Mutations in GFPT1-related congenital myasthenic syndromes are associated with synaptic morphological defects and underlie a tubular aggregate myopathy with synaptopathy

Abstract

Mutations in GFPT1 (glutamine-fructose-6-phosphate transaminase 1), a gene encoding an enzyme involved in glycosylation of ubiquitous proteins, cause a limb-girdle congenital myasthenic syndrome (LG-CMS) with tubular aggregates (TAs) characterized predominantly by affection of the proximal skeletal muscles and presence of highly organized and remodeled sarcoplasmic tubules in patients' muscle biopsies. We report here the first long-term clinical follow-up of 11 French individuals suffering from LG-CMS with TAs due to GFPT1 mutations, of which 9 are new. Our retrospective clinical evaluation stresses an evolution toward a myopathic weakness that occurs concomitantly to ineffectiveness of usual CMS treatments. Analysis of neuromuscular biopsies from 3 unrelated individuals demonstrates that the maintenance of neuromuscular junctions (NMJs) is dramatically impaired with loss of post-synaptic junctional folds and evidence of denervation-reinnervation processes affecting the 3 main NMJ components. Moreover, molecular analyses of the human muscle biopsies confirm glycosylation defects of proteins with reduced O-glycosylation and show reduced sialylation of transmembrane proteins in extrajunctional area. Altogether, these results pave the way for understanding the etiology of this rare neuromuscular disorder that may be considered as a "tubular aggregates myopathy with synaptopathy".

Clinical and research strategies for limb‐girdle congenital myasthenic syndromes

“…There is no respiratory dysfunction and complete absence of ocular and bulbar weakness. Serum creatine kinase (CK) levels are mildly elevated, and muscle biopsy shows characteristic tubular aggregates …”

GFPT1 deficiency in muscle leads to myasthenia and myopathy in mice

“…Subsarcolemmal tubular aggregates are classified as densely packed vesicular or tubular membranes derived from the sarcoplasmic reticulum (29). Not only are they present in muscle biopsies from patients with mutations in proteins involved in the glycosylation pathway (1–3,5,8,9) but are also implicated in myopathies resulting from STIM1 and ORAI1 mutations (30–32), and are present in muscle from Caveolin1 −/− and Caveolin2 −/− mice (29). Whether tubular aggregates are direct pathological components contributing to the observed phenotype, or whether they represent a compensatory mechanism to pathological events, or a bystander event that is not connected to the functional impairment is poorly understood, making it challenging to understand the molecular mechanisms that induce tubular aggregates (33).…”

“…Some patients also display a decremental response to electromyography (EMG) (2,12) and repetitive nerve stimulation (7–9), and endplates in patient biopsies display simplification of the postsynaptic membrane with fewer and poorly developed junctional folds (2,8,9,12). Until now there has only been one report of a human mutation that disrupts the GFPT1-L isoform resulting in the absence of glycosylated proteins.…”

Trouble at the junction: When myopathy and myasthenia overlap

“…Mild elevations of CK have been reported in a minority of patients with DPAGT1 ‐, GFPT1 ‐, and ALG2 ‐CMS . Muscle MRI in patients with GFPT1 ‐ and DPAGT1 ‐CMS showed a significant amount of fatty infiltration of lower limb muscles, which was not present in CMS not related to disorders of glycosylation .…”

“…Clinically, these disorders most frequently present with limb‐girdle weakness without ophthalmoparesis or bulbar weakness, a phenotype termed “limb‐girdle CMS.” All are inherited in an autosomal recessive manner. Most patients present in early childhood, although onset in the second and third decades of life has been reported in GFPT1 ‐CMS . A slow progression of weakness over time has been described in all four disorders, with a reduced responsiveness to CMS treatment over time suggesting a progressive myopathy …”