Phylogenetic and Structural Significance of Dihydrofolate Synthase (folC) Mutations in Drug-Resistant Mycobacterium tuberculosis.

The aim of this study is to investigate the mutation pattern of the folC gene in drug-resistant Mycobacterium tuberculosis (MTB) clinical isolates of global and Hong Kong cohorts. The public sequence read archives of 1,124 MTB genomes from three independent studies were retrieved and folC mutations existing solely in drug-resistant MTB strains were identified. A phylogenetic tree was constructed to analyze the segregation of mutation-related amino acid residues in the FolC structure. These mutation sites were further supported by direct Sanger sequencing of the folC gene among 254 clinical MTB isolates in a Hong Kong cohort. Homology modeling of wild-type and mutated FolC was performed, and the predicted structures were docked with hydroxydihydropteroate, the metabolic derivative of para-aminosalicylic acid (PAS), to evaluate the resultant binding affinity changes. Combining the results of three previous cohorts and our cohort, E40, I43, S150, and E153 are the most frequently affected amino acid residues in resistant isolates. Based on the distribution of mutations in the genome-based phylogenetic tree, lineage-specific mutation patterns were observed. Regarding the segregation of affected amino acid residues, the four most frequently affected residues are all in close proximity of the binding pocket for the PAS derivative. Molecular modeling results showed that mutations at E40, I43, and S150 can alter the structure of FolC putative binding pocket, causing the PAS derivative to bind outside of the now deformed pocket. This might ablate the interaction between the protein and the PAS derivative. To conclude, this study is the first comprehensive mutation pattern and bioinformatics analysis of the folC gene in MTB drug-resistant isolates. The distribution of mutations in phylogenetic lineages and protein structure is reported, analyzed, and discussed.