Moderate levels of 5-fluorocytosine cause the emergence of high frequency resistance in cryptococci

Abstract

The antifungal agent 5-fluorocytosine (5-FC) is used for the treatment of several mycoses, but is unsuitable for monotherapy due to the rapid development of resistance. Here, we show that cryptococci develop resistance to 5-FC at a high frequency when exposed to concentrations several fold above the minimal inhibitory concentration. The genomes of resistant clones contain alterations in genes relevant as well as irrelevant for 5-FC resistance, suggesting that 5-FC may be mutagenic at moderate concentrations. Mutations in FCY2 (encoding a known permease for 5-FC uptake), FCY1, FUR1, UXS1 (encoding an enzyme that converts UDP-glucuronic acid to UDP-xylose) and URA6 contribute to 5-FC resistance. The uxs1 mutants accumulate UDP-glucuronic acid, which appears to down-regulate expression of permease FCY2 and reduce cellular uptake of the drug. Additional mutations in genes known to be required for UDP-glucuronic acid synthesis (UGD1) or a transcriptional factor NRG1 suppress UDP-glucuronic acid accumulation and 5-FC resistance in the uxs1 mutants.

Raman Study of Pathogenic Candida auris: Imaging Metabolic Machineries in Reaction to Antifungal Drugs

“…Despite its severe acute and chronic toxicities (Miller and Bates, 1969 ; Al-Dhaheri and Douglas, 2010 ; Grela et al, 2018 ), AmB is still considered the gold standard for severe mycoses treatment (Ostrosky-Zeichner et al, 2003 ; Lemke et al, 2005 ). On the other hand, 5-FC is one of the oldest antifungal drugs and has a wide spectrum of activity despite a reported drawback in monotherapy due to a rapid frequency resistance in cryptococci (Chang et al, 2021 ).…”

Tunicamycin Potentiates Antifungal Drug Tolerance via Aneuploidy in Candida albicans

“…In C. albicans , aneuploidy enables adaptation to fluconazole ( 11 – 14 ) and caspofungin ( 15 – 17 ). In Cryptococcus neoformans , aneuploidy enables adaptation to fluconazole ( 18 , 19 ) and flucytosine ( 20 ). In Candida glabrata , formation of a new minichromosome is associated with fluconazole resistance ( 21 ), and in Candida auris , an emerging outbreak pathogen that can acquire resistance to all three major classes of antifungal drugs, duplication of chromosome V is associated with resistance to fluconazole ( 22 ).…”

Adaptation to Fluconazole via Aneuploidy Enables Cross-Adaptation to Amphotericin B and Flucytosine in Cryptococcus neoformans

“…In a recent study of H99, as well as other C. neoformans and C. gattii strains, 5FC selection caused Chr1 disomy that was attributed to AFR1 , a gene on Chr1 that encodes an efflux pump. However, the effect of increased AFR1 expression on 5FC tolerance was strain dependent ( 29 ). Taken together, these studies indicate that Chr1 disomy can be selected by growth on FLC or 5FC and that it confers cross-tolerance to both FLC and 5FC, irrespective of the drug used for the initial selection.…”