Evolutionary history of the mtDNA 9-bp deletion in Chinese populations and its relevance to the peopling of east and southeast Asia

Whole Mitochondrial Genome Detection and Analysis of Two- to Four-Generation Maternal Pedigrees Using a New Massively Parallel Sequencing Panel

Mitochondrial DNA (mtDNA) is an effective genetic marker in forensic practice, especially for aged bones and hair shafts. Detection of the whole mitochondrial genome (mtGenome) using traditional Sanger-type sequencing is laborious and time-consuming. Additionally, its ability to distinguish point heteroplasmy (PHP) and length heteroplasmy (LHP) is limited. The application of massively parallel sequencing in mtDNA detection helps researchers to study the mtGenome in-depth. The ForenSeq mtDNA Whole Genome Kit, which contains a total of 245 short amplicons, is one of the multiplex library preparation kits for the mtGenome. We used this system to detect the mtGenome in the blood samples and hair shafts of thirty-three individuals from eight two-generation pedigrees, one three-generation pedigree, and one four-generation pedigree. High-quality sequencing results were obtained. Ten unique mtGenome haplotypes were observed in the mothers from the ten pedigrees. A total of 26 PHPs were observed using the interpretation threshold of 6%. Eleven types of LHPs in six regions were evaluated in detail. When considering homoplasmic variants only, consistent mtGenome haplotypes were observed between the twice-sequenced libraries and between the blood and hair shafts from the same individual and among maternal relatives in the pedigrees. Four inherited PHPs were observed, and the remainder were de novo/disappearing PHPs in the pedigrees. Our results demonstrate the effective capability of the ForenSeq mtDNA Whole Genome Kit to generate the complete mtGenome in blood and hair shafts, as well as the complexity of mtDNA haplotype comparisons between different types of maternal relatives when heteroplasmy is considered.

Mitochondrial DNA 7908-8816 region mutations in maternally inherited essential hypertensive subjects in China

Background

Nuclear genes or family-based mitochondrial screening have been the focus of genetic studies into essential hypertension. Studies into the role of mitochondria in sporadic Chinese hypertensives are lacking. The objective of the study was to explore the relationship between mitochondrial DNA (mtDNA) variations and the development of maternally inherited essential hypertension (MIEH) in China.

Methods

Yangzhou residents who were outpatients or in-patients at the Department of Cardiology in Northern Jiangsu People’s Hospital (Jiangsu, China) from June 2009 to June 2015 were recruited in a 1:1 case control study of 600 gender-matched Chinese MIEH subjects and controls. Genomic DNA was isolated from whole blood cells. The most likely sites for hypertension were screened using oligodeoxynucleotides at positions 7908–8816, purified and subsequently analyzed by direct sequencing according to the revised consensus Cambridge sequence. The frequency, density, type and conservative evolution of mtDNA variations were comprehensively analyzed.

Results

We found a statistical difference between the two groups for body mass index, waist circumference, abdominal circumference, triglyceride, low-density lipoprotein cholesterol, fasting blood glucose, uric acid, creatinine and blood urea nitrogen (P < 0.05). More amino-acid changes and RNA variants were found in MIEH subjects than the controls (P < 0.01). The detection system simultaneously identified 40 different heteroplasmic or homoplasmic mutations in 4 genes: COXII, tRNA^Lys, ATP8 and ATP 6. The mtDNA variations were mainly distributed in regions of ATP6 binding sites, and the site of highest mutation frequency was m. 8414C > T. Three changes in single bases (C8414T in ATP8, A8701G in ATP6 and G8584A in ATP6) were significantly different in the MIEH patients and the controls (P < 0.001). The m.8273_8281del mutation was identified from 59 MIEH patients.

Conclusions

Our results indicate that novel mtDNA mutations may be involved in the pathological process of MIEH, and mitochondrial genetic characteristics were identified in MIEH individuals.

Electronic supplementary material

The online version of this article (10.1186/s12920-018-0408-0) contains supplementary material, which is available to authorized users.

Carriers of mitochondrial DNA macrohaplogroup R colonized Eurasia and Australasia from a southeast Asia core area

BACKGROUND

The colonization of Eurasia and Australasia by African modern humans has been explained, nearly unanimously, as the result of a quick southern coastal dispersal route through the Arabian Peninsula, the Indian subcontinent, and the Indochinese Peninsula, to reach Australia around 50 kya. The phylogeny and phylogeography of the major mitochondrial DNA Eurasian haplogroups M and N have played the main role in giving molecular genetics support to that scenario. However, using the same molecular tools, a northern route across central Asia has been invoked as an alternative that is more conciliatory with the fossil record of East Asia. Here, we assess as the Eurasian macrohaplogroup R fits in the northern path.

RESULTS

Haplogroup U, with a founder age around 50 kya, is one of the oldest clades of macrohaplogroup R in western Asia. The main branches of U expanded in successive waves across West, Central and South Asia before the Last Glacial Maximum. All these dispersions had rather overlapping ranges. Some of them, as those of U6 and U3, reached North Africa. At the other end of Asia, in Wallacea, another branch of macrohaplogroup R, haplogroup P, also independently expanded in the area around 52 kya, in this case as isolated bursts geographically well structured, with autochthonous branches in Australia, New Guinea, and the Philippines.

CONCLUSIONS

Coeval independently dispersals around 50 kya of the West Asia haplogroup U and the Wallacea haplogroup P, points to a halfway core area in southeast Asia as the most probable centre of expansion of macrohaplogroup R, what fits in the phylogeographic pattern of its ancestor, macrohaplogroup N, for which a northern route and a southeast Asian origin has been already proposed.

Investigation on maternal lineage of a Neolithic group from northern Shaanxi based on ancient DNA

A magnetic bead purification method was successfully used to extract ancient DNA from the skeletal remains of 10 specimens excavated from Wuzhuangguoliang (Wzhgl) site, which was located in northern Shaanxi. The multidimensional scaling (MDS) and analysis of molecular variance approach (AMOVA) revealed that ancient Wzhgl people bored a very high similarity to southern Han Chinese. By constructing the MJ-network of various modern people including Han Chinese and Japanese, the phylogenetic analysis indicated that the Wzhgl population had close maternal distance with ancient Shandong and Xinjiang people. These findings indicated that Wzhgl contributed to the gene pool of Han Chinese and modern Japanese. In addition, population migration and interflow between Wzhgl people and ancient Shandong or Xinjiang probably occurred in Neolithic period.

Mitochondrial intergenic COII/tRNA(Lys) 9-bp deletion, a biomarker for hepatocellular carcinoma?

The COII/tRNA(Lys) intergenic 9-bp deletion is one of the most commonly studied human mitochondrial DNA (mtDNA) polymorphisms. It consists of the loss of one of two tandemly repeated copies of the sequence CCCCCTCTA from a non-coding region located between cytochrome oxidase II (COII) and tRNA(Lys) gene. Most recently, case-control studies have shown a positive association between this deletion with hepatocellular cancer. In this study, we first performed a detailed analysis between this deletion and clinical diseases; moreover, we took the phylogenetic approach to examine the pathogenicity status of 9-bp deletion.

Asian-specific mitochondrial genome polymorphism (9-bp deletion) in Hungarian patients with mitochondrial disease

A 9-bp deletion of the mtDNA is known as an anthropological marker of people with East-Asian origin. This 9-bp mtDNA deletion was analyzed in 1073 Hungarians with suspected mitochondrial disease and in 468 healthy control individuals. Fourteen cases with the 9-bp deletion were found in the cohort of mitochondrial patients, and one individual from 468 controls. In six cases the 9-bp deletion was present together with pathogenic major deletions in the mitochondrial genome. In one patient we found a frame shift mutation in the D-loop region, and in another family a pathogenic m.8322 A > G mutation in the tRNA(Lys) gene. Although the 9-bp deletion is common in the populations of the Pacific region and Asia, it is present in the Hungarian population as well. This 9-bp deletion may induce instability of the mtDNA and may provoke the introduction of other pathogenic mutations.

The mitochondrial DNA 9-bp deletion polymorphism is a risk factor for hepatocellular carcinoma in the Chinese population

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide. Although molecular biology of carcinogenesis and tumor progression of HCC has been increasingly understood with intense research in recent years, the molecular and cellular mechanisms of HCC pathogenesis are still poorly understood. In the present study, a case-control study including 390 HCC patients and 431 healthy controls was conducted to investigate the association of HCC susceptibility with the mitochondrial DNA (mtDNA) 9-bp deletion polymorphism in Chinese population. Chi-square testing showed that frequencies of 9-bp one repeat or two repeats were significantly different between HCC and control groups. Carriage of 9-bp one repeat fragment was associated with a significantly increased risk of developing HCC (odds ratio=1.48, 95% confidence interval: 1.03-2.14, p=0.027). Stratification analysis further showed that the differences between cases and controls were more obvious in drinkers than nondrinkers. Computational modeling of the 9-bp deletion polymorphism suggests that the mtDNA sequence without the 9-bp deletion polymorphism lies within a predicted binding site (seed region) for hsa-miR-519c-5p and hsa-miR-526a. Our data suggested that the 9-bp deletion polymorphism in mitochondria may influence HCC risk, likely through specific microRNA-mediated regulation, which was possibly involved in the pathogenesis of HCC. The replication of our studies in other populations with larger sample size is warranted.

Southeast Asian origins of five Hill Tribe populations and correlation of genetic to linguistic relationships inferred with genome-wide SNP data

In Thailand, the term Hill Tribe is used to describe populations whose members traditionally practice slash and burn agriculture and reside in the mountains. These tribes are thought to have migrated throughout Asia for up to 5,000 years, including migrations through Southern China and/or Southeast Asia. There have been continuous migrations southward from China into Thailand for approximately the past thousand years and the present geographic range of any given tribe straddles multiple political borders. As none of these populations have autochthonous scripts, written histories have until recently, been externally produced. Northern Asian, Tibetan, and Siberian origins of Hill Tribes have been proposed. All purport endogamy and have nonmutually intelligible languages. To test hypotheses regarding the geographic origins of these populations, relatedness and migrations among them and neighboring populations, and whether their genetic relationships correspond with their linguistic relationships, we analyzed 2,445 genome-wide SNP markers in 118 individuals from five Thai Hill Tribe populations (Akha, Hmong, Karen, Lahu, and Lisu), 90 individuals from majority Thai populations, and 826 individuals from Asian and Oceanean HGDP and HapMap populations using a Bayesian clustering method. Considering these results within the context of results ofrecent large-scale studies of Asian geographic genetic variation allows us to infer a shared Southeast Asian origin of these five Hill Tribe populations as well ancestry components that distinguish among them seen in successive levels of clustering. In addition, the inferred level of shared ancestry among the Hill Tribes corresponds well to relationships among their languages.

Molecular adaptation of modern human populations

Modern humans have gone through varied processes of genetic adaptations when their ancestors left Africa about 100,000 years ago. The environmental stresses and the social transitions (e.g., emergence of the Neolithic culture) have been acting as the major selective forces reshaping the genetic make-up of human populations. Genetic adaptations have occurred in many aspects of human life, including the adaptation to cold climate and high-altitude hypoxia, the improved ability of defending infectious diseases, and the polished strategy of utilizing new diet with the advent of agriculture. At the same time, the adaptations once developed during evolution may sometimes generate deleterious effects (e.g., susceptibility to diseases) when facing new environmental and social changes. The molecular (especially the genome-wide screening of genetic variations) studies in recent years have detected many genetic variants that show signals of Darwinian positive selection in modern human populations, which will not only provide a better understanding of human evolutionary history, but also help dissecting the genetic basis of human complex diseases.

Human evolution in Siberia: from frozen bodies to ancient DNA

BACKGROUND

The Yakuts contrast strikingly with other populations from Siberia due to their cattle- and horse-breeding economy as well as their Turkic language. On the basis of ethnological and linguistic criteria as well as population genetic studies, it has been assumed that they originated from South Siberian populations. However, many questions regarding the origins of this intriguing population still need to be clarified (e.g. the precise origin of paternal lineages and the admixture rate with indigenous populations). This study attempts to better understand the origins of the Yakuts by performing genetic analyses on 58 mummified frozen bodies dated from the 15th to the 19th century, excavated from Yakutia (Eastern Siberia).

RESULTS

High quality data were obtained for the autosomal STRs, Y-chromosomal STRs and SNPs and mtDNA due to exceptional sample preservation. A comparison with the same markers on seven museum specimens excavated 3 to 15 years ago showed significant differences in DNA quantity and quality. Direct access to ancient genetic data from these molecular markers combined with the archaeological evidence, demographical studies and comparisons with 166 contemporary individuals from the same location as the frozen bodies helped us to clarify the microevolution of this intriguing population.

CONCLUSION

We were able to trace the origins of the male lineages to a small group of horse-riders from the Cis-Baïkal area. Furthermore, mtDNA data showed that intermarriages between the first settlers with Evenks women led to the establishment of genetic characteristics during the 15th century that are still observed today.

Maternal footprints of Southeast Asians in North India

We have analyzed 7,137 samples from 125 different caste, tribal and religious groups of India and 99 samples from three populations of Nepal for the length variation in the COII/tRNA(Lys) region of mtDNA. Samples showing length variation were subjected to detailed phylogenetic analysis based on HVS-I and informative coding region sequence variation. The overall frequencies of the 9-bp deletion and insertion variants in South Asia were 1.9 and 0.6%, respectively. We have also defined a novel deep-rooting haplogroup M43 and identified the rare haplogroup H14 in Indian populations carrying the 9-bp deletion by complete mtDNA sequencing. Moreover, we redefined haplogroup M6 and dissected it into two well-defined subclades. The presence of haplogroups F1 and B5a in Uttar Pradesh suggests minor maternal contribution from Southeast Asia to Northern India. The occurrence of haplogroup F1 in the Nepalese sample implies that Nepal might have served as a bridge for the flow of eastern lineages to India. The presence of R6 in the Nepalese, on the other hand, suggests that the gene flow between India and Nepal has been reciprocal.

Redefinition of hypervariable region I in mitochondrial DNA control region and comparing its diversity among various ethnic groups

The hypervariable region I (HVR-I) of the mitochondrial DNA control region described in the literature is variable in its 5'and 3' ends as well as in its length, causing a problem when data from different ethnic groups are to be compared. To redefine HVR-I, which should be highly polymorphic yet relatively short in length, we analyzed 1437 reported sequences distributed among 11 geographic areas in the world. The results showed that the 237-bp (nts 16126-16362) redefined HVR-I (rHVR-I) had a global genetic diversity of 0.9905 and the 154-bp (nts 16209-16362) short HVR-I (sHVR-I) had a global diversity of 0.9735. Being flanked by a stretch of highly conservative sequences, both rHVR-I and sHVR-I can be produced by PCR, even if extracted from badly degraded specimens. Comparing the genetic diversity among 3870 sequences from 25 countries, we found that the genetic diversity of rHVR-I was 0.9869+/-0.0133 in Asian countries, 0.9685+/-0.0193 in African countries, 0.9299+/-0.0664 in European countries, and 0.8477+/-0.1857 in American countries, whereas that of sHVR-I was 0.9689+/-0.0284 in Asian countries, 0.9504+/-0.0334 in African countries, 0.8721+/-0.0911 in European countries, and 0.8230+/-0.1693 in American countries. The difference in genetic diversity among these countries is consistent with the notion that genetic diversity roughly reflects the genetic history of a given ethnic group. Our results indicate that a polymorphic, short, and PCR-producible HVR-I can be defined, making the comparison among various ethnic groups possible.

CYP2C19 genetic polymorphism in Thai, Burmese and Karen populations

The genetic polymorphism of CYP2C19 was examined in three Southeast Asian populations. This study was conducted in 774 Thais, 127 Burmeses and 131 Karens. Genomic DNA was extracted from leucocytes and analyzed by the PCR-RFLP technique. Genotype analysis revealed that the allele frequencies of CYP2C19*1, CYP2C19*2 and CYP2C19*3 in the Thais were 0.68, 0.29 and 0.03, respectively, and those of the Burmese population were 0.66, 0.30 and 0.04, respectively. For Karens, the minority ethnic in Mynmar, the allele frequencies of CYP2C19*1, CYP2C19*2 and CYP2C19*3 were 0.71, 0.28 and 0.01, respectively. The prevalence of PM estimated from genotype data among these three ethnic populations were 9.2%, 11.0%, and 8.4%, respectively. The PM phenotype and the frequencies of CYP2C19 defective alleles, particularly CYP2C19*3 among these three Southeast Asian ethnics appeared to be lower than other Asian populations. Lower prevalence of CYP2C19 PM suggests that these ethnics may have different capacity to metabolize drugs that are substrates of CYP2C19. Certain drug dosage regiments should be considered differently for Asian populations.

Genotyping of eight polymorphic genes encoding drug-metabolizing enzymes and transporters using a customized oligonucleotide array

Polymorphisms in drug-metabolizing genes may lead to the production of dysfunctional proteins and consequently affect therapeutic efficacy and toxicity of drugs. Different frequencies of polymorphic alleles among the races have been postulated to account for the observed ethnic variations in drug responses. In the current study, we aimed to estimate the frequencies of 14 polymorphisms in eight genes (TPMT, NQO1, MTHFR, GSTP1, CYP1A1, CYP2D6, ABCB1, and SLC19A1) in the Singapore multiracial populations by screening 371 cord blood samples from healthy newborns. To improve genotyping efficacy, we designed an oligonucleotide array based on the principle of allele-specific primer extension (AsPEX) that was capable of detecting the 14 polymorphisms simultaneously. Cross-validation using conventional polymerase chain reaction-restriction fragment-length polymorphism (PCR-RFLP) assays demonstrated 99% concordant results. Measurements on the fluorescent intensity displayed clear distinctions among different genotypes. Statistical analyses showed significantly different allele distributions in several genes among the three races, namely Chinese, Malays, and Indians. Comparing the allelic frequencies in Chinese with previous studies in Caucasian populations, NQO1 609C>T and SLC19A1 80G>A were distinctly different, whereas close similarity was observed for MTHFR 677C>T. We have demonstrated an array-based methodology for rapid multiplex detection of genetic polymorphisms. The allelic frequencies reported in this study may have important therapeutic and prognostic implications in the clinical use of relevant drugs.

Coding region mitochondrial DNA SNPs: targeting East Asian and Native American haplogroups

We have developed a single PCR multiplex SNaPshot reaction that consists of 32 coding region SNPs that allows (i) increasing the discrimination power of the mitochondrial DNA (mtDNA) typing in forensic casework, and (ii) haplogroup assignments of mtDNA profiles in both human population studies (e.g. anthropological) and medical research. The selected SNPs target the East Asian phylogeny, including its Native American derived branches. We have validated this multiplex assay by genotyping a sample of East Asians (Taiwanese) and Native Americans (Argentineans). In addition to the coding SNP typing, we have sequenced the complete control region for the same samples. The genotyping results (control region plus SNaPshot profiles) are in good agreement with previous human population genetic studies (based on e.g. complete sequencing) and the known mtDNA phylogeny. We observe that the SNaPshot method is reliable, rapid, and cost effective in comparison with other techniques of multiplex SNP genotyping. We discuss the advantages of our SNP genotyping selection with respect to previous attempts, and we highlight the importance of using the known mtDNA phylogeny as a framework for SNP profile interpretation and as a tool to minimize genotyping errors.

High prevalence of the COII/tRNA(Lys) intergenic 9-bp deletion in mitochondrial DNA of Taiwanese patients with MELAS or MERRF syndrome

The COII/tRNA(Lys) intergenic 9-bp deletion (MIC9D) of mitochondrial DNA (mtDNA) has been established as a genetic polymorphism for Asian-Pacific populations. We investigated whether this small mtDNA deletion is co-transmitted with human diseases such as mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) and myoclonic epilepsy with ragged-red fibers (MERRF) syndromes. Forty unrelated Taiwanese families, including 12 families with MERRF and A8344G mtDNA mutation and 28 families with MELAS and A3243G mutation of mtDNA, respectively, were recruited in this study. In addition, 199 healthy subjects were recruited as control. We found that the frequency of occurrence of mtDNA with the MIC9D polymorphism in healthy subjects was 21% (41/199). However, the incidence of the MIC9D polymorphism was 67% (8/12) among the probands of all the families with MERRF syndrome (P = 0.001; OR = 8) and 39% (11/28) among the probands of the families with MELAS syndrome (P = 0.038; OR = 2). This finding indicates that the frequency of occurrence of mtDNA with the MIC9D polymorphism in patients with MERRF or MELAS syndrome is higher than that of healthy subjects. The prevalence of mitochondrial encephalomyopathies in relation to the MIC9D polymorphism of mtDNA in Taiwanese population is discussed.

Asian and non-Asian origins of Mon-Khmer- and Mundari-speaking Austro-Asiatic populations of India

In the present study, we analyzed 1,686 samples from 31 tribal populations of India for the mitochondrial DNA 9-base-pair deletion/insertion polymorphism, and characterized them based on the relevant mitochondrial DNA coding-region single nucleotide polymorphisms and hypervariable region I motifs, to test the genetic origins of the ethnically and linguistically heterogeneous Austro-Asiatic tribes of India. A comparative analysis of our results with the existing data suggests multiple origins of Austro-Asiatic tribes in India, and particularly the Asian and non-Asian origins of the Mon-Khmer and the Mundari populations. We also identified a novel subclade of haplogroup B in the Mon-Khmer Khasi tribes that distinguishes them from the Nicobarese, indicating two different waves of migration of the Mon-Khmer tribes in India.

Y-chromosome and mitochondrial DNA studies on the population structure of the Christmas Island community

Christmas Island is a remote Australian territory located close to the main Indonesian island of Java. Y-chromosome and mitochondrial DNA (mtDNA) markers were used to investigate the genetic structure of the population, which comprises communities of mixed ethnic origin. Analysis of 12 Y-chromosome biallelic polymorphisms revealed a high level of gene diversity and haplotype frequencies that were consistent with source populations in southern China and Southeast Asia. mtDNA hypervariable segment I (HVS-I) sequences displayed high levels of haplotype diversity and nucleotide diversity that were comparable to various Asian populations. Genetic distances revealed extremely low mtDNA differentiation among Christmas Islanders and Asian populations. This was supported by the relatively high proportion of sequence types shared among these populations. The most common mtDNA haplogroups were M* and B, followed by D and F, which are prevalent in East/Southeast Asia. Christmas Islanders of European descent were characterized by the Eurasian haplogroup R*, and a limited degree of admixture was observed. In general, analysis of the genetic data indicated population affinities to southern Chinese (in particular from the Yunnan Province) and Southeast Asia (Thailand, Malaysia, and Cambodia), which was consistent with historical records of settlement. The combined use of these different marker systems provides a useful and appropriate model for the study of contemporary populations derived from different ethnic origins.

Mitochondrial DNA diversity in indigenous populations of the southern extent of Siberia, and the origins of Native American haplogroups

In search of the ancestors of Native American mitochondrial DNA (mtDNA) haplogroups, we analyzed the mtDNA of 531 individuals from nine indigenous populations in Siberia. All mtDNAs were subjected to high-resolution RFLP analysis, sequencing of the control-region hypervariable segment I (HVS-I), and surveyed for additional polymorphic markers in the coding region. Furthermore, the mtDNAs selected according to haplogroup/subhaplogroup status were completely sequenced. Phylogenetic analyses of the resulting data, combined with those from previously published Siberian arctic and sub-arctic populations, revealed that remnants of the ancient Siberian gene pool are still evident in Siberian populations, suggesting that the founding haplotypes of the Native American A-D branches originated in different parts of Siberia. Thus, lineage A complete sequences revealed in the Mansi of the Lower Ob and the Ket of the Lower Yenisei belong to A1, suggesting that A1 mtDNAs occasionally found in the remnants of hunting-gathering populations of northwestern and northern Siberia belonged to a common gene pool of the Siberian progenitors of Paleoindians. Moreover, lineage B1, which is the most closely related to the American B2, occurred in the Tubalar and Tuvan inhabiting the territory between the upper reaches of the Ob River in the west, to the Upper Yenisei region in the east. Finally, the sequence variants of haplogroups C and D, which are most similar to Native American C1 and D1, were detected in the Ulchi of the Lower Amur. Overall, our data suggest that the immediate ancestors of the Siberian/Beringian migrants who gave rise to ancient (pre-Clovis) Paleoindians have a common origin with aboriginal people of the area now designated the Altai-Sayan Upland, as well as the Lower Amur/Sea of Okhotsk region.

Different population histories of the Mundari- and Mon-Khmer-speaking Austro-Asiatic tribes inferred from the mtDNA 9-bp deletion/insertion polymorphism in Indian populations

Length variation in the human mtDNA intergenic region between the cytochrome oxidase II (COII) and tRNA lysine (tRNA(lys)) genes has been widely studied in world populations. Specifically, Austronesian populations of the Pacific and Austro-Asiatic populations of southeast Asia most frequently carry the 9-bp deletion in that region implying their shared common ancestry in haplogroup B. Furthermore, multiple independent origins of the 9-bp deletion at the background of other mtDNA haplogroups has been shown in populations of Africa, Europe, Australia, and India. We have analyzed 3293 Indian individuals belonging to 58 populations, representing different caste, tribal, and religious groups, for the length variation in the 9-bp motif. The 9-bp deletion (one copy) and insertion (three copies) alleles were observed in 2.51% (2.15% deletion and 0.36% insertion) of the individuals. The maximum frequency of the deletion (45.8%) was observed in the Nicobarese in association with the haplogroup B5a D-loop motif that is common throughout southeast Asia. The low polymorphism in the D-loop sequence of the Nicobarese B5a samples suggests their recent origin and a founder effect, probably involving migration from southeast Asia. Interestingly, none of the 302 (except one Munda sample, which has 9-bp insertion) from Mundari-speaking Austro-Asiatic populations from the Indian mainland showed the length polymorphism of the 9-bp motif, pointing either to their independent origin from the Mon-Khmeric-speaking Nicobarese or to an extensive admixture with neighboring Indo-European-speaking populations. Consistent with previous reports, the Indo-European and Dravidic populations of India showed low frequency of the 9-bp deletion/insertion. More than 18 independent origins of the deletion or insertion mutation could be inferred in the phylogenetic analysis of the D-loop sequences.

Genetic structure of Hmong-Mien speaking populations in East Asia as revealed by mtDNA lineages

Hmong-Mien (H-M) is a major language family in East Asia, and its speakers distribute primarily in southern China and Southeast Asia. To date, genetic studies on H-M speaking populations are virtually absent in the literature. In this report, we present the results of an analysis of genetic variations in the mitochondrial DNA (mtDNA) hypervariable segment 1 (HVS1) region and diagnostic variants in the coding regions in 537 individuals sampled from 17 H-M populations across East Asia. The analysis showed that the haplogroups that are predominant in southern East Asia, including B, R9, N9a, and M7, account for 63% (ranging from 45% to 90%) of mtDNAs in H-M populations. Furthermore, analysis of molecular variance (AMOVA), phylogenetic tree analysis, and principal component (PC) analysis demonstrate closer relatedness between H-M and other southern East Asians, suggesting a general southern origin of maternal lineages in the H-M populations. The estimated ages of the mtDNA lineages that are specific to H-M coincide with those based on archeological cultures that have been associated with H-M. Analysis of genetic distance and phylogenetic tree indicated some extent of difference between the Hmong and the Mien populations. Together with the higher frequency of north-dominating lineages observed in the Hmong people, our results indicate that the Hmong populations had experienced more contact with the northern East Asians, a finding consistent with historical evidence. Moreover, our data defined some new (sub-)haplogroups (A6, B4e, B4f, C5, F1a1, F1a1a, and R9c), which will direct further efforts to improve the phylogeny of East Asian mtDNAs.

Mitochondrial genome variation in eastern Asia and the peopling of Japan

To construct an East Asia mitochondrial DNA (mtDNA) phylogeny, we sequenced the complete mitochondrial genomes of 672 Japanese individuals (http://www.giib.or.jp/mtsnp/index_e.html). This allowed us to perform a phylogenetic analysis with a pool of 942 Asiatic sequences. New clades and subclades emerged from the Japanese data. On the basis of this unequivocal phylogeny, we classified 4713 Asian partial mitochondrial sequences, with <10% ambiguity. Applying population and phylogeographic methods, we used these sequences to shed light on the controversial issue of the peopling of Japan. Population-based comparisons confirmed that present-day Japanese have their closest genetic affinity to northern Asian populations, especially to Koreans, which finding is congruent with the proposed Continental gene flow to Japan after the Yayoi period. This phylogeographic approach unraveled a high degree of differentiation in Paleolithic Japanese. Ancient southern and northern migrations were detected based on the existence of basic M and N lineages in Ryukyuans and Ainu. Direct connections with Tibet, parallel to those found for the Y-chromosome, were also apparent. Furthermore, the highest diversity found in Japan for some derived clades suggests that Japan could be included in an area of migratory expansion to Continental Asia. All the theories that have been proposed up to now to explain the peopling of Japan seem insufficient to accommodate fully this complex picture.

Current limitations of SNP data from the public domain for studies of complex disorders: a test for ten candidate genes for obesity and osteoporosis

BACKGROUND

Public SNP databases are frequently used to choose SNPs for candidate genes in the association and linkage studies of complex disorders. However, their utility for such studies of diseases with ethnic-dependent background has never been evaluated.

RESULTS

To estimate the accuracy and completeness of SNP public databases, we analyzed the allele frequencies of 41 SNPs in 10 candidate genes for obesity and/or osteoporosis in a large American-Caucasian sample (1,873 individuals from 405 nuclear families) by PCR-invader assay. We compared our results with those from the databases and other published studies. Of the 41 SNPs, 8 were monomorphic in our sample. Twelve were reported for the first time for Caucasians and the other 29 SNPs in our sample essentially confirmed the respective allele frequencies for Caucasians in the databases and previous studies. The comparison of our data with other ethnic groups showed significant differentiation between the three major world ethnic groups at some SNPs (Caucasians and Africans differed at 3 of the 18 shared SNPs, and Caucasians and Asians differed at 13 of the 22 shared SNPs). This genetic differentiation may have an important implication for studying the well-known ethnic differences in the prevalence of obesity and osteoporosis, and complex disorders in general.

CONCLUSION

A comparative analysis of the SNP data of the candidate genes obtained in the present study, as well as those retrieved from the public domain, suggests that the databases may currently have serious limitations for studying complex disorders with an ethnic-dependent background due to the incomplete and uneven representation of the candidate SNPs in the databases for the major ethnic groups. This conclusion attests to the imperative necessity of large-scale and accurate characterization of these SNPs in different ethnic groups.

Mitochondrial DNA sequence polymorphisms of five ethnic populations from northern China

To study the mitochondrial DNA (mtDNA) polymorphisms in a total of 232 individuals from five ethnic populations (Daur, n=45; Ewenki, n=47; Korean, n=48; Mongolian, n=48; Oroqen, n=44) in northern China, we analyzed the control region sequences and typed for a number of characteristic mutations in coding regions (especially the region 14576-16047), by direct sequencing or restriction-fragment-length-polymorphism (RFLP) analysis. With the exception of 14 individuals belonging to the European-specific haplogroups R2, H, J, and T, the mtDNAs considered could be assigned into the East Asian-specific haplogroups described recently. The polymorphisms in cytochrome b sequence were found to be very informative for defining or supporting the haplogroups status of East Asian mtDNAs in addition to the reported regions 10171-10659 and 14055-14590 in our previous study. The haplogroup distribution frequencies varied in the five ethnic populations, but in general they all harbored a large amount of north-prevalent haplogroups, such as D, G, C, and Z, and thus were in agreement with their ethnohistory of northern origin. The two populations (Ewenki and Oroqen) with small population census also show concordant features in their matrilineal genetic structures, with lower genetic diversities observed.

Theoretical analysis of mutation hotspots and their DNA sequence context specificity

Mutation frequencies vary significantly along nucleotide sequences such that mutations often concentrate at certain positions called hotspots. Mutation hotspots in DNA reflect intrinsic properties of the mutation process, such as sequence specificity, that manifests itself at the level of interaction between mutagens, DNA, and the action of the repair and replication machineries. The hotspots might also reflect structural and functional features of the respective DNA sequences. When mutations in a gene are identified using a particular experimental system, resulting hotspots could reflect the properties of the gene product and the mutant selection scheme. Analysis of the nucleotide sequence context of hotspots can provide information on the molecular mechanisms of mutagenesis. However, the determinants of mutation frequency and specificity are complex, and there are many analytical methods for their study. Here we review computational approaches for analyzing mutation spectra (distribution of mutations along the target genes) that include many mutable (detectable) positions. The following methods are reviewed: derivation of a consensus sequence, application of regression approaches to correlate nucleotide sequence features with mutation frequency, mutation hotspot prediction, analysis of oligonucleotide composition of regions containing mutations, pairwise comparison of mutation spectra, analysis of multiple spectra, and analysis of "context-free" characteristics. The advantages and pitfalls of these methods are discussed and illustrated by examples from the literature. The most reliable analyses were obtained when several methods were combined and information from theoretical analysis and experimental observations was considered simultaneously. Simple, robust approaches should be used with small samples of mutations, whereas combinations of simple and complex approaches may be required for large samples. We discuss several well-documented studies where analysis of mutation spectra has substantially contributed to the current understanding of molecular mechanisms of mutagenesis. The nucleotide sequence context of mutational hotspots is a fingerprint of interactions between DNA and DNA repair, replication, and modification enzymes, and the analysis of hotspot context provides evidence of such interactions.

Native American mtDNA prehistory in the American Southwest

This study examines the mtDNA diversity of the proposed descendants of the multiethnic Hohokam and Anasazi cultural traditions, as well as Uto-Aztecan and Southern-Athapaskan groups, to investigate hypothesized migrations associated with the Southwest region. The mtDNA haplogroups of 117 Native Americans from southwestern North America were determined. The hypervariable segment I (HVSI) portion of the control region of 53 of these individuals was sequenced, and the within-haplogroup diversity of 18 Native American populations from North, Central, and South America was analyzed. Within North America, populations in the West contain higher amounts of diversity than in other regions, probably due to a population expansion and high levels of gene flow among subpopulations in this region throughout prehistory. The distribution of haplogroups in the Southwest is structured more by archaeological tradition than by language. Yumans and Pimans exhibit substantially greater genetic diversity than the Jemez and Zuni, probably due to admixture and genetic isolation, respectively. We find no evidence of a movement of mtDNA lineages northward into the Southwest from Central Mexico, which, in combination with evidence from nuclear markers, suggests that the spread of Uto-Aztecan was facilitated by predominantly male migration. Southern Athapaskans probably experienced a bottleneck followed by extensive admixture during the migration to their current homeland in the Southwest.

Genetic relationship of Chinese ethnic populations revealed by mtDNA sequence diversity

The origin and demographic history of the ethnic populations of China have not been clearly resolved. In this study, we examined the hypervariable segment I sequences (HVSI) of the mitochondrial DNA control region in 372 individuals from nine Chinese populations and one northern Thai population. A relatively high percentage of individuals was found to share sequences with those from other populations of the same ethnogenesis. In general, the populations of southern or Pai-Yuei tribal origin showed high haplotype diversity and nucleotide diversity compared with the populations of northern or Di-Qiang tribal origin. Mismatch distributions from these populations showed concordant features. All except the northern groups Nu, Lisu, Tibetan, and Mongolian showed typical signatures of ancient population expansions in the mismatch distributions and neutrality tests. Episodes of extreme size reduction in the past are one of the likely explanations for the absence of evidence of expansion in northern populations. Small sample sizes as well as samples from isolated subpopulations contributed to the bumpy mismatch distributions observed. Phylogenetic analysis and haplotype sharing among populations suggest that current mtDNA variation in these ethnic populations could reveal their ethnohistory to some extent, but in general, linguistic and geographic classifications of the populations did not agree well with classification by mtDNA variation.

H. pylori and gastric cancer: the Asian enigma

The actual distribution of Helicobacter pylori infection and its related diseases in various Asian countries is controversial. Only limited information is available regarding this issue. We discuss the etiological role of H. pylori in gastric cancer through the Asian experience. Seroprevalence of H. pylori infection in asymptomatic subjects and the annual incidence rate of gastric cancer per 100,000 in various Asian countries are summarized from literature reviews and World Health Organization statistics, respectively. There is a large intercountry variation in incidence of gastric cancer and H. pylori seroprevalence among Asian countries. There is a strong link between H. pylori infection and gastric cancer in many countries, such as Japan. By contrast, the prevalence of H. pylori infection is high in some countries, including India and Bangladesh, but low gastric cancer rates have been reported. These disparate observations represent the Asian enigma. Factors that may influence the etiology of gastric cancer include the genetic diversity of the infecting H. pylori strains and differences in the host genetic background in various ethnic groups, including gastric acid secretion and genetic polymorphisms in proinflammatory cytokines. These factors, in addition to environmental factors, such as personal hygiene and dietary habits, reflect the multifactorial etiology of gastric cancer.

The structure of diversity within New World mitochondrial DNA haplogroups: implications for the prehistory of North America

The mitochondrial DNA haplogroups and hypervariable segment I (HVSI) sequences of 1,612 and 395 Native North Americans, respectively, were analyzed to identify major prehistoric population events in North America. Gene maps and spatial autocorrelation analyses suggest that populations with high frequencies of haplogroups A, B, and X experienced prehistoric population expansions in the North, Southwest, and Great Lakes region, respectively. Haplotype networks showing high levels of reticulation and high frequencies of nodal haplotypes support these results. The haplotype networks suggest the existence of additional founding lineages within haplogroups B and C; however, because of the hypervariability exhibited by the HVSI data set, similar haplotypes exhibited in Asia and America could be due to convergence rather than common ancestry. The hypervariability and reticulation preclude the use of estimates of genetic diversity within haplogroups to argue for the number of migrations to the Americas.

Phylogeographic differentiation of mitochondrial DNA in Han Chinese

To characterize the mitochondrial DNA (mtDNA) variation in Han Chinese from several provinces of China, we have sequenced the two hypervariable segments of the control region and the segment spanning nucleotide positions 10171-10659 of the coding region, and we have identified a number of specific coding-region mutations by direct sequencing or restriction-fragment-length-polymorphism tests. This allows us to define new haplogroups (clades of the mtDNA phylogeny) and to dissect the Han mtDNA pool on a phylogenetic basis, which is a prerequisite for any fine-grained phylogeographic analysis, the interpretation of ancient mtDNA, or future complete mtDNA sequencing efforts. Some of the haplogroups under study differ considerably in frequencies across different provinces. The southernmost provinces show more pronounced contrasts in their regional Han mtDNA pools than the central and northern provinces. These and other features of the geographical distribution of the mtDNA haplogroups observed in the Han Chinese make an initial Paleolithic colonization from south to north plausible but would suggest subsequent migration events in China that mainly proceeded from north to south and east to west. Lumping together all regional Han mtDNA pools into one fictive general mtDNA pool or choosing one or two regional Han populations to represent all Han Chinese is inappropriate for prehistoric considerations as well as for forensic purposes or medical disease studies.

Major genomic mitochondrial lineages delineate early human expansions

BACKGROUND

The phylogeographic distribution of human mitochondrial DNA variations allows a genetic approach to the study of modern Homo sapiens dispersals throughout the world from a female perspective. As a new contribution to this study we have phylogenetically analysed complete mitochondrial DNA(mtDNA) sequences from 42 human lineages, representing major clades with known geographic assignation.

RESULTS

We show the relative relationships among the 42 lineages and present more accurate temporal calibrations than have been previously possible to give new perspectives as how modern humans spread in the Old World.

CONCLUSIONS

The first detectable expansion occurred around 59,000-69,000 years ago from Africa, independently colonizing western Asia and India and, following this southern route, swiftly reaching east Asia. Within Africa, this expansion did not replace but mixed with older lineages detectable today only in Africa. Around 39,000-52,000 years ago, the western Asian branch spread radially, bringing Caucasians to North Africa and Europe, also reaching India, and expanding to north and east Asia. More recent migrations have entangled but not completely erased these primitive footprints of modern human expansions.