Polycythemia vera is caused by a genetic mutation that forces your bone marrow to produce too many red blood cells. In about 90% of cases, the culprit is a specific mutation in the JAK2 gene, a gene that controls how blood cells grow and divide. This isn’t a mutation you inherit from your parents in most cases. It develops on its own during your lifetime, in a single blood-forming stem cell that then multiplies out of control.
The JAK2 Mutation
The JAK2 gene produces a protein that acts like an on/off switch for blood cell production. Normally, your body sends chemical signals (through hormones like erythropoietin) telling your bone marrow when to make more red blood cells and when to stop. The JAK2 protein sits inside that signaling chain, relaying the message from the cell’s surface to its nucleus.
In polycythemia vera, a single amino acid swap in the JAK2 protein (a valine replaced by phenylalanine at position 617) locks that switch in the “on” position permanently. The bone marrow no longer waits for a signal to ramp up production. It churns out red blood cells, and often platelets, continuously. This is why the disease is classified as a myeloproliferative neoplasm: “myelo” for bone marrow, “proliferative” for overgrowth.
A smaller number of patients, roughly those in the remaining 10%, carry a different mutation in the same JAK2 gene, located in a region called exon 12. These patients tend to have a slightly different presentation: their red blood cell counts are elevated, but their platelet and white blood cell counts often stay closer to normal. The bone marrow looks different under a microscope too, with red cell precursors expanding while other cell lines remain relatively undisturbed.
Why This Mutation Happens
The honest answer is that no one fully understands why the JAK2 mutation arises in the first place. It’s a somatic mutation, meaning it occurs in a single cell after birth rather than being passed down through families. You can think of it the way you’d think of many cancers: a random copying error in DNA that happens to land in exactly the wrong spot. Unlike many cancers, though, polycythemia vera doesn’t appear to have strong links to environmental exposures, diet, or lifestyle choices.
There is a small hereditary component. Some families carry inherited genetic variants that make them slightly more susceptible to developing JAK2 mutations, but having these variants doesn’t mean you’ll develop the disease. Polycythemia vera itself is not directly inherited.
Who Gets Polycythemia Vera
Polycythemia vera is uncommon, affecting roughly 0.4 to 2.8 people per 100,000 each year. It’s diagnosed most often in middle age, with median diagnosis around age 50 to 53 depending on gender. Women tend to be diagnosed a few years earlier than men. The disease can occur at any age but is rare in people under 40.
What the Mutation Does to Your Blood
The immediate consequence of the JAK2 mutation is too many red blood cells. More red blood cells means thicker blood, and thicker blood flows more slowly and is more prone to clotting. This is the central danger of polycythemia vera.
Thrombosis (blood clots) is the most serious complication. Between 12% and 15% of patients have already experienced a clot before they’re even diagnosed. Arterial clots, which can cause strokes and heart attacks, are more common than venous clots: roughly 16% to 27% of patients experience arterial events before or at diagnosis, compared to 7% to 12% for venous events. Many people first discover they have polycythemia vera only after a clot sends them to the hospital.
One telltale sign that distinguishes polycythemia vera from other causes of high red blood cell counts is erythropoietin levels. Erythropoietin is the hormone your kidneys release to tell your bone marrow to make more red blood cells. In polycythemia vera, erythropoietin levels drop well below normal because the bone marrow is ignoring the normal feedback loop. In one study, 34 out of 36 polycythemia vera patients had erythropoietin below the reference range, averaging just 2.1 units per liter. Even after treatment brought their blood counts back to normal, erythropoietin stayed suppressed for years. By contrast, people with high red blood cell counts from other causes (like lung disease or living at high altitude) have erythropoietin levels that are elevated or normal.
How It Differs From Secondary Polycythemia
Not everyone with elevated red blood cell counts has polycythemia vera. Secondary polycythemia is a separate condition where the body produces extra red blood cells in response to something external, usually low oxygen levels. Common causes include chronic lung disease (like COPD or uncontrolled asthma), sleep apnea, heavy smoking, and living at high altitude. In these cases, the bone marrow is working correctly; it’s simply responding to a legitimate signal that the body needs more oxygen-carrying capacity.
The distinction matters because secondary polycythemia is treated by addressing the underlying cause (quitting smoking, treating sleep apnea, supplemental oxygen), while polycythemia vera requires ongoing management of the bone marrow itself. The erythropoietin test is one of the clearest ways to tell them apart: low erythropoietin points toward polycythemia vera, while high erythropoietin suggests a secondary cause.
How Polycythemia Vera Is Diagnosed
Diagnosis follows criteria established by the World Health Organization. The primary red flag is an elevated hemoglobin level: above 165 g/L for men or above 160 g/L for women. Alternatively, a hematocrit (the percentage of blood volume occupied by red blood cells) above 49% in men or 48% in women raises suspicion. A red cell mass more than 25% above the predicted value for your body size also qualifies.
From there, testing for the JAK2 mutation confirms the diagnosis in the vast majority of cases. If the common JAK2 V617F mutation isn’t found, doctors may test for the less common exon 12 mutations. Bone marrow biopsy and erythropoietin levels fill in the remaining picture.
Long-Term Disease Progression
Polycythemia vera is a chronic condition. Most people live with it for many years with proper management, but there is a risk of the disease evolving into something more serious over time. About 12% of patients develop myelofibrosis within 10 years, a condition where scar tissue gradually replaces healthy bone marrow. By 15 years, that number rises to roughly 25%. Transformation to acute myeloid leukemia is less common but does occur, affecting about 3.2% of patients at the 10-year mark and around 8.2% by 15 years.
These progression rates are important context for understanding what polycythemia vera is: not a static condition but a slow-moving blood cancer with the potential to change character over years or decades. Regular monitoring is a lifelong part of living with this disease.