Cause
Some people have a history of exposure to chemotherapy (especially alkylating agents such as melphalan, cyclophosphamide, busulfan, and chlorambucil) or radiation (therapeutic or accidental), or both (e.g., at the time of stem cell transplantation for another disease). Workers in some industries with heavy exposure to hydrocarbons such as the petroleum industry have a slightly higher risk of contracting the disease than the general population. Xylene and benzene exposures have been associated with myelodysplasia. Vietnam veterans exposed to Agent Orange are at risk of developing MDS. A link may exist between the development of MDS "in atomic-bomb survivors 40 to 60 years after radiation exposure" (in this case, referring to people who were in close proximity to the dropping of the atomic bombs in Hiroshima and Nagasaki during World War II). Children with Down syndrome are susceptible to MDS, and a family history may indicate a hereditary form of sideroblastic anemia or Fanconi anemia. Pathophysiology
MDS most often develops without an identifiable cause. Risk factors include exposure to an agent known to cause DNA damage, such as radiation, benzene, and certain chemotherapies; other risk factors have been inconsistently reported. Proving a connection between a suspected exposure and the development of MDS can be difficult, but the presence of genetic abnormalities may provide some supportive information. Secondary MDS can occur as a late toxicity of cancer therapy (therapy associated MDS, t-MDS).
MDS after exposure to radiation or alkylating agents such as busulfan, nitrosourea, or procarbazine, typically occurs 3–7 years after exposure and frequently demonstrates loss of chromosome 5 or 7. MDS after exposure to DNA topoisomerase II inhibitors occurs after a shorter latency of only 1–3 years and can have a 11q23 translocation. Other pre-existing bone-marrow disorders such as acquired aplastic anemia following immunosuppressive treatment and Fanconi anemia can evolve into MDS.MDS is thought to arise from mutations in the multipotent bone-marrow stem cell, but the specific defects responsible for these diseases remain poorly understood. Differentiation of blood precursor cells is impaired, and a significant increase in levels of apoptotic cell death occurs in bone-marrow cells. Clonal expansion of the abnormal cells results in the production of cells that have lost the ability to differentiate. If the overall percentage of bone-marrow myeloblasts rises over a particular cutoff (20% for WHO and 30% for FAB), then transformation to acute myelogenous leukemia (AML) is said to have occurred. The progression of MDS to AML is a good example of the multistep theory of carcinogenesis in which a series of mutations occurs in an initially normal cell and transforms it into a cancer cell.Although recognition of leukemic transformation was historically important (see History), a significant proportion of the morbidity and mortality attributable to MDS results not from transformation to AML, but rather from the cytopenias seen in all MDS patients.