Monogenic inheritance in early-onset dementia: illustration in Alzheimer's disease and frontotemporal lobar dementia.

Early-onset Alzheimer's disease (EOAD) and frontotemporal lobar dementia (FTLD) account for the majority of early-onset dementia (onset before 65 years). The high frequency of genetic forms is a common feature of EOAD and FTLD. A lot of efforts have been done to unravel the genetic bases of monogenic forms of these two diseases. PSEN1, APP and PSEN2 are the major causes of monogenic EOAD while GRN, MAPT and C9ORF72 are the most frequently mutated genes in familial FTLD. Besides, the rise of new generation sequencing technologies (NGS) during the last decade allowed a better description of the genetic architecture. A myriad of genes implicated each in a lower number of families with variable penetrance have been highlighted, especially in FTLD. The genetic heterogeneity and it contribution to the clinical variability have been described with more detailed and the process of molecular diagnostic has been modified as well. Here we propose to review old and recent findings about the contribution of genetic factors into these two major early-onset dementia diseases. The impact on the diagnostic and on the knowledge of associated pathophysiological mechanisms is also discussed.