Frequency and Phenotype of Myotubular Myopathy Amongst Danish Patients with Congenital Myopathy Older than 5 Years

Background: Centronuclear myopathy (CNM) is one of four main subtypes of congenital myopathy. X-linked myotubular myopathy (XLMTM) is considered one of the most severe forms, but survivors past infancy have been described. However, detailed information on XLMTM phenotypes in patients who survive infancy is scarce.

Objective: The aim of the study was to report the genetic findings in patients with a predominant centronuclear finding on muscle biopsy and describe the prevalence, phenotypes and the course of the disease in patients with XLMTM in a Danish cohort of patients with congenital myopathies older than five years.

Methods: Ninety-four out of 119 invited patients older than five years were included in the study and assessed by muscle tests, functional tests, muscle biopsy, plasma creatine kinase levels and genetic testing. Genes related to CNM were sequenced in patients who had centronuclear findings on muscle histology. In patients with MTM1 mutations, medical records from local hospitals were reviewed to obtain information on birth history and course of disease.

Results: Sixteen of 94 patients had CNM on muscle biopsy; three male patients, aged 14-25 years, carried a pathogenic MTM1 mutation, six patients carried a pathogenic DNM2 mutation and two carried pathogenic RYR1 mutations. The mutations have all been described before to cause CNM. The MTM phenotypes ranged from severe (classical) to mild; one patient had always been non-ambulant, one had lost ambulation, and one was still ambulant at 25 years.

Conclusions: Our findings show that CNM caused by DNM2 mutations is the most common form of CNM in Danish patients older than 5 years, but XLMTM is not negligible even past age 5 years, and the phenotype may be much milder than generally described - also in patients with the classically described infantile form of the disease.