Variants of katG, inhA and nat genes are not associated with mutations in efflux pump genes (mmpL3 and mmpL7) in isoniazid-resistant clinical isolates of Mycobacterium tuberculosis from India

To understand the impact of efflux pump genes such as mmpL3 and mmpL7 on isoniazid (INH) resistance and to correlate with presence or absence of mutations in essential genes of INH resistance (katG, inhA, and nat) in clinical isolates of Mycobacterium tuberculosis (M. tuberculosis). One hundred (75 resistant and 25 sensitive) clinical isolates of M. tuberculosis from India were selected for the study. The presence of mutations in specific regions of katG, inhA, and nat, efflux pump genes (mmpL3 and mmpL7) associated with INH resistance were analyzed using multiplex allele-specific polymerase chain reaction (MAS-PCR) and DNA sequencing methods, respectively. Substitution mutation AGC-ACC at codon 315 of the katG gene was detected in 65% of resistant isolates. Mutation (C-T at nucleotide position 15) in the inhA promoter region was seen in 22% of resistant isolates. Silent mutation (GGA to GGG) at codon 207 in the nat gene was found in three resistant isolates. No mutations were found in either of the efflux genes (mmpL3 and mmpL7) in any of the isolates. Of the 75 resistant isolates analyzed, 74% had mutation in katG and inhA genes. Thus, this report suggests that the role of mmpL3, mmpL7 and nat genes in INH resistance should not be overestimated in comparison to the primary contribution by katG and inhA in clinical isolates of M. tuberculosis. Further, this concise report is the first of its kind to our knowledge, to show the influence of efflux genes on INH resistance in relation to katG and inhA in clinical isolates of M. tuberculosis.

Molecular analysis of isoniazid-resistant clinical isolates of Mycobacterium tuberculosis from India

The presence of mutations in specific regions of katG, inhA, oxyR-ahpC and kasA associated with isoniazid (INH)-resistant clinical isolates of Mycobacterium tuberculosis from India were analysed by DNA sequencing. Point mutations in the katG gene at codon 315 and a mutation at codon 138 were detected in 64.3% (45/70) and 4% (1/25) of isolates, respectively. Polymorphisms at codon 463 of the katG gene were found both in resistant and sensitive isolates. Mutation at the inhA and oxyR-ahpC promoter regions occurred in 11.4% (8/70) and 35.0% (14/40) of the isolates, respectively. No mutation was found to occur in kasA and inhA structural gene regions. Of the 70 resistant isolates studied, 55 (78.6%) showed mutation in the regions sequenced. This is the first comprehensive molecular analysis of INH resistance in India, which suggests that point mutation rather than deletion and insertion is the major cause of INH resistance.