PROJECT SUMMARY: The development of a myelodysplastic syndrome (MDS) is a multistep process involving disease initiation, clonal progression with acquisition of cooperating mutations and a complex interplay with the microenvironment. In this proposal, we will focus specifically on the initial event in this pathway that ultimately leads to loss of chromosome 7 and MDS in children. Through our recent studies on pediatric MDS, we identified germline heterozygous mutations in two interferon-inducible genes on chromosome 7-SAMD9 and SAMD9L-that result in growth suppression when exogenously expressed in tissue culture cells and are associated with monosomy 7 in children with a range of different myeloid abnormalities. Curiously, the copy of chromosome 7 that is lost in these patients universally is the copy that harbors the mutant allele. Thus, it appears that in this context, monosomy 7 is an adaptation to the cellular effect of these mutations and while likely providing a mechanism for hematopoietic cells to grow in these patients, the loss of one copy of chromosome 7 can lead to MDS or other hematopoietic abnormalities. These genomic and clinical findings establish a strong scientific premise to investigate the cellular and functional impacts of SAMD9L mutations in human and mouse hematopoietic cells with a long-term goal to understand how these mutations can ultimately lead to MDS with monosomy 7. We hypothesize that expression of mutant SAMD9L decreases hematopoietic cell growth and differentiation, and that different SAMD9L alleles can cooperate with cell intrinsic or extrinsic factors to influence hematopoietic phenotypes. We will test our hypothesis with the following specific aims using a combination of genetic tools and functional assays in human and mouse hematopoietic cells. Specific Aim 1: We will test the in vitro impact of different SAMD9L mutations in primary hematopoietic cells. Specific Aim 2: We will determine the effect of Samd9l alleles on self-renewal and transformation. Specific Aim 3: We will test the contribution of environmental stress and SAMD9L expression on hematopoietic cell growth and differentiation in human cells. The identification of germline mutations in SAMD9 or SAMD9L in children with MDS is a significant advancement in the field of pediatric myeloid neoplasms. Our proposed studies will define the role of both mutant and wild-type SAMD9L in hematopoiesis, the results of which will ultimately impact how patients with these mutations are clinically managed. Not only will this proposal significantly enhance our knowledge of SAMD9L biology, but we will broadly address how chromosomal aneuploidy can be an adaptive response to cellular stresses, which is likely a shared mechanism across different developmental abnormalities or cancers.