Developmental rescue of an embryonic-lethal mutation in the retinoblastoma gene in chimeric mice

Patterns in the tapestry of chromatin-bound RB

The retinoblastoma protein (RB)-mediated regulation of E2F is a component of a highly conserved cell cycle machine. However, RB's tumor suppressor activity, like RB's requirement in animal development, is tissue-specific, context-specific, and sometimes appears uncoupled from cell proliferation. Detailed new information about RB's genomic distribution provides a new perspective on the complexity of RB function, suggesting that some of its functional specificity results from context-specific RB association with chromatin. Here we summarize recent evidence showing that RB targets different types of chromatin regulatory elements at different cell cycle stages. RB controls traditional RB/E2F targets prior to S-phase, but, when cells proliferate, RB redistributes to cell type-specific chromatin loci. We discuss the broad implications of the new data for RB research.

Selective Occupation by E2F and RB of Loci Expressed by RNA Polymerase III

In all cases tested, TFIIIB is responsible for recruiting pol III to its genetic templates. In mammalian cells, RB binds TFIIIB and prevents its interactions with both promoter DNA and pol III, thereby suppressing transcription. As TFIIIB is not recruited to its target genes when bound by RB, the mechanism predicts that pol III-dependent templates will not be occupied by RB; this contrasts with the situation at most genes controlled by RB, where it can be tethered by promoter-bound sequence-specific DNA-binding factors such as E2F. Contrary to this prediction, however, ChIP-seq data reveal the presence of RB in multiple cell types and the related protein p130 at many loci that rely on pol III for their expression, including RMRP, RN7SL, and a variety of tRNA genes. The sets of genes targeted varies according to cell type and growth state. In such cases, recruitment of RB and p130 can be explained by binding of E2F1, E2F4 and/or E2F5. Genes transcribed by pol III had not previously been identified as common targets of E2F family members. The data provide evidence that E2F may allow for the selective regulation of specific non-coding RNAs by RB, in addition to its influence on overall pol III output through its interaction with TFIIIB.

Monitoring the Cell Cycle of Tumor Cells in Mouse Models of Human Cancer

Cell division is obligatory to tumor growth. However, both cancer cells and noncancer cells in tumors can be found in distinct stages of the cell cycle, which may inform the growth potential of these tumors, their propensity to metastasize, and their response to therapy. Hence, it is of utmost importance to monitor the cell cycle of tumor cells. Here we discuss well-established methods and new genetic advances to track the cell cycle of tumor cells in mouse models of human cancer. We also review recent genetic studies investigating the role of the cell-cycle machinery in the growth of tumors in vivo, with a focus on the machinery regulating the G1/S transition of the cell cycle.

Histologic transformation in lung cancer: when one door shuts, another opens

Histologic transformation (HT) is a major cause of drug resistance to therapy in patients with lung cancer. HTs to small-cell lung cancer (SCLC) have been reported frequently in patients with epidermal growth factor receptor (EGFR)-mutated lung cancer. Although HTs have an impact on the clinical outcomes in patients owing to a high refractoriness to treatments, there is limited data on the prevalence, causes, mechanisms, treatment efficacy, and future treatment strategies. In this review, we assess the literature regarding HTs comprehensively, including those describing EGFR-tyrosine kinase inhibitors, other molecular targeted drugs, and immune checkpoint inhibitors. Furthermore, we discuss the mechanisms of HTs and the lineage plasticity to SCLC and squamous cell carcinoma in lung cancer. In addition, we summarize the treatment efficacy and future perspectives of HTs in patients with lung cancer, and propose better management strategies for this group of patients.

Spatiotemporally controlled overexpression of cyclin D1 triggers generation of supernumerary cells in the postnatal mouse inner ear

The retinoblastoma family of pocket proteins (pRBs), composed of Rb1, p107, and p130 are negative regulators of cell-cycle progression. The deletion of any individual pRB in the auditory system triggers hair cells' (HCs) and supporting cells' (SCs) proliferation to different extents. Nevertheless, accessing their combined role in the inner ear through conditional or complete knockout methods is limited by the early mortality of the triple knockout. In quiescent cells, hyperphosphorylation and inactivation of the pRBs are maintained through the activity of the Cyclin-D1-cdk4/6 complex. Cyclin D1 (CycD1) is expressed in the embryonic and neonatal inner ear. In the mature organ of Corti (OC), CycD1 expression is significantly downregulated, paralleling the OC mitotic quiescence. Earlier studies showed that CycD1 overexpression leads to cell-cycle reactivation in cultures of inner ear explants. Here, we characterize a Cre-activated, Doxycycline (Dox)-controlled, conditional CycD1 overexpression model, which when bred to a tetracycline-controlled transcriptional activator and the Atoh1-cre mouse lines, allow for transient CycD1 overexpression and pRBs' downregulation in the inner ear in a reversible fashion. Analyses of postnatal mice's inner ears at various time points revealed the presence of supernumerary cells throughout the length of the cochlea and in the vestibular end-organs. Notably, most supernumerary cells were observed in the inner hair cells' (IHCs) region, expressed myosin VIIa (M7a), and showed no signs of apoptosis at any of the time points analyzed. Auditory and vestibular phenotypes were similar between the different genotypes and treatment groups. The fact that no significant differences were observed in auditory and vestibular function supports the notion that the supernumerary cells detected in the adult mice cochlea and macular end-organs may not impair auditory functions.

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

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

Role of duplicate genes in determining the tissue-selectivity of hereditary diseases

A longstanding puzzle in human genetics is what limits the clinical manifestation of hundreds of hereditary diseases to certain tissues, while their causal genes are expressed throughout the human body. A general conception is that tissue-selective disease phenotypes emerge when masking factors operate in unaffected tissues, but are specifically absent or insufficient in disease-manifesting tissues. Although this conception has critical impact on the understanding of disease manifestation, it was never challenged in a systematic manner across a variety of hereditary diseases and affected tissues. Here, we address this gap in our understanding via rigorous analysis of the susceptibility of over 30 tissues to 112 tissue-selective hereditary diseases. We focused on the roles of paralogs of causal genes, which are presumably capable of compensating for their aberration. We show for the first time at large-scale via quantitative analysis of omics datasets that, preferentially in the disease-manifesting tissues, paralogs are under-expressed relative to causal genes in more than half of the diseases. This was observed for several susceptible tissues and for causal genes with varying number of paralogs, suggesting that imbalanced expression of paralogs increases tissue susceptibility. While for many diseases this imbalance stemmed from up-regulation of the causal gene in the disease-manifesting tissue relative to other tissues, it was often combined with down-regulation of its paralog. Notably in roughly 20% of the cases, this imbalance stemmed only from significant down-regulation of the paralog. Thus, dosage relationships between paralogs appear as important, yet currently under-appreciated, modifiers of disease manifestation.

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

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

Development of zebrafish medulloblastoma-like PNET model by TALEN-mediated somatic gene inactivation

Genetically engineered animal tumor models have traditionally been generated by the gain of single or multiple oncogenes or the loss of tumor suppressor genes; however, the development of live animal models has been difficult given that cancer phenotypes are generally induced by somatic mutation rather than by germline genetic inactivation. In this study, we developed somatically mutated tumor models using TALEN-mediated somatic gene inactivation of cdkn2a/b or rb1 tumor suppressor genes in zebrafish. One-cell stage injection of cdkn2a/b-TALEN mRNA resulted in malignant peripheral nerve sheath tumors with high frequency (about 39%) and early onset (about 35 weeks of age) in F0 tp53^e7/e7 mutant zebrafish. Injection of rb1-TALEN mRNA also led to the formation of brain tumors at high frequency (58%, 31 weeks of age) in F0 tp53^e7/e7 mutant zebrafish. Analysis of each tumor induced by somatic inactivation showed that the targeted genes had bi-allelic mutations. Tumors induced by rb1 somatic inactivation were characterized as medulloblastoma-like primitive neuroectodermal tumors based on incidence location, histopathological features, and immunohistochemical tests. In addition, 3' mRNA Quanti-Seq analysis showed differential activation of genes involved in cell cycle, DNA replication, and protein synthesis; especially, genes involved in neuronal development were up-regulated.

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

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

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

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

Inactivation of the retinoblastoma gene yields a mouse model of malignant colorectal cancer

The retinoblastoma gene (Rb) is mutated at significant frequency in various human epithelial tumors, including colorectal cancer, and is strongly associated with metastatic disease. However, sole inactivation of Rb in the mouse has so far failed to yield epithelial cancers. Here, we specifically inactivate Rb and/or p53 in the urogenital epithelium and the intestine. We find that loss of both tumor suppressors is unable to yield tumors in the transitional epithelium lining the bladder, kidneys and ureters. Instead, these mice develop highly metastatic tumors of neuroendocrine, not epithelial, origin within the urogenital tract to give prostate cancer in the males and vaginal tumors in the females. Additionally, we discovered that the sole inactivation of Rb in the intestine was sufficient to induce formation of metastatic colorectal adenocarcinomas. These tumors closely mirror the human disease in regard to age of onset, histological appearance, invasiveness and metastatic potential. Like most human colorectal carcinomas, our murine Rb-deficient tumors demonstrate genomic instability and they show activation of β-catenin. Deregulation of the Wnt/β-catenin pathway is specific to the intestinal tumors, as genomic instability but not activation of β-catenin was observed in the neuroendocrine tumors. To date, attempts to generate genetically engineered mouse models of colorectal cancer tumors have yielded mostly cancer of the small intestine, which rarely occurs in humans. Our system provides the opportunity to accurately model and study colorectal cancer in the mouse via a single gene mutation.

Pumilio and nanos RNA-binding proteins counterbalance the transcriptional consequences of RB1 inactivation

The ability of the retinoblastoma protein (RB) tumor suppressor to repress transcription stimulated by the E2 promoter binding factors (E2F) is integral to its biological functions. Our recent report described a conserved feedback mechanism mediated by the RNA-binding proteins Pumilio and Nanos that increases in importance following RB loss and helps cells to tolerate deregulated E2F.

Post-transcriptional gene expression control by NANOS is up-regulated and functionally important in pRb-deficient cells

Inactivation of the retinoblastoma tumor suppressor (pRb) is a common oncogenic event that alters the expression of genes important for cell cycle progression, senescence, and apoptosis. However, in many contexts, the properties of pRb-deficient cells are similar to wild-type cells suggesting there may be processes that counterbalance the transcriptional changes associated with pRb inactivation. Therefore, we have looked for sets of evolutionary conserved, functionally related genes that are direct targets of pRb/E2F proteins. We show that the expression of NANOS, a key facilitator of the Pumilio (PUM) post-transcriptional repressor complex, is directly repressed by pRb/E2F in flies and humans. In both species, NANOS expression increases following inactivation of pRb/RBF1 and becomes important for tissue homeostasis. By analyzing datasets from normal retinal tissue and pRb-null retinoblastomas, we find a strong enrichment for putative PUM substrates among genes de-regulated in tumors. These include pro-apoptotic genes that are transcriptionally down-regulated upon pRb loss, and we characterize two such candidates, MAP2K3 and MAP3K1, as direct PUM substrates. Our data suggest that NANOS increases in importance in pRb-deficient cells and helps to maintain homeostasis by repressing the translation of transcripts containing PUM Regulatory Elements (PRE).

RB1 mutation spectrum in a comprehensive nationwide cohort of retinoblastoma patients

BACKGROUND

Retinoblastoma (Rb) is a childhood cancer of the retina, commonly initiated by biallelic inactivation of the RB1 gene. Knowledge of the presence of a heritable RB1 mutation can help in risk management and reproductive decision making. We report here on RB1 mutation scanning in a unique nationwide cohort of Rb patients from the Netherlands.

METHODS

From the 1173 Rb patients registered in the Dutch National Retinoblastoma Register until January 2013, 529 patients from 433 unrelated families could be included. RB1 mutation scanning was performed with different detection methods, depending on the time period.

RESULTS

Our mutation detection methods revealed RB1 mutations in 92% of bilateral and/or familial Rb patients and in 10% of non-familial unilateral cases. Overall an RB1 germline mutation was detected in 187 (43%) of 433 Rb families, including 33 novel mutations. The distribution of the type of mutation was 37% nonsense, 20% frameshift, 21% splice, 9% large indel, 5% missense, 7% chromosomal deletions and 1% promoter. Ten per cent of patients were mosaic for the RB1 mutation. Six three-generation families with incomplete penetrance RB1 mutations were found. We found evidence that two variants, previously described as pathogenic RB1 mutations, are likely to be neutral variants.

CONCLUSIONS

The frequency of the type of mutations in the RB1 gene in our unbiased national cohort is the same as the mutation spectrum described worldwide. Furthermore, our RB1 mutation detection regimen achieves a high scanning sensitivity.

p19Ink4d is a tumor suppressor and controls pituitary anterior lobe cell proliferation

Pituitary tumors develop in about one-quarter of the population, and most arise from the anterior lobe (AL). The pituitary gland is particularly sensitive to genetic alteration of genes involved in the cyclin-dependent kinase (CDK) inhibitor (CKI)-CDK-retinoblastoma protein (Rb) pathway. Mice heterozygous for the Rb mutation develop pituitary tumors, with about 20% arising from the AL. Perplexingly, none of the CKI-deficient mice reported thus far develop pituitary AL tumors. In this study, we show that deletion of p19(Ink4d) (p19), a CKI gene, in mice results in spontaneous development of tumors in multiple organs and tissues. Specifically, more than one-half of the mutant mice developed pituitary hyperplasia or tumors predominantly in the AL. Tumor development is associated with increased cell proliferation and enhanced activity of Cdk4 and Cdk6 and phosphorylation of Rb protein. Though Cdk4 is indispensable for postnatal pituitary cell proliferation, it is not required for the hyperproliferative pituitary phenotype caused by p19 loss. Loss of p19 phosphorylates Rb in Cdk4(-/-) pituitary AL cells and mouse embryonic fibroblasts (MEFs) and rescues their proliferation defects, at least partially, through the activation of Cdk6. These results provide the first genetic evidence that p19 is a tumor suppressor and the major CKI gene that controls pituitary AL cell proliferation.

Animal models in retinoblastoma research

Advances in animal models of retinoblastoma have accelerated research in this field, aiding in understanding tumor progression and assessing therapeutic modalities. The distinct pattern of mutations and specific location of this unique intraocular tumor have paved the way for two types of models- those based on genetic mutations, and xenograft models. Retinoblastoma gene knockouts with an additional loss of p107, p130, p53 and using promoters of Nestin, Chx10, and Pax6 genes show histological phenotypic changes close to the human form of retinoblastoma. Conditional knockout in specific layers of the developing retina has thrown light on the origin of this tumor. The use of xenograft models has overcome the obstacle of time delay in the presentation of symptoms, which remains a crucial drawback of genetic models. With the advances in molecular and imaging technologies, the current research aims to develop models that mimic all the features of retinoblastoma inclusive of its initiation, progression and metastasis. The combination of genetic and xenograft models in retinoblastoma research has and will help to pave way for better understanding of retinoblastoma tumor biology and also in designing and testing effective diagnostic and treatment modalities.

E2f2 induces cone photoreceptor apoptosis independent of E2f1 and E2f3

The 'activating' E2fs (E2f1-3) are transcription factors that potently induce quiescent cells to divide. Work on cultured fibroblasts suggested they were essential for division, but in vivo analysis in the developing retina and other tissues disproved this notion. The retina, therefore, is an ideal location to assess other in vivo adenovirus E2 promoter binding factor (E2f) functions. It is thought that E2f1 directly induces apoptosis, whereas other activating E2fs only induce death indirectly by upregulating E2f1 expression. Indeed, mouse retinoblastoma (Rb)-null retinal neuron death requires E2f1, but not E2f2 or E2f3. However, we report an entirely distinct mechanism in dying cone photoreceptors. These neurons survive Rb loss, but undergo apoptosis in the cancer-prone retina lacking both Rb and its relative p107. We show that while E2f1 killed Rb/p107 null rod, bipolar and ganglion neurons, E2f2 was required and sufficient for cone death, independent of E2f1 and E2f3. Moreover, whereas E2f1-dependent apoptosis was p53 and p73-independent, E2f2 caused p53-dependent cone death. Our in vivo analysis of cone photoreceptors provides unequivocal proof that E2f-induces apoptosis independent of E2f1, and reveals distinct E2f1- and E2f2-activated death pathways in response to a single tumorigenic insult.

Retinal horizontal cells lacking Rb1 sustain persistent DNA damage and survive as polyploid giant cells

The retinoblastoma tumor susceptibility gene, Rb1, is a key regulator of the cell cycle, and mutations in this gene have been found in many human cancers. Prior studies showed that retina-specific knockout of Rb1 in the mouse results in the formation of abnormally large horizontal cells, but the development, fate, and genomic status of these cells remain unknown. In this study, we conditionally inactivate Rb1 in early retinal progenitors and show that the loss of Rb1 leads to the rapid degeneration of most retinal cells except horizontal cells, which persist as giant cells with aberrant centrosome content, DNA damage, and polyploidy/aneuploidy. We observed inappropriate cell cycle entry of Rb1-deficient horizontal cells during the first postnatal weeks, which dropped off abruptly by P30. Despite extensive DNA damage in Rb1-deficient horizontal cells, these cells can still enter mitosis. Adult Rb1-deficient horizontal cells display elevated DNA content (5N-34N) that varied continuously, suggesting the presence of aneuploidy. We also found evidence of supernumerary and disoriented centrosomes in a rare population of mitotic cells in the mutant retinas. Overall our data demonstrate that horizontal cells are a remarkably robust cell type and can survive for months despite extensive DNA damage and elevated genome content.

Critical choices for modeling breast cancer in transgenic mouse models

Modeling breast cancer in the mouse has helped to better define the heterogeneity of human breast cancer. In the recent past, it has become evident that some limitations have restricted the potential benefits that can be achieved with this approach. In this review, we highlight some key points that should be taken into account when the mouse is used, with special emphasis on transgenic models.

Inactivation of Rb in stromal fibroblasts promotes epithelial cell invasion

Stromal-derived growth factors are required for normal epithelial growth but are also implicated in tumour progression. We have observed inactivation of the retinoblastoma protein (Rb), through phosphorylation, in cancer-associated fibroblasts in oro-pharyngeal cancer specimens. Rb is well known for its cell-autonomous effects on cancer initiation and progression; however, cell non-autonomous functions of Rb are not well described. We have identified a cell non-autonomous role of Rb, using three-dimensional cultures, where depletion of Rb in stromal fibroblasts enhances invasive potential of transformed epithelia. In part, this is mediated by upregulation of keratinocyte growth factor (KGF), which is produced by the depleted fibroblasts. KGF drives invasion of epithelial cells through induction of MMP1 expression in an AKT- and Ets2-dependent manner. Our data identify that stromal fibroblasts can alter the invasive behaviour of the epithelium, and we show that altered expression of KGF can mediate these functions.

Quiescence: early evolutionary origins and universality do not imply uniformity

Cell cycle investigations have focused on relentless exponential proliferation of cells, an unsustainable situation in nature. Proliferation of cells, whether microbial or metazoan, is interrupted by periods of quiescence. The vast majority of cells in an adult metazoan lie quiescent. As disruptions in this quiescence are at the foundation of cancer, it will be important for the field to turn its attention to the mechanisms regulating quiescence. While often presented as a single topic, there are multiple forms of quiescence each with complex inputs, some of which are tied to conceptually challenging aspects of metazoan regulation such as size control. In an effort to expose the enormity of the challenge, I describe the differing biological purposes of quiescence, and the coupling of quiescence in metazoans to growth and to the structuring of tissues during development. I emphasize studies in the organism rather than in tissue culture, because these expose the diversity of regulation. While quiescence is likely to be a primitive biological process, it appears that in adapting quiescence to its many distinct biological settings, evolution has diversified it. Consideration of quiescence in different models gives us an overview of this diversity.

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.

Mouse chimeras as a system to investigate development, cell and tissue function, disease mechanisms and organ regeneration

Chimeras are organisms composed of at least two genetically distinct cell lineages originating from different zygotes. In the laboratory, mouse chimeras can be produced experimentally; various techniques allow combining different early stage mouse embryos with each other or with pluripotent stem cells. Identification of the progeny of the different lineages in chimeras permits to follow cell fate and function, enabling correlation of genotype with phenotype. Mouse chimeras have become a tool to investigate critical developmental processes, including cell specification, differentiation, patterning, and the function of specific genes. In addition, chimeras can also be generated to address biological processes in the adult, including mechanisms underlying diseases or tissue repair and regeneration. This review summarizes the different types of chimeras and how they have been generated and provides examples of how mouse chimeras offer a unique and powerful system to investigate questions pertaining to cell and tissue function in the developing and adult organism.

Rescue of myogenic defects in Rb-deficient cells by inhibition of autophagy or by hypoxia-induced glycolytic shift

The retinoblastoma tumor suppressor (pRb) is thought to orchestrate terminal differentiation by inhibiting cell proliferation and apoptosis and stimulating lineage-specific transcription factors. In this study, we show that in the absence of pRb, differentiating primary myoblasts fuse to form short myotubes that never twitch and degenerate via a nonapoptotic mechanism. The shortened myotubes exhibit an impaired mitochondrial network, mitochondrial perinuclear aggregation, autophagic degradation, and reduced adenosine triphosphate production. Bcl-2 and autophagy inhibitors restore mitochondrial function and rescue muscle degeneration, leading to formation of long, twitching myotubes that express normal levels of muscle-specific proteins and stably exit the cell cycle. A hypoxia-induced glycolytic switch also rescues the myogenic defect after either chronic or acute inactivation of Rb in a hypoxia-inducible factor-1 (HIF-1)-dependent manner. These results demonstrate that pRb is required to inhibit apoptosis in myoblasts and autophagy in myotubes but not to activate the differentiation program, and they also reveal a novel link between pRb and cell metabolism.

Dual loss of rb1 and Trp53 in the adrenal medulla leads to spontaneous pheochromocytoma

Using a Cre/loxP system, we have determined the phenotypic consequences attributable to in vivo deletion of both Rb1 and Trp53 in the mouse adrenal medulla. The coablation of these two tumor suppressor genes during embryogenesis did not disrupt adrenal gland development but resulted in the neoplastic transformation of the neural crest-derived adrenal medulla, yielding pheochromocytomas (PCCs) that developed with complete penetrance and were inevitably bilateral. Despite their typically benign status, these PCCs had profound ramifications on mouse vitality, with effected mice having a median survival of only 121 days. Evaluation of these PCCs by both immunohistochemistry and electron microscopy revealed that most Rb1(-/-):Trp53(-/-) chromaffin cells possessed atypical chromagenic vesicles that did not seem capable of appropriately storing synthesized catecholamines. The structural remodeling of the heart in mice harboring Rb1(-/-):Trp53(-/-) PCCs suggests that the mortality of these mice may be attributable to the inappropriate release of catecholamines from the mutated adrenal chromaffin cells. On the basis of the collective data from Rb1 and Trp53 knockout mouse models, it seems that the conversion of Rb1 loss-driven adrenal medulla hyperplasia to PCC can be greatly enhanced by the compound loss of Trp53, whereas the loss of Trp53 alone is generally ineffectual on adrenal chromaffin cell homeostasis. Consequently, the Trp53 tumor suppressor gene is an efficient genetic modifier of Rb1 loss in the development of PCC, and their compound loss in the adrenal medulla has a profound impact on both cellular homeostasis and animal vitality.

Melanocyte homeostasis in vivo tolerates Rb1 loss in a developmentally independent fashion

There has been uncertainty regarding the precise role that the pocket protein Rb1 plays in murine melanocyte homeostasis. It has been reported that the TAT-Cre mediated loss of exon 19 from a floxed Rb1 allele causes melanocyte apoptosis in vivo and in vitro. This is at variance with other findings showing, either directly or indirectly, that Rb1 loss in melanocytes has no noticeable effect in vivo, but in vitro leads to a semi-transformed phenotype. In this study, we show that Rb1-null melanocytes lacking exon 19 do not undergo apoptosis and survive both in vitro and in vivo, irrespective of the developmental stage at which Cre-mediated ablation of the exon occurs. Further, Rb1 loss has no serious long-term ramifications on melanocyte homeostasis in vivo, with Rb1-null melanocytes being detected in the skin after numerous hair cycles, inferring that the melanocyte stem cell population carrying the Cre-mediated deletion is maintained. Consequently, whilst Rb1 loss in the melanocyte is able to alter cellular behaviour in vitro, it appears inconsequential with respect to melanocyte homeostasis in the mouse skin.

Emerging roles of E2Fs in cancer: an exit from cell cycle control

Mutations of the retinoblastoma tumour suppressor gene (RB1) or components regulating the RB pathway have been identified in almost every human malignancy. The E2F transcription factors function in cell cycle control and are intimately regulated by RB. Studies of model organisms have revealed conserved functions for E2Fs during development, suggesting that the cancer-related proliferative roles of E2F family members represent a recent evolutionary adaptation. However, given that some human tumours have concurrent RB1 inactivation and E2F amplification and overexpression, we propose that there are alternative tumour-promoting activities for the E2F family, which are independent of cell cycle regulation.

E2F4 cooperates with pRB in the development of extra-embryonic tissues

The retinoblastoma gene, RB-1, was the first identified tumor suppressor. Rb(-/-) mice die in mid-gestation with defects in proliferation, differentiation and apoptosis. The activating E2F transcription factors, E2F1-3, contribute to these embryonic defects, indicating that they are key downstream targets of the retinoblastoma protein, pRB. E2F4 is the major pRB-associated E2F in vivo, yet its role in Rb(-/-) embryos is unknown. Here we establish that E2f4 deficiency reduced the lifespan of Rb(-/-) embryos by exacerbating the Rb mutant placental defect. We further show that this reflects the accumulation of trophectoderm-like cells in both Rb and Rb;E2f4 mutant placentas. Thus, Rb and E2f4 play cooperative roles in placental development. We used a conditional mouse model to allow Rb(-/-);E2f4(-/-) embryos to develop in the presence of Rb wild-type placentas. Under these conditions, Rb(-/-);E2f4(-/-) mutants survived to birth. These Rb(-/-);E2f4(-/-) embryos exhibited all of the defects characteristic of the Rb and E2f4 single mutants and had no novel defects. Taken together, our data show that pRB and E2F4 cooperate in placental development, but play largely non-overlapping roles in the development of many embryonic tissues.

Generation of double-knockout embryonic stem cells

Gene inactivation in mouse embryonic stem (ES) cells usually affects a single allele that is subsequently transmitted to the mouse germline. Upon breeding to homozygosity the consequences of complete gene ablation can be studied in the context of the complete organism. In many cases, it can be useful to study the consequences of gene ablation already in ES cells, for example, when a cellular phenotype is expected. This requires both alleles of a gene to be disrupted. Besides consecutive targeting by using different selectable marker genes, homozygosity for gene disruption can also be obtained by selecting cells for duplication of (part of) the chromosome carrying the targeted allele with concomitant loss of the wild-type allele.

E2f3a and E2f3b contribute to the control of cell proliferation and mouse development

The E2f3 locus encodes two Rb-binding gene products, E2F3a and E2F3b, which are differentially regulated during the cell cycle and are thought to be critical for cell cycle progression. We targeted the individual inactivation of E2f3a or E2f3b in mice and examined their contributions to cell proliferation and development. Chromatin immunoprecipitation and gene expression experiments using mouse embryo fibroblasts deficient in each isoform showed that E2F3a and E2F3b contribute to G(1)/S-specific gene expression and cell proliferation. Expression of E2f3a or E2f3b was sufficient to support E2F target gene expression and cell proliferation in the absence of other E2F activators, E2f1 and E2f2, suggesting that these isoforms have redundant functions. Consistent with this notion, E2f3a(-/-) and E2f3b(-/-) embryos developed normally, whereas embryos lacking both isoforms (E2f3(-/-)) died in utero. We also find that E2f3a and E2f3b have redundant and nonredundant roles in the context of Rb mutation. Analysis of double-knockout embryos suggests that the ectopic proliferation and apoptosis in Rb(-/-) embryos is mainly mediated by E2f3a in the placenta and nervous system and by both E2f3a and E2f3b in lens fiber cells. Together, we conclude that the contributions of E2F3a and E2F3b in cell proliferation and development are context dependent.

The role of the RB tumour suppressor pathway in oxidative stress responses in the haematopoietic system

Exposure to pro-oxidants and defects in the repair of oxidative base damage are associated with disease and ageing and also contribute to the development of anaemia, bone marrow failure and haematopoietic malignancies. This Review assesses emerging data indicative of a specific role for the RB tumour suppressor pathway in the response of the haematopoietic system to oxidative stress. This is mediated through signalling pathways that involve DNA damage sensors, forkhead box O (Foxo) transcription factors and p38 mitogen-activated protein kinases and has downstream consequences for cell cycle progression, antioxidant capacity, mitochondrial mass and cellular metabolism.

Rb and hematopoiesis: stem cells to anemia

The retinoblastoma protein, Rb, was one of the first tumor suppressor genes identified as a result of the familial syndrome retinoblastoma. In the period since its identification and cloning a large number of studies have described its role in various cellular processes. The application of conditional somatic mutation with lineage and temporally controlled gene deletion strategies, thus circumventing the lethality associated with germ-line deletion of Rb, have allowed for a reanalysis of the in vivo role of Rb. In the hematopoietic system, such approaches have led to new insights into stem cell biology and the role of the microenvironment in regulating hematopoietic stem cell fate. They have also clarified the role that Rb plays during erythropoiesis and defined a novel mechanism linking mitochondrial function to terminal cell cycle withdrawal. These studies have shed light on the in vivo role of Rb in the regulation of hematopoiesis and also prompt further analysis of the role that Rb plays in both the regulation of hematopoietic stem cells and the terminal differentiation of their progeny.

Tailoring to RB: tumour suppressor status and therapeutic response

The retinoblastoma tumour suppressor (RB) is a crucial regulator of cell-cycle progression that is invoked in response to a myriad of anti-mitogenic signals. It has been hypothesized that perturbations of the RB pathway confer a synonymous proliferative advantage to tumour cells; however, recent findings demonstrate context-specific outcomes associated with such lesions. Particularly, loss of RB function is associated with differential response to wide-ranging therapeutic agents. Thus, the status of this tumour suppressor may be particularly informative in directing treatment regimens.

Studying chromosome instability in the mouse

Aneuploidy has long been recognized as one of the hallmarks of cancer. It nonetheless remains uncertain whether aneuploidy occurring early in the development of a cancer is a primary cause of oncogenic transformation, or whether it is an epiphenomenon that arises from a general breakdown in cell cycle control late in tumorigenesis. The accuracy of chromosome segregation is ensured both by the intrinsic mechanics of mitosis and by an error-checking spindle assembly checkpoint. Many cancers show altered expression of proteins involved in the spindle checkpoint or in proteins implicated in other mitotic processes. To understand the role of aneuploidy in the initiation and progression of cancer, a number of spindle checkpoint genes have been disrupted in mice, most through conventional gene targeting (to create germ-line knockouts). We describe the consequence of these mutations with respect to embryonic development, tumor progression and an unexpected link to premature aging; readers are referred elsewhere [1] for a discussion of other cell cycle regulators.

Rb intrinsically promotes erythropoiesis by coupling cell cycle exit with mitochondrial biogenesis

Regulation of the cell cycle is intimately linked to erythroid differentiation, yet how these processes are coupled is not well understood. To gain insight into this coordinate regulation, we examined the role that the retinoblastoma protein (Rb), a central regulator of the cell cycle, plays in erythropoiesis. We found that Rb serves a cell-intrinsic role and its absence causes ineffective erythropoiesis, with a differentiation block at the transition from early to late erythroblasts. Unexpectedly, in addition to a failure to properly exit the cell cycle, mitochondrial biogenesis fails to be up-regulated concomitantly, contributing to this differentiation block. The link between erythropoiesis and mitochondrial function was validated by inhibition of mitochondrial biogenesis. Erythropoiesis in the absence of Rb resembles the human myelodysplastic syndromes, where defects in cell cycle regulation and mitochondrial function frequently occur. Our work demonstrates how these seemingly disparate pathways play a role in coordinately regulating cellular differentiation.

Functional interaction of the retinoblastoma and Ini1/Snf5 tumor suppressors in cell growth and pituitary tumorigenesis

The Ini1 subunit of the SWI/SNF chromatin remodeling complex suppresses formation of malignant rhabdoid tumors in humans and mice. Transduction of Ini1 into Ini1-deficient tumor-derived cell lines has indicated that Ini1 arrests cell growth, controls chromosomal ploidy, and suppresses tumorigenesis by regulating components of the retinoblastoma (Rb) signaling pathway. Furthermore, conditional inactivation of Ini1 in mouse fibroblasts alters the expression of various Rb-E2F-regulated genes, indicating that endogenous Ini1 levels may control Rb signaling in cells. We have reported previously that loss of one allele of Ini1 in mouse fibroblasts results only in a 15% to 20% reduction in total Ini1 mRNA levels due to transcriptional compensation by the remaining Ini1 allele. Here, we examine the effects of Ini1 haploinsufficiency on cell growth and immortalization in mouse embryonic fibroblasts. In addition, we examine pituitary tumorigenesis in Rb-Ini1 compound heterozygous mice. Our results reveal that heterozygosity for Ini1 up-regulates cell growth and immortalization and that exogenous Ini1 down-regulates the growth of primary cells in a Rb-dependent manner. Furthermore, loss of Ini1 is redundant with loss of Rb function in the formation of pituitary tumors in Rb heterozygous mice and leads to the formation of large, atypical Rb(+/-) tumor cells lacking adrenocorticotropic hormone expression. These results confirm in vivo the relationship between Rb and Ini1 in tumor suppression and indicate that Ini1 plays a role in maintaining the morphologic and functional differentiation of corticotrophic cells.

In vivo significance of the G2 restriction point

Loss of activity of the retinoblastoma pathway is a common event in human cancer. Mouse models have revealed that tumorigenesis by loss of Rb was accelerated by concomitant loss of the cell cycle inhibitor p27KIP1. This has been attributed to reduced apoptosis and weakening of the G1 checkpoint. However, the role of p27KIP1 in a recently identified G2 restriction point may offer an alternative explanation for this synergy. Here, we have investigated the significance of the G2 restriction point in Rb-deficient pituitaries. We show that Rb loss in the pituitary gland activated the G2 restriction point, as evidenced by the appearance of cyclin B1-p27KIP1 complexes. Somewhat unexpectedly, these complexes remained present in Rb-deficient tumors. These results indicate that the G2 restriction point does operate in vivo. However, in the pituitary gland, this mechanism seems to retard rather than to prevent tumor growth.

Cell lineage-specific interactions between Men1 and Rb in neuroendocrine neoplasia

Inactivation of multiple endocrine neoplasia (MEN) type 1 gene (Men1) results in development of multiple endocrine tumors in Men1(+/-) mice and in humans. Intriguingly, loss of the wild-type retinoblastoma 1 (Rb) gene also leads to MEN-like phenotype in Rb(+/-) mice. To evaluate potential genetic interactions between these genes, we prepared and characterized Men1(+/-)Rb(+/-) compound mice in parallel with their parental genotypes. Men1 and Rb did not cooperate in tumor suppression, as demonstrated by comparable survival rates of Rb(+/-) and Men1(+/-)Rb(+/-) mice, absence of tumor growth acceleration and lack of novel neoplasms. Notably, the loss of the remaining copy of the wild-type Men1 and Rb was mutually exclusive in all tumors of Men1(+/-)Rb(+/-) mice, including pituitary anterior lobe and adrenal medulla neoplasms shared by Rb- and Men1-deficient phenotypes. Down-regulation of Men1 targets p18 and p27 and increased presence of phosphorylated-Rb were observed in Men1-deficient pheochromocytomas of Men1(+/-)Rb(+/-) and Men1(+/-) mice. At the same time, the RNA interference (RNAi) knock-down of Men1 mRNA resulted in increased apoptosis of Rb-deficient medullary thyroid carcinoma cells. These results demonstrate that, depending on cell lineage context, combined Men1 and Rb deficiency may be either redundant or detrimental to neoplastic growth. Identification of cell lineage-specific interactions between Men1 and Rb may have important implications for development of rationally designed therapeutic approaches.

Cell cycle regulation in the postmitotic neuron: oxymoron or new biology?

Adult CNS neurons are typically described as permanently postmitotic but there is probably nothing permanent about the neuronal cell cycle arrest. Rather, it appears that these highly differentiated cells must constantly keep their cell cycle in check. Relaxation of this vigilance leads to the initiation of a cell cycle and entrance into an altered and vulnerable state, often leading to death. There is evidence that neurons which are at risk of neurodegeneration are also at risk of re-initiating a cell cycle process that involves the expression of cell cycle proteins and DNA replication. Failure of cell cycle regulation might be a root cause of several neurodegenerative disorders and a final common pathway for others.

Unique requirement for Rb/E2F3 in neuronal migration: evidence for cell cycle-independent functions

The cell cycle regulatory retinoblastoma (Rb) protein is a key regulator of neural precursor proliferation; however, its role has been expanded to include a novel cell-autonomous role in mediating neuronal migration. We sought to determine the Rb-interacting factors that mediate both the cell cycle and migration defects. E2F1 and E2F3 are likely Rb-interacting candidates that we have shown to be deregulated in the absence of Rb. Using mice with compound null mutations of Rb and E2F1 or E2F3, we asked to what extent either E2F1 or E2F3 interacts with Rb in neurogenesis. Here, we report that E2F1 and E2F3 are both functionally relevant targets in neural precursor proliferation, cell cycle exit, and laminar patterning. Each also partially mediates the Rb requirement for neuronal survival. Neuronal migration, however, is specifically mediated through E2F3, beyond its role in cell cycle regulation. This study not only outlines overlapping and distinct functions for E2Fs in neurogenesis but also is the first to establish a physiologically relevant role for the Rb/E2F pathway beyond cell cycle regulation in vivo.

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.

Murine bilateral retinoblastoma exhibiting rapid-onset, metastatic progression and N-myc gene amplification

Human retinoblastoma is a pediatric cancer initiated by RB gene mutations in the developing retina. We have examined the origins and progression of retinoblastoma in mouse models of the disease. Retina-specific inactivation of Rb on a p130-/- genetic background led to bilateral retinoblastoma with rapid kinetics, whereas on a p107-/- background Rb mutation caused predominantly unilateral tumors that arose with delayed kinetics and incomplete penetrance. In both models, retinoblastomas arose from cells at the extreme periphery of the murine retina. Furthermore, late retinoblastomas progressed to invade the brain and metastasized to the cervical lymph nodes. Metastatic tumors lacking Rb and p130 exhibited chromosomal changes revealed by representational oligonucleotide microarray analysis including high-level amplification of the N-myc oncogene. N-myc was found amplified in three of 16 metastatic retinoblastomas lacking Rb and p130 as well as in retinoblastomas lacking Rb and p107. N-myc amplification ranged from 6- to 400-fold and correlated with high N-myc-expression levels. These murine models closely resemble human retinoblastoma in their progression and secondary genetic changes, making them ideal tools for further dissection of steps to tumorigenesis and for testing novel therapies.

Rb is critical in a mammalian tissue stem cell population

The inactivation of the retinoblastoma (Rb) tumor suppressor gene in mice results in ectopic proliferation, apoptosis, and impaired differentiation in extraembryonic, neural, and erythroid lineages, culminating in fetal death by embryonic day 15.5 (E15.5). Here we show that the specific loss of Rb in trophoblast stem (TS) cells, but not in trophoblast derivatives, leads to an overexpansion of trophoblasts, a disruption of placental architecture, and fetal death by E15.5. Despite profound placental abnormalities, fetal tissues appeared remarkably normal, suggesting that the full manifestation of fetal phenotypes requires the loss of Rb in both extraembryonic and fetal tissues. Loss of Rb resulted in an increase of E2f3 expression, and the combined ablation of Rb and E2f3 significantly suppressed Rb mutant phenotypes. This rescue appears to be cell autonomous since the inactivation of Rb and E2f3 in TS cells restored placental development and extended the life of embryos to E17.5. Taken together, these results demonstrate that loss of Rb in TS cells is the defining event causing lethality of Rb(-/-) embryos and reveal the convergence of extraembryonic and fetal functions of Rb in neural and erythroid development. We conclude that the Rb pathway plays a critical role in the maintenance of a mammalian stem cell population.

Selective requirements for E2f3 in the development and tumorigenicity of Rb-deficient chimeric tissues

The tumor suppressor function of the retinoblastoma protein pRB is largely dependent upon its capacity to inhibit the E2F transcription factors and thereby cell proliferation. Attempts to study the interplay between pRB and the E2Fs have been hampered by the prenatal death of Rb; E2f nullizygous mice. In this study, we isolated Rb; E2f3 mutant embryonic stem cells and generated Rb(-/-); E2f3(-/-) chimeric mice, thus bypassing the lethality of the Rb(-/-); E2f3(-/-) germ line mutant mice. We show that loss of E2F3 has opposing effects on two of the known developmental defects arising in Rb(-/-) chimeras; it suppresses the formation of cataracts while aggravating the retinal dysplasia. This model system also allows us to assess how E2f3 status influences tumor formation in Rb(-/-) tissues. We find that E2f3 is dispensable for the development of pRB-deficient pituitary and thyroid tumors. In contrast, E2f3 inactivation completely suppresses the pulmonary neuroendocrine hyperplasia arising in Rb(-/-) chimeric mice. This hyperproliferative state is thought to represent the preneoplastic lesion of small-cell lung carcinoma. Therefore, our observation highlights a potential role for E2F3 in the early stages of this tumor type.

p18Ink4c, but not p27Kip1, collaborates with Men1 to suppress neuroendocrine organ tumors

Mutant mice lacking both cyclin-dependent kinase (CDK) inhibitors p18(Ink4c) and p27(Kip1) develop a tumor spectrum reminiscent of human multiple endocrine neoplasia (MEN) syndromes. To determine how p18 and p27 genetically interact with Men1, the tumor suppressor gene mutated in familial MEN1, we characterized p18-Men1 and p27-Men1 double mutant mice. Compared with their corresponding single mutant littermates, the p18(-/-); Men1(+/-) mice develop tumors at an accelerated rate and with an increased incidence in the pituitary, thyroid, parathyroid, and pancreas. In the pituitary and pancreatic islets, phosphorylation of the retinoblastoma (Rb) protein at both CDK2 and CDK4/6 sites was increased in p18(-/-) and Men1(+/-) cells and was further increased in p18(-/-); Men1(+/-) cells. The remaining wild-type Men1 allele was lost in most tumors from Men1(+/-) mice but was retained in most tumors from p18(-/-); Men1(+/-) mice. Combined mutations of p27(-/-) and Men1(+/-), in contrast, did not exhibit noticeable synergistic stimulation of Rb kinase activity, cell proliferation, and tumor growth. These results demonstrate that functional collaboration exists between p18 and Men1 and suggest that Men1 may regulate additional factor(s) that interact with p18 and p27 differently.

Cell cycle regulation in the developing lens

Regulation of cell proliferation is a critical aspect of the development of multicellular organisms. The ocular lens is an excellent model system in which to unravel the mechanisms controlling cell proliferation during development. In recent years, several cell cycle regulators have been shown to be essential for maintaining normal patterns of lens cell proliferation. Additionally, many growth factor signaling pathways and cell adhesion factors have been shown to have the capacity to regulate lens cell proliferation. Given this complexity, understanding the cross talk between these many signaling pathways and how they are coordinated are important directions for the future.