TREM2 and early-onset autosomal dominant Alzheimer disease: This suggests that TREM2 is part of a functional network involved in different neurodegenerative dementias (FTD, Alzheimer's disease, and Nasu-Hakola), but besides the fact that homozygous mutations cause an early-onset and more severe disease, whereas heterozygous variants increase the risk for a late-onset disorder, it is not possible to establish a specific correlation between genotype and phenotype.