Authors: Raheem TY, Iwalokun BA, Fowora MA, Osuolale KA, Adesesan AA.
Background: Despite the tremendous improvements in the diagnosis and treatment of tuberculosis, pulmonary tuberculosis (PTB) remains a leading cause of morbidity and mortality in adults and children worldwide. The World Health Organization (WHO) recommends detection of MDR TB using bacteriological confirmation of TB and testing for drug resistance using rapid molecular tests, culture methods or sequencing technologies. M. tuberculosis strains resistant to multiple anti-TB drugs are becoming increasingly common. Different independent mutations of rpoB, katG and InhA genes encoding either the drug target or the enzymes involved in drug activation have been found to be one of the strategies responsible for resistance to Rifampicin and Isoniazid.
Methods: This cross-sectional, multicentre study was conducted on MDRTB isolates collected from pulmonary TB patients in Lagos, Nigeria from May 2012 to October 2016. After informed consent, structured questionnaires were administered to obtain sociodemographic data. Sputum samples were collected and processed for microscopy and culture using Lowenstein-Jensen medium. Isolates were identified by biochemical and molecular methods and drug susceptibility testing was performed using MIC, proportion and line probe Assay methods.
Objectives: The study investigated the occurrence and frequency of gene mutations associated with rifampicin (RIF) and isoniazid (INH) resistance from the isolated MDRTB and assessed whether all the M. tuberculosis which elicited MDRTB phenotype had mutation markers.
Results: Of the 48 M. tuberculosis that elicited MDR phenotype by the proportion and MIC methods, 2 (4.2%) isolates lacked any of the inhA, katG and rpoBgene mutations associated with RIF and isoniazid resistance in the line probe assay method. The highest occurring rpoB mutation conferring RIF resistance among the MDR M. tuberculosis tested was 97.3% with mutation point S531L at a frequency of 60.4% followed by H526D mutation (22.9%), H536Y (8.3%) and D516V (6.3%). The S315T mutation of the katG gene was responsible for 50% of isoniazid resistance among the MDR isolates. This was followed by C-15T mutation (14.6%), S94A mutation (12.5%) and 1194A (8.3%) of inhA mutation points.
Conclusions: Different mutations of genes encoding either drug target or the enzymes involved in drug activation such as S531L, H526D, H536Y, D516V, S315T, C-15T and S94A were detected in MDRTB isolates. The study also showed that lack of inhA, katG and rpoB resistance defining mutations may not be sufficient as markers of susceptibility to anti-TB drugs. There is need for a nation-wide study of the pattern of mutations of drug/enzyme gene targets in MDRTB in Nigeria.
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