BCR-ABL and CML: Diagnosis, Treatment, and Monitoring

Chronic myeloid leukemia (CML) is driven by a single genetic abnormality called BCR-ABL, a fused gene created when pieces of chromosomes 9 and 22 swap places in a blood-forming cell. That swap produces what is known as the Philadelphia chromosome, and the protein it encodes acts as a permanently “on” growth signal that pushes white blood cells to multiply without control. The story of CML is one of the most dramatic in modern oncology: a once-fatal blood cancer transformed into a manageable chronic condition through drugs designed to block that one protein. But living well with CML depends on getting the diagnosis confirmed correctly, choosing the right targeted therapy, and tracking the disease at the molecular level for years.

How BCR-ABL Creates the Disease

In a healthy person, the ABL gene on chromosome 9 codes for a tightly regulated enzyme involved in cell growth. CML begins when a chunk of chromosome 9 breaks off and fuses with a piece of chromosome 22. The result is a shortened chromosome 22, the Philadelphia chromosome, carrying a new hybrid gene: BCR-ABL. That translocation, written in shorthand as t(9;22)(q34;q11), produces a fusion protein with tyrosine kinase activity that never shuts off.¹ The always-active kinase triggers a cascade of downstream signals that tell cells to keep dividing and to resist the normal self-destruct program that clears out damaged cells.²

Identifying the Philadelphia chromosome as the hallmark of CML was a landmark moment. Peter Nowell’s discovery in 1960 provided some of the earliest evidence that cancer could have a genetic origin.³ Decades later, researchers confirmed that BCR-ABL was not just associated with CML but was the cause of it, opening the door to a drug that could target the problem at its root.⁴

The Three Phases of CML

CML does not behave the same way at every stage. It progresses through three recognized phases, and where a patient falls on that spectrum shapes every treatment decision.

Most people are diagnosed in the chronic phase, when the disease is relatively slow-moving. White blood cell counts are elevated, the spleen may be enlarged, and patients sometimes feel fatigued, but the bone marrow still produces mostly mature cells. Without adequate treatment, a small percentage of patients each year, roughly one to four percent, progress to more dangerous territory.⁵

The accelerated phase is an intermediate stage marked by rising blast counts and additional chromosomal changes. Beyond that lies blast crisis, the most aggressive phase, which resembles acute leukemia and carries much higher risks of serious illness and death. Blast crises tend to be myeloid in most cases, though roughly a quarter are lymphoid, a distinction that matters for treatment planning.⁵ Progression to blast crisis involves more than just increasing BCR-ABL activity; additional genetic changes, problems with telomere maintenance, and epigenetic shifts all play a role.⁶ In the advanced phase of the disease, BCR-ABL-independent mechanisms also contribute to the malignant behavior of the cells, making treatment harder.²

How CML Is Diagnosed

A suspicious blood count, often discovered during routine bloodwork, is usually the first hint. The classic finding is a markedly elevated white blood cell count with an unusual spread of immature cells. From there, confirming the diagnosis means proving the Philadelphia chromosome is present.

Three main laboratory techniques are used. Conventional cytogenetics, or G-banding, examines chromosomes under a microscope to spot the characteristic shortened chromosome 22. Fluorescence in situ hybridization (FISH) uses fluorescent probes that light up when they bind to the BCR-ABL fusion, making it possible to detect the abnormality even in cells that are not actively dividing. Both tests can be performed on bone marrow or peripheral blood samples.⁷ The third technique, quantitative reverse-transcription PCR (RT-qPCR), detects BCR-ABL messenger RNA in the blood and is extremely sensitive. It confirms the diagnosis and establishes a baseline number that becomes essential for monitoring treatment.

A bone marrow biopsy is typically performed at diagnosis to assess the percentage of blast cells and look for additional chromosomal abnormalities beyond the Philadelphia chromosome. Those extra changes, if present, can signal higher risk and influence which therapy a doctor recommends first.

Tyrosine Kinase Inhibitors Changed Everything

Before the year 2001, CML was managed with interferon-alpha and chemotherapy, treatments that extended survival but rarely eliminated the disease. The arrival of imatinib, the first drug designed to block the BCR-ABL protein directly, turned CML from a near-certain death sentence into a condition most people live with for decades.⁸ Imatinib fits into the active site of the BCR-ABL kinase and prevents it from phosphorylating its downstream targets. Without those signals, the leukemic cells stop growing and die.

Second-generation tyrosine kinase inhibitors (TKIs) followed: dasatinib, nilotinib, and bosutinib. These were developed to work faster and to overcome some forms of resistance that crop up with imatinib. In patients who switched to a second-generation drug after an inadequate response to imatinib, dasatinib produced the highest rates of deep cytogenetic and molecular responses in one long-term study, followed by nilotinib and then bosutinib.⁹ All three are now approved for first-line use as well, giving doctors and patients more options from the start.

Ponatinib, a third-generation TKI, was developed specifically to address the T315I mutation, a single amino acid change in the BCR-ABL protein that blocks virtually all earlier TKIs from binding. More recently, asciminib introduced a different strategy: instead of competing for the kinase’s active site, it locks onto a separate pocket on the protein called the myristoyl-binding site. That allosteric mechanism makes it effective against many mutations that defeat other TKIs, though compound mutations (two mutations in the same protein copy) can still escape it.¹⁰

Monitoring Treatment With Molecular Testing

Prescribing a TKI is only the beginning. The real work of CML management is tracking how deeply the drug suppresses BCR-ABL over time, because the depth of molecular response guides every subsequent decision.

Monitoring relies on quantitative PCR testing of blood samples, usually every three months in the first year and at regular intervals after that. Results are reported on the International Scale (IS), a standardized system that allows BCR-ABL levels measured in one laboratory to be compared with those measured in another. Adoption of the IS was slow in the United States for years but is now used by the vast majority of testing labs.¹¹ Keeping that scale reliable requires laboratories to validate their own conversion factors against reference labs. In European standardization rounds, applying the correct conversion factor brought about 95% of results within an acceptable range of the expected value, compared to around 72% without it.¹²

The 2025 European LeukemiaNet (ELN) recommendations define specific milestones at 3, 6, and 12 months of therapy. Each milestone is a BCR-ABL level on the IS: roughly 10% at 3 months, 1% at 6 months, and 0.1% (called major molecular response, or MMR) at 12 months. Results falling below these thresholds are now labeled “favorable,” meaning there is no need to switch treatment. Results in a gray zone earn a “warning,” and results above the threshold are “unfavorable,” suggesting a switch to a different TKI is preferred.¹³ Data from the German CML Registry show that patients who hit those milestones on time are far more likely to reach deep molecular response later, with roughly 85% to 93% of milestone achievers going on to reach very low or undetectable BCR-ABL levels.¹⁴

For patients who reach very deep responses, the three-month BCR-ABL number reported by standard RT-qPCR can sit at or below the test’s detection limit, making it hard to distinguish between “very low” and “truly gone.” Digital PCR is emerging as a more sensitive alternative, able to detect BCR-ABL transcripts at levels 10 to 100 times lower than conventional methods. In one study, digital PCR and leukemic stem cell assays both found positive signals in samples that standard PCR called undetectable.¹⁵ Whether that extra sensitivity actually improves clinical decisions, particularly around stopping therapy, is still being worked out.¹⁶

When the Drug Stops Working

Not every patient responds to the first TKI indefinitely. Resistance is the primary reason treatment fails, and it falls into two broad categories.

The first and best understood is BCR-ABL-dependent resistance, driven by point mutations in the kinase domain of the protein. These mutations alter the shape of the drug-binding pocket so the TKI can no longer attach properly.¹⁷ The T315I “gatekeeper” mutation is the most notorious because it creates a bulky substitution that physically blocks most TKIs. Other mutations work more subtly, locking the kinase into a shape that certain drug classes cannot recognize.¹⁸ This is why knowing the specific mutation matters: different TKIs can overcome different mutations, and choosing the right one depends on identifying what change the leukemia has made.

Mutation testing has traditionally been done with Sanger sequencing, but this method has limited sensitivity and struggles to tell apart compound mutations (two changes in the same copy of BCR-ABL) from polyclonal mutations (different changes in different cell populations).¹⁹ Next-generation sequencing (NGS) is increasingly replacing Sanger in clinical labs. A validation study in an Indian cohort found about 95% agreement between NGS and Sanger results, with NGS picking up additional low-level mutations that Sanger missed.²⁰ The distinction between compound and polyclonal mutations is clinically relevant because compound mutations can render even the newest drugs ineffective.

Stopping Treatment and Treatment-Free Remission

For decades, the assumption was that CML patients would take a TKI for life. That assumption is changing. Treatment-free remission (TFR), the ability to stop taking the drug without the disease returning, is now a recognized goal of therapy for patients who achieve and sustain very deep molecular responses.

Current ELN guidelines consider stopping therapy for patients in chronic phase who have been on a TKI for at least five years (or four years for a second-generation drug) and have maintained a deep molecular response, ideally at a level called MR4 or deeper, for at least two years.²¹ NCCN guidelines allow a slightly shorter treatment duration of three years before considering a stop, though the deep-response requirement is similar.

Roughly half of patients who attempt TFR will eventually lose their molecular response and need to restart therapy. The DAstop2 trial, which studied patients attempting a second TKI discontinuation after relapsing from a first attempt and then regaining deep response on dasatinib, reported TFR rates of about 61% at six months, 56% at one year, and 46% at two years. No patients in the trial progressed to advanced disease, and the vast majority who restarted treatment re-achieved deep response within about three months.²² This means TFR is not an all-or-nothing gamble: patients who relapse can go back on the drug and regain control, making the attempt relatively safe under close molecular monitoring.

That monitoring is non-negotiable. After stopping a TKI, patients are typically tested monthly for the first six months and then every few months after that. Any confirmed rise in BCR-ABL triggers a restart. The need for this intensive surveillance is a practical burden, but it is what makes TFR feasible rather than reckless.

Cardiovascular Side Effects of TKIs

Turning a fatal cancer into a chronic illness means patients live long enough for long-term drug side effects to matter. Cardiovascular toxicity has emerged as one of the most important concerns, and it is not uniform across TKIs.

Dasatinib has been linked to pulmonary arterial hypertension, a condition where blood pressure in the lung arteries rises dangerously. Nilotinib and ponatinib, meanwhile, are associated with arterial vascular problems, including peripheral artery disease and, in some cases, heart attacks and strokes.²³ These risks are not abstract: they influence which TKI is chosen first, especially for patients who already have cardiovascular risk factors like diabetes, high cholesterol, or a history of vascular disease. A patient with pre-existing peripheral artery disease would typically be steered away from nilotinib or ponatinib.

Identifying who is most at risk before problems develop is an active area of research. Several cardiovascular risk prediction models have shown value for forecasting arterial events in CML patients on TKIs, and future work aims to refine these tools and explore whether any protective interventions can reduce the risk without compromising the drug’s cancer-fighting ability.²⁴ In practical terms, this means CML patients on long-term TKI therapy should have regular cardiovascular screening, and any new symptoms like leg pain, shortness of breath, or chest discomfort warrant prompt attention.

When Stem Cell Transplant Is Still Considered

With multiple generations of TKIs available, stem cell transplant has moved from the standard treatment for CML to a last resort reserved for a small group of patients. The number of transplants performed for chronic-phase CML has dropped dramatically since imatinib entered the picture.²⁵

Transplant still makes sense in specific situations: patients who have failed multiple lines of TKI therapy and have no remaining oral options, those with certain high-risk mutations that render all available drugs ineffective, and especially patients who present in or progress to blast crisis. For advanced-phase disease, transplantation remains the treatment of choice, though doctors typically start a TKI while the transplant is being arranged to control the disease in the interim.²⁶ The decision to transplant involves weighing the risk of transplant-related complications against the risk of untreatable or poorly treatable leukemia, a calculation that becomes more favorable the younger and healthier the patient.

CML in Children and Young Adults

CML is uncommon in children, but when it occurs, it tends to present more aggressively than in older adults. Kids are more likely to show up with very high white cell counts and larger spleens. The adult prognostic scoring systems that predict how someone will respond to treatment do not apply well to the pediatric population.²⁷

For a long time, pediatric CML was assumed to be biologically identical to the adult version, just occurring in a younger person. Emerging evidence challenges that assumption, pointing to some genetic differences between CML in children and CML in adults. The treatment approach still centers on TKIs, but the practical considerations differ. A ten-year-old starting imatinib faces the prospect of decades of daily medication, with unique concerns about growth, bone development, and the long-term cardiovascular effects discussed earlier. These factors make treatment-free remission an especially attractive goal in younger patients, though the eligibility criteria and success rates are still being studied specifically in this group.