Reduced survival of apolipoprotein E4 homozygotes in Down's syndrome?

AN increased frequency of apolipoprotein E (ApoE) allele epsilon 4 has been established in Alzheimer's disease (AD) subjects. Based on the observation that all individuals with Down's syndrome (DS) develop the neuropathology of AD by the age of 40 years we sought to determine whether ApoE genotype affected the clinical symptoms of AD. Unexpectedly, we failed to find any epsilon 4 homozygotes, as have other published studies: this is highly significant statistically. We suggest this is due to decreased survival or, less likely, embryonic lethality of epsilon 4 homozygotes in DS.