Epigenetics histone modifications

An important aspect of the histone code theory is that some modifications can be passed on during cell division. As a result histone modification patterns including histone acetylation serve as a means to store inheritable traits of an organism which are not DNA encoded. This kind of information is generally termed epigenetic information. 

Hake SB, Xiao A, Allis CD (2004) Linking the epigenetic language of covalent histone modifications to cancer. Br J Cancer 90 761-769... 

The biologic processes described above and cytosine methylation, fit the epigenetic definition well because they are heritable. However, whether the entire repertoire of histone modifications is heritable remains to be established. In fact, it is likely that only a subset will have epigenetic inheritance. 

Epigenetics refers to the inheritance of information from a single cell to another, independent of the DNA sequence. Epigenetic mechanisms include DNA methylation, histone modifications, and, more recently, a variety of noncoding RNAs (ncRNAs). These mechanisms have been extensively reviewed elsewhere (7). Here, we briefly summarize them and emphasize those features relevant in the field of epigenomics. 

Although ncRNAS are not strictly part of the epigenetic machinery, there is evidence that some of them may actually work as epigenetic regulators since they can interfere with the epigenetic machinery (DNA methylation and histone modifications), thus influencing gene expression (see Section 2.2) (26,27). 

From the previous sections it is now possible to define epigenomics. It refers to the study of the complete set of epigenetic modifications (DNA methylation, histone modifications, and/or specific ncRNAs) that occur across the entire genome of a cell/tissue at a specific time (28). 

Paramutation is defined as the interaction between two alleles in a particular locus in which one induces a heritable change on the other one (99,100). Importantly, this change occurs in the absence of any alteration in DNA sequence, indicating that paramutations are mediated by epigenetic modifications. Paramutations were originally described in maize and several of them have already been characterized. Recently, paramutations have been described in mammals (mice) (101). The authors provide evidence that paramutations in mice involve RNAs, since they were not able to detect changes in DNA methylation or histone modifications. Yet, it is unclear whether paramutations in mice are mediated by post-transcriptional or post-translational modifications (100). 

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