GDF15 and Alzheimer disease: Next, we assessed whether the brain age gap remained predictive of future dementia risk after adjusting for established biomarkers and risk factors, such as chronological age, cognitive function, polygenic risk score (PRS) for AD, APOE4 genotype and other AD-related proteins not included in the brain aging model, such as GDF15 (ref. 31) and APOE32 (Fig. 3e).