PSEN2 and Alzheimer disease: AD exists as two genetically distinct forms: early-onset AD, which is caused by autosomal dominant mutations in one of several genes (PSEN1, PSEN2, APP, SORL1) and typically has an onset of symptoms between the ages of 40 and 60 years [2], and the more common late-onset AD (LOAD), which is sporadic, polygenic, and typically has an onset of symptoms in the mid-60s [3].