Molecular basis of R133C Rett syndrome

Abstract

Rett syndrome is a debilitating autistic spectrum disorder affecting one in ten thousand girls. Patients develop normally for up to eighteen months before a period of regression involving stagnation in head growth, loss of speech, hand use and mobility. It is almost exclusively caused by mutation in Methyl CpG binding Protein 2 (MeCP2). MeCP2 has traditionally been thought of as a transcriptional repressor, although its exact function remains unknown and it has recently been shown that the protein can also bind to hydroxymethylation and non-CpG methylation, which occurs predominantly at CAC sites in the mature nervous system. Genotype-phenotype studies of the most common Rett-causing mutations in affected patients revealed that a missense mutation, R133C results in a milder form of Rett syndrome. The reasons for this are unclear, as the mutation lies right in the heart of the methylated DNA binding domain. Previous in vitro studies of R133C showed a severe deficit in binding to methylated cytosine. A subsequent study found that R133C binding to hydroxymethylated cytosine was specifically impaired, whereas binding to methylated cytosine was indistinguishable from wildtype. Defining the DNA binding impairment of MeCP2R133C would yield important insights into Rett disease pathophysiology and provide an explanation for the phenotypic spectrum seen in patients. To shed light on these matters, a novel mouse model of the R133C mutation was created. The R133C mouse had a phenotype that was less severe than other missense mutant mice, in terms of survival, growth, Rett-like phenotypic score and some behavioural paradigms thus recapitulating the patient data. At the molecular level in adult mouse brain, MeCP2R133C protein abundance was reduced. Immunohistochemistry showed that MeCP2R133C had an abnormal pattern of localisation in the nucleus of neurons. In vitro electrophoretic mobility shift assays suggested that MeCP2R133C binding to (hydroxy)methyl-CAC may be reduced to a greater extent than binding to mCpG. Chromatin immunoprecipitation experiments confirmed the deficit in binding to methylated sites and supported a disproportionate reduction in binding to methylation in a CAC sequence context. Analysis of adult mouse cerebellar gene expression revealed a subtle upregulation of long genes and downregulation of short genes. Based on these data, it is proposed that Rett syndrome caused by the R133C mutation results from a combination of protein instability and defective binding to methylated DNA. Methyl-CAC binding is potentially abolished. The downstream biological consequence of this is a length-dependent deregulation of gene expression in the brain.