Gametic imprinting and the manifestation of the fused gene in the house mouse

Strain-Specific Epigenetic Regulation of Endogenous Retroviruses: The Role of Trans-Acting Modifiers

Approximately 10 percent of the mouse genome consists of endogenous retroviruses (ERVs), relics of ancient retroviral infections that are classified based on their relatedness to exogenous retroviral genera. Because of the ability of ERVs to retrotranspose, as well as their cis-acting regulatory potential due to functional elements located within the elements, mammalian ERVs are generally subject to epigenetic silencing by DNA methylation and repressive histone modifications. The mobilisation and expansion of ERV elements is strain-specific, leading to ERVs being highly polymorphic between inbred mouse strains, hinting at the possibility of the strain-specific regulation of ERVs. In this review, we describe the existing evidence of mouse strain-specific epigenetic control of ERVs and discuss the implications of differential ERV regulation on epigenetic inheritance models. We consider Krüppel-associated box domain (KRAB) zinc finger proteins as likely candidates for strain-specific ERV modifiers, drawing on insights gained from the study of the strain-specific behaviour of transgenes. We conclude by considering the coevolution of KRAB zinc finger proteins and actively transposing ERV elements, and highlight the importance of cross-strain studies in elucidating the mechanisms and consequences of strain-specific ERV regulation.

Metastable epialleles and their contribution to epigenetic inheritance in mammals

Many epigenetic differences between individuals are driven by genetic variation. Mammalian metastable epialleles are unusual in that they show variable DNA methylation states between genetically identical individuals. The occurrence of such states across generations has resulted in their consideration by many as strong evidence for epigenetic inheritance in mammals, with the classic A^vy and Axin^Fu mouse models - each products of repeat element insertions - being the most widely accepted examples. Equally, there has been interest in exploring their use as epigenetic biosensors given their susceptibility to environmental compromise. Here we review the classic murine metastable epialleles as well as more recently identified candidates, with the aim of providing a more holistic understanding of their biology. We consider the extent to which epigenetic inheritance occurs at metastable epialleles and explore the limited mechanistic insights into the establishment of their variable epigenetic states. We discuss their environmental modulation and their potential relevance in genome regulation. In light of recent whole-genome screens for novel metastable epialleles, we point out the need to reassess their biological relevance in multi-generational studies and we highlight their value as a model to study repeat element silencing as well as the mechanisms and consequences of mammalian epigenetic stochasticity.

An abundant placental transcript containing an IAP-LTR is allelic to mouse pregnancy-specific glycoprotein 23 (Psg23): cloning and genetic analysis

Several families of endogenous retroviruses (ERVs) are expressed in mammalian placental tissues, and are implicated in aspects of placental development and function. We characterized the structure of abundant ERV-related transcripts in mouse placenta. In addition to the 7 kb full-length type I and 5 kb type I deleted intracisternal A-particle (IAP) transcripts, we identified and cloned an abundant 2 kb transcript encoding a novel member of the pregnancy-specific glycoprotein (Psg) gene family, which contains an IAP long terminal repeat (LTR) in the 3' untranslated region (UTR). The polyadenylation signal for the transcript is provided by the inserted LTR sequence. This sequence is allelic to Psg23 and is therefore denoted as Psg23(LTR). The transcript encodes a protein of 471 amino acids and has a domain organisation similar to previously described Psg proteins. Modelling of the protein N-domain produced a structure in good agreement with an existing crystalline structure for mouse sCEACAM1a. The LTR insertion is widely distributed among inbred mouse strains but is not found in 129/sv, CBA/2, or in wild mice. Cloning of the genomic region downstream of the LTR insertion site from the C57Bl/6J strain indicates that the insertion consists of a solo LTR without additional IAP sequence, and identified the original Psg23 polyadenylation signal sequence downstream of the insertion site. Psg23(LTR) was mapped to proximal chromosome 7 using the European collaborative interspecific mouse backcross (EUCIB) panel, and to yeast artificial chromosome (YAC) E072, which contains other members of the Psg gene family, by polymerase chain reaction (PCR). Northern blot analysis of RNA from adult and fetal mouse tissues and in situ hybridization to mid-gestation mouse embryos indicated that Psg23(LTR) is expressed predominantly in placental spongiotrophoblast. We detected a small, but statistically non-significant, bias in favour of transmission of Psg23(LTR) to the offspring of heterozygous parents. However, a larger study would be required to determine whether this allele is selectively advantageous to the developing embryo.

Alternative splicing and expressivity of the Axin(Fu) allele in mice

Variable expressivity and incomplete penetrance are common for numerous mutations. In most cases the reasons behind these phenomena remain obscure. Caused by the insertion of a murine retrotransposon into intron 6 of the Axin locus, the Axin(Fu) mutation induces alternative splicing and ultimately leads to abnormal tail development in mice. In this investigation RNase protection assay was used to investigate the potential connection between alternative splicing with the expressivity and penetrance of the mutant allele. The results reported here confirm previous observations that alternative splicing occurs in mRNA transcribed from the mutant Axin(Fu) allele. However this investigation also shows that lower levels of alternative splicing commonly take place in the wild type transcript. Correlation analysis reveals a significant connection between tail abnormalities and the ratio of correct to alternatively spliced mRNAs. Overall this paper demonstrates that higher levels of alternatively spliced mRNAs correlate with stronger expression of the mutant trait.

Monosomy for the most telomeric, gene-rich region of the short arm of human chromosome 16 causes minimal phenotypic effects

We have examined the phenotypic effects of 21 independent deletions from the fully sequenced and annotated 356 kb telomeric region of the short arm of chromosome 16 (16p13.3). Fifteen genes contained within this region have been highly conserved throughout evolution and encode proteins involved in important housekeeping functions, synthesis of haemoglobin, signalling pathways and critical developmental pathways. Although a priori many of these genes would be considered candidates for critical haploinsufficient genes, none of the deletions within the 356 kb interval cause any discernible phenotype other than alpha thalassaemia whether inherited via the maternal or paternal line. These findings contrast with previous observations on patients with larger (> 1 Mb) deletions from the 16p telomere and therefore address the mechanisms by which monosomy gives rise to human genetic disease.

Does genomic imprinting contribute to valproic acid teratogenicity?

Reciprocal outcrosses and backcrosses were made between strains of mice with different susceptibilities to valproic acid (VPA) teratogenicity. Relatively resistant C57BL/6J (C) and more susceptible SWV (S) strains of mice produced F1 hybrids in which the female parent was C and the male parent was S (CS-F1) as well as the reciprocal with S dams and C sires (SC-F1). Each was backcrossed to each strain, producing 8 types of backcross matings: CS x C, SC x C, CS x S, SC x S; C x CS, C x SC, S x CS, S x SC (for all matings dams are listed first). At 8d:12 +/- 5h of gestation, a teratogenic dose, 600 mg/kg, of aqueous VPA was injected ip into the dams. Fetuses were examined on gestation day (gd) 18 for abnormality, mortality, litter size, and weight. Genomic imprinting (imprinting) is a phenomenon at least in part involving hyper- or hypomethylation of bases in DNA, which is believed to determine whether or not the imprinted gene will be expressed. Imprinting has been reported to occur differentially in the male and female for a number of gene loci. Thus, in crosses between strains with differing susceptibility to VPA, if imprinting is occurring, the susceptibility of a fetus might be predicted to be disproportionately influenced by susceptibility of its grandparents. Significant differences in frequency (%) of occurrence of exencephaly in progeny of all backcrosses with F1 dams consistent with those expected for imprinting were found in the present study (CS-F1x C = 21.8 +/- 3.9%, SC-F1x C = 10.8 +/- 3.2%, P < 0.03; CS-F1x S = 14.8 +/- 3.1%, SC-F1x S = 6.3 +/- 2.3%, P < 0.03). SWV dams revealed the same pattern (S x SC-F1 = 50.0 +/- 8.3%, S x CS-F1 = 37.1 +/- 4.7%, P < 0.04). Differences in prenatal mortality also consistent with genomic imprinting occurred in backcrosses with pure-line SWV dams (S x SC = 64.4 +/- 8.0%, S x CS = 30.5 +/- 4.5%, P < 0.001). Fetal weight was reduced in a manner consistent with imprinting in backcrosses involving SWV (S x SC = 0.50 +/- 0.18 g, S x CS = 0.96 +/- 0.05, P < 0.01). Three of four of the parameters investigated showed differences in some of the backcrosses of reciprocal F1's consistent with those expected if genomic imprinting were occurring.

Mapping around the Fused locus on mouse chromosome 17

We have established a high-resolution genetic map of the region surrounding the Fused locus as a first step towards the molecular identification and analysis of this gene. The candidate region has been covered to a large extent by YAC and P1 contigs, and has been partly characterized by pulsed-field gel analysis.

Phenotypic expression of the fused (Fu) gene in chimaeric mice

The dominant gene Fused (Fu) produces skeletal abnormalities during embryonic development. It was previously shown that C57BL/6 mice contain a suppressor of Fu, which acts after fertilization. Chimaeras were used to study whether this gene would suppress the Fu phenotype after the 8-cell stage of embryo development. We found no effect of the suppressor gene on Fu phenotype (its degree and frequency of expression) in chimaeric mice. We conclude that either the suppressor gene from C57BL/6 mice can only influence Fu expression at the intracellular level or Fu expression is determined before the 8-cell embryonic stage.

Genomic imprinting--defusing the ovarian time bomb

Why do mammals imprint their parental genomes? Imprinting is seen in many phyla, but that in mammals is by far the most dramatic. Is there something peculiar to mammals that calls for such a striking phenomenon? We propose that imprinting is a device that protects female mammals from the potential ravages of ovarian trophoblast disease. Without imprinting, the ovarian teratomas that frequently arise from parthenogenetically activated oocytes in situ might be capable of forming malignant trophoblast. An allele that favored imprinting would spread rapidly because of the great increase in fitness associated with suppressing a lethal cancer of females.

Mechanisms of genomic imprinting in mammals

This chapter can be summarized by the following main points: Genomic imprinting results in the functional nonequivalence of the maternal and paternal genomes, thereby preventing the development of viable parthenogenotes and androgenotes in eutherian mammals. Imprinting may have arisen as a result of the specialized evolutionary requirements of the parental genomes or may have been an obligatory step in the development of placentation. A substantial proportion of transgenes and a smaller number of endogenous genes demonstrate imprinted pattern of expression in mice and humans. An analysis of DNA methylation in somatic tissues and germ cells during embryonic and postnatal development reveals dynamic changes, particularly during gametogenesis and early embryogenesis. The nature and timing of these changes suggest that DNA methylation may be involved in genomic imprinting. Imprinted genes display complex methylation patterns. Many aspects of these patterns are consistent with a role for methylation in the imprinted phenotype, although it is currently unclear whether methylation functions in the establishment of imprinting or plays a secondary role in the maintenance of the imprinted pattern of expression. Studies underway to identify new imprinted genes may help elucidate both the function and mechanism of genomic imprinting.

Genetic analysis of genomic imprinting: an Imprintor-1 gene controls inactivation of the paternal copy of the mouse Tme locus

The Thp deletion on mouse chromosome 17 is lethal when inherited from the mother, because it deletes the T-associated maternal effect (Tme) locus, the paternal copy of which is inactivated by genomic imprinting. We have found a paternally nonimprinted Tme variant in crosses of Thp females with Mus m. musculus males. The data are consistent with the existence of a single Tme-unlinked gene, Imprintor-1 (Imp-1), with two alleles, one of which only causes imprinting at the Tme locus. Imp-1 is unlinked to the gene for cation-dependent Man-6-P receptor and acts prezygotically. Although Tme and Igf2r were thought to be identical, they show different patterns of imprinting in interspecies hybrids. The apparent nonequivalence of the Igf2r gene and Tme results in occurrence of viable mice lacking an active Igf2r gene. These mice are bigger at birth than their normal littermates, in accord with the proposed function of the IGF-II/Man-6-P receptor.