The readers of this blog will have noted the frequent mention of epigenetics – a young but hot area of research which holds promise for novel therapeutic interventions for a myriad of diseases. The term epigenetic means over and above the genome. It refers to changes in gene expression caused by mechanisms other than changes in the underlying DNA sequence. It helps to think of epigenetics in terms of the charm bracelet analogy – the DNA is the charm bracelet and epigenetic modifications are the charms that can be added to or removed from the bracelet.
Every cell has about 30,000 genes. What distinguishes a heart cell from a liver cell is the pattern of gene expression (what genes are on and what genes are off). Considering the vast number of cell types in the human body it’s easy to appreciate how the instructions for turning genes on and off must be finely tuned and exquisitely calibrated. When missteps occur – disease often follows.
Epigenetic modifications catalyze gene expression changes. The best known epigenetic modification is methylation. Adding methyl “charms” to DNA leads to gene silencing. MeCP2, the protein that is mutated in Rett syndrome as well as a variety of other disorders, binds to DNA that has already been methylated and plays a role in regulating downstream genes.
The excitement surrounding epigenetics is that it’s dynamic. “Charms” can be added and they can be removed. The ability to catalyze these modifications through drugs has already led to some treatments for cancers. The hope is that these treatments are the mere tip of the iceberg.
The latest breakthrough in epigenetics comes to us from the laboratory of RSRT advisor Nat Heintz of Rockefeller University. The connection to Rett Syndrome, however, does not end there. The post-doc in the Heintz lab who made this discovery is Skirmantas Kriaucionis. As a graduate student, he spent five years in the lab of RSRT trustee Professor Adrian Bird who in 2007 announced the dramatic Rett reversal experiments. The Heintz and Kriaucionis paper was published online, April 16th, in the high profile journal Science.
We all remember the four nucleotide bases (A-adenine, T-thymine, G-guanine, C-cystosine) from high school biology. Together these four bases make up DNA. If we were taking high school biology today we would also learn about a fifth nucleotide, methylcytosine (mC) that replaces cytosine depending on whether a gene needs to be turned on or off. Kriaucionis has now added a sixth, hydroxymethylcytosine (hmC), a nucleotide base previously observed only in the simplest of organisms, bacterial viruses.
Drs. Heintz and Kriaucionis discuss their findings with Monica Coenraads.
MC: It’s not every day that a new nucleotide is discovered. Congratulations! Were you surprised by this finding?
NH: Yes, I was. I like to think that in most cases I know where my research is heading. But, in this particular case, the finding was completely unexpected. Broadly speaking, I think these experiments validate that it’s worthwhile spending time analyzing detailed properties of cells as they exist and operate in vivo, rather than studying their properties after they have been adapted for growth in culture.
Ironically, as a graduate student in David Shub’s lab I worked on bacterial viruses carrying modified nucleotides in their genomes. Now, many years later, that research has come full circle.
MC: As is often the case in science, the discovery was rather accidental. Dr. Kriaucionis, can you tell us what kind of research you were undertaking and how you stumbled on hydroxymethylcytosine (hmC)?
SK: The discovery of hmC can be traced back to several key reasons. One, we were interested in investigating chromatin structure (DNA/protein material which makes up the chromosomes) in different brain cell types. A finding which attracted our attention is that certain cell types have distinct looking chromatin. A dramatic example is Purkinje cells which have a very open chromatin configuration, in contrast to cerebellar granule cells which have very condensed looking chromatin (see above picture) We wanted to investigate this further. Second, was the availability of technological tools in the Heintz lab, which not only gave us access to these specific cells but allowed us to isolate very pure material to investigate. Thirdly, was the rather outdated technique that I was using to quantify the absolute levels of the different nucleotides. Scientists rarely use this technique now because it does not tell you where in the genome the nucleotides are. Since location gives insight into biological function this technique is rarely employed. But the decision to use this method turned out to play a large part of how I was able to identify the hmC.
At first I didn’t believe my results, and it took several months to reproduce the results many times using different experimental conditions to give us the necessary confidence that indeed, we were seeing a novel nucleotide and not an artifact of the experiment.
MC: So hmC is an example of a further modification to a nucleotide base, cytosine, that has already been modified by a methyl group. Has your discovery added more complexity to the process by which genes are regulated?
SK: Yes, indeed it does add a layer of complexity to what sort of biological message is being encoded by these modifications. The next important step will be to figure out what outcome is expected when DNA sequences have unmethylated cytosine, methylated cytosine or hydroxymethylated cystosine.
It’s interesting to note that while histone proteins have many epigenetic modifications mammalian DNA had, until now, only one – methylation. This finding is totally unexpected. Although RNA has plenty of modifications, its functional repertoire is complex as well including structural, enzymatic and coding roles.
MC: Do you have any hypothesis about what hmC might be doing?
SK: hmC is very abundant in brain so this gives us confidence that it’s doing something biologically significant. We currently have two hypotheses regarding its function. One hypothesis is that the hmC might be an intermediate step to get methylated cytosine back to its original unmethylated state.
MC: So in essence hmC might transform a silent gene (methylated) to an active gene (unmethylated).
SK: Correct. Another possibility is that hmC is a stable final modification. It could still activate a silent gene without demethylation. We see different amounts of hmC in different cell types so it may be that in some cell types hmC is an intermediate while in other cell types it’s a stable modification. And obviously we are keeping open mind as there may be an unpredicted function of hmC.
MC: There was a second paper in the same issue of Science as your paper that identified the enzyme that catalyzes the conversion from mC to hmC. Can you elaborate on the synergies between these two papers?
NH: The second paper is from the laboratory of Anjana Rao at Harvard. We were unaware of her ongoing research until very recently. Our papers complement and actually help each other quite a bit. The Rao lab identified an enzyme which catalyzes the conversion from mC to hmC. This is a very significant finding. A criticism that our paper was receiving was that hydroxylation of methyl groups could be a spontaneous reaction that was happening due to the oxidation of DNA. We addressed this question in our study, but the accompanying paper completely puts this criticism to rest since the group found the enzyme catalyzing the conversion of mC to hmC. So in that respect their paper was helpful to us. Our data strengthened their paper because we showed that hmC was highly enriched in brain, providing a link to the biology of neurons and, perhaps, neurological disease. Taken together, the two papers establish the importance of hmC in the mammalian genome, and suggest that this new epigenetic mark will provide an entry into a previously unanticipated and important field of biology.
MC: Dr. Kriaucionis, previous to your post-doc position in the Heintz lab you were in Adrian Bird’s lab for five years. Prof Bird’s lab focuses on DNA methylation and discovered MECP2, the “Rett gene” in the early 1990s. You co-authored 5 papers on MECP2 and Rett Syndrome with Prof. Bird. Do you think there is any possible connection between MECP2 and hmC?
SK: I think there is indeed a connection. As your readers know, MECP2 binds to methylated DNA. There is data showing that MECP2 resists binding to DNA that contains hmC. If these findings hold up, then hydroxylating mC could release MECP2 from the chromatin and influence for example nearby gene expression. Finding genomic MeCP2, mC and hmC distribution in neuronal cell types will provide us with valuable insights into the biological function of MECP2 and the role of DNA modifications.
NH: So Skirmantas’ hypothesis implies that hmC can modulate the distribution of where MeCP2 binds in the nucleus. There is an alternative scenario in which a different and, as of yet, unidentified protein binds to hmC. My theory is that this protein is likely not a repressor, like MeCP2, but perhaps is acting as an activator. The beauty of science is that we can test these hypotheses. I suspect we’ll have our answer in a relatively short amount of time.
MC: What are the next steps that the lab is pursuing?
SK: Of utmost importance is to answer the following question: where is hmC and what is it doing? To answer that question we are developing a new set of tools which will allow us to adequately map where hmC is in the genome. I hope that having a clear understanding of location will provide clues as to its role.
MC: Dr. Heintz, you are known in the scientific world as a “big idea man” who identifies and then develops the necessary tools to go after key neurobiological questions. Can you elaborate on the development of the novel tools to which Dr Kriaucionis just alluded?
NH: The fact that as currently applied bisulfate sequencing techniques,which are used to detect sites of methylation in the genome, do not distinguish between a cytosine that is methylated or hydroxymethylated is a major problem. We hope to develop new methodology that will allow us to map the precise sites of genomic hmC and mC separately.
Our strength as a laboratory fits in nicely with the task ahead. We have at our disposal gene expression data that is specific to a large variety of specific cell types. This information will be very useful as we begin to map where in the genome hmC is found. These two separate but very complementary sides of our lab will help elucidate not only the function of hmC but perhaps also the function of the proteins that bind to it.
MC: Dr. Heintz, your lab does not have a history of working on epigenetics and yet you made a remarkable discovery. Given this finding do you envision changing the focus of your lab in any way?
NH: Yes, I think it will change our focus somewhat. We feel this discovery is a critically important finding and the role of hmC in neurons will be of high interest both fundamentally to understand brain function and also for investigation of epigenetic influences on disease states. So we will be focusing a lot of our attention on what the biological role of hmC is in the healthy and diseased brain .
MC: You are not a “Rett researcher” yet you have kept up with the literature, attended a number of Rett scientific meetings that I have organized over the years and have been a highly respected advisor to the field. Do you have any insight on recent progress and where you think the field is heading?
NH: I’m very enthusiastic about the developments of the last few years. Adrian Bird’s reversal experiments are stunning. It has also become very clear, largely as a consequence of Huda Zoghbi’s work, that the impact of MeCP2 in different brain cell types is distinct. This means that strategies aimed at treating particular symptoms of the disease can be devised in the nearer term while approaches to reversing the entire phenotype, a much taller order, are explored.
MC: On behalf of our readers I thank you both for your time and wish you the very best for your ongoing research. I look forward to staying in touch and hearing how this work unravels. Congratulations again on your paper.