Characterization of the chromosome 4 genes that affect fluconazole-induced disomy formation in Cryptococcus neoformans

Heteroresistance in Cryptococcus neoformans is an intrinsic adaptive resistance to azoles and the heteroresistant phenotype is associated with disomic chromosomes. Two chromosome 1 (Chr1) genes, ERG11, the fluconazole target, and AFR1, a drug transporter, were reported as major factors in the emergence of Chr1 disomy. In the present study, we show Chr4 to be the second most frequently formed disomy at high concentrations of fluconazole (FLC) and characterize the importance of resident genes contributing to disomy formation. We deleted nine Chr4 genes presumed to have functions in ergosterol biosynthesis, membrane composition/integrity or drug transportation that could influence Chr4 disomy under FLC stress. Of these nine, disruption of three genes homologous to Sey1 (a GTPase), Glo3 and Gcs2 (the ADP-ribosylation factor GTPase activating proteins) significantly reduced the frequency of Chr4 disomy in heteroresistant clones. Furthermore, FLC resistant clones derived from sey1Δglo3Δ did not show disomy of either Chr4 or Chr1 but instead had increased the copy number of the genes proximal to ERG11 locus on Chr1. Since the three genes are critical for the integrity of endoplasmic reticulum (ER) in Saccharomyces cerevisiae, we used Sec61ß-GFP fusion as a marker to study the ER in the mutants. The cytoplasmic ER was found to be elongated in sey1Δ but without any discernable alteration in gcs2Δ and glo3Δ under fluorescence microscopy. The aberrant ER morphology of all three mutant strains, however, was discernable by transmission electron microscopy. A 3D reconstruction using Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) revealed considerably reduced reticulation in the ER of glo3Δ and gcs2Δ strains. In sey1Δ, ER reticulation was barely detectable and cisternae were expanded extensively compared to the wild type strains. These data suggest that the genes required for maintenance of ER integrity are important for the formation of disomic chromosomes in C. neoformans under azole stress.