Credit: Reprinted with permission of AAAS

Monozygotic twins originate from a single fertilized egg, but they are not truly 'identical'—for instance, they can develop different chronic diseases. But are the differences due to nature—subtle sequence changes in the DNA—or nurture? In a recent issue of the Proceedings of the National Academy of Sciences, Mario Fraga et al. take an alternate approach to this debate, implicating epigenetic alterations in the differences (10.1073/pnas.0500398102).

Fraga et al. analyzed DNA methylation and histone acetylation in lymphocytes of 80 twins. They found significant differences in these epigenetic modifications between siblings in 35% of the twin pairs—and the differences increased with age. To show this, they generated labeled PCR probes for each individual that reflected the distribution of methyl groups along their DNA, and hybridized these to chromosomes. Shown are the DNA methylation patterns of chromosome 1 from 3-year-old twins (left pair) and 50-year-old twins (right pair). The younger twins showed similar patterns of methylation, visualized as an overlap of green and red probes (yellow). The chromosomes from the older twins show substantial disparity in methylation patterns (distinct red and green pattern).

Do such epigenetic differences lead to variations in gene expression that might explain phenotypic differences between monozygotic twins? In this study, the answer was yes—gene expression patterns in the 3-year-old twins were virtually identical, but varied widely in the 50-year-old twins. So fear not, identical twin, you are unique.