APOA2 and early-onset autosomal dominant Alzheimer disease: Apolipoprotein A (ApoA) proteins, ApoA-I, ApoA-II, ApoA-IV, and ApoA-V, play critical roles in lipid metabolism, neuroinflammation, and blood–brain barrier integrity, making them pivotal in neurological diseases such as Alzheimer’s disease (AD), stroke, Parkinson’s disease (PD), and multiple sclerosis (MS).