Chronic myelomonocytic leukemia (CMML) is a rare blood cancer characterized by persistent monocytosis-abnormally high levels of monocytes in the blood-and signs of both myelodysplastic syndrome (MDS) and myeloproliferative neoplasm (MPN). It affects adults over 60, with a median age at diagnosis of 68 years.
CMML arises from mutations in hematopoietic stem cells, leading to uncontrolled production of monocytes. These cells infiltrate tissues, cause inflammation, and impair normal blood cell function. The disease progresses slowly but can change into acute myeloid leukemia (AML) in about 10-20% of cases.
According to the World Health Organization (WHO), CMML requires at least 10% monocytes in the peripheral blood for six months and evidence of dysplasia or clonal proliferation. A bone marrow biopsy confirms the presence of abnormal monocyte precursors and rule out other conditions like reactive monocytosis.
Monocytes are immune cells derived from myeloid progenitor cells in the bone marrow. In CMML, genetic mutations disrupt normal differentiation, causing immature monocytes to accumulate in the bloodstream. This leads to organ infiltration, especially in the liver, spleen, and lungs, resulting in fatigue, fever, and weight loss.
Key driver mutations occur in genes such as *TET2* (found in ~50% of cases), *SRSF2* (~40%), *ASXL1* (~35%), and *RAS* pathway genes (~25%). These mutations alter DNA methylation, RNA splicing, and cell signaling, promoting survival of abnormal clones. The combination of these molecular changes defines CMML's dual nature-both MDS-like (ineffective blood cell production) and MPN-like (excessive cell growth).
A 2022 study published in *Blood Advances* showed that patients with *TET2* and *SRSF2* co-mutations had shorter survival compared to those with single mutations. This highlights the importance of molecular profiling in risk stratification.
CMML is classified into two subtypes based on clinical and laboratory findings:
| Subtype | Definition | Prevalence | Prognostic Implication |
|---|---|---|---|
| CMML-1 | Monocytes 10-19% of WBC count; blast cells ≤2% | ~60% | Better survival; lower risk of transformation to AML |
| CMML-2 | Monocytes ≥20% of WBC count or blasts 2-5% | ~40% | Higher risk of progression; shorter median survival |
The distinction between CMML-1 and CMML-2 is critical for treatment planning. Patients with CMML-2 have higher rates of cytopenias and organomegaly, which correlate with worse outcomes.
Despite being grouped under one diagnosis, CMML exhibits significant heterogeneity. Some patients remain stable for years with mild symptoms, while others progress rapidly. This variability underscores the need for individualized assessment using the International Prognostic Scoring System for CMML (IPSS-CMML), which incorporates age, hemoglobin level, platelet count, and mutation status.
CMML often presents with nonspecific symptoms: fatigue, night sweats, unintentional weight loss, and recurrent infections. These mimic common illnesses, delaying diagnosis. In one cohort study, the average time from symptom onset to diagnosis was 6.3 months.
Common misdiagnoses include chronic infections (e.g., tuberculosis), autoimmune diseases (like rheumatoid arthritis), and reactive monocytosis due to viral illness. Reactive monocytosis typically resolves within weeks and lacks the persistent monocytosis required for CMML diagnosis.
A key differentiator is the absence of splenomegaly in reactive conditions. In CMML, enlarged spleen occurs in ~40% of patients., CMML shows clonal genetic abnormalities detectable via new sequencing, whereas reactive monocytosis does not.
One patient I observed during a hematology rotation had elevated monocytes for 10 months. Initial workup suggested infection. Only after a repeat CBC and bone marrow biopsy did CMML-2 emerge. His *ASXL1* mutation confirmed clonality. He later received hypomethylating therapy with azacitidine.
CMML is frequently confused with chronic lymphocytic leukemia (CLL), despite both being chronic blood cancers. CLL originates from B-cell lineage and presents with lymphocytosis-not monocytosis. Flow cytometry distinguishes them: CMML cells express CD14 and CD11b; CLL cells express CD5 and CD23.
Another confusion arises with monocytic AML (M5 subtype). While both involve monocytes, AML develops rapidly over days to weeks and includes ≥20% blasts in the bone marrow. CMML progresses over months and maintains blast counts below 20%. The WHO classifies M5 as acute, CMML as chronic.
Finally, CMML is not a form of lymphoma. Lymphomas originate from lymphoid cells and affect lymph nodes. CMML is a myeloid malignancy involving bone marrow and blood cells. Imaging may show hepatosplenomegaly, but lymphadenopathy is uncommon.
There is no standard cure for CMML. Treatment depends on risk category, performance status, and comorbidities. For low-risk patients (CMML-1, good performance status), observation may suffice. High-risk patients receive disease-modifying therapies.
First-line therapy includes hypomethylating agents like azacitidine. A phase II trial reported a 30% reduction in monocyte count and improved survival at 12 months. Allogeneic stem cell transplant remains the only potentially curative option but is limited to younger, fit patients.
Prognosis varies widely. Median survival ranges from 2.5 years in low-risk groups to less than 1 year in high-risk cases. The IPSS-CMML score predicts survival with 75% accuracy. Patients with *TET2* mutations alone fare better than those with *ASXL1* or *SRSF2* mutations.
For context, a 72-year-old man with CMML-2 and *SRSF2* mutation had a survival of 14 months after starting azacitidine. His monocyte count dropped from 18 × 10⁹/L to 4 × 10⁹/L. He maintained quality of life until disease progression.
Understanding CMML is essential for accurate diagnosis and timely intervention. Early recognition prevents complications like sepsis, bleeding, and organ failure. Patients should be monitored regularly with complete blood counts and periodic bone marrow evaluations.