Despite the relevance of the c-kit/stem cell factor (SCF) signaling pathway in mast cell (MC) diseases, the exact frequency of KIT mutations in different compartments of bone marrow (BM) hematopoietic cells of individuals with systemic mastocytosis (SM), and its different diagnostic categories, remains unknown. In this study, we prospectively analyzed the presence of KIT mutations in fluorescence-activated cell-sorting (FACS)-purified populations of BM MCs (n ؍ 113) and other BM cell compartments (n ؍ 67) from adults with SM. Our results show the presence of D816V KIT mutation in virtually all adults (93%) with indolent and aggressive forms of SM, except welldifferentiated SM (29%), while other KIT mutations were rarely (< 3%) detected. In around one-third of patients with mutated MCs, the KIT mutation was also detected in CD34 ؉ hematopoietic cells and eosinophils, and, to a lesser extent, in monocytic, neutrophil-lineage BM precursor cells and lymphocytes. Most patient with poor-prognosis SM (81%) carried the KIT mutation in 2 or more BM myeloid cell populations, while this was detected in a smaller proportion (27%) of indolent cases. These results would support the notion that KIT mutation is a hallmark of adult SM where it targets a pluripotent hematopoietic stem cell, and may contribute to explaining previously observed discrepancies in the literature.
IntroductionMast cell diseases (MCDs) are a heterogeneous group of disorders characterized by an abnormal proliferation and accumulation of mast cells (MCs) in different tissues. In a relatively high proportion of cases, the clonal nature of the disease can be established on the basis of the demonstration of gain-of-function mutations involving the tyrosine kinase domain of c-kit in lesional skin and bone marrow (BM) cells. [1][2][3][4][5][6][7][8][9] Typically, the A71763T substitution is detected, whereby an aspartate is changed for a valine at codon 816 of the c-kit protein sequence. However, other uncommon somatic (V560G, 10 D815K, 11 D816Y, 2,11,12 D816F, 2,11 D816H, 13 and D820G 14 ) and germ-line (F522C, 15 A533D, 16 K509I, 17 and del419 18 ) mutations, which may result in a ligand-independent activation of the stem cell factor receptor, have been reported, most frequently in individual cases. Presence of the KIT mutations in patients with systemic mastocytosis (SM) is not restricted to BM MCs, but it has also been sporadically reported in other non-MC hematopoietic lineages, 7,[19][20][21][22][23][24] suggesting that expansion of clonal MCs may arise from an uncommitted hematopoietic stem cell. However, attempts to demonstrate the presence of KIT mutation in purified CD34 ϩ hematopoietic precursor cells have failed so far. 25 The pathogenetic relevance of the different KIT mutations in MCDs is still not fully understood; however, their identification has become of major prognostic significance 26 due to the availability of protein kinase inhibitors such as imatinib (STI571, Gleevec; Novartis, Basel, Switzerland). These new targeted drugs hav...
