RB1 in cancer: different mechanisms of RB1 inactivation and alterations of pRb pathway in tumorigenesis

miR-1297 promotes cell proliferation by inhibiting RB1 in liver cancer

Liver cancer is the one of the most common causes of cancer-associated mortality worldwide. MicroRNAs (miRNAs or miRs) are important in various types of cancer, including liver cancer. In the present study, with miRNA expression profile data obtained from the Gene Expression Omnibus database, three independent methods were used to investigate the miRNAs that are involved in liver carcinogenesis, including Fisher's exact test, t-test and Wilcoxon test. Five differentially expressed miRNAs were identified. Among them, miR-1297 drew specific attention. Target gene analysis and Ingenuity Pathway Analysis revealed its potential impact on cell death and cell cycle. Cell Counting Kit-8 proliferation assay indicated that the HepG2 cell proliferation was promoted by miR-1297, while miR-1297 inhibitor could significantly inhibit the proliferation of HepG2 cells. Luciferase assays confirmed that miR-1297 directly bound to the 3'-untranslated region of retinoblastoma (RB)1, and western blotting demonstrated that miR-1297 suppressed the expression of RB1 at the protein level. RB1 is involved in the regulation of the human cell cycle pathway. It is possible that miR-1297 contributes to the carcinogenesis of liver cancer via downregulation of the tumor-suppressor gene RB1. Our results suggest that miR-1297 may serve as a potential therapeutic target of liver cancer.

Chromothripsis Is a Recurrent Genomic Abnormality in High-Risk Myelodysplastic Syndromes

To explore novel genetic abnormalities occurring in myelodysplastic syndromes (MDS) through an integrative study combining array-based comparative genomic hybridization (aCGH) and next-generation sequencing (NGS) in a series of MDS and MDS/myeloproliferative neoplasms (MPN) patients. 301 patients diagnosed with MDS (n = 240) or MDS/MPN (n = 61) were studied at the time of diagnosis. A genome-wide analysis of DNA copy number abnormalities was performed. In addition, a mutational analysis of DNMT3A, TET2, RUNX1, TP53 and BCOR genes was performed by NGS in selected cases. 285 abnormalities were identified in 71 patients (23.6%). Three high-risk MDS cases (1.2%) displayed chromothripsis involving exclusively chromosome 13 and affecting some cancer genes: FLT3, BRCA2 and RB1. All three cases carried TP53 mutations as revealed by NGS. Moreover, in the whole series, the integrative analysis of aCGH and NGS enabled the identification of cryptic recurrent deletions in 2p23.3 (DNMT3A; n = 2.8%), 4q24 (TET2; n = 10%) 17p13 (TP53; n = 8.5%), 21q22 (RUNX1; n = 7%), and Xp11.4 (BCOR; n = 2.8%), while mutations in the non-deleted allele where found only in DNMT3A (n = 1), TET2 (n = 3), and TP53 (n = 4). These cryptic abnormalities were detected mainly in patients with normal (45%) or non-informative (15%) karyotype by conventional cytogenetics, except for those with TP53 deletion and mutation (15%), which had a complex karyotype. In addition to well-known copy number defects, the presence of chromothripsis involving chromosome 13 was a novel recurrent change in high-risk MDS patients. Array CGH analysis revealed the presence of cryptic abnormalities in genomic regions where MDS-related genes, such as TET2, DNMT3A, RUNX1 and BCOR, are located.

Immunohistochemistry of apoptosis-related proteins in retinoblastoma

Retinoblastoma is the most common intraocular malignant neoplasia during childhood and results from the partial or total inactivity of the retinoblastoma protein (pRb). In the absence of pRb, the E2F transcription factors increase the levels of cell cycle proteins as well as some pro-apoptotic proteins. We intended to study the immunohistochemistry profile of apoptotic-related proteins in retinoblastoma. We also evaluated the association between the expression of apoptotic protein and stage of tumor or survivor after a 5year follow up. Apoptosis-related proteins (Apaf-1, Bak, Bax, Bcl-2, Bcl-xL, Bim-long, MDM2, p53, pro-caspase-3, PUMA, Smac/DIABLO and cleaved caspase-3) were evaluated using immunohistochemistry on tissue microarrays which contained samples of retinoblastoma tumors taken from ninety-three patients without any treatment previous to surgery. The immunohistochemistry reactions were evaluated using an optical microscope as well as the ACIS III^® platform. The pro-apoptotic proteins (APAF-1, Bax, p53, PUMA, Smac/DIABLO) were more frequently expressed than the anti-apoptotic proteins (Bcl-2, Bcl-xL and MDM2). The protein Bcl-xL had a negative correlation with cleaved caspase-3, a marker of cell apoptosis. Bcl-xL may be implicated in an apoptosis block.

Osteosarcoma: prognosis plateau warrants retinoblastoma pathway targeted therapy

Osteosarcoma (OS) is the most common primary bone cancer in children and adolescents, affecting ~560 young patients in the United States annually. The term OS describes a diverse array of subtypes with varying prognoses, but the majority of tumors are high grade and aggressive. Perhaps because the true etiology of these aggressive tumors remains unknown, advances in OS treatment have reached a discouraging plateau, with only incremental improvements over the past 40 years. Thus, research surrounding the pathogenesis of OS is essential, as it promises to unveil novel therapeutic targets that can attack tumor cells with greater specificity and lower toxicity. Among the candidate molecular targets in OS, the retinoblastoma (RB) pathway demonstrates the highest frequency of inactivation and thus represents a particularly promising avenue for molecular targeted therapy. This review examines the present thinking and practices in OS treatment and specifically highlights the relevance of the RB pathway in osteosarcomagenesis. Through further investigation into RB pathway-related novel therapeutic targets, we believe that a near-term breakthrough in improved OS prognosis is possible.

Novel Molecular Markers for Breast Cancer

The use of molecular biomarkers assures that breast cancer (BC) patients receive optimal treatment. Established biomarkers, such as estrogen receptor, progesterone receptor, HER2, and Ki67, have been playing significant roles in the subcategorization of BC to predict the prognosis and decide the specific therapy to each patient. Antihormonal therapy using 4-hydroxytamoxifen or aromatase inhibitors have been employed in patients whose tumor cells express hormone receptors, while monoclonal antibody to HER2 has been administered to HER2-positive BCs. Although new therapeutic agents have been developed in the past few decades, many patients still die of the disease due to relapse; thus, novel molecular markers that predict therapeutic failure and those that can be targets for specific therapy are expected. We have chosen four of such molecules by reviewing recent publications, which are cyclin E, B-Myb, Twist, and DMP1β. The oncogenicity of these molecules has been demonstrated in vivo and/or in vitro through studies using transgenic mice or siRNAs, and their expressions have been shown to be associated with shortened overall or disease-free survival of BC patients. The former three molecules have been shown to accelerate epithelial-mesenchymal transition that is often associated with cancer stem cell-ness and metastasis; all these four can be novel therapeutic targets as well. Thus, large prospective studies employing immunohistochemistry will be needed to establish the predictive values of these molecules in patients with BC.

MDM2/MDMX: Master negative regulators for p53 and RB

MDM2 (mouse double minute 2 homolog) and MDMX (double minute X human homolog, also known as MDM4) are critical negative regulators of tumor protein p53. Our recent work shows that MDMX binds to and promotes degradation of retinoblastoma protein (RB) in an MDM2-dependent manner. In a xenograft tumor growth mouse model, silencing of MDMX results in inhibition of p53-deficient tumor growth, which can be effectively reversed by concomitant RB silencing. Thus, MDMX exerts its oncogenic activity via suppression of RB.

Contextualizing the Genes Altered in Bladder Neoplasms in Pediatric andTeen Patients Allows Identifying Two Main Classes of Biological ProcessesInvolved and New Potential Therapeutic Targets

Research on bladder neoplasms in pediatric and teen patients (BNPTP) has described 21 genes, which are variously involved in this disease and are mostly responsible for deregulated cell proliferation. However, due to the limited number of publications on this subject, it is still unclear what type of relationships there are among these genes and which are the chances that, while having different molecular functions, they i) act as downstream effector genes of well-known pro- or anti- proliferative stimuli and/or interplay with biochemical pathways having oncological relevance or ii) are specific and, possibly, early biomarkers of these pathologies. A Gene Ontology (GO)-based analysis showed that these 21 genes are involved in biological processes, which can be split into two main classes: cell regulation-based and differentiation/development-based. In order to understand the involvement/overlapping with main cancer-related pathways, we performed a meta-analysis dependent on the 189 oncogenic signatures of the Molecular Signatures Database (OSMSD) curated by the Broad Institute. We generated a binary matrix with 53 gene signatures having at least one hit; this analysis i) suggests that some genes of the original list show inconsistencies and might need to be experimentally re- assessed or evaluated as biomarkers (in particular, ACTA2) and ii) allows hypothesizing that important (proto)oncogenes (E2F3, ERBB2/HER2, CCND1, WNT1, and YAP1) and (putative) tumor suppressors (BRCA1, RBBP8/CTIP, and RB1-RBL2/p130) may participate in the onset of this disease or worsen the observed phenotype, thus expanding the list of possible molecular targets for the treatment of BNPTP.

Genome-wide DNA methylation analyses in lung adenocarcinomas: Association with EGFR, KRAS and TP53 mutation status, gene expression and prognosis

BACKGROUND

DNA methylation alterations are early events in tumorigenesis and important in the regulation of gene expression in cancer cells. Lung cancer patients have in general a poor prognosis, and a deeper insight into the epigenetic landscape in lung adenocarcinoma tumors and its prognostic implications is needed.

RESULTS

We determined whole-genome DNA methylation profiles of 164 fresh frozen lung adenocarcinoma samples and 19 samples of matched normal lung tissue using the Illumina Infinium 450K array. A large number of differentially methylated CpGs in lung adenocarcinoma tissue were identified, and specific methylation profiles were observed in tumors with mutations in the EGFR-, KRAS- or TP53 genes and according to the patients' smoking status. The methylation levels were correlated with gene expression and both positive and negative correlations were seen. Methylation profiles of the tumor samples identified subtypes of tumors with distinct prognosis, including one subtype enriched for TP53 mutant tumors. A prognostic index based on the methylation levels of 33 CpGs was established, and was significantly associated with prognosis in the univariate analysis using an independent cohort of lung adenocarcinoma patients from The Cancer Genome Atlas project. CpGs in the HOX B and HOX C gene clusters were represented in the prognostic signature.

CONCLUSIONS

Methylation differences mirror biologically important features in the etiology of lung adenocarcinomas and influence prognosis.

Patterns of Chromosomal Aberrations in Solid Tumors

Chromosomal abnormalities are a defining feature of solid tumors. Such cytogenetic alterations are mainly classified into structural chromosomal aberrations and copy number alterations, giving rise to aneuploid karyotypes. The increasing detection of these genetic changes allowed the description of specific tumor entities and the associated patterns of gene expression. In fact, tumor-specific landscapes of gross genomic copy number changes, including aneuploidies of entire chromosome arms and chromosomes result in a global deregulation of the transcriptome of cancer cells. Furthermore, the molecular characterization of cytogenetic abnormalities has provided insights into the mechanisms of tumorigenesis and has, in a few instances, led to the clinical implementation of effective diagnostic and prognostic tools, as well as treatment strategies that target a specific genetic abnormality.

The BMI1 inhibitor PTC-209 is a potential compound to halt cellular growth in biliary tract cancer cells

BMI1 is a core component of the polycomb repressive complex 1 (PRC1) and is up-regulated in biliary tract cancer (BTC), contributing to aggressive clinical features. In this study we investigated the cytotoxic effects of PTC-209, a recently developed inhibitor of BMI1, in BTC cells. PTC-209 reduced overall viability in BTC cell lines in a dose-dependent fashion (0.04 - 20 µM). Treatment with PTC-209 led to slightly enhanced caspase activity and stop of cell proliferation. Cell cycle analysis revealed that PTC-209 caused cell cycle arrest at the G1/S checkpoint. A comprehensive investigation of expression changes of cell cycle-related genes showed that PTC-209 caused significant down-regulation of cell cycle-promoting genes as well as of genes that contribute to DNA synthesis initiation and DNA repair, respectively. This was accompanied by significantly elevated mRNA levels of cell cycle inhibitors. In addition, PTC-209 reduced sphere formation and, in a cell line-dependent manner, aldehyde dehydrogease-1 positive cells. We conclude that PTC-209 might be a promising drug for future in vitro and in vivo studies in BTC.

Consistent gene expression profiles in MexTAg transgenic mouse and wild type mouse asbestos-induced mesothelioma

Background

The MexTAg transgenic mouse model of mesothelioma replicates many aspects of human mesothelioma, including induction by asbestos, pathogenicity and response to cytotoxic chemotherapy, despite high levels of the SV40 large T Antigen (TAg) in the mesothelial compartment. This model enables analysis of the molecular events associated with asbestos induced mesothelioma and is utilised here to investigate the molecular dynamics of tumours induced in these mice, using gene expression patterns as a read out.

Methods

Gene expression of MexTAg mesothelioma cell lines bearing a high or low number of copies of the TAg transgene were compared to wild type mouse mesotheliomas and normal mouse mesothelial cells using Affymetrix microarray. These data were then compared to a similar published human microarray study using the same platform.

Results

The main expression differences between transgenic mouse and wild type mouse mesotheliomas occurred for genes involved in cell cycle regulation and DNA replication, as would be expected from overexpression of the TAg oncogene. Quantitative PCR confirmed that E2F and E2F regulated genes were significantly more upregulated in MexTAg mesotheliomas and MexTAg mesothelial cells compared to wild type mesotheliomas. Like human mesothelioma, both MexTAg and wild type mesotheliomas had more genes underexpressed than overexpressed compared to normal mouse mesothelial cells. Most notably, the cdkn2 locus was deleted in the wild type mouse mesotheliomas, consistent with 80 % human mesotheliomas, however, this region was not deleted in MexTAg mesotheliomas. Regardless of the presence of TAg, all mouse mesotheliomas had a highly concordant set of deregulated genes compared to normal mesothelial cells that overlapped with the deregulated genes between human mesotheliomas and mesothelial cells.

Conclusions

This investigation demonstrates that the MexTAg mesotheliomas are comparable with wild type mouse mesotheliomas in their representation of human mesothelioma at the molecular level, with some key gene expression differences that are attributable to the TAg transgene expression. Of particular note, MexTAg mesothelioma development was not dependent on cdkn2 deletion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1953-y) contains supplementary material, which is available to authorized users.

Collateral Lethality: A new therapeutic strategy in oncology

Genomic deletion of tumor suppressor genes (TSG) is a rite of passage for virtually all human cancers. The synthetic lethal paradigm has provided a framework for the development of molecular targeted therapeutics that are functionally linked to the loss of specific TSG functions. In the course of genomic events that delete TSGs, a large number of genes with no apparent direct role in tumor promotion also sustain deletion as a result of chromosomal proximity to the target TSG. In this perspective, we review the novel concept of "collateral lethality", which has served to identify cancer-specific therapeutic vulnerabilities resulting from co-deletion of passenger genes neighboring TSG. The large number of collaterally deleted genes, playing diverse functions in cell homeostasis, offers a rich repertoire of pharmacologically targetable vulnerabilities presenting novel opportunities for the development of personalized anti-neoplastic therapies.

Genomic Copy Number Variation Affecting Genes Involved in the Cell Cycle Pathway: Implications for Somatic Mosaicism

Somatic genome variations (mosaicism) seem to represent a common mechanism for human intercellular/interindividual diversity in health and disease. However, origins and mechanisms of somatic mosaicism remain a matter of conjecture. Recently, it has been hypothesized that zygotic genomic variation naturally occurring in humans is likely to predispose to nonheritable genetic changes (aneuploidy) acquired during the lifetime through affecting cell cycle regulation, genome stability maintenance, and related pathways. Here, we have evaluated genomic copy number variation (CNV) in genes implicated in the cell cycle pathway (according to Kyoto Encyclopedia of Genes and Genomes/KEGG) within a cohort of patients with intellectual disability, autism, and/or epilepsy, in which the phenotype was not associated with genomic rearrangements altering this pathway. Benign CNVs affecting 20 genes of the cell cycle pathway were detected in 161 out of 255 patients (71.6%). Among them, 62 individuals exhibited >2 CNVs affecting the cell cycle pathway. Taking into account the number of individuals demonstrating CNV of these genes, a support for this hypothesis appears to be presented. Accordingly, we speculate that further studies of CNV burden across the genes implicated in related pathways might clarify whether zygotic genomic variation generates somatic mosaicism in health and disease.

Expression Analysis of Genes Involved in the RB/E2F Pathway in Astrocytic Tumors

Astrocytic gliomas, which are derived from glial cells, are considered the most common primary neoplasias of the central nervous system (CNS) and are histologically classified as low grade (I and II) or high grade (III and IV). Recent studies have shown that astrocytoma formation is the result of the deregulation of several pathways, including the RB/E2F pathway, which is commonly deregulated in various human cancers via genetic or epigenetic mechanisms. On the basis of the assumption that the study of the mechanisms controlling the INK4/ARF locus can help elucidate the molecular pathogenesis of astrocytic tumors, identify diagnostic and prognostic markers, and help select appropriate clinical treatments, the present study aimed to evaluate and compare methylation patterns using bisulfite sequencing PCR and evaluate the gene expression profile using real-time PCR in the genes CDKN2A, CDKN2B, CDC6, Bmi-1, CCND1, and RB1 in astrocytic tumors. Our results indicate that all the evaluated genes are not methylated independent of the tumor grade. However, the real-time PCR results indicate that these genes undergo progressive deregulation as a function of the tumor grade. In addition, the genes CDKN2A, CDKN2B, and RB1 were underexpressed, whereas CDC6, Bmi-1, and CCND1 were overexpressed; the increase in gene expression was significantly associated with decreased patient survival. Therefore, we propose that the evaluation of the expression levels of the genes involved in the RB/E2F pathway can be used in the monitoring of patients with astrocytomas in clinical practice and for the prognostic indication of disease progression.

Epigenetic changes in gastroenteropancreatic neuroendocrine tumours

An understanding of epigenetic drivers of tumorigenesis has developed rapidly during the last years. The identification of these changes including DNA methylation and histone modifications in gastroenteropancreatic neuroendocrine tumours (GEP-NETs) is a step forward in trying to define underlying biologic processes in this heterogeneous disease. The reversible nature of these changes represents a potential therapeutic target. We present an overview of the current knowledge of epigenetic alterations related to GEP-NETs, focusing on the influence and impact these changes have on pathogenesis and prognosis. The potential role of demethylating agents in the management of this patient population is discussed.

The Drosophila retinoblastoma protein, Rbf1, induces a Debcl- and Drp1-dependent mitochondrial apoptosis

In accordance with its tumor suppressor role, the retinoblastoma protein pRb can ensure pro-apoptotic functions. Rbf1, the Drosophila homolog of Rb, also displays a pro-apoptotic activity in proliferative cells. We have previously shown that the Rbf1 pro-apoptotic activity depends on its ability to decrease the level of anti-apoptotic proteins such as the Bcl-2 family protein Buffy. Buffy often acts in an opposite manner to Debcl, the other Drosophila Bcl-2-family protein. Both proteins can localize at the mitochondrion, but the way they control apoptosis still remains unclear. Here, we demonstrate that Debcl and the pro-fission gene Drp1 are necessary downstream of Buffy to trigger a mitochondrial fragmentation during Rbf1-induced apoptosis. Interestingly, Rbf1-induced apoptosis leads to a Debcl- and Drp1-dependent reactive oxygen species production, which in turn activates the Jun Kinase pathway to trigger cell death. Moreover, we show that Debcl and Drp1 can interact and that Buffy inhibits this interaction. Notably, Debcl modulates Drp1 mitochondrial localization during apoptosis. These results provide a mechanism by which Drosophila Bcl-2 family proteins can control apoptosis, and shed light on a link between Rbf1 and mitochondrial dynamics in vivo.

Diffuse anterior retinoblastoma: current concepts

Diffuse anterior retinoblastoma is a rare variant of retinoblastoma seeding in the area of the vitreous base and anterior chamber. Patients with diffuse anterior retinoblastoma are older than those with the classical types, with the mean age being 6.1 years. The original cells of diffuse anterior retinoblastoma are supposed to be cone precursor. Patients most commonly present with pseudouveitis, pseudohypopyon, and increased intraocular pressure. The retina under fundus examination is likely to be normal, and the clinical features mimic the inflammation progress, which can often lead to misdiagnosis. The published diffuse anterior retinoblastoma cases were diagnosed after fine-needle aspiration biopsy running the potential risk of inducing metastasis. The most common treatment for diffuse anterior retinoblastoma is enucleation followed by systematic chemotherapy according to the patient’s presentation and clinical course. This review summarizes the recent advances in etiology (including tumorigenesis and cell origin), pathology, diagnosis, differential diagnosis, and new treatment. The challenges of early diagnosis and prospects are also discussed.

Genomic landscape of cutaneous T cell lymphoma

Cutaneous T cell lymphoma (CTCL) is a non-Hodgkin lymphoma of skin-homing T lymphocytes. We performed exome and whole genome DNA sequence and RNA sequencing on purified CTCL and matched normal cells. The results implicate mutations in 17 genes in CTCL pathogenesis, including genes involved in T cell activation and apoptosis, NFκB signaling, chromatin remodeling, and DNA damage response. CTCL is distinctive in that somatic copy number variants (SCNVs) comprise 92% of all driver mutations (mean of 11.8 pathogenic SCNVs vs. 1.0 somatic single nucleotide variants per CTCL). These findings have implications for novel therapeutics.

The Biology of Retinoblastoma

Retinoblastoma, the most common primary intraocular cancer of childhood, is a malignancy arising in the developing retina. Tumor formation usually begins with mutation in both alleles of the retinoblastoma tumor suppressor gene RB1, followed by a series of other genetic alterations that correlate with the clinical stage and pathologic findings of the tumor. Analysis of sporadic and heritable retinoblastoma led to the development of Knudson's Two-Hit Hypothesis. The tumor suppressor RB1 gene codes for the retinoblastoma protein which is a key regulator of cellular replication via its binding to the E2F family of transcription factors and chromatin remodeling proteins. Studies of preclinical models of retinoblastoma in the form of transgenic mice and xenograft animal models have significantly contributed to the development of effective therapies for this disease. Research on retinoblastoma has paved the way toward understanding many of the mechanisms in cancer genetics.

Molecular Determinants for the Inactivation of the Retinoblastoma Tumor Suppressor by the Viral Cyclin-dependent Kinase UL97

The retinoblastoma (Rb) tumor suppressor restricts cell cycle progression by repressing E2F-responsive transcription. Cellular cyclin-dependent kinase (CDK)-mediated Rb inactivation through phosphorylation disrupts Rb-E2F complexes, stimulating transcription. The human cytomegalovirus (HCMV) UL97 protein is a viral CDK (v-CDK) that phosphorylates Rb. Here we show that UL97 phosphorylates 11 of the 16 consensus CDK sites in Rb. A cleft within Rb accommodates peptides with the amino acid sequence LXCXE. UL97 contains three such motifs. We determined that the first LXCXE motif (L1) of UL97 and the Rb cleft enhance UL97-mediated Rb phosphorylation. A UL97 mutant with a non-functional L1 motif (UL97-L1m) displayed significantly reduced Rb phosphorylation at multiple sites. Curiously, however, it efficiently disrupted Rb-E2F complexes but failed to relieve Rb-mediated repression of E2F reporter constructs. The HCMV immediate early 1 protein cooperated with UL97-L1m to inactivate Rb in transfection assays, likely indicating that cells infected with a UL97-L1m mutant virus show no defects in growth or E2F-responsive gene expression because of redundant viral mechanisms to inactivate Rb. Our data suggest that UL97 possesses a mechanism to elicit E2F-dependent gene expression distinct from disruption of Rb-E2F complexes and dependent upon both the L1 motif of UL97 and the cleft region of Rb.

Disruptive chemicals, senescence and immortality

Carcinogenesis is thought to be a multistep process, with clonal evolution playing a central role in the process. Clonal evolution involves the repeated 'selection and succession' of rare variant cells that acquire a growth advantage over the remaining cell population through the acquisition of 'driver mutations' enabling a selective advantage in a particular micro-environment. Clonal selection is the driving force behind tumorigenesis and possesses three basic requirements: (i) effective competitive proliferation of the variant clone when compared with its neighboring cells, (ii) acquisition of an indefinite capacity for self-renewal, and (iii) establishment of sufficiently high levels of genetic and epigenetic variability to permit the emergence of rare variants. However, several questions regarding the process of clonal evolution remain. Which cellular processes initiate carcinogenesis in the first place? To what extent are environmental carcinogens responsible for the initiation of clonal evolution? What are the roles of genotoxic and non-genotoxic carcinogens in carcinogenesis? What are the underlying mechanisms responsible for chemical carcinogen-induced cellular immortality? Here, we explore the possible mechanisms of cellular immortalization, the contribution of immortalization to tumorigenesis and the mechanisms by which chemical carcinogens may contribute to these processes.

The Rb-E2F transcriptional regulatory pathway in tumor angiogenesis and metastasis

The retinoblastoma tumor suppressor protein Rb plays a major role in regulating G1/S transition and is a critical regulator of cell proliferation. Rb protein exerts its growth regulatory properties mainly by physically interacting with the transcriptionally active members of the E2F transcription factor family, especially E2Fs 1, 2, and 3. Given its critical role in regulating cell proliferation, it is not surprising that Rb is inactivated in almost all tumors, either through the mutation of Rb gene itself or through the mutations of its upstream regulators including K-Ras and INK4. Recent studies have revealed a significant role for Rb and its downstream effectors, especially E2Fs, in regulating various aspects of tumor progression, angiogenesis, and metastasis. Thus, components of the Rb-E2F pathway have been shown to regulate the expression of genes involved in angiogenesis, including VEGF and VEGFR, genes involved in epithelial-mesenchymal transition including E-cadherin and ZEB proteins, and genes involved in invasion and migration like matrix metalloproteinases. Rb has also been shown to play a major role in the functioning of normal and cancer stem cells; further, Rb and E2F appear to play a regulatory role in the energy metabolism of cancer cells. These findings raise the possibility that mutational events that initiate tumorigenesis by inducing uncontrolled cell proliferation might also contribute to the progression and metastasis of cancers through the mediation of the Rb-E2F transcriptional regulatory pathway. This review highlights these recent studies on tumor promoting functions of the Rb-E2F pathway.

Whole-exome sequencing characterizes the landscape of somatic mutations and copy number alterations in adrenocortical carcinoma

CONTEXT

Adrenocortical carcinoma (ACC) is a rare and lethal malignancy with a poorly defined etiology, and the molecular genetics of ACC are incompletely understood.

OBJECTIVE

To utilize whole-exome sequencing for genetic characterization of the underlying somatic mutations and copy number alterations present in ACC.

DESIGN

Screening for somatic mutation events and copy number alterations (CNAs) was performed by comparative analysis of tumors and matched normal samples from 41 patients with ACC.

RESULTS

In total, 966 nonsynonymous somatic mutations were detected, including 40 tumors with a mean of 16 mutations per sample and one tumor with 314 mutations. Somatic mutations in ACC-associated genes included TP53 (8/41 tumors, 19.5%) and CTNNB1 (4/41, 9.8%). Genes with potential disease-causing mutations included GNAS, NF2, and RB1, and recurrently mutated genes with unknown roles in tumorigenesis comprised CDC27, SCN7A, and SDK1. Recurrent CNAs included amplification at 5p15.33 including TERT (6/41, 14.6%) and homozygous deletion at 22q12.1 including the Wnt repressors ZNRF3 and KREMEN1 (4/41 9.8% and 3/41, 7.3%, respectively). Somatic mutations in ACC-established genes and recurrent ZNRF3 and TERT loci CNAs were mutually exclusive in the majority of cases. Moreover, gene ontology identified Wnt signaling as the most frequently mutated pathway in ACCs.

CONCLUSIONS

These findings highlight the importance of Wnt pathway dysregulation in ACC and corroborate the finding of homozygous deletion of Wnt repressors ZNRF3 and KREMEN1. Overall, mutations in either TP53 or CTNNB1 as well as focal CNAs at the ZNRF3 or TERT loci denote mutually exclusive events, suggesting separate mechanisms underlying the development of these tumors.

MDMX exerts its oncogenic activity via suppression of retinoblastoma protein

Inactivation of the retinoblastoma protein (RB) has a major role in the development of human malignancies. We have previously shown that MDM2, an ubiquitin E3 ligase and major negative regulator of p53, binds to and promotes proteasome-mediated degradation of RB. MDMX, a homolog of MDM2, also binds to and inhibits p53 transactivation activity, yet it does not possess intrinsic ubiquitin ligase activity. Here, we show that MDMX binds to and promotes RB degradation in an MDM2-dependent manner. Specifically, the MDMX C-terminal ring domain binds to the RB C-pocket and enhances MDM2-RB interaction. Silencing MDMX induces RB accumulation, cell cycle arrest and senescence-like phenotypes, which are reverted by simultaneous RB knockdown. Furthermore, MDMX ablation leads to significant retardation of xenograft tumor growth, concomitant with RB accumulation. These results demonstrate that MDMX exerts oncogenic activity via suppression of RB, and suggest that both MDM2 and MDMX could be chemotherapeutic targets.

The retinoblastoma tumor suppressor promotes efficient human cytomegalovirus lytic replication

The retinoblastoma (Rb) tumor suppressor controls cell cycle, DNA damage, apoptotic, and metabolic pathways. DNA tumor virus oncoproteins reduce Rb function by either inducing Rb degradation or physically disrupting complexes between Rb and its myriad binding proteins. Human cytomegalovirus (HCMV), a betaherpesvirus being investigated for potential roles in human cancers, encodes multiple lytic-phase proteins that inactivate Rb in distinct ways, leading to the hypothesis that reduced Rb levels and/or activity would benefit HCMV lytic infection. Paradoxically, we found that Rb knockdown prior to infection, whether transient or constitutive, impaired HCMV lytic infection at multiple stages, notably viral DNA replication, late protein expression, and infectious virion production. The existence of differentially modified forms of Rb, the temporally and functionally distinct means by which HCMV proteins interact with Rb, and the necessity of Rb for efficient HCMV lytic replication combine to highlight the complex relationship between the virus and this critical tumor suppressor.

IMPORTANCE

Initial work examining viral protein modulation of cell cycle progression and oncogenic transformation revealed that these proteins inactivated the function of cellular tumor suppressor proteins. However, subsequent work, including experiments described here using human cytomegalovirus, demonstrate a more nuanced interaction that includes the necessity of cellular tumor suppressors for efficient viral replication. Understanding the positive impacts that cellular tumor suppressors have on viral infections may reveal new activities of these well-studied yet incompletely understood proteins. The basis for oncolytic viral therapy is the selective replication of viruses in transformed cells in which tumor suppressor function may be compromised. Understanding how tumor suppressors support viral infections may allow for the generation of modified oncolytic viruses with greater selective tumor cell replication and killing.

Identifying core gene modules in glioblastoma based on multilayer factor-mediated dysfunctional regulatory networks through integrating multi-dimensional genomic data

The driver genetic aberrations collectively regulate core cellular processes underlying cancer development. However, identifying the modules of driver genetic alterations and characterizing their functional mechanisms are still major challenges for cancer studies. Here, we developed an integrative multi-omics method CMDD to identify the driver modules and their affecting dysregulated genes through characterizing genetic alteration-induced dysregulated networks. Applied to glioblastoma (GBM), the CMDD identified a core gene module of 17 genes, including seven known GBM drivers, and their dysregulated genes. The module showed significant association with shorter survival of GBM. When classifying driver genes in the module into two gene sets according to their genetic alteration patterns, we found that one gene set directly participated in the glioma pathway, while the other indirectly regulated the glioma pathway, mostly, via their dysregulated genes. Both of the two gene sets were significant contributors to survival and helpful for classifying GBM subtypes, suggesting their critical roles in GBM pathogenesis. Also, by applying the CMDD to other six cancers, we identified some novel core modules associated with overall survival of patients. Together, these results demonstrate integrative multi-omics data can identify driver modules and uncover their dysregulated genes, which is useful for interpreting cancer genome.

To live or let die - complexity within the E2F1 pathway

The E2F1 transcription factor is a recognized regulator of the cell cycle as well as a potent mediator of DNA damage-induced apoptosis and the checkpoint response. Understanding the diverse and seemingly dichotomous functions of E2F1 activity has been the focus of extensive ongoing research. Although the E2F pathway is frequently deregulated in cancer, the contributions of E2F1 itself to tumorigenesis, as a promoter of proliferation or cell death, are far from understood. In this review we aim to provide an update on our current understanding of E2F1, with particular insight into its novel interaction partners and post-translational modifications, as a means to explaining its diverse functional complexity.

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that multiple endocrine neoplasia type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma (MTC). Thyroid C-cells are known to express RET at high levels relative to most cell types; therefore, aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET, the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations have uncovered mutation of RAS family members and inactivation of Rb1 regulatory pathway as potential mediators of C-cell transformation. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular-targeted therapies.

Retinoblastoma. Fifty years of progress. The LXXI Edward Jackson Memorial Lecture

PURPOSE

To review the progress made in understanding the genetic basis, molecular pathology, and treatment of retinoblastoma since the previous Jackson lecture on the topic was published 50 years ago.

DESIGN

Perspective based on personal experience and the literature.

METHODS

The literature regarding retinoblastoma was reviewed since 1963. Advances in understanding the biology and treatment of retinoblastoma provided context through the author's clinical, pathologic, and research experiences.

RESULTS

Retinoblastoma was first identified in the 1500s and defined as a unique clinicopathologic entity in 1809. Until the mid-1900s, knowledge advanced sporadically, with technological developments of ophthalmoscopy and light microscopy, and with the introduction of surgical enucleation, chemotherapy, and radiation therapy. During the last 50 years, research and treatment have progressed at an unprecedented rate owing to innovations in molecular biology and the development of targeted therapies. During this time period, the retinoblastoma gene was discovered; techniques for genetic testing for retinoblastoma were developed; and plaque brachytherapy, chemoreduction, intra-arterial chemotherapy, and intraocular injections of chemotherapeutic agents were successfully introduced.

CONCLUSIONS

Nearly all patients with retinoblastoma in developed countries can now be cured of their primary cancer--a remarkable achievement for a childhood cancer that once was uniformly fatal. Much of this success is owed to deciphering the role of the Rb gene, and the benefits of targeted therapies, such as chemoreduction with consolidation as well as intra-arterial and intravitreal chemotherapies. Going forward, the main challenge will be ensuring that access to care is available for all children, particularly those in developing countries.

Pumilio and nanos RNA-binding proteins counterbalance the transcriptional consequences of RB1 inactivation

The ability of the retinoblastoma protein (RB) tumor suppressor to repress transcription stimulated by the E2 promoter binding factors (E2F) is integral to its biological functions. Our recent report described a conserved feedback mechanism mediated by the RNA-binding proteins Pumilio and Nanos that increases in importance following RB loss and helps cells to tolerate deregulated E2F.

Post-transcriptional gene expression control by NANOS is up-regulated and functionally important in pRb-deficient cells

Inactivation of the retinoblastoma tumor suppressor (pRb) is a common oncogenic event that alters the expression of genes important for cell cycle progression, senescence, and apoptosis. However, in many contexts, the properties of pRb-deficient cells are similar to wild-type cells suggesting there may be processes that counterbalance the transcriptional changes associated with pRb inactivation. Therefore, we have looked for sets of evolutionary conserved, functionally related genes that are direct targets of pRb/E2F proteins. We show that the expression of NANOS, a key facilitator of the Pumilio (PUM) post-transcriptional repressor complex, is directly repressed by pRb/E2F in flies and humans. In both species, NANOS expression increases following inactivation of pRb/RBF1 and becomes important for tissue homeostasis. By analyzing datasets from normal retinal tissue and pRb-null retinoblastomas, we find a strong enrichment for putative PUM substrates among genes de-regulated in tumors. These include pro-apoptotic genes that are transcriptionally down-regulated upon pRb loss, and we characterize two such candidates, MAP2K3 and MAP3K1, as direct PUM substrates. Our data suggest that NANOS increases in importance in pRb-deficient cells and helps to maintain homeostasis by repressing the translation of transcripts containing PUM Regulatory Elements (PRE).

Regulation of E2F1-induced apoptosis by poly(ADP-ribosyl)ation

The transcription factor adenovirus E2 promoter-binding factor (E2F)-1 normally enhances cell-cycle progression, but it also induces apoptosis under certain conditions, including DNA damage and serum deprivation. Although DNA damage facilitates the phosphorylation and stabilization of E2F1 to trigger apoptosis, how serum starvation renders cells vulnerable to E2F1-induced apoptosis remains unclear. Because poly(ADP-ribose) polymerase 1 (PARP1), a nuclear enzyme essential for genomic stability and chromatin remodeling, interacts directly with E2F1, we investigated the effects of PARP1 on E2F1-mediated functions in the presence and absence of serum. PARP1 attenuation, which increased E2F1 transactivation, induced G2/M cell-cycle arrest under normal growth conditions, but enhanced E2F1-induced apoptosis in serum-starved cells. Interestingly, basal PARP1 activity was sufficient to modify E2F1 by poly(ADP-ribosyl)ation, which stabilized the interaction between E2F1 and the BIN1 tumor suppressor in the nucleus. Accordingly, BIN1 acted as an RB1-independent E2F1 corepressor. Because E2F1 directly activates the BIN1 gene promoter, BIN1 curbed E2F1 activity through a negative-feedback mechanism. Conversely, when the BIN1-E2F1 interaction was abolished by PARP1 suppression, E2F1 continuously increased BIN1 levels. This is functionally germane, as PARP1-depletion-associated G2/M arrest was reversed by the transfection of BIN1 siRNA. Moreover, PARP-inhibitor-associated anti-transformation activity was compromised by the coexpression of dominant-negative BIN1. Because serum starvation massively reduced the E2F1 poly(ADP-ribosyl)ation, we conclude that the release of BIN1 from hypo-poly(ADP-ribosyl)ated E2F1 is a mechanism by which serum starvation promotes E2F1-induced apoptosis.

Genetic alterations and cancer formation in a European flatfish at sites of different contaminant burdens

Fish diseases are an indicator for marine ecosystem health since they provide a biological end-point of historical exposure to stressors. Liver cancer has been used to monitor the effects of exposure to anthropogenic pollution in flatfish for many years. The prevalence of liver cancer can exceed 20%. Despite the high prevalence and the opportunity of using flatfish to study environmentally induced cancer, the genetic and environmental factors driving tumor prevalence across sites are poorly understood. This study aims to define the link between genetic deterioration, liver disease progression, and anthropogenic contaminant exposures in the flatfish dab (Limanda limanda). We assessed genetic changes in a conserved cancer gene, Retinoblastoma (Rb), in association with histological diagnosis of normal, pretumor, and tumor pathologies in the livers of 165 fish from six sites in the North Sea and English Channel. The highest concentrations of metals (especially cadmium) and organic chemicals correlated with the presence of tumor pathology and with defined genetic profiles of the Rb gene, from these sites. Different Rb genetic profiles were found in liver tissue near each tumor phenotype, giving insight into the mechanistic molecular-level cause of the liver pathologies. Different Rb profiles were also found at sampling sites of differing contaminant burdens. Additionally, profiles indicated that histological "normal" fish from Dogger sampling locations possessed Rb profiles associated with pretumor disease. This study highlights an association between Rb and specific contaminants (especially cadmium) in the molecular etiology of dab liver tumorigenesis.

Coregulation of FANCA and BRCA1 in human cells

Fanconi anemia (FA) is a genetically heterogeneous syndrome associated with increased cancer predisposition. The underlying genes govern the FA pathway which functions to protect the genome during the S-phase of the cell cycle. While upregulation of FA genes has been linked to chemotherapy resistance, little is known about their regulation in response to proliferative stimuli. The purpose of this study was to examine how FA genes are regulated, especially in relation to the cell cycle, in order to reveal their possible participation in biochemical networks. Expression of 14 FA genes was monitored in two human cell-cycle models and in two RB1/E2F pathway-associated primary cancers, retinoblastoma and basal breast cancer. In silico studies were performed to further evaluate coregulation and identify connected networks and diseases. Only FANCA was consistently induced over 2-fold; FANCF failed to exhibit any regulatory fluctuations. Two tools exploiting public data sets indicated coregulation of FANCA with BRCA1. Upregulation of FANCA and BRCA1 correlated with upregulation of E2F3. Genes coregulated with both FANCA and BRCA1 were enriched for MeSH-Term id(s) genomic instability, microcephaly, and Bloom syndrome, and enriched for the cellular component centrosome. The regulation of FA genes appears highly divergent. In RB1-linked tumors, upregulation of FA network genes was associated with reduced expression of FANCF. FANCA and BRCA1 may jointly act in a subnetwork - supporting vital function(s) at the subcellular level (centrosome) as well as at the level of embryonic development (mechanisms controlling head circumference).

Differential response of normal and malignant urothelial cells to CHK1 and ATM inhibitors

While DNA damage response pathways are well characterized in cancer cells, much less is known about their status in normal cells. These pathways protect tumour cells from DNA damage and replication stress and consequently present potential therapeutic targets. Here we characterize the response of human telomerase reverse transcriptase (hTERT)-immortalized normal human urothelial (NHU) and bladder cancer cell lines to agents that disrupt the DNA damage response. Effects of replication and DNA damage response inhibitors on cell cycle progression, checkpoint induction and apoptosis were analysed in hTERT-NHU and bladder cancer cell lines. The primary signalling cascade responding to replication stress in malignant cells (ataxia telangiectasia-mutated (ATM) and Rad3-related-checkpoint kinase 1 (ATR-CHK1)) is not activated in hTERT-NHU cells after treatment with a replication inhibitor and these cells do not depend upon CHK1 for protection from apoptosis during replication stress. Instead, ATM signalling is rapidly activated under these conditions. Intriguingly, an ATM inhibitor suppressed S-phase checkpoint activation after exposure to replication inhibitors and stopped entry of cells into S-phase indicating G1 checkpoint activation. Consistent with this, hTERT-NHU cells treated with the ATM inhibitor showed increased levels of cyclin-dependent kinase inhibitor p19(INK4D), reduced levels of cyclin D1 and CDK4, and reduced phosphorylation of the retinoblastoma protein. In contrast, a bladder cancer cell line cotreated with ATM and replication inhibitors progressed more slowly through S phase and showed a marked increase in apoptosis. Taken together, our findings suggest that ATM and CHK1 signalling cascades have different roles in tumour and normal epithelial cells, confirming these as promising therapeutic targets.

Genome-wide screening of cytogenetic abnormalities in multiple myeloma patients using array-CGH technique: a Czech multicenter experience

Characteristic recurrent copy number aberrations (CNAs) play a key role in multiple myeloma (MM) pathogenesis and have important prognostic significance for MM patients. Array-based comparative genomic hybridization (aCGH) provides a powerful tool for genome-wide classification of CNAs and thus should be implemented into MM routine diagnostics. We demonstrate the possibility of effective utilization of oligonucleotide-based aCGH in 91 MM patients. Chromosomal aberrations associated with effect on the prognosis of MM were initially evaluated by I-FISH and were found in 93.4% (85/91). Incidence of hyperdiploidy was 49.5% (45/91); del(13)(q14) was detected in 57.1% (52/91); gain(1)(q21) occurred in 58.2% (53/91); del(17)(p13) was observed in 15.4% (14/91); and t(4;14)(p16;q32) was found in 18.6% (16/86). Genome-wide screening using Agilent 44K aCGH microarrays revealed copy number alterations in 100% (91/91). Most common deletions were found at 13q (58.9%), 1p (39.6%), and 8p (31.1%), whereas gain of whole 1q was the most often duplicated region (50.6%). Furthermore, frequent homozygous deletions of genes playing important role in myeloma biology such as TRAF3, BIRC1/BIRC2, RB1, or CDKN2C were observed. Taken together, we demonstrated the utilization of aCGH technique in clinical diagnostics as powerful tool for identification of unbalanced genomic abnormalities with prognostic significance for MM patients.

Gastroenteropancreatic endocrine tumors

Gastroenteropancreatic endocrine tumors (GEP-NETs) are relatively uncommon; comprising approximately 0.5% of all human cancers. Although they often exhibit relatively indolent clinical courses, GEP-NETs have the potential for lethal progression. Due to their scarcity and various technical challenges, GEP-NETs have been understudied. As a consequence, we have few diagnostic, prognostic and predictive biomarkers for these tumors. Early detection and surgical removal is currently the only reliable curative treatment for GEP-NET patients; many of whom, unfortunately, present with advanced disease. Here, we review the genetics and epigenetics of GEP-NETs. The last few years have witnessed unprecedented technological advances in these fields, and their application to GEP-NETS has already led to important new information on the molecular abnormalities underlying them. As outlined here, we expect that "omics" studies will provide us with new diagnostic and prognostic biomarkers, inform the development of improved pre-clinical models, and identify novel therapeutic targets for GEP-NET patients.

Genetic and epigenetic mutations of tumor suppressive genes in sporadic pituitary adenoma

Human pituitary adenomas are the most common intracranial neoplasms. Approximately 5% of them are familial adenomas. Patients with familial tumors carry germline mutations in predisposition genes, including AIP, MEN1 and PRKAR1A. These mutations are extremely rare in sporadic pituitary adenomas, which therefore are caused by different mechanisms. Multiple tumor suppressive genes linked to sporadic tumors have been identified. Their inactivation is caused by epigenetic mechanisms, mainly promoter hypermethylation, and can be placed into two groups based on their functional interaction with tumor suppressors RB or p53. The RB group includes CDKN2A, CDKN2B, CDKN2C, RB1, BMP4, CDH1, CDH13, GADD45B and GADD45G; AIP and MEN1 genes also belong to this group. The p53 group includes MEG3, MGMT, PLAGL1, RASSF1, RASSF3 and SOCS1. We propose that the tumor suppression function of these genes is mainly mediated by the RB and p53 pathways. We also discuss possible tumor suppression mechanisms for individual genes.

Inhibition of retinoblastoma mRNA degradation through Poly (A) involved in the neuroprotective effect of berberine against cerebral ischemia

Berberine is one kind of isoquinoline alkaloid with anti-apoptotic effects on the neurons suffering ischemia. To address the explanation for these activities, the berberine-induced cell cycle arrest during neurons suffering ischemia/reperfusion had been studied in the present study. According to the in vitro neurons with oxygen-glucose deprivation and in vivo ICR mice with cerebral ischemia/reperfusion, it was found that berberine could protect the mRNA of retinoblastoma (Rb) from degradation through its function on the poly(A) tail. The prolonged half-life of retinoblastoma 1 (gene of Rb, RB1) mRNA level secures the protein level of retinoblastoma, which facilitates cell cycle arrest of neurons in the process of ischemia/reperfusion and subsequently avoids cells entering in the apoptotic process. The poly(A) tail of RB1 mRNA, as a newly identified target of berberine, could help people focus on the interaction between berberine and mRNA to further understand the biological activities and functions of berberine.

Epigenetic alterations in osteosarcoma: promising targets

Cancer is being reinterpreted due to recent discoveries related to epigenetic regulation during development, and the importance of epigenetic mechanisms in initiation and progression of cancer has been further highlighted by the recent explosion in medical information. Osteosarcoma is highly genetically unstable, and current therapeutic regimens are subject to chemoresistance and tumor relapse. Understanding the epigenetic mechanisms in the pathogenesis of osteosarcoma will provide novel avenues for cancer therapy. In this review, we examine the epigenetic alterations in gene expression in osteosarcoma, and discuss the utilization of epigenetic regulation therapy in treatment against osteosarcoma.

Targeting the nucleolus for cancer intervention

The contribution of the nucleolus to cancer is well established with respect to its traditional role in facilitating ribosome biogenesis and proliferative capacity. More contemporary studies however, infer that nucleoli contribute a much broader role in malignant transformation. Specifically, extra-ribosomal functions of the nucleolus position it as a central integrator of cellular proliferation and stress signaling, and are emerging as important mechanisms for modulating how oncogenes and tumor suppressors operate in normal and malignant cells. The dependence of certain tumor cells to co-opt nucleolar processes to maintain their cancer phenotypes has now clearly been demonstrated by the application of small molecule inhibitors of RNA Polymerase I to block ribosomal DNA transcription and disrupt nucleolar function (Bywater et al., 2012 [1]). These drugs, which selectively kill tumor cells in vivo while sparing normal cells, have now progressed to clinical trials. It is likely that we have only just begun to scratch the surface of the potential of the nucleolus as a new target for cancer therapy, with "suppression of nucleolar stress" representing an emerging "hallmark" of cancer. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Disrupting the interaction between retinoblastoma protein and Raf-1 leads to defects in progenitor cell proliferation and survival during early inner ear development

The retinoblastoma protein (pRb) is required for cell-cycle exit of embryonic mammalian hair cells but is not required for hair cell fate determination and early differentiation, and this provides a strategy for hair cell regeneration by manipulating the pRb pathway. To reveal the mechanism of pRb functional modification in the inner ear, we compared the effects of attenuated pRb phosphorylation by an inhibitor of the Mitogen-Activated Protein (MAP) kinase pathway and an inhibitor of the Rb-Raf-1 interaction on cultured chicken otocysts. We demonstrated that the activity of pRb is correlated with its phosphorylation state, which is regulated by a newly established cell cycle-independent pathway mediated by the physical interaction between Raf-1 and pRb. The phosphorylation of pRb plays an important role during the early stage of inner ear development, and attenuated phosphorylation in progenitor cells leads to cell cycle arrest and increased apoptosis along with a global down-regulation of the genes involved in cell cycle progression. Our study provides novel routes to modulate pRb function for hair cell regeneration.