MBNL1 overexpression is not sufficient to rescue the phenotypes in a mouse model of RNA toxicity.

Myotonic dystrophy type 1 (DM1) is caused by an expanded (CTG)n tract in the 3'UTR of the DM protein kinase (DMPK) gene. The RNA transcripts produced from the expanded allele sequester or alter the function of RNA-binding proteins (MBNL1, CUGBP1, etc.). The sequestration of MBNL1 results in RNA-splicing defects that contribute to disease. Overexpression of MBNL1 in skeletal muscle has been shown to rescue some of the DM1 features in a mouse model and has been proposed as a therapeutic strategy for DM1. Here, we sought to confirm if overexpression of MBNL1 rescues the phenotypes in a different mouse model of RNA toxicity. Using an inducible mouse model of RNA toxicity in which expression of the mutant DMPK 3'UTR results in RNA foci formation, MBNL1 sequestration, splicing defects, myotonia and cardiac conduction defects, we find that MBNL1 overexpression did not rescue skeletal muscle function nor beneficially affect cardiac conduction. Surprisingly, MBNL1 overexpression also did not rescue myotonia, though variable rescue of Clcn1 splicing and other splicing defects was seen. Additionally, contrary to the previous study, we found evidence for increased muscle histopathology with MBNL1 overexpression. Overall, we did not find evidence for beneficial effects from overexpression of MBNL1 as a means to correct RNA toxicity mediated by mRNAs containing an expanded DMPK 3'UTR.

NKX2-5, a modifier of skeletal muscle pathology due to RNA toxicity.

RNA toxicity is implicated in a number of disorders; especially those associated with expanded repeat sequences, such as myotonic dystrophy (DM1). Previously, we have shown increased NKX2-5 expression in RNA toxicity associated with DM1. Here, we investigate the relationship between NKX2-5 expression and muscle pathology due to RNA toxicity. In skeletal muscle from mice with RNA toxicity and individuals with DM1, expression of Nkx2-5 or NKX2-5 and its downstream targets are significantly correlated with severity of histopathology. Using C2C12 myoblasts, we show that over-expression of NKX2-5 or mutant DMPK 3'UTR results in myogenic differentiation defects, which can be rescued by knockdown of Nkx2-5, despite continued toxic RNA expression. Furthermore, in a mouse model of NKX2-5 over-expression, we find defects in muscle regeneration after induced damage, similar to those seen in mice with RNA toxicity. Using mouse models of Nkx2-5 over-expression and depletion, we find that NKX2-5 levels modify disease phenotypes in mice with RNA toxicity.

TWEAK/Fn14, a pathway and novel therapeutic target in myotonic dystrophy.

Myotonic dystrophy type 1 (DM1), the most prevalent muscular dystrophy in adults, is characterized by progressive muscle wasting and multi-systemic complications. DM1 is the prototype for disorders caused by RNA toxicity. Currently, no therapies exist. Here, we identify that fibroblast growth factor-inducible 14 (Fn14), a member of the tumor necrosis factor receptor super-family, is induced in skeletal muscles and hearts of mouse models of RNA toxicity and in tissues from DM1 patients, and that its expression correlates with severity of muscle pathology. This is associated with downstream signaling through the NF-κB pathways. In mice with RNA toxicity, genetic deletion of Fn14 results in reduced muscle pathology and better function. Importantly, blocking TWEAK/Fn14 signaling with an anti-TWEAK antibody likewise improves muscle histopathology and functional outcomes in affected mice. These results reveal new avenues for therapeutic development and provide proof of concept for a novel therapeutic target for which clinically available therapy exists to potentially treat muscular dystrophy in DM1.

The Science of Vascular Contributions to Cognitive Impairment and Dementia (VCID): A Framework for Advancing Research Priorities in the Cerebrovascular Biology of Cognitive Decline.

The World Health Organization reports that 47.5 million people are affected by dementia worldwide. With aging populations and 7.7 million new cases each year, the burden of illness due to dementia approaches crisis proportions. Despite significant advances in our understanding of the biology of Alzheimer's disease (AD), the leading dementia diagnosis, the actual causes of dementia in affected individuals are unknown except for rare fully penetrant genetic forms. Evidence from epidemiology and pathology studies indicates that damage to the vascular system is associated with an increased risk of many types of dementia. Both Alzheimer's pathology and cerebrovascular disease increase with age. How AD affects small blood vessel function and how vascular dysfunction contributes to the molecular pathology of Alzheimer's are areas of intense research. The science of vascular contributions to cognitive impairment and dementia (VCID) integrates diverse aspects of biology and incorporates the roles of multiple cell types that support the function of neural tissue. Because of the proven ability to prevent and treat cardiovascular disease and hypertension with population benefits for heart and stroke outcomes, it is proposed that understanding and targeting the biological mechanisms of VCID can have a similarly positive impact on public health.

Modeling the human MTM1 p.R69C mutation in murine Mtm1 results in exon 4 skipping and a less severe myotubular myopathy phenotype.

X-linked myotubular myopathy (MTM) is a severe neuromuscular disease of infancy caused by mutations of MTM1, which encodes the phosphoinositide lipid phosphatase, myotubularin. The Mtm1 knockout (KO) mouse has a severe phenotype and its short lifespan (8 weeks) makes it a challenge to use as a model in the testing of certain preclinical therapeutics. Many MTM patients succumb early in life, but some have a more favorable prognosis. We used human genotype-phenotype correlation data to develop a myotubularin-deficient mouse model with a less severe phenotype than is seen in Mtm1 KO mice. We modeled the human c.205C>T point mutation in Mtm1 exon 4, which is predicted to introduce the p.R69C missense change in myotubularin. Hemizygous male Mtm1 p.R69C mice develop early muscle atrophy prior to the onset of weakness at 2 months. The median survival period is 66 weeks. Histopathology shows small myofibers with centrally placed nuclei. Myotubularin protein is undetectably low because the introduced c.205C>T base change induced exon 4 skipping in most mRNAs, leading to premature termination of myotubularin translation. Some full-length Mtm1 mRNA bearing the mutation is present, which provides enough myotubularin activity to account for the relatively mild phenotype, as Mtm1 KO and Mtm1 p.R69C mice have similar muscle phosphatidylinositol 3-phosphate levels. These data explain the basis for phenotypic variability among human patients with MTM1 p.R69C mutations and establish the Mtm1 p.R69C mouse as a valuable model for the disease, as its less severe phenotype will expand the scope of testable preclinical therapies.

A humanized Smn gene containing the SMN2 nucleotide alteration in exon 7 mimics SMN2 splicing and the SMA disease phenotype.

Proximal spinal muscular atrophy (SMA) is a neurodegenerative disease caused by low levels of the survival motor neuron (SMN) protein. In humans, SMN1 and SMN2 encode the SMN protein. In SMA patients, the SMN1 gene is lost and the remaining SMN2 gene only partially compensates. Mediated by a C>T nucleotide transition in SMN2, the inefficient recognition of exon 7 by the splicing machinery results in low levels of SMN. Because the SMN2 gene is capable of expressing SMN protein, correction of SMN2 splicing is an attractive therapeutic option. Although current mouse models of SMA characterized by Smn knock-out alleles in combination with SMN2 transgenes adequately model the disease phenotype, their complex genetics and short lifespan have hindered the development and testing of therapies aimed at SMN2 splicing correction. Here we show that the mouse and human minigenes are regulated similarly by conserved elements within in exon 7 and its downstream intron. Importantly, the C>T mutation is sufficient to induce exon 7 skipping in the mouse minigene as in the human SMN2. When the mouse Smn gene was humanized to carry the C>T mutation, keeping it under the control of the endogenous promoter, and in the natural genomic context, the resulting mice exhibit exon 7 skipping and mild adult onset SMA characterized by muscle weakness, decreased activity and an alteration of the muscle fibers size. This Smn C>T mouse represents a new model for an adult onset form of SMA (type III/IV) also know as the Kugelberg-Welander disease.

Generation of a tamoxifen inducible SMN mouse for temporal SMN replacement.

Proximal spinal muscular atrophy (SMA) is caused by low levels of the SMN protein, encoded by the Survival Motor Neuron genes (SMN1 and SMN2). Mouse models of SMA can be rescued by increased SMN expression, but the timing of SMN replacement for complete rescue is unknown. Studies in zebrafish predict restoration of SMN function during embryogenesis may be important for axonal pathfinding, while the mouse models and normal human disease progression suggest that post-natal treatment may be sufficient for amelioration of disease. To evaluate the timing for SMN replacement, we have generated a stably integrated Cre-inducible SMN mouse in which expression of full-length SMN2 occurs after tamoxifen administration. Our temporally inducible SMN transgene is able to express SMN in embryonic, neonatal, and weanling mice and as such can be utilized in severe and mild SMA mouse models to identify the therapeutic window for SMN replacement.

Intron 7 conserved sequence elements regulate the splicing of the SMN genes.

Proximal spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of the survival motor neuron (SMN) protein. In humans there are two nearly identical SMN genes, SMN1 and SMN2. The SMN2 gene generates a truncated protein, due to a C to T nucleotide alteration in exon 7, which leads to inefficient RNA splicing of exon 7. This exclusion of SMN exon 7 is central to the onset of the SMA disease. Exon 7 splicing is regulated by a number of exonic and intronic splicing regulatory sequences and the trans-factors that bind them. Here, we identify conserved intronic sequences in the SMN genes. Five regions were examined due to conservation and their proximity to exons 6 through 8. Using mutagenesis two conserved elements located in intron 7 of the SMN genes that affect exon 7 splicing have been identified. Additional analysis of one of these regions showed decreased inclusion of exon 7 in SMN transcripts when deletions or mutations were introduced. Furthermore, multimerization of this conserved region was capable of restoring correct SMN splicing. Together these results describe a novel intronic splicing enhancer sequence located in the final intron of the SMN genes. This discovery provides insight into the splicing of the SMN genes using conserved intonic sequence as a tool to uncover regions of importance in pre-messenger RNA splicing. A better understanding of the way SMN premRNA is spliced can lead to the development of new therapies.

Vascular contributions to cognitive impairment and dementia: Research consortia that focus on etiology and treatable targets to lessen the burden of dementia worldwide.

The research into vascular contributions to cognitive impairment and dementia (VCID) aims to understand the importance of cerebrovascular biology in cognitive decline. Prevention and treatment of VCID is poised to have major impact on dementia-related disease burden and is thus a critical emerging objective in dementia research. This article presents VCID consortia focused on multidisciplinary approaches to identify key pathologic targets and develop diagnostic tools with the goal of bridging the divide between basic research and clinical trials. Members of these multi-institute, multidisciplinary consortia provide a prospective on the history and emerging science of VCID and how VCID consortia can address some of the more complex questions in VCID and drive the field forward. These consortia, and others like them, are uniquely suited to tackle some of the most difficult obstacles in translating research to the clinic.

Alzheimer's Disease-Related Dementias Summit 2016: National research priorities.

Goal 1 of the National Plan to Address Alzheimer's Disease is to prevent and effectively treat Alzheimer disease and Alzheimer disease-related dementias by 2025. To help inform the research agenda toward achieving this goal, the NIH hosts periodic summits that set and refine relevant research priorities for the subsequent 5 to 10 years. This proceedings article summarizes the 2016 Alzheimer's Disease-Related Dementias Summit, including discussion of scientific progress, challenges, and opportunities in major areas of dementia research, including mixed-etiology dementias, Lewy body dementia, frontotemporal degeneration, vascular contributions to cognitive impairment and dementia, dementia disparities, and dementia nomenclature.

TWEAK Regulates Muscle Functions in a Mouse Model of RNA Toxicity.

Myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults, is caused by toxic RNAs produced from the mutant DM protein kinase (DMPK) gene. DM1 is characterized by progressive muscle wasting and weakness. Therapeutic strategies have mainly focused on targeting the toxic RNA. Previously, we found that fibroblast growth factor-inducible 14 (Fn14), the receptor for TWEAK, is induced in skeletal muscles and hearts of mouse models of RNA toxicity and that blocking TWEAK/Fn14 signaling improves muscle function and histology. Here, we studied the effect of Tweak deficiency in a RNA toxicity mouse model. The genetic deletion of Tweak in these mice significantly reduced muscle damage and improved muscle function. In contrast, administration of TWEAK in the RNA toxicity mice impaired functional outcomes and worsened muscle histopathology. These studies show that signaling via TWEAK is deleterious to muscle in RNA toxicity and support the demonstrated utility of anti-TWEAK therapeutics.

Development of an AP-FRET based analysis for characterizing RNA-protein interactions in myotonic dystrophy (DM1).

Förster Resonance Energy Transfer (FRET) microscopy is a powerful tool used to identify molecular interactions in live or fixed cells using a non-radiative transfer of energy from a donor fluorophore in the excited state to an acceptor fluorophore in close proximity. FRET can be a very sensitive tool to study protein-protein and/or protein-nucleic acids interactions. RNA toxicity is implicated in a number of disorders; especially those associated with expanded repeat sequences, such as myotonic dystrophy. Myotonic dystrophy (DM1) is caused by a (CTG)n repeat expansion in the 3' UTR of the DMPK gene which results in nuclear retention of mutant DMPK transcripts in RNA foci. This results in toxic gain-of-function effects mediated through altered functions of RNA-binding proteins (e.g. MBNL1, hnRNPH, CUGBP1). In this study we demonstrate the potential of a new acceptor photobleaching assay to measure FRET (AP-FRET) between RNA and protein. We chose to focus on the interaction between MBNL1 and mutant DMPK mRNA in cells from DM1 patients due to the strong microscopic evidence of their co-localization. Using this technique we have direct evidence of intracellular interaction between MBNL1 and the DMPK RNA. Furthermore using the AP-FRET assay and MBNL1 mutants, we show that all four zinc-finger motifs in MBNL1 are crucial for MBNL1-RNA foci interactions. The data derived using this new assay provides compelling evidence for the interaction between RNA binding proteins and RNA foci, and mechanistic insights into MBNL1-RNA foci interaction demonstrating the power of AP-FRET in examining RNA-Protein interactions in DM1.

Age of onset of RNA toxicity influences phenotypic severity: evidence from an inducible mouse model of myotonic dystrophy (DM1).

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy in adults. It is caused by an expanded (CTG)n tract in the 3' UTR of the Dystrophia Myotonica Protein Kinase (DMPK) gene. This causes nuclear retention of the mutant mRNA into ribonuclear foci and sequestration of interacting RNA-binding proteins (such as muscleblind-like 1 (MBNL1)). More severe congenital and childhood-onset forms of the disease exist but are less understood than the adult disease, due in part to the lack of adequate animal models. To address this, we utilized transgenic mice over-expressing the DMPK 3' UTR as part of an inducible RNA transcript to model early-onset myotonic dystrophy. In mice in which transgene expression was induced during embryogenesis, we found that by two weeks after birth, mice reproduced cardinal features of myotonic dystrophy, including myotonia, cardiac conduction abnormalities, muscle weakness, histopathology and mRNA splicing defects. Notably, these defects were more severe than in adult mice induced for an equivalent period of exposure to RNA toxicity. Additionally, the utility of the model was tested by over-expressing MBNL1, a key therapeutic strategy being actively pursued for treating the disease phenotypes associated with DM1. Significantly, increased MBNL1 in skeletal muscle partially corrected myotonia and splicing defects present in these mice, demonstrating the responsiveness of the model to relevant therapeutic interventions. Furthermore, these results also represent the first murine model for early-onset DM1 and provide a tool to investigate the effects of RNA toxicity at various stages of development.

Stress-induced isoforms of MDM2 and MDM4 correlate with high-grade disease and an altered splicing network in pediatric rhabdomyosarcoma.

Pediatric rhabdomyosarcoma (RMS) is a morphologically and genetically heterogeneous malignancy commonly classified into three histologic subtypes, namely, alveolar, embryonal, and anaplastic. An issue that continues to challenge effective RMS patient prognosis is the dearth of molecular markers predictive of disease stage irrespective of tumor subtype. Our study involving a panel of 70 RMS tumors has identified specific alternative splice variants of the oncogenes Murine Double Minute 2 (MDM2) and MDM4 as potential biomarkers for RMS. Our results have demonstrated the strong association of genotoxic-stress inducible splice forms MDM2-ALT1 (91.6% Intergroup Rhabdomyosarcoma Study Group stage 4 tumors) and MDM4-ALT2 (90.9% MDM4-ALT2-positive T2 stage tumors) with high-risk metastatic RMS. Moreover, MDM2-ALT1-positive metastatic tumors belonged to both the alveolar (50%) and embryonal (41.6%) subtypes, making this the first known molecular marker for high-grade metastatic disease across the most common RMS subtypes. Furthermore, our results show that MDM2-ALT1 expression can function by directly contribute to metastatic behavior and promote the invasion of RMS cells through a matrigel-coated membrane. Additionally, expression of both MDM2-ALT1 and MDM4-ALT2 increased anchorage-independent cell-growth in soft agar assays. Intriguingly, we observed a unique coordination in the splicing of MDM2-ALT1 and MDM4-ALT2 in approximately 24% of tumor samples in a manner similar to genotoxic stress response in cell lines. To further explore splicing network alterations with possible relevance to RMS disease, we used an exon microarray approach to examine stress-inducible splicing in an RMS cell line (Rh30) and observed striking parallels between stress-responsive alternative splicing and constitutive splicing in RMS tumors.

Splicing of the Survival Motor Neuron genes and implications for treatment of SMA

Proximal spinal muscular atrophy (SMA) is a neuromuscular disease caused by low levels of the survival motor neuron (SMN) protein. The reduced SMN levels are due to loss of the survival motor neuron-1 (SMN1) gene. Humans carry a nearly identical SMN2 gene that generates a truncated protein, due to a C to T nucleotide alteration in exon 7 that leads to inefficient RNA splicing of exon 7. This exclusion of SMN exon 7 is central to the onset of the SMA disease, however, this offers a unique therapeutic intervention in which corrective splicing of the SMN2 gene would restore SMN function. Exon 7 splicing is regulated by a number of exonic and intronic splicing regulatory sequences and trans-factors that bind them. A better understanding of the way SMN pre-mRNA is spliced has lead to the development of targeted therapies aimed at correcting SMN2 splicing. As therapeutics targeted toward correction of SMN2 splicing continue to be developed available SMA mouse models can be utilized in validating their potential in disease treatment.