Heterogeneity among populations for familial aggregation of blood pressure

A simple model of familial aggregation was applied to age–sex corrected systolic (SBP) and diastolic (DBP) blood pressure data from two separate studies: The Québec Family Study and the Tecumseh Community Health Study. Examination of the familial correlations suggested heterogeneity across studies in all DBP correlations, but for SBP only spouse correlations were higher in Québec. Fitting the path model to the data revealed that the only source of heterogeneity for SBP was in spouse resemblance (u), which was significant in Québec but could be dropped from the model in Tecumseh. For SBP, no further heterogeneity was detected, with the vertical transmission being compatible with (but not evidence for) purely polygenic inheritance (i.e., τ_f = τ_m = 1/2). Familiality estimates (t²) for SBP were quite high (nearly 40%), and extra sibling resemblance was significant. For DBP, heterogeneity was detected in all parameters of the model except for spouse resemblance. Common sibship environmental effects (b) were of little importance in Tecumseh but were highly significant in Québec. Vertical transmission from mothers (τ_m) could be fixed at 1/2 in Tecumseh but was only about half as large in Québec, and familiality estimates were higher in Québec (nearly 60%) than in Tecumseh (about 25%). One source of the heterogeneity for SBP was due to cohabitation, which positively impacted on sibling correlations. The source of the sample heterogeneity in DBP may be through environmental factors which have a smaller impact in Tecumseh than in Québec, although genetic heterogeneity could not be ruled out. Differences in the study designs, as well as differences in the two populations, could account for the heterogeneity detected.