View source for WBR0113

{{WBRQuestion
|QuestionAuthor=William J Gibson
|ExamType=USMLE Step 1
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry, Genetics
|SubCategory=Hematology, General Principles
|Prompt=A 46 year old man is being followed for Chronic Myelogenous Leukemia.  His oncologist would like to follow his patient’s response to imatinib and monitor for signs of relapse.  Which of the following tests would be best suited for this purpose?
|Explanation=Polymerase Chain Reaction (PCR) is a method which can be used to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence.  PCR is exquisitely sensitive and is able to detect the presence of rare template sequences of DNA.  In this case, the oncologist is seeking to 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 CML.  Only in cells in which the fusion was 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.

References: 
Tags: #Genetics #Hematology #Biochemistry
|AnswerA=Fluorescent In-Situ Hybridization
|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 fluorescently labeled DNA probes that are uniquely complimentary 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 which may signal relapse are present, they are likely very rare and would be extremely difficult to identify via FISH.
|AnswerB=Quantitative PCR with primers mapping to chromosome 8 and chromosome 14
|AnswerBExp=The t(8:14) translocation which causes MYC to be placed adjacent to the IgH locus is seen in [[Burkitt’s lymphoma]], not CML.
|AnswerC=Quantitative PCR with primers mapping to chromosome 22 and chromosome 9
|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]].
|AnswerD=Southern Blot for fusion gene with probes mapping to chromosome 22 and chromosome 9
|AnswerDExp=[[Southern blot]]s 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.
|AnswerE=Northern blot for fusion gene with probes mapping to chromosome 8 and chromosome 14
|AnswerEExp=[[Northern blots]] are used to detect the presence of certain DNA segments. While the BCR-ABL genomic fusion may be detectable in a bulk tumor via Northern blot (if the precise breakpoint was known), the detection of rare tumor cells would require a much more sensitive method.
|EducationalObjectives=Quantit
|References=First Aid 2014 page 81, First Aid 2012 page 84<br>

Michor F, Hughes TP, Iwasa Y, et al. Dynamics of chronic myeloid leukaemia. Nature. 2005;435(7046):1267-70.

Branford S, Rudzki Z, Parkinson I, et al. Real-time quantitative PCR analysis can be used as a primary screen to identify patients with CML treated with imatinib who have BCR-ABL kinase domain mutations. Blood. 2004;104(9):2926-32.
|RightAnswer=C
|WBRKeyword=Chronic myelogenous leukemia, 
|Approved=Yes
}}