Molecular genetics of Alzheimer's disease and aging.

Alzheimer's disease is a genetically complex disorder associated with multiple genetic defects, either mutational or of susceptibility. Although potentially associated with an accelerated stochastically driven aging process, Alzheimer's disease is an independent clinical entity in which the aging process exerts a deleterious effect on brain activity in conjunction with polymodal genetic factors and other pathological conditions (i.e., age-related cerebrovascular deterioration) and environmental factors (i.e., nutrition). Alzheimer's disease genetics does not explain in full the etiopathogenesis of this disease. Therefore, it is likely that environmental factors and/or epigenetic phenomena also contribute to Alzheimer's disease pathology and phenotypic expression of dementia. The genomics of Alzheimer's disease is still in its infancy, but this field is aiding the understanding of novel aspects of this disease, including genetic epidemiology, multifactorial risk factors, pathogenic mechanisms associated with genetic networks and genetically regulated metabolic cascades. Alzheimer's disease genomics is also helping to develop new strategies in pharmacogenomic research and prevention. Functional genomics, proteomics, pharmacogenomics, high-throughput methods, combinatorial chemistry and modern bioinformatics will greatly contribute to accelerate drug development for Alzheimer's disease and other complex disorders. The multifactorial genetic dysfunction in dementia includes mutational loci (APP, PS1, PS2, TAU) and diverse susceptibility loci (APOE, alpha2M, alphaACT, LRP1, IL1 alpha, TNF, ACE, BACE, BCHE, CST3, MTHFR, GSK3 beta, NOS3 and many other genes) distributed across the human genome, probably converging in a common pathogenic mechanism that leads to premature neuronal death, in which mitochondrial DNA mutations may contribute to increased genetic variability and heterogeneity. In Alzheimer's disease, multiple pathogenic events, including genetic factors, accumulation of aberrant or misfolded proteins, protofibril formation, ubiquitin-proteasome system dysfunction, excitotoxic reactions, oxidative and nitrosative stress, mitochondrial injury, synaptic failure, altered metal homeostasis, dysfunction of axonal and dendritic transport, and chaperone misoperation may converge in pathogenic pathways leading to premature death and neurodegeneration. Some of these mechanisms are common to several neurodegenerative disorders, which differ depending upon the gene(s) affected and the involvement of specific genetic networks, together with epigenetic factors and environmental events. Many genes potentially associated with Alzheimer's disease in some studies cannot be confirmed as candidate genes in replication studies, indicating that methodological problems and genomic complexity are leading to erroneous conclusions. A different approach to Alzheimer's disease functional genomics is to integrate individual genetic information in polygenic genotypes (haplotype-like model) and to investigate genotype-phenotype correlations and genotype-related pharmacogenomic behaviors. The application of functional genomics to Alzheimer's disease can be a suitable strategy for molecular diagnosis and for understanding pathophysiological mechanisms associated with Alzheimer's disease-related neurodegeneration. Furthermore, the pharmacogenomics of Alzheimer's disease may contribute in the future to optimize drug development and therapeutics, increasing efficacy and safety, and reducing side-effects and unnecessary costs.