All About Mutations
Part 12

What it means to have a mutation and what role mutations play in Huntington’s disease.

What role do environmental substances play in expansion?

Substances in the environment that cause mutations are called mutagens. Cancer causing agents (called carcinogens) are often harmful because they are either mutagens themselves or are turned into mutagens in the body. However, as of this writing (January 2002), research on Huntington’s disease has shown that, unlike the case in many cancers, mutagens do not appear to play a role in HD.

However, it does deserve to be noted that very rarely, HD “appears” in a person whose family had no history of the disease. Although these cases of HD might seem to imply that environmental substances (and not genes) are at work, this is probably not true. Instead, there is a more likely explanation: a person who has a non-HD allele with a borderline number of repeats (typically between 36 and 39 copies of CAG) may produce a sperm or egg cell that contains a slightly expanded allele. Sometimes, these few extra copies of CAG may be just enough to cause the child inheriting the allele to have an abnormal codon repeat number (40 CAGs or above). One researcher speculated that about 10% of HD cases are caused by such changes in repeat number. Thus, although environmental substances might appear to cause these rare cases of HD, small expansions of already borderline alleles provide the more likely explanation.

Last Modified: 1-28-04

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