Longitudinal Investigation of Structural and Resting-State Effective Connectivity Alterations in a Non-Human Primate Model of Huntington's Disease.

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by expanded CAG repeats in the huntingtin gene which produce a mutant huntingtin (mHTT) protein that contributes to progressive striatal, cortical, and white-matter atrophy, resulting in motor dysfunction and cognitive decline. Recently, a non-human primate (NHP) model of HD was developed via stereotaxic delivery of an adeno-associated viral vector expressing 85 CAG repeats (85Q) into the striatum. This model recapitulates several neuropathological changes and symptoms observed in people with HD (PwHD) including chorea and mild cognitive impairment. A previous longitudinal, multimodal MRI investigation in this model revealed volumetric and resting-state functional connectivity (rs-FC) changes compared to controls, in key regions involved in HD, over the course of 30 months. We aimed to study longitudinal changes in structural connectivity (SC), obtained from diffusion MRI scans from the same animals, comparing the 85Q animals to the control (Buffer) group. Additionally, going beyond the correlative rs-FC analyses, we investigated changes in causal, inter-regional functional interactions by estimating effective connectivity (EC) from rs functional MRI scans, constrained to strong structural connections. We found that the SC between basal ganglia regions and the cortex was reduced in the 85Q primates compared to the Buffer group at 14-months post virus injection, aligning with the pathological process observed in PwHD at later stages of the disease. EC from the caudate and putamen to the motor cortex was significantly reduced in the 85Q animals as early as 3-months post-injection providing novel insights into early alterations in causal functional interactions.