Role of TET dioxygenases in the regulation of both normal and pathological hematopoiesis.

The family of ten-eleven translocation dioxygenases (TETs) consists of TET1, TET2, and TET3. Although all TETs are expressed in hematopoietic tissues, only TET2 is commonly found to be mutated in age-related clonal hematopoiesis and hematopoietic malignancies. TET2 mutation causes abnormal epigenetic landscape changes and results in multiple stages of lineage commitment/differentiation defects as well as genetic instability in hematopoietic stem/progenitor cells (HSPCs). TET2 mutations are founder mutations (first hits) in approximately 40-50% of cases of TET2-mutant (TET2MT) hematopoietic malignancies and are later hits in the remaining cases. In both situations, TET2MT collaborates with co-occurring mutations to promote malignant transformation. In TET2MT tumor cells, TET1 and TET3 partially compensate for TET2 activity and contribute to the pathogenesis of TET2MT hematopoietic malignancies. Here we summarize the most recent research on TETs in regulating of both normal and pathogenic hematopoiesis. We review the concomitant mutations and aberrant signals in TET2MT malignancies. We also discuss the molecular mechanisms by which concomitant mutations and aberrant signals determine lineage commitment in HSPCs and the identity of hematopoietic malignancies. Finally, we discuss potential strategies to treat TET2MT hematopoietic malignancies, including reverting the methylation state of TET2 target genes and targeting the concomitant mutations and aberrant signals.

Mechanisms that regulate the activities of TET proteins.

The ten-eleven translocation (TET) family of dioxygenases consists of three members, TET1, TET2, and TET3. All three TET enzymes have Fe+2 and α-ketoglutarate (α-KG)-dependent dioxygenase activities, catalyzing the 1st step of DNA demethylation by converting 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), and further oxidize 5hmC to 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). Gene knockout studies demonstrated that all three TET proteins are involved in the regulation of fetal organ generation during embryonic development and normal tissue generation postnatally. TET proteins play such roles by regulating the expression of key differentiation and fate-determining genes via (1) enzymatic activity-dependent DNA methylation of the promoters and enhancers of target genes; and (2) enzymatic activity-independent regulation of histone modification. Interacting partner proteins and post-translational regulatory mechanisms regulate the activities of TET proteins. Mutations and dysregulation of TET proteins are involved in the pathogenesis of human diseases, specifically cancers. Here, we summarize the research on the interaction partners and post-translational modifications of TET proteins. We also discuss the molecular mechanisms by which these partner proteins and modifications regulate TET functioning and target gene expression. Such information will help in the design of medications useful for targeted therapy of TET-mutant-related diseases.

Ripk3 signaling regulates HSCs during stress and represses radiation-induced leukemia in mice.

Receptor-interacting protein kinase 3 (Ripk3) is one of the critical mediators of inflammatory cytokine-stimulated signaling. Here we show that Ripk3 signaling selectively regulates both the number and the function of hematopoietic stem cells (HSCs) during stress conditions. Ripk3 signaling is not required for normal homeostatic hematopoiesis. However, in response to serial transplantation, inactivation of Ripk3 signaling prevents stress-induced HSC exhaustion and functional HSC attenuation, while in response to fractionated low doses of ionizing radiation (IR), inactivation of Ripk3 signaling accelerates leukemia/lymphoma development. In both situations, Ripk3 signaling is primarily stimulated by tumor necrosis factor-α. Activated Ripk3 signaling promotes the elimination of HSCs during serial transplantation and pre-leukemia stem cells (pre-LSCs) during fractionated IR by inducing Mlkl-dependent necroptosis. Activated Ripk3 signaling also attenuates HSC functioning and represses a pre-LSC-to-LSC transformation by promoting Mlkl-independent senescence. Furthermore, we demonstrate that Ripk3 signaling induces senescence in HSCs and pre-LSCs by attenuating ISR-mediated mitochondrial quality control.

Novel oncogenes and tumor suppressor genes in hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) is a very deadly disease. HCC initiation and progression involve multiple genetic events, including the activation of proto-oncogenes and disruption of the function of specific tumor suppressor genes. Activation of oncogenes stimulates cell growth and survival, while loss-of-function mutations of tumor suppressor genes result in unrestrained cell growth. In this review, we summarize the new findings that identified novel proto-oncogenes and tumor suppressors in HCC over the past five years. These findings may inspire the development of novel therapeutic strategies to improve the outcome of HCC patients.

Leukemia Stem Cells in the Pathogenesis, Progression, and Treatment of Acute Myeloid Leukemia.

Despite the significant progress that has been made in understanding the biology of leukemia stem cells (LSCs), some key questions regarding the concept of LSCs have not as yet been satisfactorily addressed experimentally. As a result, the clinical relevance of LSCs remains less than clear due to controversies caused largely by technical limitations in efficiently identifying LSCs. This has impeded our ability to fully address the features of genetic heterogeneity and metabolic/epigenetic plasticity of pre-LSCs and LSCs. With the development and use of humanized immunocompromised mice, we are able to more precisely analyze LSCs for their functions and interaction with the bone marrow niche. In addition, some promising targets in LSCs have recently been identified, including Sonic Hedgehog (SHH) and BCL-2, which are highly expressed in AML cells. It is hopeful that new anti-LSC compounds will be tested fully in clinical trials for their efficacy in treating human leukemias.

Toll-like receptor signaling in hematopoietic homeostasis and the pathogenesis of hematologic diseases.

Toll-like receptors (TLRs), which are found in innate immune cells, are essential mediators of rapid inflammatory responses and appropriate T-cell activation in response to infection and tissue damage. Accumulating evidence suggests that TLR signaling is involved in normal hematopoiesis and specific hematologic pathologies. Particular TLRs and their downstream signaling mediators are expressed not only in terminally differentiated innate immune cells but also in early hematopoietic progenitors. Sterile activation of TLR signaling is required to generate early embryonic hematopoietic progenitor cells. In adult animals, TLR signaling directly or indirectly promotes differentiation of myeloid cells at the expense of that of lymphoid cells and the self-renewal of hematopoietic stem cells during infection and tissue damage. Activating mutations of the MyD88 gene, which codes for a key adaptor involved in TLR signaling, are commonly detected in B-cell lymphomas and other B-cell hematopathologies. Dysregulated TLR signaling contributes to the pathogenesis of many hematopoietic disorders, including bone marrow failure, myelodysplastic syndrome, and acute myeloid leukemia. Complete elucidation of the molecular mechanisms by which TLR signaling mediates the regulation of both normal and pathogenic hematopoiesis will prove valuable to the development of targeted therapies and strategies for improved treatment of hematopoietic disorders.