A 14-year-old boy presents with a 2-week history of progressive dyspnea, cough, and chest pain. Imaging reveals a large anterior mediastinal mass. Bone marrow examination shows 45% blasts that are TdT+ and CD3+ (cytoplasmic). Cytogenetics and molecular testing identify the translocation marked **A** in the diagram — t(7;9)(q34;q34.3) — which juxtaposes the TCR-beta enhancer to a truncated NOTCH1. Which of the following best describes the pathogenic consequence of this translocation?
A. Fusion of TAL1 and STIL genes resulting in aberrant transcription factor expression
B. Inactivation of FBXW7 leading to stabilization of c-MYC and cyclin E proteins
C. Constitutive ligand-independent Notch signaling driving T-cell self-renewal and proliferation via MYC and HES1 pathways
D. Loss of tumor suppressor function through CDKN2A/B deletion on chromosome 9p21
Explanation
Why "Constitutive ligand-independent Notch signaling driving T-cell self-renewal and proliferation via MYC and HES1 pathways" is right
The t(7;9)(q34;q34.3) translocation is the classical molecular lesion in T-ALL that juxtaposes the TCR-beta enhancer (on chromosome 7q34) to a truncated NOTCH1 gene (on chromosome 9q34.3). This produces constitutive, ligand-independent activation of the intracellular Notch fragment (NICD), which drives uncontrolled T-cell self-renewal and proliferation through downstream transcriptional activation of MYC, HES1, and pre-TCR signaling pathways. This is the hallmark pathogenic mechanism of NOTCH1-activated T-ALL and is present in >50% of T-ALL cases. The translocation marked A directly produces this effect.
Why each distractor is wrong
Loss of tumor suppressor function through CDKN2A/B deletion on chromosome 9p21: While CDKN2A/B deletion is a recurrent lesion in ~70% of T-ALL cases and contributes to pathogenesis, it is NOT the direct consequence of the t(7;9) translocation. CDKN2A/B deletion is a separate, cooperating genetic event.
Fusion of TAL1 and STIL genes resulting in aberrant transcription factor expression: TAL1/STIL fusion is another recurrent lesion in T-ALL but is NOT produced by t(7;9). This fusion occurs through different chromosomal rearrangements and represents an alternative, non-NOTCH1-driven pathway.
Inactivation of FBXW7 leading to stabilization of c-MYC and cyclin E proteins: FBXW7 mutations are common in T-ALL and also stabilize NICD, but FBXW7 inactivation is NOT the direct consequence of t(7;9). FBXW7 mutations are point mutations or deletions, not translocation products.
