Mouse tissue culture models of unstable triplet repeats: in vitro selection for larger alleles, mutational expansion bias and tissue specificity, but no association with cell division rates.

The expansion of CAG.CTG trinucleotide repeats has been associated with an increasing number of human diseases. Once into the expanded disease-associated range, the repeats become dramatically unstable in the germline and also throughout the soma. Instability is expansion-biased, contributing towards the unusual genetics, and most likely the tissue-specificity and progressive nature of the symptoms. Such expansions constitute a unique form of dynamic mutation whose mechanism is poorly understood. It is generally assumed that repeat length changes arise via replication slippage, yet no direct evidence exists to support this hypothesis in a mammalian system. We have previously generated transgenic mouse models of unstable CAG.CTG repeats that reconstitute the dynamic nature of somatic mosaicism observed in humans. We have now used tissues from these mice to establish in vitro cell cultures. Monitoring of repeat stability in these cells has revealed the progressive accumulation of larger alleles as a result of repeat length changes in vitro, as confirmed by single cell cloning. We also observed the selection of cells carrying longer repeats during the first few passages of the cultures and frequent additional selective sweeps at later stages. The highest levels of instability were observed in cultured kidney cells, whereas the transgene remained relatively stable in eye cells and very stable in lung cells, paralleling the previous in vivo observations. No correlation between repeat instability and the cell proliferation rate was found, rejecting a simple association between length change mutations and cell division, and confirming a role for additional cell-type specific factors.