Defective internalization and sustained activation of truncated granulocyte colony-stimulating factor receptor found in severe congenital neutropenia/acute myeloid leukemia

Acquired mutations truncating the C-terminal domain of the granulocyte colony-stimulating factor receptor (G-CSF-R) are found in about 20% of severe congenital neutropenia (SCN) patients, with this cohort of patients predisposed to acute myeloid leukemia (AML). In myeloid cells, such mutations act in a dominant-negative manner leading to hyperproliferation and lack of differentiation in response to G-CSF. However, why these truncated receptors are dominant in function over wild-type receptors has remained unclear. We report that ligand-induced internalization of truncated G-CSF-R is severely impaired compared with the wild-type receptor, which results in sustained activation of STAT proteins. Strikingly, in cells coexpressing both truncated and wild-type forms, the truncated receptors acted dominantly with regard to both internalization and sustained activation. Site-directed mutagenesis of the C-terminus showed that receptor tyrosines in this region were dispensable for internalization, whereas a di-leucine-containing motif in Box B3 played some role. However, loss of the di-leucine motif was not the critical determinant of the sustained activation status of truncated receptors. These data suggest that defective internalization, leading to extended receptor activation, is a major cause of the dominant hyperproliferative effect of truncated G-CSF receptors, which is only partially due to the loss of a di-leucine motif present in the Box B3 region of the full-length receptor.