Polymorphisms in Rv3806c (ubiA) and the upstream region of embA in relation to ethambutol resistance in clinical isolates of Mycobacterium tuberculosis from North India

Mutations at embB306 are the most prevalent polymorphisms associated with ethambutol (EMB) resistance, responsible for 40-60% of EMB resistant clinical cases of tuberculosis (TB). The present study analyzed additional mutations associated with EMB resistance in the embB, embC, embA and Rv3806c (ubiA) genes in 29 EMB resistant and 29 EMB susceptible clinical isolates of M. tuberculosis selected from 360 patients with TB. The entire ubiA gene, mutational hotspot regions of embB, embC, and upstream region of embA were screened for polymorphisms by DNA sequencing and the results correlated with minimum inhibitory concentrations (MIC) of EMB. The most common polymorphism identified in ubiA was at codon 149 (GAA to GAC), occurring in 5/29 (17.2%) resistant isolates and 7/29 (24%) susceptible isolates. Mutations in embB were most common at codon 306 (ATG to ATC/GTG), occurring only in EMB resistant isolates (20/29; 69%). Mutations in the upstream region of embA at -8, -11, -12 and -60 codons also occurred in EMB resistant strains (8/29; 27.5%) of which 6/8 (75%) were observed in isolates with EMB MIC ≥16 μg/ml. Though no polymorphisms associated with EMB resistance were identified in ubiA, polymorphisms upstream to embA may contribute to high level EMB resistance.