A novel ALS SOD1 C6S mutation with implications for aggregation related toxicity and genetic counseling

Pathogenicity classification of SOD1 variants of uncertain significance by in vitro aggregation propensity

Deposition of insoluble SOD1 aggregates in motor neurons is the hallmark of SOD1-associated ALS. Mutant SOD1 protein promotes structural instability that leads to misfolded SOD1 protein aggregates, which can be recapitulated in vitro. Therefore, aggregation propensity in cell lines can be a reliable indicator for the pathogenicity classification of SOD1 variants. Herein, we performed in vitro experiment to classify the pathogenicity of 34 SOD1 variants of uncertain significance (VUS) from 215 variants reported previously. The clinical features of 234 ALS patients with 31 SOD1 likely pathogenic (LP) variants were summarized. 31 VUS variants formed aggregates spontaneously, indicating LP variants. Missense variants were mainly located in the C-terminal of SOD1. Among patients with 31 SOD1 LP variants, 75% of patients had lower limb onset. The onset of familial ALS patients (45.7±14.0 years) is earlier than sporadic ALS patients (50.6±13.1 years). Our results expand the spectrum of SOD1 mutations and highlight the natural history of SOD1-positive ALS patients for further clinical trials in SOD1-related ALS.

A Novel Variant in Superoxide Dismutase 1 Gene (p.V119M) in Als Patients with Pure Lower Motor Neuron Presentation

Amyotrophic lateral sclerosis (ALS) is a progressive and fatal disorder characterized by degeneration of motor neurons in the cerebral cortex, brain stem, and spinal cord. Most cases of ALS appear sporadically, but 5–10% of patients have a family history of disease. Mutations in the superoxide dismutase 1 gene (SOD1) have been found in 12–23% of familial cases and in 1–2% of sporadic cases. Currently, more than 180 different SOD1 gene variants have been identified in ALS patients. Here, we describe two apparently sporadic ALS patients carrying the same SOD1 c.355G>A variant, leading to the p.V119M substitution, not previously described. Both the patients showed pure lower motor neuron phenotype. The former presented with the flail leg syndrome, a rare ALS variant, characterized by progressive distal onset weakness and atrophy of lower limbs, slow progression and better survival than typical ALS. The latter exhibited rapidly progressive weakness of upper and lower limbs, neither upper motor neuron nor bulbar involvement, and shorter survival than typical ALS. We provide an accurate description of the phenotype, and a bioinformatics analysis of the p.V119M variant on protein structure. This study may increase the knowledge about genotype-phenotype correlations in ALS and improve the approach to ALS patients.

Genotype-phenotype correlations in a chinese population with familial amyotrophic lateral sclerosis

Objective: This study aimed to determine the distribution of the most commonly mutated genes (SOD1, TARDBP, FUS/TLS, and C9ORF72) associated with familial amyotrophic lateral sclerosis (FALS) and the association between genotype and phenotype in 242 Chinese patients.Methods: A total of 58 families were screened for ALS-associated mutations in SOD1，TARDBP, FUS, and C9ORF72 hexanucleotide repeat expansion. These mutations were analyzed to evaluate the relationship between genotype and phenotype in Chinese FALS patients.Results: Partial clinical data were obtained for 242 relatives of the 58 analyzed families, with a male-to-female ratio of 1.2:1 and a mean age of disease onset of 45.9±12.0 (13-80) years. 26 mutations associated with pathogenesis were identified in 32 probands from 58 different families. Mutations in SOD1, FUS, TARDBP, and C9ORF72 accounted for 32.8%, 12.1%, 8.6%, and 1.7% of FALS, respectively. FALS patients showed longer survival times; however, bulbar-onset ALS and the male-to-female ratio for them were lower than those reported previously. The site of onset, age of onset, and lifespan differed in FALS patients with SOD1, TARDBP, and FUS mutations.Discussion: In this study, patients with SOD1 mutations exhibited heterogeneous survival times that showed a bimodal distribution, while patients with FUS mutations showed rapid disease progression. Our results showed the relative contributions of the different types of mutations associated with ALS and provided phenotype-genotype correlations with clinical features in Chinese patients.

Clinical and Molecular Landscape of ALS Patients with SOD1 Mutations: Novel Pathogenic Variants and Novel Phenotypes. A Single ALS Center Study

Mutations in the copper zinc superoxide dismutase 1 (SOD1) gene are the second most frequent cause of familial amyotrophic lateral sclerosis (ALS). Nearly 200 mutations of this gene have been described so far. We report all SOD1 pathogenic variants identified in patients followed in the single ALS center of Lyon, France, between 2010 and 2020. Twelve patients from 11 unrelated families are described, including two families with the not yet described H81Y and D126N mutations. Splice site mutations were detected in two families. We discuss implications concerning genetic screening of SOD1 gene in familial and sporadic ALS.

Oxidative misfolding of Cu/Zn-superoxide dismutase triggered by non-canonical intramolecular disulfide formation

Misfolded Cu/Zn-superoxide dismutase (SOD1) is a pathological species in a subset of amyotrophic lateral sclerosis (ALS). Oxidative stress is known to increase in affected spinal cords of ALS and is thus considered to cause damages on SOD1 leading to the misfolding and aggregation. Despite this, it still remains elusive what triggers misfolding of SOD1 under oxidizing environment. Here, we show that a thiol group of Cys111 in SOD1 is oxidized to a sulfenic acid with hydrogen peroxide and reveal that further dissociation of the bound metal ions from the oxidized SOD1 allows another free Cys residue (Cys6) to nucleophilically attack the sulfenylated Cys111. As a result, an intra-molecular disulfide bond forms between Cys6 and Cys111. Such an abnormal SOD1 with the non-canonical disulfide bond was conformationally extended with significant cytotoxicity as well as high propensity to aggregate. Taken together, we propose a new model of SOD1 misfolding under oxidizing environment, in which formation of the non-canonical intramolecular disulfide bond plays a pivotal role.

From animal models to human disease: a genetic approach for personalized medicine in ALS

Amyotrophic Lateral Sclerosis (ALS) is the most frequent motor neuron disease in adults. Classical ALS is characterized by the death of upper and lower motor neurons leading to progressive paralysis. Approximately 10 % of ALS patients have familial form of the disease. Numerous different gene mutations have been found in familial cases of ALS, such as mutations in superoxide dismutase 1 (SOD1), TAR DNA-binding protein 43 (TDP-43), fused in sarcoma (FUS), C9ORF72, ubiquilin-2 (UBQLN2), optineurin (OPTN) and others. Multiple animal models were generated to mimic the disease and to test future treatments. However, no animal model fully replicates the spectrum of phenotypes in the human disease and it is difficult to assess how a therapeutic effect in disease models can predict efficacy in humans. Importantly, the genetic and phenotypic heterogeneity of ALS leads to a variety of responses to similar treatment regimens. From this has emerged the concept of personalized medicine (PM), which is a medical scheme that combines study of genetic, environmental and clinical diagnostic testing, including biomarkers, to individualized patient care. In this perspective, we used subgroups of specific ALS-linked gene mutations to go through existing animal models and to provide a comprehensive profile of the differences and similarities between animal models of disease and human disease. Finally, we reviewed application of biomarkers and gene therapies relevant in personalized medicine approach. For instance, this includes viral delivering of antisense oligonucleotide and small interfering RNA in SOD1, TDP-43 and C9orf72 mice models. Promising gene therapies raised possibilities for treating differently the major mutations in familial ALS cases.

A 50 bp deletion in the SOD1 promoter lowers enzyme expression but is not associated with ALS in Sweden

Mutations in the superoxide dismutase (SOD1) gene have been linked to amyotrophic lateral sclerosis (ALS). A 50 base pair (bp) deletion of SOD1 has been suggested to reduce transcription and to be associated with later disease onset in ALS. This study was aimed to reveal if the 50 bp deletion influenced SOD1 enzymatic activity, occurrence and phenotype of the disease in a Swedish ALS/control cohort. Blood samples from 512 Swedish ALS patients and 354 Swedish controls without coding SOD1 mutations were analysed for the 50 bp deletion allele. The enzymatic activity of SOD1 in erythrocytes was analysed and genotype-phenotype correlations were assessed. Results demonstrated that the genotype frequencies of the 50 bp deletion were all found to be in Hardy-Weinberg equilibrium. No significant differences were found for age of onset, disease duration or site of onset. SOD1 enzymatic activity showed a statistically significant decreasing trend in the control group, in which the allele was associated with a 5% reduction in SOD1 activity. The results suggest that the 50 bp deletion has a moderate reducing effect on SOD1 synthesis. No modulating effects, however, were found on ALS onset, phenotype and survival in the Swedish population.

Disulfide scrambling in superoxide dismutase 1 reduces its cytotoxic effect in cultured cells and promotes protein aggregation

Mutations in the gene coding for superoxide dismutase 1 (SOD1) are associated with familiar forms of the neurodegenerative disease amyotrophic lateral sclerosis (ALS). These mutations are believed to result in a “gain of toxic function”, leading to neuronal degeneration. The exact mechanism is still unknown, but misfolding/aggregation events are generally acknowledged as important pathological events in this process. Recently, we observed that demetallated apoSOD1, with cysteine 6 and 111 substituted for alanine, is toxic to cultured neuroblastoma cells. This toxicity depended on an intact, high affinity Zn²⁺ site. It was therefor contradictory to discover that wild-type apoSOD1 was not toxic, despite of its high affinity for Zn²⁺. This inconsistency was hypothesized to originate from erroneous disulfide formation involving C6 and C111. Using high resolution non-reducing SDS-PAGE, we have in this study demonstrated that the inability of wild-type apoSOD1 to cause cell death stems from formation of non-native intra-molecular disulfides. Moreover, monomeric apoSOD1 variants capable of such disulfide scrambling aggregated into ThT positive oligomers under physiological conditions without agitation. The oligomers were stabilized by inter-molecular disulfides and morphologically resembled what has in other neurodegenerative diseases been termed protofibrils. Disulfide scrambling thus appears to be an important event for misfolding and aggregation of SOD1, but may also be significant for protein function involving cysteines, e.g. mitochondrial import and copper loading.

Identification of a novel missense (C7W) mutation of SOD1 in a large familial amyotrophic lateral sclerosis pedigree

Mutations of Cu-Zn superoxide dismutase (SOD1) have rarely been identified in Chinese patients with amyotrophic lateral sclerosis (ALS). We recently initiated a program to screen mutations of SOD1, TARDBP, and C9orf72 genes, the most commonly mutated genes in ALS patients in Western countries, in Chinese ALS patients. In this study, we report a novel missense SOD1 mutation with a substitution of tryptophan for cysteine at the seventh amino acid (p.C7W, traditionally named p.C6W) based on HUGO Gene Nomenclature in a familial ALS pedigree. We also found that the activities of SOD1 were significantly decreased in the C7W patient and the carriers of the family, compared with the SOD1 activities of normal family members. Compared with reported C7G and C7S patients, analysis of phenotype revealed relatively mild disease phenotypes in C7W patients, which is correlated with less deteriorated alteration in protein structure. Like those of many other familial ALS families, variable clinical phenotypes in the C7W intrafamily suggest that potential genetic modifiers may contribute to this disease.

Palmitoylation of superoxide dismutase 1 (SOD1) is increased for familial amyotrophic lateral sclerosis-linked SOD1 mutants

Mutations in Cu,Zn-superoxide dismutase (mtSOD1) cause familial amyotrophic lateral sclerosis (FALS), a neurodegenerative disease resulting from motor neuron degeneration. Here, we demonstrate that wild type SOD1 (wtSOD1) undergoes palmitoylation, a reversible post-translational modification that can regulate protein structure, function, and localization. SOD1 palmitoylation was confirmed by multiple techniques, including acyl-biotin exchange, click chemistry, cysteine mutagenesis, and mass spectrometry. Mass spectrometry and cysteine mutagenesis demonstrated that cysteine residue 6 was the primary site of palmitoylation. The palmitoylation of FALS-linked mtSOD1s (A4V and G93A) was significantly increased relative to that of wtSOD1 expressed in HEK cells and a motor neuron cell line. The palmitoylation of FALS-linked mtSOD1s (G93A and G85R) was also increased relative to that of wtSOD1 when assayed from transgenic mouse spinal cords. We found that the level of SOD1 palmitoylation correlated with the level of membrane-associated SOD1, suggesting a role for palmitoylation in targeting SOD1 to membranes. We further observed that palmitoylation occurred predominantly on disulfide-reduced as opposed to disulfide-bonded SOD1, suggesting that immature SOD1 is the primarily palmitoylated species. Increases in SOD1 disulfide bonding and maturation with increased copper chaperone for SOD1 expression caused a decrease in wtSOD1 palmitoylation. Copper chaperone for SOD1 overexpression decreased A4V palmitoylation less than wtSOD1 and had little effect on G93A mtSOD1 palmitoylation. These findings suggest that SOD1 palmitoylation occurs prior to disulfide bonding during SOD1 maturation and that palmitoylation is increased when disulfide bonding is delayed or decreased as observed for several mtSOD1s.

Clinical genetics of amyotrophic lateral sclerosis: what do we really know?

Hereditary amyotrophic lateral sclerosis (ALS) encompasses a group of genetic disorders characterized by adult-onset loss of the lower and upper motor neuron systems, often with involvement of other parts of the nervous system. Cases of hereditary ALS have been attributed to mutations in 12 different genes, the most common being SOD1, FUS and TARDBP-mutations in the other genes are rare. The identified genes explain 25-35% of cases of familial ALS, but identifying the remaining genes has proved difficult. Only a few genes seem to account for significant numbers of ALS cases, with many others causing a few cases each. Hereditary ALS can be inherited in an autosomal dominant, autosomal recessive or X-linked manner, and families with low disease penetrance are frequently observed. In such families, the genetic predisposition may remain unnoticed, so many patients carry a diagnosis of isolated or sporadic ALS. The only clinical feature that distinguishes recognized hereditary from apparently sporadic ALS is a lower mean age of onset in the former. All the clinical features reported in hereditary cases (including signs of extrapyramidal, cerebellar or cognitive involvement) have also been observed in sporadic cases. Genetic counseling and risk assessment in relatives depend on establishing the specific gene defect and the disease penetrance in the particular family.