LDB3 splicing abnormalities are specific to skeletal muscles of patients with myotonic dystrophy type 1 and alter its PKC binding affinity

Myotonic dystrophy type 1 (DM1) is caused by transcription of CUG repeat RNA, which causes sequestration of muscleblind-like 1 (MBNL1) and upregulation of CUG triplet repeat RNA-binding protein (CUG-BP1). In DM1, dysregulation of these proteins contributes to many aberrant splicing events, causing various symptoms of the disorder. Here, we demonstrate the occurrence of aberrant splicing of LIM domain binding 3 (LDB3) exon 11 in DM1 skeletal muscle. Exon array surveys, RT-PCR, and western blotting studies demonstrated that exon 11 inclusion was DM1 specific and could be reproduced by transfection of a minigene containing the CTG repeat expansion. Moreover, we found that the LDB3 exon 11-positive isoform had reduced affinity for PKC compared to the exon 11-negative isoform. Since PKC exhibits hyperactivation in DM1 and stabilizes CUG-BP1 by phosphorylation, aberrant splicing of LDB3 may contribute to CUG-BP1 upregulation through changes in its affinity for PKC.