Clinical and genetic characterisation of a large Indian congenital myasthenic syndrome cohort

Abstract

Congenital myasthenic syndromes (CMS) are a rare group of inherited disorders caused by gene defects associated with the neuromuscular junction and potentially treatable with commonly available medications such as acetylcholinesterase inhibitors and β2 adrenergic receptor agonists. In this study, we identified and genetically characterized the largest cohort of CMS patients from India to date. Genetic testing of clinically suspected patients evaluated in a South Indian hospital during the period 2014-19 was carried out by standard diagnostic gene panel testing or using a two-step method that included hotspot screening followed by whole-exome sequencing. In total, 156 genetically diagnosed patients (141 families) were characterized and the mutational spectrum and genotype-phenotype correlation described. Overall, 87 males and 69 females were evaluated, with the age of onset ranging from congenital to fourth decade (mean 6.6 ± 9.8 years). The mean age at diagnosis was 19 ± 12.8 (1-56 years), with a mean diagnostic delay of 12.5 ± 9.9 (0-49 years). Disease-causing variants in 17 CMS-associated genes were identified in 132 families (93.6%), while in nine families (6.4%), variants in genes not associated with CMS were found. Overall, postsynaptic defects were most common (62.4%), followed by glycosylation defects (21.3%), synaptic basal lamina genes (4.3%) and presynaptic defects (2.8%). Other genes found to cause neuromuscular junction defects (DES, TEFM) in our cohort accounted for 2.8%. Among the individual CMS genes, the most commonly affected gene was CHRNE (39.4%), followed by DOK7 (14.4%), DPAGT1 (9.8%), GFPT1 (7.6%), MUSK (6.1%), GMPPB (5.3%) and COLQ (4.5%). We identified 22 recurrent variants in this study, out of which eight were found to be geographically specific to the Indian subcontinent. Apart from the known common CHRNE variants p.E443Kfs*64 (11.4%) and DOK7 p.A378Sfs*30 (9.3%), we identified seven novel recurrent variants specific to this cohort, including DPAGT1 p.T380I and DES c.1023+5G>A, for which founder haplotypes are suspected. This study highlights the geographic differences in the frequencies of various causative CMS genes and underlines the increasing significance of glycosylation genes (DPAGT1, GFPT1 and GMPPB) as a cause of neuromuscular junction defects. Myopathy and muscular dystrophy genes such as GMPPB and DES, presenting as gradually progressive limb girdle CMS, expand the phenotypic spectrum. The novel genes MACF1 and TEFM identified in this cohort add to the expanding list of genes with new mechanisms causing neuromuscular junction defects.

Keywords: NGS; congenital myasthenic syndromes; genetics; neuromuscular junction; recurrent mutations.

Figure 1

Molecular diagnostic flow chart for suspected congenital myasthenic syndrome patients. CMS = congenital myasthenic syndrome; NGS = next generation sequencing.

Figure 2

Demographic, clinical features, congenital myasthenic syndrome subtypes and gene frequencies. (A) Geographic distribution by their various Indian states of origin of congenital myasthenic syndrome (CMS) patients reported in this study. (B) Clinical features of CMS patients: percentage of four common symptom groups (limb, ocular, bulbar and respiratory). (C) Pie chart showing frequencies of various CMS subtypes based on location of defect at neuromuscular junction and/or underlying pathomechanism. (D) Frequencies of individual CMS genes identified in the current cohort.

Figure 3

Clinical images of genetically confirmed congenital myasthenic syndrome patients. (A) An 8-year-old male with homozygous CHRNE c.1052C>T; p.P351L showing ptosis, ophthalmoparesis and facial weakness. (B) A 14-year-old female with homozygous COLQ c.1319G>A; p.C440Y presented with history of recurrent apneic spells, ptosis, limb weakness and respiratory distress from birth. She was ventilator-dependent until 13 years of age and showed significant improvement with salbutamol. (C) A 12-year-old male with compound heterozygous RAPSN variants c.-210A>G (5′UTR)/c.1144T>C; p.C382R with dysmorphic facial features, ptosis, open mouth and severe proximal contractures. (D) A 23-year-old male with homozygous GFPT1 c.1103C>T; p.T368I presented with limb girdle weakness and muscle wasting with no ocular or bulbar involvement mimicking the limb girdle muscular dystrophy phenotype. (E) A 14-year-old male with compound heterozygous AGRN c.5302G>A; p.A1768T/c.6057_6060delCGTGinsT p.V2022del presented with easy fatigability from childhood and profound proximal muscle weakness. (F) A 2-year-old female with homozygous VAMP1 c.97C>T (p.R33*) showing severe hypotonia, pectus excavatum, myopathic facies, hyperextensible fingers and high arched palate (3F published previously in Polavarapu et al.).