Understanding Myeloproliferative Neoplasms: A Patient's Guide to Causes, Progression, and Treatment

Table of Contents

• Key Points
• Background: What Are Myeloproliferative Neoplasms?
• How JAK2 and Cytokine Signaling Go Wrong
• The Three Driver Mutations: JAK2, CALR, and MPL
• How the Mutations Give Blood Stem Cells an Advantage
• The Timeline: When Do These Mutations Appear?
• Additional Mutations That Shape the Disease
• Inherited Factors, Sex Differences, and Age
• The Central Role of Inflammation
• Progression to Secondary Acute Myeloid Leukemia
• What Genetic Findings Mean for Treatment
• Current Treatments
• Limitations of Current Knowledge
• What This Means for Patients
• Frequently Asked Questions
• Source Information

Key Points

• MPNs are chronic blood cancers driven by mutations in JAK2, CALR, or MPL in over 90% of cases, causing overproduction of mature blood cells.
• These mutations arise decades before symptoms, even in utero, and the disease is shaped by inflammation, additional mutations, and inherited factors.
• Progression to secondary AML carries a median survival of 3 to 5 months; risk is highest in primary myelofibrosis at about 20.6%.
• Emerging therapies targeting mutant CALR and JAK2 V617F aim for durable disease modification and potentially eradicating mutated stem cells.

Background: What Are Myeloproliferative Neoplasms?

Classic myeloproliferative neoplasms (MPNs) are a group of three related chronic blood disorders. They are essential thrombocythemia (ET, too many platelets), polycythemia vera (PV, too many red blood cells), and primary myelofibrosis (PMF, scarring of the bone marrow). All three are clonal hematopoietic stem-cell (HSC) diseases, meaning they start in a single blood-forming stem cell that acquires a genetic error and then multiplies.

These disorders cause the bone marrow to overproduce mature, functional blood cells. Despite the excess cells, patients face dangerous complications including bleeding (hemorrhage), blood clots (thrombosis), and progression to secondary acute myeloid leukemia (AML, a fast-growing blood cancer).

The three disorders are also linked to one another. Essential thrombocythemia can progress to polycythemia vera. In 10 to 20% of cases — about 1 in 10 to 1 in 5 patients — either essential thrombocythemia or polycythemia vera can evolve into secondary myelofibrosis (called post–essential thrombocythemia or post–polycythemia vera myelofibrosis).

Because of this complexity, diagnostic criteria have changed substantially over the years. Doctors now recognize a fourth pathological condition called pre–primary myelofibrosis. This diagnosis is based mainly on histological features (microscopic tissue patterns) seen in a bone marrow sample. The current World Health Organization (WHO) classification of MPNs is not universally accepted and has recognized limitations.

How JAK2 and Cytokine Signaling Go Wrong

The idea that all three disorders share a common origin is supported by their shared molecular mechanism. In every case, the root problem is oncogenic activation (cancer-causing switching on) of Janus kinase 2 (JAK2), a signaling enzyme inside blood cells.

JAK2 normally sits attached to cytokine receptors (docking proteins on the cell surface) and transmits growth signals. Three genes can trigger abnormal JAK2 activation: JAK2 itself, calreticulin (CALR), and the thrombopoietin receptor (MPL). These mutations appear in more than 90% of MPN cases — that is, more than 9 in 10 patients — and are called phenotypic driver mutations because they drive the disease's visible features.

The remaining 10% of patients (about 1 in 10) are considered to have "triple-negative" disease. This complicates diagnosis, especially when a patient has thrombocytosis (too many platelets) but no evidence of clonal hematopoiesis (a single mutated stem cell taking over).

The Three Driver Mutations: JAK2, CALR, and MPL

Understanding the specific mutation matters for prognosis and treatment. Here is what each one does.

JAK2 mutations. JAK2 V617F sits in the pseudokinase domain (JH2), a region that normally keeps the enzyme switched off. JAK2 exon 12 mutations sit in the linker region between the SH2 and pseudokinase domains. The V617F change causes the enzyme to autophosphorylate and stay active without any growth signal.

Once active, JAK2 V617F turns on downstream signaling through the thrombopoietin receptor (MPL), the erythropoietin receptor (EPOR), and presumably the granulocyte colony-stimulating factor receptor. That signaling runs through the STAT, RAS–MAPK (mitogen-activated protein kinase), and PI3K pathways, driving myeloid progenitor cells to multiply.

CALR mutations. CALR mutations in exon 9 cause a +1 frameshift, a type of genetic error that shifts how the gene is read. This produces a mutant protein with a brand-new C-terminal end that lacks the KDEL motif. The KDEL motif is a tetrapeptide sequence (four-amino-acid tag) that normally keeps proteins in, or returns them to, the endoplasmic reticulum. The mutant protein also gains entirely new functions.

Mutant CALR binds firmly to MPL — and not to other cytokine receptors — inside the endoplasmic reticulum. It then travels through the normal secretory pathway to the cell surface. There, mutant CALR induces MPL dimerization (pairing up of two receptors) and activation exclusively at the cell surface. Much of the mutant CALR protein is secreted and can stimulate the mutant clone in an autocrine (self-signaling) or paracrine (neighbor-signaling) manner.

In short, mutant CALR behaves as both a rogue chaperone (a protein that improperly helps other proteins fold) and a cytokine (a growth signal). The result is oncogenic activation of the MPL–JAK2–STAT pathway. In addition, changes in the mutant CALR sequence disrupt calcium balance, cause endoplasmic reticulum stress, and trigger a cellular response called the unfolded protein response.

MPL mutations. The MPL W515 residue plays a key role in preventing the receptor from dimerizing and activating when no cytokine is bound. Substituting this amino acid induces receptor activation. Similarly, the MPL S505N mutation, located in the transmembrane domain, causes stable dimerization of the receptor, which activates JAK2 and downstream signaling.

Importantly, mutant CALR and MPL are restricted to megakaryocytic neoplasms (ET and PMF, both involving platelet-producing cells). In contrast, JAK2 V617F is found across all three subtypes (ET, PV, and PMF).

How the Mutations Give Blood Stem Cells an Advantage

All three driver mutations occur in multipotent hematopoietic stem cells (HSCs), which can give rise to both myeloid and lymphoid lineages. However, only the myeloid lineage gets amplified, though mutations can be found at low frequency in lymphocytes including T cells.

JAK2 V617F gives HSCs a slight advantage and primarily enhances lineage differentiation. Heterozygosity (having one mutated copy) may favor megakaryocyte (platelet-producing cell) differentiation. Homozygosity (having two mutated copies) favors erythroid (red blood cell) differentiation.

MPL is the key cytokine receptor responsible for the clonal advantage of MPN stem cells. Knocking out MPL is sufficient to eliminate the clonal advantage and the transplant capability of JAK2 V617F HSCs. This effect relates to direct MPL activation by JAK2 V617F, but thrombopoietin is still needed for the full disease phenotype to develop. JAK2 V617F also damages the HSC niche (the supportive environment in the bone marrow) by harming nestin-positive cells through secretion of interleukin-1β. This mechanism appears particularly important in the transition from JAK2 V617F clonal hematopoiesis to true MPN.

CALR mutations also arise in HSCs and cause substantial amplification of the megakaryocytic lineage. Some evidence suggests CALR mutations occur in megakaryocyte- or platelet-biased HSCs, or that they induce megakaryocyte bias. The clonal advantage of CALR-mutated HSCs may work essentially through MPL, though the unfolded protein response also appears involved. CALR-mutated human HSCs may have a greater clonal advantage than JAK2 V617F HSCs. This is based on the high CALR variant allele frequency (the fraction of cells carrying the mutation) in MPNs. The precise properties of MPL-mutated HSCs are not well understood in humans.

Two membrane proteins stand out as therapeutic targets. JAK2 V617F HSCs express high levels of CD123, whereas CALR mutants are associated with high levels of megakaryocyte markers such as G6B. Because normal HSCs express these proteins weakly or not at all, they can be targeted directly with treatments. Targeting MPL is also valuable because MPL is activated in different conformations in CALR-, MPL-, and JAK2 V617F–mutated MPNs than under normal conditions.

The Timeline: When Do These Mutations Appear?

MPNs can develop at any age, including in children, but they are usually diagnosed in people older than 60 years. On average, essential thrombocythemia occurs at an earlier age, while myelofibrosis occurs later.

Phylogenetic studies and mathematical models reveal something striking: JAK2 V617F arises decades before MPN emerges — even in utero. The growth rate of the JAK2 V617F clone varies widely, from 3 to 190% per year. Slower growth is seen in clonal hematopoiesis.

CALR mutations are typically acquired later than JAK2 V617F, with mean acquisition times of 25 years and 15 years, respectively. Yet CALR-mutated MPNs occur earlier, on average, than JAK2 V617F diseases. Why? Because the CALR mutant clone has greater fitness — that is, a larger clonal advantage — so it reaches clinical significance faster once it appears.

Additional Mutations That Shape the Disease

In approximately half of cases, the MPN driver mutation is the only acquired mutation. In the remaining cases, additional somatic (acquired, not inherited) driver mutations are present. These are not specific to MPN.

These additional mutations can appear before or after JAK2 V617F, but they typically follow the CALR mutation. Many of them affect gene regulation:

• Epigenetic regulation (control of which genes are switched on): TET2, DNMT3A, IDH1, IDH2, EZH2, and ASXL1
• Splicing (processing of genetic messages): SF3B1, SRSF2, and U2AF1
• Signaling: SH2B3, NRAS, and CBL
• Transcription (control of gene reading): TP53, RUNX1, and NFE2

Splicing mutations are rare in polycythemia vera but frequent in primary myelofibrosis, where they give aged HSCs a survival advantage. In polycythemia vera or essential thrombocythemia, the number of clonal fitness driver mutations is low (one or two). In primary myelofibrosis, the number is high, and mutations in SRSF2, U2AF1-Q157, ASXL1, IDH1, IDH2, and EZH2 confer a poor prognosis. Karyotypic abnormalities (chromosomal changes) are also frequent in primary myelofibrosis, and some carry a poor prognosis.

Treatment itself can shape clonal selection. For example, ruxolitinib, a JAK1 and JAK2 inhibitor, may influence which clones survive. Multihit TP53 mutations (biallelic alterations, meaning both copies of the gene are damaged) drive therapy-resistant leukemia, while the role of single TP53 mutations remains unclear.

Inherited Factors, Sex Differences, and Age

Essential thrombocythemia usually occurs at younger ages in women than in men, but progression to myelofibrosis is more common in men. Polycythemia vera has a similar incidence in men and women, but a better prognosis in women. However, young women are more prone to hepatic-vein thrombosis (a blood clot in the liver's main vein). Primary myelofibrosis is more prevalent in men.

A hormonal influence is suspected. Tamoxifen (a breast cancer drug) induces apoptosis (programmed cell death) of JAK2 V617F HSCs, though its clinical relevance is uncertain.

Familial clustering supports an inherited predisposition to MPN. Most cases involve common genetic variants with modest risk — notably the 46/1 haplotype, which favors JAK2 V617F acquisition. Rare, more penetrant familial structural or missense variants involve EPOR, RBBP6, SH2B3, and 14q32 duplication, although the mechanisms remain largely unknown.

The Central Role of Inflammation

Inflammation plays an important role in how MPN develops, contributing to clonal evolution, thrombosis, and progression toward myelofibrosis and secondary AML. The three driver mutations primarily drive inflammation, with distinct profiles among JAK2 V617F, MPL, and CALR mutations. Additional mutations that enhance clonal fitness further modulate these inflammatory profiles.

Inflammation unrelated to mutation — such as from infection, aging, or other chronic conditions — can also promote clonal dominance.

Systemic inflammation underlies many constitutional symptoms, particularly fatigue. This is especially prevalent in primary myelofibrosis. The inflammation comes from JAK2-driven STAT3 activation and cytokine release, amplified by nuclear factor κB (NF-κB) signaling in both clonal and nonclonal cells.

In the bone marrow, interleukin-1β and tumor necrosis factor α remodel the HSC niche, favoring clonal dominance and progression toward myelofibrosis. Megakaryocyte-derived transforming growth factor β1 (TGF-β1) drives the proliferation of mesenchymal stem cells and their differentiation into myofibroblasts and osteoblasts, working together with interleukin-1β and interleukin-8. JAK2 V617F further promotes fibrosis through senescent neutrophils that activate megakaryocytes and through monocytes that differentiate into fibrocytes.

Finally, inflammation raises the risk of thrombosis through several mechanisms: activation of endothelial cells, platelets, and leukocytes (particularly monocytes and granulocytes); formation of neutrophil extracellular traps; and activation of coagulation. Inflammation may even drive disease progression by inducing or selecting certain mutations. For all these reasons, targeting inflammation is an important therapeutic approach.

Progression to Secondary Acute Myeloid Leukemia

One of the most serious challenges in MPN care is progression to acute myeloid leukemia (AML). Secondary AML after MPN has a median overall survival of only 3 to 5 months. That means half of patients live less than three to five months after diagnosis.

The cumulative risk of secondary AML varies by MPN subtype:

• Essential thrombocythemia: 0.7 to 3.8% (roughly 1 in 130 to 1 in 26 patients)
• Polycythemia vera: 2.3 to 6.8% (roughly 1 in 43 to 1 in 15 patients)
• Primary myelofibrosis: 20.6% (about 1 in 5 patients), with the highest risk and shortest latency

The ability of older cytoreductive therapies (treatments that reduce blood cell counts) — such as chlorambucil, pipobroman, and radiophosphorus — to induce leukemia progression is well documented. However, the role of hydroxyurea monotherapy (used alone) is controversial.

Evolution to AML reflects the stepwise acquisition of adverse genetic lesions. Post-MPN AML differs from primary AML (AML that occurs without previous blood disease). FLT3, NPM1, and CEBPA mutations are uncommon in post-MPN AML, whereas JAK2, CALR, and MPL mutations may persist. In many cases, JAK2–STAT signaling remains active.

Interestingly, JAK2 wild-type secondary AML is frequently observed in JAK2 V617F MPNs. In contrast, CALR wild-type secondary AML is rare in CALR-mutated cases. The loss of the original driver mutation can happen in two ways:

1. The MPN driver mutation occurs in preexisting clonal hematopoiesis — usually driven by a TET2 or DNMT3A mutation — and leukemia develops in that original clone.
2. AML and MPN develop in two independent clones. Evidence suggests JAK2 V617F–driven inflammation selects the leukemic clone, which ultimately outcompetes the JAK2 V617F clone.

Common mutations associated with leukemia affect epigenetic factors (ASXL1, EZH2, IDH1, and IDH2), splicing factors (SRSF2, SF3B1, and U2AF1), and transcription factors (NFE2, RUNX1, and TP53). Cytogenetic abnormalities are frequent. Some AML cases resemble post-myelodysplastic syndrome AML (with epigenetic and splicing mutations), while others resemble therapy-related AML (with TP53 pathway mutations in 30 to 40% of cases). Multihit TP53 mutations lead to a dismal prognosis.

Allogeneic hematopoietic stem cell transplantation (receiving stem cells from a donor) is the only curative option for secondary AML. However, it is feasible for only a few patients and is rarely successful — particularly for those with multihit TP53 AML.

What Genetic Findings Mean for Treatment

The traditional classification of MPN into essential thrombocythemia, polycythemia vera, or primary myelofibrosis remains extremely important for prognosis and treatment. However, molecular details add crucial information.

The type of JAK2 V617F mutation, jAK2 exon 12 mutations, and which HSC subtype was initially targeted all interact with the bone marrow microenvironment to shape the disease. JAK2 V617F MPNs are also characterized by marked inflammation.

JAK2 exon 12 mutations give rise to polycythemia vera that is frequently characterized by isolated erythrocytosis (high red cell counts only). CALR-mutated and MPL-mutated MPNs cause essential thrombocythemia and primary myelofibrosis instead.

There are also important differences between CALR type 1 (a 52-base-pair deletion) and CALR type 2 (a 5-base-pair insertion) MPNs, even though both involve frameshift mutations:

• CALR type 1 MPNs are more likely to progress to myelofibrosis, carry few additional mutations, and occur in a high percentage of patients classified as low- or intermediate-risk.
• CALR type 2 primary myelofibrosis is rare but is associated with additional mutations and has a worse prognosis.

This molecular classification will certainly become increasingly important as new therapies targeting driver mutations become available.

Current Treatments

Traditional treatment focuses on four goals: alleviating symptoms, preventing thrombotic complications, controlling myeloproliferation (excess blood cell production), and restoring normal blood cell counts. These goals matter because they strongly affect quality of life and mortality.

Standard treatments include phlebotomy (removing blood) to quickly normalize hematocrit level (the proportion of blood made up of red cells). Phlebotomy also lowers thrombosis risk in polycythemia vera. Other standard treatments are cytoreductive treatment (primarily hydroxyurea) and low-dose aspirin.

However, two treatments have transformed MPN management.

Interferon alfa. The development of pegylated forms of interferon alfa — such as ropeginterferon alfa-2b (Besremi) — has reduced the adverse effects of earlier formulations. Pegylation is a process that attaches a molecule to the drug so it stays in the body longer and can be given less often. Current therapies primarily control symptoms, thrombosis, and splenomegaly (enlarged spleen) but have limited disease-modifying effects, except for pegylated interferon alfa and JAK2 inhibitors in some patients.

Emerging therapies that selectively target mutant CALR and JAK2 V617F — using immunotherapy and selective inhibitors — could be a breakthrough for people with MPN. The expectation is to achieve durable disease modification and potentially clonal eradication (wiping out the mutated stem cells entirely).

Limitations of Current Knowledge

Several important gaps remain in our understanding of MPNs.

• The current World Health Organization classification is not universally accepted and has limitations.
• The precise properties of MPL-mutated HSCs are not well understood in humans.
• The role of single TP53 mutations in leukemic transformation remains unclear.
• The clinical relevance of hormonal influences on MPN (such as tamoxifen killing JAK2 V617F HSCs) is uncertain.
• Whether hydroxyurea monotherapy contributes to leukemia progression is controversial.
• The mechanisms behind rare inherited MPN predisposition variants remain largely unknown.
• Allogeneic stem cell transplantation — the only curative option for secondary AML — is feasible for only a few patients and rarely successful.

What This Means for Patients

This research offers several practical takeaways for people living with MPNs.

1. Know your driver mutation. Whether you have JAK2 V617F, JAK2 exon 12, CALR (and which type), or MPL shapes your prognosis and future treatment options. Ask your hematologist which mutation you carry.
2. Understand your subtype risk. Progression to secondary AML ranges from less than 4% in essential thrombocythemia to about 21% in primary myelofibrosis. This helps guide monitoring frequency.
3. Take thrombosis prevention seriously. Blood clots are a major cause of illness in MPN. Phlebotomy, aspirin, and cytoreductive therapy are used specifically to reduce this risk.
4. Report fatigue and constitutional symptoms. These often reflect systemic inflammation, which is now recognized as a major driver of disease progression and is a target of emerging therapies.
5. Discuss new therapies with your care team. Pegylated interferon alfa and JAK2 inhibitors offer disease-modifying potential in some patients, and clinical trials of mutation-selective treatments are ongoing.
6. Consider genetic counseling if MPN runs in your family. Familial clustering occurs, and the 46/1 haplotype and rare variants can increase inherited risk.

Frequently Asked Questions

What are myeloproliferative neoplasms (MPNs)?

MPNs are chronic blood cancers that start in a single bone marrow stem cell. They cause the body to overproduce mature blood cells. The three classic types are essential thrombocythemia (too many platelets), polycythemia vera (too many red blood cells), and primary myelofibrosis (bone marrow scarring). They can lead to bleeding, blood clots, and progression to secondary acute myeloid leukemia.

What causes MPNs?

More than 90% of MPN cases are driven by mutations in one of three genes: JAK2, CALR, or MPL. These mutations switch on growth signaling permanently. They arise decades before symptoms appear, even in utero. The disease is then shaped by inflammation, additional mutations, and inherited factors. About 10% of patients have 'triple-negative' disease, meaning no mutation is found in these three genes.

What is the risk of MPN progressing to acute myeloid leukemia (AML)?

The risk of progression to secondary AML varies by subtype. In essential thrombocythemia, it is 0.7 to 3.8% (roughly 1 in 130 to 1 in 26 patients). In polycythemia vera, it is 2.3 to 6.8% (roughly 1 in 43 to 1 in 15 patients). In primary myelofibrosis, it is 20.6% (about 1 in 5 patients), with the highest risk and shortest latency.

What is the survival rate for secondary AML after MPN?

Secondary AML after MPN has a median overall survival of only 3 to 5 months. That means half of patients live less than three to five months after diagnosis. Allogeneic hematopoietic stem cell transplantation is the only curative option, but it is feasible for only a few patients and is rarely successful, particularly for those with multihit TP53 AML.

What treatments are available for MPNs?

Current treatments aim to relieve symptoms, prevent blood clots, control excess blood cell production, and restore normal blood counts. Standard options include phlebotomy, cytoreductive therapy (primarily hydroxyurea), and low-dose aspirin. Pegylated interferon alfa and JAK2 inhibitors can modify the disease in some patients. Emerging therapies that selectively target mutant CALR and JAK2 V617F are under study.

What does it mean to have a JAK2, CALR, or MPL mutation?

Knowing your driver mutation helps determine prognosis and treatment. JAK2 V617F is found in all three MPN subtypes and is linked to marked inflammation. CALR and MPL mutations are restricted to essential thrombocythemia and primary myelofibrosis. CALR type 1 is more likely to progress to myelofibrosis, while CALR type 2 primary myelofibrosis is rare but has a worse prognosis.

What should I discuss with my doctor about MPN?

Ask your hematologist which driver mutation you carry, as it shapes prognosis and future treatment options. Understand your subtype risk of progression to secondary AML, which ranges from less than 4% in essential thrombocythemia to about 21% in primary myelofibrosis. Take blood clot prevention seriously. Report fatigue and constitutional symptoms. Discuss new therapies and consider genetic counseling if MPN runs in your family.

When should someone with a myeloproliferative neoplasm seek a second opinion?

A second opinion can help when the driver mutation or MPN subtype is unclear. JAK2 V617F, JAK2 exon 12, CALR (type 1 or 2), and MPL carry different prognoses and treatment options. It is also reasonable before starting cytoreductive therapy or a JAK inhibitor, when weighing pegylated interferon alfa, or when progression to secondary myelofibrosis or AML is suspected. Because progression risk ranges from under 4% in essential thrombocythemia to about 21% in primary myelofibrosis, confirming subtype and mutation guides monitoring. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original article title: Myeloproliferative Neoplasms

Authors: Isabelle Plo, Ph.D., and William Vainchenker, M.D., Ph.D. (INSERM Unité Mixte de Recherche 1287, Gustave Roussy, Villejuif, France)

Publication: Review Article, The New England Journal of Medicine, 2026;395:788-802. DOI: 10.1056/NEJMra2507867. Copyright © 2026 Massachusetts Medical Society.

This patient-friendly article is based on peer-reviewed research.