Lack of clinical significance for the common arginine-to-leucine substitution at codon 463 of the katG gene in isoniazid-resistant Mycobacterium tuberculosis in Singapore

Novel mutations within the embB gene in ethambutol-susceptible clinical isolates of Mycobacterium tuberculosis

Genetic analysis of the embB gene revealed mutations in 17 (68%) of 25 ethambutol (EMB) resistant isolates (M306I, M306V, M306L, Q497R) but also in 4 (20%) of 20 EMB-susceptible isolates of Mycobacterium tuberculosis, namely, an ATG-->ATM substitution resulting in M306I, G406N, and the novel alterations M423I and A659T.

Novel mutations in ndh in isoniazid-resistant Mycobacterium tuberculosis isolates

Novel mutations in NADH dehydrogenase (ndh) were detected in 8 of 84 (9.5%) isoniazid (INH)-resistant isolates (T110A [n = 1], R268H [n = 7]), but not in 22 INH-susceptible isolates of Mycobacterium tuberculosis. Significantly, all eight isolates with mutations at ndh did not have mutations at katG, kasA, or the promoter regions of inhA or ahpC, except for one isolate. Mutations in ndh appear to be an additional molecular mechanism for isoniazid resistance in M. tuberculosis.

The susceptibility of Mycobacterium tuberculosis to isoniazid and the Arg-->Leu mutation at codon 463 of katG are not associated

A mutation (CCG-->CTG [Arg-->Leu]) in codon 463 of katG (catalase peroxidase) of Mycobacterium tuberculosis has been found in isoniazid (INH)-resistant strains. A PCR restriction endonuclease analysis to detect this mutation was applied to 395 M. tuberculosis isolates from patients in The Netherlands. The proportion of isolates with a detectable mutation was 32% (32 out of 100) and 29% (85 out of 295) among INH-susceptible isolates and INH-resistant or -intermediate isolates, respectively. Sequencing of five INH-susceptible isolates with such mutations showed that all five had the Arg463Leu mutation. We conclude that the Arg463Leu mutation of katG of M. tuberculosis is not a reliable indicator of INH resistance.

The genetics and biochemistry of isoniazid resistance in mycobacterium tuberculosis

Although the primary targets of activated isoniazid (INH) are proteins involved in the biosynthesis of cell wall mycolic acids, clinical resistance is dominated by specific point mutations in katG. Mutations associated with target mutations contribute to, but still cannot completely explain, resistance to INH. Despite the wealth of genetic information currently available, the molecular mechanism of cell death induced by INH remains elusive.

Genotypic characterization of drug-resistant Mycobacterium tuberculosis isolates from Peru

SETTING

Twenty-nine epidemiological unrelated and mostly multidrug-resistant Mycobacterium tuberculosis (MDR-TB) strains from Peruvian patients.

OBJECTIVE

To investigate the molecular genetics of MDR-TB strains recovered in a Latin American country.

DESIGN

Antimicrobial agent susceptibility testing, major genetic group designation, IS6110 fingerprinting, spoligotyping, and automated deoxyribonucleic acid sequencing of regions of the katG, rpoB, embB, gyrA, and pncA genes with mutations commonly associated with drug resistance.

RESULTS

Nineteen isolates were found to be multidrug-resistant by susceptibility testing. IS6110 typing showed that virtually all isolates were unique and therefore had independently acquired drug resistance. Seventy-nine percent of isoniazid-resistant strains had a Ser315Thr amino acid change in KatG. Ninety-five percent of rifampin-resistant isolates had amino acid replacements in the rifampin-resistance determining region of RpoB. Six of 11 ethambutol-resistant strains had EmbB alterations. Eleven pyrazinamide-resistant strains had distinct mutations in pncA.

CONCLUSION

Virtually all organisms evolved drug resistance independently. The types of drug resistance-associated mutations identified were very similar to changes occurring in isolates from other areas of the world. Nucleotide sequence-based strategies for rapid detection of drug resistance-conferring mutants will be applicable to organisms recovered in Peru, and potentially other areas of Latin America.

Contribution of kasA analysis to detection of isoniazid-resistant Mycobacterium tuberculosis in Singapore

Genotypic analysis of resistance to isoniazid (INH) in Mycobacterium tuberculosis is complex due to the various genes potentially involved. Mutations in ketoacyl acyl carrier protein synthase (encoded by kasA) were present in 16 of 160 (10%) INH-resistant isolates (R121K [n = 1], G269S [n = 3], G312S [n = 11], G387D [n = 1]). However, G312S was also present in 6 of 32 (19%) susceptible strains. kasA analysis contributed marginally to the performance of INH genotypic testing in Singapore. The significance of kasA polymorphisms in INH resistance should be carefully established.

A Ser315Thr substitution in KatG is predominant in genetically heterogeneous multidrug-resistant Mycobacterium tuberculosis isolates originating from the St. Petersburg area in Russia

Parts of katG and rpoB from 27 Russian Mycobacterium tuberculosis isolates were sequenced to detect mutations causing resistance to isoniazid (INH) and rifampin (RMP), respectively. All 24 INH-resistant isolates had a mutated katG, and 22 of them (91.7%) carried a mutation coding for a Ser315Thr shift. An rpoB mutation was noted for each of the 21 RMP-resistant isolates, with Ser531Leu being the most prevalent change encoded. Only two isolates had identical IS6110 fingerprints.

Mechanisms of isoniazid resistance in Mycobacterium tuberculosis

Isoniazid (INH) is a widely used front-line antituberculous agent with bacteriocidal activity at concentrations as low as 150 nM against Mycobacterium tuberculosis. INH is a prodrug and requires activation by an endogenous mycobacterial enzyme, the catalase-peroxidase KatG, before exerting toxic effects on cellular targets. Resistance to INH develops primarily through failure to activate the prodrug due to point mutations in the katG gene. In addition to mutations in katG, mutations in several other loci, such as the alkylhydroperoxidase AhpC and the enoylreductase InhA, may contribute to INH resistance. Although these markers can be used to accurately predict clinical INH resistance in a large number of cases, the molecular mechanisms involved remain largely speculative and incomplete.