Rett syndrome

Rett syndrome is a disorder of brain development that occurs almost exclusively in girls. After 6 to 18 months of apparently normal development, girls with the classic form of Rett syndrome develop severe problems with language and communication, learning, coordination, and other brain functions. Early in childhood, affected girls lose purposeful use of their hands and begin making repeated hand wringing, washing, or clapping motions. They tend to grow more slowly than other children and have a small head size (microcephaly). Other signs and symptoms can include breathing abnormalities, seizures, an abnormal curvature of the spine (scoliosis), and sleep disturbances.

Researchers have described several variants of Rett syndrome with overlapping signs and symptoms. The atypical forms of this disorder range from a mild type, in which speech is preserved, to a very severe type that has no period of normal development. A form of Rett syndrome called the early-onset seizure variant has most of the characteristic features of classic Rett syndrome, but also causes seizures that begin in infancy.

This condition affects an estimated 1 in 10,000-22,000 females.

Most cases of classic Rett syndrome are caused by mutations in the MECP2 gene. This gene provides instructions for making a protein (MeCP2) that is critical for normal brain development. The MeCP2 protein likely plays a role in forming connections (synapses) between nerve cells. Researchers believe that this protein has several functions, including regulating other genes in the brain by switching them off when they are not needed. The MeCP2 protein may also control the production of different versions of certain proteins in nerve cells. Although mutations in the MECP2 gene disrupt the normal function of nerve cells, it is unclear how these mutations lead to the signs and symptoms of Rett syndrome.

Males with mutations in the MECP2 gene often die before birth or in infancy. A small number of males with a MECP2 mutation, however, have developed signs and symptoms similar to those of classic Rett syndrome. Some of these boys have an extra X chromosome in many or all of the body's cells. The extra X chromosome contains a normal copy of the MECP2 gene, which produces enough of the MeCP2 protein for the boys to survive. Other males with features of Rett syndrome have mutations in the MECP2 gene that occur after conception and are present in only a fraction of the body's cells. In rare cases, researchers have discovered that the MECP2 gene is abnormally duplicated in boys with intellectual disability and some developmental problems characteristic of Rett syndrome.

Mutations in the CDKL5 gene cause an atypical form of Rett syndrome in females called the early-onset seizure variant. The CDKL5 gene provides instructions for making a protein that appears to be essential for normal brain development. Although the function of this protein is unknown, it may play a role in regulating the activity of other genes. Researchers are working to determine how mutations in the CDKL5 gene lead to seizures and the features of Rett syndrome in affected girls.

More than 99 percent of classic Rett syndrome cases occur in people with no history of the disorder in their family. Many of these cases result from new mutations in the MECP2 gene.

A few families have been described with more than one affected family member. These cases helped researchers determine that Rett syndrome has an X-linked dominant pattern of inheritance. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. The inheritance is dominant if one copy of the altered gene in each cell is sufficient to cause the condition.