| Candida albicans is a fungal pathogen of humans that does not undergo meiosis. Yet, genomic variability and aneuploidy (a loss or gain of whole chromosomes or segments of a chromosome), are common in this organism. I adapted the technique of comparative genome hybridization (CGH) microarrays to analyze aneuploidy in laboratory and clinical isolates of C. albicans. CGH analysis of laboratory strains demonstrated that several standard strains exhibit whole chromosome trisomy and one series of strains has a segmental monosomy that occurred as a result of transformation. CGH analysis of 90 clinical isolates identified a high level of aneuploidy (51%) in fluconazole-resistant strains relative to fluconazole-sensitive strains (12%). Furthermore, a segmental aneuploidy of chromosome 5L (Ch5L) DNA was shown to correlate with fluconazole resistance (FluR). This segmental aneuploidy arose by the formation of an isochromosome (a chromosome with two identical arms) composed of two copies of Ch5L flanking the centromere. While i(SL) is relatively stable under rich culture conditions, loss of i(5L) can occur and i(5L) loss always correlates with a loss of FluR. Expression array analysis of i(5L) strains revealed that most genes on Ch5L exhibit increased expression. Two well-characterized genes involved in drug resistance, ERG11 and TAC1, are present on i(5L) and thus are present in increased gene copies. Deletion of two extra copies of either ERG] I or TA C1 in an i(5L) strain background decreased Flu R to similar MIC values, suggesting that each gene contributes equally to FluR. Furthermore, deletion of one copy of both ERG11 and TAC1 in the same i(5L) strain reduced the fluconazole MIC to the same level as a strain that lost of an entire Ch5L arm (via telomere-mediated chromosome truncation), suggesting ERG11 and TAC1 are major contributors to FluR on Ch5L. The effects of aneuploidy on gene copy number, gene expression level, LOH, and acquired azole resistance are discussed. |
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