4-1.　Molecular mechanism of drug resistance in M. tuberculosis

Isoniazid resistance is well known to be associated with alterations in the catalase-peroxidase gene (katG)^{17, 18)} or the inhA gene¹⁹⁾, which encodes an enzyme involved in mycolic acid biosynthesis.　Mutations^{20, 21)} in the rpoB produce drug resistance by diminishing rifampin binding affinity for the RNA polymerase.　Mutations^{22, 23)} in genes encoding the ribosomal S12 protein (rpsL) and 16S rRNA (rrs) have always been detected in streptomycin-resistant M. tuberculosis.　Miscense mutations^{24, 25)} in the DNA gyrase (gyrA) gene have always been associated with increased resistance to fluoroquinolones.　Amino acid substitutions²⁶⁾ in EmbB alter the drug-protein interaction and thereby cause ethambutol resistance.　PZA resistant M. tuberculosis strains lose pyrazinamidase (PZase) activity.　There is usually a good correlation between the loss of PZase activity and development of PZA resistance.　Mutations^{26, 27)} in the pncA gene encoding the PZase of M. tuberculosis could be involved in the loss of PZase activity.

Table 4.　Frequency of Mutations in Drug-resistant M.tuberculosis Strains Isolated in mostly Asian Countries

　

4.2.　Detection of mutations in the genes involved in drug resistsnse

Various PCR-based molecular genetic techniques for the detection of mutations of the genes associated with drug resistanse in M.tuberuclosis were reported.　They the include direct sequencing²⁸⁾, single-strand conformation polymorphism analysis²⁹⁾, hetero-duplex formation assay³⁰⁾, reverse hybridization-based line probe assay³¹⁾, and molecular beacon assay³²⁾ of PCR products.　These techniques produce results within a much shorter time than the time required for conventional culture techniques.

　

References

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