Category Archives: Epigenetics
On the Trail of the Epigenetic Code: Test System on Drosophila Should Provide the Key to Histone Function
The condensation of the DNA involves a dramatic restructuring of the two metre-long DNA thread to a chromosome that has a diameter of 1.5 micrometres. The DNA is wound around the packaging proteins called “histones”. (Credit: Max Planck Society)
Test system on Drosophila should provide the key to histone function. The genetic inherited material DNA was long viewed as the sole bearer of hereditary information. The function of its packaging proteins, the histones, was believed to be exclusively structural. Additional genetic information can be stored, however, and passed on to subsequent generations through chemical changes in the DNA or histones.
Scientists from the Max Planck Institute for Biophysical Chemistry in Göttingen have succeeded in creating an experimental system for testing the function of such chemical histone modifications and their influence on the organism. Chemical modifications to the histones may constitute an “epigenetic histone code” that complements the genetic code and decides whether the information from certain regions of the DNA is used or suppressed.
The research, now available online, appears Nov. 1 in EMBO reports.
How do you get a two-metre-long DNA thread into the cell nucleus? By winding it into a ball, of course! The DNA is wound around proteins known as histones, becoming 50,000 times shorter as a result. Other proteins then aggregate on it to form chromatin and, finally, the chromosomes. The latter are the product of an ingenious packaging trick. The five types of histones (H1, H2A, H2B, H3 und H4) fulfil even more tasks, however, and this is what makes them so fascinating. Histones can have small chemical attachments, such as acetyl, methyl and phosphate groups, in different places. These cause the opening of the chromatin, for example, and hence enable the genetic information to be read. Conversely, certain areas of the DNA molecule can be deactivated and rendered unreadable through other modifications, such as the binding of proteins. Scientists refer to this process as “gene silencing.” “The histone modifications can intervene in the control of gene activity in this way and, as a result, complement the genetic code,” explains Herbert Jäckle, Director of the Max Planck Institute for Biophysical Chemistry in Göttingen.
Every time a cell divides, this modification pattern of the histones is inherited by the daughter cells. The scientists assume that this epigenetic inheritance is controlled by a cell-specific or organ-specific “histone code.” “This decides whether the cell machinery has access to the DNA-coded genes or whether the access is blocked,” says Jäckle. The scientists would very much like to crack this code: for the production of the histones, hundreds of gene copies are stored in the genome of higher organisms. Therefore, up until now, it appeared to be impossible to switch off these gene copies and replace them with genetically-modified histone variants. Researchers could only create a test system if they managed to do this: if these variants lack certain docking sites, for example for chemical groups, certain modifications to the histones could be prevented and it would then be possible to investigate the extent to which the absence of these modifications leads to diagnosable defects in the organism.
Article Continues -> http://www.sciencedaily.com/releases/2010/10/101011125957.htm
This image shows the 5,372 samples as dots colour-coded for the six major clusters identified by comparing gene expression profiles. The left and right panels of the figure are projections of the same three-dimensional shape viewed from two different perspectives. (Credit: Brazma / EMBL)
Just like members of an orchestra are active at different times although playing the same piece of music, every cell in our body contains the same genetic sequence but expresses this differently to give rise to cells and tissues with specialised properties.
By integrating gene expression data from an unprecedented variety of human tissue samples, Alvis Brazma and his team at the European Bioinformatics Institute, an outstation of the European Molecular Biology Laboratory (EMBL), and their collaborators have for the first time produced a global map of gene expression. The full analysis behind this unique view of the genetic activities determining our appearance, function and behaviour is published in Nature Biotechnology.
The analysis used data collected from 163 laboratories worldwide involving 5,372 human samples from various tissues, cell types and diseases. Most transcriptomics experiments compare gene expression in only a few cell types or conditions and although technically challenging, integrating this data on a large-scale has created a new way for scientists to explore gene expression. The analysis is visualised as a map subdividing the human gene expression space into six distinct major groups or ‘continents’.
The continents emerged by grouping samples with similar gene expression signatures. This established the identity of the six groups: brain; muscle; hematopoietic (blood related); healthy and tumour solid tissues; cell lines derived from solid tissues; and partially differentiated cells. By visualising these subsets in 3D, comparisons can be made on the degree of similarity in the gene expression profiles on each grouping. For example, analysis of the continents showed that cell lines are usually more similar to each other than to their tissue of origin.
A new bioinformatics service allowing anyone to explore this expression map has been developed by the European Bioinformatics Institute as part of the ArrayExpress Gene Expression Atlas resource (www.ebi.ac.uk/gxa/).
Adapted from materials provided by European Molecular Biology Laboratory.
- Margus Lukk, Misha Kapushesky, Janne Nikkilä, Helen Parkinson, Angela Goncalves, Wolfgang Huber, Esko Ukkonen, Alvis Brazma. A global map of human gene expression. Nature Biotechnology, 8 April 2010 DOI: 10.1038/nbt0410-322
Although the human genome sequence faithfully lists (almost) every single DNA base of the roughly 3 billion bases that make up a human genome, it doesn’t tell biologists much about how its function is regulated. (Credit: iStockphoto/Andrey Prokhorov)
Although the human genome sequence faithfully lists (almost) every single DNA base of the roughly 3 billion bases that make up a human genome, it doesn’t tell biologists much about how its function is regulated. Now, researchers at the Salk Institute provide the first detailed map of the human epigenome, the layer of genetic control beyond the regulation inherent in the sequence of the genes themselves.
“In the past we’ve been limited to viewing small snippets of the epigenome,” says senior author Joseph Ecker, Ph.D., professor and director of the Genomic Analysis Laboratory at the Salk Institute and a member of the San Diego Epigenome Center. “Being able to study the epigenome in its entirety will lead to a better understanding of how genome function is regulated in health and disease but also how gene expression is influenced by diet and the environment.”
Their study, published in the Oct. 14, 2009 advance online edition of the journal Nature, compared the epigenomes of human embryonic stem cells and differentiated connective cells from the lung called fibroblasts, revealing a highly dynamic, yet tightly controlled, landscape of chemical signposts known as methyl-groups. The head-to-head comparison brought to light a novel DNA methylation pattern unique to stem cells, which may explain how stem cells establish and maintain their pluripotent state, the researchers say.
The emergence of epigenetics has already changed the way researchers think about how disease arises and how physicians treat it. Epigenetic changes play a crucial role in the development of cancer and some drugs that directly interact with the epigenome have been approved for the treatment of lymphoma and lung cancer and are now tested against a number of other cancer types. “Unless we know how these drugs affect the entire epigenome, we don’t really understand their full mechanism of action,” says Ecker.
Recognizing the central role of the epigenome in many areas of biology and medicine the National Institutes of Health launched a five-year Roadmap Epigenomics Program in 2008. The San Diego Epigenome Center, headed by Bing Ren, Ph.D., Professor of Cellular and Molecular Medicine at the University of California, San Diego School of Medicine and head of the Laboratory of Gene Regulation at the Ludwig Institute for Cancer Research, is an integral part of the five-year, $190 million push to accelerate research into modifications that alter genetic behavior across the human genome.
The current study, to which Ren and additional members of the Center located at the University of Wisconsin and the Morgridge Institute for Research in Madison, Wisconsin, also contributed, is not only the first complete high-resolution map of an epigenome superimposed on the human genome, but also the first study to be published as a direct result of the Roadmap Epigenomics Program.
“This paper exemplifies the goals of the NIH Roadmap for Medical Research’s Epigenomics Program,” said Linda Birnbaum, Ph.D., director of the National Institute of Environmental Health Sciences, one of the NIH institutes funding this program. “The science has matured to a point that we can now map the epigenome of a cell. This paper documents the first complete mapping of the methylome, a subset of the entire epigenome, of 2 types of human cells – an embryonic stem cell and a human fibroblast line. This will help us better understand how a diseased cell differs from a normal cell, which will enhance our understanding of the pathways of various diseases.”
Epigenetic signals can tinker with genetic information in at least two ways: One targets histones, the “spools” around which DNA winds and which control access to DNA. The other is DNA methylation, a chemical modification of one letter, C (cytosine), of the four letters (A, G, C, and T) that comprise our DNA. In the last couple of years, Ecker’s laboratory started to zoom in on genomic methylation patterns, which are essential for normal development and are associated with a number of key cellular processes, including carcinogenesis.
Perfecting the technique in Arabidopsis thaliana, a plant whose genome is 25 times smaller than the human genome, Ryan Lister, Ph.D., a postdoctoral researcher in Ecker’s lab and co-first author on the current study, developed an ultra high-throughput methodology to precisely determine whether each C in the genome is methylated or not, and layer the resulting epigenomic map upon the exact genome it regulates.
He then put the brand new technology to work to map the epigenomes of differentiated fibroblast cells and human embryonic stem cells (hESCs.) “We wanted to know how the epigenome of a differentiated cell that’s programmed to perform a specific job differs from the epigenome of a pluripotent stem cell, that has the potential to turn into any other cell type,” Lister says.
Just as expected, in fibroblast cells the majority of Cs followed by a G carried a methyl-group, a pattern often referred to as CG-methylation. But much to the Salk researchers’ surprise, in embryonic stem cells about a quarter of all methylation events occurred in a different context.
“Non-CG methylation is not completely unheard of — people have seen it in dribs and drabs, even in stem cells. But nobody expected that it would be so extensive,” says postdoctoral researcher and co-first author Mattia Pelizzola, who along with Lister undertook the extensive task of extracting and analyzing the epigenome data from these vast sequence datasets. “The whole field had been focused on CG methylation, and non-CG methylation was often considered a technical artifact.”
To confirm their finding, the authors then targeted several regions in a second hESC line, as well as in fibroblast cells that had been reprogrammed into so called induced pluripotent stem (iPS) cells. “They both had the same high level of non-CG methylation, which was lost when we forced them to differentiate,” says Pelizzola.
Being able to create high resolutions maps of the human epigenome, Ecker’s group will now begin to examine how it changes during normal development as well as examining a variety of disease states. “For the first time, we will be able to see the fine details of how DNA methylation changes in stem cells and other cells as they grow and develop into new cell types,” he says. “We believe this knowledge will be extremely valuable for understanding diseases such as cancer and possibly even mental disorders. Right now we just don’t know how the epigenome changes during the aging process or how the epigenome is impacted by our environment or diet.”
This work was supported in part by grants from the Mary K. Chapman Foundation, the NIH, the California Institute for Regenerative Medicine, the Australian Research Council Centre of Excellence Program and the Morgridge Institute for Research.
Researchers who also contributed to the work include Robert H. Dowen and Joseph R. Nery in the Genomic Analysis Laboratory, Gary Hon, Leonard Lee, Zhen Ye, Que-Minh Ngo and Lee Edsall at the Ludwig Institute for Cancer Research at the University of California San Diego, Julian Tonti-Filippini and A. Harvey Millar at the ARC Center of Excellence in Plant Energy Biology in Crawley, Australia, Jessica Antosiewicz-Bourget, Ron Stewart, Victor Ruotti and James A. Thomson at the Morgridge Institute for Research and at the Genome Center of Wisconsin, both at the University of Wisconsin in Madison.
The “Nature versus Nurture” debate just got more complicated. (Well, even more complicated than the original “If you really think you can reduce all of biology to such a simplistic division you’re missing pretty much every point involved” complication.) Birds have been observed reconstructing cultural information in complete isolation, meaning that culture can be genetically encoded.
Cold Spring Harbor Laboratory scientists isolated a Zebra Finch, preventing it from learning the songs of its parents (and probably pissing off a bunch of PETA activists who genuinely don’t have anything better to do). These finches are known to learn their song from elder male relatives, which is why the scientists were surprised to see the same songs emerge from a colony of these utterly isolated birds.
They didn’t get it right immediately. The first isolated bird, cut off from its culture, emitted a cacophonous screeching about as melodious as nails being dragged down a pieces of broken blackboard which were, in turn, being dragged down an even larger blackboard. It even tried to teach its kids the same, but they obviously thought “that sucks” (in bird) and made a few improvements. After four generations, the original finch songs reappeared, meaning that either
a) Cultural information can be genetically encoded or
b) Cold Spring Harbor Laboratory has embarrassingly bad sound insulation.
We’re going to assume a) for now.
Anyone who studied a little genetics in high school has heard of adenine, thymine, guanine and cytosine – the A, T, G and C that make up the DNA code. But those are not the whole story. The rise of epigenetics in the past decade has drawn attention to a fifth nucleotide, 5-methylcytosine (5-mC), that sometimes replaces cytosine in the famous DNA double helix to regulate which genes are expressed. And now there’s a sixth: 5-hydroxymethylcytosine.
In experiments to be published online April 16 by Science, researchers reveal an additional character in the mammalian DNA code, opening an entirely new front in epigenetic research.
The work, conducted in Nathaniel Heintz’s Laboratory of Molecular Biology at The Rockefeller University, suggests that a new layer of complexity exists between our basic genetic blueprints and the creatures that grow out of them. “This is another mechanism for regulation of gene expression and nuclear structure that no one has had any insight into,” says Heintz, who is also a Howard Hughes Medical Institute investigator. “The results are discrete and crystalline and clear; there is no uncertainty. I think this finding will electrify the field of epigenetics.”
Genes alone cannot explain the vast differences in complexity among worms, mice, monkeys and humans, all of which have roughly the same amount of genetic material. Scientists have found that these differences arise in part from the dynamic regulation of gene expression rather than the genes themselves. Epigenetics, a relatively young and very hot field in biology, is the study of nongenetic factors that manage this regulation.