Somatic deletion events occur during early embryonic development and modify the extent of CAG expansion in subsequent generations

Alterations in trinucleotide repeat length during transmission are important in the pathophysiology ofHuntington’s disease (HD). However, it is not well understood where, when and by what mechanism expan-sion occurs. We have followed the fate of CAG repeats during development in mice that can [hHD(2/1)/Msh2(1/1)] or cannot [hHD(2/1)/Msh2(2/2)] expand their repeats. Here we show that long repeats are shor-tened during somatic replication early in the embryo of the progeny. Our data point to different mechanismsfor expansion and deletion. Deletions arise during replication, do not depend on the presence of Msh2 andare largely restricted to early development. In contrast, expansions depend on strand break repair, requirethe presence of Msh2 and occur later in development. Overall, these results suggest that deletions in earlydevelopment serve as a safeguard of the genome and protect against expansion of the disease-range repeatsduring transmission.INTRODUCTIONAt least eight hereditary and progressive neurodegenerativedisorders have been identiﬁed, in which the underlyingmutation is a CAG expansion within the coding sequence ofthe gene (1–3). Once the CAG length exceeds a diseasethreshold, expansion becomes the predominant change andexceeds contraction events by 3–175-fold (4–7). Increasein repeat length from generation to generation is a criticalfeature in pathophysiology, yet little is known about howand where the mutation occurs in vivo. Human data andexperiments in mouse models for disease have revealed thatexpansion must be carefully considered in the context ofmammalian development (8–14).When present in non-coding regions, somatic mosaicism oftriplet repeats is observed in the early embryo (15–17). Infragile X syndrome, for example, during transmission from afemale carrier to her male progeny, expansion occurred tothe full-mutation length in all tissues except sperm (18).When examined carefully, expansions were found in theembryo prior to germline segregation at day 5, while by day20 the repeat length in the developing germline cells wasdeleted to a normal repeat size (18,19). Whether expansionoccurred in the oocyte of the mother or early in the embryocould not be determined in these experiments.Developmental aspects of expansion in coding regions arenot as well characterized. Expansions in the sperm are foundin HD patients (10,11) and hHD transgenic mice (14).However, alterations in size of the CAG tracts found incoding sequences are small relative to either fragile X or myo-tonic dystrophy, and they are most often transmitted throughthe male line (4,20,21). Single cell analysis of testes fromHD patients has revealed that mutations occur in all testicularcell types including those that proliferate and those that differ-entiate into mature sperm (22). In hHD mice, expansion ispresent in haploid germ cells that are post-mitotic and post-meiotic and limit timing of expansion to late testicular devel-opment and differentiation (14). While the mechanism ofexpansion is not resolved from these studies, data fromhumans and mice agree that instability is present in thegerm cells and may account for the growth of the tripletrepeat tract in subsequent generations.Less is known about events post-fertilization. However,several pieces of evidence indicate that there is somaticinstability. For example, the degree of expansion observedin HD pedigrees does not correspond to that in the sperm ofHuman Molecular Genetics, Vol. 13, No. 24