[Immunohistochemical and molecular characterization of systemic mastocytoses]

The WHO has published an updated classification of mastocytosis and the criteria for the diagnosis of systemic mastocytosis (SM). These include one major criterion, compact mast cell (MC) infiltrates in extracutaneous tissues, and four minor criteria, i.e. cytomorphologic atypia with spindling of MC (>25 %), detection of the activating somatic c-kit mutation D816 V in MC, aberrant expression of CD2 and/or CD25 on MC, and an elevated serum tryptase level (>20 ng/ml). Systemic mastocytosis is diagnosed when the major plus one minor, or three minor criteria are fulfilled. In the present study, we have established methods for the detection of CD25 and the c-kit mutation D816V in paraffin-embedded bone marrow trephine biopsy specimen of 57 patients with various subtypes of mastocytoses and 239 controls. While MCs in almost all patients with SM (55/57) expressed CD25, only 2/239 of the control samples contained CD25-positive MCs. With newly designed molecular pathological methods, c-kit codon 816 mutations were detected by "peptide nucleic acid" (PNA)-mediated PCR-clamping and/or analysis of microdissected MC in 52/57 cases with SM. All cases with detectable c-kit mutations also contained CD25-positive MC. The c-kit mutation D816 V was also detected in microdissected cells of associated hematologic neoplasias in 6/15 cases. With the methods established for the investigation of paraffine-embedded tissues, the pathologist plays a central role in the diagnosis of SM.