Apolipoprotein E element 4 allele frequency is not increased in progressive supranuclear palsy

The frequency of allele 4 of apolipoprotein E (ApoE) is increased in sporadic and familial Alzheimer's disease (AD) with late onset. ^[1] The role of ApoE element 4 in the pathogenesis of AD is unknown, but the high affinity binding of ApoE element 4 to amyloid-beta protein (A beta P) and its presence in cerebral amyloid deposits prompted the hypothesis that ApoE element 4 plays a direct role in polymerization and deposition of A beta P. ^[2] An alternative proposal stems from the finding that ApoE element 3, the more common allele, binds tau protein, a promoter of microtubule assembly and the major constituent of paired helical filaments (PHFs), the prominent cytoskeletal alteration in AD neurons and the unit component of neurofibrillary tangles (NFTs). ^[3] Thus, the lack of ApoE element 3 would hamper the normal function of tau, leading to its phosphorylization and polymerization into PHFs. ^[3] To distinguish between these two hypotheses, we analyzed ApoE allele frequency and ApoE immunoreactivity in the brains of subjects with progressive supranuclear palsy (PSP), a neurodegenerative disease characterized, as is AD, by NFTs, but without A beta P deposits.

Methods.

Twelve cases of typical PSP were selected on the basis of clinical and histopathologic criteria. All subjects developed in the course of disease axial rigidity, ophthalmoplegia, and pseudobulbar palsy leading to a bedridden state in 5 to 9 years. Microscopic examination showed the presence of NFTs in the basal ganglia, brainstem nuclei, and dentate nucleus, fulfilling the NINDS pathologic criteria for PSP. ^[4] Moreover, in six of 12 cases NFTs were ultrastructurally characterized and were found to be composed of 15- to 20-nm-diameter straight filaments. Two cases showed a number of consistent A beta P plaques with the diagnosis of AD, ^[5] and they were considered a combination of PSP and AD. Formalin-fixed as well as unfixed sections of the pons from two PSP cases were processed with a goat antiserum to human ApoE (Medix Biotech), according to the peroxidase-antiperoxidase method. Adjacent sections were immunostained with an anti-tau antiserum using the same method. Genomic DNA extracted from frozen tissue according to standard procedures was amplified by PCR using primers and conditions described ^[6] and was digested with Hha I. ApoE allele frequencies of PSP cases were compared with those of a group of 134 age-matched neurologically normal subjects living in northern Italy, as well as with those of the US population. ^[7] For statistical analysis we used a nonparametric test, since there was numeric disparity between the populations examined. The PSP group is small because of the low incidence (1.39 times 100,000) of the disease. ^[8]

Results.

In PSP subjects, the antibody to human ApoE immunostained several NFTs localized in small neurons of the basis of the pons Figure 1, A. The same NFTs were recognized by the antiserum to tau in the adjacent section Figure 1, B. In parallel experiments, NFTs as well as A beta P plaques were ApoE immunoreactive in the neocortex of AD cases Figure 1, C. ApoE element 4 allele frequency in PSP cases did not significantly differ from that observed in controls (0.125 in PSP cases, 0.104 in Italian controls, and 0.136 in the US population) Table 1. After the exclusion of the two cases with associated AD changes, ApoE element 4 frequency in PSP was 0.05.

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Figure 1. Tegmentum of pons. The same taureactive neurofibrillary tangles (NFTs) (A) of progressive supranuclear palsy are ApoE positive (B) as seen in comparison with adjacent sections. (C) Neocortical NFTs of Alzheimer's disease are ApoE positive. Scale bar equals 100 micro meter.

Discussion.

The pathologic hallmark of PSP is an extensive subcortical neurofibrillary degeneration with abnormal filaments composed of hyperphosphorylated and ubiquitinated tau protein, ^[9] as is the PHF of AD. Contrary to AD, A beta P deposition does not occur in PSP. Thus, PSP is a suitable model to test the hypothesis that ApoE modulates the normal function of tau. Our finding of ApoE immunoreactivity in NFTs of PSP cases, in spite of an ApoE allele frequency similar to that of the normal population, argues against an interaction between tau and ApoE being pivotal in abnormal filament formation. This negative finding suggests that the ApoE element 4 association with AD relates to its binding with A beta P deposits. ApoE element 4 might regulate the fate of soluble A beta P in the extracellular space, leading to A beta P deposition as amyloid plaques. Alternatively, the association with ApoE element 4 could potentiate the neurotoxic effect of A beta P, since ApoE element 4 was found to decrease neurite outgrowth in vitro. ^[10]