The Ash Twins Blog

In my lab, we are studying the developmental brain disorder MECP2 duplication syndrome. It is caused by genomic duplication of the q28 region on the X chromosome, and common symptoms include severe intellectual disability and autism, progressing to severe epilepsy and early death. Genotype-phenotype analyses of patient samples suggest that the key dosage-sensitive gene in Xq28 duplication is the transcriptional regulator methyl-CpG binding protein 2 (MECP2), mutations of which have also been linked to Rett Syndrome, Angelman Syndrome, and autism. To study the neurobiology of this neurodevelopmental disorder, we use mice which have been engineered to overexpress MECP2 at twice normal levels. Analogous to humans with MECP2 duplication, these MECP2 duplication mice develop a progressive neurological phenotype with repetitive compulsive motor behaviors, abnormal social behavior and learning, anxiety, spasticity, and seizures.

A convincing physiological link between the symptoms of MECP2 duplication syndrome and its underlying genetic lesion has yet to be established, but work in mouse models and neuronal culture indicates that deranged dendritic/synaptic structure and plasticity may be an important locus of disease. The MECP2 duplication mice have a dramatically increased density of excitatory synapses in the hippocampus which is most prominent at young ages. Overexpression of MECP2 in primary slice cultures (to make a primary slice culture – a brain is harvested, sliced into slabs, and placed in a petri dish with oxygen and nutrients. They can survive for weeks and months with appropriate care)  causes a range of structural abnormalities in dendritic arbors and dendritic spines (the small protrusions from dendrites at which most excitatory synapses form). Detailed neuropathology with postmortem MECP2 duplication syndrome patient samples has yet to be performed.

In our lab, we are studying the formation, development, and pruning of these dendritic spines in MECP2 duplication mice. We cross the MECP2-duplication mice to the thy1-GFP (Green Fluorescent Protein form jellyfish) transgenic mice, which have been engineered to express bright fluorescence in a small number of layer  pyramidal neurons. These glow-in-the-dark neurons allows us to image apical dendrite and dendritic spine morphology in intact animals over days, weeks, and months, using two-photon microscopy (basically, a laser scans through the brain tissue identifying points of fluorescence in 3 dimensions). We are imaging the synaptic development from presymptomatic (4 weeks) stages to severe symptomatic (40 weeks) stages.

Our key finding so far is that the process of dendritic spine turnover (formation and pruning of synapses over time) does not undergo the normal developmental deceleration with age, but rather stays abnormally high well into adulthood, and that this elevated spine turnover is correlated with a net decrease in dendritic spine densities.