Homozygous Duplication in the CHRNE in a Family with Congenital Myasthenic Syndrome 4C: 18-Year Follow Up

Abstract

Background and objectives: Congenital myasthenic syndromes (CMSs) are rare inherited diseases characterized by muscle weakness and fatigability on exertion resulting from defects in the neuromuscular junctions. Mutations in 32 genes have been reported as the underlying causes of CMS, with mutations in the cholinergic receptor nicotinic epsilon subunit (CHRNE) being the most common cause of the disease. Methodology and Materials: This study investigated a large consanguineous family with multiple individuals suffering from abnormal fatigue and muscle weakness in the ocular and limb regions. Moreover, the affected individuals were followed up for 18 years to observe the clinical course of the disease.

Results: High-quality exome sequencing followed by bidirectional Sanger sequencing revealed a homozygous duplication variant (NM_000080.4: c.1220-8_1227dup) in the splice acceptor site of exon 11 of the CHRNE gene. This variant is predicted to cause frameshift and premature termination (p.Cys410ProfsTer51). Both parents had heterozygous duplication variants with no clinical symptoms. The personalized treatment of the affected individuals resulted in a marked improvement in the clinical symptoms. More than 80% of the disease symptoms in the affected individuals subsided after the use of pyridostigmine and salbutamol (4 mg).

Conclusions: This is the first report of long-term follow up of cases with homozygous insertion (c.1220-8_1227dup) in the CHRNE gene. Furthermore, this report expands the phenotypic symptoms associated with the CHRNE mutation.

Keywords: CHRNE mutation; congenital myasthenic syndrome; genetics; phenotypic spectrum.

Figure 1

A pedigree chart of a five-generation family segregating congenital myasthenic syndromes in an autosomal recessive manner. Affected individuals are represented with black symbols and unaffected individuals with clear symbols.

Figure 2

(A) Sanger sequence analysis of the CHRNE gene. Both parents (IV:3 and IV:4) and four unaffected siblings (V:4, V:7, V:8, and V:9) and one healthy cousin (V:2) carry a heterozygous variant (c.1220-8_1227dup). The affected individuals (V:5), (V:1), and (V:3) have a homozygous duplication variant underlined red. The unaffected individual (V:6) carries a wild-type sequence. Arrowhead shows the point of insertion. Blue line shows a splice acceptor site (B). Inserted mutation occurs in exon 11, and partial alignment of the human CHRNE amino acid in comparison with its orthologues shows a highly conserved region across species.

Figure 3

Protein–protein interaction network analysis of the CHRNE gene was conducted using the STRING database with a confidence score of 0.4. The analysis revealed a strong association between the CHRNE protein and other nicotinic acetylcholine receptor proteins, muscle-specific receptor tyrosine kinase, and neuromuscular synapses.