No abstract

The RUNX genes encode transcription factors involved in development and human disease. RUNX1 and RUNX3 are frequently associated with leukemias, yet the basis for their involvement in leukemogenesis is not fully understood. Here, we show that Runx1;Runx3 double-knockout (DKO) mice exhibited lethal phenotypes due to bone marrow failure and myeloproliferative disorder. These contradictory clinical manifestations are reminiscent of human inherited bone marrow failure syndromes such as Fanconi anemia (FA), caused by defective DNA repair. Indeed, Runx1;Runx3 DKO cells showed mitomycin C hypersensitivity, due to impairment of monoubiquitinated-FANCD2 recruitment to DNA damage foci, although FANCD2 monoubiquitination in the FA pathway was unaffected. RUNX1 and RUNX3 interact with FANCD2 independently of CBFβ, suggesting a nontranscriptional role for RUNX in DNA repair. These findings suggest that RUNX dysfunction causes DNA repair defect, besides transcriptional misregulation, and promotes the development of leukemias and other cancers.

A broad range of human leukemias carries RUNX1 and MLL genetic alterations. Despite such widespread involvements, the relationship between RUNX1 and MLL has never been appreciated. Recently, we showed that RUNX1 physically and functionally interacts with MLL, thereby regulating the epigenetic status of critical cis-regulatory elements for hematopoietic genes. This newly unveiled interaction between the two most prevalent leukemia genes has solved a long-standing conundrum: leukemia-associated RUNX1 N-terminal point mutants that exhibit no obvious functional abnormalities in classical assays for the assessment of transcriptional activities. These mutants turned out to be defective in MLL interaction and subsequent epigenetic modifications that can be examined by the histone-modification status of cis-regulatory elements in the target genes. RUNX1/MLL binding confirms the importance of RUNX1 function as an epigenetic regulator. Recent studies employing next-generation sequencing on human hematological malignancies identified a plethora of mutations in epigenetic regulator genes. These new findings would enhance our understanding on the mechanistic basis for leukemia development and may provide a novel direction for therapeutic applications. This review summarizes the current knowledge about the epigenetic regulation of normal and malignant hematopoiesis by RUNX1 and MLL.

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The transcription factor TAL1/SCL is one of the most prevalent oncogenes in T-cell acute lymphoblastic leukemia (T-ALL), a malignant disorder resulting from leukemic transformation of thymus T-cell precursors. TAL1 is normally expressed in hematopoietic stem cells (HSCs) but is silenced in immature thymocytes. We hypothesize that TAL1 contributes to leukemogenesis by activating genes that are normally repressed in immature thymocytes. Herein, we identified a novel TAL1-regulated super-enhancer controlling the GIMAP locus, which resides within an insulated chromosomal locus in T-ALL cells. The GIMAP genes are expressed in HSCs and mature T-cells but are downregulated during the immature stage of thymocyte differentiation. The GIMAP enhancer is activated by TAL1, RUNX1 and GATA3 in human T-ALL cells but is repressed by E-proteins. Overexpression of human GIMAP genes in immature thymocytes alone does not induce tumorigenesis but accelerates leukemia development in zebrafish. Our results demonstrate that aberrant activation of the GIMAP enhancer contributes to T-cell leukemogenesis.

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Hematopoietic stem cells (HSC) have the potential to replenish the blood system for the lifetime of the organism. Their two defining properties, self-renewal and differentiation, are tightly regulated by the epigenetic machineries. Here, using conditional gene knockout models, we demonstrate a critical requirement of lysine acetyltransferase 5 (Kat5, also known as Tip60) for murine HSC maintenance both in the embryonic and adult stages, which depends on its acetyltransferase activity. Genome-wide chromatin and transcriptome profiling in murine hematopoietic stem and progenitor cells revealed that Tip60 co-localizes with c-Myc and that Tip60 deletion suppress the expression of Myc target genes, which are associated with critical biological processes for HSC maintenance, cell-cycle and DNA repair. Notably, acetylated H2A.Z (acH2A.Z) was enriched at the Tip60-bound active chromatin and Tip60 deletion induced a robust reduction in the acH2A.Z / H2A.Z ratio. These results uncover a critical epigenetic regulatory layer for HSC maintenance at least in part through Tip60 dependent H2A.Z acetylation to activate Myc target genes.

Increased levels and non-telomeric roles have been reported for shelterin proteins, including RAP1 in cancers. Herein using Rap1 null mice, we provide the genetic evidence that mammalian Rap1 plays a major role in hematopoietic stem cell survival, oncogenesis and response to chemotherapy. Strikingly, this function of RAP1 is independent of its association with the telomere or with its known partner TRF2. We show that RAP1 interacts with many members of the DNA damage response (DDR) pathway. RAP1 depleted cells show reduced interaction between XRCC4/DNA Ligase IV and DNA-PK, and are impaired in DNA Ligase IV recruitment to damaged chromatin for efficient repair. Consistent with its role in DNA damage repair, RAP1 loss decreases double-strand break repair via NHEJ in vivo, and consequently reduces B cell class switch recombination. Finally, we discover that RAP1 levels are predictive of the success of chemotherapy in breast and colon cancer.