TREM2 and Parkinson disease: Rare loss of function variants in the TREM2 gene can cause Nasu-Hakola disease when both alleles are affected (Paloneva et al., 2000) or increase the risk to develop AD, PD, or frontotemporal dementia in heterozygous carriers (Guerreiro et al., 2013b, Jonsson et al., 2013).