DESCRIPTION - RNA Analysis
BCR/ABL-QUAL should be ordered to screen whole blood or bone marrow from patients suspected of Chronic Myeloid Leukemia (CML) or Acute Lymphocytic Leukemia (ALL) for the presence of the bcr/abl transcript and is part of the initial molecular testing for this translocation. This test can also be used to distinguish between the major and minor transcripts, independent of an identifiable Philadelphia chromosome, and as an adjunct to cytogenetics or FISH. The major transcript, characterized by the p210 fusion gene product, is diagnostic for 95% of CML. The minor transcript, characterized by the p190 fusion gene product, is observed in a subset of ALL (10-30%). The assay is qualitative and should not be used for monitoring patients for minimal residual disease.
The most common mechanisms of acquired resistance to Imatinib/Gleevac are BCR-ABL amplification at the genomic or transcript level and point mutations in the kinase domain. Several BCR-ABL kinase domain mutations have been reported. The Y253, E255, T315 and M351 mutations account for approximately 50-60% of those detected at the time of relapse. Structural data suggest that these mutations may interfere with imatinib binding to the ABL kinase domain by interrupting critical contact points or by inducing a conformation to which drug binding is reduced. Some mutants, such as T315I and E255K, are insensitive to imatinib at clinically achievable doses and suggest withdrawal of imatinib treatment in favor of alternative therapeutic strategies. Other mutants, such as M351T or Y253F, retain intermediate levels of sensitivity to imatinib and may be overcome by dose escalation. Thus, to optimize therapeutic response, not only the presence of a mutation but also the actual amino-acid change should be investigated in patients displaying hematologic or cytogenetic resistance to Imatinib. A recent report also suggested (a) that in late-CP and AP patients treated with Imatinib, mutations can be detected by mutation scanning before clinical evidence of resistance, thus predicting the subsequent course of the disease; (b) that mutations in the nucleotide binding loop of the kinase region (P-loop) are associated with a particularly poor prognosis; and (c) that patients with longer duration of CML before initiation of Imatinib therapy and patients who fail to achieve a major cytogenetic response (MCgR) in the first 6 months of therapy are at high risk of developing mutations and resistance and should be regularly monitored. For these reasons, routine testing for emerging mutations should be performed to assure rational therapeutic management of CML patients.
Related Tests: BCR-ABL-SCAN
| Test Order Code | BCR-ABL-QUAL |
|
CPT Codes |
83891 x 1, 83902 x 1, 83898 x 2, 83894 x 2, 83912 x 1 |
| Indications For Testing |
|
|
Turn Around Time |
Up to 5 days. Expedited analysis is available upon request for an additional charge. |
| Specimen Requirements |
Peripheral Blood (preferred) All peripheral blood collections require PAXGene® RNA collection tubes or Transgenomic’s RNA Analysis Kits only. A sample submission kit with PAXGene® RNA collection tubes and shipping information can be obtained from Transgenomic Labs by contacting the Client Services Coordinator (1-866-500-GENE).
Bone Marrow: 2-3 mL of bone marrow in a PAXGene® RNA collection tube will be also accepted for testing. |
| Specimen Kits | Contact the Client Services Coordinator (1-866-500-GENE) for details. |
| Shipping and Contact Information |
Transgenomic Molecular Laboratory 12325 Emmet Street Omaha, NE 68164 USA Phone: (866) 500-GENE / (866) 500-4363 Fax: (402) 452-5447 E-mail: labservice@transgenomiclabs.com |
| Test Submission and Patient Consent Forms | |
| Test Methodology | Reverse Transcriptase-PCR and high-sensitivity nucleic acid fragment analysis. Scanning and sequence analysis of the entire coding regions of both BCR and ABL genes. |
| Sensitivity | The test has been shown to detect BCR-ABL transcripts in sample preparations that contain 1 positive cell in a background of 1x105 cells. |
| Limitations | Extraction of RNA from samples is dependent upon the quality and quantity of the initial sample. Inadequate RNA extraction may occur from samples not prepared and shipped correctly. Results of this test must always be interpreted in the context of morphologic and other relevant data, and should not be used alone for a diagnosis of malignancy. A negative result does not preclude the presence of BCR-ABL transcripts below the limit of detection. |
| References |
OMIM Entries for:
BCR-ABL
| 608232 | LEUKEMIA, CHRONIC MYELOID; CML |
| 151140 | BREAKPOINT CLUSTER REGION; BCR |
| 189980 | ABELSON MURINE LEUKEMIA VIRAL ONCOGENE HOMOLOG 1; ABL1 |
Additional Background
Chronic Myeloid Leukemia (CML) is characterized by the presence of the Philadelphia Chromosome (Ph) in 95% of disease cases. Ph is a reciprocal translocation between the long arms of chromosomes 9 and 22 that transposes the 3’ segment of the ABL gene from 9q34 to the 5’ segment of the BCR gene on 22q11. The rearranged BCR-ABL gene, t(9;22)(q34;q11) is transcribed into chimeric BCR-ABL mRNA, and although the translocation breakpoints are widely dispersed within the ABL first intron, a great majority of CML patients exhibit spliced mRNA transcripts with Major BCR/ABL breakpoints of b2/a2 and/or b3/a2, which are either BCR exon b2 or b3 fused to ABL exon a2, respectively. Both b2a2 and b3a2 transcripts translate into a 210-kD fusion protein (p210), which leads to leukemogenesis of CML in 95% of cases and of Acute Lymphocytic Leukemia (ALL) in 10-30% of cases. The other breakpoints of b2a3, b3a3, and e6a2 are possible, but are very infrequent. The minor BCR/ABL translocation has a breakpoint that the e1 exon of BCR fuses with a2 exon of ABL to form an e1b2 transcript encoding a smaller fusion protein, p190. The minor BCR/ABL is detectable in 50 to 70% of Ph-positive acute leukemias and lymphoid blast phase of CML, but rarely in CML in chronic phase. The rare e19a2 fusion transcript is found in cases of CML with prominent neutrophilic maturation. These unusual neoplasms also have been reported in the literature by some investigators as chronic neutrophilic leukemia. An alternative breakpoint e1b3 is possible but also rare. A representation of the various BCR-ABL fusion transcripts associated with CML and ALL is shown below.
A representation of the BCR and ABL gene structures with the corresponding BCR-ABL fusion transcripts. The arrows designate three different breakpoint cluster regions in the BCR gene. Breaks in the M-BCR are most commonly associated with CML, the m-BCR with ALL, and the µ-BCR with chronic neutrophilic leukemia. ABL breakpoints usually occur between exons 1a and 2, resulting in fusion transcripts containing the tyrosine kinase domain of the ABL gene and variable portions of the BCR gene.
Other Test
BCR/ABL-SCAN Another molecular test (is appropriate for a patient diagnosed with CML characterized by the t(9;22) chromosomal translocation resulting in the formation of the BCR-ABL fusion gene, and is undergoing or considering treatment with imatinib. Mutations in the BCR-ABL fusion gene are the most commonly reported mechanism of acquired resistance to imatinib therapy. Results from this mutation scanning test will assist physicians in predicting patient relapse before it happens and help them make appropriate adjustments in treatment.
Note: The performance characteristics of this test were validated by TRANSGENOMIC Molecular Laboratory. The U.S. Food and Drug Administration (FDA) has not approved this test. However, FDA approval is currently not required for clinical use of this test. The results are not intended to be used as the sole means for clinical diagnosis or patient management decisions. TRANSGENOMIC Molecular Laboratory is authorized under Clinical Laboratory Improvement Amendments (CLIA) to perform high-complexity testing.