A 45-year-old man presents with fatigue, left upper quadrant fullness, and splenomegaly on examination. Complete blood count shows WBC 180,000/μL with left shift (myelocytes, metamyelocytes, and bands present), basophilia 8%, and hemoglobin 9.2 g/dL. Bone marrow biopsy confirms chronic myeloid leukemia with t(9;22) Philadelphia chromosome. The structure marked **C** in the diagram is responsible for the pathogenesis of this disease. Which of the following best describes the mechanism by which this structure drives unrestrained myeloid proliferation?

Explanation

## Why "Constitutive tyrosine kinase activity leading to unregulated downstream signaling via RAS, JAK-STAT, and PI3K pathways" is right The BCR-ABL1 fusion oncoprotein (marked **C**) arising from the t(9;22) Philadelphia chromosome possesses constitutive (unregulated) tyrosine kinase activity. Unlike the normal ABL1 kinase, which is tightly controlled and activated only in response to specific cellular signals, the BCR-ABL1 fusion protein is permanently "switched on." This constitutive kinase activity phosphorylates multiple downstream substrates, activating the RAS, JAK-STAT, and PI3K/AKT signaling cascades. These pathways drive unrestrained myeloid cell proliferation, survival, and reduced apoptosis—the hallmark of CML pathogenesis. This mechanism is directly supported by Robbins Pathology 10e (Ch 13) and Harrison Principles of Internal Medicine 21e (Ch 102). ## Why each distractor is wrong - **Loss of tumor suppressor function through deletion of the TP53 gene on chromosome 17**: TP53 mutations are characteristic of acute leukemias and lymphomas, not the primary driver of CML. While TP53 loss may occur in CML blast crisis, it is not the initiating mechanism of chronic-phase disease. The Philadelphia chromosome and BCR-ABL1 kinase activity, not TP53 loss, define CML pathogenesis. - **Activation of the NOTCH signaling pathway resulting in enhanced self-renewal of hematopoietic stem cells**: NOTCH pathway dysregulation is implicated in T-cell acute lymphoblastic leukemia (T-ALL) and some myeloid malignancies, but it is not the mechanism of BCR-ABL1–driven CML. The anchor structure (BCR-ABL1) operates through tyrosine kinase signaling, not NOTCH. - **Overexpression of the MYC oncogene due to translocation to the immunoglobulin heavy chain locus**: MYC translocation is the hallmark of Burkitt lymphoma (t(8;14)), not CML. While MYC may be dysregulated secondarily in CML blast crisis, it is not the primary oncogenic driver of chronic-phase disease. **High-Yield:** BCR-ABL1 constitutive tyrosine kinase activity is the defining oncogenic lesion in CML; imatinib (Gleevec), the first-generation tyrosine kinase inhibitor, competitively binds the ATP-binding site and revolutionized CML treatment from a uniformly fatal disease to a chronic manageable condition with >80% 10-year survival. [cite: Robbins and Cotran Pathologic Basis of Disease, 10e, Ch 13 (Hematopoietic and Lymphoid Neoplasms); Harrison's Principles of Internal Medicine, 21e, Ch 102 (Chronic Myeloid Leukemia)]