What is chronic myelomonocytic leukemia?
Chronic myelomonocytic leukemia (CMML) is a type of cancer that starts in blood-forming cells of the bone marrow and invades the blood.
Normal bone marrow
Bone marrow is found inside certain bones such as the skull, ribs, pelvis, and spine. It is made up of blood-forming cells, fat cells, and supporting tissues that help the blood-forming cells grow. A small fraction of the blood-forming cells are a special type of cell known as stem cells. Stem cells are needed to make new cells. When a stem cell divides it makes 2 cells: one cell that stays a stem cell and another cell that can keep changing and dividing to make blood cells.
There are 3 types of blood cells: red blood cells, white blood cells, and platelets.
Red blood cells pick up oxygen in the lungs and carry it to the rest of the body. These cells also bring carbon dioxide back to the lungs. Having too few red blood cells is called anemia. It can cause people to appear pale and feel tired and weak. Severe anemia can cause shortness of breath.
White blood cells (also known as leukocytes) are important in defending the body against infection. The 2 major types of white blood cells are lymphocytes and granulocytes.
Lymphocytes are immune cells that are found in the bone marrow, the blood, and in lymph nodes. They make the antibodies that help the body fight germs. They can also directly kill invading germs by producing toxic substances that damage the cells.
Granulocytes are a group of white blood cells that destroy bacteria. They are called granulocytes because they contain granules that can be seen under the microscope. These granules are made up of enzymes and other substances which can destroy germs that cause infections.
In the bone marrow, granulocytes develop from young cells called myeloblasts. The most common type of granulocyte is the neutrophil; this cell is crucial in fighting bacteria. Other types of granulocytes are basophils, and eosinophils. When the number of neutrophils in the blood is low, it is called neutropenia. This can lead to severe infections.
Monocytes are related to the granulocyte family. They also help protect the body against bacteria. The early cells in the bone marrow that turn into monocytes are called monoblasts. When monocytes leave the bloodstream and go into tissue, they become macrophages. Macrophages can destroy germs by surrounding and digesting them. They are also important in helping lymphocytes to recognize germs and begin producing antibodies to fight them.
Platelets are thought of as a type of blood cell, but they are actually small pieces of a cell. They start as a large cell in the bone marrow cell called the megakaryocyte. Pieces of this cell break off and enter the bloodstream as platelets. Platelets are needed for your blood to clot. They plug up damaged areas of blood vessels caused by cuts or bruises. A shortage of platelets, called thrombocytopenia, can result in excessive bleeding and bruising.
Chronic myelomonocytic leukemia
In CMML, patients have an elevated number of monocytes in the blood, with a monocyte count of at least 1,000 (per mm3). Often, the monocyte count is much higher, causing the total white blood cell count to become very high as well. Usually there are abnormal cells in the bone marrow, but the amount of blasts (very early/immature cells) is below 20%. Many patients have enlarged spleens (an organ that lies just below the left rib cage). About 15% to 30% of patients go on to develop acute myeloid leukemia.
Because CMML patients have abnormal looking (dysplastic) cells in their bone marrow, for a long time CMML was considered to be a type of myelodysplastic syndrome. Still, it didn't fit in well with other diseases in that category. That is because the major problem in myelodysplastic syndrome is having too few blood cells.
Patients with CMML may have shortages of some blood cells, but a main problem is too many of a certain type of white blood cell (the monocyte). In this way CMML is more like a myeloproliferative disease (myelo -- bone marrow, proliferative -- excessive growth). Chronic myeloid leukemia is an example of a myeloproliferative disease where the problem is an overproduction of white blood cells. Since CMML has features of both myelodysplastic syndrome and myeloproliferative disorder, experts created a new category for it: myelodysplastic/myeloproliferative diseases. CMML is the most common disease in this group. Much less common diseases in this group are atypical chronic myeloid leukemia and juvenile myelomonocytic leukemia. All of these diseases produce a lot of abnormal blood cells.
Last Medical Review: 11/08/2012
Last Revised: 11/08/2012
- What Is Leukemia - Chronic Myelomonocytic (CMML)?
- Causes, Risk Factors, and Prevention
- Early Detection, Diagnosis, and Staging
- Treating Leukemia - Chronic Myelomonocytic (CMML)
- After Treatment
- Talking With Your Doctor
- What`s New in Leukemia - Chronic Myelomonocytic (CMML) Research?
- Other Resources and References