With current treatment regimens, long-term remission rates for adult patients with high risk Myelodysplastic Syndrome (MDS) and Acute Myeloid Leukemia (AML) are 15-25%. RUNX1 is the DNA- binding subunit of the core-binding factor (CBF) complex and a master-regulator transcription factor involved in hematopoiesis. Majority of mutant (mt) RUNX1 are missense, large deletions or truncation- mutations, behaving mostly as loss of function (LOF) mutations. Presence of mtRUNX1 confers relative therapy-resistance and poorer survival in patients with MDS/AML. The germline mutations and deletions in RUNX1 cause the highly penetrant (~40%) Familial Platelet Disorder with a propensity to evolve into MDS or AML. Lack of specific targeted therapy, coupled with resistance to standard therapy may account for poorer prognosis and outcome in MDS/AML expressing somatic or germline mtRUNX1. Therefore, there is an unmet need to develop novel targeted therapies for MDS/AML expressing mtRUNX1. Our preliminary studies demonstrate that knockdown of RUNX1 induces significantly more in vitro lethality in AML blasts expressing mtRUNX1 versus wild type (wt) RUNX1. Utilizing RNA-Seq signature of RUNX1 knockdown and querying the LINCS1000-CMap (Connectivity-Mapping) datasets, we identified homoharringtonine (HHT) among the top expression mimickers (EMs). Consistent with observations that presence of mtRUNX1 impairs ribosomal biogenesis (RiBi), treatment with HHT or its semisynthetic analog omacetaxine mepesuccinate (OM), which inhibit protein translation, preferentially exerted more lethality in vitro and efficacy in vivo in models of AML expressing mtRUNX1. This was associated with repression of RUNX1 and its targets, as well as attenuation of short-lived proteins including c-Myc and MCL-1. Notably, co-treatment with OM and venetoclax (Ven) induced synergistic lethality and superior in vivo efficacy in xenograft models of AML expressing mtRUNX1. Therefore, our Overarching hypothesis motivating studies proposed is that targeted combination of OM and Ven will yield high remission rates and improved survival, correlating with specific genetic and gene-expression signatures in patients with high-risk MDS/AML expressing mtRUNX1. Specific aims of studies proposed are: AIM 1: To conduct a Phase Ib/II clinical trial of co-treatment with OM and Ven in patients with high risk MDS or AML expressing mtRUNX1. AIM 2: To determine correlates of efficacy/resistance to co-treatment with OM and Ven, including genetic-lesions architecture (via NextGen and scDNA sequencing), epigenetic and gene- expression signature (via RNA-Seq, RPPA and CyTOF analyses) and impaired RiBi features in MDS/AML cells of patients enrolled on the Phase Ib/II trial. AIM 3: To determine pre-clinical efficacy of additional OM-based combinations with BET or CDK9 inhibitor, as well as with novel targeted agents directed against druggable hits nominated through an in vitro protein domain-specific CRISPR-gRNA screen.