When Felipe Santiago-Tirado, assistant professor in the Department of Biological Sciences at the University of Notre Dame, asks his students to name a disease caused by fungal organisms, most can only think of one: athlete’s foot. That condition, of course, is merely annoying.
“People have this idea that viruses and bacteria can kill you, but fungal infections don’t,” Santiago-Tirado said. “But that’s one of the main reasons that I always highlight, whenever I publish, that fungal infections are kind of neglected.”
However, with over 300 million people afflicted with serious fungal infections globally and about 1.6 million people dying from them annually, the study of fungi is crucial for preventing infections and saving lives.
In his paper published in mBio, the journal of the American Society for Microbiology, Santiago-Tirado and his colleagues describe a gene they discovered that increases the chance for drug resistance in the fungi Cryptococcus neoformans. The gene directs the cell to develop a type of transporter protein that typically forms a channel responsible for removing toxins from the fungi’s cell. The fungi cause several diseases, most frequently in the lungs, but the disease can travel to the brain and become fatal—of 300,000 cases annually, 200,000 people die.
When present, this gene (named PDR6) forms a transporter—which is atypical at only half the size of most other transporters—that pumps antifungal medications out of the cell in the same manner as other toxins. This process renders the therapy useless.
When the gene is deleted, fluconazole, an inexpensive, oral medication, is effective in killing the fungus because there is no transporter to pump the drug out of the cell. When the gene transporter is present, only an expensive, intravenous medication works well. This limits its use in many countries, and physicians will then opt for fluconazole. Unfortunately, this increases drug resistance already common in fungal infections, partly because the organisms are similar to human cells.
“So, there are antivirals, for viruses, and antibiotics, for bacteria, but those types of cells are very different from our cells,” he said. “Whatever kills the fungi can also kill your own cells, and because of that, there are very few antifungals and they are usually toxic.”
Pdr6 belongs to a family of transporter genes that are present in cells from all kingdoms of life, but is part of a subgroup that are only present in plants and fungi. The transporters pump molecules from one side of a membrane to another, evolving with repeated exposure to medications. Plants evolved these transporter pumps to remove environmental toxins like heavy metals, and fungi are no different.
Additionally, pdr6 appears to protect the fungi, Santiago-Tirado said. Through evolution, fungi have developed methods to avoid being eaten by amoebas, which are natural predators in their environment. This ability to avoid being consumed is duplicated in the lungs, but when the fungi enter the lungs of a person with a normally functioning immune system, the lung’s macrophages, which are similar to amoebas, kill it. In people with compromised immune systems, the fungi is not as readily recognized as a threat. It then proliferates, spreads to the brain, and kills most patients from the brain infection. When PDR6 is absent, however, the infection is restricted to the lungs, Santiago-Tirado said.
“If we find a way to inactivate this gene or somehow block the function of this gene, we can keep it in the lungs where it’s easier to treat, since it is much harder to get drugs into the brain,” said Santiago-Tirado, who is also affiliated with the Eck Institute for Global Health.
He said he was able to explore the mechanisms of the gene transporter thanks to a Sean Cocchia Rare Disease Research seed grant through Notre Dame’s Warren Center for Drug Discovery. Further research will involve delving into why the transporter created in the presence of pdr6 is half the size of other transporters, as well as how that transporter works.
“The one particular gene seems to be involved with so many important cellular processes, which is always nice for drug development,” he said. “Furthermore, PDR6 seems to be conserved in a variety of other pathogenic fungi, meaning that the mechanisms of antifungal resistance discovered in Cryptococcus may be broadly applicable to other fungi.”
Other authors on the student include Christopher J. Winski, a graduate student in Santiago-Tirado’s lab, Shahriar Mobashery, Navari Family Professor in Life Sciences in the Department of Chemistry and Biochemistry, and Yuanyuan Qing, a graduate student in Mobashery’s lab.