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MBNL proteins in health, disease, and therapeutic applications
Abstract The Muscleblind-like (MBNL) family comprises evolutionarily conserved RNA-binding proteins that interact with target RNAs via zinc finger domains. MBNLs orchestrate RNA processing, particularly alternative splicing, driving the developmental fetal-to-adult isoform switch across numerous target transcripts. This transition is a cornerstone in the process of MBNL-maintained cellular homeostasis and fails in many pathological conditions associated with deregulated expression or function of specific MBNL paralogs. This review provides current insights into the roles of MBNL genes and proteins in both health and disease. We examine their genomic architecture and protein organization and synthesize key insights from animal models to delineate the selective and compensatory functions of individual MBNL paralogs in physiology. To illustrate the roles of MBNLs in disease, we outline nucleotide repeat expansion disorders marked by their functional depletion, with a primary focus on myotonic dystrophy (DM). We also highlight selected cancer studies that have demonstrated the dual roles of MBNLs in tumorigenesis, encompassing both pro-oncogenic and tumor suppressive functions. Finally, using DM as a model, we review evidence for the therapeutic potential of endogenous MBNL gene modulation and argue that analogous strategies could be adapted and tailored to restore MBNL homeostasis in other disorders involving their dysregulation.
Promoter-targeted small RNA duplexes increase MBNL1 transcription and mitigate myotonic dystrophy-associated spliceopathy
Abstract Functional depletion of Muscleblind-like (MBNL) proteins is a key trigger of myotonic dystrophy (DM)-associated alternative splicing (AltS) defects. To overcome MBNL insufficiency in DM cell models, we harnessed a conserved endogenous mechanism of RNA activation (RNAa) via rationally designed small activating RNA (saRNA) targeted to the most active promoter of MBNL1 gene. We report on two lead saRNA duplexes that stimulated MBNL1 transcription via an on-site mechanism that involves AGO2-mediated loading of the antisense strand onto target sequence, followed by recruitment of RNAPII and auxiliary RNAa pathway components. We demonstrate that neither the antisense lncRNA MBNL1-AS1 overlapping MBNL1 promoter nor promoter-associated cryptic RNAs are mechanistically involved in saRNA-induced MBNL1 gene activation. Our data highlight putative transcription factors whose binding recruitment via identified saRNAs may affect MBNL1 expression. Most importantly, we show that RNAa-based approach upregulates MBNL1 protein content in distinct DM cell models and corrects the AltS of multiple MBNL1-regulated biomarker exons, underscoring the feasibility of adapting saRNA into novel therapeutic designs. This is the first report that site-specific augmentation of the endogenous MBNL1 transcription mitigates disease-associated AltS defects and as such, it offers new perspectives into therapeutic options against DM.