Background: MeCP2, methyl-CpG-binding protein 2, binds to methylated cytosines at CpG dinucleotides, as well as to unmethylated DNA, and affects chromatin condensation. MECP2 mutations in females lead to Rett syndrome, a neurological disorder characterized by developmental stagnation and regression, loss of purposeful hand movements and speech, stereotypic hand movements, deceleration of brain growth, autonomic dysfunction and seizures. Most mutations occur de novo during spermatogenesis. Located at Xq28, MECP2 is subject to X inactivation, and affected females are mosaic. Rare hemizygous males suffer from a severe congenital encephalopathy.

Hirschsprung disease (HSCR) is a multigenic, congenital disorder that affects 1 in 5,000 newborns and is characterized by the absence of neural crest-derived enteric ganglia in the colon 1 . One of the primary genes affected in HSCR encodes the G protein-coupled endothelin receptor-B (EDNRB) 2,3 . The expression of Ednrb is required at a defined time period during the migration of the precursors of the enteric nervous system (ENS) into the colon 4 . In this study, we describe a conserved spatiotemporal ENS enhancer of Ednrb. This 1-kb enhancer is activated as the ENS precursors approach the colon, and partial deletion of this enhancer at the endogenous Ednrb locus results in pigmented mice that die postnatally from megacolon. We identified binding sites for SOX10, an SRY-related transcription factor associated with HSCR 5 , in the Ednrb ENS enhancer, and mutational analyses of these sites suggested that SOX10 may have multiple roles in regulating Ednrb in the ENS.Mice and individuals with HSCR with mutations in the EDNRB-mediated pathway have megacolon because of the absence of enteric neurons in the distal gut 1 . This regional specificity of aganglionosis could be explained by a temporal requirement for Ednrb between embryonic day (E) 11 and E12.5 (ref. 4), when vagal neural crest-derived ENS progenitors are populating the hindgut during mouse embryogenesis, such that in the absence of EDNRB the migratory wavefront is delayed near the ileocecal junction (Fig. 1a) [6][7][8] . To elucidate the molecular mechanisms for Ednrb expression in the ENS, we dissected the Ednrb genomic region. We isolated a 78-kb P1 genomic clone encompassing Ednrb (Fig. 1b) and used it to create four independent transgenic lines. When we crossed the individual transgenic lines into the Ednrb-null mice, all the lines rescued postnatal death from megacolon (Fig. 1c). Although three of the lines did not rescue the melanocyte defect in the Ednrb-null mice (Fig. 1c), one line (when homozygous with respect to the transgene) partially rescued the pigmentation defect that resembles the hypomorphic Ednrb s allele 9 (data not shown). These results suggested that the P1 clone contained the necessary information for expression of Ednrb in ENS progenitors. In addition, we mapped a main transcription start site Ednrb -/-Tg P5

Rett syndrome (RTT) is a neurodevelopmental disorder characterized by cognitive regression, loss of purposeful hand movements and speech, stereotypies, ataxia, seizures, mental retardation and acquired microcephaly. Mutations in MECP2, encoding methyl-CpG-binding protein 2, are responsible for approximately 90% of classic RTT cases. RTT displays phenotypic overlap with Angelman syndrome, a disorder caused by loss of expression of the imprinted gene UBE3A. MeCP2 binds to methylated DNA and may alter the expression of imprinted genes, thereby suggesting a mechanistic link between the two disorders. Here, we tested the hypothesis that MeCP2 deficiency affects expression of Ube3a in mouse models of RTT. As Ube3a is only imprinted in brain, we evaluated Ube3a expression in brains of 15 different litters of neonatal or 8-week-old male Mecp2 mutant mice by real-time quantitative RT-PCR and western blot analysis. We found no significant differences between Mecp2(tm1.1Bird/Y) or Mecp2(tm1.1Jae/Y) mutants and their wild-type male siblings that served as negative controls. In positive control mice carrying a maternally inherited Ube3a deletion, Ube3a sense transcript and protein levels were drastically reduced. Our data contrast with two recent reports of substantially decreased Ube3a expression in brain tissues of MeCP2-deficient mice. We, therefore, challenge the conclusion that decreased UBE3A/Ube3a expression contributes to the pathophysiology of RTT.