6. MPD and MDS

How do Myeloproliferative Disorders (MPD) and Myelodysplastic Syndrome (MDS) relate to blood cancers like leukemia and lymphoma?

Blood cancers can be classified by what cell type they originate from (myeloid or lymphoid), where they’re located (bone marrow or lymph node) and what the cancerous cells are doing (maturing or not). Both MPD and MDS fit into the box of myeloid cell origin, originating in the bone marrow, and being able to undergo maturation. Chronic myeloid leukemia is actually one type of myeloproliferative disorder.

What’s the difference between MPD and MDS?

The difference between MPD and MDS is whether the cells mature normally or abnormally. In myeloproliferative disorders, cells mature normally, but they have an abnormally high amount of proliferation – hence the name emphasizes they are proliferative. This can lead to very high numbers of cells in the blood. In myelodysplastic syndromes, the cells mature abnormally. A stem cell in the bone marrow, instead of maturing into a normal cell, matures into something else – something just weird, and often not very functional. These ‘weird’ cells are described as dysplastic (hence, myelodysplastic syndromes). These dysplastic cells are often so odd and non-functional that they die. This results in lower numbers in the blood of what cell type they should have become.

How are the symptoms of myelodysplastic syndrome produced?

Myelodysplastic syndromes may have mild or severe symptoms depending on whether cell maturation is mildly or severely abnormal. As maturation becomes more abnormal, the affected cells are less able to function or survive, further reducing cell numbers.  Also as maturation becomes more abnormal, fewer cells leave the bone marrow. This means that if maturation is quite abnormal, a lot of the bone marrow is taken up by abnormally developed cells, and little space is left for normal cell production. This can decrease the numbers of all normal cell types in the blood.  This can result in symptoms of anemia, such as fatigue and weakness, and well as bleeding, bruising and susceptibility to infection.

How can myelodysplastic syndrome transform into AML?

The only difference between myelodysplastic syndrome and acute myeloid leukemia, is that in myelodysplastic syndromes the cells can mature into something, while in acute myeloid leukemia they can’t mature at all. If in myelodysplastic syndromes, maturation stops occurring, the disease becomes the same thing as acute myeloid leukemia. This transformation can occur in severe cases. Transformation to acute myeloid leukemia is usually recognized by the larger fraction of the bone marrow taken up by the immature cells once they stop being able to mature into anything.

How does aplastic anemia relate to MDS?

Aplastic anemia is not technically a type of myelodysplastic syndromes. Aplastic anemia does, like myelodysplastic syndromes, result in low levels of blood cells, but it’s caused by different problems in the bone marrow, such as death of the stem cells, not abnormal maturation.

What are the types of myeloproliferative disorders?

There are four main types of myeloproliferative disorders: chronic myeloid leukemia, polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis.  The differences are mainly in which cell types are affected.

What happens in CML?

CML usually has very high numbers of a type of white blood cell called neutrophils (and often increases in other cell types too). Symptoms are often related to energy expenditure producing the excess cells, leading to fatigue and weakness.  CML, like MDS, can transform to AML if changes are acquired that cause the cancerous cells to no longer be able to mature.

What happens in polycythemia vera?

Polycythemia vera results in high numbers of red blood cells. Having more red blood cells makes the blood thicker, so it’s more likely to clot and block blood vessels.  Some patients might also have excess bleeding, because in sometimes in polycythemia vera the platelets do not work properly. This is an unusual disorder because both excess bleeding and excess clotting may occur.

What happens in essential thrombocythemia?

Essential thrombocythemia results in high numbers of platelets specifically. The many platelets that are produced may or may not be functional – if they are functional they can cause blood to clot inside vessels, blocking them.  If the platelets are not functional, there can be excessive bleeding. Overproduction of platelets usually results from a change in the type of cell that produces platelets, called a megakaryocyte.

What happens in idiopathic myelofibrosis?

Idiopathic myelofibrosis also results from a change in megakaryocytes, but rather than producing extra platelets, they produce extra ‘platelet derived growth factor’ (PDGF) – a molecule (not a cell) that signals fibrous tissue to fill marrow space. This leaves little room for the production of blood cells, decreasing the numbers of all types of blood cells. This can result in anemia, susceptibility to infection and bleeding. Essential thrombocythemia can turn into idiopathic myelofibrosis if the megakaryocytes start producing excess platelet derived growth factor as well as or instead of the excess platelets. Idiopathic myelofibrosis can also transform into acute myeloid leukemia if changes are acquired that block maturation.

References

Leukemia and Lymphoma Society of Canada. Disease and Support Information. July 11 2013. Available at http://www.llscanada.org/?gclid=COr5jbK34roCFQVgMgodfl4AMQ#/diseaseinformation/. Accessed Nov 2013.

Aster JC, DeAngelo DJ. Chapter 20. Myeloproliferative Disorders and Myelodysplastic Syndromes. In: Bunn H, Aster JC. eds. Pathophysiology of Blood DisordersNew York: McGraw-Hill; 2011.http://accessmedicine.mhmedical.com.ezproxy.library.ubc.ca/content.aspx?bookid=676&Sectionid=44827790. Accessed February 09, 2014

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