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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.

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.

Short tandem repeat expansions in the human genome are overrepresented in a variety of neurological disorders. It was recently shown that huntingtin (HTT) repeat expansions with full penetrance, i.e. 40 or more CAG repeats, which normally cause Huntington's disease (HD), are overrepresented in patients with amyotrophic lateral sclerosis (ALS). Whether patients carrying HTT repeat expansions with reduced penetrance, (36-39 CAG repeats), or alleles with intermediate penetrance, (27-35 CAG repeats), have an increased risk of ALS has not yet been investigated. Here, we examined the role of HTT repeat expansions in a motor neuron disease (MND) cohort, searched for expanded HTT alleles, and investigated correlations with phenotype and neuropathology. MND patients harboring C9ORF72 hexanucleotide repeat expansions (HREs) were included, to investigate whether HTT repeat expansions were more common in this group. We found a high prevalence of intermediate (range 5.63%-6.61%) and reduced penetrance (range 0.57%-0.66%) HTT gene expansions in this cohort compared to other populations of European ancestry, but no differences between the MND cohort and the control cohort were observed, regardless of C9ORF72HRE status. Upon autopsy of three patients with intermediate or reduced penetrance HTT alleles, huntingtin inclusions were observed in the caudate nucleus and frontal lobe, but no significant somatic mosaicism was detected in different parts of the nervous system. Thus, we demonstrate, for the first time, huntingtin inclusions in individuals with MND and intermediate and reduced penetrance HTT repeat expansions but more clinicopathological investigations are needed to further understand the impact of HTT gene expansion-related pleiotropy.

Mutations in the gene encoding the ubiquitously expressed free radical scavenging enzyme superoxide dismutase-1 (SOD1) are found in 2–6% of amyotrophic lateral sclerosis patients. The most frequent SOD1 mutation worldwide is D90A. Amyotrophic lateral sclerosis caused by this mutation has some unusual features: the heredity is usually recessive, the phenotype is stereotypic with slowly evolving motor symptoms beginning in the legs and may also include sensory, autonomic, and urinary bladder involvement. Furthermore, the mutant protein resembles the wild type, with normal content and enzymatic activity in the central nervous system. Here, we report neuropathological findings in nine patients homozygous for the D90A mutation. All nine had numerous small granular inclusions immunoreactive for misfolded SOD1 in motor neurons and glial nuclei in the spinal cord and brainstem. In addition to degeneration of the corticospinal tracts, all patients had degeneration of the dorsal columns. We also found intense gliosis in circumscribed cortical areas of the frontal and temporal lobes and in the insula. In these areas and in adjacent white matter, there were SOD1 staining neuropil threads. A few SOD1-immunopositive cytoplasmic neuronal inclusions were observed in cortical areas, as were glial nuclear inclusions. As suggested by the symptoms and signs and earlier neurophysiological and imaging investigations, the histopathology in patients homozygous for the D90A SOD1 extends beyond the motor system to include cognitive and sensory cortical areas. However, even in the patients that had a symptomatic disease duration of more than 2 or 3 decades and lived into their 70s or 80s, there were no SOD1-inclusion pathology and no typical dysfunction (apart from the musculature) in non-nervous organs. Thus, only specific parts of the CNS seem to be vulnerable to toxicity provoked by homozygously expressed mutant SOD1.