Among the genotypes that prevail in the modern spectrum of Mycobacterium tuberculosis strains, the Beijing genotype is the one that causes major concern, as it is geographically widespread and it is considered hypervirulent. Comparative genomic studies have shown that Beijing strains have principally evolved through mechanisms of deletion of chromosomal regions, designated regions of difference (RD), and mutations. In this paper, we aimed to determine the evolutionary history of Beijing strains through the analysis of polymorphisms generated by deletions of large specific sequences, i.e., RD105, RD181, RD150, and RD142, and by single nucleotide substitutions in genes mutT4 and mutT2, coding for DNA repair enzymes. Based on the molecular characteristics of a collection of Beijing strains recently isolated in Tuscany, Italy, we propose a phylogenetic reconstruction of the Beijing family. According to our model, the Beijing family evolved from a M. tuberculosis progenitor following deletion of the RD207 region, an event responsible for the loss of spacers 1-34 in the direct repeat (DR) locus. The major lineages of the Beijing family then evolved via subsequent deletions of regions RD105, RD181 and RD150. In the most ancient evolutionary lineages genes mutT4 and mutT2 were in wild type configuration; the mutT4 mutation was acquired subsequent to the RD181 deletion in a progenitor strain that, in turn, gave rise to a sublineage bearing the mutT2 mutation. Within the major branches of the Beijing family, deletion of additional spacers in the DR locus led to evolution of sublineages characterized by different spoligotypes. Our evolutionary model of the Beijing family provides a deeper framework than previously proposed for epidemiologic and phylogenetic studies of circulating M. tuberculosis Beijing strains, thus allowing a more systematic and comprehensive evaluation of the relevance of Beijing strain variability.
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