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Background: Spinobulbar muscular atrophy (SBMA) is an X-linked neuromuscular disorder characterized by adult-onset progressive muscle atrophy, flaccid paresis, and bulbar palsy. In addition, increasing evidence indicates that SBMA is a multisystem disorder with prominent non-motor symptoms, such as sensory neuropathy, androgen insensitivity, and glucose intolerance. This study aimed to further characterize the clinical manifestations and biomarker profile in a large Swedish SBMA cohort.

Methods: 49 genetically confirmed SBMA patients were identified from a motor neuron disease database at Umeå University Hospital, Sweden. CAG repeat length in the androgen receptor (AR) gene was assessed by RP-PCR. Blood samples were analyzed for cardiovascular and muscle biomarkers. Clinical data were collected from medical records and interviews, with autopsy findings reviewed in two cases.

Results: The mean CAG repeat length was 43.1, with a mean age at motor symptom onset of 58.6 years. Notably, 19% of patients initially presented with sensory symptoms. High prevalence of hypertonia (70%), diabetes mellitus (39%), and cardiac disease (38%) was observed. Elevated troponin levels were common, and pNfL (neurofilament light chain in plasma) was elevated in seven patients, likely reflecting combined cerebrovascular and cardiovascular comorbidity. Importantly, two of these seven patients exhibited rapid disease progression, and a concomitant diagnosis of ALS was confirmed histopathologically.

Discussion: This cohort was characterized by a relatively low number of AR gene CAG repeats and a late onset of motor symptoms. Sensory symptoms frequently occurred before motor decline. Cardiovascular disease and diabetes were common comorbidities and, in some cases, preceded neurological symptoms. These findings underscore the need for improved clinical awareness of the heterogeneous presentation of SBMA and support routine cardiovascular monitoring to reduce diagnostic delays and prevent early mortality.

Mutations in the antioxidant enzyme superoxide dismutase-1 (SOD1) are a well-established cause of amyotrophic lateral sclerosis (ALS). The mutations promote SOD1 misfolding, resulting in protein aggregation and motor neuron degeneration. SOD1 is normally a structurally stable enzyme, and the mechanisms underlying SOD1 misfolding remain poorly understood. Approximately one third of SOD1 in cerebrospinal fluid (CSF) exhibits an N-terminal truncation, the biological significance of which remains unclear. This is remarkable given the dramatic effects ALS-linked C-terminal truncations have on the enzyme. In this study, we identified the truncation site and investigated its impact on SOD1 stability and enzymatic activity. Edman degradation revealed the cleavage site between Asn-26 and Gly-27, generating a 26-residue peptide that was confirmed by mass spectrometry. We analyzed postmortem tissues from different parts of the central nervous system (CNS), including the choroid plexus, and found only trace amounts of N-terminally truncated SOD1. Biochemical characterization of the SOD1 in CSF was done by size exclusion chromatography, ion exchange chromatography, and mass spectrometry. Our findings demonstrate that SOD1 in CSF retains full enzymatic activity, that the N-terminally truncated variant is mainly present in heterodimers with native SOD1 subunits, and that the dimer remains folded and active, with both fragments of the truncated SOD1 fixed after proteolysis. Truncated SOD1 was absent in human plasma. In mice, only transgenically expressed human SOD1 underwent truncation in CSF, whereas endogenous murine SOD1 remained intact. Lastly, the N-terminal truncation does not induce misfolding, unlike the destabilizing effects observed with C-terminal truncations. The location where the truncation takes place and the underlying mechanism could not be identified. Whether the N-truncated SOD1 variant contributes to ALS pathogenesis remains to be determined. (Figure presented.).

Mutations in the antioxidant enzyme superoxide dismutase-1 (SOD1) are a well-established cause of amyotrophic lateral sclerosis (ALS). The mutations promote SOD1 misfolding, resulting in protein aggregation and motor neuron degeneration. SOD1 is normally a structurally stable enzyme, and the mechanisms underlying SOD1 misfolding remain poorly understood. Approximately one third of SOD1 in cerebrospinal fluid (CSF) exhibits an N-terminal truncation, the biological significance of which remains unclear. This is remarkable given the dramatic effects ALS-linked C-terminal truncations have on the enzyme. In this study, we identified the truncation site and investigated its impact on SOD1 stability and enzymatic activity. Edman degradation revealed the cleavage site between Asn-26 and Gly-27, generating a 26-residue peptide that was confirmed by mass spectrometry. We analyzed postmortem tissues from different parts of the central nervous system (CNS), including the choroid plexus, and found only trace amounts of N-terminally truncated SOD1. Biochemical characterization of the SOD1 in CSF was done by size exclusion chromatography, ion exchange chromatography, and mass spectrometry. Our findings demonstrate that SOD1 in CSF retains full enzymatic activity, that the N-terminally truncated variant is mainly present in heterodimers with native SOD1 subunits, and that the dimer remains folded and active, with both fragments of the truncated SOD1 fixed after proteolysis. Truncated SOD1 was absent in human plasma. In mice, only transgenically expressed human SOD1 underwent truncation in CSF, whereas endogenous murine SOD1 remained intact. Lastly, the N-terminal truncation does not induce misfolding, unlike the destabilizing effects observed with C-terminal truncations. The location where the truncation takes place and the underlying mechanism could not be identified. Whether the N-truncated SOD1 variant contributes to ALS pathogenesis remains to be determined. 

Mutations in superoxide dismutase-1 (SOD1) are a common cause of amyotrophic lateral sclerosis (ALS). Inheritance is as a rule dominant, but in carriers of the most common mutation, D90A, disease can develop in both homozygous and, more rarely, in heterozygous individuals with unexplained differences in clinical presentation. There is mounting evidence that prion-like spread of SOD1 aggregation is the primary cause of the disease. Two different strains of aggregates have been found to arise in human SOD1 (hSOD1) transgenic mouse models of ALS. Strain A is formed by most mutants including hSOD1G85R and hSOD1WT, whereas hSOD1D90A transgenic mice form a distinct strain B in addition to A. To explore the effects of aggregate strain propensities when hSOD1 variants are coexpressed, we generated digenic hSOD1G85R/WT and hSOD1G85R/D90A mice. Coexpression of hSOD1WT considerably shortened the lifespan of hSOD1G85R mice to the extent expected from the neurotoxicities of the variants alone. In contrast, coexpression of hSOD1D90A had a minimal effect on survival, far smaller than expected. Moreover, time from onset to the end stage was markedly prolonged in the hSOD1G85R/D90A mice. Aggregation of hSOD1 developed concomitantly with motor neuron disease, and the aggregates contained large amounts of both coexpressed variants in both digenic models. Our findings suggest that hSOD1WT has high a capacity to coaggregate with mutants and enhance neurotoxicity. Such interactions may be restricted by differences in strain propensities, which may contribute to the primarily recessive inheritance associated with the hSOD1D90A mutation.

Fused in sarcoma (FUS) is an RNA-binding protein implicated in juvenile amyotrophic lateral sclerosis (ALS). Mutations in the FUS gene, particularly those affecting the nuclear localization signal (NLS), impair nuclear import and lead to cytoplasmic accumulation of FUS inclusions in motor neurons. However, the pathological and clinical significance of FUS variants outside the NLS remains less understood. Here, we describe clinical and histopathological findings from three ALS patients carrying FUS variants: two with NLS-region variants (R495X and P525L), and one with a variant in the N-terminal region outside the NLS (Q23L). The patients carrying NLS variants presented with aggressive, juvenile-onset spinal and bulbar ALS, characterized primarily by lower motor neuron involvement and rapid disease progression. In contrast, the Q23L patient exhibited a slowly progressive disease course, with predominantly upper motor neuron signs. Neuropathological analysis revealed cytoplasmic FUS inclusions in motor neurons of patients with NLS variants, consistent with typical FUS pathology. In contrast, the Q23L patient lacked FUS inclusions and instead displayed pTDP-43 pathology in the hippocampus, neocortex (including the motor cortex), nucleus olivaris, lentiform nucleus, striatum, and some lower motor neurons. Taken together, these results suggest that Q23L is most likely a benign variant. As antisense oligonucleotides (ASOs) targeting FUS are currently being explored in clinical trials, further neuropathological investigations are needed to determine whether ASO-mediated FUS silencing would be effective for patients carrying FUS variants outside the NLS region.

Gene silencing therapy is an effective treatment for amyotrophic lateral sclerosis (ALS) patients carrying mutations in the superoxide dismutase-1 (SOD1) gene aiming to reduce noxious forms of SOD1 in the central nervous system (CNS). The normal steady-state level of SOD1 protein in human CNS is therefore of interest but is contested. In this work we have analyzed SOD1 protein content, total protein content, and SOD1 enzymatic activity in six areas of the CNS as well as in four peripheral tissues from sporadic and familial ALS patients and non-ALS controls. Our results show that SOD1 in the human CNS constitutes around 100 μg/g wet weight corresponding to about 0.16% of the total protein in the studied areas. Of the peripheral tissues analyzed, kidney and erythrocytes contain roughly equal amounts, liver higher, and skeletal muscle lower levels of SOD1 compared to the CNS. This data shows SOD1 protein levels around 10 times lower compared to previously published figures. However, SOD1 can still be considered an abundant protein considering that > 12 000 proteins are expressed in human cells. There was no difference in SOD1 protein content between sporadic or familial ALS patients and control individuals. The level and activity of SOD1 are not deviating in the areas of the CNS that are most vulnerable to ALS. Instead, insufficient control of SOD1 structure and aggregation could be important factors behind the vulnerability of motor areas to SOD1 proteotoxicity. (Figure presented.).

Mutations in superoxide dismutase-1 (SOD1) are a cause of hereditary amyotrophic lateral sclerosis (ALS) through a gain-of-function mechanism involving unfolded mutant SOD1. Intrathecal gene therapy using the antisense-oligo-nucleotide drug tofersen to reduce SOD1 expression delays disease progression and has recently been approved in the United States and the European Union. However, the discovery of children homozygous for inactivating SOD1 mutations developing the SOD1 Deficiency Syndrome (ISODDES) with injury to the motor system suggests that a too low SOD1 antioxidant activity may be deleterious in humans. Measuring SOD1 activity in cerebrospinal fluid (CSF) in tofersen-treated patients is recommended but difficult due to low concentration and the presence of the isoenzyme SOD3. We here present a sensitive method to assess SOD1 activity by removing SOD3 from CSF samples using highly specific immobilized antibodies and subsequent measurement of the SOD activity. We validated the method on 171 CSF samples from ALS patients with and without mutations and controls and used paired erythrocyte samples for comparison. We found that in ALS patients with wildtype SOD1, the SOD1 activity in CSF was equal to controls, but patients with mutant SOD1 show lower activity in CSF, even for patients with mutants previously reported to have full activity in erythrocytes. Activity variation in CSF was large among patients carrying the same SOD1 mutation and larger than in erythrocytes and in post-mortem nervous tissue. Additionally, we identified a discrepancy between the SOD1 activity and protein level measured with ELISA in both CSF and erythrocytes. Since antibodies used for SOD1 ELISA-quantification are raised against the natively folded wildtype SOD1, the concentration of mutant SOD1s may be underestimated. Analysis of SOD1 enzymatic activity in CSF is therefore a more reliable way to monitor the effect of SOD1-lowering drugs.

Mutations in the human Superoxide dismutase 1 (hSOD1) gene are well-established cause of the motor neuron disease ALS. Patients and transgenic (Tg) ALS model mice carrying mutant variants develop hSOD1 aggregates in the CNS. We have identified two hSOD1 aggregate strains, which both transmit spreading template-directed aggregation and premature fatal paralysis when inoculated into adult transgenic mice. This prion-like spread of aggregation could be a primary disease mechanism in SOD1-induced ALS. Human SOD1 aggregation has been studied extensively both in cultured cells and under various conditions in vitro. To determine how the structure of aggregates formed in these model systems related to disease-associated aggregates in the CNS, we used a binary epitope-mapping assay to examine aggregates of hSOD1 variants G93A, G85R, A4V, D90A, and G127X formed in vitro, in four different cell lines and in the CNS of Tg mice. We found considerable variability between replicate sets of in vitro-generated aggregates. In contrast, there was a high similarity between replicates of a given hSOD1 mutant in a given cell line, but pronounced variations between different hSOD1 mutants and different cell lines in both structures and amounts of aggregates formed. The aggregates formed in vitro or in cultured cells did not replicate the aggregate strains that arise in the CNS. Our findings suggest that the distinct aggregate morphologies in the CNS could result from a micro-environment with stringent quality control combined with second-order selection by spreading ability. Explorations of pathogenesis and development of therapeutics should be conducted in models that replicate aggregate structures forming in the CNS. (Figure presented.)

Superoxide dismutase-1 is a ubiquitously expressed antioxidant enzyme. Mutations in SOD1 can cause amyotrophic lateral sclerosis, probably via a toxic gain-of-function involving protein aggregation and prion-like mechanisms. Recently, homozygosity for loss-of-function mutations in SOD1 has been reported in patients presenting with infantile-onset motor neuron disease. We explored the bodily effects of superoxide dismutase-1 enzymatic deficiency in eight children homozygous for the p.C112Wfs∗11 truncating mutation. In addition to physical and imaging examinations, we collected blood, urine and skin fibroblast samples. We used a comprehensive panel of clinically established analyses to assess organ function and analysed oxidative stress markers, antioxidant compounds, and the characteristics of the mutant Superoxide dismutase-1. From around 8 months of age, all patients exhibited progressive signs of both upper and lower motor neuron dysfunction, cerebellar, brain stem, and frontal lobe atrophy and elevated plasma neurofilament concentration indicating ongoing axonal damage. The disease progression seemed to slow down over the following years. The p.C112Wfs∗11 gene product is unstable, rapidly degraded and no aggregates were found in fibroblast. Most laboratory tests indicated normal organ integrity and only a few modest deviations were found. The patients displayed anaemia with shortened survival of erythrocytes containing decreased levels of reduced glutathione. A variety of other antioxidants and oxidant damage markers were within normal range. In conclusion, non-neuronal organs in humans show a remarkable tolerance to absence of Superoxide dismutase-1 enzymatic activity. The study highlights the enigmatic specific vulnerability of the motor system to both gain-of-function mutations in SOD1 and loss of the enzyme as in the here depicted infantile superoxide dismutase-1 deficiency syndrome.

Increasing evidence suggests that propagation of the motor neuron disease amyotrophic lateral sclerosis (ALS) involves the pathogenic aggregation of disease-associated proteins that spread in a prion-like manner. We have identified two aggregate strains of human superoxide dismutase 1 (hSOD1) that arise in the CNS of transgenic mouse models of SOD1-mediated ALS. Both strains transmit template-directed aggregation and premature fatal paralysis when inoculated into the spinal cord of adult hSOD1 transgenic mice. This spread of pathogenic aggregation could be a potential target for immunotherapeutic intervention. Here we generated mouse monoclonal antibodies (mAbs) directed to exposed epitopes in hSOD1 aggregate strains and identified an aggregate selective mAb that targets the aa 143–153 C-terminal extremity of hSOD1 (αSOD1^143–153). Both pre-incubation of seeds with αSOD1^143–153 prior to inoculation, and weekly intraperitoneal (i.p.) administration attenuated transmission of pathogenic aggregation and prolonged the survival of seed-inoculated hSOD1^G85R Tg mice. In contrast, administration of a mAb targeting aa 65–72 (αSOD1^65–72), which exhibits high affinity towards monomeric disordered hSOD1, had an adverse effect and aggravated seed induced premature ALS-like disease. Although the mAbs reached similar concentrations in CSF, only αSOD1^143–153 was found in association with aggregated hSOD1 in spinal cord homogenates. Our results suggest that an aggregate-selective immunotherapeutic approach may suppress seeded transmission of pathogenic aggregation in ALS. However, long-term administration of αSOD1^143–153 was unable to prolong the lifespan of non-inoculated hSOD1^G85R Tg mice. Thus, spontaneously initiated hSOD1 aggregation in spinal motor neurons may be poorly accessible to therapeutic antibodies.