Rett syndrome is a rare neurological disorder affecting mainly females and very few males. It is present from conception and usually remains undetected until major regression occurs at around one year of age, when children may lose acquired skills and become withdrawn. Genetic but largely not inherited, Rett syndrome is usually caused by a fault on a gene called MECP2 which is found on the X chromosome. People with Rett syndrome have profound and multiple physical and communication disabilities and are totally reliant on others for support throughout their lives.
Rett syndrome was first described in 1966 by the Austrian doctor, Andreas Rett. It could occur in any family and affects approximately 1 in 12,000 girls born each year.
Almost all cases of Rett syndrome are caused by a mutation (change in the DNA) in the MECP2 gene, which is found on the X chromosome (one of the sex chromosomes). This gene contains instructions to make a particular protein (MeCP2) that is vital for brain development. The gene abnormality prevents nerve cells in the brain from working properly. Almost all people with Rett syndrome have no history of the disorder in their family, and the mutation has happened spontaneously.
Why is Rett syndrome almost always seen in girls?
Each person normally has one pair of sex chromosomes in each cell – females have two X chromosomes and males have one X and one Y.
Rett syndrome results when half the cells in the child’s brain and spinal cord are normal and half are abnormal (the child survives, but has a severe disability). This happens when the child has one X chromosome with the severe MECP2 abnormality and one X chromosome with the normal version of the gene. The child is almost always female as males don’t have a second X chromosome.
Rare exceptions of Rett syndrome in males occur when, for example, the male is born with an extra X chromosome so he has three sex chromosomes (XXY; known as Klinefelter’s syndrome), or when the change is found in only some of the boy’s cells (known as mosaicism).
Males who carry the abnormal MECP2 gene on their only X chromosome will usually develop a serious condition known as infantile encephalopathy and will not often survive beyond two years.
Males with less severe changes in their MECP2 gene can live a long life and are affected with learning difficulties but usually less severely than in Rett syndrome. Females who carry such (milder) changes on one of their two X chromosomes will often be perfectly healthy.
You can find more information about Rett syndrome from the NHS Choices website.