The G389R mutation in the MAPT gene presenting as sporadic corticobasal syndrome

A few patients with mutations in the microtubule-associated protein tau gene (MAPT), affected by frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17T), may clinically present with a corticobasal syndrome (CBS). We report a case of apparently sporadic CBS bearing a mutation in the MAPT gene so far associated with frontotemporal dementia (FTD) phenotype. The patient is a 41-year-old man with progressive asymmetric signs of cortical and basal ganglia involvement consistent with CBS. Magnetic resonance imaging showed asymmetric cortical atrophy and unusual corticospinal tract hyperintensity in T2-weighted images. Genetic testing revealed a heterozygous G to C mutation at the first base of codon 389 of the MAPT gene, changing glycine to arginine (G389R), in the patient and his unaffected elderly father. In conclusion, the MAPT G389R mutation shows phenotypic variability resulting in both FTD and CBS. The mutation also demonstrates incomplete penetrance. Corticospinal tract degeneration is an exceptional finding.