Colony-stimulating factor 3 receptor gene (CSF3R) T618I mutated chronic myelomonocytic leukemia: A proliferative subtype with a distinct mutational profile

Introduction/Objective Chronic myelomonocytic leukemia (CMML) is a myeloid neoplasm characterized by sustained monocytosis, ranging from cytopenia with a dysplastic subtype to leukocytosis with a proliferative subtype, with a typical mutational profile involving TET2, ASXL1, and SRSF2. Mutation in colony-stimulating factor 3 receptor gene (CSF3R) is commonly associated with chronic neutrophilic leukemia (CNL) but exceedingly rare in CMML, particularly CSF3R T618I (~10 cases described, ~30 cases of CSF3R non-T618I mutations). We report a case of CSF3R T618I mutated CMML and compare the clinicopathologic features to reported CMML cases with and without CSF3R T618I mutations. Methods/Case Report A 27-year-old woman presented for evaluation of leukocytosis, sustained monocytosis, and anemia. Peripheral blood (PB) revealed leukocytosis (white cell count 35x109/L), left-shifted and dysplastic neutrophils (myelocytes and metamyelocytes, 5%), absolute and relative monocytosis (7x109/L, 29%), anemia (Hgb 4.3 g/dL), and thrombocytopenia. Bone marrow aspirate and core biopsy demonstrated a hypercellular marrow with increased myeloblasts (~3%, immunophenotypically aberrant by flow cytometry), increased myelomonocytic cells, and multilineage dysplasia, including ring sideroblasts and hypolobated megakaryocytes. Cytogenetic and molecular studies revealed a normal karyotype and mutations in CSF3R T618I, ASXL1, SETBP1, BCORL1, KRAS, and PTPN11. Despite the presence of a CSF3R T618I mutation, CMML was diagnosed given marked monocytosis, left- shifted neutrophils in PB, multilineage dysplasia, and immunophenotypically aberrant myeloblasts. Results (if a Case Study enter NA) NA Conclusion Our case demonstrates clinicopathological features similar to those of reported CSF3R T618I mutated CMML, i.e., a proliferative subtype and less likely to have co-occurring mutations in TET2 or SRSF2, which is distinct from CSF3R non-T618I mutated CMML; the latter often has a dysplastic subtype and mutational profile of frequent TET2 and SRSF2 mutations, similar to CSF3R unmutated CMML. While additional cases with this unusual mutation need to be studied to arrive at a more definitive conclusion, the CSF3R T618I mutation seems to define a unique proliferative subtype CMML with a distinct mutational profile.