One approach researchers are pursuing for SMA focuses on protecting muscles from paralysis and increasing their strength. Although this approach does not fix the underlying genetic problem in SMA, drugs that enhance muscle function could likely be used in combination with other therapies that act on the SMN genes. Cytokinetics is developing drugs that increase the ability of the muscle to contract. These drugs have shown early promise in patients with a similar motor neuron disease called amyotrophic lateral sclerosis ALS.
The goal is to show positive results preserving muscle strength. Researchers hope that reldesemtiv can be combined with other therapies to achieve maximal therapeutic benefits for patients. SRK, developed by Scholar Rock for SMA patients, enhances muscle growth by inhibiting myostatin a muscle growth inhibitor naturally in the body. A phase 1 clinical trial of SRK in healthy volunteers resulted in no adverse effects, successful inhibition of myostatin, and a relatively long half-life. Its success supported the advancement toward a phase 2 clinical trial.
Myostatin, activins, and BMP9 are related growth factors with shared signaling pathways. Previous attempts at inhibition of myostatin have resulted in safety concerns thought to be related to off-target effects on BMP9.
Combined inhibition of both myostatin and activins but not BMP9 may boost the effect of the drug on muscle function, while improving the safety profile. Currently, BIIB is in phase 1 clinical trial. As discussed earlier, the presence of SMN2 genes is known to be a modifier of disease severity.
But in any case, early treatment is a game changer in the course of the disease as it can completely alter the prognosis. Early diagnosis and treatment is crucial. Because Spinraza nusinersen , a disease-modifying therapy, was approved by the FDA in , SMA could enter the list of diseases recommended for screening at birth.
Much work is yet to be done on this newly discovered cause of SMA. Motor neurons are the nerve cells that degenerate in SMA, leading to muscle weakness and paralysis. While some research is focused on strategies to increase SMN levels to help motor neurons, other scientists are focusing on broad neuroprotection. This research aims to prevent motor neurons from becoming dysfunctional and dying rather than altering the genetics of the SMN genes. Neuroprotective strategies could likely be used in combination with other drugs that address the underlying genetic problem in SMA.
These patients generally live into their thirties, with age of death correlating with the required amount of respiratory support. SMA type III Kugelberg-Welander patients exhibit symptoms in childhood and while they eventually become wheelchair bound, are initially able to stand and walk independently. SMA type IV is the least common and is characterized by adolescent or adult-onset of disease and often retained ambulation. Although this classification system is widely used and accepted Munsat and Davies, , the phenotypic spectrum of SMA is continuous and symptoms are variable within SMA types.
There are not adequate autopsy data available to know whether there are differences in the magnitude of MN loss that account for this vast range of clinical disease severity. However, electromyography EMG of severely affected patients shows extensive spontaneous fibrillations indicating active dennervation associated with small voluntary motor units, while EMG of more mildly affected patients displays few fibrillations and large voluntary motor units indicating that collateral axonal sprouting has occurred Crawford and Pardo, This suggests that in mild forms of SMA the distal terminal of the MN retains the ability to reinnervate neighboring, denervated myofibers, thereby maintaining muscle power and raises that possibility that therapeutic strategies to accentuate this process might be particularly effective for mild SMA patients.
In all SMA patients, the highest rate of functional decline appears to occur soon after the onset of symptoms, after which muscle power remains relatively stable in many patients Dubowitz, ; Crawford and Pardo, ; Iannaccone et al. Motor unit number estimation MUNE studies have also documented a precipitous loss of distal muscle motor units during disease progression Swoboda et al. This has raised the possibility that the most effective therapy may require delivery prior to this decline.
Figure 1. The resulting truncated transcripts encode a truncated, unstable protein product C that is rapidly degraded. The SMA disease-causing gene was mapped in to a complicated region of chromosome 5q that contains an inverted duplication Brzustowicz et al.
Lefebvre et al. These mutations are usually homozygous deletions of exons ; however, frameshift, missense, and nonsense SMN1 mutations have also been shown to cause SMA Wirth, The only functional difference between the genes appears to be a C-T transition at position 6 in exon 7 in SMN2 , which lies within an exonic enhancer sequence and leads to frequent exon 7 skipping during transcription of SMN2- derived pre-RNAs Lorson et al.
This observation along with similar observations in mice see below has focused the majority of effort in SMA therapeutics development on identifying strategies to increase SMN levels. The evolutionarily conserved 38 kDa SMN protein is ubiquitously expressed, raising the question of why motor neurons are particularly susceptible to reduced SMN expression. SMN is present in the cytoplasm and the nucleus. SMN forms a complex with Gemins and unrip and this complex regulates the assembly of small nuclear ribonuclear proteins snRNPs reviewed in Kolb et al.
Nonetheless, the mechanisms by which lowered snRNP assembly levels in all cells might cause selective motor neuron degeneration remain poorly defined. The minor spliceosome is responsible for splicing of a minority of introns that are present in only a small proportion of genes, some of which are enriched in the nervous system such as voltage gated ion channels and synaptic components.
In this way, SMN deficiency could cause missplicing of neural-specific genes resulting in preferential motor neuron degeneration. Others have argued that any splicing changes that occur in SMA mice are only evident in late-stage disease and thus are not a primary event in SMA pathogenesis Baumer et al. Another hypothesis is that it is the disruption of an alternative, neuron-specific function of SMN that underlies the specific susceptibility of motor neurons to SMN protein deficiency.
SMN is actively transported in discrete granules in the motor axon Zhang et al. It has been further postulated that these defects may lead to cytoskeletal disruptions of the nerve terminal. Interestingly, a gene encoding an actin modifying protein, Plastin 3, has been shown to be a protective modifier in human SMA Oprea et al. SMN likely has several molecular functions and the specific defect that causes motor neuron disease remains to be definitively determined.
As a result, most high throughput, cell-based therapeutic screening assays to date have screened for SMN induction, rather than for an improvement in some aspect of SMN function. Figure 2. SMA pathogenesis. Studies of SMA mouse models have uncovered multiple pathological and functional abnormalities of the motor system with particular abnormalities of both NMJ and MN synapses.
Fly, nematode, fish , and mouse models of SMA have provided key insights regarding the cellular pathogenesis of SMA and enabled in vivo preclinical studies of candidate therapeutics. The observation that complete knockout of SMN in any organism is embryonically lethal confirms that some SMN protein is required for survival of all cells Schrank et al. Embryonic lethality can be prevented by expression of some SMN protein, and SMA disease severity in mice correlates with transgenic SMN2 copy number, recapitulating the relationship between SMN expression and disease severity seen in humans Hsieh-Li et al.
In recent studies, using conditional SMA mice in which restoration of SMN expression could be temporally controlled, it has been further demonstrated that increased SMN expression postnatally is capable of improving the disease phenotype, but the timing of this restoration is critical. Increased SMN expression at postnatal day 1 in SMA mice that would typically live 2 weeks results in a near complete rescue of many mice, but delaying SMN restoration until after the first postnatal week is ineffective Le et al.
In severe SMA mice, studies have demonstrated a surprisingly modest degree of motor neuron death even at disease end-stage, although certain motor neuron pools are more severely affected than others Mentis et al.
There is also little evidence of profound distal axonal degeneration to suggest a dying back axonopathy causing muscle weakness.
Rather early stages of disease are characterized by morphological and functional abnormalities of synapses Figure 2 , both the neuromuscular junctions NMJs Chan et al. These abnormalities are associated with simplified NMJ endplates and hypotrophic myofibers that fail to mature Figure 2 Kong et al. These early structural and functional abnormalities of synapses are likely followed by synapse loss, which in the case of the NMJ may directly result in denervated, weak muscle and in the case of central synapses may lead to further failed downstream activation of muscle.
The temporal window for effective rescue in SMA mice may relate to the ability to restore connected, but dysfunctional synapses, which cannot be regained once lost. The finding of extensive synaptic abnormalities in SMA mice has also raised the question of where SMN must be delivered to restore synaptic function. Is SMN required just in motor neurons, or is it also required in other neurons or in muscle?
Studies are ongoing to address these questions. Of note, therapeutic strategies to improve muscle growth independent of SMN induction have shown only modest Rose et al. Figure 3. Therapeutic strategies for SMA. Because SMN expression levels correlate with disease severity in humans and in mice and postnatal induction of SMN expression in mice is sufficient to prevent disease, a principal focus of therapeutics development in SMA has been to identify strategies to increase SMN protein levels either by activating SMN2 gene expression, increasing inclusion of exon 7 in SMN2-derived transcripts, stabilizing SMN protein, or by replacing the SMN1 gene Figure 3.
Other efforts have centered on neuroprotection and cell replacement. Some of the therapies currently in development for the treatment of SMA are discussed below Figure 4.
Repurposed drugs: histone deacetylase inhibitors HDACis , hydroxyurea, and prolactin. HDACis activate gene expression by inhibiting histone deacetylases HDACs , which deacetylate chromatin histones thereby promoting a tightly coiled, transcriptionally repressed region of chromatin.
Ten years ago, Chang et al. Studies with these compounds are ongoing in SMA type I infants as efficacy may be improved by early delivery. Identifying such drugs that do not have prohibitive toxicity for chronic use is a challenge for this class of compounds. Figure 4. Current development status of promising SMA therapeutics. Both repurposed drugs and novel drugs designed specifically for SMA are being developed.
Hydroxyurea was also taken to clinical trials in SMA patients as it is FDA-approved for the treatment of sickle cell disease. Despite earlier work documenting modest improvements in manual muscle testing MMT scores in Taiwan Liang et al.
Stat5 was first identified as a downstream target of three compounds able to promote SMN2 acivity, TSA, aclarubicin, and sodium vanadate, and Stat5 knockdown results in decreased SMN expression Ting et al.
More recently, prolactin, a known Stat5 activator, was shown to increase the median life span of severe SMA mice from 14 to 21 days Farooq et al. Recombinant prolactin is FDA-approved for the treatment of prolactin deficiency in women and would be available for investigation in SMA clinical trials. It is unclear, however, whether the doses of recombinant prolactin typically used in clinical practice will be adequate to activate SMN2 in SMA patients.
Quinazoline derivatives were originally found to increase SMN2 promoter activity in a screen of over , compounds in an NSC cell-based screening assay Jarecki et al. After structure activity relationship studies and lead optimization, a novel 2,4-diaminoquinazoline derivative was identified that showed good CNS penetration and a long half-life following oral dosing in mice Thurmond et al.
Additionally, this enzyme has nuclear-cytoplasmic shuttling responsibilities and plays a role in first intron pre- mRNA splicing Shen et al. A lead quinazoline compound was tested in vivo , and was shown to have modest behavioral and survival benefits in SMA mice Butchbach et al. Phase I clinical trials with this compound have been initiated by the Repligen Corporation.
The detailed mechanisms by which DcpS inhibition leads to SMN2 promoter activation or provides survival and behavioral benefits in SMA mice remains under investigation. The drug was also well tolerated Angelozzi et al. Larger randomized placebo-controlled trials are needed to further evaluate the efficacy of this drug in SMA. In , Andreassi et al.
Unfortunately, this compound has prohibitive toxicities for long-term use, as would likely be required for SMA patients. PTC Therapeutics, a company focused on developing drugs that target post-transcriptional control of gene expression, has also recently identified compounds that increase exon 7 inclusion thereby increasing SMN protein expression. They have identified three different structural scaffolds that each increases SMN protein levels and extends median survival of severe SMA mice, in one case from 14 to days.
These compounds are orally bioavailable and can penetrate the blood-brain barrier, but the mechanism by which they alter SMN splicing has not yet been described Naryshkin et al. Another strategy that has been pursued to increase exon 7 inclusion is the use of antisense oligonucleotides ASOs , modified nucleotides that bind specific mRNA sequences. This binding can mark a specific mRNA for degradation or in the case of SMA therapeutics, binding to specific cis-acting splicing regulatory motifs can promote exon 7 inclusion.
Different ASOs utilizing various chemistries Williams et al. Passini et al. Furthermore, recent data show an even more striking benefit when ASO is given systemically in severe SMA mice, with some mice surviving for more than 1 year Hua et al.
The enhanced benefit with systemic delivery suggests the requirement to restore SMN in other tissues aside from the CNS. Whether this is relevant to the human disease has yet to be determined.
Frank Bennett, Ph. Repurposed drugs: aminoglycosides and proteasome inhibition. Aminoglycosides are known for their ability to induce translational readthrough of stop codons. In the case of SMN2 transcripts, it was postulated that readthrough of the initial stop codon would extend the length of the C-terminal of the truncated SMN protein thereby improving its stability. Recently, Kwon et al. Nonetheless, both aminoglycosides and proteasome inhibitors will have to overcome challenges of drug toxicity before they can be legitimate treatments for SMA patients.
This marked the first time that the NIH embarked on a mission to develop therapeutics in-house for a particular disease, with a goal of filing an IND in 5 years. SMA Project currently has two sets of compound series in development. The first, indoprofen analogs, are based on the observation that indoprofen increased SMN levels and nuclear gem counts in SMA patient-derived fibroblasts and provided a modest survival benefit to SMA mouse embryos Lunn et al.
The second set of compounds, approximately benzimidazoles, was independently generated from a high throughput screen and lead candidates are being generated from these hits personal communication, Jill Heemskerk, Ph.
Gene Therapy Figure 3D. Gene therapy provides the opportunity to restore a normal form of the SMN1 gene to SMA patients; however, effective delivery to a difficult-to-access cell such as a motor neuron has been considered an almost impossible challenge until very recently. Several groups have accomplished this using self-complementary adeno-associated virus vectors Foust et al. The first two papers outlining this work were published weeks apart and employed two different methods.
Foust et al. Both studies resulted in increased median survival of severe SMA pups compared to vehicle-treated counterparts, but interestingly delivery of the gene intravenously provided a greater survival benefit when compared to ICV injections, with 6 of 7 severe SMA mice living past days compared to a median survival of days in the ICV-injected mice Foust et al.
Other studies have also shown dramatic extensions of median lifespan using IV scAAV9 delivery, one study showing an increase from 14 to 69 days and another showing an increase from 27 to days in SMA mice Valori et al. Preclinical studies are ongoing to address the potential challenges of toxicity, delivery, and manufacturing for human clinical trials.
Repurposed drugs: riluzole and ceftriaxone. The beta-lactam antibiotic ceftriaxone was demonstrated to increase glutamate reuptake and improve the phenotype of ALS mice Rothstein et al.
Consequently, a clinical trial of ceftriaxone is ongoing in ALS patients. This drug was evaluated in severe SMA mice and shown to modestly extend survival Nizzardo et al.
A novel neuroprotective compound currently being studied for the treatment of neurodegenerative diseases including ALS and SMA is Olesoxime TRO , a cholesterol-like compound being developed by Trophos.
This drug was identified in a cell-based screen for compounds that protected rat primary motor neurons from degeneration induced by trophic factor withdrawal Bordet et al.
Although the drug mechanisms are not entirely understood, the drug has been shown to bind TSPO and VDAC, proteins located on the outer mitochondrial membrane, suggesting a role of mitochondrial signaling in the mechanism of drug action Bordet et al. Another treatment strategy that is being actively investigated in SMA is cell replacement.
Embryonic stem ES cells can be differentiated into neural stem cells and then functional MNs under stringent growth and differentiation conditions usually involving retinoic acid , sonic hedgehog , and neurotrophic factors Wichterle et al.
One group reported the in vivo benefit of neural stem cell injections in severe SMA mice using intrathecal injections after differentiating neural stem cells from mouse spinal cord neurospheres Corti et al.
SMA mice treated with these neural stem cells displayed increased survival and motor behavioral benefits, as well as increased MN number and size in the spinal cord when compared to SMA littermates Corti et al.
Work by California Stem Cell Inc. The ability to do this on a large scale is instrumental in the development of these cells as potential therapeutics. This group has also demonstrated phenotypic benefits in animal models of ALS, SMA, and spinal cord injury , further validating the potential of this therapeutic Wyatt et al. There are many ongoing efforts to identify other novel drugs for SMA. These include high throughput screening campaigns by Novartis and by Merck in collaboration with Gideon Dreyfuss.
In the latter effort, native SMN protein-inducing compounds have been identified during a high throughput screen of over one million small molecules in SMA patient-derived cells. Hits are currently being optimized and their mechanisms of action are being characterized personal communication, Gideon Dreyfuss, Ph. Two additional recent studies highlight the utility of novel screening assays. In addition, by screening compounds at varying concentrations and those with known targets and pathways instead of diverse chemical libraries, the investigators were able to provide mechanistic evidence that implicates GSK-3 kinase as an SMN regulator Makhortova et al.
In a primary screening of over , compounds, over 6, compounds were initially identified Xiao et al. They demonstrated that, in individuals with SMA, this gene was deleted or mutated.
Umrao Monani in the laboratory of Arthur Burghes, and Dr. This work was funded by Cure SMA. Prior to this, the diagnosis was made based on the visible signs and symptoms of SMA. In , just a few years after Drs. They are building on the discoveries of Drs. Our thanks to the smart and determined researchers who are working each day on behalf of our community. The Discovery of SMA.
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