Shared genetic risk between functional somatic syndromes, internalizing disorders, and immune-mediated diseases: a twin-sibling study.

Functional somatic syndromes frequently co-occur with internalizing disorders such as anxiety disorders and major depressive disorder. Both show familial associations with immune-mediated diseases. Here, we estimate genetic and environmental contributions to functional somatic syndromes and their overlap with immune-mediated diseases, with internalizing disorders included for comparison. The study sample consisted of 6,097,372 Swedish twins, full siblings, and half-siblings born between 1945 and 2003. From nationwide registers covering inpatient, outpatient and primary care, we extracted ICD diagnoses of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), fibromyalgia (FM), irritable bowel syndrome (IBS), major depression, anxiety disorders, and immune-mediated diseases (consisting of autoimmune and autoinflammatory diseases). We used bivariate twin-sibling structural equation modeling to estimate genetic and environmental correlations. We found that the heritability of functional somatic syndromes and internalizing disorders ranged from 15 to 44%, with the unique environment explaining 49-84% of the variance. We estimated the heritability of immune-mediated diseases at 37% (95% CI 36-38%), with a unique environmental component of 63% (95% CI 62-63%). Regarding the genetic correlations with immune-mediated diseases, fibromyalgia showed the strongest genetic correlation (rA = 0.52, 95% CI 0.45-0.63), IBS, ME/CFS, and major depression showed more modest genetic correlations (rA range 0.19-0.29), and anxiety disorders showed minimal genetic correlation (rA = 0.04, 95% CI 0.00-0.08). In summary, fibromyalgia, and to a lesser degree other functional somatic syndromes and major depression, share genetic risk factors with immune-mediated diseases. These findings suggest that immune-related genetic risk factors contribute to the etiology of fibromyalgia and, to a lesser extent, other functional disorders and major depression.