Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P06889 (Mol)
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Spinal muscular atrophy (SMA) is a lethal hereditary disease caused by homozygous deletion/inactivation of the survival of motoneuron 1 (SMN1) gene. The nearby SMN2 gene, despite its identical coding capacity, is only an incomplete substitute, because a single nucleotide difference impairs the inclusion of its seventh exon in the messenger RNA (mRNA). This splicing defect can be corrected (transiently) by specially designed oligonucleotides. Here we have developed a more permanent correction strategy based on bifunctional U7 small nuclear RNAs (snRNAs). These carry both an antisense sequence that allows specific binding to exon 7 and a splicing enhancer sequence that will improve the recognition of the targeted exon. When expression cassettes for these RNAs are stably introduced into cells, the U7 snRNAs become incorporated into small nuclear ribonucleoprotein (snRNP) particles that will induce a durable splicing correction. We have optimized this strategy to the point that virtually all SMN2 pre-mRNA becomes correctly spliced. In fibroblasts from an SMA patient, this approach induces a prolonged restoration of SMN protein and ensures its correct localization to discrete nuclear foci (gems).
Mol Ther 2007 Aug
PMID:Spinal muscular atrophy: SMN2 pre-mRNA splicing corrected by a U7 snRNA derivative carrying a splicing enhancer sequence. 1750 71

Spinal muscular atrophy (SMA) is caused by loss of survival motor neuron-1 (SMN1). A nearly identical copy gene called SMN2 is present in all SMA patients; however SMN2 produces low levels of functional protein due to alternative splicing. Recently a therapeutic approach has been developed referred to as trans-splicing. Conceptually, this strategy relies upon pre-messenger RNA (pre-mRNA) splicing occurring between two separate molecules: (i) the endogenous target RNA and (ii) the therapeutic RNA that provides the correct RNA sequence via a trans-splicing event. SMN trans-splicing RNAs were initially examined and expressed from a plasmid-backbone and shown to re-direct splicing from a SMN2 mini-gene as well as from endogenous transcripts. Subsequently, recombinant adeno-associated viral vectors were developed that expressed and delivered trans-splicing RNAs to SMA patient fibroblasts. In the severe SMA patient fibroblasts, SMN2 splicing was redirected via trans-splicing to produce increased levels of full-length SMN mRNA and total SMN protein levels. Finally, small nuclear ribonucleoprotein (snRNP) assembly, a critical function of SMN, was restored to SMN-deficient SMA fibroblasts following treatment with the trans-splicing vector. Together these results demonstrate that the alternatively spliced SMN2 exon 7 is a tractable target for replacement by trans-splicing.
Mol Ther 2007 Aug
PMID:Restoration of SMN function: delivery of a trans-splicing RNA re-directs SMN2 pre-mRNA splicing. 1755 1

Homozygous deletion or mutation of the survival of motor neuron 1 gene (SMN1) causes spinal muscular atrophy. SMN1 has been duplicated in humans to create SMN2, which produces a low level of functional SMN protein. However, most SMN2 transcripts lack exon 7, resulting in a non-functional protein. A single nucleotide difference near the 5' end of exon 7 largely accounts for SMN2 exon 7 skipping, an effect that has been attributed to loss of an exonic splicing enhancer (ESE) dependent on the SR protein splicing factor ASF/SF2 or to the creation of an exonic splicing silencer (ESS) element that functions by binding of the splicing repressor hnRNP A1. Our earlier experiments favored the latter mechanism and here we provide further evidence supporting the ESS model. We demonstrate that the striking effect of hnRNP A1 depletion on SMN2 exon 7 splicing is specific, as hnRNP A1 depletion has little or no effect on other inefficient splicing events tested, and ASF/SF2 depletion does not affect SMN1/2 splicing. By two different methods, we find a strong and specific interaction of hnRNPA1 with SMN2 exon 7 and only weak and equivalent interactions between ASF/SF2 and other SR proteins with the 5' ends of SMN1 and SMN2 exon 7. Finally, we describe two disease-related exon-skipping mutations that create hnRNP A1 binding sites, but show that splicing can be restored only modestly or not at all by hnRNP A1 depletion. Together our results provide strong support for the idea that SMN2 exon 7 splicing is repressed by an hnRNPA1-dependent ESS, but also indicate that creation of such elements is context-dependent.
Hum Mol Genet 2007 Dec 15
PMID:hnRNP A1 functions with specificity in repression of SMN2 exon 7 splicing. 1788 7

Proximal spinal muscular atrophy (SMA) is a common autosomal recessive childhood form of motor neuron disease. Previous studies have highlighted nerve- and muscle-specific events in SMA, including atrophy of muscle fibres and post-synaptic motor endplates, loss of lower motor neuron cell bodies and denervation of neuromuscular junctions caused by loss of pre-synaptic inputs. Here we have undertaken a detailed morphological investigation of neuromuscular synaptic pathology in the Smn-/-;SMN2 and Smn-/-;SMN2;Delta7 mouse models of SMA. We show that neuromuscular junctions in the transversus abdominis (TVA), levator auris longus (LAL) and lumbrical muscles were disrupted in both mouse models. Pre-synaptic inputs were lost and abnormal accumulations of neurofilament were present, even in early/mid-symptomatic animals in the most severely affected muscle groups. Neuromuscular pathology was more extensive in the postural TVA muscle compared with the fast-twitch LAL and lumbrical muscles. Pre-synaptic pathology in Smn-/-;SMN2;Delta7 mice was reduced compared with Smn-/-;SMN2 mice at late-symptomatic time-points, although post-synaptic pathology was equally severe. We demonstrate that shrinkage of motor endplates does not correlate with loss of motor nerve terminals, signifying that one can occur in the absence of the other. We also demonstrate selective vulnerability of a subpopulation of motor neurons in the caudal muscle band of the LAL. Paralysis with botulinum toxin resulted in less terminal sprouting and ectopic synapse formation in the caudal band compared with the rostral band, suggesting that motor units conforming to a Fast Synapsing (FaSyn) phenotype are likely to be more vulnerable than those with a Delayed Synapsing (DeSyn) phenotype.
Hum Mol Genet 2008 Apr 01
PMID:Selective vulnerability of motor neurons and dissociation of pre- and post-synaptic pathology at the neuromuscular junction in mouse models of spinal muscular atrophy. 1806 80

Spinal muscular atrophy (SMA) is caused by loss of the survival motor neuron gene (SMN1) and retention of the SMN2 gene. The copy number of SMN2 affects the amount of SMN protein produced and the severity of the SMA phenotype. While loss of mouse Smn is embryonic lethal, two copies of SMN2 prevents this embryonic lethality resulting in a mouse with severe SMA that dies 5 days after birth. Here we show that expression of full-length SMN under the prion promoter (PrP) rescues severe SMA mice. The PrP results in high levels of SMN in neurons at embryonic day 15. Mice homozygous for PrP-SMN with two copies of SMN2 and lacking mouse Smn survive for an average of 210 days and lumbar motor neuron root counts in these mice were normal. Expression of SMN solely in skeletal muscle using the human skeletal actin (HSA) promoter resulted in no improvement of the SMA phenotype or extension of survival. One HSA line displaying nerve expression of SMN did affect the SMA phenotype with mice living for an average of 160 days. Thus, we conclude that expression of full-length SMN in neurons can correct the severe SMA phenotype in mice. Furthermore, a small increase of SMN in neurons has a substantial impact on survival of SMA mice while high SMN levels in mature skeletal muscle alone has no impact.
Hum Mol Genet 2008 Apr 15
PMID:Neuronal SMN expression corrects spinal muscular atrophy in severe SMA mice while muscle-specific SMN expression has no phenotypic effect. 1817 76

Spinal muscular atrophy (SMA) is characterized by selective loss of alpha-motor neurons and is caused by homozygous loss or mutation in the survival motor neuron (SMN1) gene. Loss of SMN1 is partially compensated by the copy gene, SMN2. Currently, there are no specific treatments for SMA. Key features of SMA are modeled in mice by deletion of murine Smn, and insertion of both full length human SMN2 gene and the major aberrant splice isoform of the SMN2 gene (SMNDelta7; [Le, T.T., Pham, L.T., Butchbach, M.E., Zhang, H.L., Monani, U.R., Coovert, D.D., Gavrilina, T.O., Xing, L., Bassell, G.J., and Burghes, A.H. 2005. SMNDelta7, the major product of the centromeric survival motor neuron (SMN2) gene, extends survival in mice with spinal muscular atrophy and associates with full-length SMN. Hum Mol Genet 14: 845-857]). The present study identified moderate-throughput, quantitative behavioral tests in neonatal SMN2(+/+);SMNDelta7(+/+);Smn(-/-) mice. It also addresses methodological approaches and common interpretational challenges in a neonatal model with motor deficiencies and frequent deaths. Animals were assessed daily for body weight and survival, and every other day for neonatal well-being indices and tests of motor function such as performance on the hind-limb suspension test (a.k.a. tube test) and geotaxis. The tube test is a novel non-invasive motor function test specifically designed for neonatal rodents. We found progressive deterioration in SMA model mice for most measures studied particularly body weight, survival, body temperature and motor function with differences appearing as early as P3. Power analysis showed that body weight, survival, righting reflex, geotaxis and tube test had highest predictive power for drug efficacy studies. This multi-functional component battery of tests provides a rapid and efficient means to identify, evaluate and develop candidate therapies as a prelude to human clinical trials.
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PMID:Identification of a battery of tests for drug candidate evaluation in the SMNDelta7 neonate model of spinal muscular atrophy. 1845 59

Spinal muscular atrophy (SMA) is a common pediatric neuromuscular disorder caused by insufficient levels of the survival of motor neuron (SMN) protein. Studies involving SMA patients and animal models expressing the human SMN2 gene have yielded relatively little information about the earliest cellular consequences of reduced SMN protein. In this study, we have used severe- and mild-SMN2 expressing mouse models of SMA as well as material from human patients to understand the initial stages of neurodegeneration in the human disease. We show that the earliest structural defects appear distally and involve the neuromuscular synapse. Insufficient SMN protein arrests the post-natal development of the neuromuscular junction (NMJ), impairing the maturation of acetylcholine receptor (AChR) clusters into 'pretzels'. Pre-synaptic defects include poor terminal arborization and intermediate filament aggregates which may serve as a useful biomarker of the disease. These defects are reflected in functional deficits at the NMJ characterized by intermittent neurotransmission failures. We suggest that SMA might best be described as a NMJ synaptopathy and that one promising means of treating it could involve maintaining function at the NMJ.
Hum Mol Genet 2008 Aug 15
PMID:Reduced SMN protein impairs maturation of the neuromuscular junctions in mouse models of spinal muscular atrophy. 1849

Spinal muscular atrophy (SMA) is a progressive disease involving the degeneration of motor neurons with no currently available treatment. While valproic acid (VPA) is a potential treatment for SMA, its therapeutic mechanisms are still controversial. In this study, we investigated the mechanisms of action of VPA in the treatment of type III-like SMA mice. SMA and wild-type mice were treated with VPA from 6 to 12 months and 10 to 12 months of age, respectively. Untreated SMA littermates and age-matched wild-type mice were used for comparison. VPA-treated SMA mice showed better motor function, larger motor-evoked potentials, less degeneration of spinal motor neurons, less muscle atrophy, and better neuromuscular junction innervation than non-treated SMA mice. VPA elevated SMN protein levels in the spinal cord through SMN2 promoter activation and probable restoration of correct splicing of SMN2 pre-messenger RNA. VPA also increased levels of anti-apoptotic factors, Bcl-2 and Bcl-x(L), in spinal neurons. VPA probably induced neurogenesis and promoted astrocyte proliferation in the spinal cord of type III-like SMA mice, which might contribute to therapeutic effects by enhancing neuroprotection. Through these effects of elevation of SMN protein level, anti-apoptosis, and probable neuroprotection, VPA-treated SMA mice had less degeneration of spinal motor neurons and better motor function than untreated type III-like SMA mice.
J Mol Med (Berl) 2008 Nov
PMID:Multiple therapeutic effects of valproic acid in spinal muscular atrophy model mice. 1864 67

Spinal muscular atrophy (SMA) is caused by homozygous deletions/mutations of SMN1 gene. All SMA patients carry a nearly identical SMN2 gene. A nucleotide change in SMN2 results in exon 7 exclusion in the majority of SMN2 mRNA, thus producing low level of SMN protein. Extracellular pH change has been shown to modulate alternative splicing of several pre-mRNAs. In this study, we showed that extracellular pH change can also modulate SMN2 exon 7 splicing in SMA cells. Low extracellular pH enhances SMN2 exon 7 skipping, whereas high extracellular pH promotes its inclusion. Low extracellular pH also reduces SMN protein expression but increases hnRNP A1 expression. In addition, we tested whether intracellular pH-modulating genes could be the modifier of SMA in a SMA discordant family and found that the mRNA levels of ATP6V1B2 gene are significantly higher in two affected siblings than the unaffected one. In conclusion, our results suggest that extracellular pH change modulates SMN2 exon 7 splicing through regulation of hnRNP A1 expression in SMA cells.
Mol Cell Neurosci 2008 Oct
PMID:Extracellular pH change modulates the exon 7 splicing in SMN2 mRNA. 1867 65

Spinal muscular atrophy (SMA) is a recessive neuromuscular disorder caused by the homozygous loss of the SMN1 gene. The human SMN2 gene has a C-to-T transition at position +6 of exon 7 and thus produces exon 7-skipping mRNAs. However, we observed an unexpectedly high level of exon 7-containing SMN2 transcripts as well as SMN protein in testis of smn(-/-) SMN2 transgenic mice. Using affinity chromatography, we identified several SMN RNA-associating proteins in mouse testis and human HeLa cells, including hnRNP Q. The major hnRNP Q isoform, Q1, directly bound SMN exon 7 in the vicinity of nucleotide +6. Overexpression of hnRNP Q1 promoted the inclusion of exon 7 in SMN2, probably by activating the use of its upstream 3' splice site. However, the minor isoforms Q2/Q3 could antagonize the activity of hnRNP Q1 and induced exon 7 exclusion. Intriguingly, enhanced exon 7 inclusion was also observed upon concomitant depletion of three hnRNP Q isoforms. Thus, differential expression of hnRNP Q isoforms may result in intricate control of SMN precursor mRNA splicing. Here, we demonstrate that hnRNP Q is a splicing modulator of SMN, further underscoring the potential of hnRNP Q as a therapeutic target for SMA.
Mol Cell Biol 2008 Nov
PMID:The RNA binding protein hnRNP Q modulates the utilization of exon 7 in the survival motor neuron 2 (SMN2) gene. 1879 68


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