Many neurodegenerative disorders, including Parkinson’s, Alzheimer’s, and amyotrophic lateral sclerosis, are well known to involve the accumulation of disease-specific proteins. Less well known are the accumulations of another set of proteins, neuronal intermediate filaments (NFs), which have been observed in these diseases for decades. NFs belong to the family of cytoskeletal intermediate filament proteins (IFs) that give cells their shape; they determine axonal caliber, which controls signal conduction; and they regulate the transport of synaptic vesicles and modulate synaptic plasticity by binding to neurotransmitter receptors. In the last two decades, a number of rare disorders caused by mutations in genes that encode NFs or regulate their metabolism have been discovered. These less prevalent disorders are providing novel insights into the role of NF aggregation in the more common neurological disorders.

Spinocerebellar ataxia type 1 (SCA1) is an adult-onset neurodegenerative disease caused by a polyglutamine expansion in the protein ATXN1, which is involved in transcriptional regulation. Although symptoms appear relatively late in life, primarily from cerebellar dysfunction, pathogenesis begins early, with transcriptional changes detectable as early as a week after birth in SCA1-knockin mice. Given the importance of this postnatal period for cerebellar development, we asked whether this region might be developmentally altered by mutant ATXN1. We found that expanded ATXN1 stimulates the proliferation of postnatal cerebellar stem cells in SCA1 mice. These hyperproliferating stem cells tended to differentiate into GABAergic inhibitory interneurons rather than astrocytes; this significantly increased the GABAergic inhibitory interneuron synaptic connections, disrupting cerebellar Purkinje cell function in a non–cell autonomous manner. We confirmed the increased basket cell–Purkinje cell connectivity in human SCA1 patients. Mutant ATXN1 thus alters the neural circuitry of the developing cerebellum, setting the stage for the later vulnerability of Purkinje cells to SCA1. We propose that other late-onset degenerative diseases may also be rooted in subtle developmental derailments.

Polygenic inheritance plays a pivotal role in driving multiple sclerosis susceptibility, an inflammatory demyelinating disease of the CNS. We developed polygenic risk scores (PRS) of multiple sclerosis and assessed associations with both disease status and severity in cohorts of European descent. The largest genome-wide association dataset for multiple sclerosis to date (n = 41 505) was leveraged to generate PRS scores, serving as an informative susceptibility marker, tested in two independent datasets, UK Biobank [area under the curve (AUC) = 0.73, 95% confidence interval (CI): 0.72–0.74, P = 6.41 × 10⁻¹⁴⁶] and Kaiser Permanente in Northern California (KPNC, AUC = 0.8, 95% CI: 0.76–0.82, P = 1.5 × 10⁻⁵³). Individuals within the top 10% of PRS were at higher than 5-fold increased risk in UK Biobank (95% CI: 4.7–6, P = 2.8 × 10⁻⁴⁵) and 15-fold higher risk in KPNC (95% CI: 10.4–24, P = 3.7 × 10⁻¹¹), relative to the median decile. The cumulative absolute risk of developing multiple sclerosis from age 20 onwards was significantly higher in genetically predisposed individuals according to PRS. Furthermore, inclusion of PRS in clinical risk models increased the risk discrimination by 13% to 26% over models based only on conventional risk factors in UK Biobank and KPNC, respectively. Stratifying disease risk by gene sets representative of curated cellular signalling cascades, nominated promising genetic candidate programmes for functional characterization. These pathways include inflammatory signalling mediation, response to viral infection, oxidative damage, RNA polymerase transcription, and epigenetic regulation of gene expression to be among significant contributors to multiple sclerosis susceptibility. This study also indicates that PRS is a useful measure for estimating susceptibility within related individuals in multicase families. We show a significant association of genetic predisposition with thalamic atrophy within 10 years of disease progression in the UCSF-EPIC cohort (P < 0.001), consistent with a partial overlap between the genetics of susceptibility and end-organ tissue injury. Mendelian randomization analysis suggested an effect of multiple sclerosis susceptibility on thalamic volume, which was further indicated to be through horizontal pleiotropy rather than a causal effect. In summary, this study indicates important, replicable associations of PRS with enhanced risk assessment and radiographic outcomes of tissue injury, potentially informing targeted screening and prevention strategies.

Significance: Cellular and molecular mediators driving multiple sclerosis (MS) pathology have been discovered to a great extent. However, the early molecular events leading to aberrant immune responses remain largely unknown. In this study, we combined bisulfite sequencing with transcriptome profiling to characterize the epigenetic landscape of four peripheral immune cell populations isolated from newly diagnosed, untreated MS patients and healthy individuals. We demonstrate widespread hypomethylation in CD19 ⁺ B cells at clinical disease onset. Notably, this epigenetic signature is functionally linked with the overactivation of B cells. Altogether, our results pinpoint the role of aberrant DNA methylation in connecting defects in the periphery with central nervous system autoimmunity and corroborate the key role of B cells in the initial stages of MS.

Significance: Over 200 genomic loci have been found associated with the risk of developing multiple sclerosis (MS). Despite this important body of data, limited information exists on the cellular pathways and molecular mechanisms underlying MS genetic complexity. In this study, we report the functional characterization of the ataxin-1 encoding ATXN1 susceptibility locus. Ataxin-1 is a polyglutamine protein that is classically associated with the neurodegenerative disorder spinocerebellar ataxia type 1 (SCA1). Here, we show that ataxin-1 also exerts a protective activity against autoimmune demyelination in a preclinical model of MS. This function is associated with an immunomodulatory role mainly targeting the B cell compartment. Altogether, these findings expand our current knowledge on both MS pathogenesis and ataxin-1 biology.