The monocytes of a large proportion of bipolar patients and offspring of bipolar parents showed an inflammatory gene expression signature. This coherent set of genes opens new avenues for biomarker development with possibilities for disease prediction in individuals genetically at risk and for the subclassification of bipolar patients who could possibly benefit from anti-inflammatory treatment.

Focusing on genes controlling white matter integrity may be a fruitful strategy in the quest to discover genes implicated in bipolar disorder. Elucidating the mechanism by which lithium attenuates brain matter loss may lead to the development of neuroprotective drugs.

No abstract

Monozygotic (MZ) twins may be subject to epigenetic modifications that could result in different patterns of gene expression. Several lines of evidence suggest that epigenetic factors may underlie mental disorders such as bipolar disorder (BD) and schizophrenia (SZ). One important epigenetic modification, of relevance to female MZ twins, is X-chromosome inactivation. Some MZ female twin pairs are discordant for monogenic X linked disorders because of differential X inactivation. We postulated that similar mechanisms may also occur in disorders with more complex inheritance including BD and SZ. Examination of X-chromosome inactivation patterns in DNA samples from blood and/or buccal swabs in a series of 63 female MZ twin pairs concordant or discordant for BD or SZ and healthy MZ controls suggests a potential contribution from X-linked loci to discordance within twin pairs for BD but is inconclusive for SZ. Discordant female bipolar twins showed greater differences in the methylation of the maternal and paternal X alleles than concordant twin pairs and suggest that differential skewing of X-chromosome inactivation may contribute to the discordance observed for bipolar disorder in female MZ twin pairs and the potential involvement of X-linked loci in the disorder.

Structural neuroimaging studies suggest the presence of subtle abnormalities in the brains of patients with bipolar disorder. The influence of genetic and/or environmental factors on these brain abnormalities is unknown. To investigate the contribution of genetic and environmental factors on grey and white matter brain densities in bipolar disorder, monozygotic and dizygotic twins concordant and discordant for bipolar disorder were scanned using 1.5 Tesla magnetic resonance imaging and compared with healthy twin pairs. A total of 232 subjects: 49 affected twin pairs (8 monozygotic concordant, 15 monozygotic discordant, 4 dizygotic concordant, 22 dizygotic discordant) and 67 healthy twin pairs (39 monozygotic and 28 dizygotic) were included. After correcting for the effect of lithium, the liability for bipolar disorder was associated with decreased grey matter density in widespread areas of the brain, but most prominent in frontal and limbic regions, and with decreased white matter density in (frontal parts of) the superior longitudinal fasciculi. The genetic risk to develop bipolar disorder was related to decreased grey matter density in the right medial frontal gyrus, precentral gyrus and insula and with decreased white matter density in the superior longitudinal fasciculi bilaterally. In conclusion, pathology in the frontal lobe, especially in parts of the superior longitudinal fasciculus, may be central to the genetic risk to develop bipolar disorder, while widespread grey matter abnormalities appear related to the illness itself.