The Importance of Digging into the Genetics of SMN Genes in the Therapeutic Scenario of Spinal Muscular Atrophy

Research trends on spinal muscular atrophy from 1995 to 2023: A bibliometric analysis

BACKGROUND

Spinal muscular atrophy (SMA) is a neuromuscular disease characterized by progressive muscle weakness due to motor neuron degeneration. The discovery of the survival motor neuron 1 (SMN1) gene in 1995 revolutionized SMA research, leading to significant therapeutic advancements. This bibliometric analysis aimed to explore global trends in SMA research and therapy, with a particular focus on China.

METHODS

A comprehensive database search identified 4506 relevant publications (3812 articles, 694 reviews) published between 1995 and 2023. Bibliometric tools were used to analyze publication trends, collaborations, and research topics.

RESULTS

SMA research has experienced substantial growth, with the United States leading in publications followed by the United Kingdom and Germany. China has shown increasing engagement in this field. Key research areas include genetic and molecular mechanisms, survival motor neuron gene therapy, antisense oligonucleotides, and muscle strength-promoting factors. Chinese researchers have contributed significantly to these areas, with a higher reporting frequency of SMA-related topics compared to other countries.

CONCLUSION

This bibliometric analysis provides a comprehensive overview of global SMA research, highlighting significant advancements, and identifying future directions. The findings offer valuable insights for researchers, clinicians, and policymakers in China to ensure alignment with global medical advancements and improve the lives of individuals affected by SMA.

Long-read sequencing identifies copy-specific markers of SMN gene conversion in spinal muscular atrophy

BACKGROUND

The complex 2 Mb survival motor neuron (SMN) locus on chromosome 5q13, including the spinal muscular atrophy (SMA)-causing gene SMN1 and modifier SMN2, remains incompletely resolved due to numerous segmental duplications. Variation in SMN2 copy number, presumably influenced by SMN1 to SMN2 gene conversion, affects disease severity, though SMN2 copy number alone has insufficient prognostic value due to limited genotype-phenotype correlations. With advancements in newborn screening and SMN-targeted therapies, identifying genetic markers to predict disease progression and treatment response is crucial. Progress has thus far been limited by methodological constraints.

METHODS

To address this, we developed HapSMA, a method to perform polyploid phasing of the SMN locus to enable copy-specific analysis of SMN and its surrounding genes. We used HapSMA on publicly available Oxford Nanopore Technologies (ONT) sequencing data of 29 healthy controls and performed long-read, targeted ONT sequencing of the SMN locus of 31 patients with SMA.

RESULTS

In healthy controls, we identified single nucleotide variants (SNVs) specific to SMN1 and SMN2 haplotypes that could serve as gene conversion markers. Broad phasing including the NAIP gene allowed for a more complete view of SMN locus variation. Genetic variation in SMN2 haplotypes was larger in SMA patients. Forty-two percent of SMN2 haplotypes of SMA patients showed varying SMN1 to SMN2 gene conversion breakpoints, serving as direct evidence of gene conversion as a common genetic characteristic in SMA and highlighting the importance of inclusion of SMA patients when investigating the SMN locus.

CONCLUSIONS

Our findings illustrate that both methodological advances and the analysis of patient samples are required to advance our understanding of complex genetic loci and address critical clinical challenges.

Analysis of SMN protein in umbilical cord blood and postnatal peripheral blood of neonates with SMA: a rationale for prompt treatment initiation to prevent SMA development

BACKGROUND

Spinal muscular atrophy (SMA) is a severe genetic neuromuscular disease caused by insufficient functional survival motor neuron protein (SMN). The SMN expression level in the spinal cord is highest during the 2nd trimester of the foetal period. We previously reported the SMN spot analysis in peripheral blood using imaging flow cytometry (IFC) as a biomarker of functional SMN protein expression. In this study, we analysed neonatal cord blood, postnatal peripheral blood, and maternal peripheral blood in presymptomatic five infants whose sibling has type 1 SMA to estimate prenatal and postnatal SMN dynamics before the onset of severe SMA.

RESULTS

Data from 37 untreated patients with SMA showed that SMN-spot+ cells were significantly correlated with SMA clinical classification and the copy numbers of the SMN2 gene. The range of values for cord blood, converted from each SMN2 copy number statistics, was - 0.7 to + 2.0 standard deviation (SD) (0.1-24.0%) for SMN-spot+ cells in patients with SMA. Subsequent analyses of the peripheral blood of neonates ranged from - 0.8 to + 0.8 SD (0.4-15.2%). The analysis of each maternal blood, converted from carrier statistics, ranged from - 0.2 to + 2.4 SD (1.4-25.2%). A correlation was observed between the cord blood and maternal peripheral blood.

CONCLUSIONS

This study suggests that the status of the motor neuron pool in the spinal cord can be presumed by cord blood SMN-spot+ cells and that SMN protein depletion determines the timing of disease onset. As the SMN spot analysis values tended to decrease with time after birth, they may eventually lead to the development of SMA. Furthermore, a correlation was found between the SMN spot analysis values of neonatal cord blood and maternal blood, which predicts disease severity after birth. In other words, the SMN protein supplied from the mother to the foetus may suppress the development of SMA in the infant at birth, and depletion of the SMN protein may occur after birth, causing the infant to develop SMA. Our findings demonstrated the effectiveness of newborn screening and the potential of maternally mediated treatment strategies by providing a rationale for prompt treatment initiation in SMA.

Neonatal Screening for Spinal Muscular Atrophy and Severe T- and B-Cell Lymphopenias in Andalusia: A Prospective Study

Spinal muscular atrophy (SMA) and severe T- and/or B-cell lymphopenias (STBCL) in the form of severe combined immunodeficiencies (SCID) or X-linked agammaglobulinemia (XLA) are rare but potentially fatal pathologies. In January 2021, we initiated the first pilot study in Spain to evaluate the efficacy of a very early detection technique for SMA and SCID. RT-PCR was performed on prospectively collected dried blood spots (DBSs) from newborns in Western Andalusia (Spain). Internal and external controls (SCID, XLA and SMA) were included. The determination of SMA was relative (positive/negative) and that of TRECs and KRECs was quantitative (copies/punch). A total of 14.035 prospective samples were analysed. All controls were correctly identified while no cases of SMA or SCID/XLA were prospectively identified. DBS analysis of infants with suspected SMA or STBCL that presented to our centre showed pathological values in two cases each for SMA and SCID and one for XLA, all of them being subsequently confirmed genetically. In this prospective pilot study, no infants with SMA or STBCL were detected; however, the technique applied here was shown to be reliable and fast, further supporting the benefits and need to include SMA and SCID in national newborn screening (NBS) programs, as it will allow early supportive and curative therapy.

Efficacy and safety of Nusinersen among children with spinal muscular atrophy from North India: A prospective cohort study (NICE-SMA study)

BACKGROUND

Intra-thecal Nusinersen has been approved for the treatment of Spinal muscular atrophy (SMA). Limited data is available regarding the efficacy and safety of Nusinersen in children with SMA type 2 and 3 from North India.

OBJECTIVE

To study the efficacy and safety of Nusinersen among children with SMA type 2 and 3 from North India compared to standard of care (SOC) over 12 months.

METHODS

Children with a genetically confirmed diagnosis of SMA and ≥2 copies of the SMN2 gene were screened for enrolment in prospective study design. Revised Hammersmith score (RHS) and revised upper limb module (RULM) were assessed every three months. Compound muscle action potentials (CMAPs) at median and ulnar nerves and quality of life (QOL) were performed at baseline and 12 months. Intra-thecal procedure-related and treatment-emergent side effects in children receiving Nusinersen therapy were recorded. Outcome measures at 6 and 12 months were compared between the Nusinersen and SOC groups.

RESULTS

Forty-two children with SMA, mean age of 85 ± 6 months, including 16 in the Nusinersen group and 26 in the SOC group, were enrolled. The mean RHS score in the Nusinersen group increased from the baseline of 35 ± 18 to 38.9 ± 19, and 39.9 ± 17 at 6 and 12 months (p value-0.001), in the SOC group increased from the baseline of 28.8 ± 15, to 29.6 ± 16, and 29.9 ± 17 at 6 and 12 months respectively (p value-0.35). The mean gain in the RHS score over 12 months in the Nusinersen group was significantly higher compared to the SOC group (p-value 0.02). RULM showed significant gain in the Nusinersen group compared to the SOC group over 12 months (p value 0.03). The median and ulnar nerve CMAPs, and QOL were similar in both the groups. A total of 119 intrathecal injections of Nusinersen were given. Most adverse events were mild and related to the intra-thecal procedure.

CONCLUSION

Intra-thecal Nusinersen therapy among children with late-onset SMA from North India over 12-month duration was associated with improvement in motor abilities as measured by RHS compared to SOC. Intra-thecal Nusinersen was safe and tolerated well.

Outcomes for patients in the RESTORE registry with spinal muscular atrophy and four or more SMN2 gene copies treated with onasemnogene abeparvovec

OBJECTIVE

We describe outcomes following onasemnogene abeparvovec monotherapy for patients with ≥four survival motor neuron 2 (SMN2) gene copies in RESTORE, a noninterventional spinal muscular atrophy patient registry.

METHODS

We evaluated baseline characteristics, motor milestone achievement, post-treatment motor function, use of ventilatory/nutritional support, and adverse events as of December 22, 2022.

RESULTS

At data cutoff, 19 patients in RESTORE had ≥four SMN2 copies and were treated with onasemnogene abeparvovec monotherapy (n=12 [63.2%] four copies; n=7 [36.8%] >four copies). All patients were identified by newborn screening and were reported as asymptomatic at diagnosis. Median age at onasemnogene abeparvovec administration was 3.0 months. Median time from treatment to last recorded visit was 15.4 months, with a range of post-treatment follow-up of 0.03-39.4 months. All 12 children who were assessed for motor development achieved new milestones, including standing alone (n=2) and walking alone (n=5). Five children reported one or more treatment-emergent adverse events (one Grade 3 or greater). No deaths or use of ventilatory/nutritional support were reported.

CONCLUSIONS

Real-world findings from the RESTORE registry indicate that patients with ≥four SMN2 gene copies treated with onasemnogene abeparvovec monotherapy demonstrated improvements in motor function. Adverse events experienced by these patients were consistent with previously reported findings.

In Search of Spinal Muscular Atrophy Disease Modifiers

The 5q Spinal Muscular Atrophy (SMA) is a hereditary autosomal recessive disease caused by defects in the survival motor neuron (SMN1) gene encoding survival motor neuron (SMN) protein. Currently, it is the leading cause of infantile mortality worldwide. SMA is a progressive neurodegenerative disease with "continuum of clinical severity", which can be modulated by genetic and epigenetic factors known as disease modifiers (DMs). Individuals (even siblings) with the same defects in SMN1 gene might have strikingly different types of SMA, supposedly due to the impact of DMs. There are several therapeutic options for SMA, all of them focusing on the restoration of the SMN protein levels to normal. Determining DMs and the pathways in which they are involved might aid in enhancing existing curative approaches. Furthermore, DMs might become novel therapeutic targets or prognostic biomarkers of the disease. This narrative review provides a brief overview of the genetics and pathobiology of SMA, and its bona fide modifiers. We describe novel, emerging DMs, approaches and tools used to identify them, as well as their potential mechanisms of action and impact on disease severity. We also propose several disease-modifying molecular mechanisms which could provide a partial explanation of the staggering variability of SMA phenotypes.

Association among biomarkers, phenotypes, and motor milestones in Chinese patients with 5q spinal muscular atrophy types 1-3

Background

Biomarkers can be used to assess the severity of spinal muscular atrophy (5q SMA; SMA). Despite their potential, the relationship between biomarkers and clinical outcomes in SMA remains underexplored. This study aimed to assess the association among biomarkers, phenotypes, and motor milestones in Chinese patients diagnosed with SMA.

Methods

We collected retrospective clinical and follow-up data of disease-modifying therapy (DMT)-naïve patients with SMA at our center from 2019 to 2021. Four biomarkers were included: survival motor neuron 2 (SMN2) copies, neuronal apoptosis inhibitory protein (NAIP) copies, full-length SMN2 (fl-SMN2), and F-actin bundling protein plastin 3 (PLS3) transcript levels. Data were analyzed and stratified according to SMA subtype.

Results

Of the 123 patients, 30 were diagnosed with Type 1 (24.3%), 56 with Type 2 (45.5%), and 37 with Type 3 (30.1%). The mortality rate for Type 1 was 50%, with median survival times of 2 and 8 months for types 1a and 1b, respectively. All four biomarkers were correlated with disease severity. Notably, fl-SMN2 transcript levels increased with SMN2 copies and were higher in Type 2b than those in Type 2a (p = 0.028). Motor milestone deterioration was correlated with SMN2 copies, NAIP copies, and fl-SMN2 levels, while PLS3 levels were correlated with standing and walking function.

Discussion

Our findings suggest that SMN2 copies contribute to survival and that fl-SMN2 may serve as a valuable biomarker for phenotypic variability in SMA Type 2 subtypes. These insights can guide future research and clinical management of SMA.

Ubiquitination Insight from Spinal Muscular Atrophy-From Pathogenesis to Therapy: A Muscle Perspective

Spinal muscular atrophy (SMA) is one of the most frequent causes of death in childhood. The disease's molecular basis is deletion or mutations in the SMN1 gene, which produces reduced survival motor neuron protein (SMN) levels. As a result, there is spinal motor neuron degeneration and a large increase in muscle atrophy, in which the ubiquitin-proteasome system (UPS) plays a significant role. In humans, a paralogue of SMN1, SMN2 encodes the truncated protein SMNΔ7. Structural differences between SMN and SMNΔ7 affect the interaction of the proteins with UPS and decrease the stability of the truncated protein. SMN loss affects the general ubiquitination process by lowering the levels of UBA1, one of the main enzymes in the ubiquitination process. We discuss how SMN loss affects both SMN stability and the general ubiquitination process, and how the proteins involved in ubiquitination could be used as future targets for SMA treatment.

One-Year Pilot Study Results of Newborn Screening for Spinal Muscular Atrophy in the Republic of Croatia

Spinal muscular atrophy (SMA) is a neuromuscular and neurodegenerative disease caused by the homozygous deletion of SMN1 exon 7 in 95% of cases. The prognosis for SMA patients has improved with the development of disease-modifying therapies, all of which are available in Croatia. The best treatment outcomes occur when therapy is applied before symptoms appear, making newborn screening (NBS) for SMA a crucial factor. Since SMA NBS is the first genetic test performed in our laboratory, for successful implementation of the program, we had to overcome logistical and organizational issues. Herein, we present the results of the SMA NBS during the one-year pilot project in Croatia and verify the suitability of the Targeted qPCR™ SMA assay for SMA NBS. The pilot project started on 1 March 2023 in the Department for Laboratory Diagnostics of the University Hospital Center Zagreb. A total of 32,655 newborns were tested. Five SMA patients were detected, and their diagnoses were confirmed by the multiplex ligation-dependent probe amplification (MLPA) assay. There have been no false positive or false negative results, to our knowledge so far. The incidence of SMA determined during the pilot study is consistent with the SMA incidence data from other European countries.

Comparison of the accuracy of multiplex digital PCR versus multiplex ligation-dependent probe amplification in quantification of the survival of motor neuron genes copy numbers

For over two decades, multiplex ligation-dependent probe amplification (MLPA) has served as the gold standard for genetic testing of spinal muscular atrophy. However, there is emerging evidence questioning the reliability of MLPA in determining the copy numbers (CNs) of the survival of motor neuron (SMN) gene in certain cases. Recently, digital polymerase chain reaction (dPCR) has shown potential for better performance in copy number variant detection. This study aimed to compare MLPA and dPCR in quantifying SMN1 and SMN2 CNs, identify reasons for observed discrepancies, and explore the clinical implications of false results. A total of 733 DNA samples, previously subjected to MLPA analysis, were tested using multiplex droplet dPCR assays. Samples exhibiting inconsistent results between the two methods underwent repeated dPCR assays. When inconsistencies persisted, a third method was employed for verification. Digital PCR yielded results consistent with those of MLPA in 94.4% (692/733) of samples. Forty-one cases exhibited quantitative disparities in SMN1 and/or SMN2 CNs between the two methods. Confirmatory tests revealed that 37 inaccurate results were produced by the MLPA analysis, whereas four were attributed to the dPCR method. The dPCR technique exhibits better accuracy than MLPA and is qualified for SMA genetic testing across various clinical scenarios.

270th ENMC International Workshop: Consensus for SMN2 genetic analysis in SMA patients 10-12 March, 2023, Hoofddorp, the Netherlands

The 270th ENMC workshop aimed to develop a common procedure to optimize the reliability of SMN2 gene copy number determination and to reinforce collaborative networks between molecular scientists and clinicians. The workshop involved neuromuscular and clinical experts and representatives of patient advocacy groups and industry. SMN2 copy number is currently one of the main determinants for therapeutic decision in SMA patients: participants discussed the issues that laboratories may encounter in this molecular test and the cruciality of the accurate determination, due the implications as prognostic factor in symptomatic patients and in individuals identified through newborn screening programmes. At the end of the workshop, the attendees defined a set of recommendations divided into four topics: SMA molecular prognosis assessment, newborn screening for SMA, SMN2 copies and treatments, and modifiers and biomarkers. Moreover, the group draw up a series of recommendations for the companies manufacturing laboratory kits, that will help to minimize the risk of errors, regardless of the laboratories' expertise.

Spinal muscular atrophy type I associated with a novel SMN1 splicing variant that disrupts the expression of the functional transcript

Introduction

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by pathogenic variants in the SMN1 gene. The majority of SMA patients harbor a homozygous deletion of SMN1 exon 7 (95%). Heterozygosity for a conventional variant and a deletion is rare (5%) and not easily detected, due to the highly homologous SMN2 gene interference. SMN2 mainly produces a truncated non-functional protein (SMN-d7) instead of the full-length functional (SMN-FL). We hereby report a novel SMN1 splicing variant in an infant with severe SMA.

Methods

MLPA was used for SMN1/2 exon dosage determination. Sanger sequencing approaches and long-range PCR were employed to search for an SMN1 variant. Conventional and improved Real-time PCR assays were developed for the qualitative and quantitative SMN1/2 RNA analysis.

Results

The novel SMN1 splice-site variant c.835-8_835-5delinsG, was identified in compound heterozygosity with SMN1 exons 7/8 deletion. RNA studies revealed complete absence of SMN1 exon 7, thus confirming a disruptive effect of the variant on SMN1 splicing. No expression of the functional SMN1-FL transcript, remarkable expression of the SMN1-d7 and increased levels of the SMN2-FL/SMN2-d7 transcripts were observed.

Discussion

We verified the occurrence of a non-deletion SMN1 variant and supported its pathogenicity, thus expanding the SMN1 variants spectrum. We discuss the updated SMA genetic findings in the Cypriot population, highlighting an increased percentage of intragenic variants compared to other populations.

Structure and function analysis of Sam68 and hnRNP A1 synergy in the exclusion of exon 7 from SMN2 transcripts

Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by the absence of a functional copy of the Survival of Motor Neuron 1 gene (SMN1). The nearly identical paralog, SMN2, cannot compensate for the loss of SMN1 because exon 7 is aberrantly skipped from most SMN2 transcripts, a process mediated by synergistic activities of Src-associated during mitosis, 68 kDa (Sam68/KHDRBS1) and heterogeneous nuclear ribonucleoprotein (hnRNP) A1. This results in the production of a truncated, nonfunctional protein that is rapidly degraded. Here, we present several crystal structures of Sam68 RNA-binding domain (RBD). Sam68-RBD forms stable symmetric homodimers by antiparallel association of helices α3 from two monomers. However, the details of domain organization and the dimerization interface differ significantly from previously characterized homologs. We demonstrate that Sam68 and hnRNP A1 can simultaneously bind proximal motifs within the central region of SMN2 (ex7). Furthermore, we show that the RNA-binding pockets of the two proteins are close to each other in their heterodimeric complex and identify contact residues using crosslinking-mass spectrometry. We present a model of the ternary Sam68·SMN2 (ex7)·hnRNP A1 complex that reconciles all available information on SMN1/2 splicing. Our findings have important implications for the etiology of SMA and open new avenues for the design of novel therapeutics to treat splicing diseases.

Novel Alu-mediated deletions of the SMN1 gene were identified by ultra-long read sequencing technology in patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disease caused by biallelic variants of the survival motor neuron 1 (SMN1) gene. In this study, our aim was to make a molecular diagnosis in two patients with SMA carrying only one SMN1 copy number. Using ultra-long read sequencing (Ultra-LRS), 1415 bp deletion and 3348 bp deletion of the SMN1 gene were identified in patient 1 and the father of patient 2, respectively. Ultra-LRS revealed two novel deletions, starting from the SMN1 promoter to intron 1. It also accurately provided the location of the deletion breakpoints in the SMN1 gene: chr5 g.70,924,798-70,926,212 for a 1415 bp deletion; chr5 g.70,922,695-70,926,042 for a 3348 bp deletion. By analyzing the breakpoint junctions, we identified that these genomic sequences were composed of Alu sequences, including AluJb, AluYm1, AluSq, and AluYm1, indicating that Alu-mediated rearrangements are a mechanism of SMN1 deletion events. In addition, full-length SMN1 transcripts and SMN protein in patient 1 were significantly decreased (p < 0.01), suggesting that a 1415 bp deletion that included the transcription and translation initiation sites of the SMN1 gene had severe consequences for SMN expression. Ultra-LRS can easily distinguish highly homozygous genes compared to other detection technologies, which is useful for detecting SMN1 intragenic mutations, to quickly discover structural rearrangements and to precisely present the breakpoint positions.

TGS1 impacts snRNA 3'-end processing, ameliorates survival motor neuron-dependent neurological phenotypes in vivo and prevents neurodegeneration

Trimethylguanosine synthase 1 (TGS1) is a highly conserved enzyme that converts the 5'-monomethylguanosine cap of small nuclear RNAs (snRNAs) to a trimethylguanosine cap. Here, we show that loss of TGS1 in Caenorhabditis elegans, Drosophila melanogaster and Danio rerio results in neurological phenotypes similar to those caused by survival motor neuron (SMN) deficiency. Importantly, expression of human TGS1 ameliorates the SMN-dependent neurological phenotypes in both flies and worms, revealing that TGS1 can partly counteract the effects of SMN deficiency. TGS1 loss in HeLa cells leads to the accumulation of immature U2 and U4atac snRNAs with long 3' tails that are often uridylated. snRNAs with defective 3' terminations also accumulate in Drosophila Tgs1 mutants. Consistent with defective snRNA maturation, TGS1 and SMN mutant cells also exhibit partially overlapping transcriptome alterations that include aberrantly spliced and readthrough transcripts. Together, these results identify a neuroprotective function for TGS1 and reinforce the view that defective snRNA maturation affects neuronal viability and function.

Recommendations for Interpreting and Reporting Silent Carrier and Disease-Modifying Variants in SMA Testing Workflows

Genetic testing for SMA diagnosis, newborn screening, and carrier screening has become a significant public health interest worldwide, driven largely by the development of novel and effective molecular therapies for the treatment of spinal muscular atrophy (SMA) and the corresponding updates to testing guidelines. Concurrently, understanding of the underlying genetics of SMA and their correlation with a broad range of phenotypes and risk factors has also advanced, particularly with respect to variants that modulate disease severity or impact residual carrier risks. While testing guidelines are beginning to emphasize the importance of these variants, there are no clear guidelines on how to utilize them in a real-world setting. Given the need for clarity in practice, this review summarizes several clinically relevant variants in the SMN1 and SMN2 genes, including how they inform outcomes for spinal muscular atrophy carrier risk and disease prognosis.

Onasemnogene Abeparvovec: A Review in Spinal Muscular Atrophy

Onasemnogene abeparvovec (Zolgensma®) is a gene therapy approved for the treatment of spinal muscular atrophy (SMA). Administered as a one-time intravenous infusion, onasemnogene abeparvovec uses the adeno-associated virus vector to deliver a functional copy of the human survival motor neuron (SMN) gene to motor neuron cells. SMN1 encodes survival motor neuron protein, which is responsible for the maintenance and function of motor neurons. In clinical trials, onasemnogene abeparvovec improved event-free survival, motor function and motor milestone outcomes in patients with SMA, with these improvements maintained over the longer term (up to a median of ≈ 5 years). Onasemnogene abeparvovec was also associated with rapid age-appropriate achievement of motor milestones and improvements in motor function in children with pre-symptomatic SMA, indicating the benefit of early treatment. Onasemnogene abeparvovec was generally well tolerated. Hepatotoxicity is a known risk that can generally be mitigated with prophylactic prednisolone. In conclusion, onasemnogene abeparvovec represents an important treatment option for patients with SMA, particularly when initiated early in the course of the disease.

Experience and Perspectives in the US on the Evolving Treatment Landscape in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a rare, progressive neuromuscular disorder that, until recently, was the most common inherited cause of infant mortality. Since 2016, three disease-modifying therapies have emerged, nusinersen, onasemnogene abeparvovec-xioi, and risdiplam, leading to a transformation in the SMA treatment landscape, changes in disease trajectories, and a profound impact on clinical care. This environment poses a challenge to making informed treatment decisions, including initial treatment choice, treatment changes, and potential use of combination therapies as new data emerge. To better understand factors that influence physician-patient decision-making, a roundtable discussion was convened by Biogen (sponsor) with a panel of four US SMA experts. This report shares the panel’s opinions and clinical experiences, with the goals of helping clinicians and people with SMA and their families to better understand the factors influencing real-world treatment decisions and stimulating a broader discussion in the SMA community. The panelists highlighted that patients are often heavily involved in treatment decisions, and physicians must be aware of current data to guide patients in making the best decisions. Thus, in the absence of data from head-to-head treatment comparisons, physicians’ roles include reviewing treatment options and describing what is known of the benefits, challenges, and potential side effects of each therapy with patients and families. For infants and young children, the panelists expressed a sense of urgency for early intervention to minimize motor function loss, whereas the goal for adults is long-term disease stabilization. In the panelists’ experience, factors that influence patients’ decisions to change to an alternative therapy include convenience, administration route, novelty of therapy, and hope for improved function, while reasons for returning to a previous therapy include a perception of decreased efficacy and side effects. Ongoing clinical trials and analyses of real-world experiences should further inform treatment decisions and optimize patient outcomes.

Deep Molecular Characterization of Milder Spinal Muscular Atrophy Patients Carrying the c.859G>C Variant in SMN2

Spinal muscular atrophy (SMA) is a severe neuromuscular disorder caused by biallelic loss or pathogenic variants in the SMN1 gene. Copy number and modifier intragenic variants in SMN2, an almost identical paralog gene of SMN1, are known to influence the amount of complete SMN proteins. Therefore, SMN2 is considered the main phenotypic modifier of SMA, although genotype−phenotype correlation is not absolute. We present eleven unrelated SMA patients with milder phenotypes carrying the c.859G>C-positive modifier variant in SMN2. All were studied by a specific NGS method to allow a deep characterization of the entire SMN region. Analysis of two homozygous cases for the variant allowed us to identify a specific haplotype, Smn2-859C.1, in association with c.859G>C. Two other cases with the c.859G>C variant in their two SMN2 copies showed a second haplotype, Smn2-859C.2, in cis with Smn2-859C.1, assembling a more complex allele. We also identified a previously unreported variant in intron 2a exclusively linked to the Smn2-859C.1 haplotype (c.154-1141G>A), further suggesting that this region has been ancestrally conserved. The deep molecular characterization of SMN2 in our cohort highlights the importance of testing c.859G>C, as well as accurately assessing the SMN2 region in SMA patients to gain insight into the complex genotype−phenotype correlations and improve prognostic outcomes.