| Summary: | Key Clinical Message Congenital myasthenic syndromes‐5 (CMS5) is a rare autosomal recessive heterogeneous disorder, caused by pathogenic variants in the COLQ that lead to skeletal muscle weakness and abnormal fatigability. The onset is usually from birth to childhood. Disease‐causing variants in the collagen‐like tail subunit are the most explained etiology in synaptic CMS, causing defected acetylcholinesterase. In this study whole‐exome sequencing (WES) was performed in an affected boy with muscle weakness, ophthalmoplegia, and bilateral ptosis and gene expression assay by qRT‐PCR was performed in entire family. A homozygous nonsense variant in the COLQ [NM_005677.4:c.679C>T], (p.Arg227Ter) was identified in the proband. Segregation analysis by Sanger sequencing confirmed the homozygous state in the proband and heterozygous state in his parents and four of the siblings. The mRNA expression level in the proband was 0.02 of a healthy person, and in the carriers were 0.42 of a healthy person. This study presents an Iranian family with two affected children and eight symptomatic carriers with attenuated mRNA expression. This study provides evidence that carriers of the COLQ disease‐causing variants could become symptomatic with some yet unknown pathogenesis mechanism and underscore the importance of further investigations to elucidate this mechanism.
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