Abstract 848 Hematology-Oncology II Poster Symposium, Sunday, 5/2

Mastocytosis is characterized by abnormal infiltration of mast cells in various organs. An activating mutation in c-kit (SCF receptor) was recently described in mastocytosis. By phosphorylation studies we have previously shown that the mutation causes activation of the receptor. Herein we describe a 12-years old girl with mastocytosis and a mutation in peripheral blood and bone marrow mononuclear cells, and delineate the role of SCF and its receptor in the pathogenesis of the disease.

Methods: Clonogenic assays containing IL-3, G-CSF and erythropoietin with 4 different concentrations of SCF were performed and analyzed for colony formation, percentage of mast cells and apoptosis rate. Results were compared with marrow cells from normal controls. In order to confirm the presence of the patient's original abnormal clone (bearing isochromosome 8q abnormality) in the cultures, FISH analysis using a chromosome 8 centromere specific probe was performed.

Results: When the highest concentration of SCF (50 ng/ml) was added, CFU-GM, CFU-Bas, BFU-E colony number changed by +275%, +317%, and -85% respectively. Controls showed changes of less than 50% in CFU-GM and BFU-E respectively, and CFU-Bas remained zero. Toluidine blue stain confirmed the presence of mast cells and an increase in their number from 2 to 20% in the patient in response to added SCF, compared to 1% in controls with and without SCF. FISH analysis confirmed the presence of the patient's original abnormal clone in all the cultures: with and without SCF. Cluster/colony ration decreased in controls, but increased in the patient from 0.39% to 0.49%. Apoptosis of harvested cultured cells with SCF increased from 11% to 50% compare to constant rates of 10-20% in controls.

Conclusions: The c-kit activating mutation induces hypersensitivity of primitive progenitors to SCF, with increased production of basophil/mast cells at the expense of erythroid cells. Accumulation of abnormal cells with increasing concentrations of SCF was accompanied by higher apoptosis rates and increased cluster formation, which is consistent with the myelodysplastic nature of the disorder.