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M Azim Surani

Publications and source records attributed to M Azim Surani.

9 recordsLinked to original sources
PubMed2003

Polycomb-group proteins are involved in silencing processes caused by a transgenic element from the murine imprinted H19/Igf2 region in Drosophila.

A subset of autosomal genes undergo genomic imprinting which results in expression from only the paternal or maternal chromosome. While this phenomenon is restricted to mammals and angiosperms, the underlying silencing mechanisms appear to be evolutionarily conserved. A biallelically unmethylated DNaseI hypersensitive region (A6-A4) between the imprinted Igf2 and H19 genes is conserved in humans and mice and functions as a tissue-specific maintenance element for the imprinted growth factor IGF2. In order to analyse A6-A4 for potentially conserved transcriptional maintenance properties, we have generated transgenic Drosophila harbouring the element in a reporter construct. These flies depicted silencing of the reporter genes lacZ and mini -white. The silenced state of the mini -white gene showed variegation and sensitivity to temperature changes. In addition, two members of the conserved Polycomb group, Enhancer of zeste and Posterior sex combs, were needed for repression. Polycomb group proteins are essential for gene silencing during development. Our results indicate that Polycomb group proteins may also be involved in the regulation of mammalian imprinted genes.

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PubMed2003

Consequences of the depletion of zygotic and embryonic enhancer of zeste 2 during preimplantation mouse development.

Enhancer of zeste 2 (Ezh2), a SET domain-containing protein, is crucial for development in many model organisms, including early mouse development. In mice, Ezh2 is detected as a maternally inherited protein in the oocyte but its function at the onset of development is unknown. We have used a conditional allele of Ezh2 to deplete the oocyte of this maternal inheritance. We show that the loss of maternal Ezh2 has a long-term effect causing severe growth retardation of neonates despite 'rescue' through embryonic transcription from the paternal allele. This phenotypic effect on growth could be attributed to the asymmetric localisation of the Ezh2/Eed complex and the associated histone methylation pattern to the maternal genome, which is disrupted in Ezh2 mutant zygotes. During subsequent development, we detect distinct histone methylation patterns in the trophectoderm and the pluripotent epiblast. In the latter where Oct4 expression continues from the zygote onwards, the Ezh2/Eed complex apparently establishes a unique epigenetic state and plasticity, which probably explains why loss of Ezh2 is early embryonic lethal and obligatory for the derivation of pluripotent embryonic stem cells. By contrast, in the differentiating trophectoderm cells where Oct4 expression is progressively downregulated Ezh2/Eed complex is recruited transiently to one X chromosome in female embryos at the onset of X-inactivation. This accumulation and the associated histone methylation are also lost in Ezh2 mutants, suggesting a role in X inactivation. Thus, Ezh2 has significant and diverse roles during early development, as well as during the establishment of the first differentiated cells, the trophectoderm, and of the pluripotent epiblast cells.

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PubMed2002

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.

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PubMed2002

Methylation-dependent silencing at the H19 imprinting control region by MeCP2.

Methylation of CpG dinucleotides is correlated with transcriptional repression of genes, including imprinted genes. In the case of the imprinted H19 gene, a 2 kb imprinting control region (ICR) is subject to differential methylation, as it is methylated only on the silenced paternal allele. This region has previously been shown to act as a silencer element at the endogenous locus. The proteins that bind at the H19 differentially methylated domain (DMD) and mediate transcriptional silencing have yet to be identified, although a family of proteins containing a methyl-CpG-binding domain (MBD), of which MeCP2 is the best characterised, are obvious candidates. MeCP2 can bind to a single methylated CpG dinucleotide through its MBD and also contains a transcriptional repression domain (TRD). The TRD interacts with Sin3a and histone deacetylases (HDACs) in vivo, forming a repressive complex. Here we show that MeCP2 is recruited to the H19 DMD in vivo and can silence a reporter gene regulated by the H19 DMD in a methylation-dependent manner. This repression can be alleviated by deletion of the TRD from MeCP2 or by inhibition of HDAC activity. These data indicate that transcriptional silencing from the H19 ICR involves recruitment of MeCP2 and presumably an associated protein complex with deacetylase activity. This complex may also be recruited to the ICR in vivo, resulting in a compact, repressive chromatin structure capable of silencing the paternal H19 allele.

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PubMed2002

Epigenetic reprogramming in mouse primordial germ cells.

Genome-wide epigenetic reprogramming in mammalian germ cells, zygote and early embryos, plays a crucial role in regulating genome functions at critical stages of development. We show here that mouse primordial germ cells (PGCs) exhibit dynamic changes in epigenetic modifications between days 10.5 and 12.5 post coitum (dpc). First, contrary to previous suggestions, we show that PGCs do indeed acquire genome-wide de novo methylation during early development and migration into the genital ridge. However, following their entry into the genital ridge, there is rapid erasure of DNA methylation of regions within imprinted and non-imprinted loci. For most genes, the erasure commences simultaneously in PGCs in both male and female embryos, which is completed within 1 day of development. Based on the kinetics of this process, we suggest that this is an active demethylation process initiated upon the entry of PGCs into the gonadal anlagen. The timing of reprogramming in PGCs is crucial since it ensures that germ cells of both sexes acquire an equivalent epigenetic state prior to the differentiation of the definitive male and female germ cells in which new parental imprints are established subsequently. Some repetitive elements, however, show incomplete erasure, which may be essential for chromosome stability and for preventing activation of transposons to reduce the risk of germline mutations. Aberrant epigenetic reprogramming in the germ line would cause the inheritance of epimutations that may have consequences for human diseases as suggested by studies on mouse models.

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PubMed2002

Xist expression and macroH2A1.2 localisation in mouse primordial and pluripotent embryonic germ cells.

The molecular mechanism underlying X chromosome inactivation in female mammals involves the non-coding RNAs Xist and its antisense partner Tsix. Prior to X inactivation, these RNAs are transcribed in an unstable form from all X chromosomes, both in the early embryo and in undifferentiated embryonic stem (ES) cells. Upon differentiation, the expression of these unstable transcripts from all alleles is silenced, and Xist RNA becomes stabilised specifically on the inactivating X chromosome. This pattern of expression is then maintained throughout subsequent somatic cell divisions. Once established, the inactive state of the X chromosome is remarkably stable, the only natural case of reactivation occurring in XX primordial germ cells (PGCs) when they enter the genital ridge. To gain insight into the X reactivation process, we have analysed Xist gene expression using RNA FISH in PGCs and also in PGC-derived embryonic germ (EG) cells. XX EG cells were shown to express unstable Xist/Tsix from both X chromosomes. In contrast, no unstable Xist/Tsix transcripts were detected in XX PGCs at any stage. Instead, a proportion of XX PGCs isolated from the genital ridge between 11.5 and 13.5 dpc (the period during which X chromosome reactivation occurs) showed an accumulation of stable Xist RNA on one X. The number of these cells decreased progressively and was nearly extinguished by 13.5 dpc. As a late marker for the inactive state, we analysed localisation of the histone H2A variant macroH2A1.2. Although macroH2A1.2 expression was observed in PGCs, no significant localisation to the inactive X was detected at any stage. We discuss these results in the context of understanding X chromosome reactivation.

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PubMed2002

Novel conserved elements upstream of the H19 gene are transcribed and act as mesodermal enhancers.

The reciprocally imprinted H19 and Igf2 genes form a co-ordinately regulated 130 kb unit in the mouse controlled by widely dispersed enhancers, epigenetically modified silencers and an imprinting control region (ICR). Comparative human and mouse genomic sequencing between H19 and Igf2 revealed two novel regions of strong homology upstream of the ICR termed H19 upstream conserved regions (HUCs). Mouse HUC1 and HUC2 act as potent enhancers capable of driving expression of an H19 reporter gene in a range of mesodermal tissues. Intriguingly, the HUC sequences are also transcribed bi-allelically in mouse and human, but their expression pattern in neural and endodermal tissues in day 13.5 embryos is distinct from their enhancer function. The location of the HUC mesodermal enhancers upstream of the ICR and H19, and their capacity for interaction with both H19 and Igf2 requires critical re-evaluation of the cis-regulation of imprinted gene expression of H19 and Igf2 in a range of mesodermal tissues. We propose that these novel sequences interact with the ICR at H19 and the epigenetically regulated silencer at differentially methylated region 1 (DMR1) of Igf2.

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PubMed2002

Histone methylation defines epigenetic asymmetry in the mouse zygote.

The oocyte cytoplasm regulates and enhances the epigenetic asymmetry between parental genomes and, consequently, functional differences observed between them during development in mammals. Here we demonstrate a preferential interaction of HP1beta with the maternal genome immediately after fertilisation in the mouse zygote, which also shows a high level of lysine 9-methylated histone H3. In contrast, the paternal genome has neither HP1beta binding nor methylated histone H3 at these early stages. Paternal binding of HP1beta is only detected at the pronuclear stage, prior to the appearance of lysine 9-methylated histone H3. The early recruitment of heterochromatic factors specifically to the maternal genome could explain the preferential DNA demethylation of the paternal genome in the zygote.

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