PSEN2 and Down syndrome: Several studies have examined with FDG PET the metabolic changes in mutation carriers including APP dosage effects (trisomy 21/Down syndrome; APP duplication; exon deletion variant) as well as mutations in PSEN1, PSEN2, and APP. Villemagne et al. (2009) found that the classic PCC and biparietal pattern of hypometabolism seen in sporadic AD was not as evident in mutation carriers (n = 8; CDR 0–3.0).