Delineation of molecular characteristics of congenital myasthenic syndromes in Indian families and review of literature

Abstract

Congenital myasthenic syndromes (CMS) are rare, heterogeneous, and often treatable genetic disorders depending on the underlying molecular defect. We performed a detailed clinical evaluation of seven patients from five unrelated families. Exome sequencing was performed on five index patients. Clinically significant variants were identified in four CMS disease-causing genes: COLQ (3/7), CHRNE (2/7), DOK7 (1/7), and RAPSN (1/7). We identified two novel variants, c.930_933delCATG in DOK7 and c.1016_1032 + 2dup in CHRNE. A common pathogenic variant, c.955-2A>C, has been identified in COLQ-related CMS patients. Homozygosity mapping of this COLQ variant in patients from two unrelated families revealed that it was located in a common homozygous region of 3.2 Mb on chromosome 3 and was likely to be inherited from a common ancestor. Patients with COLQ variants had generalized muscle weakness, those with DOK7 and RAPSN variants had limb-girdle weakness, and those with CHRNE variants had predominant ocular weakness. Patients with COLQ and DOK7 variants showed improvement with salbutamol and CHRNE with pyridostigmine therapy. This study expands the mutational spectrum and adds a small but significant cohort of CMS patients from India. We also reviewed the literature to identify genetic subtypes of CMS in India.