Information for patients View other pituitary disorders
Pituitary adenomas can occur due to genetic alterations. The known genes involved with pituitary tumours can be seen on Figure 1.
The most recently identified genetic pituitary adenoma syndrome is called X-LAG standing for X-linked acrogigantism [1]. It is caused by microduplications at chromosome Xq26.3, first described in 13 subjects with early-onset gigantism. These microduplications span an area of 500 Kb containing 4 genes. Among these genes, only one, GPR101, coding for a G-protein-coupled-receptor (GPCR), has been found to be significantly overexpressed in the pituitary tissue of these patients with infant or young childhood-onset acromegaly. The pituitary pathology has been shown to be somatomammotroph adenoma or hyperplasia. Interestingly, samples from patients carrying the microduplication were characterized by increased GHRH receptor expression, compared to both sporadic somatotropinomas and normal pituitaries. GHRH has also been identified in rat testis and placenta. This data suggests that the testis and placenta are extrahypothalamic sites of expression of the human GHRH gene. The normal expression of the gene in extrahypothalamic sites may include transcription of larger mRNA species than those observed in ectopic pathologic sources of GHRH expression [2].
These new data now help to categorize children with gigantism to two phenotypes, an early childhood form of gigantism with a typical onset in infancy [1] and a form with a typical onset above the age of 5 years, sometimes in early adolescence [3]. Some of the patients in the latter group have a mutation in the AIP gene, although some pre-pubertal children have also been described with AIP mutation [4].
Moreover, independently from microduplications, a recurrent missense mutation (p.E308D) has been found in some (11 out of 248 sporadic) acromegaly patients, with the mutation found mostly in tumours. When the mutation was transfected into rat GH3 cells, it led to increased proliferation and GH production [1].
GPR101 is an orphan widely-expressed GPCR [5]. In mice GPR101 is predominantly expressed in the brain, particularly in the hypothalamus [5]. The highest expression levels are observed in the arcuate nucleus, where GHRH producing neurons are localized[5]. Microarray data in humans show that GPR101 is expressed at similar levels in the hypothalamus and the pituitary gland (Nextbio library). The endogenous ligand for GPR101 is not known. In silico prediction indicates that GPR101 is likely coupled to Gs[5]. This prediction is supported by the elevation of cyclic AMP levels and the activation of cAMP response element-luciferase reporter gene in HEK293 cells overexpressing human GPR101 [5].
Recently, microduplications at chromosome Xq26.3 have been identified in 13 subjects with early-onset acrogigantism [6]. These microduplications span an area of 500 Kb containing 4 genes. Among these genes, only one, GPR101, coding for a G-protein-coupled-receptor (GPCR), has been found to be significantly overexpressed in patients' pituitary tumours. Moreover, a recurrent missense mutation (p.E308D) has been found in some acromegaly patients, with the mutation found mostly in tumours. When the mutation was transfected into rat GH3 cells, it led to increased proliferation and GH production. Three out of 13 samples were characterized by pituitary hyperplasia, with or without an adenoma. Interestingly, samples from patients carrying the microduplication were characterised by increased GHRH receptor expression, compared to both sporadic somatotropinomas and normal pituitaries.
GPR101 has been found to be hypermethylated in 40% of colorectal cancer samples; importantly, in stage IV colorectal cancer GPR101 promoter methylation correlates with longer time-to-progression [7]. These findings suggest that GPR101 might play a role in regulating cancer progression. Human GHRH acts as a growth factor in different malignancies and GHRH antagonists have proved anti-tumour effects, both in vitro and in animal models [8-11].
The function of GPR101 is unknown. Its expression in the hypothalamus and the pituitary gland, and its role in determining gigantism, suggest this might represent a new pathway involved in the control of GH secretion.