CMT2S is a rare Charcot-Marie-Tooth disease subtype caused by immunoglobulin mu-binding protein 2 (IGHMBP2) variants that result in abnormal RNA processing leading to alpha-motor neuron degeneration. A patient was reported with variants within IGHMBP2. Whole genome sequencing revealed a paternally inherited cryptic splice site non-coding variant (c.1235+894 C>A) in intron 8, which leads to nonsense-mediated decay resulting in IGHMBP2 haploinsufficiency.

CMT2S-MNs were differentiated from an iPSC cell line generated from the patient’s fibroblasts. Patch clamp electrophysiology, phase imaging, and immunocytochemistry experiments were performed to characterize CMT2S-MNs and identify morphology differences. To determine NMJ defects, CMT2S-MNs and control iPSCs (WT-MNs) were integrated into a dual-chamber NMJ platform with wild-type iPSC-derived skeletal muscle myofibers.

Patch clamp electrophysiology revealed hyperexcitability and spontaneous firing of CMT2S-MNs. Similar characteristics can be found in amyotrophic lateral sclerosis-derived MNs (ALS-MNs), though ALS-MNs show hyperexcitability much later than CMT2S-MNs. Analysis of Na⁺ current normalized by membrane capacitance showed a significant decrease in membrane capacitance and membrane potential in CMT2S-MNs compared to WT-MNs. An NMJ fatigue index (FI) functional readout revealed low FI and quick fatigue in CMT2S. The CMT2S NMJ system presented a dominant phenotype of tetanus, followed by decay.

CMT2S-MNs revealed a hyperexcitable phenotype with spontaneous firing, comparable to amyotrophic lateral sclerosis. The hyperexcitability observed in CMT2S-MNs may be caused by the observed reduced resting membrane potential, reduced membrane capacitance, and thus reduced Na⁺ current density. NMJ FI showed quick fatigue which may correlate with a CMT2S clinical phenotype. We are further analyzing this patient-specific model to continue phenotyping CMT2S caused by IGHMBP2 variants.