Archive for the ‘MECP2’ Category

Some quick sketches that might help put the fast-growing epigenetics and cognitive development literature into context.  Visit the University of Utah’s Epigenetics training site for more background!

The genome is just the A,G,T,C bases that encode proteins and other mRNA molecules.  The “epi”genome are various modification to the DNA – such as methylation (at C residues) – and acetylation of histone proteins.   These changes help the DNA form various secondary and tertiary structures that can facilitate or block the interaction of DNA with the transcriptional machinery.

When DNA is highly methylated, it generally is less accessible for transcription and hence gene expression is reduced.  When histone proteins (purple blobs that help DNA coil into a compact shape) are acetylated, the DNA is much more accessible and gene expression goes up.

We know that proper epigenetic regulation is critical for cognitive development because mutations in MeCP2 – a protein that binds to methylated C residues – leads to Rett syndrome.  MeCP2 is normally responsible for binding to methylated DNA and recruiting histone de-acetylases (HDACs) to help DNA coil and condense into a closed form that is inaccessible for gene expression (related post here).

When DNA is accessible for gene expression, then it appears that – during brain development – there are relatively more synaptic spines produced (related post here).  Is this a good thing? Rett syndrome would suggest that – NO – too many synaptic spines and too much excitatory activity during brain development may not be optimal.  Neither is too little excitatory (too much inhibitory) activity and too few synaptic spines.  It is likely that you need just the right balance (related post here). Some have argued (here) that autism & schizophrenia are consequences of too many & too few synapses during development.

The sketch above illustrates a theoretical conjecture – not a scenario that has been verified by extensive scientific study. It tries to explain why epigenetic effects can, in practice, be difficult to disentangle from true (changes in the A,G,T,C sequence) genetic effects.  This is because – for one reason – a mother’s experience (extreme stress, malnutrition, chemical toxins) can – based on some evidence – exert an effect on the methylation of her child’s genome.  Keep in mind, that methylation is normal and widespread throughout the genome during development.  However, in this scenario, if the daughter’s behavior or physiology were to be influenced by such methylation, then she could, in theory, when reaching reproductive age, expose her developing child to an environment that leads to altered methylation (shown here of the grandaughter’s genome).  Thus, an epigenetic change would look much like there is a genetic variant being passed from one generation to the next, but such a genetic variant need not exist (related post here, here) – as its an epigenetic phenomenon.  Genes such as BDNF have been the focus of many genetic/epigenetic studies (here, here) – however, much, much more work remains to determine and understand just how much stress/malnutrition/toxin exposure is enough to cause such multi-generational effects.  Disentangling the interaction of genetics with the environment (and its influence on the epigenome) is a complex task, and it is very difficult to prove the conjecture/model above, so be sure to read the literature and popular press on these topics carefully.

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rsrtlogoIt was a delight today to chat with Monica Coenraads, Executive Director of the Rett Syndrome Research Trust.  The RSRT has teamed up with a deeply focused world-class team of research scientists to translate the fruits of basic research on Rett syndrome into viable cures.   Whether you are a scientist, student or concerned family member, you will learn a lot from exploring the RSRT website, blog as well as this short video lectureJust by a strange, unanticipated coincidence, today marks the 10-year annivesary of the identification of MeCP2 as the underlying gene for Rett syndrome. Click here for prior blog posts on Rett syndrome.  (click here for podcast)

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In previous posts, we have explored some of the basic molecular (de-repression of chromatin structure) and cellular (excess synaptogenesis) consequences of mutations in the MeCP2 gene – a.k.a the gene whose loss of function gives rise to Rett syndrome.  One of the more difficult aspects of understanding how a mutation in a lowly gene can give rise to changes in cognitive function is bridging a conceptual gap between biochemical functions of a gene product — to its effects on neural network structure and dynamics.  Sure, we can readily acknowledge that neural computations underlie our mental life and that these neurons are simply cells that link-up in special ways – but just what is it about the “connecting up part” that goes wrong during developmental disorders?

In a recent paper entitled, “Intact Long-Term Potentiation but Reduced Connectivity between Neocortical Layer 5 Pyramidal Neurons in a Mouse Model of Rett Syndrome” [doi: 10.1523/jneurosci.1019-09.2009] Vardhan Dani and Sacha Nelson explore this question in great detail.  They address the question by directly measuring the strength of neural connections between pyramidal cells in the somatosensory cortex of healthy and MeCP2 mutant mice.  In earlier reports, MeCP2 neurons showed weaker neurotransmission and weaker plasticity (an ability to change the strength of interconnection – often estimated by a property known as “long term potentiation” (LTP – see video)).   In this paper, the authors examined the connectivity of cortical cells using an electrophysiological method known as patch clamp recording and found that early in development, the LTP induction was comparable in healthy and MeCP2 mutant animals, and even so once the animals were old enough to show cognitive symptoms.  During these early stages of development, there were also no differences between baseline neurotransmission between cortical cells in normal and MeCP2 mice.  Hmmm – no differences? Yes, during the early stages of development, there were no differences between genetic groups – however – once the team examined later stages of development (4 weeks of age) it was apparent that the MeCP2 animals had weaker amplitudes of cortical-cortical excitatory neurotransmission.  Closer comparisons of when the baseline and LTP deficits occurred, suggested that the LTP deficits are secondary to baseline strength of neurotransmission and connectivity in the developing cortex in MeCP2 animals.

So it seems that MeCP2 can alter the excitatory connection strength of cortical cells.  In the discussion of the paper, the authors point out the importance of a proper balance of inhibition and excitation (yin and yang, if you will) in the construction or “connecting up part” of neural networks.  Just as Rett syndrome may arise due to such a problem in the proper linking-up of cells – who use their excitatory and inhibitory connections to establish balanced feedback loops – so too may other developmental disorders such as autism, Down’s syndrome, fragile X-linked mental retardation arise from an improper balance of inhibition and excitation.

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arinlloydCelebrities and politicians are known for their love of the spotlight.  “Me, me, me!”  are the words to get ahead by in our modern media circus.   As well, it can even be – in the unglamorous world of science – where, in characteristically geeky form, the conventional wisdom is to shout, “my hypothesis, my hypothesis, my hypothesis!”.  Once, for example, I had a grad school professor say she was not allowed by her department to teach about glial cells in her brain development class.  Another distinguished professor once told me, “don’t even bother sending a grant in,  if it is focused on white matter“.   No sir, it appears that modern neuroscience shall only focus on one main hypothesis – the neuron doctrine and not on the lowly support cells (astrocytes, oligodendrocytes & microglia) that, actually, make up more than 90% of the human brain.  Hmmm, who would have thought to find such a cult of neuronal celebrity in the halls of academia?

With this in mind, I really enjoyed the recent paper “Rett Syndrome Astrocytes Are Abnormal and Spread MeCP2 Deficiency through Gap Junctions” [doi:10.1523/jneurosci.0324-09.2009] by Maezawa and colleagues.  The authors point out several critical gaps in the literature – namely that the expression of MeCP2 (the gene that, when mutated, gives rise to Rett syndrome) in neurons does NOT account for all of the many facets of the syndrome.  For example, when MeCP2 is deleted only in neurons (in a mouse model), it results in a milder form of abnormal neural development than when deleted in all CNS cell types ( the full mouse syndrome: stereotypic forelimb motions, tremor, motor and social behavioral abnormalities, seizures, hypoactivity, anxiety-like behavior and learning/memory deficits).  Also, it is not possible to reverse or rescue these deficits when a functional version of MeCP2 is expressed under a neuron-specific promoter.  However, when re-expressed under its endogenous promoter – it is possible to rescue the syndrome (free access article).

The authors thus looked much more closely at the expression of MeCP2 and found that they could indeed visualize the expression of the MeCP2 protein in cultured ASTROCYTES – who are a very, very important type of support cell (just think of the personal secretary Lloyd to Ari Gold on the TV show “Entourage”).  The team then examined how astrocytes that lack 80% of the expression of MeCP2 might interact with neurons – the very cells they normally support with secretions of growth factors and cytokines.   It turns out that both normal and MeCP2-deficient neurons do not thrive when co-cultured with astrocytes that have weak MeCP2 expression.   The team reports that dendritic length is reduced after a day and also a fews days of co-culture,  suggesting that the MeCP2-deficient astrocytes are failing to provide the proper trophic support for their neuronal celebrity counterparts.  Short dendrites are generally considered a bad-thing since this would predict poorer connectivity, and poorer cognition across the brain.

Hence, it seems that the lowly astrocyte is far more important in understanding what goes wrong in Rett syndrome.  Ironically, in this case however, the celebrity status of the neuron remains untarnished as astrocytes can now be blamed for the consequences of MeCP2 mutations.  The authors suggest that treatment of Rett syndrome via astrocytes is a worthwhile avenue of investigation.  This new direction in the search for a cure will be an exciting story to follow!

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The cognitive and emotional impairments in the autism spectrum disorders can be difficult for parents and siblings to understand and cope with.  Here are some graphics and videos that might assist in understanding how genetic mutations and epigenetic modifications can lead to various forms of social withdrawl commonly observed in the autism spectrum disorders in children.

In this post, the focus is just on the MecP2 gene – where mutations are known to give rise to Rett Syndrome – one of the autism spectrum disorders.  I’ll try and lay out some of the key steps in the typical bare-bones-link-infested-blogger-fashion – starting with mutations in the MecP2 gene.  Disclaimer: there are several fuzzy areas and leaps of faith in the points and mouse model evidence below, and there are many other genes associated with various aspects of autism spectrum disorders that may or may not work in this fashion.  Nevertheless, still it seems one can begin to pull a mechanistic thread from gene to social behavior Stay tuned for more on this topic.

1. The MecP2 gene encodes a protein that binds to 5-Methylcytosine – very simply – a regular cytosine reside with an extra methyl group added at position 5.  Look at the extra -CH3 group on the cytosine residue in the picture at right.  See?  That’s a 5-methylcyctosine residue – and it pairs in the DNA double helix with guanosine (G) in the same fashion as does the regular cyctosine reside (C). 5methC OK, now, mutations in the gene that encode the  MecP2 gene – such as those found at Arginine residue 133 and Serine residue 134 impair the ability of the protein to bind to these 5-Methylcyctosine residues.  bindingMecP2The figure at left illustrates this, and shows how the MecP2 protein lines up with the bulky yellow 5-Methylcytosine residues in the blue DNA double helix during binding.

2. When the MecP2 protein is bound to the methylated DNA, it serves as a binding site for another type of protein – an HDAC or histone deacetylase. The binding of MecP2 and HDAC (and other proteins (see p172 section 5.3 of this online bookChromatin Structure and Gene Expression“)).  The binding of the eponymously named HDAC’s leads to the “de-acetylation” of proteins known as histones.  The movie below illustrates how histone “de-acetylation” leads to the condensation of DNA structure and repression or shutting down of gene expression (when the DNA is tightly coiled, it is inaccessible to transcription factors).  Hence: DNA methylation leads (via MecP2, HDAC binding) to a repression on gene expression.

3. When mutated forms of MecP2 cannot bind, the net result is MORE acetylation and MORE gene expression. As covered previously here, this may not be a good thing during brain development since more gene expression can induce the formation of more synapses and – possibly – lead to neural networks that fail to grow and mature in the “normal” fashion. The figure at right toomanysynapsessuggests that neural networks with too many synapses may not be appropriately connected and may be locked-in to sub-optimal architectures.  Evidence for excessive synaptogenesis is abundant within the autism spectrum disorders.  Neuroligins – a class of genes that have been implicated in autism are known to function in cell & synaptic adhesion (open access review here), and can alter the balance of excitation/inhibition when mutated – which seems consistent with this heuristic model of neural networks that can be too adhesive or sticky.

4. Cognitive and social impairment can result from poor-functioning neural networks containing, but not limited to the amygdala. The normal development of neural networks containing the forntal cortex and amygdala are important for proper social and emotional function.  The last piece of the puzzle then would be to find evidence for developmental abnormalities in these networks and to show that such abnormalities mediate social and/or emotional function.  Such evidence is abundant.

Regarding the effects of MecP2 however, we can consider the work of Adachi et al., who were able to delete the MecP2 gene – just in the amygdala – of (albeit, an adult) mouse.  Doing so, led to the disruption of various emotional behaviors – BUT NOT – of various social interaction deficits that are observed when MecP2 is deleted in the entire forebrain.  This was the case also when the team infused HDAC inhibitors into the amygdala suggesting that loss of transcriptional repression in the adult amygdala may underlie the emotional impariments seen in some autism spectrum disorders.  Hence, such emotional impairments (anxiety etc.) might be treatable in adults (more on this result later and its implications for gene-therapy).

Whew!  Admittedly, the more you know – the more you don’t know.  True here, but still amazing to see the literature starting to interlink across human-genetic, mouse-genetic, human-functional-imaging levels of analysis. Hoping this rambling was helpful.

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