Genetics of unipolar major depressive disorder

Major depressive disorder (MDD) is a heterogeneous, highly prevalent, and moderately heritable disorder. A complex and diverse genetic-environmental interplay converges to set apart a significant minority that is susceptible to MDD, from among those who experience shorter lived and less recurrent intensive and incapacitating forms of sadness. The major technological advances of deciphering the human genome reference sequence and its common gene variations are beginning to allow cost effective genetic studies of unprecedented scale, applying increasingly denser genome wide mapping to increasingly larger case control samples. This effort is now at the initial stages of unraveling the genetic architecture of several complex phenotypes. Despite a tardy beginning, MDD genetic research is maturing from modest scale candidate gene association studies to include family-based linkage studies, and will soon allow genome wide case control association studies. Replicated risk conferring gene variants discovered so far exert a modest effect size that appears to contribute to overt phenotype expression in the context of a highly intricate concert of interrelated epigenetic and epistatic modifiers. The unraveling of additional previously unimplicated MDD risk conferring genes, that will throw light on molecular mechanisms mediating such susceptibilities, is necessary for progressing beyond current generation monoamine modulating antidepressant drugs. The review outlines basic concepts and current progress briefly overviews major replicated gene findings that to date mostly stem from hypotheses driven candidates, and ends with a discussion of current directives, including sample size and phenotype considerations and advancement of systematic studies of the functional significance of implicated gene variants, beyond their current exploratory stage.