Background and Objectives As autoantibodies to contactin-1 from patients with chronic inflammatory demyelinating polyradiculoneuropathy not only bind to the paranodes where they are supposed to cause conduction failure but also bind to other neuronal cell types, we aimed to investigate the effect of anti–contactin-1 autoantibodies on contactin-1 surface expression in cerebellar granule neurons, dorsal root ganglion neurons, and contactin-1–transfected human embryonic kidney 293 cells. Methods Immunocytochemistry including structured illumination microscopy and immunoblotting was used to determine expression levels of contactin-1 and/or sodium channels after long-term exposure to autoantibodies from 3 seropositive patients. For functional analysis of sodium channels, whole-cell recordings of sodium currents were performed on dorsal root ganglion neurons incubated with anti–contactin-1 autoantibodies. Results We found a reduction in contactin-1 expression levels on dorsal root ganglion neurons, cerebellar granule neurons, and contactin-1–transfected human embryonic kidney 293 cells and decreased dorsal root ganglion sodium currents after long-term exposure to anti–contactin-1 autoantibodies. Sodium channel density did not decrease. Discussion Our results demonstrate a direct effect of anti–contactin-1 autoantibodies on the surface expression of contactin-1 and sodium currents in dorsal root ganglion neurons. This may be the pathophysiologic correlate of sensory ataxia reported in these patients.

Fabry disease (FD) is a life-limiting disorder characterized by intracellular globotriaosylceramide (Gb3) accumulations. The underlying α-galactosidase A (α-GAL A) deficiency is caused by variants in the gene GLA. Variants of unknown significance (VUS) are frequently found in GLA and challenge clinical management. Here, we investigated a 49-year old man with cryptogenic lacunar cerebral stroke and the chance finding of the VUS S126G, who was sent to our center for diagnosis and initiation of a costly and life-long FD-specific treatment. We combined clinical examination with in vitro investigations of dermal fibroblasts (HDF), induced pluripotent stem cells (iPSC), and iPSC-derived sensory neurons. We analyzed α-GAL A activity in iPSC, Gb3 accumulation in all three cell types, and action potential firing in sensory neurons. Neurological examination and small nerve fiber assessment was normal except for reduced distal skin innervation. S126G iPSC showed normal α-GAL A activity compared to controls and no Gb3 deposits were found in all three cell types. Baseline electrophysiological characteristics of S126G neurons showed no difference compared to healthy controls as investigated by patch-clamp recordings. We pioneer multi-level cellular characterization of the VUS S126G using three cell types derived from a patient and provide further evidence for the benign nature of S126G in GLA, which is of great importance in the management of such cases in clinical practice.

Small fiber neuropathy (SFN) is characterized by acral burning pain and may be associated with genetic variants of the transient receptor potential ankyrin 1 (TRPA1) channel. We studied a 43-year-old male patient with SFN with severe acral and abdominal pain carrying the TRPA1 variant NM_007332.3:c.1678C>G (His560Asp). The patient underwent a complete neurological work-up including a skin punch biopsy for the generation of induced pluripotent stem cells. Sensory-like neurons (iSN) were differentiated and characterized by immunocytochemistry, qRT-PCR, and patch-clamp recordings. hTRPA1_H560D was further transfected into human embryonic kidney 293 (HEK293) cells and characterized using Ca ²⁺ -flux assay, Ca ²⁺ -imaging, and patch-clamp under TRPA1-agonist/-antagonist conditions. Channel localization was assessed by immunocytochemistry and western blot, and cell viability by a CellTiter-Glo Assay. The clinical work-up confirmed SFN. Sensory-like neurons were electrically excitable and expressed characteristic neuronal and peripheral markers. Ca ²⁺ -flux assay and Ca ²⁺ -imaging revealed reduced sensitivity to allyl isothiocyanate in iSN ( P < 0.001) and transfected HEK293 cells ( P < 0.05). Patch-clamp recordings showed diminished agonist-evoked currents in hTRPA1_H560D-expressing cells compared with hTRPA1_WT ( P < 0.05). TRPA1_H560D reduced TRPA1 expression ( P < 0.001) and impaired membrane localization ( P < 0.05) and channel glycosylation, without affecting HEK293 cell viability. Our 2 complementary in vitro models present TRPA1_H560D as a loss-of-function variant despite its association with clinical pain. This paradox suggests pain mechanisms beyond channel hyperactivity, such as impaired membrane trafficking, e.g. because of altered glycosylation. Our findings further emphasize the complexity of genotype–phenotype relationships in SFN and highlight the need for cautious interpretation of genetic variants of uncertain significance in clinical practice.