Thymocyte proliferation induced by pre-T cell receptor signaling is maintained through polycomb gene product Bmi-1-mediated Cdkn2a repression

Thymic Bmi-1 hampers γδT17 generation and its derived RORγt-IL-17A signaling to delay cardiac aging

New immunosenescence targets for preventing senescence-associated pathological cardiac hypertrophy (SA-PCH) need to be explored. In the present study, with physiologically aged human and mouse samples, the IL-17A level increased with physiological aging, heart failure (HF), and SA-PCH and was negatively correlated with thymic Bmi-1 expression. Bmi-1f/fLckCre+ mice and Bmi-1f/f littermates were generated to determine whether Bmi-1 delayed T cell aging by maintaining thymic T cell development to prevent SA-PCH. As a result, Bmi-1 promoted thymic T cell development by upregulating Notch signaling and prevented DN1 T cells from differentiating into γδT17 cells by downregulating γδT17 cell differentiation signaling. Bmi-1 upregulated Notch signaling by inhibiting p53-mediated Ikzf1 transcription at the -1,863 to -1,849 Ikzf1 promoter region. Bmi-1-RING1B promoted RORγt ubiquitination and degradation by proteasome to inhibit the production of IL-17A in γδT17 cells. Bmi-1 also downregulated Rorc transcribed by c-Maf by trimethylating H3K27 at the -1,511 to -1,497 Rorc promoter region. Subsequently, the number of peripheral γδT17 cells infiltrating the heart tissues was reduced, while alleviating IL-17A-dependent cardiac aging, hypertrophy, dysfunction, senescence-associated secretory phenotype (SASP), and macrophage-myofibroblast transition, ultimately improving SA-PCH. The RORγt inhibitor SR1001 and IL-17A neutralizing antibody ixekizumab prevented thymic RORγt-IL-17A-dependent SA-PCH. Furthermore, RORγt bound to Bmi-1 through ARG237 and to RING1B through GLU235, which could be used as a therapeutic strategy for SA-PCH to construct binding peptides promoting Bmi-1-RING1B binding to RORγt and degrading RORγt for inhibiting γδT17 cell differentiation and IL-17A production. Thus, thymic Bmi-1 prevented IL-17A-dependent SA-PCH by decreasing γδT17 cell numbers.

Eed-dependent histone modification orchestrates the iNKT cell developmental program alleviating liver injury

Polycomb repressive complex 2 (PRC2) is an evolutionarily conserved epigenetic modifier responsible for tri-methylation of lysine 27 on histone H3 (H3K27me3). Previous studies have linked PRC2 to invariant natural killer T (iNKT) cell development, but its physiological and precise role remained unclear. To address this, we conditionally deleted Eed, a core subunit of PRC2, in mouse T cells. The results showed that Eed-deficient mice exhibited a severe reduction in iNKT cell numbers, particularly NKT1 and NKT17 cells, while conventional T cells and NKT2 cells remained intact. Deletion of Eed disrupted iNKT cell differentiation, leading to increased cell death, which was accompanied by a severe reduction in H3K27me3 levels and abnormal expression of Zbtb16, Cdkn2a, and Cdkn1a. Interestingly, Eed-deficient mice were highly susceptible to acetaminophen-induced liver injury and inflammation in an iNKT cell-dependent manner, highlighting the critical role of Eed-mediated H3K27me3 marks in liver-resident iNKT cells. These findings provide further insight into the epigenetic orchestration of iNKT cell-specific transcriptional programs.

BMI1-induced CD127+KLRG1+ memory T cells enhance the efficacy of liver cancer immunotherapy

The efficacy of HCC (hepatocellular carcinoma) immunotherapy is hindered by the limited reactivity and short duration of tumor-infiltrating T cells. These deficiencies may be ascribed to the proliferative ability of T cells. The primary objective of this study was to identify the key factor regulating tumor-infiltrating lymphocytes (TIL) proliferation within the HCC microenvironment. Through the utilization of tissue-infiltrated T cell proteomics and fraction proteomics, we analyzed the differential proteins in T cells among HCC, liver fibrosis, and hemangioma (serving as controls) groups. Additionally, we examined the differential regulatory TFs of T cells between the HCC and VH (volunteer healthy, as a control) groups. Using cyTOF and flow cytometry technologies, as well as generating CD8+ T-specific BMI1 knockout mice, we confirmed that BMI1 controls CD127+KLRG1+ memory cell differentiation. Through RNA-seq and MeRIP-seq, we verified that BMI1 regulates TCF1 expression independently of its classical function. Furthermore, by conducting Tyramide signal amplification (TSA) IHC analysis, employing a hydrodynamic mouse HCC model, and utilizing liver-specific nanoparticle targeting therapy, we demonstrated that BMI1 in HCC influences the proliferation of infiltrating CD8+T. BMI1 inhibition promotes effector T cell differentiation while suppressing memory T cell differentiation. Moreover, liver-specific BMI1 knockdown proves beneficial in ameliorating T cell dysfunction and decelerating HCC progression. Our research group has pioneered the exploration of the proteomics of HCC-infiltrated T cells, shedding light on the pivotal role of BMI1 in controlling CD127+KLRG1+ memory CD8+ T cell differentiation, which serves as the cornerstone for achieving immunotherapy efficacy in HCC.

Histone H2A deubiquitinases in the transcriptional programs of development and hematopoiesis: a consolidated analysis

Monoubiquitinated lysine 119 of histone H2A (H2AK119ub) is a highly abundant epigenetic mark, associated with gene repression and deposited on chromatin by the polycomb repressor complex 1 (PRC1), which is an essential regulator of diverse transcriptional programs in mammalian development and tissue homeostasis. While multiple deubiquitinases (DUBs) with catalytic activity for H2AK119ub (H2A-DUBs) have been identified, we lack systematic analyses of their roles and cross-talk in transcriptional regulation. Here, we address H2A-DUB functions in epigenetic regulation of mammalian development and tissue maintenance by conducting a meta-analysis of 248 genomics datasets from 32 independent studies, focusing on the mouse model and covering embryonic stem cells (ESCs), hematopoietic, and immune cell lineages. This covers all the publicly available datasets that map genomic H2A-DUB binding and H2AK119ub distributions (ChIP-Seq), and all datasets assessing dysregulation in gene expression in the relevant H2A-DUB knockout models (RNA-Seq). Many accessory datasets for PRC1-2 and DUB-interacting proteins are also analyzed and interpreted, as well as further data assessing chromatin accessibility (ATAC-Seq) and transcriptional activity (RNA-seq). We report co-localization in the binding of H2A-DUBs BAP1, USP16, and to a lesser extent others that is conserved across different cell-types, and also the enrichment of antagonistic PRC1-2 protein complexes at the same genomic locations. Such conserved sites enriched for the H2A-DUBs and PRC1-2 are proximal to transcriptionally active genes that engage in housekeeping cellular functions. Nevertheless, they exhibit H2AK119ub levels significantly above the genomic average that can undergo further increase with H2A-DUB knockout. This indicates a cooperation between H2A-DUBs and PRC1-2 in the modulation of housekeeping transcriptional programs, conserved across many cell types, likely operating through their antagonistic effects on H2AK119ub and the regulation of local H2AK119ub turnover. Our study further highlights existing knowledge gaps and discusses important directions for future work.

Stepwise progression of β-selection during T cell development involves histone deacetylation

During T cell development, the first step in creating a unique T cell receptor (TCR) is genetic recombination of the TCRβ chain. The quality of the new TCRβ is assessed at the β-selection checkpoint. Most cells fail this checkpoint and die, but the coordination of fate at the β-selection checkpoint is not yet understood. We shed new light on fate determination during β-selection using a selective inhibitor of histone deacetylase 6, ACY1215. ACY1215 disrupted the β-selection checkpoint. Characterising the basis for this disruption revealed a new, pivotal stage in β-selection, bookended by up-regulation of TCR co-receptors, CD28 and CD2, respectively. Within this "DN3b^Pre" stage, CD5 and Lef1 are up-regulated to reflect pre-TCR signalling, and their expression correlates with proliferation. These findings suggest a refined model of β-selection in which a coordinated increase in expression of pre-TCR, CD28, CD5 and Lef1 allows for modulating TCR signalling strength and culminates in the expression of CD2 to enable exit from the β-selection checkpoint.

The functions of polycomb group proteins in T cells

T cells are involved in many aspects of adaptive immunity, including autoimmunity, anti-tumor activity, and responses to allergenic substances and pathogens. T cells undergo comprehensive epigenome remodeling in response to signals. Polycomb group (PcG) proteins are a well-studied complex of chromatin regulators, conserved in animals, and function in various biological processes. PcG proteins are divided into two distinct complexes: PRC1 (Polycomb repressive complex 1) and PRC2. PcG is correlated with the regulation of T cell development, phenotypic transformation, and function. In contrast, PcG dysregulation is correlated with pathogenesis of immune-mediated diseases and compromised anti-tumor responses. This review discusses recent findings on the involvement of PcG proteins in T cell maturation, differentiation, and activation. In addition, we explore implications in the development of the immune system diseases and cancer immunity, which offers promising targets for various treatment protocols.

Realization of the T Lineage Program Involves GATA-3 Induction of Bcl11b and Repression of Cdkn2b Expression

The zinc-finger transcription factor GATA-3 plays a crucial role during early T cell development and also dictates later T cell differentiation outcomes. However, its role and collaboration with the Notch signaling pathway in the induction of T lineage specification and commitment have not been fully elucidated. We show that GATA-3 deficiency in mouse hematopoietic progenitors results in an early block in T cell development despite the presence of Notch signals, with a failure to upregulate Bcl11b expression, leading to a diversion along a myeloid, but not a B cell, lineage fate. GATA-3 deficiency in the presence of Notch signaling results in the apoptosis of early T lineage cells, as seen with inhibition of CDK4/6 (cyclin-dependent kinases 4 and 6) function, and dysregulated cyclin-dependent kinase inhibitor 2b (Cdkn2b) expression. We also show that GATA-3 induces Bcl11b, and together with Bcl11b represses Cdkn2b expression; however, loss of Cdkn2b failed to rescue the developmental block of GATA-3-deficient T cell progenitor. Our findings provide a signaling and transcriptional network by which the T lineage program in response to Notch signals is realized.

A Notch/IL-21 signaling axis primes bone marrow T cell progenitor expansion

Long-term impairment in T cell-mediated adaptive immunity is a major clinical obstacle following treatment of blood disorders with hematopoietic stem cell transplantation. Although T cell development in the thymus has been extensively characterized, there are significant gaps in our understanding of prethymic processes that influence early T cell potential. We have uncovered a Notch/IL-21 signaling axis in bone marrow common lymphoid progenitor (CLP) cells. IL-21 receptor expression was driven by Notch activation in CLPs, and in vivo treatment with IL-21 induced Notch-dependent CLP proliferation. Taking advantage of this potentially novel signaling axis, we generated T cell progenitors ex vivo, which improved repopulation of the thymus and peripheral lymphoid organs of mice in an allogeneic transplant model. Importantly, Notch and IL-21 activation were equally effective in the priming and expansion of human cord blood cells toward the T cell fate, confirming the translational potential of the combined treatment.

The histone demethylase Lsd1 regulates multiple repressive gene programs during T cell development

Analysis of the transcriptional profiles of developing thymocytes has shown that T lineage commitment is associated with loss of stem cell and early progenitor gene signatures and the acquisition of T cell gene signatures. Less well understood are the epigenetic alterations that accompany or enable these transcriptional changes. Here, we show that the histone demethylase Lsd1 (Kdm1a) performs a key role in extinguishing stem/progenitor transcriptional programs in addition to key repressive gene programs during thymocyte maturation. Deletion of Lsd1 caused a block in late T cell development and resulted in overexpression of interferon response genes as well as genes regulated by the Gfi1, Bcl6, and, most prominently, Bcl11b transcriptional repressors in CD4+CD8+ thymocytes. Transcriptional overexpression in Lsd1-deficient thymocytes was not always associated with increased H3K4 trimethylation at gene promoters, indicating that Lsd1 indirectly affects the expression of many genes. Together, these results identify a critical function for Lsd1 in the epigenetic regulation of multiple repressive gene signatures during T cell development.

Cutting Edge: TCR-β Selection Is Required at the CD4+CD8+ Stage of Human T Cell Development

T cell development is predicated on the successful rearrangement of the TCR gene loci, which encode for Ag-specific receptors. Recombination-activating gene (RAG) 2 is required for TCR gene rearrangements, which occur during specific stages of T cell development. In this study, we differentiated human pluripotent stem cells with a CRISPR/Cas9-directed deletion of the RAG2 gene (RAG2-KO) to elucidate the requirement for the TCR β-chain in mediating β-selection during human T cell development. In stark contrast to mice, human RAG2-KO T lineage progenitors progressed to the CD4⁺CD8⁺ double-positive (DP) stage in the absence of TCRβ rearrangements. Nonetheless, RAG2-KO DPs retrovirally transduced to express a rearranged TCR β-chain showed increased survival and proliferation as compared with control-transduced RAG2-KO DPs. Furthermore, transcriptomic analysis showed that TCRβ- and control-transduced RAG2-KO DPs differed in gene pathways related to survival and proliferation. Our results provide important insights as to the distinct requirement for the TCR β-chain during human T cell development.

The Route of Early T Cell Development: Crosstalk between Epigenetic and Transcription Factors

Hematopoietic multipotent progenitors seed the thymus and then follow consecutive developmental stages until the formation of mature T cells. During this process, phenotypic changes of T cells entail stage-specific transcriptional programs that underlie the dynamic progression towards mature lymphocytes. Lineage-specific transcription factors are key drivers of T cell specification and act in conjunction with epigenetic regulators that have also been elucidated as crucial players in the establishment of regulatory networks necessary for proper T cell development. In this review, we summarize the activity of transcription factors and epigenetic regulators that together orchestrate the intricacies of early T cell development with a focus on regulation of T cell lineage commitment.

Developing T cells form an immunological synapse for passage through the β-selection checkpoint

The β-selection checkpoint of T cell development tests whether the cell has recombined its genomic DNA to produce a functional T cell receptor β (TCRβ). Passage through the β-selection checkpoint requires the nascent TCRβ protein to mediate signaling through a pre-TCR complex. In this study, we show that developing T cells at the β-selection checkpoint establish an immunological synapse in in vitro and in situ, resembling that of the mature T cell. The immunological synapse is dependent on two key signaling pathways known to be critical for the transition beyond the β-selection checkpoint, Notch and CXCR4 signaling. In vitro and in situ analyses indicate that the immunological synapse promotes passage through the β-selection checkpoint. Collectively, these data indicate that developing T cells regulate pre-TCR signaling through the formation of an immunological synapse. This signaling platform integrates cues from Notch, CXCR4, and MHC on the thymic stromal cell to allow transition beyond the β-selection checkpoint.

Structural heterogeneity of the mammalian polycomb repressor complex in immune regulation

Epigenetic regulation is mainly mediated by enzymes that can modify the structure of chromatin by altering the structure of DNA or histones. Proteins involved in epigenetic processes have been identified to study the detailed molecular mechanisms involved in the regulation of specific mRNA expression. Evolutionarily well-conserved polycomb group (PcG) proteins can function as transcriptional repressors by the trimethylation of histone H3 at the lysine 27 residue (H3K27me3) and the monoubiquitination of histone H2A at the lysine 119 residue (H2AK119ub). PcG proteins form two functionally distinct protein complexes: polycomb repressor complex 1 (PRC1) and PRC2. In mammals, the structural heterogeneity of each PRC complex is dramatically increased by several paralogs of its subunit proteins. Genetic studies with transgenic mice along with RNA-seq and chromatin immunoprecipitation (ChIP)-seq analyses might be helpful for defining the cell-specific functions of paralogs of PcG proteins. Here, we summarize current knowledge about the immune regulatory role of PcG proteins related to the compositional diversity of each PRC complex and introduce therapeutic drugs that target PcG proteins in hematopoietic malignancy.

Protein complexes that suppress gene activity by remodeling chromatin, the substance that contains most of a cell’s DNA, play a critical role in regulating the immune system and provide a therapeutic target for treating blood cancers. Seok-Jin Kang and Taehoon Chun from Korea University in Seoul, South Korea, review how polycomb group proteins, best known for their function in embryonic development, also contribute to the formation of immune cells from blood stem cell precursors. Studies with stem cells and cancer cells have begun to reveal many targets of these proteins, and drug companies are evaluating candidate agents directed against some polycomb group proteins in patients with lymphoma and other cancers. More comprehensive profiling of protein function across a broad range of immune cell types could reveal new targets for additional diseases associated with immune dysfunction.

Epigenomics-Guided Drug Development: Recent Advances in Solving the Cancer Treatment "jigsaw puzzle"

The human epigenome plays a key role in determining cellular identity and eventually function. Drug discovery undertakings have focused mainly on the role of genomics in carcinogenesis, with the focus turning to the epigenome recently. Drugs targeting DNA and histone modifications are under development with some such as 5-azacytidine, decitabine, vorinostat, and panobinostat already approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMA). This expert review offers a critical analysis of the epigenomics-guided drug discovery and development and the opportunities and challenges for the next decade. Importantly, the coupling of epigenetic editing techniques, such as clustered regularly interspersed short palindromic repeat (CRISPR)-CRISPR-associated protein-9 (Cas9) and APOBEC-coupled epigenetic sequencing (ACE-seq) with epigenetic drug screens, will allow the identification of small-molecule inhibitors or drugs able to reverse epigenetic changes responsible for many diseases. In addition, concrete and sustainable innovation in cancer treatment ought to integrate epigenome targeting drugs with classic therapies such as chemotherapy and immunotherapy.

Impaired Expression of Rearranged Immunoglobulin Genes and Premature p53 Activation Block B Cell Development in BMI1 Null Mice

B cell development is a highly regulated process that requires stepwise rearrangement of immunoglobulin genes to generate a functional B cell receptor (BCR). The polycomb group protein BMI1 is required for B cell development, but its function in developing B cells remains poorly defined. We demonstrate that BMI1 functions in a cell-autonomous manner at two stages during early B cell development. First, loss of BMI1 results in a differentiation block at the pro-B cell to pre-B cell transition due to the inability of BMI1-deficient cells to transcribe newly rearranged Igh genes. Accordingly, introduction of a pre-rearranged Igh allele partially restored B cell development in Bmi1^−/− mice. In addition, BMI1 is required to prevent premature p53 signaling, and as a consequence, Bmi1^−/− large pre-B cells fail to properly proliferate. Altogether, our results clarify the role of BMI1 in early B cell development and uncover an unexpected function of BMI1 during VDJ recombination.

Cantor et al. identify a cell-autonomous role for the polycomb group protein BMI1 in early B cell development. At the pro-B cell to pre-B cell transition, BMI1 promotes the expression of newly rearranged Igh genes in pro-B cells and subsequently prevents premature p53 activation and enables large pre-B cell proliferation.

Control of Intra-Thymic αβ T Cell Selection and Maturation by H3K27 Methylation and Demethylation

In addition to transcription factor binding, the dynamics of DNA modifications (methylation) and chromatin structure are essential contributors to the control of transcription in eukaryotes. Research in the past few years has emphasized the importance of histone H3 methylation at lysine 27 for lineage specific gene repression, demonstrated that deposition of this mark at specific genes is subject to differentiation-induced changes during development, and identified enzymatic activities, methyl transferases and demethylases, that control these changes. The present review discusses the importance of these mechanisms during intrathymic αβ T cell selection and late differentiation.

EZH2 Regulates the Developmental Timing of Effectors of the Pre-Antigen Receptor Checkpoints

The histone methyltransferase EZH2 is required for B and T cell development; however, the molecular mechanisms underlying this requirement remain elusive. In a murine model of lymphoid-specific EZH2 deficiency we found that EZH2 was required for proper development of adaptive, but not innate, lymphoid cells. In adaptive lymphoid cells EZH2 prevented the premature expression of Cdkn2a and the consequent stabilization of p53, an effector of the pre-Ag receptor checkpoints. Deletion of Cdkn2a in EZH2-deficient lymphocytes prevented p53 stabilization, extended lymphocyte survival, and restored differentiation resulting in the generation of mature B and T lymphocytes. Our results uncover a crucial role for EZH2 in adaptive lymphocytes to control the developmental timing of effectors of the pre-Ag receptor checkpoints.

Conversion of T cells to B cells by inactivation of polycomb-mediated epigenetic suppression of the B-lineage program

Ikawa et al. report that the inactivation of polycomb-mediated epigenetic regulation results in the conversion of T-lineage progenitors to the B-cell fate. This arrest was almost completely cancelled by additional deletion of Pax5.

In general, cell fate is determined primarily by transcription factors, followed by epigenetic mechanisms fixing the status. While the importance of transcription factors controlling cell fate has been well characterized, epigenetic regulation of cell fate maintenance remains to be elucidated. Here we provide an obvious fate conversion case, in which the inactivation of polycomb-medicated epigenetic regulation results in conversion of T-lineage progenitors to the B-cell fate. In T-cell-specific Ring1A/B-deficient mice, T-cell development was severely blocked at an immature stage. We found that these developmentally arrested T-cell precursors gave rise to functional B cells upon transfer to immunodeficient mice. We further demonstrated that the arrest was almost completely canceled by additional deletion of Pax5. These results indicate that the maintenance of T-cell fate critically requires epigenetic suppression of the B-lineage gene program.

Lethal and nonlethal murine malarial infections differentially affect apoptosis, proliferation, and CD8 expression on thymic T cells

Although thymic atrophy and apoptosis of the double-positive (DP) T cells have been reported in murine malaria, comparative studies investigating the effect of lethal and nonlethal Plasmodium infections on the thymus are lacking. We assessed the effects of P. yoelii lethal (17XL) and nonlethal (17XNL) infections on thymic T cells. Both strains affected the thymus. 17XL infection induced DP T-cell apoptosis and a selective decrease in surface CD8 expression on developing thymocytes. By contrast, more severe but reversible effects were observed during 17XNL infection. DP T cells underwent apoptosis, and proliferation of both DN and DP cells was affected around peak parasitemia. A transient increase in surface CD8 expression on thymic T cells was also observed. Adult thymic organ culture revealed that soluble serum factors, but not IFN-γ or TNF-α, contributed to the observed effects. Thus, lethal and nonlethal malarial infections led to multiple disparate effects on thymus. These parasite-induced thymic changes are expected to impact the naïve T-cell repertoire and the subsequent control of the immune response against the parasite. Further investigations are required to elucidate the mechanism responsible for these disparate effects, especially the reversible involution of the thymus in case of nonlethal infection.

Pleiotropic Functions of H3K27Me3 Demethylases in Immune Cell Differentiation

The trimethylation of histone H3 lysine 27 (H3K27Me3) contributes to gene repression, notably through recruitment of Polycomb complexes, and has long been considered essential to maintain cell identity. Whereas H3K27Me3 was thought to be stable and not catalytically reversible, the discovery of the Utx and Jmjd3 demethylases changed this notion, raising new questions on the role of these enzymes in gene expression and cell differentiation. Recent studies have demonstrated critical roles for Utx and Jmjd3 in the development and function of immune cells, and revealed both demethylase and demethylase-independent activities of these enzymes. I review these finding here, and discuss the current understanding of the mechanisms that underlie the broad, yet highly cell- and gene-specific, impact of these enzymes in vivo.

Prostate tumorigenesis induced by PTEN deletion involves estrogen receptor β repression

The role of ERβ in prostate cancer is unclear, although loss of ERβ is associated with aggressive disease. Given that mice deficient in ERβ do not develop prostate cancer, we hypothesized that ERβ loss occurs as a consequence of tumorigenesis caused by other oncogenic mechanisms and that its loss is necessary for tumorigenesis. In support of this hypothesis, we found that ERβ is targeted for repression in prostate cancer caused by PTEN deletion and that loss of ERβ is important for tumor formation. ERβ transcription is repressed by BMI-1, which is induced by PTEN deletion and important for prostate tumorigenesis. This finding provides a mechanism for how ERβ expression is regulated in prostate cancer. Repression of ERβ contributes to tumorigenesis because it enables HIF-1/VEGF signaling that sustains BMI-1 expression. These data reveal a positive feedback loop that is activated in response to PTEN loss and sustains BMI-1.

Interplay of H2A deubiquitinase 2A-DUB/Mysm1 and the p19(ARF)/p53 axis in hematopoiesis, early T-cell development and tissue differentiation

Monoubiquitination of core histone 2A (H2A-K119u) has a critical role in gene regulation in hematopoietic differentiation and other developmental processes. To explore the interplay of histone H2A deubiquitinase Myb-like SWIRM and MPN domain containing1 (2A-DUB/Mysm1) with the p53 axis in the sequential differentiation of mature lymphocytes from progenitors, we systematically analyzed hematopoiesis and early T-cell development using Mysm1(-/-) and p53(-/-)Mysm1(-/-) mice. Mysm1(-/-) thymi were severely hypoplastic with <10% of wild-type cell numbers as a result of a reduction of early thymocyte progenitors in context with defective hematopoietic stem cells, a partial block at the double-negative (DN)1-DN2 transition and increased apoptosis of double-positive thymocytes. Increased rates of apoptosis were also detected in other tissues affected by Mysm1 deficiency, including the developing brain and the skin. By quantitative PCR and chromatin immunoprecipitation analyses, we identified p19(ARF), an important regulator of p53 tumor suppressor protein levels, as a potential Mysm1 target gene. In newly generated p53(-/-)Mysm1(-/-) double-deficient mice, anomalies of Mysm1(-/-) mice including reduction of lymphoid-primed multipotent progenitors, reduced thymocyte numbers and viability, and interestingly defective B-cell development, growth retardation, neurological defects, skin atrophy, and tail malformation were almost completely restored as well, substantiating the involvement of the p53 pathway in the alterations caused by Mysm1 deficiency. In conclusion, this investigation uncovers a novel link between H2A deubiquitinase 2A-DUB/Mysm1 and suppression of p53-mediated apoptotic programs during early lymphoid development and other developmental processes.

Polycomb proteins control proliferation and transformation independently of cell cycle checkpoints by regulating DNA replication

The ability of PRC1 and PRC2 to promote proliferation is a main feature that links polycomb (PcG) activity to cancer. PcGs silence the expression of the tumour suppressor locus Ink4a/Arf, whose products positively regulate pRb and p53 functions. Enhanced PcG activity is a frequent feature of human tumours, and PcG inhibition has been proposed as a strategy for cancer treatment. However, the recurrent inactivation of pRb/p53 responses in human cancers raises a question regarding the ability of PcG proteins to affect cellular proliferation independently from this checkpoint. Here we demonstrate that PRCs regulate cellular proliferation and transformation independently of the Ink4a/Arf-pRb-p53 pathway. We provide evidence that PRCs localize at replication forks, and that loss of their function directly affects the progression and symmetry of DNA replication forks. Thus, we have identified a novel activity by which PcGs can regulate cell proliferation independently of major cell cycle restriction checkpoints.

H2A-DUBbing the mammalian epigenome: expanding frontiers for histone H2A deubiquitinating enzymes in cell biology and physiology

Posttranslational modifications of histone H2A through the attachment of ubiquitin or poly-ubiquitin conjugates are common in mammalian genomes and play an important role in the regulation of chromatin structure, gene expression, and DNA repair. Histone H2A deubiquitinases (H2A-DUBs) are a group of structurally diverse enzymes that catalyze the removal ubiquitin from histone H2A. In this review we provide a concise summary of the mechanisms that mediate histone H2A ubiquitination in mammalian cells, and review our current knowledge of mammalian H2A-DUBs, their biochemical activities, and recent developments in our understanding of their functions in mammalian physiology.

Genetic and epigenetic control of early lymphocyte development

T, B, and NK lymphocytes are generated from pluripotent hematopoietic stem cells through a successive series of lineage restriction processes. Many regulatory components, such as transcription factors, cytokines/cytokine receptors, and signal transduction molecules orchestrate cell fate specification and determination. In particular, transcription factors play a key role in regulating lineage-associated gene programs. Recent findings suggest the involvement of epigenetic factors in the maintenance of cell fate. Here, we review the early developmental events during lymphocyte lineage determination, focusing on the transcriptional networks and epigenetic regulation. Finally, we also discuss the developmental relationship between acquired and innate lymphoid cells.

Phenylhydroquinone induces loss of thymocytes through cell cycle arrest and apoptosis elevation in p53-dependent pathway

ortho-Phenylphenol has been employed in post-harvest treatment of citrus fruits. Although o-phenylphenol has been reported to cause carcinomas in the urinary tract in rats, toxicity to the immune organs is still unknown. Herein, we report that administration of o-phenylphenol induces thymic atrophy and loss of thymocytes in female BALB/c mice. The influence seems to result from inhibition of the thymocyte development, because increased and decreased populations of the CD4⁻ CD8⁻ double-negative and CD4⁺ CD8⁺ double-positive thymocytes were observed in the o-phenylphenol-administered mice, respectively. ortho-Phenylphenol is metabolized to phenylhydroquinone by cytochrome P450 monooxygenases. Phenylhydroquinone made cell cycle of thymocytes to be arrested through reduced expression of the genes associated with G₂/M phase and through phosphorylation of p53 at Ser15. Phosphorylation of p53 at Ser15 was upregulated by activation of not only ATR but also Erk1/2 and p38, leading to increase of apoptosis. Gene expression of cytochrome P450 1A1 (CYP1A1) was promoted in thymocytes from the o-phenylphenol-administered mice. Overall, our results suggest that o-phenylphenol induces CYP1A1 expression and is metabolized into phenylhydroquinone by the expressed CYP1A1 in thymocytes. The produced phenylhydroquinone in turn induces inhibition of thymocyte development through cell cycle arrest and apoptosis in the p53-dependent pathway.

Epigenetic regulation of the Ink4a-Arf (Cdkn2a) tumor suppressor locus in the initiation and progression of Notch1-driven T cell acute lymphoblastic leukemia

Activating mutations of NOTCH1 and deletion of the INK4A-ARF (CDKN2A) tumor suppressor locus are two of the most frequent genetic alterations in T cell acute lymphoblastic leukemia (T-ALL). In a murine model of T-ALL induced by the intracellular domain of Notch1 (ICN1), the genetic interaction between ICN1 signaling and Arf inactivation is developmentally stage-specific, with a more pronounced requirement for Arf deletion in thymocytes than in bone marrow precursors targeted for transformation. In the thymus, the target cell for transformation is a CD4 and CD8 double-negative progenitor that undergoes T cell receptor beta-chain rearrangement, a cell type in which polycomb silencing of Ink4a-Arf is normally requisite. Epigenetic remodeling during tumor progression licenses Arf as a tumor suppressor and in turn provides the selective pressure for Ink4a-Arf deletion in clonal T-ALLs that emerge.

β-Selection-induced proliferation is required for αβ T cell differentiation

Proliferation and differentiation are tightly coordinated to produce an appropriate number of differentiated cells and often exhibit an antagonistic relationship. Developing T cells, which arise in the thymus from a minute number of bone-marrow-derived progenitors, undergo a major expansion upon pre-T cell receptor (TCR) expression. The burst of proliferation coincides with differentiation toward the αβ T cell lineage-but the two processes were previously thought to be independent from one another, although both were driven by signaling from pre-TCR and Notch receptors. Here we report that proliferation at this step was not only absolutely required for differentiation but also that its ectopic activation was sufficient to substantially rescue differentiation in the absence of Notch signaling. Consistently, pharmacological inhibition of the cell cycle machinery also blocked differentiation in vivo. Thus the proliferation step is strictly required prior to differentiation of immature thymocytes.

PARP1 and DNA-PKcs synergize to suppress p53 mutation and telomere fusions during T-lineage lymphomagenesis

Poly(ADP-ribose) polymerase 1 (PARP1) interacts genetically with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to suppress early-onset T-lineage lymphomas in the mouse, but the underlying mechanisms have remained unknown. To address this question, we analyzed a series of lymphomas arising in PARP1(-/-)/DNA-PKcs(-/-) (P1(-/-)/D(-/-)) mice. We found that, despite defective V(D)J recombination, P1(-/-)/D(-/-) lymphomas lacked clonal reciprocal translocations involving antigen-receptor loci. Instead, tumor cells were characterized by aneuploidy driven by two main mechanisms: p53 inactivation and abnormal chromosome disjunction due to telomere fusions (TFs). Aberrant accumulation of p53 was observed in 13/19 (68.4%) lymphomas. Sequence analysis revealed five p53 mutations: three missense point mutations (one transition in exon 8 and two transversions in exons 5 and 8, respectively), one in-frame 5-11 microindel in exon 7 and a 410-bp deletion encompassing exons 5-8, resulting in a truncated protein. Analysis of tumor metaphases using sequential telomere fluorescent in-situ hybridization and spectral karyotyping revealed that nine out of nine lymphomas contained TFs. Mutant but not wild-type p53 status was associated with frequent clonal and nonclonal TFs, suggesting that p53 normally limits the extent of telomere dysfunction during transformation. Chromosomes involved in TFs were more likely to be aneuploid than chromosomes not involved in TFs in the same metaphases, regardless of the p53 status, indicating that TFs promote aneuploidy via a mechanism that is distinct from p53 loss. Finally, analysis of radiation responses in P1(-/-)/D(-/-), and control primary cells and tissues indicates that loss of PARP1 increases in vivo radiosensitivity and genomic instability in DNA-PKcs-deficient mice without impairing p53 stabilization and effector functions, suggesting a more severe defect in double-strand break (DSB) repair in double mutants. Together, our findings uncover defective DSB repair leading to tumor suppressor inactivation and abnormal segregation of fused chromosomes as two novel mechanisms promoting tumorigenesis in thymocytes lacking PARP1 and DNA-PKcs.

Genetic regulation of thymocyte progenitor aging

The number of T cell progenitors is significantly reduced in the involuted thymus, and the growth and developmental potential of the few cells that are present is severely attenuated. This review provides an overview of how aging affects T cell precursors before and following entry into the thymus and discusses the age-related genetic changes that may occur in them. Finally, interventions that rejuvenate thymopoiesis in the elderly by targeting T cell progenitors are discussed.

Direct recruitment of polycomb repressive complex 1 to chromatin by core binding transcription factors

Polycomb repressive complexes (PRCs) play key roles in developmental epigenetic regulation. Yet the mechanisms that target PRCs to specific loci in mammalian cells remain incompletely understood. In this study we show that Bmi1, a core component of Polycomb Repressive Complex 1 (PRC1), binds directly to the Runx1/CBFβ transcription factor complex. Genome-wide studies in megakaryocytic cells demonstrate significant chromatin occupancy overlap between the PRC1 core component Ring1b and Runx1/CBFβ and functional regulation of a considerable fraction of commonly bound genes. Bmi1/Ring1b and Runx1/CBFβ deficiencies generate partial phenocopies of one another in vivo. We also show that Ring1b occupies key Runx1 binding sites in primary murine thymocytes and that this occurs via PRC2-independent mechanisms. Genetic depletion of Runx1 results in reduced Ring1b binding at these sites in vivo. These findings provide evidence for site-specific PRC1 chromatin recruitment by core binding transcription factors in mammalian cells.

The critical role of histone H2A-deubiquitinase Mysm1 in hematopoiesis and lymphocyte differentiation

Stem cell differentiation and lineage specification depend on coordinated programs of gene expression, but our knowledge of the chromatin-modifying factors regulating these events remains incomplete. Ubiquitination of histone H2A (H2A-K119u) is a common chromatin modification associated with gene silencing, and controlled by the ubiquitin-ligase polycomb repressor complex 1 (PRC1) and H2A-deubiquitinating enzymes (H2A-DUBs). The roles of H2A-DUBs in mammalian development, stem cells, and hematopoiesis have not been addressed. Here we characterized an H2A-DUB targeted mouse line Mysm1tm1a/tm1a and demonstrated defects in BM hematopoiesis, resulting in lymphopenia, anemia, and thrombocytosis. Development of lymphocytes was impaired from the earliest stages of their differentiation, and there was also a depletion of erythroid cells and a defect in erythroid progenitor function. These phenotypes resulted from a cell-intrinsic requirement for Mysm1 in the BM. Importantly, Mysm1tm1a/tm1a HSCs were functionally impaired, and this was associated with elevated levels of reactive oxygen species, γH2AX DNA damage marker, and p53 protein in the hematopoietic progenitors. Overall, these data establish a role for Mysm1 in the maintenance of BM stem cell function, in the control of oxidative stress and genetic stability in hematopoietic progenitors, and in the development of lymphoid and erythroid lineages.

Regulation of the MDM2-P53 pathway and tumor growth by PICT1 via nucleolar RPL11

PICT1 (also known as GLTSCR2) is considered a tumor suppressor because it stabilizes phosphatase and tensin homolog (PTEN), but individuals with oligodendrogliomas lacking chromosome 19q13, where PICT1 is located, have better prognoses than other oligodendroglioma patients. To clarify the function of PICT1, we generated Pict1-deficient mice and embryonic stem (ES) cells. Pict1 is a nucleolar protein essential for embryogenesis and ES cell survival. Even without DNA damage, Pict1 loss led to p53-dependent arrest of cell cycle phase G(1) and apoptosis. Pict1-deficient cells accumulated p53, owing to impaired Mdm2 function. Pict1 binds Rpl11, and Rpl11 is released from nucleoli in the absence of Pict1. In Pict1-deficient cells, increased binding of Rpl11 to Mdm2 blocks Mdm2-mediated ubiquitination of p53. In human cancer, individuals whose tumors express less PICT1 have better prognoses. When PICT1 is depleted in tumor cells with intact P53 signaling, the cells grow more slowly and accumulate P53. Thus, PICT1 is a potent regulator of the MDM2-P53 pathway and promotes tumor progression by retaining RPL11 in the nucleolus.

Functional interactions between Lmo2, the Arf tumor suppressor, and Notch1 in murine T-cell malignancies

LMO2 is a target of chromosomal translocations in T-cell tumors and was activated by retroviral vector insertions in T-cell tumors from X-SCID patients in gene therapy trials. To better understand the cooperating genetic events in LMO2-associated T-cell acute lymphoblastic leukemia (T-ALL), we investigated the roles of Arf tumor suppressor loss and Notch activation in murine models of transplantation. Lmo2 overexpression enhanced the expansion of primitive DN2 thymocytes, eventually facilitating the stochastic induction of clonal CD4(+)/CD8(+) malignancies. Inactivation of the Arf tumor suppressor further increased the self-renewal capacity of the primitive, preleukemic thymocyte pool and accelerated the development of aggressive, Lmo2-induced T-cell lympholeukemias. Notch mutations were frequently detected in these Lmo2-induced tumors. The Arf promoter was not directly engaged by Lmo2 or mutant Notch, and use of a mouse model in which activation of a mutant Notch allele depends on previous engagement of the Arf promoter revealed that Notch activation could occur as a subsequent event in T-cell tumorigenesis. Therefore, Lmo2 cooperates with Arf loss to enhance self-renewal in primitive thymocytes. Notch mutation and Arf inactivation appear to independently cooperate in no requisite order with Lmo2 overexpression in inducing T-ALL, and all 3 events remained insufficient to guarantee immediate tumor development.

Transcriptional repressors, corepressors and chromatin modifying enzymes in T cell development

Gene expression is regulated by the combined action of transcriptional activators and transcriptional repressors. Transcriptional repressors function by recruiting corepressor complexes containing histone-modifying enzymes to specific sites within DNA. Chromatin modifying complexes are subsequently recruited, either directly by transcriptional repressors, or indirectly via corepressor complexes and/or histone modifications, to remodel chromatin into either a transcription-friendly 'open' form or an inhibitory 'closed' form. Transcriptional repressors, corepressors and chromatin modifying complexes play critical roles throughout T cell development. Here, we highlight those genes that function to repress transcription and that have been shown to be required for T cell development.

The role of the basic helix-loop-helix transcription factor Dec1 in the regulatory T cells

Naturally occurring regulatory T (Treg) cells play a central role in the maintenance of immune homeostasis and in restraining the development of spontaneous inflammatory responses. However, the underlying mechanisms of Treg homeostasis remain incompletely understood. Of particular note, the IL-2Rα (CD25) is crucial for the homeostasis of Treg cells and the prevention of lymphoproliferative autoimmune disease. In this paper, we report that the basic helix-loop-helix transcription factor Dec1 is involved in the homeostasis of Treg cells and plays a role in their survival or expansion after adoptive transfer to lymphopenic recipients. Hence, it is crucial for the suppression of effector T cell-mediated inflammatory responses. Enforced expression of Dec1 upregulates CD25 expression during thymocyte development and increases the number of Treg cells in the periphery. Dec1 binds the transcription factor Runx1 and colocalizes with Runx1 in Treg cells. Specifically, we demonstrate that in Treg cells the Dec1/Runx1 complex binds to regulatory elements present in the Il-2rα locus. Collectively, these data show how Dec1 mechanistically acts in Treg cells.

Delta-like 4 differentially regulates murine CD4 T cell expansion via BMI1

BACKGROUND

Studies have shown that Notch is essential for the maintenance of a T cell Th2 phenotype in vivo. It has also been shown that Notch ligands have diverse functions during T cell activation. We chose to investigate the role of Notch ligands during the Th2 response.

PRINCIPAL FINDINGS

We studied the relationship of two Notch ligands, delta-like 4 and jagged-1, to T cell proliferation in C57 Bl/6 mice. Our findings indicate that jagged-1 does not affect the rate of T cell proliferation in any subset examined. However, delta-like 4 causes an increase in the expansion of Th2 memory cells and a decrease in effector cell proliferation. Our in vivo studies indicate that the Notch system is dynamically regulated, and that blocking one Notch ligand increases the effective concentration of other Notch ligands, thus altering the response. Examination of genes related to the Notch pathway revealed that the Notch receptors were increased in memory T cells. Expression of BMI1, a gene involved in T cell proliferation, was also higher in memory T cells. Further experiments demonstrated that Notch directly regulates the expression of the BMI1 gene in T cells and may govern T cell proliferation through this pathway.

CONCLUSIONS

From these experiments we can make several novel conclusions about the role of Notch ligands in T cell biology. The first is that delta-like 4 suppresses effector cell proliferation and enhances Th2 memory cell proliferation. The second is that blocking one Notch ligand in vivo effectively increases the concentration of other Notch ligands, which can then alter the response.

Tissue-specific p19Arf regulation dictates the response to oncogenic K-ras

The ability of oncogenes to engage tumor suppressor pathways represents a key regulatory mechanism that can limit the outgrowth of incipient tumor cells. For example, in a number of settings oncogenic Ras strongly activates the Ink4a/Arf locus, resulting in cell cycle arrest or senescence. The capacity of different cell types to execute tumor suppressor programs following expression of endogenous K-ras(G12D) in vivo has not been examined. Using compound mutant mice containing the Arf(GFP) reporter and the spontaneously activating K-ras(LA2) allele, we have uncovered dramatic tissue specificity of K-ras(G12D)-dependent p19(Arf) up-regulation. Lung tumors, which can arise in the presence of functional p19(Arf), rarely display p19(Arf) induction. In contrast, sarcomas always show robust activation, which correlates with genetic evidence, suggesting that loss of the p19(Arf)-p53 pathway is a requisite event for sarcomagenesis. Using constitutive and inducible RNAi systems in vivo, we highlight cell type-specific chromatin regulation of Ink4a/Arf as a critical determinant of cellular responses to oncogenic K-ras. Polycomb-group complexes repress the locus in lung tumors, whereas the SWI/SNF family member Snf5 acts as an important mediator of p19(Arf) induction in sarcomas. This variation in tumor suppressor induction might explain the inherent differences between tissues in their sensitivity to Ras-mediated transformation.

Poised lineage specification in multipotential hematopoietic stem and progenitor cells by the polycomb protein Bmi1

Polycomb group (PcG) proteins are essential regulators of stem cells. PcG and trithorax group proteins mark developmental regulator gene promoters with bivalent domains consisting of overlapping repressive and activating histone modifications to keep them poised for activation in embryonic stem cells. Bmi1, a component of PcG complexes, maintains the self-renewal capacity of adult stem cells, but its role in multipotency remains obscure. Here we show that Bmi1 is critical for multipotency of hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs). B cell lineage developmental regulator genes, Ebf1 and Pax5, appeared to be transcriptionally repressed by bivalent domains before lineage commitment. Loss of Bmi1 resulted in a resolution of bivalent domains at the Ebf1 and Pax5 loci, leading to their premature expression in HSC/MPPs accompanied by accelerated lymphoid specification and a marked reduction in HSC/MPPs. Thus, Bmi1 is required to reinforce bivalent domains at key developmental regulator gene loci to maintain lineage specification poised for activation in adult stem cells.

Hematopoietic stem cells and retroviral infection

Retroviral induced malignancies serve as ideal models to help us better understand the molecular mechanisms associated with the initiation and progression of leukemogenesis. Numerous retroviruses including AEV, FLV, M-MuLV and HTLV-1 have the ability to infect hematopoietic stem and progenitor cells, resulting in the deregulation of normal hematopoiesis and the development of leukemia/lymphoma. Research over the last few decades has elucidated similarities between retroviral-induced leukemogenesis, initiated by deregulation of innate hematopoietic stem cell traits, and the cancer stem cell hypothesis. Ongoing research in some of these models may provide a better understanding of the processes of normal hematopoiesis and cancer stem cells. Research on retroviral induced leukemias and lymphomas may identify the molecular events which trigger the initial cellular transformation and subsequent maintenance of hematologic malignancies, including the generation of cancer stem cells. This review focuses on the role of retroviral infection in hematopoietic stem cells and the initiation, maintenance and progression of hematological malignancies.

Early T cell differentiation lessons from T-cell acute lymphoblastic leukemia

T cells develop from bone marrow-derived self-renewing hematopoietic stem cells (HSC). Upon entering the thymus, these cells undergo progressive commitment and differentiation driven by the thymic stroma and the pre-T cell receptor (pre-TCR). These processes are disrupted in T-cell acute lymphoblastic leukemia (T-ALL). More than 70% of recurring chromosomal rearrangements in T-ALL activate the expression of oncogenic transcription factors, belonging mostly to three families, basic helix-loop-helix (bHLH), homeobox (HOX), and c-MYB. This prevalence is indicative of their importance in the T lineage, and their dominant mechanisms of transformation. For example, bHLH oncoproteins inhibit E2A and HEB, revealing their tumor suppressor function in the thymus. The induction of T-ALL, nonetheless, requires collaboration with constitutive NOTCH1 signaling and the pre-TCR, as well as loss-of-function mutations for CDKN2A and PTEN. Significantly, NOTCH1, the pre-TCR pathway, and E2A/HEB proteins control critical checkpoints and branchpoints in early thymocyte development whereas several oncogenic transcription factors, HOXA9, c-MYB, SCL, and LYL-1 control HSC self-renewal. Together, these genetic lesions alter key regulatory processes in the cell, favoring self-renewal and subvert the normal control of thymocyte homeostasis.

A histone demethylase is necessary for regeneration in zebrafish

Urodele amphibians and teleost fish regenerate amputated body parts via a process called epimorphic regeneration. A hallmark of this phenomenon is the reactivation of silenced developmental regulatory genes that previously functioned during embryonic patterning. We demonstrate that histone modifications silence promoters of numerous genes involved in zebrafish caudal fin regeneration. Silenced developmental regulatory genes contain bivalent me(3)K4/me(3)K27 H3 histone modifications created by the concerted action of Polycomb (PcG) and Trithorax histone methyltransferases. During regeneration, this silent, bivalent chromatin is converted to an active state by loss of repressive me(3)K27 H3 modifications, occurring at numerous genes that appear to function during regeneration. Loss-of-function studies demonstrate a requirement for a me(3)K27 H3 demethylase during fin regeneration. These results indicate that histone modifications at discreet genomic positions may serve as a crucial regulatory event in the initiation of fin regeneration.

Stage-specific Arf tumor suppression in Notch1-induced T-cell acute lymphoblastic leukemia

Frequent hallmarks of T-cell acute lymphoblastic leukemia (T-ALL) include aberrant NOTCH signaling and deletion of the CDKN2A locus, which contains 2 closely linked tumor suppressor genes (INK4A and ARF). When bone marrow cells or thymocytes transduced with a vector encoding the constitutively activated intracellular domain of Notch1 (ICN1) are expanded ex vivo under conditions that support T-cell development, cultured progenitors rapidly induce CD4+/CD8+ T-ALLs after infusion into healthy syngeneic mice. Under these conditions, enforced ICN1 expression also drives formation of T-ALLs in unconditioned CD-1 nude mice, bypassing any requirements for thymic maturation. Retention of Arf had relatively modest activity in suppressing the formation of T-ALLs arising from bone marrow-derived ICN1+ progenitors in which the locus is epigenetically silenced, and all resulting Arf (+/+) tumors failed to express the p19(Arf) protein. In striking contrast, retention of Arf in thymocyte-derived ICN1+ donor cells significantly delayed disease onset and suppressed the penetrance of T-ALL. Use of cultured thymocyte-derived donor cells expressing a functionally null Arf-GFP knock-in allele confirmed that ICN1 signaling can induce Arf expression in vivo. Arf activation by ICN1 in T cells thereby provides stage-specific tumor suppression but also a strong selective pressure for deletion of the locus in T-ALL.

Identification of cancer stem cells in a Tax-transgenic (Tax-Tg) mouse model of adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATL) is a malignant lymphoproliferative disorder caused by HTLV-I infection. In ATL, chemotherapeutic responses are generally poor, which has suggested the existence of chemotherapy-resistant cancer stem cells (CSCs). To identify CSC candidates in ATL, we have focused on a Tax transgenic mouse (Tax-Tg) model, which reproduces ATL-like disease both in Tax-Tg animals and also after transfer of Tax-Tg splenic lymphomatous cells (SLCs) to nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice. Using a limiting dilution transplantation, it was estimated that one CSC existed per 10(4) SLCs (0.01%). In agreement with this, we have successfully identified candidate CSCs in a side population (0.06%), which overlapped with a minor population of CD38(-)/CD71(-)/CD117(+) cells (0.03%). Whereas lymphoma did not develop after transplantation of 10(2) SLCs, 10(2) CSCs could consistently regenerate the original lymphoma. In addition, lymphoma and CSCs could also be demonstrated in the bone marrow and CD117(+) CSCs were observed in both osteoblastic and vascular niches. In the CSCs, Tax, Notch1, and Bmi1 expression was down-regulated, suggesting that the CSCs were derived from Pro-T cells or early hematopoietic progenitor cells. Taken together, our data demonstrate that CSCs certainly exist and have the potential to regenerate lymphoma in our mouse model.

Rescue of key features of the p63-null epithelial phenotype by inactivation of Ink4a and Arf

Mice lacking p63 cannot form skin, exhibit craniofacial and skeletal defects, and die soon after birth. The p63 gene regulates a complex network of target genes, and disruption of p63 has been shown to affect the maintenance of epithelial stem cells, the differentiation of keratinocytes, and the preservation of the adhesive properties of stratified epithelium. Here, we show that inactivation of p63 in mice is accompanied by aberrantly increased expression of the Ink4a and Arf tumour suppressor genes. In turn, anomalies of the p63-null mouse affecting the skin and skeleton are partially ameliorated in mice lacking either Ink4a or Arf. Rescue of epithelialization is accompanied by restoration of keratinocyte proliferative capacity both in vivo and in vitro and by expression of markers of squamous differentiation. Thus, in the absence of p63, abnormal upregulation of Ink4a and Arf is incompatible with skin development.