ENIGMA and the individual: Predicting factors that affect the brain in 35 countries worldwide.

In this review, we discuss recent work by the ENIGMA Consortium (http://enigma.ini.usc.edu) - a global alliance of over 500 scientists spread across 200 institutions in 35 countries collectively analyzing brain imaging, clinical, and genetic data. Initially formed to detect genetic influences on brain measures, ENIGMA has grown to over 30 working groups studying 12 major brain diseases by pooling and comparing brain data. In some of the largest neuroimaging studies to date - of schizophrenia and major depression - ENIGMA has found replicable disease effects on the brain that are consistent worldwide, as well as factors that modulate disease effects. In partnership with other consortia including ADNI, CHARGE, IMAGEN and others1, ENIGMA's genomic screens - now numbering over 30,000 MRI scans - have revealed at least 8 genetic loci that affect brain volumes. Downstream of gene findings, ENIGMA has revealed how these individual variants - and genetic variants in general - may affect both the brain and risk for a range of diseases. The ENIGMA consortium is discovering factors that consistently affect brain structure and function that will serve as future predictors linking individual brain scans and genomic data. It is generating vast pools of normative data on brain measures - from tens of thousands of people - that may help detect deviations from normal development or aging in specific groups of subjects. We discuss challenges and opportunities in applying these predictors to individual subjects and new cohorts, as well as lessons we have learned in ENIGMA's efforts so far.

Coevolution of genes and languages and high levels of population structure among the highland populations of Daghestan.

As a result of the combination of great linguistic and cultural diversity, the highland populations of Daghestan present an excellent opportunity to test the hypothesis of language-gene coevolution at a fine geographic scale. However, previous genetic studies generally have been restricted to uniparental markers and have not included many of the key populations of the region. To improve our understanding of the genetic structure of Daghestani populations and to investigate possible correlations between genetic and linguistic variation, we analyzed ~550,000 autosomal single nucleotide polymorphisms, phylogenetically informative Y chromosome markers and mtDNA haplotypes in 21 ethnic Daghestani groups. We found high levels of population structure in Daghestan consistent with the hypothesis of long-term isolation among populations of the highland Caucasus. Highland Daghestani populations exhibit extremely high levels of between-population diversity for all genetic systems tested, leading to some of the highest FST values observed for any region of the world. In addition, we find a significant positive correlation between gene and language diversity, suggesting that these two aspects of human diversity have coevolved as a result of historical patterns of social interaction among highland farmers at the community level. Finally, our data are consistent with the hypothesis that most Daghestanian-speaking groups descend from a common ancestral population (~6000-6500 years ago) that spread to the Caucasus by demic diffusion followed by population fragmentation and low levels of gene flow.

The Role of Recent Admixture in Forming the Contemporary West Eurasian Genomic Landscape.

Over the past few years, studies of DNA isolated from human fossils and archaeological remains have generated considerable novel insight into the history of our species. Several landmark papers have described the genomes of ancient humans across West Eurasia, demonstrating the presence of large-scale, dynamic population movements over the last 10,000 years, such that ancestry across present-day populations is likely to be a mixture of several ancient groups [1-7]. While these efforts are bringing the details of West Eurasian prehistory into increasing focus, studies aimed at understanding the processes behind the generation of the current West Eurasian genetic landscape have been limited by the number of populations sampled or have been either too regional or global in their outlook [8-11]. Here, using recently described haplotype-based techniques [11], we present the results of a systematic survey of recent admixture history across Western Eurasia and show that admixture is a universal property across almost all groups. Admixture in all regions except North Western Europe involved the influx of genetic material from outside of West Eurasia, which we date to specific time periods. Within Northern, Western, and Central Europe, admixture tended to occur between local groups during the period 300 to 1200 CE. Comparisons of the genetic profiles of West Eurasians before and after admixture show that population movements within the last 1,500 years are likely to have maintained differentiation among groups. Our analysis provides a timeline of the gene flow events that have generated the contemporary genetic landscape of West Eurasia.

Extensive genome-wide autozygosity in the population isolates of Daghestan.

Isolated populations are valuable resources for mapping disease genes, as inbreeding increases genome-wide homozygosity and enhances the ability to map disease alleles on a genetically uniform background within a relatively homogenous environment. The populations of Daghestan are thought to have resided in the Caucasus Mountains for hundreds of generations and are characterized by a high prevalence of certain complex diseases. To explore the extent to which their unique population history led to increased levels of inbreeding, we genotyped >550 000 autosomal single-nucleotide polymorphisms (SNPs) in a set of 14 population isolates speaking Nakh-Daghestanian (ND) languages. The ND-speaking populations showed greatly elevated coefficients of inbreeding, very high numbers and long lengths of Runs of Homozygosity, and elevated linkage disequilibrium compared with surrounding groups from the Caucasus, the Near East, Europe, Central and South Asia. These results are consistent with the hypothesis that most ND-speaking groups descend from a common ancestral population that fragmented into a series of genetic isolates in the Daghestanian highlands. They have subsequently maintained a long-term small effective population size as a result of constant inbreeding and very low levels of gene flow. Given these findings, Daghestanian population isolates are likely to be useful for mapping genes associated with complex diseases.

Genomic structural variants are linked with intellectual disability.

Mutations in more than 500 genes have been associated with intellectual disability (ID) and related disorders of cognitive function, such as autism and schizophrenia. Here we aimed to unravel the molecular epidemiology of non-specific ID in a genetic isolate using a combination of population and molecular genetic approaches. A large multigenerational pedigree was ascertained within a Dagestan Genetic Heritage research program in a genetic isolate of indigenous ethnics. Clinical characteristics of the affected members were based on combining diagnoses from regional psychiatric hospitals with our own clinical assessment, using a Russian translation of the structured psychiatric interviews, the Diagnostic Interview for Genetic Studies and the Family Interview for Genetic Studies, based on DSM-IV criteria. Weber/CHLC 9.0 STRs set was used for multipoint parametric linkage analyses (Simwalk2.91). Next, we checked CNVs and LOH (based on Affymetrix SNP 5.0 data) in the linked with ID genomic regions with the aim to identify candidate genes associated with mutations in linked regions. The number of statistically significant (p ≤ 0.05) suggestive linkage peaks with 1.3 < LOD < 3.0 we detected in a total of 10 genomic regions: 1q41, 2p25.3-p24.2, 3p13-p12.1, 4q13.3, 10p11, 11q23, 12q24.22-q24.31, 17q24.2-q25.1, 21q22.13 and 22q12.3-q13.1. Three significant linkage signals with LOD >3 were obtained at 2p25.3-p24.2 under the dominant model, with a peak at 21 cM flanked by loci D2S2976 and D2S2952; at 12q24.22-q24.31 under the recessive model, with a peak at -120 cM flanked by marker D12S2070 and D12S395 and at 22q12.3 under the dominant model, with a peak at 32 cM flanked by marker D22S683 and D22S445. After a set of genes had been designated as possible candidates in these specific chromosomal regions,we conducted an exploratory search for LOH and CNV based on microarray data to detect structural genomic variants within five ID-linked regions with LOD scores between 2.0 and 3.9. In these selected regions we obtained 173 ROH segments and 98 CN segments. Further analysis of region 2p25.3-p24.2 revealed deletions within genes encoding MYTL, SNTG2 and TPO among five of 21 affected cases at 2p25.3-p24.2. In the ID-linked region at 12q24.22-12q24.31 19 out of 21 ID cases carried segmental CNV and 20 of 21 them displayed ROH segments with mean size lengths for ID cases 2512 kb (500-6,472 kb) and for healthy control 682 kb (531-986 kb), including the genes MED13L, HRK, FBXW8, TESC, CDK2AP1 and SBNO1. Seven of 21 affected pedigree members displayed segmental deletions at 22q12.3 that includes the gene LARGE. Eight affected pedigree members carried ROH segments and 6 CN segments at 10p11.23-p11.21 containing the genes ZEB1, c10orf68 and EPC1. Our linkage and structural genomic variation analyses in a remote highland genetic isolate with aggregation of ID demonstrated that even highly isolated single kindred ID has oligo/polygenic pathogenesis. The results obtained implicate 10 genomic regions linked with ID that contain some of previously reported candidate genes, including HRK, FBXW8, TESC, CDK2AP1 and SBNO1 at 12q24 that were shown in recent studies as associated with brain measures derived from MRI scans.

The ENIGMA Consortium: large-scale collaborative analyses of neuroimaging and genetic data.

The Enhancing NeuroImaging Genetics through Meta-Analysis (ENIGMA) Consortium is a collaborative network of researchers working together on a range of large-scale studies that integrate data from 70 institutions worldwide. Organized into Working Groups that tackle questions in neuroscience, genetics, and medicine, ENIGMA studies have analyzed neuroimaging data from over 12,826 subjects. In addition, data from 12,171 individuals were provided by the CHARGE consortium for replication of findings, in a total of 24,997 subjects. By meta-analyzing results from many sites, ENIGMA has detected factors that affect the brain that no individual site could detect on its own, and that require larger numbers of subjects than any individual neuroimaging study has currently collected. ENIGMA's first project was a genome-wide association study identifying common variants in the genome associated with hippocampal volume or intracranial volume. Continuing work is exploring genetic associations with subcortical volumes (ENIGMA2) and white matter microstructure (ENIGMA-DTI). Working groups also focus on understanding how schizophrenia, bipolar illness, major depression and attention deficit/hyperactivity disorder (ADHD) affect the brain. We review the current progress of the ENIGMA Consortium, along with challenges and unexpected discoveries made on the way.

[Mapping genes related to early onset major depressive disorder in Dagestan genetic isolates]. / Dagistan genetik izolatlarinda erken baslangiçli majör depresif bozukluk ile iliskili gen haritalamasi.

AIM: The purpose of this study was to determine the molecular epidemiology of early onset major depressive disorder (MDD) in genetic isolates of the Caucasus Dagestan indigenous ethnic populations using molecular and statistical population-genetic approaches. METHODS: Two multigenerational pedigrees from two diverse remote highland isolates with aggregation of early onset MDD were ascertained within our long-term research program titled 'Dagestan Genetic Heritage, DGH'. The first isolate included 48 cases of MDD (19 living) with 11 suicides committed, and the second included 60 MDD cases (30 living) with 12 suicides committed. The phenotypes of the affected family members were determined using a database containing diagnoses from a regional psychiatric hospital and through our own clinical examinations, which were based on a Russian translation of DIGS software based on the DSM-IV criteria . A 10 cM genomic scan (Weber/CHLC 9.0 STRs) of the 64 affected and non-affected members of the pedigrees was performed and the data was used for multipoint parametric linkage analyses. Following this scan, selected cases were analyzed by Affymetrix 6.0 SNP arrays in order to refine the contribution of copy number variations (CNVs) to the genetic basis of MDD. RESULTS: We found a total of 18 genomic regions with nominal (LOD>1.3) linkage to MDD across the two isolates. Three genomic regions had genome-wide significant (LOD>3) linkages and were found at 2p13.2-p11.2, 14q31.12-q32.13 and 22q12.3. We also confirmed previous findings for MDD at 4q25, 11p15, 12q23-24, 13q31-32, 18q21-22 and 22q11-13. Six linkage regions were observed in both genetic isolates, while 12 other linkages demonstrated population-specific heterogeneity. We detected CNV rearrangements within 12 of the 18 linkage regions. Affected subjects had the highest rate of genomic instability within the linkage regions at 2p13.2-p11.2, 4q25-q28.2, 7p14.1, 8p23, 14q31.12-q32.13, 18q22.1 and 20p13. CONCLUSION: The results obtained in this study suggest that mapping genes of complex diseases, including MDD, across genetically homogeneous isolates can help detect linkage signals and expedite the search for susceptibility genes when combined with methods that detect structural genomic variation in linkage regions.

The dual origin of Tati-speakers from Dagestan as written in the genealogy of uniparental variants.

OBJECTIVES: Tat language is classified in an Iranian subbranch of the Indo-European family. It is spoken in the Caucasus and in the West Caspian region by populations with heterogeneous cultural traditions and religion whose ancestry is unknown. The aim of this study is to get a first insight about the genetic history of this peculiar linguistic group. METHODS: We investigated the uniparental gene pools, defined by NRY and mtDNA high-resolution markers, in two Tati-speaking communities from Dagestan: Mountain Jews or Juhur, who speak the Judeo-Tat dialect, and the Tats, who speak the Muslim-Tat dialect. The samples have been collected in monoethnic rural villages and selected on the basis of genealogical relationships. A novel approach aimed at resolving cryptic cases in the recent history of human populations, which combines the properties of uniparental genetic markers with the potential of "forward-in-time" computer simulations, is presented. RESULTS: Judeo-Tats emerged as a group with tight matrilineal genetic legacy who separated early from other Jewish communities. Tats exhibited genetic signals of a much longer in situ evolution, which appear as substantially unlinked with other Indo-Iranian enclaves in the Caucasus. CONCLUSIONS: The independent demographic histories of the two samples, with mutually reversed profiles at paternally and maternally transmitted genetic systems, suggest that geographic proximity and linguistic assimilation of Tati-speakers from Dagestan do not reflect a common ancestry.

High altitude adaptation in Daghestani populations from the Caucasus.

We have surveyed 15 high-altitude adaptation candidate genes for signals of positive selection in North Caucasian highlanders using targeted re-sequencing. A total of 49 unrelated Daghestani from three ethnic groups (Avars, Kubachians, and Laks) living in ancient villages located at around 2,000 m above sea level were chosen as the study population. Caucasian (Adygei living at sea level, N = 20) and CEU (CEPH Utah residents with ancestry from northern and western Europe; N = 20) were used as controls. Candidate genes were compared with 20 putatively neutral control regions resequenced in the same individuals. The regions of interest were amplified by long-PCR, pooled according to individual, indexed by adding an eight-nucleotide tag, and sequenced using the Illumina GAII platform. 1,066 SNPs were called using false discovery and false negative thresholds of ~6%. The neutral regions provided an empirical null distribution to compare with the candidate genes for signals of selection. Two genes stood out. In Laks, a non-synonymous variant within HIF1A already known to be associated with improvement in oxygen metabolism was rediscovered, and in Kubachians a cluster of 13 SNPs located in a conserved intronic region within EGLN1 showing high population differentiation was found. These variants illustrate both the common pathways of adaptation to high altitude in different populations and features specific to the Daghestani populations, showing how even a mildly hypoxic environment can lead to genetic adaptation.

Crohn's disease and genetic hitchhiking at IBD5.

Inflammatory bowel disease 5 (IBD5) is a 250 kb haplotype on chromosome 5 that is associated with an increased risk of Crohn's disease in Europeans. The OCTN1 gene is centrally located on IBD5 and encodes a transporter of the antioxidant ergothioneine (ET). The 503F variant of OCTN1 is strongly associated with IBD5 and is a gain-of-function mutation that increases absorption of ET. Although 503F has been implicated as the variant potentially responsible for Crohn's disease susceptibility at IBD5, there is little evidence beyond statistical association to support its role in disease causation. We hypothesize that 503F is a recent adaptation in Europeans that swept to relatively high frequency and that disease association at IBD5 results not from 503F itself, but from one or more nearby hitchhiking variants, in the genes IRF1 or IL5. To test for evidence of recent positive selection on the 503F allele, we employed the iHS statistic, which was significant in the European CEU HapMap population (P=0.0007) and European Human Genome Diversity Panel populations (P≤0.01). To evaluate the hypothesis of disease-variant hitchhiking, we performed haplotype association tests on high-density microarray data in a sample of 1,868 Crohn's disease cases and 5,550 controls. We found that 503F haplotypes with recombination breakpoints between OCTN1 and IRF1 or IL5 were not associated with disease (odds ratio [OR]: 1.05, P=0.21). In contrast, we observed strong disease association for 503F haplotypes with no recombination between these three genes (OR: 1.24, P=2.6×10(-8)), as expected if the sweeping haplotype harbored one or more disease-causing mutations in IRF1 or IL5. To further evaluate these disease-gene candidates, we obtained expression data from lower gastrointestinal biopsies of healthy individuals and Crohn's disease patients. We observed a 72% increase in gene expression of IRF1 among Crohn's disease patients (P=0.0006) and no significant difference in expression of OCTN1. Collectively, these data indicate that the 503F variant has increased in frequency due to recent positive selection and that disease-causing variants in linkage disequilibrium with 503F have hitchhiked to relatively high frequency, thus forming the IBD5 risk haplotype. Finally, our association results and expression data support IRF1 as a strong candidate for Crohn's disease causation.

The key role of patrilineal inheritance in shaping the genetic variation of Dagestan highlanders.

The Caucasus region is a complex cultural and ethnic mosaic, comprising populations that speak Caucasian, Indo-European and Altaic languages. Isolated mountain villages (auls) in Dagestan still preserve high level of genetic and cultural diversity and have patriarchal societies with a long history of isolation. The aim of this study was to understand the genetic history of five Dagestan highland auls with distinct ethnic affiliation (Avars, Chechens-Akkins, Kubachians, Laks, Tabasarans) using markers on the male-specific region of the Y chromosome. The groups analyzed here are all Muslims but speak different languages all belonging to the Nakh-Dagestanian linguistic family. The results show that the Dagestan ethnic groups share a common Y-genetic background, with deep-rooted genealogies and rare alleles, dating back to an early phase in the post-glacial recolonization of Europe. Geography and stochastic factors, such as founder effect and long-term genetic drift, driven by the rigid structuring of societies in groups of patrilineal descent, most likely acted as mutually reinforcing key factors in determining the high degree of Y-genetic divergence among these ethnic groups.

Fine-scaled human genetic structure revealed by SNP microarrays.

We report an analysis of more than 240,000 loci genotyped using the Affymetrix SNP microarray in 554 individuals from 27 worldwide populations in Africa, Asia, and Europe. To provide a more extensive and complete sampling of human genetic variation, we have included caste and tribal samples from two states in South India, Daghestanis from eastern Europe, and the Iban from Malaysia. Consistent with observations made by Charles Darwin, our results highlight shared variation among human populations and demonstrate that much genetic variation is geographically continuous. At the same time, principal components analyses reveal discernible genetic differentiation among almost all identified populations in our sample, and in most cases, individuals can be clearly assigned to defined populations on the basis of SNP genotypes. All individuals are accurately classified into continental groups using a model-based clustering algorithm, but between closely related populations, genetic and self-classifications conflict for some individuals. The 250K data permitted high-level resolution of genetic variation among Indian caste and tribal populations and between highland and lowland Daghestani populations. In particular, upper-caste individuals from Tamil Nadu and Andhra Pradesh form one defined group, lower-caste individuals from these two states form another, and the tribal Irula samples form a third. Our results emphasize the correlation of genetic and geographic distances and highlight other elements, including social factors that have contributed to population structure.

J1-M267 Y lineage marks climate-driven pre-historical human displacements.

The present day distribution of Y chromosomes bearing the haplogroup J1 M267(*)G variant has been associated with different episodes of human demographic history, the main one being the diffusion of Islam since the Early Middle Ages. To better understand the modes and timing of J1 dispersals, we reconstructed the genealogical relationships among 282 M267(*)G chromosomes from 29 populations typed at 20 YSTRs and 6 SNPs. Phylogenetic analyses depicted a new genetic background consistent with climate-driven demographic dynamics occurring during two key phases of human pre-history: (1) the spatial expansion of hunter gatherers in response to the end of the late Pleistocene cooling phases and (2) the displacement of groups of foragers/herders following the mid-Holocene rainfall retreats across the Sahara and Arabia. Furthermore, J1 STR motifs previously used to trace Arab or Jewish ancestries were shown unsuitable as diagnostic markers for ethnicity.

Culture creates genetic structure in the Caucasus: autosomal, mitochondrial, and Y-chromosomal variation in Daghestan.

BACKGROUND: Near the junction of three major continents, the Caucasus region has been an important thoroughfare for human migration. While the Caucasus Mountains have diverted human traffic to the few lowland regions that provide a gateway from north to south between the Caspian and Black Seas, highland populations have been isolated by their remote geographic location and their practice of patrilocal endogamy. We investigate how these cultural and historical differences between highland and lowland populations have affected patterns of genetic diversity. We test 1) whether the highland practice of patrilocal endogamy has generated sex-specific population relationships, and 2) whether the history of migration and military conquest associated with the lowland populations has left Central Asian genes in the Caucasus, by comparing genetic diversity and pairwise population relationships between Daghestani populations and reference populations throughout Europe and Asia for autosomal, mitochondrial, and Y-chromosomal markers. RESULTS: We found that the highland Daghestani populations had contrasting histories for the mitochondrial DNA and Y-chromosome data sets. Y-chromosomal haplogroup diversity was reduced among highland Daghestani populations when compared to other populations and to highland Daghestani mitochondrial DNA haplogroup diversity. Lowland Daghestani populations showed Turkish and Central Asian affinities for both mitochondrial and Y-chromosomal data sets. Autosomal population histories are strongly correlated to the pattern observed for the mitochondrial DNA data set, while the correlation between the mitochondrial DNA and Y-chromosome distance matrices was weak and not significant. CONCLUSION: The reduced Y-chromosomal diversity exhibited by highland Daghestani populations is consistent with genetic drift caused by patrilocal endogamy. Mitochondrial and Y-chromosomal phylogeographic comparisons indicate a common Near Eastern origin of highland populations. Lowland Daghestani populations show varying influence from Near Eastern and Central Asian populations.

Evidence of still-ongoing convergence evolution of the lactase persistence T-13910 alleles in humans.

A single-nucleotide variant, C/T(-13910), located 14 kb upstream of the lactase gene (LCT), has been shown to be completely correlated with lactase persistence (LP) in northern Europeans. Here, we analyzed the background of the alleles carrying the critical variant in 1,611 DNA samples from 37 populations. Our data show that the T(-13910) variant is found on two different, highly divergent haplotype backgrounds in the global populations. The first is the most common LP haplotype (LP H98) present in all populations analyzed, whereas the others (LP H8-H12), which originate from the same ancestral allelic haplotype, are found in geographically restricted populations living west of the Urals and north of the Caucasus. The global distribution pattern of LP T(-13910) H98 supports the Caucasian origin of this allele. Age estimates based on different mathematical models show that the common LP T(-13910) H98 allele (approximately 5,000-12,000 years old) is relatively older than the other geographically restricted LP alleles (approximately 1,400-3,000 years old). Our data about global allelic haplotypes of the lactose-tolerance variant imply that the T(-13910) allele has been independently introduced more than once and that there is a still-ongoing process of convergent evolution of the LP alleles in humans.

Genome-wide linkage scan of schizophrenia: a cross-isolate study.

Genetic isolates are exceptional resources for the detection of susceptibility genes for complex diseases because of the potential reduction in genetic and clinical heterogeneity. However, the outcome of these mapping efforts is dependent upon the demographic history of a given isolated population, with the most significant factors being a constant population size, the number of generations since founding, and the pathogenic loci and their allele frequencies among founders. Here we employed a cross-isolate genome-wide multipoint linkage study design using uniform genetic and clinical methods in four Daghestan ethnically and demographically diverse isolates with an aggregation of schizophrenia. Our previous population-genetics study showed that Daghestan has an extremely high genetic diversity between ethnic populations and a low genetic diversity within them. The isolates selected for this study include some with more than 200 and some with fewer than 100 generations of demographical history since their founding. Updated clinical data using DSM-IV criteria showed between-isolate differences in aggregation of distinct types of schizophrenia: one of the isolates had a predominant aggregation of disorganized schizophrenia, while the other three had predominantly paranoid schizophrenia. The summarized cross-isolate results indicated prominent within and between-isolate differences in clinical and genetic heterogeneity: the most ancient isolates have roughly twofold fewer incidences of distinct clinical phenotypes and fewer linked genomic regions compared to the demographically younger isolates, which exhibit higher clinical and genetic heterogeneity. Affected individuals in the demographically ancient isolate of ethnic Dargins (No. 6022) who suffered from disorganized schizophrenia showed the highest linkage evidence at 17p11-p12 (LOD=3.73), while isolates with a predominant aggregation of paranoid schizophrenia (Nos. 6005, 6011, and 6034) showed the highest linkage evidence at 22q11 (LOD=3.0 and 4.4). The unified clinical, genomic, and statistical design we used enabled us to separate the linked and unlinked pedigrees in an unbiased fashion for each genomic location. Overall maximized heterogeneity lod scores for the combined pedigrees ranging from 3.5 to 8.7 were found at 2p24, 10q26, 11q23, 12q24, 17p11-p12, 22q11, and 22q13. The cross-isolate homogeneity in linkage patterns may be ascribed to an identical-by-descent "metahaplotype" block with pathogenic loci derived from the Daghestan ethnic groups' common ancestral metapopulation, while the cross-isolate differences may reflect differences in gene drift and recombination events in the history of local isolates. The results obtained support the notion that mapping genes of any complex disease (e.g., schizophrenia) in demographically older genetic isolates may be more time and cost effective in comparison with demographically younger isolates, especially in genetically heterogeneous outbred populations, due to higher clinical and genetic homogeneity of the primary isolates. A study at higher genotyping density across the regions of interest and fluorescence in situ hybridization analyses are currently underway.

Overview of genetic-epidemiological studies in ethnically and demographically diverse isolates of Dagestan, Northern Caucasus, Russia.

AIM: To assess genetic diversity and genetic distances among isolated populations from Dagestan. METHODS: A cross-population genetic epidemiology design was applied in ethnically and demographically diverse isolates from Dagestan, some with more than 200 and some with less than 100 generations of demographical history since their founding. RESULTS: The analysis of genetic diversity showed that Dagestan ethnic populations are clearly close to European ethnic populations. The genetic data support the view of them as ancient, highly isolated populations 85%-97% the rate of the endogamy and inbreeding coefficient F=0.010-0.015. Many Dagestan populations have very high prevalence of certain complex diseases such as cardiovascular illnesses, cancer, schizophrenia, mental retardation, and progressive muscular dystrophy. Lifetime morbid risk for schizophrenia in the isolates varied from 0 to 5%. Among the relatives, the number of men with chronic schizophrenia was at least twice as high as women. The average age of onset of schizophrenia was 21.2 years for offspring of consanguineous marriages and 17.4 years for offspring of non-consanguineous marriages (P=0.033). CONCLUSION: The results support the hypothesis that cross-population design provides unique opportunities for observing reliable ancestral haplotypes with disease predisposing loci, as well as population-specific linked loci.

Mapping genes of complex psychiatric diseases in Daghestan genetic isolates.

Genetic isolates, which provide outstanding opportunities for identification of susceptibility genes for complex diseases, can be classified as primary (having an ancient demographic history in a stable environment) or secondary (having a younger demographic history) Neel [1992: Minority populations: Genetics, demography, and health, pp. 1-13]. Daghestan contains 26 out of 50 indigenous Caucasus ethnicities that have been in existence for hundreds of generations in the same highland region. The ethnic groups are subdivided into numerous primary isolates. The founder effect and gene drift in these primary isolates may have caused aggregation of specific haplotypes with limited numbers of pathogenic alleles and loci in some isolates relative to others. These are expressed as inter-population differences in lifetime prevalence and features of certain complex clinical phenotypes and in patterns of genetic linkage and linkage disequilibrium (LD). Stable highland and ethnic-cultural environments have led to increased penetrance and a reduced number of phenocopies, which typically hamper the identification of any susceptibility genes for complex diseases. Owing to these characteristics of the primary isolates, a comparative linkage study in the primary isolates allows us to define the number of susceptibility genes for any complex disease and to identify the source of variability and non-replication of linkage analysis results. As part of an ongoing study, seven extended schizophrenia and one nonspecific mental retardation kindreds have been ascertained from Daghestan isolates. Lifetime morbid risk for schizophrenia in the isolates varied from 0 to 5%. A genome scan with markers spaced 10 cM apart was carried out on these pedigrees and linkage analysis was performed using descent graph methods, as implemented in Simwalk2. To identify regions containing susceptibility genes within these kindreds, we followed up those regions with non-parametric and parametric linkage analyses, with the choice of genetic model guided by the results obtained in the NPL. While the analyses are ongoing, the most positive findings were made in different isolated pedigrees on chromosomes 17p11, 3q24, and 22q for schizophrenia and on chromosome 12q for nonspecific mental retardation.

Genetics and population history of Caucasus populations.

We describe aspects of genetic diversity in several ethnic populations of the Caucasus Mountains of Daghestan using mitochondrial DNA sequences and a sample of 100 polymorphic Alu insertion loci. The mitochondrial DNA (mtDNA) sequences are like those of Europe. Principal coordinates and nearest neighbor statistics show that there is little detectable structure in the distances among populations computed from mtDNA. The Alu frequencies of the Caucasus populations suggest that they have undergone more genetic drift than most other groups since the dispersal of modern humans. Genetic differences among these populations are not large; instead, they are of the same order as distances among populations of Europe. We compare two methods of inference about the demography of ancient colonizing populations from Africa, one based on conventional FST statistics and one based on mean Alu insertion frequencies. The two approaches agree reasonably well if we assume that there was demographic growth in Africa before the diaspora of ancestors of contemporary regional human groups outside Africa.