Dysregulation of FMR1 Splicing in Human Fragile X Syndrome

Fragile X Syndrome (FXS) is a neuro-developmental disorder caused by a CGG expansion in FMR1, leading to transcriptional silencing and loss of the encoded protein FMRP. Surprisingly, ~70% of FXS individuals express FMR1, but the RNA is mis-spliced to isoform FMR1-217, composed of exon 1 spliced to a pseudo-exon in intron 1 and cannot produce FMRP. Splice-switching ASOs rescue proper FMR1 splicing and restore FMRP. FMR1-217 mis-splicing increases with CGG repeat length and is negatively correlated with patient IQ. FMR1-217 is associated with ribosome footprints, indicating it is translated into a polypeptide that may impair cognition. R-loops form at the FMR1 locus and extend into the pseudo-exon, but splice-switching ASOs reduce FMR1-217 and elevate FMRP independently of R-loop formation. DRB-based transcription analysis identified impaired Pol II elongation at the 5 prime region of FMR1 in FXS cells, indicated by accumulation of hypophosphorylated Pol II at the transcription start site. Consistent with this, camptothecin-induced Pol II stalling increased FMR1-217 pseudo-exon inclusion. The splicing factors PTBP1 and PTBP2 regulate FMR1-217 splicing in a differentiation stage-dependent manner. Together, these findings indicate that FMR1-217 mis-splicing in FXS is associated with CGG repeat expansion, R-loop formation, impaired co-transcriptional Pol II elongation and context-dependent regulation by PTBP1/PTBP2.