Each year about 10,000 to 15,000 people are diagnosed with myelodysplastic syndrome (MDS) in the United States. At Seattle Cancer Care Alliance, we see about 200 patients every year who have MDS or related diseases.
What Is MDS?
MDS is not a single disease but a group of diseases that affect blood-cell formation. In all forms of MDS, a bone marrow problem leads to low levels of blood cells circulating in the bloodstream.
- “Myelo” refers to bone marrow, which is the body’s blood-cell factory.
- “Dysplastic” refers to abnormal growth or development.
- “Myelodysplastic” means the bone marrow produces abnormal blood cells.
To understand more about what MDS is, it’s important to understand some basics about how blood cells normally form.
Healthy Blood-Cell Formation
In a healthy person, the stem cells in the bone marrow make daughter cells. The daughter cells go through several stages of development within the marrow. Eventually they mature into red blood cells (RBCs), white blood cells (WBCs), or platelets. These get released from the marrow to circulate in the bloodstream.
The circulating cells perform important functions.
- RBCs carry oxygen throughout the body.
- WBCs fight infection.
- Platelets protect against easy bleeding by helping your blood to clot.
Blood-Cell Formation in MDS
In a person with MDS, the stem cells in the bone marrow don’t function normally. Instead of producing healthy, mature RBCs, WBCs, and platelets, the marrow makes cells that tend to remain immature and to die early.
About 80 to 85 percent of people with MDS have more cells in their marrow than healthy people do (hypercellular marrow). But these cells do not live long enough to make it out of the marrow into the bloodstream, or they are not in circulation long before they die. As a result, people with MDS have low levels of one or more types of blood cells in their bloodstream (cytopenia).
- A low level of RBCs is called anemia.
- A low level of WBCs is called leukopenia.
- A low level of platelets is called thrombocytopenia.
Low levels of blood cells, or low blood counts, cause the symptoms of MDS.
MDS progresses over time in two ways. In most people with MDS, fewer and fewer healthy blood cells are produced or survive. This can lead to severe anemia (low RBCs), increased risk of infection (due to low WBCs), or risk of severe bleeding (due to low platelets).
In about 30 percent of people with MDS (depending on the subtype), the number of very immature abnormal cells in the marrow (blast cells, or blasts) increases, and MDS transforms into acute leukemia. This is why MDS is also referred to as preleukemia or smoldering leukemia.
Subtypes of MDS
Doctors divide MDS into subtypes depending on the following:
- Whether you have increased numbers of blast cells in your bone marrow or blood, and what percentage of your marrow or blood is made up of these blasts
- Whether your marrow shows abnormal growth (dysplasia) in only one type of blood cell (unilineage dysplasia) or in more than one type of blood cell (multilineage dysplasia)
- Whether there are chromosome abnormalities in your marrow cells and, if so, which type or types of abnormalities
Doctors also look at the surface markers of MDS cells to see whether the cells express certain antigens. Antigens are substances (often proteins) that are recognized by the immune system. Researchers at Fred Hutchinson Cancer Research Center pioneered the process of detecting MDS cells by flow cytometry and developing diagnostic and prognostic models based on these surface markers.
These and other factors (in particular, any other health problems you have) may help your doctor decide which treatment options make the most sense for you and how aggressive your treatment should be.
Here is a list of MDS subtypes, according to the World Health Organization (WHO) system of MDS classification.
- Refractory cytopenia with unilineage dysplasia (RCUD). This subtype means that you have too few RBCs in your blood (you have anemia) or too few WBCs in your blood (you have neutropenia) or too few platelets in your blood (you have thrombocytopenia). Occasionally patients with RCUD are low in two of these three types of blood cells. Your marrow shows dysplastic changes in only one of the three types of blood cells. You have a low percentage of blasts in your marrow and blood. Depending on your blood counts, your condition may be called refractory anemia, refractory neutropenia, or refractory thrombocytopenia.
- Refractory anemia with ring sideroblasts (RARS). Ring sideroblasts are early stages of RBCs that have accumulated abnormal amounts of iron. The iron shows up as a ring when stained with a particular dye. This subtype typically means that you have too few RBCs in your blood (you have anemia). Your WBC and platelet counts may be normal. You have a low percentage of blasts in your marrow and blood.
- Refractory cytopenia with multilineage dysplasia (RCMD). This subtype means that your marrow shows dysplastic changes in the cells that make at least two types of blood cells: RBCs, WBCs, or platelets. You also have a low percentage of blasts in your marrow and blood. If you have multilineage dysplasia and ring sideroblasts, this is classified as RCMD-RS.
- Refractory anemia with excess blasts-1 (RAEB-1) and refractory anemia with excess blasts-2 (RAEB-2). These subtypes mean that you have at least 5 percent (RAEB-1) or at least 10 percent (RAEB-2) but less than 20 percent blasts in your marrow.
- MDS associated with isolated abnormality of chromosome 5 [del(5q)]. This subtype means that part of chromosome 5 is missing. Typically it means you have too few RBCs in your blood (you have anemia). You have a low percentage of blasts in your marrow and blood.
- MDS, unclassifiable (MDS-U). This subtype means that your marrow shows dysplastic changes in cells that make WBCs or platelets (but not in those that make RBCs). You have a normal percentage of blasts in your marrow and blood. And your MDS does not fit into one of the other subtypes.
Some problems with blood-cell formation are not only myelodysplastic (having to do with abnormal production of cells in the marrow) or only myeloproliferative (having to do with overproduction of cells in the marrow). Instead, these problems have features of both MDS and myeloproliferative neoplasms. They include the following:
- Chronic myelomonocytic leukemia (CMML). The main feature of CMML is that you have too many myelocytes and monocytes, both types of WBCs, in your blood. You may have low levels of RBCs and platelets in your blood, and you may have abnormal cells in your marrow.
- Juvenile myelomonocytic leukemia (JMML). JMML is similar to CMML, but it occurs in young children. It causes high levels of myelocytes and monocytes. There may be low levels of RBCs and platelets in the child’s blood, and there may be abnormal cells in the child’s marrow.
- Atypical chronic myeloid leukemia (aCML), BCR-ABL 1–negative. In aCML, you have too many granulocytes, a type of WBC. You may have low levels of RBCs and platelets in your blood, and you may have abnormal cells in your marrow. aCML is much like chronic myeloid leukemia (CML), except people with CML have a chromosome change called the Philadelphia chromosome that people with aCML do not have.
- MDS/MPN, unclassifiable. This means your disease has features of MDS and MPN and does not fit into one of the other subtypes. This subtype includes refractory anemia with ringed sideroblasts and thrombocytosis (RARS-T), which is like RARS, above, but with the added feature that you have too many platelets in your blood.