In this study, we found that striatal DAT availability levels in PD were influenced by APOE ε2 allele, COMT Val158Met, SNCA rs356219 and deleterious variants in GBA, whereas the development of dementia was influenced by the APOE ε4 allele and also by deleterious variants in GBA. Our results therefore suggest that APOE2, COMT and SNCA may be related to dopaminergic degeneration, while APOE4 may be related to other, non-dopaminergic degeneration mechanisms, and GBA may be implicated in both. This evidence concerns the gene COMT and dementia.