Combination of SMN trans-splicing and a neurotrophic factor increases the life span and body mass in a severe model of spinal muscular atrophy

Partial restoration of cardio-vascular defects in a rescued severe model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infantile death. Loss of a gene called Survival Motor Neuron 1 (SMN1) and, as a result, reduced levels of the Survival Motor Neuron (SMN) protein leads to SMA development. SMA is characterized by the loss of functional motor neurons in the spinal cord. However, accumulating evidence suggests the contribution of other organs to the composite SMA phenotype and disease progression. A growing number of congenital heart defects have been identified in severe SMA patients. Consistent with the clinical cases, we have recently identified developmental and functional heart defects in two SMA mouse models, occurring at embryonic stage in a severe SMA model and shortly after birth in a less severe model (SMN∆7). Our goal was to examine the late stage cardiac abnormalities in untreated SMN∆7 mice and to determine whether gene replacement therapy restores cardiac structure/function in rescued SMN∆7 model. To reveal the extent of the cardiac structural/functional repair in the rescued mice, we analyzed the heart of untreated and treated SMN∆7 model using self-complementary Adeno-associated virus (serotype 9) expressing the full-length SMN cDNA. We examined the characteristics of the heart failure such as remodeling, fibrosis, oxidative stress, and vascular integrity in both groups. Our results clearly indicate that fibrosis, oxidative stress activation, vascular remodeling, and a significant decrease in the number of capillaries exist in the SMA heart. The cardiac structural defects were improved drastically in the rescued animals, however, the level of impairment was still significant compared to the age-matched wildtype littermates. Furthermore, functional analysis by in vivo cardiac magnetic resonance imaging (MRI) revealed that the heart of the treated SMA mice still exhibits functional defects. In conclusion, cardiac abnormalities are only partially rescued in post-birth treated SMA animals and these abnormalities may contribute to the premature death of vector-treated SMA animals with seemingly rescued motor function but an average life span of less than 70 days as reported in several studies.

Direct central nervous system delivery provides enhanced protection following vector mediated gene replacement in a severe model of spinal muscular atrophy

Spinal Muscular Atrophy (SMA), an autosomal recessive neuromuscular disorder, is the leading genetic cause of infant mortality. SMA is caused by the homozygous loss of Survival Motor Neuron-1 (SMN1). SMA, however, is not due to complete absence of SMN, rather a low level of functional full-length SMN is produced by a nearly identical copy gene called SMN2. Despite SMN's ubiquitous expression, motor neurons are preferentially affected by low SMN levels. Recently gene replacement strategies have shown tremendous promise in animal models of SMA. In this study, we used self-complementary Adeno Associated Virus (scAAV) expressing full-length SMN cDNA to compare two different routes of viral delivery in a severe SMA mouse model. This was accomplished by injecting scAAV9-SMN vector intravenously (IV) or intracerebroventricularly (ICV) into SMA mice. Both routes of delivery resulted in a significant increase in lifespan and weight compared to untreated mice with a subpopulation of mice surviving more than 200days. However, the ICV injected mice gained significantly more weight than their IV treated counterparts. Likewise, survival analysis showed that ICV treated mice displayed fewer early deaths than IV treated animals. Collectively, this report demonstrates that route of delivery is a crucial component of gene therapy treatment for SMA.

Delivery of therapeutic agents through intracerebroventricular (ICV) and intravenous (IV) injection in mice

Despite the protective role that blood brain barrier plays in shielding the brain, it limits the access to the central nervous system (CNS) which most often results in failure of potential therapeutics designed for neurodegenerative disorders. Neurodegenerative diseases such as Spinal Muscular Atrophy (SMA), in which the lower motor neurons are affected, can benefit greatly from introducing the therapeutic agents into the CNS. The purpose of this video is to demonstrate two different injection paradigms to deliver therapeutic materials into neonatal mice soon after birth. One of these methods is injecting directly into cerebral lateral ventricles (Intracerebroventricular) which results in delivery of materials into the CNS through the cerebrospinal fluid. The second method is a temporal vein injection (intravenous) that can introduce different therapeutics into the circulatory system, leading to systemic delivery including the CNS. Widespread transduction of the CNS is achievable if an appropriate viral vector and viral serotype is utilized. Visualization and utilization of the temporal vein for injection is feasible up to postnatal day 6. However, if the delivered material is intended to reach the CNS, these injections should take place while the blood brain barrier is more permeable due to its immature status, preferably prior to postnatal day 2. The fully developed blood brain barrier greatly limits the effectiveness of intravenous delivery. Both delivery systems are simple and effective once the surgical aptitude is achieved. They do not require any extensive surgical devices and can be performed by a single person. However, these techniques are not without challenges. The small size of postnatal day 2 pups and the subsequent small target areas can make the injections difficult to perform and initially challenging to replicate.

IGF-1 delivery to CNS attenuates motor neuron cell death but does not improve motor function in type III SMA mice

The efficacy of administering a recombinant adeno-associated virus (AAV) vector encoding human IGF-1 (AAV2/1-hIGF-1) into the deep cerebellar nucleus (DCN) of a type III SMA mouse model was evaluated. High levels of IGF-1 transcripts and protein were detected in the spinal cord at 2 months post-injection demonstrating that axonal connections between the cerebellum and spinal cord were able to act as conduits for the viral vector and protein to the spinal cord. Mice treated with AAV2/1-hIGF-1 and analyzed 8 months later showed changes in endogenous Bax and Bcl-xl levels in spinal cord motor neurons that were consistent with IGF-1-mediated anti-apoptotic effects on motor neurons. However, although AAV2/1-hIGF-1 treatment reduced the extent of motor neuron cell death, the majority of rescued motor neurons were non-functional, as they lacked axons that innervated the muscles. Furthermore, treated SMA mice exhibited abnormal muscle fibers, aberrant neuromuscular junction structure, and impaired performance on motor function tests. These data indicate that although CNS-directed expression of IGF-1 could reduce motor neuron cell death, this did not translate to improvements in motor function in an adult mouse model of type III SMA.

Optimization of SMN trans-splicing through the analysis of SMN introns

Spinal muscular atrophy (SMA), a neurodegenerative disease, is the leading genetic cause of infantile death and is caused by the loss of survival motor neuron 1 (SMN1). Humans carry a duplicated copy gene, SMN2, which produces very low levels of functional protein due to an alternative splicing event. This splicing difference is the reason that SMN2 cannot prevent SMA development when SMN1 is deleted. SMN2 generates a transcript lacking exon 7 and consequently gives rise to an unstable truncated SMN protein that cannot protect from SMA. To increase full-length SMN protein, we utilize a strategy referred to as trans-splicing. This strategy relies upon pre-mRNA splicing occurring between two separate molecules: (1) the endogenous target RNA and (2) the therapeutic RNA that provides the correct RNA sequence via a trans-splicing event. The initial trans-splicing RNA targeted intron 6 and replaced exon 7 with the SMN1 exon 7 in SMN2 pre-mRNA. To determine the most efficient intron for SMN trans-splicing event, a panel of trans-splicing RNA molecules was constructed. Each trans-splicing RNA molecule targets a specific intron within the SMN2 pre-mRNA and based on the target intron, replaces the downstream exons including exon 7. These constructs were examined by RT-PCR, immunofluorescence, and Western blotting. We have identified intron 3 as the most efficient intron to support trans-splicing in cellular assays. The intron 3 trans-splicing construct targets intron 3 and replaces exons 4-7 and was distinguished based on its ability to produce the highest level of the trans-spliced RNA and full-length SMN protein in SMA patient fibroblasts. The efficiency of the intron 3 construct was further improved by addition of an antisense that blocks the 3' splice site at the intron 4/exon 5 junction. Most importantly, intracerebroventricular injection of the Int3 construct into SMNΔ7 mice elevated the SMN protein levels in the central nervous system. This research demonstrates an alternative platform to correct genetic defects, including SMN expression and examines the molecular basis for trans-splicing.

The protective effects of β-lactam antibiotics in motor neuron disorders

In the June issue of Experimental Neurology, Nizzardo and colleagues demonstrate that the beta-lactam antibiotic ceftriaxone is neuroprotective in a mouse model of spinal muscular atrophy. Here I review their main findings and the relevance to previous and future work on motor neuron disorders and for developing therapeutic strategies.

Developments in the discovery of drugs for spinal muscular atrophy: successful beginnings and future prospects

INTRODUCTION

Spinal muscular atrophy (SMA) is an autosomal recessive disease caused by mutations in a gene that produces a protein called survival motor neuron (SMN). SMN has an important role in snRNP assembly in all cells but that may not be its only role; the reasons for SMN deficiency resulting in neuromuscular dysfunction and motor neuron degeneration remain active areas of research. Besides increasing SMN, compensating for SMN deficiencies or neuroprotection may be therapeutic options for SMA. Age of onset and the rate of disease progression are variable and therapeutic strategies should be appropriate to subtypes of SMA patients.

AREAS COVERED

The article discusses SMA, their targets and where these targets can be found. Additionally, the article reviews small molecules identified as disease modifiers and how these small molecules were discovered. The article also describes and discusses emerging concepts regarding the disease mechanisms. The author compiled this review using scientific literature, patent databases, company and patient association and government websites.

EXPERT OPINION

Small molecules targeting various processes implicated in SMA are reaching the clinic. These molecules and targets, although not yet validated, are providing insight into the complexity of a 'simple' genetic disease such as SMA. SMA is not a single disease and so various therapeutic strategies are needed. Biomarkers and regulatory guidelines are required to select patients for clinical trials, decide when to initiate treatment and how to develop combinations of investigational drugs.

Genetic therapies for RNA mis-splicing diseases

RNA mis-splicing diseases account for up to 15% of all inherited diseases, ranging from neurological to myogenic and metabolic disorders. With greatly increased genomic sequencing being performed for individual patients, the number of known mutations affecting splicing has risen to 50-60% of all disease-causing mutations. During the past 10years, genetic therapy directed toward correction of RNA mis-splicing in disease has progressed from theoretical work in cultured cells to promising clinical trials. In this review, we discuss the use of antisense oligonucleotides to modify splicing as well as the principles and latest work in bifunctional RNA, trans-splicing and modification of U1 and U7 snRNA to target splice sites. The success of clinical trials for modifying splicing to treat Duchenne muscular dystrophy opens the door for the use of splicing modification for most of the mis-splicing diseases.