GFI1 cooperates with IKAROS/IKZF1 to activate gene expression in T-cell acute lymphoblastic leukemia

Growth factor independence-1 (GFI1) is a transcriptional repressor and master regulator of normal and malignant hematopoiesis. Repression by GFI1 is attributable to recruitment of LSD1-containing protein complexes via its SNAG domain. However, the full complement of GFI1 partners in transcriptional control is not known. We show that in T-ALL cells, GFI1 and IKAROS are transcriptional partners that co-occupy regulatory regions of hallmark T cell development genes. Transcriptional profiling reveals a subset of genes directly transactivated through the GFI1—IKAROS partnership. Among these is NOTCH3 , a key factor in T-ALL pathogenesis. Surprisingly, NOTCH3 transactivation by GFI1 and IKAROS requires the GFI1 SNAG domain but occurs independent of SNAG—LSD1 binding. GFI1 variants deficient in LSD1 binding fail to transactivate NOTCH3 , but conversely, small molecules that disrupt the SNAG—LSD1 interaction while leaving the SNAG primary structure intact stimulate NOTCH3 expression. These results identify a non-canonical transcriptional control mechanism in T-ALL which supports GFI1-mediated transactivation in partnership with IKAROS and suggest competition between LSD1-containing repressive complexes and others favoring transactivation.