Cytogenetic studies in a 70-year-old woman with chronic myelogenous leukemia (CML) demonstrate a t(9;22) chromosomal translocation. Which of the following best explains the role of this translocation in the pathogenesis of leukemia in this patient?
Cytogenetic studies in a 70-year-old woman with chronic myelogenous leukemia (CML) demonstrate a t(9;22) chromosomal translocation. Which of the following best explains the role of this translocation in the pathogenesis of leukemia in this patient?
💡 Explanation
## **Core Concept**
The question revolves around the understanding of the **Philadelphia chromosome**, a characteristic genetic abnormality resulting from a **reciprocal translocation between chromosomes 9 and 22**, denoted as t(9;22). This translocation is a hallmark of **Chronic Myelogenous Leukemia (CML)** and some cases of **Acute Lymphoblastic Leukemia (ALL)**. The translocation leads to the fusion of two genes, creating a **chimeric gene** that encodes a **tyrosine kinase** with abnormal activity.
## **Why the Correct Answer is Right**
The correct answer, , involves the creation of the **BCR-ABL fusion gene**. This gene results from the juxtaposition of parts of the **BCR (Breakpoint Cluster Region) gene** on chromosome 22 with the **ABL1 gene** on chromosome 9. The BCR-ABL fusion gene encodes a **constitutively active tyrosine kinase** that promotes **uncontrolled proliferation** and **survival of leukemic cells**. This abnormal tyrosine kinase activity is central to the pathogenesis of CML, as it leads to the **uncontrolled growth of mature and immature granulocytes**.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe the role of the t(9;22) translocation in CML. Without specifying the BCR-ABL fusion, it lacks the crucial detail about the chimeric gene and its product.
- **Option B:** This option is incorrect as it implies a different mechanism or gene involvement not related to the BCR-ABL fusion, which is the established cause of CML in the context of t(9;22).
- **Option D:** This option is incorrect because it suggests an alternative genetic mechanism or pathway not directly related to the formation of the BCR-ABL fusion gene and its consequences in CML.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **BCR-ABL tyrosine kinase** is a **druggable target**. The development of **tyrosine kinase inhibitors (TKIs)**, such as **imatinib**, has revolutionized the treatment of CML by specifically targeting the BCR-ABL protein, thereby controlling the disease in many patients.