Mutations in the gene encoding the methyl-CG binding protein MeCP2 cause several neurological disorders including Rett syndrome. The di-nucleotide methyl-CG (mCG) is the classical MeCP2 DNA recognition sequence, but additional methylated sequence targets have been reported. Here we show by in vitro and in vivo analyses that MeCP2 binding to non-CG methylated sites in brain is largely confined to the tri-nucleotide sequence mCAC. MeCP2 binding to chromosomal DNA in mouse brain is proportional to mCAC + mCG density and unexpectedly defines large genomic domains within which transcription is sensitive to MeCP2 occupancy. Our results suggest that MeCP2 integrates patterns of mCAC and mCG in the brain to restrain transcription of genes critical for neuronal function.

During development from the fertilized egg to a multicellular organism, cell fate decisions have to be taken and cell lineage or tissue-specific gene expression patterns are created and maintained. These alterations in gene expression occur in the context of chromatin structure and are controlled by chromatin modifying enzymes. Gene disruption studies in different genetic systems have shown an essential role of various histone deacetylases (HDACs) during early development and cellular differentiation. In this review, we focus on the functions of the class I enzymes HDAC1 and HDAC2 during development in different organisms and summarise the current knowledge about their involvement in neurogenesis, myogenesis, haematopoiesis and epithelial cell differentiation. KEY WORDS: HDAC1, HDAC2, chromatin, differentiation, development Histone deacetylasesPosttranslational modifications of histones cause changes in the accessibility of DNA, thereby regulating many important cellular processes including transcription. Acetylation of core histones leads to a change in the net positive charge of histone tails and local opening of chromatin structure, a feature of transcriptionally active genes. On the other hand, deacetylated histone tails interact more closely with DNA and lead to a repressive state. Histone deacetylases (HDACs) catalyse the removal of acetyl groups from histone tails and are therefore considered as transcriptional corepressors. In addition to histones, HDACs also deacetylate non-histone proteins, such as the cytoskeletal protein tubulin (Hubbert et al., 2002) or transcription factors including p53 (Luo et al., 2000), E2F1 (MartinezBalbas et al., 2000) and YY1 (Yao et al., 2001). In fact, phylogenetic analysis suggests that the enzymatic activity was initially directed against non-histone proteins in a common ancestor devoid of histones (Gregoretti et al., 2004). In mammals, 18 deacetylases have been identified so far. These enzymes have been divided into 4 classes based on sequence similarity: Classic HDACs comprise class I (Saccharomyces cerevisiae Rpd3-like), class II (Saccharomyces cerevisiae Hda1-like) and class IV (HDAC11-like) enzymes. Class III consists of NAD-dependent, functionally unrelated Sir2-like deacetylases Int. J. Dev. Biol. 53: 275-289 (2009) Abbreviations used in this paper: HAT, histone acetyltransferase; HDA, HDAC, histone deacetylase; HDI, HDAC inhibitor; NuRD, nucleosome remodelling and deacetylase complex; Rpd3, reduced potassium dependency 3.named "sirtuins". Class I, II and IV HDACs are members of an ancient enzyme family, highly conserved throughout eukaryotic and prokaryotic evolution and are found in animals, plants, fungi, archaebacteria and eubacteria (Gregoretti et al., 2004). Class I histone deacetylasesPhylogenetic analysis revealed that class I genes in animals can be grouped into HDAC1/HDAC2, HDAC3 and HDAC8-like genes. Members of each subclass have been identified in protostomia (Oger et al., 2008) and deuterostomia. Species analysed so far carry orthologs (...

Rett syndrome is caused by mutations in the X-linked MECP2 gene, which encodes a chromosomal protein that binds to methylated DNA. Mouse models mirror the human disorder and therefore allow investigation of phenotypes at a molecular level. We describe an Mecp2 allelic series representing the three most common missense Rett syndrome (RTT) mutations, including first reports of Mecp2[R133C] and Mecp2[T158M] knock-in mice, in addition to Mecp2[R306C] mutant mice. Together these three alleles comprise ∼25% of all RTT mutations in humans, but they vary significantly in average severity. This spectrum is mimicked in the mouse models; R133C being least severe, T158M most severe and R306C of intermediate severity. Both R133C and T158M mutations cause compound phenotypes at the molecular level, combining compromised DNA binding with reduced stability, the destabilizing effect of T158M being more severe. Our findings contradict the hypothesis that the R133C mutation exclusively abolishes binding to hydroxymethylated DNA, as interactions with DNA containing methyl-CG, methyl-CA and hydroxymethyl-CA are all reduced in vivo. We find that MeCP2[T158M] is significantly less stable than MeCP2[R133C], which may account for the divergent clinical impact of the mutations. Overall, this allelic series recapitulates human RTT severity, reveals compound molecular aetiologies and provides a valuable resource in the search for personalized therapeutic interventions.