Reversal of phenotypes in MECP2 duplication mice using genetic rescue or antisense oligonucleotides

Abstract

Copy number variations have been frequently associated with developmental delay, intellectual disability, and autism spectrum disorders1. MECP2 duplication syndrome is one of the most common genomic rearrangements in males2 and is characterized by autism, intellectual disability, motor dysfunction, anxiety, epilepsy, recurrent respiratory tract infections, and early death3–5. The broad range of deficits caused by methyl-CpG-binding protein 2 (MeCP2) overexpression poses a daunting challenge to traditional biochemical pathway-based therapeutic approaches. Accordingly, we sought strategies that directly target MeCP2 and are amenable to translation into clinical therapy. The first question, however, was whether the neurological dysfunction is reversible after symptoms set in. Reversal of phenotypes in adult symptomatic mice has been demonstrated in some models of monogenic loss-of-function neurological disorders6–8, including loss of MeCP2 in Rett syndrome9, indicating that, at least in some cases, the neuroanatomy may remain sufficiently intact so that correction of the molecular dysfunction underlying these disorders can restore healthy physiology. Given the absence of neurodegeneration in MECP2 duplication syndrome, we hypothesized that restoration of normal MeCP2 levels in MECP2 duplication adult mice would rescue their phenotype. Therefore, we first generated and characterized a conditional Mecp2-overexpressing mouse model and showed that correction of MeCP2 levels largely reversed the behavioral, molecular, and electrophysiological deficits. Next, we sought a translational strategy to reduce MeCP2 and turned to antisense oligonucleotides (ASOs). ASOs are small modified nucleic acids that can selectively hybridize with mRNA transcribed from a target gene and silence it10,11, and have been successfully used to correct deficits in different mouse models12–18. We found that ASO treatment induced a broad phenotypic rescue in adult symptomatic transgenic MECP2 duplication mice (MECP2-TG)19,20, and corrected MECP2 levels in lymphoblastoid cells from MECP2 duplication patients in a dose-dependent manner.

FMRP Expression Levels in Mouse Central Nervous System Neurons Determine Behavioral Phenotype

“…More importantly, abnormally increased MeCP2 expression in the Fmr1 KO CNS suggests the possibility that increased MeCP2 might play a role in the overall FXS phenotype. MeCP2 over-expressing mice display impaired synaptic plasticity and autistic behaviors, 42,43 and like both FXS and FMR1 gene duplication syndrome, duplication of the human MeCP2 gene is characterized by developmental delay, mental retardation, and seizures. 44–47 The overlapping symptomatic features of FXS, Rett syndrome, and FMR1 and MeCP2 gene duplication syndromes suggests that further studies investigating the relationships of these two key regulatory proteins is warranted.…”

Clinical and biological progress over 50 years in Rett syndrome

“…91,93 When modelled in mice, MECP2 Duplication syndrome, like RTT, has shown the potential for phenotypic reversal when MeCP2 levels are restored to normal levels. 94 Loss of MeCP2 alters the cellular levels of many gene products but the effects at the individual gene level are typically small 75,95 and likely to be cell-type specific. The fact that a wide variety of genes are affected suggests that there is not going to be a single pathogenic pathway that can act as a focus for all therapeutic interventions.…”

Loss-of-function mutation in Mirta22/Emc10 rescues specific schizophrenia-related phenotypes in a mouse model of the 22q11.2 deletion

“…Along these lines, in future experiments, it is important to determine when during the lifespan Mirta22 normalization is most effective at reversing behavioral and neurophysiological phenotypes and whether Mirta22 levels could be normalized by approaches as readily translatable into medical therapies as possible. These approaches include, for example, the application of siRNAs or shRNAs (73,74) or the use of antisense oligonucleotides to inhibit excessive Mirta22 gene expression based on their efficacy in preclinical models (75,76) and clinical studies (77,78) of neurodevelopmental or neurodegenerative disorders. Alternatively, use of genome editing via CRISPR could be another highly promising approach for modulating Mirta22 function (79,80).…”