## MEN1 Genetic Basis **Key Point:** MEN1 is caused by loss-of-function mutations in the MENIN gene located on chromosome 12q13. This gene encodes a tumor suppressor protein that acts as a transcriptional regulator. **High-Yield:** The MENIN protein interacts with histone methyltransferases and acts as a scaffold protein in chromatin remodeling complexes. Loss of MENIN function leads to dysregulation of gene expression and uncontrolled cell proliferation in endocrine tissues. ## MEN1 Clinical Triad The classic presentation follows the "3 P's": | Feature | Frequency | Pathology | |---------|-----------|----------| | **P**arathyroid adenoma/hyperplasia | 90–95% | Primary hyperparathyroidism | | **P**ancreatic neuroendocrine tumor | 60–70% | Gastrinoma (most common functional), insulinoma, non-functional | | **P**ituitary adenoma | 30–40% | Prolactinoma (most common), growth hormone-secreting | **Clinical Pearl:** Screening for MEN1 should begin in childhood (age 8 years) in affected families, even before clinical manifestations appear. Genetic testing is diagnostic and allows for early surveillance. **Mnemonic: "3 P's of MEN1"** — **P**arathyroid, **P**ancreas, **P**ituitary. Remember: MENIN = chromosome 12 (not RET, which is MEN2). 
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