ARSACS, a spastic ataxia common in northeastern Québec, is caused by mutations in a new gene encoding an 11.5-kb ORF

Profiling of pathogenic variants in Japanese patients with sarcoglycanopathy

BACKGROUND

Sarcoglycanopathies (SGPs) are limb-girdle muscular dystrophies (LGMDs) that can be classified into four types, LGMDR3, LGMDR4, LGMDR5, and LGMDR6, caused by mutations in the genes, SGCA, SGCB, SGCG, and SGCD, respectively. SGPs are relatively rare in Japan. This study aims to profile the genetic variants that cause SGPs in Japanese patients.

METHODS

Clinical course and pathological findings were retrospectively reviewed in Japanese patients with SGP. Genetic analyses were performed using a combination of targeted resequencing with a hereditary muscle disease panel, whole genome sequencing, multiplex ligation-dependent probe amplification, and long-read sequencing. The structures of transcripts with aberrant splicing were also determined by RT-PCR, RNA-seq, and in silico prediction.

RESULTS

We identified biallelic variants in SGC genes in 53 families, including three families with LGMDR6, which had not been identified in Japan so far. SGCA was the most common causative gene, accounting for 56% of cases, followed by SGCG, SGCB, and SGCD, at 17%, 21%, and 6%, respectively. Missense variants in SGCA were very frequent at 78.3%, while they were relatively rare in SGCB, SGCG, and SGCD at 11.1%, 18.2%, and 16.6%, respectively. We also analyzed the haplotypes of alleles carrying three variants found in multiple cases: c.229C > T in SGCA, c.325C > T in SGCB, and exon 6 deletion in SGCG; two distinct haplotypes were found for c.229C > T in SGCA, while each of the latter two variants was on single haplotypes.

CONCLUSIONS

We present genetic profiles of Japanese patients with SGPs. Haplotype analysis indicated common ancestors of frequent variants. Our findings will support genetic diagnosis and gene therapy.

ARSACS: Clinical Features, Pathophysiology and iPS-Derived Models

Autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an early-onset neurodegenerative disease caused by mutations in the SACS gene. The first two mutations were identified in French Canadian populations 20 years ago. The disease is now known as one of the most frequent recessive ataxias worldwide. Prominent features include cerebellar ataxia, pyramidal spasticity, and neuropathy. Neuropathological findings revealed cerebellar atrophy of the superior cerebellar vermis and the anterior vermis associated with Purkinje cell death, pyramidal degeneration, cortical atrophy, loss of motor neurons, and demyelinating neuropathy. No effective therapy is available for ARSACS patients but, in the last two decades, there have been significant advances in our understanding of the disease. New approaches in ARSACS, such as the reprogramming of induced pluripotent stem cells derived from patients, open exciting perspectives of discoveries. Several research questions are now emerging. Here, we review the clinical features of ARSACS as well as the cerebellar aspects of the disease, with an emphasis on recent fields of investigation.

Altered Cellular Metabolism Is a Consequence of Loss of the Ataxia-Linked Protein Sacsin

Mitochondrial dysfunction is implicated in the pathogenesis of the neurological condition autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS), yet precisely how the mitochondrial metabolism is affected is unknown. Thus, to better understand changes in the mitochondrial metabolism caused by loss of the sacsin protein (encoded by the SACS gene, which is mutated in ARSACS), we performed mass spectrometry-based tracer analysis, with both glucose- and glutamine-traced carbon. Comparing the metabolite profiles between wild-type and sacsin-knockout cell lines revealed increased reliance on aerobic glycolysis in sacsin-deficient cells, as evidenced by the increase in lactate and reduction of glucose. Moreover, sacsin knockout cells differentiated towards a neuronal phenotype had increased levels of tricarboxylic acid cycle metabolites relative to the controls. We also observed disruption in the glutaminolysis pathway in differentiated and undifferentiated cells in the absence of sacsin. In conclusion, this work demonstrates consequences for cellular metabolism associated with a loss of sacsin, which may be relevant to ARSACS.

Journey Through Autosomal-Recessive Spastic Ataxia of Charlevoix-Saguenay: Insights From a Case Series of Seven Patients-A Single-Center Study and Review of an Indian Cohort

OBJECTIVE

In this study, we describe the clinical and investigative profiles of 7 cases of autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS).

METHODS

We performed a retrospective chart review of genetically proven cases of ARSACS from our database. Additionally, we reviewed the literature for reported cases of ARSACS from India.

RESULTS

All 7 patients experienced disease onset within the first decade of life. According to the available data, all patients had walking difficulty (7/7), spastic ataxia (7/7), classical neuroimaging findings (7/7), sensory‒motor demyelinating polyneuropathy (6/6), abnormal evoked potentials (5/5), and a thickened retinal nerve fiber layer (3/3). Exome sequencing revealed 8 unique pathogenic/likely pathogenic variants (6 novel) in the SACS gene. An additional 21 cases (18 families) of ARSACS that could be identified from India had similar clinical and investigational findings. The most common c.8793delA variant may have a founder effect.

CONCLUSION

Our series adds to the previously reported cases of ARSACS from India and expands the genetic spectrum by adding 6 novel variants.

Thickened Retinal Nerve Fiber Layer Without Hypermyelination in Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay

Purpose: To report the retinal findings in a patient with autosomal recessive spastic ataxia of Charlevoix-Saguenay. Methods: A case was evaluated. Results: A 16-year-old male patient with a known diagnosis of autosomal recessive spastic ataxia of Charlevoix-Saguenay was referred for evaluation of retinal hypermyelination given its frequent association with the condition. The patient was asymptomatic with a best-corrected visual acuity of 20/20. Optical coherence tomography of the peripapillary retinal nerve fiber layer (RNFL) showed bilateral thickening in each eye (average thicknesses: 180 µm, right eye; 177 µm, left eye). An examination showed no myelinization of the RNFL. Conclusions: Most studies to date describe RNFL thickening secondary to hypermyelination as a characteristic finding in autosomal recessive spastic ataxia of Charlevoix-Saguenay. This case provides evidence that this thickening may be a result of hypertrophy rather than hypermyelination. Further investigation is needed to define the pathophysiologic cause of RNFL thickening in autosomal recessive spastic ataxia of Charlevoix-Saguenay.

Compound Heterozygous Mutations of SACS in a Korean Cohort Study of Charcot-Marie-Tooth Disease Concurrent Cerebellar Ataxia and Spasticity

Mutations in the SACS gene are associated with autosomal recessive spastic ataxia of Charlevoix-Saguenay disease (ARSACS) or complex clinical phenotypes of Charcot-Marie-Tooth disease (CMT). This study aimed to identify SACS mutations in a Korean CMT cohort with cerebellar ataxia and spasticity by whole exome sequencing (WES). As a result, eight pathogenic SACS mutations in four families were identified as the underlying causes of these complex phenotypes. The prevalence of CMT families with SACS mutations was determined to be 0.3%. All the patients showed sensory, motor, and gait disturbances with increased deep tendon reflexes. Lower limb magnetic resonance imaging (MRI) was performed in four patients and all had fatty replacements. Of note, they all had similar fatty infiltrations between the proximal and distal lower limb muscles, different from the neuromuscular imaging feature in most CMT patients without SACS mutations who had distal dominant fatty involvement. Therefore, these findings were considered a characteristic feature in CMT patients with SACS mutations. Although further studies with more cases are needed, our results highlight lower extremity MRI findings in CMT patients with SACS mutations and broaden the clinical spectrum. We suggest screening for SACS in recessive CMT patients with complex phenotypes of ataxia and spasticity.

Proteomics and lipidomic analysis reveal dysregulated pathways associated with loss of sacsin

Introduction

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a rare incurable neurodegenerative disease caused by mutations in the SACS gene, which codes for sacsin, a large protein involved in protein homeostasis, mitochondrial function, cytoskeletal dynamics, autophagy, cell adhesion and vesicle trafficking. However, the pathogenic mechanisms underlying sacsin dysfunction are still largely uncharacterized, and so attempts to develop therapies are still in the early stages.

Methods

To achieve further understanding of how processes are altered by loss of sacsin, we used untargeted proteomics to compare protein profiles in ARSACS fibroblasts versus controls.

Results

Our analyses confirmed the involvement of known biological pathways and also implicated calcium and lipid homeostasis in ARSACS skin fibroblasts, a finding further verified in SH-SY5Y SACS -/- cells. Validation through mass spectrometry-based analysis and comparative quantification of lipids by LC-MS in fibroblasts revealed increased levels of ceramides coupled with a reduction of diacylglycerols.

Discussion

In addition to confirming aberrant Ca2+ homeostasis in ARSACS, this study described abnormal lipid levels associated with loss of sacsin.

Phenotypical, genotypical and pathological characterization of the moonwalker mouse, a model of ataxia

We performed a comprehensive study of the morphological, functional, and genetic features of moonwalker (MWK) mice, a mouse model of spinocerebellar ataxia caused by a gain of function of the TRPC3 channel. These mice show numerous behavioral symptoms including tremor, altered gait, circling behavior, impaired motor coordination, impaired motor learning and decreased limb strength. Cerebellar pathology is characterized by early and almost complete loss of unipolar brush cells as well as slowly progressive, moderate loss of Purkinje cell (PCs). Structural damage also includes loss of synaptic contacts from parallel fibers, swollen ER structures, and degenerating axons. Interestingly, no obvious correlation was observed between PC loss and severity of the symptoms, as the phenotype stabilizes around 2 months of age, while the cerebellar pathology is progressive. This is probably due to the fact that PC function is severely impaired much earlier than the appearance of PC loss. Indeed, PC firing is already impaired in 3 weeks old mice. An interesting feature of the MWK pathology that still remains to be explained consists in a strong lobule selectivity of the PC loss, which is puzzling considering that TRPC is expressed in every PC. Intriguingly, genetic analysis of MWK cerebella shows, among other alterations, changes in the expression of both apoptosis inducing and resistance factors possibly suggesting that damaged PCs initiate specific cellular pathways that protect them from overt cell loss.

Neurofilaments: Novel findings and future challenges

Neurofilaments (NFs) are abundant cytoskeletal proteins that emerge as a critical hub for cell signalling within neurons. As we start to uncover essential roles of NFs in regulating microtubule and organelle dynamics, nerve conduction and neurotransmission, novel discoveries are expected to arise in genetics, with NFs identified as causal genes for various neurodegenerative diseases. This review will discuss how the latest advances in fundamental and translational research illuminate our understanding of NF biology, particularly their assembly, organisation, transport and degradation. We will emphasise the notion that filaments are not one entity and that future challenges will be to apprehend their diverse composition and structural heterogeneity and to scrutinize how this regulates signalling, sustains neuronal physiology and drives pathophysiology in disease.

Widening the clinical, radiological and genetic spectrum of autosomal recessive ataxia of Charlevoix-Saguenay in Indian patients

BACKGROUND

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS), classically presenting as a triad of early-onset cerebellar ataxia, lower extremity spasticity and peripheral neuropathy, is caused by mutations in SACS gene which encodes the protein sacsin.

OBJECTIVE

To provide new insight into the occurrence of SACS mutations in South India.

METHODS

Patients with three cardinal features of ARSACS-peripheral neuropathy, cerebellar ataxia, and pyramidal tract signs were included. Nine patients were clinically identified and genetically evaluated. Mutation screening of SACS by targeted sequencing of 40 recessive ataxia genes panel by next-generation sequencing was conducted. Additional investigations included magnetic resonance imaging (MRI), fundoscopy, optical coherence tomography (OCT) and nerve conduction studies (NCS). Functional disability was assessed by the Spinocerebellar Degeneration Functional Score.

RESULTS

Two hundred and fifteen cerebellar ataxia patients were screened, and 9 patients with cerebellar ataxia with spasticity, peripheral neuropathy and MRI brain characteristics, consistent with a clinical diagnosis of ARSACS were identified, of which 7 patients were identified to have mutation in the SACS gene and are detailed hereafter. Age of presentation ranged from 20 to 55 years (29.8 ± 11.9) with a mean disease duration of 12.7 years (SD-7.65, range 5-22 years). All except one had onset of symptoms in the form of an ataxic gait noticed before 20 years of age. Additional features were subnormal intelligence (4/7), slow and hypometric saccades (1/7), seizures (1/7), kyphoscoliosis (1/7) and dysmorphic facies (1/7). SDFS was 3 in 5/7 patients signifying moderate disability with independent ambulation. MRI showed cerebellar atrophy with predominant atrophy of the superior vermis (7/7), horizontal linear T2 hypointensities in the pons(7/7), hyperintensities where lateral pons merges with the middle cerebellar peduncle (MCP) (7/7) well seen in fluid-attenuated inversion recovery (FLAIR) images, thickening of MCP (3/7), symmetric lateral thalamic hyperintensities (6/7), posterior fossa arachnoid cyst (4/7),thinning of posterior mid-body of corpus callosum (7/7), marginal mineralisation of the basal ganglia (7/7), bilateral parietal atrophy (7/7) and thinning of corticospinal tract on diffusion tensor imaging (DTI) (7/7). We identified pathogenic homozygous frameshift mutations in the SACS gene in six patients (including two siblings), while one patient had a heterozygous pathogenic deletion.

CONCLUSIONS

This is the largest series of genetically confirmed ARSACS patients from India highlighting the clinical, ophthalmological, imaging and genetic features of this cohort.

Natural History of Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay: a 4-Year Longitudinal Study

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a neurologic disorder with generally well-known clinical manifestations. However, few studies assessed their progression rate using a longitudinal design. This study aimed to document the natural history of ARSACS over a 4-year period in terms of upper and lower limb functions, balance, walking capacity, performance in daily living activities, and disease severity. Forty participants were assessed on three occasions over 4 years. Participant performance was reported in raw data as well as in percentage from reference values to consider the normal aging process. Severe balance and walking capacity impairments were found, with a significant performance decrease over the 4 years. Balance reached a floor score of around 6 points on the Berg Balance Scale for participants aged >40 years, while other participants lost about 1.5 points per year. The mean loss in walking speed was 0.044 m/s per year and the mean decrease in the distance walked in 6 min was 20.8 m per year for the whole cohort. Pinch strength, balance, walking speed, and walking distance decreased over time even when reported in percentage from reference values. Major impairments and rapid progression rates were documented in the present study for upper limb coordination, pinch strength, balance, and walking capacity in the ARSACS population. A progression rate beyond the normal aging process was observed. These results provide fundamental insights regarding the disease prognosis that will help to better inform patients, develop specific rehabilitation programs, and improve trial readiness.

Hereditary Ataxias: From Bench to Clinic, Where Do We Stand?

Cerebellar ataxias are a wide heterogeneous group of movement disorders. Within this broad umbrella of diseases, there are both genetics and sporadic forms. The clinical presentation of these conditions can exhibit a diverse range of symptoms across different age groups, spanning from pure cerebellar manifestations to sensory ataxia and multisystemic diseases. Over the last few decades, advancements in our understanding of genetics and molecular pathophysiology related to both dominant and recessive ataxias have propelled the field forward, paving the way for innovative therapeutic strategies aimed at preventing and arresting the progression of these diseases. Nevertheless, the rarity of certain forms of ataxia continues to pose challenges, leading to limited insights into the etiology of the disease and the identification of target pathways. Additionally, the lack of suitable models hampers efforts to comprehensively understand the molecular foundations of disease's pathophysiology and test novel therapeutic interventions. In the following review, we describe the epidemiology, symptomatology, and pathological progression of hereditary ataxia, including both the prevalent and less common forms of these diseases. Furthermore, we illustrate the diverse molecular pathways and therapeutic approaches currently undergoing investigation in both pre-clinical studies and clinical trials. Finally, we address the existing and anticipated challenges within this field, encompassing both basic research and clinical endeavors.

Rare-variant and polygenic analyses of amyotrophic lateral sclerosis in the French-Canadian genome

PURPOSE

The genetic etiology of amyotrophic lateral sclerosis (ALS) includes few rare, large-effect variants and potentially many common, small-effect variants per case. The genetic risk liability for ALS might require a threshold comprised of a certain amount of variants. Here, we tested the degree to which risk for ALS was affected by rare variants in ALS genes, polygenic risk score, or both.

METHODS

335 ALS cases and 356 controls from Québec, Canada were concurrently tested by microarray genotyping and targeted sequencing of ALS genes known at the time of study inception. ALS genome-wide association studies summary statistics were used to estimate an ALS polygenic risk score (PRS). Cases and controls were subdivided into rare-variant heterozygotes and non-heterozygotes.

RESULTS

Risk for ALS was significantly associated with PRS and rare variants independently in a logistic regression model. Although ALS PRS predicted a small amount of ALS risk overall, the effect was most pronounced between ALS cases and controls that were not heterozygous for a rare variant in the ALS genes surveyed.

CONCLUSION

Both PRS and rare variants in ALS genes impact risk for ALS. PRS for ALS is most informative when rare variants are not observed in ALS genes.

A Novel Mutation in Sacsin, p.Val1335IIe, May Cause Late-Onset Sacsinopathy Due to Haploinsufficiency

Autosomal recessive spastic ataxia in Charlevoix-Saguenay (ARSACS) is a neurodegenerative disorder caused by mutations in the sacsin molecular chaperone protein (SACS) gene. Since the first report from Quebec in 1978, many pathogenic ARSACS variants with significantly reduced chaperone activities have been reported worldwide in adolescents, with presumably altered protein folding. In this study, a novel SACS mutation (p.Val1335IIe, Heterozygous) was identified in a Korean patient in their 50s with late-onset ARSACS characterized by cerebellar ataxia and spasticity without peripheral neuropathy. The mutation was confirmed via whole exome sequencing and Sanger sequencing and was predicted to likely cause disease using prediction software. RT-PCR and ELISA showed decreased SACS mRNA expression and sacsin protein concentrations in the proband, supporting its implications in diseases with pathogenicity and reduced chaperone function from haploinsufficiency. Our results revealed the pathogenicity of the SACS Val1335IIe mutation in the proband patient's disease manifestation, even though the symptoms had a limited correlation with the typical ARSACS clinical triad, which could be due to the reduced chaperon function from haploinsufficiency. Furthermore, our study suggests that variants of SACS heterozygosity may have diverse symptoms, with a wide range of disease onsets for late-onset sacsinopathy.

A SACS deletion variant in Great Pyrenees dogs causes autosomal recessive neuronal degeneration

ARSACS (autosomal recessive spastic ataxia of Charlevoix-Saguenay) is a human neurological disorder characterized by progressive cerebellar ataxia and peripheral neuropathy. A recently recognized disorder in Great Pyrenees dogs is similarly characterized by widespread central nervous system degeneration leading to progressive cerebellar ataxia and spasticity, combined with peripheral neuropathy. Onset of clinical signs occurred in puppies as young as 4 months of age, with slow progression over several years. A multi-generation pedigree suggested an autosomal recessive mode of inheritance. Histopathology revealed consistent cerebellar Purkinje cell degeneration, neuronal degeneration in brainstem nuclei, widespread spinal cord white matter degeneration, ganglion cell degeneration, inappropriately thin myelin sheaths or fully demyelinated peripheral nerve fibers, and normal or only mild patterns of denervation atrophy in skeletal muscles. Genome-wide single nucleotide polymorphism (SNP) genotype data was collected from 6 cases and 26 controls, where homozygosity mapping identified a 3.3 Mb region on CFA25 in which all cases were homozygous and all controls were either heterozygous or homozygous for alternate haplotypes. This region tagged the SACS gene where variants are known to cause ARSACS. Sanger sequencing of SACS in affected dogs identified a 4 bp deletion that causes a frame shift and truncates 343 amino acids from the C terminus of the encoded sacsin protein (p.Val4244AlafsTer32). Our clinical and histopathological descriptions of this canine disorder contribute to the description of human ARSACS and represents the first naturally occurring large animal model of this disorder.

JNK-interacting protein 4 is a central molecule for lysosomal retrograde trafficking

Lysosomal positioning is an important factor in regulating cellular responses, including autophagy. Because proteins encoded by disease-responsible genes are involved in lysosomal trafficking, proper intracellular lysosomal trafficking is thought to be essential for cellular homeostasis. In the past few years, the mechanisms of lysosomal trafficking have been elucidated with a focus on adapter proteins linking motor proteins to lysosomes. Here, we outline recent findings on the mechanisms of lysosomal trafficking by focusing on adapter protein c-Jun NH2 -terminal kinase-interacting protein (JIP) 4, which plays a central role in this process, and other JIP4 functions and JIP family proteins. Additionally, we discuss neuronal diseases associated with aberrance in the JIP family protein. Accumulating evidence suggests that chemical manipulation of lysosomal positioning may be a therapeutic approach for these neuronal diseases.

A mitochondrial-targeted antioxidant (MitoQ) improves motor coordination and reduces Purkinje cell death in a mouse model of ARSACS

Mitochondrial deficits have been observed in animal models of Autosomal-recessive spastic ataxia of the Charlevoix-Saguenay (ARSACS) and in patient-derived fibroblasts. We investigated whether mitochondrial function could be restored in Sacs^-/- mice, a mouse model of ARSACS, using the mitochondrial-targeted antioxidant ubiquinone MitoQ. After 10 weeks of chronic MitoQ administration in drinking water, we partially reversed motor coordination deficits in Sacs^-/- mice but did not affect litter-matched wild-type control mice. MitoQ administration led to a restoration of superoxide dismutase 2 (SOD2) in cerebellar Purkinje cell somata without altering Purkinje cell firing deficits. Purkinje cells in anterior vermis of Sacs^-/- mice normally undergo cell death in ARSACS; however, Purkinje cells numbers were elevated after chronic MitoQ treatment. Furthermore, Purkinje cell innervation of target neurons in the cerebellar nuclei of Sacs^-/- mice were also partially restored with MitoQ treatment. Our data suggest that MitoQ is a potential therapeutic treatment for ARSACS and that it improves motor coordination via increasing cerebellar Purkinje cell mitochondria function and reducing Purkinje cell death.

Digital health metrics reveal upper limb impairment profiles in ARSACS

OBJECTIVE

Adults with autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) often present with reduced upper limb coordination affecting their independence in daily life. Previous studies in ARSACS identified reduced performance in clinical assessments requiring fine and gross dexterity as well as prehension. However, the kinematic and kinetic aspects underlying reduced upper limb coordination in ARSACS have not been systematically investigated yet. In this work, we aimed to provide a detailed characterization of alterations in upper limb movement patterns and hand grip forces in 57 participants with ARSACS.

METHODS

We relied on a goal-directed technology-aided assessment task, which provides eight previously validated digital health metrics describing movement efficiency, smoothness, speed, and grip force control.

RESULTS

First, we observed that 98.3% of the participants were impaired in at least one of the metrics, that all metrics are significantly impaired on a population level, and that grip force control during precise manipulations is most commonly and strongly impaired. Second, we identified high inter-participant variability in the kinematic and kinetic impairment profiles, thereby capturing different clinical profiles subjectively observed in this population. Lastly, abnormal goal-directed task performance in ARSACS could be best explained by reduced movement speed, efficiency, and especially force control during precise manipulations, while abnormal movement smoothness did not have a significant effect.

INTERPRETATION

This work helped to refine the clinical profile of ARSACS and highlights the need for characterizing individual kinematic and kinetic impairment profiles in clinical trials in ARSACS.

AlphaFold predicted structure of the Hsp90-like domains of the neurodegeneration linked protein sacsin reveals key residues for ATPase activity

The ataxia-linked protein sacsin has three regions of partial homology to Hsp90's N-terminal ATP binding domain. Although a crystal structure for this Hsp90-like domain has been reported the precise molecular interactions required for ATP-binding and hydrolysis are unclear and it is debatable whether ATP biding is compatible with these domains. Furthermore, the Identification of a sacsin domain(s) equivalent to the middle domain of Hsp90 has been elusive. Here we present the superimposition of an AlphaFold structure of sacsin with yeast Hsp90, which provides novel insights into sacsin's structure. We identify residues within the sacsin Hsp90-like domains that are required for ATP binding and hydrolysis, including the putative catalytic arginine residues equivalent to that of the Hsp90 middle domain. Importantly, our analysis allows comparison of the Hsp90 middle domain with corresponding sacsin regions and identifies a shorter lid segment, in the sacsin ATP-binding domains, than the one found in the N-terminal domain of Hsp90. Our results show how a realignment of residues in the lid segment of sacsin that are involved in ATP binding can better match equivalent residues seen in Hsp90, which we then corroborated using molecular dynamic simulations. We speculate, from a structural viewpoint, why some ATP competitive inhibitors of Hsp90 may not bind sacsin, while others would. Together our analysis supports the hypothesis that sacsin's function is ATP-driven and would be consistent with it having a role as a super molecular chaperone. We propose that the SR1 regions of sacsin be renamed as HSP-NRD (Hsp90 N-Terminal Repeat Domain; residues 84-324) and the fragment immediately after as HSP-MRD (Hsp90 Middle Repeat Domain; residues 325-518).

A rehabilitation program to increase balance and mobility in ataxia of Charlevoix-Saguenay: An exploratory study

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is characterized by balance impairment and mobility limitations, which both increase the risk of falling. The objective of this study was to explore the effects of a rehabilitation program aimed at increasing trunk and lower limb motor control on balance and walking abilities, and accomplishment of activities of daily living. In this exploratory study, a group-supervised rehabilitation program was performed three times a week for 8 weeks (two sessions at a rehabilitation gym and one pool session). Outcome measures included the Ottawa Sitting Scale, Berg Balance Scale, modified Activities-specific Balance Confidence Scale, 30-Second Chair Stand Test, 10-Meter Walk Test, Barthel Index, and Scale for the Assessment and Rating of Ataxia. Significant improvements in balance, trunk control, maximal and self-selected walking speed difference, ataxia severity and accomplishment of specific activities of daily living were noted for the whole group at the end of the program. At the individual level, all participants improved beyond the standard error of measurement in at least two outcome measures. Also, most participants reported many perceived improvements related to balance, posture and functional mobility. This study provides encouraging results on the effects of a rehabilitation program for ambulatory people with ARSACS. Group intervention could have a positive impact on their daily lives and improve the health care service offered to this population. Future studies with larger sample sizes including control groups and other forms of ataxia are necessary to validate our results to generalize them.

Transcriptome analysis of gene expression profiling from the deep sea in situ to the laboratory for the cold seep mussel Gigantidas haimaensis

Background

The deep-sea mussel Gigantidas haimaensis is a representative species from the Haima cold seep ecosystem in the South China Sea that establishes endosymbiosis with chemotrophic bacteria. During long-term evolution, G. haimaensis has adapted well to the local environment of cold seeps. Until now, adaptive mechanisms responding to environmental stresses have remained poorly understood.

Results

In this study, transcriptomic analysis was performed for muscle tissue of G. haimaensis in the in situ environment (MH) and laboratory environment for 0 h (M0), 3 h (M3) and 9 h (M9), and 187,368 transcript sequences and 22,924 annotated differentially expressed genes (DEGs) were generated. Based on Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, these DEGs were enriched with a broad spectrum of biological processes and pathways, including those associated with antioxidants, apoptosis, chaperones, immunity and metabolism. Among these significantly enriched pathways, protein processing in the endoplasmic reticulum and metabolism were the most affected metabolic pathways. These results may imply that G. haimaensis struggles to support the life response to environmental change by changing gene expression profiles.

Conclusion

The present study provides a better understanding of the biological responses and survival strategies of the mussel G. haimaensis from deep sea in situ to the laboratory environment.

Insights into SACS pathological attributes in autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS)☆

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a rare early-onset neurodegenerative disease caused by mutations in the SACS gene, encoding Sacsin. Initial functional annotation of Sacsin was based on sequence homology, with subsequent experiments revealing the Sacsin requirement for regulating mitochondrial dynamics, along with its domains involved in promoting neurofilament assembly or resolving their bundling accumulations. ARSACS phenotypes associated with SACS loss-of-function are discussed, and how advancements in ARSACS disease models and quantitative omics approaches can improve our understanding of ARSACS pathological attributes. Lastly in the perspectives section, we address gene correction strategies for monogenic disorders such as ARSACS, along with their common delivery methods, representing a hopeful area for ARSACS therapeutics development.

Multi-omic profiling reveals the ataxia protein sacsin is required for integrin trafficking and synaptic organization

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset cerebellar ataxia caused by mutations in SACS, which encodes the protein sacsin. Cellular ARSACS phenotypes include mitochondrial dysfunction, intermediate filament disorganization, and progressive death of cerebellar Purkinje neurons. It is unclear why the loss of sacsin causes these deficits or why they manifest as cerebellar ataxia. Here, we perform multi-omic profiling in sacsin knockout (KO) cells and identify alterations in microtubule dynamics and mislocalization of focal adhesion (FA) proteins, including multiple integrins. Deficits in FA structure, signaling, and function can be rescued by targeting PTEN, a negative regulator of FA signaling. ARSACS mice possess mislocalization of ITGA1 in Purkinje neurons and synaptic disorganization in the deep cerebellar nucleus (DCN). The sacsin interactome reveals that sacsin regulates interactions between cytoskeletal and synaptic adhesion proteins. Our findings suggest that disrupted trafficking of synaptic adhesion proteins is a causal molecular deficit in ARSACS.

Circumventing the packaging limit of AAV-mediated gene replacement therapy for neurological disorders

INTRODUCTION

Gene therapy provides the exciting opportunity of a curative single treatment for devastating diseases, eradicating the need for chronic medication. Adeno-associated viruses (AAVs) are among the most attractive vector carriers for gene replacement in vivo. Yet, despite the success of recent AAV-based clinical trials, the clinical use of these vectors has been limited. For instance, the AAV packaging capacity is restricted to ~4.7 kb, making it a substantial challenge to deliver large gene products.

AREAS COVERED

In this review, we explore established and emerging strategies that circumvent the packaging limit of AAVs to make them effective vehicles for gene replacement therapy of monogenic disorders, with a particular focus on diseases affecting the nervous system. We report historical references, design remarks, as well as strengths and weaknesses of these approaches. We additionally discuss examples of neurological disorders for which such strategies have been attempted.

EXPERT OPINION

The field of AAV-gene therapy has experienced enormous advancements in the last decade. However, there is still ample space for improvement aimed at overcoming existing challenges that are slowing down the progressive trajectory of this field.

The ARSACS disease protein sacsin controls lysosomal positioning and reformation by regulating microtubule dynamics

Autosomal recessive spastic ataxia of Charlevoix-Saguenay is a fatal brain disorder featuring cerebellar neurodegeneration leading to spasticity and ataxia. This disease is caused by mutations in the SACS gene that encodes sacsin, a massive 4579-amino acid protein with multiple modular domains. However, molecular details of the function of sacsin are not clear. Here, using live cell imaging and biochemistry, we demonstrate that sacsin binds to microtubules and regulates microtubule dynamics. Loss of sacsin function in various cell types, including knockdown and KO primary neurons and patient fibroblasts, leads to alterations in lysosomal transport, positioning, function, and reformation following autophagy. Each of these phenotypic changes is consistent with altered microtubule dynamics. We further show the effects of sacsin are mediated at least in part through interactions with JIP3, an adapter for microtubule motors. These data reveal a new function for sacsin that explains its previously reported roles and phenotypes.

Integrative Organelle-Based Functional Proteomics: In Silico Prediction of Impaired Functional Annotations in SACS KO Cell Model

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an inherited neurodegenerative disease characterized by early-onset spasticity in the lower limbs, axonal-demyelinating sensorimotor peripheral neuropathy, and cerebellar ataxia. Our understanding of ARSACS (genetic basis, protein function, and disease mechanisms) remains partial. The integrative use of organelle-based quantitative proteomics and whole-genome analysis proposed in the present study allowed identifying the affected disease-specific pathways, upstream regulators, and biological functions related to ARSACS, which exemplify a rationale for the development of improved early diagnostic strategies and alternative treatment options in this rare condition that currently lacks a cure. Our integrated results strengthen the evidence for disease-specific defects related to bioenergetics and protein quality control systems and reinforce the role of dysregulated cytoskeletal organization in the pathogenesis of ARSACS.

Phenotypical spectrum of SACS variants: Neuromuscular perspective of a complex neurodegenerative disorder

OBJECTIVES

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by the SACS gene variants. Main clinical features include early-onset and progressive cerebellar ataxia, spasticity, sensorimotor polyneuropathy. However, the phenotypic spectrum expanded with the increased availability of next-generation sequencing methods.

MATERIALS AND METHODS

Herein, we describe the clinical features of nine patients from seven unrelated families with SACS variants from the cohort of the Neuromuscular Disorders Unit of the Neurology Department of the Istanbul University, Istanbul Faculty of Medicine.

RESULTS

Seven patients were male. Seven patients in our cohort had disease onset in the first decade of life. Eight patients were born to consanguineous marriages. Distal weakness in the lower limbs was a prominent feature in all of our patients. Seven patients had ataxia, and six patients had spasticity. Interestingly, one patient showed an isolated Charcot-Marie-Tooth-like phenotype. Five patients showed sensorimotor demyelinating polyneuropathy in the nerve conduction studies. Linear pontine hypointensity was the most frequent cranial magnetic resonance imaging (MRI) abnormality. Two patients with a later disease onset had a homozygous c.11542_11544delATT (p.Ile3848del) variant. The rest of the identified variants were scattered throughout the SACS gene.

CONCLUSIONS

Atypical clinical features in our patients highlight that the phenotypic spectrum of ARSACS can be observed in a wide range.

Participation and Functional Independence in Adults With Recessive Spastic Ataxia of Charlevoix-Saguenay

Background. Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a progressive disorder where upper and lower extremities motor performances may bring participation restriction. Purpose. To document participation in adults with ARSAC and explore associations with motor performances. Method. Twenty-eight participants took part in the study. Participation was assessed using the LIFE-H. Motor performance was assessed using several outcomes including the SARA, LEMOCOT, Berg Balance Scale, 10-Meter Walk Test, and Finger-to-nose Test. Findings. Participation was significantly lower in the wheelchair user subgroup. Also, for 29 activities out of 77, at least 15% of participants reported severely disrupted participation. Participation was correlated with upper and lower limbs coordination, walking ability, balance, disease severity, and fine dexterity (Spearman r = .41–0.85, p < .03). Implications. Results showed significant participation restrictions and suggest that interventions aiming to improve or compensate upper and lower limbs functions could help to decrease disease burden.

Reliability and validity of digital health metrics for assessing arm and hand impairments in an ataxic disorder

OBJECTIVES

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is the second most frequent recessive ataxia and commonly features reduced upper limb coordination. Sensitive outcome measures of upper limb coordination are essential to track disease progression and the effect of interventions. However, available clinical assessments are insufficient to capture behavioral variability and detailed aspects of motor control. While digital health metrics extracted from technology-aided assessments promise more fine-grained outcome measures, these have not been validated in ARSACS. Thus, the aim was to document the metrological properties of metrics from a technology-aided assessment of arm and hand function in ARSACS.

METHODS

We relied on the Virtual Peg Insertion Test (VPIT) and used a previously established core set of 10 digital health metrics describing upper limb movement and grip force patterns during a pick-and-place task. We evaluated reliability, measurement error, and learning effects in 23 participants with ARSACS performing three repeated assessment sessions. In addition, we documented concurrent validity in 57 participants with ARSACS performing one session.

RESULTS

Eight metrics had excellent test-retest reliability (intraclass correlation coefficient 0.89 ± 0.08), five low measurement error (smallest real difference % 25.4 ± 5.7), and none strong learning effects (systematic change η -0.11 ± 2.5). Significant correlations (ρ 0.39 ± 0.13) with clinical scales describing gross and fine dexterity and lower limb coordination were observed.

INTERPRETATION

This establishes eight digital health metrics as valid and robust endpoints for cross-sectional studies and five metrics as potentially sensitive endpoints for longitudinal studies in ARSACS, thereby promising novel insights into upper limb sensorimotor control.

Molecular Characterization of Portuguese Patients with Hereditary Cerebellar Ataxia

Hereditary cerebellar ataxia (HCA) comprises a clinical and genetic heterogeneous group of neurodegenerative disorders characterized by incoordination of movement, speech, and unsteady gait. In this study, we performed whole-exome sequencing (WES) in 19 families with HCA and presumed autosomal recessive (AR) inheritance, to identify the causal genes. A phenotypic classification was performed, considering the main clinical syndromes: spastic ataxia, ataxia and neuropathy, ataxia and oculomotor apraxia (AOA), ataxia and dystonia, and ataxia with cognitive impairment. The most frequent causal genes were associated with spastic ataxia (SACS and KIF1C) and with ataxia and neuropathy or AOA (PNKP). We also identified three families with autosomal dominant (AD) forms arising from de novo variants in KIF1A, CACNA1A, or ATP1A3, reinforcing the importance of differential diagnosis (AR vs. AD forms) in families with only one affected member. Moreover, 10 novel causal-variants were identified, and the detrimental effect of two splice-site variants confirmed through functional assays. Finally, by reviewing the molecular mechanisms, we speculated that regulation of cytoskeleton function might be impaired in spastic ataxia, whereas DNA repair is clearly associated with AOA. In conclusion, our study provided a genetic diagnosis for HCA families and proposed common molecular pathways underlying cerebellar neurodegeneration.

Genetics of Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS) and Role of Sacsin in Neurodegeneration

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an early-onset neurodegenerative disease that was originally discovered in the population from the Charlevoix-Saguenay-Lac-Saint-Jean (CSLSJ) region in Quebec. Although the disease progression of ARSACS may start in early childhood, cases with later onset have also been observed. Spasticity and ataxia could be common phenotypes, and retinal optic nerve hypermyelination is detected in the majority of patients. Other symptoms, such as pes cavus, ataxia and limb deformities, are also frequently observed in affected individuals. More than 200 mutations have been discovered in the SACS gene around the world. Besides French Canadians, SACS genetics have been extensively studied in Tunisia or Japan. Recently, emerging studies discovered SACS mutations in several other countries. SACS mutations could be associated with pathogenicity either in the homozygous or compound heterozygous stages. Sacsin has been confirmed to be involved in chaperon activities, controlling the microtubule balance or cell migration. Additionally, sacsin may also play a crucial role in regulating the mitochondrial functions. Through these mechanisms, it may share common mechanisms with other neurodegenerative diseases. Further studies are needed to define the exact functions of sacsin. This review introduces the genetic mutations discovered in the SACS gene and discusses its pathomechanisms and its possible involvement in other neurodegenerative diseases.

Molecular Identity and Location Influence Purkinje Cell Vulnerability in Autosomal-Recessive Spastic Ataxia of Charlevoix-Saguenay Mice

Patterned cell death is a common feature of many neurodegenerative diseases. In patients with autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) and mouse models of ARSACS, it has been observed that Purkinje cells in anterior cerebellar vermis are vulnerable to degeneration while those in posterior vermis are resilient. Purkinje cells are known to express certain molecules in a highly stereotyped, patterned manner across the cerebellum. One patterned molecule is zebrin, which is expressed in distinctive stripes across the cerebellar cortex. The different zones delineated by the expression pattern of zebrin and other patterned molecules have been implicated in the patterning of Purkinje cell death, raising the question of whether they contribute to cell death in ARSACS. We found that zebrin patterning appears normal prior to disease onset in Sacs^–/– mice, suggesting that zebrin-positive and -negative Purkinje cell zones develop normally. We next observed that zebrin-negative Purkinje cells in anterior lobule III were preferentially susceptible to cell death, while anterior zebrin-positive cells and posterior zebrin-negative and -positive cells remained resilient even at late disease stages. The patterning of Purkinje cell innervation to the target neurons in the cerebellar nuclei (CN) showed a similar pattern of loss: neurons in the anterior CN, where inputs are predominantly zebrin-negative, displayed a loss of Purkinje cell innervation. In contrast, neurons in the posterior CN, which is innervated by both zebrin-negative and -positive puncta, had normal innervation. These results suggest that the location and the molecular identity of Purkinje cells determine their susceptibility to cell death in ARSACS.

Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay without Spasticity

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a rare progressive neurodegenerative disease caused by either homozygous or compound heterozygous mutations in the SACS gene. The original ARSACS cases found in Quebec showed very homogenous phenotypes characterized by cerebellar ataxia, spasticity, and polyneuropathy. However, many cases with atypical phenotypes have been found in other regions and ethnic groups. We herein present a Japanese patient with atypical ARSACS who showed cerebellar ataxia and polyneuropathy, but no spasticity. She carried novel compound heterozygous mutations (p.Lys4326Glu and p.Leu1412Lysfs*16) in the SACS gene. The brain MRI findings were useful for making a diagnosis of ARSACS.

Two divergent haplogroups of a sacsin-like gene in Acropora corals

Reef-building corals are declining due to environmental changes. Sacsin is a member of the heat shock proteins and has been reported as a candidate protein associated with the stress response in Acropora corals. Recently, high nucleotide diversity and the persistence of two divergent haplogroups of sacsin-like genes in Acropora millepora have been reported. While it was not clear when the two haplogroups have split and whether the haplogroups have persisted in only A. millepora or the other lineages in the genus Acropora. In this study, we analyzed a genomic region containing a sacsin-like gene from Acropora and Montipora species. Higher nucleotide diversity in the sacsin-like gene compared with that of surrounding regions was also observed in A. digitifera. This nucleotide diversity is derived from two divergent haplogroups of a sacsin-like gene, which are present in at least three Acropora species. The origin of these two haplogroups can be traced back before the divergence of Acropora and Montipora (119 Ma). Although the link between exceptionally high genetic variation in sacsin-like genes and functional differences in sacsin-like proteins is not clear, the divergent haplogroups may respond differently to envionmental stressors and serve in the adaptive phsiological ecology of these keystone species.

Functional mobility in walking adult population with ataxia of Charlevoix-Saguenay

BACKGROUND

This study aimed to describe lower limbs impairments, balance and activity limitations related to indoor mobility in adult walkers with autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS).

RESULTS

Twenty-five participants were recruited with a mean age of 32.2 (± 10.4) years with 45.7% using a walking aid. There is a significant difference between participants with and without a walking aid in terms of lower limbs coordination, balance and mobility. Although participants who walk without a walking aid perform better than the others and they are below predictive or reference values. Despite significant mobility limitations, only mild spasticity and passive range of motion limitations were observed. However, there is a significant difference between unaffected individuals and participants with ARSACS for lower limb muscle cocontraction.

CONCLUSIONS

Results show a high level of lower limb impairments, balance and mobility limitation in adults' participants with ARSACS that are still walking, including people not using a walking aid. One of the most original finding is the presence of excessive cocontraction and a relatively mild level of spasticity in the lower limbs muscles. Results of this study better circumscribes the impairments and activities that should be the focus of intervention including rehabilitation in ARSACS.

Assessment of Sacsin Turnover in Patients With ARSACS: Implications for Molecular Diagnosis and Pathogenesis

Background and Objectives

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by variations in SACS gene encoding sacsin, a huge multimodular protein of unknown function. More than 200 SACS variations have been described worldwide to date. Because ARSACS presents phenotypic variability, previous empirical studies attempted to correlate the nature and position of SACS variations with the age at onset or with disease severity, although not considering the effect of the various variations on protein stability. In this work, we studied genotype-phenotype correlation in ARSACS at a functional level.

Methods

We analyzed a large set of skin fibroblasts derived from patients with ARSACS, including both new and already published cases, carrying variations of different types affecting diverse domains of the protein.

Results

We found that sacsin is almost absent in patients with ARSACS, regardless of the nature of the variation. As expected, we did not detect sacsin in patients with truncating variations. We found it strikingly reduced or absent also in compound heterozygotes carrying diverse missense variations. In this case, we excluded SACS mRNA decay, defective translation, or faster posttranslational degradation as possible causes of protein reduction. Conversely, our results demonstrate that nascent mutant sacsin protein undergoes cotranslational ubiquitination and degradation.

Discussion

Our results provide a mechanistic explanation for the lack of genotype-phenotype correlation in ARSACS. We also propose a new and unambiguous criterion for ARSACS diagnosis that is based on the evaluation of sacsin level. Last, we identified preemptive degradation of a mutant protein as a novel cause of a human disease.

First glance at the molecular etiology of hearing loss in French-Canadian families from Saguenay-Lac-Saint-Jean's founder population

The French-Canadian population of Saguenay-Lac-Saint-Jean is known for its homogenous genetic background. The hereditary causes of hearing loss were previously unexplored in this population. Individuals with hearing loss were referred from the otorhinolaryngology, pediatrics and family physicians' clinics to the medical genetics service at the Centre intégré universitaire de santé et de services sociaux du Saguenay-Lac-Saint-Jean between June 2015 and March 2021. A regional clinical evaluation strategy was developed. Samples from 63 individuals belonging to 41 families were sent independently to different molecular clinical laboratories and index cases were analyzed through comprehensive multigene panels, with a diagnostic rate of 54%. Sixteen hearing loss causal variants were identified in 12 genes, with eight of these variants not been previously reported in the literature. Recurrent variants were present in four genes, suggesting a possible founder effect, while GJB2 gene variants were scarce. A comprehensive multigene panel approach as part of the proposed clinical evaluation strategy offers a high diagnostic yield for this population.

Efficient Neuroprotective Rescue of Sacsin-Related Disease Phenotypes in Zebrafish

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a multisystem hereditary ataxia associated with mutations in SACS, which encodes sacsin, a protein of still only partially understood function. Although mouse models of ARSACS mimic largely the disease progression seen in humans, their use in the validation of effective therapies has not yet been proposed. Recently, the teleost Danio rerio has attracted increasing attention as a vertebrate model that allows rapid and economical screening, of candidate molecules, and thus combines the advantages of whole-organism phenotypic assays and in vitro high-throughput screening assays. Through CRISPR/Cas9-based mutagenesis, we generated and characterized a zebrafish sacs-null mutant line that replicates the main features of ARSACS. The sacs-null fish showed motor impairment, hindbrain atrophy, mitochondrial dysfunction, and reactive oxygen species accumulation. As proof of principle for using these mutant fish in high-throughput screening studies, we showed that both acetyl-DL-leucine and tauroursodeoxycholic acid improved locomotor and biochemical phenotypes in sacs^−/− larvae treated with these neuroprotective agents, by mediating significant rescue of the molecular functions altered by sacsin loss. Taken together, the evidence here reported shows the zebrafish to be a valuable model organism for the identification of novel molecular mechanisms and for efficient and rapid in vivo optimization and screening of potential therapeutic compounds. These findings may pave the way for new interventions targeting the earliest phases of Purkinje cell degeneration in ARSACS.

Genetic burden linked to founder effects in Saguenay-Lac-Saint-Jean illustrates the importance of genetic screening test availability

The Saguenay–Lac-Saint-Jean (SLSJ) region located in the province of Quebec was settled in the 19th century by pioneers issued from successive migration waves starting in France in the 17th century and continuing within Quebec until the beginning of the 20th century. The genetic structure of the SLSJ population is considered to be the product of a triple founder effect and is characterised by a higher prevalence of some rare genetic diseases. Several studies were performed to elucidate the historical, demographic and genetic background of current SLSJ inhabitants to assess the origins of these rare disorders and their distribution in the population. Thanks to the development of new sequencing technologies, the genes and the variants responsible for the most prevalent conditions were identified. Combined with other resources such as the BALSAC population database, identifying the causal genes and the pathogenic variants allowed to assess the impacts of some of these founder mutations on the population health and to design precision medicine public health strategies based on carrier testing. Furthermore, it stimulated the establishment of many public programmes.

We report here a review and an update of a subset of inherited disorders and founder mutations in the SLSJ region. Data were collected from published scientific sources. This work expands the knowledge about the current frequencies of these rare disorders, the frequencies of other rare genetic diseases in this population, the relevance of the carrier tests offered to the population, as well as the current available treatments and research about future therapeutic avenues for these inherited disorders.

Retinal Architecture in Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS): Insights into Disease Pathogenesis and Biomarkers

BACKGROUND

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) causes unique retinal abnormalities, which have not been systematically investigated.

OBJECTIVE

To deeply phenotype the retina in ARSACS in order to better understand its pathogenesis and identify potential biomarkers.

METHODS

We evaluated 29 patients with ARSACS, 66 with spinocerebellar ataxia (SCA), 38 with autosomal recessive cerebellar ataxia (ATX), 22 with hereditary spastic paraplegia (SPG), 21 cases of papilledema, and 20 healthy controls (total n = 196 subjects). Participants underwent visual acuity assessment, intraocular pressure measurement, fundoscopy, and macular and peripapillary optical coherence tomography (OCT). Macular layers thicknesses in ARSACS were compared with those of age-matched healthy controls. Ophthalmologists analyzed the scans for abnormal signs in the different patient groups. Linear regression analysis was conducted to look for associations between retinal changes and age, age at onset, disease duration, and Scale for the Assessment and Rating of Ataxia (SARA) scores in ARSACS.

RESULTS

Only patients with ARSACS exhibited peripapillary retinal striations (82%) on fundoscopy, and their OCT scans revealed foveal hypoplasia (100%), sawtooth appearance (89%), papillomacular fold (86%), and macular microcysts (18%). Average peripapillary retinal nerve fiber layer (pRNFL) was thicker in ARSACS than in SCA, ATX, SPG, and controls; a cut-off of 121 μm was 100% accurate in diagnosing ARSACS. All macular layers were thicker in ARSACS when compared to healthy controls. RNFL thickness in the inferior sector of the macula positively correlated with SARA scores.

CONCLUSIONS

Retinal abnormalities are highly specific for ARSACS, and suggest retinal hyperplasia due to abnormal retinal development. OCT may provide potential biomarkers for future clinical trials. © 2021 International Parkinson and Movement Disorder Society.

The History of Gene Hunting in Hereditary Spinocerebellar Degeneration: Lessons From the Past and Future Perspectives

Hereditary spinocerebellar degeneration (SCD) encompasses an expanding list of rare diseases with a broad clinical and genetic heterogeneity, complicating their diagnosis and management in daily clinical practice. Correct diagnosis is a pillar for precision medicine, a branch of medicine that promises to flourish with the progressive improvements in studying the human genome. Discovering the genes causing novel Mendelian phenotypes contributes to precision medicine by diagnosing subsets of patients with previously undiagnosed conditions, guiding the management of these patients and their families, and enabling the discovery of more causes of Mendelian diseases. This new knowledge provides insight into the biological processes involved in health and disease, including the more common complex disorders. This review discusses the evolution of the clinical and genetic approaches used to diagnose hereditary SCD and the potential of new tools for future discoveries.

Autosomal recessive spastic ataxia of Charlevoix-Saguenay caused by novel mutations in SACS gene: A report of two Chinese families

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a rare hereditary disease characterized by cerebellar ataxia, pyramidal signs in lower limbs, and sensorimotor neuropathy. The disease is caused by bi-allelic mutations of the SACS gene encoding the sacsin protein. Over 200 mutations have been reported worldwide. Here, we report two unrelated Chinese ARSACS patients with novel mutations revealed by whole-exome sequencing (WES). One 36-year-old female patient exhibited classical ARSACS characteristics including cerebellar ataxia, pyramidal tract signs in the lower limbs and sensorimotor neuropathy, while the other 9-year-old male showed cerebellar ataxia and peripheral neuropathy. WES identified a compound heterozygous variant in the SACS gene (c.5692 G > T, p.E1898X; c.12673-12677 del TATCA, p.Y4225D fs*6) in the female patient and another compound heterozygous variant (c.1773C > A, p.S578X; c.8088-8089 in. CA, p.M2697Q fs*43) in the male patient. All of these novel mutations were predicted to be loss-of-function which affect the expression of the two important C-terminal domains (DnaJ and HEPN). These findings add new insights into the mutational and clinical spectrum of ARSACS.

From fuzziness to precision medicine: on the rapidly evolving proteomics with implications in mitochondrial connectivity to rare human disease

Mitochondrial (mt) dysfunction is linked to rare diseases (RDs) such as respiratory chain complex (RCC) deficiency, MELAS, and ARSACS. Yet, how altered mt protein networks contribute to these ailments remains understudied. In this perspective article, we identified 21 mt proteins from public repositories that associate with RCC deficiency, MELAS, or ARSACS, engaging in a relatively small number of protein-protein interactions (PPIs), underscoring the need for advanced proteomic and interactomic platforms to uncover the complete scope of mt connectivity to RDs. Accordingly, we discuss innovative untargeted label-free proteomics in identifying RD-specific mt or other macromolecular assemblies and mapping of protein networks in complex tissue, organoid, and stem cell-differentiated neurons. Furthermore, tag- and label-based proteomics, genealogical proteomics, and combinatorial affinity purification-mass spectrometry, along with advancements in detecting and integrating transient PPIs with single-cell proteomics and transcriptomics, collectively offer seminal follow-ups to enrich for RD-relevant networks, with implications in RD precision medicine.

A novel SACS p.Pro4154GlnfsTer20 mutation in a family with autosomal recessive spastic ataxia of Charlevoix-Saguenay

Loss-of-function mutations in the sacsin (SACS) gene lead to autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS), impairing the function of sacsin. Genotype-phenotype correlations are still unclear for the different mutations reported in ARSACS. Here, we present a Turkish ARSACS family in whom the novel homozygous frameshift mutation in SACS c.12461delC (p.Pro4154GlnfsTer20) was detected by next-generation sequencing (NGS). The index patient was admitted with progressive spastic ataxia and dysarthria. Since no common mutation in autosomal recessive (AR) cerebellar ataxias, whole gene sequencing provide an advantage to detect novel mutations and may be more effective for clinical diagnosis.

Functional Network Profiles in ARSACS Disclosed by Aptamer-Based Proteomic Technology

Although the genetic basis of autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) has been uncovered, our poor understanding of disease mechanisms requires new light on functional pathways and modifying factors to improve early diagnostic strategies and offer alternative treatment options in a rare condition with no cure. Investigation of the pathologic state combining disease models and quantitative omic approach might improve biomarkers discovery with possible implications in patients' diagnoses. In this study, we analyzed proteomics data obtained using the SomaLogic technology, comparing cell lysates from ARSACS patients and from a SACS KO SH-SY5Y neuroblastoma cell model. Single-stranded deoxyoligonucleotides, selected in vitro from large random libraries, bound and quantified molecular targets related to the neuroinflammation signaling pathway and to neuronal development. Changes in protein levels were further analyzed by bioinformatics and network approaches to identify biomarkers of ARSACS and functional pathways impaired in the disease. We identified novel significantly dysregulated biological processes related to neuroinflammation, synaptogenesis, and engulfment of cells in patients and in KO cells compared with controls. Among the differential expressed proteins found in this work, we identified several proteins encoded by genes already known to be mutated in other forms of neurodegeneration. This finding suggests that common dysfunctional networks could be therapeutic targets for future investigations.