OBJECTIVE:Biallelic intronic AAGGG repeat expansions in RFC1 cause cerebellar ataxia with neuropathy and vestibular areflexia syndrome and may also contribute to isolated sensory neuropathy. The clinical significance of both heterozygous and homozygous (biallelic) RFC1 expansions in more diverse patient populations remains unclear-partly due to the absence of accurate, user-friendly computational pipelines specifically tailored for tandem repeat analysis. METHODS:To discern the relationship between RFC1 expansions and idiopathic peripheral neuropathy (iPN), we performed whole-genome sequencing (WGS) followed by polymerase chain reaction (PCR)-based confirmation in a large, well-characterized US cohort consisting of 788 patients with iPN (369 pure small fiber neuropathy (SFN), 266 sensorimotor, 144 pure sensory, and 9 pure motor). We developed an integrative pipeline combining ExpansionHunter Denovo and Expansion Hunter coupled with unsupervised clustering to reliably detect and genotype RFC1 expansions from short-read WGS data, achieving 97.2% concordance with repeat-primed PCR-based validation. RESULTS:Biallelic RFC1 expansions were present in only 1 out of 778 controls but present in 18 out of 788 (2.3%) patients with iPN (Fisher's exact p = 7 × 10-5), including 6.9% (10/144) of pure sensory, 1.1% (4/369) of SFN, and 1.5% (4/266) of sensorimotor neuropathy. These data indicate that motor nerve involvement should not exclude patients from RFC1 repeat screening. Monoallelic expansions were observed at a nominally higher frequency in iPN than in controls (9.1% vs 7.2%), but this difference did not reach statistical significance (Fisher's exact p = 0.17). We also found no evidence of second mutations or expansions on the other allele among monoallelic carriers. INTERPRETATION:Our approach provides a robust, cost-effective method for detecting RFC1 expansions from WGS data. Our findings indicate that homozygous (biallelic) AAGGG repeat expansions in RFC1 contribute to development of iPN. Heterozygous expansions may also confer disease risk, but future studies are needed to assess this observation and explore any phenotypic differences with biallelic cases. ANN NEUROL 2026;100:95-108.

OBJECTIVE:RFC1 biallelic repeat expansion is increasingly recognized as a cause of chronic idiopathic axonal polyneuropathy (CIAP), but it remains challenging to know who to test. This study aims to determine the prevalence of biallelic and monoallelic RFC1 expansions and their corresponding neuropathy phenotypes in CIAP patients and identify discriminators of RFC1 expansion status. METHODS:The cross-sectional Peripheral Neuropathy Research Registry was queried for individuals with CIAP and tested for RFC1 expansions. Associations between RFC1 biallelic and monoallelic status and clinical history, neurologic examination findings, fiber type involvement (exam-based), electrodiagnostic (EDX) patterns (sensory, sensorimotor, motor), and the presence of sensory neuronopathy (Camdessanche criteria) were determined. RESULTS:A total of 788 participants were enrolled in the study, of whom 18 had biallelic RFC1 expansions (2.3%) and 62 had monoallelic RFC1 expansions (7.9%). All biallelic RFC1 patients had a mixed fiber neuropathy, 27.8% had weakness on examination, and 44.4% had motor EDX abnormalities. Our study identified five factors associated with RFC1 biallelic status: (1) abnormal upper limb vibration (OR 4.65, 95% CI 1.20-18.13), (2) abnormal sensory EDX studies (OR 4.65, 95% CI 1.11-24.29), (3) sensory-only polyneuropathy on EDX (OR 7.17, 95% CI 1.69-38.15), (4) sensory neuronopathy (OR 3.64, 95% CI 1.16-11.19) (AUCs = 0.80-0.81 for biallelic and 0.69-0.71 for monoallelic), and (5) body mass index (OR 0.88, 95% CI 0.78-0.97). INTERPRETATION:The prevalence of biallelic RFC1 expansion is lower in this North American CIAP population compared to previous studies. Motor involvement was frequent. Discrimination of biallelic status was good, but poor for monoallelic status.

BACKGROUND AND AIMS:Recent preclinical studies have shown that nicotinamide adenine dinucleotide (NAD+) plays a critical role in molecular mechanisms of axon degeneration, and reductions in NAD+ levels are associated with axonal degeneration. Nicotinamide riboside (NR) is a safe and widely available pyridine-nucleoside form of vitamin B3 and is an NAD+ precursor. METHODS:To investigate if oral supplementation of synthetic NR can act as a therapeutic agent to prevent degeneration of small somatic sensory axons innervating the skin or promote regeneration of these same fibers in humans, we utilized a validated experimental model of cutaneous nerve degeneration and regeneration and conducted a placebo-controlled, double-blinded Phase 2 study. RESULTS:NR supplementation did not result in elevation of plasma NAD+ levels but resulted in a small increase in NAD+ in the skin samples. NR supplementation did not prevent capsaicin-induced degeneration of the epidermal sensory nerve fibers, and there was no difference in the amount of epidermal reinnervation at the 90-day visit. Although there was a small but statistically significant increase in the number of epidermal sensory nerve fibers at the 60-day visit, these results are preliminary and will need to be validated in larger studies. INTERPRETATION:At present oral NR supplementation, at the doses used in this study, cannot be recommended to prevent neuropathy or to improve nerve regeneration.

BACKGROUND:Neurofilament light chain (Nf-L) has been identified as a biomarker of neurodegeneration in many neuromuscular conditions, including several subtypes of polyneuropathies. The purpose of this research was to investigate whether Nf-L is also a promising biomarker for idiopathic peripheral neuropathy (IPN), the second most common subtype of axonal polyneuropathy. METHODS:Nf-L levels were quantified using an ultrasensitive digital immunoassay SiMoA in plasma samples from 294 subjects. Participant inclusion required a diagnosis of IPN confirmed by electrodiagnostic testing, intraepidermal nerve fiber density (IENFD), and/or neuromuscular examination. Laboratory testing recommended by the American Academy of Neurology for the evaluation of polyneuropathy was normal in all subjects. RESULTS:In our cohort, the majority of participants (78.1%, N = 228) had Nf-L levels in the age-adjusted normal range. Those with elevated Nf-L levels had higher scores on two different neuropathy severity scores and were more likely to have abnormal electrodiagnostic testing, including reduced action potential amplitude in peroneal motor and sural sensory nerves. No differences in blood Nf-L levels were observed in those participants with a short duration (≤ 1.5 years) versus long duration (≥ 5 years) of disease. Nf-L levels were also not correlated with the presence of neuropathic pain, nor the location of paresthesia. Nf-L expression had the strongest correlation with age. CONCLUSIONS:In this cohort with IPN, Nf-L levels correlated with disease severity as assessed by clinical examination and electrophysiology. However, given that Nf-L was in the normal range for the majority of subjects in our cohort, its use as a biomarker for clinical trials evaluating new treatments for IPN will be limited.

ABSTRACT Objective Determine the association between diabetes and metabolic syndrome (MetS) burden (number of MetS criteria fulfilled) and pain, neuropathy severity, and fiber type involvement in individuals with established polyneuropathy. Methods The Peripheral Neuropathy Research Registry was queried for individuals with type 1 and type 2 diabetes (DPN) and non‐diabetic peripheral neuropathy (cryptogenic sensory polyneuropathy and prediabetes) using cross‐sectional observational data. Associations between diabetes or MetS burden and pain presence (yes/no), neuropathy severity (Total Neuropathy Score reduced), and fiber type involvement (pinprick, vibration, and proprioception examination—small, large, mixed) using logistic, linear, and multinomial regression models were determined. Results A total of 1112 participants were included (265 DPN, 847 non‐diabetic peripheral neuropathy [NDPN]). Compared to NDPN, DPN participants were more likely to have pain, higher neuropathy severity, and mixed fiber involvement. In adjusted models, diabetes was associated with pain (odds ratio [OR] 1.85, CI: 1.15–3.03) and severity (point estimate [PE] 0.84, CI: 0.27–1.42), but not fiber type involvement. As the MetS burden increased, pain, neuropathy severity, and mixed fiber type involvement increased (p < 0.05 for trend). In adjusted models, MetS burden was associated with pain (OR 1.23, CI: 1.06–1.41) but not severity or fiber type involvement. Interpretation Participants with DPN were more likely to have pain, greater neuropathy severity, and possibly more mixed fiber involvement than those with NDPN. Similarly, increasing MetS burden also led to more painful neuropathy and possibly more severe neuropathy with more mixed fiber involvement.

Biallelic AAGGG expansions in Replication Factor Complex Subunit 1 ( RFC1) are associated with cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) and are increasingly recognised as a common cause of adult-onset ataxia and sensory neuropathy. However, the disease-causing mechanisms remain unclear. Here we leveraged in vitro assays, post-mortem brain tissue, patient-derived cell lines and a neuronal Drosophila model to demonstrate that AAGGG expansions are associated with tissue-specific reductions in the expression of RFC1 transcript, along with impaired RFC1 function and increased sensitivity to DNA damage from platinum-based drugs. CRISPR/Cas9 excision of the AAGGG repeat and flanking AluSx3 element normalized RFC1 expression in iPSC-derived neurons and rescued the DNA damage response, providing a framework for future therapeutic strategies. We also show that these biological findings are clinically relevant in heterozygous AAGGG expansion carriers, who display an increased risk and severity of neuropathy with platinum-based chemotherapy. ### Competing Interest Statement The authors have declared no competing interest. European Research Council, https://ror.org/0472cxd90, 101165557 National Ataxia Foundation, https://ror.org/03s9xxw55 Medical Research Council, MR/T001712/1 Fondazione Cariplo, https://ror.org/01gb56c55, 2019-1836 Charcot-Marie-Tooth Association, SR-202504 AFM-Telethon, 28813 Fondazione Regionale per la Ricerca Biomedica, 1751723 Muscular Dystrophy UK, 24GRO-PG24-0719-1 Guarantors of Brain, https://ror.org/00xkj2889

Peripheral neuropathies (PNs) affect over 20 million individuals in the United States, manifesting as a wide range of sensory, motor, and autonomic nerve symptoms. While various conditions such as diabetes, metabolic disorders, trauma, autoimmune disease, and chemotherapy-induced neurotoxicity have been linked to PN, approximately one-third of PN cases remain idiopathic, underscoring a critical gap in our understanding of these disorders. Over the years, considerable efforts have focused on unraveling the complex molecular pathways underlying PN to advance diagnosis and treatment. Traditional methods such as linkage analysis, fluorescence in situ hybridization, polymerase chain reaction, and Sanger sequencing identified initial genetic variants associated with PN. However, the establishment and application of next-generation sequencing (NGS) and, more recently, long-read/single-cell sequencing have revolutionized the field, accelerating the discovery of novel disease-causing variants and challenging previous assumptions about pathogenicity. This review traces the evolution of genomic technologies in PN research, emphasizing the pivotal role of NGS in uncovering genetic complexities. We provide a comprehensive analysis of established genomic approaches such as genome-wide association studies, targeted gene panel sequencing, and whole-exome/genome sequencing, alongside emerging multiomic technologies including RNA sequencing and proteomics. Integrating these approaches promises holistic insights into PN pathophysiology, potentially revealing new biomarkers and therapeutic targets. Furthermore, we discuss the clinical implications of genomic and multiomic integration, highlighting their potential to enhance diagnostic accuracy, prognostic assessment, and personalized treatment strategies for PN. Challenges and questions in standardizing these technologies for clinical use are raised, underscoring the need for robust guidelines to maximize their clinical utility.

Purpose This white paper provides guidance regarding the process for establishing and maintaining international collaborations to conduct oncology/neurology-focused chemotherapy-induced peripheral neurotoxicity (CIPN) research. Methods An international multidisciplinary group of CIPN scientists, clinicians, research administrators, and legal experts have pooled their collective knowledge regarding recommendations for establishing and maintaining international collaboration to foster advancement of CIPN science. Results Experts provide recommendations in 10 categories: (1) preclinical and (2) clinical research collaboration; (3) collaborators and consortiums; (4) communication; (5) funding; (6) international regulatory standards; (7) staff training; (8) data management, quality control, and data sharing; (9) dissemination across disciplines and countries; and (10) additional recommendations about feasibility, policy, and mentorship. Conclusion Recommendations to establish and maintain international CIPN research collaboration will promote the inclusion of more diverse research participants, increasing consideration of cultural and genetic factors that are essential to inform innovative precision medicine interventions and propel scientific discovery to benefit cancer survivors worldwide. Relevance to inform research policy Our suggested guidelines for establishing and maintaining international collaborations to conduct oncology/neurology-focused chemotherapy-induced peripheral neurotoxicity (CIPN) research set forth a challenge to multinational science, clinical, and policy leaders to (1) develop simple, streamlined research designs; (2) address logistical barriers; (3) simplify and standardize regulatory requirements across countries; (4) increase funding to support international collaboration; and (5) foster faculty mentorship.

Peripheral neuropathy (PN) is a common neurological condition in elderly adults. Vitamin D deficiency has been associated with diabetic and chemotherapy-induced neuropathy, but its role in idiopathic PN, in which no underlying cause of neuropathy can be identified, has not been investigated. Two hundred thirty patients with idiopathic PN enrolled in the Peripheral Neuropathy Research Registry (PNRR) at Johns Hopkins University School of Medicine had vitamin D testing information on record. Linear and logistic regressions were used to investigate the relationship between absolute vitamin D level or vitamin D insufficiency (<20 ng/mL) and both the severity of neuropathy as measured by the reduced total neuropathy score (TNSr) and severity of neuropathic pain. Sixteen (7%) patients were vitamin D insufficient (<20 ng/mL). Controlling for factors known to correlate with severity of neuropathy, there was no correlation between absolute vitamin D levels and TNSr (correlation coefficient 0.01, 95% CI −0.03 to 0.07, p  = .59) and no association between vitamin D insufficiency and TNSr (correlation coefficient 0.3, 95% CI −2.8 to 3.4, p  = .86). Vitamin D insufficiency was not associated with the presence of neuropathic pain (OR 4.1, 95% CI 0.6–26.0, p  = .13), and there was no correlation between vitamin D levels and pain score (correlation coefficient 0.01, 95% CI −0.02 to 0.03, p  = .59). In a single-center cohort of patients with idiopathic PN, there was no correlation between vitamin D levels and the severity of neuropathy or neuropathic pain.

Dominant missense mutations of the calcium-permeable cation channel TRPV4 cause Charcot-Marie-Tooth disease (CMT) type 2C and two forms of distal spinal muscular atrophy. These conditions are collectively referred to as TRPV4-related neuromuscular disease and share features of motor greater than sensory dysfunction and frequent vocal fold weakness. Pathogenic variants lead to gain of ion channel function that can be rescued by TRPV4 antagonists in cellular and animal models. As small molecule TRPV4 antagonists have proven safe in trials for other disease indications, channel inhibition is a promising therapeutic strategy for TRPV4 patients. However, the current knowledge of the clinical features and natural history of TRPV4-related neuromuscular disease is insufficient to enable rational clinical trial design. To address these issues, we developed a TRPV4 patient database and administered a TRPV4-specific patient questionnaire. Here, we report demographic and clinical information, including CMT Examination Scores (CMTES), from 68 patients with known pathogenic TRPV4 variants, 40 of whom also completed the TRPV4 patient questionnaire. TRPV4 patients showed a bimodal age of onset, with the largest peak occurring in the first 2 years of life. Compared to CMT type 1A (CMT1A) patients, TRPV4 patients showed distinct symptoms and signs, manifesting more ambulatory difficulties and more frequent involvement of proximal arm and leg muscles. Although patients reported fewer sensory symptoms, sensory dysfunction was often detected clinically. Many patients were affected by vocal fold weakness (55%) and shortness of breath (55%), and 11% required ventilatory support. Skeletal abnormalities were common, including scoliosis (64%), arthrogryposis (33%) and foot deformities. Strikingly, patients with infantile onset of disease showed less sensory involvement and less progression of symptoms. These results highlight distinctive clinical features in TRPV4 patients, including motor-predominant disease, proximal arm and leg weakness, severe ambulatory difficulties, vocal fold weakness, respiratory dysfunction and skeletal involvement. In addition, patients with infantile onset of disease appeared to have a distinct phenotype with less apparent disease progression based on CMTES. These collective observations indicate that clinical trial design for TRPV4-related neuromuscular disease should include outcome measures that reliably capture non-length dependent motor dysfunction, vocal fold weakness and respiratory disease.

Background and Aims: Why only half of the idiopathic peripheral neuropathy (IPN) patients develop neuropathic pain remains unknown. By conducting a proteomics analysis on IPN patients, we aimed to discover proteins and new pathways that are associated with neuropathic pain.Methods: We conducted unbiased mass-spectrometry proteomics analysis on blood plasma from 31 IPN patients with severe neuropathic pain and 29 IPN patients with no pain, to investigate protein biomarkers and protein-protein interactions associated with neuropathic pain. Univariate modeling was done with linear mixed modeling (LMM) and corrected for multiple testing. Multivariate modeling was performed using elastic net analysis and validated with internal cross-validation and bootstrapping.Results: In the univariate analysis, 73 proteins showed a p-value <.05 and 12 proteins showed a p-value <.01. None were significant after Benjamini-Hochberg adjustment for multiple testing. Elastic net analysis created a model containing 12 proteins with reasonable discriminatory power to differentiate between painful and painless IPN (false-negative rate 0.10, false-positive rate 0.18, and an area under the curve 0.75). Eight of these 12 proteins were clustered into one interaction network, significantly enriched for the complement and coagulation pathway (Benjamini-Hochberg adjusted p-value = .0057), with complement component 3 (C3) as the central node. Bootstrap validation identified insulin-like growth factor-binding protein 2 (IGFBP2), complement factor H-related protein 4 (CFHR4), and ferritin light chain (FTL), as the most discriminatory proteins of the original 12 identified.Interpretation: This proteomics analysis suggests a role for the complement system in neuropathic pain in IPN.

Why only half of the idiopathic peripheral neuropathy (IPN) patients develop neuropathic pain remains unknown. By conducting a proteomics analysis on IPN patients, we aimed to discover proteins and new pathways that are associated with neuropathic pain. We conducted unbiased mass-spectrometry proteomics analysis on blood plasma from 31 IPN patients with severe neuropathic pain and 29 IPN patients with no pain, to investigate protein biomarkers and protein–protein interactions associated with neuropathic pain. Univariate modeling was done with linear mixed modeling (LMM) and corrected for multiple testing. Multivariate modeling was performed using elastic net analysis and validated with internal cross-validation and bootstrapping. In the univariate analysis, 73 proteins showed a p -value <.05 and 12 proteins showed a p -value <.01. None were significant after Benjamini–Hochberg adjustment for multiple testing. Elastic net analysis created a model containing 12 proteins with reasonable discriminatory power to differentiate between painful and painless IPN (false-negative rate 0.10, false-positive rate 0.18, and an area under the curve 0.75). Eight of these 12 proteins were clustered into one interaction network, significantly enriched for the complement and coagulation pathway (Benjamini–Hochberg adjusted p -value = .0057), with complement component 3 (C3) as the central node. Bootstrap validation identified insulin-like growth factor-binding protein 2 (IGFBP2), complement factor H-related protein 4 (CFHR4), and ferritin light chain (FTL), as the most discriminatory proteins of the original 12 identified. This proteomics analysis suggests a role for the complement system in neuropathic pain in IPN.

Background and aims: Peripheral neuropathy (PN) is a common neurological condition in elderly adults. Vitamin D deficiency has been associated with diabetic and chemotherapy-induced neuropathy, but its role in idiopathic PN, in which no underlying cause of neuropathy can be identified, has not been investigated. Methods: Two hundred thirty patients with idiopathic PN enrolled in the Peripheral Neuropathy Research Registry (PNRR) at Johns Hopkins University School of Medicine had vitamin D testing information on record. Linear and logistic regressions were used to investigate the relationship between absolute vitamin D level or vitamin D insufficiency (<20 ng/mL) and both the severity of neuropathy as measured by the reduced total neuropathy score (TNSr) and severity of neuropathic pain. Results: Sixteen (7%) patients were vitamin D insufficient (<20 ng/mL). Controlling for factors known to correlate with severity of neuropathy, there was no correlation between absolute vitamin D levels and TNSr (correlation coefficient 0.01, 95% CI -0.03 to 0.07, p = .59) and no association between vitamin D insufficiency and TNSr (correlation coefficient 0.3, 95% CI -2.8 to 3.4, p = .86). Vitamin D insufficiency was not associated with the presence of neuropathic pain (OR 4.1, 95% CI 0.6-26.0, p = .13), and there was no correlation between vitamin D levels and pain score (correlation coefficient 0.01, 95% CI -0.02 to 0.03, p = .59). Interpretation: In a single-center cohort of patients with idiopathic PN, there was no correlation between vitamin D levels and the severity of neuropathy or neuropathic pain.

Dominant mutations in the calcium-permeable ion channel TRPV4 (transient receptor potential vanilloid 4) cause diverse and largely distinct channelopathies, including inherited forms of neuromuscular disease, skeletal dysplasias and arthropathy. Pathogenic TRPV4 mutations cause gain of ion channel function and toxicity that can be rescued by small molecule TRPV4 antagonists in cellular and animal models, suggesting that TRPV4 antagonism could be therapeutic for patients. Numerous variants in TRPV4 have been detected with targeted and whole exome/genome sequencing, but for the vast majority, their pathogenicity remains unclear. Here, we used a combination of clinical information and experimental structure-function analyses to evaluate 30 TRPV4 variants across various functional protein domains. We report clinical features of seven patients with TRPV4 variants of unknown significance and provide extensive functional characterization of these and an additional 17 variants, including structural position, ion channel function, subcellular localization, expression level, cytotoxicity and protein-protein interactions. We find that gain-of-function mutations within the TRPV4 intracellular ankyrin repeat domain target charged amino acid residues important for RhoA interaction, whereas ankyrin repeat domain residues outside of the RhoA interface have normal or reduced ion channel activity. We further identify a cluster of gain-of-function variants within the intracellular intrinsically disordered region that may cause toxicity via altered interactions with membrane lipids. In contrast, assessed variants in the transmembrane domain and other regions of the intrinsically disordered region do not cause gain of function and are likely benign. Clinical features associated with gain of function and cytotoxicity include congenital onset of disease, vocal cord weakness and motor-predominant disease, whereas patients with likely benign variants often demonstrated late-onset and sensory-predominant disease. These results provide a framework for assessing additional TRPV4 variants with respect to likely pathogenicity, which will yield critical information to inform patient selection for future clinical trials for TRPV4 channelopathies.