Alzheimer's disease is often referred to as the "disease of the century."
Alzheimer's disease may have several underlying genetic causes. Three autosomal dominant genes—amyloid precursor protein (APP), presenilin I (PS I), and presenilin II (PS II)—cause an early onset (less than 65 years of age) familial form of Alzheimer's disease. Another autosomal gene called apolipoprotein E (APOE) may predispose an individual to develop the late onset (older than 65 years of age) form of Alzheimer's disease. Other as yet unmapped genes may also play roles in the causation of Alzheimer's disease and may be used in molecular testing.
Most cases of Alzheimer’s disease are what are called sporadic; that is, they appear randomly in different families. However, the normal age of onset, which is about seventy‐ five years, is old enough so that other family members who might have developed the disease may have died of other causes. Some scientists argue that even these sporadic cases may be inherited but don't appear clustered in families because of the death of the at-risk relatives from other causes before Alzheimer’s onset. Dementia that includes amnesia with aphasia (loss of speech) or agraphia (loss of writing ability) is thought to be of the sporadic type. Knowing whether these sporadic cases are genetic (inherited) is probably not of concern to most families, as the actual manifestation of the disease is only occasional. However, to scientists trying to determine the cause of the disease in hope of devising effective prevention or treatment, this issue is a significant one (Kumar V, 1996).
The exact cause of Alzheimer's disease remains obscure, but several hypotheses have been put forward. Some forms of the disease are probably inherited. Genetic defects have been identified in three chromosomes involved in Alzheimer's disease, and some apolipoprotein E genotypes may have a protective effect on the development of Alzheimer's disease.......
