Triazole resistant Aspergillus fumigatus in an Israeli patient with Chronic Cavitary Pulmonary Aspergillosis due to a novel E306K Substitution in Hmg1

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.

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

Aspergillus fumigatus is a ubiquitous environmental mold that can cause severe disease in immunocompromised patients and chronic disease in individuals with underlying lung conditions. Triazoles are the most widely used class of antifungal drugs to treat A. fumigatus infections, but their use in the clinic is threatened by the emergence of triazole-resistant isolates worldwide, reinforcing the need for a better understanding of resistance mechanisms. The predominant mechanisms of A. fumigatus triazole resistance involve mutations affecting the promoter region or coding sequence of the target enzyme of the triazoles, Cyp51A. However, triazole-resistant isolates without cyp51A-associated mutations are frequently identified. In this study, we investigate a pan-triazole-resistant clinical isolate, DI15-105, that simultaneously carries the mutations hapEP88L and hmg1F262del, with no mutations in cyp51A. Using a Cas9-mediated gene-editing system, hapEP88L and hmg1F262del mutations were reverted in DI15-105. Here, we show that the combination of these mutations accounts for pan-triazole resistance in DI15-105. To our knowledge, DI15-105 is the first clinical isolate reported to simultaneously carry mutations in hapE and hmg1 and only the second with the hapEP88L mutation. IMPORTANCE Triazole resistance is an important cause of treatment failure and high mortality rates for A. fumigatus human infections. Although Cyp51A-associated mutations are frequently identified as the cause of A. fumigatus triazole resistance, they do not explain the resistance phenotypes for several isolates. In this study, we demonstrate that hapE and hmg1 mutations additively contribute to pan-triazole resistance in an A. fumigatus clinical isolate lacking cyp51-associated mutations. Our results exemplify the importance of and the need for a better understanding of cyp51A-independent triazole resistance mechanisms.

Addressing Microbial Resistance Worldwide: Challenges over Controlling Life-Threatening Fungal Infections

Fungal infections are a serious global concern because of their ability to spread and colonize host tissues in immunocompromised individuals. Such infections have been frequently reported worldwide and are currently gaining clinical research relevance owing to their resistant character, representing a bottleneck in treating affected people. Resistant fungi are an emergent public health threat. The upsurge of such pathogens has led to new research toward unraveling the destructive potential evoked by these species. Some fungi-grouped into Candida, Aspergillus, and Cryptococcus-are causative agents of severe and systemic infections. They are associated with high mortality rates and have recently been described as sources of coinfection in COVID-hospitalized patients. Despite the efforts to elucidate the challenges of colonization, dissemination, and infection severity, the immunopathogenesis of fungal diseases remains a pivotal characteristic in fungal burden elimination. The struggle between the host immune system and the physiological strategies of the fungi to maintain cellular viability is complex. In this brief review, we highlight the relevance of drug resistance phenotypes in fungi of clinical significance, taking into consideration their physiopathology and how the scientific community could orchestrate their efforts to avoid fungal infection dissemination and deaths.

OUP accepted manuscript

The increasing incidence and changing epidemiology of invasive fungal infections continue to present many challenges to their effective management. The repertoire of antifungal drugs available for treatment is still limited although there are new antifungals on the horizon. Successful treatment of invasive mycoses is dependent on a mix of pathogen-, host- and antifungal drug-related factors. Laboratories need to be adept at detection of fungal pathogens in clinical samples in order to effectively guide treatment by identifying isolates with acquired drug resistance. While there are international guidelines on how to conduct in vitro antifungal susceptibility testing, these are not performed as widely as for bacterial pathogens. Furthermore, fungi generally are recovered in cultures more slowly than bacteria, and often cannot be cultured in the laboratory. Therefore, non-culture-based methods, including molecular tests, to detect fungi in clinical specimens are increasingly important in patient management and are becoming more reliable as technology improves. Molecular methods can also be used for detection of target gene mutations or other mechanisms that predict antifungal drug resistance. This review addresses acquired antifungal drug resistance in the principal human fungal pathogens and describes known resistance mechanisms and what in-house and commercial tools are available for their detection. It is emphasized that this approach should be complementary to culture-based susceptibility testing, given the range of mutations, resistance mechanisms and target genes that may be present in clinical isolates, but may not be included in current molecular assays.

Are Point Mutations in HMG-CoA Reductases (Hmg1 and Hmg2) a Step towards Azole Resistance in Aspergillus fumigatus?

Invasive aspergillosis, mainly caused by Aspergillus fumigatus, can lead to severe clinical outcomes in immunocompromised individuals. Antifungal treatment, based on the use of azoles, is crucial to increase survival rates. However, the recent emergence of azole-resistant A. fumigatus isolates is affecting the efficacy of the clinical therapy and lowering the success rate of azole strategies against aspergillosis. Azole resistance mechanisms described to date are mainly associated with mutations in the azole target gene cyp51A that entail structural changes in Cyp51A or overexpression of the gene. However, strains lacking cyp51A modifications but resistant to clinical azoles have recently been detected. Some genes have been proposed as new players in azole resistance. In this study, the gene hmg1, recently related to azole resistance, and its paralogue hmg2 were studied in a collection of fifteen azole-resistant strains without cyp51A modifications. Both genes encode HMG-CoA reductases and are involved in the ergosterol biosynthesis. Several mutations located in the sterol sensing domain (SSD) of Hmg1 (D242Y, G307D/S, P309L, K319Q, Y368H, F390L and I412T) and Hmg2 (I235S, V303A, I312S, I360F and V397C) were detected. The role of these mutations in conferring azole resistance is discussed in this work.