Glutamine-fructose-6-phosphate
transaminase 1 (GFPT1) is the first
enzyme of the
hexosamine biosynthetic pathway. It transfers an amino group from
glutamine to
fructose-6-phosphate to yield glucosamine-6-phosphate, thus providing the precursor for
uridine diphosphate N-acetylglucosamine (
UDP-GlcNAc) synthesis.
UDP-GlcNAc is an essential substrate for all mammalian glycosylation biosynthetic pathways and N-
glycan branching is especially sensitive to alterations in the concentration of this
sugar nucleotide. It has been reported that GFPT1 mutations lead to a distinct sub-class of
congenital myasthenic syndromes (CMS) termed "limb-girdle CMS with tubular aggregates". CMS are hereditary
neuromuscular transmission disorders in which neuromuscular junctions are impaired. To investigate whether alterations in protein glycosylation at the neuromuscular junction might be involved in this impairment, we have employed mass spectrometric strategies to study the N-glycomes of myoblasts and myotubes derived from two healthy controls, three GFPT1 patients, and four patients with other
muscular diseases, namely CMS caused by mutations in DOK7,
myopathy caused by mutations in MTND5,
limb girdle muscular dystrophy type 2A (
LGMD2A), and
Pompe disease. A comparison of the relative abundances of bi-, tri-, and tetra-antennary N-
glycans in each of the cell preparations revealed that all samples exhibited broadly similar levels of branching. Moreover, although some differences were observed in the relative abundances of some of the N-
glycan constituents, these variations were modest and were not confined to the GFPT1 samples. Therefore, GFPT1 mutations in CMS patients do not appear to compromise global N-glycosylation in muscle cells.