Spinal muscular atrophy: Where are we now? Current challenges and high hopes

Spinal muscular atrophy (SMA) is one of the biggest challenges in today's medicine. This disease affects 1 in every 6,000–10,000 live births [1]. The carrier frequency varies (1:38–1:50), with Caucasian and Asian populations having the highest [2]. It is a neuro-degenerative disease of the spinal cord that causes progressive muscle wasting, weakness, respiratory distress, and paralysis [1, 2]. Furthermore, due to its severity, it has been demonstrated that it is the most common genetic cause of childhood death [3]. SMA is a neuromuscular disorder that is caused by a homozygous deletion or point mutation in the Survival of Motor Neuron 1 (SMN1) gene, which is located on the long arm of chromosome 5 (5q13). A mutation or deletion in the SMN1 gene prevents the SMN protein from being produced. The SMN protein is widely expressed and primarily found in the cytoplasm. The SMN protein in humans is encoded by two genes, SMN1 and SMN2. Both genes are found on chromosome 5 and are nearly identical (with the exception of 11 compared to SMN1), with one in non-coding exon 8 [4]. The transition in SMN2 exon 7 results in the significant exclusion of this exon from the mature transcript, contributing to the production of a predominantly nonfunctional and unstable SMNΔ7 protein [5]. However, a small number of SMN2 transcripts contain exon 7 and encode full-length, functional SMN, so when SMN1 is mutated or deleted – it provides enough SMN to prevent death but not enough to fully compensate for the loss of SMN1 expression (Fig. 1) [6]. The number of SMN2 copies in the genome ranges from 0 to 8. According to research, the more SMN2 genes a patient has, the more functional protein is produced and the milder the SMA phenotype [7]. Exon 7 SMN1 contains three exonic splicing enhancers (ESEs) that can promote exon 7 inclusion into the mature transcript. The most important appears to be ESE2, to which trans factors, such as tra2-1, attach, interact with the exon, and are required for exon 7 inclusion [8]. The C6T transition eliminates an exonic splicing enhancer (ESE) that is dependent on SF2/ASF. This motif is found in SMN1 exon 7 and displaces the inhibitory effect of the heterogeneous nuclear ribonucleoprotein (hnRNP) A1, contributing to the formation of the protein SMNΔ7 [9].

Fig. 1

The disease is characterized by symmetrical muscle weakness, which can cause movement problems and severe physical disability. SMA is classified into four clinical types based on the age of symptom onset and the severity of the disease. According to some sources, SMA type 0 is the most severe form of muscular atrophy, and symptoms appear during prenatal development [10]. SMA type I (also known as Werdnig-Hoffmann disease) manifests itself in the first weeks or months of a child's life as muscle weakness and an apparent lack of movement progress. Children's health rapidly deteriorates, and mechanical airway ventilation may be required. Children with SMA type II (also known as Dubowitz disease) can develop normally up to the age of six months and learn to sit without assistance, but as they get older, they develop symmetrical muscle weakness, swallowing difficulties, and respiratory problems [11]. Symptoms of SMA type III (also known as Kugelberg-Welander disease) appear when children are able to walk. However, the disease causes walking difficulties or difficulties climbing stairs. SMA type IV can occur in adults, but it is extremely rare. Symptoms do not manifest themselves until the age of 20 or 30. They include walking problems, difficulty rising from a squatting position, walking upstairs, and frequent falls. Patients in this group have significantly reduced mobility, but they can move around independently for many years [12].

Some medications are currently available to alleviate the symptoms of this disease, but they are prohibitively expensive. The first, nusinersen, marketed as Spinraza by Biogen (Cambridge, MA, USA), was approved by the FDA in December 2016 and the European Medicines Agency (EMA) in June 2017. The first year's cost ranged from $516,896 to $907,665, and the second year's cost ranged from $258,448 to $457,889. The price differs between the United States and Europe, and it fluctuated over a three-year period between 2017 and 2020. The second medication, onasemnogene abeparvovec, marketed by Novartis (Basel, Switzerland) as Zolgensma, was approved by the FDA in May 2019 and by the EMA in August 2020. It is known as the most expensive medication in the world, with its price of $2.1 million in the USA for a single injection. A third drug is risdiplam, marketed as Evrysdi by Roche, which was approved by the FDA in August 2020 and by EMA in March 2021 [3]. Although the FDA has approved these three drugs for use, clinical trials are still ongoing. Current SMA treatment methods are expensive and frequently insufficient. Because of the limited window, treatment time is a critical component for successful therapy. There is currently no fully successful SMA cure, and more solutions should be developed. This review describes various types of drugs, treatment-related issues, and ongoing research.

There was no efficient method of SMA therapy for a very long time. One of the most promising methods today is a treatment based on SMN protein level restoration. One of the ideas is to use oligonucleotides to modify the splicing of SMN2 exon 7. Oligonucleotides that are delivered directly to the central nervous system (CNS) show high target specificity, but also low toxicity and systemic exposure. Oligonucleotide therapy differs from other gene therapy because it is reversible, and there is no problem with permanent alteration [13]. However, only 1% of administrated oligonucleotides are capable of crossing the blood-brain barrier, which is a huge problem to resolve. Intrathecal or intracerebroventricular injections can ameliorate the transport of oligonucleotides to their destinations in the CNS, but these methods are very invasive. One of the solutions avoiding this obstacle is to administer oligonucleotides via an intranasal or oral application. The other problem is that the pharmacodynamic and pharmacokinetic properties of this treatment are not well known [14]. In 2006, the intronic splicing silencer N1 (ISS-N1) region was discovered by Singh Laboratory. It is an intronic-cis element, located in the SMN2 intron 7 downstream to the 5′ splicing site. It plays a major role in modulating alternative splicing of SMN2 exon 7 in a regulatory network connected with the pathogenesis of SMA. It recruits spliced repressors hnRNP A1 and A2 that inhibit exon inclusion [14, 15].

2.1

Mechanisms of action

Nusinersen is an orphan drug (antisense oligonucleotide) that disrupts the function of ISS-N1 in intron 7 of the SMN2 gene, which allows production of the functional SMN protein (Fig. 3B) [16]. Chemically, nusinersen is a 2′-O-methoxyethyl (2′MOE) modified antisense oligonucleotide (AO) [16] (Fig. 2A) that differs from RNA nucleotides with the addition of a methoxy group at the 2′-hydroxyl position and a phosphorothioate backbone, which increases its stability and RNA hybridization affinity by improving resistance to nuclease activity. Nusinersen boosted exon 7 inclusion by binding to a particular region in the intron downstream exon 7 of the SMN2. It is possible to convert SMN2 mRNA with exon 7 into a functional, full-length SMN protein [16]. The dosage recommended by FDA is 12 mg administrated intrathecally. Three doses are administered every 14 days, the next one 30 days after the third dose, and then one dose every four months (probably for the rest of the patient's life) [14].

Fig. 2

On May 24, 2019, FDA approved onasemnogene abeparvovec (Zolgensma) for treating pediatric patients younger than 2 years suffering from spinal muscular atrophy with bi-allelic mutations in the SMN1 gene [22]. Onasemnogene abeparvovec is a gene therapy based on using the recombinant adeno-associated virus subtype 9 (AAV9) which is designed to deliver a functional copy of the SMN1 gene encoding human SMN protein (Fig. 3E) [22, 23].

Fig. 3

Risdiplam (Evrysdi) is a SMN2 splicing modifier administered to patients at least 2 months old suffering from SMA caused by mutations in chromosome 5q that conduce to SMN protein deficiency [27]. It is a small-molecule drug with the chemical formula C₂₂H₂₃N₇O (Fig. 2B). Risdiplam is designed to increase the level of SMN protein and targets exonic splicing enhancer 2 (ESE2) located in SMN2 exon 7 and 5′ splicing site of SMN2 intron 7, causing inclusion of exon 7 and production of fully functional SMN protein in the CNS and in peripheral tissues of the body (Fig. 3C) [26, 27]. Risdiplam is taken orally, and the dosage is determined by patient age and body weight [27].

The Food and Drug Administration and the European Medicines Agency have authorized three therapies thus far: Spinraza, Zolgensma, and Evrysdi are all SMN-enhancing medications, although other medicines are being tested in clinical studies as well. Currently, therapies in clinical trials are aimed at treating the disease's symptoms rather than the cause, which in SMA are neuromuscular function, muscle weakness, and muscle fatigue. Reldesemtiv (previously CK-2127107, CK-107) is a fast skeletal muscle troponin activator of the next generation (FSMT). Nerve impulses are diminished in numerous disorders, including SMA, and this adds to muscle weakness. Damage to motor neurons reduces calcium ion release, resulting in less effective muscle activation to produce a movement [34]. The calcium sensitivity of the troponin-tropomyosin complex is increased by reldesemtiv. The rate of calcium release from troponin slows, resulting in an increase in the amount of calcium in the body. The drug is currently in phase II clinical trials for people with SMA types II, III, and IV (NCT02644668) [35].

Scholar Rock's Apitegromab (SRK-015) is a myostatin inhibitor that allows the body's muscle mass to grow. Myostatin (GDF-8, growth differentiation factor 8) is a non-active polypeptide proMyostatin that inhibits muscle cell growth and differentiation. The mature growth factor is released from the precursor in two stages. Proprotein convertases cleave proMyostatin, resulting in the production of inactive, latent myostatin. Second, pre-domains are cleaved by a BMP/Tolloid protease, such as Tolloid-like protein 2 (TLL-2) or bone morphogenetic protein 1 (BMP1), releasing the mature growth factor and allowing it to interact with its receptor [36]. The conformational flexibility of known latency-related structural elements, as well as regions adjacent to the tolloid and furin of the proteolytic cleavage site, is affected by highly specific binding to the arm region in the prodomain of pro/latent myostatin, according to structural studies on the binding of apitegromab to proMyostatin and the latent form of the protein [37]. Positive results from a phase 2 clinical trial (TOPAZ, NCT03921528) contributed to Scholar Rock's announcement of a phase 3 study named SAPPHIRE (NCT05156320), which might be the final step for approval of SRK-015 as an addition therapy to conventional SMN treatment [38].

Many different drugs have been attempted since the genetic basis of the disease was discovered, but the majority of them have failed. For instance, olesoxime (cholest-4-en-3-one, oxime) is a cholesterol-like compound that was first developed by Trophos [39, 40]. The drug was found to be safe and well-tolerated in phase 2 clinical studies in 3- to 25-year-olds with confirmed SMA type II or non-ambulatory SMA type III. However, there was no improvement in motor function when compared to those receiving placebo [41, 42]. The goal of the open-label extension study (OLEOS, NCT02628743) was to examine the safety, tolerability, and efficacy of the drug over a longer period of time. It was discovered that motor function looked to remain steady for 52 weeks, but then began to deteriorate [40, 43]. Roche decided to stop developing olesoxime in May 2018 because of technological challenges and lower-than-expected efficacy.

Novartis's Branaplam (LMI070, NVS-SM1) was in a phase II clinical trial in children with SMA type I (NCT02268552). It is a small molecule that is taken orally once a week. It was discovered during a high-throughput screening of pyridazine 2 and refined using multi-parameter lead optimization (Fig. 2C) [32]. It works by modifying SMN2 exon 7 splicing to increase the amount of functional SMN protein. Branaplam stabilizes the duplex U1:5′ splice site at the 5′ splice site SMN2 exon 7, thereby improving exon 7 inclusion (Fig. 3D). The drug has been shown in studies to improve the survival rate of a mouse model of SMA [33]. Novartis announced in July 2021 that it would no longer pursue branaplam as a treatment for SMA but would instead promote the molecule as a potential treatment for Huntington's disease.

PTC Pharmaceuticals, Roche, and the SMA Foundation collaborated to develop RG7800, a small molecule. An oral small-molecule compound was discovered through chemical screening and optimization to modify the splicing of SMN2 exon 7. A compound from the pyrazolopyrazine subclass has begun clinical trials in adults with SMA type II or type III, and the study is known as MOONFISH [42]. Initial results were promising as SMN protein levels doubled [43]. However, further studies were halted because it was discovered that the drug accumulates in the cornea of the experimental animals and is not removed quickly enough, which could lead to toxicity. The focus was on creating another chemical, RG7916, and participants in the MOONFISH (NCT02240355) research were invited to join in the JEWELFISH clinical trial of risdiplam (NCT03032172) [44].

The quinazoline derivative RG3039 was developed as an oral experimental medication to block a scavenger mRNA decapping enzyme (DcpS) and so enhance protein levels. The medication penetrated the blood-brain barrier into CNS tissues and inhibited the enzyme DcpS, however, SMN levels were low in early tests. DcpS enzyme activity is significantly suppressed in tissues from SMA mice, suggesting that enzyme activity could be used as a pharmacodynamic measure of medication activity in SMA patients' clinical trials [45]. RG3039 has also been proven to extend life and improve SMN protein levels in mice with varied degrees of illness severity [46]. Repligen, which created the drug, has secured a deal with Pfizer to continue developing RG3039. However, the contract was unexpectedly terminated in 2015, and progress on the drug was suspended.

Valproic acid (VPA) is a short-chain carboxylic acid, a drug with anticonvulsant properties and a histone deacetylase (HDAC) inhibitor with a terminal half-life (t_1/2) of 8–10 hours in human serum. The function of HDAC is deacetylation, which can alter gene transcription selectively and thus promote chromatin condensation. It was able to conclude that his-tone deacetylase inhibitor drugs increase SMN2 promoter activity by up-regulation of SMN2 expression via inhibition of HDAC2 [47]. Initial studies have shown the acid to be safe, well-tolerated with no apparent hepatotoxicity, although two people developed carnitine deficiency [48]. However, while VPA enhances motor abilities in 2- to 3-year-olds, side effects such as increased body development and gastrointestinal difficulties have been reported in the treatment group in subsequent studies [49]. A study from genetically confirmed SMA type III [50]. All these data and the results of a study in adults (ages 17–25) confirm that VPA is a promising potential treatment [51].

Patients had high hopes when research on each of these drugs began; however, none of these drugs have been brought to market for various reasons.

The current treatment for SMA is insufficient. Several children were not cured after receiving nusinersen injections during the ENDEAR clinical study; several still need breathing support, assistance with feeding, and other daily routine tasks. In rare cases, motor development was interrupted, and some individuals died as a result [18]. Furthermore, the medication's intrathecal injection may cause tissue damage, infections, weakness, and malaise [14]. A large number of patients have severe scoliosis, making intrathecal injections extremely difficult, if not impossible [52]. Chronic treatment, on the other hand, can be a major challenge to conquer. Intranasal or oral administration is one technique to tackle this problem, although it is insufficient due to the blood-brain barrier. Because of the preservation of nusinersen in peripheral tissues, which can cause substantial damage, pharmacodynamics and pharmacokinetics must also be investigated [14]. Treatment with nusinersen did not result in significant improvement in adult patients. The compound muscle action potential (CMAP) was measured, and while there was an improvement in terms of higher CMAP amplitudes, there was a decrease in CMAP upon repetitive motor stimulation. Neuromuscular defects persist after treatment, implying that nusinersen treatment is ineffective in adult patients. Furthermore, there was a link between CMAP results and motor function (as measured by the 6-minute walk), fatigue, elbow extension, shoulder abduction, and revised upper limb module. This could imply that this disease is more complex in adults, and that in their case, all disabilities are secondary effects, and that restoring the SMN protein is insufficient. There is also no correlation between CMAP decrement and disease severity, disease duration, or patient age, implying that this disease is unique in each case and that there are individual differences. Examining these differences can lead to the best treatment method for each individual, but it will also be a difficult and time-consuming process. More clinical research is needed to confirm the short- and long-term tolerability, safety, effectiveness, and peripheral administration of all SMA therapies [52].

Another significant limitation is time, which is crucial in the treatment. To begin with, the disease is very aggressive, and the sooner treatment begins, the better the chances of survival and better results. The age of the patient at the start of therapy has a high correlation with treatment success. Early SMA therapy appears to be critical for enhancing therapeutic results. Elevating the SMN protein level in a patient with advanced SMA, who has essentially no motor neurons, is ineffective [18]. Second, the cost of drugs rises in tandem with the weight of the child, which rises rapidly as the child grows. Because of the extremely high cost of this treatment, it may be discontinued. Hopefully, the FDA recently accepted the lower pricing of risdiplam, which may result in other cost reductions [52].

It is also critical to reduce the cost of all treatments. Because of the extremely high cost, the treatment is not covered by all insurance companies in the United States. They added some restrictions to the treatment's availability, such as if the disease is caused by biallelic SMN1 mutations. Other restrictions include age, symptom manifestation, and disease stage. Furthermore, the high cost of the drug is not the only financial issue; there are numerous other expenses such as rehabilitation, the purchase of specialized equipment, administration costs, and the inability to work caused by the disease in the case of adult patients or parents who have children with SMA and must care for them full-time [53, 54, 55]. As a result, access to the treatment may be severely restricted.

Despite the fact that there are several approved medicines on the market and compounds in clinical trials, researchers are always looking for new drugs to help SMA sufferers. The future direction in which neuromuscular junction (NMJ) instability can be studied is worth emphasizing. It may be able to increase NMJ protection in adults by using phenotypic modifiers, and it may also be possible to undertake alterations in embryos that result in an improved postnatal phenotype. Mice have been used in such research with great success [56].

The goals of future research will then be to solve all of the previously mentioned problems. Because SMN-based therapies will be sufficient for the majority of SMA patients, it is crucial to enhance SMN restoration to improve motor function and to carefully examine all drug action and metabolism [52].

The discovery of genetic causes of SMA has allowed for the development of disease-treating therapies. Each of these therapies, however, has advantages and disadvantages. Nusinersen has transformed the lives of SMA children and their families. In 51% of patients, the treatment reduces the risk of death while also improving the patients' quality of life [18]. According to the findings of Sansone and colleagues (2020), the use of nusinersen aids in the treatment of respiratory problems by lowering mortality and the need for respiratory support [57]. However, there are some significant obstacles to overcome and better investigate in this treatment. Treating patients with spinal muscular atrophy who have bi-allelic mutations in the SMN1 gene with onasemnogene abeparvovec is a fantastic opportunity as well as a hopeful alternative to long-term treatment with nusinersen. The results of various medical trials demonstrated that this therapeutic has the potential to improve overall survival as well as life quality by providing better motor functions. The disadvantage of this gene therapy is the patient age limit (less than 2 years old), which prevents onasemnogene abeparvovec from being available to dozens of people. It is possible that future research will change this. Despite the fact that clinical trials are still ongoing, the results of treating SMA patients with risdiplam show promising potential. In comparison to natural history studies, patients who received risdiplam had higher survival rates and improved motor functions. Risdiplam and onasemnogene abeparvovec do not have any dangerous side effects. Clinical trials are being conducted to test various drugs that increase SMN levels or with muscle-enhancing therapies. It is also important to note that, despite all of the risks and side effects associated with this therapy, patients and their families are adamant about completing the treatment at any cost. Simple things like preparing breakfast or sitting independently are sufficient milestones for participants. They admit that they are willing to go to any length for these simple things, demonstrating the importance of full participation in the treatment and its ongoing improvement [58].