Laboratory Assessment of Patients with Systemic Mastocytosis

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Abstract

Systemic mastocytosis (SM) is a heterogeneous disorder characterized by the pathological accumulation of mast cells within tissues. The majority of adult patients are classified with indolent systemic mastocytosis (ISM), which generally carries a low risk of transformation to an aggressive state and is not thought to affect lifespan. However, a subset of patients with aggressive systemic mastocytosis (ASM) have a poor prognosis and will require cytoreductive therapy. Increased mast cell burden is observed in the inflamed tissues and affected organs and tissues of patients with mast cell proliferative disorders. However, normal mast cells participate in host defense, so approaches to preferentially target clonally expanding mast cells are needed. We found that mammalian target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) are up-regulated in neoplastic and developing immature mast cells compared with their terminally differentiated counterparts. Elevated mTOR mRNA was also observed in bone marrow mononuclear cells of patients exhibiting mast-cell hyperplasia. Selective inhibition of mTORC1 and mTORC2 through genetic and pharmacologic manipulation revealed that, whereas mTORC1 may contribute to mast-cell survival, mTORC2 was only critical for homeostasis of neoplastic and dividing immature mast cells. The cytostatic effect of mTORC2 down-regulation in proliferating mast cells was determined to be via inhibition of cell-cycle progression. Because mTORC2 was observed to play little role in the homeostasis of differentiated, nonproliferating, mature mast cells, these data provide a rationale for adopting a targeted approaching selectively inhibiting mTORC2 to effectively reduce the proliferation of mast cells associated with inflammation and disorders of mast cell proliferation while leaving normal differentiated mast cells largely unaffected. Acvating mutations in the receptor tyrosine kinase KIT, most notably KIT D816V, are commonly observed in patients with systemic mastocytosis. Thus, inhibition of KIT has been a major focus for treatment of this disorder. We investigated a novel approach to such inhibition. Utilizing rational drug design, we targeted the switch pocket (SP) of KIT, which regulates its catalytic conformation. Two SP inhibitors thus identified, DP-2976 and DP-4851, were examined for effects on neoplastic mast cell proliferation and mast cell activation. Autophosphorylation of both wild-type and, where also examined, KIT D816V activation was blocked by these compounds in transfected 293T cells, HMC 1.1 and 1.2 human mast cell lines, and in CD34+-derived human mast cells activated by stem cell factor (SCF). Both inhibitors induced apoptosis in the neoplastic mast cell lines and reduced survival of primary bone marrow mast cells from patients with mastocytosis. Moreover, the SP inhibitors more selectively blocked SCF potentiation of FcRI-mediated degranulation. Overall, SP inhibitors represent an innovative mechanism of KIT inhibition whose dual suppression of KIT D816V neoplastic mast cell proliferation and SCF-enhanced mast cell activation may provide significant therapeutic benefits.