PncA

Abstract: Naturally pyrazinamide (PZA)-resistant Mycobacterium bovis and acquired PZA-resistant M. tuberculosis strains lose pyrazinamidase (PZase). To investigate the molecular mechanism of PZA resistance, we have cloned the gene (pncA) encoding M. tuberculosis PZase. Mutations in pncA were identified in both types of PZA-resistant strains, and transformation of these strains with a functional pncA gene restored PZase activity and PZA susceptibility. These findings, besides providing the basis for understanding how PZA works, should have implications for rapid detection of PZA-resistant clinical isolates of M. tuberculosis and also for rapid differentiation of M. bovis from M. tuberculosis strains.

Abstract: Pyrazinamide (PZA) is an important sterilising tuberculosis drug that helps to shorten the duration of current chemotherapy regimens for tuberculosis. When first discovered, it had activity in murine tuberculosis but no apparent in vitro activity, and its subsequent use in treatment depended largely on classic experiments at Cornell University, which showed its requirement for an acid pH for activity and its sterilising activity in the mouse. Recent studies have shown that PZA enters Mycobacterium tuberculosis by passive diffusion, is converted to pyrazinoic acid (POA) by nicotinamidase/pyrazinamidase (PZase) and is then excreted by a weak efflux pump. Protonated POA (HPOA) is reabsorbed into the bacilli under acid conditions and accumulates because the efflux pump is inefficient, causing cellular damage. Unlike other antibacterials, PZA has no defined target of action. PZA is more active against old than against actively growing cultures, probably because the energy production and efflux pump would be slowed down by low bacterial metabolism. This review deals with the activity of PZA in vitro, in macrophages and in animal models. It describes the evidence from clinical trials that it is an effective sterilising drug that acts synergistically with rifampicin. The highly diverse mutations in the PZase gene (pncA) that lead to loss of PZase activity cause PZA resistance. Methods for susceptibility determination either as tests against PZA or nicotinamide in liquid and solid media, as tests for PZase activity or for mutations in pncA, are reviewed.

Abstract: Pyrazinamide (PZA) is a first-line tuberculosis drug that plays a unique role in shortening the duration of tuberculosis chemotherapy. PZA is hydrolyzed intracellularly to pyrazinoic acid (POA) by pyrazinamidase (PZase, encoded by pncA), an enzyme frequently lost in PZA-resistant strains, but the target of POA in Mycobacterium tuberculosis has remained elusive. Here, we identify a previously unknown target of POA as the ribosomal protein S1 (RpsA), a vital protein involved in protein translation and the ribosome-sparing process of trans-translation. Three PZA-resistant clinical isolates without pncA mutation harbored RpsA mutations. RpsA overexpression conferred increased PZA resistance, and we confirmed that POA bound to RpsA (but not a clinically identified ΔAla mutant) and subsequently inhibited trans-translation rather than canonical translation. Trans-translation is essential for freeing scarce ribosomes in nonreplicating organisms, and its inhibition may explain the ability of PZA to eradicate persisting organisms.

Abstract: The emergence of multi- and extensively drug-resistant tuberculosis is a significant impediment to the control of this disease because treatment becomes more complex and costly. Reliable and timely drug susceptibility testing is critical to ensure that patients receive effective treatment and become noninfectious. Molecular methods can provide accurate and rapid drug susceptibility results. We used DNA sequencing to detect resistance to the first-line antituberculosis drugs isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB) and the second-line drugs amikacin (AMK), capreomycin (CAP), kanamycin (KAN), ciprofloxacin (CIP), and ofloxacin (OFX). Nine loci were sequenced: rpoB (for resistance to RIF), katG and inhA (INH), pncA (PZA), embB (EMB), gyrA (CIP and OFX), and rrs, eis, and tlyA (KAN, AMK, and CAP). A total of 314 clinical Mycobacterium tuberculosis complex isolates representing a variety of antibiotic resistance patterns, genotypes, and geographical origins were analyzed. The molecular data were compared to the phenotypic data and the accuracy values were calculated. Sensitivity and specificity values for the first-line drug loci were 97.1% and 93.6% for rpoB, 85.4% and 100% for katG, 16.5% and 100% for inhA, 90.6% and 100% for katG and inhA together, 84.6% and 85.8% for pncA, and 78.6% and 93.1% for embB. The values for the second-line drugs were also calculated. The size and scope of this study, in numbers of loci and isolates examined, and the phenotypic diversity of those isolates support the use of DNA sequencing to detect drug resistance in the M. tuberculosis complex. Further, the results can be used to design diagnostic tests utilizing other mutation detection technologies.