These are the first data suggesting that long-lasting agonist 5-HT2A receptor-targeting autoantibodies in plasma from patients having TBI and/or T2DM and suffering with microvascular and neurodegenerative (e.g., PD) complications cause significant modulation of expression in genes underlying neuroapoptosis, the maintenance of differentiated neuronal morphology, neurotransmitter release, synaptic plasticity, and amyloid beta precursor protein interactions. The gene discussed is APP; the disease is Parkinson disease.