Primary hyperparathyroidism (PHPT) is the most common clinical manifestation of MEN1, affecting more than 95% of all MEN1 patients 14. The age of the onset (typically between 20 and 25 years of age) of MEN1-associated parathyroid tumours is about three decades earlier than that of sporadic parathyroid adenoma; these tumours are generally characterised by multiglandular hyperplasia 16.

Symptoms of PHPT in MEN1 are the same as those of sporadic PHPT. PHPT in MEN1 manifests with hypercalcaemia as a result of overproduction of parathyroid hormone (PTH) by tumoural and supernumerary parathyroid glands. PHPT is defined as an increased serum concentration of PTH (normal range 10–60 pg/ml) 17 and an increased serum concentration of calcium (normal range 8.5–10.5 mg/dl or 2.1–2.6 mmol/l) 17. The common clinical manifestations of hypercalcaemia include:

1) central nervous system – altered mental status, including lethargy, depression, decreased alertness and confusion;

2) gastrointestinal tract – anorexia, constipation, nausea and vomiting:

3) kidneys – polyuria, nycturia, polydipsia, impaired concentrating ability, dehydration, hypercalciuria and increased risk for kidney stones;

4) skeleton – increased bone resorption and increased fracture risk, mainly in women who manifest PHPT before 35 years of age;

5) cardiovascular system – hypertension, shortened QT interval.

Moreover, hypercalcaemia may increase the secretion of gastrin from a gastrinoma.

At present, total parathyroidectomy is the proposed effective treatment for PHPT in symptomatic hypercalcaemic MEN1 patients. Subtotal parathyroidectomy results in a 50% risk of recurrence within 8–12 years after the intervention. Total parathyroidectomy is often followed by autotransplantation of resected fresh or cryopreserved normal parathyroid tissue in the forearm. To prevent late recurrence, a total parathyroidectomy followed by a life-long treatment with vitamin D analogues is used as an alternative. Intraoperative rapid parathormone monitoring aids both detection of extra glands and immediate assessment of the postresection parathormone level.

Pancreatic tumours occur in about 30–80% of MEN1 patients and are the second most frequently expressed clinical manifestation of MEN1. They are characterised by multiple nodular lesions developed at an early age 1819. The majority of these tumours produce excessive amounts of hormone (gastrin, insulin, glucagons, somatostatin, neurotensin or vasoactive intestinal polypeptide (VIP)) and are associated with distinct clinical syndromes. These hormone-secreting tumours can be detected by biochemical screening for elevated serum hormone concentrations. The most common functional pancreatic tumours are gastrinomas and insulinomas. Nevertheless, about one third of pancreatic tumours are non-functional and clinically silent. Non-functional tumours and insulinomas are located within the pancreas, while gastrinomas are often found in the soft tissue around the pancreas and in the duodenal submucosa, but not in the mucosa where the gastrin-producing G cells are located. Endoscopic ultrasonography (EUS) examination is the most sensitive imaging procedure for the detection of small (≤10 mm) pancreatic endocrine tumours in asymptomatic MEN1 patients; its sensitivity is higher than 75%. The use of EUS in association with Octreoscan scintigraphy increases the pancreatic tumoural detection rate to 90% 20; EUS allows precise localisation of the tumours, while Octreoscan scintigraphy gives much more information about the spread of the disease and detects liver metastases with a sensitivity of 92% 21.

The incidence of pituitary adenomas MEN1 patients varies from 15 to 90%. Generally, symptoms depend on the level of pituitary hormone produced and/or compression effects due to size of the tumour. Mass effects include visual field defects, headaches and blurred vision. Approximately 60% of MEN1-associated pituitary tumours secrete prolactin (prolactinomas), 25% secrete growth hormone (GH) causing gigantism in children and acromegaly in adults, 3% secrete adenocorticotrophin (ACTH) causing hypercortisolism, and the others seem to be non-functional. Pituitary tumours can be detected by computed tomography (CT) scanning and nuclear magnetic resonance imaging (MRI). Treatment of pituitary tumours consists of drug therapy and/or surgery often in association with radiotherapy of the residual unresectable tumours.

The gene locus causing MEN1 has been localised to chromosome 11q13 by studies of loss of heterozigosity (LOH) on MEN1-associated tumours and by linkage analysis in MEN1 families 27282930. The results of these studies agree with Knudson's "two hits" model for tumour development 31 and indicated that the MEN1 gene is a putative tumour suppressor gene. The mutated MEN1 allele is a germline mutation present in all cells at birth. The second mutation is a somatic mutation that occurs in the predisposed endocrine cell and leads to loss of the remaining wild type allele; it gives cells the survival advantage needed for tumour development.

In 1997 the responsible gene, MEN1, was identified by positional cloning 32. It spans about 10 Kb and consists of ten exons encoding a 610 amino acid nuclear protein, named menin. Mutation analysis revealed that the MEN1 gene was frequently, but not always, mutated in MEN1 families 32. To date, more than 400 different germline or somatic mutations have been reported in MEN1 families and sporadic cases from several international studies. Mutations are distributed over the entire coding region without showing any significant hot spot region 333435363738. Approximately 20% of mutations are nonsense mutations, about 50% are frameshift insertions and deletions, 20% are missense mutations and about 7% are splice site defects. The nonsense mutations and many of the frameshift insertions and deletions and donor-splice site mutations are truncating mutations predicting a loss-of-function of menin, and therefore supporting the hypothesis that MEN1 is a tumour suppressor gene. About 68% of identified missense mutations occur on an amino acid that is conserved among humans, mice, zebrafish and Drosophila. More than 10% of the MEN1 mutations arise de novo and may be transmitted to subsequent generations. Nevertheless, about 10–20% of MEN1 patients may not harbour mutations within the coding region of the MEN1 gene 1533343539; these individuals may have mutations in the promoter or untranslated regions (UTRs), which remain to be investigated. Moreover, because all MEN1 families investigated to date have tight linkage to 11q13, the presence of another tumour suppressor gene in this region is also a possibility 40.

MEN1 gene encodes a 610 amino acid (67 Kda) nuclear protein that is highly conserved among humans, mice (98%) and rats (97%), and more distantly among zebrafish (75%) and Drosophila (47%) 4142434445. Analysis of the menin amino acid sequence did not reveal homology to any other known protein, sequence motif or signal peptide, thus the putative function of menin could not be deduced. Since the amino acid sequence and mutation profile of menin provide a few clues to the functions of menin, most of what is known about its role is derived from in vitro studies. These studies revealed that menin is located primarily in the nucleus 46 and identified at least two independent nuclear localisation signals (NLSs) in the C-terminus of the protein. None of the MEN1 missense mutations or in-frame deletions 3153334353647484950 alter either of these NLSs. However, all truncating mutations induce a lack of at least one of these NLSs. The nuclear localisation of menin suggests that this protein may have an important role in the regulation of DNA transcription and replication, in cell cycle, or in the maintenance of genome integrity. Recent studies have demonstrated that over-expression of menin in a Ras-transformed NIH3T3 cell model reversed the transformed phenotype 51, inducing decreased proliferation, suppression of growth in soft agar and inhibition of tumour growth in nude mice. There is increasing evidence that menin may act in DNA repair or synthesis, but the exact mechanism by which menin regulates DNA synthesis or DNA repair in response to DNA damage, is currently unknown. In the last years menin has been shown to interact with several proteins of known functions.

The first identified partner of menin was JunD, a transcriptional factor belonging to the AP1 transcription complex family. Menin interacts with the N-terminus of JunD through its N-terminus and central domains (which are critical for this interaction). Wild type menin represses JunD-activated transcription maybe via a histone deacetylase-dependent mechanism 5253.

Menin interacts, directly, with three members of the nuclear factor NF-kB family of transcription regulators: NF-kB1 (p50), NF-kB2 (p52) and RelA (p65) 54. These proteins modulate the expression of various genes and are involved in the oncogenesis of numerous organs. Menin interacts with NF-kB by its central domain and represses NF-kB-mediated transcription.

Moreover, menin interferes with the Transforming Growth Factor beta (TGFβ) signalling pathway at the level of Smad3. Alteration of the TGFβ signalling pathways is important in pancreatic carcinogenesis.

Even the rodent protein Pem has been shown to bind menin directly 55. Pem is a homeobox-containing protein which plays a role in the regulation of transcription. However, since Pem sequence has no known homolog in the human genome, its direct relevance to MEN1 in humans is still controversial. Mouse and human menin are very similar and this could suggest the existence of a human protein, with a function similar to that of Pem, which binds menin and thus plays a role in the pathogenicity of MEN1 mutations.

Although menin has been identified primarily as a nuclear protein, recent studies have reported its interaction with the glial fibrillary acid protein (GFAP) and with vimentin (components of intermediate filaments (IFs)), suggesting a putative role in glial cell oncogenesis.

Finally, menin interacts with the metastasis suppressor Nm23H1 56. This interaction enables menin to act as an atypical GTPase and to hydrolyze GTP. The binding of menin to Nm23H1 may be relevant also to the control of genomic stability, as Nm23H1 is associated to the centrosome that is involved in the maintenance of chromosome integrity. This may be supported by the fact that normal cells from MEN1 patients present an elevated level of chromosome alterations 57585960 and that MEN1 tumours have more genome aberrations than equivalent tumours from non-MEN1 patients 61.