Ethambutol resistance in Mycobacterium tuberculosis: critical role of embB mutations

Abstract

Ethambutol [(S,S')-2,2'-(ethylenediimino)di-1-butanol; EMB], is a first-line drug used to treat tuberculosis. To gain insight into the molecular basis of EMB resistance, we characterized the 10-kb embCAB locus in 16 EMB-resistant and 3 EMB-susceptible genetically distinct Mycobacterium tuberculosis strains from diverse localities by automated DNA sequencing and single-stranded conformation polymorphism analysis. All 19 organisms had virtually identical sequences for the entire 10-kb region. Eight EMB-resistant organisms had mutations located in codon 306 of embB that resulted in the replacement of the wild-type Met residue with Ile or Val. Automated sequence analysis of the 5' region (1,892 bp) of embB in an additional 69 EMB-resistant and 30 EMB-susceptible M. tuberculosis isolates from diverse geographic localities and representing 70 distinct IS6110 fingerprints confirmed the unique association of substitutions in amino acid residue 306 of EmbB with EMB resistance. Six other embB nucleotide substitutions resulting in four amino acid replacements were uniquely found in resistant strains. Sixty-nine percent of epidemiologically unassociated EMB-resistant organisms had an amino acid substitution not found in susceptible strains, and most (89%) replacements occurred at amino acid residue 306 of EmbB. For strains with the Met306Leu or Met306Val replacements EMB MICs were generally higher (40 microg/ml) than those for organisms with Met306Ile substitutions (20 microg/ml). The data are consistent with the idea that amino acid substitutions in EmbB alter the drug-protein interaction and thereby cause EMB resistance.

Genetic characterisation of the ethambutol resistance-determining region in Mycobacterium tuberculosis: prevalence and significance of embB306 mutations

“…Five possible nucleotide changes (A916G, A916C, G918A, G918C and G918T) were described at this codon resulting in three possible amino acid mutations (M306V, M306L and M306I) [5,12,13].…”

Role of embCAB gene mutations in ethambutol resistance in Mycobacterium tuberculosis isolates from India

“…It acts against MTB by targeting the membrane-associated arabinosyl transferases, which are encoded by embCAB genes [1,2]. Mutations in these genes have previously been mapped in EMBresistant MTB isolates [2].…”

Rapid detection of ethambutol-resistant genes in Mycobacterium tuberculosis clinical isolates

“…Sreevatsan et al (1997) reported that in the EMB resistant isolates, the TTC(phe)→TTA (leu) mutation of the codon 285, TTC(phe)→GTC(Val) mutation of codon 330, ACC(Thr) → ATC(Ile) mutation of codon 630 of embB, the GTG(Val)→CTG(Leu) mutation of codon 981 of embC , and the GGT(Gly)→AGT(Ser) mutation of the embA could be detected. These researchers also found mutations in codon 406 and codon 497 of the embB gene, a mutation in the embC-embA intervening area, and a mutation of codon iniA501 (Alcaide et al 1997;Sreevatsan et al 1997;Jaber et al 2009). These results indicate that the EMB drug-resistance mechanism of M. tuberculosis is undoubtedly complicated.…”

“…The EMB resistance of Mycobacterium is related to a mutation in the operon of the encoding gene embCAB of the AftA. Sreevatsan et al (1997) reported that 69% of the EMB-resistant isolates of M. tuberculosis have the embB gene mutation, among which 89% are mutations to the amino acid codon 306. The embB gene is approximately 3,246 bp long, and the encoding glycosyltransferase along with embB genetic mutation gives rise to the structural change in glycosyltransferase, which influences the interaction of EMB and glycosyltransferase to produce drug resistance.…”