Triplet repeat tracts occur throughout the human genome. Expansions of a (GAA)(n)/(TTC)(n) repeat tract during its transmission from parent to child are tightly associated with the occurrence of Friedreich's ataxia. Evidence supports DNA slippage during DNA replication as the cause of the expansions. DNA slippage results in single-stranded expansion intermediates. Evidence has accumulated that predicts that hairpin structures protect from DNA repair the expansion intermediates of all of the disease-associated repeats except for those of Friedreich's ataxia. How the latter repeat expansions avoid repair remains a mystery because (GAA)(n) and (TTC)(n) repeats are reported not to self-anneal. To characterize the Friedreich's ataxia intermediates, we generated massive expansions of (GAA)(n) and (TTC)(n) during DNA replication in vitro using human polymerase beta and the Klenow fragment of Escherichia coli polymerase I. Electron microscopy, endonuclease cleavage, and DNA sequencing of the expansion products demonstrate, for the first time, the occurrence of large and growing (GAA)(n) and (TTC)(n) hairpins during DNA synthesis. The results provide unifying evidence that predicts that hairpin formation during DNA synthesis mediates all of the disease-associated, triplet repeat expansions
