RISK FACTORS FOR DEVELOPING ALZHEIMER’S DISEASE: DOWN’S SYNDROME
April 2nd, 2009It has been mentioned already that there are similar abnormal changes in the brains of older people with Down’s syndrome and sufferers from Alzheimer’s disease. In addition to the structural changes there are also similar biochemical abnormalities. Whether or not older people with Down’s syndrome are particularly likely to develop dementia is, however, a matter of contention, as with so many other aspects of Alzheimer’s disease. In one study, less than half of the group of Down’s syndrome sufferers over the age of fifty demonstrated significant intellectual decline; in others, psychological impairments characteristic of Alzheimer’s have been found in the majority if not all.
Part of the problem lies in the fact that we don’t have adequate tests to measure dementia in people who have a pre-existing intellectual problem. The existing tests on previously normal people are difficult enough to interpret, so results for people with
Down’s syndrome can be very misleading. This is particularly true if all the Down’s syndrome subjects are studied only once and a judgement made as to whether, at say the age of fifty, they are suffering from a dementia-like illness. The best way of deciding whether or not dementia is an inevitable accompaniment to increasing age in Down’s syndrome is to undertake a longitudinal study. This means regular assessment over a period of time, providing an opportunity to compare change in intellectual ability over a number of years. Further research of this type is needed, but if it is confirmed that subjects with Down’s syndrome can exhibit the abnormal changes in the brain that occur in Alzheimer’s disease, yet do not all develop dementia or significant intellectual decline, then this would have important implications for the significance of the presence of senile plaques and neurofibrillary tangles, etc. in the diagnosis of Alzheimer’s disease.
The presence of plaques and tangles in the brains of people with Down’s syndrome stimulated examination of chromosome 2 I. The chromosomes are the structures on which all genes lie, and in Down’s syndrome there is extra chromosome 2 I material. At one time it was postulated that people with Alzheimer’s disease also had extra chromosome 2 I material. It was therefore possible to conjecture that an excess of genetic material on chromosome 2 I may cause the abnormalities in the brain in both Alzheimer’s disease and Down’s syndrome. This view appeared particularly attractive when it was discovered that the gene responsible for the formation of the substance in the centre of the senile plaque was also on chromosome 2 I. It seemed as if at least part of the cause of Alzheimer’s disease had been found. We now know that it is not as simple as this; although chromosome 2 I is implicated in the development of the senile plaque protein, it seems that the earlier hypothesis that this particular gene was the Alzheimer gene is unlikely to be true.
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