Direct chemical control of the hematopoietic master transcription factor PU.1

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PROJECT SUMMARY/ABSTRACT: Direct control of the hematopoietic master transcription factor PU.1 Hematopoiesis, the process by which all lineages of blood cells are derived, is under coordinate control by a restricted group of transcription factors. Currently, factor-specific control relies heavily on genetic methods, such as RNA interference and CRISPR/Cas9, to alter the expression of transcription factors of interest. While highly selective, gene-based approaches are associated with significant latency (many hours to days) and therefore cannot access critical cellular dynamics at timescales in the minute-to-hour régime. Moreover, cytotoxicity and genotoxicity due to viral and non-viral gene delivery remain outstanding issues, particularly in therapy. Direct chemical control of specific transcription factors could address these opportunities, but a general lack of endogenous ligands for medicinal chemistry and broad structural homology challenge drug discovery. To meet this challenge, we have translated the disposition of molecular hydration in factor/DNA recognition into an orthogonal selection criterion to library screening. As proof of concept, we developed an osmotically driven phage display screen to obtain short peptides that enhance or inhibit DNA binding by PU.1, a master transcription factor in hematopoietic stem cell homeostasis and differentiation. De-regulation of PU.1 represents a major molecular lesion in several hematopoietic malignancies (e.g., acute myeloid leukemia, multiple myeloma, and Hodgkin's disease) as well as fibrosis of the lungs, liver, and kidneys. The objectives of this proposal are: to validate the biological and molecular properties of PU.1-targeted peptides, and adapting the osmotic screening technique to target other transcription factors that function in concert with PU.1. To achieve these objectives, we propose three specific aims. 1) We will define the functional profiles of PU.1-targeted peptides in cultured hematopoietic models, as well as primary murine and patient-derived leukemic and pro-fibrotic cells. Preliminary data show that PU.1-targeted peptides enter the cell nucleus and modulate the expression of major PU.1 target genes in as little as 30 min, an onset well below currently achievable limits by genetic manipulations. Our proposed studies are aimed at characterizing their transcriptional profiles and the attendant changes in cellular and disease phenotypes. 2) We will determine the molecular properties of peptide modulation of factor/DNA recognition. Detailed studies are aimed at dissecting the diverse structural and mechanistic bases of peptide/complex interactions. 3) We will expand osmotic screening to target lineage-specific transcription factors that co-regulate with PU.1, including the interferon regulatory factors IRF4 and IRF8 that bind DNA cooperatively with PU.1, as well as partners such as C/EBPα that are recruited collaboratively by low-affinity PU.1 binding. In summary, this proposal is expected to advance transcription factor pharmacology with novel targeted reagents, particularly activators, and demonstrate the combination of structural and physicochemical interrogation (library panning + osmotic pressure) as a tractable, generalizable solution to overcome current bottlenecks in chemical control.