Epigenetic and genotype-specific effects on the stability of de novo imposed methylation patterns in transgenic mice

Elevated murine HB-EGF confers sensitivity to diphtheria toxin in EGFR-mutant lung adenocarcinoma

Conditional ablation of defined cell populations in vivo can be achieved using genetically engineered mice in which the human diphtheria toxin (DT) receptor (DTR) is placed under control of a murine tissue-specific promotor, such that delivery of DT selectively ablates cells expressing this high-affinity human DTR; cells expressing only the endogenous low-affinity mouse DTR are assumed to be unaffected. Surprisingly, we found that systemic administration of DT induced rapid regression of murine lung adenocarcinomas that express human mutant EGFR in the absence of a transgenic allele containing human DTR. DT enzymatic activity was required for tumor regression, and mutant EGFR-expressing tumor cells were the primary target of DT toxicity. In FVB mice, EGFR-mutant tumors upregulated expression of HBEGF, which is the DTR in mice and humans. HBEGF blockade with the enzymatically inactive DT mutant CRM197 partially abrogated tumor regression induced by DT. These results suggest that elevated expression of murine HBEGF, i.e. the low-affinity DTR, confers sensitivity to DT in EGFR-mutant tumors, demonstrating a biological effect of DT in mice lacking transgenic DTR alleles and highlighting a unique vulnerability of EGFR-mutant lung cancers.

KRAB zinc finger protein diversification drives mammalian interindividual methylation variability

Most transposable elements (TEs) in the mouse genome are heavily modified by DNA methylation and repressive histone modifications. However, a subset of TEs exhibit variable methylation levels in genetically identical individuals, and this is associated with epigenetically conferred phenotypic differences, environmental adaptability, and transgenerational epigenetic inheritance. The evolutionary origins and molecular mechanisms underlying interindividual epigenetic variability remain unknown. Using a repertoire of murine variably methylated intracisternal A-particle (VM-IAP) epialleles as a model, we demonstrate that variable DNA methylation states at TEs are highly susceptible to genetic background effects. Taking a classical genetics approach coupled with genome-wide analysis, we harness these effects and identify a cluster of KRAB zinc finger protein (KZFP) genes that modifies VM-IAPs in trans in a sequence-specific manner. Deletion of the cluster results in decreased DNA methylation levels and altered histone modifications at the targeted VM-IAPs. In some cases, these effects are accompanied by dysregulation of neighboring genes. We find that VM-IAPs cluster together phylogenetically and that this is linked to differential KZFP binding, suggestive of an ongoing evolutionary arms race between TEs and this large family of epigenetic regulators. These findings indicate that KZFP divergence and concomitant evolution of DNA binding capabilities are mechanistically linked to methylation variability in mammals, with implications for phenotypic variation and putative paradigms of mammalian epigenetic inheritance.

Transcriptional activities of human elongation factor-1α and cytomegalovirus promoter in transgenic dogs generated by somatic cell nuclear transfer

Recent advances in somatic cell nuclear transfer (SCNT) in canines facilitate the production of canine transgenic models. Owing to the importance of stable and strong promoter activity in transgenic animals, we tested human elongation factor 1α (hEF1α) and cytomegalovirus (CMV) promoter sequences in SCNT transgenic dogs. After transfection, transgenic donor fibroblasts with the hEF1α-enhanced green fluorescence protein (EGFP) transgene were successfully isolated using fluorescence-activated cell sorting (FACS). We obtained four puppies, after SCNT, and identified three puppies as being transgenic using PCR analysis. Unexpectedly, EGFP regulated by hEF1α promoter was not observed at the organismal and cellular levels in these transgenic dogs. EGFP expression was rescued by the inhibition of DNA methyltransferases, implying that the hEF1α promoter is silenced by DNA methylation. Next, donor cells with CMV-EGFP transgene were successfully established and SCNT was performed. Three puppies of six born puppies were confirmed to be transgenic. Unlike hEF1α-regulated EGFP, CMV-regulated EGFP was strongly detectable at both the organismal and cellular levels in all transgenic dogs, even after 19 months. In conclusion, our study suggests that the CMV promoter is more suitable, than the hEF1α promoter, for stable transgene expression in SCNT-derived transgenic canine model.

In quest of genomic treasure

It should be emphasized that "129" is not simply a number but is also the designation of a mouse strain that has made a great contribution to modern biological science and technology. Embryonic stem cells derived from 129 mice were essential components of gene-targeting strategies in early research. More recently, 129 mice have provided superior donor genomes for cloning by nuclear transfer. Some factor or factors conferring genomic plasticity must exist in the 129 genome, but these remain unidentified.

Rosa26 locus supports tissue-specific promoter driving transgene expression specifically in pig

Genetically modified pigs have become a popular model system in fundamental research, agricultural and biomedical applications. However, random integration often result in unstable expression of transgene and unpredictable phenotypes. The Rosa26 locus has been widely used to produce genetic modified animals with high and consistent expressing of transgene in mouse, human and rat, as it can be targeted efficiently and is not subject to gene-silencing effects. Recently, the first case of reporter gene targeting pigs in porcine Rosa26 (pRosa26) locus was reported. In the study, full sequence of pRosa26 locus was further characterized, and the pRosa26 promoter (pR26) was cloned and we evidenced that the new porcine endogenous promoter is suitable for driving transgene expression in a high and stable manner by avoiding DNA methylation. Furthermore, elongation factor 1a promoter (EF1a) -driven GFP reporter and Myostatin promoter (MyoP)-driven Follistatin (Fst) were successfully targeted into the pRosa26 locusby traditional homologous recombination (HR) strategy. EF1a showed high activity and hypomethylation at the locus. And, muscle-specific promoter MyoP was activated strictly in muscle of the pRosa26 targeted pigs, indicating Rosa26 locus supports tissue-specific promoter driving transgene expression in its own manner. The study provided further demonstration on biomedical and agricultural applications of porcine Rosa26 promoter and locus.

The methylation of DNA repeat elements is sex-dependent and temporally different in response to X radiation in radiosensitive and radioresistant mouse strains

The effects of ionizing radiation on DNA methylation are of importance due to the role that DNA methylation plays in maintaining genome stability, and the presence of aberrant DNA methylation in many cancers. There is limited evidence that radiation-sensitivity may influence the modulation of DNA methylation by ionizing radiation, resulting in a loss of methylation. The BALB/c, CBA and C57Bl/6 strains are the most commonly utilized mouse strains in radiation research and are classified as radiation sensitive (BALB/c and CBA) or radiation resistant (C57Bl/6). We present here the first direct comparison of changes in repeat element DNA methylation (L1, B1 and Intracisternal A Particle; IAP) over time in these three mouse strains after high-dose radiation exposure. Using a high-resolution melt assay, methylation of the spleen repeat elements was investigated between 1 and 14 days after whole-body irradiation with 1 Gy X rays. Our study demonstrated that rather than a loss of methylation at the elements, all strains exhibited an early increase in L1 methylation one day after irradiation. In the most radiosensitive strain (BALB/c) the increase was also detected at 6 days postirradiation. The radioresistant C57Bl/6 strain exhibited a loss of L1 methylation at 14 days postirradiation. Less extensive changes to the B1 and IAP elements were detected at various time points, and pyrosequencing revealed that the responses of the strains were influenced by sex, with the male BALB/c and CBA mice exhibiting a greater response to the irradiation. The results of our study do not support the hypothesis that the most radiosensitive strains exhibit the greatest loss of repeat element DNA methylation after exposure to high-dose radiation. While the exact mechanism and biological outcome of the changes in DNA methylation observed here are still to be elucidated, this study provides the first evidence that radiation exposure elicits time-dependent changes in the methylation of repeat elements that are influenced by the genetic background, gender and the type of repeat element investigated. Furthermore, it suggest that any induced changes may not be persistent.

Genomic integration of adenoviral gene transfer vectors following transduction of fertilized mouse oocytes

Adenoviral vectors (AdV) are popular tools to deliver foreign genes into a wide range of cells. They have also been used in clinical gene therapy trials. Studies on AdV-mediated gene transfer to mammalian oocytes and transmission through the germ line have been reported controversially. In the present study we investigated whether AdV sequences integrate into the mouse genome by microinjecting AdV into the perivitelline space of fertilized oocytes. We applied a newly developed PCR technique (HiLo-PCR) for identification of chromosomal junctions next to the integrated AdV. We demonstrate that mouse oocytes can be transduced by different recombinant adenoviral vectors (first generation and gutless). In one transgenic mouse line using the first generation adenoviral vector, the genome has integrated into a highly repetitive cluster located on the Y chromosome. While the transgene (GFP) was expressed in early embryos, no expression was detected in adult transgenic mice. The use of gutless AdV resulted in expression of the transgene, albeit the vector was not transmitted to progeny. These results indicate that under optimized conditions fertilized mouse oocytes are transduced by AdV and give rise to transgenic founder animals. Therefore, adequate precautions should be taken in gene therapy protocols of reproductive patients since transduction of oocytes or early embryos and subsequent chromosomal integration cannot be ruled out entirely.

Efficient mammalian germline transgenesis by cis-enhanced Sleeping Beauty transposition

Heightened interest in relevant models for human disease increases the need for improved methods for germline transgenesis. We describe a significant improvement in the creation of transgenic laboratory mice and rats by chemical modification of Sleeping Beauty transposons. Germline transgenesis in mice and rats was significantly enhanced by in vitro cytosine-phosphodiester-guanine methylation of transposons prior to injection. Heritability of transgene alleles was also greater from founder mice generated with methylated versus non-methylated transposon. The artificial methylation was reprogrammed in the early embryo, leading to founders that express the transgenes. We also noted differences in transgene insertion number and structure (single-insert versus concatemer) based on the influence of methylation and plasmid conformation (linear versus supercoiled), with supercoiled substrate resulting in efficient transpositional transgenesis (TnT) with near elimination of concatemer insertion. Combined, these substrate modifications resulted in increases in both the frequency of transgenic founders and the number of transgenes per founder, significantly elevating the number of potential transgenic lines. Given its simplicity, versatility and high efficiency, TnT with enhanced Sleeping Beauty components represents a compelling non-viral approach to modifying the mammalian germline.

Transgene expression is associated with copy number and cytomegalovirus promoter methylation in transgenic pigs

Transgenic animals have been used for years to study gene function, produce important proteins, and generate models for the study of human diseases. However, inheritance and expression instability of the transgene in transgenic animals is a major limitation. Copy number and promoter methylation are known to regulate gene expression, but no report has systematically examined their effect on transgene expression. In the study, we generated two transgenic pigs by somatic cell nuclear transfer (SCNT) that express green fluorescent protein (GFP) driven by cytomegalovirus (CMV). Absolute quantitative real-time PCR and bisulfite sequencing were performed to determine transgene copy number and promoter methylation level. The correlation of transgene expression with copy number and promoter methylation was analyzed in individual development, fibroblast cells, various tissues, and offspring of the transgenic pigs. Our results demonstrate that transgene expression is associated with copy number and CMV promoter methylation in transgenic pigs.

Unexpected expression pattern of tetracycline-regulated transgenes in mice

In generating a conditional transgenic murine model based on a tetracycline-regulated system, we obtained unexpected patterns of expression due to the transcriptional inactivity of the tet-responder promoter. Here we show strong cell-type-restricted expression that was variegated to an extent determined by the number of responder transgene copies integrated into the host genome.

Regional FMRP deficits and large repeat expansions into the full mutation range in a new Fragile X premutation mouse model

Carriers of FMR1 alleles with 55-200 repeats in the 5' UTR are at risk for Fragile X associated tremor and ataxia syndrome. The cause of the neuropathology is unknown but is thought to be RNA-mediated. Maternally transmitted premutation alleles are also at risk of expansion of the repeat tract into the "full mutation" range (>200 repeats). The mechanism responsible for expansion is unknown. Full mutation alleles produce reduced amounts of the FMR1 gene product, FMRP, which leads to Fragile X mental retardation syndrome. We have developed a murine model for Fragile X premutation carriers that recapitulates key features seen in humans including a direct relationship between repeat number and Fmr1 mRNA levels, an inverse relationship with FMRP levels and Purkinje cell dropout that have not been seen in a previously described knock-in mouse model. In addition, these mice also show a differential deficit of FMRP in different parts of the brain that might account for symptoms of the full mutation that are seen in premutation carriers. As in humans, repeat instability is high with expansions predominating and, for the first time in a mouse model, large expansions into the full mutation range are seen that occur within a single generation. Thus, contrary to what was previously thought, mice may be good models not only for the symptoms seen in human carriers of FMR1 premutation alleles but also for understanding the mechanism responsible for repeat expansion, a phenomenon that is responsible for a number of neurological and neurodevelopmental disorders.

Stem cells adaptive network: mechanism and implications for evolution and disease development

During development, different cells and tissues acquire different programmes of gene expression, so that cells are related to each other through a somatic cells tree or cluster and adult pluripotential stem cells (PSC) may be defined as progenitors that we distinguish in four types according to their biological behaviour. This clustering may segregate specific pathways establishing spatial patterns of cell-cell communications. Thus, we suggest that normal somatic cells renewal is tributary of multipotential stem cells (MSC), while renewal of cells undergoing stress or abnormal death is tributary of PSC through specific pathway(s) from cluster, thus, defining the cell repertoire that will be produced. We also assume that PSC play a pivotal role in evolutionary and propose the theory of "internal clusters competition". According to the functional duality of stem cells (SC) we define a stem cells adaptive network (SCAN) which we believe is linked to the central clock and display two pathways. The diurnal pathway includes SC-somatic cells communications, while the nocturnal pathway includes inter-SC network. These alternate pathways could be activated or repressed as a consequence of change in the biological chirality. This new approach of SC may contribute to our understanding on how some diseases may develop including cancer which could be linked to "cluster illness", while demyelinating and systemic diseases could be related to "PSC locus illness" or "focalised SCAN disturbances" and it explains how any environment stress may act on organism evolution.

High-efficiency FLP and PhiC31 site-specific recombination in mammalian cells

DNA site-specific recombinases (SSRs) such as Cre, FLPe, and φC31, are powerful tools for analyzing gene function in vertebrates. While the availability of multiple high-efficiency SSRs would facilitate a wide array of genomic engineering possibilities, efficient recombination in mammalian cells has only been observed with Cre recombinase. Here we report the de novo synthesis of mouse codon-optimized FLP (FLPo) and ΦC31 (ΦC31o) SSRs, which result in recombination efficiencies similar to Cre.

Loss of imprinting at the Dlk1-Gtl2 locus caused by insertional mutagenesis in the Gtl2 5' region

BACKGROUND

The Dlk1 and Gtl2 genes define a region of mouse chromosome 12 that is subject to genomic imprinting, the parental allele-specific expression of a gene. Although imprinted genes play important roles in growth and development, the mechanisms by which imprinting is established and maintained are poorly understood. Differentially methylated regions (DMRs), which carry methylation on only one parental allele, are involved in imprinting control at many loci. The Dlk1-Gtl2 region contains three known DMRs, the Dlk1 DMR in the 3' region of Dlk1, the intergenic DMR 15 kb upstream of Gtl2, and the Gtl2 DMR at the Gtl2 promoter. Three mouse models are analyzed here that provide new information about the regulation of Dlk1-Gtl2 imprinting.

RESULTS

A previously existing insertional mutation (Gtl2lacZ), and a targeted deletion in which the Gtl2 upstream region was replaced by a Neo cassette (Gtl2Delta5'Neo), display partial lethality and dwarfism upon paternal inheritance. Molecular characterization shows that both mutations cause loss of imprinting and changes in expression of the Dlk1, Gtl2 and Meg8/Rian genes. Dlk1 levels are decreased upon paternal inheritance of either mutation, suggesting Dlk1 may be causative for the lethality and dwarfism. Loss of imprinting on the paternal chromosome in both Gtl2lacZ and Gtl2Delta5'Neo mice is accompanied by the loss of paternal-specific Gtl2 DMR methylation, while maternal loss of imprinting suggests a previously unknown regulatory role for the maternal Gtl2 DMR. Unexpectedly, when the Neo gene is excised, Gtl2Delta5' animals are of normal size, imprinting is unchanged and the Gtl2 DMR is properly methylated. The exogenous DNA sequences integrated upstream of Gtl2 are therefore responsible for the growth and imprinting effects.

CONCLUSION

These data provide further evidence for the coregulation of the imprinted Dlk1 and Gtl2 genes, and support a role for Dlk1 as an important neonatal growth factor. The ability of the Gtl2lacZ and Gtl2Delta5'Neo mutations to cause long-range changes in imprinting and gene expression suggest that regional imprinting regulatory elements may lie in proximity to the integration site.

Cardiovascular expression of the mouse WNK1 gene during development and adulthood revealed by a BAC reporter assay

Large deletions in WNK1 are associated with inherited arterial hypertension. WNK1 encodes two types of protein: a kidney-specific isoform (KS-WNK1) lacking kinase activity and a ubiquitously expressed full-length isoform (L-WNK1) with serine threonine kinase activity. Disease is thought to result from hypermorphic mutations increasing the production of one or both isoforms. However, the pattern of L-WNK1 expression remains poorly characterized. We generated transgenic mice bearing a murine WNK1 BAC containing the nlacZ reporter gene for monitoring L-WNK1 expression during development and adulthood. We observed previously unsuspected early expression in the vessels and primitive heart during embryogenesis, consistent with the early death of WNK1(-/-) mice. The generalized cardiovascular expression observed in adulthood may also suggest a possible kidney-independent role in blood pressure regulation. The second unsuspected site of L-WNK1 expression was the granular layer and Purkinje cells of the cerebellum, suggesting a role in local ion balance or cell trafficking. In the kidney, discordance between endogenous L-WNK1 and transgene expression suggests that either cis-regulatory elements important for physiological renal expression lie outside the BAC sequence or that illegitimate interactions occur between promoters. Despite this limitation, this transgenic model is a potentially valuable tool for the analysis of spatial and temporal aspects of WNK1 expression and regulation.

Extent and pattern of DNA methylation alteration in rice lines derived from introgressive hybridization of rice and Zizania latifolia Griseb

We have reported previously that introgression by Zizania latifolia resulted in extensive DNA methylation changes in the recipient rice genome, as detected by a set of pre-selected DNA segments. In this study, using the methylation-sensitive amplified polymorphism (MSAP) method, we globally assessed the extent and pattern of cytosine methylation alterations in three typical introgression lines relative to their rice parent at approximately 2,700 unbiased genomic loci each representing a recognition site cleaved by one or both of the isoschizomers, HpaII/MspI. Based on differential digestion by the isoschizomers, it is estimated that 15.9% of CCGG sites are either fully methylated at the internal Cs and/or hemi-methylated at the external Cs in the rice parental cultivar Matsumae. In comparison, a statistically significant increase in the overall level of both methylation types was detected in all three studied introgression lines (19.2, 18.6, 19.6%, respectively). Based on comparisons of MSAP profiles between the isoschizomers within the rice parent and between parent and the introgression lines, four major groups of MSAP banding patterns are recognized, which can be further divided into various subgroups as a result of inheritance of, or variation in, parental methylation patterns. The altered methylation patterns include hyper- and hypomethylation changes, as well as inter-conversion of hemi- to full-methylation, or vice versa, at the relevant CCGG site(s). Most alterations revealed by MSAP in low-copy loci can be validated by DNA gel blot analysis. The changed methylation patterns are uniform among randomly selected individuals for a given introgression line within or among selfed generations. Sequencing on 31 isolated fragments that showed different changing patterns in the introgression line(s) allowed their mapping onto variable regions on one or more of the 12 rice chromosomes. These segments include protein-coding genes, transposon/retrotransposons and sequences with no homology. Possible causes for the introgression-induced methylation changes and their implications for genome evolution and crop breeding are discussed.

Sleeping Beauty transposition in the mouse genome is associated with changes in DNA methylation at the site of insertion

The Sleeping Beauty (SB) transposon (Tn) system is a nonviral gene delivery tool that has widespread application for transfer of therapeutic genes into the mammalian genome. To determine its utility as a gene delivery system, it was important to assess the epigenetic modifications associated with SB insertion into the genome and potential inactivation of the transgene. This study investigated the DNA methylation pattern of an SB Tn as well as the flanking genomic region at insertion sites in the mouse genome. The ubiquitous ROSA26 promoter and an initial part of the eGFP coding sequence in the SB Tn exhibited high levels of CpG methylation in transgenic mouse lines, irrespective of the chromosomal loci of the insertion sites. In contrast, no detectable CpG methylation in the endogenous mouse ROSA26 counterpart was observed in the same animals. Furthermore, significant hypomethylation was detected in neighboring chromosomal sequences of two unique SB Tn insertions compared to wild-type patterns. Taken together, these results suggest that SB Tn insertions into the mouse genome can be discriminated by DNA methylation machinery from an identical endogenous DNA sequence and can profoundly alter the DNA methylation status of the transgene cargo as well as flanking host genomic regions.

De novo methylation, long-term promoter silencing, methylation patterns in the human genome, and consequences of foreign DNA insertion

This chapter presents a personal account of the work on DNA methylation in viral and mammalian systems performed in the author's laboratory in the course of the past 30 years. The text does not attempt to give a complete and meticulous account of the work accomplished in many other laboratories; in that sense it is not a review of the field in a conventional sense. Since the author is also one of the editors of this series of Current Topics in Immunology and Microbiology on DNA methylation, to which contributions by many of our colleagues in this field have been invited, the author's conscience is alleviated that he has not cited many of the relevant and excellent reports by others. The choice of viral model systems in molecular biology is well founded. Over many decades, viruses have proved their invaluable and pioneering role as tools in molecular genetics. When our interest turned to the demonstration of genome-wide patterns of DNA methylation, we focused mainly on the human genome. The following topics in DNA methylation will be treated in detail: (1) The de novo methylation of integrated foreign genomes; (2) the long-term gene silencing effect of sequence-specific promoter methylation and its reversal; (3) the properties and specificity of patterns of DNA methylation in the human genome and their possible relations to pathogenesis; (4) the long-range global effects on cellular DNA methylation and transcriptional profiles as a consequence of foreign DNA insertion into an established genome; (5) the patterns of DNA methylation can be considered part of a cellular defense mechanism against foreign or repetitive DNA; which role has food-ingested DNA played in the elaboration of this mechanism? The interest in problems related to DNA methylation has spread-like the mechanism itself-into many neighboring fields. The nature of the transcriptional programs orchestrating embryonal and fetal development, chromatin structure, genetic imprinting, genetic disease, X chromosome inactivation, and tumor biology are but a few of the areas of research that have incorporated studies on the importance of the hitherto somewhat neglected fifth nucleotide in many genomes. Even the fly researchers now have to cope with the presence of this nucleotide, in however small quantities it exists in the genome of their model organism, at least during embryonal development. The bulk of the experimental work accomplished in the author's laboratory has been shouldered by many very motivated undergraduate and graduate students and by a number of talented postdoctoral researchers. Their contributions are reflected in the list of references in this chapter. We have also had the good luck to receive funding through a number or organizations as acknowledged.

Controlled and localized genetic manipulation in the brain

Brain structure and function are determined in part through experience and in part through our inherited genes. A powerful approach for unravelling the balance between activity-dependent neuronal plasticity and genetic programs is to directly manipulate the genome. Such molecular genetic studies have been greatly aided by the remarkable progress of large-scale genome sequencing efforts. Sophisticated mouse genetic manipulations allow targeted point-mutations, deletions and additions to the mouse genome. These can be regulated through inducible promoters expressing in genetically specified neuronal cell types. However, despite significant progress it remains difficult to target specific brain regions through transgenesis alone. Recent work suggests that transduction vectors, like lentiviruses and adeno-associated viruses, may provide suitable additional tools for localized and controlled genetic manipulation. Furthermore, studies with such vectors may aid the development of human genetic therapies for brain diseases.

DNA methylation of Sleeping Beauty with transposition into the mouse genome

The Sleeping Beauty transposon is a recently developed non-viral vector that can mediate insertion of transgenes into the mammalian genome. Foreign DNA elements that are introduced tend to invoke a host-defense mechanism resulting in epigenetic changes, such as DNA methylation, which may induce transcriptional inactivation of mammalian genes. To assess potential epigenetic modifications associated with Sleeping Beauty transposition, we investigated the DNA methylation pattern of transgenes inserted into the mouse genome as well as genomic regions flanking the insertion sites with bisulfite-mediated genomic sequencing. Transgenic mouse lines were created with two different Sleeping Beauty transposons carrying either the Agouti or eGFP transgene. Our results showed that DNA methylation in the keratin-14 promoter and Agouti transgene were negligible. In addition, two different genomic loci flanking the Agouti insertion site exhibited patterns of DNA methylation similar to wild-type mice. In contrast, high levels of DNA methylation were observed in the eGFP transgene and its ROSA26 promoter. These results indicate that transposition via Sleeping Beauty into the mouse genome may result in a significant level of de novo DNA methylation. This may depend on a number of different factors including the cargo DNA sequence, chromosomal context of the insertion site, and/or host genetic background.

COX-2 inhibitors and genetic background reduce mammary tumorigenesis in cyclooxygenase-2 transgenic mice

Cyclooxygenase-2 (COX-2) overexpression is a widely recognized feature of human breast cancer and inhibitors of the enzyme have antitumor effects in a subset of tumor settings. Previously, we demonstrated that direct overexpression of COX-2 under control of the mammary-specific MMTV promoter/enhancer, was itself oncogenic and lead to the induction of mammary tumors in multiparous, outbred CD1 mice. In the present study, we provide evidence that COX-2 dependent tumor progression can also be studied in FVB/N, an inbred strain widely used for analysis of breast cancer progression. In these mice, the human COX-2 transgene was strongly induced during pregnancy/lactation and mammary tumors developed after multiple pregnancies. However, crossing the COX-2 FVB/N mice with the C57BL6 strain resulted in loss of the mammary tumorigenic phenotype despite the fact that the human COX-2 gene was induced. Treatment of the COX-2 transgenic mice in the FVB/N strain with celecoxib (1600 ppm), a COX-2 selective inhibitor, resulted significant reduction in tumor size and multiplicity when compared to transgenic mice fed with control chow. SC-560 (20 ppm), a COX-1 selective inhibitor did not have significant effect on tumorigenesis. These studies suggest that FVB/N is a susceptible mouse strain well suited to the study of COX-2 mediated tumor progression and may provide a tool for the identification of interacting genes and therapeutic treatments for this clinically important target.

On the biological significance of DNA methylation

This chapter presents a personal account of the work on DNA methylation in viral and mammalian systems performed in the author's laboratory in the course of the past thirty years. The text does not attempt to give a complete and meticulous account of the many relevant and excellent reports published by many other laboratories, so it is not a review of the field in a conventional sense. The choice of viral model systems in molecular biology is well founded. Over many decades, viruses have proven their invaluable and pioneering role as tools in molecular genetics. When our interest turned to the demonstration of genome-wide patterns of DNA methylation, we focused mainly on the human genome. The following topics in DNA methylation will be treated in detail: (i) the de novo methylation of integrated foreign genomes; (ii) the long-term gene silencing effect of sequence-specific promoter methylation and its reversal; (iii) the properties and specificity of patterns of DNA methylation in the human genome and their possible relations to pathogenesis; (iv) the long-range global effects on cellular DNA methylation and transcriptional profiles as a consequence of foreign DNA insertion into an established genome; (v) the patterns of DNA methylation can be considered part of a cellular defense mechanism against foreign or repetitive DNA; what role has food-ingested DNA played in the elaboration of this mechanism?

Influence of in vitro manipulation on the stability of methylation patterns in the Snurf/Snrpn-imprinting region in mouse embryonic stem cells

Recent work on embryonic stem (ES) cells showed that stem cell-derived tissues and embryos, cloned from ES cell nuclei, often fail to maintain epigenetic states of imprinted genes. This deregulation is frequently associated with in vitro manipulations and culture conditions which might affect the cells potential to develop into normal fetuses. Usually, epigenetic instability is reported in differentially methylated regions of mostly growth-related imprinted genes. However, little is known about the epigenetic stability of genes that function late in organogenesis. Hence, we set out to investigate the epigenetic stability of neuronal genes and analyzed DNA methylation patterns in the Snurf/Snrpn imprinted cluster in several cultured mouse ES cell lines. We also determined the effects of in vitro stress factors such as consecutive passaging, trypsination, mechanical handling, single cell cloning, centrifugation, staurosporine-induced neurogenesis and the insertion of viral (foreign) DNA into the host genome. Intriguingly, none of these in vitro manipulations interfered with the stability of the methylation patterns in the analyzed neuronal genes. These data imply that, in contrast to growth-related genes like Igf2, H19, Igf2r or Grb10, the methylation imprints of the analyzed neuronal genes in the Snurf/Snrpn cluster may be particularly stable in manipulated ES cells.

Effects of donor cell type and genotype on the efficiency of mouse somatic cell cloning

Although it is widely assumed that the cell type and genotype of the donor cell affect the efficiency of somatic cell cloning, little systematic analysis has been done to verify this assumption. The present study was undertaken to examine whether donor cell type, donor genotype, or a combination thereof increased the efficiency of mouse cloning. Initially we assessed the developmental ability of embryos that were cloned from cumulus or immature Sertoli cells with six different genotypes (i.e., 2 x 6 factorial). Significantly better cleavage rates were obtained with cumulus cells than with Sertoli cells (P < 0.005, two-way ANOVA), which probably was due to the superior cell-cycle synchrony of cumulus cells at G0/G1. After embryo transfer, there was a significant effect of cell type on the birth rate, with Sertoli cells giving the better result (P < 0.005). Furthermore, there was a significant interaction (P < 0.05) between the cell type and genotype, which indicates that cloning efficiency is determined by a combination of these two factors. The highest mean birth rate (10.8 +/- 2.1%) was obtained with (B6 x 129)F1 Sertoli cells. In the second series of experiments, we examined whether the developmental ability of clones with the wild-type genotype (JF1) was improved when combined with the 129 genotype. Normal pups were cloned from cumulus and immature Sertoli cells of the (129 x JF1)F1 and (JF1 x 129)F1 genotypes, whereas no pups were born from cells with the (B6 x JF1)F1 genotype. The present study clearly demonstrates that the efficiency of somatic cell cloning, and in particular fetal survival after embryo transfer, may be improved significantly by choosing the appropriate combinations of cell type and genotype.

Tetracycline-inducible system for regulation of skeletal muscle-specific gene expression in transgenic mice

Tightly regulated control of over-expression is often necessary to study one aspect or time point of gene function and, in transgenesis, may help to avoid lethal effects and complications caused by ubiquitous over-expression. We have utilized the benefits of an optimized tet-on system and a modified muscle creatine kinase (MCK) promoter to generate a skeletal muscle-specific, doxycycline (Dox) controlled over-expression system in transgenic mice. A DNA construct was generated in which the codon optimized reverse tetracycline transactivator (rtTA) was placed under control of a skeletal muscle-specific version of the mouse MCK promoter. Transgenic mice containing this construct expressed rtTA almost exclusively in skeletal muscles. These mice were crossed to a second transgenic line containing a bi-directional promoter centered on a tet responder element driving both a luciferase reporter gene and a tagged gene of interest; in this case the calpain inhibitor calpastatin. Compound hemizygous mice showed high level, Dox dependent muscle-specific luciferase activity often exceeding 10,000-fold over non-muscle tissues of the same mouse. Western and immunocytochemical analysis demonstrated similar Dox dependent muscle-specific induction of the tagged calpastatin protein. These findings demonstrate the effectiveness and flexibility of the tet-on system to provide a tightly regulated over-expression system in adult skeletal muscle. The MCKrtTA transgenic lines can be combined with other transgenic responder lines for skeletal muscle-specific over-expression of any target gene of interest.

Schizophrenia as a disorder of neurodevelopment

A combination of genetic susceptibility and environmental perturbations appear to be necessary for the expression of schizophrenia. In addition, the pathogenesis of the disease is hypothesized to be neurodevelopmental in nature based on reports of an excess of adverse events during the pre- and perinatal periods, the presence of cognitive and behavioral signs during childhood and adolescence, and the lack of evidence of a neurodegenerative process in most individuals with schizophrenia. Recent studies of neurodevelopmental mechanisms strongly suggest that no single gene or factor is responsible for driving a highly complex biological process. Together, these findings suggest that combinatorial genetic and environmental factors, which disturb a normal developmental course early in life, result in molecular and histogenic responses that cumulatively lead to different developmental trajectories and the clinical phenotype recognized as schizophrenia.

Altered DNA methylation: a secondary mechanism involved in carcinogenesis

This review focuses on the role that DNA methylation plays in the regulation of normal and aberrant gene expression and on how, in a hypothesis-driven fashion, altered DNA methylation may be viewed as a secondary mechanism involved in carcinogenesis. Research aimed at discerning the mechanisms by which chemicals can transform normal cells into frank carcinomas has both theoretical and practical implications. Through an increased understanding of the mechanisms by which chemicals affect the carcinogenic process, we learn more about basic biology while, at the same time, providing the type of information required to make more rational safety assessment decisions concerning their actual potential to cause cancer under particular conditions of exposure. One key question is: does the mechanism of action of the chemical in question involve a secondary mechanism and, if so, what dose may be below its threshold?

Effects of mouse strain, position of integration and tetracycline analogue on the tetracycline conditional system in transgenic mice

The tetracycline conditional system is a very powerful method for achieving control of gene expression in transgenic mice, allowing one to turn expression both off and on in the same animal. We have used it to make a tissue-specific transgenic mouse model of Charcot-Marie-Tooth disease type 1A. This disease is most commonly caused by overexpression of peripheral myelin protein 22 (PMP22) in Schwann cells of the peripheral nervous system. Here we describe the effects of position of integration of the transgene, tetracycline analogue and mouse strain in this model. The small transgenes used to express tTA, the LacZ reporter and the pmp22 cDNA were all very dependent on the position of integration with few of the transgenic lines working successfully. In contrast, the single transgenic made with the 560 kb yeast artificial chromosome construct containing the tTA open reading frame worked well. Tetracycline was found to be cleared from mice relatively fast in comparison with doxycycline and is thus useful if one wants to switch on gene expression after extended periods of administration. Finally, the initial litters were on a mixed genetic background and the level of LacZ or pmp22 expression was very variable between mice. We found that expression became uniform between mice, and occurred in a higher proportion of cells, when the transgenes were crossed onto the CBA/Ca background in comparison with the C57BL/6J background.

Generation dependent reduction of tTA expression in double transgenic NZL-2/tTA(CMV) mice

Despite the overall successful application of the tet-system to regulate gene expression in vitro and in vivo, nothing is known so far about the long-term stability of this system in transgenic mice. In this study, mice of generation F2, F3, F4, or F10 of two independent tTA(CMV) transgenic lines were bred with NZL-2 mice containing a tTA-responsive bidirectional promoter that allows the simultaneous expression of two reporter genes encoding luciferase and beta-galactosidase. Analysis of the expression of transgenes in double transgenic mice revealed a dramatic reduction of tTA transactivator mRNA over time. As a consequence, the expression of both reporter genes was gradually reduced from generation to generation in tissues of embryonic and adult NZL-2/tTA(CMV) mice. Luciferase activity in NZL-2/tTA(CMV)(F10) mice was reduced 8-10-fold compared to NZL-2/ tTA(CMV)(F2) mice, and beta-galactosidase expression was no longer detectable. In summary, we describe the long-term instability of the tet-system in our NZL-2/tTA(CMV) double transgenic mice. The molecular basis of this observation and experimental tools to overcome this limitation need to be addressed in future.

Foreign DNA integration--perturbations of the genome--oncogenesis

We have been interested in the consequences of foreign DNA insertion into established mammalian genomes and have initially studied this problem in adenovirus type 12 (Ad12)-transformed cells or in Ad12-induced hamster tumors. Since integrates are frequently methylated de novo, it appears that they might be modified by an ancient defense mechanism against foreign DNA. In cells transgenic for the DNA of Ad12 or for the DNA of bacteriophage lambda, changes in cellular methylation and transcription patterns have been observed. Thus, the insertion of foreign DNA can have important functional consequences that are not limited to the site of foreign DNA insertion. These findings appear to be relevant also for tumor biology and for the interpretation of data derived from experiments with transgenic organisms. For most animals, the main portal of entry for foreign DNA is the gastrointestinal tract. Large amounts of foreign DNA are regularly ingested with the supply of nutrients. Starting in 1987/1988, we have been investigating the fate of orally administered foreign DNA in mice. Naked DNA of bacteriophage M13 and the cloned gene for the green fluorescent protein (GFP) of Aequorea victoria have been used as test molecules. Moreover, the plant-specific gene for the ribulose-1,5-bisphosphate carboxylase (rubisco) has been followed in mice after feeding soybean leaves. At least transiently, food-ingested DNA can be traced to different organs and, after transplacental transfer, to fetuses and newborns. There is no evidence for germ line transmission or for the expression of orally administered GFP DNA.