Multiple endocrine neoplasia type 2 (MEN2), also known as Sipple syndrome, is a group of rare familial cancer syndromes involving multiple endocrine organs, most commonly thyroid, adrenal glands, and parathyroid. MEN2 was first described by Sipple in 1961 when he noticed a high association of bilateral pheochromocytomas with medullary thyroid cancer (MTC). Later, various other tissue and organ involvement was reported in areas that are not classically considered endocrine tissues like gut or skin. MEN2 is an autosomal dominant condition with very high penetrance and variable expressivity. Although MEN2 is rare, recognition is very important both for patient and family member evaluation and treatment.
MEN2 is further classified into two subcategories: MEN2A and MEN2B. Both types involve the thyroid and adrenal glands, but MEN2A also causes primary hyperparathyroidism (20 % to 30%). MEN2A is further categorized into the following four subtypes:
In both MEN2A and MEN2B, there is an occurrence of multicentric tumor formation in all organs where RET proto-oncogene is expressed.
The rearranged during transfection (RET) protein is a receptor tyrosine kinase that is localized to chromosome 10q11.2. It appears to transduce growth and differentiate signals in several tissues, particularly those arising from neural crest cells. Some cytogenetic mutations have been reported; these may involve intracellular and extracellular domains of the RET protein signaling pathway. The germline RET mutations in MEN2 result in a gain of function of this tyrosine kinase receptor. This is different from many other inherited predispositions to neoplasia that are due to heritable "loss-of-function" mutations that inactivate tumor suppressor proteins.
Structure of RET Proto-Oncogene and Associated Mutations
This RET tyrosine kinase protein consists of an extracellular part with a ligand-binding domain, a cadherin (calcium-dependent cell adhesion)-like domain, and a cysteine-rich domain close to the cell membrane. It has a single transmembrane domain and an intracellular part with two tyrosine kinase subdomains, TK1 and TK2. RET activation occurs by the binding of one of its four ligands, glial cell line-derived neurotrophic factor (GDNF), neurturin (TNT), artemin, or persephin. Interaction of these molecules results in dimerization of RET, cross-autophosphorylation, and subsequent phosphorylation of intracellular substrates. The number of different mutations causes MEN2A. The majority of the mutations in MEN2A variants occur in the cysteine-rich region of RET protein's extracellular domain (coded by the genes in exon10 and 11), while mutations in the intracellular TK2 domain cause MEN2B-associated tumors. A single 918 Met to Thr mutation (M918T) in exon 16 is responsible for over 95% of cases of MEN2B. There are other less common mutations associated with both MEN2A and MEN2B divided into high-risk, moderate-risk and low-risk categories. Identification of these specific mutations is very important as it significantly impacts screening, diagnosis, and treatment, for example, the M918T mutation in exon 16 is considered a high-risk mutation for MEN2B cases and needs aggressive screening and prophylactic treatment to decrease morbidity and mortality.
The prevalence of all MEN2 worldwide is 1 in 35,000, while in the United States, it is 1 in 30,000 to 50,000. The epidemiology of MEN2B is unknown. The prevalence of MEN2B is estimated to be between 1 in 600,000 to 1 in 4 million.
MEN2A syndrome affects 60% to 90% of MEN2 families while MEN2B affects only 5% of MEN2 families. All of these conditions are due to one of the several RET proto-oncogene mutations which play an important role in the growth and differentiation of the structures mostly evolved from neural crest cells. Following are the most commonly involved structures.
Medullar thyroid cancer (MTC) is the most common manifestation of MEN2A and MEN2B with 100% penetrance and usually the first manifestation in MEN2 patients. MTC is a neuroendocrine tumor of the thyroid gland caused by the hyperplasia of calcitonin-producing parafollicular C-cells, the only cells in the thyroid gland derived from neural crest cells. Almost 100% of patients with MEN2A and MEN2B develop MTC, particularly early in life, with the highest incidence in the third decade, while 25% of MTC cases have RET proto-oncogene mutation. MTC most commonly presents with a solitary thyroid nodule and/or cervical lymphadenopathy.
Pheochromocytoma, a typically benign adrenal medullary tumor (usually bilateral and multicentric), occurs in 40% to 50% of patients with MEN2A or MEN2B; the frequency and penetration highly depend on the specific type of mutation. The adrenal medulla is also a derivative of neural crest cells. Usually, it is identified as a part of the screening process in the patients with known or suspected MEN2. One must always take care of bilateral or extra-renal pheochromocytoma. Although it is rare for pheochromocytoma to appear before MTC, it can be the initial manifestation of MEN2 with the classic symptoms of pheochromocytoma such as paroxysmal attacks of a headache, anxiety, diaphoresis, and palpitations. The mean age of presentation is 25 to 32 years, and it may appear as early as 8 to 12 years of age.
Primary hyperparathyroidism is present is 10% to 25% of patients with MEN 2A, while it is not associated with MEN 2B. The condition is usually mild and asymptomatic.
Cutaneous lichen amyloidosis (CLA) also termed lichen planus amyloidosis (LPA), is a rare skin condition that is associated with MEN2A. It is thought to be a primary neuropathy and presents with pruritic, pigmented, scaly papules usually in extensor surfaces of extremities and interscapular region. Histology has shown amyloid deposition.
Hirschsprung disease (HD), also known as chronic aganglionic megacolon, also is associated with MEN2A. It is characterized by the absence of autonomic ganglion cells within the parasympathetic chain of the sigmoid colon, resulting in peristalsis, chronic obstruction, and megacolon.
Marfanoid habitus, kyphoscoliosis/lordosis, joint laxity, mucosal neuromas typically at lips and tongue, and intestinal ganglioneuromas are conditions exclusively associated with the MEN2B syndrome.
MEN2A and MEN2B should be suspected in any patient diagnosed with MTC or pheochromocytoma, particularly when the age of presentation is very young (younger than 35). MTC most commonly presents with a solitary thyroid nodule and/or cervical lymphadenopathy. Any patient with diagnosed MTC or family history of MTC should be tested for RET proto-oncogene mutations for both MEN2A and MEN2B. The patients who are diagnosed with pheochromocytoma at the age earlier than that of its sporadic forms should be tested for MEN2A and MEN2B. The classic symptoms of pheochromocytoma are the paroxysms of a headache, anxiety, diaphoresis and palpitations, and high blood pressure. The presence of these symptoms in the third decade, particularly in between 25 and 32 years, should prompt to screen for MEN2.
A detailed history of the presence of associated conditions (described above) in the patient or the family members should be taken. Other possible physical examination findings include marfanoid habitus (decreased upper to lower body ratio), mucosal neuromas (red papules) over lips and tongues, and joint hyperlaxity associated with MEN2B. The patients typically lack lens dislocation or aortic abnormalities, unlike Marfan syndrome. MEN2A also is suspected in patients with clinical features like purity, scaly, pigmented papules in the interscapular region as these are features of CLA that have an association with MEN2A.
Patients with classic clinical manifestations of MEN2 are evaluated for two main reasons:
As the most common presentation of both MEN2A and MEN2B is MTC, any patient presenting with a cold, solitary nodule and cervical lymphadenopathy must be evaluated further for the possibility of MTC. The clinical presentation of sporadic MTC is the same as genetic MTC except the latter appears early in life. For this reason, the patient should be biopsied by fine-needle aspiration (FNA) to identify MTC and genotyping should be done for underlying potential RET mutations in the patient as well as first-degree family members.
In an asymptomatic patient identified with high serum calcitonin level or genetic testing (tested for positive family history), MTC is often identified in a preneoplastic state such as neoplastic C-cell hyperplasia. The presence of C-cell hyperplasia is defined based upon microscopy criteria: an increased number of diffusely scattered C-cells (greater than or equal to 7 per thyroid follicle), distribution of C-cells beyond the normal anatomical location, or the follicles completely surrounded by C-cells. Neoplastic C-cell hyperplasia is differentiated from reactive or secondary C-cell hyperplasia as the nests of C cells appear to extend beyond the basement membrane to infiltrate and destroy thyroid follicles.
It is less likely for pheochromocytoma to precede MTC in MEN2. Any patient with the diagnosed pheochromocytoma must be evaluated further for associated tumors by appropriate biochemical and radiological screening and RET genotyping.
The presence of primary hyperthyroidism alone does not indicate for further testing as there is less than 20% association with MEN2A and no association with MEN2B.
The genetic testing for RET proto-oncogene is employed to diagnose and identify a specific type of mutation present in an index patient ( the first affected member of the family) such as high-risk, moderate-risk or low-risk mutations. The type of mutations not only determine the expressivity and penetrance of the disease but also spares the effort to test for all the possible mutations in all the family members and determine when to start the screening process for associated tumors and when to do the prophylactic surgery.
For an index patient with suspected MEN2A, evaluation begins with testing for the most common mutated codons in exons 10 and 11, and if negative, we move on to look for other common mutations in descending order. The risk of MTC is low if no germline mutation is found. In this case, sequencing the entire RET coding region to identify a RET mutation is an option.
The same is done for the index patient with MEN2B phenotype. The patient is initially tested for common mutations like a mutation in RET codon M918T in exon 16, and if negative, for the A883F mutation in exon 15. These two mutations are responsible for more than 95% of cases of MEN 2B. If no mutations are identified, the entire RET coding region should be sequenced.
Although, it is rare that patients with classic MEN2A and MEN2B phenotype have no identifiable RET mutation, the presence of at least two of the classical features of MEN2A (MTC, pheochromocytoma, primary hyperparathyroidism) or the presence of majority of clinical features of MEN2B (MTC, pheochromocytoma, mucosal neuromas, Marfanoid habitus, intestinal ganglioneuromas, myelinated corneal nerves) are enough to make a clinical diagnosis.
Other cases in which genetic testing can be considered:
Only a small blood sample is required for RET genotyping; therefore, it can be performed at or soon after birth. At the latest, genotyping should be done before time so that prophylactic thyroidectomy could be performed in the event of a positive result.
Biochemical and Radiological Testing
Once the RET genotyping is positive in index patients or an asymptomatic patient with positive family history, biochemical and radiological screening for other tumors is started as described below.
Medullary Thyroid Cancer
Children with certain RET mutations can develop clinically apparent MTC at an early age. The goal for screening in patients with known RET mutations but without clinically apparent disease is to perform a prophylactic thyroidectomy before MTC develops or when it is still confined to the thyroid gland.
For children tested positive for high-risk mutations, monitoring starts at three years of age, and for children with moderate risk mutations, monitoring starts at five years of age. Testing includes the following:
Serum calcitonin concentration correlates with the size of the tumor but those with small tumors or neoplastic C-cell hyperplasia, serum calcitonin levels may be normal, and specific stimulation tests like calcium or pentagastrin infusion can be used to confirm the diagnosis. The detection of a serum calcitonin level (basal or stimulated) above the upper limit of normal is an indication for surgery.
The risk of developing pheochromocytoma is also variable depending upon genotype. Due to screening programs, pheochromocytomas may be diagnosed at a young age and before symptoms are present.
For children in the high-risk categories, annual screening for pheochromocytoma should begin by age 11 years. For children in the moderate-risk category, annual screening starts at 16 years of age. Screening tests include plasma fractionated metanephrines or 24-hour urinary metanephrines and normetanephrine. If biochemical results are positive, the next step is adrenal imaging with CT or MRI. If initial imaging is unable to identify the unilateral versus bilateral disease, adrenal venous sampling can be done.
Hyperparathyroidism (only associated with MEN2A) is often mild and asymptomatic. The mean age at diagnosis was 33 years according to one study. Annual biochemical screening starts at the age of 11 years for high-risk patients and by the age of 16 for moderate risk patients. The test of choice for screening is serum calcium corrected for albumin levels. If elevated, serum parathyroid hormone (PTH) is measured, and the diagnosis is established with high or inappropriately high levels of serum PTH in the presence of hypercalcemia.
Medullary Thyroid Carcinoma (MTC)
Virtually, all patients with MEN2A and MEN2B develop MTC. The disease is usually bilateral and multicentric including its precursor neoplastic C-cell hyperplasia. The disease in MEN2B may be more aggressive than MEN2A. The aggressiveness of the disease in each syndrome depends upon the type of mutation.
For the patients with a predisposition to MEN2B, total prophylactic thyroidectomy with lymph node dissection is indicated at 1 year of age as the metastatic disease have been reported in such patients soon after age 1. The lymph nodes are removed even when there is no evidence of involvement.
For the patients with high-risk mutations of MEN2A, total thyroidectomy with lymph node dissection is indicated at age 5.
For the patients with moderate-risk mutations, the surgery can wait until late childhood or early adulthood. The treatment aims to remove the thyroid gland before it becomes inoperable. Total thyroidectomy is the only way considering bilateral and multicentric nature of the hereditary forms of MTC. Before doing the surgery, it is compulsory to rule out the presence of pheochromocytoma, and if present, it should be removed first.
Nearly 40% of the patients with MEN2A and 50% with MEN2B develop pheochromocytoma which is also likely to be bilateral and multicentric than its sporadic forms. Bilateral adrenalectomy is recommended in patients who have bilateral pheochromocytoma or if there is unilateral disease, but there is a history of an aggressive bilateral tumor in one of the family members.
The patients with unilateral pheochromocytoma and normal-appearing contralateral glands, unilateral adrenalectomy is recommended because in patients who have undergone unilateral adrenalectomy and developed a contralateral tumor, later on, the chances of metastatic pheochromocytoma were found minimal and no death has even been reported due to catecholamine crisis in patients with MEN2.
Prior to unilateral or bilateral adrenalectomy, patients should be treated with alpha-blockade preoperatively; the patient should receive glucocorticoid stress coverage while awaiting transfer to the surgery.
Primary hyperparathyroidism has only 10% to 25% association with MEN2A, and it does not occur at all in MEN2B families. Prophylactic parathyroidectomy is not recommended in asymptomatic patients as the disease is usually mild, appears late, and is often clinically obscure. Surgery is reserved for the patients who develop worsening hypercalcemia, bone loss, and renal impairment. So, instead of removing the gland, the patient is monitored with serum calcium, creatinine, and bone density every 1 or 2 years. The disease is usually multiglandular, and parathyroidectomy is done only when the disease becomes clinically apparent and progressive. Again, the patient should be screened for concurrent pheochromocytoma, and if present, adrenalectomy is done before parathyroidectomy.