The beginning of the era of genetic prediction can be dated to 1983, when Huntington's disease (HD) became the first disease to be mapped to a previously unknown genetic location through the use of restriction enzymes that cleave deoxyribonucleic acid (DNA) at sequence-specific sites (Gusella et al.). Huntington's disease is a late-onset autosomal dominant neuropsychiatric disorder. The child of an affected parent has a 50 percent chance of inheriting the genetic mutation that causes HD. Disease onset usually occurs in the fourth decade of life and is marked by a movement disorder, alterations in mood, and cognitive decline. There is no treatment or cure.
Inherited variations of these DNA sequences, which also are known as restriction fragment length polymorphisms (RFLPs), can be used as genetic markers to map diseases on chromosomes and to trace the inheritance of diseases in families. The discovery of these markers represented a significant advance in HD research. Not only did the markers provide a possible clue for finding the HD gene and understanding the mechanism by which the gene causes brain cells to die, this discovery meant that predictive testing for some individuals at risk for HD was possible through the use of a technique called linkage. Linkage testing requires the collection and analysis of blood samples from affected and elderly unaffected relatives of the at-risk individual who asks for testing to trace the pattern of inheritance of the HD gene in a specific family. Linkage testing is labor-intensive and expensive and can result in erroneous conclusions caused by incorrectly attributed paternity, misdiagnosis, and the distance between the gene and the markers used for testing. The discovery of the HD gene in 1993 (Huntington's Disease Collaborative Research Group) made testing more accurate, less expensive, faster, and possible for every person at risk for HD.
Since that time new discoveries in molecular genetics have shifted the focus from relatively rare single-gene disorders such as HD to common adult-onset disorders that cause substantial morbidity and mortality. Examples include the identification of mutations in the BRCA1 and BRCA2 genes as causes of susceptibility to breast and ovarian cancers (Miki et al.; Wooster et al.), the discovery of multiple genetic mutations associated with the risk of colorectal cancer (Laken et al.; Lynch and Lynch), the reported association between the APOE e4 allele and late-onset Alzheimer disease (Strittmatter et al.), associations between factor V Leiden and thromboembolic disease (Hille et al.; Ridker et al.; Simioni et al.), and the identification of the HFE gene for hereditary hemochromatosis (Beutler et al.; Edwards et al.). In the second decade of the twenty-first century it has been predicted that genetic tests will be available for diabetes, asthma, dyslexia, attention deficit hyperactivity disorder, obesity, and schizophrenia. These discoveries point to the potential use of genetic tests for population screening in adult populations and an increasing role in public health for genetic testing.
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