Anatomical Phenotyping in a Mouse Model of Fragile X Syndrome Using Magnetic Resonance Imaging and Computed Tomography

Background: Phenotyping in the mouse brain using Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) has been shown to be quite useful in determining specific changes in the brain and skull (Neiman et al., Physiol Genomics, 2006).  Fragile X Syndrome (FXS) is the most common cause of mental retardation and can be linked to a specific gene.  The Fragile X knockout mouse (FX KO) is the most widely used animal model of FXS (Dutch-Belgian Fragile X Consortium, Cell, 1994).

Objectives: The purpose of this study was to assess differences between FX KO and wild type (WT) mice using a variety of imaging methods.

Methods:  

Fourteen male C57/B6 fixed mice brains were examined (7 WT and 7 FX KO).

MRI Acquisition - A 7.0 Tesla MRI scanner (Varian Inc.) was used to acquire anatomical images of brains within skulls as well as Diffusion Tensor Images (DTI) to assess changes in the white matter. Total imaging time was ~11 h and 16 h for the two methods, respectively. 

CT Acquisition – Three dimensional CT data sets were acquired using a micro CT scanner (GE Medical Systems).  The computed images show calcified bone as highly intense against a relatively uniform dark background.  Imaging time was ~2.5 h. 

Data Analysis - To visualize and compare changes, the images were registered.  Volumes were calculated from the anatomical images by using deformation fields for each individual brain from the segmented population average.  From this data the volume of 62 different structures were assessed.  Changes in white matter were determined from Fractional Anisotropy (FA) maps which were created and registered from the DTI data set.  FA is a scalar measure of the degree of anisotropy in the tissue and it ranges from 0 to 1, where 0 is isotropic (i.e. the tissue has no order or spherical symmetry, gray matter) and 1 is highly anisotropic (i.e. the tissue is highly ordered, white matter).  Skeletal changes were also assessed using the registered CT images.

Results: Significant volume differences were found in 3 regions, a decrease in the arbor vita of the cerebellum and the striatum, and an increase in the cerebral cortex of the parieto-temporal lobe.  The most significant difference was the arbor vita of the cerebellum, p=0.0002, false discovery rate = 1.5%.  The cerebellum, therefore, was examined on a voxel by voxel basis to determine where the changes were localized.  The significant decreases in the cerebellum seem to be located in specific nuclei such as the dentate nucleus and the nucleus interpositus.  No significant differences in FA were found between the FX KO and WT, and in spite of the facial dimorphism that is common in human FXS we did not find any significant skeletal changes between groups.

Conclusions: This study shows specific volumetric changes within the cerebellum in Fragile X syndrome.  No changes were found in both the white matter and skeletal assessment of the FX KO mouse; however, changes may be found in other Autism Spectrum Disorders.