Origins of a 350-kilobase genomic duplication in Mycobacterium tuberculosis and its impact on virulence.

In the present study, we have investigated the evolution and impact on virulence of a 350-kb genomic duplication present in the most recently evolved members of the Mycobacterium tuberculosis East Asian lineage. In a mouse model of infection, comparing HN878 subclones HN878-27 (no duplication) and HN878-45 (with the 350-kb duplication) revealed that the latter is impaired for in vivo growth during the initial 3 weeks of infection. Furthermore, the median survival time of mice infected with isolate HN878-45 is significantly longer (77 days) than that of mice infected with HN878-27. Whole-genome sequencing of both isolates failed to reveal any mutational events other than the duplication that could account for such a substantial difference in virulence. Although we and others had previously speculated that the 350-kb duplication arose in response to some form of host-applied selective pressure (P. Domenech, G. S. Kolly, L. Leon-Solis, A. Fallow, M. B. Reed, J. Bacteriol. 192: 4562-4570, 2010, and B. Weiner, J. Gomez, T. C. Victor, R. M. Warren, A. Sloutsky, B. B. Plikaytis, J. E. Posey, P. D. van Helden, N. C. Gey van Pittius, M. Koehrsen, P. Sisk, C. Stolte, J. White, S. Gagneux, B. Birren, D. Hung, M. Murray, J. Galagan, PLoS One 7: e26038, 2012), here we show that this large chromosomal amplification event is very rapidly selected within standard in vitro broth cultures in a range of isolates. Indeed, subclones harboring the duplication were detectable after just five rounds of in vitro passage. In contrast, the duplication appears to be highly unstable in vivo and is negatively selected during the later stages of infection in mice. We believe that the rapid in vitro evolution of M. tuberculosis is an underappreciated aspect of its biology that is often ignored, despite the fact that it has the potential to confound the data and conclusions arising from comparative studies of isolates at both the genotypic and phenotypic levels.

Reduced transmissibility of East African Indian strains of Mycobacterium tuberculosis.

BACKGROUND: Mycobacterium tuberculosis (MTB) has been classified into 4 main lineages. Some reports have associated certain lineages with particular clinical phenotypes, but there is still insufficient information regarding the clinical and epidemiologic implications of MTB lineage variation. METHODS: Using large sequence polymorphisms we classified MTB isolates from a population-based study in Montreal, Canada into the 4 major lineages, and identified the associated clinical and epidemiologic features. In addition, IS6110-RFLP and spoligotyping were used as indicators of recent TB transmission. The study population was divided into a derivation cohort, diagnosed between 2001 and 2007, and a separate validation cohort, diagnosed between 1996 and 2000. RESULTS: In the derivation cohort, when compared to the other MTB lineages, the East African-Indian (EAI) lineage was associated with lower rates of TB transmission, as measured by: positive TST among close contacts of pulmonary TB cases (adjusted odds ratio 0.6: [95% confidence interval 0.4-0.9]), and clustered TB cases (0.3: [<0.001-0.6]). Severe forms of TB were also less likely among the EAI group (0.4: [<0.001-0.8]). There were no significant differences when comparing patients with the other MTB lineages. In the validation cohort, the EAI lineage was associated with lower rates of positive TST among contacts (0.5: [0.3-0.9]) and a trend towards less clustered TB cases (0.5: [0.1-1.8]) when compared to the other lineages. Disease severity among the different groups was not significantly different in the validation cohort. CONCLUSIONS: We conclude that in Montreal, EAI strains were associated with reduced transmission compared to other MTB lineages.

Massive gene duplication event among clinical isolates of the Mycobacterium tuberculosis W/Beijing family.

As part of our effort to uncover the molecular basis for the phenotypic variation among clinical Mycobacterium tuberculosis isolates, we have previously reported that isolates belonging to the W/Beijing lineage constitutively overexpress the DosR-regulated transcriptional program. While generating dosR knockouts in two independent W/Beijing sublineages, we were surprised to discover that they possess two copies of dosR. This dosR amplification is part of a massive genomic duplication spanning 350 kb and encompassing >300 genes. In total, this equates to 8% of the genome being present as two copies. The presence of IS6110 elements at both ends of the region of duplication, and in the novel junction region, suggests that it arose through unequal homologous recombination of sister chromatids at the IS6110 sequences. Analysis of isolates representing the major M. tuberculosis lineages has revealed that the 350-kb duplication is restricted to the most recently evolved sublineages of the W/Beijing family. Within these isolates, the duplication is partly responsible for the constitutive dosR overexpression phenotype. Although the nature of the selection event giving rise to the duplication remains unresolved, its evolution is almost certainly the result of specific selective pressure(s) encountered inside the host. A preliminary in vitro screen has failed to reveal a role of the duplication in conferring resistance to common antitubercular drugs, a trait frequently associated with W/Beijing isolates. Nevertheless, this first description of a genetic remodeling event of this nature for M. tuberculosis further highlights the potential for the evolution of diversity in this important global pathogen.

Strains of the East Asian (W/Beijing) lineage of Mycobacterium tuberculosis are DosS/DosT-DosR two-component regulatory system natural mutants.

As part of our ongoing efforts to uncover the phenotypic consequences of genetic variability among clinical Mycobacterium tuberculosis isolates, we previously reported that isolates of the "East Asian" or "W/Beijing" lineage constitutively overexpress the coordinately regulated transcriptional program known as the DosR regulon under standard in vitro conditions. This phenotype distinguishes the W/Beijing lineage from all other M. tuberculosis lineages, which normally induce expression of this regulon only once exposed to low oxygen or nitric oxide, both of which result in inhibition of bacterial respiration and replication. Transcription of the DosR regulon is controlled through a two-component regulatory system comprising the transcription factor DosR and two possible cognate histidine sensor kinases, DosS and DosT. Through sequence analysis of a carefully selected set of isolates representing each of the major M. tuberculosis lineages, we describe herein a naturally occurring frameshift mutation in the gene encoding the DosT sensor kinase for isolates of the most recently evolved W/Beijing sublineages. Intriguingly, the occurrence of the frameshift mutation correlates precisely with the appearance of the constitutive DosR regulon phenotype displayed by the same "modern" W/Beijing strains. However, complementation studies have revealed that the mutation in dosT alone is not directly responsible for the constitutive DosR regulon phenotype. Our data serve to highlight the evolutionary pressure that exists among distinct M. tuberculosis lineages to maintain tight control over DosR regulon expression.

Major Mycobacterium tuberculosis lineages associate with patient country of origin.

Over recent years, there has been an increasing acknowledgment of the diversity that exists among Mycobacterium tuberculosis clinical isolates. To facilitate comparative studies aimed at deciphering the relevance of this diversity to human disease, an unambiguous and easily interpretable method of strain classification is required. Presently, the most effective means of assigning isolates into a series of unambiguous lineages is the method of Gagneux et al. (S. Gagneux et al., Proc. Natl. Acad. Sci. USA 103:2869-2873, 2006) that involves the PCR-based detection of large sequence polymorphisms (LSPs). In this manner, isolates are classified into six major lineages, the majority of which display a high degree of geographic restriction. Here we describe an independent replicate of the Gagneux study carried out on 798 isolates collected over a 6-year period from mostly foreign-born patients resident on the island of Montreal, Canada. The original trends in terms of bacterial genotype and patient ethnicity are remarkably conserved within this Montreal cohort, even though the patient distributions between the two populations are quite distinct. In parallel with the LSP analysis, we also demonstrate that "clustered" tuberculosis (TB) cases defined through restriction fragment length polymorphism (RFLP) analysis (for isolates with >or=6 IS6110 copies) or RFLP in combination with spoligotyping (for isolates with <6 IS6110 copies) do not stray across the LSP-defined lineage boundaries. However, our data also demonstrate the poor discriminatory power of either RFLP or spoligotyping alone for these low-IS6110-copy-number isolates. We believe that this independent validation of the LSP method should encourage researchers to adopt this system in investigations aimed at elucidating the role of strain variation in TB.