Rett Syndrome — a severe neurodevelopmental condition caused by MECP2 loss-of-function, now with FDA-approved treatment that makes definitive molecular diagnosis directly therapeutic.
Whole genome sequencing reads the complete MECP2 gene, identifying all variant types — including large deletions and complex rearrangements that explain 5-10% of clinically diagnosed Rett syndrome cases testing MECP2-negative on standard panels.
Rett Syndrome
Rett syndrome (RTT) is a severe X-linked dominant neurodevelopmental disorder caused by de novo pathogenic variants in the MECP2 (methyl-CpG binding protein 2) gene on chromosome Xq28. Classic RTT occurs almost exclusively in females and affects approximately 1 in 10,000-15,000 live female births. After a period of apparently normal development during the first 6-18 months of life, affected girls undergo a characteristic regression phase with loss of acquired purposeful hand skills and spoken language, development of stereotypic hand movements (wringing, clapping, tapping), gait abnormalities, and acquired microcephaly. The regression is followed by a prolonged plateau phase during which some skills may partially recover.
MECP2 encodes a chromatin-associated protein that functions as a transcriptional regulator, binding methylated CpG dinucleotides and modulating gene expression across the genome. MECP2 pathogenic variants disrupt this global transcriptional regulation, affecting neuronal maturation and synaptic function. Over 95% of classic Rett syndrome cases are caused by de novo MECP2 pathogenic variants — eight recurrent variants (R106W, R133C, T158M, R168X, R255X, R270X, R294X, R306C) account for approximately 65-70% of all cases. Large deletions spanning one or more exons account for approximately 8-10% of cases and require copy number variant analysis for detection. Approximately 3-5% of patients meeting revised clinical criteria for RTT are MECP2-negative, which may reflect variants in other genes (CDKL5, FOXG1) or MECP2 variants in regions not captured by standard sequencing.
The treatment landscape for Rett syndrome has been transformed by the 2023 FDA approval of trofinetide (Daybue) — the first drug specifically approved for RTT. Trofinetide is an analog of the IGF-1 tripeptide glycine-proline-glutamate (GPE) that targets neuroinflammation and synaptic dysfunction. Additional therapies in clinical development include gene replacement therapy using AAV vectors, MECP2 reactivation approaches on the inactive X chromosome in females, and downstream pathway modulators. Each of these therapeutic strategies requires a confirmed MECP2 molecular diagnosis — making definitive genotyping not merely diagnostic but directly treatment-enabling.
Atypical Rett variants — including the preserved speech variant, early seizure variant (CDKL5), and congenital variant (FOXG1) — extend the clinical spectrum. Whole genome sequencing evaluates MECP2, CDKL5, and FOXG1 simultaneously.
Standard MECP2 sequencing detects point mutations in coding exons. Large deletions, complex rearrangements, and deep intronic variants require genome-level analysis to detect — these account for 10-15% of genetically confirmed RTT.
Large MECP2 deletions account for 8-10% of Rett syndrome and require dedicated copy number analysis
Approximately 8-10% of patients with classic Rett syndrome have large MECP2 deletions — spanning part of one exon to the entire gene — rather than point mutations. Standard Sanger sequencing of MECP2 coding exons, still used as first-line testing in some settings, does not detect deletions. Even NGS-based gene panels may have limited sensitivity for large deletions depending on the analytical pipeline. Whole genome sequencing provides simultaneous point mutation detection and copy number variant analysis across the entire MECP2 locus, resolving both common recurrent mutations and large structural variants in a single test.
The first approved treatment makes molecular diagnosis directly therapeutic — not just informational
Before trofinetide's FDA approval in 2023, MECP2 genotyping primarily served diagnostic and prognostic functions. Now, a confirmed MECP2 pathogenic variant is a prerequisite for initiating the only FDA-approved treatment specifically for Rett syndrome. This changes the clinical calculus of molecular testing: a negative MECP2 result on a standard panel — particularly one that did not evaluate large deletions or deep intronic variants — is no longer simply a diagnostic endpoint but a potential barrier to treatment access. Comprehensive MECP2 analysis that resolves all variant types ensures that no patient with a treatable molecular diagnosis is denied therapy because of a testing gap.
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A patient had spent decades in the UK healthcare system without a diagnosis. Dante data, accepted by NHS clinical teams at Queen Elizabeth University Hospital Glasgow, identified Noonan Syndrome and a RUNX1 leukemia-associated variant that had gone undetected. After 40 years, they finally had an answer.
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Common questions about whole genome sequencing.
What is the difference between whole genome sequencing and a targeted genetic test?
Targeted genetic tests — including standard hereditary cancer panels — read a pre-defined list of known variants in a specific set of genes. They are designed to find what they already know to look for. Whole genome sequencing reads your entire genome: all 6 billion base pairs, every gene, every region between genes. A Mayo Clinic study published in JAMA Oncology found that standard testing guidelines missed more than half of patients with inherited cancer mutations. Genome Test does not have a fixed list.
What will I receive when my results are ready?
Your Dante Genome delivers 200+ physician-ready reports organized by clinical category — hereditary cancer, cardiac conditions, rare diseases, pharmacogenomics, carrier status, and more. Reports are delivered to your secure Genome Manager and are formatted for direct clinical use. Your genome data is permanently retained and re-analyzed automatically as science advances.
What happens if a clinically significant variant is found?
If a pathogenic or likely-pathogenic variant is identified, it will be clearly flagged in your physician-ready report with clinical context, published evidence, and recommended next steps. We recommend sharing any clinically significant finding with your physician or a genetic counselor, who can guide decisions about surveillance, risk reduction, or cascade testing for family members.
How is this different from a consumer DNA test like 23andMe or AncestryDNA?
Consumer DNA tests use genotyping chips that read less than 0.1% of your genome — a tiny pre-selected set of common variants. They are optimized for ancestry and population-level traits, not clinical genetic findings. The Dante Genome Test sequences 100% of your genome at 30X coverage, the same standard used in clinical diagnostic settings. The two tests are not comparable in scope, methodology, or clinical utility.
How long does it take to get results, and how are they delivered?
Your collection kit ships within 48 hours of ordering. Once your sample arrives at our CLIA-certified laboratory, sequencing and analysis takes 6–8 weeks. Results are delivered securely to your Genome Manager, where you can access your reports, share them with your physician, and receive automatic updates as new findings are validated against your genome.
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