Normal somatomedin and somatomedin receptors in achondroplastic dwarfism

It has been proposed that the basic abnormality in achondroplasia may be a quantitative defect in endochondral new bone formation secondary to decreased synthesis of somatomedin (SM) or abnormal binding of SM to specific receptors. To test this hypothesis, we have measured plasma SM levels and SM receptors on circulating mononuclear cells obtained from 5 achondroplastic dwarfs and 5 age-matched controls. Plasma SM levels were 0.82 ± 0.14 U/ml (mean ± S.E.M.) for the achondroplastic dwarfs and 0.90 ± 0.12 U/ml for the controls. The specific binding of ¹²⁵I-SM to 50 X 10⁶ mononuclear cells was 7.66 ± 1.11% for the dwarf group and 7.66 ± 1.16% for the controls. Achondroplastic and control cells possessed equal numbers of receptor sites and identical receptor affinity for SM. The data indicate that plasma SM levels and SM binding to circulating mononuclear cells are normal in achondroplastic dwarfs and suggest a primary intracellular defect.