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

p53 status and the efficacy of cancer therapy in vivo

The therapeutic responsiveness of genetically defined tumors expressing or devoid of the p53 tumor suppressor gene was compared in immunocompromised mice. Tumors expressing the p53 gene contained a high proportion of apoptotic cells and typically regressed after treatment with gamma radiation or adriamycin. In contrast, p53-deficient tumors treated with the same regimens continued to enlarge and contained few apoptotic cells. Acquired mutations in p53 were associated with both treatment resistance and relapse in p53-expressing tumors. These results establish that defects in apoptosis, here caused by the inactivation of p53, can produce treatment-resistant tumors and suggest that p53 status may be an important determinant of tumor response to therapy.

Nerve growth factor regulates the expression and activity of p33cdk2 and p34cdc2 kinases in PC12 pheochromocytoma cells

In the absence of serum, nerve growth factor (NGF) promotes the survival and differentiation of the PC12 pheochromocytoma cell line. In the presence of serum, NGF acts primarily as a differentiation factor and negative regulator of cell cycling. To investigate NGF control of cell cycling, we have analyzed the regulation of cyclin dependent kinases during PC12 cell differentiation. NGF treatment leads to a reduction in the steady-state protein levels of p33cdk2 and p34cdc2, two key regulators of cell cycle progression. The decrease in p33cdk2 and p34cdc2 coincides with a decrease in the enzymatic activity of cyclinA-p34cdc2, cyclinB-p34cdc2, cyclinE-p33cdk2, and cyclinA-p33cdk2 kinases. The decline in p33cdk2 and p34cdc2 kinase activity in response to NGF is accelerated in cells that over-express the p140trk NGF receptor, suggesting that the timing of the down- regulation is dependent on the level of p140trk and the strength of the NGF signal. The level of cyclin A, a regulatory subunit of p33cdk2 and p34cdc2, is relatively constant during PC12 differentiation. Nevertheless, the DNA binding activity of the cyclinA-associated transcription factor E2F/DP decreases. Thus, NGF down-regulates the activity of cyclin dependent kinases and cyclin-transcription factor complexes during PC12 differentiation.

The retinoblastoma protein-binding region of simian virus 40 large T antigen alters cell cycle regulation in lenses of transgenic mice

Regulation of the cell cycle is a critical aspect of cellular proliferation, differentiation, and transformation. In many cell types, the differentiation process is accompanied by a loss of proliferative capability, so that terminally differentiated cells become postmitotic and no longer progress through the cell cycle. In the experiments described here, the ocular lens has been used as a system to examine the role of the retinoblastoma protein (pRb) family in regulation of the cell cycle during differentiation. The ocular lens is an ideal system for such studies, since it is composed of just two cell types: epithelial cells, which are capable of proliferation, and fiber cells, which are postmitotic. In order to inactivate pRb in viable mice, genes encoding either a truncated version of simian virus 40 large T antigen or the E7 protein of human papillomavirus were expressed in a lens-specific fashion in transgenic mice. Lens fiber cells in the transgenic mice were found to incorporate bromodeoxyuridine, implying inappropriate entry into the cell cycle. Surprisingly, the lens fiber cells did not proliferate as tumor cells but instead underwent programmed cell death, resulting in lens ablation and microphthalmia. Analogous lens alterations did not occur in mice expressing a modified version of the truncated T antigen that was mutated in the binding domain for the pRb family. These experimental results indicate that the retinoblastoma protein family plays a crucial role in blocking cell cycle progression and maintaining terminal differentiation in lens fiber cells. Apoptotic cell death ensues when fiber cells are induced to remain in or reenter the cell cycle.

The retinoblastoma protein-binding region of simian virus 40 large T antigen alters cell cycle regulation in lenses of transgenic mice

Regulation of the cell cycle is a critical aspect of cellular proliferation, differentiation, and transformation. In many cell types, the differentiation process is accompanied by a loss of proliferative capability, so that terminally differentiated cells become postmitotic and no longer progress through the cell cycle. In the experiments described here, the ocular lens has been used as a system to examine the role of the retinoblastoma protein (pRb) family in regulation of the cell cycle during differentiation. The ocular lens is an ideal system for such studies, since it is composed of just two cell types: epithelial cells, which are capable of proliferation, and fiber cells, which are postmitotic. In order to inactivate pRb in viable mice, genes encoding either a truncated version of simian virus 40 large T antigen or the E7 protein of human papillomavirus were expressed in a lens-specific fashion in transgenic mice. Lens fiber cells in the transgenic mice were found to incorporate bromodeoxyuridine, implying inappropriate entry into the cell cycle. Surprisingly, the lens fiber cells did not proliferate as tumor cells but instead underwent programmed cell death, resulting in lens ablation and microphthalmia. Analogous lens alterations did not occur in mice expressing a modified version of the truncated T antigen that was mutated in the binding domain for the pRb family. These experimental results indicate that the retinoblastoma protein family plays a crucial role in blocking cell cycle progression and maintaining terminal differentiation in lens fiber cells. Apoptotic cell death ensues when fiber cells are induced to remain in or reenter the cell cycle.

An early haematopoietic defect in mice lacking the transcription factor GATA-2

Blood cell development relies on the expansion and maintenance of haematopoietic stem and progenitor cells in the embryo. By gene targeting in mouse embryonic stem cells, we demonstrate that the transcription factor GATA-2 plays a critical role in haematopoiesis, particularly of an adult type. We propose that GATA-2 regulates genes controlling growth factor responsiveness or the proliferative capacity of early haematopoietic cells.

p53-dependent apoptosis produced by Rb-deficiency in the developing mouse lens

The retinoblastoma tumour-suppressor gene (RB) has been implicated in negative growth regulation, induction of differentiation, and inhibition of cellular transformation. Homozygous inactivation of the Rb gene in the mouse leads to mid-gestational lethality with defects in erythropoiesis and neurogenesis. Here we describe the effects of the Rb-deficient state on the development of the ocular lens. The regional compartmentalization of growth, differentiation and apoptosis in the developing lens provides an ideal system to examine more closely the relationships of these processes in vivo. We demonstrate that loss of Rb function is associated with unchecked proliferation, impaired expression of differentiation markers, and inappropriate apoptosis in lens fibre cells. In addition, we show that ectopic apoptosis in Rb-deficient lenses is dependent on p53, because embryos doubly null for Rb and p53 show a nearly complete suppression of this effect. This developmental system provides a framework for understanding the consequences of the frequent mutation of both RB and p53 in human cancer.

Dual roles of the retinoblastoma protein in cell cycle regulation and neuron differentiation

To assess the functions of the retinoblastoma protein (RB) during normal development, we have analyzed mouse embryos that lack a functional copy of the retinoblastoma gene (genotype: Rb-1 delta 20/Rb-1 delta 20). Our findings demonstrate that RB plays an important role in the regulation of the neuronal cell cycle. In mutant embryos, dividing cells are found well outside of the normal neurogenic regions in both the central and peripheral nervous systems. In addition to abnormal cell cycle regulation, however, the mutant embryos show two less expected phenotypes. First, many of the ectopically dividing cells die by apoptosis shortly after their entrance into S phase. In sensory ganglia, most nerve cells die by this process, beginning at about the same time as normal target-related neuronal death. Second, although the expression of certain differentiation markers such as N-CAM and Brn-3.0 appears to be near normal, nerve cells, especially in sensory ganglia, do not mature properly. Their morphology is stunted and expression of neuronal beta II tubulin is greatly reduced. Preferential reduction in the expression of TrkA, TrkB, and the low-affinity neurotrophin receptor p75LNGFR may be relevant to neuronal cell death and lack of neuronal differentiation seen in the mutant embryos. Primary cultures of dorsal root and trigeminal ganglion cells from later stage mutant embryos reveal a decrease in neuronal cell survival and in neurite outgrowth even in the presence of the appropriate neurotrophins. Taken together, these results suggest that the p110RB protein not only regulates progression through the cell cycle but is also important for cell survival and differentiation.

Generation of lymphohematopoietic cells from embryonic stem cells in culture

An efficient system was developed that induced the differentiation of embryonic stem (ES) cells into blood cells of erythroid, myeloid, and B cell lineages by coculture with the stromal cell line OP9. This cell line does not express functional macrophage colony-stimulating factor (M-CSF). The presence of M-CSF had inhibitory effects on the differentiation of ES cells to blood cells other than macrophages. Embryoid body formation or addition of exogenous growth factors was not required, and differentiation was highly reproducible even after the selection of ES cells with the antibiotic G418. Combined with the ability to genetically manipulate ES cells, this system will facilitate the study of molecular mechanisms involved in development and differentiation of hematopoietic cells.

Cooperative tumorigenic effects of germline mutations in Rb and p53

The tumour suppressor genes Rb and p53 are mutated in several types of human cancer, and many tumour types carry mutations in both genes. To study how these genes normally function, we and others have created mouse strains with Rb and p53 mutations. Here we describe the phenotypic effects of combined germline mutations in these two tumour suppressor genes. Mice mutant for both genes have reduced viability and exhibit novel pathology including pinealoblastomas, islet cell tumours, bronchial epithelial hyperplasia and retinal dysplasia. These data indicate that mutations in Rb and p53 can cooperate in the transformation of certain cell types in the mouse.

More to learn from gene knockouts

Gene targeting by homologous recombination in mouse embryonic stem cells is a powerful technique to determine the physiological function of any gene product in embryonic and postnatal development and in molecular pathogenesis. Although the technique is very demanding and still in its developing stage several knockout mice carrying disrupted genes, which were once thought important for the development or molecular pathogenesis of certain tissues, have given unexpected results. A gene/function redundancy or superfluous and on-functional theory has been advanced by many investigators to explain the unexpected results. These surprising results may teach us a new lesson and lead to a revision of the strongly held view that highly conserved and abundantly expressed genes have a prominent role and function in cell physiology and development. Additional, they may also support the notion that molecular cross-talk among the genes may play an important role in determining the minimal phenotype.

The oncogenic cysteine-rich LIM domain protein rbtn2 is essential for erythroid development

The LIM domain protein rbtn2 is associated with T cell acute leukemias. We demonstrate that rbtn2 is a nuclear protein expressed in the erythroid lineage in vivo, and using homologous recombination, we show that it is essential for erythroid development in mice. The homozygous rbtn2 null mutation leads to failure of yolk sac erythropoiesis and embryonic lethality around E10.5. Moreover, in vitro differentiation of yolk sac tissue from homozygous mutant mice and sequentially targeted double-mutant ES cells demonstrates a block to erythroid development. This shows a pivotal role for a LIM domain protein in lineage specification during mammalian development and suggests that RBTN2 and GATA-1 are critical at similar stages of erythroid differentiation.

Loss of heterozygosity at the retinoblastoma locus in human pituitary tumors

BACKGROUND

Recent studies using knockouts of the Retinoblastoma (Rb) gene by homologous recombination in transgenic mice have revealed that a high frequency of heterozygous animals develop pituitary tumors associated with loss of heterozygosity (LOH) at the Rb locus. The authors have determined the frequency of LOH at the Rb locus in 42 benign human pituitary tumors.

METHODS

Polymerase chain reaction- (PCR) amplification of polymorphic regions in introns 17 and 20 of the human Rb gene was used to detect heterozygosity in pituitary tumor DNA and matched control DNA samples.

RESULTS

The PCR assay was informative in 42 of 48 pituitary tumors examined, and no allelic deletion of Rb was detected in any of the tumors.

CONCLUSIONS

These studies confirmed a recent report that LOH at the Rb locus is rare in benign human pituitary tumors.

Tumour predisposition in mice heterozygous for a targeted mutation in Nf1

Human neurofibromatosis type 1 is a dominant disease caused by the inheritance of a mutant allele of the NF1 gene. In order to study NF1 function, we have constructed a mouse strain carrying a germline mutation in the murine homologue. Heterozygous animals do not exhibit the classical symptoms of the human disease, but are highly predisposed to the formation of various tumour types, notably phaeochomocytoma, a tumour of the neural crest-derived adrenal medulla, and myeloid leukaemia, both of which occur with increased frequency in human NF1 patients. The wild-type Nf1 allele is lost in approximately half of the tumours from heterozygous animals. In addition, homozygosity for the Nf1 mutation leads to abnormal cardiac development and mid-gestational embryonic lethality.

Parental origin of germ-line and somatic mutations in the retinoblastoma gene

Segregation analysis of polymorphic sites within the retinoblastoma (RB) gene and on chromosome 13, as well as the parental origin of the lost allele in the tumor, were analyzed in 24 families with RB patients. Four mutant alleles transmitted through the germ-line and seven de novo germ-line mutant alleles were identified in 11 patients with hereditary RB. Segregation analysis within the RB gene and on chromosome 13 was useful for DNA diagnosis of susceptibility to RB in relatives of hereditary patients, even if mutations were not identified. All seven de novo germ-line mutant alleles were paternally derived. The bias toward the paternal allele for de novo germ-line mutations of the RB gene was statistically significant. Seven paternal alleles and six maternal alleles were lost in 13 non-hereditary RB tumors with no bias in the parental origin of the somatic allele loss. These results suggest that the physical environment or a deficiency in DNA repair during spermatogenesis may be associated with significant risk factors for de novo germ-line mutations.

Reversal of terminal differentiation mediated by p107 in Rb-/- muscle cells

The terminal differentiation of mammalian muscle cells requires the tumor suppressor retinoblastoma protein (Rb). Unlike their wild-type counterparts, multinucleated myotubes from mouse cells deficient in Rb (Rb-/-) were induced by serum to re-enter the cell cycle. Development of the myogenic phenotype in Rb-/- cells correlated with increased expression of p107, which interacted with myogenic transcription factors. Serum-induced cell cycle reentry, on the other hand, correlated with decreased p107 expression. Thus, although p107 could substitute for Rb as a cofactor for differentiation, it could not maintain the terminally differentiated state in Rb-/- myotubes.

Gene-targeting and the p53 tumor-suppressor gene

Gene-targeting techniques are now frequently applied to embryonic stem (ES) cells to introduce mutations of endogenous genes in mice. Modifications introduced into tumor-suppressor genes by this technology have produced mice and cell lines with unique tumorigenic and growth characteristics, respectively. A number of strategies have been developed to enhance the efficiency of homologous recombination between targeting vectors and endogenous genes. This review describes recent advances in the techniques used to construct mice with a variety of genetic alterations. In addition, an application of gene-targeting is illustrated in the study of a class of genes with tumor-suppressor function. Recent findings from experiments using gene targeted mice to study the p53 tumor-suppressor gene are discussed and the potential of gene-targeting for the discovery and study of novel tumor-suppressor genes are explored.

The retinoblastoma gene and its significance

The first human tumour suppressor gene, the Retinoblastoma Susceptibility gene (RB1) was first demonstrated in retinoblastoma, a rare paediatric eye tumour which has been studied extensively over the last century. Genetic studies of retinoblastoma have yielded unique insights into familial cancer syndromes and the mechanisms of oncogenesis by tumour suppressor genes such as the RB1 gene. In this view, we will summarize past research into the genetics of retinoblastoma that led to the discovery of the RB1 gene and discuss the influence these results have had on the field of cancer research. In addition, we will discuss current research into RB1 as it relates to cancer and its potential for new therapies.

Apoptosis or retinoblastoma: alternative fates of photoreceptors expressing the HPV-16 E7 gene in the presence or absence of p53

A transgenic mouse model for retinoblastoma was produced previously by directing SV40 T antigen expression to retinal photoreceptor cells using the promoter of the interstitial retinol-binding protein (IRBP) gene. This gene becomes active prior to the terminal differentiation of photoreceptors. Because T antigen-transforming activity is attributable, at least in part, to the inactivation of the retinoblastoma (pRb) and p53 tumor suppressor proteins, we addressed the role of p53 in the development of retinoblastoma in mice. Transgenic mice expressing HPV-16 E7 under the control of the IRBP promoter were generated to inactivate pRb in photoreceptors while leaving p53 intact. Rather than developing retinoblastomas, the retinas of these mice degenerate due to photoreceptor cell death at a time in development when photoreceptors are normally undergoing terminal differentiation. The dying cells exhibit the histological and ultrastructural features of apoptosis and contain fragmented DNA. p53 is required for the induction of apoptosis in this model, because mice expressing E7 in a p53 nullizygous background develop retinal tumors instead of undergoing retinal degeneration.

Altered cell cycle regulation in the lens of HPV-16 E6 or E7 transgenic mice: implications for tumor suppressor gene function in development

Tumor suppressor proteins are believed to play a role in regulating cell cycle control during mammalian development. The E6 and E7 oncoproteins from human papillomavirus type 16 are known to affect cell growth control, at least in part, through their inactivation of cellular tumor suppressor gene products, p53 and Rb, respectively. Therefore, these viral proteins can serve as trans-dominant repressors of tumor suppressor gene function. To study the potential role of p53 and Rb in murine lens morphogenesis, we generated transgenic mice in which the expression of E6 or E7 was directed to the developing lens. Transgenic mice expressing E7 exhibited microphthalmia and cataracts, whereas transgenic mice expressing E6 exhibited cataracts without noticeable microphthalmia. Microscopic analysis of the lenses from neonatal and adult E7 transgenic mice revealed inhibition of lens fiber cell differentiation, induction of cell proliferation in spatially inappropriate regions of the lens, and apoptosis. Transgenic mice expressing a mutant E7 that is defective in Rb/p107 binding exhibited normal eyes, suggesting that the activity of Rb and/or Rb-like proteins is required for the perturbation of lens development and induction of apoptosis in E7 mice. Microscopic analysis of lenses from E6 neonatal and adult transgenic mice indicated the presence of nuclei in elongated fiber cells, suggesting that E6 inhibits lens fiber cell denucleation. Furthermore, expression of E6 inhibited the apoptotic-like DNA degradation observed in the lenses of nontransgenic 15.5-day embryos. In lenses from neonatal E6 x E7 double transgenic mice, the level of apoptosis was reduced compared with that seen in lenses from neonatal E7 mice. In adults E6 x E7 double transgenic mice, lens tumors developed, whereas in E6 or E7 only transgenic mice, tumors did not. Taken together, these results point to specific roles in lens morphogenesis for Rb and p53 and to the necessity of these tumor suppressor gene products in regulating exit from the normal cell division cycle in differentiating lens fiber cells.

Growth arrest by induction of p53 in DNA damaged keratinocytes is bypassed by human papillomavirus 16 E7

Cellular tumor suppressors p53 and Rb play an important role in controlling cell proliferation. Inactivation of these tumor suppressor proteins can occur by gene mutation or by association with oncoproteins from the small DNA tumor viruses. One function of p53 is in regulating cell cycle check-point control after DNA damage. To dissect the pathways by which p53 and Rb may act, the E6 and E7 oncogenes of human papillomavirus (HPV) types 6 and 16 were introduced into primary human epithelial cells by retroviral transfer vector, and cells were assayed for growth arrest after DNA damage induced by actinomycin D. The E6 or E7 oncogenes from the low-risk HPV6 had no affect on growth arrest, p53 protein levels increased, Rb protein levels decreased, and Rb was predominantly in the hypophosphorylated state similar to vector-infected cells. Either the E6 or the E7 oncogene from the high-risk HPV16 abrogated growth arrest. Cells expressing HPV16 E6 (16E6) were severely reduced in p53 protein levels that did not increase detectably after DNA damage, Rb protein levels did not decrease, and hyperphosphorylated Rb was present. After DNA damage in cells expressing 16E7 p53 levels increased, and Rb protein levels decreased; however, Rb was predominantly in the hyperphosphorylated state. Even though p53 protein levels increased in response to DNA damage in cells expressing 16E7, G1 growth arrest was bypassed. This suggests that the circuitry controlling the growth arrest signal after DNA damage may be interconnected between the p53 and Rb pathways.

Inactivation of the retinoblastoma gene appears to be very uncommon in myelodysplastic syndromes

Rearrangements of the retinoblastoma (RB) gene have been reported in a few cases of myelodysplastic syndromes (MDS). In addition, low or absent expression of the RB protein is found in 20-30% of cases of acute myeloid leukaemias (AML), particularly in AML with a monocytic component (M4 or M5). We performed Southern blot analysis of the RB gene in 90 cases of MDS, including 37 cases of chronic myelomonocytic leukaemia (CMML). None of them had progressed to AML at the time of study. In 37/90 patients (including 20 CMML) Northern blot analysis, study of RB protein by immunocytochemistry on bone marrow slides, and detection of point mutations in exons 20-24 of the RB gene was also made, using single strand conformation polymorphism analysis (SSCP). No abnormal Southern profile was found in any of the 90 patients. Northern blot and immunocytochemical study of RB protein were normal in the 37 cases studied. SSCP analysis detected a point mutation in 2/37 patients tested. Direct sequencing confirmed the mutation in each case, which involved intron 21 and intron 23, respectively, and was located outside splicing sites of the neighbouring exons. These findings suggest that abnormalities of the RB gene and its expression must be very rare in MDS, and play a minor role, if any, in the pathophysiology of those disorders, at least before progression to AML.

Induction versus progression of brain tumor development: differential functions for the pRB- and p53-targeting domains of simian virus 40 T antigen

The ability of simian virus 40-encoded large T antigen to disrupt the growth control of a variety of cell types is related to its ability to interfere with certain cellular proteins, such as p53 and the retinoblastoma susceptibility gene product (pRB). We have used wild-type and mutant forms of T antigen in transgenic mice to dissect the roles of pRB, p53, and other cellular proteins in tumorigenesis of different cell types. In this study, using a cell-specific promoter to target expression specifically to brain epithelium (the choroid plexus) and to B and T lymphoid cells, we characterize the tumorigenic capacity of a T-antigen fragment that comprises only the amino-terminal 121 residues. This fragment (dl1137) retains the ability to interact with pRB and p107 but lacks the p53-binding domain. While loss of the p53-binding region results in loss of the capacity to induce lymphoid abnormalities, dl1137 retains the ability to induce choroid plexus tumors that are histologically indistinguishable from those induced by wild-type T antigen. Tumors induced by dl1137 develop much more slowly, however, reaching an end point at around 8 months of age rather than at 1 to 2 months. Analysis of tumor progression indicates that tumor induction by dl1137 does not require secondary genetic or epigenetic events. Rather, the tumor growth rate is significantly slowed, indicating that the T-antigen C-terminal region contributes to tumor progression in this cell type. In contrast, the pRB-binding region appears essential for tumorigenesis as mutation of residue 107, known to disrupt pRB and p107 binding to wild-type T antigen, abolishes the ability of the dl1137 protein to induce growth abnormalities in the brain.

p53 and E2F-1 cooperate to mediate apoptosis

The tumor-suppressor protein p53 appears to function at the G1 phase of the cell cycle as a checkpoint in response to DNA damage. Mutations in the p53 gene lead to an increased rate of genomic instability and tumorigenesis. The E2F-1 transcription factor is a protein partner of the retinoblastoma-susceptibility gene product, RB. E2F-1 appears to function as a positive regulator or signal for entry into S phase. To explore possible interactions of p53 and E2F-1 in the cell cycle, a human E2F-1 expression plasmid was introduced into a murine cell line containing a temperature-sensitive p53 allele which produces a p53 protein in the wild-type conformation at 32 degrees C and the mutant form at 37.5 degrees C. Coexpression of the wild-type p53 protein and E2F-1 in these cells resulted in a rapid loss of cell viability through a process of apoptosis. Thus, the cell cycle utilizes an interacting or communicative pathway between RB-E2F-1 and p53.

Induction versus progression of brain tumor development: differential functions for the pRB- and p53-targeting domains of simian virus 40 T antigen

The ability of simian virus 40-encoded large T antigen to disrupt the growth control of a variety of cell types is related to its ability to interfere with certain cellular proteins, such as p53 and the retinoblastoma susceptibility gene product (pRB). We have used wild-type and mutant forms of T antigen in transgenic mice to dissect the roles of pRB, p53, and other cellular proteins in tumorigenesis of different cell types. In this study, using a cell-specific promoter to target expression specifically to brain epithelium (the choroid plexus) and to B and T lymphoid cells, we characterize the tumorigenic capacity of a T-antigen fragment that comprises only the amino-terminal 121 residues. This fragment (dl1137) retains the ability to interact with pRB and p107 but lacks the p53-binding domain. While loss of the p53-binding region results in loss of the capacity to induce lymphoid abnormalities, dl1137 retains the ability to induce choroid plexus tumors that are histologically indistinguishable from those induced by wild-type T antigen. Tumors induced by dl1137 develop much more slowly, however, reaching an end point at around 8 months of age rather than at 1 to 2 months. Analysis of tumor progression indicates that tumor induction by dl1137 does not require secondary genetic or epigenetic events. Rather, the tumor growth rate is significantly slowed, indicating that the T-antigen C-terminal region contributes to tumor progression in this cell type. In contrast, the pRB-binding region appears essential for tumorigenesis as mutation of residue 107, known to disrupt pRB and p107 binding to wild-type T antigen, abolishes the ability of the dl1137 protein to induce growth abnormalities in the brain.

The 1993 Walter Hubert Lecture: the role of the p53 tumour-suppressor gene in tumorigenesis

The p53 tumour-suppressor gene is mutated in 60% of human tumours, and the product of the gene acts as a suppressor of cell division. It is thought that the growth-suppressive effects of p53 are mediated through the transcriptional transactivation activity of the protein. Overexpression of the p53 protein results either in arrest in the G1 phase of the cell cycle or in the induction of apoptosis. Both the level of the protein and its transcriptional transactivation activity increase following treatment of cells with agents that damage DNA, and it is thought that p53 acts to protect cells against the accumulation of mutations and subsequent conversion to a cancerous state. The induction of p53 levels in cells exposed to gamma-irradiation results in cell cycle arrest in some cells (fibroblasts) and apoptosis in others (thymocytes). Cells lacking p53 have lost this cell cycle control and presumably accumulate damage-induced mutations that result in tumorigenesis. Thus, the role of p53 in suppressing tumorigenesis may be to rescue the cell or organism from the mutagenic effects of DNA damage. Loss of p53 function accelerates the process of tumorigenesis and alters the response of cells to agents that damage DNA, indicating that successful strategies for radiation therapy may well need to take into account the tissue of origin and the status of p53 in the tumour.

DRTF1/E2F: an expanding family of heterodimeric transcription factors implicated in cell-cycle control

During the cell cycle, the transcription of certain genes is integrated with cell-cycle progression, thus providing an important level of control. In mammalian cells, DRTF1/E2F is a transcription activity comprising a group of related heterodimeric transcription factors that function in this integration process. The primary molecules involved in generating the afferent signals that converge on DRTF1/E2F belong to a class of proteins, exemplified by the retinoblastoma tumour suppressor gene product, whose activities are, in turn, regulated by cyclin-dependent kinases. The transcriptional activity of DRTF1/E2F is therefore regulated through a pathway that links the machinery of the cell cycle to the transcription apparatus. As such, it is likely to play a pivotal role in regulating cell-cycle progression.

The cell cycle and the retinoblastoma protein family

Tumor formation results from alterations in the control of normal cell proliferation. To further our understanding of the molecular mechanisms underlying the deregulation of cell proliferation much attention, over the past decade, has been focused on the function of proto-oncogenes. Cellular oncogenes are thought to be growth promoting. More recently, a class of genes known as tumor suppressors have come under intense study. Tumor suppressors are largely thought to restrain cell proliferation. The retinoblastoma protein (Rb) is one of a growing list of tumor suppressors. Concurrent with the study of tumor suppressor genes has been a rapid increase in our understanding of the cell cycle at the molecular level. Rb and a related protein p107 are involved in the processes of cell proliferation and differentiation. Each functionally interacts with and affects the activity of the transcription factor E2F as well as other transcription factors involved in cell proliferation and differentiation. Additionally, Rb and p107 are modified by, and/or form specific complexes with, several elements of the basic cell cycle machinery. Specifically, Rb and p107 interact with and are modified by various cyclins and cyclin dependent kinases (cdk), some of which have been shown to be essential for cell cycle progression and in some cases their deregulation has been implicated in the development of cancer. This review will attempt to convey our current functional and mechanistic understanding of the biological roles Rb and p107 play in proliferation, development and differentiation. A knowledge of the interplay between these positive and negative regulators of cell proliferation and differentiation, noted above, is central to our understanding of human cancer.

In vivo and in vitro models of medulloblastomas and other primitive neuroectodermal brain tumors of childhood

Recent advances in understanding the basic biology of the neoplastic cells that populate childhood primitive neuroectodermal tumors (PNET) of the central nervous system (CNS) underline several unique properties of these common pediatric brain neoplasms. For example, studies of posterior fossa cerebellar medulloblastomas (MB), a prototypical group of brain tumors that comprise the largest class of PNET, suggest that the molecular phenotype of subpopulations of neoplastic cells in MB partially recapitulates stages in the acquisition of the neuronal phenotype by normal developing human CNS progenitor cells. However, as reviewed here, it appears that the neoplastic cells in MB exhibit one or more molecular defects in the sequence of normal maturational events that enable CNS progenitor cells to exit the cell cycle, become committed to the neuronal lineage, and undergo terminal differentiation into fully mature, permanently postmitotic CNS neurons. Indeed, since PNET emerge almost exclusively in early childhood, the induction of PNET may result from genetic lesions that arise in developing CNS progenitor cells thereby preventing these neural precursors from executing normal programs of lineage commitment and differentiation in the CNS. Clarification of how lineage commitment and maturation in PNET comprised of neuron-like tumor cells deviate from normal CNS development may clarify how oncogenes and tumor suppressor genes exert their effects in a cell type specific manner at different stages in the normal maturation of CNS cells. Recently, a number of potentially effective in vitro and in vivo model systems of PNET have been developed. Since these model systems could facilitate efforts to elucidate mechanisms of neoplastic transformation and tumor progression in the CNS, we review the potential utility of several recently described in vitro (e.g., MB cell lines) and in vivo (e.g., transgenic mice) experimental systems as models of authentic childhood CNS neoplasms.

Gene targeting and development of the nervous system

Gene targeting provides a means of directly assaying the function of specific genes during mouse nervous system development. Generation of targeted mutant mice has provided the first evidence of developmental roles for genes whose function was suggested based on their expression, but for which appropriate assay systems were lacking. In other cases, where gene function was known, targeted mutations have revealed in which cell population, and at what developmental stage, particular genes are first indispensable. The existing targeted mutants suggest that an early mechanism of pattern formation in mammals involves regional control of proliferation or survival of neural precursors, and that later general functions, such as the control of differentiation of precursors, may be performed by different genes in distinct neural lineages. As many genes display complex temporal and spatial patterns of expression, analysis of the full range of functions of such genes will require the generation of a series of alleles, including stage- and tissue-specific mutations.

Tumor suppressor genes

The mutation of tumor suppressor genes is thought to contribute to tumor growth by inactivating proteins that normally act to limit cell proliferation. Several tumor suppressor proteins have been identified in recent years, but only two of them, p53 and pRb, are understood in detail. In the past year, a role has become apparent for both of these proteins in transcription and phosphorylation events required for passage of a cell from G1 to S phase. The pRb protein appears to prevent the function of transcription factors and other proteins needed for S phase until its inactivation by cyclin-dependent kinases in late G1. Induction of p53 by DNA damage may act to cause cell cycle arrest or cell death by altering the transcription program of damaged cells. A detailed molecular understanding of these growth regulators is now emerging, and is the subject of this review.

Analysis of cell cycle-related gene expression in postmitotic neurons: selective induction of Cyclin D1 during programmed cell death

Sympathetic neurons undergo RNA and protein synthesis-dependent programmed cell death when deprived of nerve growth factor. To test the hypothesis that neuronal programmed cell death is a consequence of conflicting growth signals which cause the inappropriate activation of cell cycle genes, we have analyzed cell cycle-related genes for their expression in postmitotic neurons. Surprisingly, many of these genes are expressed in neurons, although cdc2, cdk2, and cyclin A are not. During programmed cell death, the expression of most of these genes, including several cyclins and the Rb and p53 tumor suppressor genes, decreases similar to that of neuronal genes. In contrast, cyclin D1 expression is selectively induced in dying neurons. Cyclin D1 mRNA levels peak 15-20 hr after nerve growth factor withdrawal, concurrent with the time that neurons become committed to die. These results provide an extensive characterization of cell cycle gene expression in postmitotic neurons and provide the evidence for a gene induced during neuronal programmed cell death.

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.

Mouse homologues of human hereditary disease

Details are given of 214 loci known to be associated with human hereditary disease, which have been mapped on both human and mouse chromosomes. Forty two of these have pathological variants in both species; in general the mouse variants are similar in their effects to the corresponding human ones, but exceptions include the Dmd/DMD and Hprt/HPRT mutations which cause little, if any, harm in mice. Possible reasons for phenotypic differences are discussed. In most pathological variants the gene product seems to be absent or greatly reduced in both species. The extensive data on conserved segments between human and mouse chromosomes are used to predict locations in the mouse of over 50 loci of medical interest which are mapped so far only on human chromosomes. In about 80% of these a fairly confident prediction can be made. Some likely homologies between mapped mouse loci and unmapped human ones are also given. Sixty six human and mouse proto-oncogene and growth factor gene homologies are also listed; those of confirmed location are all in known conserved segments. A survey of 18 mapped human disease loci and chromosome regions in which the manifestation or severity of pathological effects is thought to be the result of genomic imprinting shows that most of the homologous regions in the mouse are also associated with imprinting, especially those with homologues on human chromosomes 11p and 15q. Useful methods of accelerating the production of mouse models of human hereditary disease include (1) use of a supermutagen, such as ethylnitrosourea (ENU), (2) targeted mutagenesis involving ES cells, and (3) use of gene transfer techniques, with production of 'knockout mutations'.

Tumor spectrum analysis in p53-mutant mice

BACKGROUND

The p53 tumor suppressor gene is mutated in a large percentage of human malignancies, including tumors of the colon, breast, lung and brain. Individuals who inherit one mutant allele of p53 are susceptible to a wide range of tumor types. The gene encodes a transcriptional regulator that may function in the cellular response to DNA damage. The construction of mouse strains carrying germline mutations of p53 facilitates analysis of the function of p53 in normal cells and tumorigenesis.

RESULTS

In order to study the effects of p53 mutation in vivo, we have constructed a mouse strain carrying a germline disruption of the gene. This mutation removes approximately 40% of the coding capacity of p53 and completely eliminates synthesis of p53 protein. As observed previously for a different germline mutation of p53, animals homozygous for this p53 deletion mutation are viable but highly predisposed to malignancy. Heterozygous animals also have an increased cancer risk, although the distribution of tumor types in these animals differs from that in homozygous mutants. In most cases, tumorigenesis in heterozygous animals is accompanied by loss of the wild-type p53 allele.

CONCLUSION

We reaffirm that p53 function is not required for normal mouse development and conclude that p53 status can strongly influence tumor latency and tissue distribution.

Tumor suppressor genes and their roles in breast cancer

Tumor suppressor genes have been identified by the occurrence of mutations in many families with hereditary forms of cancer, exposed during development of the tumor by loss of heterozygosity. They have a number of diverse functions. For example, both the RB gene of retinoblastoma and the p53 gene, which is commonly mutated in breast and colon cancer among others, produce proteins involved in distinct steps of cell cycle control, while the nm23 product prevents metastasis. Here we review the data developed until now on the possible presence and role of mutations in these and other tumor suppressor genes in breast cancer. A more complete understanding of the tumor suppressor genes could not only provide diagnostic information, but could lead to specific gene therapy to replace suppressor functions lost in individual tumors.

Infrequency of ras, p53, WT1, or RB gene alterations in Wilms tumors

BACKGROUND

Alteration of the ras family of oncogenes and of the tumor suppressor genes p53 and RB are the most common genetic events in human tumors. Although there have been no reports of the prevalence of these alterations in Wilms tumors, overexpression of the N-myc and insulin-like growth factor-II (IGF-II) genes have been observed, and alteration of another tumor suppressor gene (WT1) has been demonstrated.

METHODS

Forty-four Wilms tumor specimens were tested for the presence of N-, K-, and H-ras mutations in codons 12, 13, and 61 by single-strand conformation polymorphism (SSCP) analysis and direct DNA sequence analysis. Sixteen tumors were tested for abnormalities of WT1 by Southern and northern blot analysis and reverse transcriptase polymerase chain reaction (RT-PCR). N-myc, c-myc, WT1, and IGF-II mRNA expression was measured in 16 tumors by Northern blot analysis. Thirty-eight tumors were screened for p53 mutations by SSCP analysis and direct DNA sequence analysis. Nine tumors were analyzed for loss of heterozygosity (LOH) of RB.

RESULTS

Although the authors confirmed that N-myc and IGF-II are overexpressed in Wilms tumors, no mutations of ras family, p53, or RB genes were identified, and no gross alterations of WT1 were detected by Southern or Northern blot analysis.

CONCLUSIONS

These findings suggest that H-ras, K-ras, N-ras, p53, and RB are not involved in the pathogenesis of Wilms tumor.

Inhibition of granulocyte differentiation by G1 cyclins D2 and D3 but not D1

Growth factor-induced signals govern the expression of three D-type cyclins, which, in turn, function as regulatory subunits of cyclin-dependent kinases (cdks) to control cell cycle transitions during the late G1 interval. 32D myeloid cells, which self-renew as uncommitted precursors in interleukin 3 (IL-3), express cyclins D2 and D3 (but not D1) in complexes with cdk4 and cdk2. When transferred to granulocyte colony-stimulating factor (G-CSF), 32D cells stop dividing and terminally differentiate to mature neutrophils. Cyclin D and cdk4 expression ceased as cells underwent growth arrest in G-CSF, but cdk2 levels were sustained. 32D cells engineered to ectopically express D-type cyclins exhibited contracted G1 intervals with a compensatory lengthening of S phase but remained IL-3 dependent for cell growth; those overexpressing cyclins D2 and D3 (but not D1) were unable to differentiate and died in G-CSF. Cyclin D2 mutants, which cannot efficiently bind to, or functionally interact with, the retinoblastoma protein (pRb) or its relatives (p107) did not block differentiation. Conversely, the introduction of a catalytically inactive cdk4 mutant into cells overexpressing cyclin D2 restored their G-CSF response. The persistence of cdk2 and its predilection to functionally interact with cyclins D2 and D3 rather than D1 might explain the specificity of the differentiation blockade.

Antioncogenes and human cancer

The antioncogenes, or tumor suppressor genes, as negative regulators of cell division, stand in contrast to oncogenes. For most human cancers, the more frequently mutated genes are the antioncogenes, the principal exception being the leukemias and lymphomas. Persons heterozygous for germ-line mutations in antioncogenes are strongly predisposed to one or more kinds of cancer, and most dominantly inherited cancer is attributable to such heterozygosity. Seven antioncogenes have been cloned through the study of these persons, and several others have been mapped. An eighth one was mapped and cloned through the investigation of tumors and is not yet known in hereditary form. Three dominantly inherited forms of cancer are not attributable to mutations in antioncogenes. The corresponding nonhereditary forms of most cancers generally reveal abnormalities of the same antioncogenes that are found in the hereditary forms but may also show additional ones. Some cancers, especially the embryonal tumors of children, have a small number of antioncogene mutations; some others, such as most sarcomas, have more, and the common carcinomas have the most, reflecting a hierarchy of controls over growth of stem cell populations. Still more members of this gene category remain to be mapped and cloned through the study of cancer families and of tumors. The genes that have been cloned act at diverse points in the signal transduction pathway in cells, from the outer cell membranes to sites of gene transcription, in some cases as negative regulators of oncogene expression.

Transgenic approaches to cancer biology

Transgenic animal models have played a major role in advancing our understanding of tumorigenesis. The most important recent advance has been the production of animals bearing targeted mutations generated by homologous recombination. For the first time, we can ask questions about loss of gene function and the consequences of gene alterations in situ. Perhaps most significantly, this approach has been applied to two of the tumour suppressor genes, Rb and p53. Homologous recombination has helped to clarify not only the normal roles of these genes, but also the mechanisms by which their dysfunction may lead to tumorigenesis.