Generalized anxiety disorder and major depressive disorder comorbidity: an example of genetic pleiotropy?

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Abstract
Generalized anxiety disorder (GAD) and major depressive disorder (MDD) are the most common type of anxiety-mood comorbidity. Up to 80% of subjects with lifetime GAD also have a comorbid mood disorder during their lifetime. Many hypotheses have been raised to explain such high comorbidity. Pleiotropy, i.e. a single genetic mutation explains (apparently) different disorders, is one of them and is hereby reviewed. Importance and reliability of GAD and MDD comorbidity (1); Evidence in favour of co-aggregation of GAD and MDD within families (the risk of one disorder in a proband increasing the risk for the other in relatives) (2); substantial heredity for both disorders according to twin studies with evidence for genetic correlation of unity between the two disorders (3); existence of numerous mechanisms (4) potentially linking the two disorders to common vulnerability genes, are all in accordance with such a hypothesis. Some examples of potentially shared mechanisms (such as CRF dysregulation or abnormal transcription factors) and possible common vulnerability genes (for example, the serotonin transporter gene) are given to highlight the pleiotropy hypothesis.