RepSox and its analog for prophylactic intervention in RUNX1 haploinsufficiency FPDMM - ABSTRACT Familial platelet disorder with associated myeloid malignancy (FPDMM) due to haploinsufficiency of the transcription factor RUNX1 is a rare orphan disorder affecting hundreds of people in the United States. FPDMM is associated with both quantitative and qualitative defects in platelets (Plts), resulting in a moderate bleeding diathesis, and an increased risk of developing myelodysplastic and myeloleukemic transformation. At present there is no treatment that reduces the risk of bleeding or developing leukemia in FPDMM. We have identified a small compound RepSox, known to block transforming-growth factor-β (TGFb) type I receptor kinase (ALK5), that corrects both the quantitative and qualitative defects in FPDMM megakaryocytes (Mks) and Plts. These findings are based on studies of both patient-derived induced-pluripotent stems cells (iPSC)- and short-hairpin (sh) RNA suppression of RUNX1 levels in healthy CD34+-hematopoietic stem and progenitor cells (shRx)-derived human (h) Mks, the terminal hematopoietic cells that gives rise to Plts. Testing other inhibitors that suppress the TGFb pathway did not replicate these findings, suggesting that RepSox affects FPDMM hMks by a separate pathway. We propose that RepSox increases RUNX1 level/activity or modifies distinct RUNX1 isoform usage, correcting both the Plt bleeding diathesis and leukemic risk. We propose the following studies to bring RepSox or a RepSox analog towards clinical application in patients with FPDMM: Specific Aim (SA) #1: Studies of the effects of RepSox on RUNX1 in FPDMM. We will examine whether RepSox affects RUNX1 expression level, specific activity and/or RUNX1 isoform usage in FPDMM hematopoietic progenitor and definitive cells. SA#2: Pharmacologic studies of RepSox and RepSox-related drugs. A series of modified RepSox compounds have already been generated and will be examined to optimize RepSox effects in FPDMM for potential therapeutic application and to gain additional insights into the role of ALK5 inhibition in this process. This will be accompanied by murine studies to define the best mode/dosage of administration, levels achieveable, excretion and toxicity. SA#3: Efficacy of RepSox and RepSox-related drugs in murine models of FPDMM. A newly- described murine model of RUNX1 haploinsufficiency develops mild thrombocytopenia and Plt defects with an increased risk of leukemic transformation, especially after a “second hit”. This Runx1R188Q/+ murine model will be the focus of in vivo therapeutic studies. RepSox and/or related compounds will be tested in this system to see if they can correct the two major phenotypes. We believe that at the end of our studies that we will have better defined how RepSox corrects Mk/Plt biology and reduce leukemic risk in patients with FPDMM and will have identified candidate drugs to move forward to clinical application utilizing a PARTNERSHIP PLAN STEERING COMMITTEE. We envision an initial clinical study of the drug in the setting of acute bleeds that will be then extended to intermediate-length studies to limit bleeding manifestations and finally to long-term studies to examine efficacy to prevent accelerated leukemic evolution in FPDMM.
