Large intergenerational repeat expansions of the CAG trinucleotide repeat in the HD gene have been well documented for the male germline. We describe a recurrent large expansion of a maternal allele with 36 CAG repeats (to 66 and 57 repeats, respectively, in two daughters) associated with onset of Huntington disease (HD) in the second and third decade in a family without history of HD. Our findings give evidence of a gonadal mosaicism in the unaffected mother. We hypothesize that large expansions also occur in the female germline and that a negative selection of oocytes with long repeats might explain the different instability behavior of the male and the female germlines.
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Laccone, F., & Christian, W. (2000). A recurrent expansion of a maternal allele with 36 CAG repeats causes Huntington disease in two sisters. American Journal of Human Genetics, 66(3), 1145–1148. https://doi.org/10.1086/302810