Epidemiological and Genomic Landscape of Azole Resistance Mechanisms in Aspergillus Fungi

Abstract

Invasive aspergillosis is a life-threatening mycosis caused by the pathogenic fungus Aspergillus. The predominant causal species is Aspergillus fumigatus, and azole drugs are the treatment of choice. Azole drugs approved for clinical use include itraconazole, voriconazole, posaconazole, and the recently added isavuconazole. However, epidemiological research has indicated that the prevalence of azole-resistant A. fumigatus isolates has increased significantly over the last decade. What is worse is that azole-resistant strains are likely to have emerged not only in response to long-term drug treatment but also because of exposure to azole fungicides in the environment. Resistance mechanisms include amino acid substitutions in the target Cyp51A protein, tandem repeat sequence insertions at the cyp51A promoter, and overexpression of the ABC transporter Cdr1B. Environmental azole-resistant strains harboring the association of a tandem repeat sequence and punctual mutation of the Cyp51A gene (TR34/L98H and TR46/Y121F/T289A) have become widely disseminated across the world within a short time period. The epidemiological data also suggests that the number of Aspergillus spp. other than A. fumigatus isolated has risen. Some non-fumigatus species intrinsically show low susceptibility to azole drugs, imposing the need for accurate identification, and drug susceptibility testing in most clinical cases. Currently, our knowledge of azole resistance mechanisms in non-fumigatus Aspergillus species such as A. flavus, A. niger, A. tubingensis, A. terreus, A. fischeri, A. lentulus, A. udagawae, and A. calidoustus is limited. In this review, we present recent advances in our understanding of azole resistance mechanisms particularly in A. fumigatus. We then provide an overview of the genome sequences of non-fumigatus species, focusing on the proteins related to azole resistance mechanisms.

Keywords: A. flavus; A. niger; Aspergillus fumigatus; Aspergillus section Fumigati; Cdr1B; Cyp51A; azole resistance; tandem repeats.

Figure 1

Basic drug resistance mechanisms and the corresponding azole resistance mechanisms in A. fumigatus. Drug resistance mechanisms are categorized into five basic types. The underlying molecular mechanisms identified in A. fumigatus are shown in red text.

Figure 2

Known azole resistance mechanisms in A. fumigatus. (A) Amino acid substitutions responsible for azole resistance. G54, P216, F219, M220, and G448 show the position at which amino acid changes resulted in azole resistance. The MICs of the strains harboring the change (to the indicated amino acid) are shown in parentheses (Mann et al., ; Mellado et al., ; Camps S. M. et al., ; Krishnan-Natesan et al., 2012). (B) The efflux transporters related to azole resistance. The MIC of the strain with overexpression of the cdr1B gene is shown in parentheses (Fraczek et al., 2013). (C) The tandem repeat sequences in a cyp51A promoter. The none-overlapping bases are shown in italics. The MICs of the strains harboring TR34 or TR46 and/or amino acid change(s) are shown in parentheses (Snelders et al., 2011, 2015). ITCZ, itraconazole; PSCZ, posaconazole; VRCZ, voriconazole.

Figure 3

The molecular genetic phylogenetic trees of the Cyp51A and Cdr1B proteins of pathogenic Aspergillus fungi. The sequences were retrieved from the AspGD and NCBI databases according to sequence similarity. The protein sequences of Cyp51A (A) and Cdr1B (B) were aligned using the ClustalW software, and the phylogenetic trees were constructed by the UPGMA method. The trees were drawn using FigTree v1.4.2 software. The IDs shown behind a species name are associated with the database from which the sequences were retrieved. (C) Amino acid sequence alignment of Cyp51A. The sequence surrounding the azole resistance-related amino acids (G54, P216, M220, and G448) is depicted. The numbers indicate amino acid position in the A. fumigatus Cyp51A protein.