BCR/ABL, p210, mRNA Detection, Reverse Transcription-PCR (RT-PCR), Quantitative, Monitoring Chronic Myelogenous Leukemia (CML)
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Chronic myelogenous leukemia (CML) is a hematopoietic stem cell neoplasm included in the broader diagnostic category of myeloproliferative neoplasms. CML is consistently associated with fusion of the breakpoint cluster region gene (BCR) at chromosome 22q11 to the Abelson gene (ABL) at chromosome 9q23. This fusion is designated BCR/ABL and may be seen on routine karyotype as the Philadelphia chromosome.
Although various breakpoints within the BCR and ABL genes have been described, >95% of CMLs contain mRNA in which either the BCR exon 13 (e13) or BCR exon 14 (e14) is fused to the ABL exon 2 (a2), yielding fusion forms e13/a2 and e14/a2, respectively. The e13/a2 and e14/a2 fusion forms produce a 210-kDa protein (p210). The p210 fusion protein is an abnormal tyrosine kinase known to be critical for the clinical and pathologic features of CML, and agents that block the tyrosine kinase activity, such as imatinib, have been used successfully for treatment. It has been shown that monitoring the level of BCR/ABL mRNA (bcr/abl) in CML patients during treatment is helpful for both prognosis and management of therapy.(1-3)
Quantitative reverse-transcription PCR (qRT-PCR) is the most sensitive method for monitoring bcr/abl levels during treatment. This test detects the 2 most common bcr/abl fusion forms found in CML (e13/a2 and e14/a2).
Monitoring response to therapy in patients with chronic myeloid leukemia who are known to have the e13/a2 or e14/a2 fusion forms
An interpretive report will be provided. When bcr/abl mRNA is present, quantitative results are converted to an international scale, which can be directly compared with results reported in the IRIS (International Randomized Study of Interferon versus STI571) trial involving newly diagnosed chronic myeloid leukemia patients. Using the international scale, a result <0.1% bcr/abl(p210):abl is equivalent to a major molecular remission. For further discussion of the international scale, see Clinical Reference number 1.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
This test detects only the e13/a2 and e14/a2 fusion forms, which code for the p210 protein. Other fusion forms are not detected, including those containing the BCR e1 exon, which codes for the p190 protein commonly found in acute lymphoblastic leukemia (ALL). If the patient is known to carry an e1/a2 (p190) fusion form, BA190 / BCR/ABL, p190, mRNA Detection, Reverse Transcription-PCR (RT-PCR), Quantitative, Monitoring Assay should be used for monitoring.
This test should not be used to screen for bcr/abl fusions at the time of diagnosis; BADX / BCR/ABL, mRNA Detection, Reverse Transcription-PCR (RT-PCR), Qualitative, Diagnostic Assay, which is designed to detect all reported and theoretical BCR/ABL fusion variants, should be ordered for this purpose.
The precision of this assay at low bcr/abl levels is relatively poor, such that inter-run variation can be as high as 0.5 log. Only level changes >0.5 log should be considered clinically significant. For example, if a result is given as 0.1% bcr/abl:abl, then any result between 0.05% and 0.5% should be considered essentially equivalent. If the results are being used to make major therapeutic decisions, significant changes during monitoring should be verified with a subsequent specimen.
In general, the results of this assay cannot be directly compared with results generated from other PCR assays, including identical assays performed in other laboratories. Monitoring should be performed using the same method and laboratory for each subsequent specimen. However, the results may be directly compared with results obtained from laboratories reporting results using the international scale.
The results of this assay cannot be directly compared with bcr/abl results obtained using FISH technology. FISH measures DNA alleles and PCR-based assays measure mRNA transcripts. Because a single DNA allele can produce many mRNA transcripts, the values are not directly comparable.
Blood is the specimen of choice for monitoring. While most patients show similar bcr/abl levels in blood and bone marrow drawn at the same time, some patients have a consistent difference in the levels in blood and bone marrow such that alternating specimen types during monitoring can lead to confusion.
Assay precision does not appear to be significantly affected by specimen transport or moderate delays in processing. However, in specimens with very low levels of bcr/abl these conditions may cause sufficient RNA degradation to produce false-negative results. Thus, specimens should be shipped as quickly as possible and specimens >3 days old at the time of receipt will be considered unacceptable.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
The presence or absence of BCR/ABL mRNA (bcr/abl) fusion form producing the p210 fusion protein is reported. If positive, the level is reported as the ratio of bcr/abl (p210) to abl with conversion to a percentage (ie, bcr/abl (p210) as a percentage of total abl). The result is then converted to the international scale.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Branford S, Fletcher L, Cross NCP, et al: Desirable performance characteristics for BCR-ABL measurement on an international reporting scale to allow consistent interpretation of individual patient response and comparison of response rates between clinical trials. Blood 2008 October 15;111(8):3330-3338
2. Hughes TP, Kaeda J, Branford S, et al: Frequency of major molecular responses to imatinib or interferon alfa plus cytarabine in newly diagnosed chronic myeloid leukemia. N Engl J Med 2003 October 9;349(15):1423-1432
3. Hughes TP, Deininger M, Hochhaus A, et al: Monitoring CML patients responding to treatment with typrosine kinase inhibitors: review and recommendations for harmonizing current methodology for detecting BCR-ABL transcripts and kinase domain mutations and for expressing results. Blood 2006 July 1;108(1):28-37
4. Radich JP, Gooley T, Bryant E, et al: The significance of bcr/abl molecular detection in chronic myeloid leukemia patients "late," 18 months or more after transplantation. Blood 2001 September 15;98(6):1701-1707
5. Olavarria E, Kanfer E, Szydlo R, et al: Early detection of BCR-ABL transcripts by quantitative reverse transcriptase-polymerase chain reaction predicts outcome after allogeneic stem cell transplantation for chronic myeloid leukemia. Blood 2001 March 15;97(6):1560-1565