Apolipoprotein E genotypes and plasma levels in mild cognitive impairment conversion to Alzheimer's disease: A follow-up study

Mild cognitive impairment (MCI) is the transition stage between the normal aging process and dementia itself. The most common clinical phenotype is amnestic MCI (aMCI) [subtypes: single domain (sMCI) and multiple domains (mMCI)], which is considered prodromal to Alzheimer’s disease (AD). The APOE (apolipoprotein E) e4 allele is the most important genetic risk factor for AD, but its association with MCI onset and conversion to AD is controversial. In this follow-up study of 88 aMCI patients (68% sMCI and 32% mMCI at baseline), we examined APOE genotypes and plasma levels in relation to MCI development and progression based on their clinical/cognitive data obtained at baseline and follow-up assessment (mean follow-up time = 6.6±3.4 years). A control sample (n=164) was collected in previous investigations. The overall conversion rate to mMCI or AD was 52.2%. The APOE e4 allele was associated with a higher risk of developing MCI (OR: 2.23; 95% CI: 1.22-4.08). The conversion rate in the e4 allele carriers (32% of the sample) was 71%, and the e4 allele was associated with a higher risk of conversion to mMCI/AD (OR: 4.1; 95% CI: 1.2-13.6). APOE e2 allele carriers were 7% (all sMCI) and none progressed to mMCI/AD. Among MCI subjects, e4 carriers had the lowest plasma apoE levels (37.8±12.5mg/l), and e2 carriers had the highest (78.6±38.1mg/l). APOE e4 is a risk allele for the development and progression of aMCI, the APOE e2 allele seems to be protective, and apoE levels associated to them are an integral part of their action.