# Clinical Cases: Signal Transduction Pathways

## Case 1: Pheochromocytoma (G-Protein and cAMP Signaling)

### Clinical Image
![Pheochromocytoma CT](case_01_image.jpg)
*Source: [Radiopaedia - Pheochromocytoma](https://radiopaedia.org/articles/phaeochromocytoma) - CC BY-NC-SA 3.0*

### Case Presentation
A 42-year-old woman presents with episodes of severe headache, palpitations, and diaphoresis occurring several times weekly. During episodes, her blood pressure is markedly elevated (220/120 mmHg). Between episodes, her blood pressure is normal. Physical examination during an episode reveals tachycardia, pallor, and tremor. 24-hour urine collection shows markedly elevated catecholamines and metanephrines. CT abdomen reveals a 4 cm right adrenal mass. She is diagnosed with pheochromocytoma. Before surgical resection, she requires alpha-adrenergic blockade (phenoxybenzamine) followed by beta-blockade (propranolol) to control hypertensive crises and prevent perioperative cardiovascular complications. The pathophysiology involves excessive catecholamine (epinephrine and norepinephrine) release from the adrenal medulla tumor. These catecholamines bind to adrenergic receptors (GPCRs), activating downstream signaling. Beta-receptors couple to Gs, activating adenylyl cyclase and increasing cAMP, which activates PKA and produces cardiac effects (increased rate and contractility). Alpha-1 receptors couple to Gq, activating phospholipase C to generate IP3 and DAG, causing calcium release and vasoconstriction. Surgical resection is curative. She is screened for hereditary syndromes (MEN2, VHL, SDH mutations) given approximately 40% of pheochromocytomas have genetic predisposition.

### Key Learning Points
- Catecholamines signal through GPCRs: beta-adrenergic receptors couple to Gs and cAMP/PKA pathway (cardiac effects), while alpha-1 receptors couple to Gq and PLC/IP3/calcium pathway (vasoconstriction)
- Beta-blockers (propranolol, metoprolol) are competitive antagonists at beta-adrenergic receptors, one of the most successful drug classes developed from understanding GPCR signaling
- Alpha-blockade must precede beta-blockade in pheochromocytoma to avoid unopposed alpha-mediated vasoconstriction and hypertensive crisis

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## Case 2: EGFR-Mutant Lung Cancer (Receptor Tyrosine Kinase Signaling)

### Clinical Image
![EGFR Mutant Lung Cancer](case_02_image.jpg)
*Source: [Wikipedia - Epidermal growth factor receptor](https://en.wikipedia.org/wiki/Epidermal_growth_factor_receptor) - CC BY-SA 3.0*

### Case Presentation
A 58-year-old never-smoking woman presents with cough and dyspnea. Chest CT reveals a large right lower lobe mass with mediastinal lymphadenopathy. CT-guided biopsy confirms lung adenocarcinoma. Molecular testing is performed and reveals an EGFR exon 19 deletion mutation. The oncologist explains that EGFR is a receptor tyrosine kinase (RTK) that normally requires ligand binding for activation. However, certain mutations cause constitutive, ligand-independent activation, driving uncontrolled proliferation through the RAS-MAPK and PI3K-AKT pathways. EGFR mutations are more common in never-smokers, women, and patients of Asian descent. She is started on osimertinib, a third-generation EGFR tyrosine kinase inhibitor (TKI) that binds the ATP-binding pocket of EGFR and blocks its kinase activity. Her tumor shows dramatic response, with significant shrinkage on follow-up imaging. After 18 months, her disease progresses. Repeat biopsy reveals an EGFR C797S resistance mutation that prevents osimertinib binding. She is transitioned to combination therapy targeting the resistance mechanism. This case illustrates both the success of targeted therapy and the challenge of acquired resistance.

### Key Learning Points
- EGFR is a receptor tyrosine kinase; ligand binding induces dimerization and trans-autophosphorylation, creating docking sites for adaptor proteins that activate RAS-MAPK and PI3K-AKT pathways
- Activating EGFR mutations cause constitutive signaling independent of ligand; EGFR TKIs (gefitinib, erlotinib, osimertinib) block kinase activity by competing for the ATP binding site
- Acquired resistance commonly develops through secondary mutations, pathway bypass, or phenotypic transformation, highlighting the need for repeat molecular testing at progression

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## Case 3: Polycythemia Vera (JAK2 V617F Mutation)

### Clinical Image
![Polycythemia Vera Blood Smear](case_03_image.jpg)
*Source: [Wikipedia - Polycythemia vera](https://en.wikipedia.org/wiki/Polycythemia_vera) - CC BY-SA 3.0*

### Case Presentation
A 65-year-old man presents with facial plethora, pruritus after bathing (aquagenic pruritus), and erythromelalgia (burning pain in his hands and feet). Complete blood count reveals hemoglobin 19.5 g/dL (elevated), hematocrit 58%, WBC 14,000/mm3, and platelets 550,000/mm3. Erythropoietin level is low. Bone marrow biopsy shows hypercellularity with trilineage proliferation. Genetic testing reveals the JAK2 V617F mutation, present in >95% of polycythemia vera cases. The hematologist explains the mechanism: cytokine receptors (including the erythropoietin receptor) lack intrinsic kinase activity and instead associate with JAK kinases. Normally, cytokine binding induces receptor dimerization, bringing JAKs together for trans-phosphorylation and activation, which then phosphorylates STATs that translocate to the nucleus to activate target genes. The V617F mutation in JAK2's pseudokinase regulatory domain causes constitutive kinase activity, leading to erythropoietin-independent proliferation of erythroid precursors. He is treated with phlebotomy to reduce hematocrit below 45%, low-dose aspirin, and eventually ruxolitinib (a JAK1/2 inhibitor) for symptom control and cytoreduction. JAK inhibitors block the constitutively active kinase, providing targeted therapy for myeloproliferative neoplasms.

### Key Learning Points
- The JAK-STAT pathway transmits signals from cytokine receptors that lack intrinsic kinase activity; JAKs phosphorylate STATs, which dimerize and translocate to the nucleus to regulate gene expression
- The JAK2 V617F mutation causes constitutive JAK2 activation, leading to cytokine-independent signaling and myeloproliferation; it is found in >95% of polycythemia vera and approximately 50% of essential thrombocythemia and myelofibrosis
- JAK inhibitors (ruxolitinib, fedratinib) provide targeted therapy for myeloproliferative neoplasms by blocking the constitutively active JAK2, demonstrating translation of signaling biology into clinical practice