Disrupted neuronal maturation in Angelman syndrome-derived induced pluripotent stem cells.

Angelman syndrome (AS) is a neurogenetic disorder caused by deletion of the maternally inherited UBE3A allele and is characterized by developmental delay, intellectual disability, ataxia, seizures and a happy affect. Here, we explored the underlying pathophysiology using induced pluripotent stem cell-derived neurons from AS patients and unaffected controls. AS-derived neurons showed impaired maturation of resting membrane potential and action potential firing, decreased synaptic activity and reduced synaptic plasticity. These patient-specific differences were mimicked by knocking out UBE3A using CRISPR/Cas9 or by knocking down UBE3A using antisense oligonucleotides. Importantly, these phenotypes could be rescued by pharmacologically unsilencing paternal UBE3A expression. Moreover, selective effects of UBE3A disruption at late stages of in vitro development suggest that changes in action potential firing and synaptic activity may be secondary to altered resting membrane potential. Our findings provide a cellular phenotype for investigating pathogenic mechanisms underlying AS and identifying novel therapeutic strategies.

The Antisense Transcript SMN-AS1 Regulates SMN Expression and Is a Novel Therapeutic Target for Spinal Muscular Atrophy.

The neuromuscular disorder spinal muscular atrophy (SMA), the most common inherited killer of infants, is caused by insufficient expression of survival motor neuron (SMN) protein. SMA therapeutics development efforts have focused on identifying strategies to increase SMN expression. We identified a long non-coding RNA (lncRNA) that arises from the antisense strand of SMN, SMN-AS1, which is enriched in neurons and transcriptionally represses SMN expression by recruiting the epigenetic Polycomb repressive complex-2. Targeted degradation of SMN-AS1 with antisense oligonucleotides (ASOs) increases SMN expression in patient-derived cells, cultured neurons, and the mouse central nervous system. SMN-AS1 ASOs delivered together with SMN2 splice-switching oligonucleotides additively increase SMN expression and improve survival of severe SMA mice. This study is the first proof of concept that targeting a lncRNA to transcriptionally activate SMN2 can be combined with SMN2 splicing modification to ameliorate SMA and demonstrates the promise of combinatorial ASOs for the treatment of neurogenetic disorders.

Antagonistic regulation of mRNA expression and splicing by CELF and MBNL proteins.

RNA binding proteins of the conserved CUGBP1, Elav-like factor (CELF) family contribute to heart and skeletal muscle development and are implicated in myotonic dystrophy (DM). To understand their genome-wide functions, we analyzed the transcriptome dynamics following induction of CELF1 or CELF2 in adult mouse heart and of CELF1 in muscle by RNA-seq, complemented by crosslinking/immunoprecipitation-sequencing (CLIP-seq) analysis of mouse cells and tissues to distinguish direct from indirect regulatory targets. We identified hundreds of mRNAs bound in their 3' UTRs by both CELF1 and the developmentally induced MBNL1 protein, a threefold greater overlap in target messages than expected, including messages involved in development and cell differentiation. The extent of 3' UTR binding by CELF1 and MBNL1 predicted the degree of mRNA repression or stabilization, respectively, following CELF1 induction. However, CELF1's RNA binding specificity in vitro was not detectably altered by coincubation with recombinant MBNL1. These findings support a model in which CELF and MBNL proteins bind independently to mRNAs but functionally compete to specify down-regulation or localization/stabilization, respectively, of hundreds of mRNA targets. Expression of many alternative 3' UTR isoforms was altered following CELF1 induction, with 3' UTR binding associated with down-regulation of isoforms and genes. The splicing of hundreds of alternative exons was oppositely regulated by these proteins, confirming an additional layer of regulatory antagonism previously observed in a handful of cases. The regulatory relationships between CELFs and MBNLs in control of both mRNA abundance and splicing appear to have evolved to enhance developmental transitions in major classes of heart and muscle genes.

Towards a therapy for Angelman syndrome by targeting a long non-coding RNA.

Angelman syndrome is a single-gene disorder characterized by intellectual disability, developmental delay, behavioural uniqueness, speech impairment, seizures and ataxia. It is caused by maternal deficiency of the imprinted gene UBE3A, encoding an E3 ubiquitin ligase. All patients carry at least one copy of paternal UBE3A, which is intact but silenced by a nuclear-localized long non-coding RNA, UBE3A antisense transcript (UBE3A-ATS). Murine Ube3a-ATS reduction by either transcription termination or topoisomerase I inhibition has been shown to increase paternal Ube3a expression. Despite a clear understanding of the disease-causing event in Angelman syndrome and the potential to harness the intact paternal allele to correct the disease, no gene-specific treatment exists for patients. Here we developed a potential therapeutic intervention for Angelman syndrome by reducing Ube3a-ATS with antisense oligonucleotides (ASOs). ASO treatment achieved specific reduction of Ube3a-ATS and sustained unsilencing of paternal Ube3a in neurons in vitro and in vivo. Partial restoration of UBE3A protein in an Angelman syndrome mouse model ameliorated some cognitive deficits associated with the disease. Although additional studies of phenotypic correction are needed, we have developed a sequence-specific and clinically feasible method to activate expression of the paternal Ube3a allele.

LincRNA-p21 activates p21 in cis to promote Polycomb target gene expression and to enforce the G1/S checkpoint.

The p53-regulated long noncoding RNA lincRNA-p21 has been proposed to act in trans via several mechanisms ranging from repressing genes in the p53 transcriptional network to regulating mRNA translation and protein stability. To further examine lincRNA-p21 function, we generated a conditional knockout mouse model. We find that lincRNA-p21 predominantly functions in cis to activate expression of its neighboring gene, p21. Mechanistically, we show that lincRNA-p21 acts in concert with hnRNP-K as a coactivator for p53-dependent p21 transcription. Additional phenotypes of lincRNA-p21 deficiency could be attributed to diminished p21 levels, including deregulated expression and altered chromatin state of some Polycomb target genes, a defective G1/S checkpoint, increased proliferation rates, and enhanced reprogramming efficiency. These findings indicate that lincRNA-p21 affects global gene expression and influences the p53 tumor suppressor pathway by acting in cis as a locus-restricted coactivator for p53-mediated p21 expression.

Alternative splicing regulates vesicular trafficking genes in cardiomyocytes during postnatal heart development.

During postnatal development the heart undergoes a rapid and dramatic transition to adult function through transcriptional and post-transcriptional mechanisms, including alternative splicing (AS). Here we perform deep RNA-sequencing on RNA from cardiomyocytes and cardiac fibroblasts to conduct a high-resolution analysis of transcriptome changes during postnatal mouse heart development. We reveal extensive changes in gene expression and AS that occur primarily between postnatal days 1 and 28. Cardiomyocytes and cardiac fibroblasts show reciprocal regulation of gene expression reflecting differences in proliferative capacity, cell adhesion functions and mitochondrial metabolism. We further demonstrate that AS plays a role in vesicular trafficking and membrane organization. These AS transitions are enriched among targets of two RNA-binding proteins, Celf1 and Mbnl1, which undergo developmentally regulated changes in expression. Vesicular trafficking genes affected by AS during normal development (when Celf1 is downregulated) show a reversion to neonatal splicing patterns after Celf1 re-expression in adults. Short-term Celf1 induction in adult animals results in disrupted transverse tubule organization and calcium handling. These results identify potential roles for AS in multiple aspects of postnatal heart maturation, including vesicular trafficking and intracellular membrane dynamics.

Nonsense-mediated decay as a terminating mechanism for antisense oligonucleotides.

Antisense oligonucleotides (ASOs) are synthetic oligonucleotides that alter expression of disease-associated transcripts via Watson-Crick hybridization. ASOs that function through RNase H or the RNA-induced silencing complex (RISC) result in enzymatic degradation of target RNA. ASOs designed to sterically block access of proteins to the RNA modulate mRNA metabolism but do not typically cause degradation. Here, we rationally design steric blocking ASOs to promote mRNA reduction and characterize the terminating mechanism. Transfection of ASOs complementary to constitutive exons in STAT3 and Sod1 results in greater than 70% reduction of mRNA and protein. The ASOs promote aberrant exon skipping and generation of premature termination codon (PTC)-containing mRNAs. We inhibit the nonsense-mediated mRNA decay (NMD) pathway and show that the PTC-containing mRNAs are recognized by the UPF1 ATPase, cleaved by the SMG6 endonuclease and degraded by the XRN1 cytoplasmic exonuclease. NMD surveillance, however, does not entirely explain the mechanism of decreased STAT3 expression. In addition to exon skipping, ASO treatment causes intron retention and reduction of chromatin-associated STAT3 mRNA. The application of steric blocking ASOs to promote RNA degradation allows one to explore more nucleotide modifications than tolerated by RNase H or RISC-dependent ASOs, with the goal of improving ASO drug properties.

The Mef2 transcription network is disrupted in myotonic dystrophy heart tissue, dramatically altering miRNA and mRNA expression.

Cardiac dysfunction is the second leading cause of death in myotonic dystrophy type 1 (DM1), primarily because of arrhythmias and cardiac conduction defects. A screen of more than 500 microRNAs (miRNAs) in a DM1 mouse model identified 54 miRNAs that were differentially expressed in heart. More than 80% exhibited downregulation toward the embryonic expression pattern and showed a DM1-specific response. A total of 20 of 22 miRNAs tested were also significantly downregulated in human DM1 heart tissue. We demonstrate that many of these miRNAs are direct MEF2 transcriptional targets, including miRNAs for which depletion is associated with arrhythmias or fibrosis. MEF2 protein is significantly reduced in both DM1 and mouse model heart samples, and exogenous MEF2C restores normal levels of MEF2 target miRNAs and mRNAs in a DM1 cardiac cell culture model. We conclude that loss of MEF2 in DM1 heart causes pathogenic features through aberrant expression of both miRNA and mRNA targets.

Alternative splicing dysregulation secondary to skeletal muscle regeneration.

OBJECTIVE: Dysregulation of alternative splicing has become a molecular hallmark of myotonic dystrophy type 1 (DM1), in which neonatal splice variants are expressed in adult skeletal muscle. Splicing dysregulation is induced by RNA containing expanded CUG repeats expressed from the expanded mutant allele by sequestration of muscleblindlike 1 (MBNL1) protein within nuclear RNA foci and increased CUGBP, ELAV-like family member 1 (CELF1) protein levels. Dysregulated splicing has also been identified in other neuromuscular disorders, suggesting either that diseases with different molecular causes share a common pathogenic mechanism or that dysregulated splicing can also be a common secondary consequence of muscle degeneration and regeneration. METHODS: In this study, we examined regulation of alternative splicing in 4 different mouse models of muscular dystrophy, including DM1, limb-girdle muscular dystrophy, congenital merosin-deficient muscular dystrophy, and Duchenne muscular dystrophy, and 2 myotoxin (cardiotoxin and notexin) muscle injury models. RESULTS: We show that DM1-like alternative splicing dysregulation and altered expression of MBNL1 and CELF1 occur in non-DM1 mouse models of muscular dystrophy and muscle injury, most likely due to recapitulation of neonatal splicing patterns in regenerating fibers. In contrast, CELF1 was elevated in nuclei of mature myofibers of the DM1 model, consistent with a primary effect of pathogenic RNA expression. INTERPRETATION: Splicing dysregulation in DM1 is a primary effect of RNA containing expanded CUG repeats. However, we conclude that splicing changes can also be observed secondary to muscle regeneration, and this possibility must be taken into account when evaluating cause-effect relationships between dysregulated splicing and disease processes.

CUGBP1 overexpression in mouse skeletal muscle reproduces features of myotonic dystrophy type 1.

The neuromuscular disease myotonic dystrophy type I (DM1) affects multiple organ systems with the major symptoms being severe muscle weakness, progressive muscle wasting and myotonia. The causative mutation in DM1 is a CTG repeat expansion in the 3'-untranslated region of the DM protein kinase (DMPK) gene. RNA transcribed from the expanded allele contains the expanded CUG repeats and leads to the nuclear depletion of Muscleblind-like 1 (MBNL1) and to the increased steady-state levels of CUG-binding protein 1 (CUGBP1). The pathogenic effects of MBNL1 depletion have previously been tested by the generation of MBNL1 knockout mice, but the consequence of CUGBP1 overexpression in adult muscle is not known. In a DM1 mouse model expressing RNA containing 960 CUG repeats in skeletal muscle, CUGBP1 up-regulation is temporally correlated with severe muscle wasting. In this study, we generated transgenic mice with doxycycline-inducible and skeletal muscle-specific expression of CUGBP1. Adult mouse skeletal muscle overexpressing CUGBP1 reproduces molecular and physiological defects of DM1 tissue. The results from this study strongly suggest that CUGBP1 has a major role in DM1 skeletal muscle pathogenesis.

The pathobiology of splicing.

Ninety-four percent of human genes are discontinuous, such that segments expressed as mRNA are contained within exons and separated by intervening segments, called introns. Following transcription, genes are expressed as precursor mRNAs (pre-mRNAs), which are spliced co-transcriptionally, and the flanking exons are joined together to form a continuous mRNA. One advantage of this architecture is that it allows alternative splicing by differential use of exons to generate multiple mRNAs from individual genes. Regulatory elements located within introns and exons guide the splicing complex, the spliceosome, and auxiliary RNA binding proteins to the correct sites for intron removal and exon joining. Misregulation of splicing and alternative splicing can result from mutations in cis-regulatory elements within the affected gene or from mutations that affect the activities of trans-acting factors that are components of the splicing machinery. Mutations that affect splicing can cause disease directly or contribute to the susceptibility or severity of disease. An understanding of the role of splicing in disease expands potential opportunities for therapeutic intervention by either directly addressing the cause or by providing novel approaches to circumvent disease processes.

A postnatal switch of CELF and MBNL proteins reprograms alternative splicing in the developing heart.

From a large-scale screen using splicing microarrays and RT-PCR, we identified 63 alternative splicing (AS) events that are coordinated in 3 distinct temporal patterns during mouse heart development. More than half of these splicing transitions are evolutionarily conserved between mouse and chicken. Computational analysis of the introns flanking these splicing events identified enriched and conserved motifs including binding sites for CUGBP and ETR-3-like factors (CELF), muscleblind-like (MBNL) and Fox proteins. We show that CELF proteins are down-regulated >10-fold during heart development, and MBNL1 protein is concomitantly up-regulated nearly 4-fold. Using transgenic and knockout mice, we show that reproducing the embryonic expression patterns for CUGBP1 and MBNL1 in adult heart induces the embryonic splicing patterns for more than half of the developmentally regulated AS transitions. These findings indicate that CELF and MBNL proteins are determinative for a large subset of splicing transitions that occur during postnatal heart development.