Expression of a retinoblastoma transgene results in dwarf mice

RB depletion is required for the continuous growth of tumors initiated by loss of RB

The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo.

Preclinical Solid Tumor Models to Study Novel Therapeutics in Brain Metastases

Metastases are the most common malignancy of the adult central nervous system and are becoming an increasingly troubling problem in oncology largely due to the lack of successful therapeutic options. The limited selection of treatments is a result of the currently poor understanding of the biological mechanisms of metastatic development, which in turn is difficult to achieve because of limited preclinical models that can accurately represent the clinical progression of metastasis. Described in this article are in vitro and in vivo model systems that are used to enhance the understanding of metastasis and to identify new therapies for the treatment of brain metastasis. © 2021 Wiley Periodicals LLC.

Heritable genetic variants in key cancer genes link cancer risk with anthropometric traits

BACKGROUND

Height and other anthropometric measures are consistently found to associate with differential cancer risk. However, both genetic and mechanistic insights into these epidemiological associations are notably lacking. Conversely, inherited genetic variants in tumour suppressors and oncogenes increase cancer risk, but little is known about their influence on anthropometric traits.

METHODS

By integrating inherited and somatic cancer genetic data from the Genome-Wide Association Study Catalog, expression Quantitative Trait Loci databases and the Cancer Gene Census, we identify SNPs that associate with different cancer types and differential gene expression in at least one tissue type, and explore the potential pleiotropic associations of these SNPs with anthropometric traits through SNP-wise association in a cohort of 500,000 individuals.

RESULTS

We identify three regulatory SNPs for three important cancer genes, FANCA, MAP3K1 and TP53 that associate with both anthropometric traits and cancer risk. Of particular interest, we identify a previously unrecognised strong association between the rs78378222[C] SNP in the 3' untranslated region (3'-UTR) of TP53 and both increased risk for developing non-melanomatous skin cancer (OR=1.36 (95% 1.31 to 1.41), adjusted p=7.62E-63), brain malignancy (OR=3.12 (2.22 to 4.37), adjusted p=1.43E-12) and increased standing height (adjusted p=2.18E-24, beta=0.073±0.007), lean body mass (adjusted p=8.34E-37, beta=0.073±0.005) and basal metabolic rate (adjusted p=1.13E-31, beta=0.076±0.006), thus offering a novel genetic link between these anthropometric traits and cancer risk.

CONCLUSION

Our results clearly demonstrate that heritable variants in key cancer genes can associate with both differential cancer risk and anthropometric traits in the general population, thereby lending support for a genetic basis for linking these human phenotypes.

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.

Cross-species genomic and epigenomic landscape of retinoblastoma

Genetically engineered mouse models (GEMMs) of human cancer are important for advancing our understanding of tumor initiation and progression as well as for testing novel therapeutics. Retinoblastoma is a childhood cancer of the developing retina that initiates with biallelic inactivation of the RB1 gene. GEMMs faithfully recapitulate the histopathology, molecular, cellular, morphometric, neuroanatomical and neurochemical features of human retinoblastoma. In this study, we analyzed the genomic and epigenomic landscape of murine retinoblastoma and compared them to human retinoblastomas to gain insight into shared mechanisms of tumor progression across species. Similar to human retinoblastoma, mouse tumors have low rates of single nucleotide variations. However, mouse retinoblastomas have higher rates of aneuploidy and regional and focal copy number changes that vary depending on the genetic lesions that initiate tumorigenesis in the developing murine retina. Furthermore, the epigenetic landscape in mouse retinoblastoma was significantly different from human tumors and some pathways that are candidates for molecular targeted therapy for human retinoblastoma such as SYK or MCL1 are not deregulated in GEMMs. Taken together, these data suggest there are important differences between mouse and human retinoblastomas with respect to the mechanism of tumor progression and those differences can have significant implications for translational research to test the efficacy of novel therapies for this devastating childhood cancer.

Transgenic and knockout mice models to reveal the functions of tumor suppressor genes

Cancer is caused by multiple genetic alterations leading to uncontrolled cell proliferation through multiple pathways. Malignant cells arise from a variety of genetic factors, such as mutations in tumor suppressor genes (TSGs) that are involved in regulating the cell cycle, apoptosis, or cell differentiation, or maintenance of genomic integrity. Tumor suppressor mouse models are the most frequently used animal models in cancer research. The anti-tumorigenic functions of TSGs, and their role in development and differentiation, and inhibition of oncogenes are discussed. In this review, we summarize some of the important transgenic and knockout mouse models for TSGs, including Rb, p53, Ink4a/Arf, Brca1/2, and their related genes.

Cell cycling and differentiation do not require the retinoblastoma protein during early Xenopus development

The retinoblastoma protein (pRb) is a central regulator of the cell cycle, controlling passage through G1 phase. Moreover, pRb has also been shown to play a direct role in the differentiation of multiple tissues, including nerve and muscle. Rb null mice display embryonic lethality, although recent data have indicated that at least some of these defects are due to placental insufficiency. To investigate this further, we have examined the role of pRb in early development of the frog Xenopus laevis, which develops without the need for a placenta. Surprisingly, we see that loss of pXRb has no effect on either cell cycling or differentiation of neural or muscle tissue, while overexpression of pXRb similarly has no effects. We demonstrate that, in fact, pXRb is maintained in a hyperphosphorylated and therefore inactive state early in development. Therefore, Rb protein is not required for cell cycle control or differentiation in early embryos, indicating unusual control of these G1/G0 events at this developmental stage.

E2F1 activation is responsible for pituitary adenomas induced by HMGA2 gene overexpression

The High Mobility Group protein HMGA2 is a nuclear architectural factor that plays a critical role in a wide range of biological processes including regulation of gene expression, embryogenesis and neoplastic transformation. Several studies are trying to identify the mechanisms by which HMGA2 protein is involved in each of these activities, and only recently some new significant insights are emerging from the study of transgenic and knock-out mice. Overexpression of HMGA2 gene leads to the onset of prolactin and GH-hormone induced pituitary adenomas in mice, suggesting a critical role of this protein in pituitary tumorigenesis. This was also confirmed in the human pathology by the finding that HMGA2 amplification and/or overexpression is present in human prolactinomas. This review focuses on recent data that explain the mechanism by which HMGA2 induces the development of pituitary adenomas in mice. This mechanism entails the activation of the E2F1 protein by the HMGA2-mediated displacement of HDAC1 from pRB protein.

Visualizing dynamic E2F-mediated repression in vivo

Although many E2F target genes have been identified recently, very little is known about how any single E2F site controls the expression of an E2F target gene in vivo. To test the requirement for a single E2F site in vivo and to learn how E2F-mediated repression is regulated during development and tumorigenesis, we have constructed a novel series of wild-type and mutant Rb promoter-LacZ transgenic reporter lines that allow us to visualize the activity of a crucial E2F target in vivo, the retinoblastoma tumor suppressor gene (Rb). Two mutant Rb promoter-LacZ constructs were used to evaluate the importance of a single E2F site or a nearby activator (Sp1/Ets) site that is found mutated in low-penetrance retinoblastomas. The activity of the wild-type Rb promoter is dynamic, varying spatially and temporally within the developing nervous system. While loss of the activator site silences the Rb promoter, loss of the E2F site stimulates its activity in the neocortex, retina, and trigeminal ganglion. Surprisingly, E2F-mediated repression of Rb does not act globally or in a static manner but, instead, is a highly dynamic process in vivo. Using neocortical extracts, we detected GA-binding protein alpha (GABPalpha, an Ets family member) bound to the activator site and both E2F1 and E2F4 bound to the repressor site of the Rb promoter in vitro. Additionally, we detected binding of both E2F1 and E2F4 to the Rb promoter in vivo using chromatin immunoprecipitation analysis on embryonic day 13.5 brain. Unexpectedly, we detect no evidence for Rb promoter autoregulation in neuroendocrine tumors from Rb+/-; RbP-LacZ mice that undergo loss of heterozygosity at the Rb locus, in contrast to the situation in human retinoblastomas where high RB mRNA levels are found. In summary, this study provides the first demonstration that loss of an E2F site is critical for target gene repression in vivo and underscores the complexity of the Rb and E2F family network in vivo.

HMGA2 induces pituitary tumorigenesis by enhancing E2F1 activity

HMGA2 gene amplification and overexpression in human prolactinomas and the development of pituitary adenomas in HMGA2 transgenic mice showed that HMGA2 plays a crucial role in pituitary tumorigenesis. We have explored the pRB/E2F1 pathway to investigate the mechanism by which HMGA2 acts. Here we show that HMGA2 interacts with pRB and induces E2F1 activity in mouse pituitary adenomas by displacing HDAC1 from the pRB/E2F1 complex-a process that results in E2F1 acetylation. We found that loss of E2F1 function (obtained by mating HMGA2 and E2F1(-/-) mice) suppressed pituitary tumorigenesis in HMGA2 mice. Thus, HMGA2-mediated E2F1 activation is a crucial event in the onset of these tumors in transgenic mice and probably also in human prolactinomas.

Generation of reversible Rb-knockdown mice

This study describes the generation of reversible Rb-knockdown mice using Tet-off system coupled with Rb-deficient mice currently available. Mice expressing pRB conditionally in Rb-/- background were generated by crossings P(hCMV)-tTA/TRE-Rb transgenic mice with conventional Rb+/- mice. Transgenic Rb was tightly controlled with reversibility and biologically effective as exemplified by cyclin E expression in a doxycycline-dependent manner in mouse embryonic fibroblasts. However, its ectopic expression was not sufficient to rescue the phenotypes of Rb-/- embryos at organismal level, suggesting the requirement of more sophisticated regulation of pRB. With all, these results demonstrate that our experimental strategy can be an alternative way to convert classical gene-disrupted mice into reversible conditional ones.

Suppression of Melanotroph Carcinogenesis Leads to Accelerated Progression of Pituitary Anterior Lobe Tumors and Medullary Thyroid Carcinomas in Rb+/− Mice

Mice with a single copy of the retinoblastoma gene (Rb+/−) develop a syndrome of multiple neuroendocrine neoplasia. They usually succumb to fast-growing, Rb-deficient melanotroph tumors of the pituitary intermediate lobe, which are extremely rare in humans. Thus, full assessment of Rb role in other, more relevant to human pathology, neoplasms is complicated. To prevent melanotroph neoplasia while preserving spontaneous carcinogenesis in other types of cells, we have prepared transgenic mice in which 770-bp fragment of pro-opiomelanocortin promoter directs expression of the human RB gene to melanotrophs (TgPOMC-RB). In three independent lines, transgenic mice crossed to Rb+/− background are devoid of melanotroph tumors but develop the usual spectrum of other neoplasms. Interestingly, abrogation of melanotroph carcinogenesis results in accelerated progression of pituitary anterior lobe tumors and medullary thyroid carcinomas. A combination of immunologic tests, cell culture studies, and tumorigenicity assays indicates that α-melanocyte–stimulating hormone, which is overproduced by melanotroph tumors, attenuates neoplastic progression by decreasing cell proliferation and inducing apoptosis. Taken together, we show that cell lineage–specific complementation of Rb function can be successfully used for refining available models of stochastic carcinogenesis and identify α-melanocyte–stimulating hormone as a potential attenuating factor during progression of neuroendocrine neoplasms.

Role of the RB tumor suppressor in cancer

Apart from their coordinated inactivation by DNA tumor viral oncoproteins, the pRB and p53 tumor suppressor pathways were not known to be connected ten years ago. Within the last decade, our appreciation of how these pathways are interconnected has grown substantially. The checks and balances that exist between pRB and p53 involve the regulation of the G1/S transition and its checkpoints, and much of this is under the control of the E2F transcription factor family. Following DNA damage, the p53-dependent induction of p21CIP1 regulates cyclin E/Cdk2 and cyclin A/Cdk2 complexes both of which phosphorylate pRB, leading to E2F-mediated activation. Similarly, E2F1-dependent induction of p19ARF antagonizes the ability of mdm2 to degrade p53, leading to p53 stabilization and potentially p53-mediated apoptosis or cell cycle arrest. From the existing mouse models discussed above, we also know that proliferation, cell death and differentiation of distinct tissues are also intimately linked through entrance and exit from the cell cycle, and thus through pRB and p53 pathways. Virtually all human tumors deregulate either the pRB or p53 pathway, and often times both pathways simultaneously, which is critical for crippling cellular defense against neoplasia. The next decade of cancer research will likely see these two tumor suppressor pathways only merge even more.

Activation of cyclin D1-Cdk4 and Cdk4-directed phosphorylation of RB protein in diabetic mesangial hypertrophy

To determine the role of cell-cycle proteins in regulating pathological renal hypertrophy, diabetes was induced in mice expressing a human retinoblastoma (RB) transgene and in wild-type littermates. Whole-kidney and glomerular hypertrophy caused by hyperglycemia was associated with specific G1 phase cell-cycle events: early and sustained increase in expression of cyclin D1 and activation of cyclin D1-cdk4 complexes, but no change in expression of cyclin E or cdk2 activity. Overexpression of RB alone likewise caused hypertrophy and increased only cyclin D1-cdk4 activity; these effects were not further augmented by high glucose. Identical observations were made when isolated mesangial cells conditionally overexpressing RB from a tetracycline-repressible system hypertrophied in response to high glucose. A mitogenic signal in the same cell-culture system, in contrast, transiently and sequentially activated both cyclin D1-cdk4 and cyclin E-cdk2. In vivo and in cultured mesangial cells, high glucose resulted in persistent partial phosphorylation of RB, an event catalyzed specifically by cyclin D1-cdk4. These data indicate that mesangial hypertrophy caused by hyperglycemia in diabetes results in sustained cyclin D1-cdk4-dependent phosphorylation of RB and maintenance of mesangial cells in the early-to-middle G1 phase of the cell cycle.

Tumor suppression by a severely truncated species of retinoblastoma protein

Rb(+/+):Rb(-/-) chimeric mice are healthy until early in adulthood when they develop lethal pituitary tumors composed solely of Rb(-/-) cells. In an effort to delineate the minimal structures of the retinoblastoma protein necessary for RB tumor suppression function, chimeric animals derived from stably transfected RB(-/-) embryonic stem (ES) cells were generated. One such ES cell transfectant expressed a human RB allele encoding a stable, truncated nuclear derivative lacking residues 1 to 378 (Delta 1-378). Others encoded either wild-type human RB or an internally deleted derivative of the Delta 1-378 mutant. All gave rise to viable chimeric animals with comparable degrees of chimerism. However, unlike control mice derived, in part, from naive Rb(-/-) ES cells or from ES cells transformed by the double RB mutant, Delta 1-378/Delta exon22, animals derived from either wild-type RB- or Delta 1-378 RB-producing ES cells failed to develop pituitary tumors. Thus, in this setting, a substantial fraction of the RB sequence is unnecessary for RB-mediated tumor suppression.

Animal models of tumor-suppressor genes

The development of cancer requires multiple genetic alterations perturbing distinct cellular pathways. In human cancers, these alterations often arise owing to mutations in tumor-suppressor genes whose normal function is to either inhibit the proliferation, apoptosis, or differentiation of cells, or maintain their genomic integrity. Mouse models for tumor suppressors frequently provide definitive evidence for the antitumorigenic functions of these genes. In addition, animal models permit the identification of previously unsuspected roles of these genes in development and differentiation. The availability of null and tissue-specific mouse mutants for tumor-suppressor genes has greatly facilitated our understanding of the mechanisms leading to cancer. In this review, we describe mouse models for tumor-suppressor genes.

The retinoblastoma gene: a prototypic and multifunctional tumor suppressor

Genome instability has been implicated in the generation of multiple somatic mutations that underlie cancer. Germline mutation in the retinoblastoma (RB) gene leads to tumor formation in both human and experimental animal models, and reintroduction of wild-type RB is able to suppress neoplastic phenotypes. Rb governs the passage of cells through the G1 phase-restriction point and this control is lost in most cancer cells. Rb has also been shown to promote terminal differentiation and prevent cell cycle reentry. Recent studies implicate Rb in mitotic progression, faithful chromosome segregation, checkpoint control, and chromatin remodeling, suggesting that Rb may function in the maintenance of genome integrity. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities. A single protein capable of performing multiple antioncogenic functions may be a common characteristic of other tumor suppressors including p53 and BRCA1/2.

Cloning of the Retinoblastoma cDNA from the Japanese medaka (Oryzias latipes) and preliminary evidence of mutational alterations in chemically-induced retinoblastomas

We have cloned a medaka homolog of the human retinoblastoma (Rb) susceptibility gene. The medaka Rb cDNA encodes a predicted protein of 909 amino acids. DNA sequence analysis with other vertebrate Rb sequences demonstrates that the medaka Rb cDNA is highly conserved in regions of functional importance. An antibody raised against an epitope of the human pRb recognizes the protein product of the medaka Rb gene, detecting a 105 kDa protein in all tissues examined and at differential levels for the stages of embryonic development studied. The sequence reported herein, combined with the high degree of conservation observed in critical domains, has also facilitated a preliminary investigation of the molecular etiology of chemically-induced retinoblastoma. The mutational alterations characterized suggest that medaka may provide a novel model and, thus, provide additional insight into the human retinoblastoma condition.

Retinoblastoma gene promoter directs transgene expression exclusively to the nervous system

In human, germ line mutations in the tumor suppressor retinoblastoma (Rb) predispose individuals to retinoblastoma, whereas somatic inactivation of Rb contributes to the progression of a large spectrum of cancers. In mice, Rb is highly expressed in restricted cell lineages including the neurogenic, myogenic, and hematopoietic systems, and disruption of the gene leads to specific embryonic defects in these tissues. The symmetry between Rb expression and the defects in mutant mice suggest that transcriptional control of Rb during embryogenesis is pivotal for normal development. We have initiated studies to dissect the mechanisms of transcriptional regulation of Rb during development by promoter lacZ transgenic analysis. DNA sequences up to 6 kilobase pairs upstream of the mouse Rb promoter, isolated from two different genomic libraries, directed transgene expression exclusively to the developing nervous system, excluding skeletal muscles and liver. Expression of the transgene in the central and peripheral nervous systems, including the retina, recapitulated the expression of endogenous Rb during embryogenesis. A promoter region spanning approximately 250 base pairs upstream of the transcriptional starting site was sufficient to confer expression in the central and peripheral nervous systems. To determine whether this expression pattern was conserved, we isolated the human Rb 5' genomic region and generated transgenic mice expressing lacZ under control of a 1.6-kilobase pair human Rb promoter. The human Rb promoter lacZ mice also expressed the transgene primarily in the nervous system in several independent lines. Thus, transgene expression directed by both the human and mouse Rb promoters is restricted to a subset of tissues in which Rb is normally expressed during embryogenesis. Our findings demonstrate that regulatory elements directing Rb expression to the nervous system are delineated within a well defined core promoter and are regionally separated from elements, yet to be identified, that are required for expression of Rb in the developing hematopoietic and skeletal muscle systems.

A paradigm for cancer treatment using the retinoblastoma gene in a mouse model

Discovery of tumor suppressor genes has provided a rational approach to cancer prevention and treatment. Loss of retinoblastoma susceptibility gene (Rb) function is a rate-limiting event in the development of human and mouse cancers. Establishment of animal models of cancer associated with Rb deficiency allowed us to develop and test long-awaited approaches to genetic correction for treating tumors in vivo. Recent studies demonstrated that (1) prevention of carcinogenesis is achieved by correction of gene copy number in Rb+/- mice, and (2) reconstitution of Rb gene functions is sufficient for suppression of neoplasia in immunocompetent mice. These results fulfill a promise of cancer treatment by reconstitution of tumor suppressor function.

A new functional classification of tumor-suppressing genes and its therapeutic implications

Cell fusion studies have demonstrated that malignancy can be suppressed by a single dose of malignancy suppressor genes (MSGs), indicating that malignancy is a recessive phenotype. Correspondingly, it is widely believed that mutational inactivation of both alleles of tumor suppressor genes (TSGs), in familial and sporadic tumors, is the formal proof of the recessive nature of malignancy. Evidence presented here, however, shows that unlike MSGs, identified solely through cell fusion studies with no gene of this class yet cloned, many well-known TSGs have gene dosage effects and inhibit cellular growth in vitro. Moreover, homozygous inactivation of a growth-inhibitory TSG (GITSG) is not directly correlated with malignancy. An alternative interpretation is provided for the loss of wild-type alleles of these genes in the tumors. It is concluded that the MSGs and the GITSGs do not belong to the same class of genes. The functional classification of tumor-suppressing genes has important implications for developing effective cancer therapies.

Temporal, spatial, and cell type-specific control of Cre-mediated DNA recombination in transgenic mice

We have developed a universal system for temporal, spatial, and cell type-specific control of gene expression in mice that (1) integrates the advantages of tetracycline-controlled gene expression and Cre-recombinase-loxP site-mediated gene inactivation, and (2) simplifies schemes of animal crosses by combination of two control elements in a single transgene. Two transgenic strains were generated in which the cell type-specific control was provided by either the retinoblastoma gene promoter or the whey acidic protein promoter. Both promoters drive the expression of the reverse tetracycline-controlled transactivator (rtTA). Placed in cis configuration to the rtTA transcription unit, the rtTA-inducible promoter directs expression of Cre recombinase. In both strains crossed with cActXstopXLacZ reporter mice, which have a loxP-stop of transcription/translation-loxP-LacZ cassette driven by chicken beta-actin promoter, Cre-loxP-mediated DNA recombination leading to LacZ expression was accurately regulated in a temporal, spatial, and cell type-specific manner. This approach can be applied to establishment of analogous mouse strains with virtually any promoter as systems to control gene regulation in a variety of cell types.

Mouse patched1 controls body size determination and limb patterning

Hedgehog (Hh) proteins control many developmental events by inducing specific cell fates or regulating cell proliferation. The Patched1 (Ptc1) protein, a binding protein for Hh molecules, appears to oppose Hh signals by repressing transcription of genes that can be activated by Hh. Sonic hedgehog (Shh), one of the vertebrate homologs of Hh, controls patterning and growth of the limb but the early embryonic lethality of ptc1(−)(/)(−) mice obscures the roles of ptc1 in later stages of development. We partially rescued ptc1 homozygous mutant embryos using a metallothionein promoter driving ptc1. In a wild-type background, the transgene causes a marked decrease in animal size starting during embryogenesis, and loss of anterior digits. In ptc1 homozygotes, a potent transgenic insert allowed survival to E14 and largely normal morphology except for midbrain overgrowth. A less potent transgene gave rise to partially rescued embryos with massive exencephaly, and polydactyly and branched digits in the limbs. The polydactyly was preceded by unexpected anterior limb bud transcription of Shh, so one function of ptc1 is to repress Shh expression in the anterior limb bud.

Targeted disruption of CDK4 delays cell cycle entry with enhanced p27(Kip1) activity

The mechanism by which cyclin-dependent kinase 4 (CDK4) regulates cell cycle progression is not entirely clear. Cyclin D/CDK4 appears to initiate phosphorylation of retinoblastoma protein (Rb) leading to inactivation of the S-phase-inhibitory action of Rb. However, cyclin D/CDK4 has been postulated to act in a noncatalytic manner to regulate the cyclin E/CDK2-inhibitory activity of p27(Kip1) by sequestration. In this study we investigated the roles of CDK4 in cell cycle regulation by targeted disruption of the mouse CDK4 gene. CDK4(-/-) mice survived embryogenesis and showed growth retardation and reproductive dysfunction associated with hypoplastic seminiferous tubules in the testis and perturbed corpus luteum formation in the ovary. These phenotypes appear to be opposite to those of p27-deficient mice such as gigantism and gonadal hyperplasia. A majority of CDK4(-/-) mice developed diabetes mellitus by 6 weeks, associated with degeneration of pancreatic islets. Fibroblasts from CDK4(-/-) mouse embryos proliferated similarly to wild-type embryonic fibroblasts under conditions that promote continuous growth. However, quiescent CDK4(-/-) fibroblasts exhibited a substantial ( approximately 6-h) delay in S-phase entry after serum stimulation. This cell cycle perturbation by CDK4 disruption was associated with increased binding of p27 to cyclin E/CDK2 and diminished activation of CDK2 accompanied by impaired Rb phosphorylation. Importantly, fibroblasts from CDK4(-/-) p27(-/-) embryos displayed partially restored kinetics of the G(0)-S transition, indicating the significance of the sequestration of p27 by CDK4. These results suggest that at least part of CDK4's participation in the rate-limiting mechanism for the G(0)-S transition consists of controlling p27 activity.

Developmental defects and tumor predisposition in Rb mutant mice

Targeted gene disruption in the mouse germline permits the introduction of gene mutations similar to those found in inherited human diseases. New advances in gene targeting that enable cell type specific gene disruption in mice further increases the utility of mouse models to study genetic defects as found in cancer. Here we review the phenotypes observed in mice carrying germline mutated copies of the retinoblastoma tumor suppressor gene. We will illustrate how methods that permit tissue-specific Rb inactivation in mice provide new and more versatile tools to gain insight into the etiology of sporadic cancer.

Cell cycle genes in chondrocyte proliferation and differentiation

Coordinated proliferation and differentiation of growth plate chondrocytes controls longitudinal growth of endochondral bones. While many extracellular factors regulating these processes have been identified, much less is known about the intracellular mechanisms transducing and integrating these extracellular signals. Recent evidence suggests that cell cycle proteins play an important role in the coordination of chondrocyte proliferation and differentiation. Our current knowledge of the function and regulation of cell cycle proteins in endochondral ossification is summarized.

RB-mediated suppression of spontaneous multiple neuroendocrine neoplasia and lung metastases in Rb+/- mice

Alterations in pathways mediated by retinoblastoma susceptibility gene (RB) product are among the most common in human cancer. Mice with a single copy of the Rb gene are shown to develop a syndrome of multiple neuroendocrine neoplasia. The earliest Rb-deficient atypical cells were identified in the intermediate and anterior lobes of the pituitary, the thyroid and parathyroid glands, and the adrenal medulla within the first 3 months of postnatal development. These cells form gross tumors with various degrees of malignancy by postnatal day 350. By age of 380 days, 84% of Rb+/- mice exhibited lung metastases from C-cell thyroid carcinomas. Expression of a human RB transgene in the Rb+/- mice suppressed carcinogenesis in all tissues studied. Of particular clinical relevance, the frequency of lung metastases also was reduced to 12% in Rb+/- mice by repeated i.v. administration of lipid-entrapped, polycation-condensed RB complementary DNA. Thus, in spite of long latency periods during which secondary alterations can accumulate, the initial loss of Rb function remains essential for tumor progression in multiple types of neuroendocrine cells. Restoration of RB function in humans may prove an effective general approach to the treatment of RB-deficient disseminated tumors.

Strain-dependent myeloid hyperplasia, growth deficiency, and accelerated cell cycle in mice lacking the Rb-related p107 gene

To investigate the function of the Rb-related p107 gene, a null mutation in p107 was introduced into the germ line of mice and bred into a BALB/cJ genetic background. Mice lacking p107 were viable and fertile but displayed impaired growth, reaching about 50% of normal weight by 21 days of age. Mutant mice exhibited a diathetic myeloproliferative disorder characterized by ectopic myeloid hyperplasia in the spleen and liver. Embryonic p107(-/-) fibroblasts and primary myoblasts isolated from adult p107(-/-) mice displayed a striking twofold acceleration in doubling time. However, cell sort analysis indicated that the fraction of cells in G1, S, and G2 was unaltered, suggesting that the different phases of the cell cycle in p107(-/-) cells was uniformly reduced by a factor of 2. Western analysis of cyclin expression in synchronized p107(-/-) fibroblasts revealed that expression of cyclins E and A preceded that of D1. Mutant embryos expressed approximately twice the normal level of Rb, whereas p130 levels were unaltered. Lastly, mutant mice reverted to a wild-type phenotype following a single backcross with C57BL/6J mice, suggesting the existence of modifier genes that have potentially epistatic relationships with p107. Therefore, we conclude that p107 is an important player in negatively regulating the rate of progression of the cell cycle, but in a strain-dependent manner.

Mutations of N-terminal regions render the retinoblastoma protein insufficient for functions in development and tumor suppression

To assess biological roles of the retinoblastoma protein (RB), four independent transgenic mouse lines expressing human RB with different deletions in the N-terminal region (RBdeltaN) were generated and compared with mice expressing identically regulated, full-length RB. Expression of both RB and RBdeltaN caused developmental growth retardation, but the wild-type protein was more potent. In contrast to wild-type RB, the RBdeltaN proteins were unable to rescue Rb-/- mice completely from embryonic lethality. Embryos survived until gestational day 18.5 but displayed defects in the terminal differentiation of erythrocytes, neurons, and skeletal muscle. In Rb+/- mice, expression of the RBdeltaN transgenes failed to prevent pituitary melanotroph tumors but delayed tumor formation or progression. These results strongly suggest that N-terminal regions are crucial for embryonic and postnatal development, tumor suppression, and the functional integrity of the entire RB protein. Furthermore, these transgenic mice provide models that may begin to explain human families with low-penetrance retinoblastoma and mutations in N-terminal regions of RB.

alpha-Adrenergic stimulation induces phosphorylation of retinoblastoma protein in neonatal rat ventricular myocytes

Mammalian cardiac myocytes become postmitotic shortly after birth, and the subsequent myocardial growth in adaptation to increasing workloads becomes primarily dependent on hypertrophy of existing myocytes. Although hypertrophic growth of cardiac myocytes has been extensively studied by using both in vitro and in vivo models, the molecular mechanism controlling the switch from hyperplastic to hypertrophic growth of cardiac myocytes is largely unknown. Since the majority of terminally differentiated cardiac myocytes are growth-arrested in G1/G0 phase, it has been hypothesized that the retinoblastoma protein (Rb) or its related pocket proteins which block G1/S transition becomes constitutively active during myocardial terminal differentiation. To test this hypothesis, we studied the regulation of Rb activity by alpha-adrenergic stimulation in neonatal rat ventricular myocytes which are mostly postmitotic in culture. Our results demonstrate that Rb is predominantly in the active hypo-phosphorylated state in control neonatal ventricular myocytes. alpha-Adrenergic stimulation activates G1/S transition in foetal but not neonatal rate ventricular myocytes. Although alpha-adrenergic stimulation does not activate G1/S transition in neonatal myocytes, it induces hyperphosphorylation of Rb to the same extent as in proliferating skeletal-muscle myoblasts or foetal ventricles. Hyper- but not hypo-phosphorylated Rb in stimulated neonatal myocytes or proliferating skeletal-muscle myoblasts fails to bind to the transcription factor, E2F, suggesting that hyper-phosphorylated Rb is inactive. Therefore F1/S transition could also be blocked at steps in addition to Rb inactivation during terminal differentiation and these blockades are refractory to alpha-adrenergic stimulation.

A thyroid hormone receptor coactivator negatively regulated by the retinoblastoma protein

The retinoblastoma protein (Rb) plays a critical role in cell proliferation, differentiation, and development. To decipher the mechanism of Rb function at the molecular level, we have systematically characterized a number of Rb-interacting proteins, among which is the clone C5 described here, which encodes a protein of 1,978 amino acids with an estimated molecular mass of 230 kDa. The corresponding gene was assigned to chromosome 14q31, the same region where genetic alterations have been associated with several abnormalities of thyroid hormone response. The protein uses two distinct regions to bind Rb and thyroid hormone receptor (TR), respectively, and thus was named Trip230. Trip230 binds to Rb independently of thyroid hormone while it forms a complex with TR in a thyroid hormone-dependent manner. Ectopic expression of the protein Trip230 in cells, but not a mutant form that does not bind to TR, enhances specifically TR-dependent transcriptional activity. Coexpression of wild-type Rb, but not mutant Rb that fails to bind to Trip230, inhibits such activity. These results not only identify a coactivator molecule that modulates TR activity, but also uncover a role for Rb in a pathway that responds to thyroid hormone.

Adenovirus-mediated retinoblastoma gene therapy suppresses spontaneous pituitary melanotroph tumors in Rb+/- mice

The retinoblastoma gene (RB) is the prototypic tumor suppressor. Studies to date have demonstrated cancer suppression with tumor cells reconstituted with RB ex vivo and implanted into immunodeficient mice, as well as with germline transmission of a human RB transgene into tumor-prone Rb +/- mice. To mimic the therapy of cancer more closely, spontaneous pituitary melanotroph tumors arising in immunocompetent Rb +/- mice were treated with a recombinant adenovirus carrying RB cDNA. Intratumoral RB gene transfer decreased tumor cell proliferation, reestablished innervation by growth-regulatory dopaminergic neurons, inhibited the growth of tumors, and prolonged the life spans of treated animals.

pRb controls proliferation, differentiation, and death of skeletal muscle cells and other lineages during embryogenesis

Mice deficient for the RB gene (RB-/-), prior to death at embryonic day 14.5, show increased cell death in all tissues that normally express RB1: the nervous system, liver, lens, and skeletal muscle precursor cells. We have generated transgenic mice (RBlox) that express low levels of pRb, driven by an RB1 minigene. RBlox/RB-/- mutant fetuses die at birth with specific skeletal muscle defects, including increased cell death prior to myoblast fusion, shorter myotubes with fewer myofibrils, reduced muscle fibers, accumulation of elongated nuclei that actively synthesized DNA within the myotubes, and reduction in expression of the late muscle-specific genes MCK and MRF4. Thus, insufficient pRb results in failure of myogenesis in vivo, manifest in two ways. First, the massive apoptosis of myoblasts implicates a role of pRb in cell survival. Second, surviving myotubes failed to develop normally and accumulated large polyploid nuclei, implicating pRb in permanent withdrawal from the cell cycle. These results demonstrate a role for pRb during terminal differentiation of skeletal muscles in vivo and place pRb at a nodal point that controls cell proliferation, differentiation, and death.

A new member of the hsp90 family of molecular chaperones interacts with the retinoblastoma protein during mitosis and after heat shock

A gene encoding a new heat shock protein that may function as a molecular chaperone for the retinoblastoma protein (Rb) was characterized. The cDNA fragment was isolated by using the yeast two-hybrid system and Rb as bait. The open reading frame of the longest cDNA codes for a protein with substantial sequence homology to members of the hsp90 family. Antibodies prepared against fusions between glutathione S-transferase and portions of this new heat shock protein specifically recognized a 75-kDa cellular protein, hereafter designated hsp75, which is expressed ubiquitously and located in the cytoplasm. A unique LxCxE motif in hsp75, but not in other hsp90 family members, appears to be important for binding to the simian virus 40 T-antigen-binding domain of hypophosphorylated Rb, since a single mutation changing the cysteine to methionine abolishes the binding. In mammalian cells, Rb formed complexes with hsp75 under two special physiological conditions: (i) during M phase, when the envelope that separates the nuclear and cytoplasmic compartments broke down, and (ii) after heat shock, when hsp75 moved from its normal cytoplasmic location into the nucleus. In vitro, hsp75 had a biochemical activity to refold denatured Rb into its native conformation. Taken together, these results suggest that Rb may be a physiological substrate for the hsp75 chaperone molecule. The discovery of a heat shock protein that chaperones Rb identifies a mechanism, in addition to phosphorylation, by which Rb is regulated in response to progression of the cell cycle and to external stimuli.

The transcription factor E2F-1 mediates the autoregulation of RB gene expression

The retinoblastoma (RB) gene is the prototype tumor suppressor gene. Mutations in this gene are often associated with the occurrence of various tumors. Several mutations have been found in the promoter region of the gene, suggesting that inappropriate transcriptional regulation of the RB gene contributes to tumorigenesis. Sequence analysis of the RB promoter has revealed a potential E2F recognition site within a region critical for RB gene transcription. By using the cloned E2F-1 gene, here we report that (i) RB expression is negatively regulated by its own gene product, (ii) E2F-1 binds specifically to an E2F recognition sequence in the RB promoter and transactivates the RB promoter, (iii) overexpression of RB suppresses E2F-1-mediated stimulation of RB promoter activity, and (iv) the expression of the RB gene is paralleled by the expression of the E2F-1 gene during cell cycle progression. These results demonstrate that expression of RB is negatively autoregulated through E2F-1.

The transcription factor E2F-1 mediates the autoregulation of RB gene expression

The retinoblastoma (RB) gene is the prototype tumor suppressor gene. Mutations in this gene are often associated with the occurrence of various tumors. Several mutations have been found in the promoter region of the gene, suggesting that inappropriate transcriptional regulation of the RB gene contributes to tumorigenesis. Sequence analysis of the RB promoter has revealed a potential E2F recognition site within a region critical for RB gene transcription. By using the cloned E2F-1 gene, here we report that (i) RB expression is negatively regulated by its own gene product, (ii) E2F-1 binds specifically to an E2F recognition sequence in the RB promoter and transactivates the RB promoter, (iii) overexpression of RB suppresses E2F-1-mediated stimulation of RB promoter activity, and (iv) the expression of the RB gene is paralleled by the expression of the E2F-1 gene during cell cycle progression. These results demonstrate that expression of RB is negatively autoregulated through E2F-1.