Study results suggest that anxiety state and trait are heritable and share some genetic factors in bipolar disorder (BD), with trait anxiety showing a stair-step distribution in family members based on their genetic proximity to the affected individual.

“If this trait is proven to be an endophenotype, it will be of help in diagnosing and treating bipolar I patients in a more reliable and biologically valid manner than our current classification allows,” say Javier Contreras (University of Texas Health Science Center at San Antonio, Texas, USA) and colleagues.

The localization of genes that predispose to BD has been difficult as some of these genes may be transmitted without expression of the categorical clinical phenotype.

“One strategy to overcome this obstacle is the use of quantitative endophenotypes, as has been done for other medical disorders,” explain the researchers in the Journal of Affective Disorders.

Contreras and team therefore computed heritability and genetic correlation of the state and trait scale from the Anxiety State and Trait Inventory in 30 bipolar I extended families, with an average family size of 10 members, and 20 unrelated healthy controls from a Costa Rican sample.

Of the 300 individuals from the extended families, 63 had BD type I, 74 major depressive disorder, 101 had no axis I disorder, with the remainder presenting other disorders.

The study showed that anxiety symptoms were underdiagnosed, with 14.4% meeting criteria for a DSM-IV categorical anxiety diagnosis compared with 26.0% of 274 individuals with no categorical anxiety diagnosis that were over the 75th percentile of the anxiety trait scale.

“This finding strongly encourages the use of anxiety quantitative measures in psychiatric genetics research,” says the team.

Patients with BD showed the highest trait anxiety scores, followed by relatives with other psychiatric disorders, healthy relatives, and healthy unrelated controls. The researchers note that although both state and trait showed significant heritability, state was less heritable and stable.

Furthermore, only anxiety trait showed normal distribution in healthy individuals, current mood status independence, and significant liability for BD type I.

“Further research is needed to evaluate if anxiety traits are specially related to bipolar I disorder in comparison with other traits such as anger, attention, or response inhibition deficit, pathological impulsivity or low self-directedness,” conclude the researchers.

MedWire (www.medwire-news.md) is an independent clinical news service provided by Current Medicine Group, a part of Springer Science+Business Media. © Current Medicine Group Ltd; 2009

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