Clinical, Electrophysiologic, and Pathologic Features of Anti-Contactin-Associated Protein 1 Autoimmune Nodopathy

Abstract

Background and objectives: The significance of anti-contactin-associated protein 1 (Caspr1) antibodies in autoimmune nodopathies (ANs) has not been fully established. The aim of this study was to elucidate the clinical profiles of Caspr1 AN.

Methods: Consecutive serum samples were included from patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) nationwide who were referred to our laboratory for antibody testing with questionnaires as part of routine clinical practice and who fulfilled definite European Federation of Neurological Societies and Peripheral Nerve Society electrodiagnostic criteria. Anti-Caspr1 immunoglobulin (Ig) G was screened using in-house ELISA and confirmed using immunohistochemistry and Western blotting. Clinical data, electrophysiologic findings, and treatment responses were retrospectively collected. The pathologic features of sural nerves were also examined. Inflammatory Neuropathy Cause and Treatment (INCAT) disability scores, antibody titers, and serum neurofilament light chain (NfL) levels were analyzed at baseline and follow-up.

Results: Of 559 screened serum samples from patients with CIDP (median age at onset 54 years, 35% female), 19 cases with anti-Caspr1 IgG were identified; the main subclass was IgG4 (17 cases). Patients with IgG4 Caspr1 AN cases exhibited an older age at disease onset (median 67 years); male predominance (82%); high proportions of limb weakness (100%), gait disturbance (100%), tremor (65%), and sensory ataxia (82%); and very high CSF protein levels (249 mg/dL). Nerve conduction studies showed prolonged F-wave and distal motor latencies and reduced sensorimotor conduction velocities in all nerves tested. Light microscopy findings of sural nerve biopsy specimens from 4 patients with Caspr1 AN indicated a loss of myelinated fiber density and myelin ovoids without macrophage-mediated demyelination or onion bulbs. Electron microscopic evaluation demonstrated axo-glial detachment. Patients with IgG4 Caspr1 AN showed a poor response to IVIg (31%) and needed combined immunotherapies. Even after correcting for age, serum NfL levels were higher in patients with IgG4 Caspr1 AN than in healthy controls (p = 0.0609) and correlated with INCAT scores (p = 0.0143). In the 7 patients with 2 consecutive serum samples, antibody titers decreased with clinical improvement.

Discussion: IgG4 Caspr1 AN presents with a similar clinical phenotype to other nodopathies (e.g., neurofascin 155 and contactin 1), but with an older age at onset. Changes in antibody titers may be a potential biomarker for monitoring disease activity.

Figure 1. Study Flowchart

AN = autoimmune nodopathy; Caspr1 = contactin-associated protein 1; CBA = cell-based assay; CIDP = chronic inflammatory demyelinating polyradiculoneuropathy; CNTN1 = contactin 1; IgG = immunoglobulin G; IHC = immunohistochemistry; NF155 = neurofascin 155; WB = Western blotting.

Figure 2. Detection of Anti-Caspr1 IgG

(A) Serum samples from patients with CIDP (n = 559), disease controls (n = 79), and healthy controls (n = 32) were tested for autoantibodies against Caspr1 using ELISA. Disease controls included participants with Guillain-Barré syndrome/Fisher syndrome (n = 49); anti–myelin-associated glycoprotein neuropathy (n = 11); Charcot-Marie-Tooth disease (n = 9); hereditary neuropathy with liability to pressure palsies (n = 1); and polyneuropathy, organomegaly, endocrinopathy, M protein, and skin changes syndrome (n = 9). The dotted line represents the mean +5 SD of healthy controls, which was the cutoff value. The number next to each dot corresponds to the patient number in the manuscript. Patients with non-IgG4 anti-Caspr1 IgG are shown in blue. Serum samples from 5 patients (patients 8, 10, 15, 19, and 20) were taken after treatment. (B) IgG subclass analysis. (C) Double immunostaining of murine teased nerve fibers with commercial anti-Caspr1 polyclonal IgG and serum samples from a healthy control or patients with anti-Caspr1 IgG. IgG from patients with IgG4 anti-Caspr1 IgG (patients 1 and 4) distinctly bound to the paranodes, whereas immunostaining was weak in patient 16 (with a low titer) and patient 13 (with non-IgG4 anti-Caspr1 IgG). Scale bars = 10 μm. (D) Western blot analysis using recombinant human Caspr1 protein. Caspr1 was detected as the target band using mouse monoclonal anti-Caspr1 IgG. Serum samples from all patients with IgG4 anti-Caspr1 IgG except patient 15 showed the same band under nonreducing conditions. Caspr1 = contactin-associated protein 1; CIDP = chronic inflammatory demyelinating polyradiculoneuropathy; HC = healthy control; IgG = immunoglobulin G; M = marker; OD = optical density; P = patient; PC = positive control.

Figure 3. Sural Nerve Biopsy Images

Patient 20 (A, C: IgG4 Caspr1 AN) and patient 8 (B, D: non-IgG4 Caspr1 AN). (A, B) Transverse sections stained with toluidine blue showed moderate sub-perineurial edema, a mild decrease in myelinated fiber density, and myelin ovoids. (C, D) Under electron microscopy, longitudinal sections clearly showed the detachment of myelin loops from the axolemma at the paranode (arrows). In contrast to that observed in (C), axo-glial detachment was mild in (D). Scale bars = 100 μm (A, B), 1 μm (C, D), and 0.5 μm (high-power magnifications of the squares in C and D). Caspr1 = contactin-associated protein 1; IgG = immunoglobulin G.

Figure 4. Changes in Clinical Scales and Biomarkers in Caspr1 AN

(A) Serum NfL levels in IgG4 Caspr1 AN and healthy controls. The number next to each dot corresponds to the patient number in the article. Serum samples from 4 patients (patients 10, 15, 19, and 20) were taken after treatment. (B) Changes in clinical scores, antibody titers, and serum NfL levels in Caspr1 AN. Follow-up samples were available for 7 patients (patients 1, 4, 5, 8, 13, 15, and 19). Clinical improvement was observed in INCAT scores after treatment. Both anti-Caspr1 IgG titers and serum NfL levels were decreased at follow-up compared with baseline. Dots and whiskers represent median values and interquartile ranges, respectively. (C) Extracted results from a subgroup of treatment-naïve patients with pretreatment and post-treatment serum samples (n = 4; patients 1, 4, 5, and 13). AN = autoimmune nodopathy; Caspr1 = contactin-associated protein 1; IgG = immunoglobulin G; INCAT = Inflammatory Neuropathy Cause and Treatment; NfL = neurofilament light chain; P = patient.