Studies of twin pairs discordant for disease can be useful for identifying risk factors for disease. Since both genetic and environmental factors are extensively shared by twins, particularly by MZ twins, case-control studies can be particularly powerful. In such a study, each twin is interviewed with regard to specific environmental factors—such as occupation, lifestyle factors, illnesses or injuries, and diet—prior to the onset of the disease in the affected twin. The presence of these factors in the twin with the disease is compared to the twin without disease. An association of an environmental factor with the disease suggests this factor may be causally related. Factors more common among the unaffected twins suggest that the factor may protect against the development of the disease.
Environmental influences on PD have been investigated by studying discordant twin pairs. PD has repeatedly been found to be more common in people who do not smoke cigarettes. Some have proposed that some people are genetically predisposed to both Parkinson's disease and smoking, while others suggest cigarette smoking somehow prevents the degeneration that leads to PD. In two studies of discordant twin pairs, cigarette smoking was more common in the twin without Parkinson's disease, especially in the MZ pairs. Because monozygotic twins are genetically identical, this pattern tips the scales in favor of a direct biological action of cigarette smoke.
As medicine focuses more on early intervention or prevention, it becomes important to identify those persons at risk for a particular condition. This can be a problem if there is no diagnostic test. In discordant twin pairs, the unaffected twin is more likely to be "at risk" for a particular condition, whether due to shared genes or environment, than would be true for two nontwins. Therefore, studying the unaffected "at risk" twin may help to clarify what features are useful for predicting those who later will develop a particular disease. For example, in the PD twin study, the unaffected twins are being studied prospectively with brain imaging tests that may show early evidence of injury to the brain area damaged in PD. If abnormalities on this test are found to precede the development of PD, this could provide a useful method of early detection. When treatments to slow or stop the onset of PD are available, individuals with imaging abnormalities may receive intervention before symptoms develop.
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