Growth hormone releasing hormone and growth hormone: genetic studies in familial growth hormone deficiency

Molecular study of human growth hormone gene cluster in three families with isolated growth hormone deficiency and similar phenotype

The growth hormone (GH) gene (hGH-N) cluster was analysed using polymerase chain reaction, Southern and polymorphism analysis in five patients (including two pairs of siblings) with extreme short stature and absence of GH secretion. Patients 1 and 2 (siblings) were homozygous for a large deletion removing four genes of the cluster: hGH-N, hCS-L, hCS-A and hGH-V. Both siblings produced high anti-GH antibody levels in response to exogenous GH therapy, followed by growth arrest a few months after starting replacement therapy. In patient 3 we detected a heterozygous deletion which involved three genes of the cluster (hCS-A, hGH-V, hCS-B) and left an intact hGH-N gene. Direct sequencing of hGH-N specific amplified fragments excluded the presence of any point mutations in exons and splicing regions. In patients 4 and 5 (sisters) our study did not demonstrate any gene deletions. Analysis of polymorphic restriction patterns in this family demonstrated that both sisters inherited the same alleles from the father but different alleles from the mother, suggesting that the defect was not linked to the hGH-N gene. These results confirm the difficulty of clinical identification of subjects with hGH-N deletion and underline the importance of DNA analysis in patients with absence of GH secretion and extreme growth retardation.

Genetic and molecular analysis of familial isolated growth hormone deficiency

Familial isolated growth hormone deficiency (IGHD) has been associated with complete deletions of the hGH-N gene encoding the pituitary growth hormone (GH) in a large number of cases. However, there is still no alternative empirical explanation for the remaining familial or non-familial IGHD cases. We studied a large kindred including five IGHD-affected first cousins to determine possible IGHD inheritance and whether the hGH-N gene was the cause of IGHD in this pedigree. Sex-linked and autosomal recessive transmission of IGHD in this kindred was rejected. Autosomal dominant inheritance was the most probable explanation according to a model of one locus with two alleles, one being dominant for IGHD, under genetic modifiers or epistasis. Southern blotting analysis (BamHI and HindIII digestions) was used to determine whether the hGH-N gene was present in the patients and their family members. Because we found that the hGH-N gene was present, five restriction fragment length polymorphisms (RFLPs; HincII, MspI-A and B, and BglII-A and B) linked to the hGH-N gene were used to try to identify the possible RFLP haplotypes in the pedigree that could be markers or associated with the abnormal hGH-N alleles responsible for IGHD. From the haplotype analysis, it appeared that other genes not linked to the hGH-N gene cluster were the cause of the IGHD phenotype in this kindred. An alternative conclusion could be that the hGH-N gene was responsible for IGHD in this kindred, if a mutation (gene conversion) at the MspI-B site or a reciprocal recombination event between the HincII and MspI-B sites occurred from generation P to F1 and a similar event took place from generation F1 to F2. The non-significant GH responses of patients to the growth releasing factor test confirmed that the hGH-N gene structural product or some step in its regulation was responsible for causing IGHD in this kindred. We suggest that genetic micromutations in the hGH-N gene are present and are responsible for IGHD. We developed a method using the polymerase chain reaction to amplify a 790-bp fragment of the hGH-N gene. The fragment spanned from the second part of the dyad symmetry region in the non-transcribed 5' end of the hGH-N gene to 9 bp before the alternative splice-acceptor site in exon 3. The expected fragment was verified by its digestion with seven diagnostic restriction endonucleases (BamHI, FspI, PstI, NdeI, BssHII, BglII and HincII).(ABSTRACT TRUNCATED AT 400 WORDS)

The little (lit) mutation cosegregates with the growth hormone releasing factor receptor on mouse chromosome 6

The little (lit) autosomal recessive mutation in the mouse causes dwarfism due to isolated growth hormone deficiency. The in vitro physiology of pituitary growth hormone release in lit/lit animals suggests that an abnormality in the growth hormone releasing factor (GRF) receptor (Ghrfr) is a very likely candidate for the lit mutation. We mapped Ghrfr to the region around lit on Chromosome (Chr) 6 in 100 chromosomes of an FVB x Czech II interspecific backcross. Molecular markers were Neuropeptide Y (Npy), homeobox (Hoxa2), immunoglobulin kappa chain (Igk), wingless-related MMTV integration site (Wnt-2), cystic fibrosis (Cftr), carboxypeptidase A (Cpa), and Ghrfr. Map order and distances were as follows: Cen-II-Wnt-2-(0 cM)-Cftr-(6.0 cM)-Cpa-(8.0 cM)-Npy-(1.0 cM)-Hoxa2-(3.0 cM)-Ghrfr-(2.0 cM)-Igk. We mapped lit (by phenotype) relative to Hoxa2 and Igk on 72 F2 chromosomes of offspring of a B6CZ lit/ + x B6FVB lit/ + intercross and 18 chromosomes of offspring of a B6FVB lit/ + intercross. Map order and distances were as follows: Hoxa2-(2.1 cM)-lit/Ghrfr-(3.7 cM)-Igk. No recombinations between lit and Ghrfr were detected. Thus, Ghrfr maps to mouse Chr 6 and may be allelic with lit. Amplification of a short segment at the 3' end of the Ghrfr mRNA by reverse transcription coupled to the polymerase chain reaction showed a greatly diminished level of GRF receptor mRNA in the pituitaries of lit/lit mice as compared with lit/ + controls. Low level of message could reflect a primary molecular defect or be due to secondary hypoplasia of somatotropes.(ABSTRACT TRUNCATED AT 250 WORDS)

Aetiology and pathogenesis of growth hormone deficiency

Growth hormone deficiency (GHD) represents an aetiologically non-uniform disorder: it can have multiple causes. Three main groups can be differentiated: (1) GHD due to manifest organic alterations of the hypothalamo-hypophyseal system; (2) so-called idiopathic cases; and (3) genetically determined forms. The first group comprises the congenital malformations, affecting the midline structures of the brain more often than the pituitary itself, and the acquired lesions, mainly tumours, which are generally suprasellar rather than intrasellar. The large majority of the so-called idiopathic cases result from perinatal damage which affects the hypothalamus more often than the pituitary proper. Births in non-cephalic positions, in particular breech deliveries, play a significant role, but other difficult parturitions of prolonged duration also lead to such damage. With improved obstetric techniques, particularly the more frequent use of caesarean section, such lesions now occur far less frequently than 30 years ago. In the last 20 years, several genetically determined disorders associated with GHD have been described, four of them with isolated GHD, three with multiple deficiencies of pituitary hormones. Most cases are inherited by autosomal recessive transmission.

Expression of the human growth hormone variant gene in cultured fibroblasts and transgenic mice

The nucleotide sequence of the human growth hormone variant gene, one of the five members of the growth hormone gene family, predicts that it encodes a growth hormone-like protein. As a first step in determining whether this gene is functional in humans, we have expressed a mouse metallothionein I/human growth hormone variant fusion gene in mouse L cells and in transgenic mice. The growth hormone variant protein expressed in transiently transfected L cells is distinct from growth hormone itself with respect to reactivity with anti-growth hormone monoclonal antibodies, behavior during column chromatography, and isoelectric point. Transgenic mice expressing the growth hormone variant protein are 1.4- to 1.9-fold larger than nontransgenic controls, suggesting that the protein has growth-promoting properties.

Receptor-associated resistance to growth hormone-releasing factor in dwarf "little" mice

Anterior pituitaries from the dwarf mouse strain "little" did not release growth hormone or accumulate adenosine 3',5'-monophosphate (cyclic AMP) in response to human and rat growth hormone-releasing factor (GRF). Dibutyryl cyclic AMP, as well as the adenylate cyclase stimulators forskolin and cholera toxin, markedly stimulated growth hormone (GH) release. The basis of the GH deficiency in the little mouse may therefore be a defect in an early stage of GRF-stimulated GH release related either to receptor binding or to the function of the hormone-receptor complex.

Regulation of human insulin gene expression in transgenic mice

Insulin is a polypeptide hormone of major physiological importance in the regulation of fuel homeostasis in animals (reviewed in refs 1,2). It is synthesized by the beta-cells of pancreatic islets, and circulating insulin levels are regulated by several small molecules, notably glucose, amino acids, fatty acids and certain pharmacological agents. Insulin consists of two polypeptide chains (A and B, linked by disulphide bonds) that are derived from the proteolytic cleavage of proinsulin, generating equimolar amounts of the mature insulin and a connecting peptide (C-peptide). Humans, like most vertebrates, contain one proinsulin gene, although several species, including mice and rats, have two highly homologous insulin genes. We have studied the regulation of serum insulin levels and of insulin gene expression by generating a series of transgenic mice containing the human insulin gene. We report here that the human insulin gene is expressed in a tissue-specific manner in the islets of these transgenic mice, and that serum human insulin levels are properly regulated by glucose, amino acids and tolbutamide, an oral hypoglycaemic agent.