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Antimicrobial Agents and Chemotherapy, October 2008, p. 3701-3709, Vol. 52, No. 10
0066-4804/08/$08.00+0     doi:10.1128/AAC.00423-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

A Nonsense Mutation in the ERG6 Gene Leads to Reduced Susceptibility to Polyenes in a Clinical Isolate of Candida glabrata{triangledown}

Patrick Vandeputte,1,2* Guy Tronchin,1 Gérald Larcher,1 Emilie Ernoult,1,{dagger} Thierry Bergès,3 Dominique Chabasse,1,2 and Jean-Philippe Bouchara1,2

Groupe d'Etude des Interactions Hôte-Pathogène, UPRES-EA 3142, Université d'Angers Angers, France,1 Laboratoire de Parasitologie-Mycologie, Centre Hospitalier Universitaire, Angers, France,2 Physiologie Moléculaire des Transporteurs de Sucre, FRE 3091, Faculté des Sciences, 86022 Poitiers Cedex, France3

Received 31 March 2008/ Returned for modification 15 May 2008/ Accepted 28 July 2008


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ABSTRACT
 
Unlike the molecular mechanisms that lead to azole drug resistance, the molecular mechanisms that lead to polyene resistance are poorly documented, especially in pathogenic yeasts. We investigated the molecular mechanisms responsible for the reduced susceptibility to polyenes of a clinical isolate of Candida glabrata. Sterol content was analyzed by gas-phase chromatography, and we determined the sequences and levels of expression of several genes involved in ergosterol biosynthesis. We also investigated the effects of the mutation harbored by this isolate on the morphology and ultrastructure of the cell, cell viability, and vitality and susceptibility to cell wall-perturbing agents. The isolate had a lower ergosterol content in its membranes than the wild type, and the lower ergosterol content was found to be associated with a nonsense mutation in the ERG6 gene and induction of the ergosterol biosynthesis pathway. Modifications of the cell wall were also seen, accompanied by increased susceptibility to cell wall-perturbing agents. Finally, this mutation, which resulted in a marked fitness cost, was associated with a higher rate of cell mortality. Wild-type properties were restored by complementation of the isolate with a centromeric plasmid containing a wild-type copy of the ERG6 gene. In conclusion, we have identified the molecular event responsible for decreased susceptibility to polyenes in a clinical isolate of C. glabrata. The nonsense mutation detected in the ERG6 gene of this isolate led to a decrease in ergosterol content. This isolate may constitute a useful tool for analysis of the relevance of protein trafficking in the phenomena of azole resistance and pseudohyphal growth.


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INTRODUCTION
 
Despite extensive efforts to identify new cellular targets, only a few antifungal drugs are currently available for the treatment or prophylaxis of invasive candidiasis. These drugs belong to only four chemical families targeting three cellular mechanisms (28). Azole drugs, such as fluconazole and voriconazole, inhibit 14-alpha-sterol demethylase, a key enzyme of the ergosterol biosynthesis pathway. Polyene drugs, mainly amphotericin B, bind to ergosterol, which induces the formation of pores in the plasma membrane (8). Pyrimidine analogs, represented exclusively by flucytosine, interfere with DNA and protein synthesis, whereas echinocandins, such as the recently developed drugs caspofungin, micafungin, and anidulafungin, inhibit the synthesis of cell wall β-glucans.

The prevalence of invasive candidiasis has increased markedly since the 1970s, mostly due to the development of intensive care procedures and the increasing number of immunocompromised patients. However, the emergence of antifungal resistance in yeast clinical isolates may also have contributed to this increase in prevalence (26). In addition to the intensive use of antifungals for prophylaxis and treatment, there has been a shift in the nature of the causal agents. Some species formerly rarely seen in medical mycology have become major pathogenic agents (26). For example, among the species of the genus Candida, Candida glabrata now ranks second as the cause of all clinical forms of candidiasis (13). Usually a commensal of the digestive tract, this opportunistic pathogen belongs to the phylum Ascomycetes and has the particularity of being genetically closer to Saccharomyces cerevisiae than to other Candida species. For example, this yeast species has a haploid genome and is unable to produce filaments under normal culture conditions (13). One major trait of C. glabrata is its relatively low susceptibility to azole antifungals, which has been suggested to be the cause of its increased incidence (9).

Given the constitutively low level of susceptibility of C. glabrata to azoles, polyene drugs may constitute a useful alternative for the treatment of infections with this fungus. Although resistance to polyenes is less common than that to azoles, it is also increasingly reported in pathogenic yeasts and filamentous fungi (12). However, the molecular mechanisms underlying this resistance remain largely unknown. We previously characterized a clinical isolate of C. glabrata and demonstrated the role of an ERG6 gene mutation in its decreased susceptibility to polyenes (32). We recently isolated a second clinical isolate of this species with decreased susceptibility to polyenes. In this study, we aimed to determine the molecular mechanisms responsible for this phenotype.


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MATERIALS AND METHODS
 
Yeast strains and culture conditions. This study was performed with a clinical isolate, designated 40407061, recovered in the Laboratory of Parasitology and Mycology of Angers University Hospital in 2004 from a 76-year-old woman admitted to the hospital for convalescence after undergoing hip surgery. In 2004, the patient was treated orally with amphotericin B (4.5 g/day) from 8 to 26 August for a nondocumented case of bronchopneumopathy. On 26 August, mycological examination of a urine sample on CHROMagar Candida (Becton Dickinson, Franklin Lakes, NJ) revealed the profuse and exclusive growth of a yeast, which was identified as Candida glabrata by using ID32C test strips (Biomérieux, Marcy-l'Etoile, France). Additionally, antifungal susceptibility, determined with Fungitest strips (Bio-Rad, Marne la Vallée, France), revealed a low level of susceptibility to amphotericin B.

The isolate was deposited at the Mycology Section of the Institute of Hygiene and Epidemiology (IHEM, Brussels, Belgium) and is publicly available under accession number 21230. As no matching susceptible isolate was available, wild-type isolate IHEM 21231, which is susceptible to polyenes, was used as a control throughout this study. Isolate IHEM 21229, which also displays decreased susceptibility to polyene drugs (32), was used for comparison in some experiments.

The isolates were maintained by regular passages on yeast extract-peptone-dextrose (YEPD) agar plates containing yeast extract at 5 g/liter, peptone at 10 g/liter, dextrose at 20 g/liter, agar at 20 g/liter, and chloramphenicol at 0.5 g/liter. They were both cryopreserved at –80°C in 20% (wt/vol) glycerol and lyophilized.

Susceptibility testing. The susceptibility of C. glabrata isolate 21230 to polyenes (amphotericin B and nystatin) and azoles (econazole, miconazole, ketoconazole, fluconazole, itraconazole, and voriconazole) was determined by the disk diffusion method with Neosensitab tablets from Rosco Diagnostica (Taastrup, Denmark), as described previously (31). Susceptibility testing was carried out on Casitone agar plates (Bacto Casitone, 9 g/liter; glucose, 20 g/liter; yeast extract, 5 g/liter; chloramphenicol, 0.5 g/liter; agar, 18 g/liter; pH 7.2).

The MICs of amphotericin B, ketoconazole, and fluconazole were also determined on Casitone agar plates by the Etest procedure (AB Biodisk, Solna, Sweden), according to the manufacturer's recommendations.

Sterol analysis. Sterols were analyzed from 50 mg lyophilized cells grown to stationary phase in YEPD broth, as described previously (31). Briefly, after extraction by saponification, the amount of ergosterol was determined from the maximum absorbance at 281.5 nm (29), and the sterol species were identified on the basis of their relative retention times on a gas chromatograph.

Nucleic acid isolation. Blastoconidia from mid-exponential-phase cultures in 10 ml of YEPD broth were harvested by centrifugation and ground in liquid nitrogen with a mortar and pestle. Total genomic DNA was then recovered by processing the sample with the DNeasy Plant minikit (Qiagen Inc., Valencia, CA), according to the manufacturer's instructions. Total RNA was recovered by phenol-chloroform extraction from blastoconidia isolated from early-exponential-phase cultures in 50 ml of YEPD broth, as described previously (31).

Gene sequencing. The ERG4, ERG5, and ERG6 genes were sequenced with the specific primers listed in Table 1. Sequencing was performed as described previously (31) by using the dideoxynucleoside triphosphate method on a CEQ8000 automatic sequencer (Beckman Coulter Inc., Fullerton, CA), and the PCR products were purified with a High Pure PCR product purification kit (Roche Diagnostics GmbH, Mannheim, Germany) and used as the template. The sequences were compared by alignment by using the ALIGNn program (http://bioinfo.hku.hk/services/analyseq/cgi-bin/alignn_in.pl).


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  		TABLE 1. Oligonucleotides used for gene sequencing, evaluation of gene expression, and fitness cost determination

Bioinformatic analysis of the ERG6 gene sequence was carried out with the Multiple Translation program (http://bioinfo.hku.hk/services/analyseq/cgi-bin/traduc_in.pl) and the Conserved Domain Database search program (17, 18), available from the NCBI website (http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi).

Real-time RT-PCR. The levels of expression of the ergosterol biosynthesis genes ERG1, ERG2, ERG3, ERG4, ERG5, and ERG6 in isolate 21230 were determined by reverse transcription (RT)-PCR and were compared with those of the genes in wild-type isolate 21231. Total RNA (1 µg) was used as the template for RT by using a Protoscript first-strand cDNA synthesis kit (New England Biolabs, Ipswich, MA) with random nonanucleotides. The cDNA obtained was diluted 10-fold, and a 4-µl aliquot was used as the template to perform real-time PCR in a LightCycler apparatus (Roche Diagnostics) by using the QuantiTect SYBR green kit (Qiagen) and the gene-specific primers (final concentration, 0.5 µM each) described in Table 1. The PCR conditions were as follows: 15 min at 95°C, followed by 40 cycles of 15 s at 94°C for denaturation, 20 s at 55°C for annealing, and 30 s at 72°C for elongation. After amplification, the PCR products were slowly denatured with continuous fluorescence acquisition to determine their melting temperatures and thus to ensure the specificity of the reaction. Three independent experiments were performed, and the data presented are mean values (± standard deviations) of the change in the level of expression relative to the level of expression of the same gene in wild-type isolate 21231. The data were normalized to the level of β-actin expression, and the change in the level of expression was calculated by use of the following formula: 2exp[(CT gene – CT actin)isolate 21230 – (CT gene – CT actin)isolate 21231], where CT is the cycle threshold, defined as the number of cycles for which the curve representing the fluorescence intensity according to the number of cycles crosses an arbitrarily defined baseline at 0.1 fluorescence units.

Morphological studies. The morphologies of the C. glabrata isolate 21230 and 21231 cells were studied microscopically by the observation of lactic blue suspensions from cultures grown for 48 h at 37°C on YEPD agar plates. The ultrastructure of blastoconidia grown on YEPD broth was also determined by transmission electron microscopy, as described previously (4), by using a JEM-2010 transmission electron microscope (Jeol, Paris, France).

Growth kinetics, viability, and vitality. The growth capabilities of isolate 21230 were investigated, as were its viability and fitness. Growth curves were drawn by monitoring the absorbance at 600 nm of three independent cultures incubated at 37°C with constant shaking (150 rpm) until stationary phase. The generation time and latency period were determined and compared to those calculated for wild-type isolate 21231. The percentage of dead cells was determined by staining of the cultures with methylene blue, which leaves the living cells unlabeled. Finally, the fitness of isolate 21230 was investigated by culturing this isolate together with wild-type isolate 21231. Briefly, 50 ml of YEPD broth was inoculated with the same quantity of cells of the two isolates (i.e., 1 ml of a suspension in water with an optical density at 600 nm of 0.1) and incubated at 37°C for 48 h with constant shaking (150 rpm). Subsequently, the mixed culture was diluted 106-fold and 100-µl aliquots were plated on YEPD agar plates, which were incubated at 37°C for 24 h. The susceptibilities of 100 randomly selected colonies to amphotericin B were determined by the disk diffusion method as described above, and for colonies with decreased susceptibility, the presence of the ERG6 mutation was verified by PCR with primer pair ERG6-3F and ERG6-4R2 (Table 1), after RNA extraction and cDNA synthesis. As the reverse primer hybridized to a nucleotide sequence located after the nonsense mutation detected in the 3' end of the ERG6 gene in isolate 21230, a negative PCR signal was observed with the mutated allele, whereas a 771-bp fragment was amplified with the wild-type allele.

Cell wall stress resistance studies. The susceptibilities of cells of isolate 21230 to cell wall-perturbing agents (i.e., calcofluor white, Congo red, sorbitol, and caffeine) in comparison with those of cells of wild-type isolate 21231 were determined. These compounds were incorporated in the YEPD agar medium at concentrations of 1 mg/ml for calcofluor white and Congo red (two markers of the cell wall polysaccharides), 2 M for sorbitol (which increases the osmolarity of the medium, thereby inducing cell wall stress), and 2 mM for caffeine (a nucleotide that modifies the expression of cell wall integrity-related genes) (15). Afterward, 10 µl of the fungal suspensions containing 103 to 107 cells per ml was spotted on the agar surface. The plates were then allowed to dry and were finally incubated for 48 h at 37°C.

Complementation study. The function of the C-24 sterol methyltransferase, the enzyme encoded by the ERG6 gene, was epigenetically restored in a ura3 derivative of isolate 21230 by transformation with a centromeric plasmid containing a wild-type copy of the ERG6 gene, as described previously (32). The complementation of C-24 sterol methyltransferase function was checked by determining the susceptibility of the transformed clone to amphotericin B and to cell wall-perturbing agents and by determining the growth capacity, viability, and fitness of the isolate. We also studied these phenotypic traits in a complemented clone of C. glabrata clinical isolate 21229. All these experiments were performed as described above, except for fitness determinations in complemented clones. For the latter experiments, the fitness of each complemented clone was investigated by the analysis of 20 colonies originated from cocultures with the wild-type isolate. To do this, the presence of the ERG6-harboring plasmid was detected by plasmid extraction with the EZNA yeast plasmid kit (Omega Bio-Tek, Doraville, GA), followed by PCR with pRS416-specific primers flanking the cloning site (Table 1). The selected primers therefore amplified a 1,552-bp fragment within the ERG6-harboring pRS416-derived plasmid, whereas they gave a negative PCR signal with pRS416 or a nontransformed colony.

Nucleotide sequence accession number. The sequences of the ERG4, ERG5, and ERG6 genes of C. glabrata isolate 21230 were deposited in the GenBank database and are available under accession numbers EU490411, EU490412, and EU310475, respectively.


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RESULTS
 
Antifungal susceptibility. Isolate 21230 was less susceptible to both amphotericin B and nystatin than wild-type isolate 21231, as shown by the disk diffusion method on Casitone agar, with the sizes of the growth inhibition zones being just over half the sizes of those for the wild type (Table 2). Isolate 21230 was also more susceptible than the wild type to all azole drugs tested (Table 2). This finding was confirmed by determining the MICs of amphotericin B, ketoconazole, and fluconazole (Table 3). The MIC of amphotericin B for isolate 21230 was more than twice that for the wild-type isolate, whereas the MICs of ketoconazole and fluconazole were lower by factors of 3 and 6, respectively.


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  		TABLE 2. Susceptibilities of C. glabrata isolates 21231 and 21230 to antifungal agentsa


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  		TABLE 3. MICs of polyene and azole drugs for C. glabrata isolates 21231 and 21230

Sterol content. Gas chromatography analysis of the sterol content of isolate 21230 revealed marked differences from wild-type isolate 21231. Indeed, ergosterol, which corresponded to the major peak in the chromatogram of the wild-type isolate, was not detectable for the poorly susceptible isolate (Fig. 1 and Table 4). Conversely, several peaks corresponding to sterol intermediates increased markedly, and some supplemental peaks were also detected for isolate 21230. However, these sterol intermediates were considered to be {Delta}5,7-dienols, which, like ergosterol, have two conjugated double bonds at C-5 and C-7, as the absorbance at 281.5 nm (the maximum absorption wavelength of the conjugation) was identical for the two isolates (data not shown).


Figure 1
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  		FIG. 1. Gas-phase sterol chromatograms of Candida glabrata isolates 21231 (A) and 21230 (B). As illustrated by the dashed line, ergosterol was the main sterol species in wild-type isolate 21231 and accounted for 52.2% of all sterols, whereas it was not detectable on the chromatogram of isolate 21230.


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  		TABLE 4. Percentages of sterol intermediates in C. glabrata isolates 21231 and 21230

ERG4, ERG5, and ERG6 gene sequences. The open reading frames (ORFs) corresponding to the ERG4, ERG5 and ERG6 genes of C. glabrata isolate 21230 were compared with the corresponding ORFs of strain CBS138, the genome of which has been fully sequenced (the sequence is available from the GenBank database). Only silent mutations were seen in the sequences of the ERG4 and ERG5 genes for isolate 21230, and a single point mutation corresponding to the substitution of a cytosine by a thymine at position +994 relative to the start (position +1) of the ORF was identified in the ERG6 gene. Analysis of the ERG6 gene sequence by use of the Multiple Translation program revealed that this point mutation leads to a stop codon and therefore to the shortening of the primary sequence of the C-24 sterol methyltransferase by 41 amino acids at the C terminus.

Levels of expression of ergosterol biosynthesis genes. RT-PCR experiments revealed for isolate 21230 increased levels of expression of most of the ergosterol biosynthesis genes tested compared to the levels of expression by wild-type isolate 21231 (Fig. 2). Indeed, with the exception of the ERG4 gene, the expression of which was unaffected (relative expression level, 1.8), all genes tested were overexpressed, with relative levels of expression of 2.3, 13.1, 17.6, 3.5, and 7.7 for the ERG1, ERG2, ERG3, ERG5, and ERG6 genes, respectively.


Figure 2
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  		FIG. 2. Relative levels of expression of ergosterol biosynthesis genes in Candida glabrata isolate 21230 compared to those in wild-type isolate 21231. RT-PCR experiments showed stronger expression of most of the ergosterol biosynthesis genes in isolate 21230. The results are expressed as the mean values of three independent experiments (± standard deviation) normalized to the β-actin mRNA level.

Cell morphology. The colonies of isolate 21230 and the wild-type isolate were similar in appearance. Similarly, light microscopy revealed that the isolate with decreased susceptibility to polyenes had a morphology typical of that of C. glabrata, with solitary, sometimes budding blastoconidia (data not shown). However, transmission electron microscopy revealed modifications of the cell wall ultrastructure for isolate 21230 that consisted of a thinner inner layer of the cell wall, accompanied by retractions of the underlying cytoplasm (Fig. 3).


Figure 3
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  		FIG. 3. Ultrastructures of Candida glabrata wild-type isolate 21231 (A) and isolate 21230 (B) cells. Transmission electron microscopy revealed important changes in the cell wall in isolate 21230, with a thinner inner layer (see insets in panels A and B) accompanied by retractions of the underlying cytoplasm (arrowheads). N, nucleus; mt, mitochondrion. Bars in panels A and B, 0.5 µm; bars in insets, 0.1 µm.

Growth kinetic parameters and cell viability and vitality. The growth capabilities of isolate 21230 were also markedly affected since the latency period was about 6 h, whereas it was 2 h for wild-type isolate 21231. Moreover, the generation time calculated during the exponential growth phase was about 8 h for isolate 21230, whereas it was only 6 h for the wild-type isolate (Fig. 4A). Similarly, the first dead cells were detected by methylene blue staining after only 6 h of incubation in YEPD broth for isolate 21230, and almost half the cells were dead after 120 h of incubation. By contrast, for the wild-type isolate, the first dead cells appeared after 48 h of incubation and almost 80% of the cells were still alive after 120 h of incubation (Fig. 4B). Furthermore, after the coculture of isolate 21230 with the wild-type isolate and subculture, only 7% of the colonies obtained displayed decreased susceptibility to amphotericin B and harbored the stop mutation in the ERG6 gene.


Figure 4
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  		FIG. 4. Growth capacity and cell viability of the isolates with reduced susceptibilities to polyenes and their complemented counterparts compared to those of the wild-type isolate. (A and C) Growth curves were drawn by monitoring the absorbance at 600 nm of three independent cultures in YEPD broth. Isolates 21230 (solid triangles) and 21229 (solid squares) isolates had longer latency periods and longer generation times than wild-type isolate 21231 (solid circles) and their complemented counterparts, isolates 21230C (open triangles) and 21229C (open squares), respectively. (B and D) Curves representing the percentage of dead cells according to the duration of cultures. The number of dead cells began to increase after only 6 h of incubation for isolate 21230 (solid triangles), and almost half the cells were dead after 120 h of incubation. Similarly, more than 60% of the cells of the isolate 21229 (solid squares) were dead after 120 h of culture. By contrast, the first dead cells appeared after only 48 h of incubation for the wild-type isolate (solid circles) and complemented clones 21230C (open triangles) and 21229C (open squares), and more than 80% of the cells remained alive after a 120-h incubation.

Susceptibility to cell wall-perturbing agents. The susceptibilities to calcofluor white (1 mg/ml), Congo red (1 mg/ml), sorbitol (2 M), and caffeine (2 mM) were determined by culturing the cells in the presence of these drugs incorporated into solid medium. The growth of the two clinical isolates with low levels of susceptibility to the polyene drugs was inhibited by the four cell wall-disrupting agents tested, although some differences between the two isolates were observed, with the growth of isolate 21229 being more strongly inhibited by 2 mM caffeine than that of isolate 21230 (Fig. 5).


Figure 5
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  		FIG. 5. Susceptibility to cell wall-perturbing drugs of Candida glabrata wild-type isolate 21231 or clinical isolates 21229 and 21230 and their complemented counterparts, isolates 21229C and 21230C, respectively. Calcofluor white (1 mg/ml), Congo red (1 mg/ml), sorbitol (2 M), and caffeine (2 mM) were incorporated into the agar medium. Afterward, 10 µl of fungal suspensions containing from 107 to 103 cells per ml (from left to right) was spotted on the agar surface. The plates were dried and incubated at 37°C for 48 h. The two clinical isolates were more susceptible than the wild-type isolate to all the cell wall-perturbing agents tested, and complementation of the two clinical isolates restored cell wall integrity. A growth control was performed by spotting the suspensions onto YEPD agar plates containing no inhibitor.

Complementation study. One clone that was able to grow on minimum medium and that was therefore efficiently transformed with the plasmid containing a wild-type copy of the ERG6 gene was subcultured, and its susceptibility to amphotericin B was determined by the disk diffusion method. The complemented clone, 21230C, had wild-type susceptibility to amphotericin B (diameters of growth inhibition zones, 29 mm for the wild type and 13 mm for clinical isolate 21230). Wild-type levels of susceptibility to calcofluor white, Congo red, sorbitol, and caffeine were also observed for this complemented clone and for complemented clone 21229C derived from isolate 21229 (Fig. 5). Similarly, complementation of the two isolates with decreased susceptibility also restored (i) wild-type growth, with latency periods of about 2 h and generation times of about 7 h for both complemented clones; (ii) cell viability, with less than 15% dead cells after 120 h of incubation for both complemented clones (Fig. 4); and (iii) fitness, as almost half the colonies obtained after coculture with the wild-type isolate contained the plasmid.


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DISCUSSION
 
Resistance to polyene drugs in yeast clinical isolates is increasingly reported, but the molecular mechanisms, unlike those involved in azole resistance, largely remain unclear. We previously investigated the molecular mechanisms responsible for decreased susceptibility to polyenes in a clinical isolate of C. glabrata with pseudohyphal growth (32). We describe here another clinical isolate of the same yeast species that was also recovered from an elderly patient treated with amphotericin B and which displayed low levels of susceptibility to polyenes on first isolation.

Antifungal susceptibility testing performed by a disk diffusion method as well as MIC determination by the Etest procedure revealed for the isolate a lower level of susceptibility to polyenes compared to that for a wild-type isolate. The isolate was also found to be more susceptible than the wild type to azole drugs, consistent with our previous observations (32), although resistance to polyenes may also be associated with azole resistance (1, 10, 14, 23, 27, 33). For example, cross-resistance to polyenes and azoles has been observed in a petite mutant of the yeast C. albicans (10). However, azole resistance due to overexpression of the CDR1 gene encoding an ATP binding cassette transporter associated with an increase in susceptibility to polyenes has been reported in most of the petite mutants studied (2, 4, 6, 7, 22).

As ergosterol is the target of polyene drugs (8), the sterol content of the isolate with decreased susceptibility to polyenes was investigated both quantitatively and qualitatively and compared with that of a wild-type isolate. No quantitative differences were observed between the two isolates. Conversely, gas-phase chromatography revealed important changes in the sterol profile for the isolate with decreased susceptibility to polyenes. Consistent with the reduced susceptibility to amphotericin B, ergosterol was not detectable on the chromatogram for isolate 21230, and as a consequence, several peaks corresponding to sterol intermediates were observed. Because of the unavailability of reference standards, these intermediates were not identified precisely, but they were considered to be nonergosterol {Delta}5,7-dienols, based on the results of spectrophotometric assays.

These results led us to sequence genes encoding enzymes catalyzing reactions occurring late in the ergosterol biosynthesis pathway. Only silent mutations were seen in the sequences of the ERG4 and ERG5 genes, which encode the C-24 sterol reductase and the C-22 sterol desaturase, respectively. By contrast, a nonsense mutation (C994T) was observed in the sequence of the ERG6 gene of isolate 21230. This mutation introduced a stop codon 41 amino acids before the C terminus of the C-24 sterol methyltransferase encoded by the ERG6 gene. The truncation of this enzyme, which catalyzes the conversion of zymosterol into fecosterol by C-24 methylation (16), may have important consequences for its function, as suggested by the search for conserved domains. Interestingly, two conserved domains may be found in the sequence of the C-24 sterol methyltransferase: the first, located between positions 134 and 222, is found in all S-adenosyl-L-methionine-dependent methyltransferases (11), whereas the second, located between positions 231 and 331, is a conserved domain found at the C termini of plant and fungal sterol methyltransferases (3). The second domain (Conserved Domain Database accession number pfam 08498), which usually comprises 137 amino acids, was 37 amino acids shorter in our clinical isolate. This shortening may have modified the C-24 sterol methyltransferase activity, thereby accounting for the observed defect in ergosterol biosynthesis. Genetic evidence for the involvement of the ERG6 mutation in the reduced susceptibility of this clinical isolate to polyenes was provided by complementing the ERG6 function, as a ura3 derivative of the clinical isolate transformed with a centromeric plasmid containing a wild-type copy of the ERG6 gene was susceptible to polyenes.

The ERG6 mutation also resulted in the overexpression of almost all the genes catalyzing late steps in the ergosterol biosynthesis pathway, consistent with the hypothesis that this metabolic pathway is blocked. Ergosterol is known to exert negative feedback on its own biosynthesis in S. cerevisiae (5, 29), particularly at the C-24 methylation step, and this feature seems to be common to all sterol methyltransferases (19). A similar phenomenon may occur in C. glabrata, a species phylogenetically closely related to S. cerevisiae (24), and the absence of ergosterol in our clinical isolate may relieve the inhibition exerted by this compound on its biosynthesis pathway.

The absence of ergosterol may have major consequences for the organization of the fungal cell wall or plasma membrane, due to the impairment of protein targeting. Such disturbances have been demonstrated for S. cerevisiae, in which a defect in the C-24 methylation of sterols ({Delta}erg6 background) prevents the targeting of the tryptophan transporter to the plasma membrane (30). It has recently been shown that ergosterol is essential for the targeting of Cdr1p to the plasma membrane in C. albicans (25). Moreover, Pasrija et al. (25) demonstrated that deletion of the ERG6 gene increases susceptibility to azoles, probably due to the mistargeting of Cdr1p to the plasma membrane, resulting in a loss of the efflux function of this protein. Thus, although it is offset by the accumulation of other {Delta}5,7-dienols, the absence of ergosterol may account for the greater susceptibility of our isolate to azole drugs, as nonergosterol {Delta}5,7-dienols are probably not as efficient as ergosterol at ensuring the normal fluidity of the membrane, which is more readily destabilized by exposure to azoles. Moreover, as in C. albicans and S. cerevisiae, the absence of ergosterol in C. glabrata may have an indirect effect on susceptibility to azoles by preventing the targeting of efflux pumps to the plasma membrane, thereby favoring the accumulation of these drugs within the cell.

However, different mutational events affecting the same gene may have different consequences for the residual activity of the encoded protein. For example, site-directed mutagenesis experiments with the ERG6 gene of S. cerevisiae have demonstrated the critical importance of certain highly conserved amino acids. Aromatic residues at positions 82, 83, 85, 87, 91, and 93, which constitute region 1 of sterol methyltransferases, are essential for the catalytic activity of the enzyme (21). Similarly, three aspartic acid residues at positions 125, 152, and 276 and a glutamic acid at position 195 are required for the binding of the S. cerevisiae sterol methyltransferase substrates S-adenosyl-L-methionine and zymosterol (20). Substitutions affecting such important residues have different consequences, depending on the nature of the amino acid replacing the original residue. The replacement of these residues by a leucine residue leads to a total loss of enzyme activity, whereas isoleucine has a weaker effect on the sterol methyltransferase. By contrast, the replacement of the tyrosine at position 83 by a phenylalanine increases the activity of Erg6p.

Two different mutational events were identified in the ERG6 gene of clinical isolates 21230 and 21229: a nonsense mutation in isolate 21230 and a missense mutation in isolate 21229 (32). This may account for the morphological differences observed between the two isolates. We previously characterized isolate 21229, which has decreased susceptibility to polyenes and pseudohyphal growth, and demonstrated that both these features were linked to mutation of the ERG6 gene. We also suggested that the absence of ergosterol in the membranes of this isolate prevented the targeting to the plasma membrane of certain enzymes required for the release of daughter cells after budding and septation. Surprisingly, clinical isolate 21230 had a normal morphology, consisting of round cells generally solitary but sometimes budding, and the MIC of amphotericin B for this isolate was1/10 that of isolate 21229. However, the examination of ultrathin sections of blastoconidia by transmission electron microscopy revealed changes in the structure of the cell wall for this isolate, with a thinning of the inner layer associated with retraction of the underlying cytoplasm. Moreover, these cell wall modifications were accompanied by an increase in susceptibility to cell wall-perturbing agents, as reported for isolate 21229. These results suggest that the cell wall organization strongly depends on the sterol composition of the membranes. The C-24 sterol methyltransferase may not be fully inactivated by the nonsense mutation, probably because the C terminus of the enzyme plays a less important role than the rest of the molecule in the activity of the enzyme. As a consequence, ergosterol is probably not entirely absent but may be masked by sterol intermediates accumulating in mutant cells and therefore not detectable on sterol chromatograms. Thus, the amount of ergosterol synthesized, although too low for wild-type levels of susceptibility to polyenes, may be sufficient to ensure the normal trafficking of proteins to the plasma membrane and to allow the cells to separate after budding and septation. It would be interesting to introduce the mutated ERG6 allele of isolate 21230 into an ERG6 null background to determine whether this allele is actually nonfunctional or whether the nonsense mutation leads, as suggested here, to a partial loss of C-24 sterol methyltransferase activity.

Consistent with these changes in the ultrastructure of the cell wall, the isolates with decreased susceptibility to polyenes had higher mortality rates, grew less well, and displayed a marked fitness cost in antifungal drug-free medium. These observations suggest that the ERG6 mutations constitute a selective disadvantage in the absence of selection pressure. However, given the haploidy of the C. glabrata genome (13) and the resulting higher probability of expression of a mutated allele, such mutations may be clinically relevant in the current therapeutic context, which is dominated by prophylaxis. Together with the constitutively low level of susceptibility to azoles of C. glabrata and the frequent acquisition of azole resistance, these mutations may account at least in part for the increasing incidence of this yeast species (9).

In conclusion, we determined the molecular mechanism responsible for the decreased susceptibility to polyenes observed in a clinical isolate of C. glabrata. This decrease in susceptibility was due to a nonsense mutation in the ERG6 gene, which encodes an enzyme involved in late steps of the ergosterol biosynthesis pathway. As this is the second isolate of this species with a low level of susceptibility to polyenes to be characterized in our laboratory, it seems that ERG6 mutants of C. glabrata, which are obtained more easily in this species than in diploid Candida species, may be selected by the prophylactic or therapeutic use of amphotericin B. Proteomic studies are under way to characterize the differences between clinical isolates with decreased susceptibilities to polyenes in protein trafficking to the plasma membrane for identification of the enzymes involved in the release of daughter cells after budding and septation. Directed mutagenesis experiments will also be carried out to determine the functions of the different domains of Erg6p.


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ACKNOWLEDGMENTS
 
During the course of this work, P.V. was the recipient of a grant from Angers Loire Métropole. This study was supported in part by a grant from the Institut de Parasitologie de l'Ouest, Rennes, France. We thank Angers University Hospital, France, for financial support.


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FOOTNOTES
 
* Corresponding author. Mailing address: Groupe d'Etude des Interactions Hôte-Pathogène, UPRES-EA 3142, Laboratoire de Parasitologie-Mycologie, Centre Hospitalier Universitaire, 4 rue Larrey, Angers Cedex 9 49933, France. Phone: 33 02 41 35 34 72. Fax: 33 02 41 35 36 16. E-mail: patrick.vandeputte{at}etud.univ-angers.fr Back

{triangledown} Published ahead of print on 11 August 2008. Back

{dagger} Present address: Centre de Recherche contre le Cancer Angers-Nantes, Centre Régional de Lutte contre le Cancer Paul Papin, INSERM U892, Angers, France. Back


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Antimicrobial Agents and Chemotherapy, October 2008, p. 3701-3709, Vol. 52, No. 10
0066-4804/08/$08.00+0     doi:10.1128/AAC.00423-08
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