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Mammalian sex chromosomes are an excellent model system to understand the pivotal roles of epigenetic programming in development. Sex chromosomes precisely recapitulate stereotypical epigenetic programming in a timely and spatially-coordinated manner. In females, one of the X chromosomes is inactivated for dosage compensation of X-linked genes. In males, sex chromosomes are inactivated from meiosis to spermiogenesis in a process that is essential for germ cell developments. Meiotic sex chromosomes provide a unique opportunity which enable us to dissect the process of epigenetic programming. Following the initiation of meiotic sex chromosome inactivation, various epigenetic modifications are established gradually on the sex chromosomes in a coordinated manner. Such epigenetic modifications on the sex chromosomes are maintained through meiotic cell division into the spermatids, implying the inheritance of an epigenetic memory following meiosis. We investigate the underlying mechanism of sex chromosome inactivation specifically focusing the roles of DNA damage response pathways.
Figure 1 Initiation mechanism of meiotic sex chromosome inactivation. (A) Schematic of sex chromosome inactivation. (B) MDC1 directs chromosome-wide spreading of gH2AX. Immunostaining of meiotic chromosome spreads using antibodies against gH2AX together with the anti-SCP3 antibody. SCP3 staining displays the status of chromosome synapsis and is used to distinguish meiotic stages. Areas surrounding sex chromosomes are highlighted in dotted rectangles and magnified in the right panels. (C) Pictorial representation about the role of MDC1 in MSCI. The first step is MDC1-independent recognition of the unsynapsed axis. The second step is MDC1-dependent spreading of gH2AX to the chromosome-wide domain. Ichijima et al Genes Dev 2011 25: 959-971
Sin, H. S., Barski, A., Zhang, F., Kartashov, A. V., Nussenzweig, A.,Chen,J., Andreassen,P. R., Namekawa, S. H.* RNF8 regulates active epigenetic modifications and escape gene activation from inactive sex chromosomes in postmeiotic spermatids. Genes Dev,in press. *Corresponding author
Ichijima, Y., Sin,H. S., Namekawa, S. H.* Sex chromosome inactivation in germ cells: Emerging roles of DNA damage response pathways. Cell. Mol. Life. Sci,Epub 2012 Mar 2. 2012. *Corresponding author
Payer, B.,* Lee, J. T., Namekawa, S. H.*X-inactivation and X-reactivation: Epigenetic hallmarks of mammalian reproduction and pluripotent stem cells. Hum Genet, Aug;130(2):265-80. 2011. Epub 2011 Jun 12. *Corresponding authors
Ichijima, Y., Ichijima,M., Lou, Z., Nussenzweig, A.,Camerini-Otero,R. D., Chen,J., Andreassen,P. R.,Namekawa, S. H.*MDC1 directs chromosome-wide silencing of the sex chromosomes in male germ cells. Genes Dev, May 1;25(9):959-71. 2011.*Corresponding author
Namekawa, S. H.,* and Lee, J. T.* Detection of nascent RNA, single-copy DNA, and protein localization by immunoFISH in murine germ cells and pre-implantation embryos. Nature Protocols. Feb;6(3):270-84. 2011. Epub 2011 Feb 10. *Corresponding authors.
Namekawa, S. H., Payer, B., Huynh K. D., Jaenisch, R., and Lee, J. T.Two-step imprinted X-inactivation: Repeat vs genic silencing in the mice. Mol Cell Biol Jul;30(13):3187-205. 2010 Epub 2010 Apr 19.
Namekawa, S. H., Park, P. J., Zhang, L. F., Shima, J. E., McCarrey, J. R., Griswold, M. D., and Lee, J. T. Postmeiotic sex chromatin in the male germline of mice. Curr Biol, 16, 660-667. 2006.
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