Modelling seeding and neuroanatomic spread of pathology in amyotrophic lateral sclerosis

Abstract

The neurodegenerative disorder amyotrophic lateral sclerosis (ALS) is characterized by the progressive loss of upper and lower motor neurons, with pathological involvement of cerebral motor and, additionally, extra-motor areas, in a clinicopathological spectrum with frontotemporal dementia (FTD). A key unresolved question is whether the distribution of pathology in ALS is driven by molecular factors such as regional gene expression, by differential network vulnerability, or is a combination of both. A system of histopathological staging of ALS based on the regional burden of TDP-43 pathology observed in post mortem brains has been supported to some extent by analysis of distribution of in vivo structural MRI changes. In this paper, computational modelling using a Network Diffusion Model (NDM) was used to investigate whether a process of focal pathological seeding followed by structural network-based spread recapitulated post mortem histopathological staging and, secondly, whether this had any relationship to the pattern of expression of a panel of genes implicated in ALS across the healthy brain. Regionally parcellated T1-weighted MRI data from ALS patients (baseline n=79) was studied in relation to a healthy control structural connectome and a database of associated regional cerebral gene expression. The NDM provided strong support for a structural network-based basis for regional pathological spread in ALS, but no simple relationship to the spatial distribution of ALS-related genes in the healthy brain. Intriguingly, the critical seed regions for spread within the model were not within the primary motor cortex but basal ganglia, thalamus and insula, where NDM recapitulated aspects of the post mortem histopathological staging system. Within the ALS-FTD clinicopathological spectrum, non-primary motor structures may be among the earliest sites of cerebral pathology.