Familial cases tend to have an earlier age at onset. Genetic defects in
familial cases have been identified on chromosomes 21, 19, 14, 12 and 1.
The
so-called 『early onset』 cases of AD in persons in their 30′s, 40′s, and
50′s may have a genetic basis. Less than 1% of early onset AD cases are
linked to a genetic defect on chromosome 21
About half of early onset AD cases are linked to mutations in the presenilin 1 gene on chromosome 14.
A presenilin 2 gene has been discovered on chromosome 1, but this defect accounts for less than 1% of cases.
The
more typical 『late onset』 cases of AD occurring after age 60 may have
underlying genetic defects. A genetic locus on chromosome 19 encodes
for a cholesterol transporter called apolipoprotein E (apoE).
A
genetic locus on chromosome 12 that encodes for alpha-2-macroglobulin
may be found in 30% of AD cases. Mutations in the tau gene which codes
for tau, a protein that is associated with microtubules, can be found in some AD cases.