Cellular and molecular abnormalities in severe congenital neutropenia predisposing to leukemia

Severe congenital neutropenia (SCN) is a rare hematological disease characterized by a selective decrease in the level of circulating neutrophils in peripheral blood, maturation arrest at the promyelocyte stage of differentiation in the bone marrow, recurrent severe infections, and evolution to acute myelogenous leukemia (AML). Cellular and molecular studies of 12 SCN patients, including 5 patients that evolved to develop AML, revealed impaired proliferative characteristics and accelerated apoptosis of bone marrow progenitor cells in SCN compared with 11 healthy controls as demonstrated by flow cytometry analysis. Sequencing analysis revealed heterozygous deletion or substitution mutations in the neutrophil elastase (NE) gene in 9 of 12 patients but not in healthy controls. Expression of various NE mutants, but not normal NE, resulted in accelerated apoptosis of human promyelocytic HL-60 progenitor cells, similar to impaired survival observed in patients' cells. Bone marrow-derived primitive CD34(+) and CD33(+)/CD34(-) progenitor cells from SCN patients evolving to AML, all with mutations in the granulocyte colony-stimulating factor receptor (G-CSFR) gene, demonstrated normal cell survival, whereas more differentiated CD15(+)/CD33(-)/CD34(-) cells negative for mutant G-CSFR gene, continue to exhibit accelerated apoptosis. These data demonstrate that impaired survival of bone marrow myeloid progenitor cells, probably driven by expression of mutant NE, is the cellular mechanism responsible for neutropenia in SCN. Furthermore, our results suggest that acquired G-CSFR mutations may initiate signaling events that override the pro-apoptotic effect of mutant NE in primitive progenitor cells, resulting in an expansion of the abnormal AML clone.