Myotubular Myopathy 1 (MTM1)
Myotubular myopathy 1 is a pan-ethnic disorder caused by pathogenic variants in the gene MTM1. It is inherited in an X-linked manner, and therefore typically only males are affected. Clinical symptoms can vary in severity. In the most severe form, which is also the most common, onset is during the prenatal period. Infants are born with severe weakness, require ventilators for respiration and gastrostomy tubes for feeding, and do not learn to crawl or walk. Most die in infancy. In the moderate form, motor milestones are delayed and patients may occasionally require ventilation for breathing assistance. Muscle weakness in the face may result in myopathic facial features. Life expectancy may be normal. In the mild form, patients learn to walk without significant delay, and following infancy may not require any respiratory ventilation. Patients may be at risk for medical complications, especially during times of illness. Life expectancy may be normal. In general, null variants result in a more severe phenotype, and variants with residual protein function (including splice site changes) result in a milder phenotype. However, there can be phenotypic variability for the same pathogenic variant, even within a family, and therefore it may not be possible to accurately predict the severity of disease based on genotype.
For information about carrier frequency and residual risk, please see the residual risk table.