PSEN2 and Alzheimer disease: The mean expression of genes implicated in Aβ synthesis in oligodendrocytes (BACE1, BACE2, PSEN1, PSEN2, APH1A, NCSTN15) was replicated in the SEA-AD data (Fig. 7d), with oligodendrocytes having the highest levels of both APP and PSEN1. Therefore, the early loss of oligodendrocytes may be attributed to these higher levels of Aβ molecules that have known cytotoxicity.