The Test Sample
What is being tested?
BCR-ABL1 refers to a gene sequence found in an abnormal chromosome 22 of some people with certain forms of leukemia. Unlike most cancers, the cause of chronic myelogenous leukemia (CML) and some other leukemias can be traced to a single, specific genetic abnormality in one chromosome. The presence of the gene sequence known as BCR-ABL1 confirms the diagnosis of CML and a form of acute lymphoblastic lymphoma (ALL).
Humans have 23 pairs of chromosomes containing inherited genetic information. Those genes contain the blueprints, in the form of DNA, for producing the proteins that our bodies rely on to function properly. While some genetic abnormalities are inherited, they can also come from changes that occur to genes or chromosomes after a person is born. This can happen through exposure to various environmental factors (e.g., radiation, certain chemicals) but more often for unknown reasons.
The BCR-ABL1 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 ABL1 region in chromosome 9 fuses with the BCR gene region in chromosome 22. This type of change is called a reciprocal translocation and is often abbreviated as t(9;22). The resulting chromosome 22 that has the BCR-ABL1 gene sequence is known as the Philadelphia (Ph) chromosome because that is where it was first discovered.
The resulting Philadelphia chromosome or BCR-ABL1 gene encode an abnormal protein that is responsible for the development of CML and a type of ALL. At diagnosis, 90-95% of cases of CML show a characteristic t(9;22) BCR-ABL1 reciprocal chromosomal translocation. About one in four adults with ALL have the translocation.
The protein formed by BCR-ABL1 is a type of enzyme called a tyrosine kinase. That enzyme is responsible for the uncontrolled growth of leukemic cells. When large numbers of abnormal leukemic cells start to crowd out the normal blood cell precursors in the bone marrow, signs and symptoms of leukemia start to emerge. Treatment of these leukemias typically involves a tyrosine kinase inhibitor (TKI).
Testing for BCR-ABL1 detects the Philadelphia chromosome and BCR-ABL1 fusion gene or its transcripts, which are the RNA copies made by the cell from the abnormal stretches of DNA. The presence of the BCR-ABL1 abnormality confirms the clinical diagnosis of CML, a type of ALL, and rarely acute myeloid leukemia (AML).
There are several different types of BCR-ABL1 tests available, including:
- Cytogenetics (chromosome analysis or karyotyping)
This test looks at chromosomes under a microscope to detect structural and/or numerical abnormalities. For example, the Philadelphia chromosome looks shorter than normal. 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 fluorescent dye-labeled probes to "light up" the BCR-ABL1 gene sequence when it is present. It can also determine the percentage of blood or bone marrow cells that contain the abnormal, fused BCR-ABL1 gene.
- Genetic molecular testing (qualitative or quantitative)
Polymerase chain reaction (PCR)-based qualitative and quantitative tests detect and measure the BCR-ABL1 gene in leukemia cells taken from blood or bone marrow samples.
- Secondary mutations within the BCR-ABL1 are known to cause resistance to therapy. These can be detected by DNA sequencing methods.
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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.