What Is CMML? Diagnosis, Genetics, and Treatment

Chronic myelomonocytic leukemia, usually called CMML, is a blood cancer in which the bone marrow overproduces monocytes, a type of white blood cell that normally helps fight infection. It sits in an unusual overlap zone between two broader families of blood disorders, carrying features of both sluggish, poorly developing blood cells and overly aggressive cell production. The disease mostly affects older adults, with a median age at diagnosis in the early-to-mid 70s, and it behaves unpredictably: some people live for years with manageable symptoms, while others progress to acute leukemia within months. That wide range of outcomes makes understanding the diagnosis, the genetics driving it, and the available treatment approaches genuinely useful for anyone navigating a CMML diagnosis.

How CMML Is Diagnosed

The hallmark of CMML is a persistent increase in monocytes circulating in the blood. Under current diagnostic criteria, the monocyte count must be at least 0.5 × 10⁹ per liter and monocytes must make up at least 10 percent of all white blood cells, sustained for longer than three months. A bone marrow biopsy must show consistent changes in how blood cells look and mature, with blasts (immature cells, including a subtype called promonocytes) staying below 20 percent. Doctors also look for genetic or chromosomal evidence that the monocyte expansion is driven by a single abnormal clone of cells rather than an infection or inflammation temporarily boosting monocyte numbers.¹

That three-month sustained monocytosis requirement matters because plenty of everyday conditions can push monocytes up temporarily. Chronic infections, autoimmune diseases, recovery from chemotherapy, and even some medications can all cause a reactive monocytosis that looks superficially similar. Working through these possibilities is a real diagnostic challenge, and monocyte counts alone are not enough to clinch the diagnosis.²

Flow Cytometry and Monocyte Subsets

One of the more helpful advances in CMML diagnosis involves looking not just at how many monocytes there are, but at what kinds. Monocytes come in subtypes: classical, intermediate, and nonclassical. In CMML, the classical monocytes dominate to an unusual degree. A multicenter validation study found that when classical monocytes make up 94 percent or more of total monocytes on flow cytometry, the test has a sensitivity of about 94 percent and specificity of roughly 90 percent for CMML.³ A separate study noted that a decrease in the nonclassical monocyte fraction was even more sensitive than the classical monocyte increase at distinguishing CMML from other causes of monocytosis.⁴

This flow cytometry approach is increasingly used in routine practice because it can flag CMML early, even when other findings are borderline. The disease has historically been tricky to pin down, partly because of an emerging concept called oligomonocytic CMML, where monocyte counts are elevated but do not yet meet the traditional threshold. Whether oligomonocytic CMML represents a genuine early stage of the disease or a separate entity remains an active debate.⁵

Dysplastic Versus Proliferative Phenotypes

CMML has long been divided into two broad clinical pictures based on the white blood cell count at diagnosis. When the total white blood cell count is below 13 × 10⁹ per liter, the disease is classified as dysplastic (sometimes abbreviated MD-CMML), resembling myelodysplastic syndromes with low blood counts, anemia, and sluggish marrow output. When white cells are above that threshold, it is considered proliferative (MP-CMML), behaving more like a myeloproliferative neoplasm with elevated counts, an enlarged spleen, and sometimes constitutional symptoms like night sweats and weight loss.

These are not just academic distinctions. In a retrospective study of 91 patients, the proliferative form carried a higher rate of progression to acute leukemia and a shorter median survival. The dysplastic form had a longer overall survival but a longer pre-leukemic phase, meaning patients lived longer with chronic disease before any transformation occurred.⁶ A separate study confirmed the pattern and added that splenomegaly, liver enlargement, lymph node involvement, and skin involvement were all more common in the proliferative group, with median survival of 11 months versus 30 months in the dysplastic group.⁷

Whether dysplastic and proliferative CMML are truly two separate diseases or represent two phases of a single disease that can evolve over time is still debated. Some patients initially present with a dysplastic picture and later develop proliferative features. The practical takeaway is that the proliferative form tends to be more aggressive and often requires treatment sooner.

The Genetic Landscape

Genetic testing has transformed how CMML is understood and managed. Chromosomal abnormalities show up in roughly 30 percent of patients, and somatic gene mutations are found in up to 90 percent.⁸ The three most commonly mutated genes are TET2, SRSF2, and ASXL1, and one large study of 275 patients found that 93 percent carried at least one mutation across nine frequently affected genes.⁹

These genes affect different parts of how cells read and regulate their DNA. TET2, the most commonly mutated, is involved in removing chemical tags (methyl groups) from DNA, which influences which genes get turned on or off. When TET2 is broken, abnormal methylation patterns accumulate. Studies have shown that patients with TET2 mutations have measurably higher levels of overall DNA methylation compared with patients whose TET2 is intact.¹⁰ SRSF2, mutated in close to half of CMML patients, affects how genetic instructions are spliced together before being used to make proteins. ASXL1 plays a role in how tightly DNA is packaged and which genes are accessible. These mutations often co-occur, and ASXL1/SRSF2 co-mutation in particular has been associated with a genetic profile that overlaps heavily with acute myeloid leukemia showing monocytic features.¹¹

Beyond these three, mutations in signaling pathway genes also matter. A study of 70 patients found that about 7 percent of those without RAS mutations instead carried BRAF mutations, both of which activate the same growth-signaling cascade.¹² Identifying these mutations can matter for choosing targeted treatments, since the specific signaling pathways involved shape which drugs might work.

Why ASXL1 Mutations Get Special Attention

Among all the mutations found in CMML, ASXL1 stands out for its prognostic weight. Patients with ASXL1 mutations consistently do worse. One study found that every patient whose CMML progressed to acute leukemia carried an ASXL1 mutation, while none of the patients without the mutation transformed during follow-up.¹³ A separate analysis in Korean patients confirmed the survival difference, with median overall survival of 11 months for those with the mutation versus 19 months without it.¹⁴

ASXL1 status is now built into several scoring systems used to estimate prognosis and guide treatment decisions. Its presence can tip the balance toward earlier, more aggressive intervention, including consideration of stem cell transplantation in patients who are fit enough for it.

DNA Methylation Patterns and What They Reveal

Beyond individual gene mutations, the overall pattern of DNA methylation across the genome carries additional information. A study that profiled methylation in CMML patients identified two main clusters. One cluster was enriched for patients with worse clinical features and poorer survival, while the other grouped patients with more favorable characteristics. Methylation patterns also differed between patients with low-risk and higher-risk chromosomal abnormalities, and between those with and without TET2 mutations.¹⁵ This is relevant clinically because the main drugs used to treat CMML, called hypomethylating agents, work by stripping methyl groups off DNA. Patients whose disease is heavily driven by methylation abnormalities may be the ones who benefit most, though the field is still working out how to use methylation profiling to select therapy in practice.

Risk Stratification

Predicting how a given patient’s CMML will behave requires pulling together clinical findings and genetic data. Several scoring systems exist, but the CPSS-Mol model is one of the most validated. It combines chromosomal abnormalities, mutations in RUNX1, NRAS, SETBP1, and ASXL1, red blood cell transfusion dependence, white blood cell count, and marrow blast percentage to sort patients into four risk groups. In the original study, median overall survival ranged from more than 144 months in the lowest-risk group down to 18 months in the highest-risk group, and the four-year cumulative risk of transforming to acute leukemia ranged from zero to 48 percent.¹⁶

A recent comparative analysis found that the CPSS-Mol and the IPSS-M (a model originally built for myelodysplastic syndromes but applied to CMML) performed equivalently and both outperformed older models that relied on fewer variables.¹⁷ The practical message is that gene panels are now considered standard care in CMML, not optional extras. Risk scores inform whether watchful waiting is reasonable, whether drug therapy should start promptly, or whether transplant should be discussed.

Autoimmune and Inflammatory Complications

One of CMML’s more surprising features is its frequent association with inflammatory and autoimmune problems. Roughly 10 to 30 percent of patients develop some form of systemic inflammatory or autoimmune disease alongside their leukemia.¹⁸ About 20 percent will have a clinically recognized autoimmune condition at some point during their disease course.¹⁹

The most common type is vasculitis, an inflammation of blood vessels. In one case series with a literature review, vasculitis accounted for the majority of autoimmune diagnoses in CMML patients. In about a third of cases, the autoimmune disease and the leukemia were diagnosed at the same time, while in nearly half the CMML came first. Steroids worked well as initial treatment, with an 85 percent response rate, though about 40 percent of patients eventually needed second-line therapy. Hypomethylating agents used for the leukemia also improved the autoimmune symptoms in about two-thirds of patients who received them.²⁰

Whether the autoimmune diseases are caused by the abnormal monocyte clone, triggered by the same underlying immune dysregulation, or are simply coincidental in an older population remains unclear. What is clear is that unexplained vasculitis or other inflammatory symptoms in an older patient should prompt a look at the blood counts and monocyte percentage.

GM-CSF Signaling and Inflammation

At the molecular level, CMML cells behave abnormally in how they respond to growth signals. One important finding is that CMML monocytes and their precursors are hypersensitive to GM-CSF, a growth factor that normally stimulates white blood cell production in measured amounts. In laboratory testing, CMML cells showed an exaggerated activation of the STAT5 signaling pathway in response to GM-CSF compared with healthy cells. This hypersensitivity was specific to GM-CSF rather than being a generalized overreaction to all growth signals, and it correlated with higher-risk disease features including higher white cell and monocyte counts.²¹

This finding is relevant because it identifies a potential therapeutic target. Early-phase studies of drugs that block GM-CSF signaling or the downstream JAK-STAT pathway have shown some promise in controlling disease activity and improving symptoms.²² Whether these approaches will become standard treatment is still being worked out in clinical trials.

Treatment for CMML

Treatment decisions in CMML depend heavily on which phenotype a patient has, their risk score, and whether they are a transplant candidate. There is no one-size-fits-all approach.

Hypomethylating Agents

For patients who need systemic therapy, hypomethylating agents (azacitidine and decitabine) are the most widely used drugs. A meta-analysis of 14 studies including 600 CMML patients found a pooled overall response rate of about 43 percent. Azacitidine and decitabine produced similar overall response rates, though decitabine was associated with a higher rate of marrow-level improvement and a higher rate of freeing patients from needing blood transfusions.²³ A 43 percent response rate is modest, and responses are usually not permanent, which underscores the need for new therapies.

Hydroxyurea and Cytoreduction

For patients with the proliferative form, particularly those with very high white blood cell counts, significant spleen enlargement, or constitutional symptoms, hydroxyurea remains a frontline tool. It is an oral drug that slows cell production and can bring counts down relatively quickly. A randomized trial showed it outperformed oral etoposide, and it remains the recommended initial cytoreductive agent. Doctors generally aim to bring the white blood cell count below 15 × 10⁹ per liter, though no hard target has been formally established.²⁴ Hydroxyurea can also be combined with hypomethylating agents in patients with higher-risk disease.²⁵

A recent analysis provided important data on what counts as adequate cytoreduction. Patients who still had a white blood cell count above 10 × 10⁹ per liter or a monocyte count above 1 × 10⁹ per liter after six treatment cycles had substantially worse overall survival and a higher rate of transformation to acute leukemia, regardless of which drug they were on. Among those treated with hydroxyurea, patients with persistent white counts above 10 had a median survival from that point of about 14 months, compared with about 45 months for those who achieved adequate count reduction.²⁶ This suggests that monitoring treatment response by checking counts after several cycles offers a useful early signal of whether the current approach is working.

Stem Cell Transplantation

Allogeneic stem cell transplantation is the only treatment with the potential to cure CMML, but it comes with serious risks. In a European study of transplanted CMML patients, the four-year estimated overall survival was 33 percent, with a 41 percent rate of transplant-related mortality and a 32 percent cumulative rate of relapse. Patients who entered transplant in complete remission fared better, which argues for doing the transplant relatively early after diagnosis or after achieving the best possible response to prior therapy.²⁷ Given the high morbidity and mortality, transplantation is typically reserved for younger, fitter patients with higher-risk disease.²⁸

Differences at the Stem Cell Level Between CMML Stages

CMML is classified into stages (CMML-1 and CMML-2) based on the proportion of blasts and promonocytes in the blood and marrow, with CMML-2 having higher percentages and a worse prognosis. Research using single-cell gene expression analysis has found that the differences between these stages go deeper than just blast counts. Stem cells from CMML-1 patients showed strong activity of genes associated with neutrophil production and MYC-driven programs, while CMML-2 stem cells exhibited a distinctly different profile dominated by interferon-related gene programs. The stem cells of the two stages also diverged in their developmental trajectories, suggesting fundamentally different biology rather than a simple linear progression from one stage to the next.²⁹ If confirmed in larger studies, this could eventually lead to stage-specific treatment strategies.

Experimental Approaches on the Horizon

The limited effectiveness of current therapies has spurred interest in new drug classes. One of the more novel approaches is STX-0712, a so-called cytotoxicity-targeting chimera that works by specifically depleting monocytes and bone marrow blasts that carry the CCR2 surface protein, a marker found on many CMML cells. The FDA has granted it a fast-track designation for relapsed or refractory CMML, and a phase 1 trial is currently enrolling patients.³⁰ Fast-track designation does not guarantee approval; it means the FDA recognizes an unmet need and is willing to expedite the review process if trial results are favorable.

Other investigational strategies include drugs targeting the JAK-STAT signaling pathway (such as ruxolitinib) and antibodies directed against GM-CSF.³¹ The disease’s strong inflammatory component and its well-characterized signaling abnormalities make it a reasonable target for precision therapies, though most of these are still in early-phase testing. For a disease that has been treated with essentially the same handful of drugs for the past two decades, even modest advances represent meaningful progress.