Expression and characterisation of Fmr1 splice variants during folliculogenesis in the rat

The FMR1 gene consists of 17 exons and codes for the FMRP protein. FMR1 is involved in four genetic disorders depending on the CGG repeats length in its 5'UTR: the full mutation is responsible for the Fragile X syndrome while the premutation is associated with the Fragile X-associated Tremor/Ataxia Syndrome, Fragile X-associated Primary Ovarian Insufficiency (FXPOI) and Fragile X-associated neuropsychiatric disorders. FMR1 presents multiple isoforms resulting from skipping of exons 12 and 14 and the use of alternative splice sites in exons 15 and 17.

To investigate the expression of Fmr1 splicing variants during folliculogenesis in the rat.

We used preantral, early antral and preovulatory follicles to isolate RNA and characterise, by fluorescent PCR followed by sequencing, all the isoforms present in the different follicular stages.

We identified two isoforms resulting from splicing of exon 12, six isoforms resulting from splicing of exon 14 and 15 and one isoform for exon 17.

The expression levels of the isoforms vary within each follicular stage but not between different stages of folliculogenesis. Importantly, we identify for the first time in rat, an isoform that contains exon 12 and two isoforms, one that includes and one that excludes exon 14 and use the third acceptor site in exon 15.

Characterisation of the different FMR1 variants expressed during folliculogenesis will help to understand the potential distinct cellular roles of each of them and the possible implication in the development of FXPOI.