Down syndrome (DS) patients, after the fourth decade of life, display some neurophatological features of the Alzheimer's disease (AD). Several hypotheses suggested that apoE4 protein, an AD risk factor, might promote amyloid formation by stabilizing an aggregated conformation of the β amyloid protein (Aβ). This peptide is the major proteinaceous component of the senile plaques either in AD or DS, and it is a proteolytic product of the amyloid precursor protein (APP). Both brain and platelets express three APP transcripts of the apparent molecular weight of 106, 110 and 130 kDa. In DS the Aβ deposits may ensue, at least in part, from the overexpression of the Aβ precursor gene located on chromosome 21. Aims of the present study were to evaluate the frequency of apoE4 isoform in DS population, and to ascertain whether the ratio between the 130 and the 106-110 kDa platelet APP isoforms is lower in DS, as seems to occur in AD patients. ApoE4 frequency was significantly lower in DS when compared to AD patients. E4 allele frequency of older DS patients was about half that of younger ones. The 130 to 106-110 kDa APP isoform ratio was similar in young DS and control subjects, and markedly lower in AD patients. Our results indicate that: i) in DS patients the early, selective accumulation of Aβ peptides is independent of the ApoE genotype, but the allele ε4 predisposes to various causes of premature death; and ii) platelet APP isoform abnormalities, which can be observed in AD patients, do not occur in young DS patients, suggesting a different processing of APP platelets in DS with respect to AD.

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