G.P.42: Mild clinical phenotype in two siblings carrying myotubular myopathy

Abstract

Among congenital myopathies, X-linked recessive myotubular myopathy is one of the most severe forms. The majority of patients present with severe hypotonia at birth and respiratory insufficiency and most of them die in the first year of life. However, there is phenotypic variability. Here, we report two patients carrying a missense mutation in MTM1 gene and presenting with a mild phenotype. The first boy was born on time with respiratory insufficiency and generalised hypotonia. He required ventilator support for 3 weeks, and then he went home without specific care. He walked at 11 month. He is now 24 and can run, practices canoe and works as a salesperson. At the exam, he presents mild proximal weakness and restrictive syndrome with no ventilator support (VC = 57%). His brother was born on time with respiratory insufficiency and hypotonia. He stayed in intensive care unit for 6 weeks, then went home without support. He walked at 18 months. Respiratory involvement was more severe than his brother with a severe restrictive syndrome (VC = 33%) but the young man refused non-invasive night-time ventilation proposed at the age of 16. Being now 20, he can walk without any difficulty but cannot run. Clinical exam reveals a mild proximal weakness. Both patients underwent muscle biopsy showing the typical centronuclear myopathy pattern. Molecular analysis of MTM1gene found a hemizygous mutation c.575A>G in exon 8 leading to a missense p.Tyr192Cys. A decreased level of the MTM1 protein was observed by western blot analysis on lymphoblasts. To date, about 300 different mutations in around 540 families have been identified in MTM1 gene, including point mutations, insertions and small and large deletions and duplication. Around 70% of patients presenting with a mild or moderate phenotype carries a missense mutation, which is also the most frequent type of mutation. Another patient has been described with the same mutation (c.575A>G) and presented also with a mild phenotype.