## RET Proto-oncogene Mutations in MEN 2A **Key Point:** MEN 2A is characterized by activating mutations in the RET proto-oncogene, most commonly in exon 11, which encodes the cysteine-rich extracellular domain. ### Mutation-Phenotype Correlation | MEN Syndrome | Most Common RET Mutation | Domain Affected | Clinical Features | | --- | --- | --- | --- | | MEN 2A | Exon 11 (codons 609, 611, 618, 620) | Cysteine-rich | MTC, Pheochromocytoma, Primary hyperparathyroidism | | MEN 2B | Exon 16 (codon 918) | Kinase domain | MTC, Pheochromocytoma, Mucosal neuromas, Marfanoid habitus | | Familial MTC | Exons 10, 11, 13, 14, 15 | Variable | MTC only | **High-Yield:** Exon 11 mutations in MEN 2A lead to cysteine substitutions that cause aberrant disulfide bonding and ligand-independent RET dimerization, resulting in constitutive kinase activation. ### Pathophysiology 1. Activating RET mutations cause gain-of-function 2. Ligand-independent receptor dimerization occurs 3. Constitutive autophosphorylation of tyrosine residues 4. Activation of downstream signaling (MAPK, PI3K/AKT pathways) 5. Neoplastic transformation of calcitonin-producing C cells **Clinical Pearl:** Genetic testing for RET mutations is mandatory in all patients with medullary thyroid carcinoma, and prophylactic thyroidectomy is recommended in childhood for carriers of activating RET mutations in MEN 2A families. **Mnemonic:** MEN 2A = **Exon 11** (cysteine), MEN 2B = **Exon 16** (kinase) — remember the numbers: 11 for 2A, 16 for 2B. [cite:Robbins 10e Ch 24] 
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