A comprehensive longitudinal study of magnetic resonance imaging identifies novel features of the Mecp2 deficient mouse brain
Rett syndrome (RTT) is a X-linked neurodevelopmental disorder which represents the leading cause of severe incurable intellectual disability in females worldwide. The vast majority of RTT cases are caused by mutations in the X-linked MECP2 gene, and preclinical studies on RTT largely benefit from the use of mouse models of Mecp2, which present a broad spectrum of symptoms phenocopying those manifested by RTT patients. Neurons represent the core targets of the pathology; however, neuroanatomical abnormalities that regionally characterize the Mecp2 deficient mammalian brain remain ill-defined. Neuroimaging techniques, such as MRI and MRS, represent a key approach for assessing in vivo anatomic and metabolic changes in brain. Being non-invasive, these analyses also permit to investigate how the disease progresses over time through longitudinal studies. To foster the biological comprehension of RTT and identify useful biomarkers, we have performed a thorough in vivo longitudinal study of MRI and MRS in Mecp2 deficient mouse brains. Analyses were performed on both genders of two different mouse models of RTT, using an automatic atlas-based segmentation tool that permitted to obtain a detailed and unbiased description of the whole RTT mouse brain. We found that the most robust alteration of the RTT brain consists in an overall reduction of the brain volume. Accordingly, Mecp2 deficiency generally delays brain growth, eventually leading, in heterozygous older animals, to stagnation and/or contraction. Most but not all brain regions participate to the observed deficiency in brain size; similarly, the volumetric defect progresses diversely in different brain areas also depending on the specific Mecp2 genetic lesion and gender. Interestingly, in some regions volumetric defects anticipate overt symptoms, possibly revealing where the pathology originates and providing a useful biomarker for assessing drug efficacy in pre-clinical studies. DOI: 10.1016/j.nbd.2023.106083
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In vivo MRI experiments were conducted on a 7-Tesla scanner for rodents, fully equipped for brain MRI/MRS (Biospec, Paravision 6.0 Software Bruker-Biospin). A dedicated mouse head coil (4-channels) was used as receiver together with a volume coil as transmitter. For cerebral anatomy analysis, T2-weighted images were acquired with a fast-spin-echo sequence (TR/TE= 3350-3500/44-45 ms, resolution of 73-76 µm, thickness of 0.65-0.7 mm, 9 averages, 9-12 minutes of acquisition). T2-weighted images were analysed the Atlas Normalization Toolbox using elasti X (ANTX, a MATLAB toolbox,https://github.com/ChariteExpMri/antx2), allowing to extract the cerebral volume of different brain areas following the Allen Mouse Brain Atlas (http://mouse.brain-map.org/). The whole procedure was completed following the guidelines described in Koch et al. (2017). Single brain region volume is stated in this dataset and they are sub-divided in: - Male BIRD (Mecp2tm1.1Bird line) mouse model; - Female BIRD (Mecp2tm1.1Bird line) mouse model; - Male Y120D (Mecp2 Y120D line) mouse model; - Female Y120D (Mecp2 Y120D line) mouse model; Male animals were scanned at P20, P30 and P40; females animals at P60, P90 and P200. The number of the animals excluded from the analysis for technical problems were clearly stated in Material and Methods of the manuscript (Carli et al., 2023). Brain sub-regions in which the volume value was not present for at least the half of each groups was excluded from the dataset. The whole procedure was completed following the guidelines described in Koch et al. (2017).