Dysregulation of cytokines synthesis is thought to play a role in the development of a number of age-related conditions, such as rheumatoid arthritis, osteoporosis, atherosclerosis, and others, but observational studies have led to contradictory results. We investigated potential familial influences on the plasma levels of IL-6 and TNF-alpha in 91 nuclear and more complex pedigrees of Caucasian ethnic origin (N=401 individuals). The maximum likelihood based variance decomposition analysis showed significant positive correlation between circulating IL-6 and age in both genders. The magnitude of these correlations in our sample ranged from 0.22 in females to 0.28 in males (P<0.001). Significant association between TNF-alpha and IL-6 (r=0.28, r=0.43; P<0.001; respectively for men and women) was also observed. Likelihood ratio test clearly revealed that additive genetic effect for TNF-alpha was highly significant (P<0.001), and accounted over 80% of its variation, adjusted for IL-6 levels and age. In contrast, heritability estimate for IL-6 adjusted for age and TNF-alpha, revealed small contribution of genetic factors (24.1 +/- 10.2%). The bivariate variance component analysis demonstrated that significant relationship between IL-6 and TNF-alpha was due to shared environment only (r(E)=0.760 +/- 0.140). As evinced from our complex segregation analysis the nature of the genetic determinant of each of these two cytokines is quite complex and it is probably oligogenic.
Copyright 2002 Elsevier Science Ltd.