Myeloproliferative neoplasm defined by the Philadelphia chromosome (BCR-ABL1) — transformed by targeted TKI therapy into a chronic, manageable disease.
Also known as: CML, chronic myeloid leukemia, chronic myelogenous leukemia, Philadelphia chromosome
Overview
Clonal myeloproliferative neoplasm of hematopoietic stem cell origin, defined by the Philadelphia chromosome — t(9;22)(q34;q11) reciprocal translocation creating the BCR-ABL1 fusion gene encoding a constitutively active tyrosine kinase. Progresses through chronic phase → accelerated phase → blast crisis if untreated.
Epidemiology
Annual incidence ~1-2 per 100,000. Median age at diagnosis ~65 years. Accounts for ~15% of adult leukemias. With TKI therapy, life expectancy approaches that of the general population.
Try two board-style Chronic Myeloid Leukemia questions
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Question 1HematologyMedium
A 54-year-old man presents with fatigue, early satiety, and a 10-pound weight loss over three months. On exam, the spleen is palpable 8 cm below the left costal margin. Laboratory studies show WBC 142,000/microL with a left-shifted differential including myelocytes, metamyelocytes, basophilia, and eosinophilia, hemoglobin 11.2 g/dL, and platelets 480,000/microL. Leukocyte alkaline phosphatase score is low. Which of the following tests will most likely confirm the diagnosis?
ABCR-ABL1 fusion testing
BJAK2 V617F mutation testing
CFlow cytometry for CD5/CD19 coexpression
DSerum protein electrophoresis
Reveal answer & full explanation
Correct answer: A — BCR-ABL1 fusion testing
ABCR-ABL1 fusion testing✓
BJAK2 V617F mutation testing
CFlow cytometry for CD5/CD19 coexpression
DSerum protein electrophoresis
Why BCR-ABL1 fusion testing is correct
This middle-aged patient has the classic chronic myelogenous leukemia (CML) presentation: massive splenomegaly, marked leukocytosis with the full spectrum of granulocyte precursors, basophilia, and eosinophilia.
A low leukocyte alkaline phosphatase score separates CML from a leukemoid reaction and from other myeloproliferative neoplasms, which typically show a normal or high score.
CML is defined by the t(9;22) Philadelphia chromosome, which fuses BCR and ABL1; detecting the BCR-ABL1 fusion by FISH, RT-PCR, or cytogenetics confirms the diagnosis and identifies the target for tyrosine kinase inhibitor therapy.
Why the others are wrong
JAK2 V617F mutation testing — JAK2 V617F is the molecular hallmark of polycythemia vera and is also seen in essential thrombocythemia and primary myelofibrosis - conditions that typically lack the marked left-shifted granulocytosis and basophilia of CML (confused-with another myeloproliferative neoplasm).
Flow cytometry for CD5/CD19 coexpression — Coexpression of CD5 and CD19 identifies chronic lymphocytic leukemia, which presents with lymphocytosis and smudge cells, not a left-shifted myeloid series (buzzword-matching 'leukemia' to the lymphoid lineage).
Serum protein electrophoresis — Serum protein electrophoresis detects monoclonal gammopathies such as multiple myeloma or MGUS and does not address a myeloid leukocytosis (premature closure on an unrelated paraproteinemia).
Question 2HematologyMedium
A 45-year-old man presents with fatigue, pallor, and splenomegaly. CBC shows WBC 88,000/µL with a shift toward myeloid precursors, Hgb 9.2 g/dL, and platelets 620,000/µL. Peripheral smear shows myelocytes, metamyelocytes, and basophilia. LDH is elevated. BCR-ABL fusion gene is detected by polymerase chain reaction (PCR). What is the first-line treatment?
ARituximab
BCytarabine and daunorubicin
CImatinib
DHydroxyurea
Reveal answer & full explanation
Correct answer: C — Imatinib
ARituximab
BCytarabine and daunorubicin
CImatinib✓
DHydroxyurea
Why Imatinib is correct
Chronic myeloid leukemia (CML) is defined by BCR-ABL (Philadelphia chromosome t(9;22))
Imatinib is a BCR-ABL tyrosine kinase inhibitor (TKI) and is first-line for chronic-phase CML
Imatinib achieves cytogenetic remission in more than 80% of patients
Why the others are wrong
Rituximab — targets B-cell malignancies (e.g., CD20+ lymphomas/leukemias); CML is a myeloid disorder
Cytarabine and daunorubicin — used in acute myeloid leukemia (AML) induction, not chronic-phase CML
Hydroxyurea — offers temporary WBC control but does not achieve cytogenetic remission and is not first-line definitive therapy
Additional high-yield points
Newer TKIs (dasatinib, nilotinib) are options for imatinib intolerance or resistance
Bone marrow transplant is reserved for blast crisis or TKI failure
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No clear familial or environmental factors otherwise
Rare association with prior chemotherapy
Not associated with chronic infection or autoimmune disease
Pathophysiology
Acquired t(9;22) translocation fuses BCR (chromosome 22) and ABL1 (chromosome 9) to form the BCR-ABL1 oncoprotein, a constitutively active tyrosine kinase that drives proliferation, blocks apoptosis, and confers genomic instability. The result is expansion of myeloid lineage with preserved differentiation in chronic phase (leukocytosis with full spectrum of mature and immature granulocytes), progressing to accelerated phase (genomic instability, secondary mutations) and blast crisis (acute leukemia phenotype, either myeloid 70% or lymphoid 30%).
Clinical presentation
Symptoms
Often asymptomatic — diagnosed on incidental leukocytosis (~40-50% of cases)
Fatigue, malaise, weight loss, low-grade fever, night sweats (constitutional symptoms)
Early satiety, abdominal fullness from splenomegaly
Bone pain (marrow expansion), splenic infarct pain
Hyperviscosity symptoms with very high WBC (>200K): visual changes, priapism, stroke, TIA
Blast crisis: bleeding, infection, organ infiltration
Signs / physical exam
Splenomegaly (massive in advanced disease; one of the largest causes of palpable spleen)
Hepatomegaly (less prominent)
Pallor (anemia in advanced disease)
Lymphadenopathy uncommon in chronic phase (suggests progression)
Sternal tenderness
Classic findings
Middle-aged adult with marked leukocytosis (often 100-500K), left-shifted granulocytosis with all stages of myeloid maturation, basophilia, and splenomegaly.
Differential diagnosis
Leukemoid reaction — Reactive granulocytosis from infection/inflammation; LAP (leukocyte alkaline phosphatase) HIGH (LOW in CML); no Philadelphia chromosome; identifiable trigger
Other myeloproliferative neoplasms (PV, ET, MF) — JAK2 V617F (or CALR, MPL) mutation; no BCR-ABL1; distinct hematologic patterns
Chronic neutrophilic leukemia — Rare; CSF3R mutation; no BCR-ABL1
Atypical CML BCR-ABL1 negative — Granulocytic proliferation with dysplasia; CSF3R or SETBP1 mutations
Acute leukemia (CML blast crisis) — >20% blasts; lymphoid or myeloid blast crisis; antecedent CML history
Diagnostic workup
Diagnostic criteria
BCR-ABL1 fusion (by cytogenetics t(9;22), FISH, or PCR) in a patient with myeloproliferative features. Phase determined by blast count and clinical criteria.
BCR-ABL1 tyrosine kinase inhibitors for CML — selection tailored to mutation status and comorbidities.
Treatment
First-line
Tyrosine kinase inhibitor (TKI) — imatinib, dasatinib, nilotinib, bosutinib (first-line options); ponatinib for T315I mutation or failure of multiple TKIs
Imatinib (Gleevec) — first-generation, well-tolerated, long-term safety data; standard initial therapy in many settings
Second-generation TKIs (dasatinib, nilotinib, bosutinib) achieve faster and deeper molecular responses; preferred for higher-risk disease
Monitoring: quantitative BCR-ABL1 IS% every 3 months; goals — major molecular response (MMR, BCR-ABL1 ≤0.1%) by 12 months; early molecular response (≤10%) by 3 months
Treatment-free remission (TFR) — TKI discontinuation possible in selected patients with deep, durable molecular response (≥MR4.5 for ≥2 years on therapy); ~40-50% maintain response off therapy
Hydroxyurea as bridging therapy for cytoreduction with very high WBC
Allopurinol for tumor lysis prevention with high WBC
Allogeneic HSCT — reserved for TKI-resistant disease or advanced phase (now rarely needed in chronic phase)
Second-line / adjunct
TKI resistance/intolerance: switch to alternative TKI; check BCR-ABL1 kinase domain mutations to guide selection (T315I → ponatinib or asciminib)
Asciminib (STAMP inhibitor) — novel allosteric BCR-ABL1 inhibitor approved for previously treated CML and T315I mutation
Accelerated phase: increase TKI dose or switch; consider HSCT
Blast crisis: combine TKI with chemotherapy appropriate to lineage (AML or ALL induction); allogeneic HSCT if remission achieved
Pregnancy: TKIs teratogenic; counsel contraception; interferon-alpha or imatinib interruption in selected cases
Manage TKI side effects: dasatinib pleural effusion, nilotinib hyperglycemia and arteriosclerosis, ponatinib vascular events, imatinib edema/weight gain
Cardiovascular events with second/third-generation TKIs
Treatment-free remission failure with molecular relapse
PANCE pearls
Leukocytosis 100K-500K with basophilia + splenomegaly + Philadelphia chromosome = CML.
LAP score is LOW in CML, HIGH in leukemoid reaction — useful when imaging or cytogenetics not immediately available.
Imatinib (Gleevec) was the first molecularly targeted cancer therapy and transformed CML from a fatal disease (median survival ~3-5 years pre-imatinib) to a chronic condition with near-normal life expectancy.
Monitor BCR-ABL1 IS% every 3 months — goal is major molecular response (MMR, ≤0.1%) by 12 months.
T315I mutation confers resistance to all standard TKIs (imatinib, dasatinib, nilotinib, bosutinib) — requires ponatinib or asciminib.
Dasatinib causes pleural effusion in ~30% — switch TKI if symptomatic or refractory.
Treatment-free remission (TKI discontinuation) is possible after sustained deep molecular response — ~40-50% maintain remission off therapy.
Allogeneic HSCT, once curative gold standard, is now reserved for TKI-resistant or advanced-phase disease.
References
ELN 2020 — European LeukemiaNet 2020 recommendations for treating CML (Hochhaus et al., Leukemia 2020)
IRIS Trial — Long-term outcomes of imatinib treatment for CML (Hochhaus et al., NEJM 2017)
DASISION — Dasatinib versus Imatinib in Newly Diagnosed Chronic-Phase CML (Kantarjian et al., NEJM 2010)
ENESTnd — Nilotinib versus Imatinib for Newly Diagnosed CML (Saglio et al., NEJM 2010)
STOP-IM/EURO-SKI — Discontinuation of TKIs in CML in deep molecular response (Saussele et al., Lancet Oncol 2018)
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