University of Washington Department of Pharmacology
Function of Muscleblind-like 3 (MBNL3) Protein in Muscle Differentiation and Myotonic Dystrophy
Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disease that is genetically linked to expansions in the noncoding region of two different genes. Myotonic dystrophy type 1 (DM1) is caused by CTG repeat expansions in the 3’-UTR of the myotonic dystrophy protein kinase (DMPK) gene. In DM2 (type 2) patients, CCTG repeats in the first intron of the ZNF9 gene are expanded. The DMPK and ZNF9 genes have no apparent functional similarities. The expression of RNA transcripts with expanded repeats is the only common feature and is thought to be essential for DM pathogenesis. The prevailing model is that the mutant transcripts accumulate in the nuclei and alter the function of RNA-binding proteins, such as members of the muscleblind-like (MBNL) family of Cys3His zinc finger proteins.
We discovered that MBNL3 antagonizes muscle differentiation and are now focused on elucidating how MBNL3 functions as an inhibitor in myogenic cells. MyoD and myocyte enhancer factor 2 (Mef2) are regulatory factors important for muscle gene transcription. The beta-exon within the transactivation domain of Mef2 transcripts is highly conserved and subject to alternative splicing. We have data indicating that MBNL3 lowered MyoD protein levels and promoted splicing of Mef2 transcripts encoding the less active (–)beta isoform. In DM skeletal muscle and in a DM1 cell culture system, higher MBNL3 expression levels were accompanied by defects in Mef2 beta-exon splicing. These data suggest that expression of CUG expanded RNAs can lead to increased MBNL3 expression and decreased Mef2D beta-exon splicing. Thus, elevating MBNL3 activity in myogenic cells could lead to muscle degeneration disorders such as myotonic dystrophy.
Binding to Mef2D intron 7 sequences was required for MBNL3 to function as a silencer of beta-exon splicing. The RNA binding and splicing activities of MBNL3 call for one or more of the highly conserved Cys3His motifs. We are interested in further exploring the importance of the zinc fingers in the antagonistic function of MBNL3 in muscle. Disrupting MBNL3-RNA interactions may represent a potential approach for treating the muscle-related symptoms of myotonic dystrophy.