Genetics may underpin psychiatric comorbidity in bipolar patients

Relatives of people with bipolar disorder are at an increased risk of developing not only bipolarity but also other psychiatric disorders, according to Swedish researchers.

Led by Jie Song, from the Karolinska Institutet in Stockholm, Sweden, the team found that first-degree relatives of people with bipolar disorder were at an almost eightfold increased risk of developing the disorder themselves compared with non-relatives.

Writing in Bipolar Disorders, the researchers say this suggests that familial risk may offer a rationale to initiate preventive family-based screening in primary care.

Song et al conducted a family-based study using data from 54,723 individuals with bipolar disorder identified in Swedish national registries.

Analysis revealed that the relatives of individuals with bipolar disorder had increased risks of bipolar disorder, and risks decreased with the distance of the biological relationships. Specifically, the risk of developing bipolar disorder increased 5.8- to 7.9-fold for first-degree relatives, 2.2- to 3.3-fold for second-degree relatives and 1.6-fold for third-degree relatives.

The researchers noted a higher risk of bipolar disorder in maternal than paternal half-siblings and a significantly increased risk of bipolar disorder in adoptees than in biologically unrelated parents with bipolar disorder. The team says this highlights the need for studies of the effects of growing up in families with parents who have bipolar disorder.

When Song et al investigated the association between bipolar disorder and other psychiatric disorders they found that, among the 54,723 patients diagnosed with bipolar disorder, a number had, at some point, also been diagnosed with schizophrenia (n=3320), anxiety disorders (n=14,563), attention deficit/hyperactivity disorder (ADHD; n=2064), drug abuse (n=5733), personality disorders (n=8473), or autism spectrum disorders (ASD; n=776).

There were substantially increased risks for all of these psychiatric disorders in individuals with bipolar disorder when compared with population controls (relative risk=9.7–22.9).

The researchers observed that full siblings of bipolar disorder patients also had significantly increased risks of developing all the psychiatric comorbidities under study, with risk increases ranging from 1.7 to 2.8-fold. Again, the risk decreased with increasing genetic distance. Song et al say that the associations between bipolar disorder and these psychiatric disorders are to a large extent explained by shared genetic factors.

“These results confirm the importance of genetic risk factors in the etiology of [bipolar disorder], as well as their pleiotropic effects for other psychiatric disorders”, the team concludes.

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