The joint effect of apolipoprotein E ε4 and MRI findings on lower- extremity function and decline in cognitive function

Background. Cognitive decline and poor physical function are risk factors for disability in old age and may occur more often in subjects with the apolipoprotein E ε4 (ApoE-ε4) allele. The objective of this study was to investigate the joint effect of ApoE-ε4 and structural changes detected on MRI brain scans on cognitive decline and lower-extremity function. Methods. Brain MRI (1.5 T), neuropsychological tests, and lower-extremity physical function tests were administered to World War II male veteran twins ages 69 to 80. Quantification of MRI scans used a previously published algorithm to segment brain images into total cerebral brain (TCB), cerebrospinal fluid (CSF), and white-matter hyperintensity (WMH) volumes. A short battery of physical performance tests was used to assess lower- extremity function. Ten-year changes in performance on the Mini-Mental State Exam (MMSE), the Benton Visual Retention Test (BVRT), and the Digit Symbol Substitution (DSS) test were used to assess cognitive decline. Results. For the sample as a whole, the comparison of subjects by median split of total cerebral brain volume found that those with brain volumes below the median performed worse on tests of gait and balance (p < .01) and experienced greater cognitive decline on the MMSE and BVRT cognitive test batteries (both p < .01). In addition, subjects with WMH volumes above the median had poor performance on the standing balance tasks and experienced greater decline on the DSS test (p < .01). Stratified analyses revealed that the joint effect of radiological findings and the ApoE-ε4 allele on cognitive decline and lower- extremity function was often greater than that expected from the separate effects combined. Conclusions. We conclude that radiological findings in conjunction with ApoE-ε4 may single out a group at higher risk for dementia. We speculate that the observed interaction effect may be due to increased susceptibility to brain injury or impaired repair mechanisms in subjects with ApoE-ε4.