SMN, the product of the spinal muscular atrophy-determining gene, is expressed widely but selectively in the developing human forebrain

Cognitive function in SMA patients with 2 or 3 SMN2 copies treated with SMN-modifying or gene addition therapy during the first year of life

BACKGROUND

Spinal muscular atrophy (SMA) is a neuromuscular disease, causing progressive muscle weakness due to loss of lower motoneurons. Since 2017, three therapies, two modifying gene transcription and one adding the defective gene, have been approved with comparable efficacy on motor outcome. Data on cognitive outcomes of treated SMA type 1 patients is limited. The aim of this study was to evaluate cognitive function in symptomatic and presymptomatic SMA type 1 patients with two or three SMN2 copies who received SMN-modifying or gene-addition therapy in the first year of life.

METHODS

Cognitive testing was performed in 20 patients, including 19 symptomatic SMA type 1 patients with up to three SMN2 copies and 1 pre-symptomatically treated patient. Children were tested using Bayley Scales of Infant Development (BSID-III) at the age of 2 or 3 years or the Wechsler Preschool and Primary Scale of Intelligence (WPSII-IV) at the of age of 5 years.

RESULTS

11/20 patients showed subnormal cognitive development. Boys had significantly lower cognitive scores. Patients requiring assisted ventilation or feeding support were more likely to have cognitive deficits. Achieving more motor milestones was associated with a better cognitive outcome.

CONCLUSION

Treated patients with SMA type 1 have heterogeneous cognitive function with 55 % of patients showing deficits. Risk factors for cognitive impairment in our cohort were male gender and need for assisted ventilation or feeding support. Therefore, cognitive assessment should be included in the standard of care to allow early identification of deficits and potential therapeutic interventions.

Impaired Neurodevelopment in Children with 5q-SMA - 2 Years After Newborn Screening

OBJECTIVE

Numerous studies have consistently found that reduced SMN protein expression does not severely affect cognitive function in SMA patients. However, the average intelligence quotient of SMA patients has ranged above to below average in different studies. The cognitive development of SMA patients identified through newborn screening remains largely unknown.

METHODS

40 of 47 eligible SMA patients (23 females/17 males) from 39 families identified through newborn screening between January 2018 and December 2020 underwent developmental testing using Bayley III (BSID) after the 2 years of age. The mean age was 29.25 months (23-42 months). 17 patients had 2, 11 patients had 3 and 12 patients had ≥4 copies of SMN2.

RESULTS

cognitive scale: mean 94.55 (SD 24.01); language scale: mean 86.09 (SD 26.41); motor scale: 81.28 (SD 28.07). Overall, the cognitive scales show that 14 children were below average, 20 children were average and 6 children were above average. 10/14 children with below average scores had 2 SMN2 copies. The post-hoc pairwise comparisons showed that the cognition main scale was significantly more sensitive to the number of SMN2 copies than the motor main scale of the BSID (MΔ= 10.27, p = 0.014). There is also evidence that cognition scored higher than the language main scale (MΔ= 7.11, p = 0.090).

CONCLUSION

The impaired cognitive development of SMA children with 2 SMN2 copies, despite early initiation of therapy, underscores the critical role of the SMN protein in the early stages of brain development.

Sleep architecture and Nusinersen therapy in children with Spinal Muscular Atrophy type 1

BACKGROUND

Spinal muscular atrophy (SMA) is a severe neuromuscular disorder, the phenotype of the disease is caused by the mutation of the SMN1 (survival motor neuron 1) gene which encodes for the SMN protein. Innovative treatments for SMA have become available and the first molecule approved is Nusinersen, an antisense oligonucleotide that increases the production of SMN protein. Nusinersen has been shown to be associated with a significant motor improvement and an increase of the event-free survival. For these reasons the aim of the present study is to assess if Nusinersen is able modify sleep architecture and microstructure and to improve sleep structure in these patients.

METHODS

Sixteen patients affected by SMA1 were enrolled in the study (4 boys, 12 girls; median age 72.5 months, intelligence quotient range 24-84). All patients underwent complete nocturnal PSG before the start of the treatment trough intrathecal injections with Nusinersen (T0) and after the fifth infusion (day 180, T180). PSG recordings were visually scored and interpreted according to the indications of the American Academy of Sleep Medicine (AASM) and and microstructure by means of the Cyclic Alternating Pattern (CAP).

RESULTS

After 6 months therapy we found a significantly reduced sleep latency and a significantly increased sleep efficiency. Regarding sleep microstructure parameters (CAP), we did not find any significant change after therapy however, it is worth mentioning that a moderate effect size was observed for the increase in CAP A3 index.

CONCLUSIONS

We observed short-term effects of Nusinersen on sleep with an improvement in sleep efficiency and reduction in sleep onset latency; regarding sleep microstructure, a moderate effect size was found for the number of CAP A3 subtypes that slightly increased, possibly indicating a slightly higher arousability. This finding points at a probably overall better sleep pattern organization associated with the treatment, but they need to be confirmed by larger studies with patients treated earlier in life and for a longer period.

Cognitive performance of adult patients with SMA before and after treatment initiation with nusinersen

BACKGROUND

Spinal muscular atrophy (SMA) is a genetic neuromuscular disease caused by mutations of the SMN1 gene. Deficient SMN protein causes irreversible degeneration of alpha motor neurons characterized by progressive muscle weakness and atrophy. Considering that SMA is a multi-systemic disorder and SMN protein was found to be expressed in cortical structures, the cognitive profile of adult patients with SMA has recently been of particular interest. With nusinersen, a novel, disease-modifying drug has been established, but its effects on neuropsychological functions have not been validated yet. Aim of this study was to investigate the cognitive profile of adult patients with SMA during treatment initiation with nusinersen and to reveal improvement or deterioration in cognitive performance.

METHODS

This monocentric longitudinal study included 23 patients with SMA type 2 and 3. All patients were assessed with the Edinburgh Cognitive and Behavioral ALS Screen (ECAS) before and after 14 months of treatment initiation with nusinersen. Additionally, motor function was evaluated by Hammersmith Functional Motor Scale Expanded (HFMSE), Revised Upper Limb Module (RULM) and Amyotrophic Lateral Sclerosis Functional Rating Scale Revised (ALSFRS-R).

RESULTS

Of the treatment-naive patients, only three were below the age- and education-matched cut-off for cognitive impairment in the ECAS total score. Significant differences between SMA type 2 and 3 were only detected in the domain of Language. After 14 months of treatment, patients showed significant improvement of absolute scores in all three ALS-specific domains, in the non-ALS-specific domain of Memory, in both subscores and in the ECAS total score. No associations were detected between cognitive and functional outcome measures.

CONCLUSIONS

In some adult patients with SMA abnormal cognitive performance in ALS-specific functions of the ECAS was evident. However, the presented results suggest no clinically significant cognitive changes during the observed treatment period with nusinersen.

Brain Magnetic Resonance Imaging (MRI) in Spinal Muscular Atrophy: A Scoping Review

BACKGROUND

5q Spinal Muscular Atrophy (SMA) is a prototypical lower motor neuron disorder. However, the characteristic early motor impairment raises the question on the scope of brain involvement with implications for further investigations on the brain as a potential therapeutic target.

OBJECTIVE

To review changes across the SMA clinical spectrum reported on brain magnetic resonance imaging (MRI).

METHODS

We conducted a scoping review of existing literature on PubMed and EMBASE. Two reviewers searched and retrieved relevant articles on magnetic resonance brain imaging in individuals with SMA censoring to April 2022. Full-text articles published in peer-reviewed journals or abstracts accepted to conferences in English and French were included.

RESULTS

Twelve articles were identified describing a total of 39 patients [age range: 11 days to 41 years old, type 0 (n = 5), type 1 (n = 4), type 2 (n = 2), type 3 (n = 22), type 4 (n = 6)]. All reported structural changes and did not explore other MRI modalities. In individuals with infantile onset SMA, cortical and subcortical brain abnormalities in white matter, basal ganglia, thalamus, hippocampus, and high intensity areas around lateral ventricles and thalami were reported over time. In individuals with later-onset SMA, reduced cerebellar and lobular volume were observed as well as increased grey matter density in motor areas.

CONCLUSIONS

Limited data on brain imaging in SMA highlights both cortical and subcortical involvement in SMA, supporting the hypothesis that changes are not restricted to lower motor neuron pathways. Further studies are needed to determine the extent and prevalence of structural and functional brain changes across SMA types.

Executive function is inversely correlated with physical function: the cognitive profile of adult Spinal Muscular Atrophy (SMA)

Background

Spinal muscular atrophy (SMA) issues from mutations in the survival of motor neuron (SMN) 1 gene. Loss or reduction of the SMN protein results in progressive muscle weakness. Whether this protein deficiency also affects cortical function remains unclear. While no data on adult patients exists so far, prior studies in children with SMA indicate cognitive abilities equal or even superior to healthy controls. This may suggest a possible compensatory—neuropsychological and interactional—process. The goal of this study was to assess the cognitive profile of adult patients with SMA, with a special focus on social cognition as a potential candidate for enhanced cognitive function through compensatory processes.

Methods

In a cross-sectional design, N = 31 adult SMA patients (types II and III) were assessed for language, verbal fluency, memory, visuospatial abilities and executive function with the Edinburgh Cognitive and Behavioural ALS Screen and for social cognition with the Reading the Mind in the Eyes Test. Physical function was evaluated using the Hammersmith Functional Motor Scale Expanded. N = 19 neurologically healthy controls were matched with patients for age, sex and years of education.

Results

In none of the abovementioned cognitive domains significant differences between SMA patients and controls were found. Among patients, no differences between type II SMA and type III SMA were detected for any domain. However, a trend towards better social cognition in patients with type II SMA, compared to those with type III SMA was observed. Furthermore, a significant inverse correlation of physical function and executive function was detected: lower motor function was associated with a better executive function.

Conclusions

This study shows cognitive abilities in adult SMA in the normal range for all assessed domains. Thus, reduction of SMN protein has no obvious negative impact on cognitive function. Executive functions are identified as the only cognitive domain correlated with disease severity. Therefore, executive functions may play a role in the adaptation to physical restrictions in SMA, making them a promising target for future research.

Cognitive Performance of Patients with Adult 5q-Spinal Muscular Atrophy and with Amyotrophic Lateral Sclerosis

Motor neuron diseases, such as spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS), share several clinical similarities while differing substantially in etiology, disease onset and progression. Cognitive dysfunction, a clinically relevant non-motor feature in a substantial proportion of ALS patients, has been less frequently investigated in SMA. In this prospective multicenter cross-sectional study, cognitive function was assessed by the Edinburgh Cognitive (and Behavioural) ALS Screen (ECAS) and a German vocabulary test (Wortschatztest, WST) in 34 adult patients with SMA types 2-4 and in 34 patients with ALS. Demographic and clinical parameters were assessed to identify factors that potentially influence cognitive function. While SMA and ALS patients were comparable in the vocabulary test, on average, SMA patients performed better than ALS patients in the cognitive domains of memory, language and executive function. Better cognitive abilities in SMA patients seemed to be related to the early onset, rather than the extent or the duration, of their physical handicap. Future studies should focus on disease-specific cognitive functions in SMA.

Effects of long-term non-invasive ventilation on sleep structure in children with Spinal Muscular Atrophy type 2

OBJECTIVE

Changes of sleep architecture have been reported in children with Spinal Muscular Atrophy type 2 (SMA2), mainly represented by a decrease of arousability. No studies have evaluated the effect of long-term ventilation on sleep parameters in these children. The aim of this study was to evaluate the effects of long-term non-invasive positive pressure ventilation (LTNPPV) on sleep architecture and to assess the residual differences from normal controls.

METHODS

Nine consecutive children with SMA2 underwent two distinct polysomnographic (PSG) studies, one in spontaneous breathing, and subsequently after LTNPPV. The results were then compared to 15 age-matched controls.

RESULTS

SMA2 patients showed only slightly modified sleep architecture on LTNPPV: increased stage N2% and decreased number of awakenings, while several significant differences persisted between SMA2 patients on LTNPPV and controls (decreased total sleep time, number of awakenings, sleep efficiency, and percentage of REM sleep). Sleep microstructure, evaluated by means of the Cyclic alternating pattern (CAP) showed only marginal changes on LTNPPV (small shortening of CAP A1 subtype duration and small increase in CAP A3 index). Conversely, CAP parameters on LTNPPV showed significant differences between SMA2 patients vs. controls, with increased A1 subtype percentage and decreased percentage of A2 and A3 subtypes.

CONCLUSIONS

This is the first study in children affected by SMA2 reporting data on sleep microstructure and their changes after LTNPPV. We found persisting, small but important changes in sleep microstructure during LTNPPV in these children, suggesting that this treatment only partially improves their arousability.

DNA Damage Response and DNA Repair in Skeletal Myocytes From a Mouse Model of Spinal Muscular Atrophy

We studied DNA damage response (DDR) and DNA repair capacities of skeletal muscle cells from a mouse model of infantile spinal muscular atrophy (SMA) caused by loss-of-function mutation of survival of motor neuron (Smn). Primary myocyte cultures derived from skeletal muscle satellite cells of neonatal control and mutant SMN mice had similar myotube length, myonuclei, satellite cell marker Pax7 and differentiated myotube marker myosin, and acetylcholine receptor clustering. DNA damage was induced in differentiated skeletal myotubes by γ-irradiation, etoposide, and methyl methanesulfonate (MMS). Unexposed control and SMA myotubes had stable genome integrity. After γ-irradiation and etoposide, myotubes repaired most DNA damage equally. Control and mutant myotubes exposed to MMS exhibited equivalent DNA damage without repair. Control and SMA myotube nuclei contained DDR proteins phospho-p53 and phospho-H2AX foci that, with DNA damage, dispersed and then re-formed similarly after recovery. We conclude that mouse primary satellite cell-derived myotubes effectively respond to and repair DNA strand-breaks, while DNA alkylation repair is underrepresented. Morphological differentiation, genome stability, genome sensor, and DNA strand-break repair potential are preserved in mouse SMA myocytes; thus, reduced SMN does not interfere with myocyte differentiation, genome integrity, and DNA repair, and faulty DNA repair is unlikely pathogenic in SMA.

Fam118B, a newly identified component of Cajal bodies, is required for Cajal body formation, snRNP biogenesis and cell viability

Cajal bodies are specialized and dynamic compartments in the nucleus that are involved in the biogenesis of small nuclear ribonucleoproteins (snRNPs). Because of the dynamic and varied roles of Cajal bodies, it is of great interest to identify the components of Cajal bodies to better understand their functions. We performed a genome-wide screen to identify proteins that colocalize with coilin, the marker protein of Cajal bodies. In this study, we identified and characterized Fam118B as a newly discovered component of Cajal bodies. Fam118B is widely expressed in a variety of cell lines derived from various origins. Overexpression of Fam118B changes the canonical morphology of Cajal bodies, whereas depletion of Fam118B disrupts the localization of components of Cajal bodies, including coilin, the survival of motor neuron protein (SMN) and the Sm protein D1 (SmD1, also known as SNRPD1). Moreover, depletion of Fam118B reduces splicing capacity and inhibits cell proliferation. In addition, Fam118B associates with coilin and SMN proteins. Fam118B depletion reduces symmetric dimethylarginine modification of SmD1, which in turn diminishes the binding of SMN to this Sm protein. Taken together, these data indicate that Fam118B, by regulating SmD1 symmetric dimethylarginine modification, plays an important role in Cajal body formation, snRNP biogenesis and cell viability.

SMN deficiency disrupts brain development in a mouse model of severe spinal muscular atrophy

Reduced expression of the survival motor neuron (SMN) gene causes the childhood motor neuron disease spinal muscular atrophy (SMA). Low levels of ubiquitously expressed SMN protein result in the degeneration of lower motor neurons, but it remains unclear whether other regions of the nervous system are also affected. Here we show that reduced levels of SMN lead to impaired perinatal brain development in a mouse model of severe SMA. Regionally selective changes in brain morphology were apparent in areas normally associated with higher SMN levels in the healthy postnatal brain, including the hippocampus, and were associated with decreased cell density, reduced cell proliferation and impaired hippocampal neurogenesis. A comparative proteomics analysis of the hippocampus from SMA and wild-type littermate mice revealed widespread modifications in expression levels of proteins regulating cellular proliferation, migration and development when SMN levels were reduced. This study reveals novel roles for SMN protein in brain development and maintenance and provides the first insights into cellular and molecular pathways disrupted in the brain in a severe form of SMA.

The role of RNA processing in the pathogenesis of motor neuron degeneration

Motor neurons are large, highly polarised cells with very long axons and a requirement for precise spatial and temporal gene expression. Neurodegenerative disorders characterised by selective motor neuron vulnerability include various forms of amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). A rapid expansion in knowledge on the pathophysiology of motor neuron degeneration has occurred in recent years, largely through the identification of genes leading to familial forms of ALS and SMA. The major emerging theme is that motor neuron degeneration can result from mutation in genes that encode factors important for ribonucleoprotein biogenesis and RNA processing, including splicing regulation, transcript stabilisation, translational repression and localisation of mRNA. Complete understanding of how these pathways interact and elucidation of specialised mechanisms for mRNA targeting and processing in motor neurons are likely to produce new targets for therapy in ALS and related disorders.

Repair of pre-mRNA splicing: prospects for a therapy for spinal muscular atrophy

Recent analyses of complete genomes have revealed that alternative splicing became more prevalent and important during eukaryotic evolution. Alternative splicing augments the protein repertoire--particularly that of the human genome--and plays an important role in the development and function of differentiated cell types. However, splicing is also extremely vulnerable, and defects in the proper recognition of splicing signals can give rise to a variety of diseases. In this review, we discuss splicing correction therapies, by using the inherited disease Spinal Muscular Atrophy (SMA) as an example. This lethal early childhood disorder is caused by deletions or other severe mutations of SMN1, a gene coding for the essential survival of motoneurons protein. A second gene copy present in humans and few non-human primates, SMN2, can only partly compensate for the defect because of a single nucleotide change in exon 7 that causes this exon to be skipped in the majority of mRNAs. Thus SMN2 is a prime therapeutic target for SMA. In recent years, several strategies based on small molecule drugs, antisense oligonucleotides or in vivo expressed RNAs have been developed that allow a correction of SMN2 splicing. For some of these, a therapeutic benefit has been demonstrated in mouse models for SMA. This means that clinical trials of such splicing therapies for SMA may become possible in the near future.

Spinal muscular atrophy: why do low levels of survival motor neuron protein make motor neurons sick?

Many neurogenetic disorders are caused by the mutation of ubiquitously expressed genes. One such disorder, spinal muscular atrophy, is caused by loss or mutation of the survival motor neuron1 gene (SMN1), leading to reduced SMN protein levels and a selective dysfunction of motor neurons. SMN, together with partner proteins, functions in the assembly of small nuclear ribonucleoproteins (snRNPs), which are important for pre-mRNA splicing. It has also been suggested that SMN might function in the assembly of other ribonucleoprotein complexes. Two hypotheses have been proposed to explain the molecular dysfunction that gives rise to spinal muscular atrophy (SMA) and its specificity to a particular group of neurons. The first hypothesis states that the loss of SMN's well-known function in snRNP assembly causes an alteration in the splicing of a specific gene (or genes). The second hypothesis proposes that SMN is crucial for the transport of mRNA in neurons and that disruption of this function results in SMA.

Unusual clinical features in infantile Spinal Muscular Atrophies

Spinal Muscular Atrophies (SMA) are a group of degenerative diseases primarily affecting the anterior horn cells of the spinal cord and resulting in muscle weakness and atrophy. Diagnostic criteria were proposed by the International SMA Consortium (ISMAC) to differentiate"classical" proximal SMA caused by homozygous deletion or conversion of the SMN1 gene (5q13) from atypical SMA unlinked to chromosome 5q (non-5q-SMA entities). The aim of our study was to emphasize the unusual clinical features encountered in infantile SMA.

PATIENTS AND METHODS

We retrospectively analyzed 63 children with SMA hospitalized between 1985 and 2006.

RESULTS

Forty-eight children suffered from classical SMA and 15 from atypical SMA, including 4 distal SMA, 2 scapuloperoneal SMA, one pontocerebellar hypoplasia type I, 7 neurogenic arthrogryposis multiplex congenita (2 of them associated with a central nervous system (CNS) involvement) and one undetermined case.

CONCLUSION

This study confirmed the clinical variety of proximal SMA and put in perspective some exclusion criteria (CNS involvement, phrenic or facial palsy). Some symptoms allowed us to anticipate the normality of the SMN1 gene: improvement of motor condition, distal predominance and, more relatively, assymetry of motor weakness. Diagnosis difficulties were especially encountered in case of predominant distal deficit, arthrogryposis multiplex congenita and associated clinical abnormalities. Detailed phenotypical description and syndromic regrouping of cases of atypical SMA lead to a better understanding of underlying physiopathological processes and to the identification of other genes involved in infantile SMA.

Cajal body number and nucleolar size correlate with the cell body mass in human sensory ganglia neurons

This paper studies the cell size-dependent organization of the nucleolus and Cajal bodies (CBs) in dissociated human dorsal root ganglia (DRG) neurons from autopsy tissue samples of patients without neurological disease. The quantitative analysis of nucleoli with an anti-fibrillarin antibody showed that all neurons have only one nucleolus. However, the nucleolar volume and the number of fibrillar centers per nucleolus significantly increase as a function of cell body size. Immunostaining for coilin demonstrated the presence of numerous CBs in DRG neurons (up to 20 in large size neurons). The number of CBs per neuron correlated positively with the cell body volume. Light and electron microscopy immunocytochemical analysis revealed the concentration of coilin, snRNPs, SMN and fibrillarin in CBs of DRG neurons. CBs were frequently associated with the nucleolus, active chromatin domains and PML bodies, but not with telomeres. Our results support the view that the nucleolar volume and number of both fibrillar centers and CBs depend on the cell body mass, a parameter closely related to transcriptional and synaptic activity in mammalian neurons. Moreover, the unusual large number of CBs could facilitate the transfer of RNA processing components from CBs to nucleolar and nucleoplasmic sites of RNA processing.