Heritability of Alzheimer’s disease estimated from twin studies is greater than heritability derived from genome-based studies, for reasons that remain unclear. We apply both approaches to the same twin sample, considering both Alzheimer’s disease polygenic risk scores (PRSs) and heritability from twin models, to provide insight into the role of measured genetic variants, and to quantify uncaptured genetic risk.
A population-based heritability and polygenic association study of Alzheimer’s disease was conducted between 1986 and 2016 and is the first study to incorporate PRSs into biometrical twin models of Alzheimer’s disease. The sample included 1586 twins drawn from the Swedish Twin Registry which were nested within 1137 twin pairs (449 complete pairs, 688 incomplete pairs) with clinically-based diagnoses and registry follow-up (Mage = 85.28, SD = 7.02; 44% male; 431 cases, 1155 controls).
We report contributions of PRSs at P < 1 × 10−05, considering a full PRS, PRS without the APOE region (PRS.no.APOE), and PRS.no.APOE plus directly measured APOE alleles. Biometric twin models estimated the contribution of environmental influences and measured (PRS) and unmeasured genes to Alzheimer’s disease risk. The full PRS and PRS.no.APOE contributed 10.1% and 2.4% to Alzheimer’s disease risk, respectively. When APOE ε4 alleles were added to the model with the PRS.no.APOE, the total contribution was 11.4% to Alzheimer’s disease risk, where APOE ε4 explained 9.3% and PRS.no.APOE dropped from 2.4% to 2.1%. The total genetic contribution to Alzheimer’s disease risk, measured and unmeasured, was 71% while environmental influences unique to each twin accounted for 29% of the risk.
The APOE region explains much of the measurable genetic contribution to Alzheimer’s disease, with smaller contributions from other measured polygenic influences. Importantly, substantial background genetic influences remain to be understood.