Induced pluripotent stem cells from a spinal muscular atrophy patient

Spinal muscular atrophy is one of the most common inherited forms of neurological disease leading to infant mortality. Patients have selective loss of lower motor neurons resulting in muscle weakness, paralysis and often death. Although patient fibroblasts have been used extensively to study spinal muscular atrophy, motor neurons have a unique anatomy and physiology which may underlie their vulnerability to the disease process. Here we report the generation of induced pluripotent stem cells from skin fibroblast samples taken from a child with spinal muscular atrophy. These cells expanded robustly in culture, maintained the disease genotype and generated motor neurons that showed selective deficits compared to those derived from the child's unaffected mother. This is the first study to show that human induced pluripotent stem cells can be used to model the specific pathology seen in a genetically inherited disease. As such, it represents a promising resource to study disease mechanisms, screen new drug compounds and develop new therapies.

Delivery of recombinant follistatin lessens disease severity in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality. SMA is caused by loss of functional survival motor neuron 1 (SMN1), resulting in death of spinal motor neurons. Current therapeutic research focuses on modulating the expression of a partially functioning copy gene, SMN2, which is retained in SMA patients. However, a treatment strategy that improves the SMA phenotype by slowing or reversing the skeletal muscle atrophy may also be beneficial. Myostatin, a member of the TGF-beta super-family, is a potent negative regulator of skeletal muscle mass. Follistatin is a natural antagonist of myostatin, and over-expression of follistatin in mouse muscle leads to profound increases in skeletal muscle mass. To determine whether enhanced muscle mass impacts SMA, we administered recombinant follistatin to an SMA mouse model. Treated animals exhibited increased mass in several muscle groups, elevation in the number and cross-sectional area of ventral horn cells, gross motor function improvement and mean lifespan extension by 30%, by preventing some of the early deaths, when compared with control animals. SMN protein levels in spinal cord and muscle were unchanged in follistatin-treated SMA mice, suggesting that follistatin exerts its effect in an SMN-independent manner. Reversing muscle atrophy associated with SMA may represent an unexploited therapeutic target for the treatment of SMA.

Identification and characterization of the porcine (Sus scrofa) survival motor neuron (SMN1) gene: an animal model for therapeutic studies

Spinal muscular atrophy (SMA) is an autosomal recessive disorder that is characterized by the degeneration of the motor neurons of the spinal cord leading to muscle atrophy. SMA is a result of a loss-of-function of the gene survival motor neuron-1 (SMN1). We have chosen to generate a transgenic swine model of SMA for the development and testing of therapeutics and evaluation of toxicology. To this end, we report the first cloning and identification of the swine SMN1 gene and show that there is significant sequence homology between swine and human SMN throughout the coding region. Reverse transcriptase-polymerase chain reaction results demonstrated slight changes in SMN RNA expression during development and in different tissues. In contrast, protein expression profiles were dramatically different based upon different tissues and developmental stages, consistent with human SMN expression. Porcine SMN localization is consistent with human SMN, localizing diffusely within the cytoplasm and in punctate nuclear structures characteristic of nuclear gems. Importantly, transient transfection of porcine SMN1 in 3813 SMA type 1 fibroblasts demonstrate that porcine SMN1 can rescue the deficiency of SMN protein and gem formation in these cells. These studies provide the first characterization of the porcine SMN1 gene and SMN protein and suggest that a transgenic swine SMA model is feasible.

The Wallerian degeneration slow (Wld(s)) gene does not attenuate disease in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a severe neuromuscular disease characterized by loss of spinal alpha-motor neurons, resulting in the paralysis of skeletal muscle. SMA is caused by deficiency of survival motor neuron (SMN) protein levels. Recent evidence has highlighted an axon-specific role for SMN protein, raising the possibility that axon degeneration may be an early event in SMA pathogenesis. The Wallerian degeneration slow (Wld(s)) gene is a spontaneous dominant mutation in mice that delays axon degeneration by approximately 2-3 weeks. We set out to examine the effect of Wld(s) on the phenotype of a mouse model of SMA. We found that Wld(s) does not alter the SMA phenotype, indicating that Wallerian degeneration does not directly contribute to the pathogenesis of SMA development.

Identification and characterisation of a nuclear localisation signal in the SMN associated protein, Gemin4

Gemin4 is a ubiquitously expressed multifunctional protein that is involved in U snRNP assembly, apoptosis, nuclear/cytoplasmic transportation, transcription, and RNAi pathways. Gemin4 is one of the core components of the Gemin-complex, which also contains survival motor neuron (SMN), the seven Gemin proteins (Gemin2-8), and Unrip. Mutations in the SMN1 gene cause the autosomal recessive disorder spinal muscular atrophy (SMA). Although the functions assigned to Gemin4 predominantly occur in the nucleus, the mechanisms that mediate the nuclear import of Gemin4 remain unclear. Here, using a novel panel of Gemin4 constructs we identify a canonical nuclear import sequence (NLS) in the N-terminus of Gemin4. The Gemin4 NLS is necessary and independently sufficient to mediate nuclear import of Gemin4. This is the first functional NLS identified within the SMN-Gemin complex.

A SMNDelta7 read-through product confers functionality to the SMNDelta7 protein

Spinal muscular atrophy (SMA) affects about 1 in every 6000 children born and is the leading genetic cause of infant death. SMA is a recessive disorder caused by the mutation or deletion of Survival Motor Neuron-1 (SMN1). SMN2, a nearly identical copy gene, has the potential to encode the same protein as SMN1 and is retained in all SMA patients. The majority of SMN2-derived transcripts are alternatively spliced and therefore encode a truncated isoform lacking exon 7 (SMNDelta7), which is a defective protein because it is unstable, has a reduced ability to self-associate and is unable to efficiently function in SMN cellular activities. However, we have shown that the SMN C-terminus functions non-specifically, since heterologous sequences can compensate for the exon 7 sequence. Several classes of compounds identified in SMN-inducing high throughput screens have been proposed to function through a read-through mechanism; however, a functional analysis of the SMNDelta7 read-through product has not been performed. In this report, the SMNDelta7 read-through product is characterized and compared to the SMNDelta7 protein. In a series of in vitro and cell based assays, SMNDelta7 read-through product is shown to increase protein stability, promote neurite outgrowths in SMN deficient neurons, and significantly elevate SMN-dependent UsnRNP assembly in extracts from SMA patient fibroblasts. Collectively, these results demonstrate that SMNDelta7 read-through product is more active than the SMNDelta7 protein and suggest that SMA therapeutics that specifically induce SMNDelta7 read-through may provide an alternative platform for drug discovery.

Induction of full-length survival motor neuron by polyphenol botanical compounds

The loss of survival motor neuron-1 (SMN1) is responsible for the development of the neurodegenerative disorder spinal muscular atrophy (SMA). A nearly identical copy of SMN1 is present on the same chromosomal region called SMN2. While SMN2 encodes a normal SMN protein, the majority of SMN2-derived transcripts are alternatively spliced, resulting in a truncated protein that lacks the 16 amino acids encoded by SMN exon 7. Numerous studies have shown that the SMN2-derived protein product, called SMNDelta7, is unstable and dysfunctional. Therefore, identifying molecules that stimulate full-length SMN expression from the SMN2 gene could lead to the development of effective therapies for a broad range of SMA patient populations. Polyphenol compounds have been shown to provide benefit in varied genetic disease contexts. For example, epigallocatechin galate (EGCG) was found to correct aberrant alternative mRNA splicing in familiar dysautonomia (FD). A series of polyphenols were screened and a subset was shown to increase full-length SMN expression from SMN2. Curcumin, EGCG, and resveratrol increased exon 7 inclusion of SMN2 transcripts in transient reporter assays. In SMA patient fibroblasts, these compounds stimulated the production of full-length SMN RNA and protein as well as the formation of SMN-containing nuclear gems. Collectively, these compounds elevated total SMN concentrations in SMA patient fibroblasts, potentially through the modulation of SMN2 exon 7 alternative splicing.

Protein phosphatase 1 binds to the RNA recognition motif of several splicing factors and regulates alternative pre-mRNA processing

Alternative splicing emerges as one of the most important mechanisms to generate transcript diversity. It is regulated by the formation of protein complexes on pre-mRNA. We demonstrate that protein phosphatase 1 (PP1) binds to the splicing factor transformer2-beta1 (tra2-beta1) via a phylogenetically conserved RVDF sequence located on the RNA recognition motif (RRM) of tra2-beta1. PP1 binds directly to tra2-beta1 and dephosphorylates it, which regulates the interaction between tra2-beta1 and other proteins. Eight other proteins, including SF2/ASF and SRp30c, contain an evolutionary conserved PP1 docking motif in the beta-4 strand of their RRMs indicating that binding to PP1 is a new function of some RRMs. Reducing PP1 activity promotes usage of numerous alternative exons, demonstrating a role of PP1 activity in splice site selection. PP1 inhibition promotes inclusion of the survival of motoneuron 2 exon 7 in a mouse model expressing the human gene. This suggests that reducing PP1 activity could be a new therapeutic principle to treat spinal muscular atrophy and other diseases caused by missplicing events. Our data indicate that the binding of PP1 to evolutionary conserved motifs in several RRMs is the link between known signal transduction pathways regulating PP1 activity and pre-mRNA processing.

Restoration of SMN function: delivery of a trans-splicing RNA re-directs SMN2 pre-mRNA splicing

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.

Novel aminoglycosides increase SMN levels in spinal muscular atrophy fibroblasts

Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality. SMA is caused by the homozygous absence of survival motor neuron-1 (SMN1). SMN2, a nearly identical copy gene, is retained in all SMA patients and encodes an identical protein as SMN1; however, SMN1 and SMN2 differ by a silent C to T transition which results in the production of an alternatively spliced isoform (SMNDelta7), which encodes a defective protein, demonstrating that the absence of the short peptide encoded by SMN exon 7 is critical in SMA development. Previously, we have shown that for some functions heterologous sequences can compensate for the exon 7 peptide, suggesting that the SMN C-terminus functions non-specifically. Consistent with this hypothesis, we now identify novel aminoglycosides that can induce SMN protein levels in patient fibroblasts. This hypothesis was supported, in part, by a novel fluorescent SMN read-through assay. Interestingly, however, through the development of a SMN exon 7-specific antibody, results suggested that levels of normal full-length SMN might also be elevated by aminoglycoside treatment. These results demonstrate that the compounds that promote read-through may provide an alternative platform for the discovery of compounds that induce SMN protein levels.

Stimulating full-length SMN2 expression by delivering bifunctional RNAs via a viral vector

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder that is the leading genetic cause of infant mortality. SMA is caused by the loss of survival motor neuron-1 (SMN1). In humans, a nearly identical copy gene is present, called SMN2. SMN2 is retained in all SMA patients and encodes an identical protein compared to SMN1. However, a single silent nucleotide difference in SMN2 exon 7 results in the production of a spliced isoform (called SMNDelta7) that encodes a nonfunctional protein. The presence of SMN2 represents a unique therapeutic target since SMN2 has the capacity to encode a fully functional protein. Here we describe an in vivo delivery system for short bifunctional RNAs that modulate SMN2 splicing. Bifunctional RNAs derive their name from the presence of two domains: an antisense RNA sequence specific to a target RNA and an untethered RNA segment that serves as a binding platform for splicing factors. Plasmid-based and recombinant adeno-associated virus vectors were developed that expressed bifunctional RNAs that stimulated SMN2 exon 7 inclusion and full-length SMN protein in patient fibroblasts. These experiments provide a mechanism to modulate splicing from a variety of genetic contexts and demonstrate directly a novel therapeutic approach for SMA.

Minute virus of mice small non-structural protein NS2 localizes within, but is not required for the formation of, Smn-associated autonomous parvovirus-associated replication bodies

The non-structural proteins NS1 and NS2 of the parvovirus minute virus of mice (MVM) are required for efficient virus replication. It has previously been shown that NS1 and NS2 interact and colocalize with the survival motor neuron (Smn) gene product in novel nuclear structures that are formed late in infection, termed Smn-associated APAR (autonomous parvovirus-associated replication) bodies (SAABs). It is not clear what molecular viral intermediate(s) contribute to SAAB formation. The current results address the role of NS2 in SAAB formation. In highly synchronized wild-type MVM infection of murine A9(2L) cells, NS2 colocalizes with Smn and other SAAB constituents. An MVM mutant that does not produce NS2 still generates SAABS, albeit with a temporal delay. The lag in SAAB formation seen in the absence of NS2 is probably related to the temporal delay in virus replication, suggesting that, whilst NS2 is required for efficient viral infection, it is dispensable for SAAB formation.

A non-sequence-specific requirement for SMN protein activity: the role of aminoglycosides in inducing elevated SMN protein levels

Spinal muscular atrophy (SMA) is caused by homozygous loss of the survival motor neuron (SMN1) gene. In virtually all SMA patients, a nearly identical copy gene is present, SMN2. SMN2 cannot fully compensate for the loss of SMN1 because the majority of transcripts derived from SMN2 lack a critical exon (exon 7), resulting in a dysfunctional SMN protein. Therefore, the critical distinction between a functional and a dysfunctional SMN protein is the inclusion or the exclusion of the exon 7 encoded peptide. To determine the role of the 16 amino acids encoded by SMN exon 7, a panel of synthetic mutations were transiently expressed in SMA patient fibroblasts and HeLa cells. Consistent with previous reports, the protein encoded by SMN exons 1-6 was primarily restricted to the nucleus. However, a variety of heterologous sequences fused to the C-terminus of SMN exons 1-6 allowed mutant SMN proteins to properly distribute to the cytoplasm and to the nuclear gems. These data demonstrate that the SMN exon 7 sequence is not specifically required, rather this region functions as a non-specific 'tail' that facilitates proper localization. Therefore, a possible means to restore additional activity to the SMNDelta7 protein could be to induce a longer C-terminus by suppressing recognition of the native stop codon. To address this possibility, aminoglycosides were examined for their ability to restore detectable levels of SMN protein in SMA patient fibroblasts. Aminoglycosides can suppress the accurate identification of translation termination codons in eukaryotic cells. Consistent with this, treatment of SMA patient fibroblasts with tobramycin and amikacin resulted in a quantitative increase in SMN-positive gems and an overall increase in detectable SMN protein. Taken together, this work describes the role of the critical exon 7 region and identifies a possible alternative approach for therapeutic intervention.

Monitoring of recombinant survival motor neuron protein using fiber-optic surface plasmon resonance

Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality. SMA is caused by the homozygous loss of the survival motor neuron 1 (SMN1) gene. A nearly identical copy gene exists known as SMN2, however, due to an aberrant splicing event, the SMN2 gene fails to produce sufficient full-length protein to protect against disease development in the absence of SMN1. While a number of compounds have recently been identified that can stimulate full-length survival motor neuron (SMN) expression from the nearly identical copy SMN2, one of the difficulties has been the lack of a highly reproducible and quantitative means to measure the levels of SMN protein. To develop a technique that allows the rapid and highly sensitive measurement of SMN protein, a Surface Plasmon Resonance (SPR) application has been developed. The ability to quantify unassociated SMN protein and monitor the binding of SMN with other proteins in solution using a SPR sensor in less than 15 min and at low ng mL(-1) levels in HEPES Buffer Saline (HBS) has been achieved. The detection limit for the specific binding of SMN in HBS pH 7.4 solution is 0.99 ng mL(-1) with non-specific binding accounting for approximately 30% of the signal. Quantification of SMN is based on an immunoassay performed on the gold surface of the SPR sensor. 16-mercaptohexadecanoic acid (MHA) was reacted with dicyclohexylcarbodiimide (DCC) and N-hydroxysuccinimide (NHS) to form a pre-activated thiol (MHA-NHS). Antibodies for SMN were then coupled to the sensor with the pre-activated thiol. Sensor specificity was examined with mixtures of myoglobin (MG) and SMN. SMN sensor response decreases by more than 60% when MG was added to SMN. The decrease in sensor response can be attributed to non-specific binding of SMN to MG, verified with a sensor for MG.

A survival motor neuron:tetanus toxin fragment C fusion protein for the targeted delivery of SMN protein to neurons

Spinal muscular atrophy (SMA) is a degenerative disorder of spinal motor neurons caused by homozygous mutations in the survival motor neuron (SMN1) gene. Because increased tissue levels of human SMN protein (hSMN) in transgenic mice reduce the motor neuron loss caused by murine SMN knockout, we engineered a recombinant SMN fusion protein to deliver exogenous hSMN to the cytosolic compartment of motor neurons. The fusion protein, SDT, is comprised of hSMN linked to the catalytic and transmembrane domains of diphtheria toxin (DTx) followed by fragment C of tetanus toxin (TTC). Following overexpression in Escherichia coli, SDT possessed a subunit molecular weight of approximately 130 kDa as revealed by both SDS-PAGE and immunoblot analyses with anti-SMN, anti-DTx, and anti-TTC antibodies. Like wild-type SMN, purified SDT showed specific binding in vitro to an RG peptide derived from Ewing's sarcoma protein. The fusion protein also bound to cultured primary neurons in amounts similar to those achieved by TTC. Unlike the case with TTC, however, immunolabeling of SDT-treated neurons with anti-TTC and anti-SMN antibodies showed staining restricted to the cell surface. Results from cytotoxicity studies in which the DTx catalytic domain of SDT was used as a reporter protein for internalization and membrane translocation activity suggest that the SMN moiety of the fusion protein is interfering with one or both of these processes. While these studies indicate that SDT may not be useful for SMA therapy, the use of the TTC:DTx fusion construct to deliver other passenger proteins to the neuronal cytosol should not be ruled out.

The Ewing's sarcoma protein interacts with the Tudor domain of the survival motor neuron protein

The survival motor neuron (SMN) gene is the spinal muscular atrophy (SMA) determining gene. Here we report that the SMN protein product interacts in vitro and in vivo with the arginine/glycine (RG)-rich RNA binding protein and transcription factor, Ewing's sarcoma (EWS). Recently, the SMN encoded Tudor domain (exon 3) and the YG-motifs (exon 6) have been shown to be involved in binding to RG-rich proteins. Here, we demonstrate that the Tudor domain encoded by SMN exon 3 is independently sufficient to mediate the interaction with EWS. Synthetic mutations within the Tudor domain, as well as a SMA patient-derived mutation within exon 3, reduced the levels of the SMN/EWS interaction. Carboxyl-terminal SMN mutations that prevent formation of SMN oligomers also indirectly reduced EWS binding. A role for arginine methylation has been observed in some RG-containing SMN-interacting proteins. Here we demonstrate that SMN interacts with non-methylated EWS and an EWS-derived RG-containing peptide. In contrast to previously reported results, symmetrical dimethylation of the EWS-derived RG-peptide results in a quantitative increase in the dissociation rate between SMN and the symmetrical dimethylated EWS RG-peptide. Consistent with the interaction data, endogenous and transiently expressed SMN co-localizes with endogenous EWS in a number of cultured cell lines, as well as rat primary neuron cultures. Anti-sense RNA experiments, however, demonstrate that EWS does not mediate the nuclear distribution of SMN or other Cajal body components.

Survival motor neuron protein in the nucleolus of mammalian neurons

Spinal muscular atrophy (SMA) is an inherited motor neuron disease caused by mutations in the survival motor neuron gene (SMN1). While it has been shown that the SMN protein is involved in spliceosome biogenesis and pre-mRNA splicing, there is increasing evidence indicating that SMN may also perform important functions in the nucleolus. We demonstrate here through the use of a previously characterized polyclonal anti-SMN antibody, abSMN, that the SMN protein shows a striking colocalization with the nucleolar protein, fibrillarin, in both nucleoli and Cajal bodies/gems of primary neurons. Immunoblot analysis with antifibrillarin and two different anti-SMN antibodies reveals that SMN and fibrillarin also cofractionate in the insoluble protein fraction of cultured cell lysates. Immunoprecipitation experiments using whole cell extracts of HeLa cells and cultured neurons revealed that abSMN coprecipitated small amounts of the U3 small nucleolar RNA (snoRNA) previously shown to be associated with fibrillarin in vivo. These studies raise the possibility that SMN may serve a function in rRNA maturation/ribosome synthesis similar to its role in spliceosome biogenesis.

Minute virus of mice small nonstructural protein NS2 interacts and colocalizes with the Smn protein

The small nonstructural protein NS2 of the minute virus of mice (MVM) is required for efficient viral replication, although its mode of action is unclear. Here we demonstrate that NS2 and survival motor neuron protein (Smn) interact in vitro and in vivo. NS2 and Smn also colocalize in infected nuclei at late times following MVM infection.

Minute virus of mice NS1 interacts with the SMN protein, and they colocalize in novel nuclear bodies induced by parvovirus infection

The human survival motor neuron (SMN) gene is the spinal muscular atrophy-determining gene, and a knockout of the murine Smn gene results in preembryonic lethality. Here we show that SMN can directly interact in vitro and in vivo with the large nonstructural protein NS1 of the autonomous parvovirus minute virus of mice (MVM), a protein essential for viral replication and a potent transcriptional activator. Typically, SMN localizes within nuclear Cajal bodies and diffusely in the cytoplasm. Following transient NS1expression, SMN and NS1 colocalize within Cajal bodies. At early time points following parvovirus infection, NS1 fails to colocalize with SMN within Cajal bodies; however, during the course of MVM infection, dramatic nuclear alterations occur. Formerly distinct nuclear bodies such as Cajal bodies, promyelocytic leukemia gene product (PML) oncogenic domains (PODs), speckles, and autonomous parvovirus-associated replication (APAR) bodies are seen aggregating at later points in infection. These newly formed large nuclear bodies (termed SMN-associated APAR bodies) are active sites of viral replication and viral capsid assembly. These results highlight the transient nature of nuclear bodies and their contents and identify a novel nuclear body formed during infection. Furthermore, simple transient expression of the viral nonstructural proteins is insufficient to induce this nuclear reorganization, suggesting that this event is induced specifically by a step in the viral infection process.

SRp30c-dependent stimulation of survival motor neuron (SMN) exon 7 inclusion is facilitated by a direct interaction with hTra2 beta 1

Proximal spinal muscular atrophy (SMA) is caused by the homozygous loss of survival motor neuron (SMN1). SMN2, a nearly identical copy gene, is present in all SMA patients; however this gene cannot provide protection from disease due to the aberrant splicing of a critical exon. SMN1-derived transcripts are exclusively full-length, whereas SMN2-derived transcripts predominantly lack SMN exon 7. A single non-polymorphic nucleotide difference (C in SMN1; T in SMN2) is responsible for the alternative splicing patterns. We have previously shown that transient expression of an SR-like splicing factor, hTra2 beta 1, stimulates inclusion of exon 7 in SMN2-derived mini-gene transcripts through an interaction with the AG-rich exonic splice enhancer within exon 7. We now demonstrate that a second splicing factor, SRp30c, can stimulate SMN exon 7-inclusion and that this activity required the same AG-rich enhancer as hTra2 beta 1. SRp30c did not directly associate with SMN exon 7; rather its association with the exonic enhancer was mediated by a direct interaction with hTra2 beta 1. In the absence of the hTra2 beta 1 binding site, SRp30c failed to complex with SMN exon 7. Taken together, these results identify SRp30c as a modulator of SMN exon 7-inclusion and provide insight into the molecular regulation of this critical exon.

A direct interaction between the survival motor neuron protein and p53 and its relationship to spinal muscular atrophy

Mutations in the SMN1 (survival motor neuron 1) gene cause spinal muscular atrophy (SMA). We now show that SMN protein, the SMN1 gene product, interacts directly with the tumor suppressor protein, p53. Pathogenic missense mutations in SMN reduce both self-association and p53 binding by SMN, and the extent of the reductions correlate with disease severity. The inactive, truncated form of SMN produced by the SMN2 gene in SMA patients fails to bind p53 efficiently. SMN and p53 co-localize in nuclear Cajal bodies, but p53 redistributes to the nucleolus in fibroblasts from SMA patients. These results suggest a functional interaction between SMN and p53, and the potential for apoptosis when this interaction is impaired may explain motor neuron death in SMA.

Regulation of murine survival motor neuron (Smn) protein levels by modifying Smn exon 7 splicing

Proximal spinal muscular atrophy (SMA) is caused by mutations in the survival motor neuron gene (SMN1). In humans, two nearly identical copies of SMN exist and differ only by a single non-polymorphic C-->T nucleotide transition in exon 7. SMN1 contains a 'C' nucleotide at the +6 position of exon 7 and produces primarily full-length SMN transcripts, whereas SMN2 contains a 'T' nucleotide and produces high levels of a transcript that lacks exon 7 and a low level of full-length SMN transcripts. All SMA patients lack a functional SMN1 gene but retain at least one copy of SMN2, suggesting that the low level of full-length protein produced from SMN2 is sufficient for all cell types except motor neurons. The murine Smn gene is not duplicated or alternatively spliced. It resembles SMN1 in that the critical exon 7 +6 'C' nucleotide is conserved. We have generated Smn minigenes containing either wild-type Smn exon 7 or an altered exon 7 containing the C-->T nucleotide transition to mimic SMN2. When expressed in cultured cells or transgenic mice, the wild-type minigene produced only full-length transcripts whereas the modified minigene alternatively spliced exon 7. Furthermore, Smn exon 7 contains a critical AG-rich exonic splice enhancer sequence (ESE) analogous to the human ESE within SMN exon 7, and subtle mutations within the mESE caused a variation in Smn transcript levels. In summary, we show for the first time that the murine Smn locus can be induced to alternatively splice exon 7. These results demonstrate that SMN protein levels can be varied in the mouse by the introduction of specific mutations at the endogenous Smn locus and thereby lay the foundation for developing animals that closely 'resemble' SMA patients.

An in vivo reporter system for measuring increased inclusion of exon 7 in SMN2 mRNA: potential therapy of SMA

Spinal muscular atrophy (SMA) is a degenerative motor neuron disorder resulting from homozygous loss of the SMN1 gene. SMN2, a nearly identical copy gene, is preserved in SMA patients. A single nucleotide difference between SMN1 and SMN2 causes exon 7 skipping in the majority of SMN2 mRNA. Gene therapy through modulation of SMN2 gene transcription in SMA patients may be possible. We constructed a series of SMN mini-genes comprised of SMN exon 6 to exon 8 sequences fused to green fluorescence protein (GFP) or luciferase reporters, to monitor SMN exon 7 splicing. These reporters recapitulated the splicing patterns of the endogenous SMN gene in stable cell lines. The SMN1-luciferase reporter was approximately 3.5-fold more active than SMN2-luciferase and SMN1-GFP intensities were visually distinguishable from SMN2-GFP. We have screened chemical inducers and inhibitors of kinase pathways using stable SMN-reporter lines and found that the phosphatase inhibitor sodium vanadate specifically stimulated exon 7 inclusion within SMN2 mRNAs. This is the first compound identified that can stimulate exon 7 inclusion into transcripts derived from the endogenous SMN2 gene. These results demonstrate that this system can be utilized to identify small molecules that regulate the splicing of SMN exon 7.