Regulation of the growth hormone (GH) receptor and GH-binding protein mRNA

Mutations of uncertain significance in heterozygous variants as a possible cause of severe short stature: a case report

Background

Linear bone growth is achieved by the division of chondrocytes at the growth plate and is regulated by endocrine and paracrine factors such as growth hormone. Mutations that negatively affect chondrogenesis can be a contributor to short stature. One such mutation can occur in the ACAN gene, causing short stature and advanced bone age. Similarly, mutations in growth hormone receptors (GHR) can lead to Laron syndrome (LS), one of the several disorders that are collectively called growth hormone insensitivity syndrome (GHI). Another example is Floating-Harbor syndrome (FHS), a rare autosomal dominant due to mutations in the SRCAP gene that can also result in short stature.

Case presentation

We report the case of a 6-year-old female with concomitant mutations in the three genes mentioned above. The mutations reported here were found on genetic studies and are usually benign, causing a variant of undetermined significance. However, our patient’s phenotype could only be explained by the compounded effects of pathogenic mutations of these genes. Some of the same mutations were also found in the patient’s father and her paternal grandfather. Both also presented with short stature, though not to the same degree as our patient. While these mutations are often reported to be insignificant, they gave rise to severe short stature and a specific phenotype in the patient when presented together. We think that even though the GHI spectrum is inherited through an autosomal recessive pattern, the sum of these heterozygous mutations resulted in severe short stature despite the limited GHI seen in our patient, the father, and the grandfather, through a rare ACAN and SRCAP mutation that, to our knowledge, has not been previously reported as a pathogenic mutation in the literature.

Conclusion

We investigated the possible synergistic effects of these variations on exacerbation or masking of the signs and symptoms of GHI with the hope of providing a better understanding of these genes and their function through our rare case.

The growth hormone signaling system: Insights into coordinating the anabolic and catabolic actions of growth hormone

Although growth hormone (GH) is a multifunctional factor that coordinates various aspects of feeding, reproduction, osmoregulation, and immune system function, perhaps two of its most studied actions are the regulation of growth and metabolism, particularly lipid metabolism. In this review, we describe the major growth-promoting and lipid metabolic actions of GH and then discuss how the GH system regulates these actions. Numerous intrinsic and extrinsic factors provide information about the metabolic status of the organism and influence the production of release of GH. The actions of GH are mediated by GH receptors (GHR), which are widely distributed among tissues. Teleosts possess multiple forms of GHRs that arose through the evolution of this group. Modulation of tissue responsiveness to GH is regulated by molecular and functional expression of GHRs, and in teleosts GHR subtypes, by various factors that reflect the metabolic and growth status of the organism, including nutritional state. The action of GH is propagated by the linkage of GHRs to several cellular effector systems, including JAK-STAT, ERK, PI3K-Akt, and PKC. The differential activation of these pathways, which is governed by nutrient status, underlies GH stimulation of growth or GH stimulation of lipolysis. Taken together, the multi-functional actions of GH are determined by the distribution and abundance of GHRs (and GHR subtypes in teleosts) as well as by the GHR-effector system linkages.

The P561T polymorphism of the growth hormone receptor gene has an inhibitory effect on mandibular growth in young children

P561T heterozygous missense mutation in the growth hormone receptor (GHR) is a candidate genetic polymorphism (single-nucleotide polymorphism) for human mandibular growth. The purpose of this study was to assess whether this mutation affects mandibular growth during early childhood. The difference in mandibular growth between P561T heterozygous and wild-type individuals was analysed by cephalometric measurements during childhood. The subjects included 33 children with mandibular protrusion (aged 3-12 years, 16 males and 17 females) and 27 normal children (aged 3-13 years, 14 males and 13 females). Genomic DNA extracted from buccal epithelial cells was genotyped for the P561T heterozygous mutation with a molecular analysis (polymerase chain reaction--restriction fragment length polymorphism method). Two of the patients with normal occlusion and five with mandibular protrusion were heterozygous for the mutation. Chi-square analysis showed that the frequency of this mutation did not differ statistically between the normal and mandibular protrusion subjects. Multilevel model analysis of the 101 cephalograms showed that the mutation reduced the linear measurements of the mandible. These findings suggest that P561T heterozygous mutation affects mandibular growth during early childhood, and this mutation in the GHR gene is hypothesized to function as an inhibitory factor in the process of mandibular growth.

Therapeutic aspects of growth hormone and insulin-like growth factor-I treatment on visceral fat and insulin sensitivity in adults

Growth hormone (GH) is generally considered to exert anti-insulin actions, whereas insulin-like growth factor I (IGF-I) has insulin-like properties. Paradoxically, GH deficient adults and those with acromegaly are both predisposed to insulin resistance, but one cannot extrapolate from these pathological conditions to determine the normal metabolic roles of GH and IGF-I on glucose homeostasis. High doses of GH treatment have major effects on lipolysis, which plays a crucial role in promoting its anti-insulin effects, whereas IGF-I acts as an insulin sensitizer that does not exert any direct effect on lipolysis or lipogenesis. Under physiological conditions, the insulin-sensitizing effect of IGF-I is only evident after feeding when the bioavailability of circulating IGF-I is increased. In contrast, many studies in GH deficient adults have consistently shown that GH replacement improves the body composition profile although these studies differ considerably in terms of age, the presence or absence of multiple pituitary hormone deficiency, and whether GH deficiency was childhood or adult-onset. However, the improvement in body composition does not necessarily translate into improvements in insulin sensitivity presumably due to the anti-insulin effects of high doses of GH therapy. More recently, we have found that a very low dose GH therapy (0.1 mg/day) improved insulin sensitivity without affecting body composition in GH-deficient adults and in subjects with metabolic syndrome, and we postulate that these effects are mediated by its ability to increase free 'bioavailable' IGF-I without the induction of lipolysis. These results raise the possibility that this low GH dose may play a role in preventing the decline of beta-cell function and the development of type 2 diabetes in these "high risk" subjects.

Impact of treatment with recombinant human GH and IGF-I on visceral adipose tissue and glucose homeostasis in adults

Supraphysiological doses of growth hormone (GH) therapy are generally thought to antagonize the effects of insulin, whereas the insulin-like growth factor I (IGF-I) potentiates insulin-like actions. Paradoxically, adults with GH deficiency and patients with acromegaly are both predisposed to glucose intolerance and insulin resistance; however, one cannot extrapolate from these pathological conditions to determine the true metabolic roles of GH and IGF-I in glucose homeostasis. Growth hormone also promotes lipolysis, which has been shown to be the principal determinant of its insulin-antagonistic properties; on the other hand, IGF-I, which acts as an insulin sensitizer, does not exert any direct effect on lipolysis or lipogenesis. Under physiological conditions, the insulin-sensitizing effect of IGF-I is evident only after feeding, when the bioavailability of circulating IGF-I is increased. In contrast to supraphysiological GH doses, low doses of GH treatment have been shown to increase circulating IGF-I levels and IGF-I bioavailability and, thus, may theoretically enhance insulin sensitivity without inducing lipolysis. We have recently reported that a fixed administration of a very low GH dose (1.7 microg/kg/day or 0.1mg/day) improved insulin sensitivity in adults with GH deficiency and increased peripheral glucose uptake in subjects with impaired glucose tolerance and the metabolic syndrome. Our data raise the possibility that this very low GH dose may play a role in maintaining beta-cell function and possibly delay the progression to type 2 diabetes in these high-risk patients.

The influence of growth hormone status on physical impairments, functional limitations, and health-related quality of life in adults

The availability of recombinant human GH and somatostatin analogs has resulted in widespread treatment for adults with GH deficiency (GHD) and those with GH excess (acromegaly). Despite being at opposite ends of the spectrum in terms of their GH/IGF-I axis, both of these populations experience overlapping somatic impairments. Adults with untreated GHD have low circulating levels of IGF-I that manifest as altered body composition with increased fat and reduced lean body and skeletal muscle mass. At the other end of the spectrum, adults with GH excess, who have elevated levels of IGF-I, also have altered body composition. Impairments that result from disorders of either GHD or GH excess are both associated with increased functional limitations, such as reduced ability to walk quickly for prolonged periods, and poorer health-related quality of life (HR-QoL). Adults with untreated GHD and GH excess both commonly complain of excessive fatigue that seems to be associated more with impaired aerobic than muscular performance. Several studies have documented that administration of GH or somatostatin analogs to adults with GHD or GH excess, respectively, ameliorates abnormal biochemical profile and the associated somatic impairments. However, whether these improvements translate into improved physical function in adults with GHD or GH excess remains largely unknown, and their impact on HR-QoL controversial. Review of placebo-controlled trials to date suggests that GH and somatostatin analogs have greater effects on gas exchange and aerobic performance than as anabolic agents on skeletal muscle mass and function. Future investigations should include dose-response studies to establish the optimal combination of pharmacological agents plus exercise required to improve not only biochemical markers but also physical function and HR-QoL in adults with GHD or GH excess.

Physiology and pathophysiology of growth hormone-binding protein: methodological and clinical aspects

Circulating GH is partly bound to a high-affinity binding protein (GHBP), which in humans is derived from cleavage of the extracellular domain of the GH receptor. The precise biological function GHBP is unknown, although a regulation of GH bioactivity appears plausible. GHBP levels are determined by GH secretory status, body composition, age, and sex hormones, but the cause-effect relationships remain unclarified. In addition to the possible in vivo significance of GHBP, the interaction between GH and GHBP has methodological implications for both GH and GHBP assays. The present review concentrates on methodological aspects of GHBP measurements, GHBP levels in certain clinical conditions with a special emphasis on disturbances in the GH-IGF axis, and discusses the possible relationship between plasma GHBP and GH receptor status in peripheral tissues.

Quantification of growth hormone receptor extra- and intracellular domain gene expression in chicken liver by quantitative competitive RT-PCR

The very sensitive competitive reverse transcription-polymerase chain reaction (RT-PCR) was used to investigate the expression of the extracellular (GHRe) and intracellular (GHRi) parts of the growth hormone receptor (GHR) in the liver tissue of chickens. Two competitors (internal standards), pGHRi MUT and pGHRe MUT, specific to the GHRi and GHRe genes, respectively, were constructed by site-specific mutagenesis. The internal standards defined PCR products of 394 bp for the pGHRi MUT and 330 bp for the GHRe MUT. These were used as competitors to the wild-type GHRi or GHRe which defined PCR products of 382 and 328 bp, respectively. Coamplification, under standardized conditions, of the native RNA in competition with serial dilutions of the mutant RNA in the same PCR reaction followed by enzymatic digestion produced the expected sizes of internal standard cDNA and predicted target cDNA. Expression levels of GHRe and GHRi were determined from standard curves generated. The method was sensitive enough to detect expressions down to picogram levels. Applying this method, the effect of GH and T(3) injection on GHRe and GHRi mRNA expression was determined in the liver of adult female Hisex birds and 1-day-old normal and dwarf chickens. Intravenous GH injection (25 microg/kg body weight) increased plasma levels of GH in Hisex birds after 10 min but rapidly decreased at 60 min followed by an increase in T(3). GH injection significantly increased the expression of the GHRe 60 min after injection but not at 10 min, when the GH level in plasma was high. In the liver of saline-treated dwarf (dw) and nondwarf (Dw) chicks, the level of expression of GHRe was similar in both strains despite disparate levels of basal GH and T(3). However, the level of GHRi was higher in Dw than in dw chicks. Although GH levels increased in both strains after intravenous GH injection (250 microg/kg body wt), the expression of GHRe in both strains was unaffected. However, the mRNA for the GHRi was significantly depressed by injection in the Dw but unaffected in dw chicks. Intravenous injection of T(3) (0.5 and 5 microg/kg body wt) increased plasma levels in both strains but caused depression of GHRi in Dw but not in dw chicks. T(3) injections had no effect on GHRe in either Dw or dw chicks. It is concluded that the expression of the GHRe in adult chickens is GH regulated either directly or indirectly. In contrast, in 1-day-old chicks, GH or T(3) had no effects on the GHRe but regulated the expression of GHRi in Dw chicks, whereas in dwarf chicks both had no effect on GHRe or GHRi expression. It is postulated that GHRe and GHRi gene expression may be regulated by different agonists/antagonists in different strains and depending on the age of the chicken.

Growth hormone and insulin-like growth factor-I enhance beta-glucuronidase gene activation by androgen in mouse kidney

Beta-glucuronidase (GUS) is a lysosomal enzyme that, in mouse kidney, is subject to control by multiple hormones: androgen, which increases GUS transcription; estrogen, which antagonizes androgen-mediated stimulation of GUS; and growth hormone (GH), which appears to be necessary for the full androgen effect. Neither estrogen nor GH affects GUS in the absence of androgen. In hypophysectomized or pituitary dwarf mice the reduced androgen stimulation of GUS can be partially restored with GH treatment. Androgen-induced GUS mRNA increased significantly with intermittent GH, compared to no GH or continuous GH. Intact mice subjected to continuous infusion of GH showed a depressed androgen effect on GUS similar to that seen in GH-deficient mice. Thus, pulsatile GH is required for the full androgen response. Insulin-like growth factor-I (IGF-I) also restored GUS induction by androgen in GH-deficient mice. We conclude that GH enhances the effect of androgen on the GUS gene via IGF-I. Using transgenic mice, we have also identified a genetic variant of the GUS gene that is insensitive to GH enhancement of the androgen effect.