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Section of Pediatric Endocrinology/Diabetology, Department of Pediatrics, James Whitcomb Riley Hospital for Children, Indiana University School of Medicine, Indianapolis, Indiana 46202-5225
Address correspondence and requests for reprints to: Erica A. Eugster, M.D., Assistant Clinical Professor of Pediatrics, Pediatric Endocrinology/Diabetology, Riley Hospital #5984, 702 Barnhill Drive, Indianapolis, Indiana 46254. E-mail: eeugster{at}iupui.edu
| Introduction |
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| Etiologies of Gigantism |
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Many cases of gigantism result from primary GH secretion by pituitary tumors comprised of somatotrophs (GH-secreting cells) or mammosomatotrophs (GH and PRL-secreting cells), either in the form of a pituitary microadenoma or, rarely, macroadenoma (6). The relative contributions of inherent pituitary defects vs. hypothalamic factors in the pathogenesis of pituitary tumors are far from resolved, however. The monoclonal nature of most pituitary adenomas (8), confirmed by X-inactivation studies, has implied that they originate from a single altered cell. The concept of an intrinsic pituitary defect is further supported by the discovery that specific molecular genetic abnormalities seem to form the basis of GH hypersecretion in many cases. In contrast, evidence also exists to suggest an important role for GHRH in disease progression because the number of GHRH messenger RNA transcripts within pituitary adenomas correlates strongly with their clinical behavior (9). The exact functional consequence of locally produced GHRH remains to be clarified, although an autocrine or paracrine role has been suggested by the finding of an elevated plasma GHRH concentration in association with a pituitary somatotroph adenoma, which normalized following surgical removal of the adenoma (10). An approach that integrates the different theories of pituitary adenoma formation has recently been proposed, in which tumor growth ensues via a multistep process. In this model, the initial event consists of genetic transformation of cells, with abnormal growth being subsequently promoted by hypophysiotrophic hormones and other growth factors (11). Identified molecular genetic abnormalities implicated in the pathogenesis of primary pituitary GH excess are discussed below.
Gs
mutations
The heterotrimeric G-proteins play an integral role in postligand
signal transduction in many endocrine cells, in which they act by
stimulating adenylyl cyclase, resulting in cAMP accumulation and
subsequent gene transcription. Activating point mutations of the
G-protein stimulatory subunit Gs
are known to form the basis for
McCune-Albright syndrome (MAS), a rare disorder characterized by the
classic triad of precocious puberty, café au lait spots, and
fibrous dysplasia of bone (12). "Constitutive activation" refers to
the autonomous and uncontrolled activation of G-protein-mediated cAMP
formation that occurs in MAS, resulting in hyperfunction of endocrine
and nonendocrine tissues. In some patients with MAS, endocrine
abnormalities include gigantism caused by the development of pituitary
mammosomatotroph adenomas or hyperplasia. The reported point mutations
observed in multiple affected tissues of patients with MAS (13),
including those with gigantism (14), involve a single amino acid
substitution within codon 201 (exon 8) or codon 227 (exon 9) of the
Gs
gene. Interestingly, these same mutations have also been
identified in somatotrophs of up to 40% of sporadic GH-secreting
pituitary adenomas (15). The resulting oncogene, gsp, is
thought to induce tumorigenesis by virtue of persistent activation of
adenylyl cyclase with subsequent GH hypersecretion (16). In contrast to
tumors without such mutations, gsp-containing pituitary
adenomas tend to be smaller, with morphologic characteristics
suggestive of slow growth, despite an absence of detectable differences
in disease progression between the two groups.
Allelic deletion of the 11q13 locus
Loss of heterozygosity (LOH) at the site of a putative tumor suppressor gene located on chromosome 11q13 represents another molecular genetic abnormality, whose association with pituitary GH excess has been firmly established. First identified within tumors from patients with MEN-1 (17), the genetic mutation was originally believed to be related to the MEN-1 gene and was thought to be the cause of the GH excess in this disease. The recent cloning of the MEN-1 gene, however, has led to the revelation that the affected locus codes for a product that is distinct from the MEN-1 gene. This has been demonstrated by the finding of an intact MEN-1 sequence in individuals from two unrelated kindreds with familial acromegaly/gigantism and 11q13 LOH (18). In addition to familial non-MEN acromegaly/gigantism (19), LOH at 11q13 has also been observed in all types of sporadically occurring pituitary adenomas (20). The exact nature of the encoded product and its role in tumor formation have yet to be clarified. Of note is the fact that LOH at 11q13 and other loci within pituitary adenomas has been correlated with an increased propensity for tumor invasiveness and biological activity (21).
Additional theoretical intrinsic pituitary defects leading to abnormal cell proliferation and excessive GH secretion might result from abnormal activation of the GHRH receptor, somatostatin receptor, pituitary transcription factors, or other growth-related signal peptides. As information regarding the complex developmental cascade of pituitary ontogenesis continues to accumulate, new light will undoubtedly be shed on the underlying mechanisms of both normal and abnormal pituitary cell growth.
Secondary GH excess
Causes of secondary GH excess include those in which there is increased secretion of hypothalamic GHRH, either from an intracranial or ectopic source, and those in which abnormal regulation of the hypothalamic-pituitary GH axis has occurred. Secondary GH excess represents an important, if poorly understood, cause of gigantism. Advances in biochemical detection assays and molecular genetic characterization should allow improved localization of the underlying hormonal abnormality in these cases.
GHRH excess
Hypothalamic GHRH excess or dysregulation has been postulated to be the most common cause of GH hypersecretion in the pediatric population. Although not definitively proven, clinical cases that support this hypothesis include congenital gigantism with massive diffuse pituitary hyperplasia, in which biochemical studies suggested central GHRH hypersecretion (5), as well as a case of mammosomatotroph hyperplasia in which systemic GHRH concentrations were found to be normal (4). The involvement of mammosomatotrophs, frequently a feature of GH excess originating in childhood (22), is further suggestive of early onset increased GHRH exposure because this cell type predominates in fetal life but is rare in the adult. Despite this evidence, the underlying mechanism of the putative abnormality in GHRH action in these cases remains unknown. Theoretical possibilities include an activating mutation in hypothalamic GHRH neurons or a decrease in somatostatin tone (see below). Another form of intracranial GHRH excess occurs in the setting of a neural tumor, such as a gangliocytoma (23, 24) or neurocytoma (25), arising within or in close proximity to the sella. Prolonged tumor secretion of GHRH leads to pituitary hyperplasia with or without adenomatous transformation, resulting in increased levels of GH and other adenohypophyseal peptides. Electron microscopy in such cases has revealed intimate contact between neurons of the tumor and pituitary GH-secreting cells (23). GHRH excess may also originate from an extracranial and ectopic neoplastic source, which represents a well-recognized cause of acromegaly (26), but has only rarely been implicated in cases of GH excess in children (3). Ectopic GHRH-secreting tumors have included carcinoid, pancreatic islet cell, and bronchial neoplasms. Recently, the first reported case of ectopic GH as the cause of acromegaly was identified, in which tumor cells from a malignant lymphoma were found to secrete high levels of pituitary GH (27).
Abnormal Somatostatin tone
Secondary GH excess may also occur from disruption of somatostatin tone. Tumor infiltration into somatostatinergic pathways has been hypothesized to form the basis for GH excess in rare cases of gigantism associated with neurofibromatosis and optic gliomas or astrocytomas (28, 29). Immunocytochemical studies in this setting have demonstrated interruption of somatostatinergic neurons, whereas neuroimaging has revealed diminished magnetic resonance signal intensity in somatostatin-rich areas of the brain (30).
| Consequences of Prolonged GH Excess |
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| Clinical Aspects of Gigantism |
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In contrast, the myriad signs and symptoms of prolonged GH excess in adults with acromegaly have been well described (35). Enlargement of facial features, excess acral growth and soft tissue swelling are essentially ubiquitous among these patients. Additional common manifestations include headaches, excessive sweating, peripheral neuropathy and arthritis. Frequently associated endocrinopathies include hypogonadism, diabetes, thyromegaly, and galactorrhea. The most common cause of death in acromegaly is from cardiovascular disease (36). Recent observations regarding other consequences of GH toxicity include a potential role for GH in normal and abnormal erythropoiesis (37) and in the pathogenesis of retinopathy (38).
Physical examination of the child presenting with growth acceleration
must include a search for evidence of other etiologies of increased
growth velocity, such as excessive sex steroid levels, as well as
careful attention to the presence of additional physical findings that
might suggest an underlying disorder, such as multiple café au
lait spots. The differential diagnosis of growth acceleration is
contained in Table
2.
Laboratory findings
An elevated IGF-I on initial screening is suggestive of GH excess, as an excellent linear dose-response correlation between plasma IGF-1 levels and 24-h mean GH secretion have been demonstrated (39). Potential confusion may arise when evaluating normal adolescents because significantly higher IGF-I levels occur during puberty than in adulthood (40), a fact that emphasizes the importance of using age-referenced norms. Although higher concentrations of IGF-I have been reported in children and adolescents with constitutional tall stature (41), no significant differences in neurosecretory dynamics of the GH-IGF-I axis have been found in healthy young adults with heights of more than three SD above the mean as compared with controls (42). The gold standard for making the diagnosis of GH excess is a failure to suppress serum GH levels to less than 5 ng/dL after a 1.75 gm/kg oral glucose challenge (maximum, 75 g). Hyperprolactinemia is an almost invariable finding in GH excess presenting in childhood, undoubtedly related to the fact that mammosomatotrophs are by far the most common type of GH-secreting cells involved in childhood gigantism. The coexistence of both GH and PRL has been clearly demonstrated within the secretory granules contained in the cytoplasm of these cells (22). Although not necessary to make the diagnosis, GH response to additional stimuli such as TRH testing is typically paradoxical. Measurement of serum GHRH levels are useful in differentiating ectopic GHRH excess from other causes of GH hypersecretion. Imaging by magnetic resonance imaging or computed tomography is an essential step in the evaluation following biochemical detection of GH excess.
Psychological aspects of tall stature/gigantism
One need only review the striking positive correlation between stature and financial/professional success in our society to be convinced that "heightism" is a true phenomenon. However, when present to an extreme degree, tallness ceases to be an advantage and may be perceived as a burden, resulting in both physical, as well as psychological, handicaps. This has prompted the pharmacological treatment of constitutionally tall adolescents with sex steroids to accelerate epiphyseal fusion, a practice that has been in existence since the 1950s (43). Whereas tall girls, in particular, often report teasing and social difficulties as a result of their size, these problems generally disappear in adulthood, when the majority of normal tall men and women indicate satisfaction with their stature (44). Because no convincing data indicating lifelong psychopathology as a result of tall stature exists (45), it may be reasonable to pursue counseling as the initial treatment of choice for otherwise healthy tall adolescents with psychosocial difficulties related to their height. In contrast, pathologic tall stature as a result of GH excess obviously results in heights that are far beyond those observed in constitutionally tall individuals. Although no in-depth information regarding the psychological profile of patients with gigantism is available, case series indicate a high incidence of severe depression, social withdrawal, and low self-esteem (3).
| Treatment of Gigantism |
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The greatest progress in recent years in the treatment of GH excess has been within the realm of medical therapy. The development of somatostatin analogs, such as octreotide, represented a major addition to the pharmacological armamentarium for GH hypersecretion. Therapeutic response to octreotide, found to be highly effective in the majority of patients with gigantism or acromegaly, may be predicted by the decrement in serum GH levels after one sc dose (48). The new sustained-release somatostatin analog preparation lanreotide given in the form of an im injection every 2 weeks, has also been shown to be successful in returning GH levels to normal in acromegalic adults with pituitary adenomas (49) as well as in those with ectopic GHRH secretion (50). Although this drug is as yet untested in children, the improved dosing schedule of lanreotide clearly represents a potentially major advance in the treatment of gigantism and disorders of glucose homeostasis in pediatric patients. Side effects of somatostatin analogues consist mainly of mild transient gastrointestinal complaints and an increased risk of gallstones. Additional pharmacological therapy consists of the dopamine agonist bromocriptine, which can provide adjuvant medical treatment of gigantism and has been found to be safe when used in a child for an extended period of time (51). An exciting new therapeutic agent has recently emerged in the form of a competitive GHRH antagonist, which has been shown to effectively suppress GH and IGF-I levels in patients with acromegaly from pituitary somatotrophic tumors as well as ectopic GHRH hypersecretion (52, 53).
| Conclusion |
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Revised September 28, 1999.
Accepted September 28, 1999.
| References |
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. J Pediatr. 123:509518.[CrossRef][Medline]
mutation at codon 201 in pituitary adenoma causing gigantism in a
6-year-old boy with McCune-Albright symdrome. J Clin Endocrinol
Metab. 81(11):38393842.
chain
of Gs and stimulate adenylyl cyclase in human pituitary tumors. Nature. 40:692696.
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