The apolipoprotein E ϵ4 allele frequency is normal in fronto-temporal dementia, but correlates with age at onset of disease

Abstract

The apolipoprotein (apoE) ϵ4 allele was studied in fronto-temporal dementia (FTD), a diagnostic category including the specific disorders Pick's disease and frontal lobe degeneration of non-Alzheimer type (FLD). These dementing diseases have neuronal and synaptic degeneration in common with Alzheimer's disease (AD), for which the presence of the apoE ϵ4 allele is a known risk factor, and lowers the age of onset of disease. Previous studies on the apoE ϵ4 allele frequency in FTD have been inconclusive. The structural hallmarks of AD, allegedly linked to apoE presentation, neuritic plaques (NP), primarily composed of aggregates of β-amyloid, and neurofibrillary tangles (NFT), primarily composed of hyperphosphorylated tau, are lacking in FTD. However, tau-positive cytoskeletal pathology is found in Pick's disease, but not in FLD. Resolving whether the ϵ4 frequency is increased in FTD or not may thus give clues to the pathogenetic mechanism of apoE in AD. We therefore studied apoE alleles in a well characterized material of FTD patients. The ϵ4 allele frequency was similar in 25 patients with FTD (14.0%) as compared with 26 healthy controls (13.5%). A post-mortem neuropathological examination was performed in 10 cases (nine had FLD and one Pick's disease). Our finding of a normal ϵ4 allele frequency in our group of FTD, principally consisting of FLD cases, support hypotheses involving differential binding of apoE to β-amyloid and/or tau, in the development of β-amyloid deposition and NP formation and/or tau hyperphosphorylation and NFT formation, for the pathogenetic role of apoE in AD. The age at onset was significantly lower ( P<0.01) in FTD patients possessing the ϵ4 allele (48.7±8.0 years) than in patients not possessing this allele (58.9±7.6 years). We conclude that, although the apoE ϵ4 allele frequency is not increase in FTD, the ϵ4 allele is not an etiological factor, but may rather be an accelerating factor in the degenerative process of FTD, thereby resulting in an earlier presentation of the disorder in individuals predisposed to develop FTD.