Synergistic roles of DYRK1A and GATA1 in trisomy 21 megakaryopoiesis
Patients with Down syndrome (DS, trisomy 21, T21) are at increased risk of transient abnormal myelopoiesis (TAM) and acute megakaryoblastic leukemia (ML-DS). Both TAM and ML-DS require prenatal somatic mutations in GATA1, resulting in the truncated isoform GATA1s.  The mechanism by which individual chromosome 21 (HSA21) genes synergize with GATA1s for leukemic transformation is challenging to study, in part due to limited human cell models with wild type GATA1 or GATA1s. HSA21-encoded DYRK1A is overexpressed in ML-DS and may be a therapeutic target. To determine how DYRK1A influences hematopoiesis in concert with GATA1s, we used gene editing to disrupt all 3 alleles of DYRK1A in isogenic T21 induced pluripotent stem cells (iPSCs) with and without the GATA1s mutation. Unexpectedly, hematopoietic differentiation revealed that DYRK1A loss combined with GATA1s leads to increased megakaryocyte proliferation and decreased maturation. This proliferative phenotype was associated with upregulation of D-type cyclins and hyperphosphorylation of Rb to allow E2F release and de-repression of its downstream targets. Notably, DYRK1A loss had no effect in T21/wtGATA1 megakaryocytes. These surprising results suggest that increased DYRK1A expression may be protective against TAM and ML-DS and that DYRK1A inhibition would not be a therapeutic option for GATA1s-associated leukemias.   Overall design: To investigate the role of DYRK1A in TAM and ML-DS, we used CRISPR/Cas9 gene editing to knock out all isoforms of human DYRK1A in isogenic T21 iPSC lines with wtGATA1 or GATA1s. We then performed gene expression profiling analysis using data obtained from RNA-seq for flow cytometry-purified CD43+/41+/235+ hematopoietic progenitors and CD41+/CD42b+ megakaryocytes from T21/GATA1s/DYRK1A+/+/+ and T21/GATA1s/DYRK1A-/-/-.