Amyloid of neurofibrillary tangles of Guamanian parkinsonism-dementia and Alzheimer disease share identical amino acid sequence.

The presence of abundant intraneuronal amyloid in the form of neurofibrillary tangles (NFT) in the brains of Guamanian parkinsonism-dementia patients and the absence of extraneuronal amyloid in the form of vascular amyloid deposits or senile plaques permit the purification of NFT without contamination with extraneuronal amyloid. Thus, we have isolated and determined the amino acid sequence of the polypeptide subunit of the amyloid fibrils of these NFT and describe their ultrastructure. The NFT, which consist of single and paired helical filaments, similar to those of Alzheimer disease, and occasionally triple helical filaments, are composed of multimeric aggregates of a polypeptide of 42 amino acids (A4 protein). The relative molecular mass of the subunit protein, 4.0-4.5 kDa, is the same as the molecular mass of the amyloid of NFT, of the amyloid plaque cores, and of vascular amyloid deposits in Alzheimer disease and Down syndrome; the sequence of 15 amino acid residues at the N-terminus of the amyloid fibrils in the NFT of Guamanian parkinsonism-dementia is identical to that of the amyloid of NFT, amyloid plaque cores, and cerebrovascular deposits in Alzheimer disease and Down syndrome. Furthermore, the heterogeneity, or variation in polypeptide length, of the N-terminus of the amyloid of Guamanian parkinsonism-dementia is the same as in Alzheimer disease and Down syndrome. Our observations indicate that the brain amyloids of these diseases have a common subunit protein, which would also indicate a common pathogenesis.