The heterogeneity of human growth hormone: novel isoforms, assay development and clinical implications.

Abstract

Human growth hormone (GH) consists of several isoforms. The multiple GH molecules arise from two related genes (GH-N: pituitary; GH-V: placenta), alternative mRNA-splicing mechanisms and posttranslational events. The most abundant GH is a 22kDa polypeptide with 191 amino acids. The 22kDa GH is the reference GH against which other non-22kDa GH isoforms are compared regarding chemical structure and biological activity. The general aim of this study was to examine the clinical implications of the GH heterogeneity. The cross-reactivity of several GH isoforms was tested in two GH immunoassays. To study the proportion of non-22kDa GH isoforms in human blood, the 22kDa GH exclusion assay (GHEA) was developed and optimised by statistical designs. The GHEA is based on extraction of 22kDa GH from serum and determination of non-22kDa GH levels by a polyclonal GH assay. The GHEA was used to evaluate the proportion of non-22kDa GH in children with normal and abnormal growth, and in acromegaly. In placenta, two transcripts derived from the GH-V gene predicting novel GH isoforms were cloned. GH isoforms showed marked differences in their cross-reactivity in the two GH immunoassays. Using the GHEA, the proportion of non-22kDa GH isoforms was increased in acromegalic men and correlated with tumour size. After surgery, the proportion of non-22kDa GH differed in acromegalics considered cured and those not cured, the former group showing normalisation of the values. In healthy prepubertal children, the proportion of non-22kDa GH was directly correlated to auxological estimates of body composition. In healthy pubertal boys, high amounts of non-22kDa GH were associated with less growth. The amount of non-22kDa GH was increased in 5 of 17 short girls with Turner s syndrome, 5 of 25 short children born small for gestational age and 4 of 24 children with idiophatic short stature. In the GH-V gene, two cDNAs predicting new GH isoforms were cloned: one encoding a placental 20kDa GH and the other encoding a 24kDa GH, named GH-V3, with a unique carboxy-terminus. In conclusion, non-22kDa GH isoforms can be present in human blood in highly variable amounts, contributing to discrepancies of GH measurements. In children, the circulating proportion of non-22kDa GH may be of biological significance for normal and abnormal growth, and for body composition. In acromegaly, the GHEA may be useful in the follow-up of the therapeutical outcome. The placental GH-V gene exhibits at least four different splicing pathways, predicting in some cases the expression of GH isoforms with distinct carboxy-termini. Key words: somatotropin, growth, molecular weight, immunoassay, acromegaly, pituitary neoplasms, growth disorders, Turner s syndrome, small for gestational age, placental hormones, alternative splicing, statistical models.