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Kathrin Muegge

Publications and source records attributed to Kathrin Muegge.

13 recordsLinked to original sources
PubMed2006

Lsh is required for meiotic chromosome synapsis and retrotransposon silencing in female germ cells.

Lymphoid specific helicase (Lsh) is a major epigenetic regulator that is essential for DNA methylation and transcriptional silencing of parasitic elements in the mammalian genome. However, whether Lsh is involved in the regulation of chromatin-mediated processes during meiosis is not known. Here, we show that Lsh is essential for the completion of meiosis and transcriptional repression of repetitive elements in the female gonad. Oocytes from Lsh knockout mice exhibit demethylation of transposable elements and tandem repeats at pericentric heterochromatin, as well as incomplete chromosome synapsis associated with persistent RAD51 foci and gammaH2AX phosphorylation. Failure to load crossover-associated foci results in the generation of non-exchange chromosomes. The severe oocyte loss observed and lack of ovarian follicle formation, together with the patterns of Lsh nuclear compartmentalization in the germ line, demonstrate that Lsh has a critical and previously unidentified role in epigenetic gene silencing and maintenance of genomic stability during female meiosis.

Animals↗
PubMed2006

Lsh is involved in de novo methylation of DNA.

Deletion of Lsh perturbs DNA methylation patterns in mice yet it is unknown whether Lsh plays a direct role in the methylation process. Two types of methylation pathways have been distinguished: maintenance methylation by Dnmt1 occurring at the replication fork, and de novo methylation established by the methyltransferases Dnmt3a and Dnmt3b. Using an episomal vector in Lsh-/- embryonic fibroblasts, we demonstrate that the acquisition of DNA methylation depends on the presence of Lsh. In contrast, maintenance of previously methylated episomes does not require Lsh, implying a functional role for Lsh in the establishment of novel methylation patterns. Lsh affects Dnmt3a as well as Dnmt3b directed methylation suggesting that Lsh can cooperate with both enzymatic activities. Furthermore, we demonstrate that embryonic stem cells with reduced Lsh protein levels show a decreased ability to silence retroviral vector or to methylate endogenous genes. Finally, we demonstrate that Lsh associates with Dnmt3a or Dnmt3b but not with Dnmt1 in embryonic cells. These results suggest that the epigenetic regulator, Lsh, is directly involved in the control of de novo methylation of DNA.

Animals↗
PubMed2005

Lsh controls silencing of the imprinted Cdkn1c gene.

Epigenetic regulation, such as DNA methylation plays an important role in the control of imprinting. Lsh, a member of the SNF2 family of chromatin remodeling proteins, controls DNA methylation in mice. To investigate whether Lsh affects imprinting, we examined CpG methylation and allelic expression of individual genes in Lsh-deficient embryos. We report here that loss of Lsh specifically alters expression of the Cdkn1c gene (also known as p57(Kip2)) but does not interfere with maintenance of imprints at the H19, Igf2, Igf2r, Zac1 and Meg9 genes. The reactivation of the silenced paternal Cdkn1c allele correlates closely with a loss of CpG methylation at the 5' DMR at the Cdkn1c promoter, whereas KvDMR1 and DMRs of other imprinted genes were not significantly changed. Chromatin immunoprecipitations demonstrate a direct association of Lsh with the 5' DMR at the Cdkn1c promoter, but not with Kv DMR1 or other imprinted loci. These data suggest that methylation of the 5' DMR plays an important role in the imprinting of the Cdkn1c gene. Furthermore, it suggests that Lsh is not required for maintenance of imprinting marks in general, but is only crucial for imprinting at distinct genomic sites.

5' Flanking Region↗
PubMed2005

Lsh, a guardian of heterochromatin at repeat elements.

Lymphoid-specific helicase (Lsh) is a crucial factor for normal embryonic development; targeted deletion of Lsh is lethal. Lsh belongs to a family of chromatin-remodeling proteins and is closely associated with pericentromeric heterochromatin. Lsh deficiency leads to abnormal heterochromatin organization, with a loss of DNA methylation, and an altered pattern of histone-tail acetylation and methylation. As a functional consequence of perturbed heterochromatin, aberrant reactivation of parasitic retroviral elements in the genome and abnormal mitosis with amplified centrosomes and genomic instability were observed. Thus, Lsh is a major epigenetic regulator crucial for normal heterochromatin structure and function.

Animals↗
PubMed2004

Lsh, an epigenetic guardian of repetitive elements.

The genome is burdened with repetitive sequences that are generally embedded in silenced chromatin. We have previously demonstrated that Lsh (lymphoid-specific helicase) is crucial for the control of heterochromatin at pericentromeric regions consisting of satellite repeats. In this study, we searched for additional genomic targets of Lsh by examining the effects of Lsh deletion on repeat regions and single copy gene sequences. We found that the absence of Lsh resulted in an increased association of acetylated histones with repeat sequences and transcriptional reactivation of their silenced state. In contrast, selected single copy genes displayed no change in histone acetylation levels, and their transcriptional rate was indistinguishable compared to Lsh-deficient cells and wild-type controls. Microarray analysis of total RNA derived from brain and liver tissues revealed that <0.4% of the 15 247 examined loci were abnormally expressed in Lsh-/-embryos and almost two-thirds of these deregulated sequences contained repeats, mainly retroviral LTR (long terminal repeat) elements. Chromatin immunoprecipitation analysis demonstrated a direct interaction of Lsh with repetitive sites in the genome. These data suggest that the repetitive sites are direct targets of Lsh action and that Lsh plays an important role as 'epigenetic guardian' of the genome to protect against deregulation of parasitic retroviral elements.

Acetylation↗
PubMed2003

Lsh, a modulator of CpG methylation, is crucial for normal histone methylation.

Methylation of histone tails and CpG methylation are involved in determining heterochromatin structure, but their cause and effect relationship has not been resolved as yet in mammals. Here we report that Lsh, a member of the SNF2 chromatin remodeling family, controls both types of epigenetic modifications. Lsh has been shown to be associated with pericentromeric heterochromatin and to be required for normal CpG methylation at pericentromeric sequences. Loss of Lsh, in Lsh-deficient mice, results in accumulation of di- and tri-methylated histone 3 at lysine 4 (H3-K4me) at pericentromeric DNA and other repetitive sequences. In contrast, di- or tri-methylation of H3-K9 and distribution of HP1 appear unchanged after Lsh deletion, suggesting independent regulatory mechanisms for H3-K4 or K9 methylation. Experimental DNA demethylation with 5'-azacytidine results in a similar increase of H3-K4me. These results support the model that loss of CpG methylation caused by Lsh deficiency antecedes elevation of H3-K4me. Thus, Lsh is crucial for the formation of normal heterochromatin, implying a functional role for Lsh in the regulation of transcription and mitosis.

Animals↗
PubMed2003

Lsh-deficient murine embryonal fibroblasts show reduced proliferation with signs of abnormal mitosis.

Genomic hypomethylation and chromosomal instability are frequent characteristics of human cancer cells. Targeted deletion of Lsh leads to a global defect in genomic methylation, and Lsh-deficient mice die at birth with a reduced body weight. Here, we examine the growth pattern of embryonal fibroblasts derived from Lsh-/- mice. The absence of Lsh leads to a severe proliferative defect of fibroblasts with lower saturation density, early signs of senescence, and a lower frequency of immortalization. The impaired growth rate in vitro may be in part responsible for the small size of Lsh-deficient mice. In addition, Lsh-/- fibroblasts accumulated high centrosome numbers, formed multipolar spindles, displayed micronuclei formation, and elevated nuclear DNA content. A similar increase in centrosome abnormalities was observed when wild-type fibroblasts were treated with a DNA-demethylating agent, suggesting that genomic hypomethylation plays an important role in mitotic defects of Lsh-/- murine embryonal fibroblasts, possibly by altering chromatin structure. Because supernumerary centrosomes are a common feature in cancer cells, this Lsh-dependent pathway has the potential to contribute to genetic instability and chromosomal aberrations during tumor progression.

Animals↗
PubMed2003

Modifications of histone cores and tails in V(D)J recombination.

The organization of chromatin and modifications to the tails of histone proteins are thought to be important in regulating the rearrangement of V, D and J gene segments, which encode immunoglobulins and T-cell receptors. A recent study shows that methylated lysine 79 in the core region of histone H3 also plays a role by providing a euchromatic 'mark' that may regulate access of the V(D)J recombinase.

Animals↗
PubMed2003

Epigenetic control during lymphoid development and immune responses: aberrant regulation, viruses, and cancer.

Methylation of cytosines controls a number of biologic processes such as imprinting and X chromosomal inactivation. DNA hypermethylation is closely associated with transcriptional silencing, while DNA hypomethylation is associated with transcriptional activation. Hypoacetylation of histones leads to compact chromatin with reduced accessibility to the transcriptional machinery. Methyl-CpG binding proteins can recruit corepressors and histone deacetylases; thus, the interplay between these epigenetic mechanisms regulates gene activation. Methylation has been implicated as an important mechanism during immune development, controlling VDJ recombination, lineage-specific expression of cell surface antigens, and transcriptional regulation of cytokine genes during immune responses. Aberrations in epigenetic machinery, either by genetic mutations or by somatic changes such as viral infections, are associated with early alterations in chronic diseases such as immunodeficiency and cancer.

Acetylation↗
PubMed2003

Association of Lsh, a regulator of DNA methylation, with pericentromeric heterochromatin is dependent on intact heterochromatin.

The eukaryotic genome is packaged into distinct domains of transcriptionally active euchromatin and silent heterochromatin. A hallmark of mammalian heterochromatin is CpG methylation. Lsh, a member of the SNF2 family, is a major regulator of DNA methylation in mice and thus crucial for normal heterochromatin formation. In order to define the molecular function of Lsh, we examined its cellular localization and its association with chromatin. Our studies demonstrate that Lsh is an exclusively nuclear protein, and we define a nuclear localization domain within the N-terminal portion of Lsh. Lsh strongly associates with chromatin and requires the internal and C-terminal regions for this interaction. Lsh accumulates at pericentromeric heterochromatin, suggesting a direct role for Lsh in the methylation of centromeric DNA sequences and the formation of heterochromatin. In search of a signal that is responsible for Lsh recruitment to pericentromeric heterochromatin, we found that histone tail modifications were critical. Prolonged treatment with histone deacetylase inhibitors has been reported to disrupt higher-order heterochromatin organization, and this was accompanied by dissociation of Lsh from pericentromeric heterochromatin. These results are consistent with a model in which Lsh is recruited by intact heterochromatin structure and then assists in maintaining heterochromatin organization by establishing CpG methylation patterns.

Animals↗
PubMed2002

IL-7 withdrawal induces a stress pathway activating p38 and Jun N-terminal kinases.

IL-7 delivers survival signals to cells at an early stage in lymphoid development. In the absence of IL-7, pro-T cells undergo programmed cell death, which has previously been associated with a decline in Bcl-2 and translocation of Bax from cytosol to mitochondria. A new, earlier feature of IL-7 withdrawal was identified using an IL-7-dependent thymocyte line. We observed that withdrawal of IL-7 induced increased expression of jun and fos family member genes including c-jun, junB, junD, c-fos and fra2. This transient response peaked 3-4 h after IL-7 was withdrawn and resulted in increased DNA-binding activity of AP-1 and in a change in the composition of the Jun/Fos family dimers shown by electrophoretic mobility shift and supershift assays. Induction of jun and fos genes and the increased DNA-binding activity of AP-1 were attributable to the phosphorylation-induced activation of the stress kinases p38 and JNK and were blocked by the chemical kinase inhibitors SB203580 and SB202190. The stress response contributed to cell death following IL-7 withdrawal as shown by blocking the activity of the stress (MAP) kinases or by blocking the production of c-Jun and c-Fos using antisense oligonucleotides.

Animals↗
PubMed2002

Bax deficiency partially corrects interleukin-7 receptor alpha deficiency.

The requirement for cytokines in hematopoiesis is partly attributable to the protection of cells from apoptosis. Since IL-7 is required for normal T cell development, we evaluated the role of Bax in vivo by generating mice deficient in both Bax and the IL-7 receptor alpha chain (IL-7R). Starting at birth, we observed complete recovery of all stages of alphabeta thymocyte development up to 4 weeks of age. However, by 12 weeks of age, thymic cellularity had reverted to that of mice deficient in IL-7R alone. The BH3 only proteins, Bad and Bim, were also part of the death pathway repressed by IL-7. Thus, in young mice, Bax emerges as an essential protein in the death pathway induced by IL-7 deficiency.

Aging↗