Genome-wide linkage scan of 98 bipolar pedigrees and analysis of clinical covariates

Association between the Arylalkylamine N-Acetyltransferase (AANAT) Gene and Seasonality in Patients with Bipolar Disorder

OBJECTIVE

Bipolar disorder (BD) is complex genetic disorder. Therefore, approaches using clinical phenotypes such as biological rhythm disruption could be an alternative. In this study, we explored the relationship between melatonin pathway genes with circadian and seasonal rhythms of BD.

METHODS

We recruited clinically stable patients with BD (n=324). We measured the seasonal variation of mood and behavior (seasonality), and circadian preference, on a lifetime basis. We analyzed 34 variants in four genes (MTNR1a, MTNR1b, AANAT, ASMT) involved in the melatonin pathway.

RESULTS

Four variants were nominally associated with seasonality and circadian preference. After multiple test corrections, the rs116879618 in AANAT remained significantly associated with seasonality (corrected p=0.0151). When analyzing additional variants of AANAT through imputation, the rs117849139, rs77121614 and rs28936679 (corrected p=0.0086, 0.0154, and 0.0092) also showed a significant association with seasonality.

CONCLUSION

This is the first study reporting the relationship between variants of AANAT and seasonality in patients with BD. Since AANAT controls the level of melatonin production in accordance with light and darkness, this study suggests that melatonin may be involved in the pathogenesis of BD, which frequently shows a seasonality of behaviors and symptom manifestations.

Familial aggregation and linkage analysis with covariates for metabolic syndrome risk factors

BACKGROUND

Mechanisms of metabolic syndrome (MetS) causation are complex, genetic and environmental factors are important factors for the pathogenesis of MetS In this study, we aimed to evaluate familial and genetic influences on metabolic syndrome risk factor and also assess association between FTO (rs1558902 and rs7202116) and CETP(rs1864163) genes' single nucleotide polymorphisms (SNP) with low HDL_C in the Tehran Lipid and Glucose Study (TLGS).

MATERIALS AND METHODS

The design was a cross-sectional study of 1776 members of 227 randomly-ascertained families. Selected families contained at least one affected metabolic syndrome and at least two members of the family had suffered a loss of HDL_C according to ATP III criteria. In this study, after confirming the familial aggregation with intra-trait correlation coefficients (ICC) of Metabolic syndrome (MetS) and the quantitative lipid traits, the genetic linkage analysis of HDL_C was performed using conditional logistic method with adjusted sex and age.

RESULTS

The results of the aggregation analysis revealed a higher correlation between siblings than between parent-offspring pairs representing the role of genetic factors in MetS. In addition, the conditional logistic model with covariates showed that the linkage results between HDL_C and three marker, rs1558902, rs7202116 and rs1864163 were significant.

CONCLUSIONS

In summary, a high risk of MetS was found in siblings confirming the genetic influences of metabolic syndrome risk factor. Moreover, the power to detect linkage increases in the one parameter conditional logistic model regarding the use of age and sex as covariates.

The genetics of early-onset bipolar disorder: A systematic review

BACKGROUND

Early-onset bipolar disorder has been associated with a significantly worse prognosis than late-onset BD and has been hypothesized to be a genetically homogenous subset of BD. A sizeable number of studies have investigated early-onset BD through linkage-analyses, candidate-gene association studies, genome-wide association studies (GWAS), and analyses of copy number variants (CNVs), but this literature has not yet been reviewed.

METHODS

A systematic review was conducted using the PubMed database on articles published online before January 15, 2015 and after 1990. Separate searches were made for linkage studies, candidate gene-association studies, GWAS, and studies on CNVs.

RESULTS

Seventy-three studies were included in our review. There is a lack of robust positive findings on the genetics of early-onset BD in any major molecular genetics method.

LIMITATIONS

Early-onset populations were quite small in some studies. Variance in study methods hindered efforts to interpret results or conduct meta-analysis.

CONCLUSIONS

The field is still at an early phase for research on early-onset BD. The largely null findings mirror the results of most genetics research on BD. Although most studies were underpowered, the null findings could mean that early-onset BD may not be as genetically homogenous as has been hypothesized or even that early-onset BD does not differ genetically from adult-onset BD. Nevertheless, clinically the probabilistic developmental risk trajectories associated with early-onset that may not be primarily genetically determined continued to warrant scrutiny. Future research should dramatically expand sample sizes, use atheoretical research methods like GWAS, and standardize methods.

The null distribution of likelihood-ratio statistics in the conditional-logistic linkage model

Olson's conditional-logistic model retains the nice property of the LOD score formulation and has advantages over other methods that make it an appropriate choice for complex trait linkage mapping. However, the asymptotic distribution of the conditional-logistic likelihood-ratio (CL-LR) statistic with genetic constraints on the model parameters is unknown for some analysis models, even in the case of samples comprising only independent sib pairs. We derive approximations to the asymptotic null distributions of the CL-LR statistics and compare them with the empirical null distributions by simulation using independent affected sib pairs. Generally, the empirical null distributions of the CL-LR statistics match well the known or approximated asymptotic distributions for all analysis models considered except for the covariate model with a minimum-adjusted binary covariate. This work will provide useful guidelines for linkage analysis of real data sets for the genetic analysis of complex traits, thereby contributing to the identification of genes for disease traits.

Genome-wide significant association between a 'negative mood delusions' dimension in bipolar disorder and genetic variation on chromosome 3q26.1

Research suggests that clinical symptom dimensions may be more useful in delineating the genetics of bipolar disorder (BD) than standard diagnostic models. To date, no study has applied this concept to data from genome-wide association studies (GWAS). We performed a GWAS of factor dimensions in 927 clinically well-characterized BD patients of German ancestry. Rs9875793, which is located in an intergenic region of 3q26.1 and in the vicinity of the solute carrier family 2 (facilitated glucose transporter), member 2 gene (SLC2A2), was significantly associated with the factor analysis-derived dimension 'negative mood delusions' (n=927; P=4.65 × 10(-8), odds ratio (OR)=2.66). This dimension was comprised of the symptoms delusions of poverty, delusions of guilt and nihilistic delusions. In case-control analyses, significant association with the G allele of rs9875793 was only observed in the subgroup of BD patients who displayed symptoms of 'negative mood delusions' (allelic χ(2) model: P(G)=0.0001, OR=1.92; item present, n=89). Further support for the hypothesis that rs9875793 is associated with BD in patients displaying 'negative mood delusions' symptom, such as delusions of guilt, was obtained from an European American sample (GAIN/TGEN), which included 1247 BD patients and 1434 controls (P(EA)=0.028, OR=1.27).

Genome-wide linkage analysis of 972 bipolar pedigrees using single-nucleotide polymorphisms

Because of the high costs associated with ascertainment of families, most linkage studies of Bipolar I disorder (BPI) have used relatively small samples. Moreover, the genetic information content reported in most studies has been less than 0.6. Although microsatellite markers spaced every 10 cM typically extract most of the genetic information content for larger multiplex families, they can be less informative for smaller pedigrees especially for affected sib pair kindreds. For these reasons we collaborated to pool family resources and carried out higher density genotyping. Approximately 1100 pedigrees of European ancestry were initially selected for study and were genotyped by the Center for Inherited Disease Research using the Illumina Linkage Panel 12 set of 6090 single-nucleotide polymorphisms. Of the ~1100 families, 972 were informative for further analyses, and mean information content was 0.86 after pruning for linkage disequilibrium. The 972 kindreds include 2284 cases of BPI disorder, 498 individuals with bipolar II disorder (BPII) and 702 subjects with recurrent major depression. Three affection status models (ASMs) were considered: ASM1 (BPI and schizoaffective disorder, BP cases (SABP) only), ASM2 (ASM1 cases plus BPII) and ASM3 (ASM2 cases plus recurrent major depression). Both parametric and non-parametric linkage methods were carried out. The strongest findings occurred at 6q21 (non-parametric pairs LOD 3.4 for rs1046943 at 119 cM) and 9q21 (non-parametric pairs logarithm of odds (LOD) 3.4 for rs722642 at 78 cM) using only BPI and schizoaffective (SA), BP cases. Both results met genome-wide significant criteria, although neither was significant after correction for multiple analyses. We also inspected parametric scores for the larger multiplex families to identify possible rare susceptibility loci. In this analysis, we observed 59 parametric LODs of 2 or greater, many of which are likely to be close to maximum possible scores. Although some linkage findings may be false positives, the results could help prioritize the search for rare variants using whole exome or genome sequencing.

A new mouse model for mania shares genetic correlates with human bipolar disorder

Bipolar disorder (BPD) is a debilitating heritable psychiatric disorder. Contemporary rodent models for the manic pole of BPD have primarily utilized either single locus transgenics or treatment with psychostimulants. Our lab recently characterized a mouse strain termed Madison (MSN) that naturally displays a manic phenotype, exhibiting elevated locomotor activity, increased sexual behavior, and higher forced swimming relative to control strains. Lithium chloride and olanzapine treatments attenuate this phenotype. In this study, we replicated our locomotor activity experiment, showing that MSN mice display generationally-stable mania relative to their outbred ancestral strain, hsd:ICR (ICR). We then performed a gene expression microarray experiment to compare hippocampus of MSN and ICR mice. We found dysregulation of multiple transcripts whose human orthologs are associated with BPD and other psychiatric disorders including schizophrenia and ADHD, including: Epor, Smarca4, Cmklr1, Cat, Tac1, Npsr1, Fhit, and P2rx7. RT-qPCR confirmed dysregulation for all of seven transcripts tested. Using a novel genome enrichment algorithm, we found enrichment in genome regions homologous to human loci implicated in BPD in replicated linkage studies including homologs of human cytobands 1p36, 3p14, 3q29, 6p21–22, 12q24, 16q24, and 17q25. Using a functional network analysis, we found dysregulation of a gene system related to chromatin packaging, a result convergent with recent human findings on BPD. Our findings suggest that MSN mice represent a polygenic model for the manic pole of BPD showing much of the genetic systems complexity of the corresponding human disorder. Further, the high degree of convergence between our findings and the human literature on BPD brings up novel questions about evolution by analogy in mammalian genomes.

Association study of serotonin pathway genes in attempted suicide

Epidemiological studies, such as family, twin, and adoption studies, demonstrate the presence of a heritable component to both attempted and completed suicide. Some of this heritability is accounted for by the presence of comorbid psychiatric disorders, but the evidence also indicates that a portion of this heritability is specific to suicidality. The serotonergic system has been studied extensively in this phenotype, but findings have been inconsistent, possibly due to the presence of multiple susceptibility variants and/or gene-gene interactions. In this study, we genotyped 174 tag and coding single nucleotide polymorphisms (SNPs) from 17 genes within the serotonin pathway on 516 subjects with a major mood disorder and a history of a suicide attempt (cases) and 515 healthy controls, with the goal of capturing the common genetic variation across each of these candidate genes. We tested the 174 markers in single-SNP, haplotype, gene-based, and epistasis analyses. While these association analyses identified multiple marginally significant SNPs, haplotypes, genes, and interactions, none of them survived correction for multiple testing. Additional studies, including assessment in larger sample sets and deep resequencing to identify rare causal variants, may be required to fully understand the role that the serotonin pathway plays in suicidal behavior.

Combinations of SNPs related to signal transduction in bipolar disorder

Any given single nucleotide polymorphism (SNP) in a genome may have little or no functional impact. A biologically significant effect may possibly emerge only when a number of key SNP-related genotypes occur together in a single organism. Thus, in analysis of many SNPs in association studies of complex diseases, it may be useful to look at combinations of genotypes. Genes related to signal transmission, e.g., ion channel genes, may be of interest in this respect in the context of bipolar disorder. In the present study, we analysed 803 SNPs in 55 genes related to aspects of signal transmission and calculated all combinations of three genotypes from the 3×803 SNP genotypes for 1355 controls and 607 patients with bipolar disorder. Four clusters of patient-specific combinations were identified. Permutation tests indicated that some of these combinations might be related to bipolar disorder. The WTCCC bipolar dataset were use for replication, 469 of the 803 SNP were present in the WTCCC dataset either directly (n = 132) or by imputation (n = 337) covering 51 of our selected genes. We found three clusters of patient-specific 3×SNP combinations in the WTCCC dataset. Different SNPs were involved in the clusters in the two datasets. The present analyses of the combinations of SNP genotypes support a role for both genetic heterogeneity and interactions in the genetic architecture of bipolar disorder.

Comparative linkage meta-analysis reveals regionally-distinct, disparate genetic architectures: application to bipolar disorder and schizophrenia

New high-throughput, population-based methods and next-generation sequencing capabilities hold great promise in the quest for common and rare variant discovery and in the search for ”missing heritability.” However, the optimal analytic strategies for approaching such data are still actively debated, representing the latest rate-limiting step in genetic progress. Since it is likely a majority of common variants of modest effect have been identified through the application of tagSNP-based microarray platforms (i.e., GWAS), alternative approaches robust to detection of low-frequency (1–5% MAF) and rare (<1%) variants are of great importance. Of direct relevance, we have available an accumulated wealth of linkage data collected through traditional genetic methods over several decades, the full value of which has not been exhausted. To that end, we compare results from two different linkage meta-analysis methods—GSMA and MSP—applied to the same set of 13 bipolar disorder and 16 schizophrenia GWLS datasets. Interestingly, we find that the two methods implicate distinct, largely non-overlapping, genomic regions. Furthermore, based on the statistical methods themselves and our contextualization of these results within the larger genetic literatures, our findings suggest, for each disorder, distinct genetic architectures may reside within disparate genomic regions. Thus, comparative linkage meta-analysis (CLMA) may be used to optimize low-frequency and rare variant discovery in the modern genomic era.

Convergent functional genomic studies of ω-3 fatty acids in stress reactivity, bipolar disorder and alcoholism

Omega-3 fatty acids have been proposed as an adjuvant treatment option in psychiatric disorders. Given their other health benefits and their relative lack of toxicity, teratogenicity and side effects, they may be particularly useful in children and in females of child-bearing age, especially during pregnancy and postpartum. A comprehensive mechanistic understanding of their effects is needed. Here we report translational studies demonstrating the phenotypic normalization and gene expression effects of dietary omega-3 fatty acids, specifically docosahexaenoic acid (DHA), in a stress-reactive knockout mouse model of bipolar disorder and co-morbid alcoholism, using a bioinformatic convergent functional genomics approach integrating animal model and human data to prioritize disease-relevant genes. Additionally, to validate at a behavioral level the novel observed effects on decreasing alcohol consumption, we also tested the effects of DHA in an independent animal model, alcohol-preferring (P) rats, a well-established animal model of alcoholism. Our studies uncover sex differences, brain region-specific effects and blood biomarkers that may underpin the effects of DHA. Of note, DHA modulates some of the same genes targeted by current psychotropic medications, as well as increases myelin-related gene expression. Myelin-related gene expression decrease is a common, if nonspecific, denominator of neuropsychiatric disorders. In conclusion, our work supports the potential utility of omega-3 fatty acids, specifically DHA, for a spectrum of psychiatric disorders such as stress disorders, bipolar disorder, alcoholism and beyond.

Genome-wide association analysis of age at onset and psychotic symptoms in bipolar disorder

Genome-wide association studies (GWAS) have identified several susceptibility loci for bipolar disorder (BP), most notably ANK3. However, most of the inherited risk for BP remains unexplained. One reason for the limited success may be the genetic heterogeneity of BP. Clinical sub-phenotypes of BP may identify more etiologically homogeneous subsets of patients, which can be studied with increased power to detect genetic variation. Here, we report on a mega-analysis of two widely studied sub-phenotypes of BP, age at onset and psychotic symptoms, which are familial and clinically significant. We combined data from three GWAS: NIMH Bipolar Disorder Genetic Association Information Network (GAIN-BP), NIMH Bipolar Disorder Genome Study (BiGS), and a German sample. The combined sample consisted of 2,836 BP cases with information on sub-phenotypes and 2,744 controls. Imputation was performed, resulting in 2.3 million SNPs available for analysis. No SNP reached genome-wide significance for either sub-phenotype. In addition, no SNP reached genome-wide significance in a meta-analysis with an independent replication sample. We had 80% power to detect associations with a common SNP at an OR of 1.6 for psychotic symptoms and a mean difference of 1.8 years in age at onset. Age at onset and psychotic symptoms in BP may be influenced by many genes of smaller effect sizes or other variants not measured well by SNP arrays, such as rare alleles.

European collaborative study of early-onset bipolar disorder: Evidence for genetic heterogeneity on 2q14 according to age at onset

Bipolar disorder has a genetic component, but the mode of inheritance remains unclear. A previous genome scan conducted in 70 European families led to detect eight regions linked to bipolar disease. Here, we present an investigation of whether the phenotypic heterogeneity of the disorder corresponds to genetic heterogeneity in these regions using additional markers and an extended sample of families. The MLS statistic was used for linkage analyses. The predivided sample test and the maximum likelihood binomial methods were used to test genetic homogeneity between early-onset bipolar type I (cut-off of 22 years) and other types of the disorder (later onset of bipolar type I and early-onset bipolar type II), using a total of 138 independent bipolar-affected sib-pairs. Analysis of the extended sample of families supports linkage in four regions (2q14, 3p14, 16p23, and 20p12) of the eight regions of linkage suggested by our previous genome scan. Heterogeneity testing revealed genetic heterogeneity between early and late-onset bipolar type I in the 2q14 region (P = 0.0001). Only the early form of the bipolar disorder but not the late form appeared to be linked to this region. This region may therefore include a genetic factor either specifically involved in the early-onset bipolar type I or only influencing the age at onset (AAO). Our findings illustrate that stratification according to AAO may be valuable for the identification of genetic vulnerability polymorphisms. © 2010 Wiley-Liss, Inc.

Genome-wide linkage scan of bipolar disorder in a Colombian population isolate replicates Loci on chromosomes 7p21-22, 1p31, 16p12 and 21q21-22 and identifies a novel locus on chromosome 12q

BACKGROUND/AIMS

Bipolar disorder (BP) is a severe psychiatric illness, characterised by alternating episodes of depression and mania, which ranks among the top ten causes of morbidity and life-long disability world-wide. We have previously performed a whole-genome linkage scan on 6 pedigrees segregating severe BP from the well-characterised population isolate of Antioquia, Colombia. We recently collected genotypes for the same set of 382 autosomal microsatellite markers in 9 additional Antioquian BP pedigrees. Here, we report the analysis of the combined pedigree set.

METHODS

Linkage analysis using both parametric and nonparametric approaches was conducted for 3 different diagnostic models: severe BP only (BPI); mood disorders (BPI, BPII and major depression); and psychosis (operationally defined by the occurrence of at least 1 episode of hallucinations and/or delusions).

RESULTS AND CONCLUSION

For BPI only, the most interesting result was obtained for chromosome 7p21.1-p22.2 under a recessive model of inheritance (heterogeneity LOD score = 2.80), a region that had previously been linked to BP in a study on Portuguese Island families. For both BPI and mood disorders, nonparametric analyses identified a locus on chromosome 12ct-q14 (nonparametric linkage = 2.55 and 2.35, respectively). This locus has not previously been reported as a candidate region for BP. Additional candidate regions were found on chromosomes 1p22-31 (mood disorders) and 21q21-22 (BPI), 2 loci that have repeatedly been implicated in BP susceptibility. Linkage analysis of psychosis as a phenotype identified candidate regions on chromosomes 2q24-31 and 16p12-q12. The finding on chromosome 16p is noteworthy because the same locus has been implicated by genome-wide association analyses of BP.

Genome-scan for bipolar disorder with sib-pair families in the Sardinian population: a new susceptibility locus on chromosome 1p22-p21?

The discovery of the genetic factors implicated in the predisposition to complex diseases may greatly profit from genetic studies in isolated populations. In this perspective, we performed a genome-wide scan using 507 microsatellite markers, with an average interval size of 7.6 cM, on a sample of 88 nuclear families with at least two affected sibs with bipolar disorder recruited in the Sardinian population. An initial analysis yielded non-parametric linkage exceeding 3.4 with P-values <0.0003 at two adjacent markers, D1S206 and D1S435 in the 1p22-p21 chromosomal region. Moreover, positive linkage ranging between 2.0 and 3.0 was obtained for other loci in several cases in regions that have already been linked to predisposition to bipolar disorder, such as 5p15.33, 8q24.13, and 11q14.3. A subsequent analysis of the 1p22-p21 region using the same set of families and a dense panel of 20 new microsatellite markers, spaced at 1.2 cM on average, reinforced the finding of suggestive linkage for this region. Interestingly, NPL values above 2.1 and P-values <0.02 were obtained for a cluster of 10 markers comprising D1S435. Thus, this study suggests that the 1p22-p21 region may contain a new locus participating to the genetic susceptibility to bipolar disorder and reproduces positive linkage for several other loci already implicated in this pathology. Since the Sardinian population presents a peculiar genetic homogeneity, these results may pave the way to further studies for replication in this population contributing to the rapid discovery of the genetic factors predisposing to bipolar disorder.

Resequencing and association analysis of arylalkylamine N-acetyltransferase (AANAT) gene and its contribution to major depression susceptibility

Circadian rhythms disruptions, including abnormalities of circadian phase position and melatonin secretion, have been described in major depression (MD). Arylalkylamine N-acetyltransferase (AANAT) is a key enzyme of the melatonin pathway involved in circadian oscillations of melatonin levels. We assessed the contribution of AANAT gene variability to susceptibility to MD considering common and rare genetic variations through a sequential sequencing and single nucleotide polymorphism (SNP)-based genotyping approach in a sample of 445 unrelated patients with MD (257 unipolar MD, 188 bipolar depression) and 440 community-based screened control subjects. We identified 17 sequence changes, thirteen of which represented novel sequence variations. We did not observe an over-representation of patients carrying rare variants compared with the healthy controls. Common variants (MAF > 2%) were included in a case-control association analysis that showed significant association after multiple testing correction of two SNPs located in the promoter region of AANAT with MD: rs3760138 (P = 0.00006) and rs4238989 (P = 0.005). Multimarker analysis found significant associations between two three-marker protective haplotypes and a susceptibility three-marker haplotype containing the rare alleles of rs3760138-rs4238989-rs8150 and MD. We present evidence of the association of genetic variability in the AANAT gene with susceptibility to MD. Our results support the hypothesis that the melatonin-signaling pathway and circadian clock mechanisms may contribute to the pathophysiology of MD.

Case-control association study of TGOLN2 in attempted suicide

Family, twin, and adoption studies provide convincing evidence for a genetic contribution to suicidal behavior. The heritability for suicidal behavior depends in part on the transmission of psychiatric disorders, such as mood disorders and substance use disorders, but is also partly independent of them. Three linkage studies using the attempted suicide phenotype in pedigrees with bipolar disorder, major depression, or alcoholism have provided consistent evidence that 2p11-12 harbors a susceptibility gene for attempted suicide. A microarray expression study using postmortem brain samples has implicated a gene from the 2p11-12 candidate region, the trans-Golgi network protein 2 (TGOLN2) gene, as being consistently up-regulated in suicide cases as compared to controls. Here, we present a TGOLN2 case-control association study using nine single nucleotide polymorphisms (SNPs). These nine SNPs, which include seven tag SNPs and two coding SNPs, have been genotyped in 517 mood disorder subjects with a history of attempted suicide and 515 normal controls. Allelic and genotypic analyses of the case-control sample did not provide evidence for association with the attempted suicide phenotype. Eight of the nine SNPs provided supportive evidence for association (P-values ranging from 0.008 to 0.03) when we compared the attempted suicide cases with a history of alcoholism to the attempted suicide cases without a history of alcoholism. However, this association finding was not replicated in an independent sample. Taken together, these analyses do not provide support for the hypothesis that common genetic variation in TGOLN2 contributes significantly to the risk for attempted suicide in subjects with major mood disorders.

A genome-wide linkage study of bipolar disorder and co-morbid migraine: replication of migraine linkage on chromosome 4q24, and suggestion of an overlapping susceptibility region for both disorders on chromosome 20p11

Migraine and Bipolar Disorder (BPAD) are clinically heterogeneous disorders of the brain with a significant, but complex, genetic component. Epidemiological and clinical studies have demonstrated a high degree of co-morbidity between migraine and BPAD. Several genome-wide linkage studies in BPAD and migraine have shown overlapping regions of linkage on chromosomes, and two functionally similar voltage-dependent calcium channels CACNA1A and CACNA1C have been identified in familial hemiplegic migraine and recently implicated in two whole genome BPAD association studies, respectively. We hypothesized that using migraine co-morbidity to look at subsets of BPAD families in a genetic linkage analysis would prove useful in identifying genetic susceptibility regions in both of these disorders. We used BPAD with co-morbid migraine as an alternative phenotype definition in a re-analysis of the NIMH Bipolar Genetics Initiative wave 4 data set. In this analysis we selected only those families in which at least two members were diagnosed with migraine by a doctor according to patients' reports. Nonparametric linkage analysis performed on 31 families segregating both BPAD and migraine identified a linkage signal on chromosome 4q24 for migraine (but not BPAD) with a peak LOD of 2.26. This region has previously been implicated in two independent migraine linkage studies. In addition we identified a locus on chromosome 20p11 with overlapping elevated LOD scores for both migraine (LOD=1.95) and BPAD (LOD=1.67) phenotypes. This region has previously been implicated in two BPAD linkage studies, and, interestingly, it harbors a known potassium dependant sodium/calcium exchanger gene, SLC24A3, that plays a critical role in neuronal calcium homeostasis. Our findings replicate a previously identified migraine linkage locus on chromosome 4 (not co-segregating with BPAD) in a sample of BPAD families with co-morbid migraine, and suggest a susceptibility locus on chromosome 20, harboring a gene for the migraine/BPAD phenotype. Together these data suggest that some genes may predispose to both bipolar disorder and migraine.

Two-dimensional genome scan identifies multiple genetic interactions in bipolar affective disorder

BACKGROUND

Bipolar disorder is a highly heritable psychiatric condition, the etiology of which remains largely unknown despite extensive efforts to identify susceptibility genes. Interactions between genes of small individual effect could partially explain the difficulties of traditional one-dimensional approaches to identify genetic risk factors.

METHODS

A nonparametric linkage (NPL) analysis of 65 Australian extended pedigrees containing 643 genotyped individuals (of whom 40% were diagnosed with affective disorder) was conducted. Chromosome-by-chromosome correlation analysis of family-specific NPL scores was conducted to detect evidence of genetic interaction. Interaction-specific multipoint NPL and permutation analysis was used to assess linkage interdependence, using family weights derived from the alternative interacting chromosome. Finally, a single nucleotide analysis of each interaction region was conducted using the publicly available genome-wide association, datasets (2933 cases, 2534 controls).

RESULTS

Significant NPL peaks were detected on chromosomes 2q24-33, 7q21-31, and 17q11-25 (Z = 3.12, 3.01, and 2.95 respectively), with four additional suggestive peaks identified. Four robust interchromosomal interaction clusters exceeding Bonferroni correction at alpha = .05 (uncorrected p < 5.38e-07) were detected on 11q23-25-2p15-12, 4q32-35-1p36, 12q23-24-4p16-15, and 20q13-9q21-22. This linkage interdependence was determined significant after permutation analysis (p = .002-.0002). A suggestive interaction was observed in the combined data on 2p14-11q23 (uncorrected p = 5.76E-10, Bonferroni corrected p = .068).

CONCLUSIONS

This study indicates a complex interplay between multiple loci underlying bipolar disorder susceptibility, and highlights the continuing usefulness of extended pedigrees in complex genetics. The challenge lies in the identification of specific gene interactions and their biological validation.

Sex differences in TTC12/ANKK1 haplotype associations with daily tobacco smoking in Black and White Americans

INTRODUCTION

The 11q23.1 genomic region has been associated with nicotine dependence in Black and White Americans.

METHODS

By conducting linkage disequilibrium analyses of 7 informative single nucleotide polymorphisms (SNPs) within the tetratricopeptide repeat domain 12 (TTC12)/ankyrin repeat and kinase containing 1 (ANKK1)/dopamine (D2) receptor gene cluster, we identified haplotype block structures in 270 Black and 368 White (n = 638) participants, from the Baltimore Epidemiologic Catchment Area cohort study, spanning the TTC12 and ANKK1 genes consisting of three SNPs (rs2303380-rs4938015-rs11604671). Informative haplotypes were examined for sex-specific associations with daily tobacco smoking initiation and cessation using longitudinal data from 1993-1994 and 2004-2005 interviews.

RESULTS

There was a Haplotype x Sex interaction such that Black men possessing the GTG haplotype who were smokers in 1993-2004 were more likely to have stopped smoking by 2004-2005 (55.6% GTG vs. 22.0% other haplotypes), while Black women were less likely to have quit smoking if they possessed the GTG (20.8%) versus other haplotypes (24.0%; p = .028). In Whites, the GTG haplotype (vs. other haplotypes) was associated with lifetime history of daily smoking (smoking initiation; odds ratio = 1.6; 95% CI = 1.1-2.4; p = .013). Moreover, there was a Haplotype x Sex interaction such that there was higher prevalence of smoking initiation with GTG (77.6%) versus other haplotypes (57.0%; p = .043).

DISCUSSION

In 2 different ethnic American populations, we observed man-woman variation in the influence of the rs2303380-rs4938015-rs11604671 GTG haplotype on smoking initiation and cessation. These results should be replicated in larger cohorts to establish the relationship among the rs2303380-rs4938015-rs11604671 haplotype block, sex, and smoking behavior.

Suggestive linkage at 9p22 in bipolar disorder weighted by alcohol abuse

Bipolar disorder (BP) is a highly heritable disorder, however attempts to map genetic risk factors are challenging. One possible reason for these difficulties is the genetic heterogeneity of BP. Hence, focusing on clinically homogeneous families to create a genetically more homogeneous sample may increase the power of finding a specific variant. Alcohol abuse (AA) and alcohol dependence (AD) are familial in BP families, and these families may carry a specific risk variant for BP. We tested this hypothesis by performing a genome-wide linkage scan in 638 pedigrees (1,835 individuals) from the National Institute of Mental Health Genetics Initiative for BP, weighting the evidence for linkage according to the family's frequency of AA or AD. Using AA weighting, we identified a linkage region on 9p22.2 with an NPL score of 3.23. The region had previously been identified in a meta-analysis of linkage in bipolar disorder. We used permutation analysis to assess if weighting by AA increased the linkage signal more than expected by chance and observed a significant P-value (P = 0.048). Therefore, the genetic risk factor for BP on 9p22.2 has an increased effect in families with high levels of AA. In summary, we present an example of using covariates such as AA and AD to define subtypes of BP, demonstrate how using such subtypes can improve the power of a linkage scan, and demonstrate statistical approaches to validate the suggested interaction.

Premorbid adjustment: a phenotype highlighting a distinction rather than an overlap between schizophrenia and bipolar disorder

BACKGROUND

Premorbid adjustment (PMA) in schizophrenia (SZ) has been widely studied and shown to be worse in individuals who develop SZ as compared to controls. It has been proposed as a predictor of clinical presentation and outcome, and may delineate a specific SZ phenotype for genetic and other biological studies. Research into PMA in BD has been scarce and inconclusive.

AIMS

The authors compared PMA in individuals suffering from BD with that of healthy controls and investigated whether levels of PMA in BD patients correlate with specific phenotypic features.

METHODS

The authors investigated 344 BD patients and 137 population-based controls. Retrospective PMA assessment was performed using the Premorbid Adjustment Scale (PAS). An overall score as well as sub-scores for age ranges and functional domains were obtained.

RESULTS

Patients had a better overall PAS score than controls and outperformed controls during early and late adolescence. They scored significantly better than controls in the functional domains "sociability and withdrawal" and "adaptation to school". No differences were observed for the other subscales and there were no differences between groups during childhood. No association was observed between PMA and any of the phenotype characteristics investigated.

CONCLUSIONS

In the largest study to date on PMA in BD, PMA was shown to be better in bipolar patients than in healthy controls. PMA in BD is not a simple proxy for commonly studied phenotypic markers of severity. PMA emerges as a phenotype in its own right, and highlights an aspect of disparity rather than overlap between SZ and BD.

Family and genetic association studies of bipolar disorder in children

The risk of bipolar disorder (BPD) (15-42%) in first-degree relatives of children with BPD are consistently larger than the 8.7% estimate of recurrence risk of BPD in first-degree relatives of adult BPD cases. There have been no family linkage studies of pediatric BPD, but secondary analyses of adult linkage samples suggest that early-onset BPD both increases the strength of associations in linkage studies. Positive associations with pediatric BPD and the BDNF gene (Vall66), the GAD1 gene (4s2241165), and the dopamine transporter gene (rs41084) have been reported but none of these associations have been replicated in independent samples. The number of informative families examined so far is quite small and studies were vastly underpowered to detect small effects. An adequately powered sample will likely require collaborative ascertainment of cases and families from multiple sites using valid and accepted measures of pediatric BPD.

Classifying mood disorders by age-at-onset instead of polarity

BACKGROUND

Polarity is the pillar of the current categorical unipolar-bipolar division of mood disorders. However, genetic studies on these polarity-based phenotypes have been largely inconclusive. Recent clinical and epidemiological studies seem to support more of a continuum than a splitting of mood disorders. A reshaping of the classification of mood disorders thus seems required. Age-at-onset and recurrence have been suggested to be more clinically and genetically useful in the phenotyping of mood disorders.

STUDY AIM

To test a classification of mood disorders based on age-at-onset, and to delineate its phenotypes.

METHODS

A total of 441 consecutive bipolar II disorder (BP-II) and 289 unipolar major depressive disorder (MDD) outpatients, presenting for treatment of a major depressive episode (MDE) in a clinical and research private practice, were assessed by a mood disorder specialist psychiatrist (FB) using a Structured Clinical Interview for the DSM-IV, modified for better probing past hypomania [Benazzi, F. Bipolar disorder-focus on bipolar II disorder and mixed depression. Lancet 2007a;369: 935-945]. The sample was divided according to age-at-onset. Age-at-onset was defined by the age at onset of the first MDE. Early-age-at-onset (EO) was defined as age at onset before 21 years, late-age-at-onset (LO) as onset at or after age 21 years. The study's current goal had not been planned when data were recorded between 1999 and 2006. Variables were compared in EO versus LO mood disorders, investigating phenotype differences. The main focus was on 'classic' diagnostic validators: MDE clinical picture, gender, course, and family history. Age, gender, BP-II, and mania/hypomania family history (possible confounding) were controlled for in the analyses. Logistic regression was used.

RESULTS

First, EO was regressed on each variable, one at a time, to find significant associations. Second, EO was regressed on all of the variables whose odds ratio (OR) was statistically significant in the previous analyses in order to find independent predictors. Independent predictors of EO mood disorder were history of hypomania, high recurrence, atypical depression, and family history of mania/hypomania. Controlling for BP-II (in addition to age and gender) did not impact the findings. The highest OR was that between EO and high recurrence (OR=4.00). Distinguishing MDE symptoms of EO mood disorder included hypersomnia and psychomotor agitation when controlling for age and gender, and, by controlling also for BP-II, hypersomnia only.

DISCUSSION

A close association among EO mood disorder, high recurrence, and bipolarity (history of hypomania, family history of mania/hypomania) was found. Compared to most previous studies testing EO versus LO in bipolar (mainly BP-I) or in unipolar MDD samples, the present study tested a mixed BP-II and MDD sample and controlled for polarity, reducing, as much as possible, the impact of polarity on the findings. EO (below age 21 years) was distinguished by hypersomnic depression, high recurrence, high history of hypomania, and high history of mania/hypomania. Replications are needed, especially in mixed samples also including BP-I. Results, if replicated, could have implications not only for clinical and genetic studies, but also for treatment (e.g., mood stabilizers could have better long-term effects than antidepressants in EO mood disorders, antidepressants could have negative long-term effects on EO).

Current research in child and adolescent bipolar disorder

Although recently more research has considered children with bipolar disorder than in the past, much controversy still surrounds the validity of the diagnosis. Furthermore, questions remain as to whether or not childhood expressions of bipolarity are continuous with adult manifestations of the illness. In order to advance current knowledge of bipolar disorders in children, researchers have begun to conduct phenomenological, longitudinal, treatment, and neuroimaging studies in youths who exhibit symptoms of bipolar illness, as well as offspring of parents with bipolar disorders. Regardless of the differences between research groups regarding how bipolar disorder in children is defined, it is agreed that pediatric bipolarity is a serious and pernicious illness. With early intervention during the period of time in which youths are exhibiting subsyndromal symptoms of pediatric bipolarity, it appears that the progression of the illness to the more malignant manifestation of the disorder may be avoided. This paper will review what is currently known and what still is left to learn about clinically salient topics that pertain to bipolar disorder in children and adolescents.

Genetics of bipolar disorder

Bipolar disorder, especially the most severe type (type I), has a strong genetic component. Family studies suggest that a small number of genes of modest effect are involved in this disorder. Family-based studies have identified a number of chromosomal regions linked to bipolar disorder, and progress is currently being made in identifying positional candidate genes within those regions, À number of candidate genes have also shown evidence of association with bipolar disorder, and genome-wide association studies are now under way, using dense genetic maps. Replication studies in larger or combined datasets are needed to definitively assign a role for specific genes in this disorder. This review covers our current knowledge of the genetics of bipolar disorder, and provides a commentary on current approaches used to identify the genes involved in this complex behavioral disorder.

The genetics of bipolar disorder: genome 'hot regions,' genes, new potential candidates and future directions

Bipolar disorder (BP) is a complex disorder caused by a number of liability genes interacting with the environment. In recent years, a large number of linkage and association studies have been conducted producing an extremely large number of findings often not replicated or partially replicated. Further, results from linkage and association studies are not always easily comparable. Unfortunately, at present a comprehensive coverage of available evidence is still lacking. In the present paper, we summarized results obtained from both linkage and association studies in BP. Further, we indicated new potential interesting genes, located in genome 'hot regions' for BP and being expressed in the brain. We reviewed published studies on the subject till December 2007. We precisely localized regions where positive linkage has been found, by the NCBI Map viewer (http://www.ncbi.nlm.nih.gov/mapview/); further, we identified genes located in interesting areas and expressed in the brain, by the Entrez gene, Unigene databases (http://www.ncbi.nlm.nih.gov/entrez/) and Human Protein Reference Database (http://www.hprd.org); these genes could be of interest in future investigations. The review of association studies gave interesting results, as a number of genes seem to be definitively involved in BP, such as SLC6A4, TPH2, DRD4, SLC6A3, DAOA, DTNBP1, NRG1, DISC1 and BDNF. A number of promising genes, which received independent confirmations, and genes that have to be further investigated in BP, have been also systematically listed. In conclusion, the combination of linkage and association approaches provided a number of liability genes. Nevertheless, other approaches are required to disentangle conflicting findings, such as gene interaction analyses, interaction with psychosocial and environmental factors and, finally, endophenotype investigations.

Defining mixed depression

BACKGROUND

Mixed depression, i.e. a major depressive episode plus co-occurring manic/hypomanic symptoms, has recently become the focus of research. However, it is still unclear if its definition should be based on specific manic symptoms or on a number/score of manic symptoms. Different definitions may have different diagnostic utility, such as treatment impacts.

STUDY AIM

Study aim was to test which definition of mixed depression was more supported, by using, as diagnostic validator, early age at onset on the basis of previous studies supporting its bipolar nature.

METHODS

Consecutive 336 Bipolar II Disorder (BP-II), and 224 Major Depressive Disorder (MDD) outpatients were cross-sectionally assessed for major depressive episode (MDE) and concurrent DSM-IV hypomanic symptoms when presenting for treatment of depression, by a mood disorder specialist psychiatrist using the Structured Clinical Interview for DSM-IV as modified by Akiskal and Benazzi (J Clin Psychiatry, 2005) and the Hypomania Interview Guide (HIG), in a private practice. Mixed depression was defined as co-occurrence of MDE and hypomanic symptoms. Early age at onset (EO) below 21 years was used as diagnostic validator.

RESULTS

Multivariable logistic regression of EO versus all within-MDE hypomanic symptoms, controlled for BP-II, showed that no specific symptom was independently associated with EO. By ROC analysis versus EO, the best combination of sensitivity and specificity, and the highest figure of correctly classified, were shown by a cutoff number >=3 symptoms, and by a cutoff HIG score >=8. Both cutoffs had similar strength of association with EO. Mixed depression defined by >=3 within-MDE hypomanic symptoms (A), or by a HIG score >=8 (B), were more frequent in EO group versus LO group (A: 70.5% versus 49.8%; B: 60.7% versus 40.9%; p<0.001), and in BP-II versus MDD (A: 72.3% versus 39.7%; p<0.001; positive predictive value for BP-II=73.1%; B: 63.9% versus 29.0%; p<0.001; positive predictive value for BP-II=76.7%).

DISCUSSION

Findings could support the diagnostic validity of a definition of mixed depression based on a cutoff number/score of within-depression hypomanic symptoms versus one based on specific symptoms, complementing and supporting previous studies using bipolar family history as validator. Diagnosing mixed depression has treatment impacts, such as careful use of antidepressants added to mood stabilising agents or no use of antidepressants, as recently shown by large naturalistic and controlled studies.

How best to identify a bipolar-related subtype among major depressive patients without spontaneous hypomania: superiority of age at onset criterion over recurrence and polarity?

BACKGROUND

History of high depressive recurrence (without history of mania/hypomania) has been proposed as a mood subtype close to bipolar disorders. Herein we test whether this is the best approach to this question.

METHODS

We systematically evaluated consecutive 224 Major Depressive (MDD) and 336 Bipolar II Disorders (BP-II) outpatients in a private practice, by the SCID for DSM-IV (modified for better probing hypomania by Akiskal and Benazzi [Akiskal, H.S., Benazzi, F., 2005. Optimizing the detection of bipolar II disorder in outpatient private practice: toward a systematization of clinical diagnostic wisdom. J. Clin. Psychiatry 66, 914-921]). We conducted univariate and multivariate analyses on such putative bipolar validators as early age at onset of first major depressive episode (before 21 years), high recurrence, family history for bipolar disorders, and depressive mixed states (mixed depression, i.e. depression plus concurrent hypomanic symptoms), in order to identify an MDD subgroup close to BP-II.

RESULTS

All bipolar validators were independent predictors of BP-II. Early onset was the only variable which identified an MDD subgroup significantly associated with all bipolar validators. This MDD subgroup was similar to BP-II on age at onset and bipolar family history, and had a high frequency of mixed depression. A dose-response relationship was found between number of bipolar validators present in MDD, and bipolar family history loading among MDD relatives.

LIMITATIONS

Study limited to outpatients.

CONCLUSIONS

From among the bipolar validators, early age at onset of first major depression (<21 years) was superior to high recurrence (>4 depressive episodes) in identifying an MDD subgroup close to BP-II, which might be subsumed under the broad bipolar spectrum. Implications of unipolar-bipolar boundaries and genetic investigations are discussed.

Genetics of bipolar disorder

Family and twin studies have consistently documented that bipolar disorder (BPD) is familial and heritable, but efforts to identify specific susceptibility genes have been complicated by the disorder's genetic and phenotypic complexity. Genetic linkage studies have implicated numerous chromosomal regions, but findings have been inconsistent. As with other complex disorders, it has become clear that linkage analysis lacks the power and precision to identify susceptibility loci for BPD. Candidate gene association studies have implicated several specific genes, but these studies have been limited by our incomplete understanding of the disorder's biology, and there have been few robustly replicated results. Within the past 2 years, a major advance in the genetics of complex disease has become feasible in the form of genome-wide association studies. Such studies, which require large sample sizes, have already proven successful in identifying susceptibility variants for a range of common medical disorders. Genome-wide association studies have begun to appear for BPD, and more are in progress. By providing an unbiased approach, this technology may reveal novel biological mechanisms underlying BPD.