Double-strand breaks in the myotonic dystrophy type 1 and the fragile X syndrome triplet repeat sequences induce different types of mutations in DNA flanking sequences in Escherichia coli

The putative role of double-strand breaks (DSBs) created in vitro by restriction enzyme cleavage in or near CGG•CCG or CTG•CAG repeat tracts on their genetic instabilities, both within the repeats and in their flanking sequences, was investigated in an Escherichia coli plasmid system. DSBs at TRS junctions with the vector generated a large number of mutagenic events in flanking sequences whereas DSBs within the repeats elicited no similar products. A substantial enhancement in the number of mutants was caused by transcription of the repeats and by the absence of recombination functions (recA(−), recBC(−)). Surprisingly, DNA sequence analyses on mutant clones revealed the presence of only single deletions of 0.4–1.6 kb including the TRS and the flanking sequence from plasmids originally containing (CGG•CCG)(43) but single, double and multiple deletions as well as insertions were found for plasmids originally containing (CTG•CAG)(n) (where n = 43 or 70). Non-B DNA structures (slipped structures with loops, cruciforms, triplexes and tetraplexes) as well as microhomologies are postulated to participate in the recombination and/or repair processes.