Robert J. Osborne

Reversal of RNA dominance by displacement of protein sequestered on triplet repeat RNA.

Resisting Repeats A set of diseases, including myotonic dystrophy, are caused by the expansion of a simple repeat in genomic DNA, which, when transcribed into RNA, can be toxic to other cellular processes. Ameliorating the effects of this toxic, repeat-laden RNA may also relieve the symptoms of the disease. Wheeler et al. (p. 336; see the Perspective by Cooper) developed an antisense morpholino oligonucleotide complementary to the expanded repeats found in the myotonic dystrophy protein kinase messenger RNA (mRNA). The morpholino bound the repeats in vitro and displaced the inappropriately bound and sequestered RNA splicing factor, Muscleblind-like 1. In an in vivo mouse model for myotonic dystrophy, local injection of the morpholino corrected a number of cellular defects in muscle, including the alternative mRNA splicing of several genes, among them the muscle-specific chloride channel, CIC1, leading to a marked reduction in the myotonia. An antisense oligonucleotide ameliorates the symptoms of myotonic dystrophy in transgenic mice. Genomic expansions of simple tandem repeats can give rise to toxic RNAs that contain expanded repeats. In myotonic dystrophy, the expression of expanded CUG repeats (CUGexp) causes abnormal regulation of alternative splicing and neuromuscular dysfunction. We used a transgenic mouse model to show that derangements of myotonic dystrophy are reversed by a morpholino antisense oligonucleotide, CAG25, that binds to CUGexp RNA and blocks its interaction with muscleblind-like 1 (MBNL1), a CUGexp-binding protein. CAG25 disperses nuclear foci of CUGexp RNA and reduces the overall burden of this toxic RNA. As MBNL1 is released from sequestration, the defect of alternative splicing regulation is corrected, thereby restoring ion channel function. These findings suggest an alternative use of antisense methods, to inhibit deleterious interactions of proteins with pathogenic RNAs.

Aberrant alternative splicing and extracellular matrix gene expression in mouse models of myotonic dystrophy

The common form of myotonic dystrophy (DM1) is associated with the expression of expanded CTG DNA repeats as RNA (CUGexp RNA). To test whether CUGexp RNA creates a global splicing defect, we compared the skeletal muscle of two mouse models of DM1, one expressing a CTGexp transgene and another homozygous for a defective muscleblind 1 (Mbnl1) gene. Strong correlation in splicing changes for ∼100 new Mbnl1-regulated exons indicates that loss of Mbnl1 explains >80% of the splicing pathology due to CUGexp RNA. In contrast, only about half of mRNA-level changes can be attributed to loss of Mbnl1, indicating that CUGexp RNA has Mbnl1-independent effects, particularly on mRNAs for extracellular matrix proteins. We propose that CUGexp RNA causes two separate effects: loss of Mbnl1 function (disrupting splicing) and loss of another function that disrupts extracellular matrix mRNA regulation, possibly mediated by Mbnl2. These findings reveal unanticipated similarities between DM1 and other muscular dystrophies.

Transcriptional and post-transcriptional impact of toxic RNA in myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is an RNA dominant disease in which mutant transcripts containing an expanded CUG repeat (CUG(exp)) cause muscle dysfunction by interfering with biogenesis of other mRNAs. The toxic effects of mutant RNA are mediated partly through sequestration of splicing regulator Muscleblind-like 1 (Mbnl1), a protein that binds to CUG(exp) RNA. A gene that is prominently affected encodes chloride channel 1 (Clcn1), resulting in hyperexcitability of muscle (myotonia). To identify DM1-affected genes and study mechanisms for dysregulation, we performed global mRNA profiling in transgenic mice that express CUG(exp) RNA, when compared with Mbnl1 knockout and Clcn1 null mice. We found that the majority of changes induced by CUG(exp) RNA in skeletal muscle can be explained by reduced activity of Mbnl1, including many changes that are secondary to myotonia. The pathway most affected comprises genes involved in calcium signaling and homeostasis. Some effects of CUG(exp) RNA on gene expression are caused by abnormal alternative splicing or downregulation of Mbnl1-interacting mRNAs. However, several of the most highly dysregulated genes showed altered transcription, as indicated by parallel changes of the corresponding pre-mRNAs. These results support the idea that trans-dominant effects of CUG(exp) RNA on gene expression in this transgenic model may occur at the level of transcription, RNA processing and mRNA decay, and are mediated mainly but not entirely through sequestration of Mbnl1.

Expression profile of FSHD supports a link between retinal vasculopathy and muscular dystrophy

Background: Facioscapulohumeral muscular dystrophy (FSHD) is caused by deletions within a tandem array of D4Z4 repeats on chromosome 4q35. In addition to muscle degeneration, most patients with FSHD develop abnormalities of the retinal vasculature. Previous work has suggested that muscle degeneration in FSHD results from increased expression of genes proximal to the deletion, including FRG1. Objectives: To reexamine this mechanism and identify pathways that are abnormally regulated early in the disease process. Methods: We prospectively studied gene expression in skeletal muscle in patients with FSHD (n = 19) vs healthy individuals (n = 30) and patients with myotonic dystrophy type 1 (n = 12). We used oligonucleotide microarrays for global analysis of gene expression and reverse transcriptase-PCR (RT-PCR) to assess expression or alternative splicing for particular genes. Results: Expression of FRG1 was not increased in patients with FSHD, either by microarray analysis or quantitative RT-PCR. Among genes on 4q35, only LRP2BP showed upregulation that was specific to FSHD. However, neither LRP2BP nor FRG1 showed imbalance of allelic expression by RT-PCR. After filtering out genes that showed similar dysregulation in other forms of muscular dystrophy, only 44 genes were specifically upregulated early in FSHD. Among these, 34 genes were characterized or partially characterized, of which 11 (32%) had a role in vascular smooth muscle or endothelial cells. Conclusion: Expression of genes on chromosome 4q35 was normally regulated in the early stages of facioscapulohumeral muscular dystrophy. Our results support a possible link between muscular dystrophy and retinal vasculopathy in facioscapulohumeral muscular dystrophy.

Splicing biomarkers of disease severity in myotonic dystrophy.

To develop RNA splicing biomarkers of disease severity and therapeutic response in myotonic dystrophy type 1 (DM1) and type 2 (DM2).

Cell-free cloning of highly expanded CTG repeats by amplification of dimerized expanded repeats

We describe conditions for producing uninterrupted expanded CTG repeats consisting of up to 2000 repeats using ϕ29 DNA polymerase. Previously, generation of such repeats was hindered by CTG repeat instability in plasmid vectors maintained in Escherichia coli and poor in vitro ligation of CTG repeat concatemers due to strand slippage. Instead, we used a combination of in vitro ligation and ϕ29 DNA polymerase to amplify DNA. Correctly ligated products generating a dimerized repeat tract formed substrates for rolling circle amplification (RCA). In the presence of two non-complementary primers, hybridizing to either strand of DNA, ligations can be amplified to generate microgram quantities of repeat containing DNA. Additionally, expanded repeats generated by rolling circle amplification can be produced in vectors for expression of expanded CUG (CUGexp) RNA capable of sequestering MBNL1 protein in cell culture. Amplification of dimerized expanded repeats (ADER) opens new possibilities for studies of repeat instability and pathogenesis in myotonic dystrophy, a neurological disorder caused by an expanded CTG repeat.