Modulating alternative splicing of MECP2: A potential therapeutic strategy for Rett syndrome

Rett syndrome (RTT) is a neurological disorder caused by loss-of-function mutations in methyl CpG binding protein 2 (MECP2), a transcriptional regulator essential for maintenance of normal neuronal function. The current FDA-approved treatment for RTT, Trofinetide, mildly alleviates some symptoms. In contrast, re-introducing MeCP2 or upregulating it through transgenesis in mouse RTT models improves most neurological phenotypes and enhances survival. Here, we devised a therapeutic strategy to moderately increase MeCP2 protein by modulating the alternative splicing of MECP2 to switch the less efficiently translated e2 to the more efficiently translated e1 isoform. We deleted Mecp2 exon 2 (unique to e2), leading to production of only e1 mRNA, and show this upregulates MeCP2 by 50-60% in mice. Next, using two independent RTT induced pluripotent stem cell (iPSC)-derived neuron models harboring mutations that reduce both MeCP2 expression and function, we show that exon 2 deletion upregulates MeCP2 and ameliorates morphological and electrophysiological deficits in these neurons. Isoform switching also significantly corrects the dysregulated transcriptome in RTT neurons. Lastly, an exon 2-skipping Morpholino upregulates MeCP2-E1 in vivo. These data set the stage for a promising therapeutic strategy using antisense oligonucleotides to promote isoform switching in patients with RTT who carry partially functioning alleles of MECP2.