157. High-Field In Vivo Neuroimaging to Determine CNS Gene Therapy Outcome and Probe Disease Pathomechanism

Abstract

Gene therapy targeting the central nervous system (CNS) is one of the most challenging gene therapies due to the blood-brain barrier (BBB). However, recombinant adeno-associated virus (rAAV) has proved to be an excellent tool to target the CNS. Another obstacle in the setting of CNS gene therapy is the non-invasive evaluation of therapeutic outcome. While biopsies and sections of the CNS are the gold-standard to assess brain pathology and response to CNS gene therapy, the invasiveness and potentially associated complications limit its frequent use in pre-clinical as well as clinical studies. Thus, it is of no surprise that non-invasive monitoring of CNS gene therapy in vivo holds great promise for longitudinal and functional assessment of treatment response. We used high-field in vivo neuroimaging to monitor intravenously (i.v.) and intracerebroventricularly (i.c.v.) administered rAAV based CNS directed gene therapy in a mouse model of Canavan disease (CD). Characteristically, Canavan disease presents with a very high NAA peak detected by magnet resonance spectrometry (MRS) and hyper intensity on T2-weighted anatomic images using magnet resonance imaging (MRI). Consequently, we first determined the efficacy of our i.v. and i.c.v. gene therapy by those two means. In congruence with motor function and pathology data, both MRI and MRS alterations have been entirely normalized by gene therapy. Another characteristic neuropathological change on Canavan brain sections is the loss of white matter tracts, which is thought to explain neurological symptoms seen in Canavan disease patients. Thus, we hypothesized that diffusion tensor imaging (DTI) enables the assessment of white matter tract degeneration and recovery upon gene therapy without brain biopsies. Selecting thalamus and corpus callosum as regions of interest (ROI), DTI indeed shows a recovery of brain white matter integrity when utilizing 3rd generation Canavan gene therapy. Furthermore, our 3rd generation gene therapy converts this CD mouse model with the severest phenotype into “supermouse”, outperforming wild-type mouse on motor function testing. We hypothesized that functional connectivity identifies brain regions that not only show response to treatment but also indicates possible explanations for this enhanced phenotype. Using resting-state functional MRI (rs-fMRI), we show that treated CD mice have a functional connectivity pattern that is more similar to or even enhanced beyond what is seen in WT brain. This suggests facilitated inter-brain-region functional connectivity, which might provide a neural mechanism that sub-serves the observed enhanced motor function. Currently, we are investigating how the identified brain regions can promote increased motor function, and how high-field brain imaging can provide biomarkers to track the disorder and treatment response in a manner that would help facilitate the prediction of CNS directed gene therapy outcome. In summary, our data show that high-field in vivo neuroimaging is a valuable tool to monitor pre-clinical CNS gene therapy and pathology in detail, that it can provide insights into pathophysiology and that it has potential implications for the use in clinical trial outcome prediction and assessment.