Background Both family aggregation and apolipoprotein E (APOE) ε4 allele are well-known risk factors for dementia, but the relation between these two factors remains unclear.

Objective To explore whether the risk of dementia and Alzheimer disease (AD) due to a positive family history is explained by APOE genotypes.

Design Community-based cohort study.

Setting The Kungsholmen district of Stockholm, Sweden.

Participants A total of 907 nondemented people 75 years or older, followed up for 6 years to detect incident dementia and AD cases according to the diagnostic criteria of the Diagnostic and Statistical Manual of Mental Disorders, Revised Third Edition.

Main Outcome Measures Risk of dementia and AD by Cox proportional hazards models after controlling for several potential confounders.

Results Subjects who had at least 2 siblings with dementia were at an increased risk of AD. Individuals with both APOE ε4 allele and at least 2 affected first-degree relatives had a higher risk of dementia and AD compared with those without these 2 factors. Similar results were obtained for history of dementia separately in parents or siblings. Among the ε4 allele carriers, subjects with 2 or more first-degree demented relatives had increased risk of dementia and AD, whereas no increased risk was detected among non–ε4 carriers.

Conclusions Family history of dementia was associated with an increased risk of dementia and AD in this very old population, but only among APOE ε4 carriers. This suggests the existence of other genetic or environmental risk factors that may be active in the presence of the APOE ε4 allele.

The role of both family history of dementia and the apolipoprotein E (APOE) gene in the development of Alzheimer disease (AD) has been extensively investigated. There is strong evidence to suggest that APOE ε4 allele carriers, as well as subjects with a family history of dementia, have an increased risk of AD.Familial aggregation and genetic risk factors appear to be most influential in AD at relatively early ages.However, there are reports supporting an effect of both familial aggregation and APOE ε4 even in late-onset AD,although a lower effect in comparison with early-onset cases has been detected.

It is hypothesized that APOE ε4 allele might explain the association between family history of dementia and AD. Previous studies have tried to evaluate this hypothesis, but to what extent familial aggregation is due to the association between the ε4 allele and AD remains equivocal. Some studies indicated that ε4-positive patients with AD tended to have a higher rate of family history of dementia than ε4-negative patients. Conversely, patients with family history of AD are also more likely to carry the ε4 allele than patients without family history.Other studies, however, showed that the APOE ε4 allele was not related to familial aggregation of AD.

Most previous analyses have been hospital-based case-control studies. Because of ascertainment bias and severe truncation of data, these studies might overestimate the effects of family history and APOE ε4 allele, especially in very old people. Only a small-scale prospective study has examined both family history of dementia and APOE ε4 allele in relation to AD risk among people 75 years or older.

In a previous study within the Kungsholmen Project, a strong familial aggregation was detected among prevalent cases of late-onset AD, but the contribution of the APOE ε4 allele was not considered. In the present study, we examined the 6-year follow-up data from the same project to explore whether the risk of dementia and AD due to a positive family history is explained by APOE genotypes.