Rapid improvements in sequencing technology have resulted in a wealth of cancer-genome data, but understanding which genomic aberrations can be targeted as sites for potential treatment remains challenging. By integrating functional and genomic analyses of primary leukemia specimens, we identified CSF3R
mutations as drivers of leukemia and also identified tyrosine kinase inhibitors that effectively target downstream CSF3R-signaling pathways. We found mutations in CSF3R
in 59% of patients with CNL or atypical CML — myeloid neoplasms for which no diseasespecific genetic markers have been identified to date. The high frequency of activating mutations in CSF3R
in these leukemias, which are characterized by high numbers of neutrophils, is consistent with its function as the receptor for the growth factor that promotes neutrophil differentiation and proliferation.16,17
The CSF3R mutations represent a biologically unifying feature of CNL and atypical CML and define a new molecular subset of hematologic cancers. The incorporation of CSF3R mutational status into current diagnostic criteria for CNL and atypical CML may help refine the molecular classification of myeloproliferative neoplasms and myeloproliferative–myelodysplastic overlap neoplasms. Although CNL and atypical CML are currently defined as separate neoplasms by the WHO, distinguishing between the two diagnoses can sometimes be challenging for clinicians and hematopathologists. The categorization relies partly on arbitrary thresholds for the total white-cell count (e.g., ≥25,000 per cubic millimeter for CNL and ≥13,000 per cubic millimeter for atypical CML), the percentage of total white cells that are immature granulocytes (<10% for CNL and ≥10% for atypical CML), and the presence or absence of dysgranulopoiesis (absent in CNL and characteristic of atypical CML).
Similar to the identification of the JAK2
V617F mutation across a spectrum of related myeloproliferative neoplasms (e.g., polycythemia vera, essential thrombocythemia, and primary myelofibrosis), the phenotype of CSF3R
mutation–positive neoplasms may be modified by additional unknown molecular abnormalities or host genetic factors, such as mutations in the gene encoding SET-binding protein 1 (SETBP1
In addition, assessment of CSF3R
mutational status may be useful for the evaluation of diseases characterized by neutrophilia in which the clinical basis is not readily apparent.
CSF3R has been shown to signal through downstream SRC family and JAK-kinase pathways, 28,29
and we have identified a novel CSF3R downstream substrate, TNK2. These downstream kinase pathways make CSF3R
mutations an attractive marker for tyrosine kinase inhibitors. The two types of CSF3R
mutations may have differential susceptibility to classes of tyrosine kinase inhibitors, with CSF3R
truncation mutations showing activation of SRC family–TNK2 kinase signaling and sensitivity to dasatinib, and CSF3R
membrane proximal mutations showing preferential activation of the JAK signaling pathway (). Our observation that a patient with a membrane proximal mutation had an excellent clinical response to the JAK inhibitor ruxolitinib, resulting in a marked decrease in the numbers of white cells and neutrophils and an increased platelet count (), constitutes a proof of concept. Although anecdotal, this observation provides an impetus for further investigation of tyrosine kinase inhibitors for the treatment of patients with neutrophilic leukemia who have CSF3R
Model for Activation and Signaling of CSF3R Mutations
truncation mutations have been shown to lead to constitutive overexpression of the receptor and ligand hypersensitivity, 22–24,37
the mechanism of action of the membrane proximal mutation does not appear to involve similar receptor overexpression, since the membrane proximal mutants do not show analogous overexpression in the Ba/F3 model (). Our data show that T618I is capable of inducing colony formation in the absence of G-CSF ligand, which suggests constitutive activation of the receptor. Data from a recent study 21
identified the same mutation in a patient with congenital neutropenia and sequential acquisition of CSF3R
mutations as the disease evolved toward AML. In our study, several patients with CNL or atypical CML had both truncation and membrane proximal mutations, and the signaling of these compound mutations and their sensitivities to tyrosine kinase inhibition also warrant characterization in future studies.
Complex genetic alterations are common in a multitude of tumor types. CSF3R
truncation mutations accelerate tumor formation in the presence of other genetic modifiers but alone are incapable of causing AML.40
mutations have been reported in patients with congenital neutropenia that progressed to AML, the prevalence of CSF3R
mutations in de novo AML is low (approximately 1%).21
It is possible that this low frequency is due to the required contribution from other genetic alterations for transformation to AML.
In conclusion, the presence of CSF3R mutations identified a distinct diagnostic subgroup of more than 50% of patients with CNL or atypical CML in our study. The oncogenic CSF3R mutations are molecular markers of sensitivity to inhibitors of SRC family–TNK2 and JAK kinases and may provide a new avenue for therapy.