Minisatellite mapping in manic depression

Recent advances in the genetics of psychiatric disorder

The prediction of risk for mental illnesses that have a strong genetic basis has usually been carried out by employing empirically derived estimates of recurrence observed in family studies. Because systematic methods of mapping the human genome with DNA polymorphisms have been developed it is possible that prenatal prediction of certain mental illnesses may eventually become a more accurate method of risk prediction than the empirical method currently used. Recombinant DNA technology as used in genetic linkage studies can also specify a mode of genetic transmission and lead on to aetiological studies of the disorder. The latter will result from the molecular cloning and sequencing of the responsible genetic mutations. The use of recombinant DNA technology has enabled single-gene effects to be found in schizophrenia, manic depression and Alzheimer's disease. In highly selected families it is now possible to calculate the risk of developing certain mental illnesses by DNA testing before birth. Failure to find linkages in separate schizophrenia, manic depression and Alzheimer's disease family samples has also been reported, indicating that there is heterogeneity of linkage with genes at several distinct genetic loci that contribute susceptibility to very similar clinical disorders. Resolution of the degree to which there is heterogeneity of linkage and a further examination of the factors which influence the variable effects of the abnormal genes predisposing to mental illness will both be necessary before accurate risk prediction can become feasible.

No evidence for a susceptibility locus predisposing to manic depression in the region of the dopamine (D2) receptor gene

Recent reports of cytogenetic abnormalities linked to psychiatric illness and the localisations of the genes for the dopamine (D2) receptor and tyrosinase on the long arm of chromosome 11 have suggested that susceptibility loci for schizophrenia and manic depression might be situated in this region. We could find no evidence for linkage in five Icelandic pedigrees between manic depression and markers in this region, and we have excluded candidate genes coding for the D2 receptor and tyrosinase. We conclude that mutations at loci in this region are not a common cause of manic depression in the population studied.

Molecular genetics and heterogeneity in manic depression

Recent research has shown that there are X-linked and possibly chromosome 11-linked forms of manic depression as well as at least one other autosomal form. Segregation analyses of large affected families and the finding of genetic linkage between chromosome specific markers and manic depression mutations provide strong evidence that bipolar as well as unipolar forms of manic depression (MD) within the same family are inherited as a dominant gene disorder. This clarification of the etiology of certain types of depression should bring changed attitudes within psychiatry and may serve to stimulate discussion of the role of evolutionary mechanisms. From a clinical point of view, it has now become possible to determine whether clinical (phenotypic) variation reflects the underlying genotypic heterogeneity of linkage. A preliminary analysis of data from four recent studies shows that there is no clear correlation between such clinical features as the ratio of unipolar to bipolar cases and the genotypic form of manic depression. Further recombinant DNA research, proven to be successful in other genetic diseases, can soon be applied to manic depression. The specific problems posed by manic depression for these techniques are discussed.