Two Epigenetic Targets Are More Effective Than One

Lysine-specific histone demethylase 1 (LSD1) via Wikimedia Commons

Epigenetics is a new and exciting territory to explore as we understand more about the role it plays in gene silencing and expression. Because epigenetic regulation of gene expression is caused by specific modification of histone proteins (e.g., methylation) that play a role in disease states like cancer, enzymes like histone deacetylases (HDACs) become viable drug targets. One drawback to inhibiting proteins that modify histones is even when selectively targeting HDACs, the effects can be far ranging with multiple HDAC-containing protein complexes found throughout the cell. These broad effects minimize the effectiveness of an inhibitor, caught between efficacy and toxicity. A recent article in Nature Communications explored how using a single compound to target two epigenetic enzymes was more effective than any individual inhibitor or combination of inhibitors. Continue reading

Rewriting the Histone Code: Searching for Treatments for Stage IV Thyroid Cancers

Chromatin fiberOften a diagnosis of thyroid cancer is associated with a good prognosis and fairly straightforward surgical treatments to remove the tumor followed by radioactive iodine ablation. Such treatment works well in tumors that have not metastasized and retain enough of their thyroid cell “identity” that they can still accumulate radioactive iodine.

However, aggressive thyroid cancers, which often metastasize and recur, do not respond to standard treatments because they are generally too dedifferentiated to accumulate iodine, so alternative treatments are needed.

One approach is to look for compounds that will reverse dedifferentiation, making tumor cells more likely to take up and concentrate radioactive iodine regardless of their location in the body. One possible target to effect dedifferentiation is epigenetic modification of histone proteins.

Histone proteins are more than the structural components of the nucleosome that organizes the chromatin inside cells. Histone proteins are subject to a host of protein modifications on their N-terminal tails such as acetylation, phosphorylation, methylation, ubiquitination and ADP-ribosylation. These various modifications are seen as creating a “histone code” that is read by other proteins and protein complexes (1). This code regulates patterns of gene expression and activity for a cell—in part resulting in a differentiated phenotype. Previous studies have suggested that some histone deacetylase (HDAC) inhibitors (e.g., valproic acid) can reverse some of the dedifferentiation associated with aggressive cancers (2).

Jang, et al. in a recent paper (3) published in Cancer Gene Therapy synthesized a group of HDAC inhibitor analogs (AB1–AB13) and tested them for their ability to inhibit growth of three aggressive human thyroid cancer cell lines and induce partial re-differentiation to the thyroid cell phenotype. Continue reading

Histones and Histone Deacetylases

Epigenetics is an increasingly big deal in biological discovery. We are regularly reading about the influence of actions peripheral to DNA in regulating DNA transcription and translation. We are learning that mice may fear what grandparent mice feared (Kelly’s blog ), due to heritable changes in DNA. In term of one of several mechanisms of epigenetic change, we are learning much about histone deacetylases and their role in gene regulation, as well as disease (Isobel’s blog ). In this blog, let’s take a step back and look at histones, and how they are influenced by acetylation/deacetylation.

The Role of Histones
Histones are proteins found in the nucleus of eukaryotic cells, where they package DNA into nucleosomes. Histones make up the main protein component of chromatin, acting as spool-like structures around which DNA wraps.

There are five major histone classes,three of these are core histones, the other two are called linker histones. Core histones comprise the core of the nucleosome, around which DNA is wrapped, while the linker histones bind at the entrance and exit sites of the DNA, so as to lock it into place. The linker histones also enable a higher order of structure. If you hold both ends of a rubber band, and twist one end, you’ll see that the rubber band twists and folds over itself; the end being held steady enables this twisting and folding: this is how the linker histones work. Histone-DNA structure is frequently represented as a beaded chain-type image (see figure).

DNA wrapping around histones in a bead and chain-like fashion.

DNA wrapping around histones in a bead and chain-like fashion.

Histone and DNA: Charged Interactions
Histone tails normally carry a positive charge due to amine groups present on their lysines and arginines. This positive charge is the means by which histone tails interact with and bind to the negatively-charged phosphate groups on the DNA backbone.

Histones are subject to post-translational modifications, primarily on their N-terminal tails, by enzymes. Such modifications include methylation, citrullination, acetylation, phosphorylation, SUMOylation, ubiquitination, and ADP-ribosylation. Such modifications can affect histone function in gene regulation. Acetylation is one of the most common post-translational modifications of histones (1). Continue reading

Decorating Histones and Their Tails

There are at least two tail stories associated with big scientific discoveries. One is Darwin’s story about the tail loss during human evolution process. The other story is associated with discovery of benzene ring structure. In his creative dream Kekule saw the snake (the linear carbon chain) eating its own tail. Even better representation of benzene structure is the comic image of six monkeys holding each other hands and tails. Nowadays, the most popular scientific story in the field of epigenetics is the story of histones and their tails. This time instead of monkey or snake, an elephant is the animal whose characteristics allegorically represent epigenetics.

elephants_web

We all know how the histone octamer wrapped with DNA represents a nucleosome – the first unit of chromatin formation. Histones, which are basic due to numerous arginines and lysines, easily attract negatively charged DNA and in that way facilitate formation of nucleosome. The nature of two materials is important but not sufficient for such complex biological function like efficient packaging of DNA and regulation of gene expression. For that reason both the DNA and histones are decorated by numerous chemical groups.

Post-translational modifications (PTMs) of histones and histone variants themselves can cause alternation of net charge, changes histone dynamics and interaction with other chromatin proteins. The extreme complexity of interactions that can be achieved by histone modifications inspired Jenuwein and Allis to launch an idea of “histone or epigenetic code”. Core histones consist of a N- terminal tail, the globular portion and a C terminus. PMTs were discovered first on the N-terminal tail of core histones. However, the logical question was: Are only the tails decorated or are there more?” Continue reading

A Scalable and Sensitive Assay for HDAC Activity

Dysfunction of histone deacetylases (HDACs) is associated with many diseases including cancers, asthma and allergies, inflammatory diseases and disorders affecting the central nervous system. Because of their involvement in such a wide range of pathologies, HDACs have become a target for drug discovery. Traditional HDAC activity assays are either isotopic or fluorescent assays using artificial substrates that are prone to artifacts or fluorescence interference. There is a need for a functional assay that is sensitive, accurate and amenable to drug-screening activities.

A recent paper by Halley et al. in the Journal of Biomolecular Screening describes the evaluation of a bioluminogenic HDAC assay, the HDAC-Glo™ I/II and SIRT-Glo™ Assays. Continue reading