Study found that extra segments added during DNA restoration caused toxicity.

Researchers discovered that a miscue of the body’s genetic repair system may cause Huntington’s disease. “We showed that when single-strand breaks in DNA caused by oxidative lesions were repaired, the Huntington’s gene continued to add extra replacement segments,” explains Cynthia McMurray, Ph.D., a Mayo Clinic molecular biologist who led the study. “Over time, this expansion—especially in nerve cells—becomes toxic.”

In their study of transgenic mice that carried the human Huntington’s gene, the researchers noted that the repeated tracts of replacement repair segments seem stable until the animals reach about four months. After that point, which represents middle age for a mouse, the segments expand and continue to do so as the animals age.

Researchers also showed that the expansion of the tracts, an inherited characteristic, caused toxicity in cells that cannot expand, such as nerve cells. The result is that cell death acceleration is directly proportional to the additional repeated lengths.

In a further step, the team eliminated a key enzyme, OGG1, related to DNA repair for oxidative lesions and found that it stopped or greatly reduced segment growth.

The findings appear in the online issue of Nature.

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