Genetic and Blood Biomarkers of Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent form of dementia. AD is highly heritable, with a complex pattern. Although clinical diagnosis is based on accurate and well defined diagnostic criteria (NINCDS-ADRDA), the definite diagnosis relies on the postmortem pathological findings. The need for measures for the early detection of AD, as well as the need to distinguish between AD and other forms of dementia, has put great emphasis on the discovery of biomarkers for Alzheimer's disease. In clinical practice, there is need for non-invasive, accurate methods for the early detection and differential diagnosis of AD. The successful identification and development of the biomarkers, depends completely on the understanding of pathology, genetic and molecular mechanisms involved in the disease. As blood is a circulating dynamic tissue and transcription reflects the ongoing changes in the system, it makes the transcriptional profiling of the blood cells, potentially, the most sensitive source for transcriptional biomarkers. A systematic comparison of the genetic and proteomic blood biomarkers in patients and healthy controls can reveal additional potential candidates for AD. In the first part of this review, we would like to discuss the most recent genetic findings and their possible involvement in the pathogenesis of AD. In the second part, we review the blood biomarkers which can be derived from peripheral blood mononuclear cells (PBMC), serum, and plasma. We discus three main category of bio-molecules, namely DNA, RNA (miRNA and mRNA), and protein as well as their possible role in the hypothetical mechanisms involved in pathogenesis of AD. A dynamic interaction between the genetic findings through the whole genome association studies and biomarkers discovery can advance our knowledge of AD pathogenesis beyond the scope of each field independently. There are two major types of Alzheimer's disease 1) Early onset AD (EOAD) with the age of onset <65 and strong familial clustering, with an autosomal dominant mode of inheritance, comprising about 5% of all cases with diagnosis of Alzheimer's disease, 2) late onset Alzheimer's disease (LOAD), comprising 95% of AD cases with the age of onset beyond 65 and no significant familial aggregation. The rare EOAD, is mostly associated with the highly penetrant mutations in APP (Amyloid Precursor Protein) on