Sequence and structural characterization of tbnat gene in isoniazid-resistant Mycobacterium tuberculosis: identification of new mutations

Xenobiotic-metabolizing enzymes in Bacillus anthracis: molecular and functional analysis of a truncated arylamine N-acetyltransferase isozyme

BACKGROUND AND PURPOSE

The arylamine N-acetyltransferases (NATs) are xenobiotic-metabolizing enzymes that play an important role in the detoxification and/or bioactivation of arylamine drugs and xenobiotics. In bacteria, NATs may contribute to the resistance against antibiotics such as isoniazid or sulfamides through their acetylation, which makes this enzyme family a possible drug target. Bacillus anthracis, a bacterial species of clinical significance, expresses three NAT isozymes with distinct structural and enzymatic properties, including an inactive isozyme ((BACAN)NAT3). (BACAN)NAT3 features both a non-canonical Glu residue in its catalytic triad and a truncated C-terminus domain. However, the role these unusual characteristics play in the lack of activity of the (BACAN)NAT3 isozyme remains unclear.

EXPERIMENTAL APPROACH

Protein engineering, recombinant expression, enzymatic analyses with aromatic amine substrates and phylogenetic analysis approaches were conducted.

KEY RESULTS

The deletion of guanine 580 (G580) in the nat3 gene was shown to be responsible for the expression of a truncated (BACAN)NAT3 isozyme. Artificial re-introduction of G580 in the nat3 gene led to a functional enzyme able to acetylate several arylamine drugs displaying structural characteristics comparable with its functional Bacillus cereus homologue ((BACCR)NAT3). Phylogenetic analysis of the nat3 gene in the B. cereus group further indicated that nat3 may constitute a pseudogene of the B. anthracis species.

CONCLUSION AND IMPLICATIONS

The existence of NATs with distinct properties and evolution in Bacillus species may account for their adaptation to their diverse chemical environments. A better understanding of these isozymes is of importance for their possible use as drug targets.

LINKED ARTICLES

This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro-organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc.

Mutation profiling for detection of isoniazid resistance in Mycobacterium tuberculosis clinical isolates

OBJECTIVES

Progress in the detection of drug-resistant TB has been underpinned by the development and implementation of new, reliable and rapid diagnostic tools. These rely mostly on the detection of specific mutations conferring resistance to anti-TB drugs. The aim of this study was to search for mutations associated with isoniazid resistance among Mycobacterium tuberculosis clinical isolates.

METHODS

A collection of 150 M. tuberculosis strains, including 50 MDR, 50 isoniazid-monoresistant and 50 pan-susceptible strains, was used. For all the strains, seven structural genes (katG, inhA, ahpC, kasA, ndh, nat and mshA) and two regulatory regions (mabA-inhA promoter and oxyR-ahpC intergenic region) were PCR amplified and sequenced in their entirety.

RESULTS

Sixty-six distinct mutations were detected at all nine loci investigated, accounting for 109 (72.7%) of the strains tested. The number of strains with any mutation among the MDR, isoniazid-monoresistant and pan-susceptible groups was 49 (98%), 37 (74%) and 23 (46%), respectively. Mutations in the katG gene predominated, with 29 different types distributed among 46 (92%) MDR, 31 (62%) isoniazid-monoresistant and 2 (4%) pan-susceptible strains. Twenty-nine and 19 mutations were found exclusively in MDR and isoniazid-monoresistant strains, respectively.

CONCLUSIONS

This study revealed 17 mutations, previously unreported, that might be of potential use as new surrogate markers of isoniazid resistance. Their diagnostic accuracy needs to be confirmed on larger strain samples and from different geographical settings. For isoniazid resistance detection, molecular approaches should still be a complement to rather than a replacement for conventional drug susceptibility testing. This is supported by the lack of mutations in any of the nine genetic loci investigated in 18 isoniazid-resistant strains from this study.

Detection of mutations associated with isoniazid resistance in multidrug-resistant Mycobacterium tuberculosis clinical isolates

OBJECTIVES

To determine the prevalence of isoniazid resistance-conferring mutations among multidrug-resistant (MDR) isolates of Mycobacterium tuberculosis from Poland.

METHODS

Nine genetic loci, including structural genes (katG, inhA, ahpC, kasA, ndh, nat and mshA) and regulatory regions (i.e. the mabA-inhA promoter and oxyR-ahpC intergenic region) of 50 MDR M. tuberculosis isolates collected throughout Poland were PCR-amplified in their entirety and screened for mutations by direct sequencing methodology.

RESULTS

Forty-six (92%) MDR M. tuberculosis isolates had mutations in the katG gene, and the katG Ser315Thr substitution predominated (72%). Eight (16%) isolates (six with a mutated katG allele) had mutations in the inhA promoter region and two such isolates also had single inhA structural gene mutations. Mutations in the oxyR-ahpC locus were found in five (10%) isolates, of which all but one had at least one additional mutation in katG. Mutations in the remaining genetic loci (kasA, ndh, nat and mshA) were detected in 12 (24%), 4 (8%), 5 (10%) and 17 (34%) MDR isolates, respectively. All non-synonymous mutants for these genes harboured mutations in katG. One isolate had no mutations in any of the analysed loci.

CONCLUSIONS

This study accentuates the usefulness of katG and inhA promoter mutations as predictive markers of isoniazid resistance. Testing only for katG 315 and inhA -15 mutations would detect isoniazid resistance in 84% of the MDR M. tuberculosis sample. This percentage would increase to 96% if the sequence analysis was extended to the entire katG gene. Analysis of the remaining genetic loci did not contribute greatly to the identification of isoniazid resistance.

Tuberculosis-spoligo-rifampin-isoniazid typing: an all-in-one assay technique for surveillance and control of multidrug-resistant tuberculosis on Luminex devices

As a follow-up of the "spoligoriftyping" development, we present here an extension of this technique which includes the detection of isoniazid resistance-associated mutations in a new 59-plex assay, i.e., tuberculosis-spoligo-rifampin-isoniazid typing (TB-SPRINT), running on microbead-based multiplexed systems. This assay improves the synergy between clinical microbiology and epidemiology by providing (i) mutation-based prediction of drug resistance profiles for patient treatment and (ii) genotyping data for tuberculosis (TB) surveillance. This third-generation microbead-based high-throughput assay for TB runs on the Luminex 200 system and on the recently launched MagPix system (Luminex, Austin, TX). Spoligotyping patterns obtained by the TB-SPRINT method were 100% (n = 85 isolates; 3,655/3,655 spoligotype data points) concordant with those obtained by microbead-based and membrane-based spoligotyping. Genetic drug susceptibility typing provided by the TB-SPRINT method was 100% concordant with resistance locus sequencing (n = 162 for rpoB gene sequencing and n = 76 for katG and inhA sequencing). Considering phenotypic drug susceptibility testing (DST) as the reference method, the sensitivity and specificity of TB-SPRINT regarding Mycobacterium tuberculosis complex (n = 162 isolates) rifampin resistance were both 100%, and those for isoniazid resistance were 90.4% (95% confidence interval, 85 to 95%) and 100%, respectively. Used routinely in national TB reference and specialized laboratories, the TB-SPRINT assay should simultaneously improve personalized medicine and epidemiological surveillance of multidrug-resistant (MDR) TB. This assay is expected to play an emerging role in public health in countries with heavy burdens of MDR TB and/or HIV/TB coinfection. Application of this assay directly to biological samples, as well as development for extensively drug-resistant (XDR) TB detection by inclusion of second-line antituberculosis drug-associated mutations, is under development. With bioinformatical methods and data mining to reduce the number of targets to the most informative ones, locally adapted formats of this technique can easily be developed everywhere.