RETT SYNDROME

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.

MECP2

About this condition

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.

Gene locus: MECP2 (Xq28)

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.

Rett Syndrome — a severe neurodevelopmental condition caused by MECP2 loss-of-function, now with FDA-approved treatment that makes definitive molecular diagnosis directly therapeutic.

Rett Syndrome

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

The first approved treatment makes molecular diagnosis directly therapeutic — not just informational

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