scientific correspondence

the unmethylated tk gene (also 4.5-fold). In mediator of the global repression associated contrast, methylated tk sequences behave with DNA . DNA methylation models like endogenous inactive genes (for exam- It has been shown that the presence of ple, the IgȔ gene), and are not preferentially methyl groups on exogenously introduced associated with acetylated (Fig. sequences makes local structure 1a). Because identical DNA sequences were less accessible to the machin- One of the main determinants of chromatin used in both transfection experiments, this ery7 and molecular probes, such as DNaseI 8. structure is histone acetylation1. Local chro- decreased level of chromatin acetylation To determine whether this inactive chro- mosomal acetylation can be regulated must be due to the presence of 5-methyl matin structure is brought about by histone dynamically, both through the involvement . These experiments therefore deacetylation, cells carrying fully methyl-8 of transactivating factors with intrinsic his- demonstrate that DNA methylation influ- ated copies of the HSV tk gene were treated tone acetylase activity, and through the ences local histone acetylation, and it has with TSA. Although remaining fully methyl- recruitment of deacetylase complexes that now been shown that the methyl-binding ated (data not shown), these sequences are repress gene expression2. Histone acetyla- MeCP2 is probably involved in this converted to a DNaseI-sensitive conforma- tion status is transiently modified from one process by recruiting histone deacetylases3,4. tion (Fig. 1c), indicating that histone state to another in response to physiological We investigated whether this under- deacetylation itself is a critical factor in the changes operating in the cell. It is not yet acetylation contributes to transcriptional generation of DNaseI-insensitive structures known, however, how the basic histone inhibition. We used the drug trichostatin A associated with methylated DNA. acetylation profiles on tissue-specific and (TSA) to increase general histone acetyla- The mammalian genome has a bimodal sequences are estab- tion in the cell5 in an attempt to bypass the methylation pattern in which CpG islands lished during normal development. Here inhibitory effects of DNA methylation (Fig. in housekeeping genes are constitutively we show that DNA methylation takes part 1b). Treatment with TSA resulted in a 2.5- unmodified, whereas most tissue-specific in this process by inducing decreased levels fold increase in transcription from a fully genes are generally methylated in every cell of chromatin acetylation. methylated chicken tk gene. This stimula- type except those that actually express the Immunoprecipitation was used to iso- tion was the resulted of a specific release gene9. This profile is established through a late specifically the highly acetylated from methyl-induced repression, rather series of de novo and events, mononucleosome fraction1 from stably than a boost in , as tran- and is perpetuated by maintenance methyl- transfected cells carrying either methylated scription from both the endogenous ȋ- ation through subsequent cell divisions10. or unmethylated copies of the thymidine and the exogenous unmethylated chicken tk We propose that this pattern helps to set up kinase gene (tk) from herpes simplex virus genes was unaffected by TSA treatment. local histone acetylation states. After repli- (HSV). This technique clearly separates the Maximal induction by TSA (7-fold) was cation, newly deposited histones in transcriptionally active acetylated nucleo- obtained on an HSV tk gene methylated unmethylated regions may undergo acetyla- some fraction from bulk chromatin. The exclusively in non- regions6. Most tion, but containing methyl- constitutively expressed ȋ-actin gene, for DNA methyl sites in the genome are not ated DNA will be subject to deacetylation example, is enriched in the acetylated frac- located within regulatory elements. Thus, throughout the cell cycle. tion (4.5-fold), as are exogenous copies of histone deacetylation may be an important Histone acetylation is a fundamental regulatory mechanism for controlling gene accessibility, and it is therefore not surpris- ing that the cell uses a variety of different molecular pathways for its modulation. Our results indicate that in higher DNA methylation may serve as a unique mechanism for setting up local histone deacetylation, and so generate maintainable epigenetic chromosomal states. S. Eden, T. Hashimshony, I. Keshet, H. Cedar Department of Cellular , Hebrew University, Ein Kerem, Jerusalem 91120, Israel e-mail: [email protected] Figure 1 DNA methylation, histone acetylation and chromatin structure. a, Micrococcal -prepared A. W. Thorne mononucleosomes from pools of L-cell colonies stably transfected with unmethylated (upper portion) or in Biophysics Laboratories, University of Portsmouth, vitro SssI-methylated (lower portion) HSV tk were precipitated with antibodies mainly specific to acetylated Portsmouth PO1 2DT, UK (ref. 1). DNA was extracted from input ( I) and bound (B) fractions and subject to quantitative poly- 1. Hebbes, T. R.et al. C. EMBO J. 13, 1823–1830 (1994). merase chain reaction (PCR) using primers for HSV tk and mouse ȋ-actin or IgȔ. Two concentrations (1- and 2. Grunstein, M. Nature 389, 348–352 (1997). 3-fold) are shown. The degree of enrichment was determined by phosporimager or gel scan analysis. Methy- 3. Nan, X. et al. Nature 393, 386–389 (1998). lated HSV tk is present in higher average copy number than the unmethylated vector. Green blobs indicate 4. Jones, P. L. et al. Nature Genet.19, 187–191 (1998). 5. Yoshida, M., Horinouchi, S. & Beppu, T. Bioassays 17, 423–430 DNA methylation. b, Cells were stably transfected with unmethylated or in vitro SssI-methylated chicken tk (1995). 6 gene or the hybrid HSV tk gene methylated in the coding region (red) but not the promoter (yellow). Quanti- 6. Keshet, I., Yisraeli, J. & Cedar, H. Proc. Natl Acad. Sci. USA 82, tative PCR conditions were established empirically for each gene and cell type, and carried out on two con- 2560–2564 (1985). 7. Buschhausen, B., Wittig, M., Graessmann, M. & Graessmann, centrations (1- and 3-fold, or 1- and 5-fold) of equivalent amounts of cDNA from untreated or TSA-treated (50 A. Proc. Natl Acad. Sci. USA 84, 1177–1181 (1987). ǁ1 ȋ ng ml , 24 h) cells. The -actin results are from one of the cell lines. c, Nuclei from a mixture of cells contain- 8. Keshet, I., Lieman-Hurwitz, J. & Cedar, H. Cell 44, 535–543 ing either the fully methylated HSV tk or the unmethylated hamster aprt gene (DŽ TSA) were digested with (1986). increasing concentrations of DNaseI and the DNA was extracted and subject to blot hybridization analysis8. 9. Yeivin, A. & Razin, A. in DNA Methylation: Molecular and Biological Significance (eds Jost, J. P. & Saluz, H.P.) 523–568 The graph (based on phosphorimaging of the blots) shows tk sensitivity to DNaseI (percentage of total DNA) (Birkhauser, Basel, 1993). as compared with aprt (average of DŽTSA samples, which were very similar) on the same blot. 10.Kafri, T. et al. Genes Dev. 6, 705–714 (1992).

842 NATURE | VOL 394 | 27 AUGUST 1998 Nature © Macmillan Publishers Ltd 1998