Growth factor independence 1B (GFI1B) coordinates assembly of transcriptional repressor complexes comprised of corepressors and histone-modifying enzymes to control gene expression programs governing lineage allocation in hematopoiesis. Enforced expression of GFI1B in K562 erythroleukemia cells favors erythroid over megakaryocytic differentiation, providing a platform to define molecular determinants of binary fate decisions triggered by GFI1B. We deployed proteome-wide proximity labeling to identify factors whose inclusion in GFI1B complexes depends upon GFI1B’s obligate effector, lysine-specific demethylase 1 (LSD1). We show that GFI1B preferentially recruits core and putative elements of the BRAF-histone deacetylase (HDAC) (BHC) chromatin-remodeling complex (LSD1, RCOR1, HMG20A, HMG20B, HDAC1, HDAC2, PHF21A, GSE1, ZMYM2, and ZNF217) in an LSD1-dependent manner to control acquisition of erythroid traits by K562 cells. Among these elements, depletion of both HMG20A and HMG20B or of GSE1 blocks GFI1B-mediated erythroid differentiation, phenocopying impaired differentiation brought on by LSD1 depletion or disruption of GFI1B-LSD1 binding. These findings demonstrate the central role of the GFI1B-LSD1 interaction as a determinant of BHC complex recruitment to enable cell fate decisions driven by GFI1B.

The molecular mechanisms leading to the establishment of durable immunological memory are inadequately understood, limiting the development of effective vaccines and durable anti-tumor immune therapies. Using a T cell-conditional knockout mouse model and a viral pathogen, we show that expression of the transcription cofactor OCA-B (Pou2af1/Bob.1/OBF-1) within T cells is necessary for proper CD4+ memory T cell formation. We also show that ectopic OCA-B expression is sufficient to drive T cells towards a memory fate, while having minimal effects on primary antiviral effector response. Bulk and single-cell gene expression profiling comparing cells transduced with OCA-B and empty vector at primary effector response identifies changes in gene expression consistent with later memory formation, including genes increased (Tbx21, Il7r, Gadd45b, Socs2) in specific subpopulations by ectopic OCA-B expression. Short-lived effector T cell compartments are expanded but show increased expression of Gadd45b and Socs2, while clusters of effector cells with memory potential show increased expression of Bcl2, Il7r, Tcf7 and Slamf6. We also describe an OCA-B-mCherry reporter mouse allele that selectively labels B and T lymphocytes, and shows high reporter expression in CD4+ TCM cells. We show that elevated OCA-B expression prospectively identifies cells with increased survival capability and memory recall potential. Cumulatively, the results demonstrate that OCA-B is necessary and sufficient to promote CD4 T cell memory in vivo.

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.