Apolipoprotein E gene allele 4 and amyloid-beta mediate tau-related network breakdown

There have been reports of altered functional connectivity in Alzheimer's disease, which is associated with the buildup of pathogenic proteins in the brain, including neurofibrillary tau tangles and amyloid-beta plaques. It is believed that the tau aggregates are the main driver of functional disconnection and resulted in cognitive decline in Alzheimer's disease. Tau propagates through connected neurons, a phenomenon often described as the 'prion-like' properties of tau, which can locally result in functional connectivity disruption. Apolipoprotein E gene allele 4 status and amyloid-beta are accelerating factors for tau-related pathological changes in Alzheimer's disease. However, the potential role of apolipoprotein E gene allele 4 and amyloid-beta in mediating the tau-related functional disconnection is not clear. I aimed to investigate the mediating effect of apolipoprotein E gene allele 4 and amyloid-beta on the local association of tau spreading on functional connections. I analysed follow-up resting-state functional MRI (fMRI) (non-baseline visit) and longitudinal tau-PET data from 211 subjects from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database and 138 healthy elderly individuals from the Harvard Aging Brain Study (HABS). The follow-up resting-state fMRI (non-baseline visit) was studied in order to study the time needed effect of tau pathology. The top 10 regions with the highest probability-weighted SUVR values using Gaussian mixture models were selected as individual-level tau-PET epicentres. I looked at how the relationship between functional connectivity to epicentres and individualized connectivity-related tau spreading was mediated by amyloid-beta status and the apolipoprotein E gene allele 4 genotype. Higher rates of tau aggregation accumulation were seen in areas with stronger connectedness (shorter distance-based connectivity) to the baseline-defined tau epicentres. Moreover, the association between functional connectivity to epicentres and tau spreading through functional connections was mediated by apolipoprotein E gene allele 4 and amyloid-beta status in both dataset's participants. Tau aggregates spread through functional connections and locally disrupt connectivity between tau epicentre and non-epicentre regions, which is mediated in apolipoprotein E gene allele 4 carriers and amyloid-beta-positive participants. These findings have implications for trial designs, proposing that apolipoprotein E gene allele 4 carriers and amyloid-beta-positive participants might need earlier intervention to attenuate tau spreading and tau relative functional disconnection.