Loss of Upc2p-Inducible ERG3 Transcription Is Sufficient To Confer Niche-Specific Azole Resistance without Compromising Candida albicans Pathogenicity

Abstract

Inactivation of sterol Δ5,6-desaturase (Erg3p) in the prevalent fungal pathogen Candida albicans is one of several mechanisms that can confer resistance to the azole antifungal drugs. However, loss of Erg3p activity is also associated with deficiencies in stress tolerance, invasive hyphal growth, and attenuated virulence in a mouse model of disseminated infection. This may explain why relatively few erg3-deficient strains have been reported among azole-resistant clinical isolates. In this study, we examined the consequences of Erg3p inactivation upon C. albicans pathogenicity and azole susceptibility in mouse models of mucosal and disseminated infection. While a C. albicans erg3Δ/Δ mutant was unable to cause lethality in the disseminated model, it induced pathology in a mouse model of vaginal infection. The erg3Δ/Δ mutant was also more resistant to fluconazole treatment than the wild type in both models of infection. Thus, complete loss of Erg3p activity confers azole resistance but also niche-specific virulence deficiencies. Serendipitously, we discovered that loss of azole-inducible ERG3 transcription (rather than complete inactivation) is sufficient to confer in vitro fluconazole resistance, without compromising C. albicans stress tolerance, hyphal growth, or pathogenicity in either mouse model. It is also sufficient to confer fluconazole resistance in the mouse vaginal model, but not in the disseminated model of infection, and thus confers niche-specific azole resistance without compromising C. albicans pathogenicity at either site. Collectively, these results establish that modulating Erg3p expression or activity can have niche-specific consequences on both C. albicans pathogenicity and azole resistance.

Loss of C-5 Sterol Desaturase Activity in Candida albicans : Azole Resistance or Merely Trailing Growth?

“…an inability to cause lethality), an erg3Δ/Δ deletion mutant was relatively resistant to fluconazole treatment in a mouse model of disseminated infection, as treatment did not significantly reduce levels of erg3Δ/Δ colonization of kidney tissue. Strikingly, in a mouse model of vaginal candidiasis, the erg3Δ/Δ mutant not only was able to readily colonize and to induce pathology but also was resistant to fluconazole treatment (18). These findings clearly confirm that loss of Erg3p activity does result in azole resistance within the mammalian host, although it also results in niche or context-specific defects in pathogenicity.…”

“…A C. albicans erg3⌬/⌬ mutant exhibits characteristics of both azole resistance and trailing growth. In this study, we used a previously reported C. albicans erg3Δ/Δ mutant and its isogenic control strain, in which a wild-type (WT) copy of ERG3 has been reintroduced (18). These strains were compared with the well-characterized azoleresistant clinical isolate TW17, which overexpresses the Mdr1p and Cdr1p efflux pumps, as well as a variant of the target enzyme Erg11p with reduced affinity for azoles (4).…”

“…Moreover, the azole susceptibility of those strains was not tested in that model of infection (13). Either way, our previous in vivo data support the idea that loss of Erg3p activity is sufficient to confer resistance to the azole antifungals within the mammalian host and, by inference, patients (18).…”

“…It is clear that, while erg3 mutants can be readily selected in vitro following exposure to azoles (10,30,31), relatively few have been reported among azole-resistant clinical isolates. Loss of Erg3p activity itself blocks ergosterol biosynthesis, resulting in membrane abnormalities, hypersensitivity to some physiological stresses, defective hyphal growth, and reduced virulence in a mouse model of disseminated infection (13,14,18,32). The resulting reduction in pathogenic fitness may result in the rapid elimination of erg3 mutants from infected patients, effectively imposing a counter-selective pressure against ERG3 inactivation.…”

Broad antifungal resistance mediated by RNAi-dependent epimutation in the basal human fungal pathogen Mucor circinelloides

“…The two front-line antifungals in clinical use against mucormycosis are the azole isavuconazole and the polyene amphotericin B. Resistance to azoles and polyenes in other pathogenic fungi, such as Candida species, can be mediated by loss of the ergosterol biosynthetic enzymes Erg3and Erg6[47][48][49][50][51][52]. Using bioinformatic analysis we have identified three candidate ERG6 homologs and one candidate ERG3 homolog in Mucor and we hypothesize that epimutation could induce silencing of these genes under appropriate drug selection, leading to acquired drug resistance.…”