DRD2 and early-onset autosomal dominant Alzheimer disease: The altered expression of brain derived neurotrophic factor (BDNF) and the D2 dopamine receptor (DRD2) is significant as the former has been implicated in Alzheimer's disease, affective disorders, posttraumatic stress disorder, schizophrenia, and substance dependence [35], while the latter has been implicated in post traumatic stress syndrome (PTSD), anxiety, social dysfunction, and depression [36].