Congenital DM1 (CDM) is not merely a severe early form of DM1 but rather a distinct clinical phenotype. CDM patients are severely affected at birth with hypotonia and generalized muscle weakness resulting in difficulty of sucking, swallowing, and breathing. Some CDM patients present symptoms even in utero, such as reduced fetal movements and hydramnios in later pregnancy. Talipes and tented upper lip (so-called carp mouth) are also seen in CDM. Despite neonatal intensive support, the mortality from respiratory failure remains high in affected infants. Surviving infants gradually improve in motor function, but nevertheless show delayed motor and mental development. Children with CDM can achieve independent walking, but develop myotonia and progressive muscle weakness in early adulthood. CDM is almost exclusively maternally inherited, even from mothers with mild forms of DM (see Sect. 3.3.2).

Although DM1 is common in populations of European and Asian descent, most DM2 patients are of northern and eastern European descent. DM2 patients also present myotonia and muscle weakness; however, the weakness typically affects the proximal muscles including the neck, elbow extension, and hip flexors. Significant muscle pain and fatigue are common in DM2. Cardiac conduction defects, cataracts, and insulin resistance are seen, whereas cognitive manifestation is rare in DM2. In contrast to DM1, a severe congenital form of DM2 has not been reported to date.

Short DNA tandem repeats are observed ubiquitously across the human genome [8]. A small fraction of these repetitive elements are prone to become mutable and highly expanded. Such genomic expansions of simple tandem repeats cause various neurogenetic disorders, including DM. DM1 is caused by an expansion of CTG repeats in the 3′ untranslated region (UTR) of the DMPK gene [2] (Fig. 3.2). The length of CTG repeats is between 50 and several thousand in DM1, whereas the normal repeat number is between 5 and 37. In general, the length of expanded CTG repeats in blood cells correlates with the age of onset in DM1 [9]. DM2 results from a similar expansion of CCTG repeats in the first intron of the CNBP/ZNF9 gene [3] (Fig. 3.2). The expanded CCTG repeat in DM2 is much longer than that in DM1, ranging from 75 to over 11,000. Importantly, unlike that seen in DM1, the size of expanded CCTG repeats does not correlate with the age of onset in DM2.

The expanded CTG and CCTG repeats in DM are not stable [10]. These mutations exhibit an exceptional degree of genetic instability in germline and somatic cells. For instance, the size of the repeat is prone to change during transmission from one generation to the next. In fact, intergenerational increments in the order of hundreds of repeats are often seen in DM1 (intergenerational instability). These mutations are also unstable in somatic cells, leading to an age-dependent growth of repeat expansion during the life of an individual (somatic instability).

Although the size of expanded CTG repeats in leukocytes correlates with the age of onset in DM1, there was no correlation between muscle weakness and CTG expansion size in either leukocytes or muscle cells [16]. This suggests that some DM1 patients may tolerate longer expansions better than others and raises the possibility that DM1 severity is modulated by additional factors, such as modifier genes, sequence interruptions in the CTG repeat tract, or epigenetic changes at the DMPK locus. The CTG expansion length in the muscle also did not correlate with that in leukocyte from the same subjects [16, 17]. However, the difference in repeat size between leukocytes and muscle is correlated with age, suggesting an age-dependent process of somatic expansion that is more pronounced in the muscle than in leukocytes.

More than 20 neurogenetic disorders are caused by unstable genomic expansions of simple tandem repeats [8]. Such unstable repeat expansion disorders fall in two categories according to the position of the repeat element within the mutant gene. In most cases the repeat tract is located in protein-coding regions, the motif is CAG, and the orientation in the reading frame produces expanded polyglutamine proteins, resulting in neurotoxicity (e.g., Huntington disease, spinocerebellar ataxia types 1, 2, 3, and 6). The second major group of unstable repeat expansion disorders results from repeats in non-protein-coding regions of genes, such as expanded CGG repeats in the 5′ UTR of FMR1 in fragile X syndrome (FXS) [18], GAA repeats in the first intron of FXN in Friedreich’s ataxia (FRDA) [19], and CTG/CCTG repeats in DM. In cases with FXS and FRDA, the critical effect of the expanded repeats is transcriptional silencing of FMR1 and FXN, respectively. However, there is no proven effect on transcription of DMPK or CNBP/ZNF9 in DM, indicating that the main pathogenic effect is a deleterious gain of function by the mutant mRNA.

Although the mutant mRNA is fully processed in DM1, repeat expansion blocks the nuclear export of mRNA with the expanded repeats of CUG (CUG^exp) [20]. Thus, the mRNA containing expanded repeats is retained in the nucleus and accumulates in discrete foci. Since DM1 is an autosomal dominant disease, nuclear retention of the mutant mRNA could theoretically lead to a 50 % reduction in DMPK protein. However, several lines of evidence strongly indicate that symptoms of DM1 result not from haploinsufficiency but from the toxic gain of function by RNA transcripts containing the CUG^exp. First, DMPK knockout mice, in which DMPK protein can be completely eliminated, show only minor symptoms in the skeletal muscle [21, 22]. Second, muscular features of DM1 can be reproduced by the expression of CUG^exp RNA at the 3′ UTR of human skeletal actin transgene in a mouse model [23]. Finally, in DM2, the highly expanded CCUG repeat in CNBP/ZNF9 is fully transcribed, but the intron containing the CCUG expansion is retained and accumulated in nuclear foci similar to that seen in DM1 [3, 24].

The mutant RNA retained in the nucleus affects at least two RNA-binding protein families: the MBNL and CELF. MBNL1, a member of the MBNL family protein, has a strong affinity for CUG^exp and CCUG^exp [25]. When CUG^exp or CCUG^exp RNA accumulates in the nucleus, MBNL1 is sequestered in nuclear RNA foci and depleted from the nucleoplasm [26, 27] (Fig. 3.4). Since MBNL1 is a regulator of alternative splicing [28], MBNL1 sequestration in foci leads to the misregulation of alternative splicing of its target exons. One clear example of the functional consequence in DM is the splicing misregulation of CLCN1 [29, 30]. This gene encodes a chloride ion channel that stabilizes the transmembrane potential in skeletal muscle. When alternative splicing of CLCN1 is misregulated in response to expression of CUG^exp RNA, the channel activity is lost, causing electrical instability of the membrane, which triggers repetitive action potentials and delay of relaxation (myotonia). The loss of MBNL1 has additional effects on gene expression, including transcriptional alterations, mRNA decay, transport of mRNA, and micro-RNA biogenesis [31–33]. Mbnl1 knockout mice exhibit myotonia and splicing alteration of Clcn1, supporting the evidence for MBNL1 dysfunction in DM1 [34]. Two other members of the MBNL family, MBNL2 and MBNL3, may also be involved in DM [34, 35]. Similar to MBNL1, MBNL2 is expressed in the skeletal muscle, heart, and brain, whereas MBNL3 is expressed mainly in the placenta. MBNL2 and MBNL3 are closely related to MBNL1 and can also be sequestered in nuclear foci. These suggest a role for the combinatorial deficiency of all three MBNL family members in DM, depending on the expression of CUG^exp or CCUG^exp RNA relative to the distinct levels of MBNL proteins in different tissues. Indeed, a recent report showed compound loss of MBNL1 and MBNL2 functions exaggerated cardiac and skeletal muscle symptoms in a mouse model [36].

Expression of CUG^exp also causes perturbations in cell signaling via several kinases, including protein kinase C (PKC), Akt kinase, cyclin-dependent kinase 4 (CDK4), and glycogen synthase kinase 3 beta (GSK3B) [37–39]. While the mechanisms of activation of these pathways have not yet been determined, upregulation of PKC enhances the phosphorylation of CELF1, a member of CELF RNA-binding protein family. The phosphorylation of CELF1 extends its half-life, resulting in the upregulation of its steady-state level [37] (Fig. 3.4). Since CELF1 is a multifunctional RNA-binding protein involved in the regulation of alternative splicing and translation, upregulation of CELF1 protein in DM1 results in misregulated alternative splicing of several pre-mRNAs, as well as altered translation or decay of mRNAs that are bound by CELF1 [40–43]. Transgenic mice overexpressing Celf1 show splicing misregulation and cardiac conduction defect similar to DM1 [44]. As CELF1 plays an important role in developmental splicing switch in the heart, increased CELF1 expression may underlie the cardiac pathology observed in DM1. However, CELF1 upregulation has not been confirmed in DM2 [27, 45].

RNA splicing is the mechanism by which intervening introns are removed, while functional exons are ligated together to form a mature mRNA transcript [46]. There are two types of RNA splicing: constitutive and alternative splicing. Alternative splicing serves to generate a wide variety of unique mRNA transcripts by selecting and pairing discrete exons, resulting in distinct but similar proteins from the same gene (Fig. 3.5).

Although many human genetic disorders have been identified as the consequences of mutations on RNA splicing, in almost all cases, these are cis-acting effects that affect the splicing of a single pre-mRNA. In contrast, DM is caused by a trans-effect of alternative splicing factors on many RNAs and now is recognized as the first example of a human genetic disease resulting from “spliceopathy” [4]. The spliceopathy in DM shares common features in splicing misregulation. First, it does not involve constitutive exons, but selectively affects a group of exons that are subject to alternative splicing. Second, it usually affects alternative exons that normally undergo a developmental splicing switch; therefore, the fetal or neonatal splicing isoforms are expressed in mature muscle fibers in DM1. Third, most target exons in the spliceopathy in DM are regulated by MBNL1, CELF1, or both.

More than 70 mis-splicing events have been reported in DM. Table 3.1 lists the transcripts known to be affected by spliceopathy in DM. In case of CLCN1, the exon 7a is negatively regulated by MBNL1 [34]. When MBNL1 is depleted in DM muscle, the exon 7a is retained in the mRNA. The inclusion of exon 7a induces a premature termination codon, which causes loss of mature CLCN1 protein in muscle membrane in DM, resulting in myotonia [29]. Misregulated splicing of insulin receptor (INSR) is also reported in DM muscle [41]. Alternative splicing of exon 11 is an important factor dictating the function of this receptor. The receptor lacking the exon 11 (non-muscle form) has a higher affinity for insulin than the receptor including the exon (muscle form). Because the inclusion of exon 11 is negatively regulated by CELF1, the non-muscle form lacking exon 11 is increased in DM muscle, leading to insulin resistance. However, whereas numerous mis-splicing events have been identified in DM, the underlying cause of progressive muscle wasting, the main disabling symptom in DM, has not been well defined. One possible candidate is mis-splicing of a group of genes regulating Ca²⁺ homeostasis, such as ryanodine receptor (RYR1), sarcoplasmic/endoplasmic reticulum calcium ATPases (SERCAs), voltage-dependent Ca²⁺ channel (CACNA1S), and T-tubule membrane scaffolding protein (BIN1) [47–49]. Because Ca²⁺ concentration is increased in DM1 cells and impaired Ca²⁺ homeostasis triggers muscle damage, misregulated splicing of these transcripts may cause muscle wasting in DM [50]. Another candidate is splicing misregulation of genes encoding cytoskeletal proteins, such as dystrophin (DMD), dystrobrevin (DTNA), titin (TTN), and ZASP (LDB3), as mutations or deletions in these genes causes other types of muscular dystrophies and myopathies [27, 51, 52].