Abstracts

Rationale: Correcting X-linked neurological disorders in women is challenging due to cellular mosaicism. An approach that augments expression of the disease gene may be simpler than gene replacement but may cause additional neurologic dysfunction in dosage sensitive disorders. Here we test the theory that “counter-balancing” X-linked genetic loss-of-function (LOF) in one mosaic cell population by genetic gain-of-function only in the complementary set of cells may suppress disease in a mouse model of Rett syndrome (RTT), an X-linked neurological disorder with epilepsy caused by mutations in MECP2 encoding the transcriptional modulator Methyl-CpG-Binding Protein 2 (MeCP2). 

Methods: We genetically combined an X-linked MeCP2 loss-of-function allele with an X-linked human MECP2 transgene (TG3) in attempt to "counter-balance" the disease pathophysiology in trans without correcting the underlying cellular deficiency (called "NT3" mice). As a control, we also combined X-linked MeCP2 loss-of-function with an autosomal human MECP2 transgene (TG1) to restore MeCP2 function in all cells (called "NT1" mice). For all experiments, we compared age- and sex-matched littermates from each group (NT3 and NT1) versus Mecp2-null/x female (N) and wild-type (WT) control animals. Immunofluorescence from adult mouse brain was performed to confirm the cellular distribution of MeCP2 in animals generated for these studies. A mouse behavioral battery was performed to assess performance in multiple domains including: locomotor activity, learning / memory, sociability and others. In vivo electrophysiology using implanted depth electrodes (targeting hippocampus, dentate gyrus) and subdural electrodes (targeting somatosensory and frontal cortex) was performed to assess multiple local within-region parameters from local field potentials (power spectral density; peak theta frequency; spiking activity) as well as far-range between-region parameters (phase synchronization).

Results: We demonstrate that combining two cellular populations, one of which is devoid of MeCP2 expression and the other containing approximately 3X normal levels of MeCP2, rescues many behavioral deficits seen in Mecp2null/x female mice. In vivo neurophysiological studies demonstrate that counter-balancing MeCP2 partially rescues select deficits related to long-range brain networks (inter-regional phase synchronization) without correcting multiple local measures of brain activity (power, peak theta frequency, epileptiform spike rate).

Conclusions: These surprising findings 1) provide insight into the complex pathophysiology of RTT, 2) demonstrate the need for caution in defining relevant preclinical outcome measures based on EEG, and 3) suggest that approaches to augment protein expression even non-cell autonomously may hold promise in RTT and possibly other X-linked neurological disorders.

Funding: None