Genomic imprinting determines methylation of parental alleles in transgenic mice

Mouse embryogenesis relies on the presence of both the maternal and the paternal genome for development to term. It has been proposed that specific modifications are imprinted onto the chromosomes during gametogenesis; these modifications are stably propagated, and their expression results in distinct and complementary contributions of the two parental genomes to the development of the embryo and the extraembryonic membranes. Genetic data further suggest that a substantial proportion of the genome could be subject to chromosomal imprinting, the molecular nature of which is unknown. We used random DNA insertions in transgenic mice to probe the genome for modified regions. The DNA methylation patterns of transgenic alleles were compared after transmission from mother or father in seven mouse strains carrying autosomal insertions of the same transgenic marker. One of these loci showed a clear difference in DNA methylation specific for its parental origin, with the paternally inherited copy being relatively undermethylated. This difference was observed in embryos on day 10 of gestation, but not in their extraembryonic membranes. Moreover, the methylation pattern was faithfully reversed upon each germline transmission to the opposite sex. Our findings provide evidence for heritable molecular differences between maternally and paternally derived alleles on mouse chromosomes.     

Isotype exclusion and transgene down-regulation in immunoglobulin-lambda transgenic mice

A given B lymphocyte makes an antibody containing either kappa- or lambda-light chains, but not both. This isotype exclusion is effected at the level of the rearrangement of the immunoglobulin gene segments, although by an unknown mechanism. An attractive possibility is that, following productive rearrangement of one of the light-chain loci, the newly synthesized light-chain polypeptide inhibits DNA rearrangement for the other isotype. To test such feedback regulation, we have created transgenic mice carrying a rearranged lambda 1-gene. By contrast with the B cells in normal newborn mice which are mainly kappa+lambda-, the B cells in the newborn transgenic mice express lambda- but not kappa-chains. We propose that the synthesis of any light chain, be it kappa or lambda, that allows expression of IgM on the cell surface results in a cessation of all V-J joining. Interestingly, the limited light-chain repertoire of the transgenic mice does not persist and most adult B cells express endogenous kappa-rearrangements and down-regulate the transgene.     

Reprogramming of genome function through epigenetic inheritance.

Most cells contain the same set of genes and yet they are extremely diverse in appearance and functions. It is the selective expression and repression of genes that determines the specific properties of individual cells. Nevertheless, even when fully differentiated, any cell can potentially be reprogrammed back to totipotency, which in turn results in re-differentiation of the full repertoire of adult cells from a single original cell of any kind. Mechanisms that regulate this exceptional genomic plasticity and the state of totipotency are being unravelled, and will enhance our ability to manipulate stem cells for therapeutic purposes.     

Genetic variation in selenoprotein S influences inflammatory response

Chronic inflammation has a pathological role in many common diseases and is influenced by both genetic and environmental factors. Here we assess the role of genetic variation in selenoprotein S (SEPS1, also called SELS or SELENOS), a gene involved in stress response in the endoplasmic reticulum and inflammation control. After resequencing SEPS1, we genotyped 13 SNPs in 522 individuals from 92 families. As inflammation biomarkers, we measured plasma levels of IL-6, IL-1beta and TNF-alpha. Bayesian quantitative trait nucleotide analysis identified associations between SEPS1 polymorphisms and all three proinflammatory cytokines. One promoter variant, -105G --> A, showed strong evidence for an association with each cytokine (multivariate P = 0.0000002). Functional analysis of this polymorphism showed that the A variant significantly impaired SEPS1 expression after exposure to endoplasmic reticulum stress agents (P = 0.00006). Furthermore, suppression of SEPS1 by short interfering RNA in macrophage cells increased the release of IL-6 and TNF-alpha. To investigate further the significance of the observed associations, we genotyped -105G --> A in 419 Mexican American individuals from 23 families for replication. This analysis confirmed a significant association with both TNF-alpha (P = 0.0049) and IL-1beta (P = 0.0101). These results provide a direct mechanistic link between SEPS1 and the production of inflammatory cytokines and suggest that SEPS1 has a role in mediating inflammation.     

A molecular programme for the specification of germ cell fate in mice

Germ cell fate in mice is induced in proximal epiblast cells by the extra-embryonic ectoderm, and is not acquired through the inheritance of any preformed germ plasm. To determine precisely how germ cells are specified, we performed a genetic screen between single nascent germ cells and their somatic neighbours that share common ancestry. Here we show that fragilis, an interferon-inducible transmembrane protein, marks the onset of germ cell competence, and we propose that through homotypic association, it demarcates germ cells from somatic neighbours. Using single-cell gene expression profiles, we also show that only those cells with the highest expression of fragilis subsequently express stella, a gene that we detected exclusively in lineage-restricted germ cells. The stella positive nascent germ cells exhibit repression of homeobox genes, which may explain their escape from a somatic cell fate and the retention of pluripotency.