A 58-year-old male smoker with hypertension presents with incidental discovery of a 6 cm left renal mass on CT imaging. The mass is solid, heterogeneously enhancing, and hypervascular. Histopathology reveals the structure marked **A** — polygonal tumor cells with clear cytoplasm arranged in nests. Which of the following best explains the molecular pathogenesis underlying this histologic appearance?

Question

Accepted Answer

C. Inactivation of the VHL tumor suppressor gene leading to HIF accumulation and constitutive VEGF transcription, resulting in the highly vascular phenotype characteristic of clear cell RCC. ## Why option 1 is right

Clear cell renal cell carcinoma arises from the proximal convoluted tubule epithelium and is characterized by inactivation of the VHL tumor suppressor gene (lost in >90% of sporadic ccRCC). Loss of VHL protein prevents degradation of hypoxia-inducible factors (HIF-1α and HIF-2α), which accumulate and constitutively transcribe target genes including VEGF, PDGF, EPO, GLUT-1, CA9, and TGF-α. This molecular cascade produces the characteristic highly vascular tumor with abundant clear cytoplasm (from glycogen and lipid content) and distinct cell membranes seen in the histology marked **A**. This VHL-HIF-VEGF axis is the rational basis for targeted therapy with tyrosine kinase inhibitors and checkpoint immunotherapy in metastatic disease (Harrison's 21e Ch 84; KEYNOTE-426, CheckMate 9ER).

## Why each distractor is wrong

- **Option 2 (PBRM1 mutation)**: While PBRM1 is a driver mutation in ccRCC and is associated with worse prognosis, it is not the primary pathogenic mechanism explaining the clear cell histology or the highly vascular phenotype. PBRM1 loss occurs as a secondary event in many ccRCC cases.
- **Option 3 (mTOR activation independent of VHL)**: mTOR inhibitors (everolimus, temsirolimus) are used as targeted therapy in metastatic ccRCC, but mTOR activation is not the primary driver of ccRCC pathogenesis. VHL loss is the initiating event in >90% of cases.
- **Option 4 (SDH complex mutations)**: SDH mutations are characteristic of paraganglioma and pheochromocytoma, not ccRCC. This represents confusion with hereditary paraganglioma syndrome and is not relevant to the pathogenesis of clear cell RCC.

**High-Yield:** VHL inactivation → HIF accumulation → VEGF/angiogenesis = the molecular basis of ccRCC's clear cell appearance and high vascularity, and the rationale for anti-VEGF and checkpoint inhibitor therapy.

[cite: Harrison's 21e Ch 84; KEYNOTE-426 NEJM 2019; CheckMate 9ER 2021]

Answer

A. Activation of mTOR signaling pathway independent of VHL loss, promoting cell proliferation and glycogen accumulation

Answer

B. Hereditary mutation in the SDH complex genes causing pseudohypoxic state and lipid accumulation in tumor cells

Answer

D. Mutation of the PBRM1 gene causing loss of chromatin remodeling function and altered cell differentiation

Explanation

Why option 1 is right

Why each distractor is wrong

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