Nov. 23, 2000 Researchers have identified a gene that allows the fungus that causes histoplasmosis to infect and persist in humans. They used a novel technique that may prove valuable in the search for drug targets in this and other disease-causing fungi.
"Histoplasmosis occurs all over the world, but the greatest concentration of cases is in the Mississippi and Ohio river valleys," says William E. Goldman, Ph.D., senior author of the paper in the Nov. 17 issue of Science. Goldman is a professor of molecular microbiology at Washington University School of Medicine in St. Louis.
About 80 percent of people who live in those areas become infected. Some have a cough or low-grade fever for a few days or longer; most experience no symptoms. But the infection may lie low for years. Then if a person's immune system is weakened by AIDS or chemotherapy, for example, the fungus may become active again. It can cause pneumonia. It also can spread from the lungs to the liver, spleen and other organs, causing serious illness or death.
Histoplasma capsulatum, the fungus that causes histoplasmosis, lives in soil. When its spores are inhaled, the higher temperatures in the respiratory system convert it to a yeast, which is taken in by white cells called macrophages. Ordinarily, macrophages kill invading pathogens, but Histoplasma is able to multiply inside them, eventually killing them.
Seeking to understand this process, Goldman's group studied a gene that codes for a calcium-grabbing protein. The gene is inactive when Histoplasma lives in soil but is switched on when the fungus converts to a yeast in the human body. The researchers thought this might explain why the yeast is able to thrive in macrophages within a special compartment that probably contains very little available calcium.
Using a new gene-altering technique, the researchers produced a mutant Histoplasma that was unable to secrete the calcium-binding protein. The mutant grew poorly inside macrophages and was unable to kill them. When inhaled by mice, it was a thousand times less effective in infecting the lungs.
The technique that showed that the calcium-binding protein is essential for infection may prove even more important than the finding itself. In microbiology research, evaluating the function of a particular gene is typically done by exchanging the original gene with a mutated version. But when this is attempted with Histoplasma, the mutated gene splices into random sites and usually fails to replace the original gene. Goldman’s group devised a recombinant DNA strategy that nearly eliminates these random events, making it possible to efficiently target a particular gene for replacement.
"This technique may prove useful in defining the roles other genes play in Histoplasma virulence and evaluating their potential as drug targets," Goldman says. "It also has a reasonable chance of working in other disease-causing fungi, many of which are understudied because of the same genetic manipulation obstacles."
Sebghati TS, Engle JT, Goldman WE. Intracellular parasitism by Histoplasma capsulatum: fungal virulence and calcium dependence. Science, Nov. 17, 2000.
This work was funded by the National Institute for Allergy and Infectious Diseases and the Burroughs-Wellcome Fund.
Map showing incidence of Histoplasmosis in U.S. is available at http://medicine.wustl.edu/~wumpa/news/2000/histomap.html.
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