Assessment of neuronal cell type specific transcriptional effects of three pathological sequence variants of the FMR1 gene promoter

FXS often results from the silencing of the FMR1 promoter triggered by a CGG triplet expansion. In 2010, Collins et al., found three variants in the FMR1 promoter which are reducing its activity to 36,2%, 29,2% and 5,9% altering the transcription factors binding sites necessary to initiate FMR1 expression and decreasing the levels of its product FMRP. FMRP is important for synaptic plasticity in particular in mGluR-dependent LTD which is exaggerated in FXS leading to impairments in learning and memory and dendritic spine morphogenesis. To validate these results, we created various plasmids controlled by the FMR1 promoter containing one of these three variants. Plasmids were transfected in HeLa cells, rat cortical neurons and human neuronal precursors. In parallel, mouse hippocampal slice cultures were shot with gold particles coated by the same plasmids. Confocal microscopy highlighted that the variant located near the first transcriptional start site was found to reduce FMR1 expression to 42.08% in human neuronal precursors and to 45.7% in mouse hippocampal slices. The cells and slices were treated with neuronal activity modulators. Bicuculline was found to rescue FMR1 expression suggesting that the inhibition of GABA receptors triggers a sufficient increase of excitatory glutamatergic activity to modulate FMR1 expression.