A possible vulnerability locus for bipolar affective disorder on chromosome 21q22.3

A genome-wide search for chromosomal loci linked to bipolar affective disorder in the Old Order Amish

The most characteristic features of bipolar affective disorder (manic-depressive illness) are episodes of mania (bipolar I, BPI) or hypomania (bipolar II, BPII) interspersed with periods of depression. Manic-depressive illness afflicts about one percent of the population, and if untreated, is associated with an approximately 20% risk of suicide. Twin, family and adoption studies provide compelling evidence for a partial genetic aetiology, but the mode(s) of inheritance has not been identified. Nonetheless, the majority of genetic linkage studies have assumed classical mendelian inheritance attributable to a single major gene. Although segregation analyses have yielded inconsistent results (with most studies rejecting a single locus inheritance model), the best single gene model is dominant inheritance if only BPI is considered. Reported linkages of bipolar affective disorder on chromosomes 11, 18, 21 and X have been difficult to substantiate, and additional studies are required for replication or exclusion of these regions. We now present the results of our genome-wide linkage analyses that provide evidence that regions on chromosomes 6, 13 and 15 harbour susceptibility loci for bipolar affective disorder, suggesting that bipolar affective disorder in the Old Order Amish is inherited as a complex trait.

Systematic chromosome examination of two families with schizophrenia and two families with manic depressive illness

Systematic and detailed chromosome analysis, combined with a semistructured interview, was performed in 2 families with schizophrenia and in 2 families with manic depressive illness. Prometaphase technique did not reveal any subtle structural chromosome abnormalities. However, in standard techniques, gain and loss of sex chromosomes were observed. This occurred in patients at a younger age than in unaffected persons. This gives rise to the suspicion that sex chromosome aneuploidy may somehow be related to the development of psychosis. But since the data set is small, especially with respect to schizophrenia, further studies are needed to elucidate this observation. In one family, cosegregation of the disease locus with a marker on chromosome 21 was seen. Therefore, further research should determine if chromosome 21 contains a gene for manic depressive illness.

Linkage analysis between pericentrometric markers on chromosome 18 and bipolar disorder: a replication test

Replication was attempted of a recent report on linkage between bipolar affective disorder and pericentrometric loci on chromosome 18. Linkage to these markers was excluded in a sample of five extended multiplex families using lod-score and affected-pedigree-member methods. In one family, however, the lod score exceeded 1.0. Although the proposed susceptibility genes are unlikely to have a major impact on the occurrence of bipolar disorder, they might modify the genetic risk in a minority of familial cases.

A molecular model for bipolar affective disorder

The biological basis of bipolar disorder is not known. Models for the illness have been proposed that were based on the neurobiological effects of pharmacological agents that affect mood. Although of great interest, these models have not adequately explained the striking clinical pattern of illness in which patients may experience either unipolar episodes or bipolar cycles of mania and depression. We now present a new model suggesting that the unique clinical heterogeneity found in patients with bipolar disorder could be explained by a defect in a 'downstream' portion of a signal transduction pathway that can regulate two or more neurotransmitter systems that have opposite effects on neuronal activity. This model may target specific candidate genes for involvement in bipolar disorder.

Genes and psychosis: old wine in new bottles?

Despite initial setbacks, linkage studies with DNA markers continue to occupy center stage in psychiatric research. Advances in molecular and statistical techniques have revived the search for disease genes, leading to a new harvest of findings. Most interest in recent years has focused on potential linkages between schizophrenia and chromosomes X-Y (the pseudoautosomal region) and 22q12-13.1, and between bipolar affective disorder and chromosomes 18 (pericentromeric region) and 21q22.3. This article provides a critical evaluation of theses studies, with implications for future research. Concerns over earlier linkage trials make this scrutiny current and topical.

Mapping genes for psychiatric disorders and behavioral traits

In the past year, some of the most exciting findings in the genetic investigation of mammalian behavior have been obtained through mapping and through gene manipulation studies in the mouse system. These include the localization of a gene for circadian periodicity in the mouse, gene knockouts of serotonin receptors, and the development of a transgenic model of Alzheimer's disease. The recent development of genetic maps covering the entire human genome and the implementation of new approaches to genetic analysis may now facilitate elucidation of complex behaviors in humans, particularly psychiatric disorders.

Evidence for a susceptibility locus for schizophrenia on chromosome 6pter-p22

We have performed linkage analysis in 186 multiplex families to search for genes that predispose to schizophrenia. Under a model with partially dominant inheritance, moderately broad disease definition and assuming locus homogeneity, a lod score of 3.2 was obtained for D6S260 on chromosome 6p23. A multipoint lod score of 3.9 (P = 2.3 x 10(-5)) was achieved when the F13A1 and D6S260 loci were analysed, allowing for locus heterogeneity. Adjusted for testing of multiple models, the multipoint lod score of 3.9 under heterogeneity has a genome wide significance of between 5-8%. The nonparametric affected pedigree member test provided results (P = 3 x 10(-4)) also supporting this finding. Our findings provide supportive evidence for a susceptibility locus for schizophrenia on distal chromosome 6p, and support a model of locus heterogeneity.