Tau and α-synuclein in susceptibility to, and dementia in, Parkinson's disease

Objective: Parkinson's disease (PD) is a neurodegenerative condition that typically presents as a movement disorder but is known to be associated with variable degrees of cognitive impairment including dementia. We investigated the genetic basis of susceptibility to and cognitive heterogeneity of this disease. Methods: In 659 PD patients, 109 of which were followed up for 3.5 years from diagnosis, and 2,176 control subjects, we studied candidate genes involved in protein aggregation and inclusion body formation, the pathological hallmark of parkinsonism: microtubule-associated protein tau (MAPT), glycogen synthase kinase-3β (GSK3B), and α-synuclein (SNCA). Results: We observed that cognitive decline and the development of PD dementia are strongly associated (p = 10^-4) with the inversion polymorphism containing MAPT. We also found a novel synergistic interaction between the MAPT inversion polymorphism and the single nucleotide polymorphism rs356219 from the 3′ region of SNCA. In our data, carrying a risk genotype at either of these loci marginally increases the risk for development of PD, whereas carrying the combination of risk genotypes at both loci approximately doubles the risk for development of the disease (p = 3 × 10^-6). Interpretation: Our data support the hypothesis that tau and α-synuclein are involved in shared or converging pathways in the pathogenesis of PD, and suggest that the tau inversion influences the development of cognitive impairment and dementia in patients with idiopathic PD. These findings have potentially important implications for understanding the interface between tau and α-synuclein pathways in neurodegenerative disorders and for unraveling the biological basis for cognitive impairment and dementia in PD.