A Naturally Occurring Proline-to-Alanine Amino Acid Change in Fks1p in <i>Candida parapsilosis</i> , <i>Candida orthopsilosis</i> , and <i>Candida metapsilosis</i> Accounts for Reduced Echinocandin Susceptibility

<jats:title>ABSTRACT</jats:title> <jats:p> <jats:italic>Candida parapsilosis</jats:italic> has emerged as a common cause of invasive fungal infection, especially in Latin America and in the neonatal setting. <jats:italic>C. parapsilosis</jats:italic> is part of a closely related group of organisms that includes the species <jats:italic>Candida orthopsilosis</jats:italic> and <jats:italic>Candida metapsilosis</jats:italic> . All three species show elevated MICs for the new echinocandin class drugs caspofungin, micafungin, and anidulafungin relative to other <jats:italic>Candida</jats:italic> 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 <jats:italic>C. parapsilosis</jats:italic> group was 1 to 2 logs less sensitive to echinocandin drugs than the reference enzyme from <jats:italic>C. albicans</jats:italic> . Furthermore, clinical isolates of <jats:italic>C. albicans</jats:italic> and <jats:italic>C. glabrata</jats:italic> which harbor mutations at this equivalent position also showed comparable 2-log decreases in target enzyme sensitivity, which correlated with increased MICs. These mutations also resulted in 2.4- to 18.8-fold-reduced <jats:italic>V</jats:italic> <jats:sub>max</jats:sub> values relative to those for the wild-type enzyme, consistent with kinetic parameters obtained for <jats:italic>C. parapsilosis</jats:italic> group enzymes. Finally, the importance of the P660A substitution for intrinsic resistance was confirmed by engineering an equivalent P647A mutation into Fks1p of <jats:italic>Saccharomyces cerevisiae</jats:italic> . The mutant glucan synthase displayed characteristic 2-log decreases in sensitivity to the echinocandin drugs. Overall, these data firmly indicate that a naturally occurring P660A substitution in Fks1p from the <jats:italic>C. parapsilosis</jats:italic> group accounts for the reduced susceptibility phenotype. </jats:p>