FAYETTEVILLE : Scientists focus on fighting fungus
Posted on Monday, November 5, 2007
FAYETTEVILLE — As Kristin McCue peers into a test tube, she immediately knows her genetically engineered yeast strain is not cooperating today.
A scientist at the University of Arkansas at Fayetteville, Mc-Cue’s ultimate goal is finding answers in unexplored areas in genetics, leading to more effective medicines to fight a fungus that can kill organ transplant patients.
Her test subjects: twin genes — and the proteins they help make — in the fungus, a yeast organism known as Candida albicans.
Learning how the genes function in the body is a long, tedious process that doesn’t always go smoothly in the laboratory.
McCue can tell her samples aren’t growing quickly because they should be cloudier. She may have to start over with a new test strain.
But even problem experiments provide clues that can one day help solve the mystery, the biological sciences researcher said.
Her quest will involve many experiments and variables.
Organ transplant patients are susceptible to a dangerous infection of Candida albicans yeast, which can enter the bloodstream and infect the heart, liver, kidneys or lungs.
McCue received an $ 85, 000 grant from the American Heart Association to study the genetics of the fungus.
She is nearly midway through the grant’s two-year research period, which will end Dec. 31, 2008.
McCue’s grant is among six new research awards that the national association has given in Arkansas this year, which range from $ 25, 000 to $ 260, 000, and is one of 13 active projects it has funded, said Terri Clark, spokesman for the American Heart Association of Arkansas.
In 2005, 2, 125 heart transplants were performed in the United States, compared with 2, 016 the year before, she said.
As a post-doctoral researcher, McCue is no longer a student and not yet teaching in university classrooms. She works in a lab run by David McNabb, an associate professor of biological sciences in UA’s Fulbright College.
McCue and McNabb say people are more familiar with Candida albicans in its localized form. It is responsible for some types of diaper rash, oral thrush and vaginal yeast infections. These milder infections can be treated with topical medications such as creams.
The yeast also can be found in the intestinal tract. Candida albicans can multiply out of control when the beneficial flora — sometimes called “good bacteria” — in the intestines have been depleted, as with use of antibiotics, or when the immune system is suppressed, McNabb said.
But once the yeast enters the blood, it can infect major organs, often with deadly results.
“That’s why we describe it as an opportunistic pathogen,” McNabb said. “Some studies have said 40 percent of patients will get the systemic infection.”
In an organ transplant patient, he continues, the immune system becomes compromised. The patient experiences trauma from the invasive surgery. The drugs given to keep the body from rejecting the new heart work by weakening the body’s natural defenses. This increases the risk for a systemic infection of the yeast.
“Systemic infections, they’re less responsive to available antifungals,” McCue said, adding that today’s antifungal medicines can have severe side effects.
McNabb said his lab devotes its work to an entire spectrum of problems surrounding Candida albicans yeast and how it becomes pathogenic, focusing on a complex of four proteins that are required for growth.
They are heme-activated proteins. Heme is an organic molecule that traps oxygen in yeast and in humans, McNabb said, adding that it is a component of blood hemoglobin, which transports oxygen through the bloodstream.
The researchers name each heme-activated protein and its corresponding gene by shortening it to “HAP” and assigning a number. The names are then logged into shared databases used by researchers around the world who are studying parts of the human genome, McCue says while demonstrating on a lab computer.
One of the four proteins, HAP 3, is somewhat unusual in that it has two different genes that code for it rather than one. McCue’s project is devoted to these two versions of HAP 3.
“We call it the HAP 31 gene, which encodes, or helps make, the HAP 31 protein,” McNabb said. The other gene McCue studies is HAP 32.
HAP 31 and HAP 32 function like twins who have a secret agreement between themselves about their division of labor, McNabb said. McCue is trying to find out the terms of this agreement, such as what task each twin has agreed to perform, or whether their duties are divided according to assigned genes.
McCue won’t know until she genetically engineers a variety of strains and runs a battery of tests on each.
“We’re interested in how genes are regulated, and how they affect the ability of Candida to cause disease,” McNabb says.
In one test strain, McCue deletes HAP 31. In another, she deletes HAP 32, and in yet another she deletes both.
Each time, she grows the mutant strains in a starter liquid containing the original strain, using recombinant DNA technology.
“We modify the DNA, basically in the test tube, and then reintroduce that modified gene into a wild cell that has all the genes, so that the new strain will take over.”
“This is our typical nutrientrich broth,” McCue said as she picked up a flask of semi-thick, honey-colored liquid. “It has lots of sugars, and amino acids and vitamins.”
Another beaker contains the same mixture with an added chemical that binds to the “free iron” in the medium, turning it a ruby red. Normally, the yeast organism lives in high iron conditions in the intestines and low iron conditions in the blood, so that’s where the high-iron, red liquid medium and the honeycolored mixture come in, as well as Petri dishes where the strains can be tested with different amounts of nutrients, such as sugar, nitrogen and acidity levels.
One of McCue’s strains didn’t grow well in the low-iron environment, which tells her that the missing gene is probably needed in low-iron conditions.
McCue’s research could have wider applications than yeast threats for transplant patients.
The Candida albicans yeast is often one of the first signs that an HIV-positive patient has developed full-blown AIDS, McNabb said.
“Usually they present with oral thrush,” McNabb said, describing this as one of the localized infections, and one that produces white mucous in the throat.
McCue’s work on HAP 31 and HAP 32 also could unlock genetic mysteries involving immunity problems among chemotherapy patients, he said, among other medical problems.
“The equivalent proteins in humans are involved in regulating about 30 percent of the human genome,” McNabb said. “So if we understand what it does in this particular organism, then that understanding of turning on and off certain genes can be applied to understanding the equivalent proteins’ behavior in higher life forms.”
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