In a survey of MDR-TB cases across India, the most common isoniazid resistance mechanism was identified. Which genetic alteration is responsible for the majority of isoniazid resistance in *Mycobacterium tuberculosis*?

Explanation

## Genetic Basis of Isoniazid Resistance in TB ### Most Common Mechanism **Key Point:** Isoniazid resistance in *Mycobacterium tuberculosis* is caused by **katG gene mutations** in ~50–80% of resistant isolates. katG encodes a catalase-peroxidase enzyme essential for converting isoniazid (pro-drug) into its active form, the isonicotinic acyl radical. ### Mechanism of Isoniazid Action & Resistance Isoniazid is a pro-drug that requires enzymatic activation by mycobacterial catalase-peroxidase (KatG). The activated form inhibits mycolic acid synthesis by targeting InhA (enoyl-ACP reductase). Loss of KatG function prevents pro-drug activation, rendering isoniazid ineffective. ### Frequency of Resistance Mechanisms | Mechanism | Gene(s) | Frequency | Phenotype | | --- | --- | --- | --- | | **katG mutations** | katG | 50–80% | High-level INH resistance | | **inhA overexpression** | inhA promoter | 15–30% | Low-level INH resistance | | **ahpC mutations** | ahpC | <5% | Rare, variable resistance | | **Other** | Various | <5% | Uncommon | **High-Yield:** katG mutations are the **most common single cause** of isoniazid resistance globally and in India. Isolates with katG mutations typically show high-level resistance (MIC >1 µg/mL). ### Clinical Pearl Isoniazid-resistant TB with katG mutations is associated with higher mortality and treatment failure compared to susceptible TB. Early detection via molecular testing (line probe assays, GeneXpert) is critical for prompt switch to second-line drugs. ### Secondary Resistance Mechanism: inhA Overexpression When katG is intact, mutations in the **inhA promoter region** (particularly the C–15T substitution) increase inhA expression, reducing drug susceptibility. This is the second most common mechanism (~15–30%) and typically confers low-level resistance. Many MDR-TB isolates carry **both** katG mutations AND inhA promoter mutations, conferring high-level resistance to both isoniazid and ethionamide. ### Why Other Options Are Less Common - **ahpC mutations:** Alkyl hydroperoxide reductase mutations are rare and do not account for the majority of isoniazid resistance. - **NAD(P)H oxidoreductase overexpression:** Not a recognized mechanism of isoniazid resistance in clinical TB. [cite:Harrison 21e Ch 158]