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Impaired Functional Connectivity in a Rat Model and Humans with Fragile X Syndrome.1Centre for Discovery Brain Sciences, The University of Edinburgh, Edinburgh, United Kingdom, 2Patrick Wild Centre, The University of Edinburgh, Edinburgh, United Kingdom, 3Simons Initiative for the Developing Brain, Edinburgh, United Kingdom, 4Centre for Clinical Brain Sciences, The University of Edinburgh, Edinburgh, United Kingdom, 5Center for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands, 6Centre for Brain Development and Repair, Institute for Stem Cell Biology and Regenerative Medicine, Bangalore, India
Synopsis
A key requirement for the effective development of novel therapies for Intellectual Disabilities is the ability to directly compare findings from basic neuroscience in rodent models with human studies. Functional magnetic resonance imaging offers a platform to overcome this translational barrier. Here, we use a parallel resting state fMRI approach in individuals with Fragile X Syndrome (FXS) and in a rat model of FXS using a 3T and 7T scanner respectively, and show that the loss of Fragile X mental retardation protein leads to a shared decrease in DMN connectivity in humans with FXS and rats that model this disorder.
Introduction
Autism Spectrum Disorders (ASD) and Intellectual Disabilities (ID), two co-occurring neurodevelopmental disorders, affect approximately 100 million individuals worldwide1, making them one of the leading cause of disability in children2. Current therapeutic interventions are restricted to managing symptoms and on behavioural therapy, rather than on acting on the underlying cause. There is therefore an urgent need for better treatments.
The study of animal models of monogenic disorders associated with a high prevalence of ASD/ID, such as Fragile X Syndrome (FXS) can help us gain a better understanding of the pathophysiology of ASD/ID and hence develop new disease-modifying therapies. However, a key requirement for effective therapeutic development is the ability to directly compare findings from basic neuroscience in rodent models with human studies. Methods that can be applied to both human and rodents would facilitate this endeavour.
Functional magnetic resonance imaging (fMRI) is directly comparable across species, providing a powerful tool to investigate how brain activity is modulated across development and disease. Resting state fMRI studies are particularly interesting as they investigate brain activity at rest and therefore do not require participants to learn a task, which can be challenging in clinical populations such as in ASD/ID. Furthermore, similar resting state networks have been found in humans and rats3, suggesting that this technique may provide a platform to compare humans and animal models of neurological conditions.
In this study, we used a parallel resting state fMRI approach in individuals with FXS and in a rat model of FXS to determine whether loss of Fragile X mental retardation protein (FMRP) leads to similar differences in brain connectivity across species.
Methods
Twenty-four 3-month old Long Evans Hooded Fmr1-/y rats and littermate controls were scanned in a 7T Agilent scanner. During scanning, animals were anaesthetised under 1% isoflurane and their physiology was constantly monitored. Structural images were acquired using a Fast Spin Echo sequence (matrix 256x256, FOV 30 mm, TR 3000 ms, echo train length 8, echo spacing 12ms, 26 slices, 1mm slice thickness). Resting state functional scans were acquired using a two-shot echo planar imaging (EPI) sequence (matrix 60x60, FOV 30mm, kzero 15, TR 2000 ms, TE 12 ms, 28 slices, 0.8mm slice thickness, no gap) with 300 volumes acquired per animal.
Five Fragile X individuals aged 18-32 were habituated in a mock-scanner before being scanned alongside 17 age-matched controls in a 3T Siemens Magnetom Verio scanner. Structural images were acquired using a T1-weighted magnetization prepared rapid acquisition gradient echo (MPRAGE) sequence (TR 2300 ms, TE 2.98 ms, 9° flip angle, FOV 256 mm, 1mm slice thickness and 160 slices per slab). Functional images were acquired using a single-shot EPI sequence (TR 1560 ms, TE 26 ms, TA 7.42 minutes, 66° flip angle, 26 axial slices, voxel size 3.4x3.4x4.0 mm, interleaved slice ordering, FOV 220 mm) with 293 volumes acquired for each participant.
MRIcro, Skullstrip and FMRIB Software Library (FSL) were used for data preprocessing. Seed-based and independent component analysis were performed in FSL and Matlab.
Results
Investigation of intrinsic brain activity in anaesthetised rats suggests a global decrease in functional connectivity in a rat model of FXS. In particular, we show a decrease in their Default Mode Network (DMN) connectivity (Figure 1). Importantly, preliminary findings suggest a similar deficit in long-range connectivity of the DMN in low-functioning individuals with FXS (Figure 2).Discussion
Our observation of a global decrease in functional connectivity in rat models of FXS is similar to the findings reported in high-functioning individuals with ASD4–6. Moreover, a decrease in DMN connectivity, which we observed both in our rat model and in individuals with severe FXS, has also been reported in high-functioning individuals with ASD7–9. The implications of these findings are two-fold; they suggest that our rat model of FXS recapitulates functional connectivity abnormalities present in Fragile X individuals and that our findings might reflect common components between FXS and the autisms. Further investigations of other single-gene disorders associated with ASD will help shed light on this.
Overall, our study demonstrates the feasibility of using resting state fMRI on individuals with moderate to severe ID and ASD. The similarity of the results between humans and rodent models points to the potential translational value of this technique.
Conclusion
Here we show that the functional connectivity abnormality observed in a rat model of FXS directly translate to individuals with severe FXS. Further experiments are now needed to assess whether therapeutic efficacy in a rodent model predicts outcome in humans, paving the way to using resting state fMRI as a biomarker for clinical trials.Acknowledgements
The authors acknowledge Dr. Maurits Jansen and Ross Lennen for their assistance in acquiring the preclinical data.
This work was supported by funds from the Simons Initiative for the Developing Brain, the Patrick Wild Centre, LouLou Foundation, MRC, Autistica, Department of Biotechnology India, Wadhwani Foundation and RS MacDonald Trust.
References
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