PCSK9 and Parkinson disease: Comparison of selected studies (DM2—diabetes mellitus type 2, HMG-CoAR—3-hydroxy-3-methyl-glutaryl-coenzyme-A reductase, PCSK9—proprotein convertase subtilisin/kexin 9, HT—hypertension, PD—Parkinson’s disease, OEA—oleoylethanolamide, PEA—palmitoylethanolamide).