Exome sequencing detection of two untranslated GFPT1 mutations in a family with limb-girdle myasthenia

• Published in: Clinical genetics Vol. 85; no. 2; pp. 166 - 171
• Main Authors: Maselli, R.A., Arredondo, J., Nguyen, J., Lara, M., Ng, F., Ngo, M., Pham, J.M., Yi, Q., Stajich, J.M., McDonald, K., Hauser, M.A., Wollmann, R.L.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.02.2014
• Subjects: 3' Untranslated regions; 4-Aminopyridine - analogs & derivatives; 4-Aminopyridine - therapeutic use; Aged; Base Sequence; congenital myasthenic syndromes; DNA Mutational Analysis; Electromyography; Epigenetics; Exome - genetics; Female; Genetic disorders; GFPT1; Glutamine-Fructose-6-Phosphate Transaminase (Isomerizing) - genetics; Glycosylation; High-Throughput Nucleotide Sequencing; Humans; limb-girdle myasthenia; Molecular Sequence Data; Mutagenesis; Myasthenia Gravis - drug therapy; Myasthenia Gravis - genetics; Myasthenia Gravis - pathology; Myasthenic Syndromes, Congenital - drug therapy; Myasthenic Syndromes, Congenital - genetics; Myasthenic Syndromes, Congenital - pathology; Neostigmine - therapeutic use; neuromuscular junction; Neuromuscular Junction - ultrastructure; Pedigree; Reverse Transcriptase Polymerase Chain Reaction; Studies; whole-exome sequencing; congenital myasthenic syndromes; neuromuscular junction; whole-exome sequencing; limb-girdle myasthenia; GFPT1
• ISSN: 0009-9163; 1399-0004
• DOI: 10.1111/cge.12118

The term ‘limb‐girdle myasthenia’ (LGM) was first used to describe three siblings with proximal limb weakness without oculobulbar involvement, but with EMG decrement and responsiveness to anticholinesterase medication. We report here that exome sequencing in the proband of this family revealed several sequence variations in genes linked to proximal limb weakness. However, the only mutations that cosegregated with disease were an intronic IVS7‐8A>G mutation and the previously reported 3′‐UTR c.*22C>A mutation in GFPT1, a gene linked to LGM. A minigene assay showed that IVS7‐8A>G activates an alternative splice acceptor that results in retention of the last seven nucleotides of intron 7 and a frameshift leading to a termination codon 13 nucleotides downstream from the new splice site. An anconeus muscle biopsy revealed mild reduction of the axon terminal size and postsynaptic fold simplification. The amplitudes of miniature endplate potentials and quantal release were also diminished. The DNA of the mildly affected father of the proband showed only the intronic mutation along with sequence variations in other genes potentially relevant to LGM. Thus, this study performed in the family originally described with LGM showed two GFPT1 untranslated mutations, which may cause disease by reducing GFPT1 expression and ultimately impairing protein glycosylation.