Rett syndrome is a genetic disorder characterized by an early phase of normal cognitive and motor development until ages 6 to 18 months, followed by a regression of learned motor and verbal skills. It affects girls almost exclusively.
The genetics of Rett syndrome
Rett syndrome is caused by a mutation in the MECP2 gene that is located on the X chromosome. This gene encodes for a protein called methyl-CpG binding protein 2 (MeCP2). MeCP2 is involved in regulating the production of several other proteins in brain cells. Mutations in the MECP2 gene, therefore, affect how some of the cells in the brain function and communicate.
How gene therapy for Rett syndrome works
Gene therapy is an experimental approach that can be used to provide a healthy copy of a mutated gene to a patient’s body.
Scientists are researching ways to deliver the correct versions of the MECP2 gene into the nucleus of cells in the brain. In order to do so, a vector or carrier is needed. This vector is usually a virus that has been modified to keep it from causing disease. A copy of the MECP2 gene is inserted into the virus, which is then  injected directly into the nervous system or bloodstream.
AVXS-201 is a gene therapy being developed by AveXis for Rett syndrome. The treatment uses a virus called AAV9 to carry the healthy MECP2 gene into the central nervous system. AAV9 is able to cross the blood-brain barrier and reach nerve cells to deliver the gene therapy.
Novartis, which purchased AveXis in May 2018, plans to submit an investigational new drug application to the U.S. Food and Drug Administration (FDA) for AVXS-201 with an estimated filing date of 2022 for approval. If the FDA accepts the new drug application, participant recruitment for clinical trials testing AVXS-201 could start in 2020.
It is important to note that gene therapy requires a careful balance of protein expression. In Rett syndrome, there is not enough correct MeCP2 protein being produced, but if the therapy leads to the production of too much of the protein it can lead to another condition known as MECP2 duplication syndrome, which also can cause severe disability.
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