Is Myelofibrosis a Myelodysplastic Syndrome?

Myelofibrosis is not a myelodysplastic syndrome. It is formally classified as a myeloproliferative neoplasm, a fundamentally different category of blood cancer, even though the two diseases share a surprising number of genetic mutations, clinical features, and treatment challenges. The confusion is understandable: both arise from defective stem cells in the bone marrow, both can cause severe anemia, and both can transform into acute leukemia. But the World Health Organization classification system has placed them in separate diagnostic bins since the 1990s, and that distinction reflects genuine differences in biology and behavior.

How the WHO Separates These Two Disease Families

The international classification of blood cancers divides myeloid neoplasms into several major groups. Myeloproliferative neoplasms, the family myelofibrosis belongs to, are defined by excessive production of blood cells driven by mutations that lock growth-signaling pathways into an “always on” state. The hallmark driver mutations in myelofibrosis involve JAK2, CALR, or MPL, genes that control how stem cells respond to growth signals. These mutations push the marrow to overproduce cells, particularly megakaryocytes (the giant cells that make platelets), which in turn trigger the scarring that gives myelofibrosis its name.¹

Myelodysplastic syndromes, by contrast, are defined by ineffective blood-cell production and abnormal cell shapes. The marrow in MDS is busy making cells, but those cells are malformed and die before they mature, leading to low blood counts (cytopenias) rather than high ones. The genetic drivers in MDS tend to cluster in a different set of genes, particularly splicing-factor genes like SF3B1, SRSF2, and U2AF1, which affect how cells process their own genetic instructions.²

In simple terms: MPNs make too many blood cells; MDS makes too few functional ones. Myelofibrosis starts on the “too many” side of that divide, even though the progressive scarring of the marrow eventually chokes off normal blood production and causes severe cytopenias that can look a lot like MDS at later stages.

Why the Two Conditions Get Confused

The clean textbook distinction between “overproduction” and “underproduction” gets muddier in real patients than classification systems suggest. Myelofibrosis, especially as it progresses, produces a clinical picture that can overlap heavily with MDS. Many people with advanced myelofibrosis develop anemia, low platelet counts, and abnormal-looking blood cells under the microscope, all features clinicians associate with MDS. When a hematologist examines a bone marrow biopsy showing fibrosis and dysplastic cells, the initial question of “is this MPN or MDS?” can be genuinely difficult to answer.

This is not just a cosmetic problem of labeling. The WHO classification recognized that some myeloid cancers sit at the boundary and cannot be comfortably assigned to either the MPN or MDS category. These are grouped into a distinct third category: myelodysplastic/myeloproliferative neoplasms (MDS/MPN). This category captures diseases that, at initial presentation, show overlapping proliferative and dysplastic features that make classification as either MDS or MPN problematic.³ Chronic myelomonocytic leukemia (CMML) is the best-known disease in this overlap zone. Myelofibrosis itself is not placed in this overlap category; it remains in the MPN group. But the fact that an entire diagnostic category exists for borderline cases tells you how porous the boundaries between these conditions really are.

The Genetic Overlap Is Larger Than Expected

One reason myelofibrosis and MDS are so easily confused is that their genetic profiles share more territory than most people realize. Primary myelofibrosis is distinguished from other MPNs (like essential thrombocythemia and polycythemia vera) partly by the fact that it carries a significantly higher frequency of mutations in genes usually associated with MDS. A meta-analysis found that 13 genes, including ASXL1, U2AF1, SRSF2, SF3B1, and ZRSR2, had significantly higher mutation frequencies in primary myelofibrosis compared with the other classic MPNs. That mutational profile actually looked more like MDS than like its supposed MPN siblings.⁴

This has led some researchers to argue that myelofibrosis is not a pure MPN at all, but rather a disease that spans a spectrum from myeloproliferative to myelodysplastic. The presence of non-driver mutations correlates with the likelihood of dysplastic features and the severity of fibrosis, suggesting that the more mutations a patient accumulates in MDS-associated genes, the more their myelofibrosis starts to resemble MDS in both appearance and behavior.⁵ The disease essentially exists along a continuum, and where any individual patient falls on that continuum depends on their particular collection of mutations.

This is an area where the science is genuinely unsettled. The classification puts myelofibrosis firmly in the MPN camp, and there are good reasons for that: the initiating driver mutations (JAK2, CALR, MPL) are distinctly proliferative. But the accumulated secondary mutations tell a more complicated story, and future classification revisions may eventually reflect that complexity more explicitly.

Fibrosis Can Also Appear in MDS

Adding to the diagnostic confusion, bone marrow fibrosis is not exclusive to myelofibrosis. MDS patients can develop significant fibrosis in their marrow too, a condition sometimes called MDS with fibrosis or MDS-MF. This presents a real clinical headache: you have a patient with dysplastic blood cells, low counts, and a fibrotic bone marrow. Is it MDS that has developed fibrosis, or myelofibrosis that has developed dysplastic features? The answer can determine treatment strategy.

Research comparing fibrosis in MDS to fibrosis in primary myelofibrosis has found striking molecular similarities. A study analyzing bone marrow biopsies from MDS patients with fibrosis found increased expression of many of the same fibrosis-promoting molecules seen in primary myelofibrosis, including transforming growth factor beta 1 (TGF-β1), thrombospondin 1, and several matrix metalloproteinases. Some MDS-MF cases also showed megakaryocytic changes resembling those in primary myelofibrosis, including increased platelet-fragment deposition in the marrow. JAK2 and SRSF2 mutations were detectable in up to 13 percent of MDS-MF cases.⁶

MDS with myelofibrosis carries a particularly poor prognosis. Case reports have described it as a rare disorder that arguably deserves its own classification subgroup, and it is associated with a higher risk of transforming into acute myeloid leukemia.⁷ The practical takeaway is that fibrosis alone does not tell you which disease you are looking at. The full diagnostic picture, including driver mutations, blood counts, and cell morphology, is needed to assign the correct label.

Why the Classification Distinction Matters for Patients

Knowing whether you have myelofibrosis (an MPN) versus an MDS is not academic hairsplitting. It changes which clinical trials you are eligible for, which drugs your physician reaches for first, and how your prognosis is calculated.

The most prominent targeted therapy in myelofibrosis is ruxolitinib, a JAK inhibitor designed to counteract the constitutively active JAK-STAT signaling that drives the disease. MDS, by contrast, is more commonly treated with hypomethylating agents like azacitidine and decitabine that address the epigenetic dysregulation underlying dysplastic cell production. The overlap syndromes in the MDS/MPN category sometimes get combination approaches. JAK2 plays a heterogeneous role across these conditions, which slowed drug development for years.⁸

Prognostic scoring systems also differ by diagnosis. In primary myelofibrosis, the DIPSS-plus and MIPSS70-plus scores incorporate factors like thrombocytopenia, unfavorable karyotype, and high-risk molecular mutations to estimate survival and risk of blast transformation. Patients with the highest-risk MIPSS70-plus scores had roughly a one-in-four chance of progressing to blast-phase disease.⁹ MDS uses its own risk-scoring tools, like the IPSS-R, that weight different variables. A patient misclassified between the two diseases could end up with an inaccurate prognosis and a suboptimal treatment plan.

Anemia as a Shared Battleground

One of the most debilitating features shared by both myelofibrosis and MDS is anemia, and the mechanisms behind it are more similar than the different disease labels might suggest. In both conditions, anemia arises from a combination of ineffective red-blood-cell production, iron metabolism gone haywire due to abnormal hepcidin signaling, and a flood of inflammatory molecules that suppresses healthy blood-cell development.¹⁰

This shared biology has created a growing area of therapeutic overlap. Luspatercept, a drug that blocks certain TGF-β signaling to restore red-cell maturation, was originally approved for lower-risk MDS and is now part of the treatment landscape for myelofibrosis-related anemia as well. Momelotinib, a JAK inhibitor with additional activity against the hepcidin pathway, was approved for myelofibrosis with anemia. Newer agents targeting the same TGF-β/BMP-SMAD axis are being developed for use across both diseases.¹⁰ Symptom-wise, fatigue driven by anemia is typically the most burdensome complaint in both conditions, and in myelofibrosis it consistently ranks as the symptom with the highest severity score on patient-reported assessments.

Stem Cell Transplant Ties the Two Together

Despite their different classification, myelofibrosis and MDS share the same only-known cure: allogeneic stem cell transplant. Both diseases are stem cell disorders at their root, meaning the defective clone originates in the earliest progenitor cells of the blood system. Transplanting healthy donor stem cells can replace the diseased marrow entirely and is currently the standard of care for eligible patients with higher-risk disease in either condition.¹¹

“Eligible” is the operative word. Transplant carries substantial risks, including graft-versus-host disease and transplant-related mortality, and is generally reserved for younger patients or those with aggressive disease features. Most people with myelofibrosis or MDS are diagnosed later in life, which limits transplant candidacy. For the majority of patients, treatment remains focused on managing symptoms, controlling blood counts, and delaying progression.¹²

How Classification Systems Have Evolved

The diagnostic categories we use today are not the ones hematologists worked with a generation ago, and they will probably look different a generation from now. The earliest diagnostic criteria for myeloproliferative disorders, published by the Polycythemia Vera Study Group in 1971, did not even require a bone marrow biopsy and focused almost entirely on blood counts and a handful of lab values.¹³ The WHO’s first attempt at classifying hematopoietic tumors in 1976 was similarly limited.

Over subsequent decades, advances in genetics and molecular testing reshaped the landscape. The discovery of the JAK2 V617F mutation in 2005 was a watershed moment for MPN diagnosis, and the growing availability of next-generation sequencing has revealed the true mutational complexity of both MPNs and MDS. Each revision of the WHO classification, most recently in 2022, has incorporated more molecular data and refined the boundaries between categories. The creation of the MDS/MPN overlap category was itself an acknowledgment that nature does not always sort diseases into tidy boxes.

The trend in classification is toward ever-greater use of molecular markers alongside traditional morphology. As genomic profiling becomes standard in clinical practice, it is plausible that future classification systems will further subdivide myelofibrosis based on its mutational burden, perhaps formally recognizing the dysplastic end of the myelofibrosis spectrum as a distinct entity rather than lumping all cases together under a single MPN label.

From Background Mutations to Blood Cancer

Both myelofibrosis and MDS can trace their origins to a phenomenon called clonal hematopoiesis, in which blood stem cells acquire mutations and begin to dominate the marrow even before any symptoms appear. The specific mutations a person’s stem cells accumulate appear to steer whether they eventually develop an MPN or an MDS. Mutations in splicing-factor genes like SF3B1, SRSF2, and U2AF1 carry a much higher risk of progressing to MDS, while driver mutations in JAK2, CALR, and MPL strongly predispose toward MPNs.²

What makes myelofibrosis unusual among MPNs is that many patients carry mutations from both columns. Their initiating event is an MPN-type driver mutation, but over time they accumulate secondary hits in MDS-associated genes. This layered mutational architecture is part of why advanced myelofibrosis can clinically mimic MDS, and why some researchers describe myelofibrosis as occupying a position between the two disease families rather than belonging cleanly to one. The answer to whether myelofibrosis is a myelodysplastic syndrome remains no by formal classification, but the biology increasingly shows these diseases are closer relatives than the organizational chart implies.