BCR-ABL1 is an abnormal gene present in certain types of leukemia. Genetic testing for BCR-ABL1 helps doctors diagnose these cancers and assess their response to treatment.

If you or someone you know has recently received a leukemia diagnosis, you may have heard about BCR-ABL1. But what exactly is BCR-ABL1? Simply put, BCR-ABL1 is an oncogene, a mutated gene that can cause cancer.

BCR-ABL1 forms when two of your chromosomes accidentally swap DNA during cell division. When this occurs, the ABL1 gene on chromosome 9 fuses with the BCR gene on chromosome 22, creating a new fused gene.

The BCR-ABL1 oncogene codes for a protein that causes cells to grow and divide uncontrollably, leading to leukemia. Healthcare professionals can test for BCR-ABL1 to diagnose and monitor the treatment of some types of leukemia.

This article looks at how the BCR-ABL1 genetic test is used to diagnose, monitor, and treat leukemia.

Doctors often recommend a BCR-ABL1 test alongside other diagnostic tests if you have signs and symptoms of leukemia. These symptoms include but aren’t limited to:

The BCR-ABL1 genetic test looks for the presence of the BCR-ABL1 oncogene in your cancer cells. To test for BCR-ABL1, a healthcare professional will collect a blood or bone marrow sample:

  • Blood sample: A blood sample is typically taken from a vein in your arm.
  • Bone marrow sample: A bone marrow sample is often collected from your hip bone, although it’s possible for other bones to be used as well. A bone marrow sample is taken during a two-step process called an aspiration and biopsy:
    • An aspiration collects a small amount of liquid bone marrow.
    • The biopsy that follows takes a solid piece of bone and bone marrow.

Three tests can look for BCR-ABL1:

  • cytogenetics
  • FISH test
  • PCR test

The sections below provide more information about these individual tests.

Cytogenetics

The DNA swap that creates the BCR-ABL1 oncogene also shortens chromosome 22, a change called the Philadelphia (Ph) chromosome. Healthcare professionals can see this change through karyotyping.

In a karyotype, cells from a sample are grown in a lab until they start dividing. An image is then taken of all of the chromosomes in a dividing cell.

Healthcare professionals can then evaluate the image to check for changes, such as the Ph chromosome, which contains the BCR-ABL1 oncogene.

FISH test

A fluorescence in situ hybridization (FISH) test uses a special florescent dye specific for BCR-ABL1. When the dye binds to the gene, it lights up under a microscope.

A healthcare professional can also quantify the amount of fluorescence to get an idea of the amount of BCR-ABL1 and, by extension, the amount of cancer present in a sample.

Because it doesn’t have to wait for cells to grow, a FISH test takes less time than cytogenetics. Due to this, it may be done alongside cytogenetics. A FISH test may also be done if cytogenetics doesn’t reveal the Ph chromosome.

PCR test

A polymerase chain reaction (PCR) test uses a very sensitive molecular probe to find and bind to the BCR-ABL1 gene in a sample. It’s often used alongside other tests, including if cytogenetics didn’t show the Ph chromosome.

Like the FISH test, the PCR test results can be quantified, giving healthcare professionals information about how much BCR-ABL1 is present. Because this test is very sensitive, it’s often used to monitor leukemia treatment.

How accurate are FISH and PCR?

FISH and PCR are both very sensitive and specific for BCR-ABL1.

Sensitivity refers to the ability of the test to accurately detect BCR-ABL1 in a sample. Conversely, specificity refers to the ability of the tests to rule out people who don’t have BCR-ABL1.

However, PCR can detect very low levels of BCR-ABL1. Because of this, it’s typically a better test for monitoring the presence of disease after treatment.

BCR-ABL1 is mainly associated with chronic myeloid leukemia (CML). In fact, an estimated 90% to 95% of people with CML have the Ph chromosome, which contains BCR-ABL1.

Although less common, BCR-ABL1 can also be present in other types of leukemia. These include acute lymphocytic leukemia (ALL) and acute myeloid leukemia (AML).

Researchers estimate that 25% of adults and 3% to 5% of children with ALL have the BCR-ABL1 oncogene.

BCR-ABL1 is much rarer in AML. For example, the Ph chromosome is only found in 0.5% to 3% of these cancers.

Mixed phenotype acute leukemia (MPAL), a type of leukemia that has features of myeloid and lymphoid leukemias, can also have BCR-ABL1. The Ph chromosome is found in about 25% of these cancers.

BCR-ABL1 testing helps doctors diagnose leukemia and assess treatment plans. Looking at the levels of BCR-ABL1 or Ph chromosome can give a doctor an idea of how your treatment is working.

This is particularly important for CML. The frontline treatment for CML is targeted therapy against the protein made by the BCR-ABL1 gene. This includes drugs like imatinib (Gleevec), dasatinib (Sprycel), and several more.

For CML, a doctor will examine three different categories to check on your treatment. Two of these categories involve levels of BCR-ABL1 or the Ph chromosome. They can use this information to determine whether your current treatment is effective or if your cancer is resistant to it:

  • Hematologic response: A hematologic response is mainly measured by complete blood count. Doctors use this information to assess the levels of different blood cells to see whether they’re returning to normal in response to treatment.
  • Cytogenetic response: A cytogenetic response is measured using cytogenetics or FISH to determine the amount of Ph chromosome or BCR-ABL1 present in a sample. If the levels of these markers decrease, it means there are fewer leukemia cells.
  • Molecular: A molecular response is assessed by using PCR to see the levels of BCR-ABL1 in a sample. Lower levels of BCR-ABL1 in a PCR test signal lower levels of cancer and a molecular response.

The goal of treatment is to establish remission. If your cancer does go into remission, your doctor will still want to follow up with you periodically to check for recurrence.

BCR-ABL1 tests are used to look for recurrence as well.

BCR-ABL1 is an oncogene that forms when DNA in two chromosomes swaps places while cells are dividing. The effects of BCR-ABL1 lead cells to grow and divide uncontrollably.

The presence of BCR-ABL1 is associated with several types of leukemia — mainly CML but also ALL and AML less commonly. Several tests, including cytogenetics, FISH, and PCR, can be used to detect BCR-ABL1.

BCR-ABL1 testing is used to help diagnose leukemia if you’re having symptoms like anemia, fever, and unintended weight loss. It’s also a vital tool for monitoring your response to treatment and checking for recurrence.