Acetylation of isoniazid - a novel mechanism of isoniazid resistance in Mycobacterium tuberculosis

Isoniazid (INH), one of the first-line drugs used for the treatment of tuberculosis, is a pro-drug which is converted into its active form by the intracellular KatG enzyme of Mycobacterium tuberculosis . The activated drug hinders cell wall biosynthesis by inhibiting InhA protein. INH resistant strains of M. tuberculosis usually have mutations in katG , inhA , ahpC , kasA , and ndh genes. However, INH resistant strains which do not have mutations in any of these genes are reported, suggesting that these strains may adopt some other mechanism to become resistant to INH. In the present study we characterized Rv2170, a putative acetyltransferase in M. tuberculosis , to elucidate its role in inactivating isoniazid. The purified recombinant protein was able to catalyze transfer of acetyl group to INH from acetyl CoA. HPLC and LC-MS analyses showed that following acetylation by Rv2170, INH is broken down into isonicotinic acid and acetylhydrazine. Drug susceptibility assay and confocal analysis showed that M. smegmatis , which is susceptible to INH, is not inhibited by INH acetylated with Rv2170. Recombinant M. smegmatis and M. tuberculosis H37Ra overexpressing Rv2170 were found to be resistant to INH at minimum inhibitory concentrations that inhibited wildtype strains. In addition, intracellular M. tuberculosis H37Ra overexpressing Rv2170 survived better in macrophages when treated with INH. Our results strongly indicate that Rv2170 acetylates INH, and this could be one of the strategies adopted by at least some M. tuberculosis strains to overcome INH toxicity.