Distal Agrin (AGRN) Congenital Myasthenic Syndrome With Mitochondrial Dysfunction

Agrin-congenital myasthenic syndrome (AGRN-CMS) is a rare, heterogeneous genetic disorder of the neuromuscular transmission that can present from infancy to adulthood. The clinical phenotype includes distal weakness mimicking distal myopathies. Additionally, electromyography and muscle biopsy may demonstrate myopathic features, increasing the diagnostic challenge. Mitochondrial dysfunction has not been reported in the muscle of AGRN-CMS patients.

We report the clinical, electrophysiological, radiological, myopathological, and genetic findings of a patient with AGRN-CMS.

A 47-year-old male presented at age 18 with sudden onset of eyelid ptosis, weakness, and fatigue, followed by dysphagia, dyspnea on exertion, exercise intolerance, and myalgias. Three sisters had similar symptoms. Neurological examination showed lower limb weakness, predominantly affecting the calf muscles with associated atrophy. Tendon reflexes were normal but absent at the ankles. Creatine kinase levels were normal. Electromyography demonstrated myopathic changes in distal muscles with fibrillation potentials in the gastrocnemius. 2 Hz-Repetitive nerve stimulation of the fibular nerve revealed a 32% decrement in the extensor digitorum brevis without facilitation. Tibialis anterior biopsy showed non-specific myopathic changes, neurogenic features, and histological evidence of mitochondrial dysfunction, as suggested by the scattered cytochrome c oxidase-negative fibers. Whole exome sequencing detected a homozygous pathogenic AGRN variant (c.5012G>A, p.Arg1671Gln). No pathogenic or suspected pathogenic variants were detected in muscle mitochondrial DNA or in nuclear genes affecting mitochondrial function.

This patient expands the pathological spectrum of AGRN-CMS to include mitochondrial dysfunction and highlights the importance of low-frequency repetitive nerve stimulation in the assessment of patients with distal weakness for differentiating myopathies from CMS.