People at high risk of developing Alzheimer's disease in later life possess hidden signs of the senile disorder decades before the onset of the illness, scientists have found.
The findings might eventually form the basis of a diagnostic test for Alzheimer's which could enable doctors to identify and treat people who would otherwise be destined to develop the progressive illness.
Abnormalities in the brain of people aged between 20 and 40, who carried a gene that is known to increase the risk of Alzheimer's, were discovered in a study by Eric Reiman and colleagues at the University of Arizona in Tucson.
The abnormalities do not appear to affect people's mental abilities but they do seem to indicate that something is beginning to happen long before the onset of the traditional symptoms of the disease, such as forgetfulness and mental distraction. The same "functional" abnormalities - low rates of glucose metabolism in specific regions of the brain - are also found in older patients with the disease, the scientists report in the journal Proceedings of the National Academy of Sciences.
Alzheimer's disease affects some 10 per cent of people over the age of 65, and almost half of those aged 85 or over.
With more people living longer, the proportion of the population affected by the disease will rise significantly in the next 20 years. A method of diagnosing Alzheimer's early in a patient's life could help doctors to develop better preventative treatments or even drugs that could delay the onset of symptoms.
Different varieties of a gene known as APOE - which is responsible for a protein called apolipoprotein E - are linked with different risks of developing Alzheimer's, with one version, known as APOE-4, resulting in the highest risk.
When people inherit two copies of the APOE-4 gene, one from their father and one from their mother, the chances of developing Alzheimer's by the time they are aged 70 rises to more than 90 per cent.
Dr Reiman wanted to assess the brains of younger people aged between 20 and 40 in order to compare those carrying both maternal and paternal copies of the APOE-4 version of the gene with those who had other versions of the gene. Using a sophisticated brain scanner that could measure the rate at which the brain uses up its supply of glucose - the "fuel" that keeps brain cells alive and active - the scientists found a way of comparing the mental activity of the two groups of volunteers in the experiment.
Dr Reiman's team found that the carriers of APOE-4 had abnormally low levels of activity in precisely the same areas of the brain seen to be low in activity in Alzheimer's patients.
"Carriers of a common Alzheimer's susceptibility gene have functional brain abnormalities in young adulthood, several decades before the possible onset of dementia," the researchers say.
"Our findings raise the possibility that functional alterations provide a foothold for the subsequent onset of neuropathology [disease] in brain regions that are preferentially vulnerable to this disorder," they say.
If this is true - and the scientists want further research to be done - then it raises the possibility that drugs or therapy may be developed to counteract the abnormalities in the way the brain functions before they develop into the full-blown disease.
A number of studies have suggested that people with active mental lives - those for instance who do crosswords or memory tests - are less prone to developing Alzheimer's. Some scientists have called this the "use it, or lose it" phenomenon.
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