The portal for rare diseases and orphan drugs

The prevalence in Europe is estimated between 1/7,700 and 1/10,400. Systemic mastocytosis (SM) preferentially affects Caucasians and there is no sex predominance.

SM is mainly observed in adults (average age at diagnosis is 60 years) and is very rare in the pediatric population. SM can be divided into five variants: indolent SM (ISM), smoldering systemic mastocytosis (SSM), aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN) and mast cell leukemia (MCL), the later 3 variants being collectively termed advanced SM (advSM). In ISM, there are no signs of high mast cell burden (B-findings) and no organ dysfunction (C-findings). SSM is a slowly progressive SM with B-findings. By contrast, advSM variants are characterized by (multiple) C-findings.

Most patients with SM present activating mutations in KIT, the receptor of the stem cell factor, in their neoplastic MCs. Of note, a recurrent KIT mutation (D816V; exon 17 of the KIT gene) located in the phosphotransferase domain of the receptor is detectable in most patients (> 80% of all SM patients), including those with advSM. Other less frequent mutations affect exons 2, 8, and 9 encoding for the extracellular domain or exons 13 and 14 encoding for Kinase Domain 1 of KIT. Whatsoever, the various KIT mutations encountered in SM result in uncontrolled activation of the receptor, making the migration, survival and activation of MCs independent of stem cell factor.

Consensus criteria for the diagnosis of systemic mastocytosis established by the WHO include one major criterion - the presence of aggregates of at least 15 MCs identified in BM or other extracutaneous organ biopsies (usually revealed after tryptase staining of biopsies) - and four minor criteria: the presence of more than 25% of MCs with atypical morphology in BM smears (spindle-shaped, degranulated and/or multinucleated MCs); aberrant immunophenotype of MCs which frequently abnormally express non-mast cell markers (CD25 and/or CD2); presence of a point mutation in codon 816 of the KIT gene in BM; peripheral blood or other extracutaneous organs; and increased level of serum tryptase (above 20 ng/mL). For diagnosis of SM, at least the major and one minor criterion or at least three minor criteria are fulfilled. The disease is then categorized, as described above, according to the presence of B-findings and C-findings.

Differential diagnoses of SM include all the other causes of mast cell activation syndromes (MCAS): primary (clonal, but not fulfilling SM diagnostic criteria) MCAS; secondary MCAS where an IgE-dependent allergy (most cases) or another reactive inflammatory disease process is present (and is considered to be the causative etiology); and idiopathic MCAS where neither clonal MC nor an IgE-dependent allergy or another underlying condition/disease can be documented. As well, differential diagnoses include other forms of mastocytosis (cutaneous mastocytosis, mast cell sarcoma), endocrine disorders (adrenal tumors, VIPoma,, Zollinger-Ellison syndrome), some gastrointestinal pathologies, allergies, other myeloproliferative diseases that affect bone marrow, histiocytosis, hypereosinophilic syndrome and Waldenström macroglobulinemia.

SM is usually a sporadic disease, although a few familial cases have been reported so far, for which no genetic counseling is available.

Management depends on the type of SM. Treatment of ISM and stable SSM is typically symptomatic (antihistamines, corticosteroids or disodium cromoglyylcate). For advanced SM treatment is symptomatic (antihistamines anti-H1 and H2) and antiproliferative with non-targeted (e.g interferon alpha) or targeted cytoreductive therapy with KIT tyrosine kinase inhibitors. Hydroxyurea (HU) is given in patients with SM-AHN. Allogeneic stem cell transplantation (allo-SCT) is rarely used and reserved for fit patients with very aggressive, life-threatening advSM and a suitable donor.

Prognosis depends on the type of SM.