Transmissible and non-transmissible neurodegenerative disease: similarities in age of onset and genetics in relation to aetiology.

In only a few cases is transmissible dementia known to have been acquired by infection from a source outside the individual; the remaining cases can be classified as sporadic, loosely familial, or autosomal dominant. Each group has a characteristic mean age of onset. A range of neurodegenerative diseases (including Alzheimer-type dementia and amyotrophic lateral sclerosis) can also be classified in this way, with similar characteristic mean ages of onset. The emergence of these diseases in later middle age, and the interdependence of age of onset and the type of familial occurrence suggest that these pathological processes are related to those genetic mechanisms which determine senescence. It is argued that the majority of cases of transmissible dementia arise, not from infection, but from the expression of endogenous virogene sequences as part of the aging processes.