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Biological Role

Chromatin Remodeling

Histone acetyltransferases serves many biological roles in the cell. Chromatin is a combination of proteins and DNA found in the nucleus, it undergoes many structural changes as different cellular events occur, such as DNA replication, DNA repair, and transcription . Chromatin in the cell can be found in two states condensed and uncondensed; the former known as Euchromatin is transcriptionally active while the latter, heterochromatin, is transcriptionally inactive , . Histones comprise the protein portion of chromatin; there are 5 different Histone proteins (H1, H2A, H2B, H3, and H4). A core histone is formed when a two of each histone subtype, excluding the H1 histone protein, form a quaternary complex. Together this octameric complex, in association with the 147 base pairs of DNA coiled around it, form the nucleosome . Histone H1 locks the nucleosome complex together, and is the last to bind in the complex. Histones tend to be positively charged proteins with N-terminal tails which stem from the core histone. The phosphodiester backbone on DNA on the other hand is negative, this allows for strong ionic interactions between histone proteins and DNA. Histone acetyltransferases transfer an acetyl group to specific lysine residues on histones neutralizing their positive charge which reduces the strong interaction between the histone and DNA . Acetlyation is also thought to perturb interactions between individual nucleosomes, as well as, act as sites of interactions for other DNA associated proteins . There can be different levels of histone acetylation, as well as, other modifications which allows the cell to have control over the level of chromatin packing during different cellular events such as replication, transcription, recombination, and repair. Acetylation is not the only regulatory post-translational modification to histones which dictate chromatin structure; methylation, phosphorylation ADP-ribosylation, and ubiquitiation have also been reported,. These combinations of different covalent modifications on the N-terminal tails has been referred to as the histone code, and it is also thought that this code may be heritable and preserved in the next cell generation . H3 and H4 histone proteins are primarily targeted by HAT’s but H2A and H2B are also acetylated. Lysines 9, 14, 18, and 23 of H3 and lysines 5, 8, 12, and 16 of H4 are targeted for acetylation ,. On histone subunit H2B lysines 5, 12, 15, and 20 are also acetylated, while only lysine 5 and 9 has been seen to be acetylated on histone H2A ,,. With so many different sites for acetylation a high level of specificity can be achieved in triggering specific responses. An example of this specificity is when histone H4 is acetylated at Lys-5 and Lys-12. This acetylation pattern has been seen during histone synthesis. Another example is acetylation of Lys-16 on H4 has been associated with dosage compensation of the male X chromosome in Drosophila melanogaster ,.

Gene Expression

Schematic showing histone acetyltransferases’ role in gene transcription

Histone modifications modulate the packing of chromatin. The level of packing of the DNA is important for gene transcription as transcriptional machinery must have access to promoter for transcription to occur. HAT’s neutralization of charged lysines allowed for the chromatin to decondense so that this machinery has access to the gene to be transcribed. Although, not all acetylation has been reported to enhance transcriptional activity; Acetylation of Lys-12 on H4 has been associated with condensed and transcriptionally inactive chromatin. Most hats act as transcriptional co-activators or gene silencers and most are often large complexes,10-20 subunits some of which are shared amongst different HAT complexes. These complexes include SAGA (for Spt/Ada/Gcn5L acetyltransferase), PCAF complex , ADA ( transcriptional adaptor), TFIID (transcription factor II D), TFTC (TBP-free TAF-containing complex), NuA3, and NuA4 (nucleosomal acetyltransferases of H3 and H4), . These complexes bring moderate the HAT specificity by bringing them to their target gene where they can then acetylate and HATs can only acetylate in these complexes. All known HAT transcriptional co-activators contain a bromodomain, a 110-amino acid module, which reconizes acetylated lysines and is functionally linked to the co-activators in the regulation of transcription.

Clinical Significance

Histone Acetyltransferases’ ability to manipulate chromatin structure and lay an epigenetic framework makes them essential in cell maintenance and survival. The process of chromatin remodeling involves several enzymes that assist in the reformation of the nucleosome and involve histone acetyltransferases and is required for DNA damage repair systems to occur . HAT has been implicated as an accessory to disease progression, specifically in neurodegenerative disorders. For instance, Huntington’s disease is a disease that affects motor skills and mental abilities. The only known mutation that has been implicated in the disease is in the N-terminal region of Huntingtin (Htt) . It has been reported that Htt directly interacts with HATs and represses HAT activity of CBP, p300, and PCAF in vitro. HATs have also been associated with control of learning and memory functions, studies have shown mice without PCAF or CBP have shown evidence of neurodegredation . Mice with PCAF deletion become incompetent of learning new things and those with the CBP knockout seemed to suffer from long-term memory loss . The misregulation of the equilibrium between acetylation and deacetylation has been associated with the manifestation of certain cancers. If histone acetyltransferases are inhibited, then damaged DNA may not be repaired, eventually leading to cell death. Controlling the chromatin remodeling process within cancer cells may provide a novel drug target for cancer research . Attacking this specific enzyme within cancer cells will cause apoptosis to occur more readily due to their high accumulation of DNA damage. One such inhibitor of histone acetyltransferase is called garcinol. Garcinol is found within the rinds of the Garcinia indica fruit, otherwise known as mangosteen. To explore the physiological effects of garcinol on histone acetyltransferase, researchers utilized HeLa cells. The cells underwent irradiation, creating double-strand breaks within the DNA, and garcinol was introduced into the cells to see if it influences the DNA damage response. If the garcinol is successful at inhibiting the non-homologous end joining, a DNA repair mechanism that shows preference in fixing double-strand breaks, then it may serve as a radiosensitizer, a molecule that increases the sensitivity of cells to radiation damage. Increase in radiosensitivity may increase the effectiveness of radiotherapy .