IDENTICAL TWINS: PINPOINTING ENVIRONMENTAL IMPACT ON THE EPIGENOME
Because identical twins develop from a single zygote, they have the same genome. This removes genetics as a variable telling scientists that the differences they observe between the individuals are caused almost solely by environmental factors. Recent studies have shown that many of these environmentally induced differences are acquired via the epigenome.
Chromosome 3 pairs in each set of twins are digitally superimposed. One twin's epigenetic tags are dyed red and the other twin's tags are dyed green. When red and green overlap, that region shows up as yellow. The 50-year old twins have more epigenetic tags in different places than do 3-year-old twins.
Nature AND Nurture
The insight we gain from studying twins helps us to better understand how nature and nurture work together. For well over a century, researchers have compared characteristics in twins in an effort to determine the extent to which certain traits are inherited, like eye color, and which traits are learned from the environment, such as language. Typically taking place in the field of Behavioral Genetics, classical twin studies have identified a number of behavioral traits and diseases that are likely to have a genetic component, and others that are more strongly influenced by the environment.
Depending on the study and the particular trait of interest, data is collected and compared from identical or fraternal twins who have been raised together or apart. Finding similarities and differences between these sets of twins is the start to determining the degree to which nature and environment play a role in the trait of interest.
Twin studies have identified some traits that have a strong genetic component, including reading disabilities like dyslexia. Other traits, like arthritis, are more likely influenced by the environment.
Identical twins (left) share all their genes and their home environment. Fraternal twins (right) also share their home environment, but only half of their genes. So a greater similarity between identical twins for a particular trait compared to fraternal twins provides evidence that genetic factors play a role.
Twin Studies Help Link Environment and Complex Traits
The advancing field of Molecular Genetics has given us new tools to use when examining traits in twins. Because they are genetically the same but their environments become more unique as they age, identical twins are an excellent model for studying how environment and genes interact. This has become increasingly important when studying complex behaviors and diseases.
For example, when only one identical twin in a pair gets a disease, researchers look for elements in the twins' environments that are different. Data is collected and compared for large numbers of affected twins and coupled with DNA and gene product analysis. These types of twin studies can help pinpoint the exact molecular mechanism of a disease and determine the extent of environmental influence. Having this information can lead to the prevention and treatment of complex diseases.
To illustrate, for twin pairs where schizophrenia occurs, in 50% of cases both identical twins in a pair develop the disease, while only 10-15% of cases in fraternal twins show this pattern. This is evidence for a strong genetic component in susceptibility to schizophrenia. However, the fact that both identical twins in a pair don't develop the disease 100% of the time indicates that there are other factors involved.
Comparing Identical and Fraternal Twins: A higher percentage of disease incidence in both identical twins is the first indication of a genetic component. Percentages lower than 100% in identical twins indicates that DNA alone does not determine susceptibility to disease.
Wong A.H., Gottesman I., Petronis A. (2005) Phenotypic differences in genetically identical organisms: the epigenetic perspective. Human Molecular Genetics, 14: Review Issue 1, R11-R18.
Fraga, M.F. et al. (2005) Epigenetic differences arise during the lifetime of monozygotic twins. PNAS, 102:10604-9.
Poulsen P., Esteller M., Vaag A., Fraga M.F. (2007) The Epigenetic Basis of Twin Discordance in Age-Related Diseases. Pediatric Research, 61: 38R-42R (subscription required).