**2.1 Chromatin structure and histone modifications**

Chromatin is the physiological template of eukaryotic genome. Its fundamental unit, the nucleosome core particle, contains ~200 bp of DNA, organized by an octamer of small, basic proteins. The protein components are histones (two copies of each highly conserved core histone protein – H2A, H2B, H3 and H4). They form an interior core; the DNA lies on the surface of the particle. Nucleosomes are an invariant component of euchromatin and heterochromatin in the interphase nucleus, and of mitotic chromosomes. The nucleosome core particle represents the first level of organization, with a packing ratio of ~6. The second level of organization is the coiling of the series of nucleosomes into a helical array

Histone Modification and Breast Cancer 323

Histone acetylation is a dynamic process directed by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Normally, Transcription factors recruit coactivators with HAT activity to regulatory DNA sites, whereas transcriptional repressors recruit corepressors with HDAC activity (Sun et al., 2001). A summary of known HAT proteins is

Many HATs have also be showed to be involved in breast cancer. Among of them, p300/CBP and NCOAs are the most important and well-characterised HAT proteins

p300 and its close homolog CBP (CREB-binding protein) are often referred to as a single entity. p300 and CBP share several conserved domains: (1) the bromodomain (Br), which is frequently found in mammalian HATs; (2) three cysteine-histidine (CH)-rich domains (CH1, CH2 and CH3); (3) a KIX domain; and (4) an ADA2-homology domain, which shows extensive similarity to Ada2p, a yeast transcriptional co-activator. The N- and C-terminal domains of p300/CBP can act as transactivation domains, and the CH1, CH3 and the KIX domains are likely to be important in mediating protein-protein interactions, and a number of cellular and viral proteins bind to these regions. The acetyl-transferase domain is located in the central region of the protein, and the Br domain could function in recognising different acetylated motifs (Fig 3A, B) (Chan et al., 2001). p300/CBP contribute to acetylation of H3-K56 and promotes the subsequent assembly of newly-synthesized DNA into chromatin (Das et al., 2009). It is a non-DNA-binding transcriptional coactivator which stimulates transcription of target genes by interacting, either directly or through cofactors, with numerous promoter-binding transcription factors such as CREB, nuclear hormone receptors, and oncoprotein-related activators such as c-Fos, c-Jun, c-Myb and AML1 (Fig 3C)

presented in Table 1 (Sterner et al., 2000; Yang, 2004; Kimura et al., 2005).

Fig. 2. Major sites of histone modifications

associated with breast cancer.

**2.2.1.1 p300/CBP** 

**2.2 Histone modifications in breast cancer 2.2.1 Histone acetylation in breast cancer** 

(Kitabayashi et al., 1998; Sterner et al., 2000).

to form the fiber with ~30 nm diameter, which is found in both interphase chromatin and mitotic chromosomes. This brings the packing ratio of DNA to ~40 in chromatin. The fiber-like structure requires additional proteins, which has not been well defined. The final packing ratio is determined by the third level of organization, the packaging of the 30 nm fiber itself. This gives a total packing ratio of ~ 1000 in euchromatin, cyclically interchangeable with packing into mitotic chromosomes to reach an overall ratio of ~10,000. Heterochromatin generally has a packing ratio -10,000 in both interphase and mitosis (Fig 1) (Lewin, 2004).

Fig. 1. Chromatin structure in eukaryotic cells

Local chromatin architecture is now generally recognized as an important factor in the regulation of gene expression. This architecture of chromatin is strongly regulated by posttranslational modifications of the N-terminal tails of the histones. Core histones are subjected to a wide range of covalent modifications including methylation, acetylation, phosphorylation, ubiquitination, sumoylation, ADP ribosylation, deamination, prolineisomerization (Fig 2) (Jovanovic et al., 2010). These modifications lead to a combinatorial histone code that demarcates chromatin regions for transcription activation or repression. Although the histone code is not fully investigated, specific marks such as lysine acetylation (H3K9ac, H3K18ac, and H4K12ac), lysine trimethylation (H3K4me3), and arginine dimethylation (H4R3me2) are generally associated with transcriptionally active gene promoters, whereas some other modifications such as lysine methylation (H3K9me2, H3K9me3 and H4K20me3) are associated with transcriptional repression. Global loss of acetylation (K16) and trimethylation (K20) of histone H4 have been shown to be characteristic of human cancer (Elsheikh et al., 2009).

to form the fiber with ~30 nm diameter, which is found in both interphase chromatin and mitotic chromosomes. This brings the packing ratio of DNA to ~40 in chromatin. The fiber-like structure requires additional proteins, which has not been well defined. The final packing ratio is determined by the third level of organization, the packaging of the 30 nm fiber itself. This gives a total packing ratio of ~ 1000 in euchromatin, cyclically interchangeable with packing into mitotic chromosomes to reach an overall ratio of ~10,000. Heterochromatin generally has a packing ratio -10,000 in both interphase and

Local chromatin architecture is now generally recognized as an important factor in the regulation of gene expression. This architecture of chromatin is strongly regulated by posttranslational modifications of the N-terminal tails of the histones. Core histones are subjected to a wide range of covalent modifications including methylation, acetylation, phosphorylation, ubiquitination, sumoylation, ADP ribosylation, deamination, prolineisomerization (Fig 2) (Jovanovic et al., 2010). These modifications lead to a combinatorial histone code that demarcates chromatin regions for transcription activation or repression. Although the histone code is not fully investigated, specific marks such as lysine acetylation (H3K9ac, H3K18ac, and H4K12ac), lysine trimethylation (H3K4me3), and arginine dimethylation (H4R3me2) are generally associated with transcriptionally active gene promoters, whereas some other modifications such as lysine methylation (H3K9me2, H3K9me3 and H4K20me3) are associated with transcriptional repression. Global loss of acetylation (K16) and trimethylation (K20) of histone H4 have been shown to be

mitosis (Fig 1) (Lewin, 2004).

Fig. 1. Chromatin structure in eukaryotic cells

characteristic of human cancer (Elsheikh et al., 2009).

Fig. 2. Major sites of histone modifications
