The levels of methyl-CpG-binding protein 2 (MeCP2) are critical for normal post-natal development and function of the nervous system. Loss of function of MeCP2, a transcriptional regulator involved in chromatin remodeling, causes classic Rett syndrome (RTT) as well as other related conditions characterized by autism, learning disabilities, or mental retardation. Increased dosage of MeCP2 also leads to clinically similar neurological disorders and mental retardation. To identify molecular mechanisms capable of compensating for altered MeCP2 levels, we generated transgenic Drosophila overexpressing human MeCP2. We find that MeCP2 associates with chromatin and is phosphorylated at serine 423 in Drosophila, as is found in mammals. MeCP2 overexpression leads to anatomical (i.e., disorganized eyes, ectopic wing veins) and behavioral (i.e., motor dysfunction) abnormalities. We used a candidate gene approach to identify genes that are able to compensate for abnormal phenotypes caused by MeCP2 increased activity. These genetic modifiers include other chromatin remodeling genes (Additional sex combs, corto, osa, Sex combs on midleg, and trithorax), the kinase tricornered, the UBE3A target pebble, and Drosophila homologues of the MeCP2 physical interactors Sin3a, REST, and N-CoR. These findings demonstrate that anatomical and behavioral phenotypes caused by MeCP2 activity can be ameliorated by altering other factors that might be more amenable to manipulation than MeCP2 itself.

Mutations in the X-linked methyl-CpG-binding protein 2 (MECP2), encoding a transcriptional repressor, cause Rett syndrome and a variety of related neurodevelopmental disorders. The vast majority of mutations associated with human disease are loss-of-function mutations, but precisely what aspect of MeCP2 function is responsible for these phenotypes remains unknown. We overexpressed wild-type human protein in transgenic mice using a large genomic clone containing the entire human MECP2 locus. Detailed neurobehavioral and electrophysiological studies in transgenic line MeCP2(Tg1), which expresses MeCP2 at approximately 2-fold wild-type levels, demonstrated onset of phenotypes around 10 weeks of age. Surprisingly, these mice displayed enhanced motor and contextual learning and enhanced synaptic plasticity in the hippocampus. After 20 weeks of age, however, these mice developed seizures, became hypoactive and approximately 30% of them died by 1 year of age. These data demonstrate that MeCP2 levels must be tightly regulated in vivo, and that even mild overexpression of this protein is detrimental. Furthermore, these results support the possibility that duplications or gain-of-function mutations in MECP2 might underlie some cases of X-linked delayed-onset neurobehavioral disorders.

Holoprosencephaly (HPE) is a common malformation of the developing forebrain and midface characterized by incomplete penetrance and variable expressivity. Familial HPE has been reported in many families with autosomal dominant inheritance in some and apparent autosomal recessive inheritance in others. We have examined 125 individuals from nine families with autosomal dominant HPE. Expression in gene carriers varied from alobar HPE and cyclopia through microforms such as microcephaly or single central incisor to normal phenotype. We performed linkage studies by either Southern blot or polymerase chain reaction analyses with DNA markers (D7S22, D7S550, and D7S483) that are deleted from some patients with sporadic HPE and flank a translocation breakpoint in 7q36 associated with HPE. The strongest support for linkage was with D7S22, which was linked with no recombination to autosomal dominant HPE in eight of nine families with a combined logarithm of odds score of 6.4 with an affected-only model-free analysis and 8.2 with a reduced-penetrance model and all phenotypes. Close linkage to this region could be excluded in one family, and there was significant evidence of genetic heterogeneity. These results show that a gene for autosomal dominant HPE is located in a chromosomal region (7q36) known to be involved in sporadic HPE with visible cytogenetic deletions. They also demonstrate genetic heterogeneity in familial HPE. We hypothesize that mutations of a gene in 7q36, designated HPE3, are responsible for both sporadic HPE and a majority of families with autosomal dominant HPE.