Gain-of-function mutations in UPC2 are a frequent cause of ERG11 upregulation in azole-resistant clinical isolates of Candida albicans.

In Candida albicans, Upc2 is a zinc-cluster transcription factor that targets genes, including those of the ergosterol biosynthesis pathway. To date, three documented UPC2 gain-of-function (GOF) mutations have been recovered from fluconazole-resistant clinical isolates that contribute to an increase in ERG11 expression and decreased fluconazole susceptibility. In a group of 63 isolates with reduced susceptibility to fluconazole, we found that 47 overexpressed ERG11 by at least 2-fold over the average expression levels in 3 unrelated fluconazole-susceptible strains. Of those 47 isolates, 29 contained a mutation in UPC2, whereas the remaining 18 isolates did not. Among the isolates containing mutations in UPC2, we recovered eight distinct mutations resulting in putative single amino acid substitutions: G648D, G648S, A643T, A643V, Y642F, G304R, A646V, and W478C. Seven of these resulted in increased ERG11 expression, increased cellular ergosterol, and decreased susceptibility to fluconazole compared to the results for the wild-type strain. Genome-wide transcriptional analysis was performed for the four strongest Upc2 amino acid substitutions (A643V, G648D, G648S, and Y642F). Genes commonly upregulated by all four mutations included those involved in ergosterol biosynthesis, in oxidoreductase activity, the major facilitator efflux pump encoded by the MDR1 gene, and the uncharacterized ATP binding cassette transporter CDR11. These findings demonstrate that gain-of-function mutations in UPC2 are more prevalent among clinical isolates than previously thought and make a significant contribution to azole antifungal resistance, but the findings do not account for ERG11 overexpression in all such isolates of C. albicans.