A systematic evaluation of linkage studies in bipolar disorder

Molecular genetics of affective disorders

Evidence for familial aggregation in Affective Disorders (AD) has been provided in classical studies. Linkage and association genetic studies have been proposed to detect genetic factors implicated in AD. However, findings from molecular genetic studies remain inconclusive. Nevertheless, current research is focusing on the phenotypes, both sub- and endophenotypes. In addition, recent advances in technology, such as microarrays, provide new tools in psychiatric genetics. These different approaches offer a new optimism era in the search of genetic factors in AD.

Psychiatric diagnoses in the context of genetic studies of bipolar disorder

Precise definition of the phenotype is an issue of critical importance for the future success of genetic studies of bipolar disorders. So far, an uncertain phenotypic spectrum and genetic heterogeneity are realities that have hampered progress in genetic studies. While recognition of a broader spectrum of related illnesses is important for some applications, for genetic studies a narrow spectrum of illness closely tied to the genotype is paramount. This paper highlights current dilemmas and trends associated with phenotype specification and traces historical approaches. Finally, we explore a number of strategic directions in the diagnostic approach to bipolar disorders that may better serve genetic studies.

Neurotransmitters and signal transduction processes in bipolar affective disorders: a synopsis

New technologies have led to tremendous progress in understanding what today we call bipolar disorders, whose clinical diagnosis has been refined continuously since Kraepelin first described them. Molecular genetic studies have produced interesting findings, but to date have failed to identify specific genes that are so far responsible for the vulnerability to bipolar disorders. Biochemical studies in combination with pharmacotherapy give hints that the neurotransmitter function and the related signal transduction may be abnormally regulated. Since all the neurotransmitter circuits are interconnected, the dysregulation may occur on different levels and it is rather improbable that one single abnormality should account for the disorder. This paper reviews these promising developments.

Molecular genetic and family studies in affective disorders: state of the art

The identification of genes responsible for mood disorders will contribute to significant advances in the awareness of diagnosis (diagnostic process and early recognition), pathophysiology, epidemiology and treatment issues. During the past two decades, the search for genes for mood disorders has mainly contributed to better understand and confirm the genetic complexities inherent to these disorders. The large amount of results available and the difficulty to digest them corroborate this observation. The major contribution of these findings should be integrated in the context of the world-wide efforts to identify the thousands of genes of the human genome. Some of these genes may be identified within the next decade. Several consistent hypotheses are currently being tested and will, hopefully, speed up the process of narrowing the important regions when the complete genome map will be available. The most promising chromosomal regions have been localized on chromosomes 4, 5, 11, 12, 18, 21 and X. A number of candidate genes have also been investigated, some of these are directly linked to neurobiological hypotheses of the aetiology of affective disorders. In parallel, specific hypotheses have been implicated, such as anticipation and dynamic mutations. Further research should concentrate on these hypotheses and confirm positive findings through interdisciplinary and multicenter projects.

Modeling the phenotype in parametric linkage analysis of bipolar disorder

The definition of phenotype is a major problem in genetic studies of psychiatric disorders. Most linkage studies in bipolar disorder have defined the phenotype as a dichotomous trait and have usually employed different hierarchical classifications in order to overcome uncertainty resulting from phenotypic variability. In this study we explored the advantages of maximizing the evidence for linkage over different phenotypic definitions when conducting parametric linkage analysis of a complex trait. The GAW10 Problem 1 was used, focusing on chromosome 18 data sets. Three major phenotypic models were analyzed: quasi-quantitative, liability-based and affection-status models. Overall, no single phenotypic model performed consistently better than the others (i.e., lod scores greater than 1.0). Each model yielded higher lod scores than the others in particular instances, suggesting that it might be useful in exploratory data analysis, where the phenotype is variable, to maximize evidence for linkage over different phenotypic models.

No evidence for significant linkage between bipolar affective disorder and chromosome 18 pericentromeric markers in a large series of multiplex extended pedigrees

Bipolar affective disorder (BP) is a major neuropsychiatric disorder with high heritability and complex inheritance. Previously reported linkage between BP and DNA markers in the pericentromeric region of chromosome 18, with a parent-of-origin effect (linkage was present in pedigrees with paternal transmission and absent in pedigrees with exclusive maternal inheritance), has been a focus of interest in human genetics. We reexamined the evidence in one of the largest samples reported to date (1,013 genotyped individuals in 53 unilineal multiplex pedigrees), using 10 highly polymorphic markers and a range of parametric and nonparametric analyses. There was no evidence for significant linkage between BP and chromosome 18 pericentromeric markers in the sample as a whole, nor was there evidence for significant parent-of-origin effect (pedigrees with paternal transmission were not differentially linked to the implicated chromosomal region). Two-point LOD scores and single-locus sib-pair results gave some support for suggestive linkage, but this was not substantiated by multilocus analysis, and the results were further tempered by multiple test effects. We conclude that there is no compelling evidence for linkage between BP and chromosome 18 pericentromeric markers in this sample.

Lack of association between bipolar disorder and tyrosine hydroxylase: a meta-analysis

Tyrosine hydroxylase (TH) is a candidate gene extensively explored in several association studies of bipolar disorder (BD). However, because of conflicting results of independent studies and low statistical power of individual studies to detect small differences between cases and controls, reliable conclusions are difficult to formulate. A method to obtain more reliable conclusions about the involvement of the TH locus in the etiology of BD is meta-analysis. We undertook a meta-analysis of studies that investigated the association between BD and TH genetic markers. The studies were identified by means of computerized searches of several databases, and the scanning of review articles and the reference lists of the primary articles identified. More than 60 publications were reviewed, and 9 relevant articles were included in this meta-analysis, with an overall sample of 1,069 subjects (547 cases and 522 normal controls). The overall odds ratio (and confidence interval) based on combining the results of the studies was 1.02 (0.68-1.54). Test of the null hypothesis that the mean log odds ratio equals zero (chi2 = 0.11; 5 df; P > 0.05) indicated that there was no overall association between bipolar disorder and tyrosine hydroxylase.

Bipolar disorder: from families to genes

BACKGROUND

Genetic factors are known to contribute to the etiology of bipolar illness, but the actual genetic mechanisms remain to be clarified.

METHODS

This paper reviews the research undertaken to establish the genetic basis of bipolar illness and to elucidate the nature of its genetic predisposition.

RESULTS

The presented findings suggest that bipolar affective disorder is a heterogeneous condition characterized by a complex relationship between the genetic susceptibility and the clinical presentation. Linkage studies have generated promising and replicated findings on chromosomes 18 and 21.

CONCLUSION

In spite of the methodological difficulties inherent in the genetic study of psychiatric disorders recent investigations have made important advances and promise to identify specific susceptibility genes.