Posterior transformation, neurological abnormalities, and severe hematopoietic defects in mice with a targeted deletion of the bmi-1 proto-oncogene

Sirt1 Promotes Osteogenic Differentiation and Increases Alveolar Bone Mass via Bmi1 Activation in Mice

Sirtuin 1 (Sirt1), a protein deacetylase, is a novel target for bone metabolism. To investigate whether overexpression of Sirt1 in mandibular mesenchymal stem cells (M-MSCs) increased alveolar bone mass in vivo, we generated Sirt1 transgenic mice (Sirt1^TG ), with Sirt1 gene expression driven by the Prx1 gene, which represents the mesenchymal lineage. Our results demonstrated that overexpression of Sirt1 in M-MSCs increased the alveolar bone volume in 1-month-old, 9-month-old, and 18-month-old Sirt1^TG mice compared with age-matched wild-type (WT) mice, and in ovariectomized Sirt1^TG mice compared with ovariectomized WT mice by stimulating M-MSC differentiation into osteoblasts. Treatment with resveratrol, a Sirt1 activator, increased Sirt1 binding with Bmi1 and reduced Bmi1 acetylation in a dose-dependent manner demonstrated in M-MSC cultures. Both treatment with resveratrol in M-MSC cultures and overexpressed Sirt1 in M-MSCs ex vivo cultures increased nuclear translocation of Bmi1. Furthermore, we demonstrated that deletion of Bmi1 blocked the increased alveolar bone volume in Sirt1^TG mice. The Sirt1 activator resveratrol inhibited human MSC senescence and promoted their differentiation into osteoblasts, which were associated with upregulating the expression levels of Sirt1 and nuclear translocation of Bmi1. The present results suggested that Sirt1 promotes MSC proliferation and osteogenic differentiation, inhibits MSC senescence to increase alveolar bone volume by promoting the deacetylation and nuclear translocation of Bmi1. Thus, our study elucidated the mechanism by which Sirt1 increases alveolar bone mass, and these findings are important for the clinical application of the Sirt1 activator resveratrol for the promotion of alveolar bone formation and prevention of alveolar bone loss. © 2019 American Society for Bone and Mineral Research.

Roles of Polycomb group proteins Enhancer of zeste (E(z)) and Polycomb (Pc) during metamorphosis and larval leg regeneration in the flour beetle Tribolium castaneum

Many organisms both undergo dramatic morphological changes during post-embryonic development and also regenerate lost structures, but the roles of epigenetic regulators in such processes are only beginning to be understood. In the present study, the functions of two histone modifiers were examined during metamorphosis and larval limb regeneration in the red flour beetle Tribolium castaneum. Polycomb (Pc), a member of Polycomb repressive complex 1 (PRC1), and Enhancer of zeste (E(z)), a member of Polycomb repressive complex 2 (PRC2), were silenced in larvae using RNA interference. In the absence of Pc, the head appendages of adults transformed into a leg-like morphology, and the legs and wings assumed a metathoracic identity, indicating that Pc acts to specify proper segmental identity. Similarly, silencing of E(z) led to homeotic transformation of legs and wings. Additional defects were also observed in limb patterning as well as eye morphogenesis, indicating that PcG proteins play critical roles in imaginal precursor cells. In addition, larval legs and antennae failed to re-differentiate when either Pc or E(z) was knocked down, indicating that histone modification is necessary for proper blastema growth and differentiation. These findings indicate that PcG proteins play extensive roles in postembryonic plasticity of imaginal precursor cells.

BCOR involvement in cancer

BCOR is a gene that encodes for an epigenetic regulator involved in the specification of cell differentiation and body structure development and takes part in the noncanonical polycomb repressive complex 1. This review provides a comprehensive summary of BCOR’s involvement in oncology, illustrating that various BCOR aberrations, such as the internal tandem duplications of the PCGF Ub-like fold discriminator domain and different gene fusions (mainly BCOR–CCNB3, BCOR–MAML3 and ZC3H7B–BCOR), represent driver elements of various sarcomas such as clear cell sarcoma of the kidney, primitive mesenchymal myxoid tumor of infancy, small round blue cell sarcoma, endometrial stromal sarcoma and histologically heterogeneous CNS neoplasms group with similar genomic methylation patterns known as CNS-HGNET-BCOR. Furthermore, other BCOR alterations (often loss of function mutations) recur in a large variety of mesenchymal, epithelial, neural and hematological tumors, suggesting a central role in cancer evolution.

Loss of the Polycomb group protein Rnf2 results in derepression of tbx-transcription factors and defects in embryonic and cardiac development

The Polycomb group (PcG) protein family is a well-known group of epigenetic modifiers. We used zebrafish to investigate the role of Rnf2, the enzymatic subunit of PRC1. We found a positive correlation between loss of Rnf2 and upregulation of genes, especially of those whose promoter is normally bound by Rnf2. The heart of rnf2 mutants shows a tubular shaped morphology and to further understand the underlying mechanism, we studied gene expression of single wildtype and rnf2 mutant hearts. We detected the most pronounced differences at 3 dpf, including upregulation of heart transcription factors, such as tbx2a, tbx2b, and tbx3a. These tbx genes were decorated by broad PcG domains in wildtype whole embryo lysates. Chamber specific genes such as vmhc, myh6, and nppa showed downregulation in rnf2 mutant hearts. The marker of the working myocard, nppa, is negatively regulated by Tbx2 and Tbx3. Based on our findings and literature we postulate that loss of Rnf2-mediated repression results in upregulation and ectopic expression of tbx2/3, whose expression is normally restricted to the cardiac conductive system. This could lead to repression of chamber specific gene expression, a misbalance in cardiac cell types, and thereby to cardiac defects observed in rnf2 mutants.

Mouse polycomb group gene Cbx2 promotes osteoblastic but suppresses adipogenic differentiation in postnatal long bones

A set of key developmental genes is essential for skeletal growth from multipotent progenitor cells at weaning. Polycomb group proteins, which regulate such genes contributes to the cell lineage commitment and subsequent differentiation via epigenetic chromatin modification and remodeling. However, it is unclear which cell lineage and gene sets are targeted by polycomb proteins during skeletal growth. We now report that mice deficient in a polycomb group gene Cbx2^cterm/cterm exhibited skeletal hypoplasia in the tibia, femur, and cranium. Long bone cavities in these mice contained fewer multipotent mesenchymal stromal cells. RNA-sequencing of bone marrow cells showed downregulation and upregulation of osteoblastic and adipogenic genes, respectively. Furthermore, the expression levels of genes specifically expressed in B-cell precursors were decreased. Forced expression of Cbx2 in Cbx2^cterm/cterm bone marrow stromal cell recovered fibroblastic colony formation and suppressed adipogenic differentiation. Collectively, our results suggest that Cbx2 controls the maintenance and adipogenic differentiation of mesenchymal stromal cells in the bone marrow.

Hypomorphic mutation of the mouse Huntington's disease gene orthologue

Rare individuals with inactivating mutations in the Huntington's disease gene (HTT) exhibit variable abnormalities that imply essential HTT roles during organ development. Here we report phenotypes produced when increasingly severe hypomorphic mutations in the murine HTT orthologue Htt, (HdhneoQ20, HdhneoQ50, HdhneoQ111), were placed over a null allele (Hdhex4/5). The most severe hypomorphic allele failed to rescue null lethality at gastrulation, while the intermediate, though still severe, alleles yielded recessive perinatal lethality and a variety of fetal abnormalities affecting body size, skin, skeletal and ear formation, and transient defects in hematopoiesis. Comparative molecular analysis of wild-type and Htt-null retinoic acid-differentiated cells revealed gene network dysregulation associated with organ development that nominate polycomb repressive complexes and miRNAs as molecular mediators. Together these findings demonstrate that Htt is required both pre- and post-gastrulation to support normal development.

Chasing Mavericks: The quest for defining developmental waves of hematopoiesis

Hematopoiesis is the process by which mature blood and immune cells are produced from hematopoietic stem and progenitor cells (HSCs and HSPCs). The last several decades of research have shed light on the origin of HSCs, as well as the heterogeneous pools of fetal progenitors that contribute to lifelong hematopoiesis. The overarching concept that hematopoiesis occurs in dynamic, overlapping waves throughout development, with each wave contributing to both continuous and developmentally limited cell types, has been solidified over the years. However, recent advances in our ability to track the production of hematopoietic cells in vivo have challenged several long-held dogmas on the origin and persistence of distinct hematopoietic cell types. In this review, we highlight emerging concepts in hematopoietic development and identify unanswered questions.

Heterochromatic genome instability and neurodegeneration sharing similarities with Alzheimer's disease in old Bmi1+/- mice

Sporadic Alzheimer’s disease (AD) is the most common cause of dementia. However, representative experimental models of AD have remained difficult to produce because of the disease’s uncertain origin. The Polycomb group protein BMI1 regulates chromatin compaction and gene silencing. BMI1 expression is abundant in adult brain neurons but down-regulated in AD brains. We show here that mice lacking one allele of Bmi1 (Bmi1+/−) develop normally but present with age cognitive deficits and neurodegeneration sharing similarities with AD. Bmi1+/− mice also transgenic for the amyloid beta precursor protein died prematurely and present aggravated disease. Loss of heterochromatin and DNA damage response (DDR) at repetitive DNA sequences were predominant in Bmi1+/− mouse neurons and inhibition of the DDR mitigated the amyloid and Tau phenotype. Heterochromatin anomalies and DDR at repetitive DNA sequences were also found in AD brains. Aging Bmi1+/− mice may thus represent an interesting model to identify and study novel pathogenic mechanisms related to AD.

A prospect of cell immortalization combined with matrix microenvironmental optimization strategy for tissue engineering and regeneration

Cellular senescence is a major hurdle for primary cell-based tissue engineering and regenerative medicine. Telomere erosion, oxidative stress, the expression of oncogenes and the loss of tumor suppressor genes all may account for the cellular senescence process with the involvement of various signaling pathways. To establish immortalized cell lines for research and clinical use, strategies have been applied including internal genomic or external matrix microenvironment modification. Considering the potential risks of malignant transformation and tumorigenesis of genetic manipulation, environmental modification methods, especially the decellularized cell-deposited extracellular matrix (dECM)-based preconditioning strategy, appear to be promising for tissue engineering-aimed cell immortalization. Due to few review articles focusing on this topic, this review provides a summary of cell senescence and immortalization and discusses advantages and limitations of tissue engineering and regeneration with the use of immortalized cells as well as a potential rejuvenation strategy through combination with the dECM approach.

BMI1 enhancer polymorphism underlies chromosome 10p12.31 association with childhood acute lymphoblastic leukemia

Genome-wide association studies of childhood acute lymphoblastic leukemia (ALL) have identified regions of association at PIP4K2A and upstream of BMI1 at chromosome 10p12.31-12.2. The contribution of both loci to ALL risk and underlying functional variants remain to be elucidated. We carried out single nucleotide polymorphism (SNP) imputation across chromosome 10p12.31-12.2 in Latino and non-Latino white ALL cases and controls from two independent California childhood leukemia studies, and additional Genetic Epidemiology Research on Aging study controls. Ethnicity-stratified association analyses were performed using logistic regression, with meta-analysis including 3,133 cases (1,949 Latino, 1,184 non-Latino white) and 12,135 controls (8,584 Latino, 3,551 non-Latino white). SNP associations were identified at both BMI1 and PIP4K2A. After adjusting for the lead PIP4K2A SNP, genome-wide significant associations remained at BMI1, and vice-versa (pmeta < 10-10 ), supporting independent effects. Lead SNPs differed by ethnicity at both peaks. We sought functional variants in tight linkage disequilibrium with both the lead Latino SNP among Admixed Americans and lead non-Latino white SNP among Europeans. This pinpointed rs11591377 (pmeta = 2.1 x 10-10 ) upstream of BMI1, residing within a hematopoietic stem cell enhancer of BMI1, and which showed significant preferential binding of the risk allele to MYBL2 (p = 1.73 x 10-5 ) and p300 (p = 1.55 x 10-3 ) transcription factors using binomial tests on ChIP-Seq data from a SNP heterozygote. At PIP4K2A, we identified rs4748812 (pmeta = 1.3 x 10-15 ), which alters a RUNX1 binding motif and demonstrated chromosomal looping to the PIP4K2A promoter. Fine-mapping chromosome 10p12 in a multi-ethnic ALL GWAS confirmed independent associations and identified putative functional variants upstream of BMI1 and at PIP4K2A.

Overexpression of Sirt1 in mesenchymal stem cells protects against bone loss in mice by FOXO3a deacetylation and oxidative stress inhibition

OBJECTIVE

B cell-specific Moloney murine leukemia virus integration site 1 (Bmi-1) deficiency (Bmi-1^-/-) leads to an osteoporotic phenotype with a significant downregulation of Sirt1 protein expression. Sirtuin 1 (Sirt1) haploinsufficiency results in a bone loss by decreased bone formation; however, it is unclear whether Sirt1 overexpression in mesenchymal stem cells (MSCs) plays an anti-osteoporotic role. The aim of the study is to identify whether the overexpression of Sirt1 in MSCs could restore skeletal growth retardation and osteoporosis in Bmi-1 deficient mice.

METHODS

We used our new generated transgenic mouse model that overexpresses Sirt1 in its MSCs (Sirt1^TG) to cross with Bmi-1^-/- mice to generate Bmi-1^-/- mice with Sirt1 overexpression in MSCs, and compared their skeletal metabolism with those of their Bmi-1^-/- and wild-type (WT) littermates (6 mice for each genotype) at 4 weeks of age using imaging, histopathological, immunohistochemical, histomorphometric, cellular, and molecular methods.

RESULTS

The levels of expression for Sirt1 were noticeably higher in the skeletal tissue of Sirt1^TG mice than in those of WT mice. In Comparison to WT mice, the body weight and size, skeletal size, bone volume, osteoblast number, alkaline phosphatase and type I collagen positive areas, osteogenic related gene expression levels were all significantly increased in the Sirt1^TG mice. Overexpression of Sirt1 in Bmi-1^-/- mouse MSCs resulted in a longer lifespan, improved skeletal growth and significantly increased bone mass by stimulating osteoblastic bone formation and inhibiting osteoclastic bone resorption in the Bmi-1^-/- mice, although the defects were not completely restored. Furthermore, Sirt1 overexpression in MSCs reduced the acetylation level of FOXO3a (Forkhead box O3a), increasing levels of expression for FOXO3a and SOD2 (Superoxide dismutase 2) in bony tissue, enhanced osteogenesis and reduced osteogenic cell senescence. We also demonstrated that nicotinamide, a Sirt1 inhibitor, blocks the effect of overexpression of Sirt1 in MSCs on osteogenesis and osteogenic cell senescence.

CONCLUSIONS

Taken together, these results demonstrate that Sirt1 overexpression in MSCs increased the osteoblastic bone formation and partially restores the defects in skeletal growth and osteogenesis in Bmi-1^-/- mice by FOXO3a deacetylation and oxidative stress inhibition. Our data support the proposal that Sirt1 is a target for promoting bone formation as an anabolic approach for the treatment of osteoporosis.

Smchd1 regulates long-range chromatin interactions on the inactive X chromosome and at Hox clusters

The regulation of higher-order chromatin structure is complex and dynamic, and a full understanding of the suite of mechanisms governing this architecture is lacking. Here, we reveal the noncanonical SMC protein Smchd1 to be a novel regulator of long-range chromatin interactions in mice, and we add Smchd1 to the canon of epigenetic proteins required for Hox-gene regulation. The effect of losing Smchd1-dependent chromatin interactions has varying outcomes that depend on chromatin context. At autosomal targets transcriptionally sensitive to Smchd1 deletion, we found increased short-range interactions and ectopic enhancer activation. In contrast, the inactive X chromosome was transcriptionally refractive to Smchd1 ablation, despite chromosome-wide increases in short-range interactions. In the inactive X, we observed spreading of trimethylated histone H3 K27 (H3K27me3) domains into regions not normally decorated by this mark. Together, these data suggest that Smchd1 is able to insulate chromatin, thereby limiting access to other chromatin-modifying proteins.

PTE, a novel module to target Polycomb Repressive Complex 1 to the human cyclin D2 (CCND2) oncogene

Polycomb group proteins are essential epigenetic repressors. They form multiple protein complexes of which two kinds, PRC1 and PRC2, are indispensable for repression. Although much is known about their biochemical properties, how mammalian PRC1 and PRC2 are targeted to specific genes is poorly understood. Here, we establish the cyclin D2 (CCND2) oncogene as a simple model to address this question. We provide the evidence that the targeting of PRC1 to CCND2 involves a dedicated PRC1-targeting element (PTE). The PTE appears to act in concert with an adjacent cytosine-phosphate-guanine (CpG) island to arrange for the robust binding of PRC1 and PRC2 to repressed CCND2 Our findings pave the way to identify sequence-specific DNA-binding proteins implicated in the targeting of mammalian PRC1 complexes and provide novel link between polycomb repression and cancer.

Cell Signaling in Neuronal Stem Cells

The defining characteristic of neural stem cells (NSCs) is their ability to multiply through symmetric divisions and proliferation, and differentiation by asymmetric divisions, thus giving rise to different types of cells of the central nervous system (CNS). A strict temporal space control of the NSC differentiation is necessary, because its alterations are associated with neurological dysfunctions and, in some cases, death. This work reviews the current state of the molecular mechanisms that regulate the transcription in NSCs, organized according to whether the origin of the stimulus that triggers the molecular cascade in the CNS is internal (intrinsic factors) or whether it is the result of the microenvironment that surrounds the CNS (extrinsic factors).

CBFβ-SMMHC Inhibition Triggers Apoptosis by Disrupting MYC Chromatin Dynamics in Acute Myeloid Leukemia

The fusion oncoprotein CBFβ-SMMHC, expressed in leukemia cases with chromosome 16 inversion, drives leukemia development and maintenance by altering the activity of the transcription factor RUNX1. Here, we demonstrate that CBFβ-SMMHC maintains cell viability by neutralizing RUNX1-mediated repression of MYC expression. Upon pharmacologic inhibition of the CBFβ-SMMHC/RUNX1 interaction, RUNX1 shows increased binding at three MYC distal enhancers, where it represses MYC expression by mediating the replacement of the SWI/SNF complex component BRG1 with the polycomb-repressive complex component RING1B, leading to apoptosis. Combining the CBFβ-SMMHC inhibitor with the BET inhibitor JQ1 eliminates inv(16) leukemia in human cells and a mouse model. Enhancer-interaction analysis indicated that the three enhancers are physically connected with the MYC promoter, and genome-editing analysis demonstrated that they are functionally implicated in deregulation of MYC expression. This study reveals a mechanism whereby CBFβ-SMMHC drives leukemia maintenance and suggests that inhibitors targeting chromatin activity may prove effective in inv(16) leukemia therapy.

Overexpressed Sirt1 in MSCs Promotes Dentin Formation in Bmi1-Deficient Mice

Sirt1 promotes odontoblastic gene expression in human dental pulp cells, whereas the inhibition of Sirt1 downregulates the expression of those genes. To investigate whether the overexpression of Sirt1 in mesenchymal stem cells (MSCs) driven by Prx1 promoter could rescue the dentin formation defects in Bmi1-deficient (Bmi1^-/-) mice, we established the MSCs overexpressing Sirt1 in Bmi1 knockout mice (Sirt1^TGBmi1^-/-). First, we used Prx1-Cre/ROSA^nTnG mice to demonstrate that Prx1 linage cells exist mainly in the pulp horns at 4 wk of age. Second, we found that 4-wk-old Sirt1^TG mice had increased tooth volume as compared with wild-type (WT) littermates. The expression level of Sirt1 was significantly higher in dental papilla mesenchymal cells of Sirt1^TG mice than WT mice. Furthermore, we demonstrated that the tooth mineralization, dental volume, dentin sialoprotein-immunopositive areas, odontoblastic gene expression, and percentage of proliferating BrdU-positive cells were significantly elevated in the Sirt1^TG mice and dramatically reduced in the Bmi1^-/- mice versus the WT littermates at 4 wk of age. However, the areas of predentin and the percentage of TUNEL-positive apoptotic cells were significantly reduced in the Sirt1^TG mice but dramatically increased in the Bmi1^-/- mice as compared with the WT littermates. All these parameters were rescued in the Sirt1^TGBmi1^-/- mice versus the Bmi1^-/- mice. Finally, by using dental papilla mesenchymal cells, we found that the overexpression of Sirt1 rescued the reduced cell proliferation and differentiation and increased the cell apoptosis caused by Bmi1 deficiency, which was associated with increased p53 deacetylation. Therefore, this study indicates that Sirt1 is a potential therapeutic target for promoting dentin formation in an anabolic approach to the treatment of dental developmental defects.

The putative tumor suppressor gene EphA7 is a novel BMI-1 target

Bmi1 was originally identified as a gene that contributes to the development of mouse lymphoma by inhibiting MYC-induced apoptosis through repression of Ink4a and Arf. It codes for the Polycomb group protein BMI-1 and acts primarily as a transcriptional repressor via chromatin modifications. Although it binds to a large number of genomic regions, the direct BMI-1 target genes described so far do not explain the full spectrum of BMI-1-mediated effects. Here we identify the putative tumor suppressor gene EphA7 as a novel direct BMI-1 target in neural cells and lymphocytes. EphA7 silencing has been reported in several different human tumor types including lymphomas, and our data suggest BMI1 overexpression as a novel mechanism leading to EphA7 inactivation via H3K27 trimethylation and DNA methylation.

Pharmacological targets of breast cancer stem cells: a review

Breast cancers contain small population of tumor-initiating cells called breast cancer stem cells (BCSCs), which are spared even after chemotherapy. Recently, BCSCs are implicated to be a cause of metastasis, tumor relapse, and therapy resistance in breast cancer. BCSCs have unique molecular mechanisms, which can be targeted to eliminate them. These include surface biomarkers, proteins involved in self-renewal pathways, drug efflux transporters, apoptotic/antiapoptotic proteins, autophagy, metabolism, and microenvironment regulation. The complex molecular mechanisms behind the survival of BCSCs and pharmacological targets for elimination of BCSCs are described in this review.

Three-step transcriptional priming that drives the commitment of multipotent progenitors toward B cells

Miyai et al. used multipotent progenitors harboring a tamoxifen-inducible form of Id3—where virtually all cells became B cells within 6 d by withdrawing 4-OHT—to identify a three-step transcription factor network model during specification of multipotent progenitors toward the B-cell lineage.

Stem cell fate is orchestrated by core transcription factors (TFs) and epigenetic modifications. Although regulatory genes that control cell type specification are identified, the transcriptional circuit and the cross-talk among regulatory factors during cell fate decisions remain poorly understood. To identify the “time-lapse” TF networks during B-lineage commitment, we used multipotent progenitors harboring a tamoxifen-inducible form of Id3, an in vitro system in which virtually all cells became B cells within 6 d by simply withdrawing 4-hydroxytamoxifen (4-OHT). Transcriptome and epigenome analysis at multiple time points revealed that ∼10%–30% of differentially expressed genes were virtually controlled by the core TFs, including E2A, EBF1, and PAX5. Strikingly, we found unexpected transcriptional priming before the onset of the key TF program. Inhibition of the immediate early genes such as Nr4a2, Klf4, and Egr1 severely impaired the generation of B cells. Integration of multiple data sets, including transcriptome, protein interactome, and epigenome profiles, identified three representative transcriptional circuits. Single-cell RNA sequencing (RNA-seq) analysis of lymphoid progenitors in bone marrow strongly supported the three-step TF network model during specification of multipotent progenitors toward B-cell lineage in vivo. Thus, our findings will provide a blueprint for studying the normal and neoplastic development of B lymphocytes.

Bmi1+ Progenitor Cell Dynamics in Murine Cornea During Homeostasis and Wound Healing

The outermost layer of the eye, the cornea, is renewed continuously throughout life. Stem cells of the corneal epithelium reside in the limbus at the corneal periphery and ensure homeostasis of the central epithelium. However, in young mice, homeostasis relies on cells located in the basal layer of the central corneal epithelium. Here, we first studied corneal growth during the transition from newborn to adult and assessed Keratin 19 (Krt19) expression as a hallmark of corneal maturation. Next, we set out to identify a novel marker of murine corneal epithelial progenitor cells before, during and after maturation, and we found that Bmi1 is expressed in the basal epithelium of the central cornea and limbus. Furthermore, we demonstrated that Bmi1+ cells participated in tissue replenishment in the central cornea. These Bmi1+ cells did not maintain homeostasis of the cornea for more than 3 months, reflecting their status as progenitor rather than stem cells. Finally, after injury, Bmi1+ cells fueled homeostatic maintenance, whereas wound closure occurred via epithelial reorganization. Stem Cells 2018;36:562-573.

HOTAIR but not ANRIL long non-coding RNA contributes to the pathogenesis of multiple sclerosis

Studies have revealed that dysregulation in gene expression is one of the main aspects of multiple sclerosis (MS) pathogenesis. Although the molecular pathways underlying the immunomodulatory role of vitamin D (VD) in MS is not completely elucidated, VD has more recently become a topic of interest in immune regulation and is widely administered to patients with MS as an immunomodulatory supplement. Long non-coding RNAs (lncRNAs) are known to play important roles in regulation of gene expression via different mechanisms. Given that VD-related genes are regulated by epigenetic mechanisms, here we aimed to evaluate the role of VD in combination with HOTAIR and ANRIL lncRNAs using in vivo, in vitro and in silico experiments in MS pathogenesis. Our data revealed that HOTAIR but not ANRIL lncRNA is probably involved in the pathogenesis of MS and experimental autoimmune encephalomyelitis through an unclear mechanism and it seems that by affecting the expression, inflammation and VD can influence HOTAIR-related mechanisms, which require further study.

Maintenance of Positional Identity of Neural Progenitors in the Embryonic and Postnatal Telencephalon

Throughout embryonic development and into postnatal life, regionally distinct populations of neural progenitor cells (NPCs) collectively generate the many different types of neurons that underlie the complex structure and function of the adult mammalian brain. At very early stages of telencephalic development, NPCs become organized into regional domains that each produce different subsets of neurons. This positional identity of NPCs relates to the regional expression of specific, fate-determining homeodomain transcription factors. As development progresses, the brain undergoes vast changes in both size and shape, yet important aspects of NPC positional identity persist even into the postnatal brain. How can NPC positional identity, which is established so early in brain development, endure the many dynamic, large-scale and complex changes that occur over a relatively long period of time? In this Perspective article, we review data and concepts derived from studies in Drosophila regarding the function of homeobox (Hox) genes, Polycomb group (PcG) and trithorax group (trxG) chromatin regulators. We then discuss how this knowledge may contribute to our understanding of the maintenance of positional identity of NPCs in the mammalian telencephalon. Similar to the axial body plan of Drosophila larvae, there is a segmental nature to NPC positional identity, with loss of specific homeodomain transcription factors causing homeotic-like shifts in brain development. Finally, we speculate about the role of mammalian PcG and trxG factors in the long-term maintenance of NPC positional identity and certain neurodevelopmental disorders.

Bmi1 and BRG1 drive myocardial repair by regulating cardiac stem cell function in acute rheumatic heart disease

Rheumatic heart disease (RHD) occurs due to the accumulation of complications associated with rheumatic fever, and it results in high morbidity and mortality. The majority of cases of RHD are diagnosed in the chronic stages, when treatment options are limited. A small reservoir of cardiac stem cells is responsible for maintaining cardiac homeostasis and repairing tissue damage. Understanding the role of cardiac stem cells and the various proteins responsible for their functions in different pathological stages of RHD is an important area of investigation. Polycomb complex protein BMI-1 (Bmi1) and transcription activator BRG1 (BRG1) are associated with the maintenance of stemness in various types of stem cells. The present study investigated the role served by Bmi1 and BRG1 in cardiac stem cells during various pathological stages of RHD through immunohistochemistry and western blotting. A rat model of RHD was established via immunization with the Group A Streptococcus M5 protein. The rat was demonstrated to develop acute RHD 2 months after the final immunization, characterized by cardiac inflammation and tissue damage. Chronic RHD was identified 4 months after the final immunization, revealed by cardiac tissue compression and shrinkage. Expression of the cardiac stem cell marker mast/stem cell growth factor receptor kit was identified to be elevated during acute RHD, but downregulated in the chronic stages of RHD. A similar pattern of expression was revealed for Bmi1 and BRG1, indicating that they serve a role in regulating cardiac stem cell proliferation during acute RHD. These results suggest that cardiac stem cells serve a supportive role in the acute, but not chronic, stages of RHD via expression of Bmi1 and BRG1.

Ubiquitylation at the crossroads of development and disease

Human development requires intricate cell specification and communication pathways that allow an embryo to generate and appropriately connect more than 200 different cell types. Key to the successful completion of this differentiation programme is the quantitative and reversible regulation of core signalling networks, and post-translational modification with ubiquitin provides embryos with an essential tool to accomplish this task. Instigated by E3 ligases and reversed by deubiquitylases, ubiquitylation controls many processes that are fundamental for development, such as cell division, fate specification and migration. As aberrant function or regulation of ubiquitylation enzymes is at the roots of developmental disorders, cancer, and neurodegeneration, modulating the activity of ubiquitylation enzymes is likely to provide strategies for therapeutic intervention.

Charting the dynamic epigenome during B-cell development

The epigenetic landscape undergoes a widespread modulation during embryonic development and cell differentiation. Within the hematopoietic system, B cells are perhaps the cell lineage with a more dynamic DNA methylome during their maturation process, which involves approximately one third of all the CpG sites of the genome. Although each B-cell maturation step displays its own DNA methylation fingerprint, the DNA methylome is more extensively modified in particular maturation transitions. These changes are gradually accumulated in specific chromatin environments as cell differentiation progresses and reflect different features and functional states of B cells. Promoters and enhancers of B-cell transcription factors acquire activation-related epigenetic marks and are sequentially expressed in particular maturation windows. These transcription factors further reconfigure the epigenetic marks and activity state of their target sites to regulate the expression of genes related to B-cell functions. Together with this observation, extensive DNA methylation changes in areas outside gene regulatory elements such as hypomethylation of heterochromatic regions and hypermethylation of CpG-rich regions, also take place in mature B cells, which intriguingly have been described as hallmarks of cancer. This process starts in germinal center B cells, a highly proliferative cell type, and becomes particularly apparent in long-lived cells such as memory and plasma cells. Overall, the characterization of the DNA methylome during B-cell differentiation not only provides insights into the complex epigenetic network of regulatory elements that mediate the maturation process but also suggests that late B cells also passively accumulate epigenetic changes related to cell proliferation and longevity.

Cancer stem cells and epithelial-mesenchymal transition in urothelial carcinoma: Possible pathways and potential therapeutic approaches

There is growing evidence of the presence of cancer stem cells in urothelial carcinoma. Cancer stem cells have the ability to self-renew and to differentiate into all cell types of the original heterogeneous tumor. A panel of diverse cancer stem cell markers might be suitable for simulation studies of urothelial cancer stem cells and for the development of optimized treatment protocols. The present review focuses on the advances in recognizing the markers of urothelial cancer stem cells and possible therapeutic targets. The commonly reported markers and pathways that were evaluated include CD44, CD133, ALDH1, SOX2 & SOX4, BMI1, EZH1, PD-L1, MAGE-A3, COX2/PGE2/STAT3, AR, and autophagy. Studies on the epithelial-mesenchymal transition-related pathways (Shh, Wnt/β-catenin, Notch, PI3K/Akt, TGF-β, miRNA) are also reviewed. Most of these markers were recognized through the expression patterns of cancer stem cell-rich side populations. Their regulative role in the development and differentiation of urothelial cancer stem cells was confirmed in vitro by functional analyses (e.g. cell migration, colony formation, sphere formation), and in vivo in xenograft experiments. Although a small number of these pathways are targeted by currently available drugs or drugs that are the currently being tested in clinical trials, a clear treatment approach has not been developed for most pathways. A greater understanding of the mechanisms that control the proliferation and differentiation of cancer stem cells is expected to lead to improvements in targeted therapy.

Activation of Wnt/β-catenin signaling by lithium chloride attenuates d-galactose-induced neurodegeneration in the auditory cortex of a rat model of aging

Degeneration of the central auditory system, which is characterized by reduced understanding of speech and source localization of sounds, is an important cause of age‐related hearing loss (presbycusis). Accumulating evidence has demonstrated that Wnt/β‐catenin signaling plays an essential role in the development of the auditory system but its potential role in presbycusis remains unclear. In this study, we used a rat model of aging, created by chronic systemic exposure to d‐galactose (d‐gal), and explored changes in Wnt/β‐catenin signaling in the auditory cortex. A decrease in Wnt/β‐catenin signaling in the auditory cortex was found in both naturally aging and d‐gal‐mimetic aging rats, as indicated by increased GSK3β activity and decreased β‐catenin activity. Moreover, lithium chloride (Licl), an activator of Wnt signaling pathway, was administered long term to 15‐month‐old d‐gal‐treated rats. Activation of Wnt/β‐catenin signaling by Licl attenuated d‐gal‐induced auditory cortex apoptosis and neurodegeneration. Bmi1, a transcription factor implicated in antiaging and resistance to apoptosis, can be modulated by β‐catenin activity. Here, we showed that the expression of Bmi1 was reduced and the expression of its downstream genes, p16^INK4a, p19^Arf, and p53 were increased in the auditory cortex both of naturally aging and d‐gal‐mimetic aging rats. In addition, Licl significantly increased Bmi1 expression and reduced p16^INK4a, p19^Arf, and p53 expression. Our results indicated that decreased Wnt/β‐catenin signaling might participate in the pathogenesis of central presbycusis through modulating the expression of Bmi1. Wnt/β‐catenin signaling might be used as a potential therapeutic target against presbycusis.

B-cell specific Moloney leukemia virus insert site 1 and peptidyl arginine deiminase IV positively regulate carcinogenesis and progression of esophageal squamous cell carcinoma

High expression of B-cell specific Moloney leukemia virus insert site 1 (Bmi-1) and peptidyl arginine deiminase IV (PADI4) is associated with esophageal carcinoma. However, few studies have investigated the association between the Bmi-1 and PADI4 genes. The aim of the present study was to evaluate the expression of Bmi-1 and PADI4 and identify the association between the Bmi-1 and PADI4 genes in esophageal squamous cell carcinoma (ESCC) tissues. Bmi-1 and PADI4 gene expression levels were measured using immunohistochemistry, western blotting and reverse transcription-quantitative polymerase chain reaction in ESCC tissues from 86 patients who had not received pre-operative chemoradiation. The results revealed that the Bmi-1 and PADI4 genes had increased expression in carcinoma tissues compared with pericarcinous tissue (P<0.05). Bmi-1 gene expression was revealed to be associated with differentiation degree, clinical stage and lymph node metastasis (P<0.05), but had no association with gender, age or depth of invasion (P>0.05). The expression of PADI4 was associated with clinical stage, depth of invasion and lymph node metastasis (P<0.05), but was not associated with gender, age or differentiation degree (P>0.05). In addition, there was a positive association between Bmi-1 and PADI4 gene expression in ESCC (P<0.05). These results indicated that Bmi-1 and PADI4 positively regulate carcinogenesis and progression of ESCC.

Mutation of a nucleosome compaction region disrupts Polycomb-mediated axial patterning

Nucleosomes play important structural and regulatory roles by tightly wrapping the DNA that constitutes the metazoan genome. The Polycomb group (PcG) proteins modulate nucleosomes to maintain repression of key developmental genes, including Hox genes whose temporal and spatial expression is tightly regulated to guide patterning of the anterior-posterior body axis. CBX2, a component of the mammalian Polycomb repressive complex 1 (PRC1), contains a compaction region that has the biochemically defined activity of bridging adjacent nucleosomes. Here, we demonstrate that a functional compaction region is necessary for proper body patterning, because mutating this region leads to homeotic transformations similar to those observed with PcG loss-of-function mutations. We propose that CBX2-driven nucleosome compaction is a key mechanism by which PcG proteins maintain gene silencing during mouse development.

The novel BMI-1 inhibitor PTC596 downregulates MCL-1 and induces p53-independent mitochondrial apoptosis in acute myeloid leukemia progenitor cells

Disease recurrence is the major problem in the treatment of acute myeloid leukemia (AML). Relapse is driven by leukemia stem cells, a chemoresistant subpopulation capable of re-establishing disease. Patients with p53 mutant AML are at an extremely high risk of relapse. B-cell-specific Moloney murine leukemia virus integration site 1 (BMI-1) is required for the self-renewal and maintenance of AML stem cells. Here we studied the effects of a novel small molecule inhibitor of BMI-1, PTC596, in AML cells. Treatment with PTC596 reduced MCL-1 expression and triggered several molecular events consistent with induction of mitochondrial apoptosis: loss of mitochondrial membrane potential, BAX conformational change, caspase-3 cleavage and phosphatidylserine externalization. PTC596 induced apoptosis in a p53-independent manner. PTC596 induced apoptosis along with the reduction of MCL-1 and phosphorylated AKT in patient-derived CD34⁺CD38^low/− stem/progenitor cells. Mouse xenograft models demonstrated in vivo anti-leukemia activity of PTC596, which inhibited leukemia cell growth in vivo while sparing normal hematopoietic cells. Our results indicate that PTC596 deserves further evaluation in clinical trials for refractory or relapsed AML patients, especially for those with unfavorable complex karyotype or therapy-related AML that are frequently associated with p53 mutations.

Polycomb complexes PRC1 and their function in hematopoiesis

Hematopoiesis, the process by which blood cells are continuously produced, is one of the best studied differentiation pathways. Hematological diseases are associated with reiterated mutations in genes encoding important gene expression regulators, including chromatin regulators. Among them, the Polycomb group (PcG) of proteins is an essential system of gene silencing involved in the maintenance of cell identities during differentiation. PcG proteins assemble into two major types of Polycomb repressive complexes (PRCs) endowed with distinct histone-tail-modifying activities. PRC1 complexes are histone H2A E3 ubiquitin ligases and PRC2 trimethylates histone H3. Established conceptions about their activities, mostly derived from work in embryonic stem cells, are being modified by new findings in differentiated cells. Here, we focus on PRC1 complexes, reviewing recent evidence on their intricate architecture, the diverse mechanisms of their recruitment to targets, and the different ways in which they engage in transcriptional control. We also discuss hematopoietic PRC1 gain- and loss-of-function mouse strains, including those that model leukemic and lymphoma diseases, in the belief that these genetic analyses provide the ultimate test for molecular mechanisms driving normal hematopoiesis and hematological malignancies.

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.

Mitochondrial BMI1 maintains bioenergetic homeostasis in cells

The polycomb complex proto-oncogene BMI1 [B lymphoma Mo-MLV insertion region 1 homolog (mouse)] is essential for self-renewal of normal and cancer stem cells. BMI1-null mice show severe defects in growth, development, and survival. Although BMI1 is known to exert its effect in the nucleus via repression of 2 potent cell-cycle regulators that are encoded by the Ink4a/Arf locus, deletion of this locus only partially rescues BMI1-null phenotypes, which is indicative of alternate mechanisms of action of BMI1. Here, we show that an extranuclear pool of BMI1 localizes to inner mitochondrial membrane and directly regulates mitochondrial RNA (mtRNA) homeostasis and bioenergetics. These mitochondrial functions of BMI1 are independent of its previously described nuclear functions because a nuclear localization-defective mutant BMI1 rescued several bioenergetic defects that we observed in BMI1-depleted cells, for example, mitochondrial respiration, cytochrome c oxidase activity, and ATP production. Mechanistically, BMI1 coprecipitated with polynucleotide phosphorylase, a ribonuclease that is responsible for decay of mtRNA transcripts. Loss of BMI1 enhanced ribonuclease activity of polynucleotide phosphorylase and reduced mtRNA stability. These findings not only establish a novel extranuclear role of BMI1 in the regulation of mitochondrial bioenergetics, but also provide new mechanistic insights into the role of this proto-oncogene in stem cell differentiation, neuronal aging, and cancer.-Banerjee Mustafi, S., Aznar, N., Dwivedi, S. K. D., Chakraborty, P. K., Basak, R., Mukherjee, P., Ghosh, P., Bhattacharya, R. Mitochondrial BMI1 maintains bioenergetic homeostasis in cells.

MOZ (KAT6A) is essential for the maintenance of classically defined adult hematopoietic stem cells

MOZ deletion in adult mice leads to a rapid loss of cells with HSC cell surface immuno-phenotype and transplantation ability. Absence of classically defined HSCs for an extended period does not substantially affect steady-state hematopoiesis.

Bmi1 Loss in the Organ of Corti Results in p16ink4a Upregulation and Reduced Cell Proliferation of Otic Progenitors In Vitro

The mature mammalian organ of Corti does not regenerate spontaneously after injury, mainly due to the absence of cell proliferation and the depletion of otic progenitors with age. The polycomb gene B lymphoma Mo-MLV insertion region 1 homolog (Bmi1) promotes proliferation and cell cycle progression in several stem cell populations. The cell cycle inhibitor p16^ink4a has been previously identified as a downstream target of Bmi1. In this study, we show that Bmi1 is expressed in the developing inner ear. In the organ of Corti, Bmi1 expression is temporally regulated during embryonic and postnatal development. In contrast, p16^ink4a expression is not detectable during the same period. Bmi1-deficient mice were used to investigate the role of Bmi1 in cochlear development and otosphere generation. In the absence of Bmi1, the postnatal organ of Corti displayed normal morphology at least until the end of the first postnatal week, suggesting that Bmi1 is not required for the embryonic or early postnatal development of the organ of Corti. However, Bmi1 loss resulted in the reduced sphere-forming capacity of the organ of Corti, accompanied by the decreased cell proliferation of otic progenitors in otosphere cultures. This reduced proliferative capacity was associated with the upregulation of p16^ink4ain vitro. Viral vector-mediated overexpression of p16^ink4a in wildtype otosphere cultures significantly reduced the number of generated otospheres in vitro. The findings strongly suggest a role for Bmi1 as a promoter of cell proliferation in otic progenitor cells, potentially through the repression of p16^ink4a.

Expression and survival significance of B-cell-specific Moloney murine leukemia virus integration site 1 and matrix metalloproteinase-9 in non-small-cell lung cancer

One of the main challenges in lung cancer research is identifying patients at high risk of progression and metastasis following surgical resection. In the present study, the prognostic significance of B-cell-specific Moloney murine leukemia virus integration site 1 (BMI1) and matrix metalloproteinase-9 (MMP9) in non-small-cell lung cancer (NSCLC) was evaluated. BMI1 and MMP9 expression in tumors from 132 surgical NSCLC patients [squamous cell carcinoma (SCC), n=79; and adenocarcinoma (AD), n=53] was evaluated by immunohistochemistry. The clinical significance was determined using multivariate Cox regression analysis, Kaplan-Meier curves and the log-rank test. High BMI1 expression was more frequent in SCC compared with that in AD (P=0.015). Comparisons between the expression of BMI1 and that of other known biological markers revealed that the expression of BMI1 was correlated with that of MMP9 (χ²=4.241, P=0.039) in SCC. Although an association was not identified between high BMI1 expression and overall survival (OS) in NSCLC or AD, high BMI1 expression was an unfavorable predictor of survival in SCC according to the survival curves (P=0.038). In addition, combined high BMI1 and MMP9 expression levels were significantly correlated with SCC nodal/distant metastasis (χ²=6.392, P=0.014). Multivariate Cox proportional model analysis demonstrated that this combined marker was an independent prognostic indicator of OS in SCC (P=0.025; hazard ratio = 12.963; 95% confidence interval: 1.142–7.637). Therefore, this study demonstrated that combined BMI1 and MMP9 expression may be used as a marker for the progression and metastasis of SCC. These results may aid in the elucidation of the potential mechanism underlying the involvement of BMI1 and MMP9 in tissue-specific SCC progression.

Interdependence of PRC1 and PRC2 for recruitment to Polycomb Response Elements

Polycomb Group (PcG) proteins are epigenetic repressors essential for control of development and cell differentiation. They form multiple complexes of which PRC1 and PRC2 are evolutionary conserved and obligatory for repression. The targeting of PRC1 and PRC2 is poorly understood and was proposed to be hierarchical and involve tri-methylation of histone H3 (H3K27me3) and/or monoubiquitylation of histone H2A (H2AK118ub). Here, we present a strict test of this hypothesis using the Drosophila model. We discover that neither H3K27me3 nor H2AK118ub is required for targeting PRC complexes to Polycomb Response Elements (PREs). We find that PRC1 can bind PREs in the absence of PRC2 but at many PREs PRC2 requires PRC1 to be targeted. We show that one role of H3K27me3 is to allow PcG complexes anchored at PREs to interact with surrounding chromatin. In contrast, the bulk of H2AK118ub is unrelated to PcG repression. These findings radically change our view of how PcG repression is targeted and suggest that PRC1 and PRC2 can communicate independently of histone modifications.

The Polycomb Group Protein Pcgf1 Is Dispensable in Zebrafish but Involved in Early Growth and Aging

Polycomb Repressive Complex (PRC) 1 regulates the control of gene expression programs via chromatin structure reorganization. Through mutual exclusion, different PCGF members generate a variety of PRC1 complexes with potentially distinct cellular functions. In this context, the molecular function of each of the PCGF family members remains elusive. The study of PCGF family member expression in zebrafish development and during caudal fin regeneration reveals that the zebrafish pcgf genes are subjected to different regulations and that all PRC1 complexes in terms of Pcgf subunit composition are not always present in the same tissues. To unveil the function of Pcgf1 in zebrafish, a mutant line was generated using the TALEN technology. Mutant pcgf1^-/- fish are viable and fertile, but the growth rate at early developmental stages is reduced in absence of pcgf1 gene function and a significant number of pcgf1^-/- fish show signs of premature aging. This first vertebrate model lacking Pcgf1 function shows that this Polycomb Group protein is involved in cell proliferation during early embryogenesis and establishes a link between epigenetics and aging.

Epigenetic control of adult stem cell function

Mammalian embryonic development is a tightly regulated process that, from a single zygote, produces a large number of cell types with hugely divergent functions. Distinct cellular differentiation programmes are facilitated by tight transcriptional and epigenetic regulation. However, the contribution of epigenetic regulation to tissue homeostasis after the completion of development is less well understood. In this Review, we explore the effects of epigenetic dysregulation on adult stem cell function. We conclude that, depending on the tissue type and the epigenetic regulator affected, the consequences range from negligible to stem cell malfunction and disruption of tissue homeostasis, which may predispose to diseases such as cancer.

Overexpression of Bmi1 in Lymphocytes Stimulates Skeletogenesis by Improving the Osteogenic Microenvironment

To investigate whether overexpression of Bmi1 in lymphocytes can stimulate skeletogenesis by improving the osteogenic microenvironment, we examined the skeletal phenotype of EμBmi1 transgenic mice with overexpression of Bmi1 in lymphocytes. The size of the skeleton, trabecular bone volume and osteoblast number, indices of proliferation and differentiation of bone marrow mesenchymal stem cells (BM-MSCs) were increased significantly, ROS levels were reduced and antioxidative capacity was enhanced in EμBmi1 mice compared to WT mice. In PTHrP1-84 knockin (Pthrp(KI/KI)) mice, the expression levels of Bmi1 are reduced and potentially can mediate the premature osteoporosis observed. We therefore generated a Pthrp(KI/KI) mice overexpressing Bmi1 in lymphocytes and compared them with Pthrp(KI/KI) and WT littermates. Overexpression of Bmi1 in Pthrp(KI/KI) mice resulted in a longer lifespan, increased body weight and improvement in skeletal growth and parameters of osteoblastic bone formation with reduced ROS levels and DNA damage response parameters. Our results demonstrate that overexpression of Bmi1 in lymphocytes can stimulate osteogenesis in vivo and partially rescue defects in skeletal growth and osteogenesis in Pthrp(KI/KI) mice. These studies therefore indicate that overexpression of Bmi1 in lymphocytes can stimulate skeletogenesis by inhibiting oxidative stress and improving the osteogenic microenvironment.

Glucagon-Like Peptide-2 Requires a Full Complement of Bmi-1 for Its Proliferative Effects in the Murine Small Intestine

The intestinal hormone, glucagon-like peptide-2 (GLP-2), stimulates growth, survival, and function of the intestinal epithelium through increased crypt cell proliferation, and a long-acting analog has recently been approved to enhance intestinal capacity in patients with short bowel syndrome. The goal of the present study was to determine whether GLP-2-induced crypt cell proliferation requires a full complement of B-cell lymphoma Moloney murine leukemia virus insertion region-1 homolog (Bmi-1), using the Bmi-1(eGFP/+) mouse model in comparison with age- and sex-matched Bmi-1(+/+) littermates. Bmi-1 is a member of the polycomb-repressive complex family that promotes stem cell proliferation and self-renewal and is expressed by both stem cells and transit-amplifying (TA) cells in the crypt. The acute (6 h) and chronic (11 d) proliferative responses to long-acting human (Gly(2))GLP-2 in the crypt TA zone, but not in the active or reserve stem cell zones, were both impaired by Bmi-1 haploinsufficiency. Similarly, GLP-2-induced crypt regeneration after 10-Gy irradiation was reduced in the Bmi-1(eGFP/+) animals. Despite these findings, chronic GLP-2 treatment enhanced overall intestinal growth in the Bmi-1(eGFP/+) mice, as demonstrated by increases in small intestinal weight per body weight and in the length of the crypt-villus axis, in association with decreased apoptosis and an adaptive increase in crypt epithelial cell migration rate. The results of these studies therefore demonstrate that a full complement of Bmi-1 is required for the intestinal proliferative effects of GLP-2 in both the physiological and pathological setting, and mediates, at least in part, the proliferation kinetics of cells in the TA zone.

Epigenetic balance of gene expression by Polycomb and COMPASS families

Epigenetic regulation of gene expression in metazoans is central for establishing cellular diversity, and its deregulation can result in pathological conditions. Although transcription factors are essential for implementing gene expression programs, they do not function in isolation and require the recruitment of various chromatin-modifying and -remodeling machineries. A classic example of developmental chromatin regulation is the balanced activities of the Polycomb group (PcG) proteins within the PRC1 and PRC2 complexes, and the Trithorax group (TrxG) proteins within the COMPASS family, which are highly mutated in a large number of human diseases. In this review, we will discuss the latest findings regarding the properties of the PcG and COMPASS families and the insight they provide into the epigenetic control of transcription under physiological and pathological settings.

Functional analysis of AEBP2, a PRC2 Polycomb protein, reveals a Trithorax phenotype in embryonic development and in ESCs

The Polycomb repressive complexes PRC1 and PRC2 are key mediators of heritable gene silencing in multicellular organisms. Here, we characterise AEBP2, a known PRC2 co-factor which, in vitro, has been shown to stimulate PRC2 activity. We show that AEBP2 localises specifically to PRC2 target loci, including the inactive X chromosome. Proteomic analysis confirms that AEBP2 associates exclusively with PRC2 complexes. However, analysis of embryos homozygous for a targeted mutation of Aebp2 unexpectedly revealed a Trithorax phenotype, normally linked to antagonism of Polycomb function. Consistent with this, we observe elevated levels of PRC2-mediated histone H3K27 methylation at target loci in Aebp2 mutant embryonic stem cells (ESCs). We further demonstrate that mutant ESCs assemble atypical hybrid PRC2 subcomplexes, potentially accounting for enhancement of Polycomb activity, and suggesting that AEBP2 normally plays a role in defining the mutually exclusive composition of PRC2 subcomplexes.

Highlighted article: Targeted mutation of the Polycomb protein AEBP2 in mouse provides evidence for a role for this factor in defining the composition and activity of PRC2 complexes.

An oncogenic Ezh2 mutation induces tumors through global redistribution of histone 3 lysine 27 trimethylation

B cell lymphoma and melanoma harbor recurrent mutations in the gene encoding the EZH2 histone methyltransferase (EZH2), but the carcinogenic role of these mutations is unclear. Here we describe a mouse model in which the most common somatic Ezh2 gain-of-function mutation (EZH2(Y646F) in human; Ezh2(Y641F) in mouse) is conditionally expressed. Expression of Ezh2(Y641F) in mouse B cells or melanocytes caused high-penetrance lymphoma or melanoma, respectively. Overexpression of the anti-apoptotic protein Bcl2, but not the oncoprotein Myc, or loss of the tumor suppressor protein p53 (encoded by Trp53 in mice) further accelerated lymphoma progression. Expression of the mutant Braf but not the mutant Nras oncoprotein further accelerated melanoma progression. Although expression of Ezh2(Y641F) globally increased the abundance of trimethylated Lys27 of histone H3 (H3K27me3), it also caused a widespread redistribution of this repressive mark, including a loss of H3K27me3 that was associated with increased transcription at many loci. These results suggest that Ezh2(Y641F) induces lymphoma and melanoma through a vast reorganization of chromatin structure, inducing both repression and activation of polycomb-regulated loci.