Rare genetic variation in Fibronectin 1 (FN1) protects againstAPOEe4in Alzheimer’s disease

Abstract

AbstractThe risk of developing Alzheimer’s disease (AD) significantly increases in individuals carrying theAPOEε4allele. Elderly cognitively healthy individuals withAPOEε4also exist, suggesting the presence of cellular mechanisms that counteract the pathological effects ofAPOEε4; however, these mechanisms are unknown. We hypothesized thatAPOEε4carriers without dementia might carry genetic variations that could protect them from developingAPOEε4-mediated AD pathology. To test this, we leveraged whole genome sequencing (WGS) data in National Institute on Aging Alzheimer’s Disease Family Based Study (NIA-AD FBS), Washington Heights/Inwood Columbia Aging Project (WHICAP), and Estudio Familiar de Influencia Genetica en Alzheimer (EFIGA) cohorts and identified potentially protective variants segregating exclusively among unaffectedAPOEε4carriers. In homozygous unaffected carriers above 70 years old, we identified 510 rare coding variants. Pathway analysis of the genes harboring these variants showed significant enrichment in extracellular matrix (ECM)-related processes, suggesting protective effects of functional modifications in ECM proteins. We prioritized two genes that were highly represented in the ECM-related gene ontology terms,(FN1)and collagen type VI alpha 2 chain (COL6A2) and are known to be expressed at the blood-brain barrier (BBB), for postmortem validation andin vivofunctional studies. The FN1 and COL6A2 protein levels were increased at the BBB inAPOEε4carriers with AD. Brain expression of cognitively unaffected homozygousAPOEε4carriers had significantly lower FN1 deposition and less reactive gliosis compared to homozygousAPOEε4carriers with AD, suggesting that FN1 might be a downstream driver ofAPOEε4-mediated AD-related pathology and cognitive decline. To validate our findings, we used zebrafish models with loss-of-function (LOF) mutations infn1b– the ortholog for humanFN1. We found that fibronectin LOF reduced gliosis, enhanced gliovascular remodeling and potentiated the microglial response, suggesting that pathological accumulation of FN1 could impair toxic protein clearance, which is ameliorated withFN1LOF. Our study suggests vascular deposition of FN1 is related to the pathogenicity ofAPOEε4, LOF variants in FN1 may reduceAPOEε4-related AD risk, providing novel clues to potential therapeutic interventions targeting the ECM to mitigate AD risk.