In a substantial number of patients with systemic mastocytosis (SM), an associated clonal haematological non-mast cell lineage disease (AHNMD) is detectable. Although most of these patients display KIT mutations, especially KIT(D816V), little is known about their exact frequency and their distribution in AHNMD subtypes. We examined 48 patients with SM-AHNMD for the presence of mutant KIT in the SM and AHNMD components of the disease. Mast cells and AHNMD cells were obtained from immunostained bone marrow sections by laser microdissection and examined by melting point analysis of nested-PCR products. KIT(D816V) was found in AHNMD cells in the vast majority of patients with SM-chronic myelomonocytic leukaemia (CMML, 89%). Unexpectedly, KIT(D816V) was far less frequently detectable in AHNMD cells in patients with SM-myeloproliferative neoplasm (MPN, 20%) and SM-acute myeloid leukaemia (AML, 30%). None of the patients with lymphoproliferative AHNMDs displayed KIT codon 816 mutations in AHNMD cells (0/8). In FIP1L1/PDGFRA-positive chronic eosinophilic leukaemia (CEL), neither the SM nor the CEL component of the disease exhibited the KIT mutation. Our findings demonstrate that KIT codon 816 mutations are variably present in AHNMD cells in patients with SM-AHNMD, depending on the subtype of AHNMD. The high frequency of KIT(D816V) in neoplastic mast cells and leukaemic myelomonocytic cells in SM-CMML may point to a common precursor in these patients, and may have implications for the biology of the disease and the development of KIT-targeting therapies.

In ϳ20 to 30% of patients with systemic mastocytosis (SM) , an associated clonal hematological nonmast cell lineage disorder (AHNMD) is diagnosed. Although SM may be considered to be closely related to the myeloproliferative disorders (MPDs) , it is unknown whether JAK2 V617F؉ MPD may occur as AHNMD in patients with SM. We here describe five patients with SM and co-existing chronic idiopathic myelofibrosis (SM-CIMF). In five of five patients , we detected the SM-related KIT mutation D816V , and in four of five patients , the MPD-related JAK2 mutation V617F. Surprisingly , JAK2 V617F was found not only in the AHMMD component of the disease but also in microdissected mast cells in all four JAK2 V617F -positive cases. Conversely , in two of the five patients, KIT D816V was found not only in neoplastic mast cells but also in microdissected CD15؉ neoplastic myeloid cells. Control experiments showed that 10 indolent SM patients without associated MPD did not carry the JAK2 mutation V617F and that 15 CIMF patients without SM did not carry the KIT mutation D816V. Altogether , these data suggest that KIT D816V؉ SM can coexist with JAK2 V617F؉ CIMF and that , in some of these SM-CIMF cases , the two mutations are present in the neoplastic cells of both disease components. (J Mol