Jesper Ekelund

Genome Scan Meta-Analysis of Schizophrenia and Bipolar Disorder, Part II: Schizophrenia

Schizophrenia is a common disorder with high heritability and a 10-fold increase in risk to siblings of probands. Replication has been inconsistent for reports of significant genetic linkage. To assess evidence for linkage across studies, rank-based genome scan meta-analysis (GSMA) was applied to data from 20 schizophrenia genome scans. Each marker for each scan was assigned to 1 of 120 30-cM bins, with the bins ranked by linkage scores (1 = most significant) and the ranks averaged across studies (R(avg)) and then weighted for sample size (N(sqrt)[affected casess]). A permutation test was used to compute the probability of observing, by chance, each bins average rank (P(AvgRnk)) or of observing it for a bin with the same place (first, second, etc.) in the order of average ranks in each permutation (P(ord)). The GSMA produced significant genomewide evidence for linkage on chromosome 2q (PAvgRnk<.000417). Two aggregate criteria for linkage were also met (clusters of nominally significant P values that did not occur in 1,000 replicates of the entire data set with no linkage present): 12 consecutive bins with both P(AvgRnk) and P(ord)<.05, including regions of chromosomes 5q, 3p, 11q, 6p, 1q, 22q, 8p, 20q, and 14p, and 19 consecutive bins with P(ord)<.05, additionally including regions of chromosomes 16q, 18q, 10p, 15q, 6q, and 17q. There is greater consistency of linkage results across studies than has been previously recognized. The results suggest that some or all of these regions contain loci that increase susceptibility to schizophrenia in diverse populations.

Replication of 1q42 linkage in Finnish schizophrenia pedigrees

Chromosome 1q has been implicated in the etiology of schizophrenia in several independent studies. However, the peak linkage findings have been dispersed over a large chromosomal region, with negative findings in this region also being reported. Our group has previously observed linkage on chromosome 1q42, maximizing within the DISC1 gene, which has also been implied in the etiology of schizophrenia based on functional studies. In the study presented here, we genotyped 300 polymorphic markers on chromosome 1 using a study sample of 70 families with multiple individuals affected with schizophrenia or related conditions, independent of the study samples in our previous reports. We again found evidence for linkage on 1q42 maximizing within the DISC1 gene (rs1000731, lod=2.70). Further, a haplotype containing the most strongly linked markers showed some evidence of association with the disease. This replicates the previous linkage finding in the same region and constitutes supportive evidence for a susceptibility gene in this region.

A haplotype within the DISC1 gene is associated with visual memory functions in families with a high density of schizophrenia

We have previously reported evidence of linkage and association between markers on 1q42 and schizophrenia in a study sample of 498 multiply affected Finnish nuclear families, leading to the recent identification of four significantly associated haplotypes that specifically implicate the Translin-Associated Factor X (TRAX) and Disrupted in Schizophrenia 1 and 2 (DISC1 and DISC2) genes in the genetic etiology of schizophrenia. Previously, the DISC genes were found to be disrupted by a balanced translocation (1;11)(q42.1;q14.3) that cosegregated with schizophrenia and related disorders in a large Scottish pedigree. Interestingly, we also reported earlier suggestive linkage between endophenotypic quantitative traits of visual and verbal memory and microsatellite markers in close proximity to TRAX/DISC, on 1q41. Here, we tested if the identified allelic haplotypes of TRAX/DISC would be associated with visual and/or verbal memory function impairments that are known to aggregate with schizophrenia in families. One haplotype of DISC1, HEP3, displayed association with poorer performance on tests assessing short-term visual memory and attention. Analysis of affected and unaffected offspring separately revealed that both samples contribute to the observed association to visual working memory. These results provide genetic support to the view that the DISC1 gene contributes to sensitivity to schizophrenia and associated disturbances and affects short-term visual memory functions. This finding should stimulate studies aiming at the molecular characterization of how the specific alleles of DISC1 affect the visual memory functions and eventually participates in the development of schizophrenia.

Genetic linkage and association between chromosome 1q and working memory function in schizophrenia

Schizophrenia is substantially heritable, but specific susceptibility genes remain to be identified. Progress in this endeavor has been hindered by non‐Mendelian transmission patterns, probable genetic heterogeneity, and an inability to detect premorbid and nonpenetrant carriers of predisposing genes. To circumvent these complexities, this study employed quantitative measures of liability, or “endophenotypes,” within a sample of twins discordant for schizophrenia, drawn from the relatively genetically isolated population of Finland. A region on the distal portion of chromosome 1 has shown evidence for linkage to schizophrenia in two prior studies using Finnish patient samples. To elucidate further the nature and location of this potential susceptibility gene, linkage and association analyses were carried out across the chromosome 1 region of interest using quantitative neuropsychological measures of liability. Analyses with a composite measure of liability yielded suggestive evidence for linkage at marker D1S2833 (P = 0.04). Follow‐up analyses of the individual trait measures showed that the Visual Span subtest of the Wechsler Memory Scale (WMS), an indicator of spatial working memory function, was uniquely sensitive to marker D1S2833 (P = 0.007). Association analysis confirmed that allelic variation in D1S2833 is associated with variation in spatial working memory performance as measured by the Visual Span subtest (P = 0.003), the significance of which was confirmed in an analysis of 10,000 Monte Carlo permutations. These data support the utility of this approach and provide evidence for a gene affecting spatial working memory function in schizophrenia patients and their unaffected co‐twins.

Replication of linkage on chromosome 7q22 and association of the regional Reelin gene with working memory in schizophrenia families

Schizophrenia is a common and complex mental disorder. Hereditary factors are important for its etiology, but despite linkage signals reported to several chromosomal regions in different populations, final identification of predisposing genes has remained a challenge. Utilizing a large family-based schizophrenia study sample from Finland, we have identified several linked loci: 1q32.2–q42, 2q, 4q31, 5q and 7q22. In this study, an independent sample of 352 nuclear schizophrenia families (n=1626) allowed replication of linkage on 7q21–32. In a sample of 245 nuclear families (n=1074) originating from the same geographical region as the families revealing the linkage, SNP and microsatellite association analyses of the four regional candidate genes, GRM3, RELN, SEMA3A and VGF, revealed no significant association to the clinical diagnosis of schizophrenia. Instead, quantifiable trait component analyses with neuropsychological endophenotypes available from 186 nuclear families (n=861) of the sample showed significant association to RELN variants for traits related to verbal (P=0.000003) and visual working memory (P=0.002), memory (P=0.002) and executive functioning (P=0.002). Trait-associated allele-positive subjects scored lower in the tests measuring working memory (P=0.0004–0.0000000004), memory (P=0.02–0.0001) and executive functioning (P=0.001). Our findings suggest that allelic variants of RELN contribute to the endophenotypes of schizophrenia.

Linkage analysis of putative schizophrenia gene candidate regions on chromosomes 3p, 5q, 6p, 8p, 20p and 22q in a population-based sampled Finnish family set.

During the past decade numerous studies have been published describing chromosomal regions potentially linked with schizophrenia. Unfortunately, none of these studies has been able to conclusively identify any specific gene that predisposes to schizophrenia. Typically evidence for linkage is seen on large chromosomal regions, as expected, containing tens or even hundreds of genes. Furthermore, attempts to replicate the findings have rarely been successful leaving a confusion about the existence of predisposing genes for schizophrenia in a particular region of the genome. We have carried out linkage analysis in a set of 62 pedigrees rising from a genetically isolated population of Finland with markers on six chromosomal regions earlier suggested to harbor predisposing genes for schizophrenia, namely 3p,1 5q,2,3 6p,4 8p,1 20p,5 and 22q.6,7 We were not able to find significant evidence for linkage on any of these chromosomal regions. However, some support for linkage was found on all studied chromosomal regions, except 3p.

Nature and nurture in novelty seeking.

A sample of children (n=92), derived from a representative population sample of healthy young Finns (n=2149), was studied from childhood to adulthood over 14 years to determine whether the childhood environment moderated the effect of dopamine receptor gene (DRD4) polymorphism on novelty seeking (NS). A significant interaction between the DRD4 alleles and environmental variables was observed. When the childhood-rearing environment was more hostile (emotionally distant, low tolerance of the childs normal activity, and strict discipline), the participants carrying any two- or five-repeat alleles of the DRD4 gene had a significantly greater risk of exhibiting NS scores that were above the 10th percentile on a population distribution of 2149 adult Finnish women and men. The genotype had no effects on NS when the childhood environment was more favorable. Although the results are preliminary, pending replication, they nevertheless provide important information on the long-term effects of nurture and nature on NS temperament.

An MDL method for finding haplotype blocks and for estimating the strength of haplotype block boundaries.

We describe a new method for finding haplotype blocks based on the use of the minimum description length principle. We give a rigorous definition of the quality of a segmentation of a genomic region into blocks, and describe a dynamic programming algorithm for finding the optimal segmentation with respect to this measure. We also describe a method for finding the probability of a block boundary for each pair of adjacent markers: this gives a tool for evaluating the significance of each block boundary. We have applied the method to the published data of Daly et al. The results are in relatively good agreement with the published results, but also show clear differences in the predicted block boundaries and their strengths. We also give results on the block structure in population isolates.

TTC12-ANKK1-DRD2 and CHRNA5-CHRNA3-CHRNB4 influence different pathways leading to smoking behavior from adolescence to mid-adulthood.

BACKGROUND CHRNA5-CHRNA3-CHRNB4 and TTC12-ANKK1-DRD2 gene-clusters influence smoking behavior. Our aim was to test developmental changes in their effects as well as the interplays between them and with nongenetic factors. METHODS Participants included 4762 subjects from a general population-based, prospective Northern Finland 1966 Birth Cohort (NFBC 1966). Smoking behavior was collected at age 14 and 31 years. Information on maternal smoking, socioeconomic status, and novelty seeking were also collected. Structural equation modeling was used to construct an integrative etiologic model including genetic and nongenetic factors. RESULTS Several single nucleotide polymorphisms in both gene-clusters were significantly associated with smoking. The most significant were in CHRNA3 (rs1051730, p = 1.1 × 10(-5)) and in TTC12 (rs10502172, p = 9.1 × 10(-6)). CHRNA3-rs1051730[A] was more common among heavy/regular smokers than nonsmokers with similar effect-sizes at age 14 years (odds ratio [95% CI]: 1.27 [1.06-1.52]) and 31 years (1.28 [1.13-1.44]). TTC12-rs10502172[G] was more common among smokers than nonsmokers with stronger association at 14 years (1.33 [1.11-1.60]) than 31 years (1.14 [1.02-1.28]). In adolescence, carriers of three-four risk alleles at either CHRNA3-rs1051730 or TTC12-rs10502172 had almost threefold odds of smoking regularly than subjects with no risk alleles. TTC12-rs10502172 effect on smoking in adulthood was mediated by its effect on smoking in adolescence and via novelty seeking. Effect of CHRNA3-rs1051730 on smoking in adulthood was direct. CONCLUSIONS TTC12-ANKK1-DRD2s seemed to influence smoking behavior mainly in adolescence, and its effect is partially mediated by personality characteristics promoting drug-seeking behavior. In contrast, CHRNA5-CHRNA3-CHRNB4 is involved in the transition toward heavy smoking in mid-adulthood and in smoking persistence. Factors related to familial and social disadvantages were strong independent predictors of smoking.

Association of variants in DISC1 with psychosis-related traits in a large population cohort

CONTEXT There is an abundance of data from human genetic studies and animal models that implies a role for the disrupted in schizophrenia 1 gene (DISC1) in the etiology of schizophrenia and other major mental illnesses. OBJECTIVE To study the effect of previously identified risk alleles of DISC1 on quantitative intermediate phenotypes for psychosis in an unselected population. DESIGN We examined 41 single-nucleotide polymorphisms within DISC1 and performed tests of association with 4 quantitative phenotypes. SETTING Academic research. PARTICIPANTS Individuals from an unselected birth cohort in Finland. Originally, everyone born in the catchment area in 1966 (N = 12 058) was included in the study. Of these, 4651 (38.6%) attended the 31-year follow-up and could be included in the study. MAIN OUTCOME MEASURES Scores on 4 psychometric instruments selected to function as proxies for positive and negative aspects of psychotic disorders, including the Perceptual Aberration Scale, Revised Social Anhedonia Scale, Revised Physical Anhedonia Scale, and Schizoidia Scale by Golden and Meehl. RESULTS Carriers of the minor allele of marker rs821577 had significantly higher scores on social anhedonia (P < .001). The minor allele of marker rs821633 was strongly associated with lower scores on social anhedonia when analyzed dependent on the absence of the minor alleles of markers rs1538979 and rs821577 (P < .001). CONCLUSIONS Variants in DISC1 affect the level of social anhedonia, a cardinal symptom of schizophrenia in the general population. DISC1 might be more central to human psychological functioning than previously thought, as it seems to affect the degree to which people enjoy social interactions.