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June 20, 2016

CRISPR Screen Identifies Potential Drug Target for Mosquito-Borne Virus

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    A team at Washington University School of Medicine in St. Louis has identified a single-gene pathway that is vital for Zika and other flaviviruses to spread infection between cells. [NIH

    Flavivirus represent a medically important genus of pathological viruses that are typically transmitted through the bite from an infected arthropod (mosquito or tick). This genus includes the viruses that cause yellow fever—for which the group gets its name, as flavus means yellow in Latin—dengue, West Nile Virus, Zika, and tick-borne encephalitis. Flaviviruses are genetically comprised of single-stranded RNA and share common physical appearances such as size and symmetry.

    Now, at a team of researchers at the Washington University School of Medicine (WUSM) has identified a single-gene pathway that is vital for viruses like Zika to spread infection between cells. Moreover, they showed that shutting down a single gene in this pathway—in both human and insect cells—does not negatively affect the cells themselves and renders flaviviruses unable to leave the infected cell, curbing the spread of infection. This new data points to a potential drug target against flaviviruses that have major impacts on public health.

    "We wanted to find out if we could identify genes present in the host cells that are absolutely required by the virus for infection," explained senior study author Michael Diamond, M.D., Ph.D., professor of medicine and WUSM. "Out of about 19,000 genes that we looked at, we only found nine key genes that the virus relies on for infection or to spread. All of them are associated with an important part of the cell that processes viral particles, which is essential to spreading the infection."

    To identify the genes that are critical to flaviviruses, the investigators used CRISPR technology to shut down individual host genes selectively. Because viruses must hijack host cells to replicate and spread, they are dependent upon the genetic material of the organisms they infect. If a cell lacks a gene that the virus requires for infection, the virus will be stopped in its tracks and the cell will survive. Such evidence indicates that the missing gene is vital to viral spread and should be studied further.

    “We performed a genome-wide CRISPR/Cas9-based screen to identify host genes that, when edited, resulted in reduced flavivirus infection,” the authors wrote. “We validated nine human genes required for flavivirus infectivity, and these were associated with endoplasmic reticulum functions including translocation, protein degradation, and N-linked glycosylation. In particular, a subset of endoplasmic reticulum-associated signal peptidase complex (SPCS) proteins was necessary for proper cleavage of the flavivirus structural proteins (prM and E) and secretion of viral particles.”

    The findings from this study were published recently in Nature in an article entitled “A CRISPR Screen Defines a Signal Peptide Processing Pathway Required by Flaviviruses.”

    The WUSM team uncovered nine key genes that when disabled, not only reduce viral infection but also appear to have no adverse effects on the cells the scientists studied. The researchers performed the first experiments on West Nile virus and then showed that the same results held true for other Flaviviridae family members, including Zika, dengue, yellow fever, and Japanese encephalitis. 

    Interestingly, while the absence of this gene shut down the spread of flaviviruses, the researchers found that eliminating the gene had no detrimental effect on other types of viruses, including alphaviruses, bunyaviruses, and rhabdoviruses.

    "Flaviviruses appear to be uniquely dependent on this particular gene to release the viral particle," Dr. Diamond noted. "In these viruses, this gene sets off a domino effect that is required to assemble and release the viral particle. Without it, the chain reaction doesn't happen, and the virus can't spread. So we are interested in this gene as a potential drug target because it disrupts the virus and does not disrupt the host."

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