An Overview of Alternative Splicing Defects Implicated in Myotonic Dystrophy Type I

Abstract

Myotonic dystrophy type I (DM1) is the most common form of adult muscular dystrophy, caused by expansion of a CTG triplet repeat in the 3' untranslated region (3'UTR) of the myotonic dystrophy protein kinase (DMPK) gene. The pathological CTG repeats result in protein trapping by expanded transcripts, a decreased DMPK translation and the disruption of the chromatin structure, affecting neighboring genes expression. The muscleblind-like (MBNL) and CUG-BP and ETR-3-like factors (CELF) are two families of tissue-specific regulators of developmentally programmed alternative splicing that act as antagonist regulators of several pre-mRNA targets, including troponin 2 (TNNT2), insulin receptor (INSR), chloride channel 1 (CLCN1) and MBNL2. Sequestration of MBNL proteins and up-regulation of CELF1 are key to DM1 pathology, inducing a spliceopathy that leads to a developmental remodelling of the transcriptome due to an adult-to-foetal splicing switch, which results in the loss of cell function and viability. Moreover, recent studies indicate that additional pathogenic mechanisms may also contribute to disease pathology, including a misregulation of cellular mRNA translation, localization and stability. This review focuses on the cause and effects of MBNL and CELF1 deregulation in DM1, describing the molecular mechanisms underlying alternative splicing misregulation for a deeper understanding of DM1 complexity. To contribute to this analysis, we have prepared a comprehensive list of transcript alterations involved in DM1 pathogenesis, as well as other deregulated mRNA processing pathways implications.

Keywords: CELF1; DMPK; MBNL; myotonic dystrophy; spliceopathy.

Figure 1

MBNL1, MBNL2 and MBNL3 paralog structures. All the exons that form the three MBNL paralogs are represented, each with its corresponding functional domains highlighted. Some of them are constitutively expressed in all transcripts but others undergone alternative splicing depending on the tissue or developmental process. ZnF = zinc finger; NLS = nuclear localization signal. Created with Biorender.com.

Figure 2

MBNL1 and CELF1 localization in neonatal development in skeletal muscle. CELF1 is the main splicing regulator in the nucleus during early neonatal development. It binds with higher affinity to single-stranded pre-mRNAs that will later pass to the endoplasmic reticulum, where mature mRNAs will be formed. MBNL1 is the main splicing component in the cytoplasm until the postnatal splicing switch when it is translocated to the nucleus, substituting CELF1 in the pre-mRNA binding, although in this case, with higher affinity to double-stranded sequences. Created with Biorender.com.

Figure 3

DMPK transcripts with CUG expansion and MBNL bound. MBNL1 proteins are attracted to the DMPK transcripts CUG expansion due to the weak pairing of U–U, resulting in MBNL depletion from the nucleoplasm and lack of functional DMPK in the cytoplasm. CELF1 is overexpressed in DM1 tissues and regulates most of the splicing processes undergone in the nucleus, which causes the expression of embryonic variants that lead to a spliceopathy. Created with Biorender.com.

Figure 4

DM1 spliceopathy overview. At DNA level, CTG repeats code for hairpin CUG structures that bind MBNL1 proteins with high affinity. Due to MBNL1 loss, CELF1 is overexpressed through PKC phosphorylation and alters the splicing of different transcripts, mainly switching to embryonic isoforms. The splicing deregulation or spliceopathy induces an aberrant protein expression that provokes the loss of cell function and viability. Different transcripts from different tissues are incorrectly spliced, causing most DM1 symptoms. Created with Biorender.com.