Bedaquiline- and clofazimine- selected Mycobacterium tuberculosis mutants: further insights on resistance driven largely by Rv0678
Drug-resistant tuberculosis is a serious global health threat. Bedaquiline (BDQ) is a relatively new core drug, targeting the respiratory chain in Mycobacterium tuberculosis (Mtb). While mutations in the BDQ target gene, atpE, are rare in clinical isolates, mutations in the Rv0678 gene, a transcriptional repressor regulating the efflux pump MmpS5-MmpL5, are increasingly observed, and have been linked to worse treatment outcomes. Nevertheless, underlying mechanisms of (cross)-resistance remain incompletely resolved. Our study aims to distinguish resistance associated variants from other polymorphisms, by assessing the in vitro onset of mutations under drug pressure, combined with their impact on minimum inhibitory concentrations (MICs) and on protein stability. For this purpose, isolates were exposed in vitro to sub-lethal concentrations of BDQ or clofazimine (CFZ). Selected colonies had BDQ- and CFZ-MICs determined on 7H10 and 7H11 agar. Sanger sequencing and additional Deeplex Myc-TB and whole genome sequencing (WGS) for a subset of isolates were used to search for mutations in Rv0678, atpE and pepQ. In silico characterization of relevant mutations was performed using computational tools.  We found that colonies that grew on BDQ medium had mutations in Rv0678, atpE or pepQ, while CFZ-exposed isolates presented mutations in Rv0678 and pepQ, but none in atpE. Twenty-eight Rv0678 mutations had previously been described among in vitro selected mutants or in patients' isolates, while 85 were new. Mutations were scattered across the Rv0678 gene without apparent hotspot. While most Rv0678 mutations led to an increased BDQ- and/or CFZ-MIC, only a part of them surpassed the critical concentration (69.1% for BDQ and 87.9% for CFZ). Among the mutations leading to elevated MICs for BDQ and CFZ, we report a synonymous Val1Val mutation in the Rv0678 start codon. Finally, in silico characterization of Rv0678 mutations suggests that especially the C46R mutant may render Rv0678 less stable.