Alternatively spliced, truncated GCSF receptor promotes leukemogenic properties and sensitivity to JAK inhibition

Abstract

Granulocyte colony-stimulating factor (GCSF) drives the production of myeloid progenitor and precursor cells toward neutrophils via the GCSF receptor (GCSFR, gene name CSF3R). Children with severe congenital neutropenia chronically receive pharmacologic doses of GCSF, and ~30% will develop myelodysplasia/acute myeloid leukemia (AML) associated with GCSFR truncation mutations. In addition to mutations, multiple isoforms of CSF3R have also been reported. We found elevated expression of the alternatively spliced isoform, class IV CSF3R in adult myelodysplastic syndrome/AML patients. Aside from its association with monosomy 7 and higher rates of relapse in pediatric AML patients, little is known about the biology of the class IV isoform. We found developmental regulation of CSF3R isoforms with the class IV expression more representative of a progenitor cell stage. Striking differences were found in phosphoprotein signaling involving Janus kinase (JAK)-signal transducer and activator of transcription (STAT) and cell cycle gene expression. Enhanced proliferation by class IV GCSFR was associated with diminished STAT3 and STAT5 activation, yet showed sensitivity to JAK2 inhibitors. Alterations in the C-terminal domain of the GCSFR result in leukemic properties of enhanced growth, impaired differentiation and resistance to apoptosis, suggesting that they can behave as oncogenic drivers, sensitive to JAK2 inhibition.

G-CSF and GM-CSF in Neutropenia

“…The G-CSFR is expressed primarily on neutrophils and bone marrow precursor cells, which undergo proliferation and eventually differentiation into mature granulocytes. G-CSF binds to G-CSFR, resulting in its dimerization, with a stoichiometry of 2:2 and with a high affinity (K D = 500 pM)(14, 15). Among the activated downstream signal transduction pathways are Janus kinase (JAK)/signal transducer and activator of transcription (STAT), Src kinases such as Lyn, Ras/Extracellular Regulated Kinase (ERK), and phosphatidylinositol 3-kinase (PI3K)(16).…”

“…Src homology 2 (SH2) containing proteins STAT5 and STAT 3 bind to Y704 and Gab2 to Y764..Grb2 couples to both Gab2 and to SOS, permitting signaling diversification, such as Ras/ERK, PI 3-kinase/Akt, and Shp2 (19, 20). An alternatively spliced isoform of G-CSFR elicits activation of a JAK-SHP2 pathway(15). The precise physiological roles of protein kinases and their downstream events in G-CSF-induced signaling remain unclear, although some clues are beginning to emerge (21, 22).…”

The cell survival pathways of the primordial RNA-DNA complex remain conserved in the extant genomes and may function as proto-oncogenes

“…The dMLF exerts its action through the Hh, RUNX (runt-related), and JNK pathways within the crystal cells [86]. In the human bone marrow, JAK-STAT mutations are the major causative factors for myelofi brosis and myelodysplasia with GCSF/CSF as their promoter, and small molecular JAK-inhibitor ruxolitinib as their suppressor [87,88].…”

A t(1;9) translocation involving CSF3R as a novel mechanism in unclassifiable chronic myeloproliferative neoplasm

“…Cells expressing isoform IV are sensitive to JAK2 inhibitors such as ruxolitinib or NVP-BSK805. 9 Based on these findings, after our patient experienced lack of clinical response to dasatinib and failure to the allogeneic transplant, treatment with the JAK2 inhibitor was proposed and approved. Ruxolitinib was started at 5 mg twice daily for 20 days and at 15 mg twice daily thereafter.…”

“…This CSF3R splicing donor site is the same as that previously described in isoform IV receptor. 8,9 The other two minor chimeric isoforms detected (B and C) were generated by another cryptic donor site in exon 18 or the canonical donor site of exon 17 from CSF3R with the canonical acceptor site of exon 50 of SPTAN1.…”