Well-differentiated systemic mastocytosis: Genetics, mast cell immunophenotypes, and KIT autophosphorylation

Background: Well-differentiated systemic mastocytosis (WDSM) is a rare myeloid neoplasm where the genetic etiology is often unknown. Objective: We aimed to assess WDSM patients for novel KIT variants, mast cell (MC) aberrant immunophenotypes, and KIT autophosphorylation patterns. Methods: Next-generation sequencing, MC immunophenotyping, and KIT autophosphorylation studies were performed. Results: Among 454 SM patients, there were 432 with KIT p.D816V+ SM and 4 with KIT p.D816Y+ SM—notably, none of these patients had WDSM. Of the remaining patients, we identified 7 with WDSM (1.5%) and 2 relatives with mastocytosis in skin. Next-generation sequencing revealed that 6 of 9 subjects carried known or novel germline KIT variants corresponding to regions outside of codon 816. Three patients had germline KIT p.K509I; 2 had germline KIT p.A533D; 1 had two germline KIT variants p.F681L and p.M541L; and 3 had no KIT mutation. Intracellular expression of CD2 and CD25 and less robust expression of CD30 was observed in MCs from WDSM patients. By developing a novel transient transfection assay in 293T cells, we found that unlike KIT p.D816F/V/Y variants that exhibit nearly exclusive intracellular localization and strong ligand-independent autophosphorylation (class II), WDSM-associated KIT variants showed enhanced ligand-dependent autophosphorylation relative to wild type (class I). Conclusions: Our study doubles the number of KIT variants identified in WDSM patients. No KIT p.D816V+ SM patient had WDSM. Intracellular CD2 and CD25 expression was more robustly detected in MCs from WDSM patients compared to CD30.