Histone post-translational modifications (PTMs), known as the “histone code”, are key mechanisms of epigenetics that modulate chromatin structures. The PTMs on histone including acetylation, methylation, phosphorylation, and novel acylations directly affect the accessibility of chromatin to transcription factors and other epigenetic regulators, altering genome stability and gene transcription. Histone acetylation, tightly controlled by the opposing action of histone acetyltransferases (HATs) and histone deacetylases (HDACs), occurs primarily at lysine residues on the N-terminal tails of histones H2A (Lys5, 9, and 15), H2B (Lys5, 12, 15, 16, and 20), H3 (Lys4, 9, 14, 18, 23, 27, and 36), and H4 (Lys5, 8, 12, 16, and 20), and plays vital roles in the regulation of gene expression, DNA damage repair, chromatin dynamics, etc.
Histones are basic nuclear proteins that are responsible for the nucleosome structure of the chromosomal fiber in eukaryotes. Two molecules of each of the four core histones (H2A, H2B, H3, and H4) form an octamer, around which approximately 146 bp of DNA is wrapped in repeating units, called nucleosomes. The linker histone, H1, interacts with linker DNA between nucleosomes and functions in the compaction of chromatin into higher order structures. This gene is intronless and encodes a replication-dependent histone that is a member of the histone H4 family. Transcripts from this gene lack polyA tails but instead contain a palindromic termination element. This gene is found in the histone microcluster on chromosome 6p21.33.
Histone post-translational modifications (PTMs), known as the “histone code”, are key mechanisms of epigenetics that modulate chromatin structures. The PTMs on histone including acetylation, methylation, phosphorylation, and novel acylations directly affect the accessibility of chromatin to transcription factors and other epigenetic regulators, altering genome stability and gene transcription. Histone acetylation, tightly controlled by the opposing action of histone acetyltransferases (HATs) and histone deacetylases (HDACs), occurs primarily at lysine residues on the N-terminal tails of histones H2A (Lys5, 9, and 15), H2B (Lys5, 12, 15, 16, and 20), H3 (Lys4, 9, 14, 18, 23, 27, and 36), and H4 (Lys5, 8, 12, 16, and 20), and plays vital roles in the regulation of gene expression, DNA damage repair, and chromatin dynamics. Recent research has identified that H2BK120ac, H3.3K18ac, and H4K77ac are significantly associated with the survival of hepatocellular carcinoma (HCC) patients. Furthermore, H4K77ac has been linked to HCC recurrence. These findings suggest that H2BK120ac, H3.3K18ac, and H4K77ac may be potential prognostic factors for HCC.
Histones are a family of basic proteins that form the core of the nucleosome – the fundamental structural unit of chromatin. In a single nucleosome, core histone proteins H2A, H2B, H3, and H4 form an octamer around which the DNA is tightly wrapped. Histone proteins not only serve to compact chromosomal DNA but also play vital roles in the dynamic and long-term regulation of genes by a wide variety of post-translational modifications (PTMs). These PTMs including acetylation, methylation, phosphorylation, and novel acylations directly affect the accessibility of chromatin to transcription factors and other epigenetic regulators, altering genome stability and gene transcription. Histone H4 is primarily acetylated at Lys5, 8, 12, and 16, and methylated at Lys20.
The amino-terminal tails of core histones undergo lysine methylation in multiple sites, termed as “histone code” or “epigenetic code”. Lysine methylation in core histones is a major determinant for the formation of active and inactive regions of the genome and therefore plays vital roles in multiple cellular events. In most species, lysine methylation occurs primarily on histones H3 (Lys4, 9, 27, 36, 79) and H4 (Lys5, 8, 12, 20) and has been implicated in both transcriptional activation and silencing. Methylation in histones modulated by specific histone methyltransferases (HMTs) and histone demethylases (HDMs) is impaired in the pathologies of cancer and other diseases and therefore, enzymes regulating histone lysine methylation have become promising targets for anti-cancer drugs.
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