WBR0113
| Author | [[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D. and Rim Halaby, M.D. [1])]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Biochemistry, MainCategory::Genetics |
| Sub Category | SubCategory::Hematology, SubCategory::General Principles |
| Prompt | Prompt::A 46-year-old man is being followed for chronic myelogenous leukemia (CML). His oncologist would like to assess his patient’s response to imatinib and monitor for early signs of relapse. Which of the following tests is optimal for this purpose? |
| Answer A | AnswerA::Fluorescent In-Situ Hybridization (FISH) |
| Answer A Explanation | [[AnswerAExp::Fluorescent in situ hybridization (FISH) is used to detect and localize the presence or absence of specific DNA sequences on chromosomes. Cells are permeabilized and exposed to fluorescent-labeled DNA probes that are complementary to a specific sequence of DNA of interest. While FISH is a specific test for chromosomal translocations, it is laborious and has poor sensitivity. If cancer cells are present, they are likely to be very rare, and would be extremely difficult to identify via FISH.]] |
| Answer B | AnswerB::Quantitative PCR with primers mapping to chromosome 8 and chromosome 14 |
| Answer B Explanation | [[AnswerBExp::The t(8:14) translocation, which causes c-MYC to be located adjacent to the IgH locus, is characteristic of Burkitt’s lymphoma.]] |
| Answer C | AnswerC::Quantitative PCR ( with primers mapping to chromosome 22 and chromosome 9 |
| Answer C Explanation | [[AnswerCExp::Quantitative PCR is a sensitive method to identify the presence and quantity of segments of DNA. In this case, the oncologist is searching for the BCR-ABL t(9:22) translocation which causes CML.]] |
| Answer D | AnswerD::Northern Blot for fusion gene with probes mapping to chromosome 22 and chromosome 9 |
| Answer D Explanation | [[AnswerDExp::Northern blots are used to detect the presence of certain RNA segments. While the BCR-ABL transcript may be detectable in a bulk tumor via southern blot (if the precise breakpoint was known), the detection of rare tumor cells would require a much more sensitive method.]] |
| Answer E | AnswerE::Southern blot for fusion gene with probes mapping to chromosome 8 and chromosome 14 |
| Answer E Explanation | [[AnswerEExp::The t(8:14) translocation, which causes c-MYC to be located adjacent to the IgH locus, is characteristic of Burkitt’s lymphoma.]] |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::Quantitative real-time reverse transcription-polymerase chain reaction (Q-RT-PCR) is a method to measure gene expression. The technique amplifies one or a small number of DNA across several orders of magnitude, producing hundreds of thousands to millions of copies of this DNA sequence. Q-RT-PCR is exquisitely sensitive and is able to detect the presence of rare template sequences of DNA.
Thus, in this case the oncologist is seeking to identify segments of DNA which carry a fusion of the BCR gene from chromosome 22 with the ABL gene from chromosome 9. This fusion gene causes chronic myelogenous leukemia (CML). Only in cells in which the fusion is found would a PCR amplify a template. When the different segments of DNA to which the primers anneal are not located on the same linear strand of DNA, no template will be amplified between the two primers. In clinical practice, Q-RT-PCR is widely used to evaluate the BCR-ABL/ABL ratio in peripheral blood of patients with CML undergoing imatinib therapy. The technique is used to assess for response and to monitor for early signs of relapse. The ratio trend is clinically helpful in identifying patients who might or might not benefit from the therapy. The use of Q-RT-PCR for the evaluation of therapeutic response among patients with CML is analogous to the use of Q-RT-PCR to detect HIV viral load among HIV-positive patients receiving anti-retroviral therapy. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Chronic myelogenous leukemia, WBRKeyword::Cancer, WBRKeyword::Leukemia, WBRKeyword::Molecular biology, WBRKeyword::Genetics, WBRKeyword::PCR, WBRKeyword::Imatinib, WBRKeyword::Quantitative PCR, WBRKeyword::Response rate, WBRKeyword::Ratio, WBRKeyword::BCR, WBRKeyword::ABL |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |