Which chromosomal abnormality is the hallmark of chronic myeloid leukemia and results from a reciprocal translocation between chromosomes 9 and 22?

Question

Accepted Answer

C. t(9;22) Philadelphia chromosome. ## Philadelphia Chromosome in CML

**Key Point:** The Philadelphia chromosome, resulting from t(9;22) translocation, is present in >95% of CML cases and is pathognomonic for the disease.

### Molecular Mechanism
The translocation juxtaposes the ABL gene (chromosome 9) with the BCR gene (chromosome 22), creating the BCR-ABL fusion gene. This produces a constitutively active tyrosine kinase that drives uncontrolled myeloid proliferation.

### Clinical Significance
- **Diagnostic criterion:** Present in chronic phase, accelerated phase, and blast crisis
- **Prognostic value:** Its presence defines CML; absence suggests atypical CML or other myeloproliferative neoplasm
- **Therapeutic target:** Tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib) specifically target BCR-ABL

**High-Yield:** The Philadelphia chromosome is the FIRST chromosomal abnormality linked to a specific malignancy and revolutionized targeted therapy in oncology.

**Clinical Pearl:** Patients with Philadelphia-negative CML have worse prognosis and different treatment response patterns compared to Philadelphia-positive CML.

Answer

A. t(15;17) translocation

Answer

B. t(8;14) translocation

Answer

D. Trisomy 12

Which chromosomal abnormality is the hallmark of chronic myeloid leukemia and results from a reciprocal translocation between chromosomes 9 and 22?

Explanation

Philadelphia Chromosome in CML

Molecular Mechanism

Clinical Significance

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