Pathogenic fungi reveal new mechanism for evolution

CAMBRIDGE, Mass. (September 29, 2005) - As medical procedures that compromise a person's immune system become more common, opportunistic infections that exploit these weaknesses are an increasing public health concern. Pathogenic fungi are chief among these invaders that prey upon organ-transplant recipients and chemotherapy patients. Not only are these infectious agents a scourge in hospitals, but they have also demonstrated an alarming ability to quickly sidestep drug treatment.

Now, researchers in the laboratory of Whitehead Member Susan Lindquist have identified a key mechanism that enables pathogenic fungi to evolve drug-resistant capabilities with such distressing rapidity.

"We are extremely excited about the potential of Hsp90 inhibitors for treating infections in the clinic, but the implications here are much broader," says Lindquist, whose paper will be published in the September 30 issue of the journal Science. "We now have a clearer understanding of the forces that shape the evolution of a new trait and the molecular mechanisms by which it is accomplished. The potential to evolve is an inherent biological property of the organism."

In a perspective piece that accompanies the paper, Joseph Heitman of Duke University Medical Center states, "The emergence of drug resistance in pathogenic microbes provides a resounding validation of Darwinian evolution."

For many years, Lindquist and her colleagues have been poring over a protein called Hsp90. Hsp is an acronym for "heat shock protein," meaning that the protein responds rapidly to certain environmental stresses, such as elevated temperatures.

But Hsp90 is also what's called a "chaperone protein," a member of a family of proteins dedicated to helping fellow proteins assume their proper shapes. Proteins are often compared to origami figures: They fold into a vast array of conformations whose precision is essential to the cell's well being. One misfold can prove toxic to the cell. Hsp90 directs this folding process in a wide range of key proteins.

In a 2002 Nature paper, Lindquist and her research team found that by lowering levels of Hsp90 in plants, they could cause sudden and unexpected changes in a vast array of traits, including pigmentation and leaf shape. (These results paralleled a 1998 Nature paper in which Lindquist and colleague Suzanne Rutherford discovered the same mechanism in fruit flies.) As a result of these findings, the team deduced that these plants must, over time, gradually accumulate genetic variations that Hsp90 somehow manages to keep in check. But if Hsp90 is compromised, perhaps due to an environmental stress, this reserve of dormant mutations is suddenly unleashed.

Leah Cowen, a postdoctoral scientist in the Lindquist lab, was interested in using the model yeast Saccharomyces cerevisiae to study how Hsp90 might contribute to evolution. Further, she wanted to investigate whether Hsp90 enabled pathogenic fungi such as Candida albicans and Aspergillus species-prime sources of hospital infections-to resist treatment.

To do this, Cowen designed experiments in which fungal cells were exposed to two common classes of antifungal drugs: azoles and echinocandins. Azoles, such as fluconazole, are the most widely prescribed antifungals, while echinocandins are the newest class of these drugs to reach the clinic.

Cowen genetically engineered fungal strains to have either high levels of Hsp90, or low levels, and then exposed them to these drugs. Results were striking.

"Strains with high levels of Hsp90 could rapidly evolve drug resistance, while the ability of strains with low levels of Hsp90 to evolve resistance was impaired," says Cowen. Further, Cowen also looked at strains that had already evolved drug resistance through many different mutations. Even in these cases, "when Hsp90 levels were reduced these new resistance traits were lost."

Cowen also found that once fungal strains became drug resistant, they could eventually evolve the ability to retain this resistance independent of Hsp90.

"Ultimately, these results establish a new role for Hsp90 in the evolution of adaptive traits," says Cowen. "Looking at this more broadly, this research also helps us to understand more clearly how evolution occurs at a molecular level."

"This has broad therapeutic implications as well," says Lindquist, who also is a professor of biology at MIT. "Drugs that inhibit Hsp90 might prove to be an effective strategy for combating these infections." Such drugs, theoretically at least, could both render these pathogens more responsive to treatment and prevent them from ever developing such resistance in the first place.

***

Leah E Cowen and Susan Lindquist. "Hsp90 Potentiates the Rapid Evolution of New Traits: Drug Resistance in Diverse Fungi." Science, Vol 309, September 30, 2005.