Tissue-specific and reversible RNA interference in transgenic mice

PTEN action in leukaemia dictated by the tissue microenvironment

PTEN encodes a lipid phosphatase that is underexpressed in many cancers owing to deletions, mutations or gene silencing^1–3. PTEN dephosphorylates phosphatidylinositol 3,4,5-triphosphate (PIP₃), thereby opposing the activity of class I phosphatidylinositol 3-kinases (PI3Ks) that mediate growth and survival factors signaling through PI3K effectors such as AKT and mTOR². To determine whether continued PTEN inactivation is required to maintain malignancy, we generated an RNAi-based transgenic mouse model that allows tetracycline-dependent regulation of PTEN in a time- and tissue-specific manner. Postnatal PTEN knockdown in the hematopoietic compartment produced highly disseminated T-cell leukemia (T-ALL). Surprisingly, reactivation of PTEN mainly reduced T-ALL dissemination but had little effect on tumor load in hematopoietic organs. Leukemia infiltration into the intestine was dependent on CCR9 G-protein coupled receptor (GPCR) signaling, which was amplified by PTEN loss. Our results suggest that in the absence of PTEN, GPCRs may play an unanticipated role in driving tumor growth and invasion in an unsupportive environment. They further reveal that the role of PTEN loss in tumor maintenance is not invariant and can be influenced by the tissue microenvironment, thereby producing a form of intratumoral heterogeneity that is independent of cancer genotype.

Generation of transgenic Drosophila expressing shRNAs in the miR-1 backbone

In Drosophila, long-term effects of RNA interference (RNAi) must be achieved by integrating into the genome a template from which an RNAi trigger is transcribed by cellular RNA polymerases, generally RNA polymerase II or III. With encoded triggers, not only can essentially permanent silencing be achieved, but control can also be exerted over the level of trigger expression, with a resulting variation in the degree to which the target is silenced. Knockdown can also be controlled in a temporal and cell-type-dependent fashion through the use of well-established transgenic methodologies and well-tested promoters. The forms of encoded triggers vary. Long double-stranded RNAs can be expressed as extended inverted repeats. The nearest equivalent of a small interfering RNA is an artificial microRNA (miRNA) or short hairpin RNA (shRNA), where a natural miRNA backbone (also called a scaffold) is remodeled to produce a different small RNA or a small inverted repeat (<30 nucleotides) is simply expressed. This protocol describes creation of transgenic Drosophila carrying shRNA inserts in a remodeled endogenous miRNA backbone. The protocol applies to the use of miRNA-based shRNAs, but most of the vectors, principles of experimental design, and methods are also applicable to long inverted repeat transgenes.

Heritable and inducible gene knockdown in astrocytes or neurons in vivo by a combined lentiviral and RNAi approach

Gene knockout by homologous recombination is a popular method to study gene functions in the mouse in vivo. However, its lack of temporal control has limited the interpretation of knockout studies because the complete elimination of a gene product often alters developmental processes, and can induce severe malformations or lethality. Conditional gene knockdown has emerged as a compelling alternative to gene knockout, an approach well-established in vitro but that remains challenging in vivo, especially in the adult brain. Here, we report a method for conditional and cell-specific gene knockdown in the mouse brain in vivo that combines Cre-mediated RNA interference (RNAi) with classical and lentivirus-mediated transgenesis. The method is based on the inducible expression of a silencing short hairpin RNA (shRNA) introduced in mice by lentivirus-mediated transgenesis, and on its activation by excision of a floxed stop EGFP reporter with an inducible Cre recombinase expressed in astrocytes or in neurons. This dual system should be of broad utility for comparative studies of gene functions in these two cell types in vivo.

Stable RNA interference rules for silencing

RNA interference has become an indispensable tool for loss-of-function studies across eukaryotes. By enabling stable and reversible gene silencing, shRNAs provide a means to study long-term phenotypes, perform pool-based forward genetic screens and examine the consequences of temporary target inhibition in vivo. However, efficient implementation in vertebrate systems has been hindered by technical difficulties affecting potency and specificity. Focusing on these issues, we analyse current strategies to obtain maximal knockdown with minimal off-target effects.

A modular and flexible ESC-based mouse model of pancreatic cancer

Saborowski et al. developed a flexible embryonic stem cell (ESC)-based mouse model for pancreatic cancer. The ESCs harbor a latent Kras mutant, a homing cassette, and other genetic elements needed for rapid insertion and conditional expression of tetracycline-controlled transgenes, including fluorescence-coupled shRNAs. This model produces a disease that follows the progression of human pancreatic cancer, and they used it to dissect temporal roles for Pten and c-Myc in pancreatic cancer development and maintenance.

Genetically engineered mouse models (GEMMs) have greatly expanded our knowledge of pancreatic ductal adenocarcinoma (PDAC) and serve as a critical tool to identify and evaluate new treatment strategies. However, the cost and time required to generate conventional pancreatic cancer GEMMs limits their use for investigating novel genetic interactions in tumor development and maintenance. To address this problem, we developed flexible embryonic stem cell (ESC)-based GEMMs that facilitate the rapid generation of genetically defined multiallelic chimeric mice without further strain intercrossing. The ESCs harbor a latent Kras mutant (a nearly ubiquitous feature of pancreatic cancer), a homing cassette, and other genetic elements needed for rapid insertion and conditional expression of tetracycline-controlled transgenes, including fluorescence-coupled shRNAs capable of efficiently silencing gene function by RNAi. This system produces a disease that recapitulates the progression of pancreatic cancer in human patients and enables the study and visualization of the impact of gene perturbation at any stage of pancreas cancer progression. We describe the use of this approach to dissect temporal roles for the tumor suppressor Pten and the oncogene c-Myc in pancreatic cancer development and maintenance.

The polycomb protein Ezh2 impacts on induced pluripotent stem cell generation

Reprogramming of somatic cells toward pluripotency involves extensive chromatin reorganization and changes in gene expression. Polycomb group (PcG) proteins are key regulators of chromatin structure, cell identity, and development. In this study, we investigated the impact of Ezh2, a core subunit of Polycomb repressive complex 2 (PRC2), on the generation of induced pluripotent stem (iPS) cells. We found that Ezh2 expression is induced during iPS cell generation and iPS cells contain high levels of Ezh2 mRNA and protein. Importantly, shRNA knockdown of Ezh2 during reprogramming severely impairs iPS cell generation. Mechanistically, Ezh2 acts during reprogramming at least in part through repressing the Ink4a/Arf locus, which represents a major roadblock for iPS cell generation. Interestingly, knockdown of Ezh2 in established pluripotent cells leaves pluripotency and self-renewal of embryonic stem cells and iPS cells unaffected. Altogether, our results demonstrate that Ezh2 is critical for efficient iPS cell generation, whereas it is dispensable for maintaining the reprogrammed iPS cell state.

RNAi-mediated knockdown of IKK1 in transgenic mice using a transgenic construct containing the human H1 promoter

Inhibition of gene expression through siRNAs is a tool increasingly used for the study of gene function in model systems, including transgenic mice. To achieve perdurable effects, the stable expression of siRNAs by an integrated transgenic construct is necessary. For transgenic siRNA expression, promoters transcribed by either RNApol II or III (such as U6 or H1 promoters) can be used. Relatively large amounts of small RNAs synthesis are achieved when using RNApol III promoters, which can be advantageous in knockdown experiments. To study the feasibility of H1 promoter-driven RNAi-expressing constructs for protein knockdown in transgenic mice, we chose IKK1 as the target gene. Our results indicate that constructs containing the H1 promoter are sensitive to the presence of prokaryotic sequences and to transgene position effects, similar to RNApol II promoters-driven constructs. We observed variable expression levels of transgenic siRNA among different tissues and animals and a reduction of up to 80% in IKK1 expression. Furthermore, IKK1 knockdown led to hair follicle alterations. In summary, we show that constructs directed by the H1 promoter can be used for knockdown of genes of interest in different organs and for the generation of animal models complementary to knockout and overexpression models.

An RNAi-based approach to down-regulate a gene family in vivo

Genetic redundancy poses a major problem to the analysis of gene function. RNA interference allows the down-regulation of several genes simultaneously, offering the possibility to overcome genetic redundancy, something not easily achieved with traditional genetic approaches. Previously we have used a polycistronic miR155-based framework to knockdown expression of three genes of the early B cell factor family in cultured cells. Here we develop the system further by generating transgenic mice expressing the RNAi construct in vivo in an inducible manner. Expression of the transgene from the strong CAG promoter is compatible with a normal function of the basal miRNA/RNAi machinery, and the miR155 framework readily allows inducible expression from the Rosa26 locus as shown by Gfp. However, expression of the transgene in hematopoietic cells does not lead to changes in B cell development and neuronal expression does not affect cerebellar architecture as predicted from genetic deletion studies. Protein as well as mRNA levels generated from Ebf genes in hetero- and homozygous animals are comparable to wild-type levels. A likely explanation for the discrepancy in the effectiveness of the RNAi construct between cultured cells and transgenic animals lies in the efficiency of the sequences used, possibly together with the complexity of the transgene. Since new approaches allow to overcome efficiency problems of RNAi sequences, the data lay the foundation for future work on the simultaneous knockdown of several genes in vivo.

A nanobody-based system using fluorescent proteins as scaffolds for cell-specific gene manipulation

Fluorescent proteins are commonly used to label cells across organisms, but the unmodified forms cannot control biological activities. Using GFP-binding proteins derived from Camelid antibodies, we co-opted GFP as a scaffold for inducing formation of biologically active complexes, developing a library of hybrid transcription factors that control gene expression only in the presence of GFP or its derivatives. The modular design allows for variation in key properties such as DNA specificity, transcriptional potency, and drug dependency. Production of GFP controlled cell-specific gene expression and facilitated functional perturbations in the mouse retina and brain. Further, retrofitting existing transgenic GFP mouse and zebrafish lines for GFP-dependent transcription enabled applications such as optogenetic probing of neural circuits. This work establishes GFP as a multifunctional scaffold and opens the door to selective manipulation of diverse GFP-labeled cells across transgenic lines. This approach may also be extended to exploit other intracellular products as cell-specific scaffolds in multicellular organisms.

A novel piggyBac transposon inducible expression system identifies a role for AKT signalling in primordial germ cell migration

In this work, we describe a single piggyBac transposon system containing both a tet-activator and a doxycycline-inducible expression cassette. We demonstrate that a gene product can be conditionally expressed from the integrated transposon and a second gene can be simultaneously targeted by a short hairpin RNA contained within the transposon, both in vivo and in mammalian and avian cell lines. We applied this system to stably modify chicken primordial germ cell (PGC) lines in vitro and induce a reporter gene at specific developmental stages after injection of the transposon-modified germ cells into chicken embryos. We used this vector to express a constitutively-active AKT molecule during PGC migration to the forming gonad. We found that PGC migration was retarded and cells could not colonise the forming gonad. Correct levels of AKT activation are thus essential for germ cell migration during early embryonic development.

Accelerating cancer modeling with RNAi and nongermline genetically engineered mouse models

For more than two decades, genetically engineered mouse models have been key to our mechanistic understanding of tumorigenesis and cancer progression. Recently, the massive quantity of data emerging from cancer genomics studies has demanded a corresponding increase in the efficiency and throughput of in vivo models for functional testing of putative cancer genes. Already a mainstay of cancer research, recent innovations in RNA interference (RNAi) technology have extended its utility for studying gene function and genetic interactions, enabling tissue-specific, inducible and reversible gene silencing in vivo. Concurrent advances in embryonic stem cell (ESC) culture and genome engineering have accelerated several steps of genetically engineered mouse model production and have facilitated the incorporation of RNAi technology into these models. Here, we review the current state of these technologies and examine how their integration has the potential to dramatically enhance the throughput and capabilities of animal models for cancer.

RUSH and CRUSH: a rapid and conditional RNA interference method in mice

RNA interference (RNAi) is a powerful approach to phenocopy mutations in many organisms. Gold standard conventional knock-out mouse technology is labor- and time-intensive; however, off-target effects may confound transgenic RNAi approaches. Here, we describe a rapid method for conditional and reversible gene silencing in RNAi transgenic mouse models and embryonic stem (ES) cells. RUSH and CRUSH RNAi vectors were designed for reversible or conditional knockdown, respectively, demonstrated using targeted replacement in an engineered ROSA26(lacZ) ES cell line and wildtype V6.5 ES cells. RUSH was validated by reversible knockdown of Dnmt1 in vitro. Conditional mouse model production using CRUSH was expedited by deriving ES cell lines from Cre transgenic mouse strains (nestin, cTnnT, and Isl1) and generating all-ES G0 transgenic founders by tetraploid complementation. A control CRUSH(GFP) RNAi mouse strain showed quantitative knockdown of GFP fluorescence as observed in compound CRUSH(GFP) , Ds-Red Cre-reporter transgenic mice, and confirmed by Western blotting. The capability to turn RUSH and CRUSH alleles off or on using Cre recombinase enables this method to rapidly address questions of tissue-specificity and cell autonomy of gene function in development.

Organic small hairpin RNAs (OshR): a do-it-yourself platform for transgene-based gene silencing

The RNA interference (RNAi) pathway in animal cells can be harnessed to silence gene expression with artificial small interfering RNAs (siRNAs) or transgenes that express small hairpin RNAs (shRNAs). The transgene-expressing shRNA approach has been adapted into large-scale resources for genome-wide loss-of-function screens, whereas focused studies on a narrow set of genes can be achieved by using individual shRNA constructs from these resources. Although current shRNA repositories generally work, they might fail in certain situations and therefore necessitate other alternatives. We detail here a new highly-accessible and rational design of custom shRNAs that utilizes a refined backbone configuration termed the 'organic' shRNA (OshR) platform. The OshR platform is 'organic' because it conforms more naturally to the endogenous vertebrate miRNAs by maintaining specific bulges and incorporating strategic mismatches to insure the desired guide strand is produced while reducing the accumulation of passenger strands that might contribute to off-target effects. We also demonstrate that the reliability of the OshR platform for gene silencing is increased when sequences target the 3' UnTranslated Region (3'UTR) of a gene. We further compare the OshR platform with the current and emerging shRNA designs, and propose that the OshR platform is a novel approach that can allow investigators to generate custom and effective shRNAs for individual gene functional studies.

Modeling distinct osteosarcoma subtypes in vivo using Cre:lox and lineage-restricted transgenic shRNA

Osteosarcoma is the most common primary cancer of bone and one that predominantly affects children and adolescents. Osteoblastic osteosarcoma represents the major subtype of this tumor, with approximately equal representation of fibroblastic and chondroblastic subtypes. We and others have previously described murine models of osteosarcoma based on osteoblast-restricted Cre:lox deletion of Trp53 (p53) and Rb1 (Rb), resulting in a phenotype most similar to fibroblastic osteosarcoma in humans. We now report a model of the most prevalent form of human osteosarcoma, the osteoblastic subtype. In contrast to other osteosarcoma models that have used Cre:lox mediated gene deletion, this model was generated through shRNA-based knockdown of p53. As is the case with the human disease the shRNA tumors most frequently present in the long bones and preferentially disseminate to the lungs; feature less consistently modeled using Cre:lox approaches. Our approach allowed direct comparison of the in vivo consequences of targeting the same genetic drivers using two different technologies, Cre:lox and shRNA. This demonstrated that the effects of Cre:lox and shRNA mediated knock-down are qualitatively different, at least in the context of osteosarcoma, and yielded distinct subtypes of osteosarcoma. Through the use of complementary genetic modification strategies we have established a model of the most common clinical subtype of osteosarcoma that was not previously represented and more fully recapitulated the clinical spectrum of this cancer.

Controlled insertional mutagenesis using a LINE-1 (ORFeus) gene-trap mouse model

A codon-optimized mouse LINE-1 element, ORFeus, exhibits dramatically higher retrotransposition frequencies compared with its native long interspersed element 1 counterpart. To establish a retrotransposon-mediated mouse model with regulatable and potent mutagenic capabilities, we generated a tetracycline (tet)-regulated ORFeus element harboring a gene-trap cassette. Here, we show that mice expressing tet-ORFeus broadly exhibit robust retrotransposition in somatic tissues when treated with doxycycline. Consistent with a significant mutagenic burden, we observed a reduced number of double transgenic animals when treated with high-level doxycycline during embryogenesis. Transgene induction in skin resulted in a white spotting phenotype due to somatic ORFeus-mediated mutations that likely disrupt melanocyte development. The data suggest a high level of transposition in melanocyte precursors and consequent mutation of genes important for melanoblast proliferation, differentiation, or migration. These findings reveal the utility of a retrotransposon-based mutagenesis system as an alternative to existing DNA transposon systems. Moreover, breeding these mice to different tet-transactivator/reversible tet-transactivator lines supports broad functionality of tet-ORFeus because of the potential for dose-dependent, tissue-specific, and temporal-specific mutagenesis.

INK4a/ARF limits the expansion of cells suffering from replication stress

Replication stress (RS) is a source of DNA damage that has been linked to cancer and aging, which is suppressed by the ATR kinase. In mice, reduced ATR levels in a model of the ATR-Seckel syndrome lead to RS and accelerated aging. Similarly, ATR-Seckel embryonic fibroblasts (MEF) accumulate RS and undergo cellular senescence. We previously showed that senescence of ATR-Seckel MEF cannot be rescued by p53-deletion. Here, we show that the genetic ablation of the INK4a/Arf locus fully rescues senescence on ATR mutant MEF, but also that induced by other conditions that generate RS such as low doses of hydroxyurea or ATR inhibitors. In addition, we show that a persistent exposure to RS leads to increased levels of INK4a/Arf products, revealing that INK4a/ARF behaves as a bona fide RS checkpoint. Our data reveal an unknown role for INK4a/ARF in limiting the expansion of cells suffering from persistent replication stress, linking this well-known tumor suppressor to the maintenance of genomic integrity.

Lentiviral miR30-based RNA interference against heparanase suppresses melanoma metastasis with lower liver and lung toxicity

Aim: To construct short hairpin RNAs (shRNAs) and miR30-based shRNAs against heparanase (HPSE) to compare their safety and their effects on HPSE down-modulation in vitro and in vivo to develop a more ideal therapeutic RNA interference (RNAi) vector targeting HPSE.

Methods: First, we constructed shRNAs and miR30-based shRNAs against HPSE (HPSE-shRNAs and HPSE-miRNAs) and packed them into lentiviral vectors. Next, we observed the effects of the shRNAs on knockdown for HPSE expression, adhesion, migration and invasion abilities in human malignant melanoma A375 cells in vitro. Furthermore, we compared the effects of the shRNAs on melanoma growth, metastasis and safety in xenograft models.

Results: Our data showed that these artificial miRNAs targeting HPSE could be effective RNAi agents mediated by Pol II promoters in vitro and in vivo, although these miRNAs were not more potent than the HPSE-shRNAs. It was noted that obvious lung injuries, rarely revealed previously, as well as hepatotoxicity could be caused by lentivirus-mediated shRNAs (LV shRNAs) rather than lentivirus-mediated miRNAs (LV miRNAs) in vivo. Furthermore, enhanced expression of pro-inflammatory cytokines IL-6 and TGF-β1 and endogenous mmu-miR-21a-5p were detected in lung tissues of shRNAs groups, whereas the expression of mmu-let-7a-5p, mmu-let-7b-5p and mmu-let-7c-5p were down-regulated.

Conclusion: These findings suggest that artificial miRNAs display an improved safety profile of lowered lung injury or hepatotoxicity relative to shRNAs in vivo. The mechanism of lung injuries caused by shRNAs may be correlated with changes of endogenous miRNAs in the lung. Our data here increase the flexibility of a miRNA-based RNAi system for functional genomic and gene therapy applications.

Mouse models of medulloblastoma

Medulloblastoma is the most common malignant pediatric brain tumor. Despite its prevalence and importance in pediatric neuro-oncology, the genes and pathways responsible for its initiation, maintenance, and progression remain poorly understood. Genetically engineered mouse models are an essential tool for uncovering the molecular and cellular basis of human diseases, including cancer, and serve a valuable role as preclinical models for testing targeted therapies. In this review, we summarize how such models have been successfully applied to the study of medulloblastoma over the past decade and what we might expect in the coming years.

Recent advances in the development of new transgenic animal technology

Transgenic animal technology is one of the fastest growing biotechnology areas. It is used to integrate exogenous genes into the animal genome by genetic engineering technology so that these genes can be inherited and expressed by offspring. The transgenic efficiency and precise control of gene expression are the key limiting factors in the production of transgenic animals. A variety of transgenic technologies are available. Each has its own advantages and disadvantages and needs further study because of unresolved technical and safety issues. Further studies will allow transgenic technology to explore gene function, animal genetic improvement, bioreactors, animal disease models, and organ transplantation. This article reviews the recently developed animal transgenic technologies, including the germ line stem cell-mediated method to improve efficiency, gene targeting to improve accuracy, RNA interference-mediated gene silencing technology, zinc-finger nuclease gene targeting technology and induced pluripotent stem cell technology. These new transgenic techniques can provide a better platform to develop transgenic animals for breeding new animal varieties and promote the development of medical sciences, livestock production, and other fields.

Variability of inducible expression across the hematopoietic system of tetracycline transactivator transgenic mice

The tetracycline (tet)-regulated expression system allows for the inducible overexpression of protein-coding genes, or inducible gene knockdown based on expression of short hairpin RNAs (shRNAs). The system is widely used in mice, however it requires robust expression of a tet transactivator protein (tTA or rtTA) in the cell type of interest. Here we used an in vivo tet-regulated fluorescent reporter approach to characterise inducible gene/shRNA expression across a range of hematopoietic cell types of several commonly used transgenic tet transactivator mouse strains. We find that even in strains where the tet transactivator is expressed from a nominally ubiquitous promoter, the efficiency of tet-regulated expression can be highly variable between hematopoietic lineages and between differentiation stages within a lineage. In some cases tet-regulated reporter expression differs markedly between cells within a discrete, immunophenotypically defined population, suggesting mosaic transactivator expression. A recently developed CAG-rtTA3 transgenic mouse displays intense and efficient reporter expression in most blood cell types, establishing this strain as a highly effective tool for probing hematopoietic development and disease. These findings have important implications for interpreting tet-regulated hematopoietic phenotypes in mice, and identify mouse strains that provide optimal tet-regulated expression in particular hematopoietic progenitor cell types and mature blood lineages.

Selective inhibition of cell death in malignant vs normal B-cell precursors: implications for cAMP in development and treatment of BCP-ALL

B-cell precursor acute lymphoblastic leukemia (BCP-ALL) is the most commonly occurring pediatric cancer. Despite its relatively good prognosis, there is a steady search for strategies to improve treatment effects and prevent the undesired side effects on normal cells. In the present paper, we demonstrate a differential effect of cyclic adenosine monophosphate (cAMP) signaling between normal BCPs and BCP-ALL blasts, pointing to a potential therapeutic window allowing for manipulation of cAMP signaling in the treatment of BCP-ALL. By studying primary cells collected from pediatric BCP-ALL patients and healthy controls, we found that cAMP profoundly decreased basal and DNA damage-induced p53 levels and cell death in malignant cells, whereas normal BCP counterparts displayed slightly augmented cell death when exposed to cAMP-increasing agents. We did not find evidence for a selection process involving generation of increased basal cAMP levels in BCP-ALL cells, but we demonstrate that paracrine signaling involving prostaglandin E2-induced cAMP generation has the potential to suppress p53 activation and cell death induction. The selective inhibitory effect of cAMP signaling on DNA damage-induced cell death in BCP-ALL cells appears to be an acquired trait associated with malignant transformation, potentially allowing the use of inhibitors of this pathway for directed killing of the malignant blasts.

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.

Mouse models for cancer research

Mouse models of cancer enable researchers to learn about tumor biology in complicated and dynamic physiological systems. Since the development of gene targeting in mice, cancer biologists have been among the most frequent users of transgenic mouse models, which have dramatically increased knowledge about how cancers form and grow. The Chinese Journal of Cancer will publish a series of papers reporting the use of mouse models in studying genetic events in cancer cases. This editorial is an overview of the development and applications of mouse models of cancer and directs the reader to upcoming papers describing the use of these models to be published in coming issues, beginning with three articles in the current issue.

Inducible and reversible regulation of endogenous gene in mouse

Methods for generating loss-of-function mutations, such as conventional or conditional gene knockout, are widely used in deciphering gene function in vivo. By contrast, inducible and reversible regulation of endogenous gene expression has not been well established. Using a mouse model, we demonstrate that a chimeric transcriptional repressor molecule (tTS) can reversibly inhibit the expression of an endogenous gene, Nmyc. In this system, a tetracycline response element (TRE) artificially inserted near the target gene’s promoter region turns the gene on and off in a tetracycline-inducible manner. Nmyc^TRE mice were generated by inserting a TRE into the first intron of Nmyc by the knockin technique. Nmyc^TRE mice were crossed to tTS transgenic mice to produce Nmyc^TRE/TRE: tTS embryos. In these embryos, tTS blocked Nmyc expression, and embryonic lethality was observed at E11.5d. When the dam was exposed to drinking water containing doxycycline (dox), normal endogenous Nmyc expression was rescued, and the embryo survived to birth. This novel genetic modification strategy based on the tTS–dox system for inducible and reversible regulation of endogenous mouse genes will be a powerful tool to investigate target genes that cause embryonic lethality or other defects where reversible regulation or temporary shutdown of the target gene is needed.

An artificial miRNA against HPSE suppresses melanoma invasion properties, correlating with a down-regulation of chemokines and MAPK phosphorylation

Ribonucleic acid interference (RNAi) based on microRNA (miRNA) context may provide an efficient and safe therapeutic knockdown effect and can be driven by ribonucleic acid polymerase II (RNAP II). In this study, we designed and synthesized miR155-based artificial miRNAs against heparanase (HPSE) constructed with BLOCK-iT™ Pol II miR RNAi Expression Vector Kit. The expression levels of HPSE declined significantly in both the mRNA and protein levels in HPSE-miRNA transfected melanoma cells that exhibited reduction of adhesion, migration, and invasion ability in vitro and in vivo. We also observed that HPSE miRNA could inhibit the expressions of chemokines of interleukin-8 (IL8) and chemokine (C-X-C motif) ligand 1 (CXCL1), at both the transcriptional and translational levels. Further study on its probable mechanism declared that down-regulation of IL8 and CXCL1 by HPSE-miRNA may be correlated with reduced growth-factor simulated mitogen-activated kinase (MAPK) phosphorylation including p38 MAPK, c-Jun N-terminal kinase (JNK) and extracellular-signal-regulated kinase (ERK) 1 and 2, which could be rescued by miRNA incompatible mutated HPSE cDNA. In conclusion, we demonstrated that artificial miRNAs against HPSE might serve as an alterative mean of therapy to low HPSE expression and to block the adhesion, invasion, and metastasis of melanoma cells. Furthermore, miRNA-based RNAi was also a powerful tool for gene function study.

Targeting antiapoptotic A1/Bfl-1 by in vivo RNAi reveals multiple roles in leukocyte development in mice

Gene-targeting studies in mice have identified the essential roles of most prosurvival Bcl-2 family members in normal physiology and under conditions of stress. The function of one member, Bcl2a1/Bfl-1/A1, is only poorly understood because of quadruplication of its gene locus in mice, hindering conventional knockout studies. To overcome this problem, we generated mouse models allowing traceable constitutive or reversible ablation of A1 in the hematopoietic system by RNA interference. Knockdown of A1 impaired early stages of T-cell differentiation, B-cell homeostasis, and sensitized transitional as well as follicular B cells to apoptosis induced by ligation of the B-cell receptor. As a consequence, B-cell proliferation in response to mitogens was severely impaired, whereas that of T cells appeared unaffected. Furthermore, depending on the extent of A1 knockdown, granulocytes showed increased spontaneous death in culture or failed to accumulate in significant numbers in vivo. These models highlight the critical role of A1 in leukocyte development and homeostasis, constituting valuable tools for investigating presumed roles of this Bcl-2 family member in immunity, tumorigenesis, and drug resistance.

The Polycomb complex PRC2 supports aberrant self-renewal in a mouse model of MLL-AF9;Nras(G12D) acute myeloid leukemia

The Trithorax and Polycomb groups of chromatin regulators are critical for cell-lineage specification during normal development; functions that often become deregulated during tumorigenesis. As an example, oncogenic fusions of the Trithorax-related protein mixed lineage leukemia (MLL) can initiate aggressive leukemias by altering the transcriptional circuitry governing hematopoietic cell differentiation, a process that requires multiple epigenetic pathways to implement. Here we used shRNA screening to identify chromatin regulators uniquely required in a mouse model of MLL-fusion acute myeloid leukemia, which revealed a role for the Polycomb repressive complex 2 (PRC2) in maintenance of this disease. shRNA-mediated suppression of PRC2 subunits Eed, Suz12 or Ezh1/Ezh2 led to proliferation arrest and differentiation of leukemia cells, with a minimal impact on growth of several non-transformed hematopoietic cell lines. The requirement for PRC2 in leukemia is partly because of its role in direct transcriptional repression of genes that limit the self-renewal potential of hematopoietic cells, including Cdkn2a. In addition to implicating a role for PRC2 in the pathogenesis of MLL-fusion leukemia, our results suggest, more generally, that Trithorax and Polycomb group proteins can cooperate with one another to maintain aberrant lineage programs in cancer.

A pipeline for the generation of shRNA transgenic mice

RNA interference (RNAi) is an extremely effective tool for studying gene function in almost all metazoan and eukaryotic model systems. RNAi in mice, through the expression of short hairpin RNAs (shRNAs), offers something not easily achieved with traditional genetic approaches-inducible and reversible gene silencing. However, technical variability associated with the production of shRNA transgenic strains has so far limited their widespread use. Here we describe a pipeline for the generation of miR30-based shRNA transgenic mice that enables efficient and consistent targeting of doxycycline-regulated, fluorescence-linked shRNAs to the Col1a1 locus. Notably, the protocol details crucial steps in the design and testing of miR30-based shRNAs to maximize the potential for developing effective transgenic strains. In all, this 14-week procedure provides a fast and cost-effective way for any laboratory to investigate gene function in vivo in the mouse.

A dynamic perspective of RNAi library development

Shortly after the dissertation of the mechanism of RNA interference (RNAi), various RNAi libraries for invertebrates, plants or mammals that enable loss-of-function genetic screens on a genome-wide scale have been developed. Joint academic and industrial effort has led to the commercial launch of many of these libraries and this field is expected to continuously evolve at incredible speed. This article comparatively reviews the principles and applications of different RNAi libraries: from earlier synthetic to recent lentiviral RNAi libraries. The unique properties and limitations of each library will be important references for instigators to choose a particular library for their specific application.

Lentiviral transgenic microRNA-based shRNA suppressed mouse cytochromosome P450 3A (CYP3A) expression in a dose-dependent and inheritable manner

Cytochomosome P450 enzymes (CYP) are heme-containing monooxygenases responsible for oxidative metabolism of many exogenous and endogenous compounds including drugs. The species difference of CYP limits the extent to which data obtained from animals can be translated to humans in pharmacodynamics or pharmacokinetics studies. Transgenic expression of human CYP in animals lacking or with largely reduced endogenous CYP counterparts is recognized as an ideal strategy to correct CYP species difference. CYP3A is the most abundant CYP subfamily both in human and mammals. In this study, we designed a microRNA-based shRNA (miR-shRNA) simultaneously targeting four members of mouse CYP3A subfamily (CYP3A11, CYP3A16, CYP3A41 and CYP3A44), and transgenic mice expressing the designed miR-shRNA were generated by lentiviral transgenesis. Results showed that the CYP3A expression level in transgenic mice was markedly reduced compared to that in wild type or unrelated miR-shRNA transgenic mice, and was inversely correlated to the miR-shRNA expression level. The CYP3A expression levels in transgenic offspring of different generations were also remarkably lower compared to those of controls, and moreover the inhibition rate of CYP3A expression remained comparable over generations. The ratio of the targeted CYP3A transcriptional levels was comparable between knockdown and control mice of the same gender as detected by RT-PCR DGGE analysis. These data suggested that transgenic miR-shRNA suppressed CYP3A expression in a dose-dependent and inheritable manner, and transcriptional levels of the targeted CYP3As were suppressed to a similar extent. The observed knockdown efficacy was further confirmed by enzymatic activity analysis, and data showed that CYP3A activities in transgenic mice were markedly reduced compared to those in wild-type or unrelated miR-shRNA transgenic controls (1.11±0.71 vs 5.85±1.74, 5.9±2.4; P<0.01). This work laid down a foundation to further knock down the remaining murine CYP3As or CYPs of other subfamilies, and a basis to generate CYP knockdown animals of other species.

Genetics in non-genetic model systems

The past few decades have seen the field of genetic engineering evolve at a rapid pace, with neuroscientists now equipped with a wide range of tools for the manipulation of an animal's genome in order to study brain function. However, the number of species to which these technologies have been applied, namely the fruit fly, C. elegans, zebrafish and mouse, remains relatively few. This review will discuss the variety of approaches to genetic modification that have been developed in such traditional 'genetic systems', and highlight the progress that has been made to translate these technologies to alternative species such as rats, monkeys and birds, where certain neurobiological questions may be better studied.

TLR regulation of SPSB1 controls inducible nitric oxide synthase induction

The mammalian innate immune system has evolved to recognize foreign molecules derived from pathogens via the TLRs. TLR3 and TLR4 can signal via the TIR domain-containing adapter inducing IFN-β (TRIF), which results in the transcription of a small array of genes, including IFN-β. Inducible NO synthase (iNOS), which catalyzes the production of NO, is induced by a range of stimuli, including cytokines and microbes. NO is a potent source of reactive nitrogen species that play an important role in killing intracellular pathogens and forms a crucial component of host defense. We have recently identified iNOS as a target of the mammalian SPSB2 protein. The SOCS box is a peptide motif, which, in conjunction with elongins B and C, recruits cullin-5 and Rbx-2 to form an active E3 ubiquitin ligase complex. In this study, we show that SPSB1 is the only SPSB family member to be regulated by the same TLR pathways that induce iNOS expression and characterize the interaction between SPSB1 and iNOS. Through the use of SPSB1 transgenic mouse macrophages and short hairpin RNA knockdown of SPSB1, we show that SPSB1 controls both the induction of iNOS and the subsequent production of NO downstream of TLR3 and TLR4. Further, we demonstrate that regulation of iNOS by SPSB1 is dependent on the proteasome. These results suggest that SPSB1 acts through a negative-feedback loop that, together with SPSB2, controls the extent of iNOS induction and NO production.

PDK1 attenuation fails to prevent tumor formation in PTEN-deficient transgenic mouse models

PDK1 activates AKT suggesting that PDK1 inhibition might suppress tumor development. However, while PDK1 has been investigated intensively as an oncology target, selective inhibitors suitable for in vivo studies have remained elusive. In this study we present the results of in vivo PDK1 inhibition through a universally applicable RNAi approach for functional drug target validation in oncogenic pathway contexts. This approach, which relies on doxycycline-inducible shRNA expression from the Rosa26 locus, is ideal for functional studies of genes like PDK1 where constitutive mouse models lead to strong developmental phenotypes or embryonic lethality. We achieved more than 90% PDK1 knockdown in vivo, a level sufficient to impact physiological functions resulting in hyperinsulinemia and hyperglycemia. This phenotype was reversible on PDK1 reexpression. Unexpectedly, long-term PDK1 knockdown revealed a lack of potent antitumor efficacy in 3 different mouse models of PTEN-deficient cancer. Thus, despite efficient PDK1 knockdown, inhibition of the PI3K pathway was marginal suggesting that PDK1 was not a rate limiting factor. Ex vivo analysis of pharmacological inhibitors revealed that AKT and mTOR inhibitors undergoing clinical development are more effective than PDK1 inhibitors at blocking activated PI3K pathway signaling. Taken together our findings weaken the widely held expectation that PDK1 represents an appealing oncology target.

An RNAi-based system for loss-of-function analysis identifies Raf1 as a crucial mediator of BCR-ABL-driven leukemogenesis

Genetic loss-of-function studies in murine tumor models have been essential in the analysis of downstream mediators of oncogenic transformation. Unfortunately, these studies are frequently limited by the availability of genetically modified mouse strains. Here we describe a versatile method allowing the efficient expression of an oncogene and simultaneous knockdown of targets of interest (TOI) from a single retroviral vector. Both oncogene and TOI-specific miR30-based shRNA are under the control of the strong viral long terminal repeat promoter, resulting in a single shared RNA transcript. Using this vector in a murine syngeneic BM transplantation model for BCR-ABL-induced chronic myeloid leukemia, we find that oncogene expression and target knockdown in primary hematopoietic cells with this vector is efficient both in vitro and in vivo, and demonstrate that Raf1, but not BRAF, modulates BCR-ABL-dependent ERK activation and transformation of hematopoietic cells. This expression system could facilitate genetic loss-of-function studies and allow the rapid validation of potential drug targets in a broad range of oncogene-driven murine tumor models.

A rapid and scalable system for studying gene function in mice using conditional RNA interference

RNA interference is a powerful tool for studying gene function, however, the reproducible generation of RNAi transgenic mice remains a significant limitation. By combining optimized fluorescence-coupled miR30-based shRNAs with high efficiency ES cell targeting, we developed a fast, scalable pipeline for the production of shRNA transgenic mice. Using this system, we generated eight tet-regulated shRNA transgenic lines targeting Firefly and Renilla luciferases, Oct4 and tumor suppressors p53, p16(INK4a), p19(ARF) and APC and demonstrate potent gene silencing and GFP-tracked knockdown in a broad range of tissues in vivo. Further, using an shRNA targeting APC, we illustrate how this approach can identify predicted phenotypes and also unknown functions for a well-studied gene. In addition, through regulated gene silencing we validate APC/Wnt and p19(ARF) as potential therapeutic targets in T cell acute lymphoblastic leukemia/lymphoma and lung adenocarcinoma, respectively. This system provides a cost-effective and scalable platform for the production of RNAi transgenic mice targeting any mammalian gene. PAPERCLIP:

Mouse models of cancer

Genetically engineered mouse models have significantly contributed to our understanding of cancer biology. They have proven to be useful in validating gene functions, identifying novel cancer genes and tumor biomarkers, gaining insight into the molecular and cellular mechanisms underlying tumor initiation and multistage processes of tumorigenesis, and providing better clinical models in which to test novel therapeutic strategies. However, mice still have significant limitations in modeling human cancer, including species-specific differences and inaccurate recapitulation of de novo human tumor development. Future challenges in mouse modeling include the generation of clinically relevant mouse models that recapitulate the molecular, cellular, and genomic events of human cancers and clinical response as well as the development of technologies that allow for efficient in vivo imaging and high-throughput screening in mice.

Reversible suppression of an essential gene in adult mice using transgenic RNA interference

RNAi has revolutionized loss-of-function genetics by enabling sequence-specific suppression of virtually any gene. Furthermore, tetracycline response elements (TRE) can drive expression of short hairpin RNAs (shRNAs) for inducible and reversible target gene suppression. Here, we demonstrate the feasibility of transgenic inducible RNAi for suppression of essential genes. We set out to directly target cell proliferation by screening an RNAi library against DNA replication factors and identified multiple shRNAs against Replication Protein A, subunit 3 (RPA3). We generated transgenic mice with TRE-driven Rpa3 shRNAs whose expression enforced a reversible cell cycle arrest. In adult mice, the block in cell proliferation caused rapid atrophy of the intestinal epithelium which led to weight loss and lethality within 8-11 d of shRNA induction. Upon shRNA withdrawal, villus atrophy and weight loss were fully reversible. Thus, shRpa3 transgenic mice provide an interesting tool to study tissue maintenance and regeneration. Overall, we have established a robust system that serves the purpose of temperature-sensitive alleles in other model organisms, enabling inducible and reversible suppression of essential genes in a mammalian system.

A bidirectional promoter architecture enhances lentiviral transgenesis in embryonic and extraembryonic stem cells

The two main challenges facing retroviral transgenesis are variable expression and epigenetic silencing. Although modern lentiviral vectors incorporate several elements to increase transgene expression and reduce position effect variegation and silencing, therapeutic research in stem cells, as well as production of transgenic animals, is still hampered by these two key problems. On the basis of recent studies demonstrating the chromatin insulating properties of divergent promoters, we sought to develop a bidirectional lentiviral vector with which to conduct RNA interference (RNAi)-based genetic screens in embryonic and extraembryonic stem cells. To this end, we designed and tested a series of synthetic bidirectional promoters, combining the mouse phosphoglycerate kinase 1 (Pgk1) promoter with other strong mammalian and viral promoters. Here, we demonstrate that a back-to-back configuration of the mouse Pgk1 and human eukaryotic translation elongation factor 1 alpha 1 promoters provided a substantive increase in both transgene expression and RNAi-based transcript depletion as compared with previous designs and other promoter combinations. Using this vector, we were able to achieve stable and robust depletion of a transfected luciferase reporter, as well as an endogenous non-coding RNA. The described constructs are an improved transgene delivery system capable of conducting RNAi screens in stem cells at single copy.

Lentiviral transgenesis

Conventional DNA injection-based methods are successful in generating transgenic animals and have remained nearly unchanged over the last few decades. Lentiviral vectors are alternative powerful tool for generating transgenic animals, in part because of their ability to incorporate into genomic DNA with high efficiency. This chapter describes lentiviral vectors used to generate transgenic mice and rats. We discuss the protocols and methods in high enough detail such that researchers who are accustomed to creating transgenic animals by pronuclear injection can smoothly transition to using lentiviral transgenesis. We will briefly outline the general principle of the lentiviral expression system and focus specifically on the methods used to generate lentiviral vectors, produce lentiviral particles, inject lentivirus into the fertilized oocytes, and transplant them into the pseudopregnant females. In addition to the surgical aspects of the experiment, we will describe methods to produce high titer lentivirus. Finally, we will discuss the limitations of lentiviral transgenesis and summarize information that will be useful for troubleshooting.

Functional identification of optimized RNAi triggers using a massively parallel sensor assay

Short hairpin RNAs (shRNAs) provide powerful experimental tools by enabling stable and regulated gene silencing through programming of endogenous microRNA pathways. Since requirements for efficient shRNA biogenesis and target suppression are largely unknown, many predicted shRNAs fail to efficiently suppress their target. To overcome this barrier, we developed a "Sensor assay" that enables the biological identification of effective shRNAs at large scale. By constructing and evaluating 20,000 RNAi reporters covering every possible target site in nine mammalian transcripts, we show that our assay reliably identifies potent shRNAs that are surprisingly rare and predominantly missed by existing algorithms. Our unbiased analyses reveal that potent shRNAs share various predicted and previously unknown features associated with specific microRNA processing steps, and suggest a model for competitive strand selection. Together, our study establishes a powerful tool for large-scale identification of highly potent shRNAs and provides insights into sequence requirements of effective RNAi.

Airway remodeling in asthma: new mechanisms and potential for pharmacological intervention

The chronic inflammatory response within the airways of asthmatics is associated with structural changes termed airway remodeling. This remodeling process is a key feature of severe asthma. The 5-10% of patients with a severe form of the disease account for the higher morbidity and health costs related to asthma. Among the histopathological characteristics of airway remodeling, recent reports indicate that the increased mass of airway smooth muscle (ASM) plays a critical role. ASM cell proliferation in severe asthma implicates a gallopamil-sensitive calcium influx and the activation of calcium-calmodulin kinase IV leading to enhanced mitochondrial biogenesis through the activation of various transcription factors (PGC-1α, NRF-1 and mt-TFA). The altered expression and function of sarco/endoplasmic reticulum Ca(2+) pump could play a role in ASM remodeling in moderate to severe asthma. Additionally, aberrant communication between an injured airway epithelium and ASM could also contribute to disease severity. Airway remodeling is insensitive to corticosteroids and anti-leukotrienes whereas the effect of monoclonal antibodies (the anti-IgE omalizumab, the anti-interleukin-5 mepolizumab or anti-tumor necrosis factor-alpha) remains to be investigated. This review focuses on potential new therapeutic strategies targeting ASM cells, especially Ca(2+) and mitochondria-dependent pathways.

How genetically engineered mouse tumor models provide insights into human cancers

Genetically engineered mouse models (GEMMs) of human cancer were first created nearly 30 years ago. These early transgenic models demonstrated that mouse cells could be transformed in vivo by expression of an oncogene. A new field emerged, dedicated to generating and using mouse models of human cancer to address a wide variety of questions in cancer biology. The aim of this review is to highlight the contributions of mouse models to the diagnosis and treatment of human cancers. Because of the breadth of the topic, we have selected representative examples of how GEMMs are clinically relevant rather than provided an exhaustive list of experiments. Today, as detailed here, sophisticated mouse models are being created to study many aspects of cancer biology, including but not limited to mechanisms of sensitivity and resistance to drug treatment, oncogene cooperation, early detection, and metastasis. Alternatives to GEMMs, such as chemically induced or spontaneous tumor models, are not discussed in this review.

Inducible gene and shRNA expression in resident hematopoietic stem cells in vivo

Hematopoietic stem cells (HSC) are probably the best understood somatic stem cells and often serve as a paradigm for other stem cells. Nevertheless, most current techniques to genetically manipulate them in vivo are either constitutive and/or induced in settings of hematopoietic stress such as after irradiation. Here, we present a conditional expression system that allows for externally controllable transgenesis and knockdown in resident HSCs, based on a lentiviral vector containing a tet-O sequence and a transgenic mouse line expressing a doxycyclin-regulated tTR-KRAB repressor protein. HSCs harvested from tTR-KRAB mice are transduced with the lentiviral vector containing a cDNA (i.e., Green Fluorescent Protein (GFP)) and/or shRNA (i.e., p53) of interest and then transplanted into lethally irradiated recipients. While the vector is effectively repressed by tTR-KRAB during homing and engraftment, robust GFP/shp53 expression is induced on doxycyclin treatment in HSCs and their progeny. Doxycylin-controllable transcription is maintained on serial transplantation, indicating that repopulating HSCs are stably modified by this approach. In summary, this easy to implement conditional system provides inducible and reversible overexpression or knock down of genes in resident HSCs in vivo using a drug devoid of toxic or activating effects.

Design of small molecule-responsive microRNAs based on structural requirements for Drosha processing

MicroRNAs (miRNAs) are prevalent regulatory RNAs that mediate gene silencing and play key roles in diverse cellular processes. While synthetic RNA-based regulatory systems that integrate regulatory and sensing functions have been demonstrated, the lack of detail on miRNA structure–function relationships has limited the development of integrated control systems based on miRNA silencing. Using an elucidated relationship between Drosha processing and the single-stranded nature of the miRNA basal segments, we developed a strategy for designing ligand-responsive miRNAs. We demonstrate that ligand binding to an aptamer integrated into the miRNA basal segments inhibits Drosha processing, resulting in titratable control over gene silencing. The generality of this control strategy was shown for three aptamer–small molecule ligand pairs. The platform can be extended to the design of synthetic miRNAs clusters, cis-acting miRNAs and self-targeting miRNAs that act both in cis and trans, enabling fine-tuning of the regulatory strength and dynamics. The ability of our ligand-responsive miRNA platform to respond to user-defined inputs, undergo regulatory performance tuning and display scalable combinatorial control schemes will help advance applications in biological research and applied medicine.

Cell- and region-specific miR30-based gene knock-down with temporal control in the rat brain

Background

RNA interference (RNAi) emerges as a powerful tool to induce loss-of-function phenotypes. In the context of the brain, gene manipulation is best targeted to specific subsets of cells in order to achieve a physiologically relevant outcome. Polymerase II-based viral expression systems can be used to cell-specifically express constructs incorporating flanking and loop sequences from endogenous microRNA (miRNA), which directs the designed hairpins into the endogenous gene silencing machinery. While many studies have documented non-cell-selective gene knock-down in the brain, it has not been tested whether different cell types or different areas of the central nervous system (CNS) are equally amenable to this approach. We have evaluated this issue using a tetracycline (Tet)-controllable and cell-specific miRNA 30 (miR30)-based short hairpin (shRNA) interference system.

Results

To achieve targeted expression two cell type-specific promoters were used; the enhanced compact glial fibrillary acidic protein (GfaABC₁D) promoter and the enhanced human synapsin-1 (SYN) promoter. Powerful luciferase (Luc) and the neuronal isoform of nitric oxide synthase (nNOS) gene knock-down were achieved both in vitro and in vivo. Administration of doxycycline (Dox) abrogated gene silencing. However, the efficacy of gene knock-down in both neurones and astrocytes in the hippocampus (HIP) was lower than that in the dorsal vagal complex of the brainstem (DVC). This was not due to regional differences in the expression of the the key enzymes involved in miRNA processing.

Conclusions

The results from the presented experiments demonstrated that selective gene knock-down in subsets of brain cells is achievable. However, there are some presently unknown regional factors which affect either the processing of miRNA-based cassettes or their potency for gene silencing.

Transgenic, inducible RNAi in megakaryocytes and platelets in mice

BACKGROUND

RNA interference (RNAi) is a powerful tool for suppressing gene function. The tetracycline (tet)-regulated expression system has recently been adapted to allow inducible RNAi in mice, however its efficiency in a particular cell type in vivo depends on a transgenic tet transactivator expression pattern and is often highly variable.

OBJECTIVE

We aimed to establish a transgenic strategy that allows efficient and inducible gene knockdown in particular hematopoietic lineages in mice.

METHODS AND RESULTS

Using a tet-regulated reporter gene strategy, we found that transgenic mice expressing the rtTA (tet-on) transactivator under control of the cytomegalovirus (CMV) promoter (CMV-rtTA mice) display inducible reporter gene expression with unusual and near-complete efficiency in megakaryocytes and platelets. To test whether the CMV-rtTA transgene can drive inducible and efficient gene knockdown within this lineage, we generated a novel mouse strain harboring a tet-regulated short hairpin RNA (shRNA) targeting Bcl-x(L) , a pro-survival Bcl-2 family member known to be essential for maintaining platelet survival. Doxycycline treatment of adult mice carrying both transgenes induces shRNA expression, depletes Bcl-x(L) in megakaryocytes and triggers severe thrombocytopenia, whereas doxycycline withdrawal shuts off shRNA expression, normalizes Bcl-x(L) levels and restores platelet numbers. These effects are akin to those observed with drugs that target Bcl-x(L) , clearly demonstrating that this transgenic system allows efficient and inducible inhibition of genes in megakaryocytes and platelets.

CONCLUSIONS

We have established a novel transgenic strategy for inducible gene knockdown in megakaryocytes and platelets that will be useful for characterizing genes involved in platelet production and function in adult mice.

Modeling human osteosarcoma in the mouse: From bedside to bench

Osteosarcoma (OS) is the most common primary tumour of bone, occurring predominantly in the second decade of life. High-dose cytotoxic chemotherapy and surgical resection have improved prognosis, with long-term survival for patients with localized (non-metastatic) disease approaching 70%. At presentation approximately 20% of patients have metastases and almost all patients with recurrent OS have metastatic disease and cure rates for patients with metastatic or recurrent disease remain poor (<20% survival). Over the past 20 years, considerable progress has been made in the understanding of OS pathogenesis, yet these insights have not translated into substantial therapeutic advances and clinical outcomes. Further progress is essential in order to develop molecularly based therapies that target both primary lesions as well as metastatic disease. The increasing sophistication with which gene expression can be modulated in the mouse, both positively and negatively in addition to temporally, has allowed for the recent generation of more faithful OS models than have previously been available. These murine OS models can recapitulate all aspects of the disease process, from initiation and establishment to invasion and dissemination to distant sites. The development and utilisation of murine models that faithfully recapitulate human osteosarcoma, complementing existing approaches using human and canine disease, holds significant promise in furthering our understanding of the genetic basis of the disease and, more critically, in advancing pre-clinical studies aimed at the rational development and trialing of new therapeutic approaches.

Shortcomings of short hairpin RNA-based transgenic RNA interference in mouse oocytes

BACKGROUND

RNA interference (RNAi) is a powerful approach to study a gene function. Transgenic RNAi is an adaptation of this approach where suppression of a specific gene is achieved by expression of an RNA hairpin from a transgene. In somatic cells, where a long double-stranded RNA (dsRNA) longer than 30 base-pairs can induce a sequence-independent interferon response, short hairpin RNA (shRNA) expression is used to induce RNAi. In contrast, transgenic RNAi in the oocyte routinely employs a long RNA hairpin. Transgenic RNAi based on long hairpin RNA, although robust and successful, is restricted to a few cell types, where long double-stranded RNA does not induce sequence-independent responses. Transgenic RNAi in mouse oocytes based on a shRNA offers several potential advantages, including simple cloning of the transgenic vector and an ability to use the same targeting construct in any cell type.

RESULTS

Here we report our experience with shRNA-based transgenic RNAi in mouse oocytes. Despite optimal starting conditions for this experiment, we experienced several setbacks, which outweigh potential benefits of the shRNA system. First, obtaining an efficient shRNA is potentially a time-consuming and expensive task. Second, we observed that our transgene, which was based on a common commercial vector, was readily silenced in transgenic animals.

CONCLUSIONS

We conclude that, the long RNA hairpin-based RNAi is more reliable and cost-effective and we recommend it as a method-of-choice when a gene is studied selectively in the oocyte.

[New advances in animal transgenic technology]

Animal transgenic technology is one of the fastest growing biotechnology in the 21st century. It is used to integrate foreign genes into the animal genome by genetic engineering technology so that foreign genes can be expressed and inherited to the offspring. The transgenic efficiency and precise control of gene expression are the key limiting factors on preparation of transgenic animals. A variety of transgenic techniques are available, each of which has its own advantages and disadvantages and still needs further study because of unresolved technical and safety issues. With the in-depth research, the transgenic technology will have broad application prospects in the fields of exploration of gene function, animal genetic improvement, bioreactor, animal disease models, organ transplantation and so on. This article reviews the recently developed animal gene transfer techniques, including germline stem cell mediated method to improve the efficiency, gene targeting to improve the accuracy, RNA interference (RNAi)-mediated gene silencing technology, and the induced pluripotent stem cells (iPS) transgenic technology. The new transgenic techniques can provide a better platform for the study of trans-genic animals and promote the development of medical sciences, livestock production, and other fields.