Search

ABSTRACTCandida albicansis an important fungal pathogen with a diploid genome that can adapt to caspofungin, a major drug from the echinocandin class, by a reversible loss of one copy of chromosome 5 (Ch5). Here, we explore a hypothesis that more than one gene for negative regulation of echinocandin tolerance is carried on Ch5. We constructed C. albicansstrains that each lacked one of the following Ch5 genes: CHT2for chitinase, PGA4for glucanosyltransferase, and CSU51, a putative transcription factor. We demonstrate that independent deletion of each of these genes increased tolerance for caspofungin and anidulafungin, another echinocandin. Our data indicate that Ch5 carries multiple genes for negative control of echinocandin tolerance, although the final number has yet to be established.

ABSTRACTCandida albicans, a prevailing opportunistic fungal pathogen of humans, has a diploid genome containing three homologous FKSgenes that are evolutionarily conserved. One of these, the essential gene FKS1, encodes the catalytic subunit of glucan synthase, which is the target of echinocandin drugs and also serves as a site of drug resistance. The other two glucan synthase-encoding genes, FKS2and FKS3, are also expressed, but their roles in resistance are considered unimportant. However, we report here that expression of FKS1is upregulated in strains lacking either FKS2or FKS3. Furthermore, in contrast to what is observed in heterozygous FKS1deletion strains, cells lacking FKS2or FKS3contain increased amounts of cell wall glucan, are more resistant to echinocandin drugs, and consistently are tolerant to cell wall-damaging agents. Our data indicate that C. albicansFKS2and FKS3can act as negative regulators of FKS1, thereby influencing echinocandin susceptibility.

ABSTRACTExpanding echinocandin use to prevent or treat invasive fungal infections has led to an increase in the number of breakthrough infections due to resistant Candidaspecies. Although it is uncommon, echinocandin resistance is well documented for Candida albicans, which is among the most prevalent bloodstream organisms. A better understanding is needed to assess the cellular factors that promote tolerance and predispose infecting cells to clinical breakthrough. We previously showed that some mutants that were adapted to growth in the presence of toxic sorbose due to loss of one chromosome 5 (Ch5) also became more tolerant to caspofungin. We found here, following direct selection of mutants on caspofungin, that tolerance can be conferred by at least three mechanisms: (i) monosomy of Ch5, (ii) combined monosomy of the left arm and trisomy of the right arm of Ch5, and (iii) an aneuploidy-independent mechanism. Tolerant mutants possessed cell walls with elevated chitin and showed downregulation of genes involved in cell wall biosynthesis, namely, FKS, located outside Ch5, and CHT2, located on Ch5, irrespective of Ch5 ploidy. Also irrespective of Ch5 ploidy, the CNB1and MID1genes on Ch5, which are involved in the calcineurin signaling pathway, were expressed at the diploid level. Thus, multiple mechanisms can affect the relative expression of the aforementioned genes, controlling them in similar ways. Although breakthrough mutations in two specific regions of FKS1have previously been associated with caspofungin resistance, we found mechanisms of caspofungin tolerance that are independent of FKS1and thus represent an earlier event in resistance development.