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Rett Syndrome, an X-linked dominant neurodevelopmental disorder characterized by regression of language and hand use, is primarily caused by mutations in methyl-CpG-binding protein 2 (MECP2). Loss of function mutations in MECP2 are also found in other neurodevelopmental disorders such as autism, Angelman-like syndrome and non-specific mental retardation. Furthermore, duplication of the MECP2 genomic region results in mental retardation with speech and social problems. The common features of human neurodevelopmental disorders caused by the loss or increase of MeCP2 function suggest that even modest alterations of MeCP2 protein levels result in neurodevelopmental problems. To determine whether a small reduction in MeCP2 level has phenotypic consequences, we characterized a conditional mouse allele of Mecp2 that expresses 50% of the wild-type level of MeCP2. Upon careful behavioral analysis, mice that harbor this allele display a spectrum of abnormalities such as learning and motor deficits, decreased anxiety, altered social behavior and nest building, decreased pain recognition and disrupted breathing patterns. These results indicate that precise control of MeCP2 is critical for normal behavior and predict that human neurodevelopmental disorders will result from a subtle reduction in MeCP2 expression.

Autism is a common neurodevelopmental disorder of complex genetic etiology. Rett syndrome, an X-linked dominant disorder caused by MECP2 mutations, and Angelman syndrome, an imprinted disorder caused by maternal 15q11-q13 or UBE3A deficiency, have phenotypic and genetic overlap with autism. MECP2 encodes methyl-CpG-binding protein 2 that acts as a transcriptional repressor for methylated gene constructs but is surprisingly not required for maintaining imprinted gene expression. Here, we test the hypothesis that MECP2 deficiency may affect the level of expression of UBE3A and neighboring autism candidate gene GABRB3 without necessarily affecting imprinted expression. Multiple quantitative methods were used including automated quantitation of immunofluorescence and in situ hybridization by laser scanning cytometry on tissue microarrays, immunoblot and TaqMan PCR. The results demonstrated significant defects in UBE3A/E6AP expression in two different Mecp2 deficient mouse strains and human Rett, Angelman and autism brains compared with controls. Although no difference was observed in the allelic expression of several imprinted transcripts in Mecp2-null brain, Ube3a sense expression was significantly reduced, consistent with the decrease in protein. A non-imprinted gene from 15q11-q13, GABRB3, encoding the beta3 subunit of the GABAA receptor, also showed significantly reduced expression in multiple Rett, Angelman and autism brain samples, and Mecp2 deficient mice by quantitative immunoblot. These results suggest an overlapping pathway of gene dysregulation within 15q11-q13 in Rett, Angelman and autism and implicate MeCP2 in the regulation of UBE3A and GABRB3 expressions in the postnatal mammalian brain.

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused by mutations in MECP2, encoding methyl-CpG-binding protein 2 (MeCP2). The onset of symptoms in RTT is delayed until 6-18 months and 4-6 months in the Mecp2(-/+) mouse model, corresponding to a dynamic and gradual accumulation of MeCP2 expression in individual neurons of the postnatal brain. Because of X chromosome inactivation (XCI), cells within RTT females are mosaic for expression of the heterozygous MECP2 mutation. Using the targeted Mecp2 mouse model, we investigated the effect of Mecp2 mutation on XCI and developmental MeCP2 expression in wild-type (wt)-expressing neurons by quantitative laser scanning cytometry. Mecp2(-/+) female mice exhibited uniform regional distribution of Mecp2 mutant-expressing cells in brain, but unbalanced XCI in the population, favoring expression of the Mecp2 wt allele. Interestingly, MeCP2 expression in Mecp2 wt-expressing cells from Mecp2(-/+) mice was significantly lower than those from Mecp2(+/+) age-matched controls. The negative effect of Mecp2 mutation on wt Mecp2 expression correlated with the percentage of Mecp2 mutant-expressing cells in the cortex. Similar results were observed in two RTT females with identical MECP2 mutations but different XCI ratios. These results demonstrate that Mecp2-mutant neurons affect the development of surrounding neurons in a non-cell-autonomous manner and suggest that environmental influences affect the level of MeCP2 expression in wt neurons. These results help in explaining the role of XCI in the pathogenesis of RTT and have important implications in designing therapies for female RTT patients.