MOLECULAR PATHOGENESIS STUDIES OF RETT SYNDROME - Summary Since discovering that loss-of-function mutations in the X-linked gene MECP2 cause Rett syndrome (RTT), my lab has sought to understand the molecular changes that create the profound, wide-ranging deficits in this disorder. RTT manifests 12-18 months after birth and affects motor, cognitive, psycho-social, and autonomic functions1. Complete loss of function (LOF) of MeCP2, as occurs in rare male patients, is lethal within the first two years of life, while gain of function, as occurs in MECP2 duplication disorder (MDS) in males, is lethal in young adulthood. Puzzlingly, MeCP2 LOF leads to down-regulation as well as up-regulation of thousands of genes, with the inverse pattern in MDS. MeCP2 was initially characterized as a repressor—it binds methylated cytosines and suppresses gene expression through its interactions with NCoR—so how does its loss of function result in half of the transcriptomic changes being downregulation? In the last funding period, we ruled out the possibility that the downregulation is due to activation of repressors, neuronal dysfunction, or displacement of transcriptional machinery to repetitive elements. We still need to test the possibility that without MeCP2, the NCoR complex is free to repress inappropriately, but we also have two lines of evidence that suggest another plausible and intriguing explanation. First, we found that MeCP2 interacts with TCF20, a context-dependent transcriptional coactivator/repressor, so MeCP2 may, in some contexts, activate transcription. Second, we and others have found that MeCP2-null neurons in RTT mice are less responsive to stimuli than wild-type, but that deep brain stimulation (DBS), training, and optogenetics in female mice can stimulate the circuit sufficiently to correct the behavioral deficits. Based on these data, we hypothesize that MeCP2 plays a specific role in the transcriptional response to stimuli. This could be why studies in mice and humans show that behavior is sensitive to modest changes in MeCP2 levels, even when MeCP2 is hypofunctional, such that overexpressing the severe RTT-causing allele (T158M) in mice significantly improved the phenotype. Given that ~60% of RTT-causing mutations reduce either function or levels of MeCP2 (or both), strategies that boost MeCP2 expression could benefit many people with RTT. We therefore propose the following three aims:1) determine the binding pattern of MeCP2 at baseline and immediately after activity; 2) determine the binding pattern of NCoR and TCF20 upon loss of MeCP2; and 3) investigate isoform-switching and miRNA regulation to augment MeCP2 levels and determine which classes of RTT-causing mutations will benefit from increasing expression of the mutant allele.
