NATUREIVol 447 24 May 2007 INSIGHT REVIEW Conclusions and outlook elop ght be a one-way street because of the somatic inhe 10 ance of epigenetic marks. Whether there is a linear relationship between ful; some key restrictions in developmental potential that are brought 12. emos g.i. tem. e. M. e Za, 2 5 326 -2d00ain main-containing proteins and histone about by epigenetic regulation might occur very early in development. Judging from somatic-cell nuclear-transfer experiments, it is far from B. O A b-et r Astea el- ike ch have marks that are more difficult for the oocyte to reprogramme Or 14. Feldman,NYet al.G9a-med Natural epigenetic reprogramming might be needed to ensure that is. Siarly embryogenesis Nature ae io. 188-19epigenetic inactivation of Oct-3/4during clear whether more-differentiated cells have more epigenetic marks development can start afresh in every new generation. Although various 16. Hochedlinger, K,Yamada, Y,Beard,C&Jaenisch,.Ectopicexpressionof Oct-4blocks mechanisms for the rapid erasure of histone modifications have recently been identified, the mechanism of DNA demethylation still needs to 17. Boiani, M. Eckardt, S. Schole, H R& McLaughlin, K 1. Oct4 distribution and level in be determined. Recent work on the erasure of DNA methylation from A. A, Hayashi, K.& Ha kova, P. Genetic and epigenetic regulators of pluripotency D M. etal. DNA methylation is a primary mechanism for silencing ell genes in both germ cell and somatic cell lineages sible either by replicating DNA in the absence of DNMTI or by breaking Opment 133.3411-3418(2006 DNA 21. Surani, A. Reik, W. in Epigenetics(eds Allis, C D, Jenuwein, T& Reinberg, D)315-327 It is fascinating to see that both transcription-factor interactions and 22. Bourc 'his, D. Bestor, T H Meiotic catastrophe and retrotransposon reactivation in male germcell lacking Dnmt3L Nature 431, 96-99(2004) maintain pluripotency in early embryos and ES cells. Indeed, experi- 23. Barlow, D. P, Methylation and imprinting from host defense togene regulation? Science mental reprogramming of differentiated nuclei without using somatic- 24. Bourc his. D. Xu, G. L, Lin, C S Bollman, B. Bestor, T. H. Dnmt3L and the establishment nuclear transfer or cell lon nas been a chieved recently, using maternal genomic imprints. Science 294, 2536-2539(2001). expression of pluripotency transcription-factor networks would also 26. Jelinic, P, Stehle, 1.C&Shaw, P. The testis-specific factor CTCFL withthe activate epigenetic reprogramming factors, but whether this occurs is gion methylation. PloS Biol. unclear. Perhaps combinations of transcription factors and epigenetic 27. Howell C Y, et a1 4 829-838c2tog disrupted by a matemal effect mutation in the reprogramming factors are needed for more complete reprogramming Jaenisch, R Role for DNA methylation in genomic imprinting. Nature 366, mental scientific and medical interest 62-36501993). 29. Sleutels, F, Zwart, R& Barlow, D P. The non-coding Air RNA is required for silencing In the animal kingdom, some epigenetic systems, such as imprinting, have evolved only in mammals. Many of the basic molecular building 30. Mancini-Dinardo, D, steele, S. J, Levorse, I M, Ingram, R S. 4 Inighman 5 M Elongation blocks for epigenetics, such as the enzymes for DNA methylation and Dev. 20. 1268-1282(2006) histone modifications, are highly conserved in vertebrates, but the regu- 31 lation of epigenetic modifiers might evolve more rapidly together with stone methylation independent of DNA methylation. Nature Genet. 36, 1291-1295 specific developmental strategies. Therefore, evolutionary epigenetics 32. Umlauf, D. et al. Imprinting along the Kcng1 domain on mouse chromosome involves and epigenomics will have an important role in discovering links repres between developmental adaptations and epigenetic regulators. There is probably a conflict between the requirement for eras 33. Kanduri, C, Thakur, N. Pandey, R.R. The length of the transcript encoded from the Kenglotl antisense promoter determines the degree of silencing. EMBOJ. 25, 2096-2106 ing epigenetic marks between generations and the requirement for (2006) 4. Lewis, A etal. Epigenetic dynamics of the Kangl imprinted domain in the early embryo. transposons. This conflict most probably underlies the observation that 35. Chaumeil J, Le Baccon, P, Wutz, A. Heard, E. A novel role for Xist RNA in the formation ing to multigenerational influences on inheritance and phenotype(see page 396). Epigenetic inheritance across generations is relatively com mon in plants, but it is still unclear how widespread this phenomenon is in mammals or whether it has any role in shaping evolution to restrict enhancer access to igf2. Proc. An exciting question for future work is whether segregation of X-chromosome inactivation independent epigenetic marks in early development has any primary role in deter mining cell and lineage commitment. For example, the mechanism by 39. Sado, T et ol X inactivation in the mouse embryo deficient for Dnmt: distinct effect 0 ough a matter ofhot debate, is not really unde的m数2b23 which the first two cell lineages are allocated in mammalian pre- implan standing fundamental problem in developmental biology W.et al. Reactivation of the paternal X chromosome in early nbryos. Science 1. Takahashi, K& moto, L, Otte, A P, Allis, C D, Reinberg, D. Heard, E. Epigenetic dy defined factors. Cell 126, 663-676(2006 mprinted X inactivation during early mouse development. Science 303, 644-649 en, K, Dean, W.& Reik, W. Epigenetic reprogramming in mammals. Hum. Mol Genet. 14. R47-R58(2005) 43. Heard, E& Disteche, C.M. Dosage compensation in mammals: fine-tuning the expression 4. Bird, A DNA methylation patterns and epigenetic memory. Genes Dev. 16, 6-21(2002) 6. Turner bx ene. 3. 662-673(200enetic reprogramming in mammalian development. 45. Haijkova, . et aL. Epigenetic reprogramming in mouse primordial germ cells. Mech. Dev. 7 5. Li E Chromatin modifi 7. Ringrose, L& Paro, R Epigenetic regulation of cellular memory by the polycomb and i germ cells. Development 129, 1807-1817(2002). yer, L A et al. Polycomb coro Rev Genet. 38, 413-443(2004) 47. Seki, Yet al. Extensive and orderly reprogramming of genome-wide chromatin evelopmental regulators in murine modifications associated with specification and early development of germ cells in mice. embryonic stem cells. Nature 441, 349-353(2006) Dev. bio.278,40-458(2005) @2007 Nature Publishing GroupConclusions and outlook Development might be a one-way street because of the somatic inherit￾ance of epigenetic marks. Whether there is a linear relationship between acquisition of epigenetic marks and developmental progression is doubt￾ful; some key restrictions in developmental potential that are brought about by epigenetic regulation might occur very early in development. Judging from somatic-cell nuclear-transfer experiments, it is far from clear whether more-differentiated cells have more epigenetic marks or have marks that are more difficult for the oocyte to reprogramme67. Natural epigenetic reprogramming might be needed to ensure that development can start afresh in every new generation. Although various mechanisms for the rapid erasure of histone modifications have recently been identified, the mechanism of DNA demethylation still needs to be determined. Recent work on the erasure of DNA methylation from imprinted plant genes shows that base-excision repair has an important role, and it is possible that this is also the case in mammals. Because of the generally accurate heritability of DNA methylation and because of its chemical stability, erasure of DNA methylation might only be pos￾sible either by replicating DNA in the absence of DNMT1 or by breaking DNA. It is fascinating to see that both transcription-factor interactions and epigenetic programming and reprogramming seem to be needed to maintain pluripotency in early embryos and ES cells. Indeed, experi￾mental reprogramming of differentiated nuclei without using somatic￾cell nuclear transfer or cell fusion has been achieved recently, using a mix of pluripotency factors1 . It could be expected that forcing the expression of pluripotency transcription-factor networks would also activate epigenetic reprogramming factors, but whether this occurs is unclear. Perhaps combinations of transcription factors and epigenetic reprogramming factors are needed for more complete reprogramming of somatic cells to a pluripotent state, and this would be of great funda￾mental scientific and medical interest. In the animal kingdom, some epigenetic systems, such as imprinting, have evolved only in mammals. Many of the basic molecular building blocks for epigenetics, such as the enzymes for DNA methylation and histone modifications, are highly conserved in vertebrates, but the regu￾lation of epigenetic modifiers might evolve more rapidly together with specific developmental strategies. Therefore, evolutionary epigenetics and epigenomics will have an important role in discovering links between developmental adaptations and epigenetic regulators. There is probably a conflict between the requirement for eras￾ing epigenetic marks between generations and the requirement for maintaining others, such as those in imprinted genes and in some transposons. This conflict most probably underlies the observation that some epigenetic marks are not erased between generations, thereby lead￾ing to multigenerational influences on inheritance and phenotype (see page 396). Epigenetic inheritance across generations is relatively com￾mon in plants, but it is still unclear how widespread this phenomenon is in mammals or whether it has any role in shaping evolution61. An exciting question for future work is whether segregation of epigenetic marks in early development has any primary role in deter￾mining cell and lineage commitment. For example, the mechanism by which the first two cell lineages are allocated in mammalian pre-implan￾tation embryos, although a matter of hot debate, is not really understood. An epigenetic hypothesis might allow us to take a fresh look at a long￾standing fundamental problem in developmental biology. ■ 1. Takahashi, K. & Yamanaka, S. 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Formation of an active tissue-specific chromatin domain initiated by epigenetic marking at the embryonic stem cell stage. Mol. Cell. Biol. 25, 1804–1820 (2005). 10. Azuara, V. et al. Chromatin signatures of pluripotent cell lines. Nature Cell Biol. 8, 532–538 (2006). 11. Bernstein, B. E. et al. A bivalent chromatin structure marks key developmental genes in embryonic stem cells. Cell125, 315–326 (2006). 12. Klose, R. J., Kallin, E. M. & Zhang, Y. JmjC-domain-containing proteins and histone demethylation. Nature Rev. Genet. 7, 715–727 (2006). 13. Ohm, J. E. et al. A stem cell-like chromatin pattern may predispose tumor suppressor genes to DNA hypermethylation and heritable silencing. Nature Genet. 39, 237–242 (2007). 14. Feldman, N. Y. et al. G9a-mediated irreversible epigenetic inactivation of Oct-3/4 during early embryogenesis. Nature Cell Biol. 8, 188–194 (2006). 15. Simpson, A. J., Caballero, O. L., Jungbluth, A., Chen, Y. T. & Old, L. J. Cancer/testis antigens, gametogenesis and cancer. Nature Rev. Cancer 5, 615–625 (2005). 16. Hochedlinger, K., Yamada, Y., Beard, C. & Jaenisch, R. Ectopic expression of Oct-4 blocks progenitor-cell differentiation and causes dysplasia in epithelial tissues. Cell121, 465–477 (2005). 17. Boiani, M., Eckardt, S., Scholer, H. R. & McLaughlin, K. J. Oct4 distribution and level in mouse clones: consequences for pluripotency. Genes Dev. 16, 1209–1219 (2002). 18. Surani, M. A., Hayashi, K. & Hajkova, P. Genetic and epigenetic regulators of pluripotency. Cell128, 747–762 (2007). 19. Ancelin, K. et al. Blimp1 associates with Prmt5 and directs histone arginine methylation in mouse germ cells. Nature Cell Biol. 8, 623–630 (2006). 20. Maatouk, D. M. et al. DNA methylation is a primary mechanism for silencing postmigratory primordial germ cell genes in both germ cell and somatic cell lineages. Development133, 3411–3418 (2006). 21. Surani, A. & Reik, W. in Epigenetics (eds Allis, C. D., Jenuwein, T. & Reinberg, D.) 315–327 (Cold Spring Harbor Laboratory Press, Woodbury, 2007). 22. Bourc’his, D. & Bestor, T. H. Meiotic catastrophe and retrotransposon reactivation in male germ cells lacking Dnmt3L. Nature 431, 96–99 (2004). 23. Barlow, D. P. Methylation and imprinting: from host defense to gene regulation? Science 260, 309–310 (1993). 24. Bourc’his, D., Xu, G. L., Lin, C. S., Bollman, B. & Bestor, T. H. Dnmt3L and the establishment of maternal genomic imprints. Science 294, 2536–2539 (2001). 25. Kaneda, M. et al. Essential role for de novo DNA methyltransferase Dnmt3a in paternal and maternal imprinting. Nature 429, 900–903 (2004). 26. Jelinic, P., Stehle, J. C. & Shaw, P. The testis-specific factor CTCFL cooperates with the protein methyltransferase PRMT7 in H19 imprinting control region methylation. PLoS Biol. [online] 4, e355 (2006) (doi:10.1371/journal.pbio.0040355). 27. Howell, C. Y. et al. Genomic imprinting disrupted by a maternal effect mutation in the Dnmt1 gene. Cell104, 829–838 (2001). 28. Li, E., Beard, C. & Jaenisch, R. Role for DNA methylation in genomic imprinting. Nature 366, 362–365 (1993). 29. Sleutels, F., Zwart, R. & Barlow, D. P. The non-coding Air RNA is required for silencing autosomal imprinted genes. Nature 415, 810–813 (2002) 30. Mancini-Dinardo, D., Steele, S. J., Levorse, J. M., Ingram, R. S. & Tilghman, S. M. Elongation of the Kcnq1ot1 transcript is required for genomic imprinting of neighboring genes. Genes Dev. 20, 1268–1282 (2006). 31. Lewis, A. et al. Imprinting on distal chromosome 7 in the placenta involves repressive histone methylation independent of DNA methylation. Nature Genet. 36, 1291–1295 (2004). 32. Umlauf, D. et al. Imprinting along the Kcnq1 domain on mouse chromosome 7 involves repressive histone methylation and recruitment of Polycomb group complexes. Nature Genet. 36, 1296–1300 (2004). 33. Kanduri, C., Thakur, N. & Pandey, R. 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