Loss of heterozygosity on chromosome 17 and mutation of the p53 gene in retinoblastoma

Retinoblastoma vulnerability to combined de novo and salvage pyrimidine ribonucleotide synthesis pharmacologic blockage

Retinoblastoma is an eye cancer that commonly affects young children. Despite significant advances, current treatments cause side effects even when administered locally, and patients may still have to undergo enucleation. This is particularly disheartening in cases of bilateral retinoblastoma. Hence, there is an urgent need for novel therapeutic strategies. Inhibitors of the enzyme dihydroorotate dehydrogenase (DHODH), which is involved in the de novo pyrimidine ribonucleotide synthesis pathway, have proven to be effective in preclinical trials against several cancers including pediatric cancers. Here we tested whether blocking pyrimidine ribonucleotide synthesis promotes retinoblastoma cell death. Cultured retinoblastoma cell lines were treated with small molecule inhibitors of DHODH alone or in combination with inhibitors of nucleoside uptake to also block the salvage pathway for pyrimidine ribonucleotide formation. On their own, DHODH inhibitors had a moderate killing effect. However, the combination with nucleoside uptake inhibitors greatly enhanced the effect of DHODH inhibition. In addition, we observed that pyrimidine ribonucleotide synthesis blockage can cause cell death in a p53 mutant retinoblastoma cell line derived from a patient with metastasis. Explaining these results, the analysis of a published patient cohort revealed that loss of chr16q22.2 (containing the DHODH gene) is amongst the most frequent alterations in retinoblastoma and that these tumors often show gains in chromosome regions expressing pyrimidine ribonucleotide salvage factors. Furthermore, these genome alterations associate with malignancy. These results indicate that targeting pyrimidine ribonucleotide synthesis may be an effective therapeutic strategy to consider as a treatment for retinoblastoma.

Genome maintenance in retinoblastoma: Implications for therapeutic vulnerabilities (Review)

Retinoblastoma (RB) is a pediatric ocular malignancy that is initiated mostly by biallelic inactivation of the RB transcriptional corepressor 1 (RB1) tumor suppressor gene in the developing retina. Unlike the prevailing prediction based on multiple studies involving RB1 gene disruption in experimental models, human RB tumors have been demonstrated to possess a relatively stable genome, characterized by a low mutation rate and a few recurrent chromosomal alterations related to somatic copy number changes. This suggests that RB may harbor heightened genome maintenance mechanisms to counteract or compensate for the risk of massive genome instability, which can potentially be driven by the early RB1 loss as a tumor-initiating event. Although the genome maintenance mechanisms might have been evolved to promote RB cell survival by preventing lethal genomic defects, emerging evidence suggests that the dependency of RB cells on these mechanisms also exposes their unique vulnerability to chemotherapy, particularly when the genome maintenance machineries are tumor cell-specific. This review summarizes the genome maintenance mechanisms identified in RB, including findings on the roles of chromatin regulators in DNA damage response/repair and protein factors involved in maintaining chromosome stability and promoting survival in RB. In addition, advantages and challenges for exploiting these therapeutic vulnerabilities in RB are discussed.

TP53/miR-129/MDM2/4/TP53 feedback loop modulates cell proliferation and apoptosis in retinoblastoma

Retinoblastoma (RB) is commonly-seen cancer in children. The p53 pathway dysfunction, which can lead to elevated MDM2 or MDM4 (p53 antagonists) protein expression, is frequently observed in almost all human cancers, including RB. The present study attempted to investigate the underlying mechanism from the perspective of non-coding RNA regulation. Here, we demonstrated that p53 and miR-129 were positively correlated with each other in RB. miR-129 directly targeted MDM2/4 to inhibit expression, therefore counteracting MDM2/4-mediated p53 signaling suppression and modulating RB cell proliferation and apoptosis. Moreover, p53 could activate the transcription of miR-129 via binding to the miR-129 promoter region, therefore forming a regulatory loop with MDM2/4 to affect RB progression. Altogether, the p53/miR-129/MDM2/4/p53 regulatory loop can modulate RB cell growth. We provide a solid experimental basis for developing novel therapies for RB.

An Epigenetic Mechanism Underlying Chromosome 17p Deletion-Driven Tumorigenesis

Chromosome copy-number variations are a hallmark of cancer. Among them, the prevalent chromosome 17p deletions are associated with poor prognosis and can promote tumorigenesis more than TP53 loss. Here, we use multiple functional genetic strategies and identify a new 17p tumor suppressor gene (TSG), plant homeodomain finger protein 23 (PHF23). Its deficiency impairs B-cell differentiation and promotes immature B-lymphoblastic malignancy. Mechanistically, we demonstrate that PHF23, an H3K4me3 reader, directly binds the SIN3-HDAC complex through its N-terminus and represses its deacetylation activity on H3K27ac. Thus, the PHF23-SIN3-HDAC (PSH) complex coordinates these two major active histone markers for the activation of downstream TSGs and differentiation-related genes. Furthermore, dysregulation of the PSH complex is essential for the development and maintenance of PHF23-deficient and 17p-deleted tumors. Hence, our study reveals a novel epigenetic regulatory mechanism that contributes to the pathology of 17p-deleted cancers and suggests a susceptibility in this disease. SIGNIFICANCE: We identify PHF23, encoding an H3K4me3 reader, as a new TSG on chromosome 17p, which is frequently deleted in human cancers. Mechanistically, PHF23 forms a previously unreported histone-modifying complex, the PSH complex, which regulates gene activation through a synergistic link between H3K4me3 and H3K27ac.This article is highlighted in the In This Issue feature, p. 1.

Clinical, Genomic, and Pharmacological Study of MYCN-Amplified RB1 Wild-Type Metastatic Retinoblastoma

An uncommon subgroup of unilateral retinoblastomas with highly aggressive histological features, lacking aberrations in RB1 gene with high-level amplification of MYCN (MCYNamplRB1+/+) has only been described as intra-ocular cases treated with initial enucleation. Here, we present a comprehensive clinical, genomic, and pharmacological analysis of two cases of MCYNamplRB1+/+ with orbital and cervical lymph node involvement, but no central nervous system spread, rapidly progressing to fatal disease due to chemoresistance. Both patients showed in common MYCN high amplification and chromosome 16q and 17p loss. A somatic mutation in TP53, in homozygosis by LOH, and high chromosomal instability leading to aneuploidy was identified in the primary ocular tumor and sites of dissemination of one patient. High-throughput pharmacological screening was performed in a primary cell line derived from the lymph node dissemination of one case. This cell line showed resistance to broad spectrum chemotherapy consistent with the patient's poor response but sensitivity to the synergistic effects of panobinostat-bortezomib and carboplatin-panobinostat associations. From these cells we established a cell line derived xenograft model that closely recapitulated the tumor dissemination pattern of the patient and served to evaluate whether triple chemotherapy significantly prolonged survival of the animals. We report novel genomic alterations in two cases of metastatic MCYNamplRB1+/+ that may be associated with chemotherapy resistance and in vitro/in vivo models that serve as basis for tailoring therapy in these cases.

Retinoblastoma: Etiology, Modeling, and Treatment

Retinoblastoma is a retinal cancer that is initiated in response to biallelic loss of RB1 in almost all cases, together with other genetic/epigenetic changes culminating in the development of cancer. RB1 deficiency makes the retinoblastoma cell-of-origin extremely susceptible to cancerous transformation, and the tumor cell-of-origin appears to depend on the developmental stage and species. These are important to establish reliable preclinical models to study the disease and develop therapies. Although retinoblastoma is the most curable pediatric cancer with a high survival rate, advanced tumors limit globe salvage and are often associated with high-risk histopathological features predictive of dissemination. The advent of chemotherapy has improved treatment outcomes, which is effective for globe preservation with new routes of targeted drug delivery. However, molecularly targeted therapeutics with more effectiveness and less toxicity are needed. Here, we review the current knowledge concerning retinoblastoma genesis with particular attention to the genomic and transcriptomic landscapes with correlations to clinicopathological characteristics, as well as the retinoblastoma cell-of-origin and current disease models. We further discuss current treatments, clinicopathological correlations, which assist in guiding treatment and may facilitate globe preservation, and finally we discuss targeted therapeutics for future treatments.

Analysis of the p53 pathway in peripheral blood of retinoblastoma patients; potential biomarkers

Loss of retinoblastoma (RB) function in the cone cells during retina development is necessary but not sufficient for retinoblastoma development. It has been reported that in the absence of RB activity, a retinoma is generated, and the onset of retina cancer occurs until the p53 pathway is altered. Unlike other types of cancer, in retinoblastoma the p53 tumour suppressor is mostly wild type, although its two primary regulators, MDMX and MDM2, are commonly dysregulated. A mutated RB form is inherited in around 35% of the cases, but normally two, somatic mutations are needed to alter the RB function. Here we investigated the mRNA levels of RB, p53, MDMX and MDM2 in peripheral blood samples of retinoblastoma patients to monitor the pathway status of p53 in somatic cells. We sought to investigate the involvement of these genes in the development of retina cancer, with the aim of identifying biomarkers for early diagnosis of this disease.

Genome-scale CRISPR-Cas9 screen identifies druggable dependencies in TP53 wild-type Ewing sarcoma

Stolte et al. use genome-scale CRISPR-Cas9 screening technology to identify druggable targets for TP53 wild-type Ewing sarcoma and discover reactivation of p53 through inhibition of MDM2, MDM4, Wip1, or USP7 as therapeutic strategies for the disease.

Ewing sarcoma is a pediatric cancer driven by EWS-ETS transcription factor fusion oncoproteins in an otherwise stable genomic background. The majority of tumors express wild-type TP53, and thus, therapies targeting the p53 pathway would benefit most patients. To discover targets specific for TP53 wild-type Ewing sarcoma, we used a genome-scale CRISPR-Cas9 screening approach and identified and validated MDM2, MDM4, USP7, and PPM1D as druggable dependencies. The stapled peptide inhibitor of MDM2 and MDM4, ATSP-7041, showed anti-tumor efficacy in vitro and in multiple mouse models. The USP7 inhibitor, P5091, and the Wip1/PPM1D inhibitor, GSK2830371, decreased the viability of Ewing sarcoma cells. The combination of ATSP-7041 with P5091, GSK2830371, and chemotherapeutic agents showed synergistic action on the p53 pathway. The effects of the inhibitors, including the specific USP7 inhibitor XL-188, were rescued by concurrent TP53 knockout, highlighting the essentiality of intact p53 for the observed cytotoxic activities.

UHRF1 depletion sensitizes retinoblastoma cells to chemotherapeutic drugs via downregulation of XRCC4

UHRF1 (ubiquitin-like with PHD and ring finger domains 1) is highly expressed in various human cancers including retinoblastoma, and associated with tumor-promoting effects such as inhibition of apoptosis and high proliferation. However, the molecular mechanisms underlying tumor-promoting functions of UHRF1 in retinoblastoma still remain elusive. Here, we show that stable knockdown of UHRF1 renders retinoblastoma cells sensitized to conventional chemotherapeutic drugs such as etoposide and camptothecin, resulting in enhanced DNA damage and apoptotic cell death. We found that UHRF1-depleted retinoblastoma cells can recognize DNA damages normally but have markedly low expression of XRCC4 (X-ray repair cross complementing 4) among the components of nonhomologous end-joining (NHEJ) repair complex. Conversely, overexpression of UHRF1 increased the XRCC4 expression and stable knockdown of XRCC4 sensitized retinoblastoma cells to etoposide treatment, suggesting that XRCC4 is a key mediator for the drug sensitivity upon UHRF1 depletion in retinoblastoma cells. Consistent with the findings, chromatin association of DNA ligase IV in response to acute DNA damage was found to be significantly reduced in UHRF1-depleted retinoblastoma cells and functional complementation for XRCC4 in UHRF1-depleted cells attenuated the drug sensitivity, demonstrating that XRCC4 downregulation in UHRF1-depleted cells impaired DNA repair and consequently induced robust apoptosis upon genotoxic drug treatment. In human primary retinoblastoma, high expression of UHRF1 and XRCC4 could be detected, and elevated XRCC4 expression correlated with reduced apoptosis markers, implying that UHRF1-mediated XRCC4 upregulation under pathophysiological conditions triggered by RB1 gene inactivation may confer protection against endogenous DNA damages that arise during retinoblastoma development. Taken together, these results present a new mechanistic insight into how UHRF1 mediates its tumor-promoting functions in retinoblastoma, and also provide a basis for UHRF1 targeting to improve the efficacy of current chemotherapy for retinoblastoma treatment.

Heterogeneity in retinoblastoma: a tale of molecules and models

Retinoblastoma, an intraocular pediatric cancer, develops in the embryonic retina following biallelic loss of RB1. However, there is a wide range of genetic and epigenetic changes that can affect RB1 resulting in different clinical outcomes. In addition, other transformations, such as MYCN amplification, generate particularly aggressive tumors, which may or may not be RB1 independent. Recognizing the cellular characteristics required for tumor development, by identifying the elusive cell-of-origin for retinoblastoma, would help us understand the development of these tumors. In this review we summarize the heterogeneity reported in retinoblastoma on a molecular, cellular and tissue level. We also discuss the challenging heterogeneity in current retinoblastoma models and suggest future platforms that could contribute to improved understanding of tumor initiation, progression and metastasis in retinoblastoma, which may ultimately lead to more patient-specific treatments.

MDM2 but not MDM4 promotes retinoblastoma cell proliferation through p53-independent regulation of MYCN translation

Retinoblastomas can arise from cone photoreceptor precursors in response to the loss of pRB function. Cone precursor-specific circuitry cooperates with pRB loss to initiate this process and subsequently contributes to the malignancy. Intrinsic high-level MDM2 expression is a key component of the cone precursor circuitry and is thought to inactivate p53-mediated tumor surveillance, which could otherwise be induced in response to pRB loss. However, the MDM2-related MDM4 has also been proposed to abrogate p53-mediated tumor surveillance in the absence of detectable MDM2 in retinoblastoma cells, bringing into question the importance of high-level MDM2 versus MDM4 expression. Here we report that high-level MDM2 but not MDM4 has a consistent critical role in retinoblastoma cell proliferation in vitro, as well as in orthotopic xenografts. Reduction of either MDM2 or MDM4 weakly induced p53, yet reduction of MDM2 but not MDM4 severely impaired proliferation and survival through a p53-independent mechanism. Specifically, MDM2 upregulated the mRNA expression and translation of another component of the cone circuitry, MYCN, in retinoblastoma cells. Moreover, MYCN was essential to retinoblastoma cell growth and tumor formation, and ectopic MYCN partially reversed the effects of MDM2 depletion, indicating that MYCN is an important MDM2 target. These findings indicate that high-level MDM2 expression is needed in order to perform a critical p53-independent function and may obviate the need for genomic alterations to the p53 pathway during retinoblastoma tumorigenesis.

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.

Association of p53 rs1042522, MDM2 rs2279744, and p21 rs1801270 polymorphisms with retinoblastoma risk and invasion in a Chinese population

Single nucleotide polymorphisms (SNPs) of p53 rs1042522, MDM2 rs2279744 and p21 rs1801270, all in the p53 pathway, which plays a crucial role in DNA damage and genomic instability, were reported to be associated with cancer risk and pathologic characteristics. This case-control study was designed to analyse the association between these SNPs and retinoblastoma (RB) in a Chinese Han population. These SNPs in 168 RB patients and 185 adult controls were genotyped using genomic DNA from venous blood. No significant difference was observed in allele or genotypic frequencies of these SNPs between Chinese RB patients and controls (all P > 0.05). However, the rs1042522 GC genotype showed a protective effect against RB invasion, as demonstrated by event-free survival (HR = 0.53, P = 0.007 for GC versus GG/CC). This effect was significant for patients with a lag time >1 month and no pre-enucleation treatment (P = 0.007 and P = 0.010, respectively), indicating an interaction between p53 rs1042522 and clinical characteristics, including lag time and pre-enucleation treatment status. Thus, the rs1042522 SNP may be associated with RB invasion in the Han Chinese population; however, further large and functional studies are needed to assess the validity of this association.

Genetics and epigenetics of human retinoblastoma

Retinoblastoma is a pediatric tumor of the developing retina from which the genetic basis for cancer development was first described. Inactivation of both copies of the RB1 gene is the predominant initiating genetic lesion in retinoblastoma and is rate limiting for tumorigenesis. Recent whole-genome sequencing of retinoblastoma uncovered a tumor that had no coding-region mutations or focal chromosomal lesions other than in the RB1 gene, shifting the paradigm in the field. The retinoblastoma genome can be very stable; therefore, epigenetic deregulation of tumor-promoting pathways is required for tumorigenesis. This review highlights the genetic and epigenetic changes in retinoblastoma that have been reported, with special emphasis on recent whole-genome sequencing and epigenetic analyses that have identified novel candidate genes as potential therapeutic targets.

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.

Nuclear expression of p53 in mature tumor endothelium of retinoblastoma

The present study aimed to investigate the p53 expression pattern in tumor cells and in mature tumor vascular endothelium of retinoblastoma. Nuclear p53 accumulation was observed in most of the tumor cells in both the human and orthotopic retinoblastoma animal models using SNUOT-Rb1 and Y79 cells. In the orthotopic animal model, some of the tumor vascular endothelium also demonstrated nuclear p53 immunoreactivity, and the ratio of p53 positivity among the total mature tumor vascular endothelium was slightly higher in the Y79 cell model when compared with the SNUOT-Rb1 cell model. In addition, in the human retinoblastoma specimens, 32.9% of the tumor vascular endothelium showed p53 nuclear staining. In conclusion, some of the mature tumor vascular endothelium in both the human and orthotopic models of retinoblastoma share the same cytogenetic abnormality (an abnormal nuclear accumulation of p53) with retinoblastoma cells.

The Mdm network and its regulation of p53 activities: a rheostat of cancer risk

The potent transcriptional activity of p53 (Trp53, TP53) must be kept in check for normal cell growth and survival. Tumors, which drastically deviate from these parameters, have evolved multiple mechanisms to inactivate TP53, the most prevalent of which is the emergence of TP53 missense mutations, some of which have gain-of-function activities. Another important mechanism by which tumors bypass TP53 functions is via increased levels of two TP53 inhibitors, MDM2, and MDM4. Studies in humans and in mice reveal the complexity of TP53 regulation and the exquisite sensitivity of this pathway to small changes in regulation. Here, we summarize the factors that impinge on TP53 activity and thus cell death/arrest or tumor development.

Pharmacologic activation of wild-type p53 by nutlin therapy in childhood cancer

A peculiar feature of several types of childhood cancer is that loss-of-function mutations of the TP53 (p53) tumor suppressor gene are uncommon, in contrast to many adult tumors. As p53 needs to be inactivated in order for tumor cells to survive and thrive, pediatric tumors typically make use of other mechanisms to keep p53 in check. One of the critical negative regulators of p53 is the MDM2 oncoprotein. Many anticancer drug development efforts in the past decade have therefore been devoted to the discovery and optimization of small molecules that selectively disrupt the interaction between MDM2 and p53, which could provide, in principle, a potent means to restore p53 function in tumor cells with wild-type p53. The nutlins are the class of selective inhibitors of the p53-MDM2 interaction that are currently most advanced in their clinical development. We review here the preclinical data that support the potential therapeutic use of nutlin drugs in the treatment of various pediatric tumors, including neuroblastoma, retinoblastoma, osteosarcoma, Ewing's sarcoma, rhabdomyosarcoma, medulloblastoma, and childhood acute lymphoblastic leukemia.

The components of Drosophila histone chaperone dCAF-1 are required for the cell death phenotype associated with rbf1 mutation

A Polycomb group protein, Posterior sex combs (Psc), was identified in a genetic screen designed to find factors that can specifically induce morphological defects in rbf1 mutant eyes. We discovered that rbf1 mutations enhance developmental phenotypes caused by Psc overexpression such as ectopic cell death and disorganized ommatidia. Our genetic analysis revealed that Psc-induced developmental defects are strongly influenced by CAF1p55, which is a shared component of several chromatin-associated complexes including a histone chaperone complex, chromatin assembly factor-1 (dCAF-1). Interestingly, the expression levels of dCAF-1 components, CAF1p105 and CAF1p180, are increased in rbf1 mutants, whereas the expression level of CAF1p55 itself remains relatively unchanged. We demonstrated that the increased levels of CAF1p105 and CAF1p180 are required for the hypersensitivity of rbf1 mutant cells to Psc-induced cell death and for the developmentally regulated cell death normally observed in rbf1 mutant eyes. We propose that Caf1p105 and Caf1p180 are important determinants of cell death sensitivity in rbf1 mutant cells and contribute to the genetic interaction between Psc and rbf1.

Polymorphisms of CDKN1A gene and risk of retinoblastoma

Retinoblastoma (RB) is a malignant neoplasia that occurs mostly in children under 5 years. Recently, CDKN1A gene has been shown to be up-regulated in a context of loss of function of pRb. This gene encodes the p21 protein, which is the bona fide effector of p53. We hypothesized whether two putatively functional single nucleotide polymorphisms (SNPs) of CDKN1A (rs1801270 C>A and rs1059234 C>T) may influence the risk and/or survival of RB patients. We genotyped both SNPs in 141 RB patients and 120 unrelated healthy individuals. Statistical analyses consisted of chi-square (χ(2)), odds ratio (OR) and survival curves by Kaplan-Meier method. We found that patients who carry the genotype CA for rs1801270 and CT for rs1059234 were associated to an increased risk of RB [OR = 2.5, 95% confidence interval (CI) = 1.38-4.53], whereas patients with CC for both polymorphisms were associated to a lower risk of developing RB (OR = 0.43, 95% CI = 0.25-0.74). On the other hand, Kaplan-Meier curves did not show statistically significant differences in survival among the studied polymorphisms. We conclude that the minor alleles of rs1801270 and rs1059234 polymorphisms may act as risk factors for the development of RB in our sample.

SUMMARY

The minor alleles of polymorphisms rs1801270 C>A and rs1059234 C>T in CDKN1A (p21) gene may act as risk factors for the development of RB; however, they do not seem to influence overall survival.

The genomic landscape of retinoblastoma: a review

Retinoblastoma is a paediatric ocular tumour that continues to reveal much about the genetic basis of cancer development. Study of genomic aberrations in retinoblastoma tumours has exposed important mechanisms of cancer development and identified oncogenes and tumour suppressors that offer potential points of therapeutic intervention. The recent development of next-generation genomic technologies has allowed further refinement of the genomic landscape of retinoblastoma at high resolution. In a relatively short period of time, a wealth of genetic and epigenetic data has emerged on a small number of tumour samples. These data highlight the inherent molecular complexity of this cancer despite the fact that most retinoblastomas are initiated by the inactivation of a single tumour suppressor gene. This review outlines the current understanding of the genomic, genetic and epigenetic changes in retinoblastoma, highlighting recent genome-wide analyses that have identified exciting candidate genes worthy of further validation as potential prognostic and therapeutic targets.

Analysis of MDM2 and MDM4 single nucleotide polymorphisms, mRNA splicing and protein expression in retinoblastoma

Retinoblastoma is a childhood cancer of the developing retina that begins in utero and is diagnosed in the first years of life. Biallelic RB1 gene inactivation is the initiating genetic lesion in retinoblastoma. The p53 gene is intact in human retinoblastoma but the pathway is believed to be suppressed by increased expression of MDM4 (MDMX) and MDM2. Here we quantify the expression of MDM4 and MDM2 mRNA and protein in human fetal retinae, primary retinoblastomas, retinoblastoma cell lines and several independent orthotopic retinoblastoma xenografts. We found that MDM4 is the major p53 antagonist expressed in retinoblastoma and in the developing human retina. We also discovered that MDM4 protein steady state levels are much higher in retinoblastoma than in human fetal retinae. This increase would not have been predicted based on the mRNA levels. We explored several possible post-transcriptional mechanisms that may contribute to the elevated levels of MDM4 protein. A proportion of MDM4 transcripts are alternatively spliced to produce protein products that are reported to be more stable and oncogenic. We also discovered that a microRNA predicted to target MDM4 (miR191) was downregulated in retinoblastoma relative to human fetal retinae and a subset of samples had somatic mutations that eliminated the miR-191 binding site in the MDM4 mRNA. Taken together, these data suggest that post-transcriptional mechanisms may contribute to stabilization of the MDM4 protein in retinoblastoma.

Cooperation between Rb and Arf in suppressing mouse retinoblastoma

Retinoblastoma is a pediatric cancer that has served as a paradigm for tumor suppressor gene function. Retinoblastoma is initiated by RB gene mutations, but the subsequent cooperating mutational events leading to tumorigenesis are poorly characterized. We investigated what these additional genomic alterations might be using human retinoblastoma samples and mouse models. Array-based comparative genomic hybridization studies revealed deletions in the CDKN2A locus that include ARF and P16INK4A, both of which encode tumor suppressor proteins, in both human and mouse retinoblastoma. Through mouse genetic analyses, we found that Arf was the critical tumor suppressor gene in the deleted region. In mice, inactivation of one allele of Arf cooperated with Rb and p107 loss to rapidly accelerate retinoblastoma, with frequent loss of heterozygosity (LOH) at the Arf locus. Arf has been reported to exhibit p53-independent tumor suppressor roles in other systems; however, our results showed no additive effect of p53 and Arf coinactivation in promoting retinoblastoma. Moreover, p53 inactivation completely eliminated any selection for Arf LOH. Thus, our data reveal important insights into the p53 pathway in retinoblastoma and show that Arf is a key collaborator with Rb in retinoblastoma suppression.

Epigenetic and copy number variation analysis in retinoblastoma by MS-MLPA

Retinoblastoma is the most common primary intraocular malignancy in children. Two step inactivation of RB1 (M1-M2) represents the key event in the pathogenesis of retinoblastoma but additional genetic and epigenetic events (M3-Mn) are required for tumor development. In the present study, we employed Methylation Specific Multiplex Ligation Probe Assay to investigate methylation status and copy number changes of 25 and 39 oncosuppressor genes, respectively. This technique was applied to analyse 12 retinoblastomas (5 bilateral and 7 unilateral) and results were compared to corresponding normal retina. We identified hypermethylation in seven new genes: MSH6 (50%), CD44 (42%), PAX5 (42%), GATA5 (25%), TP53 (8%), VHL (8%) and GSTP1 (8%) and we confirmed the previously reported hypermethylation of MGMT (58%), RB1 (17%) and CDKN2 (8%). These genes belong to key pathways including DNA repair, pRB and p53 signalling, transcriptional regulation, protein degradation, cell-cell interaction, cellular adhesion and migration. In the same group of retinoblastomas, a total of 29 copy number changes (19 duplications and 10 deletions) have been identified. Interestingly, we found deletions of the following oncosuppressor genes that might contribute to drive retinoblastoma tumorigenesis: TP53, CDH13, GATA5, CHFR, TP73 and IGSF4. The present data highlight the importance of epigenetic changes in retinoblastoma and indicate seven hypermethylated oncosuppressors never associated before to retinoblastoma pathogenesis. This study also confirms the presence of copy number variations in retinoblastoma, expecially in unilateral cases (mean 3 ± 1.3) where these changes were found more frequently respect to bilateral cases (mean 1.4 ± 1.1).

Childhood cancer and developmental biology a crucial partnership

For many years, there were relatively few research efforts that bridged the fields of developmental biology and cancer genetics. However, in the past decade, we have witnessed a dramatic shift and now these two fields are intertwined. Part of the impetus for this transition came from the discovery that regulatory pathways that were previously thought to be uniquely important for developmental processes were also perturbed in cancer. In addition, the conceptual framework for understanding how cells self-renew or undergo unidirectional changes in competence during development has proven to be very useful in cancer biology as researchers explore tumor initiation and progression. Finally, a deeper understanding of the process of terminal differentiation and how that relates to cellular plasticity may have important implications for both cancer biology and developmental biology. Here we highlight some of the important connections between developmental neurobiology and cancer biology in retinoblastoma. By bridging these fields, important advances have been made in modeling retinoblastoma in mice, elucidating the cell-of-origin for retinoblastoma and identifying novel therapeutic approaches.

Retinoblastoma has properties of a cone precursor tumor and depends upon cone-specific MDM2 signaling

Retinoblastomas result from the inactivation of the RB1 gene and the loss of Rb protein, yet the cell type in which Rb suppresses retinoblastoma and the circuitry that underlies the need for Rb are undefined. Here, we show that retinoblastoma cells express markers of postmitotic cone precursors but not markers of other retinal cell types. We also demonstrate that human cone precursors prominently express MDM2 and N-Myc, that retinoblastoma cells require both of these proteins for proliferation and survival, and that MDM2 is needed to suppress ARF-induced apoptosis in cultured retinoblastoma cells. Interestingly, retinoblastoma cell MDM2 expression was regulated by the cone-specific RXRgamma transcription factor and a human-specific RXRgamma consensus binding site, and proliferation required RXRgamma, as well as the cone-specific thyroid hormone receptor-beta2. These findings provide support for a cone precursor origin of retinoblastoma and suggest that human cone-specific signaling circuitry sensitizes to the oncogenic effects of RB1 mutations.

Expression of p14ARF, MDM2, and MDM4 in human retinoblastoma

It is still not clear whether the p53 pathway is altered in retinoblastoma development. We assessed the expression of the p53 pathway genes p14(ARF), mouse double minute 2 (MDM2), and mouse double minute 4 (MDM4) in human retinoblastoma compared to normal retina. Primary human retinoblastomas, retinoblastoma cell lines and normal retinas were assessed for p14(ARF) and MDM4 mRNA by quantitative RT-PCR. p14(ARF), MDM2, and MDM4 protein were measured by immunoblot and immunohistochemistry. Compared to retina, p14(ARF) mRNA expression was notably increased in retinoblastoma but p14(ARF) protein was undetectable. MDM2 and MDM4 proteins were expressed in 22/22 retinoblastomas. MDM2 was expressed in 3/10 retinas tested, and MDM4 in 10/10 retinas. The expression level of MDM2 protein in retinoblastomas and retina was comparable, while MDM4 protein was overexpressed in one retinoblastoma cell line Y79 and two primary retinoblastomas. We observe that overexpression of MDM2 and MDM4 is not a necessary step in retinoblastoma development. However, loss of detectable p14(ARF) protein and resultant lack of functional inactivation of these p53 inhibitors may contribute to retinoblastoma development by constitutive inhibition of p53.

One hit, two hits, three hits, more? Genomic changes in the development of retinoblastoma

The childhood eye cancer retinoblastoma is initiated by the loss of both alleles of the prototypic tumor suppressor gene, RB1. However, a large number of cytogenetic and comparative genomic hybridization (CGH) studies have shown that these M1 and M2 mutational events--although necessary for initiation--are not the only genomic changes in retinoblastoma. Some of these subsequent changes, which we have termed M3 to Mn, are likely crucial for tumor progression not only in retinoblastoma but also in other cancers. Moreover, genes showing genomic change in cancer are more stable markers and, therefore, possible therapeutic targets than genes simply differentially expressed. In this review, we provide the first comprehensive summary of the genomic evidence implicating gain of 1q, 2p, 6p, and 13q, and loss of 16q in retinoblastoma oncogenesis, including karyotype, CGH, and microarray CGH data. We discuss the search for candidate oncogenes and tumor suppressor genes within these regions, including the candidates (KIF14, MDM4, MYCN, E2F3, DEK, CDH11, and others), plus associations between genomic changes and clinical parameters. We also review studies of other regions of the retinoblastoma genome, the epigenetic changes of aberrant methylation of MGMT, RASSF1A, CASP8, and MLH1, and the roles microRNAs might play in this cancer. Although many candidate genes have yet to be functionally validated in retinoblastoma, work in this field lays out a molecular cytogenetic pathway of retinoblastoma development. Candidate cancer genes carry diagnostic, prognostic, and therapeutic implications beyond retinoblastoma.

Neuronal differentiation and synaptogenesis in retinoblastoma

Retinoblastomas initiate in the developing retina in utero and are diagnosed during the first few years of life. We have recently generated a series of knockout mouse models of retinoblastoma that recapitulate the timing, location, and progression of human retinoblastoma. One of the most important benefits of these preclinical models is that we can study the earliest stages of tumor initiation and expansion. This is not possible in human retinoblastoma because tumors initiate in utero and are not diagnosed until they are at an advanced stage. We found that mouse retinoblastoma cells exhibit a surprising degree of differentiation, which has not been previously reported for any neural tumor. Early-stage mouse retinoblastoma cells express proteins found normally in retinal plexiform layers. They also extend neurites and form synapses. All of these features, which were characterized by immunostaining, Golgi-Cox staining, scanning electron microscopy, and transmission electron microscopy, suggest that mouse retinoblastoma cells resemble amacrine/horizontal cells from the retina. As late-stage retinoblastoma cells expand and invade the surrounding tissue, they lose their differentiated morphology and become indistinguishable from human retinoblastomas. Taken together, our data suggest that neuronal differentiation is a hallmark of early-stage retinoblastoma and is lost as cells become more aggressive and invasive. We also show that rosette formation is not a hallmark of retinoblastoma differentiation, as previously believed. Instead, rosette formation reflects extensive cell-cell contacts between retinoblastoma cells in both early-stage (differentiated) and late-stage (dedifferentiated) tumors.

RB1 and TP53 pathways in radiation-induced sarcomas

The tumour suppressor genes, TP53 and RB1, and four genes involved in their regulation, INK4a, ARF, MDM2 and MDMX, were analysed in a series of 36 post-radiotherapy radiation-induced sarcomas. One-third of the tumours developed in patients carrying a germline mutation of RB1 that predisposed them to retinoblastoma and radiation-induced sarcomas. The genetic inactivation of RB1 and/or TP53 genes was frequently observed in these sarcomas. These inactivations were owing to an interplay between point mutations and losses of large chromosome segments. Radiation-induced somatic mutations were observed in TP53, but not in RB1 or in the four other genes, indicating an early role of TP53 in the radio-sarcomagenesis. RB1 and TP53 genes were biallelically coinactivated in all sarcomas developing in the context of the predisposition, indicating that both genes played a major role in the formation of these sarcomas. In the absence of predisposition, TP53 was biallelically inactivated in one-third of the sarcomas, whereas at least one allele of RB1 was wild type. In both genetic contexts, the TP53 pathway was inactivated by genetic lesions and not by the activation of the ARF/MDM2/MDMX pathway, as recently shown in retinoblastomas. Together, these findings highlight the intricate tissue- and aetiology-specific relationships between TP53 and RB1 pathways in tumorigenesis.

MDMX: from bench to bedside

The tumor suppressor protein p53 is negatively regulated by Mdm2, a ubiquitin ligase protein that targets p53 for degradation. Mdmx (also known as Mdm4) is a relative of Mdm2 that was identified on the basis of its ability to physically interact with p53. An increasing body of evidence, including recent genetic studies, suggests that Mdmx also acts as a key negative regulator of p53. Aberrant expression of MDMX could thus contribute to tumor formation. Indeed, MDMX amplification and/or overexpression occurs in several diverse tumors. Strikingly, recent work identifies MDMX as a specific chemotherapeutic target for treatment of retinoblastoma. Specific MDMX antagonists should therefore be developed as a tool to ensure activation of `dormant' p53 activity in tumors that retain wild-type p53.

Inactivation of the p53 pathway in retinoblastoma

Most human tumours have genetic mutations in their Rb and p53 pathways, but retinoblastoma is thought to be an exception. Studies suggest that retinoblastomas, which initiate with mutations in the gene retinoblastoma 1 (RB1), bypass the p53 pathway because they arise from intrinsically death-resistant cells during retinal development. In contrast to this prevailing theory, here we show that the tumour surveillance pathway mediated by Arf, MDM2, MDMX and p53 is activated after loss of RB1 during retinogenesis. RB1-deficient retinoblasts undergo p53-mediated apoptosis and exit the cell cycle. Subsequently, amplification of the MDMX gene and increased expression of MDMX protein are strongly selected for during tumour progression as a mechanism to suppress the p53 response in RB1-deficient retinal cells. Our data provide evidence that the p53 pathway is inactivated in retinoblastoma and that this cancer does not originate from intrinsically death-resistant cells as previously thought. In addition, they support the idea that MDMX is a specific chemotherapeutic target for treating retinoblastoma.

Retinoblastoma tumor suppressor and genome stability

Retinoblastoma gene (Rb) is the prototype of tumor suppressors. Germline mutation in the retinoblastoma gene is susceptible to cancer and reintroduction of wild-type Rb is able to suppress neoplastic phenotypes. The fundamental cellular functions of Rb in the control of cell growth and differentiation are important for its tumor suppression. In general, cancer susceptibility caused by inactivation of a tumor suppressor gene results from genome instability. Accordingly, Rb may function in the maintenance of chromosome stability by influencing mitotic progression, faithful chromosome segregation, and structural remodeling of mitotic chromosomes. Rb is also implicated in the regulation of replication machinery and in the control of cell cycle checkpoints in response to DNA damage, further supporting such a role for Rb. Moreover, the mechanistic basis for Rb-mediated transcriptional repression has revealed its connection to global chromatin remodeling. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities, and a theme throughout its multiple cellular functions is its central role in controlling activities that involve chromatin remodeling. A model in which Rb controls global genome fluidity is thus proposed. Finally, a recent study provides direct evidence indicating that loss of Rb function leads to genome instability. Therefore, tumor suppressors have a common role in the maintenance of genome stability, and such a role may be pivotal for their functions in tumor suppression.

Constitutive nuclear factor-kappaB activity is crucial for human retinoblastoma cell viability

Retinoblastoma (Rb) is the most common intraocular malignancy of childhood. Although systemic and intrathecal chemotherapy with local and cranial radiotherapy have improved overall survival, the prognosis for patients with central nervous system involvement is still poor. We investigated the role of the transcription factor nuclear factor (NF)-kappaB, which promotes cell survival in several other models, in the pathophysiology of Rb. The human Rb cell lines Y79 and WERI-Rb1 were treated with the cell permeable peptide SN50, that specifically inhibits the transcriptional activity of NF-kappaB by blocking its translocation into the nucleus. We found that NF-kappaB inhibition up-regulated Bax; down-regulated the anti-apoptotic proteins Bcl-2, A1, and cIAP-2; and induced loss of the mitochondrial transmembrane potential and caspase-independent, calpain-dependent apoptosis in Rb cells. Inhibition of the p38 kinase sensitized cells to SN50-induced cell death, whereas insulin-like growth factor-1 activated NF-kappaB and attenuated the proapoptotic effect of SN50. Finally, NF-kappaB inhibition sensitized Rb cells to doxorubicin. In conclusion, inhibition of NF-kappaB activity in Rb cells leads to loss of mitochondrial transmembrane potential and caspase-independent, calpain-dependent apoptosis. Therapeutic strategies targeting NF-kappaB could be beneficial in the clinical management of Rb, either alone or in combination with conventional chemotherapy.

The developmental biology of brain tumors

Tumors of the central nervous system (CNS) can be devastating because they often affect children, are difficult to treat, and frequently cause mental impairment or death. New insights into the causes and potential treatment of CNS tumors have come from discovering connections with genes that control cell growth, differentiation, and death during normal development. Links between tumorigenesis and normal development are illustrated by three common CNS tumors: retinoblastoma, glioblastoma, and medulloblastoma. For example, the retinoblastoma (Rb) tumor suppressor protein is crucial for control of normal neuronal differentiation and apoptosis. Excessive activity of the epidermal growth factor receptor and loss of the phosphatase PTEN are associated with glioblastoma, and both genes are required for normal growth and development. The membrane protein Patched1 (Ptc1), which controls cell fate in many tissues, regulates cell growth in the cerebellum, and reduced Ptc1 function contributes to medulloblastoma. Just as elucidating the mechanisms that control normal development can lead to the identification of new cancer-related genes and signaling pathways, studies of tumor biology can increase our understanding of normal development. Learning that Ptc1 is a medulloblastoma tumor suppressor led directly to the identification of the Ptc1 ligand, Sonic hedgehog, as a powerful mitogen for cerebellar granule cell precursors. Much remains to be learned about the genetic events that lead to brain tumors and how each event regulates cell cycle progression, apoptosis, and differentiation. The prospects for beneficial work at the boundary between oncology and developmental biology are great.

Early diagnosis of subependymal giant cell astrocytoma in children with tuberous sclerosis

OBJECTIVES

Intraventricular astrocytomas (subependymal giant cell astrocytomas) of tuberous sclerosis have a poor prognosis due to the obstruction of CSF flow. The aim of this study was to determine whether they could be differentiated during childhood and at an early preclinical stage, from subependymal nodules without any growing potential.

METHODS

The first two MRIs of all children referred to this neuropaediatric centre between 1987 and 1996 were retrospectively blindly reviewed.

RESULTS

Out of 60 patients, 24 disclosed subependymal nodules localised near the foramen of Monro, and eight of the 24 developed astrocytomas. Subependymal nodules were first detectable on MRI from 1 year of age in all cases and the first MRI evidence of growth occurred between 1 and 9 years (mean 4 years). At an early stage, subependymal nodules had different characteristics in patients who developed subependymal giant cell astrocytomas from those who did not. The nodules over 5 mm in diameter that were incompletely calcified and enhanced by gadolinium were at higher risk of growing, particularly in children with a familial history of tuberous sclerosis. To detect the subependymal giant cell astrocytomas earlier in tuberous sclerosis, it is advisible to systematically perform an MRI examination before 2 years of age and to repeat it every year if the patient has risk factors for developing astrocytomas.

Nuclear exclusion of wild-type p53 in immortalized human retinoblastoma cells

BACKGROUND

Retinoblastoma is the most common childhood tumor of the eye, arising from cells that are defective in both copies of the retinoblastoma susceptibility gene (RB1). Most retinoblastoma tumor cells eventually undergo programmed cell death (i.e., apoptosis); however, some cells can acquire the ability to metastasize and become immortal. Transfection of immortal retinoblastoma cells with DNA sequences encoding wild-type p53 protein induces cell death, suggesting that the loss of both RB1 and p53 functions may be required for cell immortalization. We have examined this possibility by characterizing the p53 protein and messenger RNA in six independently isolated, immortalized retinoblastoma cell lines.

METHODS

Western blotting methods were used to assess p53 protein level in each cell line, and Cleavase Fragment-Length Polymorphism analysis of complementary DNAs was used to screen for mutations in p53 messenger RNA. Localization of p53 protein in cells of the immortalized lines and in specimens of retinoblastoma tumors was achieved by means of indirect immunofluorescence and immunocytochemistry, respectively.

RESULTS

All six immortalized cell lines expressed wild-type p53 messenger RNA and high levels of p53 protein. Although p53 is normally a nuclear protein, the p53 in four of the six cell lines was located predominately in the cytoplasm; in the remaining two cell lines, p53 was localized in both the nucleus and the cytoplasm. Cytoplasmic localization of p53 in retinoblastoma tumor specimens was rare and usually restricted to cells that had invaded adjacent ocular tissues, indicative of the early stages of metastasis.

CONCLUSIONS

Some immortalized retinoblastoma cells may exhibit p53 dysfunction through nuclear exclusion of wild-type p53 protein.