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PubMed This is a summary of 23 peer-reviewed journal articles Updated
Medical Genetics

Biology, Genetics & Diagnostic Criteria

At a Glance

Atypical Rett syndrome encompasses neurodevelopmental conditions caused by genetic mutations, such as changes in the CDKL5, FOXG1, and MECP2 genes. While older clinical criteria required a child to lose skills for diagnosis, doctors now increasingly rely on precise genetic testing to identify these specific variants.

While “classic” Rett syndrome is almost always linked to mutations in the MECP2 gene, atypical Rett syndrome involves a broader range of genetic causes [1][2]. Understanding the biology of these conditions helps explain why your child may have some features of Rett syndrome but not others.

The Biological “Master Switches”

Think of genes like MECP2, CDKL5, and FOXG1 as master switches that control how a child’s brain develops and communicates.

  • MECP2: This gene produces a protein that acts as a vital regulator, telling other genes when to turn on and off [3][4]. Without it, the brain produces improper amounts of proteins (like BDNF, which is vital for nerve cell growth), leading to “static” in brain communication [4][5].
  • CDKL5: This gene creates an enzyme (a kinase) that helps brain cells (neurons) form the right shape and maintain connections (synapses) [6][7]. When CDKL5 is missing, neurons struggle to send signals correctly, which often results in early-onset seizures [8][9].
  • FOXG1: This gene acts as a transcription factor specifically focused on the early development of the forebrain [10]. Mutations here disrupt the very foundation of how the brain is built, often leading to more severe structural differences from birth [10][11].

The Three Major Genetic Subtypes

Under the umbrella of atypical Rett, doctors historically identified three specific “variants” based on symptoms and genetics:

  1. Early-Onset Seizure Variant (Hanefeld Variant): This is now primarily recognized as CDKL5 Deficiency Disorder (CDD) [10]. Unlike classic Rett, where seizures often appear after a period of regression, children with CDD typically experience seizures (infantile spasms) within the first few months of life [12][10].
  2. Congenital Variant (Rolando Variant): This is now often called FOXG1 Syndrome [10]. Children with this variant show developmental delays from the very beginning of life, rather than having a period of “normal” development followed by regression [13][10].
  3. Preserved Speech Variant (Zappella Variant): In this milder form, children may recover some words or the ability to use their hands for purposeful tasks [14]. This is often linked to specific, less-disruptive mutations in the MECP2 gene [2].

How Doctors Make the Diagnosis (Neul Criteria vs. Genetics)

In 2010, experts established clinical rules (the Neul criteria) for diagnosing Atypical Rett [15][16]. Under these rules, a child must first have a period of regression (loss of skills), followed by meeting specific main criteria (like loss of hand skills or language) and supportive criteria (like breathing issues or scoliosis) [14][17][18].

However, there is a catch: Many children with modern genetic diagnoses like CDD or FOXG1 never experience a true period of regression [10]. This contradiction is exactly why the medical field is shifting away from relying solely on these clinical criteria and moving toward precise, gene-specific diagnoses [10].

Look-Alike Conditions

Because many genetic conditions affect brain development, some can look very similar to Rett syndrome. One common “look-alike” is MEF2C Haploinsufficiency Syndrome [19]. Children with this condition also have severe intellectual disability, hand-wringing, and seizures, but it is caused by a different gene and requires distinct management [19][20].

Reading the Genetic Testing Report

When you look at your child’s genetic report, ensure the following elements are present to confirm a complete and accurate picture [21][22]:

  • [ ] Variant Classification: Is it labeled “Pathogenic” (disease-causing) or “Likely Pathogenic”? If your report says “VUS” (Variant of Uncertain Significance), the lab isn’t sure if that specific change causes the condition. Ask your geneticist if testing both parents can help clarify the result [21][22].
  • [ ] Standardized Name: Does it include the HGVS nomenclature (e.g., c.123C>T)? This is the “address” of the mutation on the gene [22].
  • [ ] Zygosity: Does it state “Heterozygous”? For Rett-related genes, this usually means the mutation is on one of the two copies of the gene [23].
  • [ ] Transcript ID: Does it have a code starting with “NM_”? This tells researchers exactly which “version” of the gene they were looking at [22].

Common questions in this guide

What is the difference between classic and atypical Rett syndrome?
Classic Rett syndrome is almost always linked to mutations in the MECP2 gene. Atypical Rett syndrome involves a broader range of genetic causes, such as CDKL5 and FOXG1, which can lead to different symptom patterns like early-onset seizures or developmental delays from birth.
What does a VUS mean on my child's genetic testing report?
VUS stands for Variant of Uncertain Significance. This means the laboratory found a genetic change but does not have enough data to determine if it actually causes the condition or if it is just a harmless variation. Testing the parents can sometimes help clarify these results.
Does a child have to lose skills to be diagnosed with atypical Rett syndrome?
Historically, clinical rules required a period of regression—where a child loses previously acquired skills—to make a diagnosis. However, many children with modern genetic diagnoses like CDKL5 or FOXG1 never experience a true period of regression.
What is the Preserved Speech or Zappella variant?
The Preserved Speech, or Zappella variant, is a milder form of atypical Rett often linked to specific MECP2 mutations. Children with this variant may recover some language skills or retain the ability to use their hands for purposeful tasks.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.According to my child's history, do they meet the formal clinical criteria for Atypical Rett, or was the diagnosis based purely on genetic testing?
  2. 2.Does the genetic report indicate a 'Pathogenic' or 'Likely Pathogenic' variant, or is it a 'Variant of Uncertain Significance' (VUS)?
  3. 3.How does my child's specific mutation (e.g., in CDKL5 or FOXG1) affect their brain's 'wiring' differently than classic Rett?
  4. 4.Should we consider testing for 'look-alike' genes like MEF2C if my child’s current genetic results are unclear?
  5. 5.What is the specific 'transcript ID' or HGVS nomenclature for my child's mutation, and why is that important for future research?

Questions For You

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References

References (23)
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This page provides educational information about atypical Rett syndrome genetics and diagnostic criteria. Always consult a medical geneticist or pediatric neurologist to accurately interpret your child's specific genetic testing results.

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