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Mastocytosis is a rare, heterogeneous group of disorders due to the abnormal clonal proliferation and accumulation of atypical mast cells in one or more organs of the body, most often affecting the skin.
- Two clinical diagnostic categories: cutaneous mastocytosis (CM) or systemic mastocytosis (SM)
- SM constitutes 10–15% of all the cases.
- Clinical symptoms are due to the action of mast cell mediators, usually manifesting as skin lesions but can also present as pruritus, abdominal pain, diarrhea, and even anaphylaxis with vascular collapse.
- Most patients have only skin involvement with characteristic of tan to red-brown macules; firm, fixed papules; or plaques called urticaria pigmentosa (UP).
- Symptoms of mastocytosis may occur with physical or pharmacologic stimulation, causing mast cell degranulation and release of chemical mediators, such as histamine, leukotrienes, and prostaglandins.
- Adult mastocytosis is a disorder related to clonal proliferation of mast cells secondary to a gain-of-function point mutation in the proto-oncogene c-kit (D816V in >95% of cases).
- Childhood mastocytosis is only associated with c-kit, D816V in 40% of cases; majority of cases are limited to the skin and spontaneously resolve around puberty.
- Incidence and prevalence are unknown due to many indolent cases.
- Bimodal peak incidence during infancy and early childhood and then in middle age (30 to 49 years)
- ~2/3 of reported CM cases are found in children, most during 1st year of life.
- Overall: male = female; slight female predominance in adults
Etiology and PathophysiologyGenetics
Gain-of-function point mutations in c-kit, in particular D816V
- C-kit is a proto-oncogene that encodes receptor tyrosine kinase (KIT or CD117) located on many hematopoietic stem cells, including mast cells.
- Autophosphorylation of tyrosine residues in the presence of gain-of-function c-kit mutations leads to unregulated proliferation of mast cells.
- C-kit mutations (mostly D816V) are detectable in >90% of adult SM, whereas they are detectable in the skin of 86% of childhood cutaneous lesions.
- Mutations in other genes may also be pathogenic in some cases. TET2 and NRAS have been suggested, but their role in pathogenesis and prognosis has yet to be determined.
- There is no known genetic transmission.
Age (bimodal); c-kit-activating mutations