BY RICHARD KESSIN, PH.D. ASSOCIATE DEAN FOR GRADUATE AFFAIRS
C. albicans presents a lot of experimental difficulties for basic scientists. Baker's yeast is haploid and has a well defined sexual cycle to manipulate and combine mutations. C. albicans is diploid and if you want to make a mutant and ask whether the affected gene and the protein it codes for are essential for pathogenicity or would make a good drug target, you have to mutate two copies of the gene, which has been hard to do. And if you make these mutations, you still can't move them from one strain to another and make different combinations because the wretched Candida has no sexual cycle.
Because colonies of Candida and Saccharomyces look alike on petri dishes, biologists had been lulled into thinking that they share a great majority of genes. They looked to Saccharomyces, whose genome and genetics have both been worked out, for inspiration about Candida. But with the Candida genome newly sequenced and annotated so that many of the genes are assigned a putative function we are left with a large number that have no relatives (homologues) in Saccharomyces. How odd! But how to get at them and how to ask which are involved in infection and which can be attacked with new drugs?
Here the laboratory of Aaron Mitchell, Ph.D., professor of microbiology, has contributed mightily, first to the annotation of the 7,000 genes that make this organism its unpleasant self and, now, a method to mutate both copies of every gene in the genome and open the way to understanding more about the mechanisms of pathogenicity. Sometimes when the easy way fails, you have to back up, develop new methods, and try again.
But what aspect of pathogenesis should one investigate? Anyone who has ever seen thrush, which is one infection caused by Candida albicans, realizes right away that these organisms do not grow in solitude, but rather form mats or biofilms. These also can be found on in-dwelling catheters. Biofilms are not just piles of cells, but rather differentiated structures that are produced in a temporally regulated way. During the process Candida extends hyphae (long filaments) and secretes an extracellular matrix of polysaccharide and protein. As in the case of bacterial biofilms, Candida structures keep out drugs and maintain the population of fungi.
Which of the 7,000 genes are dedicated to making biofilms and do any of them have an Achilles' heel? Using the new genetic techniques, Clarissa Nobile, Vincent Bruno, and postdoctoral fellow Mathias Richard have isolated mutants that are defective in the formation of biofilms. So far there are three mutants that block the beginning of biofilm formation and one that allows it to start but not to progress. The group calculates that something over 100 genes may be involved. The problem the lab now faces is to figure out what these genes do. One of the newly discovered genes is necessary for respiration, but the others are a mystery. Perhaps their exact functions can be identified and these will be sufficiently different for the processes of the host to allow novel drug development. We are, after all, starting with genes that are not present in humans or other organisms. We have not had a stellar history of success with controlling Candida or other fungi, especially in immune-compromised people, and the Mitchell laboratory is now returning to square one. Armed with the full genome sequence and their new genetic technology, they will not stay there long.
More information on Dr. Mitchell's work can be found at cumicro2.cpmc.columbia.edu/
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