Identification of a GATA Switch In Megakaryocytic Development.

Abstract 2605 GATA family transcription factors play critical roles in various mammalian developmental processes, including hematopoiesis. In particular, GATA-1 expression is necessary for proper terminal differentiation of mast cells, red blood cells, eosinophils, and megakaryocytes. GATA-2 is required for proliferation and survival of hematopoietic stem and progenitor cells, and is also expressed in erythroid precursors, mast cells, and early megakaryocytes. In developing erythrocytes, GATA-2 and GATA-1 are responsible for temporal control of a multi-factor transcriptional regulatory network that involves (a) GATA-2 positively regulating its own gene transcription, (b) GATA-2 positively regulating the expression of the Gata1 gene, (c) GATA-1 positively regulating its own gene transcription, and (d) GATA-1 negatively regulating Gata2 gene transcription. During this sequence of events, a “GATA switch” occurs, wherein GATA-1 replaces GATA-2 at canonical GATA binding sites within the regulatory regions of the Gata2 and Gata1 genes, as well as at many other genomic loci that encode genes responsible for proliferation or differentiation of erythroid progenitors. Similarly, in early megakaryocytic progenitors, GATA-2 promotes proliferation and suppresses expression of alternative-lineage genes; subsequent activation of GATA-1 precipitates terminal differentiation with concomitant downregulation of proliferative genes and activation of megakaryocyte-specific genes. The presence or role of a GATA switch in megakaryocytes has not yet been formally investigated. To address the role of the GATA switch in megakaryocytic differentiation, we performed massively parallel sequencing of chromatin immunoprecipitation (ChIP-Seq) material for GATA-2 and GATA-1 before or after GATA-1 restoration in the GATA1-null megakaryocytic progenitor cell line, G1ME. We obtained 22 million unique GATA-2 tags and 10 million unique GATA-1 tags and identified 14985 and 5102 high-confidence GATA-2 and GATA-1 binding sites, respectively. Additionally, we used 13 million tags from ChIP for H3K4me3 to identify 24909 genomic sites enriched for the presence of trimethylated lysine-4 on histone H3. Trimethylated H3K4 marks nearly half of all GATA-1 bound sites and one-third of GATA-2 bound sites. Over 40% of the sites bound by GATA-1 in differentiating G1ME cells were also bound by GATA-2 in proliferating G1ME cells, indicating that a GATA switch does indeed occur during megakaryocyte development. Coordinated analyses of these occupancy data with previously published gene expression datasets show that the lists of bound genes are significantly enriched for differentially expressed genes and the data depict a generally antagonistic relationship between GATA-2 and GATA-1. Interestingly, we find that even among genes that don't contain GATA switch sites, greater than 40% of those bound by GATA-1 were also occupied by GATA-2 at distinct sites. To further characterize the occupied loci, we surveyed the genomic regions bound by GATA-1 and GATA-2 to detect motifs enriched in the sequences surrounding the peak calls. As expected, we found that over 80% contained the canonical WGATAR binding motif. In contrast to reports of motifs enriched in GATA-1 ChIP studies in erythroid cells, we failed to observe significant enrichment of LRF binding motifs. Rather, the GATA-1 and GATA-2 bound regions in megakaryocytes are strongly enriched for motifs that match the binding sites for Ets family transcription factors. Finally, we have found that these genomic regions are indeed occupied by one or more Ets factors in proliferating G1ME cells. Together, these data establish the presence of a GATA switch in megakaryocyte development and provide novel insights into coordinated gene regulation by GATA factors and the differences between the closely related erythroid and megakaryocyte lineages. Disclosures: No relevant conflicts of interest to declare.