The result is increased
production of toxic β-amyloid fragments, which are
converted into insoluble aggregates that form senile plaques
in brain tissue (Figure 1 and Figure 2). Two other genes that
cause familial early-onset Alzheimer’s disease are presenilin-1,
located on chromosome 14, and presenilin-2, located on chromosome
1. Both of these genes are involved in the γ-secretase
complex, and mutations lead to excessive cleavage by the γ-
secretase enzyme, which results in increased production and
accumulation of β-amyloid fragments (Figure 1). These mechanisms
are not known to be associated with late-onset or sporadic
Alzheimer disease.