Efficient differentiation of Mycobacterium tuberculosis strains of the W-Beijing family from Russia using highly polymorphic VNTR loci

The W-Beijing family is a widespread Mycobacterium tuberculosis clonal lineage that frequently causes epidemic outbreaks. This family is genetically homogeneous and conserved, so ETR-VNTR (exact tandem repeat-variable number of tandem repeats) typing is insufficient for strain differentiation, due to a common ETR-A to E profile (42435). This leads to the false clustering in molecular epidemiological studies, especially in the regions of predominance of the W-Beijing family. In this study, we searched for VNTR loci with a high evolutionary rate of polymorphism in the W-Beijing genome. Here we further evaluated VNTR typing on a set of 99 Mycobacterium tuberculosis clinical isolates and reference strains. These isolates were characterized and classified into several genotype families based on three ETR loci (A, C, E) and eight additional loci [previously described as QUB (Queen's University Belfast) or MIRU (Mycobacterial Interspersed Repetitive Units) or Mtubs]. Ninety-nine strains were divided into 74 VNTR-types, 51 isolates of the W-Beijing family identified by IS6110 RFLP-typing (the restriction fragment length polymorphism-typing) and/or spoligotyping were subdivided into 30 VNTR-types. HGDI (the Hunter-Gaston discriminatory index) for all studied loci was close to that of IS6110 RFLP typing, a gold standard method for subtyping M. tuberculosis complex strains. The QUB 26 and QUB 18 loci located in the PPE genes were highly polymorphic and more discriminative than other loci (HGDI is 0.8). Statistically significant increase of tandem repeats number in loci ETR-A, -E, QUB 26, QUB 18, QUB 11B, Mtub21 was revealed in the W-Beijing group compared to genetically divergent non-W-Beijing strains. Thirty-six isolates were subjected to IS6110 RFLP typing. The congruence between results of the IS6110 RFLP typing and 11-loci VNTR typing was estimated on 23 isolates of the W-Beijing family. These isolates were subdivided into 9 IS6110-RFLP types and 13 VNTR types. The poor profiles correlation (0.767) reflects the differences in the rate and type of evolution between genome regions targeted by IS6110-RFLP and VNTR typing. VNTR typing in proposed format is powerful tool for discrimination of M. tuberculosis strains with different level of genetic relationship.