Browsing by Author "Visser, Hanri"
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- ItemMechanisms of resistance to new generation anti-TB drugs(Stellenbosch : Stellenbosch University, 2015-04) Visser, Hanri; Victor, T. C.; Paul, L. V.; Stellenbosch University. Faculty of Health Sciences. Dept. of Biomedical Sciences. Molecular Biology and Human Genetics.ENGLISH ABSTRACT: Drug resistance in Mycobacterium tuberculosis is an increasing global problem. Drug resistance is mostly caused by single nucleotide polymorphisms (SNPs) within the bacterial genome. This observed increase in global incidence of drug resistant tuberculosis (TB) has sparked the search for new anti-TB drugs and the repurposing of drugs that are currently used against other organisms or species of mycobacteria. One such repurposed drug, clofazimine (CFZ), is currently used for the treatment of leprosy, caused by Mycobacterium leprae. The mechanism of action of CFZ is not clear, but it is hypothesized that CFZ is reduced by a mycobacterial type II NADH oxidoreductase (NDH-2). The reduction of CFZ drives the production of reactive oxygen species (ROS) which is toxic to the pathogen. The aim of this study was to elucidate the mechanism of CFZ resistance. Towards this aim, spontaneous in vitro CFZ resistant mutants were selected, characterized and whole genome was used identify SNPs which may cause CFZ resistance. Mutations were identified in a transcriptional regulator encoded by Rv0678, fatty-acid-AMP ligase, or FadD28 (Rv2941) and glycerol kinase or GlpK (Rv3696c). Mutations in Rv0678 have previously been shown to play a role in both CFZ resistance and bedaquiline (BDQ) cross-resistance, while no link has been found between CFZ resistance and mutations in fadD28 and glpK. The novel, non-synonymous SNP identified in Rv0678 resulted in the replacement of an alanine residue with threonine at codon 84, which is located in the DNA binding domain. Virtual modelling of the mutated Rv0678 protein showed that the A84T mutation may influence DNA binding, possibly due to its proximity to the DNA binding domain. This mutation caused a change in hydrophobicity, which may influence binding to DNA. Previous studies showed that mutations in Rv0678 resulted in the upregulation of mmpL5, a putative efflux pump. However, the mechanism whereby CFZ resistance occurs via increased abundance of this efflux pump in the cell wall is not clear and needs further investigation. The cross-resistance between CFZ and BDQ, caused by mutations in Rv0678, is of concern and may influence the planning of anti-TB drug regimens for the future. The roles of the other two mutations identified in this study in CFZ resistance is also not clear and requires further investigation. Finally, the findings of this study support the role of Rv0678 in CFZ resistance thereby suggesting that this gene could be useful as a diagnostic marker to test for CFZ resistance in clinical isolates.
- ItemVersatility of 7-substituted coumarin molecules as antimycobacterial agents, neuronal enzyme inhibitors and neuroprotective agents(MDPI, 2017-09-30) Kapp, Erika; Visser, Hanri; Sampson, Samantha L.; Malan, Sarel F.; Streicher, Elizabeth M.; Foka, Germaine B.; Warner, Digby F.; Omoruyi, Sylvester I.; Enogieru, Adaze B.; Ekpo, Okobi E.; Zindo, Frank T.; Joubert, JacquesENGLISH ABSTRACT: A medium-throughput screen using Mycobacterium tuberculosis H37Rv was employed to screen an in-house library of structurally diverse compounds for antimycobacterial activity. In this initial screen, eleven 7-substituted coumarin derivatives with confirmed monoamine oxidase-B and cholinesterase inhibitory activities, demonstrated growth inhibition of more than 50% at 50 µM. This prompted further exploration of all the 7-substituted coumarins in our library. Four compounds showed promising MIC99 values of 8.31–29.70 µM and 44.15–57.17 µM on M. tuberculosis H37Rv in independent assays using GAST-Fe and 7H9+OADC media, respectively. These compounds were found to bind to albumin, which may explain the variations in MIC between the two assays. Preliminary data showed that they were able to maintain their activity in fluoroquinolone resistant mycobacteria. Structure-activity relationships indicated that structural modification on position 4 and/or 7 of the coumarin scaffold could direct the selectivity towards either the inhibition of neuronal enzymes or the antimycobacterial effect. Moderate cytotoxicities were observed for these compounds and slight selectivity towards mycobacteria was indicated. Further neuroprotective assays showed significant neuroprotection for selected compounds irrespective of their neuronal enzyme inhibitory properties. These coumarin molecules are thus interesting lead compounds that may provide insight into the design of new antimicrobacterial and neuroprotective agents.