Mechanisms of genetic instability caused by (CGG)n repeats in an experimental mammalian system

We developed an experimental system for studying genome instability caused by fragile X (CGG)(n) repeats in mammalian cells. Our method uses a selectable cassette carrying the HyTK gene under the control of the FMR1 promoter with (CGG)(n) repeats in its 5' UTR, which is integrated into the unique RL5 site in murine erythroid leukemia cells. Carrier-size (CGG)(n) repeats markedly elevated the frequency of reporter inactivation, making cells ganciclovir resistant. These resistant clones had a unique mutational signature: a change in repeat length concurrent with mutagenesis in the reporter gene. Inactivation of genes implicated in break-induced replication, including Pold3, Pold4, Rad52, Rad51, and Smarcal1, reduced the frequency of ganciclovir-resistant clones to the baseline level that was observed in the absence of (CGG)(n) repeats. We propose that replication fork collapse at carrier-size (CGG)(n) repeats can trigger break-induced replication, which results in simultaneous repeat length changes and mutagenesis at a distance.