P3‐120: Mis‐splicing of Tau Exon 2 and Exon 10 in myotonic dystrophy brain would result from different mechanisms

The mis-splicing of Tau is instrumental to the development of neurofibrillary degeneration (NFD) naturopathies such as FTDP-17 in which MAPTmutations lead to a defective splicing of Tau. A mis-splicing of Tau is also observed in the brain and muscle of patients affected by myotonic dystrophy of type I (DM1). Moreover, this Tau mis-splicing is associated with the development of NFD. DM1 is a neuromuscular disease characterized by an unstable CTG expansion. This mutation is responsible for a toxic gain of RNA-function, leading to the gain or loss of function of several splicing factors, including the CELF and MBNL families, respectively. It ultimately leads to an indirect mis-splicing of numerous transcripts. In regards to Tau mis-splicing, exon-2 inclusion is highly decreased in DM1 brains. In contrast, a mis-splicing of Tau exon-10 is seldom observed in DM1 but the mechanisms triggering this mis-splicing event remain unknown. We herein aimed to gain further insights into these molecular mechanisms involved in normal and pathological splicing of Tau exons 2 and 10 using cell-based models. Two among the brains of 5 DM1 patients showed an exon-10 mis-splicing whereas exon-2 inclusion was reduced in all DM1 patients. The over-expression of long stretches of CTGrepeats promoted the exclusion of both Tau exon 2 and exon 10. Interestingly, MBNL1 loss of expression efficiently repressed Tau exon 2 inclusion but failed to modulate Tau exon-10 splicing. Analyzing the splicing functions of three major CELF splicing factors, we showed that CELF 4 acts as a repressor for exon-2 or as an enhancer for exon-10 inclusion. In sharp contrast, CELF2 repressed both exons 2 and 10 inclusions whereas CELF1 was inefficient to modulate Tau splicing. In addition, 2D electrophoresis coupled to western-blotting suggest a possible change inbrain CELF2 activity induced by its hyperphosphorylation in DM1. Altogether, we showed that MBNL1 and CELF2 are implicated in the mis-splicing of Tau-exons 2 and 10, respectively, but not CELF1. Moreover, our Tau splicing study also suggests that two different mechanisms may contribute to the mis-splicing of several targets in DM1.