We report on the establishment of a human embryonic stem cell (HESC) line from a preimplantation fragile X-affected embryo and demonstrate its value as an appropriate model to study developmentally regulated events that are involved in the pathogenesis of this disorder. Fragile X syndrome results from FMR1 gene inactivation due to a CGG expansion at the 5′UTR region of the gene. Early events in FMR1 silencing have not been fully characterized due to the lack of appropriate animal or cellular models. Here we show that, despite the presence of a full mutation, affected undifferentiated HESCs express FMR1 and are DNA unmethylated. However, epigenetic silencing by DNA methylation and histone modification occurs upon differentiation. Our unique cell system allows the dissection of the sequence by which these epigenetic changes are acquired and illustrates the importance of HESCs in unraveling developmentally regulated mechanisms associated with human genetic disorders.
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The authors thank the laboratory technicians and embryologists Ariela Carmon, Tanya Cohen, and Nava Mei-Raz for their skillful assistance; Barak Blum for his assistance in the production of teratomoas; and Tova Naiman for her help with the Southern blot analysis. This research was partly funded by a grant from the Fragile X Foundation and by Israel Science Foundation grant #227/06.