Aspergillus fumigatus C-5 sterol desaturases Erg3A and Erg3B: role in sterol biosynthesis and antifungal drug susceptibility

Herbicides as fungicides: Targeting heme biosynthesis in the maize pathogen Ustilago maydis

Pathogens must efficiently acquire nutrients from host tissue to proliferate, and strategies to block pathogen access therefore hold promise for disease control. In this study, we investigated whether heme biosynthesis is an effective target for ablating the virulence of the phytopathogenic fungus Ustilago maydis on maize plants. We first constructed conditional heme auxotrophs of the fungus by placing the heme biosynthesis gene hem12 encoding uroporphyrinogen decarboxylase (Urod) under the control of nitrogen or carbon source-regulated promoters. These strains were heme auxotrophs under non-permissive conditions and unable to cause disease in maize seedlings, thus demonstrating the inability of the fungus to acquire sufficient heme from host tissue to support proliferation. Subsequent experiments characterized the role of endocytosis in heme uptake, the susceptibility of the fungus to heme toxicity as well as the transcriptional response to exogenous heme. The latter RNA-seq experiments identified a candidate ABC transporter with a role in the response to heme and xenobiotics. Given the importance of heme biosynthesis for U. maydis pathogenesis, we tested the ability of the well-characterized herbicide BroadStar to influence disease. This herbicide contains the active ingredient flumioxazin, an inhibitor of Hem14 in the heme biosynthesis pathway, and we found that it was an effective antifungal agent for blocking disease in maize. Thus, repurposing herbicides for which resistant plants are available may be an effective strategy to control pathogens and achieve crop protection.

Are there conserved biosynthetic genes in lichens? Genome-wide assessment of terpene biosynthetic genes suggests ubiquitous distribution of the squalene synthase cluster

Lichen-forming fungi (LFF) are prolific producers of functionally and structurally diverse secondary metabolites, most of which are taxonomically exclusive and play lineage-specific roles. To date, widely distributed, evolutionarily conserved biosynthetic pathways in LFF are not known. However, this idea stems from polyketide derivatives, since most biochemical research on lichens has concentrated on polyketide synthases (PKSs). Here, we present the first systematic identification and comparison of terpene biosynthetic genes of LFF using all the available Lecanoromycete reference genomes and 22 de novo sequenced ones (111 in total, representing 60 genera and 23 families). We implemented genome mining and gene networking approaches to identify and group the biosynthetic gene clusters (BGCs) into networks of similar BGCs. Our large-scale analysis led to the identification of 724 terpene BGCs with varying degrees of pairwise similarity. Most BGCs in the dataset were unique with no similarity to a previously known fungal or bacterial BGC or among each other. Remarkably, we found two BGCs that were widely distributed in LFF. Interestingly, both conserved BGCs contain the same core gene, i.e., putatively a squalene/phytoene synthase (SQS), involved in sterol biosynthesis. This indicates that early gene duplications, followed by gene losses/gains and gene rearrangement are the major evolutionary factors shaping the composition of these widely distributed SQS BGCs across LFF. We provide an in-depth overview of these BGCs, including the transmembrane, conserved, variable and LFF-specific regions. Our study revealed that lichenized fungi do have a highly conserved BGC, providing the first evidence that a biosynthetic gene may constitute essential genes in lichens.

Toxic eburicol accumulation drives the antifungal activity of azoles against Aspergillus fumigatus

Azole antifungals inhibit the sterol C14-demethylase (CYP51/Erg11) of the ergosterol biosynthesis pathway. Here we show that the azole-induced synthesis of fungicidal cell wall carbohydrate patches in the pathogenic mold Aspergillus fumigatus strictly correlates with the accumulation of the CYP51 substrate eburicol. A lack of other essential ergosterol biosynthesis enzymes, such as sterol C24-methyltransferase (Erg6A), squalene synthase (Erg9) or squalene epoxidase (Erg1) does not trigger comparable cell wall alterations. Partial repression of Erg6A, which converts lanosterol into eburicol, increases azole resistance. The sterol C5-desaturase (ERG3)-dependent conversion of eburicol into 14-methylergosta-8,24(28)-dien-3β,6α-diol, the "toxic diol" responsible for the fungistatic activity against yeasts, is not required for the fungicidal effects in A. fumigatus. While ERG3-lacking yeasts are azole resistant, ERG3-lacking A. fumigatus becomes more susceptible. Mutants lacking mitochondrial complex III functionality, which are much less effectively killed, but strongly inhibited in growth by azoles, convert eburicol more efficiently into the supposedly "toxic diol". We propose that the mode of action of azoles against A. fumigatus relies on accumulation of eburicol which exerts fungicidal effects by triggering cell wall carbohydrate patch formation.

Ergosterol Is Critical for Sporogenesis in Cryptococcus neoformans

Microbes, both bacteria and fungi, produce spores to survive stressful conditions. Spores produced by the environmental fungal pathogen Cryptococcus neoformans serve as both surviving and infectious propagules. Because of their importance in disease transmission and pathogenesis, factors necessary for cryptococcal spore germination are being actively investigated. However, little is known about nutrients critical for sporogenesis in this pathogen. Here, we found that ergosterol, the main sterol in fungal membranes, is enriched in spores relative to yeasts and hyphae. In C. neoformans, the ergosterol biosynthesis pathway (EBP) is upregulated by the transcription factor Sre1 in response to conditions that demand elevated ergosterol biosynthesis. Although the deletion of SRE1 enhances the production of mating hyphae, the sre1Δ strain is deficient at producing spores even when crossed with a wild-type partner. We found that the defect of the sre1Δ strain is specific to sporogenesis, not meiosis or basidium maturation preceding sporulation. Consistent with the idea that sporulation demands heightened ergosterol biosynthesis, EBP mutants are also defective in sporulation. We discovered that the overexpression of some EBP genes can largely rescue the sporulation defect of the sre1Δ strain. Collectively, we demonstrate that ergosterol is a critical component in cryptococcal preparation for sporulation.

The Microevolution of Antifungal Drug Resistance in Pathogenic Fungi

The mortality rates of invasive fungal infections remain high because of the limited number of antifungal drugs available and antifungal drug resistance, which can rapidly evolve during treatment. Mutations in key resistance genes such as ERG11 were postulated to be the predominant cause of antifungal drug resistance in the clinic. However, recent advances in whole genome sequencing have revealed that there are multiple mechanisms leading to the microevolution of resistance. In many fungal species, resistance can emerge through ERG11-independent mechanisms and through the accumulation of mutations in many genes to generate a polygenic resistance phenotype. In addition, genome sequencing has revealed that full or partial aneuploidy commonly occurs in clinical or microevolved in vitro isolates to confer antifungal resistance. This review will provide an overview of the mutations known to be selected during the adaptive microevolution of antifungal drug resistance and focus on how recent advances in genome sequencing technology have enhanced our understanding of this process.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Ergosterol is a critical component of fungal plasma membranes. Although many currently available antifungal compounds target the ergosterol biosynthesis pathway for antifungal effect, current knowledge regarding ergosterol synthesis remains incomplete for filamentous fungal pathogens like Aspergillus fumigatus. Here, we show for the first time that the lipid droplet-associated sterol C-24 methyltransferase, Erg6, is essential for A. fumigatus viability. We further show that this essentiality extends to additional Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Neither the overexpression of a putative erg6 paralog, smt1, nor the exogenous addition of ergosterol could rescue erg6 deficiency. Importantly, Erg6 downregulation results in a dramatic decrease in ergosterol and accumulation in lanosterol and is further characterized by diminished sterol-rich plasma membrane domains (SRDs) at hyphal tips. Unexpectedly, erg6 repressed strains demonstrate wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, repressing erg6 expression reduced fungal burden accumulation in a murine model of invasive aspergillosis. Taken together, our studies suggest that Erg6, which shows little homology to mammalian proteins, is potentially an attractive antifungal drug target for therapy of Aspergillus infections.

Impact of TR34/L98H, TR46/Y121F/T289A and TR53 Alterations in Azole-Resistant Aspergillus fumigatus on Sterol Composition and Modifications after In Vitro Exposure to Itraconazole and Voriconazole

BACKGROUND

Sterols are the main components of fungal membranes. Inhibiting their biosynthesis is the mode of action of azole antifungal drugs that are widely used to treat fungal disease including aspergillosis. Azole resistance has emerged as a matter of concern but little is known about sterols biosynthesis in azole resistant Aspergillus fumigatus.

METHODS

We explored the sterol composition of 12 A. fumigatus isolates, including nine azole resistant isolates with TR₃₄/L98H, TR₄₆/Y121F/T289A or TR₅₃ alterations in the cyp51A gene and its promoter conferring azole resistance. Modifications in sterol composition were also investigated after exposure to two azole drugs, itraconazole and voriconazole.

RESULTS

Overall, under basal conditions, sterol compositions were qualitatively equivalent, whatever the alterations in the target of azole drugs with ergosterol as the main sterol detected. Azole exposure reduced ergosterol composition and the qualitative composition of sterols was similar in both susceptible and resistant isolates. Interestingly TR₅₃ strains behaved differently than other strains.

CONCLUSIONS

Elucidating sterol composition in azole-susceptible and resistant isolates is of interest for a better understanding of the mechanism of action of these drugs and the mechanism of resistance of fungi.

Species-Specific Differences in C-5 Sterol Desaturase Function Influence the Outcome of Azole Antifungal Exposure

The azole antifungals inhibit sterol 14α-demethylase (S14DM), leading to depletion of cellular ergosterol and the synthesis of an aberrant sterol diol that disrupts membrane function. In Candida albicans, sterol diol production is catalyzed by the C-5 sterol desaturase enzyme encoded by ERG3. Accordingly, mutations that inactivate ERG3 enable the fungus to grow in the presence of the azoles. The purpose of this study was to compare the propensities of C-5 sterol desaturases from different fungal pathogens to produce the toxic diol upon S14DM inhibition and thus contribute to antifungal efficacy. The coding sequences of ERG3 homologs from C. albicans (CaERG3), Candida glabrata (CgERG3), Candida auris (CaurERG3), Cryptococcus neoformans (CnERG3), Aspergillus fumigatus (AfERG3A-C) and Rhizopus delemar (RdERG3A/B) were expressed in a C. albicans erg3Δ/Δ mutant to facilitate comparative analysis. All but one of the Erg3p-like proteins (AfErg3C) at least partially restored C-5 sterol desaturase activity and to corresponding degrees rescued the stress and hyphal growth defects of the C. albicans erg3Δ/Δ mutant, confirming functional equivalence. Each C-5 desaturase enzyme conferred markedly different responses to fluconazole exposure in terms of the MIC and residual growth observed at supra-MICs. Upon fluconazole-mediated inhibition of S14DM, the strains expressing each homolog also produced various levels of 14α-methylergosta-8,24(28)-dien-3β,6α-diol. The RdErg3A and AfErg3A proteins are notable for low levels of sterol diol production and failing to confer appreciable azole sensitivity upon the C. albicans erg3Δ/Δ mutant. These findings suggest that species-specific properties of C-5 sterol desaturase may be an important determinant of intrinsic azole sensitivity.

Identification of the ergC gene involved in polyene drug sensitivity in the Mucorales species Phycomyces blakesleeanus

BACKGROUND

A strain of Phycomyces blakesleeanus (Mucorales, Mucoromycota) that was previously isolated after ultraviolet mutagenesis has altered responses to polyene antifungal drugs, sterol profiles, and phototropism of its sporangia. In this study, the genetic basis for these changes was sought.

METHODS AND RESULTS

Two base pair substitutions were identified in the mutant within a P. blakelesleeanus gene that is homologous to others characterized from fungi, such as the Saccharomyces cerevisiae ERG3 gene, encoding sterol Δ5,6-desaturase. The polyene resistance and growth reduction phenotypes co-segregated with mutations in the gene in genetic crosses. The P. blakelesleeanus wild type ergC gene complemented a S. cerevisiae deletion strain of ERG3.

CONCLUSIONS

This gene discovery may contribute towards better antifungal use in treating mucormycoses diseases caused by related species in the order Mucorales.

The sterol C-14 reductase Erg24 is responsible for ergosterol biosynthesis and ion homeostasis in Aspergillus fumigatus

Ergosterol, a major lipid present in the fungal cell membrane, is considered as an effective antifungal drug target. A rational strategy for increasing drug reservoir relies on functionally validation of essential enzymes involved in fungal key biological pathway. Current knowledge regarding the essential genes in the ergosterol biosynthesis pathway is still limited in the opportunistic human pathogen Aspergillus fumigatus. In this study, we characterized two endoplasmic reticulum-localized sterol C-14 reductases encoded by both erg24A and erg24B homologs that are essential for the viability of A. fumigatus despite the fact that neither paralog is essential individually. Loss of one homolog of Erg24 impairs hyphal growth, conidiation, and virulence but has no effect on ergosterol biosynthesis. To investigate the functional significance of erg24, a conditional double mutant (Δerg24B niiA::erg24A) was constructed in the Δerg24B background. Strikingly, the conditional erg24 double mutant exhibited severe growth defects and accumulation of sterol intermediate. Moreover, the addition of metal ions and the overexpression of the corresponding ion transporters could rescue the growth defects of the erg24 double mutant in A. fumigatus, implying that the defective phenotype of the erg24 double mutant is tightly associated with dysregulation of ion homeostasis. Taken together, our results demonstrate the critical role of Erg24 in ergosterol biosynthesis and ion homeostasis in A. fumigatus, which may have important implications for antifungal discovery. KEY POINTS: • We characterized two endoplasmic reticulum-localized sterol C-14 reductases Erg24A and Erg24B in A. fumigatus. • Erg24A and Erg24B in combination, but not individually, are required for the viability of A. fumigatus. • Inactivation of Erg24 leads to the disruption of ion homeostasis and affects ergosterol biosynthesis.

Lathosterol Oxidase (Sterol C-5 Desaturase) Deletion Confers Resistance to Amphotericin B and Sensitivity to Acidic Stress in Leishmania major

Sterols are essential membrane components in eukaryotes, and sterol synthesis inhibitors can have potent effects against pathogenic fungi and trypanosomatids. Understanding the roles of sterols will facilitate the development of new drugs and counter drug resistance. LSO is required for the formation of the C-5–C-6 double bond in the sterol core structure in mammals, fungi, protozoans, plants, and algae. Functions of this C-5–C-6 double bond are not well understood. In this study, we generated and characterized a lathosterol oxidase-null mutant in Leishmania major. Our data suggest that LSO is vital for the structure and membrane-stabilizing functions of leishmanial sterols. In addition, our results imply that while mutations in lathosterol oxidase can confer resistance to amphotericin B, an important antifungal and antiprotozoal agent, the alteration in sterol structure leads to significant defects in stress response that could be exploited for drug development.

Lathosterol oxidase (LSO) catalyzes the formation of the C-5–C-6 double bond in the synthesis of various types of sterols in mammals, fungi, plants, and protozoa. In Leishmania parasites, mutations in LSO or other sterol biosynthetic genes are associated with amphotericin B resistance. To investigate the biological roles of sterol C-5–C-6 desaturation, we generated an LSO-null mutant line (lso⁻) in Leishmania major, the causative agent for cutaneous leishmaniasis. lso⁻ parasites lacked the ergostane-based sterols commonly found in wild-type L. major and instead accumulated equivalent sterol species without the C-5–C-6 double bond. These mutant parasites were replicative in culture and displayed heightened resistance to amphotericin B. However, they survived poorly after reaching the maximal density and were highly vulnerable to the membrane-disrupting detergent Triton X-100. In addition, lso⁻ mutants showed defects in regulating intracellular pH and were hypersensitive to acidic conditions. They also had potential alterations in the carbohydrate composition of lipophosphoglycan, a membrane-bound virulence factor in Leishmania. All these defects in lso⁻ were corrected upon the restoration of LSO expression. Together, these findings suggest that the C-5–C-6 double bond is vital for the structure of the sterol core, and while the loss of LSO can lead to amphotericin B resistance, it also makes Leishmania parasites vulnerable to biologically relevant stress.

IMPORTANCE Sterols are essential membrane components in eukaryotes, and sterol synthesis inhibitors can have potent effects against pathogenic fungi and trypanosomatids. Understanding the roles of sterols will facilitate the development of new drugs and counter drug resistance. LSO is required for the formation of the C-5–C-6 double bond in the sterol core structure in mammals, fungi, protozoans, plants, and algae. Functions of this C-5–C-6 double bond are not well understood. In this study, we generated and characterized a lathosterol oxidase-null mutant in Leishmania major. Our data suggest that LSO is vital for the structure and membrane-stabilizing functions of leishmanial sterols. In addition, our results imply that while mutations in lathosterol oxidase can confer resistance to amphotericin B, an important antifungal and antiprotozoal agent, the alteration in sterol structure leads to significant defects in stress response that could be exploited for drug development.

Aspergillus fumigatus Mitochondrial Acetyl Coenzyme A Acetyltransferase as an Antifungal Target

Ergosterol plays an important role in maintaining cell membrane sterol homeostasis in fungi, and as such, it is considered an effective target in antifungal chemotherapy. In yeast, the enzyme acetyl-coenzyme A (CoA) acetyltransferase (ERG10) catalyzes the Claisen condensation of two acetyl-CoA molecules to acetoacetyl-CoA in the ergosterol biosynthesis pathway and is reported as being critical for cell viability. Using yeast ERG10 for alignment, two orthologues, AfERG10A (AFUB_000550) and AfERG10B (AFUB_083570), were discovered in the opportunistic fungal pathogen Aspergillus fumigatus Despite the essentiality of AfERG10B having been previously validated, the biological function of AfERG10A remains unclear. In this study, we have characterized recombinant AfERG10A as a functional acetyl-CoA acetyltransferase catalyzing both synthetic and degradative reactions. Unexpectedly, AfERG10A localizes to the mitochondria in A. fumigatus, as shown by C-terminal green fluorescent protein (GFP) tag fusion. Both knockout and inducible promoter strategies demonstrate that Aferg10A is essential for the survival of A. fumigatus The reduced expression of Aferg10A leads to severe morphological defects and increased susceptibility to oxidative and cell wall stresses. Although the catalytic mechanism of acetyl-CoA acetyltransferase family is highly conserved, the crystal structure of AfERG10A and its complex with CoA are solved, revealing four substitutions within the CoA binding site that are different from human orthologues. Taken together, our combination of genetic and structural studies demonstrates that mitochondrial AfERG10A is essential for A. fumigatus cell viability and could be a potential drug target to feed the antifungal drug development pipeline.IMPORTANCE A growing number of people worldwide are suffering from invasive aspergillosis caused by the human opportunistic fungal pathogen A. fumigatus Current therapeutic options rely on a limited repertoire of antifungals. Ergosterol is an essential component of the fungal cell membrane as well as a target of current antifungals. Approximately 20 enzymes are involved in ergosterol biosynthesis, of which acetyl-CoA acetyltransferase (ACAT) is the first enzyme. Two ACATs in A. fumigatus are AfErg10A and AfErg10B. However, the biological function of AfErg10A is yet to be investigated. In this study, we showed that AfErg10A is localized in the mitochondria and is essential for A. fumigatus survival and morphological development. In combination with structural studies, we validated AfErg10A as a potential drug target that will facilitate the development of novel antifungals and improve the efficiency of existing drugs.

Facilitators of adaptation and antifungal resistance mechanisms in clinically relevant fungi

Opportunistic fungal pathogens can cause a diverse range of diseases in humans. The increasing rate of fungal infections caused by strains that are resistant to commonly used antifungals results in difficulty to treat diseases, with accompanying high mortality rates. Existing and newly emerging molecular resistance mechanisms rapidly spread in fungal populations and need to be monitored. Fungi exhibit a diversity of mechanisms to maintain physiological resilience and create genetic variation; processes which eventually lead to the selection and spread of resistant fungal pathogens. To prevent and anticipate this dispersion, the role of evolutionary factors that drive fungal adaptation should be investigated. In this review, we provide an overview of resistance mechanisms against commonly used antifungal compounds in the clinic and for which fungal resistance has been reported. Furthermore, we aim to summarize and elucidate potent generators of genetic variability across the fungal kingdom that aid adaptation to stressful environments. This knowledge can lead to recognizing potential niches that facilitate fast resistance development and can provide leads for new management strategies to battle the emerging resistant populations in the clinic and the environment.

Expression of C-5 sterol desaturase from an edible mushroom in fisson yeast enhances its ethanol and thermotolerance

Bioethanol is an environment friendly and renewable source of energy produced by the fermentation of agricultural raw material by a variety of microorganisms including yeast. Obtaining yeast strains that are tolerant to stresses like high levels of ethanol and high temperature is highly desirable as it reduces cost and increases yield during bioethanol production. Here, we report that heterologous expression of C-5 Sterol desaturase (FvC5SD)-an ergosterol biosynthesis enzyme from an edible mushroom Flammulina velutipes in fission yeast, not only imparts increased thermotolerance but also tolerance towards high ethanol concentration and low pH. This tolerance could be attributed to an increase of ≈1.5 fold in the level of ergosterol and oleic acid (C-18 unsaturated fatty acid) as analysed by gas chromatography- mass spectrometry. FvC5SD is a membrane localized iron binding enzyme that introduces double bond at C-5 position into the Δ7-sterol substrates to yield Δ5, 7- sterols as products. In F. velutipes, FvC5SD transcript was observed to be upregulated by ≈5 fold under low pH condition and by ≈ 9 folds and ≈5 fold at 40°C and 4°C respectively when compared to normal growth temperature of 23°C. Besides, susceptibility to cell wall inhibiting drugs like Congo red and Calcoflour white was also found to increase in FvC5SD expressing S. pombe strain. Alteration in membrane sterol and fatty acid composition could also lead to increase in susceptibility to cell wall inhibiting drugs. Thus, this study has immense industrial application and can be employed to ensure competitiveness of fermentation process.

Erg4A and Erg4B Are Required for Conidiation and Azole Resistance via Regulation of Ergosterol Biosynthesis in Aspergillus fumigatus

Ergosterol, a fungus-specific sterol enriched in cell plasma membranes, is an effective antifungal drug target. However, current knowledge of the ergosterol biosynthesis process in the saprophytic human fungal pathogen Aspergillus fumigatus remains limited. In this study, we found that two endoplasmic reticulum-localized sterol C-24 reductases encoded by both erg4A and erg4B homologs are required to catalyze the reaction during the final step of ergosterol biosynthesis. Loss of one homolog of Erg4 induces the overexpression of the other one, accompanied by almost normal ergosterol synthesis and wild-type colony growth. However, double deletions of erg4A and erg4B completely block the last step of ergosterol synthesis, resulting in the accumulation of ergosta-5,7,22,24(28)-tetraenol, a precursor compound of ergosterol. Further studies indicate that erg4A and erg4B are required for conidiation but not for hyphal growth. Importantly, the Δerg4A Δerg4B mutant still demonstrates wild-type virulence in a compromised mouse model but displays remarkable increased susceptibility to antifungal azoles. Our data suggest that inhibitors of Erg4A and Erg4B may serve as effective candidates for adjunct antifungal agents with azoles.

IMPORTANCE

Knowledge of the ergosterol biosynthesis pathway in the human opportunistic pathogen A. fumigatus is useful for designing and finding new antifungal drugs. In this study, we demonstrated that the endoplasmic reticulum-localized sterol C-24 reductases Erg4A and Erg4B are required for conidiation via regulation of ergosterol biosynthesis. Moreover, inactivation of both Erg4A and Erg4B results in hypersensitivity to the clinical guideline-recommended antifungal drugs itraconazole and voriconazole. Therefore, our finding indicates that inhibition of Erg4A and Erg4B might be an effective approach for alleviating A. fumigatus infection.

Regulation of Sterol Biosynthesis in the Human Fungal Pathogen Aspergillus fumigatus: Opportunities for Therapeutic Development

Sterols are a major component of eukaryotic cell membranes. For human fungal infections caused by the filamentous fungus Aspergillus fumigatus, antifungal drugs that target sterol biosynthesis and/or function remain the standard of care. Yet, an understanding of A. fumigatus sterol biosynthesis regulatory mechanisms remains an under developed therapeutic target. The critical role of sterol biosynthesis regulation and its interactions with clinically relevant azole drugs is highlighted by the basic helix loop helix (bHLH) class of transcription factors known as Sterol Regulatory Element Binding Proteins (SREBPs). SREBPs regulate transcription of key ergosterol biosynthesis genes in fungi including A. fumigatus. In addition, other emerging regulatory pathways and target genes involved in sterol biosynthesis and drug interactions provide additional opportunities including the unfolded protein response, iron responsive transcriptional networks, and chaperone proteins such as Hsp90. Thus, targeting molecular pathways critical for sterol biosynthesis regulation presents an opportunity to improve therapeutic options for the collection of diseases termed aspergillosis. This mini-review summarizes our current understanding of sterol biosynthesis regulation with a focus on mechanisms of transcriptional regulation by the SREBP family of transcription factors.

Analysis of cytochrome b(5) reductase-mediated metabolism in the phytopathogenic fungus Zymoseptoria tritici reveals novel functionalities implicated in virulence

Septoria tritici blotch (STB) caused by the Ascomycete fungus Zymoseptoria tritici is one of the most economically damaging diseases of wheat worldwide. Z. tritici is currently a major target for agricultural fungicides, especially in temperate regions where it is most prevalent. Many fungicides target electron transfer enzymes because these are often important for cell function. Therefore characterisation of genes encoding such enzymes may be important for the development of novel disease intervention strategies. Microsomal cytochrome b5 reductases (CBRs) are an important family of electron transfer proteins which in eukaryotes are involved in the biosynthesis of fatty acids and complex lipids including sphingolipids and sterols. Unlike the model yeast Saccharomyces cerevisiae which possesses only one microsomal CBR, the fully sequenced genome of Z. tritici bears three possible microsomal CBRs. RNA sequencing analysis revealed that ZtCBR1 is the most highly expressed of these genes under all in vitro and in planta conditions tested, therefore ΔZtCBR1 mutant strains were generated through targeted gene disruption. These strains exhibited delayed disease symptoms on wheat leaves and severely limited asexual sporulation. ΔZtCBR1 strains also exhibited aberrant spore morphology and hyphal growth in vitro. These defects coincided with alterations in fatty acid, sphingolipid and sterol biosynthesis observed through GC-MS and HPLC analyses. Data is presented which suggests that Z. tritici may use ZtCBR1 as an additional electron donor for key steps in ergosterol biosynthesis, one of which is targeted by azole fungicides. Our study reports the first functional characterisation of CBR gene family members in a plant pathogenic filamentous fungus. This also represents the first direct observation of CBR functional ablation impacting upon fungal sterol biosynthesis.

Medically important fungi respond to azole drugs: an update

The increased numbers of patients with compromised immune systems in the last three decades have increased the chances of life-threatening fungal infections. Numerous antifungal drugs have been developed in the last 20 years to treat these infections. The largest group, the azoles, inhibits the synthesis of fungal sterols. The use of these fungistatic azoles has subsequently led to the emergence of acquired azole resistance. The most common mechanisms that result in azole resistance include the overexpression or mutation of the azole target enzyme, and overexpression of drug transporters that are responsible for azole efflux from cells. Additional, less-frequent mechanisms have also been identified. Understanding azole resistance mechanisms is crucial for current antifungal treatment and for the future development of new treatment strategies.

Ergosterol biosynthesis in Aspergillus fumigatus: its relevance as an antifungal target and role in antifungal drug resistance

Ergosterol, the major sterol of fungal membranes, is essential for developmental growth and the main target of antifungals that are currently used to treat fatal fungal infections. Emergence of resistance to existing antifungals is a current problem and several secondary resistance mechanisms have been described in Aspergillus fumigatus clinical isolates. A full understanding of ergosterol biosynthetic control therefore appears to be essential for improvement of antifungal efficacy and to prevent antifungal resistance. An ergosterol biosynthesis pathway in A. fumigatus has been proposed with 14 sterol intermediates resulting in ergosterol and another secondary final compound C-24 ethyl sterol. Transcriptomic analysis of the A. fumigatus response to host-imposed stresses or antifungal agents is expanding our understanding of both sterol biosynthesis and the modes of action of antifungal drugs. Ultimately, the identification of new targets for novel drug design, or the study of combinatorial effects of targeting sterol biosynthesis together with other metabolic pathways, is warranted.

Discovery of a HapE mutation that causes azole resistance in Aspergillus fumigatus through whole genome sequencing and sexual crossing

Azole compounds are the primary therapy for patients with diseases caused by Aspergillus fumigatus. However, prolonged treatment may cause resistance to develop, which is associated with treatment failure. The azole target cyp51A is a hotspot for mutations that confer phenotypic resistance, but in an increasing number of resistant isolates the underlying mechanism remains unknown. Here, we report the discovery of a novel resistance mechanism, caused by a mutation in the CCAAT-binding transcription factor complex subunit HapE. From one patient, four A. fumigatus isolates were serially collected. The last two isolates developed an azole resistant phenotype during prolonged azole therapy. Because the resistant isolates contained a wild type cyp51A gene and the isolates were isogenic, the complete genomes of the last susceptible isolate and the first resistant isolate (taken 17 weeks apart) were sequenced using Illumina technology to identify the resistance conferring mutation. By comparing the genome sequences to each other as well as to two A. fumigatus reference genomes, several potential non-synonymous mutations in protein-coding regions were identified, six of which could be confirmed by PCR and Sanger sequencing. Subsequent sexual crossing experiments showed that resistant progeny always contained a P88L substitution in HapE, while the presence of the other five mutations did not correlate with resistance in the progeny. Cloning the mutated hapE gene into the azole susceptible akuB(KU80) strain showed that the HapE P88L mutation by itself could confer the resistant phenotype. This is the first time that whole genome sequencing and sexual crossing strategies have been used to find the genetic basis of a trait of interest in A. fumigatus. The discovery may help understand alternate pathways for azole resistance in A. fumigatus with implications for the molecular diagnosis of resistance and drug discovery.

Transcriptomic and proteomic analyses of the Aspergillus fumigatus hypoxia response using an oxygen-controlled fermenter

BACKGROUND

Aspergillus fumigatus is a mold responsible for the majority of cases of aspergillosis in humans. To survive in the human body, A. fumigatus must adapt to microenvironments that are often characterized by low nutrient and oxygen availability. Recent research suggests that the ability of A. fumigatus and other pathogenic fungi to adapt to hypoxia contributes to their virulence. However, molecular mechanisms of A. fumigatus hypoxia adaptation are poorly understood. Thus, to better understand how A. fumigatus adapts to hypoxic microenvironments found in vivo during human fungal pathogenesis, the dynamic changes of the fungal transcriptome and proteome in hypoxia were investigated over a period of 24 hours utilizing an oxygen-controlled fermenter system.

RESULTS

Significant increases in transcripts associated with iron and sterol metabolism, the cell wall, the GABA shunt, and transcriptional regulators were observed in response to hypoxia. A concomitant reduction in transcripts was observed with ribosome and terpenoid backbone biosynthesis, TCA cycle, amino acid metabolism and RNA degradation. Analysis of changes in transcription factor mRNA abundance shows that hypoxia induces significant positive and negative changes that may be important for regulating the hypoxia response in this pathogenic mold. Growth in hypoxia resulted in changes in the protein levels of several glycolytic enzymes, but these changes were not always reflected by the corresponding transcriptional profiling data. However, a good correlation overall (R(2) = 0.2, p < 0.05) existed between the transcriptomic and proteomics datasets for all time points. The lack of correlation between some transcript levels and their subsequent protein levels suggests another regulatory layer of the hypoxia response in A. fumigatus.

CONCLUSIONS

Taken together, our data suggest a robust cellular response that is likely regulated both at the transcriptional and post-transcriptional level in response to hypoxia by the human pathogenic mold A. fumigatus. As with other pathogenic fungi, the induction of glycolysis and transcriptional down-regulation of the TCA cycle and oxidative phosphorylation appear to major components of the hypoxia response in this pathogenic mold. In addition, a significant induction of the transcripts involved in ergosterol biosynthesis is consistent with previous observations in the pathogenic yeasts Candida albicans and Cryptococcus neoformans indicating conservation of this response to hypoxia in pathogenic fungi. Because ergosterol biosynthesis enzymes also require iron as a co-factor, the increase in iron uptake transcripts is consistent with an increased need for iron under hypoxia. However, unlike C. albicans and C. neoformans, the GABA shunt appears to play an important role in reducing NADH levels in response to hypoxia in A. fumigatus and it will be intriguing to determine whether this is critical for fungal virulence. Overall, regulatory mechanisms of the A. fumigatus hypoxia response appear to involve both transcriptional and post-transcriptional control of transcript and protein levels and thus provide candidate genes for future analysis of their role in hypoxia adaptation and fungal virulence.

Mevalonate governs interdependency of ergosterol and siderophore biosyntheses in the fungal pathogen Aspergillus fumigatus

Aspergillus fumigatus is the most common airborne fungal pathogen for humans. In this mold, iron starvation induces production of the siderophore triacetylfusarinine C (TAFC). Here we demonstrate a link between TAFC and ergosterol biosynthetic pathways, which are both critical for virulence and treatment of fungal infections. Consistent with mevalonate being a limiting prerequisite for TAFC biosynthesis, we observed increased expression of 3-hydroxy-3-methyl-glutaryl (HMG)-CoA reductase (Hmg1) under iron starvation, reduced TAFC biosynthesis following lovastatin-mediated Hmg1 inhibition, and increased TAFC biosynthesis following Hmg1 overexpression. We identified enzymes, the acyl-CoA ligase SidI and the enoyl-CoA hydratase SidH, linking biosynthesis of mevalonate and TAFC, deficiency of which under iron starvation impaired TAFC biosynthesis, growth, oxidative stress resistance, and murine virulence. Moreover, inactivation of these enzymes alleviated TAFC-derived biosynthetic demand for mevalonate, as evidenced by increased resistance to lovastatin. Concordant with bilateral demand for mevalonate, iron starvation decreased the ergosterol content and composition, a phenotype that is mitigated in TAFC-lacking mutants.

Probing the role of point mutations in the cyp51A gene from Aspergillus fumigatus in the model yeast Saccharomyces cerevisiae

Azole-resistant strains of Aspergillus fumigatus have been detected and the underlying molecular mechanisms of resistance characterized. Point mutations in the cyp51A gene have been proved to be related to azole resistance in A. fumigatus clinical strains and with different resistance profiles depending on the amino acid change (G54E, G54V, G54R, G54W, M220V, M220K, M220T, M220I). The aim of this work was to express A. fumigatus cyp51A genes in the yeast Saccharomyces cerevisiae in order to better assess the contribution of each independent amino acid substitution to resistance. A tetracycline regulatable system allowing repression of the endogenous essential ERG11 gene was used. The expression of Aspergillus cyp51A alleles could efficiently restore the absence of ERG11 in S. cerevisiae. In general, S. cerevisiae clones expressing. A. fumigatus cyp51A alleles from azole-resistant isolates showed higher MICs to all azoles tested than those expressing alleles from susceptible isolates. The azole susceptibility profiles obtained in S. cerevisiae upon expression of specific cyp51A alleles recapitulated susceptibility profiles observed from their A. fumigatus origins. In conclusion this work supports the concept that characteristics of specific A. fumigatus cyp51A alleles could be investigated in the heterologous host S. cerevisiae.

Expression, purification, and characterization of Aspergillus fumigatus sterol 14-alpha demethylase (CYP51) isoenzymes A and B

Aspergillus fumigatus sterol 14-α demethylase (CYP51) isoenzymes A (AF51A) and B (AF51B) were expressed in Escherichia coli and purified. The dithionite-reduced CO-P450 complex for AF51A was unstable, rapidly denaturing to inactive P420, in marked contrast to AF51B, where the CO-P450 complex was stable. Type I substrate binding spectra were obtained with purified AF51B using lanosterol (K(s), 8.6 μM) and eburicol (K(s), 22.6 μM). Membrane suspensions of AF51A bound to both lanosterol (K(s), 3.1 μM) and eburicol (K(s), 4.1 μM). The binding of azoles, with the exception of fluconazole, to AF51B was tight, with the K(d) (dissociation constant) values for clotrimazole, itraconazole, posaconazole, and voriconazole being 0.21, 0.06, 0.12, and 0.42 μM, respectively, in comparison with a K(d) value of 4 μM for fluconazole. Characteristic type II azole binding spectra were obtained with AF51B, whereas an additional trough and a blue-shifted spectral peak were present in AF51A binding spectra for all azoles except clotrimazole. This suggests two distinct azole binding conformations within the heme prosthetic group of AF51A. All five azoles bound relatively weakly to AF51A, with K(d) values ranging from 1 μM for itraconazole to 11.9 μM for fluconazole. The azole binding properties of purified AF51A and AF51B suggest an explanation for the intrinsic azole (fluconazole) resistance observed in Aspergillus fumigatus.

C-5(6) sterol desaturase from tetrahymena thermophila: Gene identification and knockout, sequence analysis, and comparison to other C-5(6) sterol desaturases

The gene coding for a C-5(6) sterol desaturase in Tetrahymena thermophila, DES5A, has been identified by the knockout of the TTHERM_01194720 sequence. Macronucleus transformation was achieved by biolistic bombardment and gene replacement through phenotypic assortment, using paromomycin as the selective agent. A knockout cell line (KO270) showed a phenotype consistent with that of the DES5A deletion mutant. KO270 converted only 6% of the added sterol into the C-5 unsaturated derivative, while the wild type accumulated 10-fold larger amounts under similar conditions. The decreased desaturation activity is specific for the C-5(6) position of lathosterol and cholestanol; other desaturations, namely C-7(8) and C-22(23), were not affected. Analysis by reverse transcription-PCR reveals that DES5A is transcribed both in the presence and absence of cholestanol in wild-type cells, whereas the transcribed gene was not detected in KO270. The growth of KO270 was undistinguishable from that of the wild-type strain. Des5Ap resembles known C-5(6) sterol desaturases, displaying the three typical histidine motifs, four hydrophobic transmembrane regions, and two other highly conserved domains of unknown function. A phylogenetic analysis placed T. thermophila's enzyme and Paramecium orthologues in a cluster together with functionally characterized C-5 sterol desaturases from vertebrates, fungi, and plants, although in a different branch.

A sterol-regulatory element binding protein is required for cell polarity, hypoxia adaptation, azole drug resistance, and virulence in Aspergillus fumigatus

At the site of microbial infections, the significant influx of immune effector cells and the necrosis of tissue by the invading pathogen generate hypoxic microenvironments in which both the pathogen and host cells must survive. Currently, whether hypoxia adaptation is an important virulence attribute of opportunistic pathogenic molds is unknown. Here we report the characterization of a sterol-regulatory element binding protein, SrbA, in the opportunistic pathogenic mold, Aspergillus fumigatus. Loss of SrbA results in a mutant strain of the fungus that is incapable of growth in a hypoxic environment and consequently incapable of causing disease in two distinct murine models of invasive pulmonary aspergillosis (IPA). Transcriptional profiling revealed 87 genes that are affected by loss of SrbA function. Annotation of these genes implicated SrbA in maintaining sterol biosynthesis and hyphal morphology. Further examination of the SrbA null mutant consequently revealed that SrbA plays a critical role in ergosterol biosynthesis, resistance to the azole class of antifungal drugs, and in maintenance of cell polarity in A. fumigatus. Significantly, the SrbA null mutant was highly susceptible to fluconazole and voriconazole. Thus, these findings present a new function of SREBP proteins in filamentous fungi, and demonstrate for the first time that hypoxia adaptation is likely an important virulence attribute of pathogenic molds.

The incidence of potentially lethal infections caused by normally benign molds has increased tremendously over the last two decades. One disease in particular, invasive pulmonary aspergillosis (IPA), caused by the common mold Aspergillus fumigatus, has become the leading cause of death due to invasive mycoses. Currently, we have a limited understanding of how this opportunistic pathogen causes disease in immunocompromised patients. In this study, we discover a previously unexplored mechanism required by this mold to cause disease, hypoxia (low oxygen) adaptation. We report that hypoxia adaptation in A. fumigatus is mediated in part by a highly conserved transcription factor, SrbA, a protein in the sterol regulatory element binding protein family. A null mutant of SrbA was unable to grow in hypoxia, displayed increased susceptibility to the azole class of antifungal drugs, and was avirulent in two distinct murine models of IPA. Importantly, we report the discovery of a novel function of SrbA in molds related to maintenance of cell polarity. The finding that SrbA regulates resistance to the azole class of antifungal drugs presents an opportunity to uncover new mechanisms of antifungal drug resistance in A. fumigatus.

Mechanisms of resistance to antifungal agents: yeasts and filamentous fungi

Failure to respond to antifungal therapy could be due to in vitro resistance (intrinsic or developed during therapy) or clinical resistance; the latter is associated with numerous factors related to the host, the antifungal agent, or the infecting isolate. Recently, a susceptible MIC breakpoint ( < or =2 microg/ml) was designed for Candida spp. to all three available echinocandins, anidulafungin (Pfizer), caspofungin (Merck) and micafungin (Astellas) and treatment failures have been associated with MICs > 2 microg/ml. In some of these cases, clinical failure was associated with the genetic mutations described below. Azole and flucytosine breakpoints, and the echinocandin susceptible breakpoint, are useful when isolates are tested by CLSI standardized methods; breakpoints are also available by the EUCAST method. More recently, in vitro resistant MIC breakpoints have been assigned for filamentous fungi (moulds) vs. five antifungal agents, but these categories are not based on correlations of in vitro with in vivo response to therapy. However, itraconazole (Janssen), amphotericin B (Bristol-Myers) and voriconazole (Pfizer) clinical failures in aspergillosis have been correlated with MICs > 2 microg/ml. This article provides a review of reported resistance molecular mechanisms to antifungal agents since 2005; previous related reviews are also listed.

Proteomic and transcriptomic analysis of Aspergillus fumigatus on exposure to amphotericin B

Amphotericin B (AMB) is the most widely used polyene antifungal drug for the treatment of systemic fungal infections, including invasive aspergillosis. It has been our aim to understand the molecular targets of AMB in Aspergillus fumigatus by genomic and proteomic approaches. In transcriptomic analysis, a total of 295 genes were found to be differentially expressed (165 upregulated and 130 downregulated), including many involving the ergosterol pathway, cell stress proteins, cell wall proteins, transport proteins, and hypothetical proteins. Proteomic profiles of A. fumigatus alone or A. fumigatus treated with AMB showed differential expression levels for 85 proteins (76 upregulated and 9 downregulated). Forty-eight of them were identified with high confidence and belonged to the above-mentioned categories. Differential expression levels for Rho-GDP dissociation inhibitor (Rho-GDI), secretory-pathway GDI, clathrin, Sec 31 (a subunit of the exocyst complex), and RAB GTPase Ypt51 in response to an antifungal drug are reported here for the first time and may represent a specific response of A. fumigatus to AMB. The expression of some of these genes was validated by real-time reverse transcription-PCR. The AMB responsive genes/proteins observed to be differentially expressed in A. fumigatus may be further explored for novel drug development.

Cloning and functional identification of C-4 methyl sterol oxidase genes from the penicillin-producing fungus Penicillium chrysogenum

Two C-4 methyl sterol oxidase genes (Pcerg25A and Pcerg25B) that are involved in ergosterol biosynthesis have been cloned from the penicillin-producing fungus Penicillium chrysogenum. cDNAs of both Pcerg25A and Pcerg25B have an ORF 885 bp in length, encoding a peptide of 295 residues. The deduced amino acid sequences of PcErg25A and PcErg25B show 86% identity, and have high identities to the characterized C-4 methyl sterol oxidases from Candida albicans and Saccharomyces cerevisiae. The function of Pcerg25A and Pcerg25B was identified by complementation of a yeast erg25-deficient strain. Pcerg25A is located in the DNA region containing the penicillin gene cluster, and thus its copy number is dependent on the patterns of the cluster region. Up to eight copies of Pcerg25A were found in the high-productivity strain NCPC 10086. By contrast, Pcerg25B was present in just a single copy in all tested P. chrysogenum genomes. Differences in the transcript level of either Pcerg25A or Pcerg25B were observed in different P. chrysogenum strains by real-time quantitative reverse transcriptase PCR analysis.

Multiple functions of ergosterol in the fission yeast Schizosaccharomyces pombe

Sterols are a major class of membrane lipids in eukaryotes. In Schizosaccharomyces pombe, sterol 24-C-methyltransferase (Erg6p), C-8 sterol isomerase (Erg2p), C-5 sterol desaturase (Erg31p, Erg32p), C-22 sterol desaturase (Erg5p) and C-24 (28) sterol reductase (Sts1p/Erg4p) have been predicted, but not yet determined, to catalyse a sequence of reactions from zymosterol to ergosterol. Disruption mutants of these genes were unable to synthesize ergosterol, and most were tolerant to the polyene drugs amphotericin B and nystatin. Disruption of erg31(+) or erg32(+) did not cause ergosterol deficiency or tolerance to polyene drugs, indicating that the two C-5 sterol desaturases have overlapping functions. GFP-tagged DRM (detergent-resistant membrane)-associated protein Pma1p localized to the plasma membrane in ergDelta mutants. DRM fractionation revealed that the association between Pma1-GFP and DRM was weakened in erg6Delta but not in other erg mutants. Several GFP-tagged plasma membrane proteins were tested, and an amino acid permease homologue, SPBC359.03c, was found to mislocalize to intracellular punctate structures in the ergDelta mutants. These results indicate that these proteins are responsible for ergosterol biosynthesis in fission yeast, similar to the situation in Saccharomyces cerevisiae. Furthermore, in fission yeast, ergosterol is important for plasma membrane structure and function and for localization of plasma membrane proteins.

Ergosterol biosynthesis pathway in Aspergillus fumigatus

The sterol composition of Aspergillus fumigatus for the biosynthesis of ergosterol is of interest since this pathway is the target for many antifungal drugs in clinical use. The sterol composition of this fungal species was analyzed by gas chromatography-mass spectrometry in different strains (susceptible and resistant to azole drugs). Also, sterols were analyzed in several A. fumigatus mutant strains deficient in enzymatic steps of the ergosterol biosynthesis pathway such as 14-alpha sterol demethylases (Cyp51A and Cyp51B) and C-5 sterol desaturases (Erg3A, Erg3B and Erg3C). All sterols identified from azole-resistant A. fumigatus strains were qualitatively and quantitatively similar to the susceptible strain (CM-237). However, sterol composition of mutants strains were different depending on the lacking enzyme. The analysis of the sterol composition in these mutant strains led to a better understanding of the ergosterol biosynthesis pathway in this important fungus.

Aspergillus section Fumigati: antifungal susceptibility patterns and sequence-based identification

This study analyzed 28 Aspergillus strains belonging to the section Fumigati that were isolated from clinical samples in Spain. All isolates sporulated slowly and were unable to grow at 48 degrees C. Phylogenetic analysis based on sequencing of partial sequences of the beta-tubulin and rodlet A genes was used to classify the 28 strains into six different clades (Neosartorya hiratsukae, Neosartorya pseudofischeri, Aspergillus viridinutans, Aspergillus lentulus, Aspergillus fumigatiaffinis, and Aspergillus fumisynnematus). Antifungal susceptibility testing showed heterogeneous patterns and grouped the strains together by species. Most A. lentulus and A. fumigatiaffinis isolates showed high MICs of amphotericin B (geometric mean [GM] MICs, >or=4.5 microg/ml), itraconazole (GM MICs, >or=6 microg/ml), voriconazole (GM MICs, >or=3 microg/ml), and ravuconazole (GM MICs, >or=3 microg/ml); N pseudofischeri and A. viridinutans showed high MICs of itraconazole (GM MICs, >or=8 microg/ml), voriconazole (GM MICs, >or=3.33 microg/ml), and ravuconazole (GM MICs, >or=2 microg/ml); and N. hiratsukae and A. fumisynnematus were susceptible to all the antifungals tested. In conclusion, a number of different species whose morphological features resemble those of Aspergillus fumigatus could succeed in producing invasive infections in the susceptible host. In addition, some of them showed high MICs for most of the antifungals available for the treatment of patients infected with these organisms. The epidemiology and clinical relevance of these species should therefore be addressed.

Analysis of the ambruticin and jerangolid gene clusters of Sorangium cellulosum reveals unusual mechanisms of polyketide biosynthesis

Ambruticins and jerangolids are structurally related antifungal polyketides produced by Sorangium cellulosum strains. Comparative analysis of the gene clusters and characterization of compounds produced by gene knockout strains suggested hypothetical schemes for biosynthesis of these compounds. Polyketide synthase (PKS) architecture suggests that the pyran ring structure common to ambruticins and jerangolids forms by an intramolecular reaction on a PKS-bound intermediate. Disrupting ambM, encoding a discrete enzyme homologous to PKS C-methyltransferase domains, gave 15-desmethylambruticins. Thus, AmbM is required for C-methylation, but not pyran ring formation. Several steps in the post-PKS modification of ambruticin involve new enzymology. Remarkably, the methylcyclopropane ring and putative carbon atom excision during ambruticin biosynthesis apparently occur on the PKS assembly line. The mechanism probably involves a Favorskii rearrangement, but further work is required to elucidate these complex events.

A new Aspergillus fumigatus resistance mechanism conferring in vitro cross-resistance to azole antifungals involves a combination of cyp51A alterations

Fourteen Aspergillus fumigatus clinical isolates that exhibited a pattern of reduced susceptibility to triazole drugs were analyzed. The sequences of the cyp51A gene from all isolates showed the presence of a point mutation at t364a, which led to the substitution of leucine 98 for histidine (L98H), together with the presence of two copies of a 34-bp sequence in tandem in the promoter of the cyp51A gene. Quantitative expression analysis (real-time PCR) showed up to an eightfold increase in the level of expression of the cyp51A gene compared to that by the susceptible strain. Three PCR fragments of one azole-resistant strain (strain CM2627) that included the promoter with the tandem repeat and part of cyp51A with the t364a mutation or PCR fragments with only one of the modifications were used to replace the cyp51A gene of an azole drug-susceptible A. fumigatus wild-type strain (strain CM237). Only transformants which had incorporated the tandem repeat in the promoter of the cyp51A gene and the L98H amino acid substitution exhibited similarly reduced patterns of susceptibility to all triazole agents and similarly increased levels of cyp51A expression, confirming that the combination of both alterations was responsible for the azole-resistant phenotype.

Role of antifungal susceptibility testing in patient management

PURPOSE OF REVIEW

The review focuses on the evolving role of antifungal susceptibility testing in the clinical management of patients with invasive fungal infections. It will clarify how to clinically interpret available susceptibility testing methods.

RECENT FINDINGS

Fluconazole susceptibility testing for yeasts has now shown a relationship between fluconazole resistance and therapeutic failure. Of increasing concern are the reported cases of fluconazole cross-resistance to the newer triazoles. The correlation between amphotericin B and echinocandin susceptibility testing for yeasts, resistance, and outcomes is unknown. Although there are reports of clinical failures with echinocandins, resistance is uncommon and susceptibility testing is unhelpful. There are standardized methods of testing for the triazoles and amphotericin B against molds; however, resistance has not been correlated with outcomes due to host and laboratory factors. Knowing the intrinsic resistance of molds to particular antifungals is of greater importance. Posaconazole is a potent new antifungal agent against many difficult-to-treat molds including fusariosis and zygomycosis.

SUMMARY

Antifungal susceptibility testing of yeasts can assist in treating patients with prior antifungal exposure in determining resistance or cross-resistance. Mold susceptibility testing is of limited clinical benefit, while posconazole is an exciting new agent for treating difficult invasive fungal infections.