Volumetric and Microstructural Differences in a Mouse Model of Rett Syndrome

Background: Rett syndrome is a neurodevelopmental disorder caused by sporadic mutations in Mecp2 leading to early-life disruptions in the brain and behaviour. Magnetic resonance imaging (MRI) of human Rett’s patients and Mecp2mouse models have found large-scale volume losses within the brain, which have been attributed to the severe structural impairments of neurons and synapses found in Rett’s (Saywell et al., 2006). However, a direct association between regional volume and neuronal morphology has not been studied across the entire brain.

Objectives: The objective of this study is to perform an analysis of the anatomical and cellular structural differences within the brain of the Mecp2³⁰⁸mouse model of Rett’s.  Anatomical MRI was used to assess the regional volume differences between groups. Diffusion-weighted MRI was used to explore the organization and order of the cellular environment in the brain using a measure of water diffusion, called fractional anisotropy (FA). Finally, a Golgi stain will be performed to measure neuronal morphology.

Methods: Mice – Two separate cohorts of male hemizygous Mecp2³⁰⁸ and C57BL/6 mice (Age = ~ 60 days) were used in the study. One cohort was used for anatomical and diffusion-weighted imaging, the second was used for the Golgi stain.

MRI Acquisition- A multi-channel 7.0 Tesla MRI scanner (Varian) was used to acquire images of the brain. To acquire images for an anatomical analysis, a T2-weighted, 3-D fast spin-echo sequence was used. To measure water diffusion through the micro-structural, cellular environment, a diffusion-weighted 3D Fast spin echo sequence was used. 

Data Analysis– To visualize and compare the volumetric and water diffusion differences, the brains for each MR sequence were registered together. For the volume measurements, the individual volumes of 62 different structures were calculated. For the white matter structural changes, the FA intensity differences were measured in the same 62 structures as well as on a voxel by voxel basis. Group differences in volume or FA were calculated while controlling for multiple comparisons using the False Discovery Rate.

Results: Volume differences were found in many regions of the brain when comparing Mecp2³⁰⁸ with wild-type mice. Of particular interest was the greater than 4% decrease in volume in the parieto-temporal cortex, corpus callosum, internal capsule, striatum and thalamus. Interestingly, although not consistent in every region, there seems to be a negative relationship between volume and fractional anisotropy, where a decrease in volume is associated with an increase in FA. We are currently in the process of quantifying neuronal morphology with the Golgi stain in these regions to determine the relationship between volume, FA and neuronal morphology.

Conclusions: This study provides whole-brain coverage of the volumetric and FA differences in the Mecp2³⁰⁸ brain. Furthermore, we expect brain regions with increases in FA to house neurons with reduced complexity due to the lack of dendritic branches that will restrict water diffusion. These findings will demonstrate that these whole brain imaging modalities can be use as biomarkers for monitoring treatment success in future studies that aim to reverse the structural impairments of neurons in Rett’s.