EVIDENCE THAT THE AGE AT DIAGNOSIS OF IDDM IS GENETICALLY-DETERMINED

OBJECTIVE - The aim of this study was to determine the impact of genet ic or environmental factors on the age or time of onset of IDDM by stu dying pairs of twins and siblings concordant for the disease. RESEARCH DESIGN AND METHODS - From 404 twin pairs referred to a diabetic twin study we selected pairs concordant for IDDM: 1) 116 identical pairs wi th an index twin diagnosed diabetic under age 60 years and 2) 12 ident ical and 12 nonidentical matched twin pairs. From 972 families referre d to a population-based diabetic family study, we selected sibling pai rs with IDDM: 33 pairs with an index case diagnosed diabetic under age 21 years. Twin and sibling pairs were analyzed for intraclass correla tions for age and time of diagnosis. RESULTS - Of twins concordant for IDDM, the age at diagnosis correlated 1) in 116 identical pairs (R = 0.94; P < 0.000001) and 2) more closely in 12 identical twins (R = 0.9 6, P < 0.000001) than 12 nonidentical twins (R = 0.59, P = 0.046). Of 33 sibling pairs with IDDM, the age, but not the time, of diagnosis wa s correlated (R = 0.53, P = 0.0016). CONCLUSIONS - Correlations within pairs of twins and siblings for age, not time, at diagnosis suggest t hat much of the variability of the age at diagnosis of IDDM is genetic ally determined.