Characterization of the katG and inhA genes of isoniazid-resistant clinical isolates of Mycobacterium tuberculosis.

Resistance to isoniazid in Mycobacterium tuberculosis has been associated with mutations in genes encoding the mycobacterial catalase-peroxidase (katG) and the InhA protein (inhA). Among the 26 isoniazid-resistant clinical isolates evaluated in this study, mutations in putative inhA regulatory sequences were identified in 2 catalase-positive isolates, katG gene alterations were detected in 20 strains, and 4 isolates had wild-type katG and inhA genes. Mutations in the katG gene were detected in all 11 catalase-negative isolates: one frameshift insertion, two partial gene deletions, and nine different missense mutations were identified. An arginine-to-leucine substitution at position 463 was detected in nine catalase-positive isolates. However, site-directed mutagenesis experiments demonstrated that the presence of a leucine at codon 463 did not alter the activity of the M. tuberculosis catalase-peroxidase and did not affect the capacity of this enzyme to restore isoniazid susceptibility to isoniazid-resistant, KatG-defective Mycobacterium smegmatis BH1 cells. These studies further support the association between katG and inhA gene mutations and isoniazid resistance in M. tuberculosis, while also suggesting that other undefined mechanisms of isoniazid resistance exist.