Tau-positive fine granules in the cerebral white matter: a novel finding among the tauopathies exclusive to parkinsonism-dementia complex of Guam.

We examined the autopsied brains of cases of 6 types of tauopathy: parkinsonism-dementia complex of Guam (PDC), corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), Pick disease, Alzheimer disease (AD), and myotonic dystrophy together with Guamanian controls. Light microscopy sections of these brains were examined using anti-tau antibodies. Tau-positive fine granules (TFGs) were globe-shaped, and 3 to 6 mum in diameter, were observed predominantly in the frontal white matter in 30 of the 35 patients with PDC. However, no TFGs were found in association with PSP, myotonic dystrophy, Pick disease, AD, or CBD. Western blot analysis of frozen brain tissue taken from the PDC cases revealed that the frontal cortex was hyperphosphorylated and contained 6 tau isoforms (3R+4R tau). However, in the present study, it was revealed that the novel TFGs in the white matter of patients with PDC was composed of 4R tau. Western blot analysis of sarkosyl-insoluble tau from the white matter of the PDC cases showed 2 major bands of 60 and 64 kDa and one minor band of 67 kDa. After dephosphorylation, these bands resolved into one major band of 4-repeat (4R) tau isoform and 3 minor bands of 3-repeat (3R) and 4R tau isoforms. Moreover, the TFGs observed in cases in which the number of neurofibrillary tangles (NFTs) was higher than the threshold level were not correlated with the presence of cortical NFTs. In conclusion, these novel TFGs were found almost exclusively in PDC brains and could therefore be considered as a characteristic neuropathologic marker of this particular tauopathy. The TFGs were hyperphosphorylated tau-positive structures that may be formed by a different mechanism from that used to produce cortical NFTs.