Discovered by Austrian doctor Andreas Rett, Rett syndrome is a genetic pervasive developmental disorder in the autism spectrum.

Rett syndrome is the most prevalent cause of serious mental retardation in girls. Incidence rates are thought to be between 1 in 10,000 and 1 in 15,000 live female births.

The disorder is caused by a mutation on the MECP2 gene on the long leg of the X chromosome. Because the disorder is X-linked, boys carrying the trait (with only a single, mutant X chromosome) rarely survive past infancy, and the disorder is commonly observed only in girls.

The protein coded by a normal MECP2 gene binds to methylated DNA and acts as a transcription repressor (i.e., it "turns off" certain genes). It is not known which specific genes are repressed by MECP2, but it is safe to assume that the symptoms of Rett syndrom are due in part to a failure of normal gene repression.

Patients exhibiting symptoms very similar to those of Rett's syndrome have also been characterized by deletions within the CDKL5 gene; these are characterized as atypical Rett syndrome cases.

Rett syndrome is characterized by a period of normal development followed by a rapid deterioration in fine motor skills and spoken language between ages 1-4. Girls with Rett syndrome develop a characteristic abnormal gait when moving and become prone to tremors and repetitive hand-washing or hand-wringing movements. They also exhibit typical autism-like social dysfunction.

After the rapid deterioration, many girls experience some improvement in social behavior and alertness during their school years. Their communication skills improve and they seem happier. Motor problems continue and often become worse after age 10, such that the child may lose the ability to walk.

It is also possible to be a carrier of an MECP2 mutation while experiencing less severe symptoms. Tests can be done at birth to determine if a child has the mutation.

No cure yet exists for Rett syndrome or any other autistic spectrum disorder. Children with Rett syndrome do retain some ability to communicate and understand communication attempts, and thus may benefit from various behavioral therapies used in other autism-spectrum cases. Exposure to age-appropriate toys and other forms of play can be beneficial even if the child does not seem to respond normally.

A mouse model of the disease (an MECP2 knockout) has been created, and further research may lead to improved therapies and treatments.

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