Gain of function mutations in CgPDR1 of Candida glabrata not only mediate antifungal resistance but also enhance virulence

A yeast cell-based system for screening Candida glabrata multidrug resistance reversal agents and selection of loss-of-function pdr1 mutants

In the pathogenic yeast Candida glabrata, multidrug resistance is associated with the overexpression of drug efflux pumps caused by gain-of-function mutations in the CgPDR1 gene. CgPdr1p transcription factor, which activates the expression of several drug efflux transporter genes, is considered to be a promising target for compounds sensitizing the multidrug-resistant yeast cells. Here, we describe a cell-based screening system for detecting the inhibitory activity of compounds interfering with the CgPdr1p function in a heterologous genetic background of the hypersensitive Saccharomyces cerevisiae mutant strain. The screening is based on the ability to abrogate the growth defect of cells suffering from the galactose-induced and CgPdr1p-driven overexpression of a dominant lethal pma1(D378N) allele placed under the control of the ScPDR5 promoter. The system allows rapid identification of multidrug resistance reversal agents inhibiting the CgPdr1p activity or loss-of-function Cgpdr1 mutations, and is amenable to high-throughput screening on solid or liquid media.

Regulation of the CgPdr1 transcription factor from the pathogen Candida glabrata

Candida glabrata is an opportunistic human pathogen that is increasingly associated with candidemia, owing in part to the intrinsic and acquired high tolerance the organism exhibits for the important clinical antifungal drug fluconazole. This elevated fluconazole resistance often develops through gain-of-function mutations in the zinc cluster-containing transcriptional regulator C. glabrata Pdr1 (CgPdr1). CgPdr1 induces the expression of an ATP-binding cassette (ABC) transporter-encoding gene, CgCDR1. Saccharomyces cerevisiae has two CgPdr1 homologues called ScPdr1 and ScPdr3. These factors control the expression of an ABC transporter-encoding gene called ScPDR5, which encodes a homologue of CgCDR1. Loss of the mitochondrial genome (ρ(0) cell) or overexpression of the mitochondrial enzyme ScPsd1 induces ScPDR5 expression in a strictly ScPdr3-dependent fashion. ScPdr3 requires the presence of a transcriptional Mediator subunit called Gal11 (Med15) to fully induce ScPDR5 transcription in response to ρ(0) signaling. ScPdr1 does not respond to either ρ(0) signals or ScPsd1 overproduction. In this study, we employed transcriptional fusions between CgPdr1 target promoters, like CgCDR1, to demonstrate that CgPdr1 stimulates gene expression via binding to elements called pleiotropic drug response elements (PDREs). Deletion mapping and electrophoretic mobility shift assays demonstrated that a single PDRE in the CgCDR1 promoter was capable of supporting ρ(0)-induced gene expression. Removal of one of the two ScGal11 homologues from C. glabrata caused a major defect in drug-induced expression of CgCDR1 but had a quantitatively minor effect on ρ(0)-stimulated transcription. These data demonstrate that CgPdr1 appears to combine features of ScPdr1 and ScPdr3 to produce a transcription factor with chimeric regulatory properties.

From Saccharomyces cerevisiae to Candida glabratain a few easy steps: important adaptations for an opportunistic pathogen

The opportunistic human fungal pathogen Candida glabrata is closely related to Saccharomyces cerevisiae, yet it has evolved to survive within mammalian hosts. Which traits help C. glabrata to adapt to this different environment? Which specific responses are crucial for its survival in the host? The main differences seem to include an extended repertoire of adhesin genes, high drug resistance, an enhanced ability to sustain prolonged starvation and adaptations of the transcriptional wiring of key stress response genes. Here, we discuss the properties of C. glabrata with a focus on the differences to related fungi.

Microarray and molecular analyses of the azole resistance mechanism in Candida glabrata oropharyngeal isolates

DNA microarrays were used to analyze Candida glabrata oropharyngeal isolates from seven hematopoietic stem cell transplant recipients whose isolates developed azole resistance while the recipients received fluconazole prophylaxis. Transcriptional profiling of the paired isolates revealed 19 genes upregulated in the majority of resistant isolates compared to their paired susceptible isolates. All seven resistant isolates had greater than 2-fold upregulation of C. glabrata PDR1 (CgPDR1), a master transcriptional regulator of the pleiotropic drug resistance (PDR) network, and all seven resistant isolates showed upregulation of known CgPDR1 target genes. The altered transcriptome can be explained in part by the observation that all seven resistant isolates had acquired a single nonsynonymous mutation in their CgPDR1 open reading frame. Four mutations occurred in the regulatory domain (L280P, L344S, G348A, and S391L) and one in the activation domain (G943S), while two mutations (N764I and R772I) occurred in an undefined region. Association of azole resistance and the CgPDR1 mutations was investigated in the same genetic background by introducing the CgPDR1 sequences from one sensitive isolate and five resistant isolates into a laboratory azole-hypersusceptible strain (Cgpdr1 strain) via integrative transformation. The Cgpdr1 strain was restored to wild-type fluconazole susceptibility when transformed with CgPDR1 from the susceptible isolate but became resistant when transformed with CgPDR1 from the resistant isolates. However, despite the identical genetic backgrounds, upregulation of CgPDR1 and CgPDR1 target genes varied between the five transformants, independent of the domain locations in which the mutations occurred. In summary, gain-of-function mutations in CgPDR1 contributed to the clinical azole resistance, but different mutations had various degrees of impact on the CgPDR1 target genes.

Molecular analysis of Candida glabrata clinical isolates

Candida glabrata is an important human pathogen, and an understanding of the genetic relatedness of its clinical isolates is essential for the prevention and control of fungal infections. In this study, we determined the relatedness of 38 Candida glabrata clinical isolates originating from two teaching hospitals in Slovakia. The 14 different genotypes were found by using microsatellite marker analysis (RPM2, MTI and Cg6) and DNA sequencing for analysis of the entire ERG11 gene. Subsequent sequencing of amplified DNA fragments of the PDR1, NMT1, TRP1 and URA3 loci in ten selected clinical isolates revealed identical DNA sequence profiles in five of them. They displayed the same microsatellite marker sizes and contained the same H576Y amino acid substitution recently described in the Pdr1p multidrug resistance transcription factor responsible for azole resistance. These results demonstrate the genetic diversity of C. glabrata clinical isolates in our hospitals and indicate a common clonal origin of some drug resistant ones.

Oropharyngeal candidiasis in the era of antiretroviral therapy

Oropharyngeal candidiasis (OPC) remains a common problem in the HIV-infected population despite the availability of antiretroviral therapy (ART). Although Candida albicans is the most frequently implicated pathogen, other Candida species also may cause infection. The emergence of antifungal resistance within these causative yeasts, especially in patients with recurrent oropharyngeal infection or with long-term use of antifungal therapies, requires a working knowledge of alternative antifungal agents. Identification of the infecting organism and antifungal susceptibility testing enhances the ability of clinicians to prescribe appropriate antifungal therapy. Characterization of the responsible mechanisms has improved our understanding of the development of antifungal resistance and could enhance the management of these infections. Immune reconstitution has been shown to reduce rates of OPC, but few studies have evaluated the current impact of ART on the epidemiology of OPC and antifungal resistance in these patients. Preliminary results from an ongoing clinical study showed that in patients with advanced AIDS, oral yeast colonization was extensive, occurring in 81.1% of the 122 patients studied, and symptomatic infection occurred in one-third. In addition, resistant yeasts were still common, occurring in 25.3% of patients colonized with yeasts or with symptomatic infection. Thus, OPC remains a significant infection in advanced AIDS, even with ART. Current knowledge of the epidemiology, pathogenesis, clinical presentation, treatment, and mechanisms of antifungal resistance observed in OPC are important in managing patients with this infection and are the focus of this review.

Antifungal drug resistance mechanisms in fungal pathogens from the perspective of transcriptional gene regulation

Fungi are primitive eukaryotes and have adapted to a variety of niches during evolution. Some fungal species may interact with other life forms (plants, insects, mammals), but are considered as pathogens when they cause mild to severe diseases. Chemical control strategies have emerged with the development of several drugs with antifungal activity against pathogenic fungi. Antifungal agents have demonstrated their efficacy by improving patient health in medicine. However, fungi have counteracted antifungal agents in several cases by developing resistance mechanisms. These mechanisms rely on drug resistance genes including multidrug transporters and drug targets. Their regulation is crucial for the development of antifungal drug resistance and therefore transcriptional factors critical for their regulation are being characterized. Recent genome-wide studies have revealed complex regulatory circuits involving these genetic and transcriptional regulators. Here, we review the current understanding of the transcriptional regulation of drug resistance genes from several fungal pathogens including Candida and Aspergillus species.

Functional analysis of cis- and trans-acting elements of the Candida albicans CDR2 promoter with a novel promoter reporter system

Azole resistance in Candida albicans can be mediated by the upregulation of the ATP binding cassette transporter genes CDR1 and CDR2. Both genes are regulated by a cis-acting element called the drug-responsive element (DRE), with the consensus sequence 5'-CGGAWATCGGATATTTTTTT-3', and the transcription factor Tac1p. In order to analyze in detail the DRE sequence necessary for the regulation of CDR1 and CDR2 and properties of TAC1 alleles, a one-hybrid system was designed. This system is based on a P((CDR2))-HIS3 reporter system in which complementation of histidine auxotrophy can be monitored by activation of the reporter system by CDR2-inducing drugs such as estradiol. Our results show that most of the modifications within the DRE, but especially at the level of CGG triplets, strongly reduce CDR2 expression. The CDR2 DRE was replaced by putative DREs deduced from promoters of coregulated genes (CDR1, RTA3, and IFU5). Surprisingly, even if Tac1p was able to bind these putative DREs, as shown by chromatin immunoprecipitation, those from RTA3 and IFU5 did not functionally replace the CDR2 DRE. The one-hybrid system was also used for the identification of gain-of-function (GOF) mutations either in TAC1 alleles from clinical C. albicans isolates or inserted in TAC1 wild-type alleles by random mutagenesis. In all, 17 different GOF mutations were identified at 13 distinct positions. Five of them (G980E, N972D, A736V, T225A, and N977D) have already been described in clinical isolates, and four others (G980W, A736T, N972S, and N972I) occurred at already-described positions, thus suggesting that GOF mutations can occur in a limited number of positions in Tac1p. In conclusion, the one-hybrid system developed here is rapid and powerful and can be used for characterization of cis- and trans-acting elements in C. albicans.

Reliability of the Vitek 2 yeast susceptibility test for detection of in vitro resistance to fluconazole and voriconazole in clinical isolates of Candida albicans and Candida glabrata

The Vitek 2 yeast susceptibility test was evaluated by testing 122 Candida isolates against fluconazole and voriconazole. Excellent categorical agreement with the CLSI broth microdilution method was observed (97.5% for both the azoles). Moreover, the Vitek 2 system was able to identify all but 2 of 59 investigated fluconazole-resistant organisms.

Hypersusceptibility to azole antifungals in a clinical isolate of Candida glabrata with reduced aerobic growth

Petite mutations have been described in Saccharomyces cerevisiae and pathogenic yeasts. However, previous studies of the phenotypic traits of these petite mutants reported that they express azole resistance. We describe a clinical isolate of Candida glabrata with a striking association between increased susceptibility to azoles and respiratory deficiency. This isolate was obtained from a urine sample together with a respiration-competent C. glabrata isolate which exhibited azole resistance. The respiratory status of the two isolates was confirmed by cultivation on glycerol-containing agar and oxygraphy. Flow cytometry revealed the normal incorporation of rhodamine 123, and mitochondrial sections with typical cristae were seen by transmission electron microscopy for both isolates. Together, these results suggested a nuclear origin for the reduced respiratory capacity of the hypersusceptible isolate. The sterol contents of these isolates were similar to the sterol content of a reference strain. Sequencing of the ERG11 and PDR1 genes revealed that the sequences were identical in the two isolates, demonstrating their close relatedness. In addition to silent mutations, they carried a nonsense mutation in PDR1 that led to the truncation of transcription factor Pdr1p. They also overexpressed both PDR1 and one of its targets, CDR1, providing a possible explanation for the azole resistance of the respiration-competent isolate. In conclusion, in addition to azole resistance, which is a common feature of C. glabrata mitochondrial petite mutants, the mutation of a nuclear gene affecting aerobic growth may lead to azole hypersusceptibility; however, the mechanisms underlying this phenotype remain to be determined.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.