Nuclear RNA Foci in the Heart in Myotonic Dystrophy

Abstract

Abstract-The disease mechanism underlying myotonic dystrophy type 1 (DM1) pathogenesis in skeletal muscle may involve sequestration of RNA binding proteins in nuclear foci of expanded poly(CUG) RNA. Here we report evidence for a parallel mechanism in the heart. Accumulation of expanded poly(CUG) RNA in nuclear foci is associated with sequestration of muscleblind proteins and abnormal regulation of alternative splicing in DM1 cardiac muscle. A toxic effect of RNA with an expanded repeat may contribute to cardiac disease in DM1. (Circ Res. 2005;97:1152-1155.)Key Words: myotonic dystrophy Ⅲ CUG repeats Ⅲ RNA foci Ⅲ muscleblind Ⅲ alternative splicing C ardiac disease in myotonic dystrophy type 1 (DM1) is characterized by interstitial fibrosis and fatty infiltration with preferential involvement of the specialized conduction tissue. 1,2 This degenerative process results in progressive failure of the conduction system, atrial and ventricular tachyarrhythmia, and mild diastolic dysfunction. 3 Sudden cardiac death ranks second after respiratory failure as a cause of death among patients with DM1. 4 The genetic basis for this multisystem, dominantly inherited disorder is an unstable expansion of CTG repeats in the 3Ј untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. 5 Variability of the DM1 phenotype is explained partly, but not entirely, by variation in length of the expanded repeat. 6,7 The tract of expanded triplet repeats is highly unstable in somatic cells and may reach massive lengths (Ͼ10 kb) in skeletal and cardiac muscle. 8 DM1 is the most common cause of muscular dystrophy in adults. Recent evidence indicates that the skeletal myopathy of DM1 results from a novel RNA-mediated disease mechanism. 9 The mutant DMPK mRNA, which contains an expanded CUG repeat, is retained in nuclear foci. 10 Accumulation of this mutant RNA in the nucleus interferes with the alternative splicing of other pre-mRNAs. 11 Only a subset of alternatively spliced pre-mRNAs is susceptible to this trans-dominant effect. For example, misregulated splicing of the CLCN1 chloride ion channel leads to reduced chloride conductance and repetitive action potentials (myotonia) in skeletal muscle. 12 Although this effect on alternative splicing is thought to reflect an interaction of the expanded poly(CUG) RNA with specific splicing factors, the exact mechanism is unknown.DMPK mRNA is expressed most highly in the heart, 5 raising the possibility that a similar RNA-mediated disease process may occur in cardiac muscle. Indeed, the initial observation of misregulated alternative splicing was made in cardiac tissue. Phillips et al observed that alternative splicing of cardiac troponin T (TNNT2) was abnormal in DM1 heart, and they postulated that altered activity of CUG binding protein 1 (CUGBP1) was the cause. 11 Among other functions, CUGBP1 is a splicing factor that binds to short (CUG) 8 oligonucleotides in vitro. 13 In skeletal muscle, attempts to show interaction of CUGPB1 with expanded poly(CUG) in vivo, by colocalizing the...

Reversible model of RNA toxicity and cardiac conduction defects in myotonic dystrophy

“…4 online). Notably, we also did not find any obvious splicing abnormalities in the heart for previously reported targets 19 (data not shown). Furthermore, CUG-BP1 in mice that reverted tended to return to levels found in mice in which transgene expression was not induced, correlating with the correction of molecular and phenotypic defects in skeletal muscle (Fig.…”

Muscleblind-Like Proteins

“…24 Transfected MBNL1, MBNL2, and MBNL3 colocalize with the expanded CUG/CCUG ribonuclear inclusions in DM cells. 19,23,25,26 Several studies have reported the colocalization of endogenous MBNL1 with ribonuclear foci, 19,20,24,[27][28][29][30][31] and one study suggests that MBNL1 is required for focus formation. 32 In addition, a mouse functional knockout of MBNL1 shows DM features, such as myotonia, abnormal myofibers, cataracts and aberrant splicing of chloride channel, cardiac troponin T, and fast skeletal troponin T. 33 At least some of the pathological features of DM are thought to be due to misregulated alternative splicing of RNA.…”

Stochastic and reversible aggregation of mRNA with expanded CUG-triplet repeats

“…Altered mRNA splicing has been reported in more than 20 genes in DM1 patients (Kalsotra et al, 2008;Lin et al, 2006). In skeletal and heart muscle cells, and neurons of DM1 patients, the recruitment of Mbnl1 into CUGrepeat RNA foci is so extensive that it is depleted from the nucleoplasm (Cardani et al, 2006;Fardaei et al, 2001;Jiang et al, 2004;Mankodi et al, 2005;Mankodi et al, 2003;Miller et al, 2000;Wheeler et al, 2007). In support of the Mbnl1-sequestration hypothesis, disruption of the Mbnl1 gene in mice reproduces both the spliceopathy found in DM1 and the characteristic symptoms of myotonia and myopathy (Kanadia et al, 2003).…”