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Antimicrobial Agents and Chemotherapy, July 2004, p . 2747-2750, Vol . 48, No . 7
Substitutions at Methionine 220 in the 14
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| ABSTRACT |
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Five clinical isolates of Aspergillus fumigatus that exhibited similar patterns of reduced susceptibility to itraconazole and other triazole drugs were analyzed . Sequence analysis of genes (cyp51A and cyp51B) encoding the 14
-sterol demethylases revealed that all five strains harbored mutations in cyp51A resulting in the replacement of methionine at residue 220 by valine, lysine, or threonine . When the mutated cyp51A genes were introduced into an A . fumigatus wild-type strain, the transformants exhibited reduced susceptibility to all triazole agents, confirming that the mutations were responsible for the resistance phenotype .
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Aspergillus fumigatus is one of the most prevalent airborne fungal pathogens causing infection worldwide, with high mortality and morbidity in the immunocompromised host (14) . Although A . fumigatus is intrinsically resistant to fluconazole, the newer triazoles such as itraconazole (ITC), posaconazole (POS), and voriconazole (VRC) are active both in vitro and in vivo against this species (2, 11, 24) . However, a number of A . fumigatus isolates with in vitro ITC resistance have been described previously (1, 4, 8, 9, 16, 19, 20); in some cases, the resistance detected in vitro has been confirmed in animal models of infection (3, 7) . Resistance to VRC has been recently detected in clinical and laboratory strains (E . Manavathu, I . Baskaran, S . Krishnan, G . Alangaden, and P . Chandrasekar, Abstr . 43rd Intersci . Conf . Antimicrob . Agents Chemother., abstr . M-471, page 444, 2003, and E . Manavathu, A . Espinel-Ingroff, G . Alangaden, and P . Chandrasekar, Abstr . 43rd Intersci . Conf . Antimicrob . Agents Chemother., abstr . M-392, page 440, 2003) . In addition, it appears that some clinical Aspergillus strains are starting to exhibit reduced susceptibility resulting in high drug MICs when new triazole agents such as POS and ravuconazole (RVC) are tested (19, 24) .
The triazoles inhibit the ergosterol biosynthesis pathway via the inhibition of 14-sterol demethylase (Cyp51), an enzyme that removes the methyl group at position C-14 of precursor sterols . In A . fumigatus, there are two distinct but related Cyp51 proteins encoded by cyp51A and cyp51B (18) . Two molecular mechanisms of resistance to azoles have been proposed in A . fumigatus: the first is reduced intracellular accumulation due to either increased expression of efflux pumps (8, 30) or reduced penetration of the drug (15) . The other mechanism of resistance is through the modification of Cyp51 (8, 22) . To date the most prevalent mechanism of resistance in A . fumigatus appears to be the modification of Cyp51, specifically mutations in cyp51A . These mutations, in both clinical strains and laboratory-generated spontaneous mutants, have been associated with two different susceptibility profiles: (i) cross-resistance to ITC and POS has been associated with amino acid substitutions at glycine 54 (G54) (9, 16, 20); (ii) cross-resistance to VRC and RVC has been associated with amino acid substitutions at G448 (Manavathu et al., 43rd ICAAC, abstr . M-471) . A third pattern of azole resistance was recently reported (9, 19) . This new pattern is characterized by high ITC, VRC, RVC, and POS MICs . The majority of the strains exhibiting this susceptibility profile harbor amino acid substitutions at methionine 220 (M220), an area of Cyp51A that was not previously associated with amino acid substitutions causing reduced susceptibility to azoles (9, 16, 20) .
The objective of this work was to determine whether the amino acid substitutions at M220 were sufficient to confer reduced susceptibility to azoles in A . fumigatus . To this end, cyp51A alleles encoding the different point mutations were introduced into a wild-type (azole-susceptible) A . fumigatus strain (CM-237); all three amino acid substitutions conferred reduced susceptibility to azoles .
Fungal strains and antifungal susceptibility testing.
Eight clinical A . fumigatus strains from the Mycelial Collection of the Spanish National Centre for Microbiology (CNM-CM) were analyzed (Table 1) . (i) Strains for which all azole drugs exhibited elevated MICs were as follows: CNM-CM-1252 (AF-90), CNM-CM-1245 (AF-91), CNM-CM-2158 (AF-1422), CNM-CM-2159 (F/6919), and CNM-CM-2164 (SO/3829) . (ii) Strains susceptible to all four triazoles (drug MICs
0.5 µg/ml) were as follows: CNM-CM-2739, CNM-CM-1369, and CNM-CM-237 . In addition, CM-237 was utilized for describing the sequence of the genes cyp51A and cyp51B (18) . A . flavus ATCC 204304 and A . fumigatus ATCC 204305 were used as quality control strains for susceptibility testing .
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Broth microdilution susceptibility testing was performed as described in the NCCLS document M38-P (21) with the modifications described previously (2, 23, 26) . ITC (Janssen Pharmaceutical, Madrid, Spain), VRC (Pfizer S.A., Madrid, Spain), RVC (Bristol-Myers Squibb, Madrid, Spain), and POS (Schering-Plough Research Institute, Kenilworth, N.J.) were obtained as powders from their respective manufacturers . The drugs were dissolved in dimethyl sulfoxide (Sigma, Madrid, Spain) at 1,600 µg/ml; the final concentration range assayed was 8.0 to 0.015 µg/ml . Visual readings were performed using a microtiter reading mirror; the MIC was defined as the lowest concentration of drug that completely inhibited fungal growth after 48 h of incubation at 35°C . Susceptibility tests were performed at least twice with each strain on different days . Susceptibility results are shown in Table 1 . The five strains exhibited reduced susceptibility to all four triazoles, with various drug MICs depending on the antifungal drug and strain combination; POS was the most active in vitro compound .
PCR amplification and sequence analysis of the cyp51A and cyp51B genes. Conidia from each strain were inoculated into 3 ml of GYEP broth (2% glucose, 0.3% yeast extract, 1% peptone) and grown overnight at 37°C . Mycelial mats were recovered and subjected to a DNA extraction protocol (18) . The full coding sequences of cyp51A and cyp51B were PCR amplified as previously described (9) . To rule out the possibility that any sequence changes identified were due to PCR-induced errors, each mutant was independently analyzed twice .
Sequence analysis of cyp51A revealed a number of point mutations . The five ITC-resistant strains (CM-1252, CM-1245, CM-2158, CM-2159, and CM-2164) each had a single nucleotide substitution in codon 220 (encodes methionine), resulting in the introduction of valine, lysine, or threonine (Table 1) . The five ITC-resistant strains also harbored point mutations in cyp51B; however, two of the three mutations detected were silent, and the one missense mutation (resulting in the replacement of Asp by Glu at codon 387) was not conserved across the five isolates (Table 1) . Sequence alignments of the region encompassing M220 revealed that this residue is not an amino acid strictly conserved between all yeasts and moulds, although this position is conserved in many fungal species and it is on the edge of a highly conserved region (Fig . 1) . Moreover, the fact that five ITC-resistant strains harbored the same mutation strongly suggests that the substitution is associated with azole resistance . In this regard, the region encompassing the mutation was sequenced in 22 A . fumigatus ITC-susceptible clinical strains; none of the 22 strains had mutations at codon 220 .
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Replacement of the wild-type cyp51A gene with cyp51A alleles that encode substitutions at codon 220.
The cyp51A alleles from strains CM-1252, CM-2159, and CM-2164 were PCR amplified, and each was individually electroporated into the wild-type A . fumigatus strain CM-237 . Electroporation was carried out using a protocol previously described for A . nidulans (28) and subsequently adapted for A . fumigatus (31); transformants were selected on medium containing ITC as described previously (9) . ITC-resistant transformants appeared after 2 to 7 days of incubation . The transformants were labeled with a "T" followed by a roman numeral (Table 1) . The numbers of ITC-resistant transformants obtained using the cyp51A genes from strains CM-2159, CM-1252, and CM-2164 were four (T-III, T-VI, T-VII and T-XI), one (T-XII), and one (T-XXII), respectively . To confirm that each transformant contained only one copy of the cyp51A gene we performed a Southern blot analysis . Chromosomal DNA was extracted from each transformant, digested with either SalI or BamH1 (Amersham Biosciences, Madrid, Spain), and resolved by gel electrophoresis . Southern blots (27) were probed with a labeled fragment of cyp51A (18); in every case, only a single band hybridized to the probe (data not shown) . The cyp51A and cyp51B genes from the six transformants were sequenced; all six appeared to have incorporated the mutated cyp51A allele . With the exception of the original mutation at codon 220, none of the transformants had any other mutations in either cyp51A or cyp51B . The susceptibility of the transformants to triazoles was determined as described above; in general, the transformants exhibited susceptibility profiles similar to those of the original clinical isolates (Table 1) .
To identify the gene replacement events we used a direct selection procedure . Consequently, there is a possibility that the mutations identified in the transformants arose spontaneously and were selected through the inclusion of ITC in the transformation plates . However, three observations argue against this possibility . First, the three alleles used in the transformations each encoded a different substitution at residue 220 . The mutations identified in the six transformants matched those present in the donor cyp51A allele; such congruence would be highly unlikely if the mutations had arisen spontaneously . Second, in control experiments in which the donor DNA was replaced by water, no resistant isolates were detected . Finally, there have been no prior reports of spontaneous mutations arising in this area of the A . fumigatus Cyp51A protein in laboratory-selected mutants (16, 20; Manavathu et al., 43rd ICAAC, abstr . M-471) .
To date there have been a number of reports that have identified polymorphisms in the cyp51 gene from clinical Candida albicans isolates that are responsible for and/or associated with azole antifungal resistance to fluconazole (10, 17, 29) . In some filamentous fungi, one mutation (Y136F) has been correlated with resistance to different demethylase inhibitors (5, 6) . In A . fumigatus, specific mutations in cyp51A have been associated with decreases in susceptibility to ITC and POS (9, 16) . In this study, substitutions at residue 220 were detected in five independent ITC-resistant isolates . The minor differences in the ways the mutations impact the susceptibility to specific azoles presumably reflect differences in the ways the azoles interact with the target protein . The clinical significance of these differences, if any, has yet to be determined .
The precise manner in which substitutions at M200 impact triazole binding is not immediately obvious . Recently, a three-dimensional model of 14-demethylase from C . albicans was built using the crystallographic coordinates of four prokaryotic P450 enzymes (12) . The docking of both the substrate and triazoles into the active site of the enzyme was explored . It was postulated that the long side chain of ITC interacted with the residues in the substrate access channel . This channel is lined by hydrophobic and aromatic residues such as F228, I231, F233, V234, and F235 (corresponding to F214, I217, F219, M220, and L221 in Cyp51A from A . fumigatus) . Some of these residues are predicted to have direct interactions with either the substrate or the antifungal drug (12) . From this model the substitutions at methionine 220 might be expected to disrupt drug binding . Recently, a homology model of Cyp51A from A . fumigatus was constructed on the basis of the X-ray structure of the CYP51 orthologue from Mycobacterium tuberculosis (32) . Some of the residues in the predicted A . fumigatus Cyp51 F-G loop (T215 and P230) are predicted to have contact with ITC and POS (32) but not directly with VRC . Podust et al . crystallized the M . tuberculosis Cyp51 in the presence of fluconazole (25) . They suggested that substitutions causing azole resistance in fungi are located in regions of the protein involved in conformational changes associated with the catalytic cycle rather than in residues that directly contact the drug (25) . Also, it has been previously pointed out that it seems more important to look at the role of specific residue mutations in relation with their local environments (13) . The gathering of further evidence will have to wait until the crystal structure for A . fumigatus Cyp51s is obtained in conjunction with the antifungal drugs . In the meantime, the identification of amino acid substitutions responsible for azole resistance continues to provide new insights into the way the drugs interact with Cyp51A . Such knowledge may aid in the development of more active molecules . Further investigations into the functional analysis of the Cyp51A and Cyp51B proteins of A . fumigatus are presently in process in our laboratory .
| ACKNOWLEDGMENTS |
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This work was supported in part by grants MPY1120/03 from Instituto de Salud Carlos III and SAF2002-02089 from the Ministry of Science and Technology . E . Mellado held a Ramón y Cajal contract from Ministry of Science and Technology . L . Alcazar-Fuoli has a predoctoral fellowship from Instituto de Salud Carlos III .
We are grateful to J.P . Latge for helpful suggestions . We thank D . W . Denning and J . Mosquera, University of Manchester (United Kingdom), for providing strains AF-90, AF-91, AF-1422, F/6919, and SO/3829 and Gema del Rio for invaluable technical assistance . We thank Pfizer, Janssen Pharmaceutical, Bristol-Myers Squibb, and Schering Plough for supplying antifungal powders . We also thank Paul M . McNicholas (Schering-Plough Research Institute) for critically reading the manuscript and helpful editing suggestions .
| FOOTNOTES |
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* Corresponding author . Mailing address: Servicio de Micología, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Carretera Majadahonda-Pozuelo Km2, 28220 Madrid, Spain . Phone: 34 91 509 7961 . Fax: 34 91 509 7966 . E-mail: emellado{at}isciii.es .
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