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Polymorphisms in the ethR gene as determinants of ethionamide resistance in Mycobacterium tuberculosis

Rodrigo Fernandes Machado*; Dr. Teca Calcagno Galvão*; Dr. Luis Caetano Martha Antunes**.
*Fundação Oswaldo Cruz, Laboratório de Genômica Funcional e Bioinformática, Instituto Oswaldo Cruz, Rio de Janeiro, Brazil;
***Fundação Oswaldo Cruz, Laboratório de Pesquisa em Infecção Hospitalar, Instituto Oswaldo Cruz, Rio de Janeiro, Brazil.

Treatment for tuberculosis caused by multidrug-resistant Mycobacterium tuberculosis is still a global challenge. Ethionamide (ETH) inhibits the synthesis of mycolic acids, essential components of the M. tuberculosis cell wall, conferring bactericidal properties to this drug. ETH is a prodrug that is activated by EthA, whose gene is regulated by the TetR repressor encoded by the adjacent gene, ethR. An increasing number of mutations have been described in the ethA promoter region (PethA) and in the ethR coding region in clinical isolates resistant to ETH. However, studies evaluating their role in resistance are incipient. Overexpression of EthR confers resistance to ETH and, therefore, mutations in ethR could affect the expression of ethA in clinical isolates and cause resistance. We previously demonstrated that the F110L mutation in EthR increased the DNA binding capacity compared to wildtype EthR. In this work, we evaluated the impact of two EthR mutations (F110L and A95T) from M. tuberculosis clinical isolates, previously identified by our group and in the literature, in the regulation of PethA activity using transcriptional fusions of PethA and a reporter gene using fluorescence assays. The results obtained showed that the F110L and A95T mutations in EthR decrease the activity of PethA when compared to wildtype EthR, leading to levels close to those caused by the t-11c mutation in PethA, known to cause resistance to ETH in M. tuberculosis. Our results suggest that EthR mutations may play a decisive role in clinical resistance to ETH due to reduced levels of ethA promoter activity.