October 30 – November 1, 2011, The Westin Boston Waterfront, Boston, MA, USA

The field of epigenetics has come to the fore in recent years, making its mark in both basic research and also fields relevant to human disease, such as stem cells and cancer. “Epigenetics” has become synonymous with modifications to DNA and associated molecules that influence whether genes are on or off. But are these epigenetic modifications self-propagating and inherited like DNA, the physical manifestation of Mendel’s gene? Given the great interest in the topic and huge public and private investment made in epigenetics, Cell Press has decided to tackle this and other aspects of epigenetics in a multi-day meeting. In addition to fostering critical discussions on the proposed mechanisms of epigenetic inheritance and the challenges that lay ahead in understanding these mechanisms, the meeting will focus on more well established epigenetic mechanisms and how they are implemented in cell and developmental biology, particularly the maintenance of cell states, such as occurs during X-inactivation and polycomb-mediated gene silencing. The scope will be broad, giving the interested attendee a glimpse of epigenetics at both the molecular as well as the organism level.

As the field of epigenetics gains momentum, this is the perfect time to take a breather, stand back, and take a critical look at the claims, the issues to be addressed, and how we move ahead.

Topic List:
- Transgenerational Epigenetic Inheritance
- Inheritance of cellular states: x-inactivation, imprinting, and lambda phage
- Replication of chromatin
- RNA and epigenetic inheritance
- Mechanisms of polycomb-mediated gene silencing

The mission of the Encyclopedia of DNA Elements (ENCODE) Project is to enable the scientific and medical communities to interpret the human genome sequence and apply it to understand human biology and improve health. The ENCODE Consortium is integrating multiple technologies and approaches in a collective effort to discover and define the functional elements encoded in the human genome, including genes, transcripts, and transcriptional regulatory regions, together with their attendant chromatin states and DNA methylation patterns. In the process, standards to ensure high-quality data have been implemented, and novel algorithms have been developed to facilitate analysis. Data and derived results are made available through a freely accessible database. Here we provide an overview of the project and the resources it is generating and illustrate the application of ENCODE data to interpret the human genome.

Image via Wikipedia

DNA methylation is THE key driver of epigenetic regulation.  Where goest CpG methylation, then followest chromatin remodelling … NOT the other way around.

“The heritability of genomic methylation patterns clearly shows that once established, DNA methylation is dominant over chromatin modifications.”

Some neurodevelopmental processes (here) seem to depend on DNA methylation, but, is this the main purpose of all this methylation?

Nope.

Our genome is a huge junk pile.  That’s right … we are built from a genome, of which some 40%, are old retroviruses, transposons and other broken legacies of foreign DNA that inserted themselves into the genomes of our mammalian ancestors.  These ancient viruses can be very dangerous and wreak havoc if they are allowed to be transcribed.  DNA methylation helps keep this from happening.  Its a HUGE job … some 60% of all CpG’s are methylated … likely THE main purpose of DNA methylation.

“The lack of cell-type-specific methylation at either enhancers or promoters indicates that DNA methylation is likely to have a negligible or very small role in development, and that the methylation changes seen at some low-CpG promoters are likely to be a result of transcriptional activation rather than a cause.”

“The data indicate that the bulk of the genome is methylated as the default state, and unmethylated regions are protected from a promiscuous DNA methylating system by a combination of very high CpG densities and histone modifications and variants that repel DNA methyltransferase complexes.”

So, we must keep in mind when reading the epigenetic literature (a methyl group here or there contributes to less anxiety) that there is a much more vital process happening (ie., lack of a methyl group here or there can lead to a lethal viral attack). Occasionally, in the process of keeping us alive, our physiological systems can make life difficult.  C’est la vie!

Also, it appears that methylation is like an enormous fire-hose spraying methyl groups everywhere in the genome to dampen the ground and prevent any small fires (viruses) from igniting. How much stock can you put in research findings that hinge on the appearance/disappearance of 1 or 2 errant methyl groups?

“… transgenic mice with increased Setdb1 expression in adult forebrain neurons show antidepressant-like phenotypes in behavioral paradigms for anhedonia, despair and learned helplessness.”

SETDB1 is a protein that helps methylate lysine #9 on the histone H3 DNA binding protein … which leads to DNA CpG methylation … which leads to repression of the NMDA receptor subunit, NR2B/Grin2b … which leads to the anti-depressant-like phenotype.

Recall that 60% of CpGs are methylated and that, in the brain (unlike other terminally differentiated tissues), these methyl groups are popping on and off a lot … perhaps reflecting an ongoing, constant tuning of the inhibition/excitation balance.

thanks for the pic whaddap.

“For example, have you thought about epigenetic effects that might be environmentally induced and can be transmitted across multiple subsequent generations?  Genotypes of individuals in previous generations might even be a better predictor of phenotype than an individual’s own genotype.”

“I know that Copy-Number Polymorphic (CNP) duplications are highly variable among individual and are considered inaccessible by most existing genotyping and sequencing technologies, but I’m still getting my genome sequenced anyway.”

“Can you please help Eric understand that rare variants and large variants (deletions, duplications and inversions) are individually rare, but collectively common in the human population might account for much more of heritability than common variation.  Nothing is known about these rare variants!”

“Yeah, Eric doesn’t realize that a very large number of closely linked genes can exhibit context-dependent and non-additive effects.”

“Gene by environment innnterraaaaactiiooon … coooool.”

–real science here.

Playa with gold NY Yankees hat worn sideways:  Man, I’ve got mad feva for the flava of these chips.

Hipster girl with multicolor wool sherpa hat:  You better watch out playa, you’ll pass on some ill health to your kids.

Playa:  Kids! I ain’t tryin’ to have no kids.  Besides, that’s some Lamarckian shit you’re talkin’.  Dads can’t pass on stuff they get from eatin’ junk food … only girls can.

Girl:  You ever hear of epigenetic reprogramming?

Playa:  You buggin’ gurrrl.  How are my sperm cells supposed to carry all that “past history” and shit to my kids.  I mean the fucked up cheeto-eating fat cells are in my ass, not my balls.  My sperm cells ain’t got nuthin’ but some nekkid DNA coiled up in them – no room for the epigenome in MY sperm babe.  Did I say my DNA was naaaked?

Girl:  You’re balls ain’t as dumb as you think.

Playa:  Oooh Shit!  Say that again!  Please!  Tell me about my sperm cells too!

Girl:  Slow down playa.  Read the paper by Carone et al., “Paternally Induced Transgenerational Environmental Reprogramming of Metabolic Gene Expression in Mammals” [DOI 10.1016/j.cell.2010.12.008].  They show that mouse fathers can pass on all kinds of crazy changes to their offspring’s liver function depending on the dad’s diet.

Playa:  Damn!  So I have to think about what I’m eating now? what I’m puttin’ into my sperm cells?

Girl:  If you want your nekkid DNA to be with me … ha ha!

Playa: Shit, that re-programming shit is messed UP!

Girl:  Don’t hate the playa, just hate the game – the epigenetic game!

from Ye et al., 2009:

HDAC1/2 genes encode proteins that modify the epigenome (make it less accessible for gene expression).

When HDAC1/2 functions around the HES5 and ID2/4 (repressors of white matter development) genes, the epigenetic changes (less acetylation of chromatin) helps to repress the repressors.

This type of epigenetic repression of gene expression (genes that repress white matter development) is essential for white matter development.

Most cells in your adult body are “terminally differentiated” – meaning that they have developed from stem cells into the final liver, or heart, or muscle or endothelial cell that they were meant to be.  From that point onward, cells are able to “remember” to stay in this final state – in part – via stable patterns of DNA methylation that reinforce the regulation of “the end state” of gene expression for that cell.  As evidence for this role of DNA methylation, it has been observed that levels of DNA methyl transferase (DNMT) decline when cells are fully differentiated and thus, cannot modify or disrupt their patterns of methylation.

NOT the case in the brain! Even though neurons in the adult brain are fully differentiated, levels of methyl transferases – DO NOT decline.  Why not? Afterall, we wouldn’t want our neurons to turn into liver cells, or big toe cells, would we?

One hypothesis, suggested by David Sweatt and colleagues is that neurons have more important things to “remember”.   They suggest in their fee and open research article, “Evidence That DNA (Cytosine-5) Methyltransferase Regulates Synaptic Plasticity in the Hippocampus” [doi: 10.1074/jbc.M511767200] that:

DNA methylation could have lasting effects on neuronal gene expression and overall functional state. We hypothesize that direct modification of DNA, in the form of DNA (cytosine-5) methylation, is another epigenetic mechanism for long term information storage in the nervous system.

By measuring methylated vs. unmethylated DNA in the promoter of the reelin and BDNF genes and relating this to electrophysiological measures of synaptic plasticity, the research team finds correlations between methylation status and synaptic plasticity.  More specifically, they find that zebularine (an inhibitor of DNMT) CAN block long-term potentiation (LTP), but NOT block baseline synaptic transmission nor the ability of synapses to fire in a theta-burst pattern (needed to induce LTP).

This suggests that the epigenetic machinery used for DNA methylation may have a role in the formation of cellular memory – but not in the same sense as in other cells in the body – where cells remember to remain in a terminally differentiated state.

In the brain, this epigenetic machinery may help cells remember stuff that’s more germane to brain function … you know … our memories and stuff.

Here’s the problem. In order to USE the BRAIN (to learn and remember stuff) we have to also USE the GENOME (to encode the proteins that synapses use in the process of memory formation).  When we’re thinking, we have to take our precious DNA out of its protective supercoiled, proteinaceous shell and allow the double helix to melt into single strands and expose their naked A’s, G’s, T’s and C’s to the chemical milieu (to the start the transcription process).  This is risky business damage to DNA can lead to cell death!

One might imaging that its best to carry out this precarious act quickly and in proximity to DNA repair enzymes (I’d think).  A very important job that includes: uncoiling chromatin superstructures, transcribing DNA (that encodes proteinaceous building blocks that synapses use to strengthen and weaken themselves) – and then – making sure there was no damage incurred along the way.  A BIG job that MUST get done each and every time my cells engage in learning.  Wow!  I didn’t realize that learning new stuff means I’m exposing my DNA to damage?  Hmm … I wonder if that PhD was worth it?

To perform this important job, it seems there is an amazing handyman of a molecule named poly(ADP-ribose) polymerase-1 (PARP-1).  Amazing, because it – itself – can function in many of the steps involved in uncoiling chromatin structures, transcription initiation and DNA repair.  The protein that can “do it all” … get the job done quickly and even fix any errors made along the way! It is known to function in the so-called base excision repair (BER) pathway and is also known have a role in transcription through remodeling of chromatin by ADP-ribosylating histones and relaxing chromatin structure, thus allowing transcription to occur (click here for a great open review of PARP-1).  Nice!

According to OMIM, earlier studies by Cohen-Armon et al. (2004) found that poly(ADP-ribose) polymerase-1 is activated in neurons that mediate several forms of long-term memory in Aplysia. Because poly(ADP-ribosyl)ation of nuclear proteins is a response to DNA damage in virtually all eukaryotic cells (indeed, PARP-1 knock-out mice are more sensitive to DNA damage), it was surprising that activation of the polymerase occurred during learning and was required for long-term memory. Cohen-Armon et al. (2004) suggested that the fast and transient decondensation of chromatin structure by poly(ADP-ribosyl)ation enables the transcription needed to form long-term memory without strand breaks in DNA.

A recent article in Journal of Neuroscience seems to confirm this function -  now in the mouse brain.  Histone H1 Poly[ADP]-Ribosylation Regulates the Chromatin Alterations Required for Learning Consolidation [doi:10.1523/JNEUROSCI.3010-10.2010] by Fontán-Lozano et al., examined cells in the hippocampus at different times during the learning of an object recognition paradigm.  They confirm (using a PARP-1 antagonist) that PARP-1 is needed to establish object memory and also that PARP-1 seems to contribute during the paradigm and up to 2 hours after the training session.  They suggest that the poly(ADP-ribosyl)ation of histone H1 influences whether H1 is bound or unbound and thus helps regulate the opening and closing of the chromatin so that transcription can take place. 

Nice to know that PARP-1 is on the job!  Still am wondering if the PhD was worth all the learning.  Are there trade-offs at play here?  MORE learning vs. LESS something?   Perhaps. Check out the paper by Grube and Bürkle (1992) - Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span. This gene may influence life span!