Molecular Mechanisms of Fluconazole Resistance in Candida parapsilosis Isolates from a U.S. Surveillance System

Abstract

Candida parapsilosis is the second or third most common cause of candidemia in many countries. The Infectious Diseases Society of America recommends fluconazole as the primary therapy for C. parapsilosis candidemia. Although the rate of fluconazole resistance among C. parapsilosis isolates is low in most U.S. institutions, the resistance rate can be as high as 7.5%. This study was designed to assess the mechanisms of fluconazole resistance in 706 incident bloodstream isolates from U.S. hospitals. We sequenced the ERG11 and MRR1 genes of 122 C. parapsilosis isolates with resistant (30 isolates; 4.2%), susceptible dose-dependent (37 isolates; 5.2%), and susceptible (55 isolates) fluconazole MIC values and used real-time PCR of RNA from 17 isolates to investigate the regulation of MDR1. By comparing these isolates to fully fluconazole-susceptible isolates, we detected at least two mechanisms of fluconazole resistance: an amino acid substitution in the 14-␣-demethylase gene ERG11 and overexpression of the efflux pump MDR1, possibly due to point mutations in the MRR1 transcription factor that regulates MDR1. The ERG11 single nucleotide polymorphism (SNP) was found in 57% of the fluconazole-resistant isolates and in no susceptible isolates. The MRR1 SNPs were more difficult to characterize, as not all resulted in overexpression of MDR1 and not all MDR1 overexpression was associated with an SNP in MRR1. Further work to characterize the MRR1 SNPs and search for overexpression of other efflux pumps is needed.

Fluconazole and Voriconazole Resistance in Candida parapsilosis Is Conferred by Gain-of-Function Mutations in MRR1 Transcription Factor Gene

“…Subsequent analysis of MRR1 from the voriconazole-resistant isolate detected an additional mutation, A3191C, causing a change from glutamine to a proline (Q1064P) in Mrr1p. Other MRR1 mutations were recently described in fluconazole-resistant clinical isolates; however, their impact in fluconazole resistance was not conclusive, since the same polymorphisms were also found in susceptible clinical isolates, with the one exception being the G2337T mutation (6).…”

“…In a large surveillance study of fluconazole-resistant clinical isolates of C. parapsilosis in the United States, the most common underlying molecular mechanisms were associated with polymorphisms in ERG11 and MRR1 (6). According to these authors, polymorphisms in MRR1 are common, and only some are associated with overexpression of MDR1.…”

“…Geographically, C. parapsilosis is the second most common agent of fungal infection in Mediterranean, South American, and Asian countries (1)(2)(3). Studies regarding the drug susceptibility profile of C. parapsilosis isolates have been carried out worldwide, and apart from their reduced susceptibility to echinocandins (4), their level of resistance to azoles has not been alarming so far (5)(6)(7). However, with the extensive use of azoles as therapeutic and prophylactic drugs prescribed in C. parapsilosis infections, it is expected that azole resistance will emerge more frequently.…”

“…Interestingly, fluconazole resistance rates vary from 70%, in isolates found in Atlanta hospitals, to 0%, in isolates coming from Portland, OR. The study of the molecular mechanisms underlying fluconazole resistance revealed the first mutations described in clinical isolates, either in the MRR1 transcription factor or in the azole target, the ERG11 gene (6).…”

“…However, with the extensive use of azoles as therapeutic and prophylactic drugs prescribed in C. parapsilosis infections, it is expected that azole resistance will emerge more frequently. This concern was recently the focus in a study of C. parapsilosis fluconazole surveillance carried out in clinical isolates from several hospitals in the United States (6). Fluconazole susceptibility was assessed in 706 C. parapsilosis isolates, collected in 80 hospitals.…”