[Gene-environment interactions in affective disorders]

Kindling and behavioural sensitization were probably the first among the animal models of affective disorders, to suggest that genes-environment interactions were likely to be involved in the pathophysiology of these disorders. Cross-sensitization among stressors, drugs of abuse and illness episodes was deemed to be supported by the induction of a series of transcription factors, such as the proto-oncogene c-fos that subsequently alter gene expression by binding at DNA sites and inducing mRNAs for substances that may exert effects over long time periods. This was an anticipation of epigenetics which is currently defined as a functional modification to the DNA that does not involve an alteration of sequence. Epigenetic modifications are most commonly regulated by DNA methylation and histone acetylation which are usually associated with the silencing and activation of gene transcription, respectively. In animal models, it was shown that parents can actively remodel epigenetic marks, and thus affect patterns of gene expression in the offspring, whereas environmental adversity decreases parental investment in the offspring and thus alters phenotypic development. In line with this, some laboratories have sought to identify changes in gene expression in post mortem brain samples of humans with affective disorders. Finally, gene-environment interactions have been directly studied, both in animals and humans, by testing how a functional polymorphism in candidate genes would moderate the influence of stressful life events on behavioural expression. Interesting results have been found and replicated for unipolar depression, however date are scarce for bipolar disorder. Findings from these studies allow the building of more sophisticated models for unipolar and bipolar genetics.