CSF3R compound mutations produce a hyperproliferative leukemia driven by overexpression of PIM kinases

Colony Stimulating Factor 3 Receptor (CSF3R) mutations are the molecular hallmark of chronic neutrophilic leukemia (CNL). A subset of these patients (~25%) have two mutations in CSF3R on the same allele. These "compound" CSF3R mutations include both an activating point mutation and a cytoplasmic truncating mutation. While compound mutations are prevalent, we have a limited understanding of their molecular characteristics. In this study, we interrogated the mechanisms driving the evolution of compound mutations. We found that CSF3R compound mutations induce a hyperproliferative leukemia compared to single mutations. Compound mutations also markedly enhance activation of STAT5 and ERK1/2 downstream of CSF3R. Utilizing a proteomics approach, we identified enrichment of MYC and mTOR pathways in compound mutant-cells. The most differentially expressed protein in this dataset was the kinase PIM1. We find that in CSF3R compound mutant-cells with high STAT activity there is increased transcription of PIM kinases. We further validated that PIM1 transcript levels are high in a human CSF3R mutant-cohort. High PIM levels drive stabilization of MYC protein, and the downstream activation of MYC-driven leukemogenic transcriptional programs. Indeed, inhibitors of PIM reverse MYC stabilization. Finally, we show promising therapeutic potential for PIM-inhibition in CSF3R mutant-disease using both human cell lines and CNL patient samples. Together, these findings uncover a novel and targetable vulnerability in CSF3R compound mutant-CNL, thus opening a new avenue of treatment for patients with this devastating disease.