For parasites like those that cause malaria to thrive in the human body, they must produce proteins that drive their growth, reproduction, and nutrition. Existing malaria drugs work by blocking parts of this protein machinery, but the pathogens frequently become resistant to these drugs by evolving ways to block the drugs from binding to or passing through their external membranes.
Now, Yale researchers have discovered a different approach that works against malaria parasites and other pathogens by destroying the genetic material that encodes some of these pathogens’ most vital proteins.
Over the past few years, Sidney Altman, Ph.D., Sterling Professor of Molecular, Cellular, and Developmental Biology, has developed a technique to destroy specific pieces of a cell’s messenger RNA (mRNA) with a compound called a peptide-morpholino oligomer (PMO) conjugate. He and his colleagues have shown that PMO conjugates can kill bacteria, including E. coli and staph bacteria, by stopping the production of proteins the pathogens require to survive.When the PMO binds its targeted mRNA sequence, this structure is recognized by an enzyme in human red blood cells that destroys the mRNA, preventing it from being transcribed into proteins in the pathogen.
“We think that this technique has general importance and has lots of applications,” says Altman, who shared the 1989 Nobel Prize in Chemistry for helping to establish that RNA can act as an enzyme as well as a carrier of genetic information.
For the new work, Altman collaborated with malaria expert Choukri Ben Mamoun, Ph.D., associate professor of internal medicine and microbial pathogenesis, and Ben Mamoun’s postdoctoral fellow Yoann Augagneur, Ph.D., to test whether the PMO conjugates are effective against Plasmodium falciparum, the parasitic species that causes the most severe cases of malaria. As a proof of concept that PMO conjugates could enter P. falciparum cells and cause its mRNA to degrade, the team of scientists developed a PMO that binds the mRNA coding for an enzyme known as gyrase, the same type of sequence that Altman had targeted in other pathogens.
“There are genes that we know are essential to the parasite but we can’t knock them out using classical genetics,” says Ben Mamoun. “But using this oligomer, we can block the genes and study what happens.” As reported in the April 2 issue of the Proceedings of the National Academy of Sciences, when the gene for gyrase was blocked, the malaria parasites died. “The next step is to test new targets and optimize the conjugates,” says Augagneur. “We can use different conjugates against the same mRNA targets and improve the activity by two- to three-fold, and we can also find new targets.” The drugs are an optimal way to target the malaria parasite, he says, because resistance, a problem with many other drugs, is unlikely to develop against the PMOs. “In principle, there wouldn’t be resistance because you would need several mutations right next to each other in order to stop the conjugate from working.”
And because you can selectively choose where on a stretch of mRNA the PMO binds, scientists can select sequences that don’t resemble any human genes, so the PMO won’t interfere with normal body processes.
“All existing anti-malarial drugs have side effects and some of them are very severe,” says Ben Mamoun. “But, although we haven’t done human trials yet, we would predict that these oligomers would be quite safe because of their specificity and selectivity.”
While Ben Mamoun and Augagneur pursue new ways to use the PMOs against malaria parasites, members of Altman’s lab plan to move on to other types of pathogens.
“There are lots of parasites that affect human cells and we just have to find out which ones are attractive as targets of this drug,” says Altman. “It’s very easy to redesign our compound to attack anything we want to look at.”