11 February 2007. In the February 9 Science, Adrian Bird and colleagues at Edinburgh University, Scotland, report striking results of a “cure” for Rett syndrome mice. Rett’s (RTT) is an X-linked autism spectrum disorder that afflicts 1 in 10,000 girls and is caused by mutations in the methyl-CpG binding protein 2 (MeCP2) gene. Mice with an MeCP2 gene deletion recapitulate many of the features of RTT, and the new results show that restoring expression of MeCP2 in the mice after symptoms have developed normalizes their behavior and neuronal function. The results indicate that RTT is not a developmental disorder, but instead results from aberrant function of mature neurons, which is rapidly corrected once the normal protein is present. They also raise the tantalizing question of whether Rett syndrome can eventually be treated later in its course than was generally assumed, that is once the girls already suffer from its symptoms.
This is good news for the Rett’s community, and for others. MeCP2 mutations appear in multiple disorders including learning disability, neonatal encephalopathy, autism, and X-linked mental retardation. The results suggest that these conditions might be treated in the future by replacing or restoring MeCP2 gene function. The MeCP2 mutation has been linked with a case of childhood-onset schizophrenia (Cohen et al., 2002), though this connection remains unconfirmed. However, the reversibility of symptoms echoes a recent report on prion disease (see Alzheimer Research Forum related news story), and it serves as a hopeful reminder that the nervous system can have the capacity to bounce back from seemingly overwhelming neurological deficits. Moreover, the study of MeCP2’s molecular biology is unraveling a new area of neuroscience that is well worth following.
To make a reversible MeCP2 mutation, first author Jacky Guy inserted a removable stop cassette in the endogenous mouse gene, which like the human is located on the X chromosome. Male mice carrying the disrupted gene developed symptoms of inertia, abnormal gait, tremor, and irregular breathing at 6 weeks, and died by 17 weeks.
By introducing an estrogen-regulated Cre recombinase transgene into the mice, the researchers showed they could use tamoxifen treatment to delete the stop cassette and restore expression of the MeCP2 protein from the endogenous gene. Gradual reactivation of the gene 3-4 weeks after birth prevented the onset of symptoms, and some of the mice are still alive 15 months later. Most surprisingly, the restoration of MeCP2 resulted in mice whose behavior was indistinguishable from control mice even when tamoxifen was started much later, at 12-17 weeks, when the mice showed advanced symptoms.
While the male mice show a dramatic MeCP2-null phenotype, female heterozygous animals more closely match the human disease. Their symptoms are milder and do not cause early death. In these mice, too, all symptoms reverted when adult animals were treated to reactivate MeCP2 expression. A direct assessment of neuronal function in the female mice further demonstrated the benefits of MeCP2 restoration: electrophysiological recordings revealed a defect in long-term potentiation in these mice, which improved in the tamoxifen-treated mice. The results strongly support the idea that MeCP2 function is required to maintain the function of adult neurons, but not for their initial development.
While immediate application of these results to humans is unlikely—it is not clear how one would go about repairing or replacing the MeCP2 gene—the experiments are important, the authors write, because they “establish the principle of reversibility in a mouse model and therefore raise the possibility that neurological defects seen in this and related human disorders are not irrevocable.”
Restoration of function depends on understanding the function of MeCP2, and that is the topic of another Bird paper this week, this one published in PNAS online. The work from first author Xinsheng Nan and others in the Bird lab identifies a new partner for MeCP2, which may contribute to mental retardation associated with some mutations.
Bird’s previous work, and work from Huda Zoghbi at Baylor College of Medicine in Houston, Texas, has revealed MeCP2 to be a multi-dimensional regulator of both gene transcription and mRNA splicing (see SRF related news story). The new work shows that the MeCP2 protein interacts with ATRX, a DNA helicase/ATPase that is itself mutated in a separate X-linked mental retardation syndrome. In mouse cells, both proteins were localized to heterochromatin, tightly packed, and transcriptionally inactive regions of DNA. The ATRX protein lost its targeting in MeCP2-null mice. Several mutations in MeCP2 that cause RTT or X-linked mental retardation inhibited interaction with ATRX in vitro and its localization in cells. The investigators propose that disruption of the ATRX- MeCP2 interaction somehow contributes to mental retardation. They are now looking at the effect of ATRX on the activity of MeCP2 target genes, including brain-derived neurotrophic factor (BDNF, see SRF related news story).—Pat McCaffrey.
Guy J, Gan J, Selfridge J, Cobb S, Bird A. Reversal of Neurological Defects in a Mouse Model of Rett Syndrome. Science. 2007 Feb 8; [Epub ahead of print] Abstract
Nan X, Hou J, Maclean A, Nasir J, Lafuente MJ, Shu X, Kriaucionis S, Bird A. Interaction between chromatin proteins MECP2 and ATRX is disrupted by mutations that cause inherited mental retardation. Proc Natl Acad Sci U S A. Epub pending. Abstract