Progression of muscle histopathology but not of spliceopathy in myotonic dystrophy type 2

Abstract

Myotonic dystrophy type 2 (DM2) is an autosomal dominant progressive disease involving skeletal and cardiac muscle and brain. It is caused by a tetranucleotide repeat within the first intron of the CNBP gene that leads to an alteration of the alternative splicing of several genes. To understand the molecular mechanisms that play a role in DM2 progression, the evolution of skeletal muscle histopathology and biomolecular findings in successive biopsies have been studied. Biceps brachii biopsies from 5 DM2 patients who underwent two successive biopsies at different years of age have been used. Muscle histopathology has been assessed on sections immunostained with fast or slow myosin. FISH in combination with MBNL1-immunofluorescence has been performed to evaluate ribonuclear inclusion and MBNL1 foci dimensions in myonuclei. Gene and protein expression and alteration of alternative splicing of several genes have been evaluated over time. All DM2 patients examined show a worsening of muscle histopathology and an increase of foci dimensions over time. The progressive worsening of myotonia in DM2 patients may be due to the decrease of CLCN1 mRNA observed in all patients examined. However, a worsening of alternative splicing alterations has not been evidenced over time. The data obtained in this study confirm that DM2 is a slow progression disease since histological and biomolecular alterations observed in skeletal muscle are minimal even after 10-year interval. The data indicate that muscle morphological alterations evolve more rapidly over time than the molecular changes thus indicating that muscle biopsy is a more sensitive tool than biomolecular markers to assess disease progression at muscle level.

Myotonic Dystrophy Type 2: An Update on Clinical Aspects, Genetic and Pathomolecular Mechanism

“…Moreover, this study also demonstrated that the affected genes are involved in numerous pathways and networks important for muscle physio-pathology, suggesting that the identified variants may contribute to DM2 pathogenesis [85]. In DM2 patients, symptoms such as muscle weakness and myotonia undergo progressive worsening with increasing age, however, this aggravation is not accompanied by a worsening of alternative splicing of several genes [39]. …”

“…Moreover, in DM2 muscle biopsy central nucleation selectively affects type 2 fibers and the atrophic nuclear clumps express fast myosin isoform (type 2 fiber) indicating that DM2 is predominantly a disease of type 2 myofibers [37]. Recently, Cardani et al [39] have studied the progression of the muscular involvement in relation to the evolution of skeletal muscle histopathology and biomolecular findings to better prognosticate patients with DM2. Data confirm that disease progression in DM2 is slow since histological and biomolecular alterations observed in skeletal muscle are minimal even after a 10-year interval.…”

“…Data confirm that disease progression in DM2 is slow since histological and biomolecular alterations observed in skeletal muscle are minimal even after a 10-year interval. However, muscle morphological alterations evolve more rapidly over time than the molecular changes thus indicating that muscle biopsy is a sensitive tool to assess disease progression at muscular level [39]. …”

“…While it is clear that MBNL1 is depleted from nucleoplasm through recruitment into ribonuclear inclusions both in DM1 and DM2 even when clinical symptoms and muscle alterations are very mild, CUGBP1 overexpression has been clearly demonstrated in DM1 but not in DM2 muscle biopsies [69–73]. Thus, the role of CUGBP1 in DM2 is particularly intriguing with contradictory results being reported [39, 69, 71, 74, 75]. CUGBP1 may have a role in the pathogenesis of splicing abnormalities because it has been demonstrated that is overexpressed in DM1 myoblasts, skeletal muscle and heart tissues [30, 71, 72] due to PKC-mediated hyperphosphorylation and subsequent protein stabilisation and upregulation [73].…”

“…However, while Dmpk knockout young mice do not develop a multisystemic phenotype mimicking myotonic dystrophy [80–82], reduction of CNBP levels is sufficient to produce multiorgan symptoms resembling those of DM as observed in heterozygous Cnbp+/− knockout mice [83]. Some studies also suggest that CCTG expansions cause reduction of CNBP protein in DM2 patients, but there are conflicting data on this point [39, 69, 75, 76, 78]. Reduction of CNBP expression has been reported in skeletal muscle tissue and cells from DM2 patients compared with those from non-DM2 subjects, including patients with DM1, thus explaining some of the phenotypic disparities between the two types of DM [69, 75, 78].…”