Severe congenital neutropenia (SCN) is characterized by a selective decrease in the number of circulating1 neutrophils associated with recurrent fevers, chronic oropharyngeal inflammation and repeated severe infections. Characteristically these patients have monocytosis, thrombocytosis and mild anemia, unless they have a severe infection. SCN is usually diagnosed in early childhood and may evolve to acute myeloid leukemia. The severity of symptoms and risk of serious infections are in general inversely proportional to absolute neutrophil counts. Neutropenia occurs because of impaired formation and reduced delivery of neutrophils from the bone marrow to the peripheral circulation. We have recently described mutations in neutrophil elastase (NE) in 21 of 24 patients with SCN . All patients with family history of SCN and more than 90 percent of sporadic cases carry mutations in neutrophil elastase gene observed that bone marrow myeloid progenitor cells from SCN patients are characterized by a significantly accelerated rate of apoptosis and a transition block from G1 to S phase of the cell cycle . We have also demonstrated that expression of mutant NE in hematopoietic progenitor cells triggers apoptotic cell death. Recently, we identified novel and previously reported point mutations in granulocyte colony-stimulating factor receptor (G-CSFR) in 6 of 7 SCN patients with elastase mutation who evolved to develop acute myeloid leukemia (AML). The research goal of this proposal is to understand the molecular mechanism of accelerated cell death and abnormal cell cycle progression in pathogenesis of SCN and its evolution to AML. We will expand our current studies of the cellular mechanism of SCN by examining apoptosis and cell cycle arrest of hematopoietic progenitor cells from a larger number of patients with SCN and SCN/AML with autosomal dominant and autosomal recessive mode of inheritance including patients from the original Kostmanns family. We will further delineate the cellular defect in SCN by focusing on more primitive bone marrow progenitor cell subpopulations and will determine subcellular localization of mutant genes in the bone marrow cells from patients with SCN and SCN/AML. Diversity of NE and G-CSFR mutations in this cohort of patients will also be examined. We will establish myeloid progenitor cell lines with inducible expression of mutant genes, which will enable a comprehensive investigation of the molecular events mediating dysregulatory effects of mutant neutrophil elastase and mutant G-CSFR. These models will be useful for development of novel therapeutic strategies for treatment of severe congenital neutropenia and prevention of its evolution to acute myeloid leukemia.