Myotonic dystrophies 1 and 2: complex diseases with complex mechanisms

Abstract

Two multi-system disorders, Myotonic Dystrophies type 1 and type 2 (DM1 and DM2), are complex neuromuscular diseases caused by an accumulation of expanded, non-coding RNAs, containing repetitive CUG and CCUG elements. Similarities of these mutations suggest similar mechanisms for both diseases. The expanded CUGn and CCUGn RNAs mainly target two RNA binding proteins, MBNL1 and CUGBP1, elevating levels of CUGBP1 and reducing levels of MBNL1. These alterations change processing of RNAs that are regulated by these proteins. Whereas overall toxicity of CUGn/CCUGn RNAs on RNA homeostasis in DM cells has been proven, the mechanisms which make these RNAs toxic remain illusive. A current view is that the toxicity of RNA CUGn and CCUGn is associated exclusively with global mis-splicing in DM patients. However, a growing number of new findings show that the expansion of CUGn and CCUGn RNAs mis-regulates several additional pathways in nuclei and cytoplasm of cells from patients with DM1 and DM2. The purpose of this review is to discuss the similarities and differences in the clinical presentation and molecular genetics of both diseases. We will also discuss the complexity of the molecular abnormalities in DM1 and DM2 caused by CUG and CCUG repeats and will summarize the outcomes of the toxicity of CUG and CCUG repeats.

Fig. (1)

Synopsis of clinical signs in DM1 and in DM2.

Fig. (2)

Common clinical presentation in DM1 and in DM2. (A) Grip myotonia in a DM2 patient. Reduced speed of first opening and weakness of the finger flexors. (B) Electromyographic recording of the tibialis anterior muscle from a DM1 patient with a classic myotonic discharge. Note the decrescendo character of the spontaneous motor unit activity. (C) Representative posterior iridescent cataract in a DM2 patient.

Fig. (3)

Differences in clinical presentation of adult DM1 and DM2. A classic forearm atrophy is shown for patient with DM1 (A) but not with DM2 (B). The “core” characteristic of DM2 is a typical predominant lower leg weakness and atrophy (B).

Fig. (4)

A hypothetical model outlining the role of CUG expansion in the dys-regulation of gene expression at several levels. In the nucleus of DM1 patients, double-stranded CUG repeat RNA binds to MBNL1. CUGBP1 binds to the opened ends of the CUG-helix. Triangles show hypothetical positions of other misregulated RNA-binding proteins, which interact with CUG repeats, such as hnRNP H. Un-aggregated CUG repeats bind to CUGBP1 and TFs affecting splicing of the CUGBP1 targets and reducing transcription. In cytoplasm of DM1 cells, the un-aggregated CUG repeats stabilize CUGBP1 increasing levels of CUGBP1 protein. The elevated CUGBP1 alters translation and stability of mRNAs. CUG repeats also change signal transduction pathways by unknown mechanisms affecting CUGBP1 activity and stability.

Fig. (5)

Hypothetic model for the role of CCUG repeats in DM2 pathology. Like in patients with DM1, CCUG repeats form nuclear aggregates presumably containing double-stranded CCUG repeats. This aggregated CCUG RNA sequesters MBNL1 changing splicing of mRNAs regulated by MBNL1. It is unknown if CUGBP1 contributes to the alterations in splicing of mRNAs in DM2 cells. In cytoplasm, the un-aggregated CCUG repeats bind to two multi-protein complexes containing CUGBP1-eIF2 and the 20S proteasome affecting translation and stability of proteins. CCUG repeats also reduce cytoplasmic levels of ZNF9 by unknown mechanism. Since ZNF9 regulates several TOP-containing mRNAs, encoding proteins of translational apparatus, the reduction of cytoplasmic ZNF9 causes the reduction of the rate of global protein synthesis.

Fig. (6)

The translational CUGBP1–eIF2 complex is increased in control and DM2 myotubes, but not in DM1 myotubes. Cytoplasmic extracts from normal, DM1 and DM2 myoblasts (Mb) and myotubes (Mt) were incubated with the C/EBPβ RNA probe and separated by native gel electrophoresis (EMSA). The upper fragment of the gel containing the CUGBP1-eIF2 complexes is shown. The presence of CUGBP1-eIF2 complex in DM2 myotubes suggests that despite the increase of CUGBP1 in both DM1 and in DM2, CUGBP1 translational activity might be normal in DM2 myotubes.

Fig. (7)

The role of signal-transduction pathways in dysregulation of activity of CUGBP1 in patients with DM1. CUGBP1 structure with three RNA-binding domains (RBD) is shown. Two sites (Ser28 and Ser302) phosphorylated by Akt and cyclin D/cdk4 correspondingly are shown. Amino acid residues predicted to be phosphorylated by PKCδ are also shown. Phosphorylation of CUGBP1 at Ser28 increases binding of CUGBP1 to cyclin D1 mRNA, promoting proliferation in myoblasts. Phosphorylation of CUGBP1 by PKCδ contributes to the stabilization of CUGBP1 in DM1. Site-specific phosphorylation of CUGBP1 by cdk4 increases interactions of the CUGBP1 with p21 mRNA leading to the elevation of p21 and promotion of skeletal muscle differentiation. In DM1 myogenesis, the reduction of cyclin D3 inhibits CUGBP1 binding to p21 mRNA and reduces p21 levels leading to a delay of DM1 differentiation.