Elimination of background recombination: somatic induction of Cre by combined transcriptional regulation and hormone binding affinity

Notch1 mutations drive clonal expansion in normal esophageal epithelium but impair tumor growth

NOTCH1 mutant clones occupy the majority of normal human esophagus by middle age but are comparatively rare in esophageal cancers, suggesting NOTCH1 mutations drive clonal expansion but impede carcinogenesis. Here we test this hypothesis. Sequencing NOTCH1 mutant clones in aging human esophagus reveals frequent biallelic mutations that block NOTCH1 signaling. In mouse esophagus, heterozygous Notch1 mutation confers a competitive advantage over wild-type cells, an effect enhanced by loss of the second allele. Widespread Notch1 loss alters transcription but has minimal effects on the epithelial structure and cell dynamics. In a carcinogenesis model, Notch1 mutations were less prevalent in tumors than normal epithelium. Deletion of Notch1 reduced tumor growth, an effect recapitulated by anti-NOTCH1 antibody treatment. Notch1 null tumors showed reduced proliferation. We conclude that Notch1 mutations in normal epithelium are beneficial as wild-type Notch1 favors tumor expansion. NOTCH1 blockade may have therapeutic potential in preventing esophageal squamous cancer.

Membrane tethering of CreER decreases uninduced cell labeling and cytotoxicity while maintaining recombination efficiency

Genetic lineage tracing is indispensable to unraveling the origin, fate, and plasticity of cells. However, the intrinsic leakiness in the CreER-loxP system raises concerns on data interpretation. Here, we reported the generation of a novel dual inducible CreER-loxP system with superior labeling characteristics. This two-component system consists of membrane localized CreER (mCreER: CD8α-FRB-CS-CreER) and TEV protease (mTEVp: CD8α-FKBP-TEVp), which are fusion proteins incorporated with the chemically induced dimerization machinery. Rapamycin and tamoxifen induce sequential dimerization of FKBP and FRB, cleavage of CreER from the membrane, and translocation into the nucleus. The labeling leakiness in Ad293 cells reduced dramatically from more than 70% to less than 5%. This tight labeling feature depends largely on the association of mCreER with HSP90, which conceals the TEV protease cutting site between FRB and CreER and thus preventing uninduced cleavage of the membrane-tethering CreER. Membrane-bound CreER also diminished significantly cytotoxicity. Our studies showed mCreER under the control of the rat insulin promoter increased labeling specificity in MIN6 islet beta-cells. Viability and insulin secretion of MIN6 cells remained intact. Our results demonstrate that this novel system can provide more stringent temporal and spatial control of gene expression and will be useful in cell fate probing.

Murine models of clonal hematopoiesis to assess mechanisms of cardiovascular disease

Clonal hematopoiesis (CH) is a phenomenon whereby somatic mutations confer a fitness advantage to hematopoietic stem and progenitor cells (HSPC) and thus facilitate their aberrant clonal expansion. These mutations are carried into progeny leukocytes leading to a situation whereby a substantial fraction of an individual's blood cells originate from the HSPC mutant clone. Although this condition rarely progresses to a hematological malignancy, circulating blood cells bearing the mutation have the potential to affect other organ systems as they infiltrate into tissues under both homeostatic and disease conditions. Epidemiological and clinical studies have revealed that CH is highly prevalent in the elderly and is associated with an increased risk of cardiovascular disease and mortality. Recent experimental studies in murine models have assessed the most commonly mutated "driver" genes associated with CH, and have provided evidence for mechanistic connections between CH and cardiovascular disease. A deeper understanding of the mechanisms by which specific CH mutations promote disease pathogenesis is of importance, as it could pave the way for individualized therapeutic strategies targeting the pathogenic CH gene mutations in the future. Here, we review the epidemiology of CH and the mechanistic work from studies using murine disease models, with a particular focus on the strengths and limitations of these experimental systems. We intend for this review to help investigators select the most appropriate models to study CH in the setting of cardiovascular disease.

Recent Advances in Implantation-Based Genetic Modeling of Biliary Carcinogenesis in Mice

Simple Summary

Biliary tract cancer (BTC) is often refractory to conventional therapeutics and is difficult to diagnose in the early stages. In addition, the pathogenesis of BTC is not fully understood, despite recent advances in cancer genome analysis. To address these issues, the development of fine disease models is critical for BTC. Although still limited in number, there are various platforms for genetic models of BTC owing to newly emerging technology. Among these, implantation-based models have recently drawn attention for their convenience, flexibility, and scalability. To highlight the relevance of this approach, we comprehensively summarize the advantages and disadvantages of BTC models developed using diverse approaches. Currently available research data on intra- and extrahepatic cholangiocarcinoma and gallbladder carcinoma are presented in this review. This information will likely help in selecting the optimal models for various applications and develop novel innovative models based on these technologies.

Abstract

Epithelial cells in the biliary system can develop refractory types of cancers, which are often associated with inflammation caused by viruses, parasites, stones, and chemicals. Genomic studies have revealed recurrent genetic changes and deregulated signaling pathways in biliary tract cancer (BTC). The causal roles have been at least partly clarified using various genetically engineered mice. Technical advances in Cre-LoxP technology, together with hydrodynamic tail injection, CRISPR/Cas9 technology, in vivo electroporation, and organoid culture have enabled more precise modeling of BTC. Organoid-based genetic modeling, combined with implantation in mice, has recently drawn attention as a means to accelerate the development of BTC models. Although each model may not perfectly mimic the disease, they can complement one another, or two different approaches can be integrated to establish a novel model. In addition, a comparison of the outcomes among these models with the same genotype provides mechanistic insights into the interplay between genetic alterations and the microenvironment in the pathogenesis of BTCs. Here, we review the current status of genetic models of BTCs in mice to provide information that facilitates the wise selection of models and to inform the future development of ideal disease models.

Heterogeneity in clone dynamics within and adjacent to intestinal tumours identified by Dre-mediated lineage tracing

Somatic models of tissue pathology commonly use induction of gene-specific mutations in mice mediated by spatiotemporal regulation of Cre recombinase. Subsequent investigation of the onset and development of disease can be limited by the inability to track changing cellular behaviours over time. Here, a lineage-tracing approach based on ligand-dependent activation of Dre recombinase that can be employed independently of Cre is described. The clonal biology of the intestinal epithelium following Cre-mediated stabilisation of β-catenin reveals that, within tumours, many new clones rapidly become extinct. Surviving clones show accelerated population of tumour glands compared to normal intestinal crypts but in a non-uniform manner, indicating that intra-tumour glands follow heterogeneous dynamics. In tumour-adjacent epithelia, clone sizes are smaller than in the background epithelia, as a whole. This suggests a zone of ∼seven crypt diameters within which clone expansion is inhibited by tumours and that may facilitate their growth.

The art of lineage tracing: From worm to human

Reconstructing the genealogy of every cell that makes up an organism remains a long-standing challenge in developmental biology. Besides its relevance for understanding the mechanisms underlying normal and pathological development, resolving the lineage origin of cell types will be crucial to create these types on-demand. Multiple strategies have been deployed towards the problem of lineage tracing, ranging from direct observation to sophisticated genetic approaches. Here we discuss the achievements and limitations of past and current technology. Finally, we speculate about the future of lineage tracing and how to reach the next milestones in the field.

K-Ras-Activated Cells Can Develop into Lung Tumors When Runx3-Mediated Tumor Suppressor Pathways Are Abrogated

K-RAS is frequently mutated in human lung adenocarcinomas (ADCs), and the p53 pathway plays a central role in cellular defense against oncogenic K-RAS mutation. However, in mouse lung cancer models, oncogenic K-RAS mutation alone can induce ADCs without p53 mutation, and loss of p53 does not have a significant impact on early K-RAS-induced lung tumorigenesis. These results raise the question of how K-RAS-activated cells evade oncogene surveillance mechanisms and develop into lung ADCs. RUNX3 plays a key role at the restriction (R)-point, which governs multiple tumor suppressor pathways including the p14ARF-p53 pathway. In this study, we found that K-RAS activation in a very limited number of cells, alone or in combination with p53 inactivation, failed to induce any pathologic lesions for up to 1 year. By contrast, when Runx3 was inactivated and K-RAS was activated by the same targeting method, lung ADCs and other tumors were rapidly induced. In a urethane-induced mouse lung tumor model that recapitulates the features of K-RAS-driven human lung tumors, Runx3 was inactivated in both adenomas (ADs) and ADCs, whereas K-RAS was activated only in ADCs. Together, these results demonstrate that the R-point-associated oncogene surveillance mechanism is abrogated by Runx3 inactivation in AD cells and these cells cannot defend against K-RAS activation, resulting in the transition from AD to ADC. Therefore, K-RAS-activated lung epithelial cells do not evade oncogene surveillance mechanisms; instead, they are selected if they occur in AD cells in which Runx3 has been inactivated.

Epidermal Tissue Adapts to Restrain Progenitors Carrying Clonal p53 Mutations

Aging human tissues, such as sun-exposed epidermis, accumulate a high burden of progenitor cells that carry oncogenic mutations. However, most progenitors carrying such mutations colonize and persist in normal tissue without forming tumors. Here, we investigated tissue-level constraints on clonal progenitor behavior by inducing a single-allele p53 mutation (Trp53R245W; p53∗/wt), prevalent in normal human epidermis and squamous cell carcinoma, in transgenic mouse epidermis. p53∗/wt progenitors initially outcompeted wild-type cells due to enhanced proliferation, but subsequently reverted toward normal dynamics and homeostasis. Physiological doses of UV light accelerated short-term expansion of p53∗/wt clones, but their frequency decreased with protracted irradiation, possibly due to displacement by UV-induced mutant clones with higher competitive fitness. These results suggest multiple mechanisms restrain the proliferation of p53∗/wt progenitors, thereby maintaining epidermal integrity.

Inducible disruption of the c-myb gene allows allogeneic bone marrow transplantation without irradiation

Allogeneic bone marrow (BM) transplantation enables the in vivo functional assessment of hematopoietic cells. As pre-conditioning, ionizing radiation is commonly applied to induce BM depletion, however, it exerts adverse effects on the animal and can limit experimental outcome. Here, we provide an alternative method that harnesses conditional gene deletion to ablate c-myb and thereby deplete BM cells, hence allowing BM substitution without other pre-conditioning. The protocol results in a high level of blood chimerism after allogeneic BM transplantation, whereas immune cells in peripheral tissues such as resident macrophages are not replaced. Further, mice featuring a low chimerism after initial transplantation can undergo a second induction cycle for efficient deletion of residual BM cells without the necessity to re-apply donor cells. In summary, we present an effective c-myb-dependent genetic technique to generate BM chimeras in the absence of irradiation or other methods for pre-conditioning.

Wnt ligands influence tumour initiation by controlling the number of intestinal stem cells

Many epithelial stem cell populations follow a pattern of stochastic stem cell divisions called 'neutral drift'. It is hypothesised that neutral competition between stem cells protects against the acquisition of deleterious mutations. Here we use a Porcupine inhibitor to reduce Wnt secretion at a dose where intestinal homoeostasis is maintained despite a reduction of Lgr5+ stem cells. Functionally, there is a marked acceleration in monoclonal conversion, so that crypts become rapidly derived from a single stem cell. Stem cells located further from the base are lost and the pool of competing stem cells is reduced. We tested whether this loss of stem cell competition would modify tumorigenesis. Reduction of Wnt ligand secretion accelerates fixation of Apc-deficient cells within the crypt leading to accelerated tumorigenesis. Therefore, ligand-based Wnt signalling influences the number of stem cells, fixation speed of Apc mutations and the speed and likelihood of adenoma formation.

Seeds in the liver

The liver is a crucial organ for homeostasis and has a tremendous self-renewal and regenerative capacity. It has long been believed that the self-renewal and repair of the liver within a given physiological condition or its repopulation in chronic liver diseases, when hepatocyte proliferation is impaired, will primarily be conducted by the proliferating duct cells, termed "oval cells" or hepatic progenitor cells (HPCs). In addition, numerous studies have revealed that HPCs are the initial tumor cells of liver cancer under certain micro-environments. However, benefit from the extensive application of lineage tracing strategies using the Cre/LoxP system, researchers have redefined the fate of these bipotential cells, raising obvious controversies regarding the capacity of liver cells to control their own biology and differentiation. Here, we review the relevant articles, focusing on cell-lineage tracing to better understanding seed cells and their distinct fate in the liver.

HUWE1 is a critical colonic tumour suppressor gene that prevents MYC signalling, DNA damage accumulation and tumour initiation

Cancer genome sequencing projects have identified hundreds of genetic alterations, often at low frequencies, raising questions as to their functional relevance. One exemplar gene is HUWE1, which has been found to be mutated in numerous studies. However, due to the large size of this gene and a lack of functional analysis of identified mutations, their significance to carcinogenesis is unclear. To determine the importance of HUWE1, we chose to examine its function in colorectal cancer, where it is mutated in up to 15 per cent of tumours. Modelling of identified mutations showed that they inactivate the E3 ubiquitin ligase activity of HUWE1. Genetic deletion of Huwe1 rapidly accelerated tumourigenic in mice carrying loss of the intestinal tumour suppressor gene Apc, with a dramatic increase in tumour initiation. Mechanistically, this phenotype was driven by increased MYC and rapid DNA damage accumulation leading to loss of the second copy of Apc The increased levels of DNA damage sensitised Huwe1-deficient tumours to DNA-damaging agents and to deletion of the anti-apoptotic protein MCL1. Taken together, these data identify HUWE1 as a bona fide tumour suppressor gene in the intestinal epithelium and suggest a potential vulnerability of HUWE1-mutated tumours to DNA-damaging agents and inhibitors of anti-apoptotic proteins.

A single dividing cell population with imbalanced fate drives oesophageal tumour growth

Understanding the cellular mechanisms of tumour growth is key for designing rational anticancer treatment. Here we used genetic lineage tracing to quantify cell behaviour during neoplastic transformation in a model of oesophageal carcinogenesis. We found that cell behaviour was convergent across premalignant tumours, which contained a single proliferating cell population. The rate of cell division was not significantly different in the lesions and the surrounding epithelium. However, dividing tumour cells had a uniform, small bias in cell fate so that, on average, slightly more dividing than non-dividing daughter cells were generated at each round of cell division. In invasive cancers induced by Kras(G12D) expression, dividing cell fate became more strongly biased towards producing dividing over non-dividing cells in a subset of clones. These observations argue that agents that restore the balance of cell fate may prove effective in checking tumour growth, whereas those targeting cycling cells may show little selectivity.

Potential Pitfalls of the Mx1-Cre System: Implications for Experimental Modeling of Normal and Malignant Hematopoiesis

Conditional knockout mice are commonly used to study the function of specific genes in hematopoiesis. Different promoters that drive Cre expression have been utilized, with the interferon-inducible Mx1-Cre still being the most commonly used "deleter strain" in experimental hematology. However, different pitfalls associated with this system could lead to misinterpretation in functional studies. We present here two of these issues related to the use of Mx1-Cre: first, a high spontaneous recombination rate when applying commonly used techniques in experimental hematology, and second, undesired short-term consequences of the use of polyinosinic:polycytidylic acid, including changes in cellular phenotypes that, however, resolve within days. Our studies emphasize therefore that proper controls are crucial when modeling gene deletion using the Mx1-Cre transgene.

Bcl-2 is a critical mediator of intestinal transformation

Intestinal tumour formation is generally thought to occur following mutational events in the stem cell pool. However, active NF-κB signalling additionally facilitates malignant transformation of differentiated cells. We hypothesized that genes shared between NF-κB and intestinal stem cell (ISCs) signatures might identify common pathways that are required for malignant growth. Here, we find that the NF-κB target Bcl-2, an anti-apoptotic gene, is specifically expressed in ISCs in both mice and humans. Bcl-2 is dispensable in homeostasis and, although involved in protecting ISCs from radiation-induced damage, it is non-essential in tissue regeneration. Bcl-2 is upregulated in adenomas, and its loss or inhibition impairs outgrowth of oncogenic clones, because Bcl-2 alleviates apoptotic priming in epithelial cells following Apc loss. Furthermore, Bcl-2 expression in differentiated epithelial cells renders these cells amenable to clonogenic outgrowth. Collectively, our results indicate that Bcl-2 is required for efficient intestinal transformation following Apc-loss and constitutes a potential chemoprevention target.

Mitochondrial Dynamics is a Distinguishing Feature of Skeletal Muscle Fiber Types and Regulates Organellar Compartmentalization

Skeletal muscle fibers differentiate into specific fiber types with distinct metabolic properties determined by their reliance on oxidative phosphorylation (OXPHOS). Using in vivo approaches, we find that OXPHOS-dependent fibers, compared to glycolytic fibers, contain elongated mitochondrial networks with higher fusion rates that are dependent on the mitofusins Mfn1 and Mfn2. Switching of a glycolytic fiber to an oxidative IIA type is associated with elongation of mitochondria, suggesting that mitochondrial fusion is linked to metabolic state. Furthermore, we reveal that mitochondrial proteins are compartmentalized to discrete domains centered around their nuclei of origin. The domain dimensions are dependent on fiber type and are regulated by the mitochondrial dynamics proteins Mfn1, Mfn2, and Mff. Our results indicate that mitochondrial dynamics is tailored to fiber type physiology and provides a rationale for the segmental defects characteristic of aged and diseased muscle fibers.

Protocols for Analyzing the Role of Paneth Cells in Regenerating the Murine Intestine using Conditional Cre-lox Mouse Models

The epithelial surface of the mammalian intestine is a dynamic tissue that renews every 3 - 7 days. Understanding this renewal process identified a population of rapidly cycling intestinal stem cells (ISCs) characterized by their expression of the Lgr5 gene. These are supported by a quiescent stem cell population, marked by Bmi-1 expression, capable of replacing them in the event of injury. Investigating the interactions between these populations is crucial to understanding their roles in disease and cancer. The ISCs exist within crypts on the intestinal surface, these niches support the ISC in replenishing the epithelia. The interaction between active and quiescent ISCs likely involves other differentiated cells within the niche, as it has previously been demonstrated that the ‘‘stemness’’ of the Lgr5 ISC is closely tied to the presence of their neighboring Paneth cells. Using conditional cre-lox mouse models we tested the effect of deleting the majority of active ISCs in the presence or absence of the Paneth cells. Here we describe the techniques and analysis undertaken to characterize the intestine and demonstrate that the Paneth cells play a crucial role within the ISC niche in aiding recovery following substantial insult.

E-cadherin can limit the transforming properties of activating β-catenin mutations

Wnt pathway deregulation is a common characteristic of many cancers. Only colorectal cancer predominantly harbours mutations in APC, whereas other cancer types (hepatocellular carcinoma, solid pseudopapillary tumours of the pancreas) have activating mutations in β-catenin (CTNNB1). We have compared the dynamics and the potency of β-catenin mutations in vivo. Within the murine small intestine (SI), an activating mutation of β-catenin took much longer to achieve Wnt deregulation and acquire a crypt-progenitor cell (CPC) phenotype than Apc or Gsk3 loss. Within the colon, a single activating mutation of β-catenin was unable to drive Wnt deregulation or induce the CPC phenotype. This ability of β-catenin mutation to differentially transform the SI versus the colon correlated with higher expression of E-cadherin and a higher number of E-cadherin:β-catenin complexes at the membrane. Reduction in E-cadherin synergised with an activating mutation of β-catenin resulting in a rapid CPC phenotype within the SI and colon. Thus, there is a threshold of β-catenin that is required to drive transformation, and E-cadherin can act as a buffer to sequester mutated β-catenin.

Cell-of-Origin of Cancer versus Cancer Stem Cells: Assays and Interpretations

A tumor originates from a normal cell that has undergone tumorigenic transformation as a result of genetic mutations. This transformed cell is the cell-of-origin for the tumor. In contrast, an established clinical tumor is sustained by subpopulations of self-renewing cancer cells operationally called cancer stem cells (CSC) that can generate, intraclonally, both tumorigenic and nontumorigenic cells. Identifying and characterizing tumor cell-of-origin and CSCs should help elucidate tumor cell heterogeneity, which, in turn, should help understand tumor cell responses to clinical treatments, drug resistance, tumor relapse, and metastatic spread. Both tumor transplantation and lineage-tracing assays have been helpful in characterizing these cancer cell populations, although each system has its strengths and caveats. In this article, we briefly review and summarize advantages and limitations of both assays in support of a combinatorial approach to accurately define the roles of both cancer-initiating and cancer-propagating cells. As an aside, we also wish to clarify the definitions of cancer cell-of-origin and CSCs, which are often interchangeably used by mistake.

Determining the fate of hepatic cells by lineage tracing: facts and pitfalls

Slow renewal of the epithelial cells by proliferation ensures homeostasis of the liver, but extensive proliferation may occur upon injury. When proliferation is impaired, transdifferentiation of mature cells or differentiation of stem cells allows production of new hepatocytes and cholangiocytes. While lineage tracings using cyclization recombinase (Cre) recombinase-mediated cell labeling represent the gold standard for defining cell fate, there are more variables than was initially realized. This led to controversies about the capacity of liver cells to switch their fate. Here, I review how cells are traced in the liver and highlight the experimental pitfalls that may cause misinterpretations and controversies.

HDAC1 and HDAC2 collectively regulate intestinal stem cell homeostasis

Histone deacetylases (HDACs) are posttranslational modifiers that deacetylate proteins. Despite their crucial role in numerous biological processes, the use of broad-range HDAC inhibitors (HDACi), has shown clinical efficacy. However, undesired side effects highlight the necessity to better understand the biology of different HDACs and target the relevant HDACs. Using a novel mouse model, in which HDAC1 and HDAC2 can be simultaneously deleted in the intestine of adult mice, we show that the simultaneous deletion of HDAC1 and HDAC2 leads to a rapid loss of intestinal homeostasis. Importantly, this deletion cannot be sustained, and 8 days after initial ablation, stem cells that have escaped HDAC1 or HDAC2 deletion swiftly repopulate the intestinal lining. In vitro ablation of HDAC1 and HDAC2 using intestinal organoid cultures resulted in a down-regulation of multiple intestinal stem cell markers and functional loss of clonogenic capacity. Importantly, treatment of wild-type organoids with class I-specific HDACi MS-275 also induced a similar loss of stemness, providing a possible rationale for the gastrointestinal side effects often observed in HDACi-treated patients. In conclusion, these data show that HDAC1 and HDAC2 have a redundant function and are essential to maintain intestinal homeostasis.

Application of a novel regulatable Cre recombinase system to define the role of liver and gut metabolism in drug oral bioavailability

The relative contribution of hepatic compared with intestinal oxidative metabolism is a crucial factor in drug oral bioavailability and therapeutic efficacy. Oxidative metabolism is mediated by the cytochrome P450 mono-oxygenase system to which cytochrome P450 reductase (POR) is the essential electron donor. In order to study the relative importance of these pathways in drug disposition, we have generated a novel mouse line where Cre recombinase is driven off the endogenous Cyp1a1 gene promoter; this line was then crossed on to a floxed POR mouse. A 40 mg/kg dose of the Cyp1a1 inducer 3-methylcholanthrene (3MC) eliminated POR expression in both liver and small intestine, whereas treatment at 4 mg/kg led to a more targeted deletion in the liver. Using this approach, we have studied the pharmacokinetics of three probe drugs--paroxetine, midazolam, nelfinavir--and show that intestinal metabolism is a determinant of oral bioavailability for the two latter compounds. The Endogenous Reductase Locus (ERL) mouse represents a significant advance on previous POR deletion models as it allows direct comparison of hepatic and intestinal effects on drug and xenobiotic clearance using lower doses of a single Cre inducing agent, and in addition minimizes any cytotoxic effects, which may compromise interpretation of the experimental data.

HIRA orchestrates a dynamic chromatin landscape in senescence and is required for suppression of neoplasia

Cellular senescence is a stable proliferation arrest that suppresses tumorigenesis. Cellular senescence and associated tumor suppression depend on control of chromatin. Histone chaperone HIRA deposits variant histone H3.3 and histone H4 into chromatin in a DNA replication-independent manner. Appropriately for a DNA replication-independent chaperone, HIRA is involved in control of chromatin in nonproliferating senescent cells, although its role is poorly defined. Here, we show that nonproliferating senescent cells express and incorporate histone H3.3 and other canonical core histones into a dynamic chromatin landscape. Expression of canonical histones is linked to alternative mRNA splicing to eliminate signals that confer mRNA instability in nonproliferating cells. Deposition of newly synthesized histones H3.3 and H4 into chromatin of senescent cells depends on HIRA. HIRA and newly deposited H3.3 colocalize at promoters of expressed genes, partially redistributing between proliferating and senescent cells to parallel changes in expression. In senescent cells, but not proliferating cells, promoters of active genes are exceptionally enriched in H4K16ac, and HIRA is required for retention of H4K16ac. HIRA is also required for retention of H4K16ac in vivo and suppression of oncogene-induced neoplasia. These results show that HIRA controls a specialized, dynamic H4K16ac-decorated chromatin landscape in senescent cells and enforces tumor suppression.

The epigenetic regulator CXXC finger protein 1 is essential for murine hematopoiesis

CXXC finger protein 1 (Cfp1), encoded by the Cxxc1 gene, binds to DNA sequences containing an unmethylated CpG dinucleotide and is an epigenetic regulator of both cytosine and histone methylation. Cxxc1-null mouse embryos fail to gastrulate, and Cxxc1-null embryonic stem cells are viable but cannot differentiate, suggesting that Cfp1 is required for chromatin remodeling associated with stem cell differentiation and embryogenesis. Mice homozygous for a conditional Cxxc1 deletion allele and carrying the inducible Mx1-Cre transgene were generated to assess Cfp1 function in adult animals. Induction of Cre expression in adult animals led to Cfp1 depletion in hematopoietic cells, a failure of hematopoiesis with a nearly complete loss of lineage-committed progenitors and mature cells, elevated levels of apoptosis, and death within two weeks. A similar pathology resulted following transplantation of conditional Cxxc1 bone marrow cells into wild type recipients, demonstrating this phenotype is intrinsic to Cfp1 function within bone marrow cells. Remarkably, the Lin- Sca-1+ c-Kit+ population of cells in the bone marrow, which is enriched for hematopoietic stem cells and multi-potential progenitor cells, persists and expands in the absence of Cfp1 during this time frame. Thus, Cfp1 is necessary for hematopoietic stem and multi-potential progenitor cell function and for the developmental potential of differentiating hematopoietic cells.

HIF-1α can act as a tumor suppressor gene in murine acute myeloid leukemia

Self-renewal of hematopoietic stem cells (HSCs) and leukemia-initiating cells (LICs) has been proposed to be influenced by low oxygen tension (hypoxia). This signaling, related to the cellular localization inside the bone marrow niche and/or influenced by extrinsic factors, promotes the stabilization of hypoxia-inducible factors (HIFs). Whether HIF-1α can be used as a therapeutic target in the treatment of myeloid malignancies remains unknown. We have used 3 different murine models to investigate the role of HIF-1α in acute myeloid leukemia (AML) initiation/progression and self-renewal of LICs. Unexpectedly, we failed to observe a delay or prevention of disease development from hematopoietic cells lacking Hif-1α. In contrast, deletion of Hif-1α resulted in faster development of the disease and an enhanced leukemia phenotype in some of the investigated models. Our results therefore warrant reconsideration of the role of HIF-1α and, as a consequence, question its generic therapeutic usefulness in AML.

Brg1 loss attenuates aberrant wnt-signalling and prevents wnt-dependent tumourigenesis in the murine small intestine

Tumourigenesis within the intestine is potently driven by deregulation of the Wnt pathway, a process epigenetically regulated by the chromatin remodelling factor Brg1. We aimed to investigate this interdependency in an in vivo setting and assess the viability of Brg1 as a potential therapeutic target. Using a range of transgenic approaches, we deleted Brg1 in the context of Wnt-activated murine small intestinal epithelium. Pan-epithelial loss of Brg1 using VillinCreERT2 and AhCreERT transgenes attenuated expression of Wnt target genes, including a subset of stem cell-specific genes and suppressed Wnt-driven tumourigenesis improving animal survival. A similar increase in survival was observed when Wnt activation and Brg1 loss were restricted to the Lgr5 expressing intestinal stem cell population. We propose a mechanism whereby Brg1 function is required for aberrant Wnt signalling and ultimately for the maintenance of the tumour initiating cell compartment, such that loss of Brg1 in an Apc-deficient context suppresses adenoma formation. Our results highlight potential therapeutic value of targeting Brg1 and serve as a proof of concept that targeting the cells of origin of cancer may be of therapeutic relevance.

The APC network regulates the removal of mutated cells from colonic crypts

Self-renewal is essential for multicellular organisms but carries the risk of somatic mutations that can lead to cancer, which is particularly critical for rapidly renewing tissues in a highly mutagenic environment such as the intestinal epithelium. Using computational modeling and in vivo experimentation, we have analyzed how adenomatous polyposis coli (APC) mutations and β-catenin aberrations affect the maintenance of mutant cells in colonic crypts. The increasing abundance of APC along the crypt axis forms a gradient of cellular adhesion that causes more proliferative cells to accelerate their movement toward the top of the crypt, where they are shed into the lumen. Thus, the normal crypt can efficiently eliminate β-catenin mutant cells, whereas APC mutations favor retention. Together, the molecular design of the APC/β-catenin signaling network integrates cell proliferation and migration dynamics to translate intracellular signal processing and protein gradients along the crypt into intercellular interactions and whole-crypt physiological or pathological behavior.

Epithelial-specific loss of PTEN results in colorectal juvenile polyp formation and invasive cancer

Cowden syndrome (CS) is a rare autosomal dominant cancer-prone disorder caused by germ-line mutation of the phosphatase and tensin homolog mutated on chromosome 10 (PTEN) tumor-suppressor gene. Affected patients commonly develop juvenile polyps, and show an elevated risk of developing colorectal cancers. The etiology of these peculiar polyps remains unclear, although previous work has suggested somatic PTEN alterations in the stroma of juvenile polyps. After a long latency period, we find epithelial-specific PTEN deletion to cause formation of juvenile polyps in the colorectum without stromal PTEN loss. More important, we find that these lesions closely recapitulate all of the characteristic histopathological features of juvenile polyps seen in patients with CS, including stromal alterations and dysplastic transformation to colorectal carcinoma. The stromal alterations we identify after epithelial-specific PTEN loss suggest that PTEN may be involved in altered epithelial-mesenchymal cross talk, which, in turn, predisposes to colorectal neoplasia and polyposis. Our transgenic model is the first to recapitulate colorectal juvenile polyposis in patients with CS. We conclude that stromal PTEN loss is not a prerequisite for the formation of juvenile polyps, and that colorectal juvenile polyps in CS are bona fide neoplastic precursor lesions.

Lineage analysis of epidermal stem cells

Lineage tracing involves labeling cells to track their subsequent behavior within the normal tissue environment. The advent of genetic lineage tracing and cell proliferation assays, together with high resolution three-dimensional (3D) imaging and quantitative methods to infer cell behavior from lineage-tracing data, has transformed our understanding of murine epidermal stem and progenitor cells. Here, we review recent insights that reveal how a progenitor cell population maintains interfollicular epidermis, whereas stem cells, quiescent under homeostatic conditions, are mobilized in response to wounding. We discuss progress in understanding how the various stem cell populations of the hair follicle sustain this complex and highly dynamic structure, and recent analysis of stem cells in sweat and sebaceous glands. The extent to which insights from mouse studies can be applied to human epidermis is also considered.

Defining stem cell dynamics in models of intestinal tumor initiation

Cancer is a disease in which cells accumulate genetic aberrations that are believed to confer a clonal advantage over cells in the surrounding tissue. However, the quantitative benefit of frequently occurring mutations during tumor development remains unknown. We quantified the competitive advantage of Apc loss, Kras activation, and P53 mutations in the mouse intestine. Our findings indicate that the fate conferred by these mutations is not deterministic, and many mutated stem cells are replaced by wild-type stem cells after biased, but still stochastic events. Furthermore, P53 mutations display a condition-dependent advantage, and especially in colitis-affected intestines, clones harboring mutations in this gene are favored. Our work confirms the previously theoretical notion that the tissue architecture of the intestine suppresses the accumulation of mutated lineages.

Brg1 is required for stem cell maintenance in the murine intestinal epithelium in a tissue-specific manner

Brg1 is a chromatin remodeling factor involved in mediation of a plethora of signaling pathways leading to its participation in various physiological processes both during development and in adult tissues. Among other signaling pathways, the Wnt pathway has been proposed to require Brg1 for transactivation of its target genes. Given the pivotal role of the Wnt pathway in the maintenance of normal intestinal homeostasis, we aimed to investigate the effects of Brg1 loss on the intestinal physiology. To this end, we deleted Brg1 in the murine small and large intestinal epithelia using a range of transgenic approaches. Pan-epithelial loss of Brg1 in the small intestine resulted in crypt ablation, while partial Brg1 deficiency led to gradual repopulation of the intestinal mucosa with wild-type cells. In contrast, Brg1 loss in the large intestinal epithelium was compensated by upregulation of Brm. We propose that while Brg1 is dispensable for the survival and function of the progenitor and differentiated cells in the murine intestinal epithelium, it is essential for the maintenance of the stem cell population in a tissue-specific manner.

Cardiac fibrosis in mice expressing an inducible myocardial-specific Cre driver

Tamoxifen-inducible Cre-mediated manipulation of animal genomes has achieved wide acceptance over the last decade, with numerous important studies heavily relying on this technique. Recently, a number of groups have reported transient complications of using this protocol in the heart. In the present study we observed a previously unreported focal fibrosis and depressed left-ventricular function in tamoxifen-treated αMHC-MerCreMer-positive animals in a Tβ4shRNAflox × αMHC-MerCreMer cross at 6-7 weeks following standard tamoxifen treatment, regardless of the presence of the floxed transgene. The phenotype was reproduced by treating mice from the original αMHC-MerCreMer strain with tamoxifen. In the acute phase after tamoxifen treatment, cell infiltration into the myocardium was accompanied by increased expression of pro-inflammatory cytokines (IL-1β, IL-6, TNFα, IFNγ, Ccl2) and markers of hypertrophy (ANF, BNP, Col3a1). These observations highlight the requirement for including tamoxifen-treated MerCreMer littermate controls to avert misinterpretation of conditional mutant phenotypes. A survey of the field as well as the protocols presented here suggests that controlling the parameters of tamoxifen delivery is important in avoiding the chronic MerCreMer-mediated cardiac phenotype reported here.

PTEN loss and KRAS activation cooperate in murine biliary tract malignancies

Carcinomas of the biliary tract are aggressive malignancies in humans. Loss of the tumour suppressor PTEN has previously been associated with cholangiocarcinoma development in a murine model. Activation of KRAS is reported in up to one-third of human cholangiocarcinomas and 50% of gall bladder carcinomas. In this study we aimed to test the potential interaction between PTEN and KRAS mutation in biliary tract malignancy. We used an inducible Cre-LoxP-based approach to coordinately delete PTEN and activate KRAS within the adult mouse biliary epithelium. We found that activation of KRAS alone has little effect upon biliary epithelium. Loss of PTEN alone results in the development of low-grade neoplastic lesions, following long latency and at low incidence. Combination of both mutations causes rapid development of biliary epithelial proliferative lesions, which progress through dysplasia to invasive carcinoma. We conclude that activation of the PI3'K pathway following loss of PTEN is sufficient to drive slow development of low-grade biliary lesions in mice. In contrast, mutational activation of KRAS does not result in a similar phenotype, despite a prediction that this should activate both the RAF-MEK-ERK and PI3'-kinase pathways. However, mutation of both genes results in rapid tumourigenesis, arguing that PTEN normally functions as a 'brake' on the PI3'-kinase pathway, limiting the influence of KRAS activation. Mutation of both genes creates a 'permissive' environment, allowing the full effects of both mutations to be manifested. These data reveal an in vivo synergy between these mutations and provides a new mouse model of biliary tract malignancy.

Acute WNT signalling activation perturbs differentiation within the adult stomach and rapidly leads to tumour formation

A role for WNT signalling in gastric carcinogenesis has been suggested due to two major observations. First, patients with germline mutations in adenomatous polyposis coli (APC) are susceptible to stomach polyps and second, in gastric cancer, WNT activation confers a poor prognosis. However, the functional significance of deregulated WNT signalling in gastric homoeostasis and cancer is still unclear. In this study we have addressed this by investigating the immediate effects of WNT signalling activation within the stomach epithelium. We have specifically activated the WNT signalling pathway within the mouse adult gastric epithelium via deletion of either glycogen synthase kinase 3 (GSK3) or APC or via expression of a constitutively active β-catenin protein. WNT pathway deregulation dramatically affects stomach homoeostasis at very short latencies. In the corpus, there is rapid loss of parietal cells with fundic gland polyp (FGP) formation and adenomatous change, which are similar to those observed in familial adenomatous polyposis. In the antrum, adenomas occur from 4 days post-WNT activation. Taken together, these data show a pivotal role for WNT signalling in gastric homoeostasis, FGP formation and adenomagenesis. Loss of the parietal cell population and corresponding FGP formation, an early event in gastric carcinogenesis, as well as antral adenoma formation are immediate effects of nuclear β-catenin translocation and WNT target gene expression. Furthermore, our inducible murine model will permit a better understanding of the molecular changes required to drive tumourigenesis in the stomach.

Evidence for a crucial role of paneth cells in mediating the intestinal response to injury

The identification of the intestinal stem cell (ISC) markers Lgr5 and Bmi-1 has furthered our understanding of how they accomplish homeostasis in this rapidly self-renewing tissue. Recent work indicates that these markers identify a cycling Lgr5(+) ISC which can be replaced by a quiescent Bmi-1(+) ISC. Currently, there is little data on how these cells interact to control intestinal crypt homeostasis and regeneration. This interaction likely involves other differentiated cells within the niche as it has previously been demonstrated that the "stemness" of the Lgr5 ISC is closely tied to the presence of their neighboring Paneth cells. To investigate this, we used two conditional mouse models to delete the transcription factor β-catenin within the intestinal crypt. Critically these differ in their ability to drive recombination within Paneth cells and therefore allow us to compare the effect of deleting the majority of active ISCs in the presence or absence of the Paneth cells. After gene deletion, the intestines in the model in which Paneth cells were retained showed a rapid recovery and repopulation of the crypt-villus axis presumably from either a spared ISC or the hypothetical quiescent ISCs. However, in the absence of Paneth cells the recovery ability was compromised resulting in complete loss of intestinal epithelial integrity. This data indicates that the Paneth cells play a crucial role within the in vivo ISC niche in aiding recovery following substantial insult.

Reversed argininosuccinate lyase activity in fumarate hydratase-deficient cancer cells

BACKGROUND

Loss of function of fumarate hydratase (FH), the mitochondrial tumor suppressor and tricarboxylic acid (TCA) cycle enzyme, is associated with a highly malignant form of papillary and collecting duct renal cell cancer. The accumulation of fumarate in these cells has been linked to the tumorigenic process. However, little is known about the overall effects of the loss of FH on cellular metabolism.

METHODS

We performed comprehensive metabolomic analyses of urine from Fh1-deficient mice and stable isotopologue tracing from human and mouse FH-deficient cell lines to investigate the biochemical signature of the loss of FH.

RESULTS

The metabolomics analysis revealed that the urea cycle metabolite argininosuccinate is a common metabolic biomarker of FH deficiency. Argininosuccinate was found to be produced from arginine and fumarate by the reverse activity of the urea cycle enzyme argininosuccinate lyase (ASL), making these cells auxotrophic for arginine. Depleting arginine from the growth media by the addition of pegylated arginine deiminase (ADI-PEG 20) decreased the production of argininosuccinate in FH-deficient cells and reduced cell survival and proliferation.

CONCLUSIONS

These results unravel a previously unidentified correlation between fumarate accumulation and the urea cycle enzyme ASL in FH-deficient cells. The finding that FH-deficient cells become auxotrophic for arginine opens a new therapeutic perspective for the cure of hereditary leiomyomatosis and renal cell cancer (HLRCC).

Tracking cells in their native habitat: lineage tracing in epithelial neoplasia

For tumours to develop, mutations must disrupt tissue homeostasis in favour of deregulated proliferation. Genetic lineage tracing has uncovered the behaviour of proliferating cells that underpins the maintenance of epithelial tissues and the barriers that are broken in neoplastic transformation. In this Review, we focus on new insights revealed by quantifying the behaviour of normal, preneoplastic and tumour cells in epithelia in transgenic mice and consider their potential importance in humans.

New insights into mechanisms of stem cell daughter fate determination in regenerative tissues

Stem cells can self-renew and differentiate over extended periods of time. Understanding how stem cells acquire their fates is a central question in stem cell biology. Early work in Drosophila germ line and neuroblast showed that fate choice is achieved by strict asymmetric divisions that can generate each time one stem and one differentiated cell. More recent work suggests that during homeostasis, some stem cells can divide symmetrically to generate two differentiated cells or two identical stem cells to compensate for stem cell loss that occurred by direct differentiation or apoptosis. The interplay of all these factors ensures constant tissue regeneration and the maintenance of stem cell pool size. This interplay can be modeled as a population-deterministic dynamics that, at least in some systems, may be described as stochastic behavior. Here, we overview recent progress made on the characterization of stem cell dynamics in regenerative tissues.

Beyond knockouts: cre resources for conditional mutagenesis

With the effort of the International Phenotyping Consortium to produce thousands of strains with conditional potential gathering steam, there is growing recognition that it must be supported by a rich toolbox of cre driver strains. The approaches to build cre strains have evolved in both sophistication and reliability, replacing first-generation strains with tools that can target individual cell populations with incredible precision and specificity. The modest set of cre drivers generated by individual labs over the past 15+ years is now growing rapidly, thanks to a number of large-scale projects to produce new cre strains for the community. The power of this growing resource, however, depends upon the proper deep characterization of strain function, as even the best designed strain can display a variety of undesirable features that must be considered in experimental design. This must be coupled with the parallel development of informatics tools to provide functional data to the user and facilitated access to the strains through public repositories. We discuss the current progress on all of these fronts and the challenges that remain to ensure the scientific community can capitalize on the tremendous number of mouse resources at their disposal.

Conditional gene expression systems in the transgenic rat brain

BACKGROUND

Turning gene expression on and off at will is one of the most powerful tools for the study of gene function in vivo. While several conditional systems were successful in invertebrates, in mice the Cre/loxP recombination system and the tet-controlled transcription activation system are predominant. Both expression systems allow for spatial and temporal control of gene activities, and, in the case of tet regulation, even for the reversible activation/inactivation of gene expression. Although the rat is the principal experimental model in biomedical research, in particular in studies of neuroscience, conditional rat transgenic systems are exceptionally rare in this species.

RESULTS

We addressed this lack of technology, and established and thoroughly characterized CreERT2 and tTA transgenic rats with forebrain-specific transgene expression, controlled by the CaMKII alpha promoter. In addition, we developed new universal rat reporter lines for both transcription control systems and established inducible and efficient reporter gene expression in forebrain neurons.

CONCLUSIONS

We demonstrate that conditional genetic manipulations in the rat brain are both feasible and practicable and outline advantages and limitations of the Tet and Cre/loxP system in the rat brain.

Activation of Wnt signalling promotes development of dysplasia in Barrett's oesophagus

Barrett's oesophagus is a precursor of oesophageal adenocarcinoma, via intestinal metaplasia and dysplasia. Risk of cancer increases substantially with dysplasia, particularly high-grade dysplasia. Thus, there is a clinical need to identify and treat patients with early-stage disease (metaplasia and low-grade dysplasia) that are at high risk of cancer. Activated Wnt signalling is critical for normal intestinal development and homeostasis, but less so for oesophageal development. Therefore, we asked whether abnormally increased Wnt signalling contributes to the development of Barrett's oesophagus (intestinal metaplasia) and/or dysplasia. Forty patients with Barrett's metaplasia, dysplasia or adenocarcinoma underwent endoscopy and biopsy. Mice with tamoxifen- and β-naphthoflavone-induced expression of activated β-catenin were used to up-regulate Wnt signalling in mouse oesophagus. Immunohistochemistry of β-catenin, Ki67, a panel of Wnt target genes, and markers of intestinal metaplasia was performed on human and mouse tissues. In human tissues, expression of nuclear activated β-catenin was found in dysplasia, particularly high grade. Barrett's metaplasia did not show high levels of activated β-catenin. Up-regulation of Ki67 and Wnt target genes was also mostly associated with high-grade dysplasia. Aberrant activation of Wnt signalling in mouse oesophagus caused marked tissue disorganization with features of dysplasia, but only selected molecular indicators of metaplasia. Based on these results in human tissues and a mouse model, we conclude that abnormal activation of Wnt signalling likely plays only a minor role in initiation of Barrett's metaplasia but a more critical role in progression to dysplasia.

Lineage tracing

Lineage tracing is the identification of all progeny of a single cell. Although its origins date back to developmental biology of invertebrates in the 19(th) century, lineage tracing is now an essential tool for studying stem cell properties in adult mammalian tissues. Lineage tracing provides a powerful means of understanding tissue development, homeostasis, and disease, especially when it is combined with experimental manipulation of signals regulating cell-fate decisions. Recently, the combination of inducible recombinases, multicolor reporter constructs, and live-cell imaging has provided unprecedented insights into stem cell biology. Here we discuss the different experimental strategies currently available for lineage tracing, their associated caveats, and new opportunities to integrate lineage tracing with the monitoring of intracellular signaling pathways.

Cdx2 determines the fate of postnatal intestinal endoderm

Knock out of intestinal Cdx2 produces different effects depending upon the developmental stage at which this occurs. Early in development it produces histologically ordered stomach mucosa in the midgut. Conditional inactivation of Cdx2 in adult intestinal epithelium, as well as specifically in the Lgr5-positive stem cells, of adult mice allows long-term survival of the animals but fails to produce this phenotype. Instead, the endodermal cells exhibit cell-autonomous expression of gastric genes in an intestinal setting that is not accompanied by mesodermal expression of Barx1, which is necessary for gastric morphogenesis. Cdx2-negative endodermal cells also fail to express Sox2, a marker of gastric morphogenesis. Maturation of the stem cell niche thus appears to be associated with loss of ability to express positional information cues that are required for normal stomach development. Cdx2-negative intestinal crypts produce subsurface cystic vesicles, whereas untargeted crypts hypertrophy to later replace the surface epithelium. These observations are supported by studies involving inactivation of Cdx2 in intestinal crypts cultured in vitro. This abolishes their ability to form long-term growing intestinal organoids that differentiate into intestinal phenotypes. We conclude that expression of Cdx2 is essential for differentiation of gut stem cells into any of the intestinal cell types, but they maintain a degree of cell-autonomous plasticity that allows them to switch on a variety of gastric genes.

V600EBraf induces gastrointestinal crypt senescence and promotes tumour progression through enhanced CpG methylation of p16INK4a

The majority of human colorectal cancers (CRCs) are initiated by mutations arising in the adenomatous polyposis coli (APC) tumour suppressor gene. However, a new class of non-APC mutated CRCs has been defined that have a serrated histopathology and carry the ^V600EBRAF oncogene. Here we have investigated the pathogenesis of serrated CRCs by expressing ^V600EBraf in the proliferative cells of the mouse gastrointestinal tract. We show that the oncogene drives an initial burst of Mek-dependent proliferation, leading to the formation of hyperplastic crypts. This is associated with β-catenin nuclear localization by a mechanism involving Mapk/Erk kinase (Mek)-dependent, Akt-independent phosphorylation of Gsk3β. However, hyperplastic crypts remain dormant for prolonged periods due to the induction of crypt senescence accompanied by upregulation of senescence-associated β-galactosidase and p16^Ink4a. We show that tumour progression is associated with down-regulation of p16^Ink4a through enhanced CpG methylation of exon 1 and knockout of Cdkn2a confirms this gene is a barrier to tumour progression. Our studies identify ^V600EBRAF as an early genetic driver mutation in serrated CRCs and indicate that, unlike APC-mutated cancers, this subtype arises by the bypassing of a ^V600EBraf driven oncogene-induced senescence programme.

Lkb1 and Pten synergise to suppress mTOR-mediated tumorigenesis and epithelial-mesenchymal transition in the mouse bladder

The AKT/PI3K/mTOR pathway is frequently altered in a range of human tumours, including bladder cancer. Here we report the phenotype of mice characterised by deletion of two key players in mTOR regulation, Pten and Lkb1, in a range of tissues including the mouse urothelium. Despite widespread recombination within the range of epithelial tissues, the primary phenotype we observe is the rapid onset of bladder tumorigenesis, with median onset of approximately 100 days. Single deletion of either Pten or Lkb1 had no effect on bladder cell proliferation or tumour formation. However, simultaneous deletion of Lkb1 and Pten led to an upregulation of the mTOR pathway and the hypoxia marker GLUT1, increased bladder epithelial cell proliferation and ultimately tumorigenesis. Bladder tissue also exhibited characteristic features of epithelial-mesenchymal transition, with loss of the epithelial markers E-cadherin and the tight junction protein ZO-1, and increases in the mesenchymal marker vimentin as well as nuclear localization of epithelial-mesenchymal transition (EMT) regulator Snail. We show that these effects were all dependent upon mTOR activity, as rapamycin treatment blocked both EMT and tumorigenesis. Our data therefore establish clear synergy between Lkb1 and Pten in controlling the mTOR pathway within bladder epithelium, and show that loss of this control leads to the disturbance of epithelial structure, EMT and ultimately tumorigenesis.

β-Catenin activation synergizes with PTEN loss to cause bladder cancer formation

Although deregulation of the Wnt signalling pathway has been implicated in urothelial cell carcinoma (UCC), the functional significance is unknown. To test its importance, we have targeted expression of an activated form of β-catenin to the urothelium of transgenic mice using Cre-Lox technology (UroIICRE(+) β-catenin(exon3/+)). Expression of this activated form of β-catenin led to the formation of localized hyperproliferative lesions by 3 months, which did not progress to malignancy. These lesions were characterized by a marked increase of the phosphatase and tensin homologue (PTEN) tumour suppressor protein. This appears to be a direct consequence of activating Wnt signalling in the bladder as conditional deletion of the adenomatous polyposis coli (Apc) gene within the adult bladder led rapidly to coincident β-catenin and PTEN expression. This PTEN expression blocked proliferation. Next, we combined PTEN deficiency with β-catenin activation and found that this caused papillary UCC. These tumours had increased pAKT signalling and were dependent on mammalian target of rapamycin (mTOR). Importantly, in human UCC, there was a significant correlation between high levels of β-catenin and pAKT (and low levels of PTEN). Taken together these data show that deregulated Wnt signalling has a critical role in promoting UCC, and suggests that human UCC that have high levels of Wnt and PI3 kinase signalling may be responsive to mTOR inhibition.

Defining the role of APC in the mitotic spindle checkpoint in vivo: APC-deficient cells are resistant to Taxol

Mutations in the adenomatous polyposis coli (APC) tumour suppressor are the key initiating event of colorectal cancer. Although the control of WNT signalling is well established as a central tumour-suppressive function, the significance of APC in regulating chromosome instability is less well established. In this study, we test whether APC-deficient cells have a functional spindle assembly checkpoint (SAC) in vivo by examining the response of these cells to Taxol and Vinorelbine. We also show for the first time that APC deficiency compromises the arrest response to Taxol in vivo. This effect is independent of the role that APC has in WNT signalling. At higher levels of Taxol, APC-deficient cells arrest as efficiently as wild-type cells. Importantly, this dose of Taxol strongly suppresses intestinal tumourigenesis in models of benign (APC(Min/+) mouse) and invasive (AhCreER(+)APC(fl/+)PTEN(fl/fl)) cancer. In contrast to intestinal enterocytes with a general SAC defect because of Bub1 (budding uninhibited by benzimidazole 1) deletion, APC-deficient enterocytes arrest equivalently to wild type when treated with Vinorelbine. This suggests that the failed arrest in response to Taxol is because of a specific defect in microtubule stabilization following Taxol treatment rather than a general role of the APC protein in the mitotic spindle checkpoint. In summary, this study clarifies the role of APC as a mitotic spindle checkpoint protein in vivo and shows that APC-deficient cells have a compromised response to Taxol.

The ordered architecture of murine ear epidermis is maintained by progenitor cells with random fate

Typical murine epidermis has a patterned structure, seen clearly in ear skin, with regular columns of differentiated cells overlying the proliferative basal layer. It has been proposed that each column is a clonal epidermal proliferative unit maintained by a central stem cell and its transit amplifying cell progeny. An alternative hypothesis is that proliferating basal cells have random fate, the probability of generating cycling or differentiated cells being balanced so homeostasis is achieved. The stochastic model seems irreconcilable with an ordered tissue. Here we use lineage tracing to reveal that basal cells generate clones with highly irregular shapes that contribute progeny to multiple columns. Basal cell fate and cell cycle time is random. Cell columns form due to the properties of postmitotic cells. We conclude that the ordered architecture of the epidermis is maintained by a stochastic progenitor cell population, providing a simple and robust mechanism of homeostasis.