Azole Resistance by Loss of Function of the Sterol Δ5,6-Desaturase Gene (ERG3) in Candida albicans Does Not Necessarily Decrease Virulence

Abstract

ABSTRACTThe inactivation ofERG3, a gene encoding sterol Δ5,6-desaturase (essential for ergosterol biosynthesis), is a known mechanism ofin vitroresistance to azole antifungal drugs in the human pathogenCandida albicans. ERG3inactivation typically results in loss of filamentation and attenuated virulence in animal models of disseminated candidiasis. In this work, we identified aC. albicansclinical isolate (VSY2) with high-level resistance to azole drugsin vitroand an absence of ergosterol but normal filamentation. Sequencing ofERG3in VSY2 revealed a double base deletion leading to a premature stop codon and thus a nonfunctional enzyme. The reversion of the double base deletion in the mutant allele (erg3-1) restored ergosterol biosynthesis and full fluconazole susceptibility in VSY2, confirming thatERG3inactivation was the mechanism of azole resistance. Additionally, the replacement of bothERG3alleles byerg3-1in the wild-type strain SC5314 led to the absence of ergosterol and to fluconazole resistance without affecting filamentation. In a mouse model of disseminated candidiasis, the clinicalERG3mutant VSY2 produced kidney fungal burdens and mouse survival comparable to those obtained with the wild-type control. Interestingly, while VSY2 was resistant to fluconazole bothin vitroandin vivo, theERG3-derived mutant of SC5314 was resistant onlyin vitroand was less virulent than the wild type. This suggests that VSY2 compensated for thein vivofitness defect ofERG3inactivation by a still unknown mechanism(s). Taken together, our results provide evidence that contrary to previous reports inactivation ofERG3does not necessarily affect filamentation and virulence.