U-shaped association between paternal age and bipolar risk

By Eleanor McDermid, Senior medwireNews Reporter

Having a father who is relatively young or old may increase a person’s risk of developing bipolar disorder, a study suggests.

The increased risk was significant for people born to fathers who were in their early 20s or aged at least 50 years. Maternal age, however, was not significantly associated with bipolar disorder risk.

This U-shaped relationship with paternal age implies different mechanisms underlying bipolar risk in the children of young or older fathers, say study author Andre Sourander (University of Turku, Finland) and co-workers.

It is thought that de novo mutations may contribute to the bipolar risk associated with having an older father, because of the increasing number of divisions spermatocytes undergo as a man ages, they say.

By contrast, environmental factors, such as socioeconomic status, education and health behaviours, may account for the risk associated with having a young father, suggests the team in Bipolar Disorders.

The study involved 1861 patients with bipolar disorder from the Finnish Prenatal Study of Bipolar Disorders, as well as 3643 age- and gender-matched controls without bipolar disorder or schizophrenia.

Previous studies have produced conflicting results on the effects of parental age on bipolar disorder risk but several have shown a link with advanced paternal age. Likewise, the current study found that fathers aged at least 50 years were 2.84-fold more likely than those aged 30 to 34 years to have a child with bipolar disorder. This association was independent of mother’s age, parental psychiatric history, parental educational level and place of birth.

Fathers aged 20 to 24 years also had an increased likelihood of having a child with bipolar disorder, at a 1.35-fold increased risk. There was no association for teenage fathers, but the numbers were very small, with 1.4% of cases and 1.0% of controls having a father in this age group.

There was no clear association between maternal age and bipolar disorder, with the increased risk seen for women who were teenagers or aged 20 to 24 years disappearing after adjustment for variables including the father’s age.

“Future studies are needed to further clarify the effects of the environment and the role of gene–environment interactions in addition to the hereditary influence in the development of [bipolar disorder],” conclude the researchers.

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