The Evil Twin is a staple of the soap opera genre—you know, the long-lost, villainous sister who kidnaps her virtuous twin and assumes her life, only to be discovered years later when the Good Twin escapes from imprisonment on an alien-inhabited desert isle. But in real life, how can identical twins with the same genes sometimes exhibit drastic differences in personality or behavior? To study epigenetic effects on behavior, Mark D. Alter, Rene Hen, and co-workers at Columbia University and the Translational Genomics Research Institute examined variability in hippocampal gene expression in genetically identical mice.
The researchers studied the behavior of inbred mice in an “open field” test, in which mice were placed in an open area and monitored for activity and exploratory behavior. The mice were classified into “high activity” and “low activity” groups, and global gene expression levels in the hippocampus were analyzed by a microarray-based approach. The mice in the high activity group had a significantly increased variance in log-transformed gene expression levels; in other words, the high activity mice showed a larger difference in expression levels between high- and low-expressed genes than did the low activity group. Furthermore, when newborn mice were exposed to postnatal handling (a daily 15-minute separation from their mother), the mice were more active in the open field test and showed a corresponding significant increase in the variance of log-transformed gene expression levels in the hippocampus.
The widened gap in expression levels between high- and low-expressed genes in the high activity mice suggests that resources for gene expression were shifted from low- to high-expressed genes, which translated to behavioral differences. These results challenge the traditional emphasis on gene-specific epigenetic differences and indicate that global epigenetic changes can lead to large-scale differences in hippocampal gene expression, which in turn can influence behavior. For all the details head to PLoS ONE, October 2008