Lack of methoxy-mycolates characterizes the geographically restricted lineage 7 of Mycobacterium tuberculosis complex

16 páginas, 6 figuras, 2 tablas
Lineage 7 (L7) emerged in the phylogeny of the Mycobacterium tuberculosis complex (MTBC) subsequent to the branching of 'ancient' lineage 1 and prior to the Eurasian dispersal of 'modern' lineages 2, 3 and 4. In contrast to the major MTBC lineages, the current epidemiology suggests that prevalence of L7 is highly confined to the Ethiopian population, or when identified outside of Ethiopia, it has mainly been in patients of Ethiopian origin. To search for microbiological factors that may contribute to its restricted distribution, we compared the genome of L7 to the genomes of globally dispersed MTBC lineages. The frequency of predicted functional mutations in L7 was similar to that documented in other lineages. These include mutations characteristic of modern lineages - such as constitutive expression of nitrate reductase - as well as mutations in the VirS locus that are commonly found in ancient lineages. We also identified and characterized multiple lineage-specific mutations in L7 in biosynthesis pathways of cell wall lipids, including confirmed deficiency of methoxy-mycolic acids due to a stop-gain mutation in the mmaA3 gene that encodes a methoxy-mycolic acid synthase. We show that the abolished biosynthesis of methoxy-mycolates of L7 alters the cell structure and colony morphology on selected growth media and impacts biofilm formation. The loss of these mycolic acid moieties may change the host-pathogen dynamic for L7 isolates, explaining the limited geographical distribution of L7 and contributing to further understanding the spread of MTBC lineages across the globe.
This study was supported by the MRC Confidence in Concept Fund and the ISSF Wellcome Trust grants 105603/Z/14/Z (G.L-M) and 204833/Z/16/Z (S.J.W). E.H., A.A. and S.J.W. would like to acknowledge funding from the Association of Physicians of Great Britain and Ireland Links with Developing Countries Scheme, and the Human Hereditary and Health in Africa (H3Africa) ‘TBGEN: an integrated approach to unravelling susceptibility to tuberculosis in Africa’ [H3A/18/003]. H3Africa is managed by the Science for Africa (SFA) Foundation in partnership with the Wellcome Trust, the NIH (National Institutes of Health) and African Society of Human genetics (AfSHG). The views expressed herein are those of the authors and not necessarily those of the SFA Foundation and its partners.