Cytochrome P450 14-alpha-Sterol Demethylase Mutation Dependent Triazole Cross-Resistance in aspergillus fumigatus.
To investigate triazole cross-resistance in A. fumigatus we selected 18 itraconazole-resistant (ITZR), 23 voriconazole-resistant (VCZR), 11 posaconazole-resistant (PCZR) and 24 ravuconazole-resistant (RCZR) isolates in the laboratory from the susceptible clinical isolate W73355. VCZR isolates were equally resistant to RCZ and vice versa, but showed only low-level resistance to ITZ and PCZ. ITZR isolates were similarly resistant to PCZ and vice versa, but were highly susceptible to VCZ and RCZ. None showed cross-resistance to amphotericin B, caspofungin and micafungin. The genes encoding P450 14a-sterol demethylases A (CYP51A) and B (CYP51B) were characterized from 6 representatives from each group and the amino acid (aa) sequences of the corresponding proteins (CYP51Ap and CYP51Bp) were deduced. A comparison of the aa sequences of CYP51Ap and CYP51Bp from resistant isolates to those of the parent showed aa changes at G54 or G138 or G448 of CYP51Ap, but none on CYP51Bp. Five of the six VCZR and all RCZR isolates had G448S variation whereas one VCZR isolate had G138R change. All ITZR and PCZR isolates had G54E and G54R variation, respectively. Thus, aa change at position 54 results in ITZ and PCZ resistance with no cross-resistance to VCZ and RCZ whereas aa variation at positions 138 and 448 results in VCZ and RCZ resistance with low level resistance to ITZ and PCZ. G54 and G448 are located in highly conserved regions of CYP51Ap, namely, the membrane anchoring region (MAR), and the heme-binding region(HBR), respectively. Molecular modeling by others suggests that the HBR is part of the active site of CYP51p, and any aa change at the active site make the organism resistant to multiple triazoles to a lesser or greater degree. In contrast, aa variation at the MAR confers resistance to triazoles with long aliphatic tail region. These results thus suggest that cross-resistance to triazoles in A. fumigatus is at least partly dependent on region-specific aa variation of CYP51Ap.
Full conference title:
43rd Interscience Conference on Antimicrobial Agents