3. Genetics of Bipolar Disorder
3.1 Heritability of Bipolar Disorder Bipolar disorder is a highly heritable disease, which has been proven by many twin and adoption studies for decades. Heritability is as high as 80-85% [Cardno et al. 1999; McGuffin 2003]. Concordance rates between monozygotic twins are 40- 70% and 5-10% for dizygotic twins. The children of two affected parents have a lifetime-risk of 50-65% to fall ill with bipolar disorder, while children with one parent with bipolar disorder only show a risk of 25% to get bipolar disorder. If one first-degree relative suffers from bipolar disorder, one has the risk of 5-10% to develop bipolar disorder. In contrast people without related affected individu- als show a risk of 1% [Rothenhäusler et al. 2004]. This assumes that the genetic constitution is very important for development of manic-depressive disease, but not the only cause. Genetic and environmental factors are the most probable reasons for the pathogenesis of bipolar disorder [Kieseppä et al. 2004]. 3.2 Candidate Genes at One View Since the main model of affective disorders has been dominated by the “neuro- transmitter imbalance theory” for decades, it is not surprising that many highly in- vestigated candidate genes have belonged to neurotransmitter systems (serotoner- gic, noradrenergic, dopaminergic, GABAergic and glutamatergic system) and biogenic amine modulation (e.g. genes encoding for Monoamine-oxidase-A (MAOA), Catechol-O-methyltransferase (COMT) and the Typtophan Hydroxylase 1 (TPH1)). Circadian rhythms are often disturbed in bipolar disorder, so “Clock genes” have been hot spots of research likewise. The growth hormones are another promising chapter of...
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