hapE and hmg1 Mutations Are Drivers of cyp51A-Independent Pan-Triazole Resistance in an Aspergillus fumigatus Clinical Isolate

Molecular Diagnostics for Invasive Fungal Diseases: Current and Future Approaches

Invasive fungal diseases (IFDs) comprise a growing healthcare burden, especially given the expanding population of immunocompromised hosts. Early diagnosis of IFDs is required to optimise therapy with antifungals, especially in the setting of rising rates of antifungal resistance. Molecular techniques including nucleic acid amplification tests and whole genome sequencing have potential to offer utility in overcoming limitations with traditional phenotypic testing. However, standardisation of methodology and interpretations of these assays is an ongoing undertaking. The utility of targeted Aspergillus detection has been well-defined, with progress in investigations into the role of targeted assays for Candida, Pneumocystis, Cryptococcus, the Mucorales and endemic mycoses. Likewise, whilst broad-range polymerase chain reaction assays have been in use for some time, pathology stewardship and optimising diagnostic yield is a continuing exercise. As costs decrease, there is also now increased access and experience with whole genome sequencing, including metagenomic sequencing, which offers unparalleled resolution especially in the investigations of potential outbreaks. However, their role in routine diagnostic use remains uncommon and standardisation of techniques and workflow are required for wider implementation.

A secondary mechanism of action for triazole antifungals in Aspergillus fumigatus mediated by hmg1

Triazole antifungals function as ergosterol biosynthesis inhibitors and are frontline therapy for invasive fungal infections, such as invasive aspergillosis. The primary mechanism of action of triazoles is through the specific inhibition of a cytochrome P450 14-α-sterol demethylase enzyme, Cyp51A/B, resulting in depletion of cellular ergosterol. Here, we uncover a clinically relevant secondary mechanism of action for triazoles within the ergosterol biosynthesis pathway. We provide evidence that triazole-mediated inhibition of Cyp51A/B activity generates sterol intermediate perturbations that are likely decoded by the sterol sensing functions of HMG-CoA reductase and Insulin-Induced Gene orthologs as increased pathway activity. This, in turn, results in negative feedback regulation of HMG-CoA reductase, the rate-limiting step of sterol biosynthesis. We also provide evidence that HMG-CoA reductase sterol sensing domain mutations previously identified as generating resistance in clinical isolates of Aspergillus fumigatus partially disrupt this triazole-induced feedback. Therefore, our data point to a secondary mechanism of action for the triazoles: induction of HMG-CoA reductase negative feedback for downregulation of ergosterol biosynthesis pathway activity. Abrogation of this feedback through acquired mutations in the HMG-CoA reductase sterol sensing domain diminishes triazole antifungal activity against fungal pathogens and underpins HMG-CoA reductase-mediated resistance.

Epidemiology and Azole Resistance of Clinical Isolates of Aspergillus fumigatus from a Large Tertiary Hospital in Ningxia, China

Purpose

The objective of this study was to determine the clinical distribution, in vitro antifungal susceptibility and underlying resistance mechanisms of Aspergillus fumigatus (A. fumigatus) isolates from the General Hospital of Ningxia Medical University between November 2021 and May 2023.

Methods

Antifungal susceptibility testing was performed using the Sensititre YeastOne YO10, and isolates with high minimal inhibitory concentrations (MICs) were further confirmed using the standard broth microdilution assays established by the Clinical and Laboratory Standards Institute (CLSI) M38-third edition. Whole-Genome Resequencing and RT-qPCR in azole-resistant A. fumigatus strains were performed to investigate the underlying resistance mechanisms.

Results

Overall, a total of 276 A. fumigatus isolates were identified from various clinical departments, showing an increasing trend in the number of isolates over the past 3 years. Two azole-resistant A. fumigatus strains (0.72%) were observed, one of which showed overexpression of cyp51A, cyp51B, cdr1B, MDR1/2, artR, srbA, erg24A, and erg4B, but no cyp51A mutation. However, the other strain harbored two alterations in the cyp51A sequences (L98H/S297T). Therefore, we first described two azole-resistant clinical A. fumigatus strains in Ningxia, China, and reported one azole-resistant strain that has the L98H/S297T mutations in the cyp51A gene without any tandem repeat (TR) sequences in the promoter region.

Conclusions

This study emphasizes the importance of enhancing attention and surveillance of azole-resistant A. fumigatus, particularly those with non-TR point mutations of cyp51A or non-cyp51A mutations, in order to gain a better understanding of their prevalence and spread in the region.

Occurrence of Aspergillus fumigatus azole resistance in soils from Switzerland

Aspergillus fumigatus is a fungal species causing diverse diseases in humans. The use of azoles for treatments of A. fumigatus diseases has resulted in azole resistance. Azoles are also widely used in the environment for crop protection, which resulted in azole resistance. Resistance is primarily due to mutations in cyp51A, which encodes the target protein for azoles. Here we addressed the occurrence of azole resistance in soils from a vast part of Switzerland. We aimed to associate the use of azoles in the environment with the occurrence of azole resistance. We targeted sample sites from different agricultural environments as well as sites with no agricultural practice (natural sites and urban sites). Starting from 327 sites, 113 A. fumigatus isolates were recovered (2019-2021), among which 19 were azole-resistant (15 with TR34/L98H and four with TR46/Y121F/T289A resistance mutations in cyp51A). Our results show that azole resistance was not associated with a specific agricultural practice. Azoles could be chemically detected in investigated soils, however, their presence was not associated with the occurrence of azole-resistant isolates. Interestingly, genetic markers of resistance to other fungicides were detected but only in azole-resistant isolates, thus reinforcing the notion that A. fumigatus cross-resistance to fungicides has an environmental origin. In conclusion, this study reveals the spreading of azole resistance in A. fumigatus from the environment in Switzerland. The proximity of agricultural areas to urban centers may facilitate the transmission of resistant strains to at-risk populations. Thus, vigilant surveillance is required to maintain effective treatment options for aspergillosis.