Methylation and sumoylation regulate LSD1-Corest recruitment and activity to control transcriptional and cellular functions of GFI1

Proper cell fate decisions require precise and coordinated changes in gene expression. Alterations in gene expression proceed through the functions of transcription factors, their associated coregulators and histone modifying enzymes. However, how these complex and diverse groups of proteins and enzymes coordinate their respective functions at target genes remains largely unknown. To gain additional insights into the coordinated activities of transcription factors and histone modifying enzymes, we studied how the transcription factor growth factor independence 1 (GFI1) carries out transcriptional repression through interaction with coregulator histone modifying enzymes. GFI1 is a transcriptional repressor and master regulator of normal and malignant hematopoiesis. GFI1 is comprised of a transcriptionally repressive N-terminal Snail/Slug/GFI1 (SNAG) domain, a C-terminal concatemer of DNA binding zinc fingers and a linker region which separates them. The relatively simple protein domain structure of GFI1 makes it an ideal transcription factor for studying mechanisms of transcriptional repression. We describe here two novel mechanisms of transcriptional repression by GFI1, both of which occur through posttranslational modification. First, we identify and characterize a SUMOylation event carried out by the SUMO2/3, UBC9, and PIAS3 SUMOylation machinery. SUMOylation occurs at K239 within a type I SUMO consensus element in the linker region of GFI1. We find that SUMO iv defective GFI1 derivatives fail to complement Gfi1 depletion phenotypes in zebrafish developmental erythropoiesis and in granulocyte differentiation in cultured human cells. SUMO defective GFI1 derivatives also display impaired LSD1/CoREST binding and fail to repress the GFI1 target gene MYC during granulocyte differentiation and enforced MYC expression blocks GFI1 mediated granulocyte differentiation. Second, we show SMYD2 mediated methylation at K8 within the GFI1 SNAG domain is a critical determinant of GFI1 transcriptional repression and contributes to GFI1 hematopoietic differentiation and leukemia cell survival functions. The methylation defective GFI1 SNAG domain lacks repressor function due to a failure of LSD1 recruitment and accumulation of promoter H3K4 dimethyl marks. Our findings here suggest GFI1 SUMOylation and methylation are part of a series of regulatory inputs that regulate GFI1 function. From these data we propose SNAG domain methylation and linker region SUMOylation coordinate LSD1/CoREST recruitment and enable CoREST dependent activation of LSD1 H3K4 demethylase activity for repression of GFI1 target genes. Our findings add GFI1 to the growing roster of transcription factors regulated by posttranslational modification and provides a rare mechanistic understanding into how these modifications regulate transcription factor functions through the recruitment histone modifying enzyme effectors.