Rett syndrome is a genetic disorder, caused by mutations in a gene called MECP2 on the X chromosome. In 99.5% of cases, the mutation is not inherited from either parent. Instead, it occurs spontaneously (called a “de novo” mutation) during the formation of sperm or egg cells or very early in embryonic development. This means that even though Rett syndrome is genetic in origin, most families have no prior history of the condition.
The MECP2 Gene and What Goes Wrong
The MECP2 gene provides instructions for making a protein called MeCP2, which acts as a master regulator during brain development. This protein attaches to specific spots on DNA and controls whether other genes are switched on or off. It plays a particularly important role in how neurons mature and respond to stimulation, influencing the activity of genes involved in brain growth and signaling.
When MECP2 is mutated, the resulting protein either doesn’t work properly or isn’t produced in the right amounts. Some mutations change a single building block in the protein, reducing its ability to bind to DNA. Others cut the protein short, leaving it incomplete. In either case, the brain’s neurons can’t properly regulate the network of genes they depend on for normal function. This leads to the progressive neurological symptoms that define Rett syndrome, including loss of hand skills, loss of spoken language, gait abnormalities, and repetitive hand movements.
Why It Almost Always Affects Girls
Rett syndrome affects roughly 7 in every 100,000 females, and the reason it overwhelmingly appears in girls comes down to the X chromosome. Girls have two X chromosomes, while boys typically have one X and one Y. In each cell of a girl’s body, one X chromosome is randomly switched off. This means that in some cells, the copy carrying the MECP2 mutation is active, while in others, the healthy copy is active. That mix of healthy and affected cells allows girls to survive, though it also explains why severity varies so widely from person to person.
Boys with only one X chromosome don’t have a backup copy. When that single MECP2 gene is mutated, every cell in the body is affected. As a result, most males with the mutation are stillborn or do not survive past infancy. The rare exceptions include boys who have a milder mutation, boys with Klinefelter syndrome (who carry two X chromosomes plus a Y), or boys with mosaicism, where only some cells carry the mutation.
How X-Chromosome Inactivation Shapes Severity
The random silencing of one X chromosome in each cell is a major reason why two girls with the exact same MECP2 mutation can have very different symptoms. If, by chance, a higher proportion of cells have silenced the X carrying the mutation (leaving the healthy copy active), symptoms tend to be milder. If more cells are running on the mutated copy, symptoms are more severe.
Recent research has added nuance to this picture. The relationship between X-chromosome inactivation patterns and symptom severity appears to depend on the specific type of MECP2 mutation involved. For mutations already associated with more severe disease, the inactivation pattern has a clearer influence on outcomes. For milder mutations, the pattern matters less. Age also plays a role, as clinical severity tends to increase over time regardless of inactivation patterns.
Genes Behind Atypical Forms
Classic Rett syndrome accounts for the majority of cases and is tied to MECP2. But atypical forms exist, and they involve mutations in different genes. These variants are classified by their distinct clinical features:
- Early seizure variant: Linked to mutations in the CDKL5 gene. Children with this form develop seizures earlier than in classic Rett syndrome.
- Congenital variant: Associated with mutations in the FOXG1 gene, the rarest of the three. Children show delayed development from birth, severe intellectual disability, absence of language, and structural brain differences.
- Preserved speech variant: Also linked to MECP2, but children retain some ability to speak.
All three genes play roles in brain development, and mutations in any of them lead to overlapping but distinguishable patterns of symptoms. FOXG1 variants remain the least studied due to their rarity.
Recurrence Risk for Families
Because the mutation arises spontaneously in nearly all cases, parents of a child with Rett syndrome face a recurrence risk of less than 1% for future pregnancies. Research on sporadic cases has found that the new mutation almost exclusively originates on the paternal X chromosome, meaning it typically arises in the father’s sperm cells rather than the mother’s eggs.
Rare familial cases do exist, where a mother carries the MECP2 mutation but is unaffected or only mildly affected due to favorable X-chromosome inactivation. In those situations, she can pass the mutation to multiple children. Genetic testing of both parents after a child is diagnosed can clarify whether the family falls into this small category.
How Rett Syndrome Is Diagnosed
Despite being a genetic condition, Rett syndrome is diagnosed primarily through clinical observation, not genetic testing alone. The revised diagnostic criteria require a period of developmental regression followed by stabilization, plus a specific combination of core features: loss of purposeful hand skills, loss of spoken language, repetitive hand movements, and gait problems. For a classic diagnosis, all four must be present. Atypical Rett syndrome requires at least two of these core features plus five of eleven supportive signs, which include breathing irregularities, scoliosis, teeth grinding, sleep disturbances, and intense eye communication.
An important clinical point: finding an MECP2 mutation is neither necessary nor sufficient for a Rett syndrome diagnosis. Some people carry MECP2 mutations without developing the syndrome, and a small number of clinically diagnosed cases have no identifiable MECP2 mutation. The diagnosis always rests on the pattern of developmental regression and characteristic symptoms, with genetic testing serving as confirmation.
Gene Therapy in Development
Because Rett syndrome traces back to a single gene, it is a strong candidate for gene therapy. A treatment called NGN-401, currently in a Phase 3 clinical trial, delivers a working copy of the MECP2 gene directly into the brain’s fluid-filled spaces using a modified virus as a carrier. The therapy is designed to produce therapeutic levels of the MeCP2 protein while avoiding overproduction, which can itself cause neurological problems (MECP2 duplication syndrome). It is given as a single treatment under general anesthesia, and participants in the trial are being followed for three years after treatment, with long-term monitoring planned for up to twelve years. The trial is actively recruiting as of 2025.