Genome screening for linkage disequilibrium in a Costa Rican sample of patients with bipolar-I disorder: a follow-up study on chromosome 18

Genetic substrates of bipolar disorder risk in Latino families

Genetic studies of bipolar disorder (BP) have been conducted in the Latin American population, to date, in several countries, including Mexico, the United States, Costa Rica, Colombia, and, to a lesser extent, Brazil. These studies focused primarily on linkage-based designs utilizing families with multiplex cases of BP. Significant BP loci were identified on Chromosomes 18, 5 and 8, and fine mapping suggested several genes of interest underlying these linkage peaks. More recently, studies in these same pedigrees yielded significant linkage loci for BP endophenotypes, including measures of activity, sleep cycles, and personality traits. Building from findings in other populations, candidate gene association analyses in Latinos from Mexican and Central American ancestry confirmed the role of several genes (including CACNA1C and ANK3) in conferring BP risk. Although GWAS, methylation, and deep sequencing studies have only begun in these populations, there is evidence that CNVs and rare SNPs both play a role in BP risk of these populations. Large segments of the Latino populations in the Americas remain largely unstudied regarding BP genetics, but evidence to date has shown that this type of research can be successfully conducted in these populations and that the genetic underpinnings of BP in these cohorts share at least some characteristics with risk genes identified in European and other populations.

Neuropsychological and dimensional behavioral trait profiles in Costa Rican ADHD sib pairs: Potential intermediate phenotypes for genetic studies

Attention deficit hyperactivity disorder (ADHD) is associated with substantial functional impairment in children and in adults. Many individuals with ADHD have clear neurocognitive deficits, including problems with visual attention, processing speed, and set shifting. ADHD is etiologically complex, and although genetic factors play a role in its development, much of the genetic contribution to ADHD remains unidentified. We conducted clinical and neuropsychological assessments of 294 individuals (269 with ADHD) from 163 families (48 multigenerational families created using genealogical reconstruction, 78 affected sib pair families, and 37 trios) from the Central Valley of Costa Rica (CVCR). We used principal components analysis (PCA) to group neurocognitive and behavioral variables using the subscales of the Child Behavior Checklist (CBCL) and 15 neuropsychological measures, and created quantitative traits for heritability analyses. We identified seven cognitive and two behavioral domains. Individuals with ADHD were significantly more impaired than their unaffected siblings on most behavioral and cognitive domains. The verbal IQ domain had the highest heritability (92%), followed by auditory attention (87%), visual processing speed and problem solving (85%), and externalizing symptoms (81%). The quantitative traits identified here have high heritabilities, similar to the reported heritability of ADHD (70-90%), and may represent appropriate alternative phenotypes for genetic studies. The use of multigenerational families from a genetically isolated population may facilitate the identification of ADHD risk genes in the face of phenotypic and genetic heterogeneity.

A Pilot Study for Discovering Candidate Genes of Chromosome 18q21 in Methamphetamine Abusers: Case-control Association Study

Objective

It was previously suggested that the malic enzyme 2 (ME2) as the candidate gene for psychosis in fine mapping of chromosome 18q21. Chromosome 18q21 is also one of the possible regions that can contribute to addiction.

Methods

We performed a pilot study for discovering candidate gene of chromosome 18q21 in the methamphetamine abusers for elucidating the candidate gene for methamphetamine addiction leading to psychosis. We have selected 30 unrelated controls (16 males, 14 females; age=59.8±10.4) and 37 male methamphetamine abusers (age=43.3±7.8). We analyzed 20 single nucleotide polymorphisms (SNPs) of 7 neuronal genes in chromosome 18q21 for DNA samples that was checked for the data quality and genotype error. The association between the case-control status and each individual SNP was measured using multiple logistic regression models (adjusting for age and sex as covariates). And we controlled false discovery rate (FDR) to deal with multiple testing problem.

Results

We found 3 significant SNPs of 2 genes in chromosome 18q21 (p-value<0.05; adjusting for age as covariate) in methamphetamine abusers compared to controls. We also found 2 significant SNPs of 1 gene (p-value<0.05; adjusting for age and sex as covariates) (rs3794899, rs3794901:MAPK4). Two SNPs in MAPK4 gene were significant in both statistical groups.

Conclusion

MAPK4, the gene for mitogen-activated protein kinase 4, is one of the final 6 candidate genes including ME2 in 18q12-21 in our previous finemapping for psychosis. Our results suggest that MAPK4 can be a candidate gene that contribute to the methamphetamine addiction leading to psychosis.

Mood disorders in individuals with distal 18q deletions

We examined 36 participants at least 4 years old with hemizygous distal deletions of the long arm of Chromosome 18 (18q-) for histories of mood disorders and to characterize these disorders clinically. Since each participant had a different region of 18q hemizygosity, our goal was also to identify their common region of hemizygosity associated with mood disorders; thereby identifying candidate causal genes in that region. Lifetime mood and other psychiatric disorders were determined by semi-structured interviews of patients and parents, supplemented by reviews of medical and psychiatric records, and norm-referenced psychological assessment instruments, for psychiatric symptoms, cognitive problems, and adaptive functioning. Sixteen participants were identified with lifetime mood disorders (ages 12-42 years, 71% female, 14 having had unipolar depression and 2 with bipolar disorders). From the group of 20 who did not meet criteria for a mood disorder; a comparison group of 6 participants were identified who were matched for age range and deletion size. Mood-disordered patients had high rates of anxiety (75%) and externalizing behavior disorders (44%), and significant mean differences from comparison patients (P < 0.05), including higher overall and verbal IQs and lower autistic symptoms. A critical region was defined in the mood-disordered group that included a hypothetical gene, C18orf62, and two known genes, ZADH2 and TSHZ1. We conclude that patients having terminal deletions of this critical region of the long arm of Chromosome 18 are highly likely to have mood disorders, which are often comorbid with anxiety and to a lesser extent with externalizing disorders.

An association study between the dymeclin gene and schizophrenia in the Japanese population

Many gene variants are involved in the susceptibility to schizophrenia and some of them are expected to be associated with other human characters. Recently reported meta-analysis of genetic associations revealed nucleotide variants in synaptic vesicular transport/Golgi apparatus genes with schizophrenia. In this study, we selected the dymeclin gene (DYM) as a candidate gene for schizophrenia. The DYM gene encodes dymeclin that has been identified to be associated with the Golgi apparatus and with transitional vesicles of the reticulum-Golgi interface. A three-step case-control study of total of 2105 Japanese cases of schizophrenia and 2087 Japanese control subjects was carried out for tag single-nucleotide polymorphisms (SNPs) in the DYM gene and an association between an SNP, rs833497, and schizophrenia was identified (allelic P=2 × 10(-5), in the total sample). DYM is the causal gene for Dyggve-Melchior-Clausen syndrome and this study shows the second neuropsychiatric disorder in which the DYM gene is involved. The present data support the involvement of Golgi function and vesicular transport in the presynapse in schizophrenia.

Psychiatric syndromes in individuals with chromosome 18 abnormalities

Chromosome 18 abnormalities are associated with a range of physical abnormalities such as short stature and hearing impairments. Psychiatric manifestations have also been observed. This study focuses on the presentations of psychiatric syndromes as they relate to specific chromosomal abnormalities of chromosome 18. Twenty-five subjects (13 with an 18q deletion, 9 with 18p tetrasomy, and 3 with an 18p deletion), were interviewed by psychiatrists (blind to specific chromosomal abnormality) using the DIGS (subjects 18 and older) or KSADS-PL (subjects under 18). A consensus best estimation diagnostic process was employed to determine psychiatric syndromes. Oligonucleotide Array Comparative Genomic Hybridization (Agilent Technologies) was utilized to define specific regions of chromosome 18 that were deleted or duplicated. These data were further analyzed to determine critical regions of the chromosome as they relate to phenotypic manifestations in these subjects. 58.3% of the chromosome 18q- deletion subjects had depressive symptoms, 58.3% had anxiety symptoms, 25% had manic symptoms, and 25% had psychotic symptoms. 66.6% of the chromosome 18p- deletion subjects had anxiety symptoms, and none had depressive, manic, or psychotic symptoms. Fifty percent of the chromosome 18p tetrasomy subjects had anxiety symptoms, 12.5% had psychotic symptoms, and 12.5% had a mood disorder. All three chromosomal disorders were associated with high anxiety rates. Psychotic, manic and depressive disorders were seen mostly in 18q- subjects and this may be helpful in narrowing regions for candidate genes for these psychiatric conditions.

TGFB-induced factor (TGIF): a candidate gene for psychosis on chromosome 18p

Schizophrenia (SC) and bipolar disorder (BP) share many clinical features, among them psychosis. We previously identified a putative gene locus for psychosis on chromosome 18p in a sample from the Central Valley of Costa Rica (CVCR) population. The present study replicated the association to a specific allele of microsatellite marker D18S63 on 18p11.3, using a newly collected sample from the CVCR. A combined analysis of both samples, plus additional subjects, showed that this specific allele on D18S63, which lies within an intron on the TGFB-induced factor (TGIF) gene, is strongly associated (P-value=0.0005) with psychosis. Eleven additional SNP markers, spanning five genes in the region, were analyzed in the combined sample from the CVCR. Only the four SNPs within the TGIF gene were in strong linkage disequilibrium with D18S63 (D'=1.00). A specific haplotype for all five markers within the TGIF gene showed evidence of association (P-value=0.011) to psychosis. A second, distinct haplotype, containing a newly identified nonsynonymous polymorphism in exon 5 of the TGIF gene, showed a nonsignificant trend towards association to psychosis (P-value=0.077). TGIF is involved in neurodevelopment, neuron survival and controls the expression of dopamine receptors. Altogether, our results point to the possible involvement of TGIF in the pathophysiology of psychotic disorders in the CVCR population.

A genome-wide scan for schizophrenia and psychosis susceptibility loci in families of Mexican and Central American ancestry

Schizophrenia is a complex psychiatric disorder, likely to be caused in part by multiple genes. In this study, linkage analyses were performed to identify chromosomal regions most likely to be associated with schizophrenia and psychosis in multiplex families of Mexican and Central American origin. Four hundred and fifty-nine individuals from 99 families, containing at least two siblings with hospital diagnoses of schizophrenia or schizoaffective disorder, were genotyped. Four hundred and four microsatellite markers were genotyped for all individuals and multipoint non-parametric linkage analyses were performed using broad (any psychosis) and narrow (schizophrenia and schizoaffective disorder) models. Under the broad model, three chromosomal regions (1pter-p36, 5q35, and 18p11) exhibited evidence of linkage with non-parametric lod (NPL) scores greater than 2.7 (equivalent to empirical P values of less than 0.001) with the peak multipoint NPL = 3.42 (empirical P value = 0.00003), meeting genomewide evidence for significant linkage in the 1pter-p36 region. Under the narrow model, the same three loci showed (non-significant) evidence of linkage. These linkage findings (1pter-p36, 18p11, and 5q35) highlight where genes for psychosis and schizophrenia are most likely to be found in persons of Mexican and Central American ancestry, and correspond to recent linkages of schizophrenia or psychosis in other populations which were formed in part from emigrants from the Spanish empire of the 15th and 16th centuries.

Malic enzyme 2 and susceptibility to psychosis and mania

Previous studies have identified a putative gene locus for both schizophrenia and bipolar disorder in the chromosome 18q21 region. To identify candidate genes associated with these disorders we completed fine mapping analyses (using microsatellite markers) in 152 families from the Central Valley of Costa Rica (CVCR) (376 total subjects, 151 with a history of psychosis, 97 with a history of mania). Microsatellite analyses showed evidence of association at two contiguous markers, both located at the same genetic distance and spanning approximately 11 known genes. In a corollary gene expression study, one of these genes, malic enzyme 2 (ME2), showed levels of gene expression 5.6-fold lower in anterior cingulate tissue from post-mortem bipolar brains. Subsequent analysis of individual SNPs in strong linkage disequilibrium with the ME2 gene revealed one SNP and one haplotype associated with the phenotype of psychosis in the CVCR sample. ME2 interacts directly with the malate shuttle system, which has been shown to be altered in schizophrenia and bipolar disorder, and has roles in neuronal synthesis of glutamate and gamma-amino butyric acid. The present study suggests that genetic variation in or near the ME2 gene is associated with both psychotic and manic disorders, including schizophrenia and bipolar disorder.

Chasing genes for mood disorders and schizophrenia in genetically isolated populations

Major affective disorders and schizophrenia are among the most common brain diseases worldwide and their predisposition is influenced by a complex interaction of genetic and environmental factors. So far, traditional linkage mapping studies for these complex disorders have not achieved the same success as the positional cloning of genes for Mendelian diseases. The struggle to identify susceptibility genes for complex disorders has stimulated the development of alternative approaches, including studies in genetically isolated populations. Since isolated populations are likely to have both a reduced number of genetic vulnerability factors and environmental background and are therefore considered to be more homogeneous compared to outbred populations, the use of isolated populations in genetic studies is expected to improve the chance of finding susceptibility loci and genes. Here we review the role of isolated populations, based on linkage and association studies, in the identification of susceptibility genes for bipolar disorder and schizophrenia.

Suicide in Ethiopian immigrants in Israel: a case for study of the genetic-environmental relation in suicide

Immigration is a major life stress event, which has been associated with increased levels of mental health problems. The linkage between immigration and suicide has been studied in various societies, with conflicting results. Extremely high rates of suicide have been found repeatedly among the population of the Ethiopian immigrants in Israel. These rates were significantly higher than other immigrant populations in the country. Possible explanations for this disparity are discussed, and future directions for further study of this area are suggested.

Genetic analysis of psychiatric disorders in humans

Psychiatric disorders place a large burden not only on affected individuals and their families but also on societies and health services. Current treatment is only effective in a proportion of the patients, so considerable effort has been put into the development of new medications. The susceptibility to all major psychiatric disorders is, at least in part, genetic. Knowledge of the genes that underlie this susceptibility may lead to the identification of new drug targets and the development of more effective treatments. Therefore, numerous genetic studies in search for the genes involved in psychiatric disorders have been performed. Although results of both linkage and association studies have been inconsistent, several promising gene regions and candidate genes have been identified recently. In this article, we will review the strategies that proved to be successful in detecting genes for psychiatric disorders and we will provide some recommendations to increase the probability of detecting susceptibility genes in genetic studies of different designs.

A genome-wide search for alleles and haplotypes associated with autism and related pervasive developmental disorders on the Faroe Islands

The involvement of genetic factors in the etiology of autism has been clearly established. We undertook a genome-wide search for regions containing susceptibility genes for autism in 12 subjects with childhood autism and related pervasive developmental disorders (PDDs) and 44 controls from the relatively isolated population of the Faroe Islands. In total, 601 microsatellite markers distributed throughout the human genome with an average distance of 5.80 cM were genotyped, including 502 markers in the initial scan. The Faroese population structure and genetic relatedness of cases and controls were also evaluated. Based on a combined approach, including an assumption-free test as implemented in CLUMP, Fisher's exact test for specific alleles and haplotypes, and IBD(0) probability calculations, we found association between autism and microsatellite markers in regions on 2q, 3p, 6q, 15q, 16p, and 18q. The most significant finding was on 3p25.3 (P(T1)=0.00003 and P(T4)=0.00007), which was also supported by other genetic studies. Furthermore, no evidence of population substructure was found, and a higher degree of relatedness among cases could not be detected, decreasing the risk of inflated P-values. Our data suggest that markers in these regions are in linkage disequilibrium with genes involved in the etiology of autism, and we hypothesize susceptibility genes for autism and related PDDs to be localized within these regions.

Screening of PIP5K2A promoter region for mutations in bipolar disorder and schizophrenia

OBJECTIVE

To analyze the promoter region of PIP5K2A, a phosphatidylinositol 4-phosphate 5-kinase that maps to 10p in a region linked to both bipolar disorder and schizophrenia.

METHODS

The promoter region was screened by single-strand conformation polymorphism analysis and DNA sequencing. Allele frequencies were determined in a case-control study. Functional significance of a promoter variant was determined by electromobility gel shift assays.

RESULTS

Homozygosity for a rare putative promoter variant, -1007C-->T, was found in only two patients with schizophrenia and in no controls or bipolar patients. The variant forms a 7/8 base match for the binding site of Oct-1, a member of the POU homeodomain family. Electromobility gel shift assays revealed increased binding of a brain-specific nuclear protein to the -1007T allele compared with -1007C.

CONCLUSION

The data suggest that homozygosity for -1007T could be a rare genetic factor in the development of schizophrenia.

Evidence of genetic overlap of schizophrenia and bipolar disorder: linkage disequilibrium analysis of chromosome 18 in the Costa Rican population

The long-standing concept that schizophrenia (SC) and bipolar disorder (BP) represent two distinct illnesses has been recently challenged by findings of overlap of genetic susceptibility loci for these two diseases. We report here the results of a linkage disequilibrium (LD) analysis of chromosome 18 utilizing subjects with SC from the Central Valley of Costa Rica. Evidence of association (P < 0.05) was obtained in three chromosomal regions: 18p11.31 (D18S63), 18q12.3 (D18S474), and 18q22.3-qter (D18S1161, D18S70), all of which overlap or are in close proximity with loci previously shown to be in LD with BP, type I in this population. Since both the SC and bipolar samples contained cases with a history of mania and almost all cases of SC and BP had a history of psychosis, we performed an alternative phenotyping strategy to determine whether presence or absence of mania, in the context of psychosis, would yield distinct linkage patterns along chromosome 18. To address this issue, a cohort of psychotic patients (including a range of DSMIV diagnoses) was divided into two groups based on the presence or absence of mania. Regions that showed association with SC showed segregation of association when the sample was stratified by history of mania. Our results are compared with previous genetic studies of susceptibility to SC or BP, in Costa Rica as well as in other populations. This study illustrates the importance of detailed phenotype analysis in the search for susceptibility genes influencing complex psychiatric disorders in isolated populations and suggests that subdivision of psychoses by presence or absence of past mania syndromes may be useful to define genetic subtypes of chronic psychotic illness.

Genetic studies of neuropsychiatric disorders in Costa Rica: a model for the use of isolated populations

The importance of genetics in understanding the etiology of mental illness has become increasingly clear in recent years, as more evidence has mounted that almost all neuropsychiatric disorders have a genetic component. It has also become clear, however, that these disorders are etiologically complex, and multiple genetic and environmental factors contribute to their makeup. So far, traditional linkage mapping studies have not definitively identified specific disease genes for neuropsychiatric disorders, although some potential candidates have been identified via these methods (e.g. the dysbindin gene in schizophrenia; Straub et al., 2002; Schwab et al., 2003). For this reason, alternative approaches are being attempted, including studies in genetically isolated populations. Because isolated populations have a high degree of genetic homogeneity, their use may simplify the process of identifying disease genes in disorders where multiple genes may play a role. Several areas of Latin America contain genetically isolated populations that are well suited for the study of neuropsychiatric disorders. Genetic studies of several major psychiatric illnesses, including bipolar disorder, major depression, schizophrenia, Tourette Syndrome, alcohol dependence, attention deficit hyperactivity disorder, and obsessive-compulsive disorder, are currently underway in these regions. In this paper we highlight the studies currently being conducted by our groups in the Central Valley of Costa Rica to illustrate the potential advantages of this population for genetic studies.

Linkage disequlibrium in the DTNBP1 (dysbindin) gene region and on chromosome 1p36 among psychotic patients from a genetic isolate in Israel: findings from identity by descent haplotype sharing analysis

Several genes have been reported recently to be associated with schizophrenia and bipolar disorder. Because of the complexity of the inheritance of these disorders, there is an urgent need to replicate these findings and to search for additional candidate genes. The study of genetic isolates is a powerful technique that may overcome some of the obstacles caused by genetic heterogeneity and ambiguity of phenotype definition. Identity by descent (IBD) haplotype sharing analysis in these populations may be used to detect mutations within shared haplotypes in smaller samples of affected individuals. In this study, we used IBD haplotype sharing analysis to replicate positive linkage and association findings in psychotic disorders, and to identify other regions of interest. Fifty-two patients with major psychiatric disorders from a genetically isolated village in Israel were studied. By studying eight Y chromosome markers, we were able to confirm the oral tradition of members of this isolate regarding a common paternal origin. Three hundred fifty nine microsatellite markers on 9 candidate chromosomes were genotyped, and haplotypes were reconstructed using information from family members. Two highly significant (P < 0.0001) peaks of haplotype sharing were found. One was for psychotic patients with any diagnosis at the location of dysbindin, a gene previously associated with schizophrenia. The other peak was for patients with schizophrenia on chromosome 1p36. Thus, this study both replicates an earlier finding and points to a novel region of interest, which might be unique to this population.

Identification of PIK3C3 promoter variant associated with bipolar disorder and schizophrenia

BACKGROUND

Genes involved in phosphoinositide (PI) lipid metabolism are excellent candidates to consider in the pathogenesis of bipolar disorder (BD) and schizophrenia (SZ). One is PIK3C3, a member of the phosphatidylinositide 3-kinase family that maps closely to markers on 18q linked to both BD and SZ in a few studies.

METHODS

The promoter region of PIK3C3 was analyzed for mutations by single-strand conformation polymorphism analysis and sequencing. A case-control association study was conducted to determine the distribution of variant alleles in unrelated patients from three cohorts. Electromobility gel shift assays (EMSA) were performed to assess the functional significance of variants.

RESULTS

Two polymorphisms in complete linked disequilibrium with each other were identified, -432C- > T and a "C" insert at position -86. The -432T allele occurs within an octamer containing an ATTT motif resembling members of the POU family of transcription factors. In each population analyzed, an increase in -432T was found in patients. EMSAs showed that a -432T containing oligonucleotide binds to brain proteins that do not recognize -432C.

CONCLUSIONS

A promoter mutation in a PI regulator affecting the binding of a POU-type transcription factor may be involved in BD and SZ in a subset of patients.

Possible evidence for a common risk locus for bipolar affective disorder and schizophrenia on chromosome 4p16 in patients from the Faroe Islands

Patients with schizophrenia (n=11) and bipolar affective disorder (n=17) from the relatively isolated population of the Faroe Islands were genotyped for 34 polymorphic markers on chromosome 4 in a search for allelic association and haplotype sharing among distantly related patients. When considering bipolar patients only, there was no clearcut support for any region on chromosome 4. The two-marker segment D4S394-D4S2983 at 4p16.1 was, however, supported by a P-value of 0.0162. For patients with schizophrenia, there was reasonable support for 4p16.1 as marker D4S2281 (P=0.0019), a two-marker segment (D4S2281-D4S1605, P=0.0009) and a three-marker segment (D4S2923-D4S2928-D4S1582, P-0.0005) appeared to be associated with schizophrenia, with some alleles/haplotypes occurring with different frequencies in patients compared to controls. When combining both psychiatric disorders, chromosome 4p16.1 received further support from five partially overlapping two- and three-marker segments (D4S394-D4S2983, P=0.0039; D4S2281-D4S1605, P=0.0027 and D4S394-D4S2983-D4S2923, P=0.006; D4S2923-D4S2928-D4S1582, P=0.00007; D4S1582-D4S1599-D4S2281, P=0.005). Increased haplotype sharing in patients with schizophrenia and in the combined data set was partly supported by Fisher's exact test and tests based on the genealogy. Our study yields support for a common risk gene for schizophrenia and bipolar affective disorder on the short arm of chromosome 4, as suggested by previous findings in the neighbouring Scottish population.

Genetic linkage in bipolar disorder

Bipolar disorder is an etiologically complex syndrome that is clearly heritable. Multiple genes, working singly or in concert, are likely to cause susceptibility to bipolar disorder. Bipolar disorder genetics has progressed rapidly in the last few decades. However, specific causal genetic mutations for bipolar disorder have not been identified. Both candidate gene studies and complete genome screens have been conducted. They have provided compelling evidence for several potential bipolar disorder susceptibility loci in several regions of the genome. The strongest evidence suggests that bipolar disorder susceptibility loci may lie in one or more genomic regions on chromosomes 18, 4, and 21. Other regions of interest, including those on chromosomes 5 and 8, are also under investigation. New approaches, such as the use of genetically isolated populations and the use of endophenotypes for bipolar disorder, hold promise for continued advancement in the search to identify specific bipolar disorder genes.

Polymorphism screening of PIP5K2A: a candidate gene for chromosome 10p-linked psychiatric disorders

Lithium is a potent noncompetitive inhibitor of inositol monophosphatases, enzymes involved in phosphoinositide (PI) and inositol phosphate metabolism. A critical component of the PI pathway is phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), which is hydrolyzed to second messengers and has a direct role in synaptic vesicle function. Interestingly, a number of genes involved in the synthesis and dephosphorylation of PtdIns(4,5)P(2) are found in regions of the genome previously mapped in bipolar disorder (BD) including 10p12, 21q22, and 22q11, among others. Some of these regions overlap with loci mapped in schizophrenia (SZ). One gene involved in PI metabolism that maps to a region of interest is 10p12-linked PIP5K2A, a member of the phosphatidylinositol 4-phosphate 5-kinase family. Polymorphism screening revealed the existence of an imperfect CT repeat polymorphism located near the exon 9-intron 9 splice donor site. A modest difference was found in the distribution of alleles from this highly polymorphic variant when bipolar and schizophrenic subjects were compared with controls; relatively rare short repeat variants were found more commonly in patients and homozygosity for a common long repeat variant was found more commonly in controls. These data suggest that the imperfect CT repeat in PIP5K2A intron 9 should be further investigated as a possible candidate allele for 10p12-linked psychiatric disorders.

A new essential hypertension susceptibility locus on chromosome 2p24-p25, detected by genomewide search

Essential hypertension (EH) is a complex disorder that results from the interaction of a number of susceptibility genes and environmental factors. We studied an isolated Sardinian village (Talana) in which the prevalence of hypertension is comparable to that in most Western populations. Talana exhibits features, such as slow demographic growth, high inbreeding, a low number of founders, stable lifestyle and culture, and accurate genealogical records, that make it suitable for the study of complex disorders. Clinical assessment of the entire adult population (N= approximately 1,000) identified approximately 100 hypertensive subjects. For our study, we selected the individuals with the most-severe EH (i.e., diastolic blood pressure >100 mm Hg), belonging to a single deep-rooted pedigree (12 generations), whose common ancestors lived in the 17th century. We performed a three-stage genomewide search using 36 affected individuals, by means of parametric linkage and allele-sharing approaches. LOD scores >1 were observed on chromosomes 1, 2, 13, 15, 17, and 19 (stage I). The most striking result was found in a 7.57-cM region on chromosome 2p24-p25. All five nonparametric linkage statistics estimated by the SimWalk2 program lie above the significance threshold of P<.008 for the whole region. Similar significance was obtained for 2p24-25 when parametric linkage (LOD score 1.99) and linkage disequilibrium mapping (P=.00006) were used, suggesting that a hypertension-susceptibility locus is located between D2S2278 and D2S168. This finding is strengthened by a recent report of linkage with marker D2S168 in a hypertensive sib-pair sample from China.

Genomewide linkage disequilibrium mapping of severe bipolar disorder in a population isolate

Genomewide association studies may offer the best promise for genetic mapping of complex traits. Such studies in outbred populations require very densely spaced single-nucleotide polymorphisms. In recently founded population isolates, however, extensive linkage disequilibrium (LD) may make these studies feasible with currently available sets of short tandem repeat markers, spaced at intervals as large as a few centimorgans. We report the results of a genomewide association study of severe bipolar disorder (BP-I), using patients from the isolated population of the central valley of Costa Rica. We observed LD with BP-I on several chromosomes; the most striking results were in proximal 8p, a region that has previously shown linkage to schizophrenia. This region could be important for severe psychiatric disorders, rather than for a specific phenotype.

Genome-wide scan for linkage to schizophrenia in a Spanish-origin cohort from Costa Rica

Genetic isolates have been useful cohorts in which to search for genes underlying disorders of unknown pathology. One such cohort is thought to exist in the Central Valley of Costa Rica surrounding the city of San Jose. Previous investigators identified a rare dominant gene for hereditary deafness in this population, and a suggestive linkage of severe bipolar psychosis has been reported in another study. Ninety-nine families with at least one pair of siblings affected with schizophrenia or a schizophrenia-spectrum diagnosis had clinical evaluations and DNA collected for genotyping. The Marshfield Medical Research Foundation (NHLBI) Mammalian Genotyping Service performed all genotyping using 404 short-tandem repeat polymorphic markers (STRPs) spaced on average 10 cM apart. Data were analyzed using the nonparametric program, GeneHunterPlus. The population structure was investigated using the STRUCT program. No region was found with genome-wide significance for linkage. Using a phenotype of schizophrenia plus schizoaffective disorder, the highest maximum likelihood score (MLS) observed was 1.78 (P < 0.004) at 176.6 cM from pter on chromosome 5q, an area previously implicated by some other groups. In addition, five regions on chromosomes 1p, 2p, 2q, 14p, and 8p had MLSs above 1.0. All other regions produced scores below 1.0. Population genetic analysis reveals no evidence for population substructure, for admixture with other populations, such as Amerindians, or for inbreeding in the parental generation. The latter casts some doubt on this population being an isolate, although there was evidence of inbreeding among the offspring.

Genetics of population isolates

Genetic isolates, as shown empirically by the Finnish, Old Order Amish, Hutterites, Sardinian and Jewish communities among others, represent a most important and powerful tool in genetically mapping inherited disorders. The main features associated with that genetic power are the existence of multigenerational pedigrees which are mostly descended from a small number of founders a short number of generations ago, environmental and phenotypic homogeneity, restricted geographical distribution, the presence of exhaustive and detailed records correlating individuals in very well ascertained pedigrees, and inbreeding as a norm. On the other hand, the presence of a multifounder effect or admixture among divergent populations in the founder time (e.g. the Finnish and the Paisa community from Colombia) will theoretically result in increased linkage disequilibrium among adjacent loci. The present review evaluates the historical context and features of some genetic isolates with emphasis on the basic population genetic concepts of inbreeding and genetic drift, and also the state-of-the-art in mapping traits, both Mendelian and complex, on genetic isolates.

Search for common haplotypes on chromosome 22q in patients with schizophrenia or bipolar disorder from the Faroe Islands

Chromosome 22q may harbor risk genes for schizophrenia and bipolar affective disorder. This is evidenced through genetic mapping studies, investigations of cytogenetic abnormalities, and direct examination of candidate genes. Patients with schizophrenia and bipolar affective disorder from the Faroe Islands were typed for 35 evenly distributed polymorphic markers on 22q in a search for shared risk genes in the two disorders. No single marker was strongly associated with either disease, but five two-marker segments that cluster within two regions on the chromosome have haplotypes occurring with different frequencies in patients compared to controls. Two segments were of most interest when the results of the association tests were combined with the probabilities of identity by descent of single haplotypes. For bipolar patients, the strongest evidence for a candidate region harboring a risk gene was found at a segment of at least 1.1 cM including markers D22S1161 and D22S922 (P=0.0081 in the test for association). Our results also support the a priori evidence of a susceptibility gene to schizophrenia at a segment of at least 0.45 cM including markers D22S279 and D22S276 (P=0.0075). Patients were tested for the presence of a missense mutation in the WKL1 gene encoding a putative cation channel close to segment D22S1161--D22S922, which has been associated with schizophrenia. We did not find this mutation in schizophrenic or bipolar patients or the controls from the Faroe Islands.

Population isolates: their special value for locating genes for bipolar disorder

OBJECTIVES

Population isolates offer several advantages for those hoping to identify predisposition genes for bipolar disorder (BP). In this review article, the rationale for performing gene mapping studies in this type of population and the results of genetic mapping studies performed to date in population isolates are presented.

METHODS

This article begins with a brief review of the concepts involved in mapping genes for BP. The concept of populations that show some degree of historical isolation and their special utility for certain types of gene mapping is presented. Methods of statistical analysis particularly relevant for gene mapping of complex diseases like BP are presented. Finally, several BP gene studies conducted to date in several population isolates are reviewed.

RESULTS

Genetic mapping studies of BP have occurred thus far in several isolates or sub-isolates, including the Amish population, Costa Ricans, Finnish, and Canadians (in Quebec), and significant linkage scores have been identified in the latter three isolates.

CONCLUSIONS

Possible greater homogeneity and greater consistency of diagnosis are factors that have been cited in several studies of BP done in isolates to date. Another special advantage of working in certain types of population isolate is their appropriateness for using certain types of association or linkage disequilibrium-based approaches at both the genome screening and fine mapping stages. These tests include mapping by linkage disequilibrium analyses, an approach that allows mapping to occur at the population, rather than the pedigree, level.

Fine-scale mapping of a locus for severe bipolar mood disorder on chromosome 18p11.3 in the Costa Rican population

We have searched for genes predisposing to bipolar disorder (BP) by studying individuals with the most extreme form of the affected phenotype, BP-I, ascertained from the genetically isolated population of the Central Valley of Costa Rica (CVCR). The results of a previous linkage analysis on two extended CVCR BP-I pedigrees, CR001 and CR004, and of linkage disequilibrium (LD) analyses of a CVCR population sample of BP-I patients implicated a candidate region on 18p11.3. We further investigated this region by creating a physical map and developing 4 new microsatellite and 26 single-nucleotide polymorphism markers for typing in the pedigree and population samples. We report the results of fine-scale association analyses in the population sample, as well as evaluation of haplotypes in pedigree CR001. Our results suggest a candidate region containing six genes but also highlight the complexities of LD mapping of common disorders.