Genetic and environmental components of interindividual variation in circulating levels of IGF-I, IGF-II, IGFBP-1, and IGFBP-3.

We assessed the magnitude of the genetic component in the variation of circulating levels of insulin-like growth factors I and II (IGF-I and IGF-II), and their binding proteins IG- FBP-1 and IGFBP-3 by measuring their serum concentra- tions in 32 monozygotic and 47 dizygotic adult twin pairs of the same sex. The intrapair correlation for the IGF-I levels was r 5 0.41 ( P , 0.009) for monozygotic twins and r 5 0.12 ( P , 0.22) for dizygotic twins. For the IGF-II concentration the intrapair correlations were r 5 0.66 ( P , 0.0001) for the monozygotic and r 5 0.34 ( P , 0.01) for the dizygotic twins. No significant intrapair correlation was found for IGFBP-1 levels in either group. The correlations for IGFBP-3 concen- tration were r 5 0.65 ( P , 0.0001) and r 5 0.23 ( P , 0.06) for monozygotic and dizygotic twins, respectively. Women had higher IGF-II levels than men (635 6 175 vs. 522 6 144 m g/liter; P , 0.0001) and IGFBP-3 levels were also higher in women compared with men (5441 6 1018 vs. 4496 6 1084 m g/ liter; P , 0.001). The proportion of variance attributable to genetic effects was 38% for the IGF-I concentration, 66% for the IGF-II concentration, and 60% for the IGFBP-3 concen- tration. No significant heritability was found for the IGFBP-1 concentrations. Our results show that, in adults, there is a substantial genetic contribution responsible for interindivid- ual variation of the circulating levels of IGF-I, IGF-II, and IGFBP-3, but not for the IGFBP-1 levels. ( J. Clin. Invest. 1996. 98:2612-2615.) Key words: twinsmonozygotic • dizygoticgenetic regulationinsulin-like growth factor