Congenital mirror movement disorder

Description

Congenital mirror movement disorder is a condition in which intentional movements of one side of the body are mirrored by involuntary movements of the other side. For example, when an affected individual makes a fist with the right hand, the left hand makes a similar movement. The mirror movements in people with this disorder primarily affect the arms, and the hands and fingers are always involved. This pattern of movements is present from infancy or early childhood and usually persists throughout a person's life.

People with congenital mirror movement disorder can have some difficulty with certain daily activities, particularly with those that require different movements in each hand, such as typing on a keyboard. They may experience discomfort or pain in their arms during prolonged use of the hands.

The extent of the mirror movements in people with congenital mirror movement disorder can vary, even within the same family. In most cases, the involuntary movements are noticeable but less pronounced than the corresponding voluntary movements. The extent of the movements typically stay the same throughout the lifetime of an affected individual.

In general, mirror movements are the only feature associated with this condition. In some cases, however, people with congenital mirror movement disorder can have partial or complete absence (agenesis) of the tissue that connects the left and right halves of the brain (corpus callosum).

Mirror movements can also occur in people who do not have congenital mirror movement disorder. Mild mirror movements are common during the normal development of young children, and they typically disappear before age 7. They can also develop later in life in people with neurodegenerative disorders such as Parkinson's disease. Mirror movements may also be present in people with certain conditions that have a wider range of signs and symptoms (syndromes).

Frequency

Congenital mirror movement disorder is a very rare disorder. Its prevalence is thought to be less than 1 in 1 million individuals. Researchers suggest that it may be more common as some mildly affected individuals may never be diagnosed.

Causes

Congenital mirror movement disorder is caused by variants (also called mutations) in at least three genes: DCC, RAD51, and NTN1.

The DCC gene provides instructions for making a protein called the netrin-1 receptor. This receptor can be attached (bound) to a protein called netrin-1, which is produced from the NTN1 gene. These two proteins fit together like a lock and key. Within the developing nervous system, the binding of netrin-1 to its receptor helps direct the growth of specialized nerve cell extensions called axons. Axons transmit nerve impulses that signal muscle movement. Normally, signals from each half of the brain control movements on the opposite side of the body.

Variants in the DCC or NTN1 gene result in an impaired or missing netrin-1 receptor or netrin-1 protein. A shortage of either of these proteins prevents axons from growing properly during nervous system development. When netrin-1 cannot bind to its receptor, axons cannot develop in ways that would carry movement signals normally. As a result, movement signals from each half of the brain are transmitted to both sides of the body, leading to mirror movements.

The RAD51 gene provides instructions for making a protein that plays role in DNA repair, but it is also likely involved in the development of nervous system functions that control movement. Variants in the RAD51 gene result in an altered or impaired RAD51 protein. It is thought that decreased function of the RAD51 protein affects the development of the early nervous system, resulting in the signs and symptoms of congenital mirror movement disorder.

There are some people with congenital mirror movement disorder who do not have an identified variant in any of these three genes. The cause of the disorder in these individuals is unknown.

Inheritance

Congenital mirror movement disorder is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person has one parent with the altered gene. Some people who have the altered gene never develop congenital mirror movement disorder; this is known as reduced penetrance.

Autosomal dominant inheritance. Credit: U.S. National Library of Medicine.

In very rare cases, this condition may be inherited in an autosomal recessive pattern, which means both copies of the gene in each cell must have a variant to cause the disorder. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition.

Autosomal recessive inheritance. Credit: U.S. National Library of Medicine.

Other Names for This Condition

• CMM
• Congenital mirror movements
• Familial congenital controlateral synkinesia
• Familial congenital mirror movements
• Hereditary congenital controlateral synkinesia
• Hereditary congenital mirror movements
• Isolated congenital controlateral synkinesia
• Isolated congenital mirror movements
• Mirror movements 1

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