ACS DIVISION OF BIOLOGICAL CHEMISTRY

January 8th, 2014 by Maxson, J. E., Luty, S. B., MacManiman, J., Abel, M. L., Druker, B. J., Tyner, J. W.

Mutations in the CSF3 (GCSF) receptor, CSF3R have recently been found in a large percentage of patients with chronic neutrophilic leukemia (CNL) and more rarely in other types of leukemia. These CSF3R mutations fall into two distinct categories: membrane proximal mutations and truncation mutations. Although both classes of mutation have exhibited the capacity for cellular transformation, several aspects of this transformation including the kinetics, requirement for ligand, and dysregulation of downstream signaling pathways have all been shown to be discrepant between the mutation types, suggesting distinct mechanisms of activation. CSF3R truncation mutations induce overexpression and ligand hypersensitivity of the receptor, likely due to removal of motifs necessary for endocytosis and degradation. In contrast, little is known about the mechanism of activation of membrane proximal mutations, which are much more commonly observed in CNL. In contrast to CSF3R truncation mutations, membrane proximal mutations do not exhibit overexpression and are capable of signaling in the absence of ligand. We show that the T615 and T618 sites of membrane proximal mutations are part of an O-linked glycosylation cluster. Mutation at these sites prevents O- glycosylation of CSF3R, and increases receptor dimerization. This increased dimerization explains the ligand-independent activation of CSF3R membrane proximal mutations. Cytokine receptor activation through loss of O- glycosylation represents a novel avenue of aberrant signaling. Finally, combination of the CSF3R membrane proximal and truncation mutations, as has been reported in some patients, leads to enhanced cellular transformation when compared to either mutation alone, underscoring their distinct mechanisms of action.