Alzheimer's disease: an evolutionary approach.

Alzheimer's disease (AD) is a complex disease associated with advanced age whose causes are still not fully known. Approaching the disease from an evolutionary standpoint may help in understanding the root cause of human vulnerability to the disease. AD is very common in humans and extremely uncommon in other mammals, which suggests that the genetic changes underlying the alterations in cerebral structure or function that have taken place over the course of the evolution of the genus Homo have left specific neurons in the human brain particularly vulnerable to factors which trigger the disease. Most of the genes whose mutation leads to AD are involved in synaptic plasticity. Evidence has also been found relating AD to neuronal oxidative stress. Neurons in certain association areas of the human brain retain juvenile characteristics into adulthood, such as the increased expression of genes related to synaptic activity and plasticity, incomplete myelination and elevated aerobic metabolism, which can cause an increase in oxidative stress in these neurons. Oxidative stress can cause myelin breakdown and epigenetic changes in the promoter region of genes related to synaptic plasticity, reducing their expression. These changes may in some cases induce hyperphosphorylation of tau and β-amyloid deposits, which are characteristic of AD. The adaptation of humans to the cognitive niche probably required an increase in synaptic plasticity and activity and neuronal metabolism in neurons in areas related to certain cognitive functions such as autobiographical memory, social interaction and planning. The cost of these changes may have been the brain's increased vulnerability to factors which can trigger AD. This vulnerability may have resulted from the evolutionary legacies that have occurred over the course of the evolution of the human brain, making AD a possible example of antagonistic pleiotropy. The evolutionary approach allows apparently unrelated data from different disciplines to be combined in a manner that may lead to an improved understanding of complex diseases such as Alzheimer's.