Trinucleotide repeat instability in human embryonic stem cells: the role of the mismatch repair machinery

Introduction
Trinucleotide repeat (TNR) instability is the mechanism responsible for numerous inherited human disorders, including Huntington's disease (HD) and myotonic dystrophy (DM1). The TNRs have different patterns of instability according to the disorder as well as the cell type investigated. It is known that the mismatch repair (MMR) machinery is involved in TNR instability, and particularly MSH2 and MSH3 seem to be required for the intergenerational and somatic CTG and CAG expansions. Most of these results have been obtained from studies on transgenic mice, due to the lack of suitable human models. The aim of this work is to study the behaviour of the TNRs causing HD and DM1 in human embryonic stem cells (hESCs), both in the undifferentiated and differentiated state, and its relationship to the MMR machinery, in order to evaluate the potential of these cells as a model in the human.
Materials and methods
HESC lines were established in our laboratory using spare IVF/PGD embryos. VUB03_DM1 carries a CTG expansion in the DMPK1 gene and VUB05_HD carries a CAG expansion in the huntingtin gene. VUB02 was used as a control line. The lines were kept in standard culture conditions, on mouse embryonic fibroblasts, and samples were taken at different passages. A well-established protocol was used to differentiate hESCs into osteoprogenitor-like cells (OPL). Whenever possible, samples for RNA and DNA extraction were taken after each passage. TNR stability was studied by PCR followed by southern blotting and/or analysis on a DNA sequencer. The MMR machinery was studied by quantitative real-time PCR (qRT-PCR) of the MSH2, MSH3, MLH3 and PMS2 genes.
Results
While the HD repeat showed a stable pattern in undifferentiated hESCs over more than 80 passages, the DM1 repeat was unstable from the first passages onwards, the TNR mostly increasing in size, leading to a mosaic culture. In the OPLs, the HD TNR remained stable, whereas the DM1 repeat showed a tendency towards a clonal growth (only a few alleles remained detectable), and a stabilization in the behaviour of the repeat. The results of the qRT-PCR showed a downregulation of all the studied genes in the OPLs, in comparison to the undifferentiated hESCs.
Conclusions
The data on VUB05_HD show that the behaviour of this TNR in hESC follows the same pattern as its somatic behaviour in vivo, in which only striatal neurons show TNR instability. Further experiments on in-vitro differentiated neuronal tissue from hESC are planned. In the case of DM1, the results suggest a correlation between the downregulation of the MMR machinnery and the stabilization of the TNR, which will be further studied by siRNA experiments.