Genetic segregation analysis of recurrent, early-onset major depression: evidence for single major locus transmission.

Coordinated efforts are now underway to identify susceptibility genes for unipolar major depressive disorder (MDD) and related disorders. These studies have focused on recurrent, early-onset MDD (RE-MDD), thought to be the most familial form of this disorder. The goal of this study was to conduct a complex segregation analysis of recurrent MDD and other major mood disorders aggregating in families identified by probands with RE-MDD. Eighty-one families were identified through probands over the age of 18 who met criteria for recurrent (> or =2 episodes), early-onset (< or =25 years), nonpsychotic, unipolar MDD (RE-MDD) and included 407 first-degree relatives and 835 extended relatives. Psychiatric diagnoses for probands and their family members who provided blood samples were formulated from structured personal interviews, structured family history assessments, and available medical records. The remaining family members who participated and those who were deceased were evaluated through the family history method augmented by available medical records. Best-estimate diagnoses were made during a consensus conference according to established diagnostic criteria. Segregation analyses were performed using the REGD routine in S.A.G.E. release 4.0. The segregation analysis of recurrent MDD supported a sex-independent Mendelian codominant model. Analysis of major mood disorders supported a sex-independent Mendelian dominant model. Interestingly, inclusion of spousal residual correlations provided better fitting models for recurrent MDD but not the broader phenotype of major mood disorders. Unlike unipolar MDD, the lifetime prevalence of bipolar I disorder in this sample of families did not exceed the reported population prevalence [Zubenko et al., 2001]. Our results suggest that a major locus contributes to the expression of recurrent MDD and possibly other major mood disorders within families identified by probands with RE-MDD. Due to the limitations of the segregation analysis model, our results cannot address whether the same major locus is segregating across families in our sample or whether multiple major loci are involved (genetic heterogeneity). The absence of aggregation of bipolar I disorder in these families strongly suggests that while the genetic determinants of unipolar and bipolar disorders may overlap, they are not identical. Our findings illustrate the advantage of employing families identified by probands with RE-MDD in studies designed to detect susceptibility loci for unipolar MDD and related disorders.