Biomarkers

Background: New techniques have been developed to estimate the age when someone converted to amyloid positivity (EAOA) from PET, oftentimes offering information Aβout a participant decades before they joined a research study. EAOA is variable across populations but we do not know the causes for these differences. This study aims to validate APOE associations with EAOA and explore genetic and sex-based factors with EAOA.

Methods: Data from six cohorts were analyzed. Our analysis included 4220 non-Hispanic white people (57.6% women; 86.7% cognitively unimpaired at baseline scan). Amyloid PET data were harmonized using gaussian mixture models. EAOA was calculated using the sampled iterative local approximation (SILA) algorithm. Sex differences in EAOA were compared using t-tests amongst amyloid positive individuals. A genome-wide association study of EAOA was performed. Gene analyses were conducted using MAGMA.

Results: Average EAOA was 81.1 years across all individuals regardless of amyloid status. APOE e2 homozygotes had slightly later EAOA than e3/e3 homozygotes. APOE e4 homozygotes converted to amyloid positivity 8.2 years before e3/e4 heterozygotes and over two decades earlier than e3 homozygotes. APOE e2/e4 converted to positivity roughly three years later than e3/e4 and nearly ten years earlier than e3 homozygotes. APOE genotype differences in EAOA described were statistically significant (p < .01). There were significant sex differences between men and women when examining amyloid positivity. Men converted to amyloid positivity over 2 years later than women (65.3 vs 63.2 years, p=3.23x10-5). The rs12981369 polymorphism in ABCA7 was associated with EAOA (β = 2.14, p=9.27×10-9). Brain eQTL databases indicate associations between rs12981369 and gene expression of ABCA7. Gene-level analyses revealed significant associations for ABCA7, HMHA1, and KIF13B.

Conclusion: This study further describes the role of APOE and reveals roles for ABCA7 and KIF13B on amyloid onset. We identified a novel variant on chromosome 19 correlating with later amyloid onset conversion and highlight important differences between sexes. These findings highlight EAOA as a powerful endophenotype of AD and offer insights into potential drug-targetable mechanisms for early AD intervention.