Influence of mutation type and location on phenotype in 123 patients with Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder that almost exclusively affects girls. It is caused by mutations in the MECP2 gene that encodes the methyl-CpG-binding protein 2 (MeCP2). In this study we correlated mutation type and location with the severity of the phenotype in 123 girls with RTT. The ability to sit, walk, speak, hand function, head growth, occurrence of epilepsy and a combined severity score were assessed in all girls at 5 years of age and then statistically correlated with the results of the molecular genetic tests. We found that patients who carry either missense mutations or deletions located within the hotspot for deletions, an area between the base pairs (bp) 1030 and 1207 of the MECP2 gene, present with a milder phenotype than other patients. We correlated the location of the mutations with the phenotype and found that all mutations that lead to either a complete or partial truncation of the region coding for the nuclear localisation signal (NLS) are associated with a more severe phenotype than other truncating mutations (p = 0.001). We did not find a significant difference between the patients with mutations in the methyl-CpG-binding domain (MBD) and those with mutations in the transcriptional repression domain (TRD). We conclude that mutation type and location correlate with the phenotype in Rett syndrome. All mutations that impair the nuclear localisation signal (NLS) are associated with more severe phenotypes.