Imprinting defects in mouse embryos: stochastic errors or polymorphic phenotype?

Dynamics of response to asynapsis and meiotic silencing in spermatocytes from Robertsonian translocation carriers

Failure of homologous synapsis during meiotic prophase triggers transcriptional repression. Asynapsis of the X and Y chromosomes and their consequent silencing is essential for spermatogenesis. However, asynapsis of portions of autosomes in heterozygous translocation carriers may be detrimental for meiotic progression. In fact, a wide range of phenotypic outcomes from meiotic arrest to normal spermatogenesis have been described and the causes of such a variation remain elusive. To better understand the consequences of asynapsis in male carriers of Robertsonian translocations, we focused on the dynamics of recruitment of markers of asynapsis and meiotic silencing at unsynapsed autosomal trivalents in the spermatocytes of Robertsonian translocation carrier mice. Here we report that the enrichment of breast cancer 1 (BRCA1) and histone γH2AX at unsynapsed trivalents declines during the pachytene stage of meiosis and differs from that observed in the sex body. Furthermore, histone variant H3.3S31, which associates with the sex chromosomes in metaphase I/anaphase I spermatocytes, localizes to autosomes in 12% and 31% of nuclei from carriers of one and three translocations, respectively. These data suggest that the proportion of spermatocytes with markers of meiotic silencing of unsynapsed chromatin (MSUC) at trivalents depends on both, the stage of meiosis and the number of translocations. This may explain some of the variability in phenotypic outcomes associated with Robertsonian translocations. In addition our data suggest that the dynamics of response to asynapsis in Robertsonian translocations differs from the response to sex chromosomal asynapsis in the male germ line.

Somatic reactivation of expression of the silent maternal Mest allele and acquisition of normal reproductive behaviour in a colony of Peg1/Mest mutant mice

Genomic imprinting confers allele-specific expression in less than 1% of genes, in a parent-of-origin specific fashion. In humans and mice the Peg1/Mest gene (Mest) is maternally repressed, and paternally expressed. Mest is expressed in embryogenic mesoderm-derived tissues and in adult brain, and paternal mutations in Mest lead to growth retardation and defective maternal behaviour. Despite our current understanding of mechanisms associated with the establishment of imprinting of Mest and other imprinted genes, it is unclear to what extent Mest imprinting needs to be maintained in adult tissues. Aberrations of imprinting are known to occur in certain rare syndromes, and involve either inherited mutations, or constitutive epigenetic alterations occurring soon after fertilization. Imprinting abnormalities may also occur in the aging somatic tissues of adult individuals. Here we report an occurrence of post-embryonic somatic variability of Mest allelic expression in a colony of mice where heterozygotes at the imprinted Mest locus for a mutation inherited from the father spontaneously expressed the normally silenced allele from the mother. In addition, a newly acquired ability to overcome the deficit in maternal reproductive behaviour had occurred in the mutant mice, but this appeared not to be directly linked to the Mest mutation. Our results suggest that at least one allele of Mest expression is required in the somatic tissues of adult individuals and that under certain conditions (such as in the presence of a Mest insertional mutation or in an altered genetic background), somatically acquired alterations of allelic expression at the Mest locus may occur.

Domain-specific response of imprinted genes to reduced DNMT1

Imprinted genes are expressed in a monoallelic, parent-of-origin-specific manner. Clusters of imprinted genes are regulated by imprinting control regions (ICRs) characterized by DNA methylation of one allele. This methylation is critical for imprinting; a reduction in the DNA methyltransferase DNMT1 causes a widespread loss of imprinting. To better understand the role of DNA methylation in the regulation of imprinting, we characterized the effects of Dnmt1 mutations on the expression of a panel of imprinted genes in the embryo and placenta. We found striking differences among imprinted domains. The Igf2 and Peg3 domains showed imprinting perturbations with both null and partial loss-of-function mutations, and both domains had pairs of coordinately regulated genes with opposite responses to loss of DNMT1 function, suggesting these domains employ similar regulatory mechanisms. Genes in the Kcnq1 domain were less sensitive to the absence of DNMT1. Cdkn1c exhibited imprinting perturbations only in null mutants, while Kcnq1 and Ascl2 were largely unaffected by a loss of DNMT1 function. These results emphasize the critical role for DNA methylation in imprinting and reveal the different ways it controls gene expression.

Is it genomic imprinting or preferential expression?

Imprinted genes are monoallelically expressed in a parent-of-origin-specific manner, but for many genes reported to be imprinted, the occurrence of preferential expression--where both alleles are expressed but one is expressed more strongly than the other in a parent-of-origin-specific way--has been reported. This preferential expression found in genes described as imprinted has not been thoroughly addressed in genomic imprinting studies. To study this phenomenon, 50 genes, reported to be imprinted in the mouse, were chosen for investigation. Preferential expression was observed for 21 of 27 maternally expressed genes. However, only 5 of 23 paternally expressed genes showed preferential expression. Recently, it has been reported that a remarkable proportion of non-imprinted genes show differential allelic expression. If there is overlap between non-imprinted genes that are differentially expressed and imprinted genes that are preferentially expressed, we need to set new definitions of imprinted genes that, in turn, would probably lead to reassessments of the total number of imprinted genes in mammalian species.

Aberrant DNA methylation of imprinted loci in superovulated oocytes

BACKGROUND

There is an increased incidence of rare imprinting disorders associated with assisted reproduction technologies (ARTs). The sex-specific epigenetic modifications that are imposed during gametogenesis act as a primary imprint to distinguish maternal and paternal alleles. The most likely candidate for the gametic mark is DNA methylation. However, the timing of DNA methylation acquisition in adult oocytogenesis and the effects of superovulation are unknown.

METHODS

We examined the maternal methylation of PEG1(MEST), LIT1(KCNQ1OT1) and ZAC(PLAGL1) and the paternal methylation of H19 in adult growing oocytes of humans and mice and compared them with the methylation status of mouse neonatal growing oocytes by using bisulphite sequencing. Furthermore, we examined the effects of superovulation in the human and mouse.

RESULTS

Maternal methylation of these genes has already been initiated to some extent in adult human and mouse non-growing oocytes but not in mouse neonates. In addition, the methylation dynamics during adult human and mouse oocyte development changed more gradually than those during neonatal oocyte development. Furthermore, we found the demethylation of PEG1 in growing oocytes from some ART-treated infertile women and a gain in the methylation of H19. We also detected methylation changes in superovulated mice.

CONCLUSION

Our studies in the human and mouse suggest that superovulation can lead to the production of oocytes without their correct primary imprint and highlight the need for more research into ARTs.

Additional evidence for the genomic imprinting model of sex determination in the haplodiploid wasp Nasonia vitripennis: isolation of biparental diploid males after X-ray mutagenesis

The primary sex-determining signal in the haplodiploid wasp Nasonia vitripennis is not known. In haplodiploid reproduction, unfertilized eggs typically develop into uniparental haploid males and fertilized eggs into biparental diploid females. Although this reproductive strategy is common to all Hymenoptera, sex-determination is not strictly specified by the number of genome copies inherited. Furthermore, primary sex-determining signals differ among haplodiploid species. In the honeybee, for example, the primary signal is the genotype at a single, polymorphic locus: diploid animals that are homozygous develop into males while heterozygotes develop into females. Sex determination in Nasonia cannot be explained by this mechanism. Various lines of evidence show that the inheritance of a paternal genome is required for female sexual development and suggest a genomic imprinting mechanism involving an imprinted gene, expressed only from a paternal copy, that triggers female sexual development. In this model, haploid or diploid uniparental embryos develop into males due to a maternal imprint that silences this locus. The genomic imprinting model predicts that a loss-of-function mutation in the paternal copy of the imprinted gene would result in male sexual development in a biparental diploid embryo. In support of this model, we have identified rare biparental diploid males in the F1 progeny of X-ray mutagenized haploid males. Although uniparental diploid male progeny of virgin triploid females have been previously described, this is the first report of biparental diploid males in Nasonia. Our work provides a new, independent line of evidence for the genomic imprinting model of Nasonia sex determination.

Widespread disruption of genomic imprinting in adult interspecies mouse (Mus) hybrids

Mammalian interspecies hybrids exhibit parent-of-origin effects in that offspring of reciprocal matings, even though genetically identical, frequently exhibit opposite phenotypes, especially in growth. This was also observed in hybridization with the genus Mus. These parent-of-origin effects suggested that imbalance in the expression of imprinted genes, which are expressed differentially, depending on their transmission through the maternal or paternal germline, and/or differential loss-of-imprinting (LOI) could underlie these opposite growth phenotypes in reciprocal mammalian hybrids. Here we report that tissue-specific LOI occurs in adult Mus hybrids. Contrary to expectations, LOI patterns were not consistent with a direct influence of altered expression levels of imprinted genes on growth. Bisulfite sequencing revealed that reactivation of maternal alleles of Peg3 and Snrpn in specific tissues was accompanied by partial demethylation at their potential imprinting control regions. We propose that abnormal reprogramming after fertilization and during preimplantation development is in part responsible for hybrid dysgenesis, for which a strong epigenetic basis has been demonstrated.

Methylation of DNA — One of the Major Epigenetic Markers

Regulation of gene expression is a complex process. It includes a great number of steps from control of mRNA synthesis to posttranslational modification of proteins. Epigenetic events play essential roles in regulation of transcription. In this review, we concentrate on methylation of DNA as one of the important epigenetic marks. It is well known that DNA methylation is associated with closed chromatin state and, therefore, repressed, inactive genes. Here we describe major processes that depend on DNA methylation: imprinting, X-inactivation, and oncogenesis. Also we describe a number of known methyl-DNA-binding proteins and links between methylation of DNA and higher-order chromatin structure.

Intralocus sexual conflict can drive the evolution of genomic imprinting

Genomic imprinting is a phenomenon whereby the expression of an allele differs depending upon its parent of origin. There is an increasing number of examples of this form of epigenetic inheritance across a wide range of taxa, and imprinting errors have also been implicated in several human diseases. Various hypotheses have been put forward to explain the evolution of genomic imprinting, but there is not yet a widely accepted general hypothesis for the variety of imprinting patterns observed. Here a new evolutionary hypothesis, based on intralocus sexual conflict, is proposed. This hypothesis provides a potential explanation for much of the currently available empirical data, and it also makes new predictions about patterns of genomic imprinting that are expected to evolve but that have not, as of yet, been looked for in nature. This theory also provides a potential mechanism for the resolution of intralocus sexual conflict in sexually selected traits and a novel pathway for the evolution of sexual dimorphism.

The individuality of mice

Mutant mice simulating human CNS disorders are used as models for therapeutic drug development. Drug evaluation requires a coherent correlation between behavioral phenotype and drug status. Variations in behavioral responses could mask such correlations, a problem highlighted by the three-site studies of Crabbe et al. (1999) and Wahlsten et al. (2003a). Factors contributing to variation are considered, focusing on differences between individual animals. Genetic differences due to minisatellite variation suggest that each mouse is genetically distinct. Effects during gestation, including maternal stress, influence later life behavior; while endocrine exchanges between fetus and parent, and between male and female fetuses dependent on intrauterine position, also contribute. Pre and perinatal nutrition and maternal attention also play a role. In adults, endocrine cyclicity in females is a recognized source of behavioral diversity. Notably, there is increasing recognition that groups of wild and laboratory mice have complex social structures, illustrated through consideration of Crowcroft (1966). Dominance status can markedly modify behavior in test paradigms addressing anxiety, locomotion and aggressiveness, to an extent comparable to mutation or drug status. Understanding how such effects amplify the behavioral spectrum displayed by otherwise identical animals will improve testing.

Epigenetic reprogramming in mammalian nuclear transfer

With the exception of lymphocytes, the various cell types in a higher multicellular organism have basically an identical genotype but are functionally and morphologically different. This is due to tissue-specific, temporal, and spatial gene expression patterns which are controlled by genetic and epigenetic mechanisms. Successful cloning of mammals by transfer of nuclei from differentiated tissues into enucleated oocytes demonstrates that these genetic and epigenetic programs can be largely reversed and that cellular totipotency can be restored. Although these experiments indicate an enormous plasticity of nuclei from differentiated tissues, somatic cloning is a rather inefficient and unpredictable process, and a plethora of anomalies have been described in cloned embryos, fetuses, and offspring. Accumulating evidence indicates that incomplete or inappropriate epigenetic reprogramming of donor nuclei is likely to be the primary cause of failures in nuclear transfer. In this review, we discuss the roles of various epigenetic mechanisms, including DNA methylation, chromatin remodeling, imprinting, X chromosome inactivation, telomere maintenance, and epigenetic inheritance in normal embryonic development and in the observed abnormalities in clones from different species. Nuclear transfer represents an invaluable tool to experimentally address fundamental questions related to epigenetic reprogramming. Understanding the dynamics and mechanisms underlying epigenetic control will help us solve problems inherent in nuclear transfer technology and enable many applications, including the modulation of cellular plasticity for human cell therapies.