Neomorphic agouti mutations in obese yellow mice

Genetically regulated epigenetic transcriptional activation of retrotransposon insertion confers mouse dactylaplasia phenotype

Dactylaplasia, characterized by missing central digital rays, is an inherited mouse limb malformation that depends on two genetic loci. The first locus, Dac, is an insertional mutation around the dactylin gene that is inherited as a semidominant trait. The second locus is an unlinked modifier, mdac/Mdac, that is polymorphic among inbred strains. Mdac dominantly suppresses the dactylaplasia phenotype in mice carrying Dac. However, little is known about either locus or the nature of their interaction. Here we show that Dac is a LTR retrotransposon insertion caused by the type D mouse endogenous provirus element (MusD). This insertion exhibits different epigenetic states and spatiotemporally expresses depending on the mdac/Mdac modifier background. In dactylaplasia mutants (Dac/+ mdac/mdac), the LTRs of the insertion contained unmethylated CpGs and active chromatin. Furthermore, MusD elements expressed ectopically at the apical ectodermal ridge of limb buds, accompanying the dactylaplasia phenotype. On the other hand, in Dac mutants carrying Mdac (Dac/+ Mdac/mdac), the 5' LTR of the insertion was heavily methylated and enriched with inactive chromatin, correlating with inhibition of the dactylaplasia phenotype. Ectopic expression was not observed in the presence of Mdac, which we refined to a 9.4-Mb region on mouse chromosome 13. We report a pathogenic mutation caused by MusD. Our findings indicate that ectopic expression from the MusD insertion correlates with the dactylaplasia phenotype and that Mdac acts as a defensive factor to protect the host genome from pathogenic MusD insertions.

Epigenetic modulation and cancer: effect of metabolic syndrome?

The importance of epigenetics in the etiology of disease, including cancer development and progression, is increasingly being recognized. However, the relevance of epigenetics to the metabolic syndrome, and how it may affect cancer, is only beginning to capture the interest of the scientific community. This review focuses on data supporting the hypothesis that, in addition to the "thrifty genotype" and "thrifty phenotype" hypotheses, diet-induced changes in "epigenetic programming" during fetal and postnatal development may precipitate the metabolic syndrome. Thus, epigenetics may bridge both the thrifty genotype and thrifty phenotype hypotheses and provide a link between genes and the environment concerning disease predisposition to metabolic syndrome and its associated diseases.

Maternal nutrient supplementation counteracts bisphenol A-induced DNA hypomethylation in early development

The hypothesis of fetal origins of adult disease posits that early developmental exposures involve epigenetic modifications, such as DNA methylation, that influence adult disease susceptibility. In utero or neonatal exposure to bisphenol A (BPA), a high-production-volume chemical used in the manufacture of polycarbonate plastic, is associated with higher body weight, increased breast and prostate cancer, and altered reproductive function. This study shows that maternal exposure to this endocrine-active compound shifted the coat color distribution of viable yellow agouti (Avy) mouse offspring toward yellow by decreasing CpG (cytosine-guanine dinucleotide) methylation in an intracisternal A particle retrotransposon upstream of the Agouti gene. CpG methylation also was decreased at another metastable locus, the CDK5 activator-binding protein (CabpIAP). DNA methylation at the Avy locus was similar in tissues from the three germ layers, providing evidence that epigenetic patterning during early stem cell development is sensitive to BPA exposure. Moreover, maternal dietary supplementation, with either methyl donors like folic acid or the phytoestrogen genistein, negated the DNA hypomethylating effect of BPA. Thus, we present compelling evidence that early developmental exposure to BPA can change offspring phenotype by stably altering the epigenome, an effect that can be counteracted by maternal dietary supplements.

Diet-induced hypermethylation at agouti viable yellow is not inherited transgenerationally through the female

The effects of nonmutagenic environmental exposures can sometimes be transmitted for several generations, suggesting transgenerational inheritance of induced epigenetic variation. Methyl donor supplementation of female mice during pregnancy induces CpG hypermethylation at the agouti viable yellow (A(vy)) allele in A(vy)/a offspring. Epigenetic inheritance occurs at A(vy); when passed through the female germ line, A(vy) epigenotype is not completely "reset." We therefore tested whether diet-induced epigenetic alterations at A(vy) are inherited transgenerationally. Female A(vy)/a mice were weaned onto either control (n=6) or a methyl-supplemented diet (n=5). These F0 dams were mated with a/a males. All F1 and F2 A(vy)/a females were weaned onto the same diet as their mothers, then mated with a/a males. F1, F2, and F3 A(vy)/a offspring were classified for coat color, an indicator of A(vy) methylation. In total, 62 F1, 98 F2, and 209 F3 A(vy)/a mice were studied. As expected, average A(vy)/a coat color was darker in the supplemented group (P<0.01). However, there was no cumulative effect of supplementation across successive generations. These results suggest that, in the female germ line, diet-induced A(vy) hypermethylation occurs in the absence of additional epigenetic modifications that normally confer transgenerational epigenetic inheritance at the locus.

The role of leptin in the regulation of neuroendocrine function and CNS development

Leptin, a hormone produced by adipocytes in proportion to fat stores, signals the sufficiency of energy reserves to the brain to control feeding and metabolism. Leptin represents a vital link between metabolic and neuroendocrine pathways, and adequate circulating leptin levels are required to permit the expenditure of energy on reproduction, growth, and other energy-intensive endocrine outputs. Leptin mediates its effects by acting upon a distributed network of CNS neurons that express the signaling form of the leptin receptor (LRb). Nutritional status early in development influences a lifelong metabolic program that modulates risk for diabetes, obesity and other elements of the metabolic syndrome. Recent evidence has demonstrated a number of important roles for leptin in the regulation of neural development and metabolic programming. In this review, we discuss leptin action, the neural circuits on which leptin acts, and our nascent understanding of how early leptin exposure may influence neural development and the predisposition to metabolic diseases.

Metastable epialleles, imprinting, and the fetal origins of adult diseases

Epigenetics is the study of the heritable changes in gene expression that occur without a change in the DNA sequence itself. These heritable epigenetic changes include chromatin folding and attachment to the nuclear matrix, packaging of DNA around nucleosomes, histone modifications, and DNA methylation. The epigenome is particularly susceptible to dysregulation during gestation, neonatal development, puberty, and old age. Nevertheless, it is most vulnerable to environmental factors during embryogenesis because the DNA synthetic rate is high, and the elaborate DNA methylation patterning and chromatin structure required for normal tissue development is established during early development. Metastable epialleles are alleles that are variably expressed in genetically identical individuals due to epigenetic modifications established during early development and are thought to be particularly vulnerable to environmental influences. The viable yellow agouti (A(vy)) allele, whose expression is correlated to DNA methylation, is a murine metastable epiallele, which has been used as an epigenetic biosensor for environmental factors affecting the fetal epigenome. In this review, we introduce epigenetic gene regulation, describe important epigenetic phenomenon in mammals, summarize literature linking the early environment to developmental plasticity of the fetal epigenome, and promote the necessity to identify epigenetically labile genes in the mouse and human genomes.

Modifiers of epigenetic reprogramming show paternal effects in the mouse

There is increasing evidence that epigenetic information can be inherited across generations in mammals, despite extensive reprogramming both in the gametes and in the early developing embryo. One corollary to this is that disrupting the establishment of epigenetic state in the gametes of a parent, as a result of heterozygosity for mutations in genes involved in reprogramming, could affect the phenotype of offspring that do not inherit the mutant allele. Here we show that such effects do occur following paternal inheritance in the mouse. We detected changes to transcription and chromosome ploidy in adult animals. Paternal effects of this type have not been reported previously in mammals and suggest that the untransmitted genotype of male parents can influence the phenotype of their offspring.

Imprinted status of pleomorphic adenoma gene-like I and paternal expression gene 10 genes in pigs1

Genomic imprinting is theorized to exist in all placental mammals and some marsupials. Imprinted genes play important roles in the regulation of fetal growth, development, and postnatal behavior, but the study of imprinted genes has been limited in livestock. In this study, the polymorphism-based approach was used to detect the expression patterns of the porcine pleomorphic adenoma gene-like I (PLAGL1) and paternal expression gene 10 (PEG10) genes. Single nucleotide polymorphisms in the exons were detected between the Meishan and Large White breeds in the PLAGL1 and PEG10 genes. The polymorphisms were used to determine the monoallelic or biallelic expression with reverse transcription-PCR-RFLP in 44 tissues from 4 heterozygous pigs (based on SNP). Imprinting analysis indicated that the PLAGL1 and PEG10 genes were both paternally expressed in all tissues tested (heart, liver, spleen, lung, kidney, stomach, small intestine, skeletal muscle, fat, uterus, and ovary). Our study showed that the method of identifying polymorphic transcripts with reverse transcription-PCR-RFLP may be beneficial for detecting the imprinting status of some candidate imprinted genes.

Environmental epigenomics and disease susceptibility

Epidemiological evidence increasingly suggests that environmental exposures early in development have a role in susceptibility to disease in later life. In addition, some of these environmental effects seem to be passed on through subsequent generations. Epigenetic modifications provide a plausible link between the environment and alterations in gene expression that might lead to disease phenotypes. An increasing body of evidence from animal studies supports the role of environmental epigenetics in disease susceptibility. Furthermore, recent studies have demonstrated for the first time that heritable environmentally induced epigenetic modifications underlie reversible transgenerational alterations in phenotype. Methods are now becoming available to investigate the relevance of these phenomena to human disease.

Cancer Susceptibility: Epigenetic Manifestation of Environmental Exposures

Cancer is a disease that results from both genetic and epigenetic changes. Discordant phenotypes and varying incidences of complex diseases such as cancer in monozygotic twins as well as genetically identical laboratory animals have long been attributed to differences in environmental exposures. Accumulating evidence indicates, however, that disparities in gene expression resulting from variable modifications in DNA methylation and chromatin structure in response to the environment also play a role in differential susceptibility to disease. Despite a growing consensus on the importance of epigenetics in the etiology of chronic human diseases, the genes most prone to epigenetic dysregulation are incompletely defined. Moreover, neither the environmental agents most strongly affecting the epigenome nor the critical windows of vulnerability to environmentally induced epigenetic alterations are adequately characterized. These major deficits in knowledge markedly impair our ability to understand fully the etiology of cancer and the importance of the epigenome in diagnosing and preventing this devastating disease.

Epigenetic variation and inheritance in mammals

What determines phenotype is one of the most fundamental questions in biology. Historically, the search for answers had focused on genetic or environmental variants, but recent studies in epigenetics have revealed a third mechanism that can influence phenotypic outcomes, even in the absence of genetic or environmental heterogeneity. Even more surprisingly, some epigenetic variants, or epialleles, can be inherited by the offspring, indicating the existence of a mechanism for biological heredity that is not based on DNA sequence. Recent work from mouse models, human monozygotic twin studies, and large-scale epigenetic profiling suggests that epigenetically determined phenotypes and epigenetic inheritance are more common than previously appreciated.

Germ-line epigenetic modification of the murine A vy allele by nutritional supplementation

Environmental effects on phenotype can be mediated by epigenetic modifications. The epigenetic state of the murine A vy allele is highly variable, and determines phenotypic effects that vary in a mosaic spectrum that can be shifted by in utero exposure to methyl donor supplementation. We have asked if methyl donor supplementation affects the germ-line epigenetic state of the A vy allele. We find that the somatic epigenetic state of A vy is affected by in utero methyl donor supplementation only when the allele is paternally contributed. Exposure to methyl donor supplementation during midgestation shifts A vy phenotypes not only in the mice exposed as fetuses, but in their offspring. This finding indicates that methyl donors can change the epigenetic state of the A vy allele in the germ line, and that the altered state is retained through the epigenetic resetting that takes place in gametogenesis and embryogenesis. Thus a mother's diet may have an enduring influence on succeeding generations, independent of later changes in diet. Although other reports have suggested such heritable epigenetic changes, this study demonstrates that a specific mammalian gene can be subjected to germ-line epigenetic change.

The insertion of a full-length Bos taurus LINE element is responsible for a transcriptional deregulation of the Normande Agouti gene

Mammalian pigmentation is controlled by the concerted action of Tyr, Tyrp1 and Dct producing eumelanin and/or pheomelanin in melanocytes. The ratio of these two pigments is determined by the agonist alpha-melanocyte stimulating hormone and the antagonist Agouti protein acting on the Mc1r. Here we show that the Agouti gene is over-expressed in Normande breed compared with Prim'Holstein breed. The Normande cattle have a characteristic coat color phenotype with a variable presence of black (eumelanin) hair over a red/brown background. We have found a previously undescribed full-length L1-BT element inserted in the 5'-genomic sequence of the Agouti gene in Normande cattle which promotes the over-expression of alternative transcripts. The variable expression of the alternative transcript directed by the long interspersed nuclear element promoter may be the origin of the brindle coat color pattern of the Normande breed. This new bovine Agouti allele isolated in Normande breed has been named Abr. Finally, as ectopic over-expression of Agouti in Ay mice is responsible for the obesity syndrome, we discuss the possible consequences of Abr for meat and milk production in cattle.

Assessing the effects of high methionine intake on DNA methylation

Methylation of DNA occurs at cytosines within CpG (cytosine-guanine) dinucleotides and is 1 of several epigenetic mechanisms that serve to establish and maintain tissue-specific patterns of gene expression. The methyl groups transferred in mammalian DNA methylation reactions are ultimately derived from methionine. High dietary methionine intake might therefore be expected to increase DNA methylation. Because of the circular nature of the methionine cycle, however, methionine excess may actually impair DNA methylation by inhibiting remethylation of homocysteine. Although little is known regarding the effect of dietary methionine supplementation on mammalian DNA methylation, the available data suggest that methionine supplementation can induce hypermethylation of DNA in specific genomic regions. Because locus-specific DNA hypomethylation is implicated in the etiology of various cancers and developmental syndromes, clinical trials of "promethylation" dietary supplements are already under way. However, aberrant hypermethylation of DNA could be deleterious. It is therefore important to determine whether dietary supplementation with methionine can effectively support therapeutic maintenance of DNA methylation without causing excessive and potentially adverse locus-specific hypermethylation. In the viable yellow agouti (Avy) mouse, maternal diet affects the coat color distribution of offspring by perturbing the establishment of methylation at the Avy metastable epiallele. Hence, the Avy mouse can be employed as a sensitive epigenetic biosensor to assess the effects of dietary methionine supplementation on locus-specific DNA methylation. Recent developments in epigenomic approaches that survey locus-specific DNA methylation on a genome-wide scale offer broader opportunities to assess the effects of high methionine intake on mammalian epigenomes.

Confirmation of sable QTL that modifies the effects of the A(y) allele on yellow coat color on mouse chromosome 1

F1-A(y) mice between RR (aabbCC) and C57BL/6J-A(y) (A(y)aBBCC) have a much darker yellow coat color than do C57BL/6J-A(y) . Quantitative trait locus (QTL) analysis was carried out to identify genes responsible for the darker modification of the yellow coat color (this has been traditionally termed "sable"). A significant sable QTL was identified on chromosome 1 (Dmyaq4, LOD score 15.5 for lightness, and 13.4 for color difference), in a chromosomal position similar to that of Dmyaq2, a sable QTL previously identified in C3H/HeJ. Another significant sable QTL was identified on chromosome 4 (Dmyaq5, LOD score 5.6 for lightness, and 4.3 for color difference) near the tyrosinase-related protein 1 (Tyrp1) locus. The effect of Dmyaq5 was significant only in the presence of the RR allele at Dmyaq4, suggesting that the Dmyaq4 (as well as Dmyaq2) is a novel coat color gene that may act up-stream of Tyrp1 signaling to increase eumelanin production.

Maternal genistein alters coat color and protects Avy mouse offspring from obesity by modifying the fetal epigenome

Genistein, the major phytoestrogen in soy, is linked to diminished female reproductive performance and to cancer chemoprevention and decreased adipose deposition. Dietary genistein may also play a role in the decreased incidence of cancer in Asians compared with Westerners, as well as increased cancer incidence in Asians immigrating to the United States. Here, we report that maternal dietary genistein supplementation of mice during gestation, at levels comparable with humans consuming high-soy diets, shifted the coat color of heterozygous viable yellow agouti (A(vy/a) offspring toward pseudoagouti. This marked phenotypic change was significantly associated with increased methylation of six cytosine-guanine sites in a retrotransposon upstream of the transcription start site of the Agouti gene. The extent of this DNA methylation was similar in endodermal, mesodermal, and ectodermal tissues, indicating that genistein acts during early embryonic development. Moreover, this genistein-induced hypermethylation persisted into adulthood, decreasing ectopic Agouti expression and protecting offspring from obesity. Thus, we provide the first evidence that in utero dietary genistein affects gene expression and alters susceptibility to obesity in adulthood by permanently altering the epigenome.

The neuronal histamine H(1) and pro-opiomelanocortin-melanocortin 4 receptors: independent regulation of food intake and energy expenditure

Hypothalamic neuronal histamine and its H(1) receptor (H(1)-R) form part of the leptin signaling pathway in the brain, and regulate body weight and adiposity by affecting food intake and energy expenditure. The pro-opiomelanocortin (POMC)-melanocortin 4 receptor (MC4-R) is also important for leptin signaling. We investigated whether and how these two neuronal pathways interact in regulating energy metabolism. From studies of agouti yellow (A(y)/a) obese mice, a model of a defect in POMC-MC4-R signaling, we concluded that the histamine H(1)-R signaling pathway is independent of the POMC-MC4-R complex in regulating food intake, energy metabolism, and adiposity.

Feeding effects of melanocortin ligands--a historical perspective

The process of energy homeostasis is a highly regulated process involving interacting signals between a variety of anorexigenic and orexigenic peptides, proteins and signaling molecules. The melanocortin system is an important component of this complex regulatory network. Involvement of the melanocortin pathway in the control of food intake and body weight regulation has been studied extensively in the past two decades. Previous studies that involve central administration of melanocortin molecules and examination of molecules that effect food intake in melanocortin knockout (KO) mice (MC3R, MC4R, POMC, AGRP and NPY) have been examined. In this review, we have summarized feeding studies that have resulted in the recognition of the melanocortin system as a major contributor to the complex neuroendocrine system regulating energy homeostasis.

Dose-response effects of ectopic agouti protein on iron overload and age-associated aspects of the Avy/a obese mouse phenome

Isogenic and congenic offspring from matings of inbred black a/a dams by sibling (or non-sibling from another inbred strain) yellow agouti Avy/a sires provide an animal model of obese yellow agouti Avy/a and isogenic lean pseudoagouti Avy/a mice exhibiting two different in vivo concentrations (high, very low) of ectopic agouti protein (ASP) with congenic lean black a/a mice as null controls. This makes it possible to differentiate between the high and very low dose levels of ectopic ASP with respect to interactions with diverse physiological and molecular pathways. Assay of differential responses to 12 or 24 months of carbonyl iron overload assessed the possible suitability of this animal model for the study of hemochromatosis. Agouti A/a B6C3F1 mice were used as non-congenic null controls. The age-related waxing and waning of body weight, food consumption, and caloric efficiency, as well as associated changes in pancreatic islets and islet cells, and formation of liver tumors were assayed. While the hypothesis that these mice might serve as a tool for investigating hemochromatosis was not confirmed, the data did provide evidence that even the very low levels of ASP in pseudoagouti Avy/a mice affect the network of molecular/metabolic/physiological response pathways that comprises the yellow agouti obese phenome. We suggest that the combination of yellow agouti Avy/a, pseudoagouti Avy/a, and black a/a congenic mice provides a practical tool for applying a dose-response systems biology approach to understanding the dysregulatory influence of ectopic ASP on the molecular-physiological matrix of the organism.

Micronutrients and cancer

Adverse dietary factors account for approximately 30% of all cancers. Overconsumption of energy is undoubtedly one of the major risk factors, but dietary composition is also very important. In particular, a low consumption of fruits and vegetables appears to double the risk of carcinomas of the lung and alimentary tract. Epidemiological studies suggest that high plasma levels of Se, carotenoids and ascorbic acid are protective against cancer. However, intervention studies with antioxidant nutrients have given mixed results, and it has not been established that the benefits of a high intake of fruits and vegetables are invariably related to the prevention of oxidative DNA damage. Folic acid supplementation appears to protect against colo-rectal neoplasia, probably by preventing mutations associated with the repair of uracil mis-incorporation. However, there are indications from animal studies that exposure to high levels of folic acid at certain stages of development may lead to epigenetic effects that are, as yet, poorly understood. There seems little doubt that micronutrients contribute to the protective effects of plant foods against cancers of the lung and alimentary tract, but it has not been established that these benefits can be achieved using supplements.

Genome-wide prediction of imprinted murine genes

Imprinted genes are epigenetically modified genes whose expression is determined according to their parent of origin. They are involved in embryonic development, and imprinting dysregulation is linked to cancer, obesity, diabetes, and behavioral disorders such as autism and bipolar disease. Herein, we train a statistical model based on DNA sequence characteristics that not only identifies potentially imprinted genes, but also predicts the parental allele from which they are expressed. Of 23,788 annotated autosomal mouse genes, our model identifies 600 (2.5%) to be potentially imprinted, 64% of which are predicted to exhibit maternal expression. These predictions allowed for the identification of putative candidate genes for complex conditions where parent-of-origin effects are involved, including Alzheimer disease, autism, bipolar disorder, diabetes, male sexual orientation, obesity, and schizophrenia. We observe that the number, type, and relative orientation of repeated elements flanking a gene are particularly important in predicting whether a gene is imprinted.

An N-ethyl-N-nitrosourea screen for genes involved in variegation in the mouse

We have developed a sensitized screen to identify genes involved in gene silencing, using random N-ethyl-N-nitrosourea mutagenesis on mice carrying a variegating GFP transgene. The dominant screen has produced six mutant lines, including both suppressors and enhancers of variegation. All are semidominant and five of the six are homozygous embryonic lethal. In one case, the homozygous lethality depends on sex: homozygous females die at midgestation and display abnormal DNA methylation of the X chromosome, whereas homozygous males are viable. Linkage analysis reveals that the mutations map to unique chromosomal locations. We have studied the effect of five of the mutations on expression of an endogenous allele known to be sensitive to epigenetic state, agouti viable yellow. In all cases, there is an effect on penetrance, and in most cases, parent of origin and sex-specific effects are detected. This screen has identified genes that are involved in epigenetic reprogramming of the genome, and the behavior of the mutant lines suggests a common mechanism between X inactivation and transgene and retrotransposon silencing. Our findings raise the possibility that the presence or absence of the X chromosome in mammals affects the establishment of the epigenetic state at autosomal loci by acting as a sink for proteins involved in gene silencing. The study demonstrates the power of sensitized screens in the mouse not only for the discovery of novel genes involved in a particular process but also for the elucidation of the biology of that process.

Complex patterns of transcription at the insertion site of a retrotransposon in the mouse

Here we report that transcriptional effects of the insertion of a retrotransposon can occur simultaneously both upstream and downstream of the insertion site. We have identified an intra-cisternal A particle (IAP) retrotransposon in intron 6 of a gene that we have named Cabp (CDK5 activator binding protein). The presence of the IAP is associated with an aberrant transcript initiating from a cryptic promoter in the IAP, reading out into the adjacent Cabp gene sequence. The expression of this transcript is highly variable among isogenic mice within the C57BL/6J strain and so Cabp(IAP) can be classified as a metastable epiallele. As expected, the presence or absence of the transcript correlates with differential DNA methylation of the 5' LTR of the IAP. More surprisingly, in mice where the retrotransposon is unmethylated and presumably transcriptionally active, we find a number of short Cabp transcripts which initiate at the normal 5' end of the gene but terminate prematurely, just 5' of the retrotransposon. This is the first report of a retrotransposon having both upstream and downstream effects on transcription at the site of insertion and it suggests that alternative polyadenylation may sometimes be caused by a downstream convergent transcription unit.

Negative and positive effects of an IAP-LTR on nearby Pcdaα gene expression in the central nervous system and neuroblastoma cell lines

Intracisternal A-particles (IAPs) are defective retrovirions encoded by members of a large family of endogenous proviral elements in the murine genome. An intact IAP element was found in the protocadherin alpha (Pcdhalpha) gene cluster of five laboratory mouse strains. However, IAP insertion was not detected in three wild mouse strains we investigated. This IAP insertion caused the disruption of one variable exon of laboratory mouse and down-regulated expression of the Pcdhalpha v8 exon, which is located just downstream of the IAP in the brain following the methylation of 5' regulatory region of Pcdhalpha v8. In contrast, the Pcdhalpha v8 exon was highly expressed in mouse neuroblastoma cell lines. This suggested that the IAP insertion activates the expression of the nearby Pcdhalpha v8 exon in these cell lines. In fact, the Pcdhalpha v8 exon expression was driven by the IAP-long terminal repeat (LTR) following the de-methylation of 5' regulatory region of Pcdhalpha v8. To investigate the promoter activity of the IAP, we constructed an IAP-LTR-ECFP reporter gene and introduced it into neuroblastoma, melanoma, lymphoma, and plasmacytoma cell lines. Interestingly, ECFP-positive cells were observed only in the neuroblastoma cell lines. Moreover, there were no differences in the promoter activities of the IAP-LTR whether it was in the sense or complimentary orientation. Thus, this IAP-LTR has negative and positive regulation on near by gene expression in the brain and neuroblastoma cell lines.

Identification and properties of imprinted genes and their control elements

Imprinted genes have the unusual characteristic that the copy from one parent is destined to remain inactive. Though few in number they nonetheless constitute a functionally important part of the mammalian genome. With their memory of parental origin, imprinted genes represent an important model for the epigenetic regulation of gene function and will provide invaluable paradigms to test whether we can predict epigenetic state from DNA sequence. Since their first discovery, systematic screens and some good fortune have led to identification of over seventy imprinted genes in the mouse and human: recent microarray analysis may reveal many more. With a significant number of imprinted genes now identified and completion of key mammalian genome sequences, we are able systematically to examine the organization of imprinted loci, properties of their control elements and begin to recognize common themes in imprinted gene regulation.

Liver-specific expression of the agouti gene in transgenic mice promotes liver carcinogenesis in the absence of obesity and diabetes

Background

The agouti protein is a paracrine factor that is normally present in the skin of many species of mammals. Agouti regulates the switch between black and yellow hair pigmentation by signalling through the melanocortin 1 receptor (Mc1r) on melanocytes. Lethal yellow (A^y) and viable yellow (A^vy) are dominant regulatory mutations in the mouse agouti gene that cause the wild-type protein to be produced at abnormally high levels throughout the body. Mice harboring these mutations exhibit a pleiotropic syndrome characterized by yellow coat color, obesity, hyperglycemia, hyperinsulinemia, and increased susceptibility to hyperplasia and carcinogenesis in numerous tissues, including the liver. The goal of this research was to determine if ectopic expression of the agouti gene in the liver alone is sufficient to recapitulate any aspect of this syndrome. For this purpose, we generated lines of transgenic mice expressing high levels of agouti in the liver under the regulatory control of the albumin promoter. Expression levels of the agouti transgene in the liver were quantified by Northern blot analysis. Functional agouti protein in the liver of transgenic mice was assayed by its ability to inhibit binding of the α-melanocyte stimulating hormone (αMSH) to the Mc1r. Body weight, plasma insulin and blood glucose levels were analyzed in control and transgenic mice. Control and transgenic male mice were given a single intraperitoneal injection (10 mg/kg) of the hepatocellular carcinogen, diethylnitrosamine (DEN), at 15 days of age. Mice were euthanized at 36 or 40 weeks after DEN injection and the number of tumors per liver and total liver weights were recorded.

Results

The albumin-agouti transgene was expressed at high levels in the livers of mice and produced a functional agouti protein. Albumin-agouti transgenic mice had normal body weights and normal levels of blood glucose and plasma insulin, but responded to chemical initiation of the liver with an increased number of liver tumors compared to non-transgenic control mice.

Conclusions

The data demonstrate that liver-specific expression of the agouti gene is not sufficient to induce obesity or diabetes, but, in the absence of these factors, agouti continues to promote hepatocellular carcinogenesis.

Hypothalamic melanocortin system regulates sympathetic nerve activity in brown adipose tissue

To clarify the neuronal mechanism of the hypothalamic melanocortin system in regulating energy metabolism, we investigated the effects of centrally administered alpha-melanocyte-stimulating hormone (alpha-MSH) and agouti-related protein (AGRP), an agonist and an antagonist for the melanocortin 4 receptor (MC4-R), respectively, on the activity of sympathetic nerves innervating brown adipose tissue (BAT) and on BAT temperature. A bolus infusion of alpha-MSH (1 nmol) into the third cerebral ventricle (i3vt) significantly increased sympathetic nerve activity and elevated BAT temperature (P<0.05). The i3vt infusion of AGRP (1 nmol) gradually suppressed BAT sympathetic nerve activity and was accompanied by a significant reduction in BAT temperature (P<0.05). In conclusion, the hypothalamic melanocortin system may regulate peripheral energy expenditure, as well as thermogenesis, through its influence on BAT sympathetic nerve activity.

Dnmt1 expression in pre- and postimplantation embryogenesis and the maintenance of IAP silencing

The methylation of intracisternal A-type particle (IAP) sequences is maintained during mouse embryogenesis. Methylation suppresses IAP expression and the potential for mutagenesis by retrotransposition, but it is not clear how methylation of these elements is maintained during the embryonic stages when the bulk of the genome is being demethylated. It has been suggested that the high levels of DNA methyltransferase-1 (Dnmt1) present during cleavage could be important for keeping IAPs methylated. To test this hypothesis, we combined mutant alleles of Dnmt1 with an agouti allele (A(iapy)), which provided a coat color readout for the methylation status of the IAP insertion in the agouti locus. We found that reduction in Dnmt1 levels directly impacted methylation at this locus, leading to stable transcriptional activation of the agouti gene in the adult. Specifically, the short maternal Dnmt1 protein was important in maintaining methylation at the A(iapy) locus in cleavage embryos, whereas the longer Dnmt1 isoform found in somatic cells was important in maintaining IAP methylation during the postimplantation stage. These results underscore the importance of maintaining proper maintenance of methylation patterns during gestation and suggest that interference with this process may stably affect gene expression patterns in the adult and may have profound phenotypic consequences.

Retrotransposition of limited deletion type of intracisternal A-particle elements in the myeloid leukemia Clls of C3H/He mice

The murine genome has about 1,000 copies of DNA elements for the intracisternal A-particle (IAP) that resembles a retrovirus. We previously reported that the genomic DNA of the cells from radiation-induced acute myeloid leukemia (AML) lines derived from C3H/He inbred mice was frequently rearranged by the integration of the IAP element. In this study, 8 IAP elements from the characteristic integration sites in 6 cell lines of radiation-induced AML from different mice were characterized and compared in structure with 114 IAP elements isolated from the normal C3H/He genome. One of the 8 elements was a full-length type I IAP, and 7 were of type-I Delta 1 with a common deletion site. Although the type I Delta 1 form is a minor population accounting for about 6% of total genomic IAP elements, it is predominantly retrotransposed in the AML cells from different C3H/He mice. This indicates that limited populations of the IAP elements contribute to the unique retrotransposition in AML cells.

Retrotransposon-derived elements in the mammalian genome: a potential source of disease

The plethora of genomic information gathered by the sequencing of the human and mouse genomes has paved the way for a new era of genetics. While in the past we focused mainly on the small percentage of DNA that codes for proteins, we can now concentrate on the remainder, i.e. the noncoding sequences that interrupt and separate genes. This portion of the genome is made up, in most part, of repetitive DNA sequences including DNA transposons, long terminal repeat (LTR) retrotransposons, LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements). Some of these elements are transcriptionally active and can transpose or retrotranspose around the genome, resulting in insertional mutagenesis that can cause disease. In these cases, insertions have occurred in the coding sequence. However, recent evidence suggests that the main effect of these elements is their ability to influence transcription of neighbouring genes. The elements themselves contain promoters that can initiate transcription of flanking genomic DNA. Furthermore, they are susceptible to epigenetic silencing, which is often stochastic and incomplete, resulting in complex patterns of transcription. This review discusses some diseases in both human and mouse that are caused by these repetitive elements.

Recent developments in our understanding of melanocortin system in the regulation of food intake

Obesity has become one of the most significant public health problems facing the world today. However, the pathogenesis of obesity is multifactorial and involves the interaction of genetic and environmental factors. There is a pressing need to better understand the biochemical pathways that control energy intake and expenditure. In the last few years, a number of important signalling molecules have been identified that play important roles in obesity. One family of these molecules is the melanocortin system, which consists of several components: (1) melanocortin peptides; (2) the five seven-transmembrane G-protein coupled melanocortin receptors (MCRs); (3) the endogenous MCR antagonists, agouti and agouti-related protein; (4) the endogenous melanocortin mediators, mahogany, and syndecan. This system plays a key role in the central nervous system control of feeding behaviour and energy expenditure. This article will provide an overview of the anatomy, physiology, and molecular biology of the melanocortin system, and recent developments in our understanding of this system in obesity.

A natural allele of Nxf1 suppresses retrovirus insertional mutations

Endogenous retroviruses have shaped the evolution of mammalian genomes. Host genes that control the effects of retrovirus insertions are therefore of great interest. The modifier-of-vibrator-1 locus (Mvb1) controls levels of correctly processed mRNA from genes mutated by endogenous retrovirus insertions into introns, including the Pitpn(vb) tremor mutation and the Eya1(BOR) model of human branchiootorenal syndrome. Positional complementation cloning identifies Mvb1 as the nuclear export factor Nxf1, providing an unexpected link between the mRNA export receptor and pre-mRNA processing. Population structure of the suppressive allele in wild Mus musculus castaneus suggests selective advantage. A congenic Mvb1(CAST) allele is a useful tool for modifying gene expression from existing mutations and could be used to manipulate engineered mutations containing retroviral elements.

Increased beta-cell apoptosis prevents adaptive increase in beta-cell mass in mouse model of type 2 diabetes: evidence for role of islet amyloid formation rather than direct action of amyloid

Nondiabetic obese humans adapt to insulin resistance by increasing beta-cell mass. In contrast, obese humans with type 2 diabetes have an approximately 60% deficit in beta-cell mass. Recent studies in rodents reveal that beta-cell mass is regulated, increasing in response to insulin resistance through increased beta-cell supply (islet neogenesis and beta-cell replication) and/or decreased beta-cell loss (beta-cell apoptosis). Prospective studies of islet turnover are not possible in humans. In an attempt to establish the mechanism for the deficit in beta-cell mass in type 2 diabetes, we used an obese versus lean murine transgenic model for human islet amyloid polypeptide (IAPP) that develops islet pathology comparable to that in humans with type 2 diabetes. By 40 weeks of age, obese nontransgenic mice did not develop diabetes and adapted to insulin resistance by a 9-fold increase (P < 0.001) in beta-cell mass accomplished by a 1.7-fold increase in islet neogenesis (P < 0.05) and a 5-fold increase in beta-cell replication per islet (P < 0.001). Obese transgenic mice developed midlife diabetes with islet amyloid and an 80% (P < 0.001) deficit in beta-cell mass that was due to failure to adaptively increase beta-cell mass. The mechanism subserving this failed expansion was a 10-fold increase in beta-cell apoptosis (P < 0.001). There was no relationship between the extent of islet amyloid or the blood glucose concentration and the frequency of beta-cell apoptosis. However, the frequency of beta-cell apoptosis was related to the rate of increase of islet amyloid. These prospective studies suggest that the formation of islet amyloid rather than the islet amyloid per se is related to increased beta-cell apoptosis in this murine model of type 2 diabetes. This finding is consistent with the hypothesis that soluble IAPP oligomers but not islet amyloid are responsible for increased beta-cell apoptosis. The current studies also support the concept that replicating beta-cells are more vulnerable to apoptosis, possibly accounting for the failure of beta-cell mass to expand appropriately in response to obesity in type 2 diabetes.

Discordance between genetic and epigenetic defects in pseudohypoparathyroidism type 1b revealed by inconsistent loss of maternal imprinting at GNAS1

Although the molecular basis of pseudohypoparathyroidism type 1b (PHP type 1b) remains unknown, a defect in imprinting at the GNAS1 locus has been suggested by the consistent finding of paternal-specific patterns of DNA methylation on maternally inherited GNAS1 alleles. To characterize the relationship between the genetic and epigenetic defects in PHP type 1b, we analyzed allelic expression and methylation of CpG islands within exon 1A of GNAS1 in patients with sporadic PHP type 1b and in affected and unaffected individuals from five multigenerational kindreds with PHP type 1b. All subjects with resistance to parathyroid hormone (PTH) showed loss of methylation of the exon 1A region on the maternal GNAS1 allele and/or biallelic expression of exon 1A-containing transcripts, consistent with an imprinting defect. Paternal transmission of the disease-associated haplotype was associated with normal patterns of GNAS1 methylation and PTH responsiveness. We found that affected and unaffected siblings in one kindred had inherited the same GNAS1 allele from their affected mother, evidence for dissociation between the genetic and epigenetic GNAS1 defects. The absence of the epigenetic defect in subjects who have inherited a defective maternal GNAS1 allele suggests that the genetic mutation may be incompletely penetrant, and it indicates that the epigenetic defect, not the genetic mutation, leads to renal resistance to PTH in PHP type 1b.

Transposable elements: targets for early nutritional effects on epigenetic gene regulation

Early nutrition affects adult metabolism in humans and other mammals, potentially via persistent alterations in DNA methylation. With viable yellow agouti (A(vy)) mice, which harbor a transposable element in the agouti gene, we tested the hypothesis that the metastable methylation status of specific transposable element insertion sites renders them epigenetically labile to early methyl donor nutrition. Our results show that dietary methyl supplementation of a/a dams with extra folic acid, vitamin B(12), choline, and betaine alter the phenotype of their A(vy)/a offspring via increased CpG methylation at the A(vy) locus and that the epigenetic metastability which confers this lability is due to the A(vy) transposable element. These findings suggest that dietary supplementation, long presumed to be purely beneficial, may have unintended deleterious influences on the establishment of epigenetic gene regulation in humans.

Transplantable melanomas in gerbils (Meriones unguiculatus). II: melanogenesis

We characterized the melanogenic apparatus in a family of transplantable gerbil melanomas (melanotic and amelanotic) using a combination of biophysical, ultrastructural and biochemical methods. Melanotic melanomas produced pure eumelanin but in vesiculo-globular melanosomes ('pheomelanosomes'); the eumelanosomes, characteristically ellipsoidal in shape with fibrillar or fibrillo-lamelar matrix, were never noticed. Melanotic melanomas also had significant tyrosinase activity and Zn, Pb/S, Ca and P content; all higher than in the amelanotic variants. The amelanotic variant, which was devoid of melanin pigment and melanosomes, had clearly detectable tyrosinase activity (albeit at 20% of that in the melanotic variant). Thus, with these multidirectional approaches we demonstrate that pure eumelanin can be synthesized in organelles ultrastructurally defined as pheomelanosomes, but a defect in the formation of melanosomes can prevent in vivo melanin synthesis despite the presence of detectable tyrosinase activity. We conclude that this melanoma system provides an excellent experimental model for the study of molecular components determining pheo- and/or eumelanogenesis. The information generated can be used for defining the roles of melanogenesis and of tyrosinase expression in the regulation of melanoma behavior and the effect of their modification on the course of the disease.

Inverse agonism gains weight

Inverse agonism is emerging as a new endogenous principle for receptor regulation. Agouti-related protein (AgRP), following its release in the brain, stimulates food intake. AgRP binds to brain melanocortin receptors, which are involved in the regulation of body weight. In addition to antagonizing the effects of the melanocortin receptor agonist alpha-melanocyte-stimulating hormone (alpha-MSH), AgRP suppresses the constitutive activity of melanocortin MC(3) and MC(4) receptors, which characterizes AgRP as an inverse agonist rather than a neutral antagonist. The balance between the activity of AgRP-containing neurons and alpha-MSH-containing neurons determines the extent of activation of melanocortin receptors in neurons onto which they project. The identification of AgRP as an endogenous inverse agonist provides physiological relevance to inverse agonism in the control of body weight.

Neuronal histamine regulates food intake, adiposity, and uncoupling protein expression in agouti yellow (A(y)/a) obese mice

Hypothalamic neuronal histamine and its H(1) receptor (H(1)-R) form a part of the leptin-signaling pathway in the brain and have been shown to regulate body weight and adiposity in diabetic (db/db) and diet-induced obese mice by affecting food intake and uncoupling protein mRNA expression. The proopiomelanocortin (POMC) melanocortin-4 receptor (MC-4R) is also important for leptin signaling. The present study had two aims: first, to clarify the antiobesity action of neuronal histamine in agouti yellow (A(y)/a) obese mice, a model of obesity in which POMC/MC-4R signaling is disrupted by blockade of MC-4R and second, to investigate the functional relationship between neuronal histamine and POMC/MC-4R signaling. Central administration of histamine into the lateral cerebroventricle decreased cumulative food intake and body weight in A(y)/a obese mice. Histamine treatment also decreased mRNA expression of ob gene in epididymal white adipose tissue and up-regulated uncoupling protein 1 mRNA expression in brown adipose tissue. These effects were attenuated in A(y)/a obese mice with histamine H(1)-receptor (H(1)-R) knockout. Histamine treatment induced c-Fos-like immunoreactivity in both paraventricular and arcuate nucleus. There was no significant difference in histamine-induced c-Fos-like immunoreactivity in the hypothalamus between A(y)/a obese mice and lean littermates, indicating histamine signaling was not disrupted at the hypothalamic level in A(y)/a obese mice. These results suggest that neuronal histamine have an antiobese action, even in A(y)/a obese mice despite a deficiency in POMC/MC-4R signaling. In addition, it appears that the histamine H(1)-R signaling pathway may be independent or downstream of the POMC/MC-4R signaling.

Transgenerational inheritance of epigenetic states at the murine Axin(Fu) allele occurs after maternal and paternal transmission

Phenotypic variation that cannot be explained by genetic or environmental heterogeneity has intrigued geneticists for decades. The molecular basis of this phenomenon, however, is largely a mystery. Axin-fused (Axin(Fu)), first identified in 1937, is a classic example of a mammalian allele displaying extremely variable expression states. Here we demonstrate that the presence or absence of its characteristic phenotype, a kinked tail, correlates with differential DNA methylation at a retrotransposon within Axin(Fu) and identify mutant transcripts arising adjacent to the retrotransposon LTR that are likely to be causative of the phenotype. Furthermore, the epigenetic state at Axin(Fu) can be inherited transgenerationally after both maternal and paternal transmission. This is in contrast to epigenetic inheritance at the murine agouti-viable yellow (A(vy)) allele, which occurs through the female only. Unlike the egg, the sperm contributes very little (if any) cytoplasm to the zygote, and therefore paternal inheritance at Axin(Fu) argues against the possibility that the effects are due to cytoplasmic or metabolic influences. Consistent with the idea of transgenerational inheritance of epigenetic marks, we find that the methylation state of Axin(Fu) in mature sperm reflects the methylation state of the allele in the somatic tissue of the animal, suggesting that it does not undergo epigenetic reprogramming during gametogenesis. Finally, we show that epigenetic inheritance is influenced by strain background. These findings enable us to propose a model for transgenerational epigenetic inheritance in mammals.

Regulation of yellow pigment formation in mice: a historical perspective

Pigment synthesis by hair follicle melanocytes is modulated by a large number of environmental and genetic factors, many of which are discussed in this review. Eumelanic (non-yellow) pigment is produced by hair follicle melanocytes following the binding of alpha-melanocyte stimulating hormone to melanocortin receptor 1. Binding of this hormone to the melanocyte membrane is blocked by agouti signaling protein (ASP) which is encoded by the agouti locus and results in the synthesis of yellow pigment, instead of non-yellow (black/brown) pigment. The cyclical release of ASP by hair follicle cells results in a black/brown hair with a subapical yellow band. This is the wild-type coat color pattern of many mammals and is called agouti. Several dominant mutations at the agouti locus in mice, induced by retrotransposon-like intracisternal A particles, result in ectopic over-expression of ASP and animals with much higher proportions of all-yellow hairs. This abnormal presence of ASP in essentially all body cells results in the 'yellow agouti obese mouse syndrome.' The obesity has been associated with binding of ASP to melanocortin receptor 4 inactivating the latter. The syndrome also includes hyperinsulinemia, increased somatic growth, and increased susceptibility to hyperplasia and carcinogenesis. The physiologic and molecular bases for these syndrome components have not yet been elucidated. This historically orientated review is subdivided, where applicable, into pre- and post-1992 subsections to emphasize the impact of the cloning of the agouti and extension loci and their protein products on the identification of the molecular and physiological pathways modulating the manifold aspects of pheomelanogenesis.

Hague (Hag). A new mouse hair mutation with an unstable semidominant allele

A spontaneous mouse hair mutation was identified in a C3H/HeN colony. The mode of inheritance of the mutation was semidominant, with incomplete penetrance when heterozygous. The trait is controlled by a single locus hague (Hag), which was mapped to the telomeric region of chromosome 15. This mutation was shown to be unstable, since its transmission could be switched from semidominant to recessive. To identify the causative gene and the nature of the mutation, hague was introduced into a high-resolution and high-density molecular genetic map. Over 2000 meioses were analyzed and the mutation was mapped to the keratin 2 complex genes. A YAC and BAC physical map of the critical region was then constructed and the gene involved was located in a 600- to 800-kb-long segment. Fourteen genes were mapped to this region; of these, 11 were expressed in the skin (5 epidermic cytokeratin and 6 hard keratin genes), but none were mutated in hague mice.

Gene silencing in mammalian cells and the spread of DNA methylation

Aberrant gene silencing in mammalian cells is associated with promoter region methylation, but the sequence of these two events is not clear. This review will consider the possibility that gene silencing is not a single event, but instead a series of events that begins with a dramatic drop in transcription potential and ends with its complete cessation. This transition will be portrayed as a chaotic process that ensues when transcription levels drop and DNA methylation begins spreading haltingly towards the diminished promoter. According to this view, silencing is stabilized when the promoter region is 'captured' by the spread of DNA methylation near or into its transcription factor binding sites.

Maternal methyl supplements in mice affect epigenetic variation and DNA methylation of offspring

This study was designed to determine if maternal dietary methyl supplements increase DNA methylation and methylation-dependent epigenetic phenotypes in mammalian offspring. Female mice of two strains were fed two levels of dietary methyl supplement or control diet prior to and during pregnancy. Offspring of these mice vary in phenotype, which is epigenetically determined and affects health and 2-y survival. Phenotype and DNA methylation of a long terminal repeat (LTR) controlling expression of the agouti gene were assayed in the resulting offspring. Methyl supplements increase the level of DNA methylation in the agouti LTR and change the phenotype of offspring in the healthy, longer-lived direction. This shows that methyl supplements have strong effects on DNA methylation and phenotype and are likely to affect long-term health. Optimum dietary supplements for the health and longevity of offspring should be intensively investigated. This should lead to public policy guidance that teaches optimal, rather than minimal, dose levels of maternal supplements.

Metastable epialleles in mammals

There are some mammalian alleles that display the unusual characteristic of variable expressivity in the absence of genetic heterogeneity. It has recently become evident that this is because the activity of these alleles is dependent on their epigenetic state. Interestingly, the epigenetic state is somewhat labile, resulting in phenotypic mosaicism between cells (variegation) and also between individuals (variable expressivity). The establishment of the epigenetic state occurs during early embryogenesis and is a probabilistic event that is influenced by whether the allele is carried on the paternal or maternal alleles. In addition, the epigenetic state determines whether these alleles are dominant. We propose that mammalian alleles with such characteristics should be termed metastable epialleles to distinguish them from traditional alleles. At this stage, it is unclear how common these alleles are, but an appreciation of their existence will aid in their identification.

Hyperleptinemia in A(y)/a mice upregulates arcuate cocaine- and amphetamine-regulated transcript expression

The effects of leptin on cocaine- and amphetamine-regulated transcript (CART) and agouti-related protein (AGRP) expression in the hypothalamic arcuate nucleus of obese A(y)/a mice were investigated. CART mRNA expression was upregulated by 41% and AGRP mRNA downregulated by 78% in hyperleptinemic A(y)/a mice relative to levels in lean a/a mice. The mRNA expression of these neuropeptides in either young nonobese A(y)/a mice or rats treated with SHU-9119, a synthetic melanocortin-4 receptor (MC4R) antagonist, did not differ significantly from that in the corresponding controls. After a 72-h fast, which decreased the concentration of serum leptin, CART and AGRP mRNA expression decreased and increased, respectively, in A(y)/a mice. The expression levels of these neuropeptides in leptin-deficient A(y)/a ob/ob double mutants were comparable to those in a/a ob/ob mice. Leptin thus modulates both CART and AGRP mRNA expression in obese A(y)/a mice, whereas leptin signals are blocked at the MCR4R level. Taken together, the present findings indicate that differential expression of these neuropeptides in A(y)/a and ob/ob mice results in dissimilar progression toward obesity.