Candida parapsilosis has emerged as a common cause of invasive fungal infection, especially in Latin America and in the neonatal setting. C. parapsilosis is part of a closely related group of organisms that includes the species C. orthopsilosis and C. metapsilosis. All three species show elevated MIC values to the new echinocandin class drugs, caspofungin, micafungin and anidulafungin, relative to other Candida species. Despite potential impacts on therapy, the mechanism behind this reduced echinocandin susceptibility has not been determined. In this report, we investigated the role of a naturally occurring Pro to Ala substitution at amino acid position 660 (P660A) immediately distal to the highly conserved hot spot 1 region of Fks1p in the reduced echinocandin susceptibility phenotype. Kinetic inhibition studies demonstrated that glucan synthase from the C. parapsilosis group was 1- to 2-logs less sensitive to echinocandin drugs than reference enzyme from C. albicans. Furthermore, clinical isolates of C. albicans and C. glabrata, which harbor mutations at this equivalent position, also showed a comparable 2-log decrease in target enzyme sensitivity, which correlated with increased MIC values. These mutations also resulted in 2.4- to 18.8-fold reduced Vmaxrelative to wild-type enzyme, consistent with kinetic parameters obtained for C. parapsilosis group enzymes. Finally, the importance of the P660A substitution for intrinsic resistance was confirmed by engineering an equivalent P647A mutation into Fks1p of Saccharomyces cerevisiae. The mutant glucan synthase displayed a characteristic 2-log decrease in sensitivity to the echinocandin drugs. Overall, these data firmly indicate that a naturally occurring P660A substitution in Fks1p from C. parapsilosis group accounts for the reduced susceptibility phenotype.
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