Management and Therapeutic Strategies for Spinal Muscular Atrophy

Spinal muscular atrophy is an autosomal recessive neuromuscular disorder characterized by progressive muscle weakness and atrophy. It is one of the most common single-gene disorders with an incidence rate of approximately 1 in 10,000 live births. The clinical manifestations are progressive hypotonia and muscle weakness due to the degeneration of alpha neurons in the anterior horn cells of the spinal cord and motor nuclei in the lower brain stem. Depending on the severity of the symptoms, SMA has five subtypes. Supportive measures can be offered for respiratory, gastrointestinal, and musculoskeletal complications. Carrier testing for all couples is recommended and this can be done by Multiplex Ligation-dependent Probe Amplification (MLPA). Prenatal diagnosis can be offered to carrier couples. Therapies must be given within the newborn period for maximum benefit and before the loss of motor neurons. It is achieved by identifying the SMA babies through Newborn screening. Several new FDA-approved drugs can reduce the progression of symptoms in SMA. However, they cannot offer a definite cure. Clinical follow-up and Neurological assessment demonstrate that SMA children can attain developmental milestones after receiving treatment, which is never normally attained in untreated cases. In utero SMA treatment with Zolgensma would enhance the survival rate and favorable neurological outcomes in the future. Base editing and Gene editing with CRISPR-Cas technologies to target the mutations and restore functional and stable SMN protein levels are the future hopes for a permanent cure of SMA..

SMA is an autosomal recessive neuromuscular disorder characterized by progressive muscle weakness and atrophy and it affects both males and females. SMA was the commonest single gene disorder leading to infant mortality before the FDA approval of disease-modifying treatments [1]. This genetic disorder has an incidence rate of approximately 1 in 6000 - 10,000 live births and with a carrier frequency of 1 in 40 - 60 [1-3]. In the USA, the incidence rate of SMA has been reported to be 1 in 10,000 with a carrier frequency of 1 in 50. A recent Indian study revealed that SMA carrier frequency was 1 in 38 and an incidence of 1 in 10,000 live births [4].

The first two cases of SMA were reported by Austrian neurologist Guido Werdnig in 1891 [5]. It was affirmed as SMA Type 2 after correlating with the current classification of SMA. Several other phenotypes have been reported later. The severest one is fetal-onset form and the mildest is adult-onset form [6,7]. In 1990, the SMA loci were mapped to chromosome 5q13. The genes, SMN1 and SMN2 (SMA-modifier) were identified in 1995. It has led to a great leap in SMA research. Since then diagnosis has been confirmed based on the genetic testing of SMN1 deletion/duplication or mutation instead of muscle biopsy. SMA has been recognized as a progressive neurological disease with high infant mortality since the 19^th century; data remains the same even after the introduction of new drugs at the beginning of the 21^st century. Several new FDA-approved drugs can reduce the progression of symptoms in SMA. However, they cannot offer a definite cure. In this review, we focus on future management strategies like Base editing and Gene editing with CRISPR-Cas technologies to target the mutations and restore functional and stable SMN protein levels for a permanent cure.

Classification

SMA is classified into five subtypes.

1. Type 0 is the most severe one with prenatal onset and severe hypotonia and respiratory failure soon after birth.

2. Type I Werdnig–Hoffmann disease with hypotonia, symmetrical proximal muscle weakness, inability to sit alone, and onset before 6 months of age.

3. Type II is Dubowitz disease with the inability to stand or walk and onset before 18 months of age.

4. Type III isKugelberg–Welander disease with an onset after 18 months of age. 

5. Type IV is the mildest form with onset after 30 years of age.

Clinical features and natural progression of SMA are detailed in Table 1 [6-9].

Multidisciplinary management in SMA

Progressive hypotonia and muscle weakness are the first signs of SMA. It is due to the degeneration of alpha neurons in the anterior horn cells of the Spinal cord and motor nuclei in the lower brain stem. Severe gross motor developmental delay has been noticed in SMA infants. They are prone to develop Pulmonary, Gastrointestinal, and Musculoskeletal complications and psychosocial problems. The quality of life of SMA patients has been achieved through a multidisciplinary approach to management.

Pulmonarycare: Children with SMA have difficulty breathing and coughing due to weak respiratory muscles and rib cage deformities. They are more prone to having sucking and swallowing difficulties and respiratory complications as of bulbar involvement. Facial and ocular muscles are usually spared. Respiratory muscle weakness leads to difficulty in clearing lower respiratory secretions and hypoventilation while sleeping. They are at increased risk of acute respiratory decompensations. The major cause of mortality in infants and children with type 1 and 2 SMA is usually respiratory failure. It’s also mandatory to administer recommended vaccinations to reduce their risk of infection. Chest physiotherapy with postural drainage and early intervention with noninvasive respiratory support is beneficial for SMA type 1 [10-12].

Nutritional care: Aspiration of food and failure to thrive occur because of bulbar dysfunction. It can be manifest as tongue weakness, difficulty in opening the mouth, and poor head control. Other associated problems like gastrointestinal reflux, delayed gastric emptying, and constipation are also seen. Symptoms worsened over a period leading to severe malnutrition.

We can advise changing the consistency of food to increase the intake and thus prevent aspiration. Naso gastric feeding techniques have been advised. Early gastrostomy is another option.

Musculoskeletal care: In immobilized cases, contractures are common. Physiotherapy and spinal bracing can be advised for musculoskeletal complications. It can delay the development of progressive scoliosis that is caused by muscle weakness [13]. The main goal of therapy is to prevent contractures. Other activities such as swimming, aquatic therapy, and adaptive sports are also beneficial.

Manual or motorized wheelchairs can be given as early as 18 months - 24 months of age.

Psychosocial support: Cure SMA India is a parent-led organization for the welfare of SMA families. They are conducting awareness programs to provide psychosocial support and care initiatives on compassionate grounds. They focus on multidisciplinary management through unconditional support to achieve the best possible quality of life and to increase the survival rate.

Genetics of SMA

SMA inheritance: As discussed earlier, SMA is an autosomal recessive neuromuscular disorder. An SMA carrier is an asymptomatic person with a functional copy of the SMN1 gene and an anon-functional copy of SMN1 - ie. (1 + 0) classical carrier. If the partner’s test is negative, the chance of having an affected child is extremely low. If the partner is also a carrier for SMA, there is a 25% risk of having a baby with SMA in each pregnancy. If two parents with no family history of SMA are found to be carriers of routine screening, it is not possible to predict whether they are at risk of having children with the severe form of SMA or one of the less severe forms of SMA. Accordingly, parents of SMA cases are not always carriers, and a small proportion of them are considered de novo or germinal mosaic cases. In that scenario explain the final risk for a given couple. Usually, SMA carrier testing implies the performance of a quantitative method that detects one copy of SMN1 in classical 1/0 carriers. A small proportion of carriers 3% - 4% have two SMN1 copies in cis and none in the other allele 2/0 carriers [14-18]. 

Chromosomal location (Figure 1): SMA-related genes, SMN1 and SMN2are paralogs and located in q13 region of human chromosome 5. SMN1 gene is located in the telomeric side and SMN2 gene is located in the centromeric side. The SMN1 gene, located in the telomeric side, is the gene responsible for SMA; its loss or defect causes SMA with different phenotypes. The SMN2 gene, located in the centromeric side, is a modifier gene for SMA; its copy number is associated with the severity of the disease.

SMA has been confirmed by genetic testing either due to deletion/duplication of the SMN 1 gene or SMN mutation. 95% of cases of SMA are due to a homozygous deletion or mutation of the Survival Motor Neuron 1 (SMN1) gene. SMN1 and the SMA-modifier gene, SMN2 were identified in 1995 [19]. SMN1 gene and SMN2 gene are located within an inverted duplication on chromosome 5q13.2. SMN1 lies telomeric of SMN2. 

The SMN1 and SMN2 genes are almost identical and the only difference is a single nucleotide change at the beginning of exon 7 C for SMN1 and T for SMN2 gene. The exon 7 with C nucleotide plays a crucial role in the production of fully functional and stable SMN protein. The exon 7 with T nucleotide in SMN2 cannot fully produce functional and stable SMN protein. Thus SMN1 gene produces 85% to 90% of functional SMN protein and the rest around 10% to 15% by the SMN2 gene [20,21].

The deletions or mutations in the SMN1 gene on chromosome 5q are the most common form of SMA, but rare non-5q SMA cases have also been reported. These point mutations also cause the production of non-functional or unstable SMN proteins. The severity of SMA depends on the number of SMN2 gene copies. More copies of the SMN2 gene mean less severe form of SMA. Four or more copies of SMN2 are present in milder phenotype [22]. 

While the most common forms of SMA about 95% are caused by deletions or mutations in the SMN1 on chromosome 5q (ie, 5q SMAs), about 5% are compound heterozygotes: they have a deletion or mutation on one of their chromosome 5, and a point mutation on the other chromosome 5, Some rare non-5q spinal muscular atrophies have also been reported [23-25]. The non-5q SMAs are genetically and clinically different.

Genetic testing for spinal muscular atrophy

Carrier testing can be done through a simple blood test. Multiplex ligation-dependent probe amplification (MLPA) is one of the most popular methods used as an initial test as it is convenient, highly sensitive, and capable of determining both SMN1 and SMN2 copy numbers and it aids in the identification of types of SMA. With 95% of affected individuals having a homozygous deletion of SMN1 exon 7, screening for the loss of exon 7 is the first tier in diagnostic testing [26-37]. Another 5% of cases will be caused by other mutations in the SMN1 gene [38,39]. They are compound heterozygotes and have been shown to have a variety of different types of SMN1 mutations including missense mutations [40], nonsense mutations [41], splice site mutations [42,43], insertions, and small deletions. There are rare SMA-affected patients with a single copy of SMN1 and an unidentified second mutation [44-46].

Carrier analysis in family

For asymptomatic siblings of SMA children, genetic testing is offered for age groups more than 18 years. Therefore, genetic counseling is given when they reach reproductive age to ensure that the decision to undergo a genetic test is taken by themselves with adequate information and understanding [47]. If the sibling has one mutated SMN1 copy, then carrier testing must be offered to the partner also. Once the partner also confirms as a carrier, then explain the risk of 25% to their offspring.

Genetic reports are confidential and the genetic counselor/Geneticist can guide the disclosure of the genetic report after getting the patient’s consent.

Prenatal testing for carriers 

The American College of Medical Genetics recommends carrier testing for all couples regardless of race or ethnicity [48,49]. The American College of Obstetricians and Gynaecologists (ACOG) emphasized that screening for SMA should be done for all women before pregnancy [50-52]. 

Detailed genetic counseling is given by a Geneticist/Genetic counselor. During counseling sessions, various options have been given to the couples to make a decision pre-conceptionally and plan future pregnancies. 

Genetic counseling: Genetic counseling is recommended for 

1. Those who are married in relations – Consanguineous marriage. 

2. Those who have a positive family history with SMA or a family member known to be a carrier. 

3. An individual without a family history of SMA can be a carrier for this condition as well. About 1 in 40, regardless of ethnic background are also carriers of SMA.

Prenatal diagnosis: It is possible by performing Chorionic villus sampling (CVS) at 11 to 13 weeks or Amniocentesis at 16 to 20 weeks to determine if the fetus is affected or not.

Other options which can be offered are:

1. In Intrauterine Insemination (IUI), sperm from a donor who is not a carrier would be used.

2. In vitro Fertilization (IVF) with their eggs or sperm/ can use donor eggs or sperm. 

3. Adoption.

4. PGD-Preimplantation Genetic Diagnosis.

Preimplantation Genetic Diagnosis (PGD) is an option to prevent the transfer of an affected embryo during the IVF.PGD is done on one or two single blastomeres taken from the Day 3 embryo after fertilization [53]. We can select and transfer an unaffected embryo to the uterus.PGD of SMA can be offered to those having an affected child with homozygous exon 7 deletion [54].

Therapeutic approaches

SMA has been considered an incurable disease even at the beginning of the 21^st century. Even though the new drugs have reduced the progression of symptoms in SMA, they cannot offer a definite cure. 

Recent therapeutic strategies: Disease-modifying therapy (DMT) with nusinersen, onasemnogeneabeparvovec, and risdiplam has been available recently. The Route of administration is different for each therapy.

Nusinersen(Spinraza) is administered by intrathecal injection with maintenance doses once in four months after the loading doses, Loading doses are given as 4 doses, each one in 2 weeks over 8 weeks duration. It is an antisense oligonucleotide that modifies the splicing of the SMN2 gene, thus increasing the availability of functional full-length survival motor neuron protein [55-59].

Risdiplam is administered orally. Risdiplam is a small molecule that alters SMN2 splicing to increase functional SMN protein. It was approved by the FDA in 2020 and by the Drug Controller General of India (DCGI) in 2021. The government of Kerala has started issuing Risdiplam for children less than 5 years of age since July 2022. Clinical follow-up and Neurological assessment demonstrate that SMA children can attain developmental milestones after receiving treatment, which is never normally attained in untreated SMA patients and late-onset patients [60-63].

Gene therapy: Onasemnogeneabeparvovec (Zolgensma) is administered as a single-dose intravenous infusion. Onasemnogeneabeparvovec (Zolgensma) is a gene therapy that utilizes an adeno-associated virus serotype 9 vector to increase the low functional SMN protein levels. In 2019, the U.S. Food and Drug Administration approved Zolgensma, which alters the underlying genetic cause of spinal muscular atrophy and helps to stop the disease’s progression. It has been approved for treating SMA cases with homozygous mutation in SMN1 [64,65].

Future perspectives

Therapies must be given within the newborn period for maximum benefit and before the loss of motor neurons. It can achieved by identifying the SMA in infants through Newborn screening. Newborn Screening (NBS) is intended to find out those newborns who are at increased risk of certain genetic disorders. Newborn screening can be done on dried blood samples to confirm homozygous deletion of exon 7 in SMN1. The most beneficial response to SMA treatments has been documented for the treatment of pre-symptomatic SMA cases. Newborn screening and adequate pre-test genetic counseling are essential for providing precise information to the families. In utero SMA treatment with Zolgensma would enhance the survival rate and favorable neurological outcomes in the future. We can assess the acceptance of parents with SMA for prenatal diagnosis, fetal therapies with gene therapy being favored, and clinical trials. Future research and follow-up are needed to understand the long-term outcome of treatment with single therapy or combination therapies [66-68].

Gene editing - Innovative therapeutic strategy (CRISPR and base editing)

The Clustered Regularly Interspaced Short Palindromic Repeat sequences (CRISPR) and the associated Cas nuclease system are the most popular genome editing platforms due to their simplicity, cost-effectiveness, and specificity in gene editing by introducing specific nucleotides. The mutated DNA sequence is edited with a guide RNA – Cas protein complex and this complex is introduced into the cells for gene editing with a viral vector or a nanoparticle. The guide RNA will have some length of homology with the gene to be edited/corrected. CRISPR/Cas facilitates nonhomologous end joining, homology-directed synthesis, nonhomologous end joining, and single base exchanges. At present a variety of CRISPR/Cas based therapeutics are being used in gene therapy in several human genetic diseases. Pluripotent stem cells of placental origin or extracted from patients following gene editing using CRISPR/Cas can be reintroduced into the patients [69]. Great progress has been made in recent times in genome editing. Apart from CRISPR/Cas, several nucleases capable of genome editing like zinc finger nuclease, transcription-activated effector nucleases, etc. are available. However, they lack the versatility, simplicity, and utility of gene therapy. Before undertaking gene manipulation therapy, several issues such as the ethical, legal, and social implications of gene editing technologies should be considered.

A recent study at Harvard Medical School and Massachusetts General Hospital showed that fibroblasts from SMA patients as well as a mouse model of the disease could have their levels of the Survival Motor Neuron (SMN) protein restored by using Adenosine Base Editors (ABEs). The results proved that using base editing or other CRISPR-based techniques to target the genetic mutations restored functional and stable SMN protein levels leading to permanent cure in SMA patients [70].

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