Histones are subject to a wide variety of posttranslational modifications including but not limited to, lysine acetylation, lysine and arginine methylation, serine and threonine phosphorylation, and lysine ubiquitination and sumoylation (Vasquero 2003). These modifications occur primarily within the histone amino-terminal tails protruding from the surface of the nucleosome as well as on the globular core region (Cosgrove 2004).
Histone modifications are proposed to affect chromosome function through at least two distinct mechanisms. The first mechanism suggests modifications may alter the electrostatic charge of the histone resulting in a structural change in histones or their binding to DNA. The second mechanism proposes that these modifications are binding sites for protein recognition modules, such as the bromodomains or chromodomains, that recognize acetylated lysines or methylated lysine, respectively.
The existence of these modifications and recognition modules led to a well established “histone code” hypothesis proposed by Strahl and Allis (2000). Overall, posttranslational modifications of histones create an epigenetic mechanism for the regulation of a variety of normal and disease-related processes.
Histones are the chief protein components of chromatin. They act as spools around which DNA winds and they play a role in gene regulation.
Classes
Six major histone classes are known:
H1 (sometimes called the linker histone or H5.)
H2A
H2B
H3
H4
Archaeal histones
Two each of the class H2A, H2B, H3 and H4, so-called core histones, assemble to form one octameric nucleosome core particle by wrapping 146 base pairs of DNA around the protein spool in 1.65 left-handed super-helical turn. The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order structure. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA spaced between each nucleosome (also referred to as linker DNA). Higher order structures include the 30 nm fiber (forming an irregular zigzag) and 100 nm fiber, these being the structures found in normal cells. During meiosis, through the combination of nucleosome interactions with other proteins, the chromosome is assembled. The assembled histones and DNA is called chromatin.
Two each of the class H2A, H2B, H3 and H4, so-called core histones, assemble to form one octameric nucleosome core particle by wrapping 146 base pairs of DNA around the protein spool in 1.65 left-handed super-helical turn. The linker histone H1 binds the nucleosome and the entry and exit sites of the DNA, thus locking the DNA into place and allowing the formation of higher order structure. The most basic such formation is the 10 nm fiber or beads on a string conformation. This involves the wrapping of DNA around nucleosomes with approximately 50 base pairs of DNA spaced between each nucleosome (also referred to as linker DNA). Higher order structures include the 30 nm fiber (forming an irregular zigzag) and 100 nm fiber, these being the structures found in normal cells. During meiosis, through the combination of nucleosome interactions with other proteins, the chromosome is assembled. The assembled histones and DNA is called chromatin.
Structure
The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer, forming two nearly symmetrical halves by tertiary structure (C2 symmetry; one macromolecule is the mirror image of the other). The H2A-H2B dimers and H3-H4 tetramer also show pseudodyad symmetry.
The 4 'core' histones (H2A, H2B, H3 and H4) are relatively similar in structure and are highly conserved through evolution, all featuring a 'helix turn helix turn helix' motif (which allows the easy dimerisation). They also share the feature of long 'tails' on one end of the amino acid structure - this being the location of post-transcriptional modification
In all, histones make five types of interactions with DNA:
Helix-dipoles from alpha-helices in H2B, H3, and H4 cause a net positive charge to accumulate at the point of interaction with negatively charged phosphate groups on DNA.
Hydrogen bonds between the DNA backbone and the amine group on the main chain of histone proteins.
Nonpolar interactions between the histone and deoxyribose sugars on DNA.
Salt links and hydrogen bonds between side chains of basic amino acids (especially lysine and arginine) and phosphate oxygens on DNA.
Non-specific minor groove insertions of the H3 and H2B N-terminal tails into two minor grooves each on the DNA molecule.
The highly basic nature of histones, aside from facilitating DNA-histone interactions, contributes to the water solubility of histones.
Histones are subject to posttranslational modification by enzymes primarily on their N-terminal tails, but also in their globular domains. Such modifications include methylation, citrullination, acetylation, phosphorylation, Sumoylation, ubiquitination, and ADP-ribosylation. This affects their function of gene regulation
In general, genes that are active have less bound histone, while inactive genes are highly associated with histones during interphase. It also appears that the structure of histones have been evolutionarily conserved, as any deleterious mutations would be severely maladaptive.
Functions
Packing proteins
Histones act as spools around which DNA winds. This enables the compaction necessary to fit the large genomes of eukaryotes inside cell nuclei: the compacted molecule is 50,000 times shorter than an unpacked molecule.
Histone modifications in chromatin regulation
Histones undergo posttranslational modifications which alter their interaction with DNA and nuclear proteins. The H3 and H4 histones have long tails protruding from the nucleosome which can be covalently modified at several places. Modifications of the tail include methylation, acetylation, phosphorylation, ubiquitination, sumoylation, citrullination, and ADP ribosylation. The core of the histones (H2A and H3) can also be modified. Combinations of modifications are thought to constitute a code, the so-called "histone code". Histone modifications act in diverse biological processes such as gene regulation, DNA repair and chromosome condensation (mitosis)
Dr Joel Gottesfeld's lab in La Jolla, California (USA), is also working on heterochromatin modifiers as a potential treatment for FRDA, and Dr Gottesfeld started his talk by referencing a paper published by Richard Festenstein's group in 2003 (Nature) as the inspiration. His own work in California has supported the theory that heterochromatin gene silencing occurs in FRDA. The interaction between the active form of the frataxin gene and the inactive (silenced) form is mediated by enzymes called histone deacetylases (HDAC). To see if targeting these enzymes would result in an increase in frataxin, the researchers tested various commercially available HDAC inhibitors and found only one that was slightly active on the frataxin gene (compound BML-210). However they were then able to study derivatives of this compound and identified a compound (4B) which is active at increasing levels of frataxin mRNA and frataxin protein in cells. The next stage was to test the compound in a mouse model of FRDA, and they found that it was able to cross the blood brain barrier and enter the nervous system; it also inhibited HDAC in the brain. There are different classes of HDACs, based on whether they are zinc dependent or non-zinc dependent, and different HDAC inhibitors act on each of the different classes. It is important for research to identify which enzymes are the target, and this gives clues as to the desired chemical properties of the inhibitor. Chemical studies that his team has carried out have now suggested that the class 1 HDAC enzymes are the target for HDAC inhibitors which work on the frataxin gene, and they have identified HDAC3 as the primary target.
The pharmaceutical company Repligen is now working with the active HDAC inhibitors that have been identified, doing pharmacokinetic and toxicology studies to learn more and identify the most effective compounds. A library of derivatives that can be screened for potential treatments has been established, and toxicity studies are going well.
