Summary Myelodysplastic Syndromes (MDS) are chronic hematopoietic disorders characterized by dysplasia, inefficient hematopoiesis, and the propensity to transform into acute myeloid leukemia (AML). Recent advances in genomic sequencing revealed a large number of mutations associated with the disease, which can be roughly grouped into three classes: (1) genes involved in signaling (i.e. FLT3, JAK2, KRAS), (2) genes functioning at the levels of chromatin and pre-mRNA splicing (i.e. RUNX1, ASXL1, SRSF2, U2AF1), and (3) genes responsible for establishing/maintaining the genome methylome (i.e. DNMT3a, TET2, IDH1/2). Given MDS is highly heterogeneous in its clinical features, a fundamental question is whether individual mutations cause the disease via distinct mechanisms or whether many mutations function in some converging pathways. Support of the latter possibility is the co-occurrence of many of these causal mutations in MDS patients. As disease-oriented (Zhang) and mechanism-central (Fu) labs, we have been taking advantage of our combined expertise to work together under this funded R01 to attack some pressing questions in the field, focusing on RUNX1 and SRSF2. In the past funding cycle (9/2013-present), we have made two conceptual breakthroughs. First, by linking specific mutations to splicing responses in MDS patients, we found that non- overlapping responses induced by splicing factor mutations are converged to the common pathways of cell cycle and DNA damage response. Second, we unexpectedly uncovered that, besides their traditional roles in splicing, all causal mutations in key splicing factors trigger excessive R-loop formation, leading to replication stress and cell cycle checkpoint activation. These findings point to dysregulation of the DNA damage response as a common ground for MDS etiology. Importantly, such elucidated common ground has laid a critical foundation for our next phase of investigation, which is to understand the contribution of individual mutations to MDS and potential synergy among them, despite their diverse roles in regulating gene expression. Building upon both our published and unpublished results, we propose to pursue the following specific aims in the next phase: Aim 1. Function of RUNX1 and its synergy with SRSF2 in preventing DNA damage; Aim 2. Mutant SRSF2 and epigenetic regulators to synergistically drive aberrant gene expression; Aim 3. Potential mechanism for bypassing R-loop-induced cell cycle checkpoint activation.