GFI1 Cooperates with IKZF1/IKAROS 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-acute lymphoblastic leukemia (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 expression by GFI1 and IKAROS requires the GFI1 SNAG domain but occurs independent of SNAG-LSD1 binding. GFI1 variants deficient in LSD1 binding fail to activate 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 noncanonical 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. IMPLICATIONS: Combinatorial diversity and cooperation between DNA binding proteins and complexes assembled by them can direct context-dependent transcriptional outputs to control cell fate and may offer new insights for therapeutic targeting in cancer.