A mechanism for deletion formation in DNA by human cell extracts: the involvement of short sequence repeats.

DNA molecules carrying a site-specific double-strand break were exposed to nuclear extracts from human cell lines. It was shown previously that breaks could be rejoined correctly by human extracts, but that a proportion of the rejoined molecules had suffered deletions and insertions. The 'mis-rejoined' proportion was higher with cell extracts from an individual with the disorder ataxia-telangiectasia than with normal cell extracts. We now show by sequence analysis that deletions in extract-treated molecules occur exclusively between short direct repeats (2-6 base pairs). A mis-rejoined molecule containing an insertion of 300 bp also had a repeat-based deletion at the same site. A number of different direct repeats are involved; however, some clustering of these occurs especially on the upstream side of the initial breakpoint. These data are most simply interpreted in terms of a model of deletion formation involving single-strand exposure and repair, perhaps with the action of other DNA-metabolising enzymes influencing the frequency with which some repeats are involved.