Summary here.Researchers tested a total of 19,150 individual genes for their effect on amyloid beta levels and ruled out all but one: calcium and integrin-binding protein 1 (CIB1). Cells without functional CIB1 genes produced abnormally high levels of amyloid beta protein...
Additional experiments in mouse cells allowed researchers to track how CIB1 regulates gamma secretase. In healthy cells, CIB1 is not directly involved in gamma secretase's activity to process amyloid beta, but CIB1 is attached to gamma secretase both in the internal compartment and at the cell surface membrane.
In cells without CIB1, gamma secretase remains in the internal compartment inside the cell and does not move to the surface membrane. More time in that internal compartment allows gamma secretase to produce more amyloid beta proteins.
Full text, very detailed for those of you who like the gritty details.