Assessing genetic overlap and causality between blood plasma proteins and Alzheimer’s Disease

Background: Blood plasma proteins are modifiable and have been associated with Alzheimer9s disease (AD), but understanding which proteins are on the causal pathway remains challenging. Objective: Investigate the genetic overlap between candidate proteins and AD using polygenic risk scores (PRS) and interrogate their causal relationship using bi-directional Mendelian Randomization (MR). Methods: Following a literature review, 31 proteins were selected for PRS analysis. PRS were constructed for prioritised proteins with and without the apolipoprotein E region (APOE+/- PRS) and tested for association with AD status across three cohorts (n=6244). An AD PRS was also tested for association with protein levels in one cohort (n=410). Proteins showing association with AD were taken forward for MR. Results: For APOE e3, apolipoprotein B-100, and C-reactive protein (CRP), protein APOE+ PRS were associated with AD below Bonferroni significance (pBonf, p-value <0.00017). No protein APOE- PRS or AD PRS (APOE+/-) passed pBonf. However, vitamin D-binding protein (protein PRS APOE-, p-value=0.009) and insulin-like growth factor-binding protein 2 (AD APOE- PRS p-value=0.025, protein APOE- PRS p-value=0.045) displayed suggestive signals and were selected for MR. In bi-directional MR, none of the 5 proteins demonstrated a causal association (p-value<0.05) in either direction. Conclusion: Apolipoproteins and CRP PRS are associated with AD and provide a genetic signal linked to a specific, modifiable risk factor. Whilst evidence of causality was limited, this study was conducted in a moderate sample size and provides a framework for larger samples with greater statistical power.