LAUSR

ALG2 mutations are extremely rare causes of congenital myasthenic syndromes (CMS). The clinical phenotype and treatment response is therefore not well described. We present the case of a baby who immediately after birth presented with pronounced truncal hypotonia, proximal muscle weakness and feeding difficulties. Single fibre electromyography showed neuromuscular transmission failure and salbutamol and ephedrine treatment improved both muscle weakness and neuromuscular transmission. Genetic analysis revealed a likely pathogenic variant c.1040del, p.(Gly347Valfs*27) in exon 2 and a variant of uncertain significance, c.239G>A, p.(Gly80Asp) in exon 1 of the ALG2 gene. Western blot in whole cell lysates of HEK293 cells transfected with p.Gly80Asp, or p.Gly347Valfs*27 expression constructs indicated that p.Gly347Valfs*27 is likely a null allele and p.Gly80Asp is pathogenic through marked reduction of ALG2 expression. This case highlights the utility of functional studies in clarifying variants of unknown significance, in suspected cases of CMS.