Mice deficient for Rb are nonviable and show defects in neurogenesis and haematopoiesis

CRISPR/Cas9-Mediated Knockout of Rb1 in Xenopus tropicalis

At this time, no molecular targeted therapies exist for treatment of retinoblastoma. This can be, in part, attributed to the lack of animal models that allow for both rapid identification of novel therapeutic targets and hypothesis driven drug testing. Within this scope, we have recently reported the first genuine genetic nonmammalian retinoblastoma cancer model within the aquatic model organism Xenopus tropicalis (Naert et al., Sci Rep 6: 35263, 2016). Here we describe the methods to generate rb1 mosaic mutant Xenopus tropicalis by employing the CRISPR/Cas9 technology. In depth, we discuss short guide RNA (sgRNA) design parameters, generation, quality control, quantification, and delivery followed by several methods for assessing genome editing efficiencies. As such the reader should be capable, by minor changes to the methods described here, to (co-) target rb1 or any one or multiple gene(s) within the Xenopus tropicalis genome by multiplex CRISPR/Cas9 methodology.

Developmental stage-specific proliferation and retinoblastoma genesis in RB-deficient human but not mouse cone precursors

Most retinoblastomas initiate in response to the inactivation of the RB1 gene and loss of functional RB protein. The tumors may form with few additional genomic changes and develop after a premalignant retinoma phase. Despite this seemingly straightforward etiology, mouse models have not recapitulated the genetic, cellular, and stage-specific features of human retinoblastoma genesis. For example, whereas human retinoblastomas appear to derive from cone photoreceptor precursors, current mouse models develop tumors that derive from other retinal cell types. To investigate the basis of the human cone-specific oncogenesis, we compared developmental stage-specific cone precursor responses to RB loss in human and murine retina cultures and in cone-specific Rb1-knockout mice. We report that RB-depleted maturing (ARR3⁺) but not immature (ARR3^-) human cone precursors enter the cell cycle, proliferate, and form retinoblastoma-like lesions with Flexner-Wintersteiner rosettes, then form low or nonproliferative premalignant retinoma-like lesions with fleurettes and p16^INK4A and p130 expression, and finally form highly proliferative retinoblastoma-like masses. In contrast, in murine retina, only RB-depleted immature (Arr3^-) cone precursors entered the cell cycle, and they failed to progress from S to M phase. Moreover, whereas intrinsically highly expressed MDM2 and MYCN contribute to RB-depleted maturing (ARR3⁺) human cone precursor proliferation, ectopic MDM2 and Mycn promoted only immature (Arr3^-) murine cone precursor cell-cycle entry. These findings demonstrate that developmental stage-specific as well as species- and cell type-specific features sensitize to RB1 inactivation and reveal the human cone precursors' capacity to model retinoblastoma initiation, proliferation, premalignant arrest, and tumor growth.

Transcription Regulators and Hormones Involved in the Development of Brown Fat and White Fat Browning: Transcriptional and Hormonal Control of Brown/Beige Fat Development

The high prevalence of obesity and related metabolic complications has inspired research on adipose tissues. Three kinds of adipose tissues are identified in mammals: brown adipose tissue (BAT), beige or brite adipose tissue and white adipose tissue (WAT). Beige adipocytes share some characteristics with brown adipocytes such as the expression of UCP1. Beige adipocytes can be activated by environmental stimuli or pharmacological treatment, and this change is accompanied by an increase in energy consumption. This process is called white browning, and it facilitates the maintenance of a lean and healthy phenotype. Thus, promoting beige adipocyte development in WAT shows promise as a new strategy in treating obesity and related metabolic consequences. In this review, we summarized the current understanding of the regulators and hormones that participate in the development of brown fat and white fat browning.

Epigenetic regulators Rbbp4 and Hdac1 are overexpressed in a zebrafish model of RB1 embryonal brain tumor, and are required for neural progenitor survival and proliferation

In this study, we used comparative genomics and developmental genetics to identify epigenetic regulators driving oncogenesis in a zebrafish retinoblastoma 1 (rb1) somatic-targeting model of RB1 mutant embryonal brain tumors. Zebrafish rb1 brain tumors caused by TALEN or CRISPR targeting are histologically similar to human central nervous system primitive neuroectodermal tumors (CNS-PNETs). Like the human oligoneural OLIG2+/SOX10+ CNS-PNET subtype, zebrafish rb1 tumors show elevated expression of neural progenitor transcription factors olig2, sox10, sox8b and the receptor tyrosine kinase erbb3a oncogene. Comparison of rb1 tumor and rb1/rb1 germline mutant larval transcriptomes shows that the altered oligoneural precursor signature is specific to tumor tissue. More than 170 chromatin regulators were differentially expressed in rb1 tumors, including overexpression of chromatin remodeler components histone deacetylase 1 (hdac1) and retinoblastoma binding protein 4 (rbbp4). Germline mutant analysis confirms that zebrafish rb1, rbbp4 and hdac1 are required during brain development. rb1 is necessary for neural precursor cell cycle exit and terminal differentiation, rbbp4 is required for survival of postmitotic precursors, and hdac1 maintains proliferation of the neural stem cell/progenitor pool. We present an in vivo assay using somatic CRISPR targeting plus live imaging of histone-H2A.F/Z-GFP fusion protein in developing larval brain to rapidly test the role of chromatin remodelers in neural stem and progenitor cells. Our somatic assay recapitulates germline mutant phenotypes and reveals a dynamic view of their roles in neural cell populations. Our study provides new insight into the epigenetic processes that might drive pathogenesis in RB1 brain tumors, and identifies Rbbp4 and its associated chromatin remodeling complexes as potential target pathways to induce apoptosis in RB1 mutant brain cancer cells.

This article has an associated First Person interview with the first author of the paper.

Summary: This study shows that chromatin remodelers that are overexpressed in a zebrafish model of RB1 mutant brain cancer are required for neural progenitor proliferation and survival, providing insight into potential mechanisms that drive tumor growth.

Off and back-on again: a tumor suppressor's tale

Tumor suppressor genes play critical roles orchestrating anti-cancer programs that are both context dependent and mechanistically diverse. Beyond canonical tumor suppressive programs that control cell division, cell death, and genome stability, unexpected tumor suppressor gene activities that regulate metabolism, immune surveillance, the epigenetic landscape, and others have recently emerged. This diversity underscores the important roles these genes play in maintaining cellular homeostasis to suppress cancer initiation and progression, but also highlights a tremendous challenge in discerning precise context-specific programs of tumor suppression controlled by a given tumor suppressor. Fortunately, the rapid sophistication of genetically engineered mouse models of cancer has begun to shed light on these context-dependent tumor suppressor activities. By using techniques that not only toggle "off" tumor suppressor genes in nascent tumors, but also facilitate the timely restoration of gene function "back-on again" in disease specific contexts, precise mechanisms of tumor suppression can be revealed in an unbiased manner. This review discusses the development and implementation of genetic systems designed to toggle tumor suppressor genes off and back-on again and their potential to uncover the tumor suppressor's tale.

Placentation defects are highly prevalent in embryonic lethal mouse mutants

Large-scale phenotyping efforts have demonstrated that approximately 25-30% of mouse gene knockouts cause intra-uterine lethality. Analysis of these mutants has largely focussed on the embryo but not the placenta, despite the critical role of this extra-embryonic organ for developmental progression. Here, we screened 103 embryonic lethal and subviable mouse knockout lines from the Deciphering the Mechanisms of Developmental Disorders programme (https://dmdd.org.uk) for placental phenotypes. 68% of lines that are lethal at or after mid-gestation exhibited placental dys-morphologies. Early lethality (E9.5-E14.5) is almost always associated with severe placental malformations. Placental defects strongly correlate with abnormal brain, heart and vascular development. Analysis of mutant trophoblast stem cells and conditional knockouts suggests primary gene function in trophoblast for a significant number of factors that cause embryonic lethality when ablated. Our data highlight the hugely under-appreciated importance of placental defects in contributing to abnormal embryo development and suggest key molecular nodes governing placentation.

The Complex Link between Apoptosis and Autophagy: a Promising New Role for RB

Physiological processes, as autophagy, proliferation and apoptosis are affected during carcinogenesis. Restoring cellular sensitivity to apoptotic stimuli, such as the antineoplastic cocktails, has been explored as a strategy to eliminate cancer cells. Autophagy, a physiological process of recycling organelles and macromolecules can be deviated from homeostasis to support cancer cells survival, proliferation, escape from apoptosis, and therapy resistance. The relationship between autophagy and apoptosis is complex and many stimuli can induce both processes. Most chemotherapeutic agents induce autophagy and it is not clear whether and how this chemotherapy-induced autophagy might contribute to resistance to apoptosis. Here, we review current strategies to sensitize cancer cells by interfering with autophagy. Moreover, we discuss a new link between autophagy and apoptosis: the tumor suppressor retinoblastoma protein (RB). Inactivation of RB is one of the earliest and more frequent hallmarks of cancer transformation, known to control cell cycle progression and apoptosis. Therefore, understanding RB functions in controlling cell fate is essential for an effective translation of RB status in cancer samples to the clinical outcome.

Non-cell autonomous control of precerebellar neuron migration by Slit and Robo proteins

During development, precerebellar neurons migrate tangentially from the dorsal hindbrain to the floor plate. Their axons cross it but their cell bodies stop their ventral migration upon reaching the midline. It has previously been shown that Slit chemorepellents and their receptors, Robo1 and Robo2, might control the migration of precerebellar neurons in a repulsive manner. Here, we have used a conditional knockout strategy in mice to test this hypothesis. We show that the targeted inactivation of the expression of Robo1 and Robo2 receptors in precerebellar neurons does not perturb their migration and that they still stop at the midline. The selective ablation of the expression of all three Slit proteins in floor-plate cells has no effect on pontine neurons and only induces the migration of a small subset of inferior olivary neurons across the floor plate. Likewise, we show that the expression of Slit proteins in the facial nucleus is dispensable for pontine neuron migration. Together, these results show that Robo1 and Robo2 receptors act non-cell autonomously in migrating precerebellar neurons and that floor-plate signals, other than Slit proteins, must exist to prevent midline crossing.

Retinoblastoma 1 protects T cell maturation from premature apoptosis by inhibiting E2F1

T lymphocytes are key cellular components of an acquired immune system and play essential roles in cell-mediated immunity. T cell development occurs in the thymus where 95% of immature thymocytes are eliminated via apoptosis. It is known that mutation of Zeb1, one of the retinoblastoma 1 (Rb1) target genes, results in a decrease in the number of immature T cells in mice. E2F1, an RB1-interacting protein, has been shown to regulate mature T cell development by interfering with thymocyte apoptosis. However, whether Rb1 regulates thymocyte development in vivo still needs to be further investigated. Here, we use a zebrafish model to investigate the role of Rb1 in T cell development. We show that Rb1-deficient fish exhibit a significant reduction in T cell number during early development that it is attributed to the accelerated apoptosis of immature T cells in a caspase-dependent manner. We further show that E2F1 overexpression could mimic the reduced T lymphocytes phenotype of Rb1 mutants, and E2F1 knockdown could rescue the phenotype in Rb1-deficient mutants. Collectively, our data indicate that the Rb1-E2F1-caspase axis is crucial for protecting immature T cells from apoptosis during early T lymphocyte maturation.

Regulation of T3 Availability in the Developing Brain: The Mouse Genetics Contribution

Alterations in maternal thyroid physiology may have deleterious consequences on the development of the fetal brain, but the underlying mechanisms remain elusive, hampering the development of appropriate therapeutic strategies. The present review sums up the contribution of genetically modified mouse models to this field. In particular, knocking out genes involved in thyroid hormone (TH) deiodination, transport, and storage has significantly improved the picture that we have of the economy of TH in the fetal brain and the underlying genetic program. These data pave the way for future studies to bridge the gap in knowledge between thyroid physiology and brain development.

Cell Cycle Regulators and Lineage-Specific Therapeutic Targets for Cushing Disease

Cell cycle proteins are critical to pituitary development, but their contribution to lineage-specific tumorigenesis has not been well-elucidated. Emerging evidence from in vitro human tumor analysis and transgenic mouse models indicates that G1/S-related cell cycle proteins, particularly cyclin E, p27, Rb, and E2F1, drive molecular mechanisms that underlie corticotroph-specific differentiation and development of Cushing disease. The aim of this review is to summarize the literature and discuss the complex role of cell cycle regulation in Cushing disease, with a focus on identifying potential targets for therapeutic intervention in patients with these tumors.

Structural changes of the macula and optic nerve head in the remaining eyes after enucleation for retinoblastoma: an optical coherence tomography study

BACKGROUND

To describe objectively the possible structural changes of the macula and optic nerve head in the free eyes of unilateral cured retinoblastoma patients and, also after enucleation using spectral domain optical coherence tomography.

METHODS

A cross sectional study involving 60 patients subdivided into three groups; 15 unilateral RB patients in whom enucleation was indicated as a sole treatment performed earlier in life [(study group (I)], 15 unilateral RB patients who had completely regressed disease with a preserved eye [(study group (II)] and 30 age and sex matched healthy controls. The remaining and free eyes in study groups and right eyes of control group had full ophthalmological examination, static automated perimetry and optical coherence tomography of the macula and optic nerve head.

RESULTS

In study group (II); a significant thinning of total macula, central fovea, ganglion cell layer (GCL), ganglion cell complex (GCC), and some sectors of outer nuclear layer (P- values ≤0.05) was found with no significant difference in peripapillary nerve fiber layer (pRNFL) thickness and optic nerve head parameters compared to the control group and the study group (I). A significantly thickened total macula, GCL, GCC, and pRNFL in study group (I) compared to study group (II). Thickened pRNFL was significantly correlated to standard automated perimetry pattern deviations. No significant difference was found between study group (I) and control group.

CONCLUSION

Retinoblastoma eyes characterized by thinning of central fovea, GCL, GCC compared to the control group. After unilateral enucleation, increased GCC and pRNFL thicknesses were detected compared to retinoblastoma group.

Defining key concepts of intestinal and epithelial cancer biology through the use of mouse models

Over the past 20 years, huge advances have been made in modelling human diseases such as cancer using genetically modified mice. Accurate in vivo models are essential to examine the complex interaction between cancer cells, surrounding stromal cells, tumour-associated inflammatory cells, fibroblast and blood vessels, and to recapitulate all the steps involved in metastasis. Elucidating these interactions in vitro has inherent limitations, and thus animal models are a powerful tool to enable researchers to gain insight into the complex interactions between signalling pathways and different cells types. This review will focus on how advances in in vivo models have shed light on many aspects of cancer biology including the identification of oncogenes, tumour suppressors and stem cells, epigenetics, cell death and context dependent cell signalling.

Mouse models of metastasis: progress and prospects

Metastasis is the spread of cancer cells from a primary tumor to distant sites within the body to establish secondary tumors. Although this is an inefficient process, the consequences are devastating as metastatic disease accounts for >90% of cancer-related deaths. The formation of metastases is the result of a series of events that allow cancer cells to escape from the primary site, survive in the lymphatic system or blood vessels, extravasate and grow at distant sites. The metastatic capacity of a tumor is determined by genetic and epigenetic changes within the cancer cells as well as contributions from cells in the tumor microenvironment. Mouse models have proven to be an important tool for unraveling the complex interactions involved in the metastatic cascade and delineating its many stages. Here, we critically appraise the strengths and weaknesses of the current mouse models and highlight the recent advances that have been made using these models in our understanding of metastasis. We also discuss the use of these models for testing potential therapies and the challenges associated with the translation of these findings into the provision of new and effective treatments for cancer patients.

Cell cycle transcription control: DREAM/MuvB and RB-E2F complexes

The precise timing of cell cycle gene expression is critical for the control of cell proliferation; de-regulation of this timing promotes the formation of cancer and leads to defects during differentiation and development. Entry into and progression through S phase requires expression of genes coding for proteins that function in DNA replication. Expression of a distinct set of genes is essential to pass through mitosis and cytokinesis. Expression of these groups of cell cycle-dependent genes is regulated by the RB pocket protein family, the E2F transcription factor family, and MuvB complexes together with B-MYB and FOXM1. Distinct combinations of these transcription factors promote the transcription of the two major groups of cell cycle genes that are maximally expressed either in S phase (G1/S) or in mitosis (G2/M). In this review, we discuss recent work that has started to uncover the molecular mechanisms controlling the precisely timed expression of these genes at specific cell cycle phases, as well as the repression of the genes when a cell exits the cell cycle.

Osteosarcoma: Molecular Pathogenesis and iPSC Modeling

Rare hereditary disorders provide unequivocal evidence of the importance of genes in human disease pathogenesis. Familial syndromes that predispose to osteosarcomagenesis are invaluable in understanding the underlying genetics of this malignancy. Recently, patient-derived induced pluripotent stem cells (iPSCs) have been successfully utilized to model Li-Fraumeni syndrome (LFS)-associated bone malignancy, demonstrating that iPSCs can serve as an in vitro disease model to elucidate osteosarcoma etiology. We provide here an overview of osteosarcoma predisposition syndromes and review recently established iPSC disease models for these familial syndromes. Merging molecular information gathered from these models with the current knowledge of osteosarcoma biology will help us to gain a deeper understanding of the pathological mechanisms underlying osteosarcomagenesis and will potentially aid in the development of future patient therapies.

Preclinical screening methods in cancer

Cancer, a group of diseases of unregulated cell proliferation, is a leading cause of death worldwide. More than 80% of compounds which have shown promising effects in preclinical studies could not get through Phase II of clinical trials. Such high attrition rate is due to improper or selective use of preclinical modalities in anticancer drug screening. The various preclinical screening methods available such as in vitro human cancer cell lines, in vivo tumor xenograft model, or genetically engineered mouse model have their respective pros and cons. Scrupulous use of these preclinical screening methods vis-à-vis efficacy of potential anticancer compound with diverse mechanism of action can help in bringing down the rate of failure of anticancer compound at clinical phase. This article provides an insight into the various preclinical methods used in anticancer studies along with their advantages and disadvantages.

TALENs and CRISPR/Cas9 fuel genetically engineered clinically relevant Xenopus tropicalis tumor models

The targeted nuclease revolution (TALENs, CRISPR/Cas9) now allows Xenopus researchers to rapidly generate custom on-demand genetic knockout models. These novel methods to perform reverse genetics are unprecedented and are fueling a wide array of human disease models within the aquatic diploid model organism Xenopus tropicalis (X. tropicalis). This emerging technology review focuses on the tools to rapidly generate genetically engineered X. tropicalis models (GEXM), with a focus on establishment of genuine genetic and clinically relevant cancer models. We believe that due to particular advantageous characteristics, outlined within this review, GEXM will become a valuable alternative animal model for modeling human cancer. Furthermore, we provide perspectives of how GEXM will be used as a platform for elucidation of novel therapeutic targets and for preclinical drug validation. Finally, we also discuss some future prospects on how the recent expansions and adaptations of the CRISPR/Cas9 toolbox might influence and push forward X. tropicalis cancer research.

A mouse model of MYCN-driven retinoblastoma reveals MYCN-independent tumor reemergence

The most frequent focal alterations in human retinoblastoma are mutations in the tumor-suppressor gene retinoblastoma (RB) and amplification of the oncogene MYCN. Whether MYCN overexpression drives retinoblastoma has not been assessed in model systems. Here, we have shown that Rb inactivation collaborates strongly with MYCN overexpression and leads to retinoblastoma in mice. Overexpression of human MYCN in the context of Rb inactivation increased the expression of MYC-, E2F-, and ribosome-related gene sets, promoted excessive proliferation, and led to retinoblastoma with anaplastic changes. We then modeled responses to MYCN-directed therapy by suppressing MYCN expression in MYCN-driven retinoblastomas. Initially, MYCN suppression led to proliferation arrest and partial tumor regression with loss of anaplasia. However, over time, retinoblastomas reemerged, typically without reactivation of human MYCN or amplification of murine Mycn. A subset of returning retinoblastomas showed genomic amplification of a Mycn target gene encoding the miR cluster miR-17~92, while most retinoblastomas reemerged without clear genetic alterations in either Mycn or known Mycn targets. This Rb/MYCN model recapitulates key genetic driver alterations seen in human retinoblastoma and reveals the emergence of MYCN independence in an initially MYCN-driven tumor.

Genomic landscape of retinoblastoma in Rb-/- p130-/- mice resembles human retinoblastoma

Several murine retinoblastoma models have been generated by deleting the genes encoding for retinoblastoma susceptibility protein pRb and one of its family members p107 or p130. In Rb^-/- p107^-/- retinoblastomas, somatic copy number alterations (SCNAs) like Mdm2 amplification or Cdkn2a deletion targeting the p53-pathway occur, which is uncommon for human retinoblastoma. In our study, we determined SCNAs in retinoblastomas developing in Rb^-/- p130^-/- mice and compared this to murine Rb^-/- p107^-/- tumors and human tumors. Chimeric mice were made by injection of 129/Ola-derived Rb^-/- p130^-/- embryonic stem cells into wild type C57BL/6 blastocysts. SCNAs of retinoblastoma samples were determined by low-coverage (∼0.5×) whole genome sequencing. In Rb^-/- p130^-/- tumors, SCNAs included gain of chromosomes 1 (3/23 tumors), 8 (1/23 tumors), 10 (1/23 tumors), 11 (2/23 tumors), and 12 (4/23 tumors), which could be mapped to frequently altered chromosomes in human retinoblastomas. While the altered chromosomes in Rb^-/- p130^-/- tumors were similar to those in Rb^-/- p107^-/- tumors, the alteration frequencies were much lower in Rb^-/- p130^-/- tumors. Most of the Rb^-/- p130^-/- tumors (16/23 tumors, 70%) were devoid of SCNAs, in strong contrast to Rb^-/- p107^-/- tumors, which were never (0/15 tumors) SCNA-devoid. Similarly, to human retinoblastoma, increased age at diagnosis significantly correlated with increased SCNA frequencies. Additionally, focal loss of Cdh11 was observed in one Rb^-/- p130^-/- tumor, which enforces studies in human retinoblastoma that identified CDH11 as a retinoblastoma suppressor. Moreover, based on a comparison of genes altered in human and murine retinoblastoma, we suggest exploring the role of HMGA1 and SRSF3 in retinoblastoma development. © 2016 Wiley Periodicals, Inc.

Mouse Models of Human Familial Cancer Syndromes

As many as 5% of human cancers appear to be of hereditable etiology. Of the more than 50 characterized familial cancer syndromes, most involve disease affecting multiple organs and many can be traced to one or more abnormalities in specific genes. Studying these syndromes in humans is a difficult task, especially when it comes to genes that may manifest themselves early in gestation. It has been made somewhat easier with the development of genetically engineered mice (GEM) that phenotypically mimic many of these inheritable human cancers. The past 15 years has seen the establishment of mouse lines heterozygous or homozygous null for genes known or suspected of being involved in human cancer syndromes, including APC, ATM, BLM, BRCA1, BRCA2, LKB1, MEN1, MLH, MSH, NF1, TP53, PTEN, RB1, TSC1, TSC2, VHL, and XPA. These lines not only provide models for clinical disease and pathology, but also provide avenues to investigate molecular pathology, gene-gene and protein-tissue interaction, and, ultimately, therapeutic intervention. Possibly of even greater importance, they provide a means of looking at placental and fetal tissues, where genetic abnormalities are often first detected and where they may be most easily corrected. We will review these mouse models, examine their usefulness in medical research, and furnish sources of animals and references.

CRISPR/Cas9 mediated knockout of rb1 and rbl1 leads to rapid and penetrant retinoblastoma development in Xenopus tropicalis

Retinoblastoma is a pediatric eye tumor in which bi-allelic inactivation of the Retinoblastoma 1 (RB1) gene is the initiating genetic lesion. Although recently curative rates of retinoblastoma have increased, there are at this time no molecular targeted therapies available. This is, in part, due to the lack of highly penetrant and rapid retinoblastoma animal models that facilitate rapid identification of targets that allow therapeutic intervention. Different mouse models are available, all based on genetic deactivation of both Rb1 and Retinoblastoma-like 1 (Rbl1), and each showing different kinetics of retinoblastoma development. Here, we show by CRISPR/Cas9 techniques that similar to the mouse, neither rb1 nor rbl1 single mosaic mutant Xenopus tropicalis develop tumors, whereas rb1/rbl1 double mosaic mutant tadpoles rapidly develop retinoblastoma. Moreover, occasionally presence of pinealoblastoma (trilateral retinoblastoma) was detected. We thus present the first CRISPR/Cas9 mediated cancer model in Xenopus tropicalis and the first genuine genetic non-mammalian retinoblastoma model. The rapid kinetics of our model paves the way for use as a pre-clinical model. Additionally, this retinoblastoma model provides unique possibilities for fast elucidation of novel drug targets by triple multiplex CRISPR/Cas9 gRNA injections (rb1 + rbl1 + modifier gene) in order to address the clinically unmet need of targeted retinoblastoma therapy.

Repression by RB1 characterizes genes involved in the penultimate stage of erythroid development

Retinoblastoma-1 (RB1), and the RB1-related proteins p107 and p130, are key regulators of the cell cycle. Although RB1 is required for normal erythroid development in vitro, it is largely dispensable for erythropoiesis in vivo. The modest phenotype caused by RB1 deficiency in mice raises questions about redundancy within the RB1 family, and the role of RB1 in erythroid differentiation. Here we show that RB1 is the major pocket protein that regulates terminal erythroid differentiation. Erythroid cells lacking all pocket proteins exhibit the same cell cycle defects as those deficient for RB1 alone. RB1 has broad repressive effects on gene transcription in erythroid cells. As a group, RB1-repressed genes are generally well expressed but downregulated at the final stage of erythroid development. Repression correlates with E2F binding, implicating E2Fs in the recruitment of RB1 to repressed genes. Merging differential and time-dependent changes in expression, we define a group of approximately 800 RB1-repressed genes. Bioinformatics analysis shows that this list is enriched for terms related to the cell cycle, but also for terms related to terminal differentiation. Some of these have not been previously linked to RB1. These results expand the range of processes potentially regulated by RB1, and suggest that a principal role of RB1 in development is coordinating the events required for terminal differentiation.

Mouse models of liver cancer: Progress and recommendations

To clarify the pathogenesis of hepatocellular carcinoma (HCC) and investigate the effects of potential therapies, a number of mouse models have been developed. Subcutaneous xenograft models are widely used in the past decades. Yet, with the advent of in vivo imaging technology, investigators are more and more concerned with the orthotopic models nowadays. Genetically engineered mouse models (GEM) have greatly facilitated studies of gene function in HCC development. Recently, GEM of miR-122 and miR-221 provided new approaches for better understanding of the in vivo functions of microRNA in hepatocarcinogenesis. Chemically induced liver tumors in animals share many of the morphological, histogenic, and biochemical features of human HCC. Yet, the complicated and obscure genomic alternation restricts their applications. In this review, we highlight both the frequently used mouse models and some emerging ones with emphasis on their merits or defects, and give advises for investigators to chose a “best-fit” animal model in HCC research.

Neuronal Survival and p73/p63/p53: A Family Affair

Elucidating the mechanisms that regulate the life versus death of mammalian neurons is important not only for our understanding of the normal biology of the nervous system but also for our efforts to devise approaches to maintain neuronal survival in the face of traumatic injury or neurodegenerative disorders. Here, we review the emerging evidence that a key survival/death checkpoint in both peripheral and central neurons involves the p53 tumor suppressor and its newly discovered family members, p73 and p63. The full-length isoforms of these proteins function as proapoptotic proteins, whereas naturally occurring N-terminal truncated variants of p73 and p63 act as prosurvival proteins, at least partially by antagonizing the full-length family members. The authors propose that together, these isoforms comprise an upstream rheostat that sums different environmental cues to ultimately determine neuronal survival during development, during neuronal maintenance in adult animals, and even following traumatic injury.

Regulation of the Embryonic Cell Cycle During Mammalian Preimplantation Development

The preimplantation development stage of mammalian embryogenesis consists of a series of highly conserved, regulated, and predictable cell divisions. This process is essential to allow the rapid expansion and differentiation of a single-cell zygote into a multicellular blastocyst containing cells of multiple developmental lineages. This period of development, also known as the germinal stage, encompasses several important developmental transitions, which are accompanied by dramatic changes in cell cycle profiles and dynamics. These changes are driven primarily by differences in the establishment and enforcement of cell cycle checkpoints, which must be bypassed to facilitate the completion of essential cell cycle events. Much of the current knowledge in this area has been amassed through the study of knockout models in mice. These mouse models are powerful experimental tools, which have allowed us to dissect the relative dependence of the early embryonic cell cycles on various aspects of the cell cycle machinery and highlight the extent of functional redundancy between members of the same gene family. This chapter will explore the ways in which the cell cycle machinery, their accessory proteins, and their stimuli operate during mammalian preimplantation using mouse models as a reference and how this allows for the usually well-defined stages of the cell cycle to be shaped and transformed during this unique and critical stage of development.

Evaluating Effects of Hypomorphic Thoc1 Alleles on Embryonic Development in Rb1 Null Mice

The Rb1 tumor suppressor protein is a molecular adaptor that physically links transcription factors like E2f with various proteins acting on DNA or RNA to repress gene expression. Loss of Rb1 liberates E2f to activate the expression of genes mediating resulting phenotypes. Most Rb1 binding proteins, including E2f, interact through carboxyl-terminal protein interaction domains, but genetic evidence suggests that an amino-terminal protein interaction domain is also important. One protein that binds Rb1 through the amino-terminal domain is encoded by Thoc1, a required component of the THO ribonucleoprotein complex important for RNA processing and transport. The physiological relevance of this interaction is unknown. Here we tested whether Thoc1 mediates effects of Rb1 loss on mouse embryonic development. We found that Thoc1 deficiency delays embryo death, and this delay correlates with reduced apoptosis in the brain. E2f protein levels are reduced in Rb1:Thoc1-deficient brain tissue. Expression of apoptotic regulatory genes regulated by E2f, like Apaf1 and Bak1, is also reduced. These observations suggest that Thoc1 is required to support increased expression of E2f and apoptotic regulatory genes that trigger apoptosis upon Rb1 loss. These findings implicate Rb1 in the regulation of the THO ribonucleoprotein complex.

Deletions of Retinoblastoma 1 (Rb1) and Its Repressing Target S Phase Kinase-associated protein 2 (Skp2) Are Synthetic Lethal in Mouse Embryogenesis

Tumor suppressor pRb represses Skp2, a substrate-recruiting subunit of the SCF(Skp2) ubiquitin ligase. Rb1(+/-) mice incur "two-hit" pituitary tumorigenesis; Skp2(-/-);Rb1(+/-) mice do not. Rb1(-/-) embryos die on embryonic day (E) 14.5-15.5. Here, we report that Skp2(-/-);Rb1(-/-) embryos died on E11.5, establishing an organismal level synthetic lethal relationship between Rb1 and Skp2 On E10.5, Rb1(-/-) placentas showed similarly active proliferation and similarly inactive apoptosis as WT placenta, whereas Rb1(-/-) embryos showed ectopic proliferation without increased apoptosis in the brain. Combining Skp2(-/-) did not reduce proliferation or increase apoptosis in the placentas but induced extensive apoptosis in the brain. We conditionally deleted Rb1 in neuronal lineage with Nes-Cre and reproduced the brain apoptosis in E13.5 Nes-Cre;Rb1(lox/lox);Skp2(-/-) embryos, demonstrating their synthetic lethal relationship at a cell autonomous level. Nes-Cre-mediated Rb1 deletion increased expression of proliferative E2F target genes in the brains of Skp2(+/+) embryos; the increases rose higher with activation of expression of apoptotic E2F target genes in Skp2(-/-) embryos. The brain apoptosis was independent of p53 but coincident with proliferation. The highly activated expression of proliferative and apoptotic E2F target genes subsided with gradually reduced roles of Skp2 in preventing p27 protein accumulation in the brain in late gestation, allowing the embryos to reach full term with normally sized brains. These findings establish that Rb1 and Skp2 deletions are synthetic lethal and suggest how this lethal relationship might be circumvented, which could help design better therapies for pRb-deficient cancer.

Deletion of Rb1 induces both hyperproliferation and cell death in murine germinal center B cells

The retinoblastoma gene (RB1) has been implicated as a tumor suppressor in multiple myeloma (MM), yet its role remains unclear because in the majority of cases with 13q14 deletions, un-mutated RB1 remains expressed from the retained allele. To explore the role of Rb1 in MM, we examined the functional consequences of single- and double-copy Rb1 loss in germinal center B cells, the cells of origin of MM. We generated mice without Rb1 function in germinal center B cells by crossing Rb1(Flox/Flox) with C-γ-1-Cre (Cγ1) mice expressing the Cre recombinase in class-switched B cells in a p107(-/-) background to prevent p107 from compensating for Rb1 loss (Cγ1-Rb1(F/F)-p107(-/-)). All mice developed normally, but B cells with two copies of Rb1 deleted (Cγ1-Rb1(F/F)-p107(-/-)) exhibited increased proliferation and cell death compared with Cγ1-Rb1(+/+)-p107(-/-) controls ex vivo. In vivo, Cγ1-Rb1(F/F)-p107(-/-) mice had a lower percentage of splenic B220+ cells and reduced numbers of bone marrow antigen-specific secreting cells compared with control mice. Our data indicate that Rb1 loss induces both cell proliferation and death in germinal center B cells. Because no B-cell malignancies developed after 1 year of observation, our data also suggest that Rb1 loss is not sufficient to transform post-germinal center B cells and that additional, specific mutations are likely required to cooperate with Rb1 loss to induce malignant transformation.

Cancerous inhibitor of protein phosphatase 2A contributes to human papillomavirus oncoprotein E7-induced cell proliferation via E2F1

Cancerous inhibitor of protein phosphatase 2A (CIP2A) is a recently identified oncoprotein that is overexpressed in many human malignant tumors including cervical cancer. Human papillomavirus (HPV) oncoprotein E7 is the key transformation factor in cervical cancer. Our previous data showed a positive association of CIP2A and HPV-16E7 protein levels; however, how CIP2A is regulated by HPV-E7 and the roles of CIP2A in HPV-E7-mediated cell proliferation are unknown. In this study, we demonstrated that HPV-16E7 protein significantly upregulating CIP2A mRNA and protein expression depended on retinoblastoma protein pRb rather than p130. CIP2A siRNA knockdown in HPV-E7-expressing cells inhibited cell proliferation, DNA synthesis and G1/S cell cycle progression. CIP2A siRNA decreased the protein levels of cyclin-dependent kinase 1 (Cdk1), Cdk2 and their partner cyclin A2, with no change in levels of Cdk4, Cdk6 and their partner cyclin D1. The downregulation of Cdk1 and Cdk2 was independent of c-Myc; instead, E2F1 was the main target of CIP2A in this process, as overexpression of E2F1 rescued the inhibitory effects of CIP2A siRNA knockdown on cell proliferation and G1 arrest of HPV-E7-expressing cells. Our studies reveal a novel function of CIP2A in HPV-16E7-mediated cell proliferation.

Loss of E2F1 Extends Survival and Accelerates Oral Tumor Growth in HPV-Positive Mice

The Human Papillomavirus (HPV) is associated with several human cancers, including head and neck squamous cell carcinomas (HNSCCs). HPV expresses the viral oncogene E7 that binds to the retinoblastoma protein (RB1) in order to activate the E2F pathway. RB1 can mediate contradictory pathways-cell growth and cell death via E2F family members. Here, we assessed the extent to which E2F1 mediates lethality of HPV oncogenes. Ubiquitous expression of the HPV oncogenes E6 and E7 caused lethality in mice that was associated with focal necrosis in hepatocytes and pancreatic tissues. Furthermore, all organs expressing HPV oncogenes displayed up-regulation of several E2F1 target genes. The E2F1 pathway mediated lethality in HPV-positive mice because deletion of E2F1 increased survival of mice ubiquitously expressing HPV oncogenes. E2F1 similarly functioned as a tumor suppressor in HPV-positive oral tumors as tumors grew faster with homozygous loss of E2F1 compared to tumors with heterozygous loss of E2F1. Re-expression of E2F1 caused decreased clonogenicity in HPV-positive cancer cells. Our results indicate that HPV oncogenes activated the E2F1 pathway to cause lethality in normal mice and to suppress oral tumor growth. These results suggest that selective modulation of the E2F1 pathway, which is activated in HPV tumors, may facilitate tumor regression.

Recombinase-based conditional and reversible gene regulation via XTR alleles

Synthetic biological tools that enable precise regulation of gene function within in vivo systems have enormous potential to discern gene function in diverse physiological settings. Here we report the development and characterization of a synthetic gene switch that, when targeted in the mouse germline, enables conditional inactivation, reports gene expression and allows inducible restoration of the targeted gene. Gene inactivation and reporter expression is achieved through Cre-mediated stable inversion of an integrated gene-trap reporter, whereas inducible gene restoration is afforded by Flp-dependent deletion of the inverted gene trap. We validate our approach by targeting the p53 and Rb genes and establishing cell line and in vivo cancer model systems, to study the impact of p53 or Rb inactivation and restoration. We term this allele system XTR, to denote each of the allelic states and the associated expression patterns of the targeted gene: eXpressed (XTR), Trapped (TR) and Restored (R).

A Mouse Model for Imprinting of the Human Retinoblastoma Gene

The human RB1 gene is imprinted due to integration of the PPP1R26P1 pseudogene into intron 2. PPP1R26P1 harbors the gametic differentially methylated region of the RB1 gene, CpG85, which is methylated in the female germ line. The paternally unmethylated CpG85 acts as promoter for the alternative transcript 2B of RB1, which interferes with expression of full-length RB1 in cis. In mice, PPP1R26P1 is not present in the Rb1 gene and Rb1 is not imprinted. Assuming that the mechanisms responsible for genomic imprinting are conserved, we investigated if imprinting of mouse Rb1 can be induced by transferring human PPP1R26P1 into mouse Rb1. We generated humanized Rb1_PPP1R26P1 knock-in mice that pass human PPP1R26P1 through the mouse germ line. We found that the function of unmethylated CpG85 as promoter for an alternative Rb1 transcript and as cis-repressor of the main Rb1 transcript is maintained in mouse tissues. However, CpG85 is not recognized as a gametic differentially methylated region in the mouse germ line. DNA methylation at CpG85 is acquired only in tissues of neuroectodermal origin, independent of parental transmission of PPP1R26P1. Absence of CpG85 methylation in oocytes and sperm implies a failure of imprint methylation establishment in the germ line. Our results indicate that site-specific integration of a proven human gametic differentially methylated region is not sufficient for acquisition of DNA methylation in the mouse germ line, even if promoter function of the element is maintained. This suggests a considerable dependency of DNA methylation induction on the surrounding sequence. However, our model is suited to determine the cellular function of the alternative Rb1 transcript.

Releasing 'brakes' to nerve regeneration: intrinsic molecular targets

Restoring critical neuronal architecture after peripheral nerve injury is challenging. Although immediate regenerative responses to peripheral axon injury involve the synthesis of regeneration-associated proteins in neurons and Schwann cells, an unfavorable balance between growth facilitatory and growth inhibitory signaling impairs the growth continuum of injured peripheral nerves. Molecules involved with the signaling network of tumor suppressors play crucial roles in shifting the balance between growth and restraint during axon regeneration. An understanding of the molecular framework of tumor suppressor molecules in injured neurons and its impact on stage-specific regeneration events may expose therapeutic intervention points. In this review we discuss how signaling networks of the specific tumor suppressors PTEN, Rb1, p53, p27 and p21 are altered in injured peripheral nerves and how this impacts peripheral nerve regeneration. Insights into the roles and importance of these pathways may open new avenues for improving the neurological deficits associated with nerve injury.

Transcriptional control of stem cell fate by E2Fs and pocket proteins

E2F transcription factors and their regulatory partners, the pocket proteins (PPs), have emerged as essential regulators of stem cell fate control in a number of lineages. In mammals, this role extends from both pluripotent stem cells to those encompassing all embryonic germ layers, as well as extra-embryonic lineages. E2F/PP-mediated regulation of stem cell decisions is highly evolutionarily conserved, and is likely a pivotal biological mechanism underlying stem cell homeostasis. This has immense implications for organismal development, tissue maintenance, and regeneration. In this article, we discuss the roles of E2F factors and PPs in stem cell populations, focusing on mammalian systems. We discuss emerging findings that position the E2F and PP families as widespread and dynamic epigenetic regulators of cell fate decisions. Additionally, we focus on the ever expanding landscape of E2F/PP target genes, and explore the possibility that E2Fs are not simply regulators of general ‘multi-purpose’ cell fate genes but can execute tissue- and cell type-specific gene regulatory programs.

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.

Direct involvement of retinoblastoma family proteins in DNA repair by non-homologous end-joining

Deficiencies in DNA double-strand break (DSB) repair lead to genetic instability, a recognized cause of cancer initiation and evolution. We report that the retinoblastoma tumor suppressor protein (RB1) is required for DNA DSB repair by canonical non-homologous end-joining (cNHEJ). Support of cNHEJ involves a mechanism independent of RB1's cell-cycle function and depends on its amino terminal domain with which it binds to NHEJ components XRCC5 and XRCC6. Cells with engineered loss of RB family function as well as cancer-derived cells with mutational RB1 loss show substantially reduced levels of cNHEJ. RB1 variants disabled for the interaction with XRCC5 and XRCC6, including a cancer-associated variant, are unable to support cNHEJ despite being able to confer cell-cycle control. Our data identify RB1 loss as a candidate driver of structural genomic instability and a causative factor for cancer somatic heterogeneity and evolution.

Generation of a Retinoblastoma (Rb)1-inducible dominant-negative (DN) mouse model

Retinoblastoma 1 (Rb1) is an essential gene regulating cellular proliferation, differentiation, and homeostasis. To exert these functions, Rb1 is recruited and physically interacts with a growing variety of signaling pathways. While Rb1 does not appear to be ubiquitously expressed, its expression has been confirmed in a variety of hematopoietic and neuronal-derived cells, including the inner ear hair cells (HCs). Studies in transgenic mice demonstrate that complete germline or conditional Rb1 deletion leads to abnormal cell proliferation, followed by massive apoptosis; making it difficult to fully address Rb1's biochemical activities. To overcome these limitations, we developed a tetracycline-inducible TetO-CB-myc6-Rb1 (CBRb) mouse model to achieve transient and inducible dominant-negative (DN) inhibition of the endogenous RB1 protein. Our strategy involved fusing the Rb1 gene to the lysosomal protease pre-procathepsin B (CB), thus allowing for further routing of the DN-CBRb fusion protein and its interacting complexes for proteolytic degradation. Moreover, reversibility of the system is achieved upon suppression of doxycycline (Dox) administration. Preliminary characterization of DN-CBRb mice bred to a ubiquitous rtTA mouse line demonstrated a significant inhibition of the endogenous RB1 protein in the inner ear and in a number of other organs where RB1 is expressed. Examination of the postnatal (P) DN-CBRb mice inner ear at P10 and P28 showed the presence of supernumerary inner HCs (IHCs) in the lower turns of the cochleae, which corresponds to the described expression domain of the endogenous Rb1 gene. Selective and reversible suppression of gene expression is both an experimental tool for defining function and a potential means to medical therapy. Given the limitations associated with Rb1-null mice lethality, this model provides a valuable resource for understanding RB1 activity, relative contribution to HC regeneration and its potential therapeutic application.

Aberrant activation of p53 due to loss of MDM2 or MDMX causes early lens dysmorphogenesis

Although forming a heterodimer or heterooligomer is essential for MDM2 and MDMX to fully control p53 during early embryogenesis, deletion of either MDM2 or MDMX in specific tissues using the loxp-Cre system reveals phenotypic diversity during organ morphogenesis, which can be completely rescued by loss of p53, suggesting the spatiotemporal independence and specificity of the regulation of p53 by MDM2 and MDMX. In this study, we investigated the role of the MDM2-MDMX-p53 pathway in the developing lens that is a relatively independent region integrating cell proliferation, differentiation and apoptosis. Using the mice expressing Cre recombinase specifically in the lens epithelial cells (LECs) beginning at E9.5, we demonstrated that deletion of either MDM2 or MDMX induces apoptosis of LEC and reduces cell proliferation, resulting in lens developmental defect that finally progresses into aphakia. Specifically, the lens defect caused by MDM2 deletion was evident at E10, occurring earlier than that caused by MDMX deletion. These lens defects were completely rescued by loss of two alleles of p53, but not one allele of p53. These results demonstrate that both MDM2 and MDMX are required for monitoring p53 activity during lens development, and they may function independently or synergistically to control p53 and maintain normal lens morphogenesis.

Cell cycle and cell fate in the developing nervous system: the role of CDC25B phosphatase

Deciphering the core machinery of the cell cycle and cell division has been primarily the focus of cell biologists, while developmental biologists have identified the signaling pathways and transcriptional programs controlling cell fate choices. As a result, until recently, the interplay between these two fundamental aspects of biology have remained largely unexplored. Increasing data show that the cell cycle and regulators of the core cell cycle machinery are important players in cell fate decisions during neurogenesis. Here, we summarize recent data describing how cell cycle dynamics affect the switch between proliferation and differentiation, with an emphasis on the roles played by the cell cycle regulators, the CDC25 phosphatases.

The pro-apoptotic activity of Drosophila Rbf1 involves dE2F2-dependent downregulation of diap1 and buffy mRNA

The retinoblastoma gene, rb, ensures at least its tumor suppressor function by inhibiting cell proliferation. Its role in apoptosis is more complex and less described than its role in cell cycle regulation. Rbf1, the Drosophila homolog of Rb, has been found to be pro-apoptotic in proliferative tissue. However, the way it induces apoptosis at the molecular level is still unknown. To decipher this mechanism, we induced rbf1 expression in wing proliferative tissue. We found that Rbf1-induced apoptosis depends on dE2F2/dDP heterodimer, whereas dE2F1 transcriptional activity is not required. Furthermore, we highlight that Rbf1 and dE2F2 downregulate two major anti-apoptotic genes in Drosophila: buffy, an anti-apoptotic member of Bcl-2 family and diap1, a gene encoding a caspase inhibitor. On the one hand, Rbf1/dE2F2 repress buffy at the transcriptional level, which contributes to cell death. On the other hand, Rbf1 and dE2F2 upregulate how expression. How is a RNA binding protein involved in diap1 mRNA degradation. By this way, Rbf1 downregulates diap1 at a post-transcriptional level. Moreover, we show that the dREAM complex has a part in these transcriptional regulations. Taken together, these data show that Rbf1, in cooperation with dE2F2 and some members of the dREAM complex, can downregulate the anti-apoptotic genes buffy and diap1, and thus promote cell death in a proliferative tissue.

Targeting tumor suppressor networks for cancer therapeutics

Cancer is a consequence of mutations in genes that control cell proliferation, differentiation and cellular homeostasis. These genes are classified into two categories: oncogenes and tumor suppressor genes. Together, overexpression of oncogenes and loss of tumor suppressors are the dominant driving forces for tumorigenesis. Hence, targeting oncogenes and tumor suppressors hold tremendous therapeutic potential for cancer treatment. In the last decade, the predominant cancer drug discovery strategy has relied on a traditional reductionist approach of dissecting molecular signaling pathways and designing inhibitors for the selected oncogenic targets. Remarkable therapies have been developed using this approach; however, targeting oncogenes is only part of the picture. Our understanding of the importance of tumor suppressors in preventing tumorigenesis has also advanced significantly and provides a new therapeutic window of opportunity. Given that tumor suppressors are frequently mutated, deleted, or silenced with loss-of-function, restoring their normal functions to treat cancer holds tremendous therapeutic potential. With the rapid expansion in our knowledge of cancer over the last several decades, developing effective anticancer regimens against tumor suppressor pathways has never been more promising. In this article, we will review the concept of tumor suppression, and outline the major therapeutic strategies and challenges of targeting tumor suppressor networks for cancer therapeutics.

Fish is Fish: the use of experimental model species to reveal causes of skeletal diversity in evolution and disease

Fishes are wonderfully diverse. This variety is a result of the ability of ray-finned fishes to adapt to a wide range of environments, and has made them more specious than the rest of vertebrates combined. With such diversity it is easy to dismiss comparisons between distantly related fishes in efforts to understand the biology of a particular fish species. However, shared ancestry and the conservation of developmental mechanisms, morphological features and physiology provide the ability to use comparative analyses between different organisms to understand mechanisms of development and physiology. The use of species that are amenable to experimental investigation provides tools to approach questions that would not be feasible in other 'non-model' organisms. For example, the use of small teleost fishes such as zebrafish and medaka has been powerful for analysis of gene function and mechanisms of disease in humans, including skeletal diseases. However, use of these fish to aid in understanding variation and disease in other fishes has been largely unexplored. This is especially evident in aquaculture research. Here we highlight the utility of these small laboratory fishes to study genetic and developmental factors that underlie skeletal malformations that occur under farming conditions. We highlight several areas in which model species can serve as a resource for identifying the causes of variation in economically important fish species as well as to assess strategies to alleviate the expression of the variant phenotypes in farmed fish. We focus on genetic causes of skeletal deformities in the zebrafish and medaka that closely resemble phenotypes observed both in farmed as well as natural populations of fishes.

Analyses of tumor-suppressor genes in germline mouse models of cancer

Tumor-suppressor genes are critical regulators of growth and functioning of cells, whose loss of function contributes to tumorigenesis. Accordingly, analyses of the consequences of their loss of function in genetically engineered mouse models have provided important insights into mechanisms of human cancer, as well as resources for preclinical analyses and biomarker discovery. Nowadays, most investigations of genetically engineered mouse models of tumor-suppressor function use conditional or inducible alleles, which enable analyses in specific cancer (tissue) types and overcome the consequences of embryonic lethality of germline loss of function of essential tumor-suppressor genes. However, historically, analyses of genetically engineered mouse models based on germline loss of function of tumor-suppressor genes were very important as these early studies established the principle that loss of function could be studied in mouse cancer models and also enabled analyses of these essential genes in an organismal context. Although the cancer phenotypes of these early germline models did not always recapitulate the expected phenotypes in human cancer, these models provided the essential foundation for the more sophisticated conditional and inducible models that are currently in use. Here, we describe these "first-generation" germline models of loss of function models, focusing on the important lessons learned from their analyses, which helped in the design and analyses of "next-generation" genetically engineered mouse models.

Animal models in osteosarcoma

Osteosarcoma (OS) is the most common non-hematologic primary tumor of bone in children and adults. High-dose cytotoxic chemotherapy and surgical resection have improved prognosis, with long-term survival for non-metastatic disease approaching 70%. However, most OS tumors are high grade and tend to rapidly develop pulmonary metastases. Despite clinical advances, patients with metastatic disease or relapse have a poor prognosis. Toward a better understanding of the molecular pathogenesis of human OS, several genetically modified OS mouse models have been developed and will be reviewed here. However, better animal models that more accurately recapitulate the natural progression of the disease are needed for the development of improved prognostic and diagnostic markers as well as targeted therapies for both primary and metastatic OS.

Reduced expression of the retinoblastoma protein shows that the related signaling pathway is essential for mediating the antineoplastic activity of erufosine

Erufosine is a new antineoplastic agent of the group of alkylphosphocholines, which interferes with signal transduction and induces apoptosis in various leukemic and tumor cell lines. The present study was designed to examine for the first time the mechanism of resistance to erufosine in malignant cells with permanently reduced expression of the retinoblastoma (Rb) protein. Bearing in mind the high number of malignancies with reduced level of this tumor-suppressor, this investigation was deemed important for using erufosine, alone or in combination, in patients with compromised RB1 gene expression. For this purpose, clones of the leukemic T-cell line SKW-3 were used, which had been engineered to constantly express differently low Rb levels. The alkylphosphocholine induced apoptosis, stimulated the expression of the cyclin dependent kinase inhibitor p27^Kip1 and inhibited the synthesis of cyclin D3, thereby causing a G₂ phase cell cycle arrest and death of cells with wild type Rb expression. In contrast, Rb-deficiency impeded the changes induced by eru-fosine in the expression of these proteins and abrogated the induction of G₂ arrest, which was correlated with reduced antiproliferative and anticlonogenic activities of the compound. In conclusion, analysis of our results showed for the first time that the Rb signaling pathway is essential for mediating the antineoplastic activity of erufosine and its efficacy in patients with malignant diseases may be predicted by determining the Rb status.

Inactivation of Rb and E2f8 synergizes to trigger stressed DNA replication during erythroid terminal differentiation

Rb is critical for promoting cell cycle exit in cells undergoing terminal differentiation. Here we show that during erythroid terminal differentiation, Rb plays a previously unappreciated and unorthodox role in promoting DNA replication and cell cycle progression. Specifically, inactivation of Rb in erythroid cells led to stressed DNA replication, increased DNA damage, and impaired cell cycle progression, culminating in defective terminal differentiation and anemia. Importantly, all of these defects associated with Rb loss were exacerbated by the concomitant inactivation of E2f8. Gene expression profiling and chromatin immunoprecipitation (ChIP) revealed that Rb and E2F8 cosuppressed a large array of E2F target genes that are critical for DNA replication and cell cycle progression. Remarkably, inactivation of E2f2 rescued the erythropoietic defects resulting from Rb and E2f8 deficiencies. Interestingly, real-time quantitative PCR (qPCR) on E2F2 ChIPs indicated that inactivation of Rb and E2f8 synergizes to increase E2F2 binding to its target gene promoters. Taken together, we propose that Rb and E2F8 collaborate to promote DNA replication and erythroid terminal differentiation by preventing E2F2-mediated aberrant transcriptional activation through the ability of Rb to bind and sequester E2F2 and the ability of E2F8 to compete with E2F2 for E2f-binding sites on target gene promoters.