen as a huge breakthrough in genetic manipulation technology; with it, scientists and engineers were able to manipulate and alter DNA with immensely improved accuracy and substantially reduced costs.
CRISPR/Cas9 has already been used to make wild improvements in medicine and genetic engineering. For instance, one team used the technique to remove HIV DNA from human cells implanted in a mouse, apparently permanently curing the animal. Another team used the technology to tamper with a mosquito’s fertility to reduce its ability to spread. 2018 is set for even more exciting CRISPR news, with human trials for combating several diseases planned in the US and Europe.
However, a slight dampening has been put the CRISPR excitement: a new, not-yet-peer reviewed study suggests that some humans may have an immunity to CRISPR/Cas9, and that this immunity may pose an safety and efficacy issue moving forward.
What is CRISPR/Cas9?
The words CRISPR, Cas9, and CRISPR/Cas9 all refer to the same technology in the public vernacular. However, CRISPR is really a type of DNA sequence while Cas9 is a “DNA Scissors” protein that uses CRISPR to determine where to cut.
Cas9 is an enzyme used by bacteria to identify and destroy viruses. This enzyme works by taking a “mugshot” of part of a virus and seeking out DNA with a matching genetic sequence. If it finds a match, Cas9 cuts up the virus, disabling it and rendering it inert.
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is the DNA “mugshot” that Cas9 uses. It has some unique properties, as given by the name: the sequences are short, palandromic, repeat with some variation, and are regularly spaced in clusters around the bacteria’s DNA. The DNA sequences themselves have been copied from previously-identified viruses, which the bacteria had killed and incorporated into its genetic structure so that there is a record for Cas9 to use. Together, CRISPR and Cas9 are a critical immune function for many bacteria.
The CRISPR biotech revolution has come about as scientists learned to re-purpose the CRISPR system for their own benefit. By making their own DNA sequences, researchers can cause Cas9 to target a very precise location on a genome. The Cas9 slices the DNA open at that point, allowing genes to be added, removed, turned on and off, or altered.
The Immune Response
While many different types of bacteria have been shown to have a CRISPR/Cas9 style defence system, the two most well-developed systems come from Staphylococcus aureus and Streptococcus pyogenes. Both of these bacteria frequently infect humans and, as a result, some humans have developed immunity responses to the bacteria. This, the researchers of the aforementioned paper suspected, could cause problems if CRISPR was used on living human subjects.
One line of defence in the humans against bacterial infection is antibodies. The body produces a range of antibodies that freely circulate in the body, looking for some particular antigen to latch onto. When that antigen is found, the antibody attaches to it. A larger immune response is triggered, including the creation of more antibodies of the same type.
According to the performed research, more than half of human blood samples tested reacted to the Cas9 protein from both S. aureus and S. pyogenes. This means that these people’s bodies have identified Cas9 proteins as foreign and have created antibodies to disable them. As a result, injecting free-floating Cas9 into a patient for gene therapy will frequently cause an immune response. Furthermore, the protein will not reach its host cells and cause the desired genetic change.
The second immunoresponse that the researchers considered is the body’s ability to identify cells “infected” with the gene therapy. As part of the immune system, cells take parts of their contents and display them on their cell surface. Other cells, called T-Cells, migrate around the body, searching for cells that are expressing inappropriate contents. If such a cell is found, the T-Cell identifies it as infected and kills it.
The study tested samples from thirteen individuals to see if this immune response was also present. They found that half the samples had an immune response to cells infected with the S. aureus Cas9, but none responded to the S. pyogenes enzyme. Given the number of samples considered, the study cannot conclude that there is no human resistance to the Cas9 of S. pyogenes. Nevertheless, the findings suggest that the human immune response may identify and kill any cells that are successfully treated using Cas9 gene therapy techniques.
The finding are almost certainly not an unavoidable roadblock in the application of CRISPR to humans; many possible workaround have already been proposed. For instance, CRISPR could be used on cells that are extracted from the body, then re-inject once the Cas9 is no longer expressed on the surface of the cells. The protein could also be modified, or a similar protein used, to prevent an immune response.
The most important part of this research is not that new techniques might be required for gene therapy in humans; the importance is that CRISPR is capable of provoking an immune response in humans. Previous studies, using less-precise viruses as the gene vector, have provoked immune responses with unfortunate results. In general, trials show no therapeutic benefit if the patient has an immune response. More importantly, in 1999 a patient named Jesse Gelsinger died after the viral vector he was given caused a massive immune response.
CRISPR is still a revolutionary tool that has, and is expected to continue to see, widespread success as a medical and technological marvel. However, this research is a stark reminder that gene therapy, like any tool, has risks. With proper due-diligence and additional research, CRISPR can undoubtedly become a safe treatment with incredibly widespread applications that is routinely administered to great effect.