FOG-1 Requires NuRD to Promote Hematopoiesis and Maintain Lineage Fidelity within the Megakaryocytic–Erythroid Compartment.

Abstract 702

Nuclear factors regulate the development of complex tissues by promoting the formation of one cell lineage over another. Transcription factors GATA-1 and GATA-2 co-operate with the cofactor FOG-1 to promote erythroid and megakaryocyte differentiation. In contrast, FOG-1 is inhibitory to the function of GATA factors in the context of mast cells. The mechanisms by which FOG-1 functions distinctly as co-activator and co-repressor are incompletely defined.

Purification of FOG-1-associated proteins led to the identification of the nucleosome remodeling and histone deacetylase NuRD complex (Hong, et al., EMBO J. 2005). To investigate the in vivo function of this interaction, we generated mice bearing point mutations in FOG-1 that disrupt NuRD binding. The phenotype of homozygous (Fog1^ki/ki) mice revealed an essential role of NuRD for normal FOG-1 function in a broad range of cell types, including erythroid cells and megakaryocytes. Here we report that defects in hematopoietic development arise as early as the bipotent megakaryocyte-erythroid progenitor in adult mice. Strikingly, Fog1^ki/ki megakaryocyte-erythroid progenitors (MEP) retained multi-lineage capacity, capable of generating mast cells and other myeloid lineages. We did not observe an increase in mast cell potential in E10.5 yolk sacs, as has been previously reported in Fog1^null embryos, indicating a stage-specific role for FOG-1's interaction with NuRD.

Genome wide transcriptome analysis of purified Fog1^ki/ki MEP revealed inappropriate expression of several mast cell-specific genes including Fcer1a, Fcer1b, Mcpt2, and Cpa3. Notably, aberrant mast cell gene expression persisted in committed Fog1^ki/ki megakaryocyte and erythroid cells. Thus FOG-1 and NuRD are essential to constrain mast cell gene expression throughout erythroid and megakaryocyte ontogeny. A direct mechanism of repression is suggested as select mast cell genes are occupied by GATA-1 and FOG-1 in erythroid precursor cells. Furthermore, a mutant form of GATA-1 that is impaired for FOG-1 binding (GATA-1-V205M) failed to repress mast cell gene expression. Together, these observations suggest that a simple heritable epigenetic silencing mechanism is insufficient to permanently repress mast cell genes. Instead, the continuous presence of GATA-1, FOG-1 and NuRD is required to maintain lineage fidelity throughout megakaryocyte-erythroid ontogeny.