Context: A monocyte pro-inflammatory state has previously been reported in bipolar disorder (BD).
Objective: To determine the contribution of genetic and environmental influences on the association between monocyte pro-inflammatory state and BD.
Design: A quantitative polymerase chain reaction case-control study of monocytes in bipolar twins. Determination of the influence of additive genetic, common, and unique environmental factors by structural equation modeling (ACE).
Setting: Dutch academic research center.
Participants: Eighteen monozygotic BD twin pairs, 23 dizygotic BD twin pairs, and 18 monozygotic and 16 dizygotic healthy twin pairs.
Main outcome measures: Expression levels of monocytes in the previously reported coherent set of 19 genes (signature) reflecting the pro-inflammatory state.
Results: The familial occurrence of the association between the monocyte pro-inflammatory gene-expression signature and BD found in the within-trait/cross-twin correlations (twin correlations) was due to shared environmental factors (ie, both monozygotic and dizygotic ratios in twin correlations approximated 1; ACE modeling data: 94% [95% confidence interval, 53%-99%] explained by common [shared] environmental factors). Although most individual signature genes followed this pattern, there was a small subcluster of genes in which genetic influences could dominate.
Conclusion: The association of the monocyte pro-inflammatory state with BD is primarily the result of a common shared environmental factor.