To help diagnose and monitor the treatment of chronic myeloid leukaemia (CML) and a type of acute lymphoblastic leukaemia (ALL)
BCR-ABL refers to a gene sequence found in an abnormal chromosome 22 of some people with certain forms of leukaemia. Humans normally have 23 pairs of chromosomes, including 22 pairs of non-sex-determining chromosomes (also known as autosomes) and 1 pair of sex chromosomes (XX for females, XY for males). Chromosomes contain a person's inherited genetic information. The genes that reside there form the blueprints for the production of countless proteins. Sometimes changes can occur to a person's chromosomes and/or genes during their lifetime, because of exposures to radiation, toxins, or for unknown reasons.
The BCR-ABL gene sequence is one such acquired change that is formed when pieces of chromosome 9 and chromosome 22 break off and switch places. When this occurs, the ABL region in chromosome 9 fuses with the BCR gene region in chromosome 22. This is referred to as reciprocal translocation and this particular one is commonly expressed as t(9;22). A derivative chromosome 22 that has the BCR-ABL gene sequence is known as the Philadelphia (Ph) chromosome. It is strongly associated with chronic myeloid leukaemia (CML) and, less commonly with one type of acute lymphoblastic leukaemia (ALL) and rarely with acute myeloid leukaemia. At diagnosis, around 90-95% of cases of CML are found to have the Philadelphia chromosome. All cases have the BCR-ABL fusion gene, including the Philadelphia chromosome-negative cases. In this small fraction of cases the genes have fused on a more microscopic scale, not involving large chromosomal segments.
These BCR-ABL gene sequences at the fusion site encode an abnormal protein. The abnormal BCR-ABL protein is a tyrosine kinase enzyme, a normal signalling protein which has become permanently switched “on” and is responsible for the development of CML and a type of ALL. When large numbers of abnormal leukaemic cells start to crowd out the normal blood cell precursors in the bone marrow, signs and symptoms of leukaemia start to emerge. Treatment of these leukaemias typically involves a tyrosine kinase inhibitor (TKI), given as daily tablets.
Testing for BCR-ABL detects the Ph chromosome (the derivative chromosome 22) and BCR-ABL fusion gene or its transcripts, the RNA copies made by the cell from the abnormal stretches of DNA. The presence of the BCR-ABL abnormality confirms the clinical diagnosis in CML or a type of ALL. These are distinguished by the typical appearances of the cells through a microscope and, in the case of ALL, by flow cytometry (or “immunophenotyping”), which looks at molecules expressed inside the cells or more commonly expressed on the leukaemic cell surfaces.
There are several different types of BCR-ABL test available, including:
- Cytogenetics (Chromosome analysis or Karyotyping)
This test looks at chromosomes under a microscope to detect structural and/or numerical abnormalities. Cells in a sample of blood or bone marrow are grown in the laboratory and then examined to determine if the Philadelphia chromosome is present. Other chromosomal abnormalities can also be detected.
- Fluorescence in situ hybridization (FISH)
This test method uses differently-coloured fluorescent dye-labelled probes to "light up" the BCR and the ABL gene sequences. Cells are examined under a microscope to determine the proportion if any where the coloured dots are fused together, indicating a BCR-ABL translocation.
- Genetic molecular testing, qualitative or quantitative
The polymerase chain reaction (PCR)-based qualitative and quantitative tests detect and measure BCR-ABL gene transcripts, or gene product units, in a patient's blood and/or bone marrow samples. This is the most sensitive test for BCR-ABL and is used to diagnose the minority of cases negative for the Philadelphia chromosome. It is also used to check the response to tyrosine kinase inhibitors (TKIs). Variant mutations of BCR-ABL can also be analysed if indicated. These mutations are responsible for treatment resistance to TKIs. If the response to treatment is suboptimal, mutational analysis may tell your doctor which TKIs will be more effective.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm or a bone marrow sample is collected using a bone marrow aspiration and/or biopsy procedure.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
BCR-ABL testing is requested to detect the Philadelphia (Ph) chromosome and BCR-ABL gene sequence. It is used to help diagnose chronic myeloid leukaemia (CML) and a type of acute lymphocytic leukaemia (ALL) in which the BCR-ABL gene sequence is present (BCR-ABL positive), to monitor response to treatment, check for treatment-resistant mutations and to monitor for disease recurrence.
A chromosome analysis, the qualitative BCR-ABL molecular genetic test, and/or (FISH) may be requested to help establish the initial diagnosis of CML or Ph-positive ALL. It is often performed along with other tests if a doctor suspects that a patient has leukaemia and is trying to diagnose or rule out CML and Ph-positive ALL. The chromosomal analysis and FISH can also help to determine what percentage of patient's blood or bone marrow cells is affected.
The qualitative BCR-ABL test can also help determine the breakpoint variant of BCR-ABL that is being produced. The size and weight of the BCR-ABL protein produced depends upon where the break in chromosome 22 occurred. In CML, the breakpoint in BCR is almost always in the major breakpoint cluster region (M-BCR), leading to the production of BCR-ABL protein of a larger size (p210). Breaks in the minor breakpoint cluster region (m-BCR) leads to a shorter fusion protein (p190), which is most frequently associated with Ph-positive ALL. Knowing whether a patient has a break in the major or minor BCR is important because the quantitative BCR-ABL molecular genetic test may be set up to measure a specific variant, p210 or p190, but not both.
The quantitative BCR-ABL molecular test is requested once the BCR-ABL gene sequence has been detected and the breakpoint variant established. It may be requested at the time of the initial diagnosis to establish a baseline value and then used periodically to monitor the person's response to treatment and, if the person achieves remission, to monitor for recurrence.
When is it requested?
BCR-ABL testing is requested when a doctor suspects that a person has CML or Ph-positive ALL. Initial testing may be indicated when a person has non-specific symptoms such as fatigue, weight loss and/or an enlarged spleen or as a follow-up to abnormal findings on a FBC. Early in the disease, a person may have few or no symptoms. As time passes and normal blood cells are crowded out of the bone marrow and the number of abnormal leukaemic cells increases, a person may experience anaemia, prolonged bleeding, and recurrent infections.
Once CML has been diagnosed, BCR-ABL quantitative genetic testing is requested periodically (typically every 3 months) to monitor the response to treatment and monitor for recurrence. If disease levels become very low and stable (a major molecular response) then the frequency of testing may change to 3-6 months though this will be determined by your doctor. If treatment resistance or disease recurrence occurs, the BCR-ABL kinase domain mutation analysis should be performed to guide further treatment.
For patients with Ph chromosome-positive ALL, BCR-ABL quantitative testing will be requested periodically to coincide with treatment and as specified in any clinical trial that the patient may have enrolled in.
What does the test result mean?
If a person has abnormal white blood cells in the bone marrow and has the Ph chromosome and BCR-ABL gene sequence, then that person has CML or Ph-positive ALL. Of those who have CML, 90-95% have the Ph chromosome by cytogenetics and 100% have BCR-ABL gene by FISH and/or qualitative BCR-ABL molecular testing. About 25% of adult ALL and 3% of childhood ALL are positive for the Ph chromosome and/or BCR-ABL gene sequence.
A small percentage of patients with CML will have the BCR-ABL gene sequence but not the Ph chromosome. These cases either have variant translocations that involve a third or even a fourth chromosome in addition to 9 and 22, or have a hidden translocation involving 9 and 22 that cannot be identified by routine chromosomal analysis. Since the treatment for BCR-ABL-related leukaemias specifically targets the tyrosine kinase protein produced, these patients can still be monitored with quantitative BCR-ABL molecular testing.
In general, if the amount of BCR-ABL in the blood or bone marrow decreases over time, then the person is responding to treatment. If the quantity of BCR-ABL drops below the test's detection level and the person's blood cell counts are normal, then the patient is considered to be in remission. If BCR-ABL levels rise, then it indicates disease progression or recurrence. It may also indicate that the person has become resistant to the tyrosine kinase inhibitor (TKI) being used to treat them. Additional genetic testing is often performed to detect the development of BCR-ABL kinase domain mutations associated with resistance to different TKIs. If a patient is determined to be resistant to their TKI tablet, a different tyrosine kinase inhibitor may be given.
If a patient with ALL is not positive for the Ph chromosome and BCR-ABL gene sequence, then that person will not be given a tyrosine kinase inhibitor drug and BCR-ABL molecular testing cannot be used to monitor the patient.
Is there anything else I should know?
Recognition of disease progression and transformation is important for prognosis and treatment. CML goes through three phases:
- Chronic phase—most patients with CML are diagnosed in chronic phase, which usually has an insidious onset. The chronic phase may last for several years. This is the phase when there are few or no symptoms and also the time period when treatment is most successful.
- Accelerated phase—changes include but are not limited to increasing white blood cell (WBC) counts, unstable platelet counts, blood basophils equal to or greater than 20%, additional cytogenetic changes, an increase in blasts in blood and/or bone marrow (but less than 20%) and lack of therapeutic response to standard treatment.
- Blast phase—when blasts equal or are greater than 20% of the blood white cells and/or of the nucleated (containing a nucleus) cells in the bone marrow, or when there is blast proliferation outside the bone marrow (apart from in the spleen).
Both blood and bone marrow are often evaluated as part of the initial diagnosis, but the majority of follow-up monitoring is performed on blood samples. There is significant test variability among laboratories using different test platforms. Therefore, for a given patient, the quantitative BCR-ABL molecular testing should be done by the same laboratory or referred to a reference or accredited laboratory that follows universal reporting criteria. Rising and falling levels of BCR-ABL are usually more important than a single test result.
Why would a chromosome analysis be done more than once?
If I have the Ph chromosome and BCR-ABL gene, should my close family members be tested?
Should everyone with leukaemia be tested?
Can BCR-ABL testing be done in my doctor's surgery?
What is the difference between chronic myelogenous leukaemia, chronic granulocytic leukaemia and chronic myeloid leukaemia?
On This Site
Tests: Philadelphia chromosome; Fluorescent In Situ Hybridisation (FISH); FBC; Bone Marrow Aspiration and Biopsy; Blood Film; Differential
Features: The Universe of Genetic Testing
Elsewhere On The Web
MacMillan Cancer Suppport – Chronic Myeloid Leukaemia
Leukaemiacare.org.uk – Chronic Myeloid Leukaemia Information Booklet
Blood Cancer UK – Chronic Myeloid leukaemia