Host-mediated modification of PvuII restriction in Mycobacterium tuberculosis

DNA methylation impacts gene expression and ensures hypoxic survival of Mycobacterium tuberculosis

DNA methylation regulates gene expression in many organisms. In eukaryotes, DNA methylation is associated with gene repression, while it exerts both activating and repressive effects in the Proteobacteria through largely locus-specific mechanisms. Here, we identify a critical DNA methyltransferase in M. tuberculosis, which we term MamA. MamA creates N⁶-methyladenine in a six base pair recognition sequence present in approximately 2,000 copies on each strand of the genome. Loss of MamA reduces the expression of a number of genes. Each has a MamA site located at a conserved position relative to the sigma factor −10 binding site and transcriptional start site, suggesting that MamA modulates their expression through a shared, not locus-specific, mechanism. While strains lacking MamA grow normally in vitro, they are attenuated in hypoxic conditions, suggesting that methylation promotes survival in discrete host microenvironments. Interestingly, we demonstrate strikingly different patterns of DNA methyltransferase activity in different lineages of M. tuberculosis, which have been associated with preferences for distinct host environments and different disease courses in humans. Thus, MamA is the major functional adenine methyltransferase in M. tuberculosis strains of the Euro-American lineage while strains of the Beijing lineage harbor a point mutation that largely inactivates MamA but possess a second functional DNA methyltransferase. Our results indicate that MamA influences gene expression in M. tuberculosis and plays an important but strain-specific role in fitness during hypoxia.

Tuberculosis is a disease with a devastating impact on public health, killing over 1.5 million people each year around the globe. Tuberculosis is caused by the bacterium Mycobacterium tuberculosis, which over millennia has evolved the ability to survive and persist for decades in the harsh environment inside its human host. Regulation of gene expression is critical for adaptation to stressful conditions. To successfully tackle M. tuberculosis, we therefore need to understand how it regulates its genes and responds to environmental stressors. In this work, we report the first investigation of the role of DNA methylation in gene regulation and stress response in M. tuberculosis. We have found that DNA methylation is important for survival of hypoxia, a stress condition present in human infections, and furthermore that DNA methylation affects the expression of several genes. In contrast to methylation-regulation systems reported in other bacteria, in which the effects of methylation vary from one gene to the next, M. tuberculosis appears to use a concerted mechanism to influence multiple genes. Our findings identify a novel mechanism by which M. tuberculosis modulates gene expression in response to stress.

Discriminatory power and reproducibility of novel DNA typing methods for Mycobacterium tuberculosis complex strains

In recent years various novel DNA typing methods have been developed which are faster and easier to perform than the current internationally standardized IS6110 restriction fragment length polymorphism typing method. However, there has been no overview of the utility of these novel typing methods, and it is largely unknown how they compare to previously published methods. In this study, the discriminative power and reproducibility of nine recently described PCR-based typing methods for Mycobacterium tuberculosis were investigated using the strain collection of the interlaboratory study of Kremer et al. This strain collection contains 90 M. tuberculosis complex and 10 non-M. tuberculosis complex mycobacterial strains, as well as 31 duplicated DNA samples to assess reproducibility. The highest reproducibility was found with variable numbers of tandem repeat typing using mycobacterial interspersed repetitive units (MIRU VNTR) and fast ligation-mediated PCR (FLiP), followed by second-generation spoligotyping, ligation-mediated PCR (LM-PCR), VNTR typing using five repeat loci identified at the Queens University of Belfast (QUB VNTR), and the Amadio speciation PCR. Poor reproducibility was associated with fluorescent amplified fragment length polymorphism typing, which was performed in three different laboratories. The methods were ordered from highest discrimination to lowest by the Hunter-Gaston discriminative index as follows: QUB VNTR typing, MIRU VNTR typing, FLiP, LM-PCR, and spoligotyping. We conclude that both VNTR typing methods and FLiP typing are rapid, highly reliable, and discriminative epidemiological typing methods for M. tuberculosis and that VNTR typing is the epidemiological typing method of choice for the near future.

IS6110 fingerprinting of sensitive and resistant strains (1991-1992) of Mycobacterium tuberculosis in Colombia

The standardized method to study the polymorphism of IS 6110 was used to characterize 53 isolates of Mycobacterium tuberculosis obtained during 1991-1992 from 14 regions in Colombia. In Valle region cluster rate was 25% (4/16). The mean number of IS6110 band was 10 +/- 3. Similarity between strains was of 60% in 81% of strains and this tended to be correlated with geographic origin. For the first time M. tuberculosis without IS6110 bands in restriction fragment length polymorphism analysis was found in Colombia. Additional studies are necessaries in order to best characterize the situation in relation to human immunodeficiency virus epidemic and recent changes in tuberculosis control program.

Calculation of the stability of the IS6110 banding pattern in patients with persistent Mycobacterium tuberculosis disease

The interpretation of molecular epidemiologic data of Mycobacterium tuberculosis infection is dependent on the understanding of the stability and evolutionary characteristics of the DNA fingerprinting marker used to classify clinical isolates. This study investigated the stability of the IS6110 banding pattern in serial tuberculosis isolates collected from patients resident in an area with a high incidence of tuberculosis. Evolutionary changes were observed in 4% of the strains, and a half-life (t(1/2)) of 8.74 years was calculated, assuming a constant rate of change over time. This rate may be composed of a high rate of change seen during the early disease phase (t(1/2) = 0.57 years) and a low rate of change seen in the late disease phase (t(1/2) = 10.69 years). The early rate probably reflects change occurring during active growth prior to therapy, while the low late rate may reflect change occurring during or after treatment. We demonstrate that the calculation of these rates is strongly influenced by the time interval between onset of disease and sputum sampling. These calculations are further complicated by partial replacement of the original strain population, resulting in the sporadic appearance of clonal variants in sputum specimens. Therefore, the true extent of genetic diversity may be underestimated within each host, thereby influencing molecular epidemiological data used to establish transmission chains.

Evolution of the IS6110-based restriction fragment length polymorphism pattern during the transmission of Mycobacterium tuberculosis

Interpretation of the molecular epidemiological data of Mycobacterium tuberculosis is dependent on the validity of the assumptions that have been made. It is assumed that the IS6110 banding pattern is sufficiently stable to define epidemiological events representing ongoing transmission. However, molecular epidemiological data also support the observation that the IS6110 banding pattern may change over time. Factors affecting this rate may include the nature and duration of disease in a host and the opportunity to experience different host environments during the transmission cycle. To estimate the rate of IS6110 change occurring during the process of transmission, M. tuberculosis isolates from epidemiologically linked patients were genotypically characterized by restriction fragment length polymorphism (RFLP) analysis. The identification of IS6110 banding pattern changes during ongoing transmission suggested that a rate could be estimated. IS6110 change was significantly associated with strains with >5 IS6110 elements (P = 0.013) and was not observed in low-copy-number isolates. The minimum rate of appearance of variant strains was calculated to be 0.14 variant cases per source-case per year. This data suggest that clustering of isolates based on identical RFLP patterns is expected to underestimate transmission in patients infected with high-copy-number isolates. A model based on the rate of appearance of both variant and invariant strains demonstrates that the genotypically defined population structure may change by 18.6% during the study period of approximately 6.5 years. The implications for the use of RFLP data for epidemiologic study are discussed.

Molecular evolution of Mycobacterium tuberculosis: phylogenetic reconstruction of clonal expansion

SETTING

M. tuberculosis isolates were collected from patients attending health clinics in a high incidence urban community and in a low incidence rural setting in South Africa.

OBJECTIVE

To reconstruct the evolutionary history of a group of closely related M. tuberculosis isolates using IS6110, DRr and MTB484(1) restriction fragment length polymorphism (RFLP) data.

DESIGN

Mycobacterium tuberculosis isolates containing an average of ten IS6110 elements, with a similarity index of > or = 65% were genotypically classified by DNA fingerprinting using the IS6110 derived probes IS-3' and IS-5', as well as the DRr and MTB484(1) probes, in combination with PvuII or Hinfl endonuclease digestion. These RFLP data were subjected to phylogenetic analysis using both genetic distance and parsimony algorithms.

RESULTS

Phylogenetic analysis predicted the existence of two independently evolving lineages, possibly evolving from a common ancestral strain. The topology of the phylogenetic tree was supported by comprehensive bootstrapping and the specific partitioning of DNA methylation phenotypes. The observed difference in the branch lengths of the two lineages may suggest differential evolutionary rates. Isolates collected from different geographical regions demonstrate independent evolution, suggesting that it is highly unlikely that strains have been recently transmitted between the two regions. The number of evolutionary events identified in this strain family differs significantly from that of previously characterized strain families, implying that evolutionary rate may be strain family dependent.

CONCLUSION

Based on this analysis we propose that the algorithm used to calculate recent epidemiological events should be revised to incorporate the evolutionary characteristics of individual strain families, thereby enhancing the accuracy of molecular epidemiological calculations.

Molecular evidence for independent occurrence of IS6110 insertions at the same sites of the genome of Mycobacterium tuberculosis in different clinical isolates

Several characteristics of Mycobacterium tuberculosis (e.g., conserved genome and low growth rate) have severely restricted the study of the microorganism. The discovery of IS6110 raised hopes of overcoming these obstacles. However, our knowledge of this IS element is relatively limited; even its two basic characteristics (transposition mechanism and target site selection) are far from well understood. In this study, IS6110 insertions in ipl loci (iplA and iplB) in two collections of clinical isolates of M. tuberculosis from different geographic locations, one from Scotland and the other from Thailand, were investigated. Five different IS6110 insertions in the loci were identified: ipl-4::IS6110, ipl-5::IS6110, ipl-11::IS6110, ipl-12::IS6110, and ipl-13::IS6110. An attempt to establish the phylogenetic relationship of the isolates containing these insertions was unsuccessful, suggesting that some of these insertions may have arisen from more than one event. This possibility is further supported by the observation that IS6110 copies existed in the same site but with different orientations in different isolates, and the insertion site of ipl-1::IS6110 harbored IS6110 copies in both iplA and iplB in different strains. All these suggest the independent occurrence of IS6110 insertions at the same sites of the genome of M. tuberculosis in different clinical isolates. The implications of this finding are discussed.

Mapping of IS6110 flanking regions in clinical isolates of Mycobacterium tuberculosis demonstrates genome plasticity

Southern hybridization was used in combination with IS6110 insertion-locus-specific probes in a comparative study to determine the structure of chromosomal domains flanking IS6110 elements in clinical isolates of Mycobacterium tuberculosis. The resulting restriction fragment length polymorphism (RFLP) data demonstrated three mutational mechanisms responsible for the polymorphisms observed: IS6110 insertion, chromosomal mutation and deletion. The frequency of IS6110 insertion within many of the chromosomal regions demonstrates that preferential integration regions are common in M. tuberculosis. Mapping the IS6110 insertion positions and chromosomal deletions in relation to the M. tuberculosis H37Rv and M. bovis BCG genome sequences reveals numerous disruptions of predicted open reading frames (ORFs). A phylogenetic tree, based on the mutational data, showed a number of independently evolving lineages of M. tuberculosis, while analysis of the mutational events occurring at each branch point suggests both divergent and convergent evolution. A significant positive correlation was demonstrated between the mutation rate and the frequency of occurrence of different isolates in families of strains, suggesting that evolution may impact on strain 'fitness' or that strain proliferation may increase the chance of mutation. We conclude that the genome of clinical isolates of M. tuberculosis continues to evolve.

Comparison of methods based on different molecular epidemiological markers for typing of Mycobacterium tuberculosis complex strains: interlaboratory study of discriminatory power and reproducibility

In this study, the currently known typing methods for Mycobacterium tuberculosis isolates were evaluated with regard to reproducibility, discrimination, and specificity. Therefore, 90 M. tuberculosis complex strains, originating from 38 countries, were tested in five restriction fragment length polymorphism (RFLP) typing methods and in seven PCR-based assays. In all methods, one or more repetitive DNA elements were targeted. The strain typing and the DNA fingerprint analysis were performed in the laboratory most experienced in the respective method. To examine intralaboratory reproducibility, blinded duplicate samples were included. The specificities of the various methods were tested by inclusion of 10 non-M. tuberculosis complex strains. All five RFLP typing methods were highly reproducible. The reliability of the PCR-based methods was highest for the mixed-linker PCR, followed by variable numbers of tandem repeat (VNTR) typing and spoligotyping. In contrast, the double repetitive element PCR (DRE-PCR), IS6110 inverse PCR, IS6110 ampliprinting, and arbitrarily primed PCR (APPCR) typing were found to be poorly reproducible. The 90 strains were best discriminated by IS6110 RFLP typing, yielding 84 different banding patterns, followed by mixed-linker PCR (81 patterns), APPCR (71 patterns), RFLP using the polymorphic GC-rich sequence as a probe (70 patterns), DRE-PCR (63 patterns), spoligotyping (61 patterns), and VNTR typing (56 patterns). We conclude that for epidemiological investigations, strain differentiation by IS6110 RFLP or mixed-linker PCR are the methods of choice. A strong association was found between the results of different genetic markers, indicating a clonal population structure of M. tuberculosis strains. Several separate genotype families within the M. tuberculosis complex could be recognized on the basis of the genetic markers used.