Retinoblastoma protein enhances the fidelity of chromosome segregation mediated by hsHec1p

Humanization reveals pervasive incompatibility of yeast and human kinetochore components

Kinetochores assemble on centromeres to drive chromosome segregation in eukaryotic cells. Humans and budding yeast share most of the structural subunits of the kinetochore, whereas protein sequences have diverged considerably. The conserved centromeric histone H3 variant, CenH3 (CENP-A in humans and Cse4 in budding yeast), marks the site for kinetochore assembly in most species. A previous effort to complement Cse4 in yeast with human CENP-A was unsuccessful; however, co-complementation with the human core nucleosome was not attempted. Previously, our lab successfully humanized the core nucleosome in yeast; however, this severely affected cellular growth. We hypothesized that yeast Cse4 is incompatible with humanized nucleosomes and that the kinetochore represented a limiting factor for efficient histone humanization. Thus, we argued that including the human CENP-A or a Cse4-CENP-A chimera might improve histone humanization and facilitate kinetochore function in humanized yeast. The opposite was true: CENP-A expression reduced histone humanization efficiency, was toxic to yeast, and disrupted cell cycle progression and kinetochore function in wild-type (WT) cells. Suppressors of CENP-A toxicity included gene deletions of subunits of 3 conserved chromatin remodeling complexes, highlighting their role in CenH3 chromatin positioning. Finally, we attempted to complement the subunits of the NDC80 kinetochore complex, individually and in combination, without success, in contrast to a previous study indicating complementation by the human NDC80/HEC1 gene. Our results suggest that limited protein sequence similarity between yeast and human components in this very complex structure leads to failure of complementation.

Genetics in ophthalmology: molecular blueprints of retinoblastoma

This review presents current knowledge on the molecular biology of retinoblastoma (RB). Retinoblastoma is an intraocular tumor with hereditary and sporadic forms. 8,000 new cases of this ocular malignancy of the developing retina are diagnosed each year worldwide. The major gene responsible for retinoblastoma is RB1, and it harbors a large spectrum of pathogenic variants. Tumorigenesis begins with mutations that cause RB1 biallelic inactivation preventing the production of functional pRB proteins. Depending on the type of mutation the penetrance of RB is different. However, in small percent of tumors additional genes may be required, such as MYCN, BCOR and CREBBP. Additionally, epigenetic changes contribute to the progression of retinoblastoma as well. Besides its role in the cell cycle, pRB plays many additional roles, it regulates the nucleosome structure, participates in apoptosis, DNA replication, cellular senescence, differentiation, DNA repair and angiogenesis. Notably, pRB has an important role as a modulator of chromatin remodeling. In recent years high-throughput techniques are becoming essential for credible biomarker identification and patient management improvement. In spite of remarkable advances in retinoblastoma therapy, primarily in high-income countries, our understanding of retinoblastoma and its specific genetics still needs further clarification in order to predict the course of this disease and improve therapy. One such approach is the tumor free DNA that can be obtained from the anterior segment of the eye and be useful in diagnostics and prognostics.

Emerging roles of RB family: new defense mechanisms against tumor progression

The retinoblastoma (RB) family of proteins, including RB1/p105, retinoblastoma-like 1 (RBL1/p107), and retinoblastoma-like 2 (RBL2/p130), is principally known for its central role on cell cycle regulation. The inactivation of RB proteins confers a growth advantage and underlies multiple types of tumors. Recently, it has been shown that RB proteins have other important roles, such as preservation of chromosomal stability, induction and maintenance of senescence and regulation of apoptosis, cellular differentiation, and angiogenesis. RB proteins are involved in many cellular pathways and act as transcriptional regulators able to bind several transcription factors, thus antagonizing or potentiating their functions. Furthermore, RB proteins might control the expression of specific target genes by recruiting chromatin remodeling enzymes. Although many efforts have been made to dissect the different functions of RB proteins, it remains still unclear which are necessary for cancer suppression and the role they play at distinct steps of carcinogenesis. Moreover, RB proteins can behave differently in various cell types or cell states. Elucidating the intricate RB protein network in regulating cell fate might provide the knowledge necessary to explain their potent tumor suppressor activity and to design novel therapeutic strategies.

RB: mitotic implications of a tumour suppressor

RB, a well known tumour suppressor that functions in the control of cell cycle progression and proliferation, has recently been shown to have additional functions in the maintenance of genomic stability, such that inactivation of RB family proteins promotes chromosome instability (CIN) and aneuploidy. Several studies have provided potential explanations for these phenomena that occur following RB loss, and they suggest that this new function of RB may contribute to its role in tumour suppression.

pRB, a tumor suppressor with a stabilizing presence

The product of the retinoblastoma tumor-susceptibility gene (RB1) is a key regulator of cell proliferation and this function is thought to be central to its tumor suppressive activity. Several studies have demonstrated that inactivation of pRB not only allows inappropriate proliferation but also undermines mitotic fidelity, leading to genome instability and ploidy changes. Such properties promote tumor evolution and correlate with increased resistance to therapeutics and tumor relapse. These observations suggest that inactivation of pRB could contribute to both tumor initiation and progression. Further characterization of the role of pRB in chromosome segregation will provide insight into processes that are misregulated in human tumors and could reveal new therapeutic targets to kill or stall these chromosomally unstable lesions. We review the evidence that pRB promotes genome stability and discuss the mechanisms that probably contribute to this effect.

Immunodetection of retinoblastoma-related protein and its phosphorylated form in interphase and mitotic alfalfa cells

Plant retinoblastoma-related (RBR) proteins are primarily considered as key regulators of G(1)/S phase transition, with functional roles in a variety of cellular events during plant growth and organ development. Polyclonal antibody against the C-terminal region of the Arabidopsis RBR1 protein also specifically recognizes the alfalfa 115 kDa MsRBR protein, as shown by the antigen competition assay. The MsRBR protein was detected in all cell cycle phases, with a moderate increase in samples representing G(2)/M cells. Antibody against the human phospho-pRb peptide (Ser807/811) cross-reacted with the same 115 kDa MsRBR protein and with the in vitro phosphorylated MsRBR protein C-terminal fragment. Phospho-MsRBR protein was low in G(1) cells. Its amount increased upon entry into the S phase and remained high during the G(2)/M phases. Roscovitine treatment abolished the activity of alfalfa MsCDKA1;1 and MsCDKB2;1, and the phospho-MsRBR protein level was significantly decreased in the treated cells. Colchicine block increased the detected levels of both forms of MsRBR protein. Reduced levels of the MsRBR protein in cells at stationary phase or grown in hormone-free medium can be a sign of the division-dependent presence of plant RBR proteins. Immunolocalization of the phospho-MsRBR protein indicated spots of variable number and size in the labelled interphase nuclei and high signal intensity of nuclear granules in prophase. Structures similar to phospho-MsRBR proteins cannot be recognized in later mitotic phases. Based on the presented western blot and immunolocalization data, the possible involvement of RBR proteins in G(2)/M phase regulation in plant cells is discussed.

Tumor suppressor function of BCSC-1 in nasopharyngeal carcinoma

BCSC-1 is dramatically upregulated in CNE-2L2 human nasopharyngeal carcinoma cells with reduced malignancy (AS cells) and is proposed to be a candidate tumor suppressor gene. We therefore examined the effect of BCSC-1 expression on malignant behaviors of CNE-2L2 cells. Growth in vitro and tumorigenesis in nude mice of wild-type CNE-2L2 cells (W cells) were inhibited by ectopic BCSC-1, and those of AS cells were promoted by BCSC-1 suppression. The tumor suppressor function of BCSC-1 was further confirmed by a study showing that intratumor BCSC-1 injection caused growth suppression of the tumor from W cells inoculated in nude mice. Immunohistochemistry exhibited marked reduction of BCSC-1 expression in 11 of 39 human nasopharyngeal carcinoma specimens. Because BCSC-1 expression was as rich as that in normal cells in the rest of the carcinoma specimens and was poor in CNE-2L2 cells, HNE-1 human nasopharyngeal carcinoma cells with rich BCSC-1 expression were used as a control in the study. No effect of BCSC-1 transfection on growth of the cells was observed. The data suggest that BCSC-1 suppression might play roles in tumorigenesis of some nasopharyngeal carcinomas and that BCSC-1 might be a potential gene therapy target in nasopharyngeal carcinomas with poor BCSC-1 expression. Enhanced aggregation of cells together with increased E-cadherin and alpha-catenin expression and reduced Wnt signaling might be involved in the mechanisms of tumor suppressor function of BCSC-1.

mars and tousled-like kinase act in parallel to ensure chromosome fidelity in Drosophila

Background

High levels of Hepatoma Up-Regulated Protein (HURP) and Tousled-Like Kinase (TLK) transcripts are found in hepatocellular carcinoma. HURP overexpression induces anchorage-independent growth of 293-T cells and enhances a rough-eye phenotype resulting from tlk overexpression in Drosophila. In addition, both HURP and Mars, a Drosophila HURP sequence homologue, promote polymerization of mitotic spindles. Thus, the genetic interaction of mars with tlk might be required for accurate chromosome segregation.

Methods

To reveal whether chromosome fidelity was decreased, the frequency of gynandromorphy, an individual with both male and female characteristics, and of non-disjunction were measured in the progeny from parents with reduced mars and/or tlk activities and analyzed by Student's t-test. To show that the genetic interaction between mars and tlk is epistatic or parallel, a cytological analysis of embryos with either reduced or increased activities of mars and/or tlk was used to reveal defects in mitotic-spindle morphology and chromosome segregation.

Results

A significant but small fraction of the progeny from parents with reduced mars activity showed gynandromorphy and non-disjunction. Results of cytological analysis revealed that the decrease in chromosome fidelity was a result of delayed polymerization of the mitotic spindle, which led to asynchronous chromosome segregation in embryos that had reduced mars activity. By removing one copy of tousled-like kinase (tlk) from flies with reduced mars activity, chromosome fidelity was further reduced. This was indicated by an increased in the non-disjunction rate and more severe asynchrony. However, the morphology of the mitotic spindles in the embryos at metaphase where both gene activities were reduced was similar to that in mars embryos. Furthermore, tlk overexpression did not affect the morphology of the mitotic spindles and the cellular localization of Mars protein.

Conclusion

Chromosome fidelity in progeny from parents with reduced mars and/or tlk activity was impaired. The results from cytological studies revealed that mars and tlk function in parallel and that a balance between mars activity and tlk activity is required for cells to progress through mitosis correctly, thus ensuring chromosome fidelity.

Hec1 overexpression hyperactivates the mitotic checkpoint and induces tumor formation in vivo

Hec1 (Highly Expressed in Cancer 1) is one of four proteins of the outer kinetochore Ndc80 complex involved in the dynamic interface between centromeres and spindle microtubules. Its overexpression is seen in a variety of human tumors and correlates with tumor grade and prognosis. We show here that the overexpression of Hec1 in an inducible mouse model results in mitotic checkpoint hyperactivation. As previously observed with overexpression of the Mad2 gene, hyperactivation of the mitotic checkpoint leads to aneuploidy in vitro and is sufficient to generate tumors in vivo that harbor significant levels of aneuploidy. These results underscore the role of chromosomal instability as a result of mitotic checkpoint hyperactivation in the initiation of tumorigenesis.

Structure-function analysis of the retinoblastoma tumor suppressor protein - is the whole a sum of its parts?

Biochemical analysis of the retinoblastoma protein's function has received considerable attention since it was cloned just over 20 years ago. During this time pRB has emerged as a key regulator of the cell division cycle and its ability to block proliferation is disrupted in the vast majority of human cancers. Much has been learned about the regulation of E2F transcription factors by pRB in the cell cycle. However, many questions remain unresolved and researchers continue to explore this multifunctional protein. In particular, understanding how its biochemical functions contribute to its role as a tumor suppressor remains to be determined. Since pRB has been shown to function as an adaptor molecule that links different proteins together, or to particular promoters, analyzing pRB by disrupting individual protein interactions holds tremendous promise in unraveling the intricacies of its function. Recently, crystal structures have reported how pRB interacts with some of its molecular partners. This information has created the possibility of rationally separating pRB functions by studying mutants that disrupt individual binding sites. This review will focus on literature that investigates pRB by isolating functions based on binding sites within the pocket domain. This article will also discuss the prospects for using this approach to further explore the unknown functions of pRB.

The ATM-dependent DNA damage signaling pathway

Many of the insights that we have gained into the mechanisms involved in cellular DNA damage response pathways have come from studies of human cancer susceptibility syndromes that are altered in DNA damage responses. ATM, the gene mutated in the disorder, ataxia-telangiectasia, is a protein kinase that is a central mediator of responses to DNA double-strand breaks in cells. Recent studies have elucidated the mechanism by which DNA damage activates the ATM kinase and initiates these critical cellular signaling pathways. The SMC1 protein appears to be a particularly important target of the ATM kinase, playing critical roles in controlling DNA replication forks and DNA repair after the damage. A major role for the NBS1 and BRCA1 proteins appears to be in the recruitment of an activated ATM kinase molecule to the sites of DNA breaks so that ATM can phosphorylate SMC1. Generation of mice and cells that are unable to phosphorylate SMC1 demonstrated the importance of SMC1 phosphorylation in the DNA-damage-induced S-phase checkpoint, in determining rates of repair of chromosomal breaks, and in determining cell survival after DNA damage. Focusing on ATM and SMC1, the molecular controls of these pathways is discussed.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Melanocytes in conditional Rb-/- mice are normal in vivo but exhibit proliferation and pigmentation defects in vitro

The function of the retinoblastoma tumour suppressor (Rb1), and the pocket protein family in general, has been implicated as an important focal point for deregulation in many of the molecular pathways mutated in melanoma. We have focused on the role of Rb1 in mouse melanocyte homeostasis using gene targeting and Cre/loxP mediated tissue-specific deletion. We show that constitutive Cre-mediated ablation of Rb1 exon 2 prevents the production of Rb1 and recapitulates the phenotype encountered in other Rb1 knockout mouse models. Mice with conditional melanocyte-specific ablation of Rb1 manifest overtly normal pigmentation and are bereft of melanocytic hyperproliferative defects or apoptosis-induced depigmentation. Histologically, these mice have melanocyte morphology and distribution comparable with control littermates. In contrast, Rb1-null melanocytes removed from their in vivo micro-environment and cultured in vitro display some of the characteristics associated with a transformed phenotype. They proliferate at a heightened rate when compared with control melanocytes and have a decreased requirement for mitogens. With progressive culture the cells depigment at relatively early passage and display a gross morphology which, whilst reminiscent of early passage melanocytes, is generally different to equivalent passage control cells. These results indicate that Rb1 is dispensable for in vivo melanocyte homeostasis when its ablation is targeted from the melanoblast stage onwards, however, when cultured in vitro, Rb1 loss increases melanocyte growth but the cells are not fully transformed.

The RB protein family in retinal development and retinoblastoma: new insights from new mouse models

The Rb gene was isolated almost 20 years ago, but fundamental questions regarding its role in retinal development and retinoblastoma remain. What is the normal function of RB protein in retinogenesis? What is the cell-of-origin of retinoblastoma? Why do retinoblastoma tumors have recurrent genetic lesions other than Rb inactivation? Why is retinoblastoma not induced by defects in cell cycle regulators other than Rb? Why is the retina so sensitive to Rb loss? Recently developed conditional Rb knockout models provide new insight into some of these issues. The data suggest that RB protein may not control the rate of progenitor division, but is critical for cell cycle exit when dividing retinal progenitors differentiate into postmitotic transition cells. This finding focuses attention on the ectopically dividing transition cell, rather than the progenitor, as the cell-of-origin. Cell-specific analyses in the RB-deficient retina reveal that ectopically dividing photoreceptors, bipolar and ganglion cells die, but amacrine, horizontal and Muller cells survive and stop dividing when they terminally differentiate. Rare amacrine transition cells escape cell cycle exit and generate tumors. These data suggest that post-Rb mutations are required to overcome growth arrest associated with terminal differentiation, rather than apoptosis as previously suggested. To explain why perturbing cell cycle regulators other than RB does not initiate retinoblastoma, we speculate that mutations in other components of the RB pathway perturb cell cycle arrest, but only RB loss triggers genome instability in retinal transition cells, which may be critical to facilitate post-Rb mutations necessary for transformation. Cell-specific differences in the effect of Rb loss on genome stability may contribute to the tremendous sensitivity of retinal transition cells to tumorigenesis. The new mouse models of retinoblastoma will be invaluable for testing these possibilities.

Induction of tetrasomy by human papillomavirus type 16 E7 protein is independent of pRb binding and disruption of differentiation

We have demonstrated previously that high-risk human papillomaviruses (HPVs) induce tetrasomy in low-grade squamous intraepithelial lesions of the cervix. In this study we show that the E6 and E7 genes of high-risk HPV-16, but not those of low-risk HPV-6, are independently able to induce tetrasomy when constitutively expressed in proliferating monolayer cultures of primary human keratinocytes. Of seven HPV-16 E7 mutants analysed (H2P, Delta6-10, Delta21-24, C24G, S31G/S32G, A50S and S71I), five were severely impaired in their ability to induce tetrasomy in monolayer and raft culture. Only mutant C24G induced tetrasomy to levels comparable with wild-type E7 in monolayer and raft culture. This mutant shows strongly reduced binding to the retinoblastoma gene product pRb. The casein kinase II phosphorylation defective mutant S31G/S32G induced tetrasomy to levels comparable with wild-type E7 in raft culture, but not in monolayer culture, and induction of tetrasomy did not correlate with raft morphology. These results indicate that pRb protein binding is not required for HPV-16 E7 associated tetrasomy and that tetrasomy is not directly related to the ability of this protein to disrupt keratinocyte differentiation.

Retinoblastoma tumor suppressor and genome stability

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

Phosphorylation of the mitotic regulator protein Hec1 by Nek2 kinase is essential for faithful chromosome segregation

Hec1 (highly expressed in cancer) plays essential roles in chromosome segregation by interacting through its coiled-coil domains with several proteins that modulate the G(2)/M phase. Hec1 localizes to kinetochores, and its inactivation either by genetic deletion or antibody neutralization leads to severe and lethal chromosomal segregation errors, indicating that Hec1 plays a critical role in chromosome segregation. The mechanisms by which Hec1 is regulated, however, are not known. Here we show that human Hec1 is a serine phosphoprotein and that it binds specifically to the mitotic regulatory kinase Nek2 during G(2)/M. Nek2 phosphorylates Hec1 on serine residue 165, both in vitro and in vivo. Yeast cells are viable without scNek2/Kin3, a close structural homolog of Nek2 that binds to both human and yeast Hec1. When the same yeasts carry an scNek2/Kin3 (D55G) or Nek2 (E38G) mutation to mimic a similar temperature-sensitive nima mutation in Aspergillus, their growth is arrested at the nonpermissive temperature, because the scNek2/Kin3 (D55G) mutant binds to Hec1 but fails to phosphorylate it. Whereas wild-type human Hec1 rescues lethality resulting from deletion of Hec1 in Saccharomyces cerevesiae, a human Hec1 mutant or yeast Hec1 mutant changing Ser(165) to Ala or yeast Hec1 mutant changing Ser(201) to Ala does not. Mutations changing the same Ser residues to Glu, to mimic the negative charge created by phosphorylation, partially rescue lethality but result in a high incidence of errors in chromosomal segregation. These results suggest that cell cycle-regulated serine phosphorylation of Hec1 by Nek2 is essential for faithful chromosome segregation.

Aberrant expression of cell-cycle regulator cyclin D1 in breast cancer is related to chromosomal genomic instability

To account for the accumulation of genomic alterations required for tumor progression, it has been suggested that the genomes of cancer cells are unstable and that this instability results from defective mutators (the "mutator phenotype" theory). To examine the hypothesis that abnormal cell-cycle regulators act as the mutators contributing to genomic instability, the present study, based on primary tumor tissues from 71 patients with breast cancer, was performed to determine whether there was an association between aberrant expression of cell-cycle regulators (cyclin A, cyclin D1, cyclin E, RB1, p21, and p27) and chromosomal instability. Comparative genomic hybridization was used to measure chromosomal changes, reflecting genomic instability in individual tumors, whereas immunohistochemistry was used to detect aberrant expression of cell-cycle regulators. Overexpression of cyclin D1 was found to be significantly correlated with increased chromosomal instability (defined as harboring more than 7 chromosomal changes), with 63% of tumors overexpressing and 27% of tumors not overexpressing, with cyclin D1 showing chromosomal instability (P < 0.05). Interestingly, this relationship was independent of cell outgrowth (as detected by the proliferation marker Ki-67) and was particularly significant in tumors not expressing p27 or in tumors with detectable RB1. These results suggest that cyclin D1 plays an alternative role in the regulation of genomic stability.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Molecular analysis of kinetochore-microtubule attachment in budding yeast

The complex series of movements that mediates chromosome segregation during mitosis is dependent on the attachment of microtubules to kinetochores, DNA-protein complexes that assemble on centromeric DNA. We describe the use of live-cell imaging and chromatin immunoprecipitation in S. cerevisiae to identify ten kinetochore subunits, among which are yeast homologs of microtubule binding proteins in animal cells. By analyzing conditional mutations in several of these proteins, we show that they are required for the imposition of tension on paired sister kinetochores and for correct chromosome movement. The proteins include both molecular motors and microtubule associated proteins (MAPs), implying that motors and MAPs function together in binding chromosomes to spindle microtubules.

The large subunit of replication factor C promotes cell survival after DNA damage in an LxCxE motif- and Rb-dependent manner

Retinoblastoma (Rb) protein promotes cell survival after DNA damage. We show here that the LxCxE binding site in Rb mediates both cell survival and cell-cycle arrest after DNA damage. Replication factor C (RF-C) complex plays an important role in DNA replication. We describe a novel function of the large subunit of RF-C in promoting cell survival after DNA damage. RF-Cp145 contains an LxCxE motif, and mutation of this motif abolishes the protective effect of RF-Cp145. The inability of wild-type RF-Cp145 to promote cell survival in Rb-null cells is rescued by Rb but not by Rb mutants defective in binding LxCxE proteins. RF-C thus enhances cell survival after DNA damage in an Rb-dependent manner.

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.