Genetic and host factors for dementia in Down's syndrome.

Background: The high risk for dementia in adults with Down's syndrome has been attributed to triplication and overexpression of the gene for amyloid precursor protein (APP). But the wide variation in age at onset must be due to other risk factors.

Aims: To identify factors which influence age at onset of dementia in Down's syndrome.

Method: Studies of factors which influence formation of beta-amyloid (Abeta) were reviewed, including atypical karyotypes, susceptibility genotypes, gender and oestrogen deficiency, and individual differences in Abeta peptide levels.

Results: The apolipoprotein E epsilon4 allele, oestrogen deficiency and high levels of Abeta1-42 peptide are associated with earlier onset of dementia, while atypical karyotypes and the apolipoprotein E epsilon2 allele are associated with reduced mortality and reduced risk of dementia.

Conclusions: Factors which influence Abeta levels, rather than overexpression of APP, may account for the differences in age at onset of dementia in Down's syndrome.