Modeling Fragile X Syndrome in Human Pluripotent Cells

Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability in males, affecting approximately one in every 4,000 boys and one in 8,000 girls worldwide. A tri-nucleotide CGG expansion at the 5′ untranslated region of the fragile X mental retardation 1 (FMR1) gene leads to CpG methylation of the region and is accompanied by epigenetic changes, resulting in the silencing of the gene. The product of the FMR1 gene is the fragile X mental retardation protein (FMRP). FMRP is most abundant in the brain and testes and is known to play a major role in synaptic plasticity. Due to the high evolutionary conservation of FMRP, several animal models were created to study the disease. Work on fly and mouse models collected valuable data regarding aberrant pathways associated with the syndrome. Although these models were successful in understanding some aspects of the disease, there is no animal model for the study of neither the CGG expansion, nor the epigenetic silencing due to the unique molecular mechanism underlying the syndrome in humans. Work on human tissues was initially restricted to chorion villus taken from fetuses or brain samples taken from postmortem-affected individuals. The generation of human embryonic fragile X cells (FXS-ESCs) and FXS derived induced pluripotent stem cells (FXS-iPSCs) opened the way to study new aspects of the disease and better understand the initial processes which are at the base of the syndrome. Work on FXS-ESCs revealed that silencing of the FMR1 gene happens during differentiation of pluripotent cells into somatic cells and that the silencing process is initiated by histone modifications followed by DNA methylation and heterochromatization. Studies on both FXS-ESCs and FXS-iPSCs revealed aberrant neural differentiation already at the early stages of neurogenesis. FX-iPSCs and their derivatives hold great potential for drug screening. Future work will search for new small molecules for the reactivation of the gene or the major pathways it plays a part in. As many questions regarding the effects caused by the silencing of FMR1 and the expansion mechanism remain unanswered, pluripotent stem cells will serve as a valuable source for FXS disease modeling both as a general model and in a patient-specific manner.