X-linked myotubular myopathy

X-linked myotubular myopathy is a condition that primarily affects muscles used for movement (skeletal muscles) and occurs almost exclusively in males.

People with this condition have muscle weakness (myopathy) and decreased muscle tone (hypotonia) that are usually evident at birth. These muscle problems impair the development of motor skills such as sitting, standing, and walking. Affected infants may also have difficulties with feeding due to muscle weakness. Individuals with X-linked myotubular myopathy frequently do not have the muscle strength to breathe unassisted and must be supported with a machine to help them breathe (mechanical ventilation). Some individuals need breathing assistance only periodically, typically during sleep, while others require it continuously. Affected individuals may also have weakness in the muscles that control eye movement (ophthalmoplegia), weakness in other muscles of the face, absent reflexes (areflexia), an abnormal curvature of the spine (scoliosis), joint deformities (contractures) of the hips and knees, and a large head with a narrow and elongated face.

Because of their severe breathing problems, individuals with X-linked myotubular myopathy usually survive only into early childhood; however, some people with this condition have lived into adulthood.

X-linked myotubular myopathy is a member of a group of disorders called centronuclear myopathies. In centronuclear myopathies, the nucleus is found at the center of many rod-shaped muscle cells instead of at either end, where it is normally located.

The incidence of X-linked myotubular myopathy is estimated to be 1 in 50,000 newborn males worldwide.

Mutations in the MTM1 gene cause X-linked myotubular myopathy. The MTM1 gene provides instructions for producing an enzyme called myotubularin. Myotubularin is thought to be involved in the development and maintenance of muscle cells.

MTM1 gene mutations probably disrupt myotubularin's role in muscle cell development and maintenance, causing muscle weakness and other signs and symptoms of X-linked myotubular myopathy.

Read more about the MTM1 gene.

This condition is inherited in an X-linked recessive pattern. The gene associated with this condition is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

In X-linked recessive inheritance, a female with one altered copy of the gene in each cell is called a carrier. She can pass on the gene, but generally does not experience signs and symptoms of the disorder. In rare cases, however, carrier females have experienced some muscle weakness associated with X-linked myotubular myopathy.