A Case of Leprechaunism with the Disorders of Insulin-like Growth Factor-I (IGF-I) /Somatomedin C (SMC) Binding Protein and Its Receptor

Abstract

Leprechaunism is a rare genetic disorder characterized by physical abnormalities, intrauterine and postnatal growth retardation, poorly developed subcutaneous fat and muscle at birth, and early death. This patient, who was a 1.5 year-old female with typical clinical features of leprechaunism, had relatively high levels of plasma GH and IGF-I/SMC but no glucose intolerance or insulin resistance. Studies were undertaken to elucidate (1) the differences among some kinds of methods for IGF-I/SMC measurement, (2) the distribution patterns of IGF-I/SMC between two kinds of its binding protein (SMBP) in plasma, and (3) the dynamics of IGF-I/SMC receptor in her erythrocytes and liver microsomal membranes. The results were as follows: (1) The level of IGF-I/SMC measured by Nichols radioimmunoassay kit was 1.33U/ml, which was higher than that of infants the same age. Conversely, it was lower than that of the control which was measured by radioimmunoassay using recombinant IGF-I/SMC after acid-ethanol or Seppak C18 extraction. (2) By Sephadex G150 gel-chromatography, immunoreactive IGF-I/SMC was eluted predominantly in 150K region, and two apparent peaks of unsaturated somatomedin binding protein (USBP) were determined in a neonatal infant (appropriate to date), a normal adult and an infant of the same age as this patient. On the other hand, immunoreactive IGF-I/SMC was located only in the fractions corresponding to 40K region, and only one peak of USBP could be estimated in the region of 40K dalton. (3) The IGF-I/SMC receptor in the patient's erythrocytes possessed significantly lower binding affinity but higher binding capacity in comparison with that of the normal neonate and adult. In addition, the receptor in liver microsomal membranes obtained from this patient at autopsy also indicated lower affinity but higher capacity than that of fetuses at more than 19 weeks of gestation. This was coincident to that of fetuses less than 19 weeks of gestation. These results suggested that this patient resembled the intrauterine fetus before midgestation not only in the co-relationship among GH, IGF-I/SMC and its binding proteins, but also in the characteristics of its receptor. The severe growth retardation existing in this patient may be, at least partly, due to the abnormality and/or immaturity of IGF-I/SMC function. It is speculated that leprechaunism could be classified in relation to fetal growth mechanism by aspects of biological functions of IGF-I/SMC during development.