Genetic Testing and Prenatal Diagnosis

Genetic testing can be used to aid in or confirm a patient’s diagnosis of Rett syndrome, a progressive neurodevelopmental condition that affects females almost exclusively. It also can be used in pregnancy screenings to enable a prenatal diagnosis.

Rett is mostly caused by mutations in the MECP2 gene situated on the X chromosome. This gene carries the information necessary to make MeCP2 protein. A healthy MeCP2 helps to maintain communication between nerve cells and regulates the activity of several genes in the brain. Mutations in the MECP2 gene can result in a decreased production of the protein, or production of dysfunctional protein, which affects the relay of messages from the brain to the rest of the body.

Oftentimes, a Rett diagnosis is made based on clinical symptoms, and on the medical and family history of the patient. Genetic testing is performed to detect MECP2 mutations and can be used to confirm a diagnosis. It also can be used as a screening tool during pregnancy.

A simple blood test is used for genetic testing and no hospital stay is required.

Genetic testing for diagnosis

A mutation associated with Rett only results in diagnosis if the patient fits the clinical criteria.

About 80-97% of people with a clinical diagnosis of Rett have a disease-causing change in the MECP2 gene. Some cases of atypical Rett syndrome may be caused by mutations in other genes such as the CDKL5 and FOXG1. A genetic test also can be used to detect these mutations.

Genetic testing for screening

The genetic mutation that causes Rett is rarely inherited. Instead, in the majority of reported cases, changes in the MECP2 gene occur de novo — that is, the mutation is spontaneous in the fetus.

Less than 1% of Rett cases are caused by a mutation inherited from the parents; some female relatives of an individual affected by Rett syndrome may not show any clinical symptoms even if they have the MECP2 mutation — they are asymptomatic carriers. Therefore, once a Rett diagnosis has been confirmed, female family members can undergo genetic testing to determine whether they have the same mutation.

Screening is more relevant in females because males, as they have only one copy of the X chromosome, typically have severe complications that do not go undiagnosed.

Prenatal screening

Prenatal screening is intended to determine whether the fetus has any genetic abnormality.

Several types of invasive and non-invasive prenatal diagnostic tests are available. Two commonly used methods are amniocentesis and chorionic villus sampling (CVS). Amniocentesis is the process of drawing a sample of amniotic fluid, or the fluid surrounding the fetus, from the amniotic sac of the pregnant mother, usually between 15 to 20 weeks of pregnancy. CVS involves taking a small sample of the placenta either through the mother’s cervix or abdomen. It can be performed as early as the 11th week of pregnancy.

The two methods carry small risks, which include miscarriage, infection or injury to the fetus, leaking of amniotic fluid, and bleeding from the vagina. They are typically done when women are 35 or older by the time the baby is due, when another screening assessment suggested a problem, or when either biological parent is a known carrier of a genetic disorder.

Prenatal screening is especially useful in families with a known history of Rett.

Overall, the results of prenatal screening help in making informed decisions during pregnancy and after the baby is born.

 

Last updated: Jan. 17, 2022, by Teresa Carvalho MS

 


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