Major depressive disorder (MDD) is believed to have a genetic factor in its pathogenesis. On the basis of studies in MDD showing brain energy depletion and maternal inheritance in some families, we hypothesize that some of the genetic factor is likely maternally inherited on the mitochondrial DNA (mtDNA). Six hundred and seventy-two pedigrees from the Genetics of Recurrent Early-Onset Depression project were analyzed for matrilineal/nonmatrilineal pairs. Pairs were constructed to control for sex, age and autosomal gene contribution (e.g. maternal vs. paternal aunts). Individuals with and without any mood disorder were tallied and compared across five different pairs. Matrilineal relatives (with the same mtDNA sequence as the proband) were significantly more likely to suffer from a mood disorder than were nonmatrilineal relatives (with another mtDNA sequence; odds ratio 2.0, 95% confidence interval: 1.5-2.6, P = 3 x 10(-6)). Our data show a modest maternal bias in the susceptibility towards the development of depression, suggesting that predisposing genetic factors likely reside on the mtDNA. Thus, our data strengthen the hypothesis that energy metabolism may be involved in the pathogenesis of depression.

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