A 58-year-old Indian male presents with painless lymphadenopathy and splenomegaly. Lymph node biopsy shows follicular lymphoma with t(14;18) translocation. The structure marked **A** in the diagram (Bcl-2 family proteins) is overexpressed due to placement of the BCL2 gene under the control of the immunoglobulin heavy chain promoter. Which of the following best explains the pathogenic consequence of this translocation and the rationale for targeted therapy?

Explanation

## Why option 1 is correct The t(14;18) translocation in follicular lymphoma places the BCL2 gene under the control of the immunoglobulin heavy chain promoter, resulting in constitutive overexpression of Bcl-2. Bcl-2 is a canonical anti-apoptotic protein that prevents mitochondrial outer membrane permeabilization (MOMP) by inhibiting pro-apoptotic Bcl-2 family members (Bax, Bak). This blockade prevents cytochrome c release from mitochondria, which is essential for formation of the apoptosome and activation of caspase 9 in the intrinsic pathway. By preventing apoptosis, Bcl-2 overexpression allows malignant B-cells to survive indefinitely, resulting in indolent but incurable follicular lymphoma. Venetoclax is a selective Bcl-2 inhibitor that binds the BH3-binding groove of Bcl-2, displacing pro-apoptotic proteins and restoring mitochondrial permeability and apoptotic capacity. This mechanism is supported by Robbins 10e Ch 2 (cell death and adaptation) and Ch 13 (lymphoid neoplasms). ## Why each distractor is wrong - **Option 2**: Bcl-2 overexpression does NOT enhance caspase 3 activation; rather, it blocks the intrinsic pathway upstream of caspase 3 by preventing cytochrome c release and apoptosome formation. Caspase 3 is an executioner caspase (structure D in the diagram) that is activated downstream of Bcl-2 inhibition, not by Bcl-2 overexpression. - **Option 3**: The extrinsic pathway (Fas/FasL, structure C) is not the primary mechanism of Bcl-2-mediated lymphomagenesis. Follicular lymphoma arises from dysregulation of the intrinsic (mitochondrial) pathway. While Bcl-2 can also inhibit extrinsic pathway caspases, the t(14;18) translocation specifically affects intrinsic pathway regulation. - **Option 4**: While p53 mutations are common in lymphomas, they are not the direct consequence of Bcl-2 overexpression from t(14;18). The translocation does not cause p53 mutations; rather, Bcl-2 overexpression is sufficient to prevent apoptosis independently of p53 status. Venetoclax works by inhibiting Bcl-2 directly, not by restoring p53 function. **High-Yield:** t(14;18) → Bcl-2 overexpression → blocked MOMP → no cytochrome c release → no apoptosome → follicular lymphoma; venetoclax restores apoptosis by inhibiting Bcl-2. [cite: Robbins and Cotran Pathologic Basis of Disease, 10th edition, Ch 2 (Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death) and Ch 13 (Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus)]