Although some cases of myelodysplastic syndrome (MDS) are linked to known risk factors, for most, the cause is unknown.

Over the past few years, scientists made great progress in understanding how certain changes in DNA of bone marrow cells may cause MDS to develop. DNA is the chemical that carries the instructions for nearly everything our cells do. We usually look like our parents because they are the source of our DNA. However, DNA affects more than the way we look.

Some genes (parts of DNA) contain instructions for controlling a cell's growth and division process. Certain genes that promote cell division are called oncogenes. Other genes called tumor suppressor genes can slow down cell division or even cause cells to die at an appropriate time. Cancers can be caused by DNA mutations (gene defects) that turn on oncogenes or turn off tumor suppressor genes.

Exposure to radiation or certain chemicals can cause mutations that lead to MDS. Sometimes these gene changes occur for no apparent reason. Every time a cell prepares to divide into 2 new cells, it must copy its DNA. This process is not perfect, and copying errors can occur. Fortunately, cells have repair enzymes that read and fix DNA. However, some errors may slip past, especially if the cells are growing rapidly.

Human DNA is packaged in 23 pairs of chromosomes. Often, MDS cells contain altered chromosomes. Tests to identify these chromosome problems can help predict the prognosis of patients with MDS. Sometimes part of one chromosome attaches to a different chromosome. This is called a translocation. Like mutations, translocations can turn on oncogenes or turn off tumor suppressor genes. Translocations that develop during life are quite common in some forms of leukemia and MDS. Another chromosome abnormality that can be seen in MDS is called a deletion. This means part or all of a chromosome is lost. Another type of chromosome abnormality is called a duplication. This means there is an extra copy of part or all of a chromosome.