Large-scale expansions and replication stalling of Friedreich's ataxia GAA repeats in an experimental mammalian system
1Human disease Friedreichs ataxia (FRDA) is caused by large-scale expansions of (GAA)n repeats in the first intron of the FXN gene. While repeat expansions during intergenerational transmissions are causative for the disease development, somatic expansions additionally contribute to the disease progression. We and others have previously shown that (GAA)n repeats transiently pause the replication fork progression in cultured human cells. However, whether and by which mechanisms fork stalling underlies repeat expansions remained unclear. Here we developed a new genetically tractable experimental system to simultaneously analyze repeat-mediated fork stalling and large-scale repeat expansions in cultured human cells. It is based on a mammalian/yeast shuttle vector that can transiently replicate from the SV40 replication origin in human HEK-293T cells or be stably maintained in S. cerevisiae utilizing ARS4-CEN6; it also contains a cassette for selecting repeat expansions in yeast. Repeat expansions accumulate in mammalian cells and are then detected upon plasmid transformation into yeast. We found that large-scale expansions of (GAA)n repeats do occur in this experimental mammalian system. Further, we observed that repeat expansions frequency depends on several previously implicated proteins in replication fork stalling, reversal, and restart. These proteins include SHPRH, RAD52, ZRANB3, DDX11, SMARCAL1, HLTF, RECQ1 and WRN. Therefore, we propose that GAA repeat expansions might occur as a consequence of deregulated replication fork regression and restoration process.