Expansion and deletion of triplet repeat sequences in Escherichia coli occur on the leading strand of DNA replication

Expansions and deletions of triplet repeat sequences that cause human hered itary neurological diseases were previously suggested to be mediated by the formation of DNA hairpins on the lagging strand during replication. The re plication properties of CTG.CAG, CGG.CCG, and TTC.GAA repeats were studied in Escherichia coil using an in vivo phagemid system as a model for continu ous leading strand synthesis. The repeats were substantially deleted when t he CTG, CGG, and GAA repeats were the templates for rolling circle replicat ion from the fl phage origin, The deletions may be mediated by hairpins for med by these repeat tracts. The distributions of the deletion products of t he CTG.CAG and CGG.CCG tracts indicated that hairpins of discrete sizes med iate deletions during complementary strand synthesis. Deletions during roll ing circle synthesis are caused by larger hairpins of specific sizes. Thus, most deletion products were of defined lengths, suggesting a preference fo r specific hairpin intermediates. Small expansions of the CTG.CAG and CGG.C CG repeats were also observed, presumably due to the formation of CTG and C GG hairpins on the nascent complementary strand. Since rolling circle repli cation has been established in vitro as a model for leading strand synthesi s, we conclude that triplet repeat instability can also occur on the leadin g strand of DNA replication.