Major factors influencing linkage disequilibrium by analysis of different chromosome regions in distinct populations: demography, chromosome recombination frequency and selection

Associations of + 138 Ins/del A and + 5665 G/T polymorphisms of endothelin-1 gene with hypertension in Burmese people in Magway, Myanmar

Background

Hypertension is one of the major public health problems worldwide, and is one of the recognized causes of premature deaths every year in the world. The purpose of this study was to investigate the associations between the + 138 insertion/deletion of adenine (Ins/del A) and + 5665 guanine-to-thymine (G/T) polymorphisms of the endothelin-1 gene and hypertension in the residents of Magway Township, Myanmar.

Methods

This study was a cross-sectional comparative study including 60 hypertensive patients and 60 control subjects in Magway Township, Myanmar. The inclusion criterion for hypertension was blood pressure ≥ 140/90 mmHg or previous diagnosis by a physician as hypertension and/or taking antihypertensive drugs. The control group had blood pressure < 140/90 mmHg and no previous diagnosis of hypertension. The genotyping was done by polymerase chain reaction and restriction fragment length polymorphism method.

Results

In this study, the genotype distribution of the + 138 Ins/del A variant was significantly different between hypertensive patients and the control group, especially in the 3A4A genotype (odds ratio [OR], 2.451; 95% confidence interval [CI], 1.138–5.280; P = 0.022). Adenine insertion genotypes (3A4A and 4A4A) were significantly associated with hypertension in the dominant model (OR, 2.494; 95% CI, 1.179–5.276; P = 0.017). In addition, there was a significant association between the 4A allele and hypertension (OR, 1.771; 95% CI, 1.026–3.056; P = 0.040). The genotype and allelic distributions of the + 5665 G/T polymorphism were not significantly different between the hypertensive patients and the control group (P > 0.05). In this study, there was no significant association between the genotype and allele frequency, and hypertension (P > 0.05). The linkage disequilibrium was weak between the + 138 Ins/del A and + 5665 G/T loci (D’ = 0.108, r² = 0.009).

Conclusions

This study provides evidence that the + 138 Ins/del A rather than + 5665 G/T polymorphism is associated with hypertension in Burmese people.

Synergistic epistasis of the deleterious effects of transposable elements

The replicative nature and generally deleterious effects of transposable elements (TEs) raise an outstanding question about how TE copy number is stably contained in host populations. Classic theoretical analyses predict that, when the decline in fitness due to each additional TE insertion is greater than linear, or when there is synergistic epistasis, selection against TEs can result in a stable equilibrium of TE copy number. While several mechanisms are predicted to yield synergistic deleterious effects of TEs, we lack empirical investigations of the presence of such epistatic interactions. Purifying selection with synergistic epistasis generates repulsion linkage between deleterious alleles. We investigated this population genetic signal in the likely ancestral Drosophila melanogaster population and found evidence supporting the presence of synergistic epistasis among TE insertions, especially TEs expected to exert large fitness impacts. Even though synergistic epistasis of TEs has been predicted to arise through ectopic recombination and TE-mediated epigenetic silencing mechanisms, we only found mixed support for the associated predictions. We observed signals of synergistic epistasis for a large number of TE families, which is consistent with the expectation that such epistatic interaction mainly happens among copies of the same family. Curiously, significant repulsion linkage was also found among TE insertions from different families, suggesting the possibility that synergism of TEs' deleterious fitness effects could arise above the family level and through mechanisms similar to those of simple mutations. Our findings set the stage for investigating the prevalence and importance of epistatic interactions in the evolutionary dynamics of TEs.

The pattern of linkage disequilibrium in Dual-Purpose Belgian Blue cattle

Quantifying the level of linkage disequilibrium (LD), non-random association of alleles at two or more loci, is important to determine the number of markers needed for genomic selection. The aims of this study were to evaluate the extent of LD in Dual-Purpose Belgian Blue (DPBB) and to compare the level of LD in DPBB with that of Walloon Holstein. Data of 28,427 single nucleotide polymorphisms (SNP), located on 29 Bos taurus autosomes (BTA), of 639 DPBB and 398 Holstein bulls were used. The level of LD between pairwise SNPs separated by up to 10 Mb was evaluated, separately for each breed, using the squared correlation of the alleles at two loci. The analysis of molecular variance showed that the percentage of variation within populations (85.48%) was higher than between populations (14.52%). However, permutation tests showed a significant genetic differentiation between the two studied populations (p < .01). The average LD found between adjacent SNP pairs in DPBB (0.16 (SD = 0.22)) was generally lower than in Holstein (0.23 (SD = 0.27)). The proportion of SNPs in useful LD (r²  > 0.30) within a genomic distance of ≤0.10 Mb between SNPs was 18.58% and 28.23% in DPBB and Holstein bulls, respectively. In both breeds, the effective population size decreased over generations; however, the decline was greater in DPBB than that in Holstein. Based on results, it can be concluded that at least 68,000 SNPs are needed for implementing genomic selection in DPBB cattle with enough accuracy.

From old markers to next generation: reconstructing the history of the peopling of Sardinia

CONTEXT

For many years the Sardinian population has been the object of numerous studies because of its unique genetic structure. Despite the extreme abundance of papers, various aspects of the peopling and genetic structure of Sardinia still remain uncertain and sometimes controversial.

OBJECTIVE

We reviewed what has emerged from different studies, focussing on some still open questions, such as the origin of Sardinians, their relationship with the Corsican population, and the intra-regional genetic heterogeneity.

METHODS

The various issues have been addressed through the analysis of classical markers, molecular markers and, finally, genomic data through next generation sequencing.

RESULTS AND CONCLUSIONS

Although the most ancient human remains date back to the end of the Palaeolithic, Mesolithic populations brought founding lineages that left evident traces in the modern population. Then, with the Neolithic, the island underwent an important demographic expansion. Subsequently, isolation and genetic drift contributed to maintain a significant genetic heterogeneity, but preserving the overall homogeneity on a regional scale. At the same time, isolation and genetic drift contributed to differentiate Sardinia from Corsica, which saw an important gene flow from the mainland. However, the isolation did not prevent gene flow from the neighbouring populations whose contribution are still recognisable in the genome of Sardinians.

Y-chromosome and Surname Analyses for Reconstructing Past Population Structures: The Sardinian Population as a Test Case

Many anthropological, linguistic, genetic and genomic analyses have been carried out to evaluate the potential impact that evolutionary forces had in shaping the present-day Sardinian gene pool, the main outlier in the genetic landscape of Europe. However, due to the homogenizing effect of internal movements, which have intensified over the past fifty years, only partial information has been obtained about the main demographic events. To overcome this limitation, we analyzed the male-specific region of the Y chromosome in three population samples obtained by reallocating a large number of Sardinian subjects to the place of origin of their monophyletic surnames, which are paternally transmitted through generations in most of the populations, much like the Y chromosome. Three Y-chromosome founding lineages, G2-L91, I2-M26 and R1b-V88, were identified as strongly contributing to the definition of the outlying position of Sardinians in the European genetic context and marking a significant differentiation within the island. The present distribution of these lineages does not always mirror that detected in ancient DNAs. Our results show that the analysis of the Y-chromosome gene pool coupled with a sampling method based on the origin of the family name, is an efficient approach to unravelling past heterogeneity, often hidden by recent movements, in the gene pool of modern populations. Furthermore, the reconstruction and comparison of past genetic isolates represent a starting point to better assess the genetic information deriving from the increasing number of available ancient DNA samples.

Genomic history of the Sardinian population

The population of the Mediterranean island of Sardinia has made important contributions to genome-wide association studies of complex disease traits and, based on ancient DNA (aDNA) studies of mainland Europe, Sardinia is hypothesized to be a unique refuge for early Neolithic ancestry. To provide new insights on the genetic history of this flagship population, we analyzed 3,514 whole-genome sequenced individuals from Sardinia. We find Sardinian samples show elevated levels of shared ancestry with Basque individuals, especially samples from the more historically isolated regions of Sardinia. Our analysis also uniquely illuminates how levels of genetic similarity with mainland aDNA samples varies subtly across the island. Together, our results indicate within-island sub-structure and sex-biased processes have substantially impacted the genetic history of Sardinia. These results give new insight to the demography of ancestral Sardinians and help further the understanding of sharing of disease risk alleles between Sardinia and mainland populations.

Genetic-Driven Druggable Target Identification and Validation

Choosing the right biological target is the critical primary decision for the development of new drugs. Systematic genetic association testing of both human diseases and quantitative traits, along with resultant findings of coincident associations between them, is becoming a powerful approach to infer drug targetable candidates and generate in vitro tests to identify compounds that can modulate them therapeutically. Here, we discuss opportunities and challenges, and infer criteria for the optimal use of genetic findings in the drug discovery pipeline.

Effects of Demographic History on the Detection of Recombination Hotspots from Linkage Disequilibrium

In some species, meiotic recombination is concentrated in small genomic regions. These "recombination hotspots" leave signatures in fine-scale patterns of linkage disequilibrium, raising the prospect that the genomic landscape of hotspots can be characterized from sequence variation. This approach has led to the inference that hotspots evolve rapidly in some species, but are conserved in others. Historic demographic events, such as population bottlenecks, are known to affect patterns of linkage disequilibrium across the genome, violating population genetic assumptions of this approach. Although such events are prevalent, demographic history is generally ignored when making inferences about the evolution of recombination hotspots. To determine the effect of demography on the detection of recombination hotspots, we use the coalescent to simulate haplotypes with a known recombination landscape. We measure the ability of popular linkage disequilibrium-based programs to detect hotspots across a range of demographic histories, including population bottlenecks, hidden population structure, population expansions, and population contractions. We find that demographic events have the potential to greatly reduce the power and increase the false positive rate of hotspot discovery. Neither the power nor the false positive rate of hotspot detection can be predicted without also knowing the demographic history of the sample. Our results suggest that ignoring demographic history likely overestimates the power to detect hotspots and therefore underestimates the degree of hotspot sharing between species. We suggest strategies for incorporating demographic history into population genetic inferences about recombination hotspots.

Allele and haplotype diversity of 12 X-STRs in Sardinia

The analysis of clusters of tightly linked X-chromosome short tandem repeat (STR) markers can assist the interpretation of complex kinship cases. However, when linkage disequilibrium (LD) is present in the population of origin of tested individuals, haplotype rather than allele frequencies should be used in likelihood calculations. The diversity of twelve X-STRs arranged in four linkage groups (I: DXS10148-DXS10135-DXS8378; II: DXS7132-DXS10079-DXS10074; III: DXS10103-HPRTB-DXS10101; IV: DXS10146-DXS10134-DXS7423) was tested in a Sardinian population sample (n=516) including three open populations from the Northern, Central and Southern part of the island, and three isolates (Benetutti, Desulo, Carloforte). Evidence of LD was detected in Sardinia within each linkage group. Significant differences in haplotype and allele frequency distribution of X-STR markers was seen between isolates and open populations, which on the contrary appeared highly homogeneous. The percentage of Sardinian haplotypes previously unobserved in a similar dataset compiled for the Italian population was: 76.3% (linkage group I), 61.3% (linkage group II), 54.1% (linkage group III), 58.9% (linkage group IV). Significant pairwise genetic differences were seen between mainland Italy, the three Sardinian isolates, and the open population of Southern Sardinia. The study confirms the presence of high levels and complex patterns of LD along the X chromosome in Sardinia, and provides population-specific haplotype data for biostatistical evaluation in kinship testing.

Overexpression of the Cytokine BAFF and Autoimmunity Risk

BACKGROUND

Genomewide association studies of autoimmune diseases have mapped hundreds of susceptibility regions in the genome. However, only for a few association signals has the causal gene been identified, and for even fewer have the causal variant and underlying mechanism been defined. Coincident associations of DNA variants affecting both the risk of autoimmune disease and quantitative immune variables provide an informative route to explore disease mechanisms and drug-targetable pathways.

METHODS

Using case-control samples from Sardinia, Italy, we performed a genomewide association study in multiple sclerosis followed by TNFSF13B locus-specific association testing in systemic lupus erythematosus (SLE). Extensive phenotyping of quantitative immune variables, sequence-based fine mapping, cross-population and cross-phenotype analyses, and gene-expression studies were used to identify the causal variant and elucidate its mechanism of action. Signatures of positive selection were also investigated.

RESULTS

A variant in TNFSF13B, encoding the cytokine and drug target B-cell activating factor (BAFF), was associated with multiple sclerosis as well as SLE. The disease-risk allele was also associated with up-regulated humoral immunity through increased levels of soluble BAFF, B lymphocytes, and immunoglobulins. The causal variant was identified: an insertion-deletion variant, GCTGT→A (in which A is the risk allele), yielded a shorter transcript that escaped microRNA inhibition and increased production of soluble BAFF, which in turn up-regulated humoral immunity. Population genetic signatures indicated that this autoimmunity variant has been evolutionarily advantageous, most likely by augmenting resistance to malaria.

CONCLUSIONS

A TNFSF13B variant was associated with multiple sclerosis and SLE, and its effects were clarified at the population, cellular, and molecular levels. (Funded by the Italian Foundation for Multiple Sclerosis and others.).

Genome-Wide Association Study Identifies Candidate Loci Underlying Agronomic Traits in a Middle American Diversity Panel of Common Bean

Common bean ( L.) breeding programs aim to improve both agronomic and seed characteristics traits. However, the genetic architecture of the many traits that affect common bean production are not completely understood. Genome-wide association studies (GWAS) provide an experimental approach to identify genomic regions where important candidate genes are located. A panel of 280 modern bean genotypes from race Mesoamerica, referred to as the Middle American Diversity Panel (MDP), were grown in four US locations, and a GWAS using >150,000 single-nucleotide polymorphisms (SNPs) (minor allele frequency [MAF] ≥ 5%) was conducted for six agronomic traits. The degree of inter- and intrachromosomal linkage disequilibrium (LD) was estimated after accounting for population structure and relatedness. The LD varied between chromosomes for the entire MDP and among race Mesoamerica and Durango-Jalisco genotypes within the panel. The LD patterns reflected the breeding history of common bean. Genome-wide association studies led to the discovery of new and known genomic regions affecting the agronomic traits at the entire population, race, and location levels. We observed strong colocalized signals in a narrow genomic interval for three interrelated traits: growth habit, lodging, and canopy height. Overall, this study detected ∼30 candidate genes based on a priori and candidate gene search strategies centered on the 100-kb region surrounding a significant SNP. These results provide a framework from which further research can begin to understand the actual genes controlling important agronomic production traits in common bean.

Detecting a hierarchical genetic population structure via Multi-InDel markers on the X chromosome

Detecting population structure and estimating individual biogeographical ancestry are very important in population genetics studies, biomedical research and forensics. Single-nucleotide polymorphism (SNP) has long been considered to be a primary ancestry-informative marker (AIM), but it is constrained by complex and time-consuming genotyping protocols. Following up on our previous study, we propose that a multi-insertion-deletion polymorphism (Multi-InDel) with multiple haplotypes can be useful in ancestry inference and hierarchical genetic population structures. A validation study for the X chromosome Multi-InDel marker (X-Multi-InDel) as a novel AIM was conducted. Genetic polymorphisms and genetic distances among three Chinese populations and 14 worldwide populations obtained from the 1000 Genomes database were analyzed. A Bayesian clustering method (STRUCTURE) was used to discern the continental origins of Europe, East Asia, and Africa. A minimal panel of ten X-Multi-InDels was verified to be sufficient to distinguish human ancestries from three major continental regions with nearly the same efficiency of the earlier panel with 21 insertion-deletion AIMs. Along with the development of more X-Multi-InDels, an approach using this novel marker has the potential for broad applicability as a cost-effective tool toward more accurate determinations of individual biogeographical ancestry and population stratification.

Zinc and Other Metals Deficiencies and Risk of Type 1 Diabetes: An Ecological Study in the High Risk Sardinia Island

Background

Type 1 diabetes incidence presents a decreasing gradient in Europe from the Nordic countries to the Mediterranean ones. Exception to this gradient is represented by Sardinia, the second largest Mediterranean island whose population shows the highest incidence in Europe, after Finland. The genetic features of this population have created a fertile ground for the epidemic of the disease, however, as well as being strikingly high, the incidence rate has suddenly presented a continuous increase from the ‘50s, not explainable by accumulation of new genetic variants. Several environmental factors have been taken into account, possibly interacting with the genetic/epigenetic scenario, but there are no strong evidences to date.

Methods

The present study investigated the hypothesis that geochemical elements could create permissive environmental conditions for autoimmune diabetes. An ecological analysis was performed to test possible correlations between the values of eight elements in stream sediments and type 1 diabetes incidence rate in Sardinia.

Results

Analyses revealed negative associations between elements, such as Co, Cr, Cu, Mn, Ni, Zn, and type 1 diabetes incidence.

Conclusions

The results suggest a possible protective role of some elements against the onset of the disease.

Genome sequencing elucidates Sardinian genetic architecture and augments association analyses for lipid and blood inflammatory markers

We report ∼17.6 million genetic variants from whole-genome sequencing of 2,120 Sardinians; 22% are absent from previous sequencing-based compilations and are enriched for predicted functional consequences. Furthermore, ∼76,000 variants common in our sample (frequency >5%) are rare elsewhere (<0.5% in the 1000 Genomes Project). We assessed the impact of these variants on circulating lipid levels and five inflammatory biomarkers. We observe 14 signals, including 2 major new loci, for lipid levels and 19 signals, including 2 new loci, for inflammatory markers. The new associations would have been missed in analyses based on 1000 Genomes Project data, underlining the advantages of large-scale sequencing in this founder population.

Interactions between genetic variants and dietary lipid composition: effects on circulating LDL cholesterol in children

BACKGROUND

Elevated serum low-density lipoprotein (LDL) cholesterol is a predictor of cardiovascular disease events, and the quality of dietary fat is known to influence serum concentrations of LDL cholesterol in children. Interindividual differences in response to diet exist, but the underlying genetic factors remain largely unknown.

OBJECTIVE

We aimed to identify genetic variants that modify the variation in serum lipid response to dietary fat quality.

DESIGN

We used data from 2 longitudinal Finnish cohorts designed to study risk factors for cardiovascular diseases. Large-scale genotyping was performed with Metabochip in a long-term randomized controlled dietary intervention trial, the Special Turku Coronary Risk Factor Intervention Project (STRIP), for discovery of genetic polymorphisms. The observational Cardiovascular Risk in Young Finns Study (YFS) with genome-wide genetic data was used as a replication sample for the initial findings. Dietary records were used to calculate the ratio of unsaturated to saturated fats. Interaction models and multiple follow-ups were used in the analysis.

RESULTS

In the STRIP cohort, a variant within the PARK2 locus, rs9364628, showed moderate interaction with dietary fat quality and a consistent direction of effect in both scans on serum LDL-cholesterol concentration in children aged 5 and 7 y (P < 0.0084 and P < 0.0057, respectively). In the YFS cohort, we were unable to replicate the initial discovery signal, but rs12207186 within the PARK2 locus and dietary lipid quality had a stronger interaction effect on serum LDL-cholesterol concentration (P < 9.44 × 10(-5)) than did rs9364628 in children aged 6 y.

CONCLUSION

This genotyping study involving 2 cohorts of healthy Finnish children indicates a possible interaction between PARK2 variants and dietary fat quality on serum LDL-cholesterol concentration. This trial was registered at clinicaltrials.gov as NCT00223600.

Demarcation of informative chromosomes in tropical sweet corn inbred lines using microsatellite DNA markers

A study of genetic variation among 10 pairs of chromosomes extracted from 13 tropical sweet corn inbred lines, using 99 microsatellite markers, revealed a wide range of genetic diversity. Allelic richness and the number of effective alleles per chromosome ranged from 2.78 to 4.33 and 1.96 to 3.47, respectively, with respective mean values of 3.62 and 2.73. According to the Shannon’s information index (I) and Nei’s gene diversity coefficient (Nei), Chromosome 10 was the most informative chromosome (I = 1.311 and Nei = 0.703), while Chromosome 2 possessed the least (I = 0.762 and Nei = 0.456). Based on linkage disequilibrium (LD) measurements for loci less than 50 cM apart on the same chromosome, all loci on Chromosomes 1, 6 and 7 were in equilibrium. Even so, there was a high proportion of genetic variation in Chromosomes 4, 5, 8, 9 and 10, thereby revealing their appropriateness for use in the genetic diversity investigations among tropical sweet corn lines. Chromosome 4, with the highest number of loci in linkage disequilibrium, was considered the best for marker-phenotype association and QTL mapping, followed by Chromosomes 5, 8, 9 and 10.

rs9939609 in the FTO gene is associated with obesity but not with several biochemical parameters in Sardinian obese children

Several studies have reported an association of the intronic single nucleotide polymorphism (SNP) rs9939609 of the fat mass and obesity-associated (FTO) gene with obesity and with a number of obesity-related features. We studied the association of rs9939609 with obesity in 912 obese children and adolescents (426 males and 486 females, mean ± SD age 10.5 ± 3.3 years) and in 543 normal weight subjects. A number of biochemical and clinical parameters was also evaluated in 700 of these patients. In the obese group, mean body mass index standard deviation score (BMI-SDS) was similar between the three genotypes. The A allele was present in 55% of the patients' and in 43% of controls' chromosomes. The distribution of heterozygotes was similar between patients and controls (47%), while the distribution of AA homozygotes was significantly higher in patients (31% vs. 20%). Logistic regression analysis on the genotypes yielded a χ(2) of 35.5 with an odds ratio of 1.6 (CI = 1.3-1.8), P < 1 × 10(-5) . None of the clinical and metabolic parameters tested was associated with the genotype. In conclusion, we have confirmed the strong association between FTO and obesity, and shown that only AA homozygotes are predisposed to develop obesity while TT homozygotes might be protected. Finally, we found no association between rs9939609 and a number of obesity-related abnormalities.

Role of major histocompatibility complex and ethnicity in acute renal graft rejection

BACKGROUND

The major histocompatibility complex (MHC) plays a main role in antigen presentation. Class I, II, and III genes form defined "blocks" of conserved DNA sequences (conserved extended haplotypes) that are useful to follow the ancestry of a population. Each variant encodes a specific peptide that determines a particular individual's immune response. In addition, differential expression of HLA antigens in certain physiological and pathological conditions may participate in the pathogenesis of allograft rejection versus tolerance.

OBJECTIVES

The aim of this study was to determine whether the specific HLA ancestry was associated with acute renal graft rejection among the Mexican mestizo population.

MATERIALS AND METHODS

We studied 544 Mexican mestizo renal donors and their respective recipients for their serologically determined HLA and based on antigens haplotype assignments. The acute rejection group was compared with the nonrejection group among donors and recipients, correspondingly.

RESULTS

Frequent Mexican alleles were observed in this study. Moreover, HLA-B*61/-DR*04, HLA-A*35/-DR*06 (Amerindian ancestry), HLA-A*68/-DR*01, HLA-A*28/-B*65/-DR*01 (African ancestry), and HLA-A*33/-B*65 (Caucasian ancestry) in donors were associated with acute renal graft rejection episodes.

CONCLUSION

Knowing the ancestry of a donor's HLA molecules may help to individualize immunosuppressive therapy for posttransplant surveillance, because they may be more membrane-exposed in parenchymal cells, making them more susceptible of being recognized by the recipient's immune system.

Variation in crossover rates across a 3-Mb contig of bread wheat (Triticum aestivum) reveals the presence of a meiotic recombination hotspot

In bread wheat (Triticum aestivum L.), initial studies using deletion lines indicated that crossover (CO) events occur mainly in the telomeric regions of the chromosomes with a possible correlation with the presence of genes. However, little is known about the distribution of COs at the sequence level. To investigate this, we studied in detail the pattern of COs along a contig of 3.110 Mb using two F2 segregating populations (Chinese Spring × Renan (F2-CsRe) and Chinese Spring × Courtot (F2-CsCt)) each containing ~2,000 individuals. The availability of the sequence of the contig from Cs enabled the development of 318 markers among which 23 co-dominant polymorphic markers (11 SSRs and 12 SNPs) were selected for CO distribution analyses. The distribution of CO events was not homogeneous throughout the contig, ranging from 0.05 to 2.77 cM/Mb, but was conserved between the two populations despite very different contig recombination rate averages (0.82 cM/Mb in F2-CsRe vs 0.35 cM/Mb in F2-CsCt). The CO frequency was correlated with the percentage of coding sequence in Cs and with the polymorphism rate between Cs and Re or Ct in both populations, indicating an impact of these two factors on CO distribution. At a finer scale, COs were found in a region covering 2.38 kb, spanning a gene coding for a glycosyl transferase (Hga3), suggesting the presence of a CO hotspot. A non-crossover event covering at least 453 bp was also identified in the same interval. From these results, we can conclude that gene content could be one of the factors driving recombination in bread wheat.

Brief communication: patterns of linkage disequilibrium and haplotype diversity at Xq13 in six Native American populations

Comparative studies of linkage disequilibrium (LD) can provide insights into human demographic history. Here, we characterize LD in six Native American populations using seven microsatellite markers in Xq13, a region of the genome extensively studied in populations around the world. Native Americans show relatively low diversity and high LD, in agreement with recent genome-wide survey and a scenario of sequential founder effects accompanying human population dispersal around the globe. LD in Native Americans is similar to that observed in some recently described small population isolates and higher than in large European isolates (e.g., Finns), which have been extensively analyzed in medical genetics studies. Haplotype analyses are consistent with a colonization of the New World by a differentiated East Asian population, followed by extensive genetic drift in the Americas.

Linkage disequilibrium patterns and genetic structure of Amerindian and non-Amerindian Brazilian populations revealed by long-range X-STR markers

The extent of X-chromosome linkage disequilibrium (LD) was studied in a southern Brazilian population, and in a pool of samples from Amerindian populations. For this purpose, 11 microsatellites, located mostly in a Xq region comprising approximately 86 Mb was investigated. The lower Amerindian gene diversity associated with significant differences between the populations studied indicated population structure as the main cause for the higher LD values in the Amerindian pool. On the other hand, the LD levels of the non-Amerindian Brazilian sample, although less extensive than that of the Amerindians, were probably determined by admixture events. Our results indicated that different demographic histories have significant effects on LD levels of human populations, and provide a first approach to the X-chromosome ancestry of Amerindian and non-Amerindian Brazilian populations, being valuable for future studies involving mapping and population genetic studies.

Exploring the diabetogenicity of the HLA-B18-DR3 CEH: independent association with T1D genetic risk close to HLA-DOA

The objective of this study was to identify additional diabetes susceptibility markers in the MHC that could be responsible for the differential diabetogenicity of different HLA-DR3 CEHs. High-resolution SNP genotyping of the MHC was carried out in 15 type 1 diabetes (T1D) patients and 39 non-diabetic controls, homozygous for DR3-DQ2 and with one copy of the A(*)30-B(*)18-MICA(*)4-F1C30-DRB1(*)0301-DQB1(*)0201-DPB1(*)0202 HLA haplotype. Significantly associated SNPs were replicated in an independent sample of 554 T1D patients and 841 controls without HLA matching. Electrophoretic mobility shift assay was used to show a functional effect of an associated SNP. Seven SNPs showed evidence of association in the initial discovery experiment. Upon replication, only rs419434 (upstream HLA-DOA gene) remained significant. A functional variant (rs432375) in complete LD with rs419434 was shown to affect USF-1 binding and could be responsible for the association signal in the region. We have identified a new susceptibility locus within the MHC with a modest contribution to T1D (OR=1.93; CI: 1.52-2.44; P=10(-8)) that is independent of HLA-DRB1 locus.

High differentiation among eight villages in a secluded area of Sardinia revealed by genome-wide high density SNPs analysis

To better design association studies for complex traits in isolated populations it's important to understand how history and isolation moulded the genetic features of different communities. Population isolates should not “a priori” be considered homogeneous, even if the communities are not distant and part of a small region. We studied a particular area of Sardinia called Ogliastra, characterized by the presence of several distinct villages that display different history, immigration events and population size. Cultural and geographic isolation characterized the history of these communities. We determined LD parameters in 8 villages and defined population structure through high density SNPs (about 360 K) on 360 unrelated people (45 selected samples from each village). These isolates showed differences in LD values and LD map length. Five of these villages show high LD values probably due to their reduced population size and extreme isolation. High genetic differentiation among villages was detected. Moreover population structure analysis revealed a high correlation between genetic and geographic distances. Our study indicates that history, geography and biodemography have influenced the genetic features of Ogliastra communities producing differences in LD and population structure. All these data demonstrate that we can consider each village an isolate with specific characteristics. We suggest that, in order to optimize the study design of complex traits, a thorough characterization of genetic features is useful to identify the presence of sub-populations and stratification within genetic isolates.

Analysis of meiotic recombination products from human sperm

Traditional methods for surveying meiotic recombination in humans are limited to pedigree and linkage disequilibrium analyses. We have developed assays that allow the direct detection of crossover and gene conversion molecules in batches of sperm DNA. To date, we have characterized 26 recombination hotspots by allele-specific PCR and selectively amplified recombinant DNA molecules from these regions. These analyses have revealed that meiotic crossover hotspots in humans are highly localized and flanked by DNA segments where recombination is suppressed. The centers of crossover hotspots are also active in noncrossover recombination, displaying short conversion tracts.

Linkage mapping of CVD risk traits in the isolated Norfolk Island population

To understand the underlying genetic architecture of cardiovascular disease (CVD) risk traits, we undertook a genome-wide linkage scan to identify CVD quantitative trait loci (QTLs) in 377 individuals from the Norfolk Island population. The central aim of this research focused on the utilization of a genetically and geographically isolated population of individuals from Norfolk Island for the purposes of variance component linkage analysis to identify QTLs involved in CVD risk traits. Substantial evidence supports the involvement of traits such as systolic and diastolic blood pressures, high-density lipoprotein-cholesterol, low-density lipoprotein-cholesterol, body mass index and triglycerides as important risk factors for CVD pathogenesis. In addition to the environmental influences of poor diet, reduced physical activity, increasing age, cigarette smoking and alcohol consumption, many studies have illustrated a strong involvement of genetic components in the CVD phenotype through family and twin studies. We undertook a genome scan using 400 markers spaced approximately 10 cM in 600 individuals from Norfolk Island. Genotype data was analyzed using the variance components methods of SOLAR. Our results gave a peak LOD score of 2.01 localizing to chromosome 1p36 for systolic blood pressure and replicated previously implicated loci for other CVD relevant QTLs.

Y-chromosome based evidence for pre-neolithic origin of the genetically homogeneous but diverse Sardinian population: inference for association scans

The island of Sardinia shows a unique high incidence of several autoimmune diseases with multifactorial inheritance, particularly type 1 diabetes and multiple sclerosis. The prior knowledge of the genetic structure of this population is fundamental to establish the optimal design for association studies in these diseases. Previous work suggested that the Sardinians are a relatively homogenous population, but some reports were contradictory and data were largely based on variants subject to selection. For an unbiased assessment of genetic structure, we studied a combination of neutral Y-chromosome variants, 21 biallelic and 8 short tandem repeats (STRs) in 930 Sardinian males. We found a high degree of interindividual variation but a homogenous distribution of the detected variability in samples from three separate regions of the island. One haplogroup, I-M26, is rare or absent outside Sardinia and is very common (0.37 frequency) throughout the island, consistent with a founder effect. A Bayesian full likelihood analysis (BATWING) indicated that the time from the most recent common ancestor (TMRCA) of I-M26, was 21.0 (16.0–25.5) thousand years ago (KYA) and that the population began to expand 14.0 (7.8–22.0) KYA. These results suggest a largely pre-Neolithic settlement of the island with little subsequent gene flow from outside populations. Consequently, Sardinia is an especially attractive venue for case-control genome wide association scans in common multifactorial diseases. Concomitantly, the high degree of interindividual variation in the current population facilitates fine mapping efforts to pinpoint the aetiologic polymorphisms.

Linkage disequilibrium analysis in the genetically isolated Norfolk Island population

Norfolk Island is a human genetic isolate, possessing unique population characteristics that could be utilized for complex disease gene localization. Our intention was to evaluate the extent and strength of linkage disequilibrium (LD) in the Norfolk isolate by investigating markers within Xq13.3 and the NOS2A gene encoding the inducible nitric oxide synthase. A total of six microsatellite markers spanning approximately 11 Mb were assessed on chromosome Xq13.3 in a group of 56 men from Norfolk Island. Additionally, three single nucleotide polymorphisms (SNPs) localizing to the NOS2A gene were analyzed in a subset of the complex Norfolk pedigree. With the exception of two of the marker pairs, one of which is the most distantly spaced marker, all the Xq13.3 marker pairs were found to be in significant LD indicating that LD extends up to 9.5-11.5 Mb in the Norfolk Island population. Also, all SNPs studied showed significant LD in both Norfolk Islanders and Australian Caucasians, with two of the marker pairs in complete LD in the Norfolk population only. The Norfolk Island study population possesses a unique set of characteristics including founder effect, geographical isolation, exhaustive genealogical information and phenotypic data of use to cardiovascular disease risk traits. With LD extending up to 9.5-11 Mb, the Norfolk isolate should be a powerful resource for the localization of complex disease genes.

Identification of a founder BRCA2 mutation in Sardinian breast cancer families

The population of Sardinia is characterized by a relatively low level of genetic heterogeneity: therefore 'founder mutations' can be expected to be found. We analysed 17 probands from families with high incidence of breast cancer or breast and ovarian cancer by sequencing the full-length coding regions of BRCA1 and BRCA2 genes. A novel BRCA2 frame-shift mutation, 3951del3insAT, which produces a protein truncated at codon 1258, was observed in six patients with BC from the same village. The mutation was not found in unaffected females (matched on basis of ethnicity and age) with no family history of cancer. Haplotype analysis strongly suggests that all affected persons had a common ancestor. The identification of this clinically significant founder mutation may facilitate screening/testing for inherited risk of breast cancer.

The genetic study of three population microisolates in South Tyrol (MICROS): study design and epidemiological perspectives

BACKGROUND

There is increasing evidence of the important role that small, isolated populations could play in finding genes involved in the etiology of diseases. For historical and political reasons, South Tyrol, the northern most Italian region, includes several villages of small dimensions which remained isolated over the centuries.

METHODS

The MICROS study is a population-based survey on three small, isolated villages, characterized by: old settlement; small number of founders; high endogamy rates; slow/null population expansion. During the stage-1 (2002/03) genealogical data, screening questionnaires, clinical measurements, blood and urine samples, and DNA were collected for 1175 adult volunteers. Stage-2, concerning trait diagnoses, linkage analysis and association studies, is ongoing. The selection of the traits is being driven by expert clinicians. Preliminary, descriptive statistics were obtained. Power simulations for finding linkage on a quantitative trait locus (QTL) were undertaken.

RESULTS

Starting from participants, genealogies were reconstructed for 50,037 subjects, going back to the early 1600s. Within the last five generations, subjects were clustered in one pedigree of 7049 subjects plus 178 smaller pedigrees (3 to 85 subjects each). A significant probability of familial clustering was assessed for many traits, especially among the cardiovascular, neurological and respiratory traits. Simulations showed that the MICROS pedigree has a substantial power to detect a LOD score > or = 3 when the QTL specific heritability is > or = 20%.

CONCLUSION

The MICROS study is an extensive, ongoing, two-stage survey aimed at characterizing the genetic epidemiology of Mendelian and complex diseases. Our approach, involving different scientific disciplines, is an advantageous strategy to define and to study population isolates. The isolation of the Alpine populations, together with the extensive data collected so far, make the MICROS study a powerful resource for the study of diseases in many fields of medicine. Recent successes and simulation studies give us confidence that our pedigrees can be valuable both in finding new candidates loci and to confirm existing candidate genes.

Distinct linkage disequilibrium (LD) runs of single nucleotide polymorphisms and microsatellite markers; implications for use of mixed marker haplotypes in LD-based mapping

It has been suggested that the haplotypic relationship between microsatellite markers and single nucleotide polymorphisms (SNPs) is of considerable importance, as microsatellite markers can potentially be incorporated into haplotypes containing SNPs to increase marker density across a region of interest. However, SNPs and microsatellite markers have different mutation rates and durations, and it is conceivable that the linkage disequilibrium (LD) patterns between the genetic markers may considerably differ. We assessed the LD patterns using 1,661 SNPs and 65 microsatellite markers along chromosome 22 and investigated whether common patterns of LD between the two genetic markers are deduced from the results. The results demonstrated that the patterns of LD among microsatellite markers varied considerably and the LD runs of SNPs and microsatellite markers showed distinct patterns. Microsatellite markers have a much higher mutation rate and the evolution of microsatellite markers is a more complex process which has distinct mutation properties from those of SNPs. We consider that these might contribute to the different LD patterns between the two genetic markers. Therefore, it would seem inadvisable to make assumptions about persistence of LD across even a relatively small genetic distance among microsatellite markers and to construct mixed marker haplotypes/LD maps employing microsatellite markers.

Association between the ACCN1 gene and multiple sclerosis in Central East Sardinia

Multiple genome screens have been performed to identify regions in linkage or association with Multiple Sclerosis (MS, OMIM 126200), but little overlap has been found among them. This may be, in part, due to a low statistical power to detect small genetic effects and to genetic heterogeneity within and among the studied populations. Motivated by these considerations, we studied a very special population, namely that of Nuoro, Sardinia, Italy. This is an isolated, old, and genetically homogeneous population with high prevalence of MS. Our study sample includes both nuclear families and unrelated cases and controls. A multi-stage study design was adopted. In the first stage, microsatellites were typed in the 17q11.2 region, previously independently found to be in linkage with MS. One significant association was found at microsatellite D17S798. Next, a bioinformatic screening of the region surrounding this marker highlighted an interesting candidate MS susceptibility gene: the Amiloride-sensitive Cation Channel Neuronal 1 (ACCN1) gene. In the second stage of the study, we resequenced the exons and the 3′ untranslated (UTR) region of ACCN1, and investigated the MS association of Single Nucleotide Polymorphisms (SNPs) identified in that region. For this purpose, we developed a method of analysis where complete, phase-solved, posterior-weighted haplotype assignments are imputed for each study individual from incomplete, multi-locus, genotyping data. The imputed assignments provide an input to a number of proposed procedures for testing association at a microsatellite level or of a sequence of SNPs. These include a Mantel-Haenszel type test based on expected frequencies of pseudocase/pseudocontrol haplotypes, as well as permutation based tests, including a combination of permutation and weighted logistic regression analysis. Application of these methods allowed us to find a significant association between MS and the SNP rs28936 located in the 3′ UTR segment of ACCN1 with p = 0.0004 (p = 0.002, after adjusting for multiple testing). This result is in tune with several recent experimental findings which suggest that ACCN1 may play an important role in the pathogenesis of MS.

Variation of the myelin oligodendrocyte glycoprotein gene is not primarily associated with multiple sclerosis in the Sardinian population

BACKGROUND

Multiple sclerosis (MS) is consistently associated with particular HLA-DRB1-DQB1 haplotypes. However, existing evidence suggests that variation at these loci does not entirely explain association of the HLA region with the disease. The MOG locus is a prime positional and functional candidate for such additional predisposing effects but the analysis is complicated by the strong, albeit labyrinthine pattern of linkage disequilibrium in the region. Here we have assessed the association of MOG variation with MS in the Sardinian population to see if it represents an independent contributor to MS predisposition.

RESULTS

After re-sequencing the MOG gene in 21 healthy parents of MS patients we detected 134 variants, 33 of which were novel. A set of 40 informative SNPs was then selected and assessed for disease association together with 1 intragenic microsatellite in an initial data set of 239 MS families. This microsatellite and 11 SNPs were found to be positively associated with MS, using the transmission disequilibrium test, and were followed up in an additional 158 families (total families analysed = 397). While in these 397 families, 8 markers showed significant association with MS, through conditional tests we determined that these MOG variants were not associated with MS independently of the main DRB1-DQB1 disease associations.

CONCLUSION

These results indicate that variation within the MOG gene is not an important independent determinant of MS-inherited risk in the Sardinian population.

Campora: a young genetic isolate in South Italy

Genetic isolates have been successfully used in the study of complex traits, mainly because due to their features, they allow a reduction in the complexity of the genetic models underlying the trait. The aim of the present study is to describe the population of Campora, a village in the South of Italy, highlighting its properties of a genetic isolate. Both historical evidence and multi-locus genetic data (genomic and mitochondrial DNA polymorphisms) have been taken into account in the analyses. The extension of linkage disequilibrium (LD) regions has been evaluated on autosomes and on a region of the X chromosome. We defined a study sample population on the basis of the genealogy and exogamy data. We found in this population a few different mitochondrial and Y chromosome haplotypes and we ascertained that, similarly to other isolated populations, in Campora LD extends over wider region compared to large and genetically heterogeneous populations. These findings indicate a conspicuous genetic homogeneity in the genome. Finally, we found evidence for a recent population bottleneck that we propose to interpret as a demographic crisis determined by the plague of the 17th century. Overall our findings demonstrate that Campora displays the genetic characteristics of a young isolate.

High frequency of the TCRBV20S1 null allele in the Sardinian population

Single nucleotide polymorphisms (SNPs) in the T-cell receptor (TCR) gene segments might play a role in shaping the TCR repertoire. Three polymorphisms have been described for the TCRBV20S1 gene segment, one of which is responsible for a nucleotide substitution at position 524, resulting in the introduction of a stop codon. Individuals homozygous for this inactivating polymorphism ("null allele") are unable to express TCRBV20 gene products. Using DNA restriction digestion analysis, we investigated the frequency of this polymorphism in 111 healthy Sardinian subjects. Inhabitants of the Mediterranean island of Sardinia are considered to represent a genetically isolated population. Our analyses revealed an incidence of 19.8% of homozygosity for the null allele, corresponding to an allele frequency of 0.45. Such an incidence, significantly higher than the one detected in 83 non-Sardinian Caucasians (6%), is the most elevated so far reported in the literature. BV20 is a single member subfamily and the null allele produces a gap in the potential TCR repertoire. Therefore, it is possible that an undetermined selective pressure could have played a role in determining the high frequency of this inactivating polymorphism in Sardinians. Alternatively, this finding could be related to a founder effect in this ancient island population.

LRRK2 mutations and Parkinson's disease in Sardinia—A Mediterranean genetic isolate

The Leucine-Rich Repeat Kinase 2 (LRRK2) Gly2019Ser mutation is frequent among Parkinson's disease (PD) patients from the Arab, Jewish, and Iberian populations, while another mutation, Arg1441Gly, is common in the Basque population. We studied the prevalence of these mutations in Sardinia, a Mediterranean genetic isolate with peculiar structure and similarities with the Basque population. Among 98 Sardinian PD probands we detected one heterozygous Gly2019Ser carrier. This mutation was also found in one of 55 Sardinian controls, an 85-year-old man, later shown to have a positive family history of parkinsonism. No carriers of Arg1441Gly, Arg1441Cys, or Arg1441His mutations were found among cases and controls. Our results suggest that the "Basque"LRRK2 mutation is absent or very rare in Sardinia. The Gly2019Ser mutation is present but its frequency is lower than that in Iberian, Arab, or Jewish populations. The identification of an 85-year-old, healthy Gly2019Ser carrier supports the concept that this mutation displays incomplete penetrance.

Population isolates in South Tyrol and their value for genetic dissection of complex diseases

The study of genetic isolates is a promising approach for the study of complex genetic traits. The small and constant population size, lack of migration, and multiple relationships between individuals in the isolate population could reduce the genetic diversity, and lead to increased levels of linkage disequilibrium (LD). We studied the extent of LD on Xq13 in six population isolates from South Tyrol in the Eastern Italian Alps. We found different levels of LD in our study samples, probably reflecting their degrees of isolation and their demographic histories. The highest values were obtained in Val Gardena (ranking among the highest levels of LD in Europe) and in Stelvio, which qualified as a microisolate according to historical information, and biodemographic and genealogical criteria. Phylogenetic analysis revealed that the two Ladin-speaking populations are genetically distant from each other, and from their German-speaking neighbours, and are characterized by a smaller effective population size than the neighbouring valleys. These peculiar characteristics suggest that South Tyrol could be a unique resource for the study of complex diseases, showing all the characteristics of isolated populations with the advantage of including, in a fairly homogeneous environment, two genetically differentiated sub-populations. This could allow investigators to gain an insight into the contribution of genetic heterogeneity in complex diseases.

Analysis of association between maternal tumor necrosis factor-alpha promoter polymorphism (-308), tumor necrosis factor concentration, and preterm birth

OBJECTIVE

This study was undertaken to investigate the association of tumor necrosis factor-alpha (TNF-alpha) single nucleotide polymorphism (G-308>A) and risk of preterm birth by performing a systematic review and a meta-analysis of available studies. In addition, association between this variant and TNF-alpha concentration in amniotic fluid (AF) in preterm birth was also investigated.

STUDY DESIGN

Articles were chosen based on a Medline and EMBASE searches (1990-2005) with no language restrictions. An ongoing case-control study conducted in Nashville, TN, was also included. Articles evaluating the association between G-308>A and preterm birth were screened according to specific inclusion criteria. Meta-analysis was performed by using a random effect model. Association between maternal -308 genotype and AF-TNF-alpha concentration was determined by sandwich immunoassays.

RESULTS

Titles and abstracts of 6851 citations identified through the search were screened. Including our own study, a total of 7 studies were included for meta-analysis. Only 2 reported a statistically significant increased risk based on -308 genotype. Meta-analysis of the case-control studies on a pooled dataset (a total of 1846 subjects, 638 cases, and 1208 controls) showed no significant association between the -308 genotype and the risk of preterm birth (odds ratio [OR] 1.41; CI 0.90-2.19). A nonsignificant increase of AF TNF-alpha concentration was demonstrated with the GG genotype in cases compared with the presence of allele A.

CONCLUSION

Meta-analysis of available evidence documented no statistically significant association between a single nucleotide polymorphism in the TNF-alpha gene (G-308>A) and preterm birth. Analyses of AF-TNF-alpha concentration demonstrated no increase in TNF-alpha in the presence of the minor allele (A). These results suggest that this single nucleotide polymorphism does not independently associate strongly with preterm birth.

Comparative study of the linkage disequilibrium of an ENCODE region, chromosome 7p15, in Korean, Japanese, and Han Chinese samples

The extent and pattern of linkage disequilibrium (LD) in the human genome provide important information for disease gene mapping. Previous studies have shown that LDs vary depending on chromosomal regions and populations. As the Asian samples of the International HapMap Project consisted of Japanese and Chinese populations, it was of interest whether we could use the HapMap data as a reference to carry out association studies of common complex diseases in a closely related population, such as Koreans. We have compared the LD and recombination patterns defined by single-nucleotide polymorphisms (SNPs) in ENCODE region ENm010, chromosome 7p15.2, in Korean, Japanese, and Chinese samples and further tested the robustness of tagSNPs among the Asian samples. We genotyped 792 SNPs in 500 kb (chromosome 7: 26699793-27199792, NCBI build 34) from 90 unrelated Koreans by fluorescence polarization detection and compared the data with Asian data from the HapMap project. Despite some differences in the position of high LD region boundaries, the overall patterns of LD were remarkably similar across the three samples, reflecting strong genetic affinities among them. Furthermore, the haplotype tag SNP transferability across the three samples was greater than 90%. Our results support the initial suggestion that the populations genotyped in the HapMap project might serve as reference populations for the selection of tagSNPs in association studies.

Localized breakdown in linkage disequilibrium does not always predict sperm crossover hot spots in the human MHC class II region

To investigate the relationship between meiotic crossover hot spots and block-like linkage disequilibrium (LD), we have extended our high-resolution studies of the human MHC class II region to a 90-kb segment upstream of the HLA-DOA gene. LD blocks in this region are not as well defined as in the neighboring 210-kb DNA segment but do show two regions of LD breakdown in which coalescent analysis indicates substantial historical recombination. Sperm crossover analysis of one region revealed a novel localized hot spot similar in intensity and morphology to most other MHC hot spots. Crossovers at this hot spot are not obviously affected by a large insertion/deletion polymorphism near the hot spot. The second region of LD breakdown, within the DPB1 gene, shows an extremely low level of sperm crossover activity and does not contain a sperm crossover hot spot. These results highlight the complexity of LD patterns and the importance of experimentally verifying crossover hot spots.

Prospects and pitfalls in whole genome association studies

Recent large-scale studies of common genetic variation throughout the human genome are making it feasible to conduct whole genome studies of genotype-phenotype associations. Such studies have the potential to uncover novel contributors to common complex traits and thus lead to insights into the aetiology of multifactorial phenotypes. Despite this promise, it is important to recognize that the availability of genetic markers and the ability to assay them at realistic cost does not guarantee success of this approach. There are a number of practical issues that require close attention, some forms of allelic architecture are not readily amenable to the association approach with even the most rigorous design, and doubtless new hurdles will emerge as the studies begin. Here we discuss the promise and current challenges of the whole genome approach, and raise some issues to consider in interpreting the results of the first whole genome studies.

Evaluating HapMap SNP data transferability in a large-scale genotyping project involving 175 cancer-associated genes

One of the many potential uses of the HapMap project is its application to the investigation of complex disease aetiology among a wide range of populations. This study aims to assess the transferability of HapMap SNP data to the Spanish population in the context of cancer research. We have carried out a genotyping study in Spanish subjects involving 175 candidate cancer genes using an indirect gene-based approach and compared results with those for HapMap CEU subjects. Allele frequencies were very consistent between the two samples, with a high positive correlation (R) of 0.91 (P<<1x10(-6)). Linkage disequilibrium patterns and block structures across each gene were also very similar, with disequilibrium coefficient (r (2)) highly correlated (R=0.95, P<<1x10(-6)). We found that of the 21 genes that contained at least one block larger than 60 kb, nine (ATM, ATR, BRCA1, ERCC6, FANCC, RAD17, RAD50, RAD54B and XRCC4) belonged to the GO category "DNA repair". Haplotype frequencies per gene were also highly correlated (mean R=0.93), as was haplotype diversity (R=0.91, P<<1x10(-6)). "Yin yang" haplotypes were observed for 43% of the genes analysed and 18% of those were identical to the ancestral haplotype (identified in Chimpazee). Finally, the portability of tagSNPs identified in the HapMap CEU data using pairwise r (2) thresholds of 0.8 and 0.5 was assessed by applying these to the Spanish and current HapMap data for 66 genes. In general, the HapMap tagSNPs performed very well. Our results show generally high concordance with HapMap data in allele frequencies and haplotype distributions and confirm the applicability of HapMap SNP data to the study of complex diseases among the Spanish population.

Shaking the tree: mapping complex disease genes with linkage disequilibrium

Much effort and expense are being spent internationally to detect genetic polymorphisms contributing to susceptibility to complex human disease. Concomitantly, the technology for detecting and genotyping single nucleotide polymorphisms (SNPs) has undergone rapid development, yielding extensive catalogues of these polymorphisms across the genome. Population-based maps of the correlations amongst SNPs (linkage disequilibrium) are now being developed to accelerate the discovery of genes for complex human diseases. These genomic advances coincide with an increasing recognition of the importance of very large sample sizes for studying genetic effects. Together, these new genetic and epidemiological data hold renewed promise for the identification of susceptibility genes for complex traits. We review the state of knowledge about the structure of the human genome as related to SNPs and linkage disequilibrium, discuss the potential applications of this knowledge to mapping complex disease genes, and consider the issues facing whole genome association scanning using SNPs.

Increased level of linkage disequilibrium in rural compared with urban communities: a factor to consider in association-study design

Few studies have investigated genetic differentiation within nonisolate European populations, despite the initiation of large national sample collections such as U.K. Biobank. Here, we used short tandem repeat markers to explore fine-scale genetic structure and to examine the extent of linkage disequilibrium (LD) within national subpopulations. We studied 955 unrelated individuals of local ancestry from nine Scottish rural regions and the urban center of Edinburgh, as well as 96 unrelated individuals from the general U.K. population. Despite little overall differentiation on the basis of allele frequencies, there were clear differences among subpopulations in the extent of pairwise LD, measured between a subset of X-linked markers, that reflected presumed differences in the depths of the underlying genealogies within these subpopulations. Therefore, there are strategic advantages in studying rural subpopulations, in terms of increased power and reduced cost, that are lost by sampling across regions or within urban populations. Similar rural-urban contrasts are likely to exist in many other populations with stable rural subpopulations, which could influence the design of genetic association studies and national biobank data collections.