Uncommon polymorphism in the presenilin genes in human familial Alzheimer's disease: not to be mistaken with a pathogenic mutation.

In this article, we studied the frequency of mutations in the presenilin (PSEN) 1, PSEN2 and amyloid precursor protein genes in a group of patients with late-onset Alzheimer's disease (AD). No pathogenic mutations were found, but a rare non-conservative single-nucleotide polymorphism was detected in the PSEN2 gene (P334R) in a large kindred with familial late-onset AD. No cosegregation was observed in this family. Uncommon polymorphisms can be easily mistaken as a pathogenic mutation when segregation is not analyzed. Family segregation study represents an essential point in considering the pathogenicity of these mutations. These uncommon single-nucleotide polymorphisms should be always taken into account in the genetic testing of AD. They may well have important implications for genetic counselling in AD.