Serum growth hormone levels measured by radioimmunoassay and radioreceptor assay: a useful diagnostic tool in children with growth disorders?

Growth hormone variants: a potential avenue for a better diagnostic characterization of growth hormone deficiency in children

Human GH (hGH) is a heterogeneous protein hormone consisting of several isoforms. This heterogeneity is the consequence of multiple hGH genes, mRNA splicing, post-translational modifications, and peripheral metabolism, and it represents one important reason for the disparity among GH assay results from different laboratories. However, other factors are involved: a) interference from endogenous GH binding proteins; b) different specificities of anti- GH (monoclonal and polyclonal) antibodies; c) different matrix effects among the calibrators; d) the use of different calibrators. The measurement of GH levels in response to provocative testing is an essential part of the diagnosis of GH deficiency. For this purpose, an accurate, reproducible and universally valid GH measurement would be highly desirable, but, despite a huge number of efforts in clinical biochemistry, this goal remains elusive.

Biologically inactive growth hormone caused by an amino acid substitution

Short stature caused by biologically inactive growth hormone (GH) is characterized by lack of GH action despite high immunoassayable GH levels in serum and marked catch-up growth to exogenous GH administration. We found a heterozygous single-base substitution (A-->G) in exon 4 of the GH-1 gene of a girl with short stature, clinically suspected to indicate the presence of bioinactive GH and resulting in the substitution of glycine for aspartic acid at codon 112. We confirmed the presence of mutant GH in the serum using isoelectric focusing analysis. The locus of mutation D112G was found within site 2 of the GH molecule in binding with GH receptor (GHR)/GH binding protein (GHBP). The expressed recombinant mutant GH tended to form a 1:1 instead of the 1:2 GH-GHBP complex normally produced by wild-type GH. The formation of a 1:2 GH-GHBP complex is compatible with the dimerization of GHRs by GH, a crucial step in GH signal transduction. Mutant GH was less potent than wild-type GH not only in phosphorylation of tyrosine residues in GHR, janus kinase 2 (JAK2), and signal transducers and activators of transcription 5 (STAT5) in IM-9 cells, but also in metabolic responses of BaF/GM cells, a stable clone transfected with cDNA of the chimera of the extracellular domain of human GHR, the transmembrane and the cytoplasmic domain of the human thrombopoietin receptor. These results indicate that the D112G mutation in the GH-1 gene causes production of bioinactive GH, which prevents dimerization of GHR and is therefore responsible for the patient's short stature.

Short stature and low IGF-1 and IGFBP-3 despite normal growth hormone secretion in a 4 year-old girl with primary empty sella syndrome

A 4.1 year-old girl presented with short stature, cleft lip and additional upper incisor. Magnetic resonance imaging showed an empty sella due to a hypoplastic anterior pituitary. IGF-I and IGFBP-3 were at and below the 1st percentile, respectively. In contrast, normal spontaneous growth hormone (GH) secretion (5.5-h nocturnal sampling) was observed and pharmacological provocation raised (GH to levels between 15.4 and 53 micrograms/l. GH-binding protein levels were normal (210 pM). GH therapy led to an increase of growth velocity from 4.5 to 10.8 cm/year and a normalization of IGF-I and IGFBP-3 levels. The findings may imply an abnormal GH secretion pattern or a bioinactive GH in our patient. The data indicate that measurements of IGF-1 and IGFBP-3 may be a more sensitive test for integrative GH activity than GH testing itself.

Ontogeny of pituitary and serum growth hormone in growing turkeys as measured by radioimmunoassay and radioreceptor assay

Some mammalian studies have revealed a wide discrepancy in pituitary and circulating growth hormone (GH) measurements determined by immunological and biological assay methods. Recent studies demonstrating that avian GH exists in numerous isoforms raise concerns that immunological measurement of GH may not accurately reflect the amount of biologically active hormone present. We sampled eight different male turkeys of a commercial strain weekly until 6 wk of age, and then biweekly until 20 wk. Total pituitary GH content and serum GH concentration were measured by avian GH RIA and radioreceptor assay (RRA). The highest mean serum GH concentration occurred at 3 wk, and the ontogeny of serum GH content from 1 to 8 wk was similar whether measured by RIA or RRA. Pituitary GH content was highest at 6 wk, but RIA and RRA estimates differed markedly throughout the study. Pituitary content of biologically active GH, as estimated by RRA, exceeded that of immunologically active GH from 2 to 10 wk, whereas the reverse was true at 14, 18, and 20 wk. We conclude that this avian GH RIA accurately measures bioactive circulating turkey GH, but that the pituitary of the young turkey may contain bioactive GH isoforms that have poor immunological activity in our RIA.

Assessment of growth hormone insulin like growth factor-I axis in Down's syndrome

As GH therapy has been reported to increase growth velocity in children with Down's syndrome (DS), we studied the GH-IGF-I axis in some DS patients affected by growth retardation without serious congenital malformation, malnutrition or pathological thyroid or adrenal function. IGF-I and IGF-II were evaluated in 39 patients in basal conditions. The patients were subsequently divided into two groups with respect to the IGF-I basal value: Group 1 (GR 1) consisting of patients with abnormally low basal IGF-I concentration as compared to age matched control subjects, group 2 (GR 2) consisting of patients with IGF-I in the normal range. In 6 GR 1 patients and 12 GR 2 patients we evaluated GH and IGF-I concentrations after stimulation with arginine (0.5 g/kg bw), and recombinant GH (4 IU im). In the same patients, GH radioreceptor assay and serum GH-binding protein were evaluated. In all patients IGF-II proved normal (534 +/- 23 ng/ml; mean +/- SE), while IGF-I was pathological in 36% of subjects. The cause of the defective IGF-I secretion in these patients does not seem to depend on an impaired GH axis, as no significant difference in arginine-stimulated GH peak values was seen between GR 1 (29.6 +/- 5.3 ng/ml) and GR 2 (15.1 +/- 2.24 ng/ml). IGF-I concentration evaluated 12, 24, and 48 h after arginine stimulation was significantly increased only in GR 2 patients (peak value: 0.95 +/- 0.1, p = 0.0003 vs baseline; GR 1: 0.34 +/- 0.05 U/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Pseudohypopituitary syndromes

In a child with short stature, the finding of normal or elevated GH levels in the presence of low concentrations of IGF-I raises the following possibilities. (1) A modification of the GH molecule, which is still detected by RIA, but inactive biologically. Therefore, an RRA or bioassay for hGH should result in considerably lower GH measurements compared with RIA determinations in the same sample. As both bioassays as well as RRAs are not widely available and are hampered by several difficulties, few children with this presumptive diagnosis have been described. So far, it has not been possible to define a specific molecular defect in one of these patients. (2) Abnormalities of the GH receptor or postreceptor mechanisms lead to a GH insensitivity syndrome. Laron-type dwarfism is usually due to a deletion in the gene for hepatic GH receptors: the serum binding protein for GH is absent. In three additional populations, the Pygmies of Zaire, the little women of Loja in Ecuador and the Mountain Ok people in Papua New Guinea, alterations of GH receptor function have been described. Finally, some reports describe patients with normal or elevated serum levels of both growth hormone and IGF-I in whom resistance to IGF has been implied in the pathogenesis of small stature.

Diagnosis of growth hormone deficiency

Many ways of evaluating the physiological state of hGH secretion exist, some of which have been touched upon and none of which has as yet proven infallible. Apart from important clinical features like history, physical data and growth rate, the diagnosis of altered pituitary function is based on tests and their interpretation. The physician responsible has to be informed on their effectiveness and pitfalls. Results should be interpreted in relation to developmental age (bone age) rather than chronological age. Research is under way to try to facilitate the diagnosis of varying degrees of alterations of hGH secretion. Reliability in predicting the effect of therapy with hGH is the ultimate aim in order to prevent unnecessary cost and disappointment for the patients. With the help of doctors involved in child care, such as physicians at kindergarten or school, it should be possible to start the slow process of investigating growth disorders at an early age.