Biallelic PIGB Variants Cause Motor Neuropathy with Conduction Blocks and Peripheral Nerve Hyperexcitability

OBJECTIVE: Glycosylphosphatidylinositol (GPI)-anchored proteins play critical roles in nervous system function. Pathogenic variants in genes involved in GPI-anchor biosynthesis cause early-onset multisystem disorders known as inherited GPI deficiencies. We describe a novel neuromuscular phenotype associated with PIGB deficiency. METHODS: Patients with neuromuscular disease carrying PIGB variants were identified, and clinical data collected. GPI-anchored protein and free-GPI expression were assessed by flow cytometry and variants validated in a PIGB knockout cellular model. RESULTS: Biallelic PIGB variants were identified in 12 patients (median age 32 years, range 15-52 years) from 9 independent families. Of 15 variants, 13 were novel, including a de novo inversion generating a PIGB::RAB27A gene fusion. Of 12 patients, 10 presented with distal lower-limb weakness, and neurophysiological studies demonstrated a motor-predominant neuropathy with frequent conduction blocks in all patients. Signs of nerve hyperexcitability were observed in 9 patients, with electromyography neuromyotonic discharges and myokymia compatible with peripheral nerve hyperexcitability in 6. A total of 3 patients showed a decremental response on repetitive nerve stimulation; 2 were symptomatic and responded to pyridostigmine, consistent with neuromuscular junction transmission defect. Neurodevelopmental features were inconsistent. Flow cytometry revealed variable reductions of GPI-anchored proteins, but consistent reduction of free GPI in blood cells. Novel variants impaired GPI-anchor expression in vitro. INTERPRETATION: We expand the phenotypic spectrum of inherited GPI deficiencies with a novel neuromuscular syndrome encompassing motor neuropathy with conduction blocks, peripheral nerve hyperexcitability, and occasional neuromuscular junction defects associated with PIGB variants. PIGB and other GPI-anchor biosynthesis genes should be considered in motor neuropathy with conduction blocks. ANN NEUROL 2026.