Depression, apolipoprotein E genotype, and the incidence of mild cognitive impairment: a prospective cohort study

BACKGROUND: It remains unknown whether depression and apolipoprotein E genotype are risk factors for incident mild cognitive impairment (MCI). OBJECTIVE: To determine whether elderly individuals with depression (measured by the short Geriatric Depression Scale) are at increased risk of developing incident MCI. DESIGN: Prospective cohort study. SETTING: Primary care clinic. PARTICIPANTS: A cohort of 840 cognitively normal elderly subjects without depression at recruitment who were followed up prospectively for a median of 3.5 years (range, 0.4-12.8 years). Subjects who developed depression (score of >/=6 on the short Geriatric Depression Scale; depression cohort) were compared with all remaining subjects (referent cohort). MAIN OUTCOME MEASURES: Incidence of MCI (primary outcome) and incidence of MCI or dementia (composite secondary outcome). RESULTS: Individuals in the depression cohort were at significantly increased risk of subsequent incident MCI (hazard ratio [HR], 2.2; 95% confidence interval [CI], 1.2-4.1) after adjusting for age (time scale), sex, and education, and considering dementia as a competing outcome. The association was stronger in men but did not vary by severity of depression. We observed a synergistic interaction between apolipoprotein E genotype (epsilon3/epsilon4 or epsilon4/epsilon4) and depression (joint effect HR, 5.1; 95% CI, 1.9-13.6; test for additive interaction, P = .03). We found a similar association between depression and the subsequent composite outcome of incident MCI or dementia (HR, 2.6; 95% CI, 1.6-4.3). CONCLUSIONS: Cognitively normal elderly individuals who develop depression are at increased risk of subsequent MCI. We found a synergistic interaction between depression and apolipoprotein E genotype.