Two ENU-induced mutations in Rasgrf1 and early mouse growth retardation

Viable offspring derived from single unfertilized mammalian oocytes

In mammals, parthenogenesis is limited because of problems arising from genomic imprinting. Here, we report live mammalian offspring derived from single unfertilized eggs. This was achieved by the targeted DNA methylation rewriting of seven imprinting control regions. By designing guide RNAs with protospacer adjacent motif (PAM) sequences matching one allele but not the other, dCas9-Dnmt3a or dCpf1-Tet1 enables targeted DNA methylation editing in an allele-specific manner. The success of parthenogenesis in mammals opens many opportunities in agriculture, research, and medicine.

In mammals, a new life starts with the fusion of an oocyte and a sperm cell. Parthenogenesis, a way of generating offspring solely from female gametes, is limited because of problems arising from genomic imprinting. Here, we report live mammalian offspring derived from single unfertilized oocytes, which was achieved by targeted DNA methylation rewriting of seven imprinting control regions. Oocyte coinjection of catalytically inactive Cas9 (dCas9)-Dnmt3a or dCpf1-Tet1 messenger RNA (mRNA) with single-guide RNAs (sgRNAs) targeting specific regions induced de novo methylation or demethylation, respectively, of the targeted region. Following parthenogenetic activation, these edited regions showed maintenance of methylation as naturally established regions during early preimplantation development. The transfer of modified parthenogenetic embryos into foster mothers resulted in significantly extended development and finally in the generation of viable full-term offspring. These data demonstrate that parthenogenesis can be achieved by targeted epigenetic rewriting of multiple critical imprinting control regions.

Impaired contextual fear conditioning in RasGRF2 mutant mice is likely Ras-ERK-dependent

Ras/Raf/MEK/ERK (Ras-ERK) signaling has been shown to play an important role in fear acquisition. However, little information is known regarding the mechanisms that contribute to the regulation of this pathway in terms of the learning of conditioned fears. Ras Guanine Nucleotide Releasing Factor 2 (RasGRF2) is one of two guanine nucleotide exchange factors (GEF) that regulates the Ras-ERK signaling pathway in a Ca²⁺-dependent manner via control of the cycling of Ras isoforms between an inactive and active state. Here we sought to determine the role of RasGRF2 on contextual fear conditioning in RasGRF2 knockout (KO) and their wild type (WT) counterparts. Male KO and WT mice underwent a single session of contextual fear conditioning (12 min, 4 unsignaled shocks), followed by either daily 12-min retention trials or the molecular analysis of Ras activation and pERK1/2 activity. KO mice showed an impaired acquisition of contextual fear, as demonstrated by reduced freezing during fear conditioning and 24-hr retention tests relative to WT mice. Ras analysis following fear conditioning demonstrated a reduction in Ras activation in the hippocampus as well as a reduction in pERK1/2 in the CA1 region of the hippocampus in KO mice, suggesting that the decrease in fear conditioning in KO mice is at least in part due to the impairment of Ras-ERK signaling in the hippocampus during learning. These data indicate a role for RasGRF2 in contextual fear conditioning in mice that may be Ras-ERK-dependent.

Genomic imprinting and its effects on postnatal growth and adult metabolism

Imprinted genes display parent-of-origin-specific expression with this epigenetic system of regulation found exclusively in therian mammals. Historically, defined imprinted gene functions were almost solely focused on pregnancy and the influence on the growth parameters of the developing embryo and placenta. More recently, a number of postnatal functions have been identified which converge on resource allocation, both for animals in the nest and in adults. While many of the prenatal functions of imprinted genes that have so far been described adhere to the "parental conflict" hypothesis, no clear picture has yet emerged on the functional role of imprints on postnatal metabolism. As these roles are uncovered, interest in the potential for these genes to influence postnatal metabolism and associated adult-onset disease outcomes when dysregulated has gathered pace. Here, we review the published data on imprinted genes and their influence on postnatal metabolism, starting in the nest, and then progressing through to adulthood. When observing the functional effects of these genes on adult metabolism, we must always be careful to acknowledge the influence both of direct expression in the relevant metabolic tissue, but also indirect metabolic programming effects caused by their modulation of both in utero and postnatal growth trajectories.

DNA methylation analysis and editing in single mammalian oocytes

Mammalian oocytes carry specific nongenetic information, including DNA methylation to the next generation, which is important for development and disease. However, evaluation and manipulation of specific methylation for both functional analysis and therapeutic purposes remains challenging. Here, we demonstrate evaluation of specific methylation in single oocytes from its sibling first polar body (PB1) and manipulation of specific methylation in single oocytes by microinjection-mediated dCas9-based targeted methylation editing. We optimized a single-cell bisulfite sequencing approach with high efficiency and demonstrate that the PB1 carries similar methylation profiles at specific regions to its sibling oocyte. By bisulfite sequencing of a single PB1, the methylation information regarding agouti viable yellow (A ^vy )-related coat color, as well as imprinting linked parthenogenetic development competency, in a single oocyte can be efficiently evaluated. Microinjection-based dCas9-Tet/Dnmt-mediated methylation editing allows targeted manipulation of specific methylation in single oocytes. By targeted methylation editing, we were able to reverse A ^vy -related coat color, generate full-term development of bimaternal mice, and correct familial Angelman syndrome in a mouse model. Our work will facilitate the investigation of specific methylation events in oocytes and provides a strategy for prevention and correction of maternally transmitted nongenetic disease or disorders.

The role of genomic imprinting in biology and disease: an expanding view

Genomic imprinting is an epigenetic phenomenon that results in monoallelic gene expression according to parental origin. It has long been established that imprinted genes have major effects on development and placental biology before birth. More recently, it has become evident that imprinted genes also have important roles after birth. In this Review, I bring together studies of the effects of imprinted genes from the prenatal period onwards. Recent work on postnatal stages shows that imprinted genes influence an extraordinarily wide-ranging array of biological processes, the effects of which extend into adulthood, and play important parts in common diseases that range from obesity to psychiatric disorders.

Perturbations to the IGF1 growth pathway and adult energy homeostasis following disruption of mouse chromosome 12 imprinting

Aim

Disruption to insulin-like growth factor (IGF) signalling pathways during early life causes growth retardation and defects of developing metabolic organs that can alter set points of energy homeostasis for a lifetime. Inheritance of two maternal copies of human chromosome 14q32.2 (Temple syndrome) causes severe foetal growth retardation and post-natal failure to thrive. Disruption of imprinted gene dosage in the orthologous region on mouse chromosome 12 also affects growth. Here, we investigated whether altering chromosome 12-imprinted gene dosage can affect IGF signalling.

Methods

We investigated mice with a transgene insertion at the imprinted domain of chromosome 12. This lesion causes misexpression of neighbouring genes such that the expression of non-coding RNAs is elevated, and levels of delta-like homologue 1 (Dlk1), retrotransposon-like 1 (Rtl1) and deiodinase 3 (Dio3) transcripts are reduced.

Results

We observed three key phenotypes in these mice: (i) embryonic growth retardation associated with altered expression of IGF1 binding proteins, (ii) peri-natal failure to thrive accompanied by hypothyroidism and low serum IGF1. Unexpectedly this phenotype was growth hormone independent. (iii) Adult animals had reduced glucose tolerance as a result of endocrine pancreatic insufficiency.

Conclusions

We propose that all of these phenotypes are attributable to impaired IGF action and show for the first time that the chromosome 12 cluster in the mouse is an imprinted locus that modulates the IGF signalling pathway. We propose that growth retardation observed in human Temple syndrome might have a similar cause.

Behavioral methods for the study of the Ras-ERK pathway in memory formation and consolidation: passive avoidance and novel object recognition tests

Memory is a high-level brain function that enables organisms to adapt their behavioral responses to the environment, hence increasing their probability of survival. The Ras-ERK pathway is a key molecular intracellular signalling cascade for memory consolidation. In this chapter we will describe two main one-trial behavioral tests commonly used in the field of memory research in order to assess the role of Ras-ERK signalling in long-term memory: passive avoidance and object recognition. Passive avoidance (PA) is a fear-motivated instrumental learning task, designed by Jarvik and Essman in 1960, in which animals learn to refrain from emitting a behavioral response that has previously been associated with a punishment. We will describe here the detailed protocol and show some examples of how PA can reveal impairments or enhancements in memory consolidation following loss or gain of function genetic manipulations of the Ras-ERK pathway. The phenotypes of global mutants as Ras-GRF1 KO, GENA53, and ERK1 KO mice, as well as of conditional region-specific mutants (striatal K-CREB mice), will be illustrated as examples. Novel object recognition (NOR), developed by Ennaceur and Delacour in 1988, is instead a more recent and highly ecological test, which relies on the natural tendency of rodents to spontaneously approach and explore novel objects, representing hence a useful non-stressful tool for the study of memory in animals without the employment of punishments or starvation/water restriction regimens. Careful indications will be given on how to select the positions for the novel object, in order to counterbalance for individual side preferences among mice during the training. Finally, the methods for calculating two learning indexes will be described. In addition to the classical discrimination index (DI) that measures the ability of an animal to discriminate between two different objects which are presented at the same time, we will describe the formula of a new index that we present here for the first time, the recognition index (RI), which quantifies the ability of an animal to recognize a same object at different time points and that, by taking into account the basal individual preferences displayed during the training, can give a more accurate measure of an animal's actual recognition memory.

Rho guanine nucleotide exchange factors: regulators of Rho GTPase activity in development and disease

The aberrant activity of Ras homologous (Rho) family small GTPases (20 human members) has been implicated in cancer and other human diseases. However, in contrast to the direct mutational activation of Ras found in cancer and developmental disorders, Rho GTPases are activated most commonly in disease by indirect mechanisms. One prevalent mechanism involves aberrant Rho activation via the deregulated expression and/or activity of Rho family guanine nucleotide exchange factors (RhoGEFs). RhoGEFs promote formation of the active GTP-bound state of Rho GTPases. The largest family of RhoGEFs is comprised of the Dbl family RhoGEFs with 70 human members. The multitude of RhoGEFs that activate a single Rho GTPase reflects the very specific role of each RhoGEF in controlling distinct signaling mechanisms involved in Rho activation. In this review, we summarize the role of Dbl RhoGEFs in development and disease, with a focus on Ect2 (epithelial cell transforming squence 2), Tiam1 (T-cell lymphoma invasion and metastasis 1), Vav and P-Rex1/2 (PtdIns(3,4,5)P3 (phosphatidylinositol (3,4,5)-triphosphate)-dependent Rac exchanger).

Imprinted genes in mouse placental development and the regulation of fetal energy stores

Imprinted genes, which are preferentially expressed from one or other parental chromosome as a consequence of epigenetic events in the germline, are known to functionally converge on biological processes that enable in utero development in mammals. Over 100 imprinted genes have been identified in the mouse, the majority of which are both expressed and imprinted in the placenta. The purpose of this review is to provide a summary of the current knowledge regarding imprinted gene function in the mouse placenta. Few imprinted genes have been assessed with respect to their dosage-related action in the placenta. Nonetheless, current data indicate that imprinted genes converge on two key functions of the placenta, nutrient transport and placental signalling. Murine studies may provide a greater understanding of certain human pathologies, including low birth weight and the programming of metabolic diseases in the adult, and complications of pregnancy, such as pre-eclampsia and gestational diabetes, resulting from fetuses carrying abnormal imprints.

A catalogue of validated single nucleotide polymorphisms in bovine orthologs of mammalian imprinted genes and associations with beef production traits

Genetic (or 'genomic') imprinting, a feature of approximately 100 mammalian genes, results in monoallelic expression from one of the two parentally inherited chromosomes. To date, most studies have been directed on imprinted genes in murine or human models; however, there is burgeoning interest in the effects of imprinted genes in domestic livestock species. In particular, attention has focused on imprinted genes that influence foetal growth and development and that are associated with several economically important production traits in cattle, sheep and pigs. We have re-sequenced regions in 20 candidate bovine imprinted genes in order to validate single nucleotide polymorphisms (SNPs) that may influence important production traits in cattle. Putative SNPs detected via re-sequencing were subsequently re-formatted for high-throughput SNP genotyping in 185 cattle samples comprising 138 performance-tested European Bos taurus (all Limousin bulls), 29 African B. taurus and 18 Indian B. indicus samples. Analysis of the resulting genotypic data identified 117 validated SNPs. Preliminary genotype-phenotype association analyses using 83 SNPs that were polymorphic in the Limousin samples with minor allele frequencies ⩾0.05 revealed significant associations between two candidate bovine imprinted genes and a range of important beef production traits: average daily gain, average feed intake, live weight, feed conversion ratio, residual feed intake and residual gain. These genes were the Ras protein-specific guanine nucleotide releasing factor gene (RASGRF1) and the zinc finger, imprinted 2 gene (ZIM2). Despite the relatively small sample size used in these analyses, the observed associations with production traits are supported by the purported biological function of the RASGRF1 and ZIM2 gene products. These results support the hypothesis that imprinted genes contribute significantly to important complex production traits in cattle. Furthermore, these SNPs may be usefully incorporated into future marker-assisted and genomic selection breeding schemes.

Mice Lacking Ras-GRF1 Show Contextual Fear Conditioning but not Spatial Memory Impairments: Convergent Evidence from Two Independently Generated Mouse Mutant Lines

Ras-GRF1 is a neuronal specific guanine exchange factor that, once activated by both ionotropic and metabotropic neurotransmitter receptors, can stimulate Ras proteins, leading to long-term phosphorylation of downstream signaling. The two available reports on the behavior of two independently generated Ras-GRF1 deficient mouse lines provide contrasting evidence on the role of Ras-GRF1 in spatial memory and contextual fear conditioning. These discrepancies may be due to the distinct alterations introduced in the mouse genome by gene targeting in the two lines that could differentially affect expression of nearby genes located in the imprinted region containing the Ras-grf1 locus. In order to determine the real contribution of Ras-GRF1 to spatial memory we compared in Morris Water Maze learning Brambilla’s mice with a third mouse line (GENA53) in which a non-sense mutation was introduced in the Ras-GRF1 coding region without additional changes in the genome and we found that memory in this task is normal. Also, we measured both contextual and cued fear conditioning, which were previously reported to be affected in Brambilla’s mice, and we confirmed that contextual learning but not cued conditioning is impaired in both mouse lines. In addition, we also tested both lines for the first time in conditioned place aversion in the Intellicage, an ecological and remotely controlled behavioral test, and we observed normal learning. Finally, based on previous reports of other mutant lines suggesting that Ras-GRF1 may control body weight, we also measured this non-cognitive phenotype and we confirmed that both Ras-GRF1 deficient mutants are smaller than their control littermates. In conclusion, we demonstrate that Ras-GRF1 has no unique role in spatial memory while its function in contextual fear conditioning is likely to be due not only to its involvement in amygdala functions but possibly to some distinct hippocampal connections specific to contextual learning.

Regulation of Neuronal Function by Ras-GRF Exchange Factors

Ras-GRF1 (GRF1) and Ras-GRF2 (GRF2) constitute a family of guanine nucleotide exchange factors (GEFs). The main isoforms, p140-GRF1 and p135-GRF2, have 2 GEF domains that give them the capacity to activate both Ras and Rac GTPases in response to signals from a variety of neurotransmitter receptors. GRF1 and GRF2 proteins are found predominantly in adult neurons of the central nervous system, although they can also be detected in a limited number of other tissues. p140-GRF1 and p135-GRF2 contain calcium/calmodulin-binding IQ domains that allow them to act as calcium sensors to mediate the actions of NMDA-type and calcium-permeable AMPA-type glutamate receptors. p140-GRF1 also mediates the action of dopamine receptors that signal through cAMP. Although p140-GRF1 and p135-GRF2 have similar functional domains, studies of GRF knockout mice show that they can play strikingly different roles in regulating MAP kinase family members, neuronal synaptic plasticity, specific forms of learning and memory, and behavioral responses to psychoactive drugs. In addition, the function of GRF proteins may vary in different regions of the brain. Alternative splice variants yielding smaller GRF1 gene isoforms with fewer functional domains also exist; however, their distinct roles in neurons have not been revealed. Continuing studies of these proteins should yield important insights into the biochemical basis of brain function as well as novel concepts to explain how complex signal transduction proteins, like Ras-GRFs, integrate multiple upstream signals into specific downstream outputs to control brain function.

Imprinted Rasgrf1 expression in neonatal mice affects olfactory learning and memory

Rasgrf1 is genomically imprinted; only the paternally inherited allele is expressed in the neonatal mouse brain until weaning, at which time expression becomes biallelic. Whereas Rasgrf1 has been implicated in learning and memory via knockout studies in adult mice, the effect of its normal imprinted expression on these phenotypes has not yet been examined. Neonatal mice with experimentally manipulated patterns of imprinted Rasgrf1 expression were assessed on an associative olfactory task. Neonates lacking the normally expressed wild-type paternal allele exhibited significant impairment in olfactory associative memory. Adult animals in which neonatal imprinting had been manipulated were also behaviorally assessed; while neonatal imprinting significantly affects body weight even into adulthood, no learning and memory phenotype attributable to imprinting was observed in adults. Additional analyses of neonates showed imprinted Rasgrf1 transcript selective to olfactory bulb even in mice that were null for Rasgrf1 in the rest of the brain and showed that Rasgrf1 affects Ras and Rac activation in the brain. Taken together, these results indicate that Rasgrf1 expression from the wild-type paternal allele contributes to learning and memory in neonatal mice.

The RasGrf family of mammalian guanine nucleotide exchange factors

RasGrf1 and RasGrf2 are highly homologous mammalian guanine nucleotide exchange factors which are able to activate specific Ras or Rho GTPases. The RasGrf genes are preferentially expressed in the central nervous system, although specific expression of either locus may also occur elsewhere. RasGrf1 is a paternally-expressed, imprinted gene that is expressed only after birth. In contrast, RasGrf2 is not imprinted and shows a wider expression pattern. A variety of isoforms for both genes are also detectable in different cellular contexts. The RasGrf proteins exhibit modular structures composed by multiple domains including CDC25H and DHPH motifs responsible for promoting GDP/GTP exchange, respectively, on Ras or Rho GTPase targets. The various domains are essential to define their intrinsic exchanger activity and to modulate the specificity of their functional activity so as to connect different upstream signals to various downstream targets and cellular responses. Despite their homology, RasGrf1 and RasGrf2 display differing target specificities and non overlapping functional roles in a variety of signaling contexts related to cell growth and differentiation as well as neuronal excitability and response or synaptic plasticity. Whereas both RasGrfs are activatable by glutamate receptors, G-protein-coupled receptors or changes in intracellular calcium concentration, only RasGrf1 is reported to be activated by LPA, cAMP, or agonist-activated Trk and cannabinoid receptors. Analysis of various knockout mice strains has uncovered a specific functional contribution of RasGrf1 in processes of memory and learning, photoreception, control of post-natal growth and body size and pancreatic β-cell function and glucose homeostasis. For RasGrf2, specific roles in lymphocyte proliferation, T-cell signaling responses and lymphomagenesis have been described.

Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1) controls activation of extracellular signal-regulated kinase (ERK) signaling in the striatum and long-term behavioral responses to cocaine

BACKGROUND

Ras-extracellular signal-regulated kinase (Ras-ERK) signaling is central to the molecular machinery underlying cognitive functions. In the striatum, ERK1/2 kinases are co-activated by glutamate and dopamine D1/5 receptors, but the mechanisms providing such signaling integration are still unknown. The Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1), a neuronal specific activator of Ras-ERK signaling, is a likely candidate for coupling these neurotransmitter signals to ERK kinases in the striatonigral medium spiny neurons (MSN) and for modulating behavioral responses to drug abuse such as cocaine.

METHODS

We used genetically modified mouse mutants for Ras-GRF1 as a source of primary MSN cultures and organotypic slices, to perform both immunoblot and immunofluorescence studies in response to glutamate and dopamine receptor agonists. Mice were also subjected to behavioral and immunohistochemical investigations upon treatment with cocaine.

RESULTS

Phosphorylation of ERK1/2 in response to glutamate, dopamine D1 agonist, or both stimuli simultaneously is impaired in Ras-GRF1-deficient striatal cells and organotypic slices of the striatonigral MSN compartment. Consistently, behavioral responses to cocaine are also affected in mice deficient for Ras-GRF1 or overexpressing it. Both locomotor sensitization and conditioned place preference are significantly attenuated in Ras-GRF1-deficient mice, whereas a robust facilitation is observed in overexpressing transgenic animals. Finally, we found corresponding changes in ERK1/2 activation and in accumulation of FosB/DeltaFosB, a well-characterized marker for long-term responses to cocaine, in MSN from these animals.

CONCLUSIONS

These results strongly implicate Ras-GRF1 in the integration of the two main neurotransmitter inputs to the striatum and in the maladaptive modulation of striatal networks in response to cocaine.

Imprint switch mutations at Rasgrf1 support conflict hypothesis of imprinting and define a growth control mechanism upstream of IGF1

Rasgrf1 is imprinted and expressed preferentially from the paternal allele in neonatal mouse brain. At weaning, expression becomes biallelic. Using a mouse model, we assayed the effects of perturbing imprinted Rasgrf1 expression in mice with the following imprinted expression patterns: monoallelic paternal (wild type), monoallelic maternal (maternal only), biallelic (both alleles transcribed), and null (neither allele transcribed). All genotypes exhibit biallelic expression around weaning. Consequences of this transient imprinting perturbation are manifested as overall size differences that correspond to the amount of neonatal Rasgrf1 expressed and are persistent, extending into adulthood. Biallelic mice are the largest and overexpress Rasgrf1 relative to wild-type mice, null mice are the smallest and underexpress Rasgrf1 as neonates, and the two monoallelically expressing genotypes are intermediate and indistinguishable from one another, in both size and Rasgrf1 expression level. Importantly, these data support one of the key underlying assumptions of the "conflict hypothesis" that describes the evolution of genomic imprinting in mammals and supposes that equivalent amounts of imprinted gene expression produce equivalent phenotypes, regardless of which parental allele is transcribed. Concordant with the difference in overall body size, we identify differences in IGF-1 levels, both in serum protein and as liver transcript, and identify additional differential expression of components upstream of IGF-1 release in the GH/IGF-1 axis. These data suggest that imprinted Rasgrf1 expression affects GH/IGF-1 axis function, and that the consequences of Rasgrf1 inputs to this axis persist beyond the time period when expression is restricted via epigenetic mechanisms, suggesting that proper neonatal Rasgrf1 expression levels are critical for development.

Effects of ooplasm manipulation on DNA methylation and growth of progeny in mice

New techniques to boost male and female fertility are being pioneered at a rapid pace in fertility clinics to increase the efficiency of assisted reproduction methods in couples in which natural conception has not been achieved. This study investigates the possible epigenetic effects of ooplasm manipulation methods on postnatal growth and development using a mouse genetic model, with particular emphasis on the possible effects of intergenotype manipulations. We performed interstrain and control intrastrain maternal pronuclear transfers, metaphase-II spindle transfers, and ooplasm transfer between C57BL/6 and DBA/2 mice, and found no major, long-term growth defects or epigenetic abnormalities, in either males or females, associated with intergenotype transfers. Ooplasm transfer itself was associated with reduced viability, and additional subtle effects of ooplasm strain of origin were observed. Both inter- and intrastrain ooplasm transfer were associated with subtle, transient effects on growth early in life. We also performed inter- and intrastrain germinal vesicle transfers (GVTs). Interstrain GVT females, but not males, had significantly lower body weights at birth and thereafter compared with the intrastrain GVT and non-GVT controls. No GVT-associated changes were observed in DNA methylation of the Mup1, Rasgrf1, H19, Snrpn, or Peg3 genes, nor any difference in expression of the imprinted Rasgrf1, Igf2r, or Mest genes. These results indicate that some ooplasm manipulation procedures may exert subtle effects on growth early in life, while intergenotype GVT can result in significant growth deficiencies after birth.

What are imprinted genes doing in the brain?

As evidence for the existence of brain-expressed imprinted genes accumulates, we need to address exactly what they are doing in this tissue, especially in terms of organisational themes and the major challenges posed by reconciling imprinted gene action in brain with current evolutionary theories attempting to explain the origin and maintenance of genomic imprinting. We are at the beginning of this endeavor and much work remains to be done but already it is clear that imprinted genes have the potential to influence diverse behavioral processes via multiple brain mechanisms. There are also grounds to believe that imprinting may contribute to risk of mental and neurological disease. As well as being a source of basic information about imprinted genes in the brain (e.g., via the newly established website, www.bgg.cardiff.ac.uk/imprinted_tables/index. html), we have used this chapter to identify and focus on a number of key questions. How are brain-expressed imprinted genes organised at the molecular and cellular levels? To what extent does imprinted action depend on neurodevelopmental mechanisms? Do imprinted gene effects interact with other epigenetic influences, especially early on in life? Are imprinted effects on adult behaviors adaptive or just epiphenomena? If they are adaptive, what areas of brain function and behavior might be sensitive to imprinted effects? These are big questions and, as shall become apparent, we need much more data, arising from interactions between behavioral neuroscientists, molecular biologists and evolutionary theorists, if we are to begin to answer them.

High-frequency generation of viable mice from engineered bi-maternal embryos

Mammalian development to adulthood typically requires both maternal and paternal genomes, because genomic imprinting places stringent limitations on mammalian development, strictly precluding parthenogenesis. Here we report the generation of bi-maternal embryos that develop at a high success rate equivalent to the rate obtained with in vitro fertilization of normal embryos. These bi-maternal mice developed into viable and fertile female adults. The bi-maternal embryos, distinct from parthenogenetic or gynogenetic conceptuses, were produced by the construction of oocytes from fully grown oocytes and nongrowing oocytes that contain double deletions in the H19 differentially methylated region (DMR) and the Dlk1-Dio3 intergenic germline-derived DMR. The results provide conclusive evidence that imprinted genes regulated by these two paternally methylated imprinting-control regions are the only paternal barrier that prevents the normal development of bi-maternal mouse fetuses to term.

Spectrum of ENU-induced mutations in phenotype-driven and gene-driven screens in the mouse

N-ethyl-N-nitrosourea (ENU) mutagenesis in mice has become a standard tool for (i) increasing the pool of mutants in many areas of biology, (ii) identifying novel genes involved in physiological processes and disease, and (iii) in assisting in assigning functions to genes. ENU is assumed to cause random mutations throughout the mouse genome, but this presumption has never been analyzed. This is a crucial point, especially for large-scale mutagenesis, as a bias would reflect a constraint on identifying possible genetic targets. There is a significant body of published data now available from both phenotype-driven and gene-driven ENU mutagenesis screens in the mouse that can be used to reveal the effectiveness and limitations of an ENU mutagenesis approach. Analysis of the published data is presented in this paper. As expected for a randomly acting mutagen, ENU-induced mutations identified in phenotype-driven screens were in genes that had higher coding sequence length and higher exon number than the average for the mouse genome. Unexpectedly, the data showed that ENU-induced mutations were more likely to be found in genes that had a higher G + C content and neighboring base analysis revealed that the identified ENU mutations were more often directly flanked by G or C nucleotides. ENU mutations from phenotype-driven and gene-driven screens were dominantly A:T to T:A transversions or A:T to G:C transitions. Knowledge of the spectrum of mutations that ENU elicits will assist in positional cloning of ENU-induced mutations by allowing prioritization of candidate genes based on some of their inherent features.

Genomic imprinting, growth control and the allocation of nutritional resources: consequences for postnatal life

PURPOSE OF REVIEW

Genes subject to genomic imprinting are predominantly expressed from one of the two parental chromosomes, are often clustered in the genome, and their activity and repression are epigenetically regulated. The role of imprinted genes in growth control has been apparent since the discovery of imprinting in the early 1980s.

RECENT FINDINGS

Drawing from studies in the mouse, we propose three distinct classes of imprinted genes - those expressed, imprinted and acting predominantly within the placenta, those with no associated foetal growth effects that act postnatally to regulate metabolic processes, and those expressed in the embryo and placenta that programme the development of organs participating in metabolic processes. Members of this latter class may interact in functional networks regulating the interaction between the mother and the foetus, affecting generalized foetal well-being, growth and organ development; they may also coordinately regulate the development of particular organ systems.

SUMMARY

The mono-allelic behaviour and sensitivity to changes in regional epigenetic states renders imprinted genes adaptable and vulnerable; in all cases, their perturbed dosage can compromise prenatal and/or postnatal control of nutritional resources. This finding has implications for understanding the relationships between prenatal events and diseases later in life.

Interactions between imprinting effects: summary and review

Mice with uniparental disomies (uniparental duplications) for defined regions of certain chromosomes, or certain disomies, show a range of developmental abnormalities most of which affect growth. These defects can be attributed to incorrect dosages of maternal or paternal copies of imprinted genes lying within the regions involved. Combinations of certain partial disomies result in interactions between the imprinting effects that seemingly independently affect foetal and/or placental growth in different ways or modify neonatal and postnatal development. The findings are generally in accord with the 'conflict hypothesis' for the evolution of genomic imprinting but do not demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that favours embryonic/foetal development, at consequent cost to the mother, will have been subject to evolutionary selection for only paternal allele expression. Reciprocally, any gene that reduces embryonic/foetal growth to limit disadvantage to the mother will have been selected for only maternal allele expression. It is concluded that survival of the placenta is core to the evolution of imprinting.

Interactions between imprinting effects in the mouse

Mice with uniparental partial or complete disomies for any one of 11 identified chromosomes show abnormal phenotypes. The abnormalities, or imprinting effects, can be attributable to an incorrect dosage of maternal or paternal copies of imprinted gene(s) located within the regions involved. Here we show that combinations of partial disomies may result in interactions between imprinting effects that seemingly independently affect fetal and/or placental growth in different ways or modify neonatal and postnatal imprinting effects. Candidate genes within the regions have been identified. The findings are generally in accord with the "conflict hypothesis" for the evolution of genomic imprinting but do not clearly demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that represses or limits embryonic/fetal growth to the advantage of the mother--by any developmental means--will have been subject to evolutionary selection for paternal allele repression. Likewise, any gene that favors embryonic/fetal development at consequent cost to the mother--by any developmental means--will have faced selection for maternal allele repression. The classical Igf2-Igf2r axis may therefore be unique. The findings involve reinterpretation of older imprinting data and consequently revision of the mouse imprinting map.