Quantile-Dependent Expressivity of Serum Interleukin-6 Concentrations as a Possible Explanation of Gene-Disease Interactions, Gene-Environment Interactions, and Pharmacogenetic Effects

Interleukin 6 (IL-6) is a moderately heritable pleiotropic cytokine whose elevated concentrations in coronary artery disease, peripheral arterial disease, pulmonary arterial hypertension, Eales' disease, Sjogren's syndrome, osteoarthritis, adenocarcinoma, neuroblastoma, polymyalgia rheumatica, pulmonary tuberculosis, and enterovirus 71 infection, and following coronary artery bypass graft show larger genetic effects than in unaffected low IL-6 controls. We hypothesize that genetic effects may depend upon whether average IL-6 concentrations are high or low, i.e., quantile-dependent expressivity. Quantile-specific offspring-parent (beta(OP)) and full-sib regression slopes (beta(FS)) were estimated by applying quantile regression to the age- and sex-adjusted serum IL-6 concentrations in families surveyed in the Framingham Heart Study. Quantile-specific heritabilities were calculated as h(2) = 2 beta(OP) / (1 + r(spouse)) and h(2) = {(1 + 8r(spouse)beta(FS))(0.5) -1} / (2r(spouse))). Heritability (h(2) +/- SE) of IL-6 concentrations increased from 0.01 +/- 0.01 at the 10th percentile (NS), 0.02 +/- 0.01 at the 25th (P = 0.009), 0.03 +/- 0.01 at the 50th (P = 0.007), 0.04 +/- 0.02 at the 75th (P = 0.004), and 0.13 +/- 0.05 at the 90th percentile (P = 0.03), or 0.0005 +/- 0.0002 for each 1% increase in the offspring's phenotype distribution (P-linear trend = 0.02) when estimated from beta(OP) and from 0.02 +/- 0.02 at the 10th (NS), 0.02 +/- 0.02 at the 25th (NS), 0.06 +/- 0.02 at the 50th (P = 0.01), 0.12 +/- 0.04 at the 75th (P = 0.001), and 0.30 +/- 0.03 at the 90th percentile (P < 10(-16)), or 0.0015 +/- 0.0007 for each 1% increase in the sibling phenotype distribution (P-linear trend = 0.02) when estimated from beta(FS). Thus the heritability of serum IL-6 concentrations is quantile dependent, which may contribute in part to the larger genetic effect size reported for diseases and environmental conditions that elevate IL-6 concentrations vis-a-vis unaffected controls.

Automated summary

Heritability of IL-6 concentrations is quantile-dependent, showing larger genetic effects in diseases and conditions that elevate IL-6 concentrations compared to unaffected controls.