The solved and unsolved mysteries of the genetics of early-onset Alzheimer's disease.

Approximately half of the Alzheimer's disease (AD) cases that are associated with early onset appear to be transmitted as a pure genetic, autosomal dominant trait. Genetic analyses of these pedigrees have found three causal genes: betaAPP, presenilin 1 (PS1), and presenilin 2 (PS2). This review provides an update on the pathological consequences of mutations in early-onset AD genes, the phenotypic heterogeneity of those cases, and future directions for research and clinical practice.