Role of the aryl hydrocarbon receptor-interacting protein in familial isolated pituitary adenoma

60 YEARS OF NEUROENDOCRINOLOGY: Acromegaly

Acromegaly (ACM) is a chronic, progressive disorder caused by the persistent hypersecretion of GH, in the vast majority of cases secreted by a pituitary adenoma. The consequent increase in IGF1 (a GH-induced liver protein) is responsible for most clinical features and for the systemic complications associated with increased mortality. The clinical diagnosis, based on symptoms related to GH excess or the presence of a pituitary mass, is often delayed many years because of the slow progression of the disease. Initial testing relies on measuring the serum IGF1 concentration. The oral glucose tolerance test with concomitant GH measurement is the gold-standard diagnostic test. The therapeutic options for ACM are surgery, medical treatment, and radiotherapy (RT). The outcome of surgery is very good for microadenomas (80-90% cure rate), but at least half of the macroadenomas (most frequently encountered in ACM patients) are not cured surgically. Somatostatin analogs are mainly indicated after surgical failure. Currently their routine use as primary therapy is not recommended. Dopamine agonists are useful in a minority of cases. Pegvisomant is indicated for patients refractory to surgery and other medical treatments. RT is employed sparingly, in cases of persistent disease activity despite other treatments, due to its long-term side effects. With complex, combined treatment, at least three-quarters of the cases are controlled according to current criteria. With proper control of the disease, the specific complications are partially improved and the mortality rate is close to that of the background population.

The AIP (aryl hydrocarbon receptor-interacting protein) gene and its relation to the pathogenesis of pituitary adenomas

Pituitary adenomas are monoclonal neoplasms that may secrete excessive quantities of their endogenous hormones, or may not be associated with any obvious syndrome, in which case they are known as non-functioning pituitary adenomas. Around 2 % have been said to occur in a familial setting, in the absence of any other tumor, now described as familial isolated pituitary adenomas (FIPA). Some 15-30 % of such families harbor inactivating germ-line mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene, along with 20 % of pediatric seemingly sporadic cases. AIP mutants are referred to as having pituitary adenoma predisposition, and present with early onset, aggressive macroadenomas, most of which secrete somatotropin. Evidence from transfection studies implies that AIP acts as a tumor suppressor; although whether this is mediated through an interaction with the aryl hydrocarbon receptor, phosphodiesterases, or with cell cycle regulators such as survivin or RET remains controversial. However, at present an interaction with the cyclic AMP pathway seems most plausible. Recently, evidence has shown that AIP may act at the cell surface, causing changes in integrin function. The presence of AIP mutations in a significant proportion of FIPA families as well as in apparently sporadic cases, particularly in young patients, suggests a need to screen such patients for AIP mutations to enable better clinical management. However, the absence of AIP mutations in over half of such cases highlights the need to search for further gene mutations.

Novel pathway for somatostatin analogs in patients with acromegaly

Acromegaly is a chronic disease with increased morbidity and mortality, where usually multiple treatment modalities are used. The somatostatin analogs (SSAs) are the mainstay of medical therapy but, in many patients, including those with a germline mutation in the aryl hydrocarbon receptor-interacting protein (AIP) gene, disease activity cannot be controlled with these drugs. Previous data have suggested the involvement of the tumor-suppressor gene ZAC1 in the mechanism of action of SSAs, and more recent findings suggested that SSAs could regulate AIP, which in turn can stimulate ZAC1, therefore suggesting the existence of a SSA-AIP-ZAC1-somatostatin effect pathway. The current review discusses these novel observations, highlighting their significance in the treatment of sporadic and familial somatotroph adenomas.

Familial isolated pituitary adenomas: an emerging clinical entity

Familial pituitary tumors are increasingly recognized. While some of these cases are related to wellknown syndromic conditions such as multiple endocrine neoplasia type 1 (MEN1) or Carney complex, others belong to the familial isolated pituitary adenoma (FIPA) patient group. The discovery of heterozygous, loss-of-function germline mutations in the gene encoding the aryl hydrocarbon receptor interacting protein (AIP) in 2006 has subsequently enabled the identification of a mutation in this gene in 20% of FIPA families and 20% of childhood-onset simplex soma- totroph adenomas. The exact mechanism by which the lack of AIP leads to pituitary adenomas is not clear. AIP mutations cause a low penetrance autosomal dominant disease with often a distinct phenotype characterized by young-onset, aggressive, large GH, mixed GH and PRL or PRL-secreting adenomas. This review aims to summarize currently available clinical data on AIP mutation-positive and negative FIPA patients.

AIP and its interacting partners

Germline mutations in the aryl hydrocarbon receptor-interacting protein gene (AIP) predispose to young-onset pituitary tumours, most often to GH- or prolactin-secreting adenomas, and most of these patients belong to familial isolated pituitary adenoma families. The molecular pathway initiated by the loss-of-function AIP mutations leading to pituitary tumour formation is unknown. AIP, a co-chaperone of heat-shock protein 90 and various nuclear receptors, belongs to the family of tetratricopeptide repeat (TPR)-containing proteins. It has three antiparallel α-helix motifs (TPR domains) that mediate the interaction of AIP with most of its partners. In this review, we summarise the known interactions of AIP described so far. The identification of AIP partners and the understanding of how AIP interacts with these proteins might help to explain the specific phenotype of the families with heterozygous AIP mutations, to gain deeper insight into the pathological process of pituitary tumour formation and to identify novel drug targets.

[Familial isolated pituitary adenoma syndrome]

Familial pituitary adenomas occur in multiple endocrine neoplasia type 1, Carney complex, as well as in familial isolated pituitary adenoma syndrome. Familial isolated pituitary adenoma syndrome is an autosomal dominant disease with incomplete penetrance. Pituitary adenomas occur in familial setting but without any other specific tumors. In 20-40% of families with this syndrome, mutations have been identified in the aryl hydrocarbon receptor interacting protein gene while in the rest of the families the causative gene or genes have not been identified. Families carrying aryl hydrocarbon receptor interacting protein gene mutations have a distinct phenotype with younger age at diagnosis and a predominance of somatotroph and lactotroph adenomas. Germline mutations of the aryl hydrocarbon receptor interacting protein gene can be occasionally identified in usually young-onset seemingly sporadic cases. Genetic and clinical testing of relatives of patients with aryl hydrocarbon receptor interacting protein gene mutations can lead to earlier diagnosis and treatment at an earlier stage of the pituitary tumor.