Project Summary/Abstract The goal of the proposed research is to elucidate the molecular pathogenesis of severe congenital neutropenia (SCN) due to mutations of caseinolytic peptidase B (CLPB). SCN is an inborn disorder of granulopoiesis characterized by severe chronic neutropenia from birth, premature death secondary to infectious complications, and transformation to myeloid malignancy. Through exome sequencing of a large SCN cohort, the candidate has recently identified heterozygous missense mutations in CLPB as a new and frequent cause of SCN. CLPB is a nuclear-encoded protein that resides within the inner mitochondrial membrane space where it functions as a molecular chaperone to disaggregate and facilitate re-folding of misfolded proteins. However, the mechanisms linking impaired CLPB function to a defect in granulocyte formation are unclear. In this proposal, the principle investigator will test the hypothesis that mutant CLPB acts in a dominant fashion to disrupt the chaperone function of CLPB, resulting in impaired mitochondrial stress responses and induction of apoptosis in promyelocytes. To test this hypothesis, the following specific aims are proposed: Aim 1) to determine whether CLPB mutations impair the mitochondrial response to endoplasmic reticulum stress in granulocytic precursors; Aim 2) to examine the impact of CLPB mutations on the switch from glycolysis to oxidative phosphorylation in early granulocytic precursors. The proposed studies should provide an understanding of the molecular pathophysiology of CLPB- mutant SCN. Ultimately, a better understanding of normal and SCN-related granulopoiesis may suggest new therapeutic approaches to treat or prevent neutropenia in patients with SCN, and in patients receiving myeloablative chemotherapy. The long-term goal of this physician-scientist candidate is to establish a productive and independent laboratory at a major academic institution studying normal and malignant hematopoiesis. The primary mentor is Dr. Daniel Link, a distinguished scientist who is also an experienced and committed mentor. A panel of senior investigators with complementary scientific and translational expertise will serve on a formal K99 mentorship committee to provide both scientific and career guidance. Washington University is an exceptional environment to train junior investigators, especially those interested in hematopoiesis. There is ready access to numerous core facilities and a strong intellectual environment with experts in stem cell biology, neutrophil biology, mitochondrial biology, and cellular models of hematopoiesis. In addition to taking courses in biostatistics and bioinformatics, the candidate will take advantage of the broad portfolio of workshops offered at Washington University to help junior investigators establish and run an independent laboratory. Washington University has committed to providing laboratory space and resources to facilitate the candidate’s transition to independence.