Stag2-dependent chromatin remodeling enforces the erythroid-specific Gata1 cistrome

The transcription factor GATA1 has pleiotropic hematopoietic functions, particularly in erythroid and megakaryocytic ontogeny. Although mechanistic investigations have uncovered many facets of GATA1 biology, how GATA1 coregulates divergent cell fates remains incompletely characterized. We previously described that loss of Stag2, a member of the cohesin complex and a recurrent mutational target in myelodysplastic syndrome (MDS) and myeloid leukemia of Down syndrome, results in altered chromatin accessibility, transcription factor function, and cell differentiation. Therefore, we hypothesized that chromatin accessibility determines GATA1 cistrome specificity and lineage fate decisions. To understand the connection between chromatin accessibility and GATA1, we comprehensively studied erythropoiesis in Stag2Δ mice. Defects in Stag2-deficient hematopoiesis included reduced numbers of erythroid progenitors (EryPs), impaired terminal erythroid differentiation, increased number of megakaryocyte progenitors, and increased megakaryocytes. RNA- and assay for transposase-accessible chromatin-sequencing of EryPs revealed altered patterns of Gata1 target gene expression with altered accessibility in conjunction with loss of expression of erythroid targets and gain of megakaryocyte targets. Despite unchanged Gata1 expression, Gata1 occupancy was reprogrammed from erythroid to megakaryocyte targets with Fli1 motifs enriched at Stag2Δ Gata1 binding sites. Functionally, we observed that Stag2-deficient EryPs have a diminished erythroid output and augmented megakaryocyte output in orthogonal differentiation assays, which was partially reversed with Fli1 knockdown. Human models and patients with primary MDS recapitulated the essential phenotypic and molecular features of our in vivo murine MDS model. Collectively, this study establishes chromatin accessibility as a determinant of transcription factor binding specificity, revealing an accessibility-driven Gata1 retargeting mechanism underlying MDS dyserythropoiesis.