Researchers believe they have identified the gene that allowstuberculosis bacteria to resist the effect of isoniazid and otheranti-tuberculosis drugs.
In an article published in the the current issue of Science,Asesh Banerjee, et al. conclude that a gene they are callinginhA encodes the target of isoniazid (isonicotinic acidhydrazide, or INH, one of the most common TB drugs). If thatgene is mutated, they explain, INH's usual binding site ischanged and the Mycobacterium tuberculosis foils the attemptsof INH to stop it from multiplying.
Complicating the researchers' task was the fact that the actionof INH is not well understood. The team relied on biochemicalevidence suggesting that the drug blocks the synthesis ofmycolic acid, a fatty acid in the cell wall of M. tuberculosis. Thetarget protein of INH, however, has not been identified.
Banerjee, et al. took a two-pronged approach in identifyinginhA as the culpable gene. Experimenting with drug-resistantstrains of M. smegmatis and M. bovis (bacteria that are similarto tuberculosis in their activity but grow more quickly, makingthem more practical for research), the researchers first isolatedvarious gene fragments from the mutated bacteria strains andtransferred them into otherwise normal, non-drug resistant M.bovis and M. smegmatis bacteria. By process of eliminationthey were able to discover which fragments conferred drugresistance on the normal bacteria. One of the open readingframes (ORFs) implicated this process, the researchers said, andalso exhibited sequence similarity to proteins involved in thebacterias' fatty acid biosynthesis. This finding, they said, isconsistent with the hypothesis that INH works by blocking thesynthesis of the fatty acids.
Banerjee et al. also used a second approach, transferringmultiple copies of normal inhA genes into normal bacteria. Thisalso caused drug resistance. The researchers theorized that theoverexpression of INH's target protein from the multiple inhAgenes stopped the INH from shutting down the fatty acidsynthesis. A similar phenomenon that has been encounteredfor other antibiotics, they said.
The inhA gene from normal M. tuberculosis is 100 percentidentical to the inhA of normal, INH-sensitive M. bovis, theresearchers stressed. "The ability to produce large quantities ofthe inhA protein will facilitate studies of the nature of inhAactivity and the mode of action of INH and ETH" (ethionamide,a second line tuberculosis drug), they conclude.
"Further investigation of this target may lead to thedevelopment of new agents for treating tuberculosis," includingdrug-resistant strains.
-- Karl A. Thiel Associate Editor
(c) 1997 American Health Consultants. All rights reserved.