RISK FACTORS FOR DEVELOPING ALZHEIMER’S DISEASE: HEREDITY
There have been many studies of the inheritance patterns of Alzheimer’s disease in families. Some of these studies are more reliable than others; in many, the criteria upon which the diagnosis has been made have been rather loosely defined. The most important studies are those whose data is based on post-mortem confirmation of Alzheimer’s disease. Studies that rely on the correct diagnosis being made during life are less satisfactory, not being backed up by pathological confirmation. In the best centres the diagnosis is probably only made with an accuracy of around 80 per cent, and in some it is no better than 50 per cent.
The results of many of the studies that have been undertaken are conflicting. There is, however, little doubt that in some families, where the disease strikes predominantly younger people, there is a very strong hereditary factor. Whether or not a person develops the disease is dependent upon the genetic material that is present within the cells of the body. This genetic material is divided up into small sub-units called genes; the more powerful genes are described as dominant and the less powerful ones as recessive. Even if the Alzheimer’s disease is controlled by a dominant gene in a family with a strong pattern of the illness, the risk of a child of an affected individual developing this type of dementia is less than 50 per cent. In many cases it is considerably less than this and once the gene becomes diluted even further, to grandchildren, nephews and nieces, etc., the risks are even smaller.
In most instances the disease is not caused by a single dominant gene. In practical terms this means that someone who is a member of a family in which several people have suffered from Alzheimer’s disease, even if the onset was before the age of sixty-five, faces a risk of developing the condition almost certainly less than 50 per cent, maybe very much less.
In most cases of Alzheimer’s disease it is very probable that heredity doesn’t play any part at all. If the family history contains only a single older person who has had this diagnosis made, there is virtually no increased risk for others in the family.
The risk is further diminished if it turns out to be true that it is not the inheritance of the gene itself that matters, but whether the gene makes us more sensitive to outside agents. An example of this is the relationship between smoking and lung cancer. We all know of people who smoke sixty cigarettes a day and yet manage to survive into their nineties without developing cancer. On the other hand we know also of other smokers who succumbed to lung cancer in their fifties or sixties. It is possible that within the genetic make-up of all of us, our genes determine how sensitive we are to the action of the chemicals in tobacco smoke. If we inherit a gene that makes us very sensitive, then we are more likely to develop lung cancer than another person who has inherited a gene that makes him resistant to the chemicals in cigarettes.
It could well be the same with Alzheimer’s disease, and for that matter the other dementias. Whether we do or do not develop the condition may depend not just upon the inheritance of a particular gene, but whether we are also exposed to something which that gene makes us particularly sensitive to. This external agent could be anything from the chemicals that we put in our food or spray on our crops to a particular virus or the way in which we react to a particular virus.
My own feeling is that the increased risk of a person developing Alzheimer’s disease is very small if the only member of the family with this diagnosis is an elderly grandparent. If two grandparents have been affected the risk rises but is not of undue concern. On the other hand, if three or four members of the family have developed the disease before the age of sixty-five there is indeed a significant risk of other members of the family being affected. The odds are, however, still likely to be better than 50 per cent in favour of any individual not contracting the condition.
It is possible that in some families a single dominant gene may be responsible for Alzheimer’s, but in most cases, if there is a genetic basis at all, it will more likely result from the interplay of several genes or the interaction between a person’s genetic makeup and other factors in the environment.
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Tags: General health