Unipolar depressive disorders have a common genotype

Background: The purpose of this study was to estimate the contribution of genetic, common- and unique environmental factors in the aetiology of unipolar major depression (MD), and to investigate whether the unipolar depressive disorders: MD, atypical depression/depression NOS, dysthymia and depressive adjustment disorder can be viewed as various expressions of an underlying genetic commonality. Methods: A sample consisting of same-sexed mono- and dizygotic twins was drawn from in- and outpatient hospital registers (N = 303). DSM-III-R criteria were assessed by personal interviews. One hundred and forty-three of the probands fulfilled the criteria for one or another unipolar depressive disorder. Cross-tabulations were used to compare concordance rates for MD and different combinations of MD and other unipolar depressive disorders. Correlations in liability and estimations of the heritability (h(2)) with biometrical model fitting were performed. Results: Concordance rates were higher among MZ- than among DZ pairs for both MD and all the different combinations of MD and other unipolar depressive disorders. Cross-concordance between MD and other unipolar disorders was observed. In all instances, except for the situation when MD was considered alone, the correlations in liability among MZ pairs were more than twice the correlations in liability among DZ pairs. The heritability of MD was 0.42, of MD + atypical depression 0.51, of MD + atypical depression + dysthymia 0.45 and of MD + atypical depression + dysthymia + depressive adjustment disorder 0.46. Limitation: Probands were not sampled from the general population. Most often the same person interviewed both twins in a pair. Conclusion: Unipolar MD is moderately heritable without significant shared family environmental effects. Unipolar depressive disorders taken together are moderately heritable without any detectable shared family environmental effects. The tendency is towards higher heritability estimates for the combined groups compared to MD alone. The study suggests that the disorders in the unipolar depressive spectrum may be different manifestations of the same genetic liability. (C) 2008 Elsevier B.V. All rights reserved.