Effect of APOE and CD33 on Cognitive Decline

Objective An Alzheimer's disease (AD) diagnosis is preceded by a long period of cognitive decline. We previously demonstrated increased risk of decline among individuals possessing one or more APOE epsilon 4 alleles together with a family history of AD. The objective of this study is to investigate the possibility that such an increased risk might be due to AD risk genes with small effects in combination with APOE. Methods Participants in the Health and Retirement Study (HRS) over the age of 65, who contributed DNA, and had two or more evaluations with an abbreviated version of the modified Telephone Interview for Cognitive Status (TICS-m) were eligible for the study (n = 7451). A genetic risk score (g-score) was derived using AD risk genes' meta-analyses data, assigning risk according to the number of risk alleles and summed over all the risk genes. Trajectories of cognitive function were modeled in four groups of Caucasian participants with and without one or more APOE epsilon 4 alleles and either a high or low g-score: APOE epsilon 4-/low g-score; APOE epsilon 4-/high g-score; APOE epsilon 4+/low g-score; and APOE epsilon 4+/high g-score. Post hoc analyses evaluated interactions between individual genes and APOE. Results Individuals in the APOE epsilon 4+/high g-score group exhibited the greatest cognitive decline over time (p<.0001). This risk appeared to be greater than the sum of the effects of either high g-score or APOE epsilon 4 alone. When gene interactions were individually tested with APOE, a statistically significant interaction with CD33 was discovered (p = 0.04) although the interaction was no longer significant when adjusted for multiple comparisons. Conclusions Individuals with multiple AD risk genes in addition to having one or more APOE epsilon 4 alleles are at greater risk of cognitive decline than individuals with either APOE epsilon 4 or a high genetic risk score. Among those with one or more APOE epsilon 4 alleles, having one or more copies of the CD33 C (risk) allele may further increase the risk of cognitive decline.