Spinal muscular atrophy: advances in research and consensus on care of patients

An early Transcriptomic Investigation in Adult Patients with Spinal Muscular Atrophy Under Treatment with Nusinersen

Spinal muscular atrophy (SMA) is a rare degenerative disorder with loss of motor neurons caused by mutations in the SMN1 gene. Nusinersen, an antisense oligonucleotide, was approved for SMA treatment to compensate the deficit of the encoded protein SMN by modulating the pre-mRNA splicing of SMN2, the centromeric homologous of SMN1, thus inducing the production of a greater amount of biologically active protein. Here, we reported a 10-month transcriptomics investigation in 10 adult SMA who received nusinersen to search for early genetic markers for clinical monitoring. By comparing their profiles with age-matched healthy controls (HC), we also analyzed the changes in miRNA/mRNAs expression and miRNA-target gene interactions possibly associated with SMA. A multidisciplinary approach of HT-NGS followed by bioinformatics/biostatistics analysis was applied. Within the study interval, those SMA patients who showed some clinical improvements were characterized by having the SMN2/SMN1 ratio slightly increased over the time, while in the stable ones the ratio decreased, suggesting that the estimation of SMN2/SMN1 expression may be an early indicator of nusinersen efficacy. On the other hand, the expression of 38/147 genes/genetic regions DE at T0 between SMA and HC like TRADD and JUND resulted "restored" at T10. We also confirmed the dysregulation of miR-146a(-5p), miR-324-5p and miR-423-5p in SMA subjects. Of interest, miR-146a-5p targeted SMN1, in line with experimental evidence showing the key role of astrocyte-produced miR-146a in SMA motor neuron loss. Molecular pathways such as NOTCH, NF-kappa B, and Toll-like receptor signalings seem to be involved in the SMA pathogenesis.

The SMN Complex at the Crossroad between RNA Metabolism and Neurodegeneration

In the cell, RNA exists and functions in a complex with RNA binding proteins (RBPs) that regulate each step of the RNA life cycle from transcription to degradation. Central to this regulation is the role of several molecular chaperones that ensure the correct interactions between RNA and proteins, while aiding the biogenesis of large RNA-protein complexes (ribonucleoproteins or RNPs). Accurate formation of RNPs is fundamentally important to cellular development and function, and its impairment often leads to disease. The survival motor neuron (SMN) protein exemplifies this biological paradigm. SMN is part of a multi-protein complex essential for the biogenesis of various RNPs that function in RNA metabolism. Mutations leading to SMN deficiency cause the neurodegenerative disease spinal muscular atrophy (SMA). A fundamental question in SMA biology is how selective motor system dysfunction results from reduced levels of the ubiquitously expressed SMN protein. Recent clarification of the central role of the SMN complex in RNA metabolism and a thorough characterization of animal models of SMA have significantly advanced our knowledge of the molecular basis of the disease. Here we review the expanding role of SMN in the regulation of gene expression through its multiple functions in RNP biogenesis. We discuss developments in our understanding of SMN activity as a molecular chaperone of RNPs and how disruption of SMN-dependent RNA pathways can contribute to the SMA phenotype.

Spinal muscular atrophy carrier frequency in Saudi Arabia

BACKGROUND

Spinal Muscular Dystrophy (SMA) is one of the leading causes of death in infants and young children from heritable diseases. Although no large-scale popultion-based studies have been done in Saudi Arabia, it is reported that the incidence of SMA is higher in the Saudi population partly because of the high degree of consanguineous marriages.

METHODS

The final analysis included 4198 normal volunteers aged between 18 and 25 years old, 54.7% males, and 45.3% females. Whole blood was spotted directly from finger pricks onto IsoCode StixTM and genomic DNA was isolated using one triangle from the machine. To discern the SMN1 copy number independently from SMN2, Multiplex PCR with Dral restriction fragment analysis was completed. We used the carrier frequency and population-level data to estimate the prevalence of SMA in the population using the life-table method.

RESULTS

This data analysis showed the presence of one copy of the SMN1 gene in 108 samples and two copies in 4090 samples, which resulted from a carrier frequency of 2.6%. The carrier frequency was twofold in females reaching 3.7% compared to 1.6% in males. 27% of participants were children of first-cousin marriages. We estimated the birth incidence of SMA to be 32 per 100,000 birth and the total number of people living with SMA in the Kingdom of Saudi Arabia to be 2265 of which 188 are type I, 1213 are type II, and 8,64 are type III.

CONCLUSION

The SMA carrier rate of 2.6% in Saudi control subjects is slightly higher than the reported global frequency of 1.25 to 2% with links to the high degree of consanguinity.

Understanding European patient expectations towards current therapeutic development in spinal muscular atrophy

Following the 2017 approval of a first spinal muscular atrophy (SMA) treatment by the European Medicines Agency, SMA Europe launched a Europe-wide survey with the goal of understanding patients' treatment expectations, realities of daily living and access to clinical trials and therapy, and how this varied according to parameters such as age and disease severity. A response rate of 31% yielded 1474 completed surveys from 26 European countries. In line with findings from a 2015 SMA Europe-led survey, participants considered stabilization of their condition to be progress. Notably, responses indicated that the current classification of SMA at diagnosis by 'type' often does not reflect current mobility level. Large gaps in treatment access were identified that varied in particular between age and disease severity groups, yet there was high interest in clinical trial participation. In addition, alternative treatment options, including combination therapies, are now expectations. These perspectives should be central considerations through the research and development processes of new SMA therapies, through data generation and discussions on access to therapies. Results from this survey indicate that collaboration between stakeholders is essential to the foundation upon which innovative approaches for SMA treatments and access can be explored.

Gene therapies for axonal neuropathies: Available strategies, successes to date, and what to target next

Nearly one-hundred loci in the human genome have been associated with different forms of Charcot-Marie-Tooth disease (CMT) and related inherited neuropathies. Despite this wealth of gene targets, treatment options are still extremely limited, and clear "druggable" pathways are not obvious for many of these mutations. However, recent advances in gene therapies are beginning to circumvent this challenge. Each type of CMT is a monogenic disorder, and the cellular targets are usually well-defined and typically include peripheral neurons or Schwann cells. In addition, the genetic mechanism is often also clear, with loss-of-function mutations requiring restoration of gene expression, and gain-of-function or dominant-negative mutations requiring silencing of the mutant allele. These factors combine to make CMT a good target for developing genetic therapies. Here we will review the state of relatively established gene therapy approaches, including viral vector-mediated gene replacement and antisense oligonucleotides for exon skipping, altering splicing, and gene knockdown. We will also describe earlier stage approaches for allele-specific knockdown and CRIPSR/Cas9 gene editing. We will next describe how these various approaches have been deployed in clinical and preclinical studies. Finally, we will evaluate various forms of CMT as candidates for gene therapy based on the current understanding of their genetics, cellular/tissue targets, validated animal models, and availability of patient populations and natural history data.

A Pathogenic Missense Variant (c.1617G>A, p.Met539Ile) in UBA1 Causing Infantile X-Linked Spinal Muscular Atrophy (SMAX2)

Background: Infantile X-linked spinal muscular atrophy (SMAX2) is a rare type of spinal muscular atrophy associated with UBA1 variants. Methods: Clinical imaging and neurophysiological tests were performed on a Chinese patient with SMAX2. Further, focused panel sequencing of UBA1 was carried out on samples of both the proband and his maternal relatives. Results: The proband, a 4-year-old boy with the SMAX2 phenotype, suffered from reduced exercise capacity since infancy. His other symptoms included speech difficulties, severe nasal tone, reduced distal muscle strength, areflexia, and inadequate sucking ability. The brain MRI of the proband's showed normal results but the electromyography results showed multiple peripheral neurogenic lesions. Five male members of the proband's family were affected with the SMAX2 phenotype. They presented similar symptoms and had experienced a long and autonomous life. Molecular analysis revealed a novel missense variant (c.1617G>A, p.Met539Ile) in the exon 15 of UBA1. The proband's mother, as well as grandmother, carried the heterozygous missense UBA1 variant; whereas, the male patients from the family carried the hemizygotic variant. Conclusions: The affected members in this Chinese family showed unique features such as extended life span, no fractures, and cramps as compared with previously reported SMAX2 cases. The novel missense variant (c.1617G>A (p.Met539Ile) in UBA1 highlights the critical role of this gene in causing SMAX2 phenotype.

Motor neuron biology and disease: A current perspective on infantile-onset spinal muscular atrophy

Infantile-onset spinal muscular atrophy (SMA) is a prototypical disease in which to investigate selective neurodegenerative phenotypes. Caused by low levels of the ubiquitously expressed Survival Motor Neuron (SMN) protein, the disease mainly targets the spinal motor neurons. This selective phenotype remains largely unexplained, but has not hindered the development of SMN repletion as a means to a treatment. Here we chronicle recent advances in the area of SMA biology. We provide a brief background to the disease, highlight major advances that have shaped our current understanding of SMA, trace efforts to treat the condition, discuss the outcome of two promising new therapies and conclude by considering contemporary as well as new challenges stemming from recent successes within the field.

Newborn genetic screening for spinal muscular atrophy in the UK: The views of the general population

Background

Spinal muscular atrophy (SMA) is an inherited neuromuscular disorder and a leading genetic cause of infant death worldwide. However, there is no routine screening program for SMA in the UK. Lack of treatments and the inability of screening tests to accurately predict disease severity are among the key reasons implementation of screening has faltered in the UK. With the recent release of the first therapy for SMA (Nusinersen), calls are being made for a reconsideration of this stance; however, very little is known about the views of the general public.

Methods

An online survey was administered to 232 individuals with no prior relationship with SMA to assess their attitudes toward a newborn screening program for it. Results are compared with previously gathered data on the views of SMA‐affected families toward screening.

Results

Eighty‐four percent of participants were in favor of newborn screening. Key reasons for support were a belief that it would lead to better healthcare and life expectancy for affected infants and facilitate informed decision‐making for future pregnancies. Key reasons for nonsupport were a belief in the potential for significant negative impact on the family unit in terms of bonding and stress.

Conclusions

Public acceptability is a key component in the evaluation of any potential screening program in the UK. This study demonstrates that newborn screening for SMA is viewed largely positively by people unfamiliar with the condition. The importance of early identification overrode all other social and ethical concerns about screening for the majority of participants.

Disease impact on general well-being and therapeutic expectations of European Type II and Type III spinal muscular atrophy patients

Spinal muscular atrophy (SMA) is a neurodegenerative disorder showing a broad clinical spectrum and no cure to date. To design and select evaluation criteria for the potential assessment of drugs currently being developed, the patient's perspective is critical. A survey, aiming to obtain a view on the current clinical state of European Type II and Type III SMA patients, the impact of this situation on their quality of life and their expectations regarding clinical development, was carried out by SMA-Europe member organizations in July 2015. A questionnaire was set up, translated into 8 European languages and sent out directly via electronic mailing to the targeted SMA patient population by the respective European patient organizations. We were able to collect 822 valid replies in less than two weeks. The questionnaire captured the current abilities of the respondents, their perception of the disease burden which appeared very similar across Europe despite some regional variations in care. According to the great majority of the respondents, stabilization of their current clinical state would represent a therapeutic progress for a compelling majority of the respondents to the questionnaire.

Commonality amid diversity: Multi-study proteomic identification of conserved disease mechanisms in spinal muscular atrophy

The neuromuscular disease spinal muscular atrophy (SMA) is a leading genetic cause of infant mortality, resulting from low levels of full-length survival motor neuron (SMN) protein. Despite having a good understanding of the underlying genetics of SMA, the molecular pathways downstream of SMN that regulate disease pathogenesis remain unclear. The identification of molecular perturbations downstream of SMN is required in order to fully understand the fundamental biological role(s) for SMN in cells and tissues of the body, as well as to develop a range of therapeutic targets for developing novel treatments for SMA. Recent developments in proteomic screening technologies have facilitated proteome-wide investigations of a range of SMA models and tissues, generating novel insights into disease mechanisms by highlighting conserved changes in a range of molecular pathways. Comparative analysis of distinct proteomic datasets reveals conserved changes in pathways converging on GAP43, GAPDH, NCAM, UBA1, LMNA, ANXA2 and COL6A3. Proteomic studies therefore represent a leading tool with which to dissect the molecular mechanisms of disease pathogenesis in SMA, serving to identify potentially attractive targets for the development of novel therapies.

Spinal muscular atrophies

Spinal muscular atrophies (SMA) are genetic disorders characterized by degeneration of lower motor neurons. The most frequent form is caused by mutations of the survival motor neuron 1 gene (SMN1). The identification of this gene greatly improved diagnostic testing and family-planning options of SMA families. SMN plays a key role in metabolism of RNA. However, the link between RNA metabolism and motor neuron degeneration remains unknown. A defect in mRNA processing likely generates either a loss of function of some critical RNA or abnormal transcripts with toxic property for motor neurons. Mutations of SMN in various organisms highlighted an essential role of SMN in motor axon and neuromuscular junction development or maintenance. The quality of life of patients has greatly improved over recent decades through the improvement of care and management of patients. In addition, major advances in translational research have been made in the field of SMA. Various therapeutic strategies have been successfully developed aiming at acting on SMN2, a partially functional copy of the SMN1 gene which remains present in patients. Drugs have been identified and some are already at preclinical stages. Identifying molecules involved in the SMA degenerative process should represent additional attractive targets for therapeutics in SMA.

Raising obstetricians' awareness of spinal muscular atrophy: towards early detection and reproductive planning

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder that is caused by degeneration of α motor neurons in the spinal cord anterior horns. This degeneration can lead to progressive atrophy of proximal muscles, weakness, respiratory failure and death in severe cases. SMA is the most common neuromuscular disease of childhood and one of the main causes of infant death, with no cure in sight. This review highlights the impact of the disease in families, summarizes genetics and ultrasound advances, discusses how obstetricians can work towards its early detection and explores the options for reproductive planning.

Aquatic therapy for a child with type III spinal muscular atrophy: a case report

Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by degeneration of alpha motor neurons. This case report describes an aquatic therapy program and the outcomes for a 3-year-old girl with type III SMA. Motor skills were examined using the 88-item Gross Motor Function Measure (GMFM), the Peabody Developmental Motor Scales (PDMS-2), and the GAITRite system. The child received aquatic therapy twice per week for 45-min sessions, for 14 weeks. The intervention included aquatic activities designed to improve gross motor skills and age-appropriate functional mobility. The GMFM total score improved by 11% following the intervention. The Standing Dimension score improved by 28% and the Walking, Running, and Jumping Dimension score improved by 18%. The gross motor quotient for the PDMS-2 improved from 66 to 74. The child's gait showed improvement in walking velocity, stride length, and single-limb support time as a percentage of the gait cycle. The outcomes of this case report demonstrate the successful improvement of gross motor function and gait in a 3-year-old child with SMA. This study provides clinical information for therapists utilizing aquatic therapy as a modality for children with neuromuscular disorders.

Sperm and oocyte communication mechanisms controlling C. elegans fertility

During sexual reproduction in many species, sperm and oocyte secrete diffusible signaling molecules to help orchestrate the biological symphony of fertilization. In the Caenorhabditis elegans gonad, bidirectional signaling between sperm and oocyte is important for guiding sperm to the fertilization site and inducing oocyte maturation. The molecular mechanisms that regulate sperm guidance and oocyte maturation are being delineated. Unexpectedly, these mechanisms are providing insight into human diseases, such as amyotrophic lateral sclerosis, spinal muscular atrophy, and cancer. Here we review sperm and oocyte communication in C. elegans and discuss relationships to human disorders.

Spinal muscular atrophy: mechanisms and therapeutic strategies

Spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disorder and a leading genetic cause of infantile mortality. SMA is caused by mutation or deletion of Survival Motor Neuron-1 (SMN1). The clinical features of the disease are caused by specific degeneration of alpha-motor neurons in the spinal cord, leading to muscle weakness, atrophy and, in the majority of cases, premature death. A highly homologous copy gene (SMN2) is retained in almost all SMA patients but fails to generate adequate levels of SMN protein due to its defective splicing pattern. The severity of the SMA phenotype is inversely correlated with SMN2 copy number and the level of full-length SMN protein produced by SMN2 ( approximately 10-15% compared with SMN1). The natural history of SMA has been altered over the past several decades, primarily through supportive care measures, but an effective treatment does not presently exist. However, the common genetic etiology and recent progress in pre-clinical models suggest that SMA is well-suited for the development of therapeutic regimens. We summarize recent advances in translational research that hold promise for the progression towards clinical trials.

Clinical outcome measures in spinal muscular atrophy

Spinal muscular atrophy is one of the most devastating neurological diseases of childhood. Affected infants and children suffer from often severe muscle weakness caused by degeneration of lower motor neurons in the spinal cord and brainstem. Identification of the causative genetic mutation in most cases has resulted in development of potential treatment strategies. To test these new drugs, clinically feasible outcomes are needed. Several different assessments, validated in spinal muscular atrophy or similar disorders, are being used by national and international research groups; however, their sensitivity to detect change is unknown. Acceptance of a few standardized, easily administered, and functionally meaningful outcomes, applicable to the phenotypic spectrum of spinal muscular atrophy, is needed. Consensus is imperative to facilitate collaboration and explore the ability of these measures to identify the therapeutic effect of disease-modifying agents. Following is an evidence-based review of available clinical outcome measures in spinal muscular atrophy.

Oligonucleotide-mediated survival of motor neuron protein expression in CNS improves phenotype in a mouse model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by homozygous mutation or deletion of the SMN1 gene encoding survival of motor neuron (SMN) protein, resulting in the selective loss of alpha-motor neurons. Humans typically have one or more copies of the SMN2 gene, the coding region of which is nearly identical to SMN1, except that a point mutation causes splicing out of exon 7 and production of a largely nonfunctional SMNDelta7 protein. The development of drugs that mitigate aberrant SMN2 splicing is an attractive therapeutic approach for SMA. A steric block antisense oligonucleotide (AO) has recently been developed that blocked an intronic splice suppressor element, and enhanced SMN2 exon 7 inclusion in SMA patient fibroblasts. Here, we show that periodic intracerebroventricular (ICV) delivery of this AO resulted in increased SMN expression in brain and spinal cord to as much as 50% of the level of healthy littermates. Real-time PCR of SMN2 transcripts confirmed the AO-mediated increase in full-length SMN. The AO-derived increase in SMN expression led to a concomitant improvement in bodyweight throughout the lifespan of the SMA animals. Treatment of SMA mice with AO also provided partial correction of motor deficits, manifest as improved righting response. Injections of a scrambled oligonucleotide had no effect on SMN expression or phenotype in the SMA mice. Our results validate that AOs that abrogate aberrant splicing of SMN2 are promising compounds for treating SMA.