• Huntington’s Chorea

Huntington’s Chorea: Evolution and Genetic Disease (3 of 3)

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Persistence
These mechanisms of evolution, mutation and selection, can help us understand the persistence of Huntington’s in populations. In general, Huntington’s is rare — 30-70 cases per million people in most Western countries¹ — but it is not entirely eliminated because selection does a relatively poor job of weeding these alleles out, while mutation continues creating new ones.

Dr. Nancy Wexler (shown at right tracking geneologies) has been studying the remarkably high frequency of Huntington’s in Lake Maracaibo since the 1970s. She has found that the high incidence of this disease there is explained by an evolutionary event called the founder effect. About 200 years ago, a single woman who happened to carry the Huntington’s allele bore 10 children—and today, many residents of Lake Maracaibo trace their ancestry (and their disease-causing gene) back to this lineage. A simple fluke of history, high-birth rates, and weak selection are responsible for the genetic burden shouldered by this population.

Solutions?
Currently, physicians don’t have any cures for Huntington’s disease—there’s no miracle pill that will stop the progress of the disease. However, understanding the evolutionary history of the disease—a recurrent mutation that is often “missed” by natural selection—points out a way to reduce the frequency of the disease in the long term: allowing people to make more informed reproductive choices.

Comparing the banding patterns in a genetic test can tell researchers whether a person carries an allele that is likely to cause Huntington's.

Today, genetic testing can identify people who carry a Huntington’s allele long before the onset of the disease and before they have made their reproductive choices. The genetic test that identifies the Huntington's allele works sort of like DNA fingerprinting. A DNA sample is copied and cut into pieces. The pieces are then spread out on a gel (see right). The banding pattern can tell researchers whether a person carries an allele that is likely to cause Huntington's.

Having this information could allow people to make more-informed reproductive decisions. For example, at Lake Maracaibo, researchers and health workers have tried to make contraception available to the local population so that they can make reproductive choices based on their own family history with the disease. But whatever people eventually decide to do with this knowledge, a deep understanding of the disease would not be possible without the historical perspective offered by evolution.

A more technical approach to current research on Huntington’s may be found on the OMIM site (Johns Hopkins).

¹Palo et al. (1987)
• Dr. Wexler photo © 1986 by Steve Uzzell