A novel tau mutation in exon 9 (1260V) causes a four-repeat tauopathy.

A novel mutation in exon 9 of tau, I260V, is associated with a clinical syndrome consistent with frontotemporal dementia with extensive tau pathology; however, neurofibrillary tangles and Pick bodies are absent. Significantly, Sarkosyl-insoluble tau extracted from affected brain tissue consisted almost exclusively of four-repeat isoforms. Consistent with these findings, in vitro biochemical assays demonstrated that the I260V mutation causes a selective increase in tau aggregation and a decrease in tau-induced microtubule assembly with four-repeat isoforms only. The contrasting pathology and biochemical effects of this mutation suggest a different disease mechanism from the other exon 9 mutations and demonstrates the critical role for the first microtubule-binding domain in tau-promoted microtubule assembly and the pathogenic aggregation of tau.