Hematopoietic Stem Cells: Myeloproliferative Neoplasms — CML, Essential Thrombocythemia, and Primary Myelofibrosis

Myeloproliferative Neoplasms: The Driver-Mutation Framework

The myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell disorders defined by specific, acquired activating mutations that drive the overproduction of functional, mature myeloid elements. Because differentiation is preserved, the marrow and blood become crowded with cells that largely still work — the clinical problem is excess rather than failure, which separates the MPNs from the myelodysplastic syndromes and the bone marrow failure syndromes discussed elsewhere. The World Health Organization recognizes eight chronic MPNs: chronic myeloid leukemia (CML), polycythemia vera (PV), primary myelofibrosis (PMF), and essential thrombocythemia (ET), together with rarer entities such as chronic neutrophilic leukemia and chronic eosinophilic leukemia, mastocytosis, and unclassifiable cases.

The most useful organizing principle is the driver mutation. Each MPN maps to a characteristic activating lesion, and detecting that lesion is simultaneously diagnostic, prognostic, and — increasingly — the basis for choosing targeted therapy. A clean two-way split exists: CML (and a few rare myeloid-phenotype disorders) is driven by the BCR-ABL1 fusion kinase, whereas PV, PMF, and ET are driven by mutations that directly or indirectly constitutively activate JAK2 signaling, the tyrosine kinase essential for erythropoietin and thrombopoietin receptor function. This distinction echoes in the natural histories: BCR-ABL1-driven disease traditionally runs a course measured in years with a substantial risk of leukemic transformation if untreated, while the JAK2-pathway MPNs usually evolve over decades, transform to acute leukemia uncommonly in the absence of cytotoxic therapy, and can evolve into one another (ET or PV can progress to myelofibrosis; PV, ET, and PMF share the same three canonical drivers).

*PV is discussed in this chapter's companion material; it is shown here to complete the driver-mutation map.

Across subtypes, MPNs share a common clinical vocabulary: constitutional symptoms (fever, night sweats, weight loss, pruritus), splenomegaly (often prominent), an increased risk of both thrombosis and bleeding, and a shared potential to transform into more aggressive myeloid malignancies, including acute myeloid leukemia (AML). The rest of this lesson follows the three MPNs most relevant to general internal medicine practice: CML, ET, and PMF.

Chronic Myeloid Leukemia

Pathobiology: BCR-ABL1 and the Philadelphia Chromosome

CML is a clonal myeloproliferative disorder characterized by abnormal production and proliferation of granulocytes with generally preserved differentiation. It is defined by the Philadelphia (Ph) chromosome, the reciprocal translocation t(9;22)(q34.1;q11.2) that fuses the ABL1 proto-oncogene on chromosome 9 with the BCR gene on chromosome 22. The resulting BCR-ABL1 oncoprotein — usually the 210-kDa p210 form in CML — is a constitutively active tyrosine kinase that switches on proliferative and anti-apoptotic signaling (RAS/MAPK, JAK-STAT, PI3K pathways), giving CML cells a growth advantage while gradually suppressing normal hematopoiesis. CML accounts for roughly 15% of adult leukemias, presents at a median age of 55–65 years, and occurs at about 2 cases per 100,000 persons per year.

BCR-ABL1 is both the diagnostic hallmark and the therapeutic target. In about 10% of patients with otherwise classic CML the Ph chromosome is not visible on standard G-banding (variant or “masked” translocations), but fluorescence in situ hybridization (FISH) or polymerase chain reaction (PCR) will still detect BCR-ABL1 — and these patients respond to tyrosine kinase inhibitor (TKI) therapy just like Ph-positive patients. Conversely, a CML-like picture without any detectable BCR-ABL1 should prompt a search for alternative diagnoses (atypical CML, chronic myelomonocytic leukemia, MDS/MPN overlap syndromes), which do not respond to TKIs and carry a worse prognosis.

Presentation and Diagnosis

In settings with routine laboratory screening, half or more of patients are diagnosed incidentally from an asymptomatic neutrophilia; the rest present with fatigue, malaise, weight loss, or symptoms of splenomegaly — early satiety, abdominal fullness, or left upper quadrant discomfort (splenomegaly is the most common physical finding). Rarely, extreme leukocytosis or thrombocytosis produces hyperviscosity or thrombotic presentations. The peripheral smear shows the full spectrum of myeloid maturation — bands, metamyelocytes, myelocytes, promyelocytes, and a small percentage of blasts — typically accompanied by thrombocytosis and a mild normocytic anemia.

The diagnosis rests on demonstrating BCR-ABL1, which can be detected with routine cytogenetics, FISH, or reverse-transcriptase PCR. At diagnosis, peripheral blood testing is as accurate as marrow for establishing the fusion. A baseline bone marrow examination is still often performed to quantify blast and basophil percentages and to look for additional chromosomal abnormalities, which carry prognostic weight and may change management. Molecular testing at diagnosis should also document the transcript type, so that rare atypical transcripts are not later misread as “undetectable” disease.

Disease Phases and Response Milestones

Untreated CML classically runs a biphasic or triphasic course: an indolent chronic phase, an accelerated phase, and a terminal blast phase. Most patients (about 90%) present in chronic phase, which is highly responsive to therapy. The blast phase is, in effect, a secondary acute leukemia — myeloid in most cases but lymphoid in roughly one quarter, a distinction that matters because lymphoid blast phase responds to ALL-type chemotherapy combined with TKIs. The newest WHO classification has proposed eliminating the accelerated phase as a separate entity (grouping such patients as high-risk CML), though many clinicians continue to recognize it because outcomes remain meaningfully worse than in chronic phase.

Once therapy begins, management is guided by molecular response rather than symptoms. BCR-ABL1 transcript levels are measured by PCR on peripheral blood and reported on the International Scale (IS). A complete cytogenetic response (no Ph-positive metaphases) corresponds roughly to BCR-ABL1 ≤1% IS (MR2) and is the milestone most consistently tied to survival; a major molecular response (MMR, MR3; ≤0.1% IS) further reduces the risk of relapse and transformation; and a deep molecular response (DMR — MR4 ≤0.01% IS, or MR4.5) is the gateway to attempting treatment-free remission (TFR).

Treatment: The TKI Era

TKIs revolutionized CML: before imatinib, median survival was 3–7 years; today the estimated 10-year survival exceeds 85% — approaching that of the age-matched general population — and annual mortality has fallen from 10–20% to about 1–2%. Every patient requires treatment at diagnosis, and TKIs now control symptoms, prevent progression, and have largely removed transplantation from first-line care.

Choice of frontline TKI is individualized: patient age and goals (long-term survival versus TFR), drug cost and access (generic imatinib is now inexpensive in many regions), comorbidities, and each agent's toxicity signature. In general, all four classic frontline TKIs produce similar survival; second-generation agents achieve deeper and faster molecular responses (useful when TFR is the goal or in high-risk disease) but have distinct adverse-effect profiles:

Beyond these agent-specific effects, TKIs as a class can cause fluid retention, rash, and QT prolongation, and they carry drug–drug interactions. A modern refinement of practice: for patients already in good molecular response, dose reduction — rather than switching — often manages mild-to-moderate toxicity without sacrificing efficacy.

Resistance, Treatment-Free Remission, and Transplantation

True resistance to imatinib occurs in only about 10% of patients at 10 years. When transcripts rise or responses are lost, the first step is checking adherence; the next is ABL1 kinase-domain mutation testing, which identifies a targetable mechanism in about half of resistant cases. More than 100 such mutations are described. The T315I “gatekeeper” mutation blocks imatinib and all second-generation TKIs and requires ponatinib or asciminib, with serious consideration of transplantation; other mutations steer the choice between remaining agents (for example, Y253H/E255K/V/F359V favor dasatinib or bosutinib, while V299L/T315A/F317L favor nilotinib). Switching from one second-generation TKI to another after resistance rarely works unless a guiding mutation is present — ponatinib is usually the next step.

For patients who achieve and sustain deep molecular response, discontinuation is a realistic goal: stopping TKIs after more than 2–3 years of DMR yields durable TFR in roughly 40–60% of patients, rising above 80% after 5 or more years of DMR. TFR attempts require intensive molecular monitoring (monthly-to-bimonthly PCR early on), and TKIs are restarted only if transcripts rise above the MMR threshold on confirmatory testing.

Allogeneic hematopoietic stem cell transplantation (HSCT) — curative but carrying 5–30% early mortality and substantial late morbidity — is now reserved for later lines: chronic-phase patients with resistance to second-generation TKIs or T315I-mutated disease, and patients presenting in or evolving to accelerated or blast phase, in whom TKI-based combinations (with chemotherapy for blast phase) are used to re-induce remission as a bridge to prompt transplantation.

Essential Thrombocythemia

Driver Mutations and the Obligation to Exclude Reactive Thrombocytosis

ET is a clonal stem cell disorder expressed as sustained overproduction of platelets. JAK2 V617F is present in roughly half to 60% of patients; most of the remainder carry CALR mutations (~30–40%) or MPL mutations (~4–8%), and up to 10% lack any canonical driver (“triple-negative”), a group that generally follows a benign course. ET can occur at any adult age, has a female predominance, and is most often found incidentally when a routine blood count reveals an elevated platelet count — sometimes one that had been present, and overlooked, for years.

Clinical Features and Complications

Most patients are asymptomatic. When symptoms occur they are vasomotor — headaches, erythromelalgia (intermittently red, hot, painful extremities), and visual disturbances — or thrombotic (arterial or venous: TIA/stroke, myocardial infarction, deep venous thrombosis). Hemorrhage becomes more likely at extreme platelet counts. Two examination and laboratory pearls: splenomegaly is not a feature of ET and should redirect the workup toward another MPN (PV, PMF, or CML); and very large platelet masses can produce spurious hyperkalemia (platelet potassium released during clotting in the tube) — a laboratory artifact without ECG changes. Long-term, ET survival approaches that of the general population, and progression to myelofibrosis or AML is less common than in the other MPNs; notably, leukemic evolution in ET is more often a consequence of cytotoxic therapy than of the disease itself.

Risk-Stratified Treatment

Therapy is determined by thrombotic risk rather than by the platelet number alone. The practical split: low-risk patients — younger than 60, no history of venous or arterial thrombosis, and JAK2-negative — can be observed without therapy or given low-dose aspirin for vasomotor symptoms; high-risk patients — older than 60, JAK2-positive, or with prior thrombosis — should receive aspirin plus cytoreductive therapy, usually hydroxyurea. Interferon alfa is the preferred platelet-lowering agent in younger patients and during pregnancy (hydroxyurea is potentially teratogenic). Anagrelide lowers platelet counts effectively but is reserved for patients who cannot tolerate or fail hydroxyurea. Patients who suffer an acute venous thromboembolism generally require lifelong anticoagulation in addition to ET-directed therapy.

Primary Myelofibrosis

Pathobiology and Clinical Features

PMF is a clonal stem cell disorder characterized by bone marrow fibrosis, extramedullary hematopoiesis, and splenomegaly. The fibrosis itself is reactive, not neoplastic: clonal megakaryocytes release cytokines (including transforming growth factor β-1) that recruit polyclonal fibroblasts to lay down reticulin and collagen, progressively impairing normal hematopoiesis. PMF is the least common of the classic MPNs and typically afflicts older adults. It shares the canonical drivers — JAK2 V617F in ~55–60%, CALR in ~35–40%, MPL in ~4–9% — with roughly 10% of patients triple-negative.

Constitutional symptoms are common (fatigue, fever, night sweats, generalized pruritus, weight loss). Splenomegaly is characteristic and often massive, causing abdominal fullness, pain, and early satiety; exuberant extramedullary hematopoiesis can occasionally produce ascites, portal hypertension, or other compressive syndromes. Laboratory results vary widely: anemia is common, while leukocyte and platelet counts may be elevated, normal, or depressed. Hyperuricemia and elevated LDH are frequent companions of the high cell turnover.

Diagnosis and Prognosis

The peripheral smear is leukoerythroblastic — teardrop-shaped red cells, nucleated red cells, and immature myeloid forms (myelocytes, promyelocytes, occasional blasts) reflecting blood production outside the marrow. Bone marrow aspiration typically fails (“dry tap”) because of the fibrosis; the biopsy shows a hypercellular marrow with clustered, atypical megakaryocytes and extensive reticulin/collagen deposition. Diagnosis is, importantly, one of exclusion: secondary causes of myelofibrosis — other MPNs including CML, metastatic carcinoma, granulomatous infections such as miliary tuberculosis, and others — must be ruled out, because several have entirely different, effective treatments. Cytogenetics and molecular testing both exclude CML and refine prognosis.

Prognosis is estimated with the International Prognostic Scoring System (IPSS, applied at diagnosis) and its dynamic successors DIPSS and DIPSS-Plus (usable at any point in the disease). Core risk factors are age over 65, anemia (hemoglobin <10 g/dL), leukocytosis (>25,000/µL), circulating blasts ≥1%, and constitutional symptoms; DIPSS-Plus adds unfavorable karyotype, platelet count <100,000/µL, and transfusion dependence. The spread in outcomes is stark: low-risk disease carries an estimated overall survival in the 15-year range, while high-risk disease averages only about 16 months. Roughly 10% of patients transform to an aggressive acute leukemia for which therapy is usually ineffective, and mutational analysis (ASXL1, EZH2, SRSF2, IDH1/2) further refines risk beyond the clinical scores.

Treatment: Supportive Care, JAK Inhibition, and Transplantation

Management is tailored to symptoms, risk category, and transplant candidacy; asymptomatic low-risk patients can be monitored. No drug yet alters the natural history of PMF, and the anemia is multifactorial and hard to treat — transfusions, androgens such as danazol, erythropoiesis-stimulating agents (ineffective when the serum erythropoietin level exceeds ~125 mU/L and potentially worsening splenomegaly), glucocorticoids, and low-dose thalidomide all have limited and inconsistent benefit. Splenectomy is avoided: it carries significant operative morbidity and mortality in this population (hemorrhage, mesenteric thrombosis, rebound counts, hepatic extramedullary hematopoiesis) and may increase the risk of blastic transformation; splenic irradiation is at best temporarily palliative.

The treatment armamentarium for myelofibrosis has grown beyond ruxolitinib and fedratinib: pacritinib is particularly useful when thrombocytopenia limits other JAK inhibitors, and newer agents now target anemia and the fibrotic clone itself.

Viewed across the three diseases, the MPNs illustrate what a mutation-first framework delivers in hematology: a single fusion kinase turned CML from a fatal illness into a near-normal-life-expectancy disease with a realistic treatment-free future; JAK2-pathway biology produced an expanding family of JAK inhibitors for myelofibrosis; and mutation-informed risk stratification keeps ET management proportional to a largely benign natural history. The continuing challenge — preventing or reversing fibrosis, eliminating the leukemic stem cell, and making TFR safe and predictable — is now being pursued with combination strategies built on exactly this foundation.