Comparative genomic and phylogeographic analysis of … · Leprosy Mission Nepal, Anandaban...
1 Comparative genomic and phylogeographic analysis of Mycobacterium leprae Marc Monot 1* , Nadine Honoré 1* , Thierry Garnier 1 , Nora Zidane 1 , Diana Sherafi 1 , Alberto Paniz-Mondolfi 2 , Masanori Matsuoka 3 , G. Michael Taylor 4 , Helen D. Donoghue 4 , Abi Bouwman 5 ,Simon Mays 6 , Claire Watson 7 , Diana Lockwood 7 , Ali Khamispour 8 , Yahya Dowlati 8 , Shen Jianping 9 , Thomas H. Rea 10 , Lucio Vera-Cabrera 11 , Mariane M. Stefani 12 , Sayera Banu 13 , Murdo Macdonald 14 , Bishwa Raj Sapkota 14 , John S. Spencer 15 , Jérôme Thomas 16 , Keith Harshman 16 , Pushpendra Singh 17 , Philippe Busso 17 , Alexandre Gattiker 18 , Jacques Rougemont 18 , Patrick J. Brennan 15 , and Stewart T. Cole 17 1 Institut Pasteur, Paris, France 2 Instituto de Biomedicina, Caracas, Venezuela 3 Leprosy Research Centre, National Institute of Infectious Diseases, Tokyo, Japan 4 Centre for Infectious Diseases and International Health, Windeyer Institute, University College London, London W1T 4JF, UK 5 Faculty of Life Sciences, Manchester Interdisciplinary Biocentre, University of Manchester, Manchester M1 7DN, UK 6 Ancient Monuments Laboratory, English Heritage Centre for Archaeology, Fort Cumberland, Portsmouth, UK. 7 London School of Hygiene and Tropical Medicine, Keppel St, London WC1E 7HT, UK 8 CRTSDL, Tehran University of Medical Sciences, Teheran, Iran 9 National Center for Leprosy Control, China CDC, Nanjing (210042), P. R. China 10 Department of Dermatology, Keck School of Medicine, University of Southern California, Los Angeles 90033, USA 11 Servicio de Dermatología, Hospital Universitario "José E. González", Monterrey, N.L., México 12 Tropical Pathology and Public Health Institute at Federal University of Goias, Rua 235 esq. c/1ª Avenida, S/N. Setor Universitario. Goiania, Goias, Brazil 13 International Centre for Diarrhoeal Disease Research, Bangladesh, Dhaka-1000, Bangladesh 14 Leprosy Mission Nepal, Anandaban Hospital, Kathmandu, Nepal 15 Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort Collins, Colorado 80523-1682, USA 16 DNA Array Facility, Center for Integrative Genomics, University of Lausanne, CH-1015 Lausanne, Switzerland
Transcript of Comparative genomic and phylogeographic analysis of … · Leprosy Mission Nepal, Anandaban...
Comparative genomic and phylogeographic analysis ofMycobacterium leprae
Marc Monot1*, Nadine Honoré1*, Thierry Garnier1, Nora Zidane1, Diana Sherafi1,
Alberto Paniz-Mondolfi2, Masanori Matsuoka3, G. Michael Taylor4, Helen D.
Donoghue4, Abi Bouwman5, Simon Mays6, Claire Watson7, Diana Lockwood7, Ali
Khamispour8, Yahya Dowlati8, Shen Jianping9, Thomas H. Rea10, Lucio Vera-Cabrera11,
Mariane M. Stefani12, Sayera Banu13, Murdo Macdonald14, Bishwa Raj Sapkota14, John
S. Spencer15, Jérôme Thomas16, Keith Harshman16, Pushpendra Singh17, Philippe
Busso17, Alexandre Gattiker18, Jacques Rougemont18, Patrick J. Brennan15, and Stewart
T. Cole17
1 Institut Pasteur, Paris, France
2 Instituto de Biomedicina, Caracas, Venezuela
3 Leprosy Research Centre, National Institute of Infectious Diseases, Tokyo, Japan
4 Centre for Infectious Diseases and International Health, Windeyer Institute, University College
London, London W1T 4JF, UK
5 Faculty of Life Sciences, Manchester Interdisciplinary Biocentre, University of Manchester,
Manchester M1 7DN, UK
6 Ancient Monuments Laboratory, English Heritage Centre for Archaeology, Fort Cumberland,
Portsmouth, UK.
7 London School of Hygiene and Tropical Medicine, Keppel St, London WC1E 7HT, UK
8 CRTSDL, Tehran University of Medical Sciences, Teheran, Iran
9 National Center for Leprosy Control, China CDC, Nanjing (210042), P. R. China
10Department of Dermatology, Keck School of Medicine, University of Southern California, Los
Angeles 90033, USA
11 Servicio de Dermatología, Hospital Universitario "José E. González", Monterrey, N.L., México
12Tropical Pathology and Public Health Institute at Federal University of Goias, Rua 235 esq. c/1ª
Avenida, S/N. Setor Universitario. Goiania, Goias, Brazil
13 International Centre for Diarrhoeal Disease Research, Bangladesh, Dhaka-1000, Bangladesh
14Leprosy Mission Nepal, Anandaban Hospital, Kathmandu, Nepal
15Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort
Collins, Colorado 80523-1682, USA
16 DNA Array Facility, Center for Integrative Genomics, University of Lausanne, CH-1015 Lausanne,
17 Global Health Institute, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne,
18 Bioinformatics and Biostatistics Core Facility, Ecole Polytechnique Fédérale de Lausanne, CH-
1015 Lausanne, Switzerland
* These authors contributed equally.
Correspondence to: Prof. S.T. Cole, Global Health Institute, EPFL, Station 15, CH-1015Lausanne, Switzerland Phone: +41 21 693 18 51. Fax: +41 21 693 17 90. E-mail:[email protected]
Reductive evolution and massive pseudogene formation have shaped the 3.31 Mb
genome of Mycobacterium leprae, an unculturable obligate pathogen, which causes
leprosy in humans. The complete genome sequence of strain Br4293 from Brazil was
obtained by conventional methods (6X coverage) and Illumina resequencing technology
was used to obtain those of strains Thai 53 (38X coverage) and NHDP63 (46X coverage)
from Thailand and the USA, respectively. Whole-genome comparisons with the
previously sequenced TN strain from India revealed that the four strains share 99.995%
sequence identity and only differ in 215 polymorphic sites, mainly SNPs, and by 5
pseudogenes. Sixteen interrelated SNP subtypes were defined by genotyping both extant
and extinct strains of M. leprae from around the world. The 16 SNP subtypes showed a
strong geographical association that reflects the migration patterns of early humans and
trade routes, with the Silk Road linking Europe to China having contributed to the
spread of leprosy.
Leprosy is a chronic, dermatological and neurological disease that results from infection with
the unculturable pathogen Mycobacterium leprae1 and causes nerve damage that can lead to
severe disabilities. There is no known reservoir for M. leprae other than human beings. New
opportunities for understanding the transmission of the leprosy bacillus and its phylogeny
have arisen following the determination of the complete 3.3-Mb genome sequence of the TN
strain, from Tamil Nadu, India2.
A notable feature of the M. leprae genome is the exceptionally large number of
pseudogenes, which occupy almost half of the TN chromosome2. The resulting loss of
function most likely accounts for the exceptionally slow growth rate of the bacillus and for
researchers' failure to culture it in vitro. Given this extensive genome decay, one might
expect to find more genetic variability between different isolates of M. leprae, but initial
analysis of SNPs demonstrated that these were very rare, occurring roughly once every 28 kb.
Furthermore, all extant isolates of M. leprae were nearly indistinguishable, belonging to one
of only four SNP types, and are derived from a single clone3. Variable number tandem
repeats (VNTRs) have also been investigated in M. leprae3-8 and, in some cases, have proved
useful for countrywide epidemiological surveys9. However, owing to variability in VNTR
profiles in samples taken from different sites on the same patient, their utility may be limited7,
The emerging discipline of microbial phylogeography is a powerful means of
monitoring not only the spread of microbes but also the movement of their hosts. For
instance, compelling associations were found between the genotypes of Helicobacter pylori
strains and their places of origin, and the migration and ethnicity of their human hosts12-14. M.
leprae is also proving useful in this respect, with its spread reflecting the migrations of early
humans3, and similar studies with tuberculosis patients suggest that Mycobacterium
tuberculosis lineages have also adapted to particular human populations15.
In the present work, the complete genome sequence of Br4923, a Brazilian strain of M.
leprae, has been determined and compared with that of the TN isolate, leading to the
discovery of a total of only 155 SNPs, of which 78 are informative. To deepen the
comparison and avoid possible ascertainment bias16, the genomes of strains from North
America and Thailand were resequenced using Illumina (Solexa) technology, revealing
comparably low levels of diversity. For phylogeographic purposes, the presence of the 78
informative SNPs was subsequently surveyed in ~400 isolates, enabling classification of M.
leprae into 16 SNP subtypes of limited geographic distribution that correlate with the patterns
of human migrations and trade routes.
RESULTSComplete genome sequence of Br4923
The Br4923 strain of M. leprae was chosen for complete genome analysis because it was
originally isolated from a patient in Brazil, the country with the second highest leprosy
burden, and because Brazil is geographically remote from India1, 17. The genome comprises
3,268,071 bp and is thus 141 bp smaller than that of the TN strain2. No evidence was found
for DNA inversions, translocations or duplications, and no transposition or amplification of
either the defective imsertion sequence elements or the four families of dispersed repeats was
detected18, consistent with the findings of an earlier quantitative PCR study3.
On alignment of the two genomes, 194 polymorphic sites were uncovered: these
correspond to 155 SNPs, 31 VNTR regions and eight insertion or deletion (indel) events. On
verification of the original sequence data, nine SNPs were found to stem from sequencing
errors2. The distribution of the true SNPs were as follows: 52 in genes encoding proteins, 39
in pseudogenes, 26 in noncoding regions (or as-yet-unidentified pseudogenes) and 38 in
dispersed repeats. The majority of the 31 VNTR regions were found to affect homopolymeric
tracts (HPT), di- and trinucleotide repeats, as well as some longer sequences, the longest of
which is 52 bp in length and present in two and three copies in the TN and Br4923 strains,
respectively. Variation in the di- and trinucleotide repeats is now well-documented4, 5, 8-11. It
is noteworthy that, at the VNTR loci, the repeat copy number was consistently higher in the
TN strain than in Br4923, as 23 of the 31 Brazilian VNTR were shorter than their Indian
counterparts. Six of the eight indels occured in repetitive elements and the others were in an
intergenic region and the gene ML0825c.
Recombination between dispersed repeats?
The SNPs associated with dispersed repeats deserve some comment, as they provide evidence
for genome plasticity in M. leprae. Variation between different copies of repeat family
members had previously been reported18, 19, but analysis of two complete genomes provided a
richer, more comprehensive dataset. Although all four repeat families (RLEP, REPLEP,
LEPRPT and LEPREP) were present in the same copy number and location in both genomes,
roughly half of the family members displayed sequence polymorphisms when pair-wise
comparisons were performed (Fig. 1). The number of polymorphic sites ranged from one in
LEPRPT and REPLEP to six in RLEP. With one exception, these resulted from G-A
transitions in the RLEP, LEPRPT and LEPREP elements or single-base indels in LEPREP or
REPLEP. The polymorphic sites tend to be occupied by A in the TN strain and by G in
Br4923. Variation in REPLEP occurs at position 636, which is occupied either by GGG or
GG (Fig. 1). Almost 25% of the total SNPs (38/155) occur in these repeats, which account
for a mere 1.16% of the genome. The over-representation of SNPs in these elements may
indicate that recombination events between different copies of the repetitive elements result in
the dispersal of a particular SNP. This interpretation is supported by the strain-specific bias
for A and G in the TN and Br4923 strains, respectively, and the finding that more differences
are found toward the center of the element rather than near its ends. In turn, these combined
findings render polymorphic sites in repetitive DNA unattractive as potential epidemiological
Search for informative SNPs
For phylogenetic and phylogeographic purposes, we determined which SNPs had been
inherited vertically and which were informative compared to those restricted to a single strain.
To do this, seven well-characterized strains of M. leprae (two of which were later
resequenced), representing all four known SNP types were examined for the presence of 117
SNPs (that is, all SNPs except those occurring in dispersed repeats). This systematic analysis
revealed that 78 SNPs were informative (66%) and 39 SNPs were uninformative (33%).
Identification of informative indels and VNTR
A similar approach was taken to uncover indels and VNTR offering phylogenetic potential,
and this resulted in the identification of two informative indels (InDel-1476519 and InDel-
978589, TN genome positions) and four informative homopolymeric tracts (HPT-741133,
HPT-3244472, HPT-1414666 and HPT-3041556). Given that we had failed to find a reliable
relationship between the SNP type and the pattern of six VNTR earlier, and in light of the
inherent instability of VNTRs10, 11, we did not pursue them further.
Resequencing isolates from North America and Thailand
Owing to the exceptionally high level of genome conservation, M. leprae is particularly
suitable for genome resequencing using Illumina technology. Deep coverage was obtained of
the genomes of the M. leprae strains Thai-53 (38X) and NHDP63 (46X) from Thailand and
the United States, respectively, and the reads were mapped onto the TN consensus sequence.
The entire genome was covered by the assembly except for the dispersed repeats that could
not be distinguished owing to the short read length. This analysis led to the identification of a
combined total of 201 SNPs and 14 single-base indels, many of which were shared with the
TN or Br4923 strains (Supplementary Table 1), and it uncovered five pseudogenes (Fig. 2a).
A distance matrix was then established (Fig. 2b) and used to construct a phylogenetic tree by
neighbour-joining20. The topology of the tree (Fig. 2c) is fully consistent with the previous
scheme derived by SNP typing3.
Synonymous and non-synonymous substitutions
The ratio of nonsynonymous to synonymous SNPs (dN/dS) is a popular method used to
predict whether any purifying selection is operating21. When the four genomes were
compared, we found 43 (42%) synonymous and 59 (58%) nonsynonymous changes in the
protein-coding genes (Supplementary Table 1); these are values similar to those reported for
the M. tuberculosis complex (38% and 62%, rspectively), in which SNPs are >10-fold more
abundant22. The dN/dS ratios were calculated within the M. leprae genes through pairwise
comparisons revealing an average value of 0.70. A ratio close to 1 is indicative of no, or
weak, selection against nonsynonymous SNPs as recently described for Salmonella typhi
(with a dN/dS =0.66)23. Like S. typhi, another obligate human pathogen, M. leprae also seems
to show little genetic drift, probably because of its small effective population size21.
Phylogenetic trees were constructed using the nonsynonymous and synonymous SNPs24, and
these had similar topology to that shown in Fig. 2c.
Phylogeographic survey of extant M. leprae
In our previous study, we had genotyped 175 strains, from 21 different geographic origins, but
found only four phylogenetic groups: SNP-types 1 – 4 (ref. 3). A close relationship was
established between the geographical origin of the strain and its genotype, giving a first
impression of how leprosy may have disseminated around the globe as humans migrated.
However, certain parts of the world were unrepresented, particularly the Middle East and
Europe. Because these regions were of historical importance in the migration of human
populations – notably the Middle East region, where early humans are thought to have arrived
from Africa and from whence the European and Asian migrations began25 – we made
intensive efforts to obtain samples from these settings. Samples were obtained from Iran and
Turkey in the Middle East, and from China, Korea and Japan in the Far East.
The 84 informative polymorphic sites were used in our survey (Fig. 3a,
Supplementary Table 2), and these sites enabled us to reconstruct the movement of leprosy,
between peoples and countries. A total of 400 samples from 28 regions were successfully
screened, resulting in the definition of 16 differentM. Leprae subtypes, as compared to the
four originally described3; these are referred to as A–P (where A corresponds to the TN isolate
while Br4923 is P, Fig. 3a). SNP-type 1 is now subdivided into four subtypes (A–D), SNP
type 2 into four subtypes (E–H), SNP type 3 into five subtypes (I–M) and SNP type 4 into
three subtypes (N–P). For instance, subtypes A and B differ from each other at one HPT and
21 SNP. Likewise, although there are no SNP differences within members of SNP type 4,
these can be subdivided based on differences in their indel or HPT profiles.
To exclude the possible effects of ascertainment bias16, we challenged this
evolutionary model by phylogenetic analysis using other actinobacteria, such as M.
tuberculosis, as an outgroup. Inspection of the positions corresponding to the M. leprae SNPs
in other genome sequences allowed the likely ancestral base to be deduced (Fig. 3b,
Supplementary Table 2). This was successful for 13 of the 15 groups of markers and
enabled us to establish a consensus ancestral sequence, which was then used in either
neighbor-joining or maximum likelihood analysis to produce a phylogenetic tree (Fig. 3c).
The topology of the trees was fully consistent with that of the scheme and the phylogenetic
tree for the four fully sequenced M. leprae strains (Figs. 2c and 3a), thereby confirming its
robustness.
An additional phylogenetic analysis was performed using maximum likelihood
analysis26, 27 to probe the relationship between the genotype of the M. leprae isolates and the
affected individual's country of origin. For completeness, an example of each genotype found
within a country was included, resulting in a sample size of 61. Overall there was a consistent
trend in the relationship between genotype and the geographic region (Fig. 4), and this trend
gives an indication of the likely direction of gene flow and dissemination of the leprosy
bacillus.
The global distribution of the 16 SNP subtypes was then plotted to show the frequency
of each genotype per country. Again, with the exception of islands3, this plot showed a
reasonably tight correlation between the country of origin of an individual, the M. leprae
genotype of the strain affecting the individual, and the known pattern of human migration
(Fig. 5). It is of interest that the M. leprae strains from both Turkey and Iran fall into the
same two groups, F and K. This is consistent with F being a precursor to strains that migrated
eastward with human populations, later giving rise to SNP type A found in India and
Southeast Asia, whereas group K may have been an ancestor for M. leprae associated with
westward migrations of humans that led to genotype M, associated with Europe and the
Americas, and genotype P, characteristic of South America. Findings with Chinese isolates of
M. leprae9 are also important, as, unlike many samples from Southeast Asia, these are not
genotype 1 but genotype 3, subtype K.
Phylogeography of ancient M. leprae
Studying ancient DNA (a-DNA) is a valuable yet challenging, approach, as this not only
enables us to obtain samples from countries where leprosy is extinct but also provides
information about different time periods and past epidemics. Skeletal remains were obtained
from leprosy graveyards in Croatia, Denmark, Egypt, England, Hungary and Turkey, and all
showed clear osteological evidence of lepromatous leprosy (Table 1). These remains were
first shown to be positive for M. leprae DNA by single-round RLEP PCR28 and then were
used to generate PCR products for sequence analysis of all three SNP typing loci, and, when
possible, the appropriate subtypes. In all 13 cases, the a-DNA samples were found to be of
SNP type 3, and seven of these were successfully subtyped. The a-DNA analysis revealed
that M. leprae from the UK belonged to SNP subtype 3I, whereas samples from Hungary were
of SNP subtypes 3K or 3M (Table 1). Both extinct and extant samples of M. leprae from
Turkey belonged to type 3K, whereas the 1,500-year-old Egyptian sample was found to be
SNP type 3. All of the archaeological cases exhibited different VNTR profiles, which assisted
in authentication of the a-DNA analysis (data not shown).
Here, we describe the complete genome sequence and comparative analysis of Br4923, a
Brazilian strain of M. leprae, and its use for the discovery of SNPs and other polymorphic
markers with phylogeographic potential. This finding was then complemented by genome
resequencing of strains from Thailand and the United States. When the four genome
sequences were compared, remarkably little genomic diversity was uncovered, consistent with
the hypothesis that leprosy has arisen from infection with a single clone, that has passed
through a recent evolutionary bottleneck2, 3. The four strains, which came from widely
separated countries, have genomes that are 99.995% identical. In terms of the diagnosis,
treatment and prevention of leprosy, this is extremely encouraging, as it means that antigenic
drift in M. leprae should be negligible and the sequences of drug targets will not vary.
Indeed, only 49 of the estimated 1,614 proteins in M. leprae show any amino acid change
(Supplementary Table 1).
In a recent comparative study with M. tuberculosis and Streptomyces coelicolor, it was
estimated that most of the mutations (56%) associated with pseudogenes or noncoding regions
in M. leprae could be attributed to transitions, mainly of the CT or GA types29. When the
noncoding or pseudogene-containing regions were compared in the two completely sequenced
M. leprae genomes, the frequency of these transitional mutations (59%) was found to be very
close to that value, whereas in the coding sequences, the frequency was 63.5%. As outlined
above, the same mutations are also dominant in the repetitive elements, where they account
for 97% of the changes. However, as these elements appear to undergo recombination (Fig.
1), this particular value may be misleading. Nonetheless, a decrease in the G+C content of a
genome is thought to be a hallmark of reductive evolution30, 31, and at 57.8%, M. leprae
clearly conforms to this rule, as the G+C content of most sequenced mycobacterial genomes is
~66%.
Five new pseudogenes were uncovered by this work (Fig. 2a), and their orthologs in
M. tuberculosis are all known or predicted to be nonessential32. The most noteworthy of these
pseudogenes was found in Br4923, where the counterpart of ML0825c in the TN strain has
acquired a frameshift mutation as the result of a thymidine insertion in codon 16 (TN position
978629). ML0825c encodes a transcriptional regulator, belonging to the ArsR family that is
predicted to repress target gene expression in the absence of heavy metal cations. In M.
tuberculosis, the ML0825c ortholog, rv2358 is the proximal gene in a bicistronic operon with
zur (furB), whose expression is repressed by rv2358 in a zinc-dependent manner33, 34. In an M.
tuberculosis zur mutant, expression of 32 genes was found to be upregulated35. It is not known
whether loss of ML0825c has any phenotypic consequences, but it is conceivable that, as a
result of polarity from the frameshift mutation, Zur may not be made, thus leading to
derepression of the Zur regulon. This mutation is found only in M. leprae strains belonging to
SNP subtypes 4O and 4P. The present study has also uncovered another useful marker for
SNP subtype 4P, Del-1476519, a single-base deletion restricted to strains from South America
or the Caribbean. We can conclude that loss of the corresponding thymidine must have
occurred within the last 500 years, since the introduction to Brazil of the ancestral strain from
West Africa during the slave trade.
Through genome comparisons, some authors have attempted to reconstruct the
evolution of M. leprae and to predict whether pseudogene formation was a gradual or
stochastic event36, 37. Using a novel approach based on the number of nonsynonymous
substitutions per site in pseudogenes, one group estimated that M. leprae and M. tuberculosis
diverged 66 million years ago and that a single pseudogenization event must have occurred in
the leprosy bacillus in the last 10 –20 million years36. Although we cannot predict the precise
date of pseudogene formation here, our data indicate that this still occurs.
The main goal of this study was to discover new polymorphic markers in order to
improve the resolution of epidemiological and phylogeographic studies of M. leprae. This
was successfully achieved, and interrogation of the 84 informative sites has led to the four
SNP types3 being resolved into 16 subtypes. Together with an increase in the number of
countries surveyed, this study has resulted in a much deeper level of understanding of the
global spread of leprosy, with confirmation of the relationship between SNP type and
geographical origin of the strains.
Paleomicrobiology was particularly helpful in this respect, and the analysis of ancient
M. leprae DNA, present in skeletal remains from countries where leprosy has been eradicated,
not only enabled us to expand our geographical coverage but also provided insight into the
genotypes of strains circulating in Europe, Turkey and Egypt as long as 1,500 years ago.
From the initial scheme for the evolution of the different M. leprae genotypes, it was
predicted that European isolates should belong to SNP type 3, and this was indeed found to be
the case (Table 1). Thanks to the new tools developed here, it was also possible to subtype
the isolates in some cases, despite the limiting amounts and extensive fragmentation of the a-
DNA. Examination of B116 (Table 1), the Egyptian skeleton from the Kellis-2 site in the
Dakhleh Oasis38, deserves some comment, as this specimen comes from a region close to the
proposed origin of both M. leprae and Homo sapiens. Based on genotype data from East
African strains of M. leprae, one might expect to find SNP type 2 in Egypt, whereas in reality
we found type 3. B116, who has a calibrated radiocarbon date of 445 + 50 years AD, in the
Roman period, was buried in close proximity to another individual with leprosy, termed B6.
Dietary studies using 15N stable isotopic analysis showed that both burials were outliers
compared to other inhumations at Kellis 2 and may therefore not have been of Egyptian
origin. Alternatively, from the phylogeny shown in Figure 3c, it is possible that when the
ancestor of M. leprae separated from the other mycobacteria, its genotype was between SNP
types 2 and 3.
The present phylogeographic scheme, based on results obtained with ~400 samples of
M. leprae from 28 different locations, extends considerably the hypothesis proposed earlier in
which the progenitor strain may have originated in East Africa and was SNP type 2 (ref. 3).
This then gave rise to SNP type 1, which spread eastward with humans into Asia and SNP
type 3, which disseminated westward into the Middle East and Europe before spawning the
type 4 strains that are found in West Africa and countries linked to West Africa by the slave
trade. Two new conclusions can be drawn from interpretation of the refined scheme (Figs.
3a, 4 and 5).
First, leprosy appears to have been introduced into Asia by two different routes: a
southern route, associated with the SNP type 1 strains encountered in the Indian subcontinent,
Indonesia and the Philippines, and a more northerly route starting in the Eastern
Mediterranean region and extending via Turkey and Iran to China and from there to Korea
and Japan. Here, strains with the 3K SNP subtype are preponderant, and the trade route
between Europe and Asia known as the Silk Road appears likely to have been a means of
transport and disease transmission (Fig. 5). The Black Death, caused by Yersinia pestis39, is
thought to have reached Europe in the fourteenth century from China via the Silk Road,
carried by humans and their fleas. For leprosy (Figure 3a) the opposite route of transmission
may have operated, with the disease originating in Europe or the Middle East and then
spreading to the Far East. Leprosy was thought to have reached China from India, in about
500 BC, and then to have spread from China to Japan40. This proposition is incompatible with
the data presented here.
Second, it seems unlikely that leprosy was introduced into the Americas by early
humans via the Bering straits; rather, it appears more probable that it was brought by
immigrants from Europe, as most of the M. leprae strains found in North, Central and South
America have the 3I genotype found in the European leprosy cases. This interpretation is
consistent with paleological findings because skeletons with signs of leprosy are limited to the
postcolonial period41.
Finally, it is worth discussing the enormous discrepancy between the period at which
pseudogene formation is thought to have arisen and the origin of early humans. It has been
estimated recently that the bulk of the pseudogenes in M. leprae arose no earlier than 9
million years ago36. Pseudogene formation is an indicator of radical change in the lifestyle of
the host bacterium, such as from the free-living to pathogenic state or of adaptation to life
within a particular tissue or cell type30, 31. In the case of M. leprae, obligate parasitism of
humans or another primate species would represent such a change. Although modern humans
represented by H. sapiens have existed only since approximately 250,000 years ago and left
Africa within the last 100,000 years to settle other regions, earlier hominids are thought to
have diverged from chimpanzees over 5 million years ago42. Reconciliation of the estimated
time of pseudogene formation with human evolution could be achieved if an ancestor of M.
leprae infected an early primate and then underwent genome decay, and was subsequently
transmitted vertically – although this seems unlikely, given that more genetic diversity among
M. leprae isolates would be expected if this were true. Alternatively, the genome decay could
well be ancient, but M. leprae may have only recently become a human pathogen. For
instance, it is conceivable that an ancestral form of M. leprae infected an invertebrate host
such as an insect, which later acted as a vector for transmitting the bacillus to humans.
Support for the latter scenario is provided by studies with the related pathogen Mycobacterium
ulcerans, which is at an early stage of reductive evolution43 and appears to be transmitted to
humans by water bugs and/or mosquitoes44, 45. Further insight into the timing of pseudogene
formation in M. leprae will be provided by microbiology and paleomicrobiology and by
deeper genome sequence analysis.
Methods and any associated references are available in the online version of the paper at
http://www.nature.com/naturegenetics/.
Accession codes. The Br4293 genome sequence, GenBank FM21192; reads from Thai53 and
NHDP63 resequencing analysis, SRP001064 (trace depository).
Note: Supplementary information is available on the Nature Genetics website
We thank the study participants and staff from the various leprosy clinics for their invaluable
participation and support; J. Molto, A. Marcsik, Y. Selim Erdal, E. Popescu, M. Slaus and J.
Boldsen for their encouragement and help with a-DNA samples; and L. Frangeul, A.
Kapopoulou and S. Uplekar for assistance with bioinformatics. This work received the
financial support of the Fondation Raoul Follereau, the Génopole programme, and the US
National Institutes of Health, National Institute of Allergy and Infectious Diseases (grant
RO1-AI47197-01A1 and contract NO1-AI25469).
M.M., N.H., P.J.B. and S.T.C. designed the study; A.P-M, M.Matsuoka, A.K., Y.D., S.J.,
T.H.R., L.V.-C., M.M.S, S.B., M.MacDonald, B.R.S. and J.S.S. contributed sources of M.
leprae DNA and demographic information; N.H., M.Monod, N.Z., D.S., T.G., J.T., K.H,
A.G., J.R., P.S. and P.B. performed DNA sequencing and bioinformatics; G.M.T., H.D.D.,
A.B., S.M., C.W. and D.L. provided, sequenced and analyzed a-DNA and contributed
archaeological insight; and S.T.C. wrote the manuscript with contributions from M.Monot,
N.H. and G.M.T. plus comments from all authors.
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Figure legends
Figure 1 SNP in dispersed repeats. The sites of SNP located within the RLEP, LEPREP,
REPLEP and LEPRPT elements present in the genomes of the TN and Br4923 strains are
shown and color coded. The position of the SNP is given above and the repeat identifier at
left. When occupied by the same nucleotide in both strains, the coloring is continuous: for
example, light blue for G. If the same change occurs within a repeat family, a different
continuous color is used: for example, light orange for A. When a site is polymorphic
between strains, discontinuous darker colors are used. Deletions are identified by *.
Figure 2 Polymorphisms and phylogeny. (a) Polymorphisms associated with new
pseudogenes with the position referring to the TN genome. (b) Diversity matrix based on
polymorphisms found in nonrepetitive sequences. Note that there are 21, 23, 65 and 56 SNP
and/or single-base indels restricted to the TN, Thai53, NHDP63 and Br4923 strains,
respectively. (c) Phylogenetic tree from neighbor joining based on polymorphisms from (b).
Figure 3 Typing system and phylogeny based on genome polymorphisms. (a) The panel
shows the four SNP types (1–4) and the 16 subtypes (A–P) used to classify strains of M.
leprae. Classification is based on SNP (green), HPT (blue) and indels (orange) with the bases
indicated representing one of only two possibilities found. Indels are denoted by –. The bases
shown are those associated with a given genotype. For instance, subtypes 1A and 1B differ
by 21 SNP and 1 HPT, for a total difference in 22 biodiversity markers. Note that for each
subtype the markers are present en bloc. (b) Scheme explaining how the ancestral versions of
polymorphic sites were deduced by comparison with an outgroup, in this case M. tuberculosis.
(c) Phylogenetic tree made by neighbor joining based on extant M. leprae sequences and
rooted with the ancestral sequence from (b) and Supplementary Table 2.
Figure 4. Maximum likelihood analysis and geographical distribution. The tree was rooted
between genotypes 2 and 3, and a single member of each genotype present per country was
included. For each replicate, tree leaves were attributed to 1 of 11 broad geographical
locations and a maximum likelihood inference of ancestral (geographical) states is shown.
Branch lengths are arbitrary and nodes with bootstrap support values above 60 are indicated
together with the distribution of ancestral states at each internal node. Vertical bars indicate
major geographical locations for each cluster.
Figure 5. Dissemination of leprosy throughout the world. Pillars are located in the country of
origin of the M. leprae sample and color coded according to the scheme for the 16 SNP
subtypes shown in Figure 3 (note this differs from the color scheme in Fig. 4). The thickness
of the pillar corresponds to the number of samples (1–5, thin; 6–29, intermediate; >30, broad).
The gray arrows indicate the migration routes of humans, with the estimated time of migration
in years shown25, 46. The red dots indicate the location of the Silk Road in the first century, and
* denotes result obtained from a-DNA.
Table 1 Ancient M. leprae DNA, details of cases studied and bone samples taken from archaeological sites
Country(Site)
Burial no.or
Age atdeath
Sex Samples Period(century
AD)
SNPtype
SNPsubtype
Croatia 2A3A
Both rhino-maxillary
8th-9th 33
Denmark G483 25–30 F Palatine 11th -15th 3 I/J –b
Egypt(Dakhleh oasis)
K2-B116 ~23 M 4th - 5th 3 K/L/M 38
England(Blackfriars,
Ipswich)1914 35–40 M
Maxillarypalatineprocess
13th -16th 3 I 28
England 112871150311784
Child30–4030–40
TibiaFoot bones
Hungary(Püspökladany)
Nasal regionNasal region
Hungary(Kiskundorozsma)
KD271 50–60 M Skull andlower limbs
7th 3 K 48
Turkey(Kovuklukaya)
KK 20/1 Mature F Nasal region 8th - 9th 3 K 49
a Not yet established. bC. Watson, D. Lockwood, personal communication.
ONLINE METHODS
Complete sequence of Br4923 and informatics. Br4923 was originally isolated in 1996
from the skin biopsy of a Brazilian patient, inoculated into nude mice, and its whole genome
sequence obtained from a pcDNA2.1 shotgun library. Briefly, 5µg of purified DNA was
sheared by nebulisation and cloned in the pcDNA2.1 vector using adapters as described
previously50. The sequencing template was obtained with a Templiphi kit (GE Healthcare),
submitted to BigDye Terminator v.3.1 cycle sequencing method and run in an ABI3730 DNA
sequencer (Applied Biosystems). The sequence was assembled from 21,300 reads, analyzed
and annotated using the Staden package (Trev, Gap4), Blast, Act and Artemis51-55, as
described previously2, 56.
Genome resequencing of Thai 53 and NHDP63 and informatics. The strains Thai53 and
NHDP63 were originally from a patient from Thailand and a native-born American from
Louisiana, United States, respectively. Genomic DNA fragment sequencing libraries were
prepared using the DNA Sample Prep Kit (Illumina) according to the protocol supplied with
the reagents and using 5µg of genomic DNA. DNA fragment libraries were loaded into one
(Thai53) or two lanes (NHDP63) of a flow cell and sequenced on the Genome Analyzer II
(Illumina) using the 36 Cycle Sequencing Kit version 1. Data were processed using the
Illumina Pipeline Software package version 1.0 and reads either mapped onto the TN
consensus sequence using MAQ57 or assembled using Edena58 and aligned to the reference
genome using blastn55. Data were managed using GAP4 and variations reported by direct
comparisons.
Phylogeny. Multiple alignments were generated using ClustalW59 and phylogenetic trees
calculated by the neighbor-joining or maximum likelihood methods using web-based software
(see URL). To calculate dN/dS ratios, we used the START 2 package60 with the Nei-
Gojobori method and the Jukes-Cantor correction61. For the phylogeographic analysis we
selected one SNP per linkage disequilibrium block (n=25) for each combination of
geographical location and genotype (n=61), excluding samples from islands. These sequences
were input to the RaxML software 26, 27, which produced a maximum likelihood tree and 200
bootstrap replicates. Trees were rooted between genotypes 2 and 3. For each replicate, the tree
leaves were attributed 1 of 11 broad geographical locations and a maximum likelihood
inference of ancestral (geographical) states was performed using the ape package under R 62.
The inferred transition rates were averaged over all replicates and applied to the maximum
likelihood tree (Fig. 4).
Sample preparation from extant M. leprae. Details of the specimens examined and their
origin may be found in Supplementary Table 3. In this study we performed PCR
amplification with >600 samples of M. leprae comprising purified DNA, fresh skin biopsies,
paraffin-embedded biopsy samples and slit-skin smears from microscope slides. M. leprae
cells and/or DNA were prepared differently according to the sample source. Fresh skin
biopsies were treated as described previously and DNA released by "freeze-boiling"63.
Paraffin-embedded specimens were heated at 60°C for 3 h, then the paraffin was removed by
two extractions with 1 ml of xylene for 15 min, which was followed by one extraction with
100% ethanol for 15 min. Tissue was progressively rehydrated by soaking in 30% ethanol
and, finally, in water. Then biopsies were minced with fine scissors followed by shearing in a
Qiagen lyser with 3 mm glass beads (10 min, maximum power). The supernatant was
transferred to an Eppendorf tube and 'freeze-boiled'. After removal of debris, the supernatant
was used directly in PCR. For slit-skin samples we used the Qiaamp DNA micro kit (Qiagen,
Inc.) to recover DNA from stained microscope slides.
Standard PCR, SNP analysis and sequence reactions. The seven well-characterized strains
of M. leprae used as sources of DNA for initial work were TN, India 2 and Thai53 (all SNP
type 1), Africa (SNP type 2), NHDP63 and NHDP98 (both SNP type 3) and Br4923 (SNP
type 4)3. Details of the primers used for genotyping may be found in Supplementary Table
4. Reactions (20 µl) typically contained M. leprae DNA from different samples, 10 mM Tris-
HCL (pH 9.0), 50 mM KCl, 1.5 mM MgCl2, 1.25 U Taq DNA polymerase (Q-Biogene) and
200 nM of forward and reverse primers. PCR was carried out for 45 cycles consisting of
denaturation at 94°C for 1 min, annealing at 55°C for 1 min and extension at 72°C for 2 min,
with a final extension at 72°C for 10 min in a thermocycler (PTC-100, MJ Research, Inc.). If
amplification failed, the PCR reaction was repeated after adding 1µl of T4GP32 (Q-Biogene).
After enzymatic treatment with Exonuclease I (USB, Corp.) and Shrimp Alkaline
Phosphatase (USB, Corp.), PCR products were submitted to BigDye Terminator version 3.1
cycle sequencing and analyzed using an ABI3100 DNA sequencer (Applied Biosystems).
Sequence data were analyzed as above51, 54.
Ancient DNA (a-DNA) studies. Details of samples taken for the bio-archaeological part of
the study are shown in Table 1. Measures to prevent cross-over contamination26 were
followed from the time of sampling. The strategy followed included (i) the use of multiple
extraction and template blanks, (ii) reproducibility, (iii) appropriate molecular behavior, (iv)
confirmation of product identity with sequencing, and (v) replication of key findings at a
separate center. One of the samples, burial 1914 from Ipswich, was sampled twice to provide
sufficient material for analysis at center 2, Manchester University. Gloves were worn and
changed between handling different skeletal components. Samples of bone were removed
from the skeleton using sterile disposable scalpel blades and transferred into sterile plastic
containers for transport to the laboratories. The work surface was cleaned between samples,
using a proprietary multisurface cleaner containing bleach.
a-DNA extraction, PCR and sequencing. At center 1, University College London, samples
of bone or scrapings were finely ground in autoclaved pestles and mortars and DNA from
bone powder extracted using the NucliSens kit from bioMérieux, as previously described28.
DNA extracts were stored at –20°C until assayed. At center 2, Manchester University, DNA
from burial 1914 was extracted independently using a modification of the method of Yang64,
which has previously been shown to be the most efficient of five tested methods for extraction
of a-DNA from bones65.
The presence of residual M. leprae DNA was first confirmed using a sensitive PCR
method, which amplifies the 37-copy repetitive element RLEP18, 19, 28. SNP typing methods
were developed to amplify fragmented DNA likely to persist in skeletal remains and applied
to the extracts. The oligonucleotide primers used for this and the key PCR conditions are
listed in Supplementary Table 5.
The Excite core kit (BioGene) was used for all PCR amplifications. This is a uracil-N-
glycosylase-ready kit suitable for real-time and routine hot-start PCR applications. SYBR
green was included in the PCR master mixes at a final dilution of 1/55,000, and reactions
were performed and monitored on the Corbett RotorGene 3000 real-time PCR platform
(Corbett Research) in a final volume of 25 l. Forty-five cycles of amplification were
performed for all methods. Melt analysis was performed using the RotorGene software and,
additionally, all products were run on 3% agarose gels. Template blanks containing water in
place of bone extract were alternated with samples in the RotorGene chamber and monitored
to ensure absence of contamination. Positive control samples were not amplified during the
course of the bioarchaeological study.
PCR products were separated on 3% (wt/vol) low-melting-point agarose (Invitrogen),
and bands were excised with a sterile scalpel blade and purified using a Geneclean III DNA
isolation kit (Fisher Life Sciences), then sequenced using Big Dye terminators, as outlined
URLs. [email protected], http://mobyle.pasteur.fr; NCBI short read archive,
http://www.ncbi.nlm.nih.gov/Traces/sra_sub/sub.cgi.
50. Garnier, T. et al. The complete genome sequence of Mycobacterium bovis. Proc Natl
Acad Sci U S A 100, 7877-82 (2003).
51. Bonfield, J.K., Smith, K.F. & Staden, R. A new DNA sequence assembly program.
Nucleic Acids Res 24, 4992-4999 (1995).
52. Carver, T.J. et al. ACT: the Artemis Comparison Tool. Bioinformatics 21, 3422-3
53. Rutherford, K. et al. Artemis: sequence visualization and annotation. Bioinformatics
16, 944-5 (2000).
54. Altschul, S.F., Gish, W., Miller, W., Myers, E.W. & Lipman, D.J. Basic local
alignment search tool. J Mol Biol 215, 403-10 (1990).
55. Altschul, S.F. et al. Gapped BLAST and PSI-BLAST: a new generation of protein
database search programs. Nucleic Acids Res 25, 3389-3402 (1997).
56. Stinear, T.P. et al. Insights from the complete genome sequence of Mycobacterium
marinum on the evolution of Mycobacterium tuberculosis. Genome Res 18, 729-41
57. Li, H., Ruan, J. & Durbin, R. Mapping short DNA sequencing reads and calling
variants using mapping quality scores. Genome Res 18, 1851-8 (2008).
58. Hernandez, D., Francois, P., Farinelli, L., Osteras, M. & Schrenzel, J. De novo
bacterial genome sequencing: millions of very short reads assembled on a desktop
computer. Genome Res 18, 802-9 (2008).
59. Thompson, J.D., Higgins, D.G. & Gibson, T.J. CLUSTAL W: improving the
sensitivity of progressive multiple sequence alignment through sequence weighting,
position-specific gap penalties and weight matrix choice. Nucleic Acids Res 22, 4673-
60. Jolley, K.A., Feil, E.J., Chan, M.S. & Maiden, M.C. Sequence type analysis and
recombinational tests (START). Bioinformatics 17, 1230-1 (2001).
61. Nei, M. & Gojobori, T. Simple methods for estimating the numbers of synonymous
and nonsynonymous nucleotide substitutions. Mol Biol Evol 3, 418-26 (1986).
62. Paradis, E., Claude, J. & Strimmer, K. APE: Analyses of Phylogenetics and Evolution
in R language. Bioinformatics 20, 289-90 (2004).
63. Woods, S.A. & Cole, S.T. A rapid method for the detection of potentially viable
Mycobacterium leprae in human biopsies: a novel application of PCR. FEMS
Microbiol Lett 53, 305-9 (1989).
64. Yang, D.Y., Waye, J.S., Dudar, J.C. & Saunders, S.R. Technical note: improved DNA
extraction from ancient bones using silica-based spin columns. American Journal of
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65. Bouwman, A.S. & Brown, T.A. Comparison between silica-based methods for the
extraction of DNA from human bones from 18th-19th century London. Ancient
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