Molecular circuitry of T-cell Development and Leukemogenesis

Abstract T-cell Acute lymphoblastic leukemia (T-ALL) is an aggressive disease with only a 10% 5-year survival rate for patients over 60, due to genomic instability. We have shown that β-catenin and TCF-1 cooperate in DP thymocytes to promote genomic instability leading to lymphomagenesis. We have also established that TCF-1 and HEB cooperate in the DP stage. Now, we aim to determine the molecular underpinnings of T-ALL and thymic development, by elucidating the interplay of all three factors, β-catenin, TCF-1, and HEB. Aberrant activation of β-catenin promotes the development of Lymphoma/Leukemia equivalent to the human disease T-ALL and blocks thymocyte development in the early DP stage. Ablation of TCF-1 in thymocytes with activated β-catenin rescues from lymphomagenesis, but the developmental block persists. Antithetically, developmental progression is achieved only when HEB is conditionally knocked out. In an effort to explain this phenomenon, we found that genes associated with the transition of DP thymocytes from DPsmall to CD69+ are downregulated by activating β-catenin (e.g. Runx-3, Ccr-7, TOX-2, etc.) and revert upon deletion of HEB. This is associated with changes in the binding of TCF-1 to DNA. Genomic sites that acquire TCF-1 binding upon β-catenin activation forfeit this binding when HEB is deleted, while at the same time novel TCF-1 binding sites emerge. These new binding sites are significantly more enriched for the conserved TCF-1 motif than TCF-1-bound sites in WT thymocytes. This observation promotes the notion that HEB guides TCF-1 towards alternative binding sites, thus altering gene expression. These findings highlight the complex interactive relationship of TCF-1 and HEB in both normal development and disease. NIH/NIAID R01 AI147652-01A1