Is Rett Syndrome Hereditary? What Parents Need to Know

When a child is diagnosed with Rett syndrome and parents learn it is caused by a genetic mutation, it is completely natural to ask: Did I pass this on to my child? The short answer is almost always no. While Rett syndrome is a genetic condition, over 99% of cases are caused by a de novo (spontaneous) mutation ^[1][2]. This means the genetic change happened randomly and was not inherited from either parent.

Understanding De Novo Mutations

Rett syndrome is primarily caused by mutations in the MECP2 gene ^[3][4]. In the vast majority of children with Rett syndrome, this mutation is a brand-new, spontaneous event that occurred either in the sperm or the egg before conception, or very shortly after fertilization ^[1][2]. While many of these spontaneous mutations happen on the X chromosome that came from the father, it is simply a random biological accident ^[5][6].

Because the mutation is spontaneous, there is nothing a parent did before or during pregnancy to cause it, and it could not have been prevented. For over 99% of families, neither parent carries the MECP2 mutation in their own DNA.

Rare Cases of Inherited Rett Syndrome

In extremely rare instances (less than 1% of cases), an MECP2 mutation can be inherited. When this happens, it is usually passed down from a mother who is an “asymptomatic carrier” ^[7].

The MECP2 gene is located on the X chromosome ^[4][3]. Because females have two X chromosomes, their bodies naturally “turn off” or inactivate one of them in every cell. In some women who carry an MECP2 mutation, a process called skewed X-inactivation causes the body to turn off the X chromosome with the mutation in almost all of their cells ^[7][3]. This effectively protects the mother from having any symptoms of Rett syndrome, but she can still pass the mutated gene to her children.

If an asymptomatic carrier mother passes the mutated gene to a daughter, the daughter will likely develop Rett syndrome. If she passes it to a son (a 50% chance for each male pregnancy), it typically leads to a very severe and often fatal neurological condition, because males only have one X chromosome and do not have a “healthy” second X chromosome to rely on.

What This Means for Siblings and Future Pregnancies

Because these rare inherited cases exist, doctors highly recommend that both parents (especially mothers) undergo genetic testing after a child is diagnosed ^[8]. This testing looks for the exact same MECP2 mutation found in the child.

If the parents’ blood tests come back negative (which is overwhelmingly the most common result), it confirms the mutation was de novo. Even with a confirmed de novo mutation, there remains a very tiny risk (usually less than 1%) of having another child with Rett syndrome ^[2][6]. This is due to germline mosaicism, a rare situation where a parent carries the mutation only in a small percentage of their reproductive cells (eggs or sperm) but nowhere else in their body.

If a parent is found to be a carrier, a genetic counselor will discuss the risks for future pregnancies. Crucially, they will also advise whether any other existing children in the family need to be tested to see if they might also be asymptomatic carriers.

A genetic counselor can help you understand your child’s specific test results, arrange testing for you and your partner, and help lift the unnecessary burden of guilt that so many parents carry.