To date,
all 3 known causative gene mutations (changes that almost always
result in Alzheimer disease if the patient lives long
enough) lead to the early-onset form
3 The first of these genes
to be identified was the amyloid precursor protein gene, located
on chromosome 21. Mutations in this gene cause excessive
cleavage by the β- and γ-secretase enzymes, instead of
normal cleavage by the α-secretase enzyme.