Anticipation in Swedish families with bipolar affective disorder.

Anticipation describes an inheritance pattern within a pedigree with an increase in disease severity or decrease in age at onset or both in successive generations. The phenomenon of anticipation has recently been shown to be correlated with the expansion of trinucleotide repeat sequences in different disorders. We have studied differences of age at onset and disease severity between two generations in 14 families with unilinear inheritance of bipolar affective disorder (BPAD). There was a significant difference in age at onset (p < 0.008), in episodes per year with (p < 0.006) and without (p < 0.03) lithium treatment, and in total episodes per year (p < 0.002) between generations I and II. Furthermore, there was a highly significant correlation (p < 0.001) in age at onset between generations I and II. No evidence for specific paternal or maternal inheritance was found. We found evidence of anticipation and could rule out ascertainment bias or some other artefact. Anticipation is thus an inheritance pattern in BPAD which suggests that the expansion of trinucleotide repeat sequences is a possible mode of inheritance in BPAD.