6.11 Multiple endocrine neoplasia type 1

Multiple endocrine neoplasia (1, 2) is characterized by the occurrence of tumours involving two or more endocrine glands within a single patient. The disorder has previously been referred to as multiple endocrine adenopathy (MEA) or the pluriglandular syndrome. However, glandular hyperplasia and malignancy may also occur in some patients and the term multiple endocrine neoplasia (MEN) is now preferred. There are two major forms of multiple endocrine neoplasia, referred to as type 1 and type 2, and each form is characterized by the development of tumours within specific endocrine glands (Table 6.11.1). Thus, the combined occurrence of tumours of the parathyroid glands, the pancreatic islet cells, and the anterior pituitary is characteristic of multiple endocrine neoplasia type 1 (MEN 1), which is also referred to as Wermer’s syndrome. However, in multiple endocrine neoplasia type 2 (MEN 2), which is also called Sipple’s syndrome, medullary thyroid carcinoma (MTC) occurs in association with phaeochromocytoma, and three clinical variants, referred to as MEN 2a, MEN 2b and MTC-only, are recognized (Table 6.11.1). Although MEN 1 and MEN 2 usually occur as distinct and separate syndromes as outlined above, some patients occasionally may develop tumours that are associated with both MEN 1 and MEN 2. For example, patients suffering from islet cell tumours of the pancreas and phaeochromocytomas or from acromegaly and phaeochromocytoma have been described, and these patients may represent ‘overlap’ syndromes. All these forms of MEN may either be inherited as autosomal dominant syndromes or they may occur sporadically, i.e. without a family history. However, this distinction between sporadic and familial cases may sometimes be difficult as in some sporadic cases the family history may be absent because the parent with the disease may have died before developing symptoms. In this chapter, the main clinical features and molecular genetics of the MEN 1 syndrome will be discussed.

Parathyroid, pancreatic, and pituitary tumours constitute the major components of MEN 1 (Fig. 6.11.1). In addition to these tumours adrenal cortical, carcinoid, facial angiofibromas, collagenomas, and lipomatous tumours may also occur in some patients (2, 3).

Primary hyperparathyroidism is the most common feature of MEN 1 and occurs in more than 95% of all MEN 1 patients (1, 3). Patients may present with asymptomatic hypercalcaemia, or nephrolithiasis, or osteitis fibrosa cystica, or vague symptoms associated with hypercalcaemia, for example polyuria, polydipsia, constipation, malaise, or occasionally with peptic ulcers. Biochemical investigations reveal hypercalcaemia, usually in association with raised circulating parathyroid hormone concentrations. The hypercalcaemia is usually mild, and severe hypercalcaemia resulting in crisis or parathyroid carcinoma are rare occurrences. Additional differences in the primary hyperparathyroidism of MEN 1 patients from that in non-MEN 1 patients include an earlier age of onset (20 to 25 years versus 55 years), and an equal male:female ratio (1:1 versus 1:3). Primary hyperparathyroidism in MEN 1 patients is unusual before the age of 15 years, and the age of conversion from being unaffected to affected has been observed to be between 20 and 21 years in some individuals (3). No effective medical treatment for primary hyperparathyroidism is generally available and surgical removal of the abnormally overactive parathyroids is the definitive treatment. However, all four parathyroid glands are usually affected with multiple adenomas or hyperplasia, although this histological distinction may be difficult, and total parathyroidectomy has been proposed as the definitive treatment for primary hyperparathyroidism in MEN 1, with the resultant lifelong hypocalcaemia being treated with oral calcitriol (1,25 dihydroxyvitamin D₃). It is recommended that such total parathyroidectomy should be reserved for the symptomatic hypercalcaemic patient with MEN 1, and that the asymptomatic hypercalcaemic MEN 1 patient should not have parathyroid surgery but have regular assessments for the onset of symptoms and complications, when total parathyroidectomy should be undertaken.

The incidence of pancreatic islet cell tumours in MEN 1 patients varies from 30 to 80% in different series (1, 3). The majority of these tumours produce excessive amounts of hormone, for example gastrin, insulin, glucagon, or vasoactive intestinal polypeptide (VIP), and are associated with distinct clinical syndromes.

These gastrin-secreting tumours represent over 50% of all pancreatic islet cell tumours in MEN 1 and approximately 20% of patients with gastrinomas will have MEN 1. Gastrinomas are the major cause of morbidity and mortality in MEN 1 patients. This is due to the recurrent, severe multiple peptic ulcers which may perforate. This association of recurrent peptic ulceration, marked gastric acid production, and non-β-islet cell tumours of the pancreas is referred to as the Zollinger–Ellison syndrome. Additional prominent clinical features of this syndrome include diarrhoea and steatorrhoea. The diagnosis is established by demonstration of a raised fasting serum gastrin concentration in association with an increased basal gastric acid secretion (4). Medical treatment of MEN 1 patients with the Zollinger–Ellison syndrome is directed to reducing basal acid output to less than 10 mmol/l, and this may be achieved by the parietal cell H⁺-K⁺-ATPase inhibitor, e.g. omeprazole. The ideal treatment for a nonmetastatic gastrinoma is surgical excision of the gastrinoma. However, in patients with MEN 1 the gastrinomas are frequently multiple or extrapancreatic and the role of surgery has been controversial (5). For example, in one study (5), only 16% of MEN 1 patients were free of disease immediately after surgery, and at 5 years this had declined to 6%; the respective outcomes in non-MEN 1 patients were better at 45 and 40%. The treatment of disseminated gastrinomas is difficult and hormonal therapy with human somatostatin analogues, e.g. Octreotide chemotherapy with streptozotocin and 5-fluoroaracil, hepatic artery embolization, and removal of all resectable tumour have all occasionally been successful (1).

These β-islet cell tumours secreting insulin represent one-third of all pancreatic tumours in MEN 1 patients (1, 3). Insulinomas also occur in association with gastrinomas in 10% of MEN 1 patients, and the two tumours may arise at different times. Insulinomas occur more often in MEN 1 patients who are below the age of 40 years, and many of these arise in individuals before the age of 20 years (3), whereas in non-MEN 1 patients insulinomas generally occur in those above the age of 40 years. Insulinomas may be the first manifestation of MEN 1 in 10% of patients and approximately 4% of patients presenting with insulinoma will have MEN 1. Patients with an insulinoma present with hypoglycaemic symptoms, which develop after a fast or exertion and improve after glucose intake. Biochemical investigations reveal raised plasma insulin concentrations in association with hypoglycaemia. Circulating concentrations of C-peptide and proinsulin, which are also raised, may be useful in establishing the diagnosis, as may an insulin suppression test. Medical treatment, which consists of frequent carbohydrate feeds and diazoxide, may be useful in the short-term, with surgery being the definitive treatment. Most insulinomas are multiple and small and preoperative localization with computed tomography scanning, coeliac axis angiography, and preoperative percutaneous transhepatic portal venous sampling is difficult and success rates have varied. Surgical treatment, which ranges from enucleation of a single tumour to a distal pancreatectomy or partial pancreatectomy, has been curative in some patients. Chemotherapy, which consists of streptozotocin or octreotide, is used for metastatic disease.

These α-islet cell, glucagon-secreting pancreatic tumours occur in less than 3% of MEN 1 patients (1, 3). The characteristic clinical manifestations of a skin rash (necrolytic migratory erythyema), weight loss, anaemia, and stomatitis may be absent and the presence of the tumour is indicated only by glucose intolerance and hyperglucagonaemia. The tail of the pancreas is the most frequent site for glucagonomas and surgical removal of these is the treatment of choice. However, treatment may be difficult as 50% of patients have metastases at the time of diagnosis. Medical treatment of these with somatostatin analogues, or with streptozotocin has been successful in some patients.

Patients with VIPomas, which are VIP-secreting pancreatic tumours, develop watery diarrhoea, hypokalaemia, and achlorhydria, referred to as the WDHA syndrome. This clinical syndrome has also been referred to as the Verner–Morrison syndrome or the VIPoma syndrome. VIPomas have been reported in only a few MEN 1 patients and the diagnosis is established by documenting a markedly raised plasma VIP concentration (1). Surgical management of VIPomas, which are mostly located in the tail of the pancreas, has been curative. However, in patients with unresectable tumour, treatment with somatostatin analogues, streptozotocin, corticosteroids, indomethicin, metoclopramide, and lithium carbonate has proved beneficial.

These tumours, which secrete pancreatic polypeptide (PP) are found in a large number of patients with MEN 1 (1, 6). No pathological sequelae of excessive pancreatic polypeptide secretion are apparent and the clinical significance of pancreatic polypeptide is unknown, although the use of serum pancreatic polypeptide measurements has been suggested for the detection of pancreatic tumours in MEN 1 patients.

The incidence of pituitary tumours in MEN 1 patients varies from 15 to 90% in different series (1, 3). Approximately 60% of MEN 1 associated pituitary tumours secrete prolactin, less than 25% secrete growth hormone, 5% secrete ACTH, and the remainder appear to be nonfunctioning. Prolactinomas may be the first manifestation of MEN 1 in less than 10% of patients and somatotrophinomas occur more often in patients over the age of 40 years (3). Less than 3% of patients with anterior pituitary tumours will have MEN 1. The clinical manifestations depend upon the size of the pituitary tumour and its product of secretion. Enlarging pituitary tumours may compress adjacent structures such as the optic chiasm or normal pituitary tissue and cause bitemporal hemianopia or hypopituitarism, respectively. The tumour size and extension are radiologically assessed by CT scanning and MRI. Treatment of pituitary tumours in MEN 1 patients is similar to that in non-MEN 1 patients and consists of medical therapy or selective hypophysectomy by the transphenoidal approach if feasible, with radiotherapy being reserved for residual unresectable tumour.

Patients with MEN 1 may have tumours involving glands other than the parathyroids, pancreas, and pituitary. Thus carcinoid, adrenal cortical, facial angiofibromas, collagenomas, thyroid, and lipomatous tumours have been described in association with MEN 1 (1, 3).

Carcinoid tumours, which occur in more than 3% of patients with MEN 1, may be inherited as an autosomal dominant trait in association with MEN 1. The carcinoid tumour may be located in the bronchi, the gastrointestinal tract, the pancreas, or the thymus (7). Bronchial carcinoids in MEN 1 patients predominantly occur in women (M:F = 1:4) whereas thymic carcinoids predominantly occur in men, with cigarette smokers having a higher risk of developing tumours. Most patients are asymptomatic and do not suffer from the flushing attacks and dyspnoea associated with the carcinoid syndrome, which usually develops after the tumour has metastasized to the liver. Somatostatin analogues have been successfully used to treat symptoms and may in some patients result in regression of gastric carcinoids (8).

The incidence of asymptomatic adrenal cortical tumours in MEN 1 patients has been reported to be as high as 40% (9). The majority of these tumours are nonfunctioning. However, functioning adrenal cortical tumours in MEN 1 patients have been documented to cause hypercortisolaemia and Cushing’s syndrome, and primary hyperaldosteronism, as in Conn’s syndrome (1, 3).

Lipomas may occur in more than 33% of patients (2, 10), and frequently they are multiple. In addition, pleural or retroperitoneal lipomas may also occur in patients with MEN 1.

Thyroid tumours consisting of adenomas, colloid goitres, and carcinomas have been reported to occur in over 25% of MEN 1 patients (1, 2). However, the prevalence of thyroid disorders in the general population is high and it has been suggested that the association of thyroid abnormalities in MEN 1 patients may be incidental and not significant.

Multiple facial angiofibromas, which are similar to those observed in patients with tuberous sclerosis, have been observed in 88% of MEN 1 patients (2, 10) and collagenomas have been reported in over 70% of MEN 1 patients (2, 10).

The gene causing MEN 1 was localized to chromosome 11q13 by genetic mapping studies that investigated MEN 1 associated tumours for loss of heterozygosity (LOH) and by segregation studies in MEN 1 families (11, 12). The results of these studies, which were consistent with Knudson’s model for tumour development (13), indicated that the MEN1 gene represented a putative tumour suppressor gene. Further genetic mapping studies defined a less than 300-Kb region as the minimal critical segment that contained the MEN1 gene and characterization of genes from this region led to the identification, in 1997, of the MEN1 gene (14, 15), which consists of 10 exons with a 1830-bp coding region (Fig. 6.11.2) that encodes a novel 610-amino acid protein, referred to as ‘MENIN’ (14). Over 1100 germline and over 200 somatic mutations of the MEN1 gene have been identified, and the majority (>70%) of these are inactivating, and are consistent with its role as a tumour suppressor gene (16). These mutations are diverse in their types and approximately 25% are nonsense mutations, approximately 40% are frameshift deletions or insertions, approximately 5% are in-frame deletions or insertions, approximately 10% are splice site mutations, approximately 20% are missense mutations, and less than 1% are whole or partial gene deletions. More than 10% of the MEN1 mutations arise de novo and may be transmitted to subsequent generations (16–18). It is also important to note that between 5% and 10% of MEN 1 patients may not harbour mutations in the coding region of the MEN1 gene (16), and that these individuals may have mutations in the promoter or untranslated regions, which remain to be investigated. The mutations are not only diverse in their types but are also scattered throughout the 1830-bp coding region of the MEN1 gene with no evidence for clustering as observed in MEN 2 (see Chapter 6.12). Correlations between the MEN1 mutations and the clinical manifestations of the disorder appear to be absent (16). Tumours from MEN 1 patients and non-MEN 1 patients have been observed to harbour the germ line mutation together with a somatic LOH involving chromosome 11q13, as expected from Knudson’s model and the proposed role of the MEN1 gene as a tumour suppressor (16). MENIN has been shown to have three nuclear localization sites (NLSs) and to be located predominantly in the nucleus (16, 19).

Studies of protein–protein interactions have revealed that MENIN interacts with several proteins involved in transcriptional regulation, genome stability, cell division, and proliferation (Fig. 6.11.2) (16). Thus, in transcriptional regulation, MENIN has been shown to interact with: the activating protein-1 transcription factor JunD and to suppress Jun-mediated transcriptional activation members (e.g. p50, p52, and p65) of the NF-κB family of transcriptional regulators to repress NF-κB-mediated transcriptional activation; members of the Smad family, Smad3 and the Smad 1/5 complex, which are involved in the transforming growth factor-β (TGFβ) and the bone morphogenetic protein-2 (BMP-2) signalling pathways, respectively; Runx2, also called cbfa1, which is a common target of TGFβ and BMP-2 in differentiating osteoblasts; and the mouse placental embryonic (Pem) expression gene, which encodes a homeobox-containing protein. Additional studies have shown that the interaction of MENIN with JunD may be mediated by a histone deacetylase-dependent mechanism, via recruitment of an mSin3A-histone deacetylase complex to repress JunD transcriptional activity. Recently, the forkhead transcription factor CHES1 has been shown to be a component of this transcriptional repressor complex and to interact with MENIN in an S-phase checkpoint pathway related to DNA damage response. MENIN uncouples ELK-1, JunD, and c-Jun phosphorylation from mitogen-activated protein kinase (MAPK) activation and suppresses insulin-induced c-Jun-mediated transactivation in CHO-1R cells (16).

A wider role in transcription regulation has also been suggested, as MENIN has been shown to be an integral component of histone methyltransferase complexes that contain members from the mixed-lineage leukaemia (MLL) and trithorax protein family. These can methylate the lysine 4 residue of histone H3 (H3K4) and H3K4 trimethylation is linked to activation of transcription. MENIN, as a component of this MLL complex, regulates the expression of genes such as the Hox homeobox genes and the genes for cyclin-dependent kinase inhibitors, p27 and p18. MENIN has been shown to directly interact with the nuclear receptor for oestrogen (ERα) and to act as a coactivator for ERα–mediated transcription, linking the activated oestrogen receptor to histone H3K4 trimethylation. MENIN has also been shown to bind to a broad range of gene promoters, independently of the histone methyltransferase complex, suggesting that MENIN functions as a general transcriptional regulator that helps maintain stable gene expression, perhaps by cooperating with other, currently unknown, proteins. MENIN also directly binds to doubled-stranded DNA and this is mediated by the positively charged residues in the NLSs in the carboxyl terminus of MENIN. The NLSs appear to be necessary for MENIN to repress the expression of the insulin-like growth factor binding protein-2 (IGFBP-2) gene by binding to the IGFBP-2 promoter. In addition, each of the NLSs has also been reported to be involved in MENIN-mediated induction of caspase 8 expression. The NLSs may therefore have roles in controlling gene transcription as well as targeting MENIN into the nucleus (16).

A role for MENIN in controlling genome stability (16) has been proposed because of its interactions with: a subunit of replication protein (RPA2), which is a heterotrimeric protein required for DNA replication, recombination, and repair; and the FANCD2 protein, which is involved in DNA repair and mutations of which result in the inherited cancer-prone syndrome of Fanconi’s anaemia. MENIN also has a role in regulating cell division as it interacts with: the nonmuscle myosin II-A heavy chain (NMHC II-A), which participates in mediating alterations in cytokinesis and cell shape during cell division and the glial fibrillary acidic protein (GFAP) and vimentin, which are involved in the intermediate filament network. MENIN also has a role in cell cycle control as it interacts with: the tumour metastases suppressor NM23H1/nucleoside diphosphate kinase, which induces guanosine triphosphatase activity and the activator of S-phase kinase (ASK), which is a component of the Cdc7/ASK kinase complex that is crucial for cell proliferation. Indeed, MENIN has been shown to completely repress ASK-induced cell proliferation.

The functional role of MENIN as a tumour suppressor also has been investigated, and studies in human fibroblasts have revealed that MENIN acts as a repressor of telomerase activity via hTERT (a protein component of telomerase) (16). Furthermore, overexpression of MENIN in the human endocrine pancreatic tumour cell line (BON1) resulted in an inhibition of cell growth which was accompanied by up-regulation of JunD expression but down-regulation of delta-like protein 1/preadipocyte factor-1, proliferating cell nuclear antigen, and QM/Jif-1, which is a negative regulator of c-Jun. These findings of growth suppression by MENIN were observed in other cell types. Thus, expression of MENIN in the RAS-transformed NIH3T3 cells partially suppressed the RAS-mediated tumour phenotype in vitro and in vivo. Overexpression of MENIN in CHO-IR cells also suppressed insulin-induced activating protein-1 transactivation, and this was accompanied by an inhibition of c-Fos induction at the transcriptional level. Furthermore, MENIN re-expression in Men1-deficient mouse Leydig tumour cell lines induced cell cycle arrest and apoptosis. In contrast, depletion of MENIN in human fibroblasts resulted in their immortalization. Thus, MENIN appears to have a large number of functions through interactions with proteins, and these mediate alterations in cell proliferation.

I am grateful to the Medical Research Council (MRC), UK, for support and to Mrs Tracey Walker for expert secretarial assistance.