The Evolution of Azole Resistance in Candida albicans Sterol 14α-Demethylase (CYP51) through Incremental Amino Acid Substitutions

In Candida albicans and Aspergillus fumigatus, the Y132F substitution in CaERG11 or Y121F substitution in Afcyp51A confers increased resistance to all triazoles except posaconazole.

“In both Candida albicans and Aspergillus fumigatus, the mutation encoding a tyrosine to phenylalanine amino acid substitution at a specific residue localized to the sterol-demethylase catalytic domain (Y132F in C. albicans CaERG11 and Y121F in A. fumigatus Afcyp51A) confers increased resistance to all triazoles except for posaconazole (4–7).”

Mutations in sterol-demethylase have variable and agent-specific effects on the MICs of triazole antifungals.

“Frequently reported in association with triazole resistance in both yeast and molds, mutations in sterol-demethylase have been observed to have variable and agent specific effects on the MICs of triazole antifungals, potentially as a result of agent-specific stabilizing interactions with sterol-demethylase residues (4, 5).”

The Y132 position is within the predicted catalytic site of sterol-demethylase and the Y132F substitution is predicted to interfere with fluconazole binding.

“The Y132 position is within the predicted catalytic site of sterol-demethylase and the Y132F substitution is predicted to interfere with fluconazole binding.”

The K143 position resides in a region proximal to the heme presumed to be involved in electron transfer from the P450 reductase and the K143R substitution affects catalytic efficiency.

“The K143 position resides in a region proximal to the heme presumed to be involved in electron transfer from the P450 reductase and the K143R substitution affects catalytic efficiency (4).”

In C. albicans, the majority of fluconazole resistance-associated ERG11 mutations are found within three hot spot regions corresponding to amino acids 105 to 165, 266 to 287, and 405 to 488.

“Among pathogenic fungi, the role of mutations in the gene encoding sterol-demethylase in antifungal resistance has been most comprehensively studied in C. albicans, where a wide repertoire of mutations have been observed among fluconazole-resistant isolates, the majority of which are found within three hot spot regions corresponding to amino acids 105 to 165, 266 to 287, and 405 to 488 (4, 5).”

Single ERG11 point mutations do not provide the same degree of POS cross-resistance in C. albicans as in Aspergillus spp.

“single ERG11 point mutations do not provide the same degree of POS cross-resistance in C. albicans (MacCallum et al. 2010; Sanglard and Coste 2016; Warrilow et al. 2019).”

Erg11 variants differ substantially in their baseline catalytic turnover and affinity for azole binding.

“Erg11 variants differ substantially in their baseline catalytic turnover and affinity for azole binding.”

Some Erg11 substitutions show relatively loose azole binding but significant contribution to azole resistance in in vitro C. albicans susceptibility testing.

“Despite some substitutions showing relatively loose azole binding, their observed contribution to azole resistance in in vitro C. albicans susceptibility testing was significant”