Bmi1 and cell of origin determinants of brain tumor phenotype

Combined aberrant expression of Bmi1 and EZH2 is predictive of poor prognosis in glioma patients

BACKGROUND AND OBJECTIVES

Bmi1 and EZH2 are involved in tumorigenesis of gliomas. However, clinicopathologic significance of their expression in gliomas is unknown; especially, the prognostic value of combined expression of Bmi1 and EZH2 has not been explored.

METHODS

Bmi1 and EZH2 expression in human gliomas and nonneoplastic brain tissues was measured by immunohistochemistry.

RESULTS

Both Bmi1 and EZH2 expressions in glioma tissues were significantly higher than those in corresponding nonneoplastic brain tissues (both P<0.001). Additionally, the upregulations of Bmi1 and EZH2 proteins were both significantly associated with advanced WHO grades (both P<0.001) and low KPS (P=0.008 and 0.01, respectively). Moreover, the overall survival of patients with high Bmi1 protein expression (P=0.006) or high EZH2 protein expression (P=0.01) was obviously lower than those with low expressions. More interestingly, glioma patients with combined overexpression of Bmi1 and EZH2 proteins had the shortest overall survival (P<0.001). Furthermore, multivariate analysis showed that Bmi1n expression (P=0.02), EZH2 expression (P=0.03), and combined expression of Bmi1 and EZH2 (P=0.008), were all independent prognostic factors for overall survival in glioma patients.

CONCLUSIONS

Our data suggest for the first time that the combination of Bmi1 and EZH2 overexpression may be a highly sensitive marker for the prognosis in glioma patients.

Tumor suppressive pathways in the control of neurogenesis

The generation of specialized neural cells in the developing and postnatal central nervous system is a highly regulated process, whereby neural stem cells divide to generate committed neuronal progenitors, which then withdraw from the cell cycle and start to differentiate. Cell cycle checkpoints play a major role in regulating the balance between neural stem cell expansion and differentiation. Loss of tumor suppressors involved in checkpoint control can lead to dramatic alterations of neurogenesis, thus contributing to neoplastic transformation. Here we summarize and critically discuss the existing literature on the role of tumor suppressive pathways and their regulatory networks in the control of neurogenesis and transformation.

A microRNA Link to Glioblastoma Heterogeneity

Glioblastomas (GBM) are one of the most malignant adult primary brain tumors. Through decades of research using various model systems and GBM patients, we have gained considerable insights into the mechanisms regulating GBM pathogenesis, but have mostly failed to significantly improve clinical outcome. For the most part GBM heterogeneity is responsible for this lack of progress. Here, we have discussed sources of cellular and microenvironmental heterogeneity in GBMs and their potential regulation through microRNA mediated mechanisms. We have focused on the role of individual microRNAs (miRNA) through their specific targets and miRNA mediated RNA-RNA interaction networks with the potential to influence various aspects of GBM heterogeneity including tumor neo-vascularization. We believe a better understanding of such mechanisms for regulation of GBM pathogenesis will be instrumental for future therapeutic options.

Genetic modeling of gliomas in mice: new tools to tackle old problems

The recently published comprehensive profiles of genomic alterations in glioma have led to a refinement in our understanding of the molecular events that underlie this cancer. Using state-of-the-art genomic tools, several laboratories have created and characterized accurate genetically engineered mouse models of glioma based on specific genetic alterations observed in human tumors. These in vivo brain tumor models faithfully recapitulate the histopathology, etiology, and biology of gliomas and provide an exceptional experimental system to discover novel therapeutic targets and test therapeutic agents. This review focuses on mouse models of glioma with a special emphasis on genetically engineered models developed around key genetic glioma signature mutations in the PDGFR, EGFR, and NF1 genes and pathways. The resulting animal models have provided insight into many fundamental and mechanistic facets of tumor initiation, maintenance and resistance to therapeutic intervention and will continue to do so in the future.

Establishment of prognostic models for astrocytic and oligodendroglial brain tumors with standardized quantification of marker gene expression and clinical variables

Background

Prognosis models established using multiple molecular markers in cancer along with clinical variables should enable prediction of natural disease progression and residual risk faced by patients. In this study, multivariate Cox proportional hazards analyses were done based on overall survival (OS) of 100 glioblastoma multiformes (GBMs, 92 events), 49 anaplastic astrocytomas (AAs, 33 events), 45 gliomas with oligodendroglial features, including anaplastic oligodendroglioma (AO, 13 events) and oligodendraglioma (O, 9 events). The modeling included two clinical variables (patient age and recurrence at the time of sample collection) and the expression variables of 13 genes selected based on their proven biological and/or prognosis functions in gliomas (ABCG2, BMI1, MELK, MSI1, PROM1, CDK4, EGFR, MMP2, VEGFA, PAX6, PTEN, RPS9, and IGFBP2). Gene expression data was a log-transformed ratio of marker and reference (ACTB) mRNA levels quantified using absolute real-time qRT-PCR.

Results

Age is positively associated with overall grade (4 for GBM, 3 for AA, 2_1 for AO_O), but lacks significant prognostic value in each grade. Recurrence is an unfavorable prognostic factor for AA, but lacks significant prognostic values for GBM and AO_O. Univariate models revealed opposing prognostic effects of ABCG2, MELK, BMI1, PROM1, IGFBP2, PAX6, RPS9, and MSI1 expressions for astrocytic (GBM and AA) and oligodendroglial tumors (AO_O). Multivariate models revealed independent prognostic values for the expressions of MSI1 (unfavorable) in GBM, CDK4 (unfavorable) and MMP2 (favorable) in AA, while IGFBP2 and MELK (unfavorable) in AO_O. With all 13 genes and 2 clinical variables, the model R² was 14.2% (P = 0.358) for GBM, 45.2% (P = 0.029) for AA, and 62.2% (P = 0.008) for AO_O.

Conclusion

The study signifies the challenge in establishing a significant prognosis model for GBM. Our success in establishing prognosis models for AA and AO_O was largely based on identification of a set of genes with independent prognostic values and application of standardized gene expression quantification to allow formation of a large cohort in analysis.

Epigenetic choreographers of neurogenesis in the adult mammalian brain

Epigenetic mechanisms regulate cell differentiation during embryonic development and also serve as important interfaces between genes and the environment in adulthood. Neurogenesis in adults, which generates functional neural cell types from adult neural stem cells, is dynamically regulated by both intrinsic state-specific cell differentiation cues and extrinsic neural niche signals. Epigenetic regulation by DNA and histone modifiers, non-coding RNAs and other self-sustained mechanisms can lead to relatively long-lasting biological effects and maintain functional neurogenesis throughout life in discrete regions of the mammalian brain. Here, we review recent evidence that epigenetic mechanisms carry out diverse roles in regulating specific aspects of adult neurogenesis and highlight the implications of such epigenetic regulation for neural plasticity and disorders.

Clinicopathological significance of Bmi-1 and S100A4 expression in esophageal squamous cell carcinoma

AIM: To detect the expression of Bmi-1 and S100A4 proteins in esophageal squamous cell carcinoma (ESCC) and to analyze their clinicopathological significance.

METHODS: The expression of Bmi-1 and S100A4 proteins was detected by immunohistochemistry in 68 ESCC specimens, 45 tumor-adjacent dysplastic tissue specimens and 36 normal esophageal mucosa specimens. Statistical analysis was performed using the chi-square test.

RESULTS: The positive rates of Bmi-1 and S100A4 protein expression differed significantly among ESCC, dysplasia and normal esophageal mucosa (Bmi-1: 57.4%, 48.9%, and 25.0%; S100A4: 48.6%, 26.7%, and 13.9%; all P < 0.01). Overexpression of Bmi-1 and S100A4 protein was related with lymph node metastasis and TNM stage in ESCC (all P < 0.05). S100A4 protein expression was found to be correlated with the depth of tumor invasion (P < 0.05). A positive correlation was also noted between the expression of Bmi-1 and that of S100A4 in ESCC (r = 0.302, P < 0.05).

CONCLUSION: Overexpression of Bmi-1 and S100A4 proteins is associated with ESCC invasion and metastasis. Combined detection of Bmi-1 and S100A4 protein expression is helpful for evaluation of the prognosis of ESCC.

Methylation and silencing of miRNA-124 by EVI1 and self-renewal exhaustion of hematopoietic stem cells in murine myelodysplastic syndrome

By expressing EVI1 in murine bone marrow (BM), we previously described a myelodysplastic syndrome (MDS) model characterized by pancytopenia, dysmegakaryopoiesis, dyserythropoiesis, and BM failure. The mice invariably died 11-14 months after BM transplantation (BMT). Here, we show that a double point mutant EVI1-(1+6Mut), unable to bind Gata1, abrogates the onset of MDS in the mouse and re-establishes normal megakaryopoiesis, erythropoiesis, BM function, and peripheral blood profiles. These normal features were maintained in the reconstituted mice until the study was ended at 21 months after BMT. We also report that EVI1 deregulates several genes that control cell division and cell self-renewal. In striking contrast, these genes are normalized in the presence of the EVI1 mutant. Moreover, EVI1, but not the EVI1 mutant, seemingly deregulates these cellular processes by altering miRNA expression. In particular, the silencing of miRNA-124 by DNA methylation is associated with EVI1 expression, but not that of the EVI1 mutant, and appears to play a key role in the up-regulation of cell division in murine BM cells and in the hematopoietic cell line 32Dcl3. The results presented here demonstrate that EVI1 induces MDS in the mouse through two major pathways, both of which require the interaction of EVI1 with other factors: one, results from EVI1-Gata1 interaction, which deregulates erythropoiesis and leads to fatal anemia, whereas the other occurs by interaction of EVI1 with unidentified factors causing perturbation of the cell cycle and self-renewal, as a consequence of silencing miRNA-124 by EVI1 and, ultimately, ensues in BM failure.

Bmi-1 regulates self-renewal, proliferation and senescence of human fetal neural stem cells in vitro

Knockout and knockdown studies have shown that the polycomb gene Bmi-1 is important for mouse postnatal and prenatal neural stem cells (NSCs) self-renewal and proliferation. Different downstream targets of Bmi-1 gene have been identified in mouse, including Ink4a/Arf locus in adult NSCs and p21 gene in embryonic NSCs. However, little is known regarding the role of Bmi-1 in human NSCs. Here, using lentiviral-delivered shRNA knockdown and over-expression techniques, we examined whether Bmi-1 is required for the self-renewal and proliferation of human fetal NSCs (hfNSCs) in vitro. Our results showed that shRNA-mediated Bmi-1 reduction profoundly impaired hfNSCs self-renewal and proliferation, whereas Bmi-1 over-expression promoted hfNSCs self-renewal capacity. Interestingly, different from mouse embryonic NSCs, Bmi-1 repressed Ink4a/Arf locus instead of p21 gene in human fetal NSCs. Moreover, Bmi-1 knockdown induced obvious senescence phenotype in hfNSCs. Further studies on the Bmi-1 pathways would help to understand the molecular mechanisms underlying hfNSCs self-renewal and human brain development.

From birth till death: neurogenesis, cell cycle, and neurodegeneration

Neurogenesis in the embryo involves many signaling pathways and transcriptional programs and an elaborate orchestration of cell cycle exit in differentiating precursors. However, while the neurons differentiate into a plethora of different subtypes and different identities, they also presume a highly polar structure with a particular morphology of the cytoskeleton, thereby making it almost impossible for any differentiated cell to re-enter the cell cycle. It has been observed that dysregulated or forced cell cycle reentry is closely linked to neurodegeneration and apoptosis in neurons, most likely through changes in the neurocytoskeleton. However, proliferative cells still exist within the nervous system, and adult neural stem cells (NSCs) have been identified in the Central Nervous System (CNS) in the past decade, raising a great stir in the neuroscience community. NSCs present a new therapeutic potential, and much effort has since gone into understanding the molecular mechanisms driving differentiation of specific neuronal lineages, such as dopaminergic neurons, for use in regenerative medicine, either through transplanted NSCs or manipulation of existing ones. Nevertheless, differentiation and proliferation are two sides of the same coin, just like tumorigenesis and degeneration. Tumor formation may be regarded as a de-differentiation of tissues, where cell cycle mechanisms are reactivated in differentiated cell types. It is thus important to understand the molecular mechanisms underlying various brain tumors in this perspective. The recent Cancer Stem Cell (CSC) hypothesis also suggests the presence of Brain Tumor Initiating Cells (BTICs) within a tumor population, although the exact origin of these rare and mostly elusive BTICs are yet to be identified. This review attempts to investigate the correlation of neural stem cells/precursors, mature neurons, BTICs and brain tumors with respect to cell cycle regulation and the impact of cell cycle in neurodegeneration.

The novel tumor-suppressor Mel-18 in prostate cancer: its functional polymorphism, expression and clinical significance

Mel-18 is a member of the polycomb group (PcG) proteins, which are chromatin regulatory factors and play important roles in development and oncogenesis. This study was designed to investigate the clinical and prognostic significance of Mel-18 in patients with prostate cancer. A total of 539 native Japanese subjects consisting of 393 prostate cancer patients and 146 controls were enrolled in this study. Mel-18 genotyping was analyzed using a PCR-RFLP method and an automated sequencer using the GENESCAN software. Immunohistochemistry revealed that Mel-18 expression was diminished in high grade and high stage prostate cancers. Moreover, patients with positive Mel-18 expression had significantly longer PSA recurrence-free survival than patients negative for Mel-18 expression (p=0.038). A Mel-18 1805A/G SNP was located in the 3' untranslated region and was predicted to alter the secondary structure of the mRNA. Mel-18 mRNA expression of the 1805A allele was clearly higher than expression of the 1805G allele by allele specific quantitative RT-PCR. In multivariate analysis, a homozygous G allele genotype and negative Mel-18 expression were independent risk factors predicting high PSA recurrence after radical prostatectomy, with HRs of 2.757 (p=0.022) and 2.271 (p=0.045), respectively. Moreover, the G allele was also an independent predictor of poor cancer-specific survival with an HR of 4.658 (p=0.019) for patients with stage D2 prostate cancer. This is the first study to provide important evidence demonstrating that Mel-18 is a tumor suppressor and possible therapeutic target, as well as a diagnostic marker for poor prognosis in prostate cancer patients.

Molecular epigenetics and genetics in neuro-oncology

Gliomas arise through genetic and epigenetic alterations of normal brain cells, although the exact cell of origin for each glioma subtype is unknown. The alteration-induced changes in gene expression and protein function allow uncontrolled cell division, tumor expansion, and infiltration into surrounding normal brain parenchyma. The genetic and epigenetic alterations are tumor subtype and tumor-grade specific. Particular alterations predict tumor aggressiveness, tumor response to therapy, and patient survival. Genetic alterations include deletion, gain, amplification, mutation, and translocation, which result in oncogene activation and tumor suppressor gene inactivation, or in some instances the alterations may simply be a consequence of tumorigenesis. Epigenetic alterations in brain tumors include CpG island hypermethylation associated with tumor suppressor gene silencing, gene-specific hypomethylation associated with aberrant gene activation, and genome-wide hypomethylation potentially leading to loss of imprinting, chromosomal instability, and cellular hyperproliferation. Other epigenetic alterations, such as changes in the position of histone variants and changes in histone modifications are also likely to be important in the molecular pathology of brain tumors. Given that histone deacetylases are targets for drugs that are already in clinical trial, surprisingly little is known about histone acetylation in primary brain tumors. Although a majority of epigenetic alterations are independent of genetic alterations, there is interaction on specific genes, signaling pathways and within chromosomal domains. Next-generation sequencing technology is now the method of choice for genomic and epigenome profiling, allowing more comprehensive understanding of genetic and epigenetic contributions to tumorigenesis in the brain.

Association between Bmi1 and clinicopathological status of esophageal squamous cell carcinoma

AIM: To investigate the clinicopathological roles of Bmi1 in esophageal squamous cell carcinoma (ESCC).

METHODS: Quantitative real-time polymerase chain reaction and immunohistochemical staining for Bmi1 were performed in cancerous and adjacent non-cancerous paraffin-embedded esophageal specimens.

RESULTS: The Bmi1 expression level was unaffected by gender and age. The level of Bmi1 mRNA in ESCC was significantly higher than that in the adjacent non-cancerous tissues (2.181 ± 2.158 vs 0.931 ± 0.894, P = 0.0152), and its over-expression was aggressively associated with lymph node metastasis (3.580 ± 2.487 vs 1.703 ± 0.758, P = 0.0003), poorer cell differentiation (P = 0.0000) and advanced pathological stage (3.827 ± 2.673 vs 1.590 ± 0.735, P = 0.0001). The patients were divided into high-expression and low-expression groups based on the median expression level of Bmi1 mRNA, and a shorter overall survival time in the former group was observed. Immunohistochemistry for Bmi1 oncoprotein showed diffusely positive, focally positive and negative expression in 44, 16 and 10 of 70 ESCC cases, respectively, compared with three, two and five of 10 adjacent non-cancerous cases (P = 0.027). The positive rate of the oncoprotein in samples of histological grade III was higher than that of grade II (P = 0.031), but its expression had no relation to the lymph node metastasis and pathological staging. In 70 ESCC samples, Bmi1 showed high intense expression in the cytoplasm and less or even no expression in the nucleus.

CONCLUSION: Bmi1 was over-expressed in ESCC. Increased Bmi1 mRNA expression was significantly associated with ESCC progression, and the oncoprotein was largely distributed in the cytoplasm of tumor cells.

Turning cancer stem cells inside out: an exploration of glioma stem cell signaling pathways

Tumors are complex collections of heterogeneous cells with recruited vasculature, inflammatory cells, and stromal elements. Neoplastic cells frequently display a hierarchy in differentiation status. Recent studies suggest that brain tumors have a limited population of neoplastic cells called cancer stem cells with the capacity for sustained self-renewal and tumor propagation. Brain tumor stem cells contribute to therapeutic resistance and tumor angiogenesis. In this minireview, we summarize recent data regarding critical signaling pathways involved in brain tumor stem cell biology and discuss how targeting these molecules may contribute to the development of novel anti-glioma therapies.

Identification of a polymorphism in the RING finger of human Bmi-1 that causes its degradation by the ubiquitin-proteasome system

Bmi-1 is a polycomb protein that plays an important role in tumor cell development and maintaining stem cell populations of many cell lineages. Here we identify a polymorphism in human Bmi-1 that changes a cysteine within its RING domain to tyrosine. This C18Y polymorphism is associated with a significant decrease in Bmi-1 level and its elevated ubiquitination, suggesting that it is being destroyed by the ubiquitin-proteasome system. Consistent with this, treating cells with the proteasome inhibitor MG-132 significantly increases C18Y Bmi-1 levels. This is the first example of a polymorphism in Bmi-1 that reduces levels of this important protein.

Progress on potential strategies to target brain tumor stem cells

The identification of brain tumor stem cells (BTSCs) leads to promising progress on brain tumor treatment. For some brain tumors, BTSCs are the driving force of tumor growth and the culprits that make tumor revive and resistant to radiotherapy and chemotherapy. Therefore, it is specifically significant to eliminate BTSCs for treatment of brain tumors. There are considerable similarities between BTSCs and normal neural stem cells (NSCs), and diverse aspects of BTSCs have been studied to find potential targets that can be manipulated to specifically eradicate BTSCs without damaging normal NSCs, including their surface makers, surrounding niche, and aberrant signaling pathways. Many strategies have been designed to kill BTSCs, and some of them have reached, or are approaching, effective therapeutic results. Here, we will focus on advantages in the issue of BTSCs and emphasize on potential therapeutic strategies targeting BTSCs.