Tuberculosis (TB) is a re-emerging disease in the European Union and is rampant in the young republics of Eastern Europe. Faced with the twin problems of multidrug resistance and synergy with the HIV-pandemic, there is now an urgent need for new and improved therapies. Recent experiments have indicated that the oxidative response of Mycobacterium tuberculosis plays a central role in the mechanism of action of the major TB drug, isoniazid (INH), and in the development of INH resistance mechanisms, and that it also has a significant impact on the virulence of the tubercle bacillus. Two key enzymes in this process are the catalase-peroxidase, KatG, and alkyl hydroperoxidase, AhpC. It is the objective of this proposal to understand how these enzymes mediate drug susceptibility and resistance, to determine their respective roles in mycobacterial virulence and use this
knowledge to design new and improved therapies.
KatG converts INH to a toxic agent that is believed to inhibit mycolic acid synthesis by binding to the InhA protein, an essential fatty acid synthetase. Resistance to INH results from mutations which reduce or abolish the activity of KatG, or lead to overproduction or modification of InhA. INH-resistant mutants, which have lost KatG activity show greatly reduced virulence in animal models whereas those with altered inhA genes are unaffected. In some INH-resistant strains, loss of KatG activity is accompanied by increased production of the alkyl hydroperoxidase AhpC. This suggests that KatG and AhpC may act on common substrates and that overexpression of AhpC can compensate for diminished KatG activity. The overexpression of AhpC might also increase the virulence of KatG deficient
strains, as it could help the bacterium to cope with the host defense. Furthermore, AhpC might be involved in the detoxification of INH. KatG and AhpC thus not only play a central role in INH susceptibility, resistance mechanisms and mycobacterial virulence but also represent ideal drug targets, as their inactivation leads to a severe growth disadvantage. It is also anticipated that further studies on the activation of INH by KatG will reveal the precise structure of its bioactive form and will allow for the design of improved inhibitors of InhA. The research program will focus on the following tasks:
crystallise KatG and AhpC
develop new methods to investigate the enzymology of KatG and AhpC
elucidate the precise role of KatG in the mechanism of action of INH
characterise the inhibition of InhA by activated INH
investigate the role of KatG and AhpC in development of drug resistance
investigate the role of KatG and AhpC in mycobacterial virulence
identify inhibitors of KatG, AhpC and InhA by rational design and combinatorial methods
It is anticipated that this program will significantly add to our understanding of the biology and pathogenesis of M. tuberculosis and that its implications are of great scientific and pharmaceutical relevance.
Funding SchemeCSC - Cost-sharing contracts
NW7 1AA London