Alzheimer’s disease is the most common form of dementia, affecting more than 32 million people worldwide. The relationship between the disease and genetics is a matter of great concern to the families of sufferers who wonder about the likelihood of inheriting Alzheimer’s disease. The fact that a parent has had this type of dementia does not necessarily mean that their children will develop it, but is Alzheimer’s hereditary?
“Familial” Alzheimer’s: when genetics is a determining factor
In less than 1% of cases, the onset of the disease is genetically determined by the transmission of a determinant gene from one generation to the next. These are rare pathogenic variants that follow an autosomal dominant pattern of inheritance, so that these cases of Alzheimer’s are inherited from both the mother and the father. This type of disease usually has an early onset and may debut before the age of 50. In these cases of hereditary early Alzheimer’s disease, mutations in three genes have been identified as the cause of the disease: the amyloid precursor protein (APP) gene and the presenilin 1 and 2 genes (PSEN1 and PSEN2). Although the onset is usually earlier and the progression faster, the symptoms of hereditary Alzheimer’s do not differ from those of other cases.
“Sporadic” Alzheimer’s disease and the APOE gene
The vast majority of Alzheimer’s cases do not have a genetic cause and are not due to familial inheritance. Studies carried out so far have shown that the development of the disease is conditioned by a combination of genetic and environmental factors, the most important of which is advanced age. However, in many cases, there is a family history of affected individuals, which highlights the contribution of genetics to the risk of developing the disease.
Recent genome-wide association studies or GWAS have identified dozens of associated genetic variants, among which the APOE gene, which codes for apolipoprotein E and for which we can find three forms: APOE2, APOE3 and APOE4. Each of us has two copies of each gene: one that we inherit from our mother and one from our father. While the APOE2 and APOE3 variants do not confer a greater predisposition, having at least one APOE4 copy, inherited from either parent, increases the risk of Alzheimer’s disease by 3 to 4 times. Despite its important contribution, being a carrier of this allele does not imply that the disease will develop, as there are many other contributing risk factors. Some are modifiable such as lifestyle or keeping the mind active, and others are not modifiable such as genetic factors or age.
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