Combination of Single Nucleotide Polymorphism and Variable-Number Tandem Repeats for Genotyping a Homogenous Population of Mycobacterium tuberculosis Beijing Strains in China

The standard 15- and 24-locus variable-number tandem repeat (VNTR) genotyping methods have demonstrated adequate discriminatory power and a small homoplasy effect for tracing tuberculosis (TB) transmission and predicting Mycobacterium tuberculosis lineages in European and North American countries. However, its validity for the definition of transmission in homogenous M. tuberculosis populations in settings with high TB burdens has been questioned. Here, we genotyped a population-based collection of 191 Beijing strains based on standard 15-locus VNTR (VNTR-15) and 8 single nucleotide polymorphisms (SNPs) in Shanghai, China. Limited discriminatory power and high rates of VNTR homoplasy were observed in the homogenous population of evolutionarily "modern" Beijing strains. Additional typing of three hypervariable loci (VNTR3820, VNTR4120, and VNTR3232) was performed for VNTR-15-based clusters. High variations of hypervariable alleles were observed in clusters with inconsistent SNP sublineages. We concluded that SNPs and hypervariable VNTR loci are helpful to enhance the discriminatory power and decrease the VNTR homoplasy effect for defining clusters. We recommend the combination of standard VNTR-15 and SNPs as first-line typing methods and the hypervariable loci for second-line typing of clustered strains for molecular epidemiology studies of homogenous M. tuberculosis populations.