Resistance to itraconazole in Aspergillus nidulans and Aspergillus fumigatus is conferred by extra copies of the A. nidulans P-450 14α-demethylase gene, pdmA

Triazoles selectively inhibit the cytochrome P-450-dependent C-14 lanosterol α-demethylase (P-450 14 α DM), a key enzyme in ergosterol biosynthesis in fungi. To investigate mechanisms of triazole resistance in a mould, we used Aspergillus nidulans, a genetically amenable model fungus closely related to more pathogenic members of the genus. We selected for genes that would give resistance to itraconazole following transformation with a high copy genomic library of A. nidulans. In all the resistant colonies that we isolated, resistance was conferred by extra copies of the A. nidulans P-450 14 α DM gene, pdmA. We determined that in A. nidulans, extra copies of pdmA increase the MIC for itraconazole 36 times over wild-type controls. Similarly, transformation of an Aspergillus fumigatus strain with pITZR1 resulted in increased resistance to itraconazole. Our results indicate that triazole resistance in clinical isolates of moulds may result from amplication or overexpression of the P-450 14 α DM and demonstrate the utility of A. nidulans as a promising model fungus for the analysis of drug resistance and susceptibility in the pathogenic fungus A. fumigatus.