The MastAttack 107: The Layperson’s Guide to Understanding Mast Cell Diseases, Part 37

44. What is a myeloproliferative neoplasm? Is that what mast cell disease is?

First, let’s pull this term apart.

“Myelo” means marrow, like bone marrow. In this context, it refers to a specific group of blood cells that are made in the bone marrow. These cells are called myeloid or myelogenous cells. These cells all start as one kind of cell called a myeloid progenitor cell. Mast cells and eosinophils are myeloid cells. There are other myeloid cells, too.

“Proliferative” means making lots of cells quickly. In this case, it means making many cells too quickly. When too many cells are made too quickly, the cells are often not made correctly so they don’t work right.

“Myeloproliferative” means making too many myeloid cells very quickly, producing cells that often don’t work right.

“Neo” means new.

“Plasm” means the substance that makes up something living, like what makes up a cell or a tissue. “Plasm” is part of many words used in biology.

“Neoplasm” means the body growing new things, things that don’t belong there. For example, cancers are neoplasms. (Although not all neoplasms are cancers).

Myeloproliferative neoplasm means your body making too many myeloid cells that don’t work correctly.

Speaking generally, any condition where the body makes too many of these myeloid cells when they shouldn’t is a myeloproliferative neoplasm. This means all form of mastocytosis and mast cell tumors (mast cell sarcoma and mastocytoma) are myeloproliferative neoplasms.

However, when people ask if mast cell diseases are myeloproliferative neoplasms, they are usually asking about the WHO (World Health Organization) classification of mast cell disease, which is a little different.

The WHO puts out an exhaustive list of diseases for reference. They group similar diseases together under one category. This list is also revised periodically as new data becomes available or experts request it.

Under the 2008 WHO guidelines, mast cell diseases were classified as myeloproliferative neoplasms along with several other diseases. The other diseases also included in this category make too many myeloid cells too quickly, like essential thrombocythemia, in which the body makes too many platelets.

The mast cell diseases classified as myeloproliferative neoplasms were cutaneous mastocytosis: maculopapular cutaneous mastocytosis (MPCM), diffuse cutaneous mastocytosis (DCM), and solitary mastocytoma of the skin; systemic mastocytosis: indolent systemic mastocytosis (ISM), systemic mastocytosis with associated hematologic disease (SM-AHD), aggressive systemic mastocytosis (ASM), and mast cell leukemia (MCL); and mast cell sarcoma. Smoldering systemic mastocytosis (SSM) was mentioned as a provisional category rather than a formal category, meaning that the WHO did not agree that this diagnosis was different enough from ISM to warrant its own category. Neither monoclonal mast cell activation syndrome (MMAS) or mast cell activation syndrome (MCAS) were classified anywhere in the 2008 WHO Guidelines as they were not yet recognized by the WHO as diseases.

Last year, the WHO revised the classification of myeloproliferative neoplasms. It removed all forms of mast cell disease from the myeloproliferative neoplasm category and made a different category for mast cell diseases. This was done because the WHO recognized that mast cell diseases differed from the other myeloproliferative neoplasms in specific ways. They also recognized that mast cell activation syndrome has a ton in common with other mast cell diseases even though it’s not a neoplastic disease. (Mast cell activation syndrome is not from the body making too many mast cells).

So all mast cell diseases were put together. In the new category, the following mast cell diseases were included: cutaneous mastocytosis: maculopapular cutaneous mastocytosis (MPCM), diffuse cutaneous mastocytosis (DCM), and solitary mastocytoma of the skin; systemic mastocytosis: indolent systemic mastocytosis (ISM), systemic mastocytosis with associated clonal hematologic non-mast cell lineage disease (SM-AHNMD), aggressive systemic mastocytosis (ASM), and mast cell leukemia (MCL); mast cell sarcoma; monoclonal mast cell activation syndrome (MMAS); and mast cell activation syndrome (MCAS).