Why do identical twins (who possess the same genes) differ from each other? New studies show that:
Human cells have tens of thousands of genes in them, each with its own job, such as producing energy. But only certain genes are active at any given time or in any cell type while the rest are appropriately dormant — a grand orchestration that adds up to a smooth-running life.
The new research, led by Mario Fraga and Manel Esteller of the Spanish National Cancer Center in Madrid, focused on two biological mechanisms that influence gene activity. In one, called DNA methylation, enzymes inside a cell attach a minuscule molecular decoration to a gene, deactivating that gene. In the other, called histone acetylation, a dormant gene is made active again.
These altered genetic settings can last a lifetime and can be important if, say, the gene turned off is one that protects against cancer.
In the new work, described in the current issue of Proceedings of the National Academy of Sciences, researchers measured the extent to which twins of various ages, from 3 to 74, differed in the number and variety of genes that had been either turned on or shut down by epigenetic processes. They found young twins had almost identical epigenetic profiles but that with age their profiles became more and more divergent.
In a finding that scientists said was particularly groundbreaking, the epigenetic profiles of twins who had been raised apart or had especially different life experiences — including nutritional habits, history of illness, physical activity, and use of tobacco, alcohol and drugs — differed more than those who had lived together longer or shared similar environments and experiences.
Small epigenetic events before birth probably account for many of the minor distinguishing differences in the appearance, personality and general health of young twins, Esteller said, and a lifetime of further epigenetic changes gradually increases individuality.