Mutations in <i>TAC1B</i> : a Novel Genetic Determinant of Clinical Fluconazole Resistance in Candida auris

<i>Candida auris</i> has emerged as a multidrug-resistant pathogen of great clinical concern. Approximately 90% of clinical <i>C. auris</i> isolates are resistant to fluconazole, the most commonly prescribed antifungal agent, and yet it remains unknown what mechanisms underpin this fluconazole resistance. To identify novel mechanisms contributing to fluconazole resistance in <i>C. auris</i>, fluconazole-susceptible <i>C. auris</i> clinical isolate AR0387 was passaged in media supplemented with fluconazole to generate derivative strains which had acquired increased fluconazole resistance <i>in vitro</i> Comparative analyses of comprehensive sterol profiles, [³H]fluconazole uptake, sequencing of <i>C. auris</i> genes homologous to genes known to contribute to fluconazole resistance in other species of <i>Candida</i>, and relative expression levels of <i>C. auris</i><i>ERG11</i>, <i>CDR1</i>, and <i>MDR1</i> were performed. All fluconazole-evolved derivative strains were found to have acquired mutations in the zinc-cluster transcription factor-encoding gene <i>TAC1B</i> and to show a corresponding increase in <i>CDR1</i> expression relative to the parental clinical isolate, AR0387. Mutations in <i>TAC1B</i> were also identified in a set of 304 globally distributed <i>C. auris</i> clinical isolates representing each of the four major clades. Introduction of the most common mutation found among fluconazole-resistant clinical isolates of <i>C. auris</i> into fluconazole-susceptible isolate AR0387 was confirmed to increase fluconazole resistance by 8-fold, and the correction of the same mutation in a fluconazole-resistant isolate, AR0390, decreased fluconazole MIC by 16-fold. Taken together, these data demonstrate that <i>C. auris</i> can rapidly acquire resistance to fluconazole <i>in vitro</i> and that mutations in <i>TAC1B</i> significantly contribute to clinical fluconazole resistance.<b>IMPORTANCE</b><i>Candida auris</i> is an emerging multidrug-resistant pathogen of global concern, known to be responsible for outbreaks on six continents and to be commonly resistant to antifungals. While the vast majority of clinical <i>C. auris</i> isolates are highly resistant to fluconazole, an essential part of the available antifungal arsenal, very little is known about the mechanisms contributing to resistance. In this work, we show that mutations in the transcription factor <i>TAC1B</i> significantly contribute to clinical fluconazole resistance. These studies demonstrated that mutations in <i>TAC1B</i> can arise rapidly <i>in vitro</i> upon exposure to fluconazole and that a multitude of resistance-associated <i>TAC1B</i> mutations are present among the majority of fluconazole-resistant <i>C. auris</i> isolates from a global collection and appear specific to a subset of lineages or clades. Thus, identification of this novel genetic determinant of resistance significantly adds to the understanding of clinical antifungal resistance in <i>C. auris</i>.