Kit Signaling Regulates Mitf Expression in Mastocytosis.

Abstract

Mastocytosis is a heterogeneous disease arising from abnormal proliferation of mast cells. Activating mutations in codon D816 of the tyrosine kinase receptor, c-kit, are found in the majority of adult patients with systemic mastocytosis, an aggressive form of the disease. Constitutive activation of the Kit signaling pathway is critical to the transformed phenotype, and thus understanding how this pathway regulates downstream events is of great importance. A number of transcription factors are also essential to mast cell development, including the Microphthalmia-associated transcription factor (Mitf). We examined Mitf expression in bone marrow biopsies from nine patients with systemic mastocytosis by immunohistochemistry; we found that Mitf is highly expressed in all cases with the D816V mutation. In contrast, Mitf is not highly expressed in non-malignant mast cells in the bone marrow from patients with aplastic anemia and leukemia, suggesting thatMitf expression is regulated by Kit-dependent signalsMitf may play a role in the transformed phenotype of mastocytosis.We show that in normal mast cells, Kit signaling markedly upregulates Mitf expression. In both normal and malignant mast cells, pharmacologic inhibitors of Kit, and the downstream kinase, PI3K, block Mitf expression. To examine whether Mitf is required for transformed phenotype from constitutive Kit signaling in mast cells, we have used a shRNA-expressing lentivirus to knockdown Mitf expression in mastocytosis cell lines. We found that silencing of Mitf markedly impaired growth in proliferation and colony forming cell assays. This work demonstrates a link between two critical factors, Kit and Mitf, in the development of malignant mast cell disease.