What Does VUS Mean on a Rett Syndrome Genetic Test?

A “Variant of Uncertain Significance” (VUS) means that the genetic testing laboratory found a change (or variant) in the MECP2 gene, but there is currently not enough scientific evidence to know if this specific change causes Rett syndrome or if it is just a harmless, normal variation ^[1][2]. Receiving a VUS result is common and can be incredibly frustrating, but it does not mean your child’s symptoms are in your head, nor does it rule out a Rett syndrome diagnosis.

Rett Syndrome is a Clinical Diagnosis

The most important thing to know is that Rett syndrome is fundamentally a clinical diagnosis, meaning it is based on the symptoms and signs a doctor observes, rather than solely on a genetic test ^[3].

Doctors use a specific set of symptom guidelines known as the Neul criteria (established in 2010) to diagnose Rett syndrome ^[3]. If your child meets these clinical criteria—such as a period of regression, loss of purposeful hand skills, or the development of repetitive hand movements—they can be diagnosed with classic or atypical Rett syndrome even if their genetic test only shows a VUS, or even if the test is completely normal ^[3][4]. The genetic test is a tool used to support the diagnosis, but the clinical symptoms are what truly define it.

Why Do VUS Results Happen?

We all have thousands of genetic variations that make us unique without causing any health problems. Sometimes, a genetic variant acts like a small “typo” in the DNA rather than a missing chapter, making its impact hard to predict. When a laboratory finds a change in the MECP2 gene that hasn’t been seen often (or ever) in other people with Rett syndrome, they must label it a VUS until more data is available ^[1].

In females, the picture is further complicated by a biological process called X-chromosome inactivation (XCI). Because females have two X chromosomes (and the MECP2 gene is on the X chromosome), the body randomly “turns off” one X chromosome in every cell. Depending on which chromosome is turned off, a female with a genetic change might have severe symptoms, mild symptoms, or no symptoms at all ^[5][6].

(Note: While it is natural to wonder if doctors can just test your child to see which X chromosome is active, testing blood for XCI patterns isn’t always reliable because the pattern in the blood might not match the pattern in the brain ^[7]. For now, this biology mostly explains why interpreting these variants is so difficult for scientists.)

What Are the Next Steps?

A VUS is not a permanent label. Your medical team, usually led by a genetic counselor or a specialized neurologist, will recommend steps to help clarify what this variant means for your child:

• Parental Testing: A genetic counselor will likely coordinate testing for both parents’ DNA to see where the variant came from ^[8]. If the variant is de novo (meaning it is new in your child and neither parent has it), it increases the suspicion that it could be causing their symptoms ^[8]. However, because of X-chromosome inactivation, a healthy mother can sometimes carry and pass on a disease-causing MECP2 variant without showing symptoms herself ^[5].
• Focus on Symptom Management: Regardless of the genetic report, your child’s care should focus on their actual symptoms. Medical management involves a multidisciplinary team to address developmental delays, seizures, or other issues related to suspected Rett syndrome ^[9].
• Access to Targeted Therapies: While day-to-day symptom management remains the same, it is important to know that a VUS might temporarily impact your child’s eligibility for certain clinical trials or FDA-approved targeted therapies, which sometimes require a confirmed “pathogenic” variant. Your doctor can help you navigate these requirements.
• Re-evaluating the Genetics: Because genetic databases are constantly growing, a VUS can be reclassified over time ^[10]. As more people worldwide are tested, scientists may gather enough evidence to reclassify your child’s VUS as either “pathogenic” (disease-causing) or “benign” (harmless) ^[11]. It is highly recommended to check back with your genetic counselor every year ^[10]. You can also ask them about enrolling your child in a patient registry, which helps scientists gather the data needed to resolve these uncertain classifications faster.