Analysis of the role of genes ethA, ethR, ndh, mshA, inhA and the inhA-promoter in resistance to ethionamide, a second-line antituberculosis drug
Abstract number: P2057
Objectives: Ethionamide (ETH) is a prodrug which is activated by an enzyme (EthA) and targets the InhA protein involved in biosynthesis of the mycobacterial cell wall. Resistance to ETH has been previously reported to be linked to mutations in genes ethA and its transcriptional regulator ethR, as well as in inhA and its promoter inhA-pro. More recently, two additional genes have been suggested to be involved in resistance to ETH: ndh encoding a NADH dehydrogenase and mshA encoding an enzyme involved in the biosynthesis of mycothiols. Our goal was to investigate the presence of mutations in these genes in a set of clinical isolates showing different levels of resistance to ETH.
Methods: 87 M. tuberculosis clinical strains collected in 20032004 at the National Reference Center for Mycobacteria were included, 47 ETH-resistant (ETH-R), 24 ETH-susceptible (ETH-S) and 16 showing an intermediate level of resistance to ETH (ETH-I). The drug susceptibility testing was performed on Lowenstein-Jensen medium with the standard proportions method. The genes ethA, ethR, inhA, inhA-pro, ndh and mshA were sequenced.
Results: Of the 47 ETH-R strains, 23 were mutated in ethA and/or ethR, and 29 in inhA and/or inhA-pro (14 in both). The 9/47 strains without mutation in these genes were shown to have no mutation in ndh and mshA. Regarding the ETH-I strains, 7/16 were mutated in ethA, while wild-type ndh and mshA genes were found in 9/9 and 8/9, respectively, of the ETH-I isolates showing no mutation in ethA/ethR, one strain having a N111S mutation in mshA. Finally, of the 24 ETH-S strains, 23 had no mutation in ethA-ethR and inhA/inhA-pro, one displaying a rare mutation (-47 g®c) in inhA-pro.
Conclusions: These results strongly suggest that ndh and mshA are not implicated in resistance to ETH in clinical strains of Mycobacterium tuberculosis. In 81% of the ETH-R M. tuberculosis clinical isolates analyzed in the present study, resistance to ETH resulted from the combination of mutations (1) in ethA-ethR and inhA-pro and (2) in inhA and inhA-pro. In the ETH-I strains, the intermediate level of resistance to ETH was mainly associated with mutations in ethA (7/16) which were all different from those found in the ETH-R isolates. For 17/63 ETH-R or ETH-I strains, the mechanism of resistance to ETH remained unknown as no mutation was found in ethA, ethR, inhA-pro, inhA, ndh and mshA, highlighting the complexity of the mechanisms of resistance to ETH in M. tuberculosis.
|Session name:||Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases|
|Location:||Vienna, Austria, 10 - 13 April 2010|
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