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PubMed This is a summary of 19 peer-reviewed journal articles Updated
Hematology

The Biology of CML: Why Your Cells Grow Out of Control

At a Glance

Chronic Myeloid Leukemia (CML) is driven by the Philadelphia chromosome, a genetic swap that creates the abnormal BCR-ABL1 gene. This gene constantly signals bone marrow to overproduce white blood cells. Doctors confirm the diagnosis using specialized tests like Karyotyping, FISH, and RT-PCR.

Chronic Myeloid Leukemia (CML) is fundamentally different from many other cancers because its biological cause is remarkably specific. While most cancers involve a chaotic mess of many different genetic mutations, CML is almost always driven by a single, well-defined event: the creation of the Philadelphia chromosome [1][2]. Understanding this biology is key to understanding why CML is so treatable today.

The Biological “Glitch”: BCR-ABL1

Your body normally uses proteins called tyrosine kinases to send signals that tell cells when to grow and divide. In a healthy person, these “switches” are carefully regulated—they turn on when a new cell is needed and off when the job is done.

In CML, two pieces of DNA from different chromosomes (9 and 22) break off and swap places, forming the Philadelphia chromosome [1][3]. This swap fuses two genes together into a new, abnormal gene called BCR-ABL1 [4]. This fusion gene produces a “broken” tyrosine kinase that is permanently stuck in the “on” position [1]. It relentlessly signals your bone marrow to produce an endless supply of white blood cells, which eventually crowd out healthy blood cells [5][6].

How CML Differs from Other Leukemias

It is helpful to distinguish CML from other common types of leukemia, as the treatment and outlook are very different:

  • CML vs. Acute Leukemias (AML/ALL): Acute leukemias involve a sudden buildup of very immature cells called blasts that cannot function at all [7][8]. In contrast, CML is a “chronic” or “indolent” disease. In the early stages, the white blood cells produced are mostly mature and can still perform some of their jobs, which is why CML often progresses much more slowly [9][10].
  • CML vs. CLL: Chronic Lymphocytic Leukemia (CLL) involves a different type of white blood cell (lymphocytes) and does not involve the BCR-ABL1 gene [8].

Confirming the Diagnosis: The Three Key Tests

Because CML looks like other blood disorders under a microscope, doctors must use specialized genetic tests to confirm the presence of the BCR-ABL1 “glitch.” For details on interpreting these results, see Reading Your “Scorecard”.

  1. G-banded Karyotyping: This is a visual map of your chromosomes. Doctors look through a microscope at your bone marrow cells to see if the physical swap of chromosomes 9 and 22 is visible [3][11]. This test is vital because it can also spot other chromosomal changes that might affect your prognosis [12].
  2. FISH (Fluorescence In Situ Hybridization): This test uses glowing “probes” that stick to the BCR and ABL1 genes. If the two colors overlap, it confirms they have fused together [13]. FISH is faster than karyotyping and can find the fusion even if the chromosomes look normal under a microscope [14][15].
  3. RT-PCR (Reverse-Transcription Polymerase Chain Reaction): This is the most sensitive test. It can detect even tiny amounts of the BCR-ABL1 “instruction manual” (the transcript) in your blood [16][17]. Once you start treatment, this test becomes your primary indicator to see how well the medicine is working [18][19].

By identifying the BCR-ABL1 gene, your medical team can confirm your diagnosis and immediately start the targeted therapy that specifically turns off this biological switch.

Common questions in this guide

What causes Chronic Myeloid Leukemia (CML)?
CML is almost always caused by a specific genetic change called the Philadelphia chromosome. This occurs when pieces of chromosomes 9 and 22 swap places, creating a new, abnormal gene known as BCR-ABL1.
How is CML different from other types of leukemia?
Unlike acute leukemias that cause a sudden buildup of non-functioning cells, CML is a slow-growing cancer where the body overproduces white blood cells that still partially function. It also differs from CLL because it is specifically driven by the BCR-ABL1 gene mutation.
What tests are used to confirm a CML diagnosis?
Doctors use three main tests to confirm CML: G-banded karyotyping to visually map your chromosomes, FISH testing to spot the fused genes, and RT-PCR to detect tiny amounts of the BCR-ABL1 gene in your blood.
Why do I need an RT-PCR test for CML?
RT-PCR is a highly sensitive molecular test used to find the BCR-ABL1 gene in your blood. Once you begin targeted therapy, this test is used regularly to measure your molecular response and show exactly how well the medication is working.
Why does my doctor look for 'blasts' in my bone marrow?
Blasts are immature white blood cells. Measuring the exact percentage of blasts in your bone marrow helps doctors determine the specific stage of your CML and decide on the most appropriate treatment plan.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my diagnosis involve any additional chromosomal abnormalities beyond the Philadelphia chromosome?
  2. 2.Which specific molecular test (RT-PCR, FISH, or Karyotyping) will we use to track my progress every three months?
  3. 3.Is my RT-PCR being reported on the 'International Scale' (IS) so we can compare my results accurately over time?
  4. 4.Are there any rare fusion transcript variants in my case that might make RT-PCR testing less accurate?
  5. 5.What was the exact percentage of 'blasts' (immature cells) in my bone marrow, and what does that tell us about my stage of CML?

Questions For You

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References

References (19)
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    Exploration of treatment-free remission in CML, based on molecular monitoring.

    Zhang Z, Zhou X, Zhou X, et al.

    Cancer medicine 2024; (13(1)):e6849 doi:10.1002/cam4.6849.

    PMID: 38133525
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    An interesting case of chronic myeloid leukemia (CML) with T315I mutation raising suspicion of de novo AML, a diagnostic conundrum.

    Iqbal P, Shahzad A, Shahid Z, et al.

    Clinical case reports 2023; (11(5)):e5908 doi:10.1002/ccr3.5908.

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This page explains the biology and diagnostic testing of CML for educational purposes only. Always consult your hematologist or oncologist for medical advice and to discuss your specific test results.

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