Journal Article

Length-Dependent Gametic CAG Repeat Instability in the Huntington's Disease Knock-in Mouse

Vanessa C. Wheeler, Wojtek Auerbach, Jacqueline K. White, Jayalakshmi Srinidhi, Anna Auerbach, Angela Ryan, Mabel P. Duyao, Vladimir Vrbanac, Meredith Weaver, James F. Gusella, Alexandra L. Joyner and Marcy E. MacDonald

in Human Molecular Genetics

Volume 8, issue 1, pages 115-122
Published in print January 1999 | ISSN: 0964-6906
Published online January 1999 | e-ISSN: 1460-2083 | DOI:
Length-Dependent Gametic CAG Repeat Instability in the Huntington's Disease Knock-in Mouse

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The CAG repeats in the human Huntington's disease (HD) gene exhibit striking length-dependent intergenerational instability, typically small size increases or decreases of one to a few CAGs, but little variation in somatic tissues. In a subset of male transmissions, larger size increases occur to produce extreme HD alleles that display somatic instability and cause juvenile onset of the disorder. Initial efforts to reproduce these features in a mouse model transgenic for HD exon 1 with 48 CAG repeats revealed only mild intergenerational instability (∼2% of meioses). A similar pattern was obtained when this repeat was inserted into exon 1 of the mouse Hdh gene. However, lengthening the repeats in Hdh to 90 and 109 units produced a graded increase in the mutation frequency to >70%, with instability being more evident in female transmissions. No large jumps in CAG length were detected in either male or female transmissions. Instead, size changes were modest increases and decreases, with expansions typically emanating from males and contractions from females. Limited CAG variation in the somatic tissues gave way to marked mosaicism in liver and striatum for the longest repeats in older mice. These results indicate that gametogenesis is the primary source of inherited instability in the Hdh knock-in mouse, as it is in man, but that the underlying repeat length-dependent mechanism, which may or may not be related in the two species, operates at higher CAG numbers. Moreover, the large CAG repeat increases seen in a subset of male HD transmissions are not reproduced in the mouse, suggesting that these arise by a different fundamental mechanism than the small size fluctuations that are frequent during gametogenesis in both species.

Journal Article.  7442 words.  Illustrated.

Subjects: Genetics and Genomics

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