Myelomonocytic leukemia is a blood cancer in which the bone marrow overproduces two related families of white cells: granulocytes (the myeloid line that makes neutrophils) and monocytes. The name actually covers three different diseases. The acute form, acute myelomonocytic leukemia, is a subtype of acute myeloid leukemia (AML) that develops over weeks. Chronic myelomonocytic leukemia (CMML) is a slower condition mostly seen in older adults, and juvenile myelomonocytic leukemia (JMML) is a rare disease of young children.
These insights into myelomonocytic leukemia cover how the three forms differ, what causes them, how they are diagnosed, and how they are treated.
What Is Myelomonocytic Leukemia?
All blood cells start as stem cells in the bone marrow. One branch, the myeloid line, gives rise to both neutrophils and monocytes through a shared parent cell. In myelomonocytic leukemia, a genetic error in these early cells causes both branches to multiply abnormally. The leukemic cells crowd the marrow and push out normal red cells, platelets, and functioning white cells.
Monocytes normally circulate in the blood and then move into tissues, where they become macrophages that clear debris and fight infection. Because leukemic monocytes keep this tendency to invade tissues, myelomonocytic leukemias can involve the gums, skin, spleen, and sometimes the nervous system.
The Three Types Compared
| Feature | Acute myelomonocytic leukemia (AMML) | Chronic myelomonocytic leukemia (CMML) | Juvenile myelomonocytic leukemia (JMML) |
|---|---|---|---|
| Who is affected | Any age, mostly adults | Mainly older adults, usually over 60 | Infants and young children |
| Pace | Rapid, weeks | Slow, months to years | Variable, often aggressive |
| Blasts in marrow or blood | 20% or more (the AML threshold) | Under 20% | Under 20% |
| Hallmark finding | Mixed myeloblasts and monocytic cells | Persistent raised monocyte count | Raised monocytes, large spleen |
| Classification | Subtype of AML (formerly FAB M4) | Myelodysplastic/myeloproliferative neoplasm | Myelodysplastic/myeloproliferative neoplasm of childhood |
The acute form is covered in more depth in our clinician’s guide to acute myelomonocytic leukemia.
Causes and Risk Factors
Like other leukemias, myelomonocytic leukemia is caused by acquired mutations in blood stem cells. These are changes that happen during a person’s lifetime, not usually inherited defects. In most patients, no single trigger is found.
Genetic changes by type
- AMML: mutations in genes such as NPM1 and FLT3 are common in AML generally. A specific subtype carries a chromosome change called inversion 16, or inv(16), and is marked by abnormal eosinophils in the marrow. This subtype usually responds well to chemotherapy.
- CMML: mutations in TET2, SRSF2, and ASXL1 are typical. Mutations in the RAS signaling pathway also occur.
- JMML: nearly all cases involve the RAS pathway, through genes such as PTPN11, NRAS, KRAS, NF1, or CBL. Children with neurofibromatosis type 1 or Noonan syndrome have a higher risk.
Other risk factors
- Older age (for AMML and CMML)
- Previous chemotherapy or radiotherapy for another cancer, which can cause “therapy-related” myeloid disease
- Heavy exposure to benzene or high doses of ionizing radiation
- A prior myelodysplastic syndrome, which can progress to AML
Beyond gene mutations, epigenetic changes also play a part. These alter how genes are switched on and off without changing the DNA code, mainly through DNA methylation. That is the reason drugs that reverse methylation have a role in treatment.
Symptoms and Clinical Presentation
Most symptoms come from the marrow failing to make enough normal cells:
- Anemia: tiredness, breathlessness, pale skin
- Low platelets: easy bruising, nosebleeds, bleeding gums, small red skin spots (petechiae)
- Low working neutrophils: frequent or severe infections and fevers
Features that point specifically to monocytic involvement include swollen, overgrown gums (gingival hypertrophy), skin nodules or rashes from leukemic cell infiltration (leukemia cutis), and an enlarged spleen or liver. CMML is often found by chance on a routine blood count showing high monocytes, before any symptoms appear. JMML typically presents with a markedly enlarged spleen, poor growth, rash, and fever in a young child.
Diagnosis and Testing
Diagnosis starts with a complete blood count and a blood film reviewed under the microscope. The key test is a bone marrow aspirate and biopsy, which counts the proportion of blasts and monocytic cells and supplies samples for specialized testing.
| Test | What it shows |
|---|---|
| Blood count and film | Raised monocytes, low hemoglobin or platelets, circulating blasts |
| Bone marrow examination | Blast percentage, which separates acute from chronic forms |
| Cytochemical stains | Myeloperoxidase marks myeloid cells; nonspecific esterase marks monocytic cells |
| Flow cytometry | Surface markers such as CD14 and CD64 that identify monocytic cells |
| Cytogenetics | Chromosome changes such as inv(16) |
| Molecular testing (PCR, next-generation sequencing) | Mutations such as NPM1, FLT3, TET2, ASXL1, RAS pathway genes |
For CMML, the defining feature is a persistently raised monocyte count, classically at least 1 × 10⁹/L (1,000 per microliter) and at least 10% of white cells, with blasts under 20%. Newer classification systems accept somewhat lower monocyte counts when other evidence supports the diagnosis. Doctors must also exclude reactive causes of monocytosis, such as chronic infection, and other blood cancers like chronic myeloid leukemia.
Treatment Options
Acute myelomonocytic leukemia
Fit patients are treated like other AML subtypes, with intensive induction chemotherapy, most commonly the “7+3” combination of cytarabine and an anthracycline. Patients with a FLT3 mutation may also receive midostaurin. Those who are older or less fit often receive a hypomethylating agent (azacitidine or decitabine) combined with venetoclax. An allogeneic stem cell transplant from a donor is offered to suitable patients at higher risk of relapse.
CMML
Treatment depends on symptoms and risk. Patients with stable counts may simply be monitored. Hydroxyurea helps control very high white counts and an enlarged spleen, while hypomethylating agents are used for more advanced disease. Allogeneic transplant is the only treatment that can cure CMML, but it is suitable for a minority of patients because of age and other health conditions.
JMML
Allogeneic stem cell transplant is the main curative treatment for most children. A small number with certain mutations, particularly some CBL or NRAS cases, can have milder disease that settles without transplant, so specialist pediatric centers individualize care.
Supportive care matters across all three forms: transfusions, antibiotics and antifungals, and prompt treatment of fever. Newer approaches, including immune-based therapies and targeted drugs, are being studied in clinical trials, but for myeloid leukemias they remain largely investigational.
Key Takeaways
- “Myelomonocytic leukemia” covers three diseases: acute (a subtype of AML), chronic (CMML), and juvenile (JMML).
- The blast percentage in the marrow, at or above 20% versus below it, separates the acute form from the chronic ones.
- Gum swelling, skin involvement, and an enlarged spleen hint at monocytic disease.
- Genetic testing guides both prognosis and the choice of targeted treatment.
- Stem cell transplant is the main curative option for high-risk AMML, CMML, and most JMML.
Frequently Asked Questions
Is myelomonocytic leukemia the same as AML?
Acute myelomonocytic leukemia is a subtype of AML, so in that sense yes. Chronic and juvenile myelomonocytic leukemia are different diseases, classified as overlap conditions between myelodysplastic and myeloproliferative neoplasms.
Can CMML turn into acute leukemia?
Yes. A proportion of patients with CMML progress to AML over time, and this is one reason doctors monitor blood counts and blast levels closely. Higher blast counts and certain mutations, such as ASXL1, are linked to a greater risk.
Why do gums swell in myelomonocytic leukemia?
Leukemic monocytes tend to leave the bloodstream and settle in tissues. When they infiltrate the gums, the tissue becomes thickened and can bleed easily. Swollen gums with abnormal blood counts should prompt an urgent medical review.
Is myelomonocytic leukemia inherited?
Usually not. The mutations are acquired during life. The exception is JMML, where children with some inherited conditions, such as neurofibromatosis type 1 or Noonan syndrome, carry a higher risk.
Read more about the wider disease family in our leukemia guide.
