A 38-year-old Indian woman presents with symptomatic hypercalcemia (serum calcium 11.8 mg/dL, PTH 145 pg/mL) and is found to have parathyroid hyperplasia on imaging. Genetic testing reveals a germline loss-of-function mutation in the gene located at the locus marked **B** in the diagram. Which of the following best describes the molecular mechanism by which this mutation predisposes to tumor development in MEN1 syndrome?

Question

Accepted Answer

C. Inactivation of a nuclear scaffolding protein that normally interacts with histone-modifying complexes and acts as a tumor suppressor, requiring a somatic second-hit for tumorigenesis (two-hit model). ## Why option 1 is correct

The locus marked **B** (chromosome 11q13) encodes the MEN1 gene, which produces menin—a nuclear scaffolding protein that interacts with histone-modifying complexes (notably MLL/KMT2A-COMPASS), JunD, and other transcription factors to regulate gene expression. Menin functions as a classic tumor suppressor. Germline loss-of-function mutations in MEN1 follow Knudson's two-hit model: the inherited mutation is present in all cells, but tumorigenesis requires a somatic second-hit (typically loss of heterozygosity) that inactivates the remaining wild-type allele. This biallelic inactivation is what drives the development of the characteristic tumors (parathyroid hyperplasia, pancreatic neuroendocrine tumors, and pituitary adenomas) seen in MEN1 syndrome. This mechanism is the defining molecular feature of MEN1 and is directly cited in Harrison 21e and the Endocrine Society MEN1 CPG 2012.

## Why each distractor is wrong

- **Option 2**: RET mutations on chromosome 10q12.1 cause MEN2 syndrome (medullary thyroid carcinoma and pheochromocytoma), not MEN1. This is a common confusion between the MEN syndromes, but the diagram explicitly marks **B** as chromosome 11q13 (MEN1), not the RET locus.
- **Option 3**: TP53 mutations (chromosome 17p13, marked **C**) cause Li–Fraumeni syndrome and are associated with a broad spectrum of cancers, not the specific MEN1 triad. While TP53 loss impairs apoptosis, it is not the primary defect in MEN1.
- **Option 4**: VHL mutations (chromosome 3p21, marked **D**) cause von Hippel–Lindau syndrome, characterized by renal cell carcinoma, pheochromocytoma, and hemangioblastomas. VHL loss leads to HIF-1α accumulation, not the menin-mediated transcriptional dysregulation seen in MEN1.

**High-Yield:** MEN1 = menin (chromosome 11q13) = nuclear scaffolding protein + tumor suppressor + two-hit model; MEN2 = RET (chromosome 10q12.1) = gain-of-function + medullary thyroid carcinoma.

[cite: Harrison 21e Ch 384; Endocrine Society MEN1 CPG 2012]

Answer

A. Loss of p53 function leading to impaired cell-cycle checkpoint control and apoptosis

Answer

B. Defective von Hippel–Lindau protein resulting in constitutive HIF-1α signaling and renal cell carcinoma

Answer

D. Constitutive activation of a receptor tyrosine kinase (RET) that drives medullary thyroid carcinoma and pheochromocytoma

Explanation

Why option 1 is correct

Why each distractor is wrong

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