Quantitative genetic modeling of regional brain volumes and cognitive performance in older male twins

As part of an ongoing longitudinal twin study, data from both MRI brain scanning and from neuropsychological testing were obtained from 139 male-male twin pairs (72 monozygotic [MZ] and 67 dizygotic [DZ]), 69-80 years old at the time of examination. For descriptive purposes, we examined the MZ and DZ intraclass correlations (ICC) of four lobar brain volumes (frontal, temporal, parietal, and occipital), two cerebrospinal fluid (CSF) volumes (lateral ventricle and temporal horn of the lateral ventricles), and two measures of cognitive functioning (verbal memory and executive function). We found that for lobar brain and CSF space volumes, the MZ ICC were significantly greater than zero (r=0.37-0.77) and greater than the corresponding DZ correlations (r=0.02-0.49). Similarly, within-pair correlations for the two neuropsychological factors were statistically significant and significantly larger in MZ twin pairs than in DZ pairs, suggesting the presence of genetic variance. Bivariate genetic analysis revealed that while close to 60% of individual differences in neuropsychological performance were due to genetic influences, less than 50% of genetic effects were in common with those influencing brain volumes. These data may shed light on the genetic liability for brain diseases that affect the elderly.