Two families have been described that segregate dominant
EOAD mutations in a homozygous state, a Italian family
with the APP mutation p.A713T [54] and a Colombian
family with the PSEN1 mutation p.E280A [55]. In both families,
the homozygous carriers did not seem to be more severely affected [54,55]. The major implication of these
findings is that the homozygous state of autosomal
dominant mutations in AD is not lethal as initially
hypothesized. This may also suggest the possible presence
of additional protective genetic factors, which may modify
the disease presentation.