Eosinophils are derived from the neoplastic clone in patients with systemic mastocytosis and eosinophilia.

Twenty to thirty-three percent of systemic mast cell disease (SMCD) patients have some degree of peripheral blood and/or bone marrow eosinophilia. While the clonality of SMCD was established by identification of the pathogenic c-kit Asp816 to Val (D816V) mutation in sorted cells of different hematopoietic lineages, this mutation has not previously been demonstrated in eosinophils from SMCD patients. In the current study, we demonstrate the D816V mutation in purified eosinophils from two patients with aggressive, eosinophilia-associated SMCD, thus proving that the eosinophils are clonal and not reactive in nature. The clinical implications of this finding are discussed.