Cyp51A Dysfunction Leads to Higher Susceptibility to Azoles Including Fluconazole in Aspergillus fumigatus.

Background: Azoles target Cyp51A and Cyp51B in Aspergillus fumigatus. Mutations in cyp51A are known as the primary mechanisms of azole resistance. However, not all of them cause azole resistance. Among them, mutations related to improved susceptibility have not been reported so far. We found that two isolates that carry frameshift or nonsense mutations in cyp51A are more susceptible to azoles, even to fluconazole (FLCZ) (IC₅₀: frameshift, 32 μg/mL; nonsense, 32 μg/mL) compared to other azole-susceptible strains (IC₅₀: > 256 μg/mL).

Objectives: We investigated the contribution of these two mutations to azole sensitivity and their effect on Cyp51A functions.

Methods: We transformed an experimental strain, AfS35, by replacing cyp51A^WT with each of the mutated cyp51A and measured its MICs to azoles. We also evaluated the functions of mutated Cyp51A after suppression of Cyp51B, based on the notion that Cyp51A and Cyp51B complement each other.

Results: Induction of mutated cyp51A in AfS35 led to higher susceptibility to FLCZ (IC₅₀: frameshift, 32-64 μg/mL; nonsense, 32 μg/mL). Transformants carrying either of the mutated cyp51A could not survive when cyp51B was suppressed, indicating that these cyp51A mutations result in Cyp51A dysfunction. Furthermore, a cyp51A-deleted mutant strain also showed increased susceptibility to FLCZ (IC₅₀: 32 μg/mL), similar to cyp51A dysfunctional strains, while a cyp51B-deleted mutant strain showed unchanged susceptibility (IC₅₀: > 256 μg/mL) from AfS35.

Conclusions: It was suggested that FLCZ can inhibit Cyp51B rather than Cyp51A and that this unequal inhibition leads to higher azole susceptibility of the two isolates harbouring Cyp51A dysfunction.