Analysis of the serotonin transporter polymorphism (5-HTTLPR) in Brazilian patients affected by dysthymia, major depression and bipolar disorder

The role of 5-HTTLPR in autism spectrum disorder: New evidence and a meta-analysis of this polymorphism in Latin American population with psychiatric disorders

The autism spectrum disorder (ASD) is a complex disorder encompassing a broad phenotypic and genotypic variability. The short (S)/long (L) 5-HTTLPR polymorphism has a functional role in the regulation of extracellular serotonin levels and both alleles have been associated to ASD. Most studies including European, American, and Asian populations have suggested an ethnical heterogeneity of this polymorphism; however, the short/long frequencies from Latin American population have been under-studied in recent meta-analysis. Here, we evaluated the 5-HTTLPR polymorphism in Colombian individuals with idiopathic ASD and reported a non-preferential S or L transmission and a non-association with ASD risk or symptom severity. Moreover, to recognize the allelic frequencies of an under-represented population we also recovered genetic studies from Latin American individuals and compared these frequencies with frequencies from other ethnicities. Results from meta-analysis suggest that short/long frequencies in Latin American are similar to those reported in Caucasian population but different to African and Asian regions.

Associations of the serotonin transporter promoter polymorphism (5-HTTLPR) with bipolar disorder and treatment response: A systematic review and meta-analysis

BACKGROUND

Associations of the serotonin transporter promoter polymorphism (5-HTTLPR) with bipolar disorder (BPD) and treatment response in bipolar patients were not conclusive. This study not only assessed the association between the 5-HTTLPR and BPD with accumulating relevant studies, but also in the first time evaluated the effect of the 5-HTTLPR on both anti-depressive and anti-manic treatment responses in bipolar patients.

METHODS

PubMed, Embase, PsycINFO, Cochrane Library and Cochrane Control Trials databases were systematically searched before February 2017. This meta-analysis followed the PRISMA guidelines.

RESULTS

A total of 32 population-based studies (5567 cases and 6993 controls) and 9 family-based studies (837 trios) were finally screened out and statistically joined into a single meta-analysis that revealed an association between S allele and an increased risk of BPD (OR = 1.06, p = .038). Pooled analysis of the 32 population-based studies indicated an association of S-carrier genotypes with an increased risk of BPD (OR = 1.10, p = .029). Meanwhile, the association remained significant in Caucasians (OR = 1.15, p = .004), which could provide an enough power (88%) to detect a significant association. Regarding the treatment response studies, 6 studies reporting the relationship of the 5-HTTLPR in anti-depressive remission rate (1034 patients) and 7 studies reporting in response rate (1098 patients) were included for pooled analyses. We observed a significant association of S-carrier genotypes with a reduced anti-depressive remission rate (OR = 0.64, p = .006) but not with anti-depressive response rate. The association between the 5-HTTLPR with anti-manic response rate was not observed in the included 6 studies (676 patients).

CONCLUSIONS

The present study supported the presence of a marginal but detectable effect of the 5-HTTLPR on susceptibility to BPD. Moreover, the detected association in Caucasian was statistically reliable. Besides, the 5-HTTLPR was identified as a useful predictor for anti-depressive remission but not for anti-depressive or anti-manic response.

The interplay of genetic and environmental factors in shaping well-being across the lifespan: Evidence from the serotonin transporter gene

BACKGROUND

Converging evidence suggests that well-being plays an important role in promoting and maintaining mental health across the life span. It has been shown that well-being has a considerable heritable component, but little is known about the specific genes involved.

METHODS

In this study, we investigated a healthy sample (N = 298) that was genotyped for the serotonin transporter-linked polymorphic region (5-HTTLPR). We hypothesized that 5-HTTLPR gene variation would influence well-being, and additionally investigated interaction effects with age and the environmental influence of early life stress (ELS).

RESULTS

Using multiple regression, our results showed a significant three-way interaction between genotype, ELS, and age. Exploration of this interaction showed that young subjects had decreased levels of well-being if they were exposed to ELS and homozygous for the short variant of 5-HTTLPR. This relationship was reversed in old age: subjects that were exposed to ELS and carried the long variant of 5-HTTLPR had decreased levels of well-being.

CONCLUSION

Our results indicate that genetic and environmental factors have joint effects on well-being that are susceptible to profound changes across the life span.

Associations of 5HTTLPR polymorphism with major depressive disorder and alcohol dependence: A systematic review and meta-analysis

OBJECTIVE

The neurotransmitter serotonin is understood to control mood and drug response. Carrying a genetic variant in the serotonin transporter gene (5HTT) may increase the risk of major depressive disorder and alcohol dependence. Previous estimates of the association of the S allele of 5HTTLPR polymorphism with major depressive disorder and alcohol dependence have been inconsistent.

METHODS

For the systematic review, we used PubMed MEDLINE and Discovery of The University of Melbourne to search for all relevant case-control studies investigating the associations of 5HTTLPR polymorphism with major depressive disorder and alcohol dependence. Summary odds ratios (OR) and their 95% confidence intervals (CI) were estimated. To investigate whether year of publication, study population or diagnostic criteria used were potential sources of heterogeneity, we performed meta-regression analyses. Publication bias was assessed using Funnel plots and Egger's statistical tests.

RESULTS

We included 23 studies of major depressive disorder without alcohol dependence containing 3392 cases and 5093 controls, and 11 studies of alcohol dependence without major depressive disorder containing 2079 cases and 2273 controls. The summary OR for homozygote carriers of the S allele of 5HTTLPR polymorphism compared with heterozygote and non-carriers combined (SS vs SL+LL genotype) was 1.33 (95% CI = [1.19, 1.48]) for major depressive disorder and 1.18 (95% CI = [1.01, 1.38]) for alcohol dependence. The summary OR per S allele of 5HTTLPR polymorphism was 1.16 (95% CI = [1.08, 1.23]) for major depressive disorder and 1.12 (95% CI = [1.01, 1.23]) for alcohol dependence. Meta-regression models showed that the associations did not substantially change after adjusting for year of publication, study population and diagnostic criteria used. There was no evidence for publication bias of the studies included in our meta-analysis.

CONCLUSIONS

Our meta-analysis confirms that individuals with the homozygous S allele of 5HTTLPR polymorphism are at increased risks of major depressive disorder as well as alcohol dependence. Further studies are required to investigate the association between 5HTTLPR polymorphism and the comorbidity of major depressive disorder and alcohol dependence as well as gene × environmental interactions.

Meta-analysis confirms a functional polymorphism (5-HTTLPR) in the serotonin transporter gene conferring risk of bipolar disorder in European populations

The serotonin transporter (5-HTT) is a candidate risk gene for bipolar disorder, and a functional polymorphism of 44-bp insertion/deletion (5-HTTLPR) located in the promoter region of this gene has been investigated for the association with the illness extensively among worldwide populations, but overall results were inconsistent and its role in the disorder remains unclear. The present study attempts to find its potential association with bipolar disorder using meta-analyzes that maximize the statistical power. We applied meta-analysis techniques by combining all available case-control studies of 5-HTTLPR and bipolar disorder in samples of European ancestry (with a total of 3778 cases and 4997 controls), and we assessed the evidence for allelic associations, heterogeneity among different studies, influence of each single study, and potential publication bias. The short allele (S allele) of 5-HTTLPR showed a significant association with bipolar disorder in our meta-analysis (odds ratio=1.10, p-value=0.005), suggesting it is likely a risk polymorphism for the illness, and the observed OR is consistent with other susceptibility loci identified through recent large-scale genetic association studies on bipolar disorder, which could be regarded simply as a small but detectable effects.

Searching for new genetic variations in expression databases for the GABAergic and glutamatergic systems

Changes in gene expression and genetic variations in coding regions have likely functional impact, potentially associated with complex diseases, such as neuropsychiatric conditions. A current need for high throughput analysis of genomic data is leading to the development and improvement of sophisticated bioinformatics approaches, which allows the processing of large amounts of sequence and gene expression data. In this study, we identified new potential genetic variations prioritizing genes related to glutamatergic and GABAergic systems, using different bioinformatics resources. The CLCbio Workbench Combined platform was initially used to build expressed sequence tags and mRNA files retrieved, respectively, from the Goldenpath and National Center for Biotechnology Information databases and latter to perform multiple batches of Smith-Waterman alignments. The PMUT software was used to increase an accurate association between potential variations and pathogenic predictions. The annotation revealed various classes of variations and most of them are deletions ranging from 1 to 7 bp. Bioinformatic pipelines seem to be useful approaches to help screening for genetic variations with potential impact in gene expression. Further analysis will foster this aim to provide celerity at the massive analysis of data currently generated in large scale high throughput experiments.

Exploring the implications of INDELs in neuropsychiatric genetics: challenges and perspectives

The decade passed after publishing the Human Genome first draft faced an enormous growth at the understanding of the genomic variation among different subjects, populations, and groups of patients. Single nucleotide polymorphisms (SNPs) and insertion or deletions (INDELs) have been increasingly recognized as a major type of genetic variations, with potential impact in protein activities and gene expression changes observed in complex genetic traits, like neuropsychiatric diseases. INDELs represent the second most common class of variations after SNPs, but there is still an important gap between the number of INDELs reported and the actual knowledge about the functional implications of such variations. There are approximately 10 million SNPs already reported, and the human populations are expected to collectively harbor at least 1.6-2.5 million INDELs. One of the major challenges is to find better platforms to screen for INDELs in a high throughput manner. The discordance in between the data from different studies might be explained by the diverse approaches employed to sequence the genomes with variable platforms. Short INDEL variations increased the scope of genetic markers in human genetic diseases, and various studies showed that common microdeletions and smaller INDELs might be highly associated with neuropsychiatric diseases such as schizophrenia, autism, mental retardation, and Alzheimer disease. The rapidly increasing amount of resequencing, genotyping, and personal genome data generated by large-scale genetic human projects require the development of integrated bioinformatics tools able to efficiently manage and analyze these genetic data. Our group is currently dealing with different approaches that might optimize sequencing and bioinformatics analyses of short INDELs to broaden our research capabilities of identifying those intriguing genetic variations. Hopefully, INDELs might become a new trend in association studies in neuropsychiatric genetics since so far the level of significant and positive associations with the standard SNPs reported presents limited predictive application.

A review of psychiatric genetics research in the Brazilian population

OBJECTIVE AND METHOD: A large increase in the number of Brazilian studies on psychiatric genetics has been observed in the 1970's since the first publications conducted by a group of researchers in Brazil. Here we reviewed the literature and evaluated the advantages and difficulties of psychiatric genetic studies in the Brazilian population. CONCLUSION: The Brazilian population is one of the most heterogeneous populations in the world, formed mainly by the admixture between European, African and Native American populations. Although the admixture process is not a particularity of the Brazilian population, much of the history and social development in Brazil underlies the ethnic melting pot we observe nowadays. Such ethnical heterogeneity of the Brazilian population obviously brings some problems when performing genetic studies. However, the Brazilian population offers a number of particular characteristics that are of major interest when genetic studies are carried out, such as the presence of isolated populations. Thus, differences in the genetic profile and in the exposure to environmental risks may result in different interactions and pathways to psychopathology.

Effect of the 5-HTTLPR polymorphism in the serotonin transporter gene on major depressive disorder and related comorbid disorders

OBJECTIVES

The serotonin transporter gene (5-HTT) has been proposed as a candidate gene for major depressive disorder (MDD). Association studies, however, have revealed inconsistent results. This could be because of the phenotypic heterogeneity of MDD, as it often presents with comorbid disorders such as generalized anxiety disorder (GAD), alcohol-related disorders, and dysthymia.

METHODS

In this exploratory study, we performed regression analyses with generalized estimating equations in patients with familial MDD (n=233) in order to explore whether a polymorphism in the serotonin transporter gene (5-HTTLPR) is differentially associated with MDD and a comorbid disorder compared with MDD without that particular comorbidity. As in general, GAD is more common in females and alcohol-related disorders are more common in males, the analyses were stratified for sex.

RESULTS

Comorbid dysthymia was less common in s-allele carriers with MDD (P<0.05) than in patients homozygous for the long allele. In the sex-specific analyses, an association between the 5-HTTLPR and comorbid alcohol use disorders was observed in females, with s-carriers reporting significantly less alcohol use disorders. The relationship with comorbid GAD differed by sex with male s-carriers reporting more comorbid GAD than female s-carriers.

CONCLUSION

The effect of the 5-HTTLPR polymorphism on MDD is co-dependent on the presence of comorbid disorders and sex. In this study, the s-allele of the 5-HTTLPR polymorphism was associated with significantly lower rates of particular lifetime comorbid disorders. Therefore, the presence of comorbid psychiatric disorders should be taken into account to clarify the association of the 5-HTTLPR polymorphism with MDD phenotypes.

Meta-analyses of genetic studies on major depressive disorder

The genetic basis of major depressive disorder (MDD) has been investigated extensively, but the identification of MDD genes has been hampered by conflicting results from underpowered studies. We review all MDD case-control genetic association studies published before June 2007 and perform meta-analyses for polymorphisms that had been investigated in at least three studies. The study selection and data extraction were performed in duplicate by two independent investigators. The 183 papers that met our criteria studied 393 polymorphisms in 102 genes. Twenty-two polymorphisms (6%) were investigated in at least three studies. Seven polymorphisms had been evaluated in previous meta-analyses, 5 of these had new data available. Hence, we performed meta-analyses for 20 polymorphisms in 18 genes. Pooled odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Statistically significant associations were found for the APOE varepsilon2 (OR, 0.51), GNB3 825T (OR, 1.38), MTHFR 677T (OR, 1.20), SLC6A4 44 bp Ins/Del S (OR, 1.11) alleles and the SLC6A3 40 bpVNTR 9/10 genotype (OR, 2.06). To date, there is statistically significant evidence for six MDD susceptibility genes (APOE, DRD4, GNB3, MTHFR, SLC6A3 and SLC6A4).

Population-based and family-based studies on the serotonin transporter gene polymorphisms and bipolar disorder: a systematic review and meta-analysis

The serotonin transporter (5-HTT) is a candidate gene for bipolar disorder (BPD). It has been investigated for association with the illness in a series of studies, but overall results have been inconsistent and its role in the disorder remains controversial. Systematic reviews using meta-analytical techniques are a useful method for objectively and reproducibly assessing individual studies and generating combined results. We performed two meta-analyses of published studies--both population-based and family-based studies--investigating the association between BPD and the 5-HTT gene-linked polymorphic region (5-HTTLPR) and the intron 2 variable numbers of tandem repeats (VNTR) polymorphisms. The literature was searched using Medline and Embase to identify studies for inclusion. We statistically joined population-based and family-based studies into a single meta-analysis. For both polymorphisms, our review revealed significant pooled odds ratios (ORs): 1.12 (95% CI 1.03-1.21) for the 5-HTTLPR and 1.12 (95% CI 1.02-1.22) for the intron 2 VNTR. Meta-regression showed that neither the study type (population-based vs family-based; P=0.41 for the 5-HTTLPR and P=0.91 for the intron 2 VNTR) nor the sample ethnicity (Caucasian vs non-Caucasian; P=0.35 for the 5-HTTLPR and P=0.66 for the intron 2 VNTR) significantly contributed to the heterogeneity of the meta-analyses. The observed ORs could be regarded simply as a very small but detectable effect of the 5-HTT, which has an additive effect when combined with other susceptibility loci. Alternative hypotheses on this finding were also discussed: a stronger effect of the haplotypes involving the two polymorphisms or other SNP markers; a more direct effect of these polymorphisms on specific phenotypes of BPD; and the presence of gene-environment interaction as a mediator of the genetic effects of 5-HTT.

Meta-analysis of the association between two polymorphisms in the serotonin transporter gene and affective disorders

Family, twin, and adoption studies show that psychiatric diseases including bipolar disorder (BP) and unipolar disorder (UP) have a substantial genetic component. For these illnesses, both positive and negative associations have been reported for two polymorphisms located in the serotonin transporter gene (5-HTT) on chromosome 17: a 17-base-pair (bp) variable-number tandem-repeat (VNTR) in intron 2 and a 44-bp insertion/deletion in the promoter region. Thus, associations between these 5-HTT polymorphisms and affective disorders remain unclear. The present work investigates these potential associations in meta-analyzes that maximize the power to find associations between each disease and the two 5-HTT polymorphisms. We applied meta-analysis techniques to case-control studies of two 5-HTT polymorphisms and two affective disorders (BP and UP), resulting in four meta-analyzes. For each polymorphism, we assessed the evidence for allelic associations, heterogeneity among studies, the influence of individual studies, and the potential for publication bias. The short allele(s) of the 44-bp insertion/deletion polymorphism showed a significant association for BP (odds ratio (OR) = 1.13, P = 0.001) but not UP. For the 17-bp VNTR, an increase in the number of tandem repeats had no significant association with any of the disorders. The small but significant effects of the 44-bp insertion/deletion polymorphism for BP is consistent with being one of many genes that contributes to the multi-factorial nature of these psychiatric disorders.

Serotonin gene polymorphisms and bipolar I disorder: focus on the serotonin transporter

The pathogenesis of bipolar disorder may involve, at least in part, aberrations in serotonergic neurotransmission. Hence, serotonergic genes are attractive targets for association studies of bipolar disorder. We have reviewed the literature in this field. It is difficult to synthesize results as only one polymorphism per gene was typically investigated in relatively small samples. Nevertheless, suggestive associations are available for the 5HT2A receptor and the serotonin transporter genes. With the availability of extensive polymorphism data and high throughput genotyping techniques, comprehensive evaluation of these genes using adequately powered samples is warranted. We also report on our investigations of the serotonin transporter, SLC6A4 (17q11.1-q12). An insertion/deletion polymorphism (5HTTLPR) in the promoter region of this gene has been investigated intensively. However, the results have been inconsistent. We reasoned that other polymorphism/s may contribute to the associations and the inconsistencies may be due to variations in linkage disequilibrium (LD) patterns between samples. Therefore, we conducted LD analyses, as well as association and linkage using 12 polymorphisms, including 5HTTLPR. We evaluated two samples. The first sample consisted of 135 US Caucasian nuclear families having a proband with bipolar I disorder (BDI, DSM IV criteria) and available parents. For case-control analyses, the patients from these families were compared with cord blood samples from local Caucasian live births (n = 182). Our second, independent sample was recruited through the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD, 545 cases, 548 controls). No significant associations were detected at the individual polymorphism or haplotype level using the case-control or family-based analyses. Our analyses do not support association between SLC6A4 and BDI families. Further studies using sub-groups of BDI are worthwhile.

Analysis of IL-1alpha, IL-1beta, and IL-1RA [correction of IL-RA] polymorphisms in dysthymia

Investigators of independent studies reported alterations in cytokine serum levels in patients with different mood disorders. Several polymorphisms associated with neuropsychiatric disorders such as schizophrenia and Alzheimer's disease have been reported at the interleukin-1 (IL-1) panel. Here we report the results of three specific polymorphisms at the IL-1alpha, IL-1beta, and IL-1RA genes, which were analyzed in 128 Brazilian subjects: 59 dysthymic patients and 69 normal controls. We found a statistically significant difference (p = 0.002) in the frequency of haplotypes with alleles 2+ (IL-1RA), T+ (IL-1alpha), and C+ (IL-1beta) in patients as compared to controls. We also observed that haplotype IL-1RA1.2/IL-1alpha CT/IL-1beta CC, present in 6 dysthymic patients (10%) was absent in the normal control group (p = 0.012). These results suggest that these polymorphisms might confer a greater susceptibility to develop dysthymia in Brazilian patients. However, to validate these data it will be of great interest to repeat this study in larger samples and other ethnic groups.

Alleles of a functional serotonin transporter promoter polymorphism are associated with major depression in alcoholics

BACKGROUND

Serotonergic neurotransmission has been implicated in the pathogenesis of both alcohol dependence and mood disorders and may therefore be important in understanding the pathophysiology of comorbid alcohol dependence and major depression. Studies of the association of these disorders with a functional polymorphism in the promoter region of the gene encoding the serotonin transporter protein (locus SLC6A4) have yielded inconsistent results. Because the convergence of these disorders may provide a refined phenotype, we examined the association of serotonin (5-HT) transporter linked polymorphic region (5-HTTLPR) alleles to comorbid alcohol dependence and major depression.

METHODS

A sample of 296 European American and 16 African American patients with comorbid alcohol dependence and major depression was recruited from treatment studies. The control group included 260 European Americans and 43 African Americans; all were screened to exclude the presence of a mood or substance use disorder. DNA isolated from whole blood was polymerase chain reaction-amplified, and genotypes were assigned on the basis of agarose gel size fractionation.

RESULTS

The frequency of the short allele in the patient group was in the range of those previously reported for samples with unipolar depression but was significantly more common than among controls (short allele frequency of cases, 45.8%; controls, 39.8%; chi(2)(1) = 4.02; p = 0.045).

CONCLUSIONS

With respect to the frequency of the short allele at the SLC6A4 locus (5-HTTLPR), major depression in alcoholics is similar to major depression in nonalcoholics. Further efforts to characterize depressed alcoholics and to examine genetic predictors of response to antidepressant treatment seem warranted.

Serotonergic polymorphisms and psychotic disorders in populations from North Spain

There is strong biological evidence relating alterations in the serotonergic system with mental disorders. These alterations may be originated at the DNA level by sequence mutations that alter the functioning of serotonin receptors and transporter. To test this hypothesis we investigated three genetic variants of the 5-HT2A receptor (-1438G/A, 102T/C and His452Tyr) and two variants of the serotonin transporter (a VNTR in the second intron and a 44 bp insertion/delition in the promoter region of the gene) in a clinical sample recruited in a human isolate and in surrounding areas in Northern Spain (N = 257) and in ethnically matched controls (N = 334). No clear association was found between 5-HT2A variants and psychosis. However, marginal associations were observed between the 5-HTT LPR and VNTR variants and psychosis (P < or = 0.05) indicating a minor contribution to psychosis of genetic alterations in this gene.

A systematic review of association studies investigating genes coding for serotonin receptors and the serotonin transporter: I. Affective disorders

The different 5-HT (serotonin) receptors including the serotonin transporter (5-HTT) are candidate genes for affective disorders such as major depressive disorder (MDD) and bipolar disorder (BD). They have been investigated in a number of allelic association studies where the individual results have been inconsistent, and therefore, definite conclusions are difficult to make. Systematic reviews using meta-analytical techniques are a reliable method for objectively and reproducibly assessing individual studies and generating combined result. This study aimed at reviewing published studies investigating the association between affective disorders (MDD and BD) and variation at genes coding for serotonin receptors and the serotonin transporter. We performed National Library of Medicine database searches to identify potential studies. More than 430 articles were reviewed and 86 studies met the inclusion criteria for participation in our review. Of these, 41 studies investigated 45 different 5-HT receptor variants and 45 studies investigated at least one of two commonly studied 5-HTT polymorphisms in MDD. Many studies investigated the association between MDD and BD with the 5-HT2A 102 T/C, the 5-HTT promoter 44 bp insertion/deletion and the intron 2 VNTR polymorphisms, and thus, these could be pooled using meta-analytic techniques. The overall odds ratio (OR) for the combined individual results was significant for BD and the two 5-HTT polymorphisms: Mantel-Haenszel weighted OR=1.14, CI: 1.03-1.26, P=0.015 for the promoter locus (N=3467) and Mantel-Haenszel Weighted odds ratio OR=1.18, CI: 1.05-1.32, P=0.004 for the VNTR locus (N=3620). However, sensitivity analysis indicated that, in each case, the overall positive association could be mostly attributed to the large effect of one individual study. Therefore, these results suggest that, although promising, further studies are required to assess appropriately the evidence suggesting an association between BD and 5-HTT.

Association analysis of the insertion/deletion polymorphism in serotonin transporter gene in patients with affective disorder

A polymorphism of serotonin transporter was studied in 226 patients with affective disorders (n = 132 for bipolar, n = 94 for unipolar affective disorder) and in 213 healthy subjects. Consensus diagnosis by at least two psychiatrists, according to the ICD-10 and DSM-IV criteria was made for each patient using SCID (Structured Clinical Interview for DSM-IV Axis I Disorders). A functional polymorphism in the promoter region of serotonin transporter gene, where 44 bp are either inserted (long allele) or deleted (short allele) was analysed. Genotype s/s was significantly more frequent in patients comparing to the control group (P = 0.011 for bipolar and P = 0.003 for unipolar affective disorder)--the most marked association was found in males with bipolar and unipolar illness. The allele frequencies also differ significantly between patients and controls (P = 0.003 for bipolar and P = 0.001 for unipolar affective disorder). The frequency of the low activity (short) allele was higher in patients than in controls (51.1% in bipolar, and 54.3 in unipolar vs 39.4% in controls). We suggest that the presence of allele s may increase the susceptibility to occurrence of affective disorder.

Association studies of bipolar disorder

Association studies in outbred populations represent an important paradigm for investigation of complex traits, such as bipolar disorder, both to follow-up regions of interest from linkage studies (by systematic linkage disequilibrium mapping and positional candidate studies) and for pure functional studies. The advantages of the association method include its relative robustness to genetic heterogeneity and the ability to detect much smaller effect sizes than are detectable using feasible sample sizes in linkage studies. The candidate gene approach is potentially very powerful, particularly when used within the context of a VAPSE (variation affecting protein structure or expression) paradigm, but a major problem is that the efficiency in the choice of candidates is inevitably a function of the level of previous understanding of disease pathophysiology. To date, most candidate gene studies in bipolar disorder have focussed on the major neurotransmitter systems that are influenced by medication used in clinical management of the disorder. Early studies often used anonymous markers in the hope of detecting linkage disequilibrium but recently direct examination of polymorphisms of known or presumed functional relevance has become more usual. Most studies in the literature have been of the unrelated case-control design with samples rarely exceeding 200-300 subjects. No definitive findings have yet emerged although there have been some interesting preliminary findings including those with polymorphisms within the genes encoding catechol-o-methyl transferase (COMT), monoamine oxidase A (MAOA) and the serotonin transporter (hSERT; 5-HTT). In this article we critically review the current status of the literature within the context of the important methodological issues and limitations inherent in the use of association studies for genetic dissection of bipolar disorder. We conclude by examining likely future directions and developments in the field.