br Introduction Myotonic dystrophy dystrophia myotonica DM i
Introduction Myotonic dystrophy (dystrophia myotonica, DM) is an autosomal dominant disorder and the most common form of inherited muscular dystrophy in adults . DM is characterised by a wide range of symptoms, including myotonia, progressive muscle loss, cataracts, cardiac conduction defects, insulin resistance and cognitive impairments . Two forms of DM, DM1 and DM2, have been described to date. In DM1, disease symptoms result from an aberrant expansion of the CTG trinucleotide repeat in the 3′ untranslated region (UTR) in myotonic dystrophy protein kinase (DMPK) on chromosome 19 , , . In DM2, disease is caused by an expansion of a CCTG tetranucleotide repeat in intron 1 of the CCHC-type zinc finger, nucleic 7-Ethyl-10-hydroxycamptothecin binding protein (CNBP) gene on chromosome 3 . Among patients with DM, several lines of evidence have suggested a relationship between transcribed RNA CUG or CCUG repeats and disease symptoms. First, the number of CUG repeats correlates with the severity of symptoms . Second, in cells derived from patients with DM, expanded CUG and CCUG repeats accumulate in the nucleus , , . Third, HSALR transgenic mice expressing an expanded CUG repeat inserted in the human skeletal muscle actin (HSA) gene manifest myotonia and abnormal muscle histology . In addition to sequence repeats, abnormalities in RNA metabolism have also been found in patients with DM. Aberrant splicing has been reported in multiple genes, including chloride channel 1 (CLCN1), insulin receptor (INSR), bridging integrator 1 (BIN1), myomesin 1 (MYOM1) and actin-binding LIM protein 1 (ABLIM1), among others , , , , , leading to a variety of symptoms in these patients. This aberrant splicing is thought to be driven by two families of splicing factors, muscleblind-like (MBNL) and CUG binding protein/ELAV-like family (CELF). MBNL proteins MBNL1–3 bind CHG/CHHG (H: A, C and U) sequences of RNA and co-localise with mRNAs containing CUG expanded repeats . This process leads to a decrease in the intracellular concentrations of functionally available MBNL proteins. Alternatively, CELF proteins, especially CELF1 and CELF2, are activated by CUG repeats, although the pathways regulating this process have not been fully elucidated . This imbalance of MBNL and CELF in turn leads to abnormal splicing of downstream genes, further exacerbating DM symptoms. Here, we examine the effects of MBNL1-3 and CELF1-6 on DM. CLCN1 is thought to be responsible for myotonia, the most characteristic symptom in DM1 . Many studies have been performed using mouse Clcn1. In these models, a frameshift occurs following the insertion of exon 7A (79 bp) between exon 6 and 7 in Clcn1; these immature mRNA transcripts are then degraded by the nonsense-mediated mRNA decay (NMD) machinery, resulting in lower steady state levels of CLCN1 protein. Correction of the abnormal splicing patterns in HSALR transgenic mice has been shown to rescue this phenotype, leading to recovery from myotonia . Moreover, in Clcn1, MBNL1–3 decrease the insertion of exon 7A, whereas CELF3–6 increase it . However, chloride channelopathy in DM1 has been reported to be due to downregulation of CLCN1 transcription . So, the mechanism of myotonia in DM1 is controversial. Far less is known regarding the function of human CLCN1, as mouse and human CLCN1 exhibit distinctly different splicing patterns. CLCN1 encodes for two additional exons, 6B (55 bp) not present in the mouse orthologue and 7A (79 bp) between exons 6 and 7; insertion of either or both of these exons into Clcn1 results in a frameshift mutation. Alternative splicing of these exons has been shown to produce numerous variants in the skeletal muscle of patients with DM, including variants such as 5–6B–7A–7–8, 5–6–6B–7A–7–8 and 5–8 . Beyond these two additional exons, another splicing pattern characterised by a three base pair (TAG triplet) extension of exon 7 has also been detected . This inserted TAG sequence is thought to act as a stop codon, resulting in the production of the immature mRNA.