Mis-spliced FMR1 transcripts in human fragile X syndrome neural progenitors and neurons

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by loss of fragile X messenger ribonucleoprotein (FMRP). In most cases, this results from a CGG expansion exceeding 200 repeats in the 5' untranslated region of the fragile X messenger ribonucleoprotein 1 (FMR1) gene, known as the "full mutation". While the trinucleotide expansion has long been thought to induce epigenetic silencing of this locus, studies have shown that many males with a full mutation still express FMR1 mRNA. However, these individuals produce little to no FMRP protein, due to mechanisms that remain unclear. Mis-splicing of FMR1 transcripts with an expanded CGG tract has recently been proposed as a potential mechanism underlying the absence of FMRP in FXS tissues despite the presence of gene transcripts.

We used human neural progenitors and neurons differentiated from FXS human pluripotent stem cells and RNA-seq to examine splicing patterns of expanded FMR1 transcripts. We analyzed transcript structure and protein expression to validate mis-splicing mechanisms.

We demonstrate an enrichment in levels of mis-spliced transcripts in human neural progenitors and neurons differentiated from FXS human pluripotent stem cells. Our findings confirm that expanded transcripts undergo aberrant splicing, which may contribute to the absence of FMRP despite transcriptional activity. We further show that pharmacological reactivation of the FMR1 locus results in expression of mis-spliced transcripts and that FMRP loss alone, in the absence of an expanded CGG tract, causes only a modest increase in splicing defect.

These results suggest that mis-splicing may represent one of several mechanisms contributing to the absence of FMRP in FXS, specifically in cases where transcriptional silencing of the FMR1 locus is incomplete and expanded transcripts are still produced. In these cases, expanded FMR1 transcripts are produced but undergo aberrant processing that prevents functional protein production. These findings have important implications for understanding FXS pathogenesis and developing therapeutic strategies targeting the expanded locus.