# Multiple Endocrine Neoplasia Syndromes

## Overview

| Syndrome | Gene | Chromosome | Inheritance | Key Tumors | Unique Features |
|---|---|---|---|---|---|
| MEN1 (Wermer) | MEN1 (menin) | 11q13 | AD, >95% penetrance by age 50 | Parathyroid hyperplasia (95%), pNET (40-70%), pituitary adenoma (30-40%) | Facial angiofibromas, thymic carcinoid (males) |
| MEN2A (Sipple) | RET (gain-of-function) | 10q11.2 | AD, strong genotype-phenotype correlation | MTC (95-100%), pheochromocytoma (50%), PHPT (20-30%) | Cutaneous lichen amyloidosis (codon 634); Hirschsprung disease (7%) |
| MEN2B | RET M918T (>95%) | 10q11.2 | AD; 50% de novo | MTC (100%, earliest/most aggressive), pheochromocytoma (50%) | Mucosal neuromas, marfanoid habitus, NO hyperparathyroidism |
| MEN4 | CDKN1B (p27) | 12p13 | AD; rare | Parathyroid, pituitary, pNET (less frequent) | MEN1-like phenotype; consider if MEN1 gene-negative |

The multiple endocrine neoplasia (MEN) syndromes are a group of hereditary disorders characterized by the development of tumors in two or more endocrine glands. They follow autosomal dominant inheritance patterns with high penetrance, meaning that most mutation carriers will develop clinical disease during their lifetime. The advent of genetic testing has enabled presymptomatic diagnosis and prophylactic management, fundamentally altering the natural history of these conditions. The three major syndromes are MEN1, MEN2A, and MEN2B, with MEN4 and other related hereditary endocrine tumor syndromes recognized as distinct entities.

## MEN1 (Wermer Syndrome)

### Genetics

MEN1 is caused by mutations in the MEN1 gene on chromosome 11q13, which encodes the menin protein, a tumor suppressor. Over 1,500 mutations have been identified. The syndrome follows autosomal dominant inheritance with penetrance exceeding 95% by age 50. De novo mutations account for approximately 10% of cases, occurring without family history. The menin protein is a nuclear scaffold protein involved in transcriptional regulation, DNA repair, and chromatin modification. Tumorigenesis follows the classical two-hit model, requiring loss of heterozygosity at the MEN1 locus. Approximately 10-25% of patients with a clinical MEN1 phenotype do not harbor MEN1 mutations (phenocopies), and in these cases, mutations in CDKN1B (MEN4), AIP, or CDC73 should be considered.

### Clinical Manifestations (Classic Triad: 3 Ps)

#### Primary Hyperparathyroidism (95% by Age 50)

PHPT is usually the first manifestation of MEN1, presenting in the second to third decade of life. Unlike sporadic PHPT, MEN1-associated PHPT involves multigland hyperplasia affecting all four parathyroid glands rather than a single adenoma. The disease presents earlier and tends to be more severe than sporadic PHPT. The surgical approach differs accordingly: subtotal parathyroidectomy (removing 3.5 glands) or total parathyroidectomy with forearm autotransplantation is recommended. Despite subtotal resection, recurrence rates remain high at 40-60% because all parathyroid tissue harbors the genetic predisposition to hyperplasia. Cinacalcet offers a medical alternative for patients who are not surgical candidates.

#### Pancreatic/Duodenal Neuroendocrine Tumors (40-70%)

Gastrinoma is the most common functional pancreatic neuroendocrine tumor in MEN1, occurring in approximately 40% of patients. These tumors are usually located in the duodenum (over 80%) and cause Zollinger-Ellison syndrome, characterized by gastric acid hypersecretion leading to multiple peptic ulcers, diarrhea, and severe GERD. Diagnosis relies on fasting gastrin exceeding 10 times the upper limit of normal or the secretin stimulation test demonstrating a paradoxical gastrin rise exceeding 120 pg/mL. PPI therapy controls acid-related symptoms but does not prevent tumor progression.

Insulinoma, the second most common functional pNET, occurs in approximately 10% of MEN1 patients and presents with hypoglycemia (Whipple triad). These tumors are often multiple, unlike sporadic insulinomas. Diagnosis requires the 72-hour supervised fast demonstrating glucose below 45 mg/dL with insulin at or above 3 mU/mL, C-peptide at or above 0.6 ng/mL, and proinsulin at or above 5 pmol/L.

Non-functioning pNETs are the most common pancreatic tumors overall in MEN1 (20-55%). They are often multiple and may secrete pancreatic polypeptide or chromogranin A without causing a clinical syndrome. Their malignant potential correlates with size, with tumors exceeding 2 cm carrying higher risk, and they represent the main cause of disease-specific mortality in MEN1. Screening involves annual fasting biochemistry (gastrin, insulin, glucose, chromogranin A, pancreatic polypeptide) combined with cross-sectional imaging (CT or MRI) every 1-3 years, with endoscopic ultrasound being the most sensitive modality for detecting small tumors. Surgery is indicated for insulinoma (symptomatic), gastrinoma exceeding 2 cm, and non-functioning pNETs exceeding 2 cm or demonstrating growth. Surgical cure of gastrinoma is rare in MEN1 due to the characteristically multiple, small duodenal tumors.

#### Pituitary Adenomas (30-40%)

Prolactinoma is the most common pituitary tumor (60%), followed by GH-secreting adenomas (25%) and non-functioning adenomas (15%). ACTH-secreting adenomas are rare. These tumors are often macroadenomas at presentation, behaving more aggressively than their sporadic counterparts. Treatment follows the same principles as sporadic tumors: dopamine agonists for prolactinomas and transsphenoidal surgery for others. Screening includes annual prolactin and IGF-1 measurements with pituitary MRI every 3-5 years or when biochemical abnormalities are detected.

#### Other Manifestations

Adrenal cortical tumors develop in 20-40% of MEN1 patients, are usually non-functioning, and are bilateral in 10-15%. Rarely, they may secrete cortisol (producing Cushing syndrome) or, very rarely, represent ACC. Adrenalectomy is considered when tumors exceed 4 cm or are functioning. Thymic carcinoid tumors (2-8%) occur predominantly in males, are aggressive with 25% mortality, and may present as an anterior mediastinal mass. Prophylactic thymectomy at the time of parathyroidectomy should be considered in male patients. Bronchial carcinoid tumors (2-8%) are more common in females and are generally less aggressive. Cutaneous manifestations include facial angiofibromas (85%), collagenomas (70%), and lipomas (30%), which may precede endocrine tumors and aid in clinical recognition. Meningiomas and ependymomas occur rarely.

<image>A comprehensive MEN1 syndrome infographic. Central figure showing a human body outline with annotated affected organs. Head: pituitary adenoma (shown as a small tumor in sella on sagittal view; label prolactinoma most common). Neck: four parathyroid glands all enlarged (multigland hyperplasia). Chest: thymic carcinoid (mediastinal mass), bronchial carcinoid. Abdomen: multiple pancreatic/duodenal NETs (show pancreas with multiple small tumors; label gastrinoma, insulinoma, non-functioning). Adrenal glands: bilateral cortical tumors. Skin: facial angiofibromas, collagenomas shown as insets. Include a genetics box: MEN1 gene, chromosome 11q13, menin protein, autosomal dominant. Include a screening schedule table on the side. Use anatomical illustration style with labeled callouts.</image>

## MEN2A (Sipple Syndrome)

### Genetics

MEN2A is caused by gain-of-function mutations in the RET proto-oncogene on chromosome 10q11.2, following autosomal dominant inheritance. Genotype-phenotype correlations in MEN2 are remarkably strong, among the most robust in oncology, and directly guide clinical management. The most common mutations occur at codon 634 (exon 11), which carries the highest penetrance for MTC, pheochromocytoma, and PHPT. Other important mutations affect codons 618, 620, and 804.

### Clinical Manifestations

#### Medullary Thyroid Carcinoma (95-100%)

MTC in MEN2A is virtually universal and characteristically bilateral and multifocal. C-cell hyperplasia precedes invasive carcinoma, with the rate of progression determined by the specific mutation. The specific RET codon mutation determines the recommended age for prophylactic thyroidectomy, representing one of the most effective cancer prevention strategies in all of medicine. | ATA Risk Category | RET Mutations | Prophylactic Thyroidectomy Timing | Metanephrine Screening Start | Ca/PTH Screening Start |
| --- | --- | --- | --- | --- | --- |
| Highest | M918T (MEN2B) | By 6 months of age | Age 8 | Not applicable (no PHPT in MEN2B) |  |
| High | Codon 634, A883F | By age 5 (or earlier if calcitonin elevated) | Age 8 | Annual from diagnosis |  |
| Moderate | Codons 609, 611, 618, 620, 630, 804, 891 | Guided by calcitonin monitoring; consider age 5-10 | Age 16 | Age 20 |  |

For ATA-High risk mutations (codon 634, A883F), thyroidectomy is recommended by age 5 or earlier if calcitonin becomes elevated. For ATA-Moderate risk mutations (codons 609, 611, 618, 620, 630, 804, 891), thyroidectomy can be delayed if calcitonin remains normal and neck ultrasound is unremarkable, with consideration by age 5-10 or when calcitonin begins rising. Post-thyroidectomy monitoring involves calcitonin and CEA measurement every 6-12 months, with calcitonin doubling time serving as a prognostic indicator: less than 6 months suggests aggressive disease, while more than 24 months indicates indolent behavior.

#### Pheochromocytoma (50%)

Pheochromocytomas in MEN2A are bilateral in 50-80% of cases (simultaneous or metachronous) and characteristically epinephrine-predominant, reflecting their adrenal medullary origin. They are generally benign with malignancy rates below 5%. Screening involves annual plasma free metanephrines, beginning at age 8 for high-risk mutations and age 16 for moderate-risk mutations. Cortical-sparing adrenalectomy is preferred to preserve adrenal cortical function and avoid lifelong steroid replacement, though the 10-15% recurrence risk in remaining adrenal tissue mandates ongoing surveillance. Critically, pheochromocytoma must always be treated before thyroidectomy to prevent intraoperative catecholamine crisis.

#### Primary Hyperparathyroidism (20-30%)

PHPT in MEN2A is typically mild, presenting as multigland hyperplasia or adenoma. It is most commonly associated with codon 634 mutations and is usually managed at the time of thyroidectomy if clinically indicated.

#### Cutaneous Lichen Amyloidosis

This pruritic lichenoid skin lesion over the upper back (interscapular region) is associated with codon 634 mutations and is pathognomonic for MEN2A when present. It may precede other manifestations and serve as a clinical clue.

#### Hirschsprung Disease

Hirschsprung disease is present in approximately 7% of MEN2A patients, associated with specific RET mutations that paradoxically cause both gain and loss of function in different tissue types.

## MEN2B

### Genetics

MEN2B is caused by the RET M918T mutation (exon 16) in over 95% of cases, with the RET A883F mutation (exon 15) accounting for fewer than 5%. Notably, 50% of MEN2B cases represent de novo mutations with no family history, making clinical recognition essential.

### Clinical Manifestations

MTC occurs in virtually 100% of MEN2B patients and is the earliest and most aggressive form of MTC seen in any MEN syndrome, potentially developing in infancy. Prophylactic thyroidectomy is recommended within the first year of life, ideally by 6 months for the M918T mutation. Pheochromocytoma occurs in approximately 50% at rates similar to MEN2A. Importantly, MEN2B does not include hyperparathyroidism, a key distinguishing feature from MEN2A.

The characteristic phenotype of MEN2B includes mucosal neuromas on the lips, tongue, buccal mucosa, and eyelids, which are virtually pathognomonic. Marfanoid habitus is present with tall, thin stature, long extremities, high-arched palate, and pectus excavatum, but without the lens subluxation or aortic root dilation seen in Marfan syndrome. Ganglioneuromatosis of the GI tract produces constipation, megacolon, and diarrhea. Corneal nerve thickening is visible on slit-lamp examination. MEN2B is the most aggressive form of the MEN syndromes, with MTC as the leading cause of death and prognosis worse than MEN2A.

## MEN4

### Genetics

MEN4 is caused by mutations in the CDKN1B (p27/Kip1) gene, which encodes a cyclin-dependent kinase inhibitor functioning as a tumor suppressor. It follows autosomal dominant inheritance but is rare. The phenotype overlaps substantially with MEN1 and is often referred to as "MEN1-like."

### Clinical Manifestations

Primary hyperparathyroidism is the most common manifestation. Pituitary adenomas occur, with ACTH-secreting tumors potentially more common than in MEN1. Pancreatic NETs are less frequent than in MEN1. Other tumors may involve the thyroid, adrenal, gonads, and stomach (gastric carcinoid). CDKN1B testing should be considered in patients with a clinical MEN1 phenotype who test negative for MEN1 gene mutations, as CDKN1B accounts for approximately 1-3% of MEN1-like presentations.

## Other Hereditary Endocrine Tumor Syndromes

### Von Hippel-Lindau (VHL)

VHL is caused by mutations in the VHL tumor suppressor gene on chromosome 3p25, following autosomal dominant inheritance. Pheochromocytomas develop in 10-20% of carriers, characteristically bilateral, norepinephrine-predominant, and with low malignancy risk. Other manifestations include CNS hemangioblastomas (cerebellar, spinal, retinal), clear cell renal cell carcinoma (40-70%), pancreatic NETs and cysts, and endolymphatic sac tumors. Screening includes annual metanephrines from age 5, annual ophthalmoscopy, MRI of brain and spine every 2 years, and annual abdominal imaging.

### Carney Complex

Carney complex results from PRKAR1A gene mutations (encoding the regulatory subunit of protein kinase A), following autosomal dominant inheritance. Primary pigmented nodular adrenocortical disease (PPNAD) produces ACTH-independent Cushing syndrome through bilateral micronodular adrenal hyperplasia, with a characteristic paradoxical increase in cortisol during the high-dose dexamethasone (Liddle) test. Cardiac myxomas can cause embolic stroke and sudden death, necessitating echocardiographic screening. Additional features include skin pigmentation (lentigines, blue nevi), large cell calcifying Sertoli cell tumors of the testes, thyroid nodules, GH-secreting pituitary adenomas, breast myxomas, and psammomatous melanotic schwannomas.

### Hyperparathyroidism-Jaw Tumor Syndrome (HPT-JT)

HPT-JT is caused by CDC73 (HRPT2) gene mutations following autosomal dominant inheritance. Its cardinal feature is PHPT carrying a 15-20% risk of parathyroid carcinoma, far exceeding the risk in MEN1 or sporadic PHPT. Associated findings include ossifying fibromas of the mandible and maxilla, renal tumors (Wilms tumor, cysts, hamartomas), and uterine fibroids.

<image>A comparison table of MEN syndromes presented as a visual grid. Rows: MEN1, MEN2A, MEN2B, MEN4. Columns: Gene/Chromosome, Inheritance, Key tumor types (with icons), Unique features, Screening recommendations. For MEN1: MEN1/11q13, parathyroid + pNET + pituitary, cutaneous lesions. For MEN2A: RET/10q11, MTC + PHEO + PHPT, lichen amyloidosis. For MEN2B: RET M918T, MTC + PHEO (no PHPT), mucosal neuromas + marfanoid habitus. For MEN4: CDKN1B/12p13, parathyroid + pituitary + pNET, MEN1-like phenotype. Use color-coded cells and small organ icons for visual clarity. Include a side box comparing prophylactic thyroidectomy timing for RET mutations.</image>

## Genetic Testing and Surveillance

### Who to Test

Genetic testing should be performed for all patients with MTC (germline RET testing), all patients with pheochromocytoma or paraganglioma (RET plus SDHx plus VHL panel), patients with PHPT under age 40 or with multigland disease or family history, first-degree relatives of known mutation carriers, and patients with clinical phenotypes suggestive of MEN syndromes.

### Recommended Screening Protocols

#### MEN1 Mutation Carriers

Beginning at age 5, annual calcium and PTH should be measured. Starting at age 8, annual fasting glucose, insulin, gastrin, chromogranin A, prolactin, and IGF-1 are added, with pituitary MRI every 3-5 years. From age 10-15, cross-sectional imaging (CT or MRI) of the pancreas and abdomen is performed every 1-3 years, or endoscopic ultrasound. Starting at age 15-20, chest CT every 1-2 years screens for thymic and bronchial carcinoid.

#### MEN2 RET Mutation Carriers

For ATA-Highest risk (M918T), prophylactic thyroidectomy is recommended by 6 months of age, with annual metanephrines from age 8. For ATA-High risk (634, A883F), prophylactic thyroidectomy is recommended by age 5, with annual metanephrines from age 8 and annual calcium/PTH. For ATA-Moderate risk, thyroidectomy timing is guided by calcitonin monitoring and can be delayed if calcitonin remains normal, with annual metanephrines from age 16 and annual calcium/PTH from age 20.

## Key Clinical Pearls

- In MEN1, non-functioning pancreatic NETs >2 cm are the leading cause of disease-specific mortality; surveillance with imaging and surgical resection when tumors reach 2 cm is a critical management decision
- RET genotype-phenotype correlation in MEN2 is among the strongest in all of oncology; the specific RET codon mutation determines recommended age for prophylactic thyroidectomy, which is one of the most effective cancer prevention strategies in medicine
- Always screen for and treat pheochromocytoma BEFORE performing thyroidectomy in MEN2; undiagnosed pheochromocytoma during surgery can cause a fatal catecholamine crisis
- MEN2B (RET M918T) is the most aggressive form; 50% are de novo mutations with no family history; suspect in any child with mucosal neuromas, marfanoid habitus, or early-onset MTC
- Subtotal parathyroidectomy in MEN1 has a high recurrence rate (40-60%) because all parathyroid tissue is predisposed to hyperplasia; patients must be counseled about the likelihood of reoperation
- Consider MEN4 (CDKN1B mutations) in patients with a clinical MEN1 phenotype who test negative for MEN1 gene mutations; CDKN1B accounts for ~1-3% of MEN1-like presentations

## References

1. Thakker RV, et al. "Clinical Practice Guidelines for Multiple Endocrine Neoplasia Type 1 (MEN1)." J Clin Endocrinol Metab. 2012;97(9):2990-3011.
2. Wells SA, et al. "Revised American Thyroid Association Guidelines for the Management of Medullary Thyroid Carcinoma." Thyroid. 2015;25(6):567-610.
3. Brandi ML, et al. "Guidelines for Diagnosis and Therapy of MEN Type 1 and Type 2." J Clin Endocrinol Metab. 2001;86(12):5658-5671.
4. Pellegata NS, et al. "Germ-Line Mutations in p27Kip1 Cause a Multiple Endocrine Neoplasia Syndrome in Rats and Humans." Proc Natl Acad Sci USA. 2006;103(42):15558-15563.
5. Amodru V, et al. "MEN1: A Century-Old Disease and New Perspectives." Endocr Rev. 2024;45(1):1-34.