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

Modeling sacsin depletion in Danio Rerio offers new insight on retinal defects in ARSACS

Biallelic mutations in the SACS gene, encoding sacsin, cause early-onset autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS), a neurodegenerative disease also characterized by unique and poorly understood retinal abnormalities. While two murine models replicate the phenotypic and neuronal features observed in patients, no retinal phenotype has been described so far. In a zebrafish knock-out strain that faithfully mirrors the main aspects of ARSACS, we observed impaired visual function due to photoreceptor degeneration, likely caused by cell cycle defects in progenitor cells. RNA-seq analysis in embryos revealed dysfunction in proteins related to fat-soluble vitamins (e.g., TTPA, RDH5, VKORC) and suggested a key role of neuroinflammation in driving the retinal defects. Our findings indicate that studying retinal pathology in ARSACS could be crucial for understanding the impact of sacsin depletion and may offer insights into halting disease progression.

An overview of early-onset cerebellar ataxia: a practical guideline

Early onset ataxias (EOAs) are a heterogeneous group of rare neurological disorders that not only involve the central and peripheral nervous system but also involve other organs. They are mainly manifested by degeneration or abnormal development of the cerebellum occurring before the age of 25 years and typically the pattern of inheritance is autosomal recessive.The diagnosis of autosomal recessive cerebellar ataxias (ARCAs) is confirmed by the clinical, laboratory, electrophysiological examination, neuroimaging findings, and mutation analysis when the causative gene is detected. Correct diagnosis is crucial for appropriate genetic counseling, estimating the prognosis, and, in some cases, pharmacological intervention. The wide variety of genotypes with a heterogeneous phenotypic manifestation makes the diagnostic work-up challenging, time-consuming, and expensive, not only for the clinician but also for the children and their parents. In this review, we focused on the step-by-step approach in which cerebellar ataxia is a prominent sign. We also outline the most common disorders in ataxias with early-onset manifestations.

Driving Mitochondrial Fission Improves Cognitive, but not Motor Deficits in a Mouse Model of Ataxia of Charlevoix-Saguenay

Autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by loss-of-function mutation in the SACS gene, which encodes sacsin, a putative HSP70-HSP90 co-chaperone. Previous studies with Sacs knock-out (KO) mice and patient-derived fibroblasts suggested that SACSIN mutations inhibit the function of the mitochondrial fission enzyme dynamin-related protein 1 (Drp1). This in turn resulted in mitochondrial hyperfusion and dysfunction. We experimentally tested this hypothesis by genetically manipulating the mitochondrial fission/fusion equilibrium, creating double KO (DKO) mice that also lack positive (PP2A/Bβ2) and negative (PKA/AKAP1) regulators of Drp1. Neither promoting mitochondrial fusion (Bβ2 KO) nor fission (Akap1 KO) influenced progression of motor symptoms in Sacs KO mice. However, our studies identified profound learning and memory deficits in aged Sacs KO mice. Moreover, this cognitive impairment was rescued in a gene dose-dependent manner by deletion of the Drp1 inhibitor PKA/Akap1. Our results are inconsistent with mitochondrial dysfunction as a primary pathogenic mechanism in ARSACS. Instead, they imply that promoting mitochondrial fission may be beneficial at later stages of the disease when pathology extends to brain regions subserving learning and memory.

Investigating the genetic basis of hereditary spastic paraplegia and cerebellar Ataxia in Pakistani families

BACKGROUND

Hereditary Spastic Paraplegias (HSPs) and Hereditary Cerebellar Ataxias (HCAs) are progressive neurodegenerative disorders encompassing a spectrum of neurogenetic conditions with significant overlaps of clinical features. Spastic ataxias are a group of conditions that have features of both cerebellar ataxia and spasticity, and these conditions are frequently clinically challenging to distinguish. Accurate genetic diagnosis is crucial but challenging, particularly in resource-limited settings. This study aims to investigate the genetic basis of HSPs and HCAs in Pakistani families.

METHODS

Families from Khyber Pakhtunkhwa with at least two members showing HSP or HCA phenotypes, and who had not previously been analyzed genetically, were included. Families were referred for genetic analysis by local neurologists based on the proband's clinical features and signs of a potential genetic neurodegenerative disorder. Whole Exome Sequencing (WES) and Sanger sequencing were then used to identify and validate genetic variants, and to analyze variant segregation within families to determine inheritance patterns. The mean age of onset and standard deviation were calculated to assess variability among affected individuals, and the success rate was compared with literature reports using differences in proportions and Cohen's h.

RESULTS

Pathogenic variants associated with these conditions were identified in five of eight families, segregating according to autosomal recessive inheritance. These variants included previously reported SACS c.2182 C > T, p.(Arg728*), FA2H c.159_176del, p.(Arg53_Ile58del) and SPG11 c.2146 C > T, p.(Gln716*) variants, and two previously unreported variants in SACS c.2229del, p.(Phe743Leufs*8) and ZFYVE26 c.1926_1941del, p.(Tyr643Metfs*2). Additionally, FA2H and SPG11 variants were found to have recurrent occurrences, suggesting a potential founder effect within the Pakistani population. Onset age among affected individuals ranged from 1 to 14 years (M = 6.23, SD = 3.96). The diagnostic success rate was 62.5%, with moderate effect sizes compared to previous studies.

CONCLUSIONS

The findings of this study expand the genotypic and phenotypic spectrum of HSPs and HCAs in Pakistan and emphasize the importance of utilizing exome/genome sequencing for accurate diagnosis or support accurate differential diagnosis. This approach can improve genetic counseling and clinical management, addressing the challenges of diagnosing neurodegenerative disorders in resource-limited settings.

MRI-ARSACS: An Imaging Index for Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay (ARSACS) Identification Based on the Multicenter PROSPAX Study

BACKGROUND

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) and hereditary spastic paraplegia type 7 (SPG7) represent the most common genotypes of spastic ataxia (SPAX). To date, their magnetic resonance imaging (MRI) features have only been described qualitatively, and a pure neuroradiological differential diagnosis between these two conditions is difficult to achieve.

OBJECTIVES

To test the performance of MRI measures to discriminate between ARSACS and SPG7 (as an index of common SPAX disease).

METHODS

In this prospective multicenter study, 3D-T1-weighted images of 59 ARSACS (35.4 ± 10.3 years, M/F = 33/26) and 78 SPG7 (54.8 ± 10.3 years, M/F = 51/27) patients of the PROSPAX Consortium were analyzed, together with 30 controls (45.9 ± 16.9 years, M/F = 15/15). Different linear and surface measures were evaluated. A receiver operating characteristic analysis was performed, calculating area under the curve (AUC) and corresponding diagnostic accuracy parameters.

RESULTS

The pons area proved to be the only metric increased exclusively in ARSACS patients (P = 0.02). Other different measures were reduced in ARSACS and SPG7 compared with controls (all with P ≤ 0.005). A cut-off value equal to 1.67 of the pons-to-superior vermis area ratio proved to have the highest AUC (0.98, diagnostic accuracy 93%, sensitivity 97%) in discriminating between ARSACS and SPG7.

CONCLUSIONS

Evaluation of the pons-to-superior vermis area ratio can discriminate ARSACS from other SPAX patients, as exemplified here by SPG7. Hence, we hereby propose this ratio as the Magnetic Resonance Index for the Assessment and Recognition of patients harboring SACS mutations (MRI-ARSACS), a novel diagnostic tool able to identify ARSACS patients and useful for discriminating ARSACS from other SPAX patients undergoing MRI. © 2024 International Parkinson and Movement Disorder Society.

Clinical and genetic variability among Bulgarian patients with autosomal recessive spastic ataxia of Charlevoix-Saguenay

BACKGROUND

Autosomal recessive spastic ataxia ofCharlevoix-Saguenay (ARSACS) is a rare neurodegenerative disorder characterizedby early-onset cerebellar ataxia, peripheral sensorimotor neuropathy, and lowerlimb spasticity. We present clinical andgenetic data of the first Bulgarian patients diagnosed with ARSACS by wholeexome sequencing (WES).

METHODS

Variant filtering was performed usinglocally established pipeline and the selected variants were analysed by Sangersequencing. All patients underwent clinical examination and testingincluding the standard rating scales for spastic paraplegia and ataxia.

RESULTS

Five different SACS gene variants, three of which novel, have been identified inpatients from three different ethnic groups. In addition to the classicalclinical triad, brain MRI revealed cerebellar atrophy, linear pontineT2-hypointensities, and hyperintense rim lateral tothalamus combined with retinal nerve fiber layer thickening on opticcoherence tomography (OCT).

CONCLUSION

We expand the mutation, geographic, and phenotypic spectrum of ARSACS, adding Bulgaria to the world map of the disease, and drawing attention to the fact that it is still misdiagnosed. We demonstrated that brain MRI and OCT are necessary clinical tests for ARSACS diagnosis, even if one of the cardinal clinical features is lacking.

Jiao's style scalp acupuncture combined with physiotherapy for autosomal recessive spastic ataxia of Charlevoix-Saguenay type: A case report

We present a case study of an 8-year-old girl with autosomal recessive spastic ataxia of Charlevoix-Saguenay, who experienced gait imbalance since the age of two. Magnetic resonance imaging of the brain and whole spine, as well as electroencephalography, revealed no abnormalities. However, genetic testing identified a likely pathogenic variant and an uncertain significance in the heterozygous state of the Sacsin Molecular Chaperone gene. Despite treatment with epileptic and antiparkinsonian medications, along with supplements, no significant improvements were observed. Subsequently, the patient underwent eight sessions of physiotherapy before starting with 14 sessions of combined Jiao's style scalp acupuncture targeting the motor and chorea-tremor areas with physiotherapy treatment. Positive changes were noted in the Trunk Control Measurement Scale (TCMS) and Pediatric Balance Scale (PBS) after three sessions of combined treatments from 25 to 36 and 21 to 43 respectively. Further combined treatments showed consistent improvements where the TCMS reached a peak of 57 out of 58 and PBS showed a peak of 54 out of 58 at the 6th month of combined treatment. This suggests that the combination of scalp acupuncture with physiotherapy treatment may provide improvement in the balance and gait of patients with ARSACS. More similar cases should be documented to better understand the potential benefits and synergies of both treatments of ARSACS.

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.

A genomic hotspot of diversifying selection and structural change in the hoary bat (Lasiurus cinereus)

Background

Previous work found that numerous genes positively selected within the hoary bat (Lasiurus cinereus) lineage are physically clustered in regions of conserved synteny. Here I further validate and expand on those finding utilizing an updated L. cinereus genome assembly and additional bat species as well as other tetrapod outgroups.

Methods

A chromosome-level assembly was generated by chromatin-contact mapping and made available by DNAZoo (www.dnazoo.org). The genomic organization of orthologous genes was extracted from annotation data for multiple additional bat species as well as other tetrapod clades for which chromosome-level assemblies were available from the National Center for Biotechnology Information (NCBI). Tests of branch-specific positive selection were performed for L. cinereus using PAML as well as with the HyPhy package for comparison.

Results

Twelve genes exhibiting significant diversifying selection in the L. cinereus lineage were clustered within a 12-Mb genomic window; one of these (Trpc4) also exhibited diversifying selection in bats generally. Ten of the 12 genes are landmarks of two distinct blocks of ancient synteny that are not linked in other tetrapod clades. Bats are further distinguished by frequent structural rearrangements within these synteny blocks, which are rarely observed in other Tetrapoda. Patterns of gene order and orientation among bat taxa are incompatible with phylogeny as presently understood, implying parallel evolution or subsequent reversals. Inferences of positive selection were found to be robust to alternative phylogenetic topologies as well as a strong shift in background nucleotide composition in some taxa.

Discussion

This study confirms and further localizes a genomic hotspot of protein-coding divergence in the hoary bat, one that also exhibits an increased tempo of structural change in bats compared with other mammals. Most genes in the two synteny blocks have elevated expression in brain tissue in humans and model organisms, and genetic studies implicate the selected genes in cranial and neurological development, among other functions.

Driving mitochondrial fission improves cognitive, but not motor deficits in a mouse model of Ataxia of Charlevoix-Saguenay

Autosomal-recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by loss-of-function mutation in the SACS gene, which encodes sacsin, a putative HSP70-HSP90 co-chaperone. Previous studies with Sacs knock-out (KO) mice and patient-derived fibroblasts suggested that SACSIN mutations inhibit the function of the mitochondrial fission enzyme dynamin-related protein 1 (Drp1). This in turn resulted in mitochondrial hyperfusion and dysfunction. We experimentally tested this hypothesis by genetically manipulating the mitochondrial fission/fusion equilibrium, creating double KO (DKO) mice that also lack positive (PP2A/Bβ2) and negative (PKA/AKAP1) regulators of Drp1. Neither promoting mitochondrial fusion (Bβ2 KO) nor fission (Akap1 KO) influenced progression of motor symptoms in Sacs KO mice. However, our studies identified profound learning and memory deficits in aged Sacs KO mice. Moreover, this cognitive impairment was rescued in a gene dose-dependent manner by deletion of the Drp1 inhibitor PKA/Akap1. Our results are inconsistent with mitochondrial dysfunction as a primary pathogenic mechanism in ARSACS. Instead, they imply that promoting mitochondrial fission may be beneficial at later stages of the disease when pathology extends to brain regions subserving learning and memory.

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.

Autophagy and neurodegeneration: Unraveling the role of C9ORF72 in the regulation of autophagy and its relationship to ALS-FTD pathology

The proper functioning of the cell clearance machinery is critical for neuronal health within the central nervous system (CNS). In normal physiological conditions, the cell clearance machinery is actively involved in the elimination of misfolded and toxic proteins throughout the lifetime of an organism. The highly conserved and regulated pathway of autophagy is one of the important processes involved in preventing and neutralizing pathogenic buildup of toxic proteins that could eventually lead to the development of neurodegenerative diseases (NDs) such as Alzheimer’s disease or Amyotrophic lateral sclerosis (ALS). The most common genetic cause of ALS and frontotemporal dementia (FTD) is a hexanucleotide expansion consisting of GGGGCC (G4C2) repeats in the chromosome 9 open reading frame 72 gene (C9ORF72). These abnormally expanded repeats have been implicated in leading to three main modes of disease pathology: loss of function of the C9ORF72 protein, the generation of RNA foci, and the production of dipeptide repeat proteins (DPRs). In this review, we discuss the normal physiological role of C9ORF72 in the autophagy-lysosome pathway (ALP), and present recent research deciphering how dysfunction of the ALP synergizes with C9ORF72 haploinsufficiency, which together with the gain of toxic mechanisms involving hexanucleotide repeat expansions and DPRs, drive the disease process. This review delves further into the interactions of C9ORF72 with RAB proteins involved in endosomal/lysosomal trafficking, and their role in regulating various steps in autophagy and lysosomal pathways. Lastly, the review aims to provide a framework for further investigations of neuronal autophagy in C9ORF72-linked ALS-FTD as well as other neurodegenerative diseases.

In Vitro Characterization of Motor Neurons and Purkinje Cells Differentiated from Induced Pluripotent Stem Cells Generated from Patients with Autosomal Recessive Spastic Ataxia of Charlevoix-Saguenay

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is an early-onset neurodegenerative disease mainly characterized by spasticity in the lower limbs and poor muscle control. The disease is caused by mutations in the SACS gene leading in most cases to a loss of function of the sacsin protein, which is highly expressed in motor neurons and Purkinje cells. To investigate the impact of the mutated sacsin protein in these cells in vitro, induced pluripotent stem cell- (iPSC-) derived motor neurons and iPSC-derived Purkinje cells were generated from three ARSACS patients. Both types of iPSC-derived neurons expressed the characteristic neuronal markers β3-tubulin, neurofilaments M and H, as well as specific markers like Islet-1 for motor neurons, and parvalbumin or calbindin for Purkinje cells. Compared to controls, iPSC-derived mutated SACS neurons expressed lower amounts of sacsin. In addition, characteristic neurofilament aggregates were detected along the neurites of both iPSC-derived neurons. These results indicate that it is possible to recapitulate in vitro, at least in part, the ARSACS pathological signature in vitro using patient-derived motor neurons and Purkinje cells differentiated from iPSCs. Such an in vitro personalized model of the disease could be useful for the screening of new drugs for the treatment of ARSACS.

Clinical and genetic characterization of a Taiwanese cohort with spastic paraparesis combined with cerebellar involvement

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurodegenerative disorders clinically characterized by progressive lower-limb spasticity. Cerebellar ataxia commonly co-occurs with complicated HSPs. HSP with concurrent cerebellar ataxia has significant clinical and genetic overlaps with hereditary cerebellar ataxia (HCA) and other inherited neurological diseases, adding to the challenge of planning genetic testing for the disease. In this study, we characterized clinical features of a cohort of 24 patients (male/female: 15/9) from 22 families who presented spastic paraparesis combined with cerebellar involvement, with a median disease onset age 20.5 (range 5–53) years. Aside from the core phenotype, 18 (75%) patients had additional neuropsychiatric and systemic manifestations. A stepwise genetic testing strategy stratified by mode of inheritance, distinct neuroimaging features (e.g., thin corpus callosum), population-specific prevalence and whole-exome sequencing was utilized to investigate the genetic etiology. Causative mutations in up to 10 genes traditionally related to HSP, HCA and other neurogenetic diseases (autosomal recessive spastic ataxia of Charlevoix-Saguenay, neurodegeneration with brain iron accumulation, and progressive encephalopathy with brain atrophy and thin corpus callosum) were detected in 16 (73%) of the 22 pedigrees. Our study revealed the genetic complexity of HSP combined with cerebellar involvement. In contrast to the marked genetic diversity, the functions of the causative genes are restricted to a limited number of physiological themes. The functional overlap might reflect common underlying pathogenic mechanisms, to which the corticospinal tract and cerebellar neuron circuits may be especially vulnerable.

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.