A newborn with spinal muscular atrophy type 0 presenting with a clinicopathological picture suggestive of myotubular myopathy

Spinal Muscular Atrophy: An Evolving Scenario through New Perspectives in Diagnosis and Advances in Therapies

Spinal muscular atrophy (SMA) linked to 5q is a recessive motor neuron disease characterized by progressive and diffuse weakness and muscular atrophy. SMA is the most common neurodegenerative disease in childhood with an incidence of approximately 1 in 6000-10,000 live births, being long considered a leading cause of hereditary mortality in infancy, worldwide. The classification of SMA is based on the natural history of the disease, with a wide clinical spectrum of onset and severity. We are currently in a new therapeutic era, that, thanks to the widespread use of the newly approved disease-modifying therapies and the possibility of an early administration, should lead to a deep change in the clinical scenario and, thus, in the history of SMA. With the aim to achieve a new view of SMA, in this review we consider different aspects of this neuromuscular disease: the historical perspective, the clinical features, the diagnostic process, the psychological outcome, innovation in treatments and therapies, the possibility of an early identification of affected infants in the pre-symptomatic phase through newborn screening programs.

Preliminary insights into RNA in CSF of pediatric SMA patients after 6 months of nusinersen

BACKGROUND

Spinal muscular atrophy (SMA) is a rare autosomal-recessive neurodegenerative disorder caused by mutations in survival motor neuron 1 (SMN1) gene, and consequent loss of function of SMN protein, which results in progressive loss of lower motor neurons, and muscular wasting. Antisense oligonucleotide (ASO) nusinersen (Spinraza®) modulates the pre-mRNA splicing of the SMN2 gene, allowing rebalance of biologically active SMN. It is administered intrathecally via lumbar puncture after removing an equal amount of cerebrospinal fluid (CSF). Its effect was proven beneficial and approved since 2017 for SMA treatment. Given the direct effect of nusinersen on RNA metabolism, the aim of this project was to evaluate cell-free RNA (cfRNA) in CSF of SMA patients under ASOs treatment for biomarker discovery.

METHODS

By RNA-sequencing approach, RNA obtained from CSF of pediatric SMA type 2 and 3 patients was processed after 6 months of nusinersen treatment, at fifth intrathecal injection (T6), and compared to baseline (T0).

RESULTS

We observed the deregulation of cfRNAs in patients at T6 and we were able to classify these RNAs into disease specific, treatment specific and treatment dependent. Moreover, we subdivided patients into "homogeneous" and "heterogeneous" according to their gene expression pattern. The "heterogeneous" group showed peculiar activation of genes coding for ribosomal components, meaning that in these patients a different molecular effect of nusinersen is observable, even if this specific molecular response was not referable to a clinical pattern.

CONCLUSIONS

This study provides preliminary insights into modulation of gene expression dependent on nusinersen treatment and lays the foundation for biomarkers discovery.

Spinal muscular atrophy: Broad disease spectrum and sex-specific phenotypes

Spinal muscular atrophy (SMA) is one of the major genetic disorders associated with infant mortality. More than 90% of cases of SMA result from deletions of or mutations in the Survival Motor Neuron 1 (SMN1) gene. SMN2, a nearly identical copy of SMN1, does not compensate for the loss of SMN1 due to predominant skipping of exon 7. The spectrum of SMA is broad, ranging from prenatal death to infant mortality to survival into adulthood. All tissues, including brain, spinal cord, bone, skeletal muscle, heart, lung, liver, pancreas, gastrointestinal tract, kidney, spleen, ovary and testis, are directly and/or indirectly affected in SMA. Accumulating evidence on impaired mitochondrial biogenesis and defects in X chromosome-linked modifying factors, coupled with the sexual dimorphic nature of many tissues, point to sex-specific vulnerabilities in SMA. Here we review the role of sex in the pathogenesis of SMA.

New and Developing Therapies in Spinal Muscular Atrophy: From Genotype to Phenotype to Treatment and Where Do We Stand?

Spinal muscular atrophy (SMA) is a congenital neuromuscular disorder characterized by motor neuron loss, resulting in progressive weakness. SMA is notable in the health care community because it accounts for the most common cause of infant death resulting from a genetic defect. SMA is caused by low levels of the survival motor neuron protein (SMN) resulting from SMN1 gene mutations or deletions. However, patients always harbor various copies of SMN2, an almost identical but functionally deficient copy of the gene. A genotype–phenotype correlation suggests that SMN2 is a potent disease modifier for SMA, which also represents the primary target for potential therapies. Increasing comprehension of SMA pathophysiology, including the characterization of SMN1 and SMN2 genes and SMN protein functions, has led to the development of multiple therapeutic approaches. Until the end of 2016, no cure was available for SMA, and management consisted of supportive measures. Two breakthrough SMN-targeted treatments, either using antisense oligonucleotides (ASOs) or virus-mediated gene therapy, have recently been approved. These two novel therapeutics have a common objective: to increase the production of SMN protein in MNs and thereby improve motor function and survival. However, neither therapy currently provides a complete cure. Treating patients with SMA brings new responsibilities and unique dilemmas. As SMA is such a devastating disease, it is reasonable to assume that a unique therapeutic solution may not be sufficient. Current approaches under clinical investigation differ in administration routes, frequency of dosing, intrathecal versus systemic delivery, and mechanisms of action. Besides, emerging clinical trials evaluating the efficacy of either SMN-dependent or SMN-independent approaches are ongoing. This review aims to address the different knowledge gaps between genotype, phenotypes, and potential therapeutics.

Spinal muscular atrophy

Autosomal-recessive proximal spinal muscular atrophy (Werdnig-Hoffmann, Kugelberg-Welander) is caused by mutation of the SMN1 gene, and the clinical severity correlates with the number of copies of a nearly identical gene, SMN2. The SMN protein plays a critical role in spliceosome assembly and may have other cellular functions, such as mRNA transport. Cell culture and animal models have helped to define the disease mechanism and to identify targets for therapeutic intervention. The main focus for developing treatment has been to increase SMN levels, and accomplishing this with small molecules, oligonucleotides, and gene replacement has been quite. An oligonucleotide, nusinersen, was recently approved for treatment in patients, and confirmatory studies of other agents are now under way.

Spinal muscular atrophy: journeying from bench to bedside

Spinal muscular atrophy (SMA) is a frequently fatal neuromuscular disorder and the most common inherited cause of infant mortality. SMA results from reduced levels of the survival of motor neuron (SMN) protein. Although the disease was first described more than a century ago, a precise understanding of its genetics was not obtained until the SMA genes were cloned in 1995. This was followed in rapid succession by experiments that assigned a role to the SMN protein in the proper splicing of genes, novel animal models of the disease, and the eventual use of the models in the pre clinical development of rational therapies for SMA. These successes have led the scientific and clinical communities to the cusp of what are expected to be the first truly promising treatments for the human disorder. Yet, important questions remain, not the least of which is how SMN paucity triggers a predominantly neuromuscular phenotype. Here we review how our understanding of the disease has evolved since the SMA genes were identified. We begin with a brief description of the genetics of SMA and the proposed roles of the SMN protein. We follow with an examination of how the genetics of the disease was exploited to develop genetically faithful animal models, and highlight the insights gained from their analysis. We end with a discussion of ongoing debates, future challenges, and the most promising treatments to have emerged from our current knowledge of the disease.

SMN-targeted therapeutics for spinal muscular atrophy: are we SMArt enough yet?

Spinal muscular atrophy (SMA) remains one of the most common and lethal autosomal recessive diseases. Homozygous deletion of survival of motor neuron 1 (SMN1) and resulting SMN protein deficiency manifests predominantly with motor neuron degeneration; however, a wealth of emerging data supports a broader influence of SMN deficiency in disease pathogenesis. In this issue of the JCI, Kariya and colleagues demonstrate the relatively selective impact of SMN depletion on the distal motor unit using a series of SMN2-expressing transgenic mice in which constitutive SMN knockdown follows variable periods of normal development. Their observations provide further insights regarding the temporal requirements for SMN in mice, renewing speculation about when and where repletion of SMN is necessary for optimal outcomes in SMA patients.

Requirement of enhanced Survival Motoneuron protein imposed during neuromuscular junction maturation

Spinal muscular atrophy is a common motor neuron disease caused by low survival motoneuron (SMN), a key protein in the proper splicing of genes. Restoring the protein is therefore a promising therapeutic strategy. Implementation of this strategy, however, depends on defining the temporal requirements for SMN. Here, we used controlled knockdown of SMN in transgenic mice to determine the precise postnatal stage requirements for this protein. Reducing SMN in neonatal mice resulted in a classic SMA-like phenotype. Unexpectedly, depletion of SMN in adults had relatively little effect. Insensitivity to low SMN emerged abruptly at postnatal day 17, which coincided with establishment of the fully mature neuromuscular junction (NMJ). Mature animals depleted of SMN eventually exhibited evidence of selective neuromuscular pathology that was made worse by traumatic injury. The ability to regenerate the mature NMJ in aged or injured SMN-depleted mice was grossly impaired, a likely consequence of the inability to meet the surge in demand for motoneuronal SMN that was seen in controls. Our results demonstrate that relative maturity of the NMJ determines the temporal requirement for the SMN protein. These observations suggest that the use of potent but potentially deleterious SMN-enhancing agents could be tapered in human patients once the neuromuscular system matures and reintroduced as needed to enhance SMN for remodeling aged or injured NMJs.

Neurodegeneration in spinal muscular atrophy: from disease phenotype and animal models to therapeutic strategies and beyond

Of the numerous inherited diseases known to afflict the pediatric population, spinal muscular atrophy (SMA) is among the most common. It has an incidence of approximately one in 10,000 newborns and a carrier frequency of one in 50. Despite its relatively high incidence, SMA remains somewhat obscure among the many neurodegenerative diseases that affect humans. Nevertheless, the last two decades have witnessed remarkable progress in our understanding of the pathology, underlying biology and especially the molecular genetics of SMA. This has led to a genuine expectation within the scientific community that a robust treatment will be available to patients before the end of the decade. The progress made in our understanding of SMA and, therefore, towards a viable therapy for affected individuals is in large measure a consequence of the simple yet fascinating genetics of the disease. Nevertheless, important questions remain. Addressing these questions promises not only to accelerate the march towards a cure for SMA, but also to uncover novel therapies for related neurodegenerative disorders. This review discusses our current understanding of SMA, considers the challenges ahead, describes existing treatment options and highlights state-of-the-art research being conducted as a means to a better, safer and more effective treatment for the disease.

New therapeutic approaches to spinal muscular atrophy

Bench to bedside progress has been widely anticipated for a growing number of neurodegenerative disorders. Of these, spinal muscular atrophy (SMA) is perhaps the best poised to capitalize on advances in targeted therapeutics development over the next few years. Several laboratories have achieved compelling success in SMA animal models using sophisticated methods for targeted delivery, repair, or increased expression of the survival motor neuron protein, SMN. The clinical community is actively collaborating to identify, develop, and validate outcome measures and biomarkers in parallel with laboratory efforts. Innovative trial design and synergistic approaches to maximize proactive care in conjunction with treatment with one or more of the promising pharmacologic and biologic therapies currently in the pipeline will maximize our chances to achieve meaningful outcomes for patients. This review highlights recent promising scientific and clinical advances bringing us ever closer to effective treatment(s) for our patients with SMA.

Survival analysis of spinal muscular atrophy type I

Purpose

The life expectancy of patients with spinal muscular atrophy (SMA) type I is generally considered to be less than 2 years. Recently, with the introduction of proactive treatments, a longer survival and an improved survival rate have been reported. In this study, we analyzed the natural courses and survival statistics of SMA type I patients and compared the clinical characteristics of the patients based on their survival periods.

Methods

We reviewed the medical records of 14 pediatric patients diagnosed with SMA type I during a 9-year period. We examined the demographic and clinical characteristics of these patients, calculated their survival probabilities, and plotted survival curves as on the censoring date, January 1, 2010. We also compared the characteristics of the patients who died before the age of 24 months (early-death, ED group) and those who survived for 24 months or longer (long-survival, LS group).

Results

The mean survival time was 22.8±2.0 months. The survival probabilities at 6 months, 12 months, 18 months, 24 months, and 30 months were 92.9%, 92.9%, 76.0%, 76.0%, and 65.1%, respectively. Birth weight was the only factor that showed a statistically significant difference between the ED and LS groups (P=0.048).

Conclusion

In this study, the survival probabilities at 2 years were far greater than expected. Because of the limited number of patients and information in this study, the contribution of improved supportive care on longer survival could not be clarified; this may be elucidated in larger cohort studies.

Fetal akinesia deformation sequence with delayed skeletal muscle maturation and polymicrogyria: evidence for a hypoxic/ischemic pathogenesis

Multiple congenital contractures, also known as fetal akinesia deformation sequence (FADS) and related terms, result from decreased fetal movement. The underlying etiologies are diverse and include central nervous system (CNS) dysgeneses and primary myopathies. Persistent central nuclei or the presence of myotubes is often regarded as evidence of a primary myopathic etiology; however, these findings are also associated with impaired fetal innervation. We report 7 fetuses, estimated gestational age 20 to 23 weeks, with persistent myotubular morphology, a change that could be (mis)interpreted as a primary myopathy. In 4 of the patients, CNS histology showed hypoxic/ischemic injury, polymicrogyria, mineralized neurons, and microinfarcts with or without loss of anterior horn neurons. FADS cases with polymicrogyria have frequently been interpreted as a consequence of a primary brain malformation. Only a few descriptions of FADS associate polymicrogyria with CNS hypoxic/ischemic injury, however, and do not describe skeletal muscle maturation delay. We hypothesize that this combination of neural and muscular pathology is an under-recognized pattern in FADS, which results from diffuse hypoxic/ischemic injury involving the brain and spinal cord during early to middle gestation.

Proximal muscle weakness in a 15-year-old boy

Proximal muscle weakness, often indicative of a myopathy, has a broad differential diagnosis in children. We present a case of an adolescent boy with proximal weakness and a mildly elevated creatine kinase. He was found to have spinal muscular atrophy type III rather than a myopathy.

Congenital myopathies

PURPOSE OF REVIEW

The present review aims to discuss the pathological and clinical heterogeneity of congenital myopathies, and the overlap between the different variants highlighted by recent studies.

RECENT FINDINGS

The spectrum of pathological changes associated with known gene defects has widened, and new genes responsible for rare structural defects have been identified. The complexity of the classification of these conditions is highlighted by the realization that defects in the same gene can result in diverse phenotypes, including disorders traditionally classified as congenital myopathies with structural abnormalities, adult-onset disorders, conditions characterized by distal weakness and wasting, or distal arthrogryposis. There is a wider appreciation of the complexities of inheritance and of the value of noninvasive assessment, such as muscle MRI. New animal models provide a better understanding of pathogenesis and are highlighting therapeutic possibilities.

SUMMARY

The overlap of clinical and pathological features in the congenital myopathies has led to the recognition that diverse disorders are often associated with the same causative gene, and is challenging traditional classifications. Identification of further causative genes and development of new models will further the understanding of pathogenesis and development of therapies.

Reduced SMN protein impairs maturation of the neuromuscular junctions in mouse models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a common pediatric neuromuscular disorder caused by insufficient levels of the survival of motor neuron (SMN) protein. Studies involving SMA patients and animal models expressing the human SMN2 gene have yielded relatively little information about the earliest cellular consequences of reduced SMN protein. In this study, we have used severe- and mild-SMN2 expressing mouse models of SMA as well as material from human patients to understand the initial stages of neurodegeneration in the human disease. We show that the earliest structural defects appear distally and involve the neuromuscular synapse. Insufficient SMN protein arrests the post-natal development of the neuromuscular junction (NMJ), impairing the maturation of acetylcholine receptor (AChR) clusters into 'pretzels'. Pre-synaptic defects include poor terminal arborization and intermediate filament aggregates which may serve as a useful biomarker of the disease. These defects are reflected in functional deficits at the NMJ characterized by intermittent neurotransmission failures. We suggest that SMA might best be described as a NMJ synaptopathy and that one promising means of treating it could involve maintaining function at the NMJ.