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Causes
By Mayo Clinic staffMost cases of Rett syndrome are caused by mutations in the MECP2 gene, which is involved in making a protein essential for normal brain development. The MECP2 gene is situated on the X chromosome — one of two chromosomes that determine a person's sex.
Girls have two X chromosomes in every cell. Because a cell needs only one working copy of the gene, each cell automatically inactivates one of its two X chromosomes. The more cells that have the mutated gene inactivated, the less severe the symptoms of Rett syndrome will be.
About 20 percent of girls with Rett syndrome don't have a typical MECP2 mutation. Researchers think that these children may have mutations in a different part of the gene or in genes that haven't yet been identified.
Rett syndrome in boys
The story is very different for most boys. Unlike girls, boys normally have one Y and one X chromosome in each cell. Because the single X chromosome can't be inactivated, boys who have the genetic mutation that causes Rett syndrome aren't protected from the devastating effects of the disease. Most of them die before birth or in early infancy.
A few boys have an extra X chromosome in many or all of their cells and may survive if they have an MECP2 mutation. A very small number of boys have mutations in the MECP2 gene in a few cells. They, too, are likely to live to adulthood.
What causes the genetic mutation?
Rett syndrome is a genetic disorder, but in only a few cases is it inherited. Instead, the genetic mutation that causes the disease occurs randomly and spontaneously — often when sperm are formed and occasionally after conception.
