Caspofungin resistance in Candia auris due to mutations in Fks1 with adjunctive role of chitin and key cell wall stress response pathway genes
The emergence of echinocandin resistance in C. auris has become a major concern. Point mutations in Fks1 subunit of {beta}-D-glucan synthase is the primary mechanism in echinocandin resistance. However, resistant isolates with wild type Fks1 are not infrequent. We screened 199 clinical C. auris isolates from 30 centres across India for echinocandin resistance. The cohort also contained six sequential isolates from a liver transplant recipient. Eleven isolates (5.7%) from 11 patients, and those six serial isolates had elevated echinocandin minimum inhibitory concentrations (MIC). Three of these 17 isolates carried S639F mutation in hot spot 1 region of Fks1. A novel Fks1 mutation, F635Y was identified in two resistant isolates, and a related F635L mutation was detected in four of the six sequential isolates. Resistant isolates (MIC[≥]2 mg/L) and those with intermediate caspofungin susceptibility (MIC, 1.0 mg/L) demonstrated higher induction of chitin synthase gene, Chs1 [resistant, 2.2(1.3-5.8); intermediate, 6(2.5-11.2)] compared to susceptible isolates [1.2(0.8-2), P<0.05]. However, the expression of the Fks1 subunit of {beta}-1, 3-glucan synthase was higher only in intermediate group [3.4(2-8.5), P<0.01]. HOG1 MAP kinase showed higher inducible expression in intermediate isolates, while those of HSP90-like protein and CnaB were comparable in resistant and intermediate groups. In one isolate pan-echinocandin resistance mediated by S639F mutation coupled with high basal chitin content was noted. This study reports novel mutation F635Y/L in Fks1 in C. auris, contributing to echinocandin resistance and suggests the possible adjunctive roles of chitin synthase, Fks1, and cell wall-remodeling pathway gene upregulation in caspofungin resistance.