Genetics
In humans, the gene for APP is located on chromosome 21 and contains at least 18 exons in 240 kilobases. Several alternative splicing isoforms of APP have been observed in humans, ranging in length from 365 to 770 amino acids, with certain isoforms preferentially expressed in neurons; changes in the neuronal ratio of these isoforms have been associated with Alzheimer's disease. Homologous proteins have been identified in other organisms such as Drosophila (fruit flies), C. elegans (roundworms), and all mammals. The amyloid beta region of the protein, located in the membrane-spanning domain, is not well conserved across species and has no obvious connection with APP's native-state biological functions.
Mutations in critical regions of Amyloid Precursor Protein, including the region that generates amyloid beta (Aβ), cause familial susceptibility to Alzheimer's disease. For example, several mutations outside the Aβ region associated with familial Alzheimer's have been found to dramatically increase production of Aβ.
A mutation (A673T) in the APP gene protects against Alzheimer’s disease. This substitution is a adjacent to the beta secretase cleavage site and results in a 40% reduction in the formation of amyloid beta in vitro.
Read more about this topic: Amyloid Precursor Protein