"Mitotic drive" of expanded CTG repeats in myotonic dystrophy type 1 (DM1)

Immortalized human myotonic dystrophy type 1 muscle cell lines to address patient heterogeneity

Historically, cellular models have been used as a tool to study myotonic dystrophy type 1 (DM1) and the validation of therapies in said pathology. However, there is a need for in vitro models that represent the clinical heterogeneity observed in patients with DM1 that is lacking in classical models. In this study, we immortalized three DM1 muscle lines derived from patients with different DM1 subtypes and clinical backgrounds and characterized them at the genetic, epigenetic, and molecular levels. All three cell lines display DM1 hallmarks, such as the accumulation of RNA foci, MBNL1 sequestration, splicing alterations, and reduced fusion. In addition, alterations in early myogenic markers, myotube diameter and CTCF1 DNA methylation were also found in DM1 cells. Notably, the new lines show a high level of heterogeneity in both the size of the CTG expansion and the aforementioned molecular alterations. Importantly, these immortalized cells also responded to previously tested therapeutics. Altogether, our results show that these three human DM1 cellular models are suitable to study the pathophysiological heterogeneity of DM1 and to test future therapeutic options.

Huge Leiomyomas Arising from Bilateral Uterine Remnants in a Mayer-Rokitansky-Küster-Hauser Syndrome Patient with Coexisting Myotonic Dystrophy Type 1: A Case Report and Literature Review

Mayer-Rokitansky-Küster-Hauser syndrome (MRKHS) is a rare congenital anomaly of the genital tract. Since the secretion of sex hormones from the ovaries is preserved, leiomyomas and adenomyomas, which are estrogen-dependent diseases, may develop from the uterine remnant. In contrast, patients with myotonic dystrophy type 1 (DM1), the most common dystrophy in adults, are considered to be at high risk for benign tumors of the female reproductive system, such as uterine leiomyomas and ovarian cysts. A rare case of huge leiomyomas arising from bilateral uterine remnants in a woman with MRKHS with coexisting DM1 is presented. Her chief complaint was abdominal distension. On pelvic magnetic resonance imaging (MRI), two solid pelvic masses showing low signal intensity on T2-weighted imaging were seen. Both the uterine corpus and cervix were unclear, but bilateral ovaries were observed normally on MRI. Two uterine leiomyoma-like masses connected by a band of fibrous tissue were found by laparotomy. As with the MRI findings, the uterine cervix and vagina could not be detected macroscopically. Normal bilateral adnexa and round ligaments were identified. All of her symptoms improved after hysterectomy.

Protein Phosphorylation Alterations in Myotonic Dystrophy Type 1: A Systematic Review

Among the most common muscular dystrophies in adults is Myotonic Dystrophy type 1 (DM1), an autosomal dominant disorder characterized by myotonia, muscle wasting and weakness, and multisystemic dysfunctions. This disorder is caused by an abnormal expansion of the CTG triplet at the DMPK gene that, when transcribed to expanded mRNA, can lead to RNA toxic gain of function, alternative splicing impairments, and dysfunction of different signaling pathways, many regulated by protein phosphorylation. In order to deeply characterize the protein phosphorylation alterations in DM1, a systematic review was conducted through PubMed and Web of Science databases. From a total of 962 articles screened, 41 were included for qualitative analysis, where we retrieved information about total and phosphorylated levels of protein kinases, protein phosphatases, and phosphoproteins in DM1 human samples and animal and cell models. Twenty-nine kinases, 3 phosphatases, and 17 phosphoproteins were reported altered in DM1. Signaling pathways that regulate cell functions such as glucose metabolism, cell cycle, myogenesis, and apoptosis were impaired, as seen by significant alterations to pathways such as AKT/mTOR, MEK/ERK, PKC/CUGBP1, AMPK, and others in DM1 samples. This explains the complexity of DM1 and its different manifestations and symptoms, such as increased insulin resistance and cancer risk. Further studies can be done to complement and explore in detail specific pathways and how their regulation is altered in DM1, to find what key phosphorylation alterations are responsible for these manifestations, and ultimately to find therapeutic targets for future treatments.

Modifiers of CAG/CTG Repeat Instability: Insights from Mammalian Models

At fifteen different genomic locations, the expansion of a CAG/CTG repeat causes a neurodegenerative or neuromuscular disease, the most common being Huntington's disease and myotonic dystrophy type 1. These disorders are characterized by germline and somatic instability of the causative CAG/CTG repeat mutations. Repeat lengthening, or expansion, in the germline leads to an earlier age of onset or more severe symptoms in the next generation. In somatic cells, repeat expansion is thought to precipitate the rate of disease. The mechanisms underlying repeat instability are not well understood. Here we review the mammalian model systems that have been used to study CAG/CTG repeat instability, and the modifiers identified in these systems. Mouse models have demonstrated prominent roles for proteins in the mismatch repair pathway as critical drivers of CAG/CTG instability, which is also suggested by recent genome-wide association studies in humans. We draw attention to a network of connections between modifiers identified across several systems that might indicate pathway crosstalk in the context of repeat instability, and which could provide hypotheses for further validation or discovery. Overall, the data indicate that repeat dynamics might be modulated by altering the levels of DNA metabolic proteins, their regulation, their interaction with chromatin, or by direct perturbation of the repeat tract. Applying novel methodologies and technologies to this exciting area of research will be needed to gain deeper mechanistic insight that can be harnessed for therapies aimed at preventing repeat expansion or promoting repeat contraction.

Distribution patterns of microsatellites and development of its marker in different genomic regions of forest musk deer genome based on high throughput sequencing

Forest musk deer (Moschus berezovskii, FMD) is an endangered artiodactyl species, male FMD produce musk. We have sequenced the whole genome of FMD, completed the genomic assembly and annotation, and performed bioinformatic analyses. Our results showed that microsatellites (SSRs) displayed nonrandomly distribution in genomic regions, and SSR abundances were much higher in the intronic and intergenic regions compared to other genomic regions. Tri- and hexanucleotide perfect (P) SSRs predominated in coding regions (CDSs), whereas, tetra- and pentanucleotide P-SSRs were less abundant. Trifold P-SSRs had more GC-contents in the 5′-untranslated regions (5'UTRs) and CDSs than other genomic regions, whereas mononucleotide P-SSRs had the least GC-contents. The repeat copy numbers (RCN) of the same mono- to hexanucleotide P-SSRs had different distributions in different genomic regions. The RCN of trinucleotide P-SSRs had increased significantly in the CDSs compared to the transposable elements (TEs), intronic and intergenic regions. The analysis of coefficient of variability (CV) of P-SSRs showed that the RCN of mononucleotide P-SSRs had relative higher variation in different genomic regions, followed by the CV pattern of RCN: dinucleotide P-SSRs > trinucleotide P-SSRs > tetranucleotide P-SSRs > pentanucleotide P-SSRs > hexanucleotide P-SSRs. The CV variations of RCN of the same mono- to hexanucleotide P-SSRs were relative higher in the intron and intergenic regions, followed by that in the TEs, and the relative lower was in the 5'UTR, CDSs and 3'UTRs. 58 novel polymorphic SSR loci were detected based on genotyping DNA from 36 captive FMD and 22 SSR markers finally showed polymorphism, stability, and repetition.

FXS-Like Phenotype in Two Unrelated Patients Carrying a Methylated Premutation of the FMR1 Gene

Fragile X syndrome (FXS) is mostly caused by two distinct events that occur in the FMR1 gene (Xq27.3): an expansion above 200 repeats of a CGG triplet located in the 5′UTR of the gene, and methylation of the cytosines located in the CpG islands upstream of the CGG repeats. Here, we describe two unrelated families with one FXS child and another sibling presenting mild intellectual disability and behavioral features evocative of FXS. Genetic characterization of the undiagnosed sibling revealed mosaicism in both the CGG expansion size and the methylation levels in the different tissues analyzed. This report shows that in the same family, two siblings carrying different CGG repeats, one in the full-mutation range and the other in the premutation range, present methylation mosaicism and consequent decreased FMRP production leading to FXS and FXS-like features, respectively. Decreased FMRP levels, more than the number of repeats seem to correlate with the severity of FXS clinical phenotypes.

Functional KCa1.1 channels are crucial for regulating the proliferation, migration and differentiation of human primary skeletal myoblasts

Myoblasts are mononucleated precursors of myofibers; they persist in mature skeletal muscles for growth and regeneration post injury. During myotonic dystrophy type 1 (DM1), a complex autosomal-dominant neuromuscular disease, the differentiation of skeletal myoblasts into functional myotubes is impaired, resulting in muscle wasting and weakness. The mechanisms leading to this altered differentiation are not fully understood. Here, we demonstrate that the calcium- and voltage-dependent potassium channel, KCa1.1 (BK, Slo1, KCNMA1), regulates myoblast proliferation, migration, and fusion. We also show a loss of plasma membrane expression of the pore-forming α subunit of KCa1.1 in DM1 myoblasts. Inhibiting the function of KCa1.1 in healthy myoblasts induced an increase in cytosolic calcium levels and altered nuclear factor kappa B (NFκB) levels without affecting cell survival. In these normal cells, KCa1.1 block resulted in enhanced proliferation and decreased matrix metalloproteinase secretion, migration, and myotube fusion, phenotypes all observed in DM1 myoblasts and associated with disease pathogenesis. In contrast, introducing functional KCa1.1 α-subunits into DM1 myoblasts normalized their proliferation and rescued expression of the late myogenic marker Mef2. Our results identify KCa1.1 channels as crucial regulators of skeletal myogenesis and suggest these channels as novel therapeutic targets in DM1.

Haplotype analysis and LD detection at DM1 locus

Myotonic dystrophy type 1 (DM1) is the most common form of muscular dystrophy affecting adults and is due to trinucleotide sequence (CTG) in the 3' UTR region of DMPK gene located at 19q13.3 chromosome. Several neighboring genes (markers) located on the same chromosomes that are statistically associated and transmitted together (haplotype), influence the disease pathogenesis as caused by mutated DMPK. The intention of the study was to investigate the population genetic characteristics and to identify founder haplotypes from Northern India. Clinically diagnosed and molecularly confirmed DM1 patients (=27) and their family members (=76) were included in the study. PCR-RFLP analysis was performed for intron 5 (C/T)/HhaI, DMPK (G/T) intron 9/HinfI, Bpm1 and CKMM genetic polymorphism. The SNP Stat Online Software was used to construct haplotype group and for linkage-disequilibrium analysis. In all DM chromosomes: allele 2 had higher frequency in HhaI and HinfI while allele 1 had higher frequency in BpmI and CKMM. Total 11, 7, 10 and 11 haplotype groups had been formed in proband (patients), proband's father, proband's mother and in combined group respectively. Haplotype combination 2 (HhaI)/2 (HinfI)/1 (BpmI)/1 (CKMM TaqI)/1 (CKMM Nco1) had higher frequency, 0.4096 and 0.2867 in patients and combined group respectively. The haplotype combination 1/1/1/1/1 and 2/1/1/1/1 was most common for patient's father and mother respectively. The polymorphic markers HhaI & HinfI; HinfI & BpmI; and HinfI & CKMM TaqI showed significant LD. In comparison to other population, HhaI and HinfI have common origin of mutation.

Assessment of Premutation in Myotonic Dystrophy Type 1 Affected Family Members by TP-PCR and Genetic Counseling

Myotonic dystrophy type 1 (DM1) is caused by the expansion of an unstable CTG repeat located in the 3′-UTR of (DMPK) the DM protein kinase gene. Patients with DM1 have expansions of greater than 50 repeats and up to many thousands. In the present study we aimed to evaluate the utility of TP-PCR in diagnostics as well as the assessment of premutation carriers in proband families. Twenty-seven DM1 cases were enrolled (from twenty-six families) and the 13 families of these cases came forward for family screening. The patient group constitute 22 males and 5 females and the average age of onset was 32.8 years (range 17 to 52). All clinically diagnosed DM1 cases and their family members DNA samples were analyzed by TP-PCR. All the cases were found to be positive for the CTG repeat expansion. Among those five families, four had at least an asymptomatic carrier. In the remaining one family other than the proband none was found to be neither affected nor asymptomatic. We reconfirmed the utility of PCR based screening for DM1 as being reliable and rapid molecular test and it should be used as an initial screening test for all patients with DM and their family members for initial screening purpose.

Application of a reliable and rapid polymerase chain reaction based method in the diagnosis of myotonic dystrophy type 1 (DM1) in India

DM1 is caused by CTG repeat expansion in the 3′-UTR of the DMPK gene. DM1 patients have expansions of greater than 50 repeats and up to many thousands. The intention of the present study is the establishment of reliable and rapid polymerase chain reaction methodology in early screening of DM1 patients and their family members. PCR followed by TP-PCR was assessed for screening of 27 cases (from 26 families) and 75 family members and 300 control samples. All patients had CTG repeat expansion while forty seven (63%) and twenty eight (37%), out of seventy five family members were heterozygous and homozygous respectively. Similarly, two hundred thirty (76.77%) and seventy (23.33%), out of three hundred control subjects were heterozygous and homozygous respectively and the number of repeats varied from 5 to 35. Thirteen complete family screenings were done. Thus, TP-PCR is a reliable and rapid molecular technique for the detection of CTG repeat expansion in DM1.

Myotonic dystrophy type 1 (DM1): a triplet repeat expansion disorder

Myotonic dystrophy is a progressive multisystem genetic disorder affecting about 1 in 8000 people worldwide. The unstable repeat expansions of (CTG)n or (CCTG)n in the DMPK and ZNF9 genes cause the two known subtypes of myotonic dystrophy: (i) myotonic dystrophy type 1 (DM1) and (ii) myotonic dystrophy type 2 (DM2) respectively. There is currently no cure but supportive management helps equally to reduce the morbidity and mortality and patients need close follow up to pay attention to their clinical problems. This review will focus on the clinical features, molecular view and genetics, diagnosis and management of DM1.

Myotonic dystrophy types 1 and 2

Myotonic dystrophies (dystrophia myotonica, or DM) are inherited disorders characterized by myotonia and progressive muscle degeneration, which are variably associated with a multisystemic phenotype. To date, two types of myotonic dystrophy, type 1 (DM1) and type 2 (DM2), are known to exist; both are autosomal dominant disorders caused by expansion of an untranslated short tandem repeat DNA sequence (CTG)(n) and (CCTG)(n), respectively. These expanded repeats in DM1 and DM2 show different patterns of repeat-size instability. Phenotypes of DM1 and DM2 are similar but there are some important differences, most conspicuously in the severity of the disease (including the presence or absence of the congenital form), muscles primarily affected (distal versus proximal), involved muscle fiber types (type 1 versus type 2 fibers), and some associated multisystemic phenotypes. The pathogenic mechanism of DM1 and DM2 is thought to be mediated by the mutant RNA transcripts containing expanded CUG and CCUG repeats. Strong evidence supports the hypothesis that sequestration of muscle-blind like (MBNL) proteins by these expanded repeats leads to misregulated splicing of many gene transcripts in corroboration with the raised level of CUG-binding protein 1. However, additional mechanisms, such as changes in the chromatin structure involving CTCN-binding site and gene expression dysregulations, are emerging. Although treatment of DM1 and DM2 is currently limited to supportive therapies, new therapeutic approaches based on pathogenic mechanisms may become feasible in the near future.

Bidirectional transcription stimulates expansion and contraction of expanded (CTG)*(CAG) repeats

More than 12 neurogenetic disorders are caused by unstable expansions of (CTG)•(CAG) repeats. The expanded repeats are unstable in germline and somatic cells, with potential consequences for disease severity. Previous studies have shown that contractions of (CAG)(95) are more frequent when the repeat tract is transcribed. Here we determined whether transcription can promote repeat expansion, using (CTG)•(CAG) repeat tracts in the size range that is typical for myotonic dystrophy type 1. We derived normal human fibroblasts having single-copy genomic integrations of 800 CTG repeats. The repeat tract showed modest instability when it was not transcribed, yielding an estimated mutation rate of 0.28% per generation. Instability was enhanced several-fold by transcription in the forward or reverse transcription, and 30-fold by bidirectional transcription, yielding many expansions and contractions of more than 200 repeats. These results suggest that convergent bidirectional transcription, which has been reported at several disease loci, could contribute to somatic instability of highly expanded (CTG)•(CAG) repeats.

Replication-dependent instability at (CTG) x (CAG) repeat hairpins in human cells

Instability of (CTG)•(CAG) microsatellite trinucleotide repeat (TNR) sequences is responsible for more than one dozen neurological or neuromuscular diseases. TNR instability during DNA synthesis is thought to involve slipped strand or hairpin structures in template or nascent DNA strands, although direct evidence for hairpin formation in human cells is lacking. We have used targeted recombination to create a series of isogenic HeLa cell lines in which (CTG)•(CAG) repeats are replicated from an ectopic copy of the c-myc replication origin. In this system the tendency of chromosomal (CTG)•(CAG) tracts to expand or contract was affected by origin location and the leading or lagging strand replication orientation of the repeats, and instability was enhanced by prolonged cell culture, increasing TNR length, and replication inhibition. Hairpin cleavage by synthetic zinc finger nucleases in these cells has provided the first direct evidence for the formation of hairpin structures during replication in vivo.

CTG repeat instability in a human embryonic stem cell line carrying the myotonic dystrophy type 1 mutation

Human embryonic stem cells (hESC) are considered to be an indefinite source of self-renewing cells that can differentiate into all types of cells of the human body and could be used in regenerative medicine, drug discovery and as a model for studying early developmental biology. hESC carrying disease-causing mutations hold promise as a tool to investigate mechanisms involved in the pathogenesis of the disease. In this report, we describe the behaviour of an expanded CTG repeat in the 3' untranslated region of the DMPK gene in VUB03_DM1, a hESC line carrying the myotonic dystrophy type 1 (DM1) mutation compared with the normal CTG repeat in two hESC lines VUB01 and VUB04_CF. Expanded CTG repeats were detected by small amount PCR, small pool PCR and Southern blot analysis in consecutive passages of VUB03_DM1. An important instability of the CTG repeat was detected during prolonged in vitro culture, showing stepwise increases of the repeat number in consecutive passages as well as a higher range of variability. This variability was present in cells of different colonies of the same passage and even within single colonies. The high repeat instability is in contrast to the previously observed stability of the repeat in preimplantation embryos and in fetuses during the first trimester of pregnancy. This in vitro culture of affected hESC represents a valuable model for studying the biology of repeat instability.

Ectopic expression of cyclin D3 corrects differentiation of DM1 myoblasts through activation of RNA CUG-binding protein, CUGBP1

Differentiation of myocytes is impaired in patients with myotonic dystrophy type 1, DM1. CUG repeat binding protein, CUGBP1, is a key regulator of translation of proteins that are involved in muscle development and differentiation. In this paper, we present evidence that RNA-binding activity of CUGBP1 and its interactions with initiation translation complex eIF2 are differentially regulated during myogenesis by specific phosphorylation and that this regulation is altered in DM1. In normal myoblasts, Akt kinase phosphorylates CUGBP1 at Ser28 and increases interactions of CUGBP1 with cyclin D1 mRNA. During differentiation, CUGBP1 is phosphorylated by cyclinD3-cdk4/6 at Ser302, which increases CUGBP1 binding with p21 and C/EBPbeta mRNAs. While cyclin D3 and cdk4 are elevated in normal myotubes; DM1 differentiating cells do not increase these proteins. In normal myotubes, CUGBP1 interacts with cyclin D3/cdk4/6 and eIF2; however, interactions of CUGBP1 with eIF2 are reduced in DM1 differentiating cells and correlate with impaired muscle differentiation in DM1. Ectopic expression of cyclin D3 in DM1 cells increases the CUGBP1-eIF2 complex, corrects expression of differentiation markers, myogenin and desmin, and enhances fusion of DM1 myoblasts. Thus, normalization of cyclin D3 might be a therapeutic approach to correct differentiation of skeletal muscle in DM1 patients.

Increased SK3 expression in DM1 lens cells leads to impaired growth through a greater calcium-induced fragility

Although cataract is a characteristic feature of myotonic dystrophy type 1 (DM1), little is known of the underlying mechanisms. We generated four lens epithelial cell lines derived from DM1 cataracts and two from age-matched, non-DM cataracts. Small-pool PCR revealed typical large triplet repeat expansions in the DM1 cells. Furthermore, real-time PCR analysis showed reduced SIX5 expression and increased expression of the Ca(2+)-activated K(+) channel SK3 in the DM1 cells. These cells also exhibited longer population doubling times which did not arise through reduced proliferation, but rather increased cell death as shown by increased release of lactate dehydrogenase (LDH). Using (86)Rb(+) as a tracer for K(+), we found no difference in the resting K(+) influx or efflux kinetics. In all cases, the ouabain sensitive component of the influx contributed approximately 50% of the total. However, stimulating internal Ca(2+) by exposure to ionomycin not only caused greater stimulation of K(+) ((86)Rb) efflux in the DM1 cells but also induced a higher rate of cell death (LDH assay). Since both the hyper-stimulation of K(+) efflux and cell death were reduced by the highly specific SK inhibitor apamin, we suggest that increased expression of SK3 has a critical role in the increased Ca(2+)-induced fragility in DM1 cells. The present data, therefore, both help explain the lower epithelial cell density previously observed in DM1 cataracts and provide general insights into mechanisms underlying the fragility of other DM1-affected tissues.

Chemical modifiers of unstable expanded simple sequence repeats: what goes up, could come down

A mounting number of inherited human disorders, including Huntington disease, myotonic dystrophy, fragile X syndrome, Friedreich ataxia and several spinocerebellar ataxias, have been associated with the expansion of unstable simple sequence DNA repeats. Despite a similar genetic basis, pathogenesis in these disorders is mediated by a variety of both loss and gain of function pathways. Thus, therapies targeted at downstream pathology are likely to be disease specific. Characteristically, disease-associated expanded alleles in these disorders are highly unstable in the germline and somatic cells, with a tendency towards further expansion. Whereas germline expansion accounts for the phenomenon of anticipation, tissue-specific, age-dependent somatic expansion may contribute towards the tissue-specificity and progressive nature of the symptoms. Thus, somatic expansion presents as a novel therapeutic target in these disorders. Suppression of somatic expansion should be therapeutically beneficial, whilst reductions in repeat length could be curative. It is well established that both cis- and trans-acting genetic modifiers play key roles in the control of repeat dynamics. Importantly, recent data have revealed that expanded CAG.CTG repeats are also sensitive to a variety of trans-acting chemical modifiers. These data provide an exciting proof of principle that drug induced suppression of somatic expansion might indeed be feasible. Moreover, as our understanding of the mechanism of expansion is refined more rational approaches to chemical intervention in the expansion pathway can be envisioned. For instance, the demonstration that expansion of CAG.CTG repeats is dependent on the Msh2, Msh3 and Pms2 genes, highlights components of the DNA mismatch repair pathway as therapeutic targets. In addition to potential therapeutic applications, the response of expanded simple repeats to genotoxic assault suggests such sequences could also have utility as bio-monitors of environmentally induced genetic damage in the soma.

Chemotherapeutic deletion of CTG repeats in lymphoblast cells from DM1 patients

Myotonic dystrophy type 1 (DM1) is caused by the expansion of a (CTG).(CAG) repeat in the DMPK gene on chromosome 19q13.3. At least 17 neurological diseases have similar genetic mutations, the expansion of DNA repeats. In most of these disorders, the disease severity is related to the length of the repeat expansion, and in DM1 the expanded repeat undergoes further elongation in somatic and germline tissues. At present, in this class of diseases, no therapeutic approach exists to prevent or slow the repeat expansion and thereby reduce disease severity or delay disease onset. We present initial results testing the hypothesis that repeat deletion may be mediated by various chemotherapeutic agents. Three lymphoblast cell lines derived from two DM1 patients treated with either ethylmethanesulfonate (EMS), mitomycin C, mitoxantrone or doxorubicin, at therapeutic concentrations, accumulated deletions following treatment. Treatment with EMS frequently prevented the repeat expansion observed during growth in culture. A significant reduction of CTG repeat length by 100-350 (CTG).(CAG) repeats often occurred in the cell population following treatment with these drugs. Potential mechanisms of drug-induced deletion are presented.

Altered activity of signaling pathways in diaphragm and tibialis anterior muscle of dystrophic mice

Duchenne muscular dystrophy is a musculoskeletal disease caused by mutations in the dystrophin gene. The purpose of this study was to use the mouse model of muscular dystrophy (mdx) to determine if the progression of the dystrophic phenotype in the diaphragm (costal) versus limb skeletal muscle (tibialis anterior) is associated with specific changes in extracellular regulated kinase (ERK1/2), p70 S6 kinase (p70(S6k)), or p38 signaling pathways. The studies detected that consistent with an earlier dystrophic phenotype, phosphorylation of p70(S6k) is elevated by 40% in the diaphragm with no change in limb muscle. In addition, phosphorylation of p38 kinase was decreased by 33% in the mdx diaphragm muscle. Levels of ERK1/2 as well as phosphorylation states were elevated in the diaphragm and limb muscle of mdx mice compared with age-matched control muscles. These results indicate that distinct signaling pathways are differentially activated in skeletal muscle of mdx mice. The specificity of these responses, particularly in the diaphragm, provides insight for potential targets for blunting the progression of the muscular dystrophy phenotype.

Mutagenic stress modulates the dynamics of CTG repeat instability associated with myotonic dystrophy type 1

The molecular basis of the myotonic dystrophy type 1 is the expansion of a CTG repeat at the DMPK locus. The expanded disease-associated repeats are unstable in both somatic and germ lines, with a high tendency towards expansion. The rate of expansion is directly related to the size of the pathogenic allele, increasing the size heterogeneity with age. It has also been suggested that additional factors, including as yet unidentified environmental factors, might affect the instability of the expanded CTG repeats to account for the observed CTG size dynamics over time. To investigate the effect of environmental factors in the CTG repeat instability, three lymphoblastoid cell lines were established from two myotonic dystrophy patients and one healthy individual, and parallel cultures were concurrently expanded in the presence or absence of the mutagenic chemical mitomycin C for a total of 12 population doublings. The new alleles arising along the passages were analysed by radioactive small pool PCR and sequencing gels. An expansion bias of the stepwise mutation was observed in a (CTG)124 allele of a cell line harbouring two modal alleles of 28 and 124 CTG repeats. Interestingly, this expansion bias was clearly enhanced in the presence of mitomycin C. The effect of mitomycin C was also evident in the normal size alleles in two cell lines with alleles of 13/13 and 12/69 repeats, where treated cultures showed new longer alleles. In conclusion, our results indicate that mitomycin C modulates the dynamics of myotonic dystrophy-associated CTG repeats in LBCLs, enhancing the expansion bias of long-pathogenic repeats and promoting the expansion of normal length repeats.

Somatic and germline instability of the ATTCT repeat in spinocerebellar ataxia type 10

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant disorder characterized by ataxia, seizures, and anticipation. It is caused by an expanded ATTCT pentanucleotide repeat in intron 9 of a novel gene, designated "SCA10." The ATTCT expansion in SCA10 represents a novel class of microsatellite repeat and is one of the largest found to cause human diseases. The expanded ATTCT repeat is unstably transmitted from generation to generation, and an inverse correlation has been observed between size of repeat and age at onset. In this multifamily study, we investigated the intergenerational instability, somatic and germline mosaicism, and age-dependent repeat-size changes of the expanded ATTCT repeat. Our results showed that (1) the expanded ATTCT repeats are highly unstable when paternally transmitted, whereas maternal transmission resulted in significantly smaller changes in repeat size; (2) blood leukocytes, lymphoblastoid cells, buccal cells, and sperm have a variable degree of mosaicism in ATTCT expansion; (3) the length of the expanded repeat was not observed to change in individuals over a 5-year period; and (4) clinically determined anticipation is sometimes associated with intergenerational contraction rather than expansion of the ATTCT repeat.

Chemically induced increases and decreases in the rate of expansion of a CAG*CTG triplet repeat

Somatic mosaicism of repeat length is prominent in repeat expansion disorders such as Huntington disease and myotonic dystrophy. Somatic mosaicism is age-dependent, tissue-specific and expansion-biased, and likely contributes toward the tissue-specificity and progressive nature of the symptoms. We propose that therapies targeted at somatic repeat expansion may have general utility in these disorders. Specifically, suppression of somatic expansion would be expected to be therapeutic, whilst reversion of the expanded mutant repeat to within the normal range would be predicted to be curative. However, the effects of genotoxic agents on the mutational properties of specific nuclear genes are notoriously difficult to define. Nonetheless, we have determined that chronic exposure over a three month period to a number of genotoxic agents can alter the rate of triplet repeat expansion in whole populations of mammalian cells. Interestingly, high doses of caffeine increased the rate of expansion by approximately 60%. More importantly, cytosine arabinoside, ethidium bromide, 5-azacytidine and aspirin all significantly reduced the rate of expansion by from 35 to 75%. These data establish that drug induced suppression of somatic expansion is possible. These data also suggest that highly unstable expanded simple sequence repeats may act as sensitive reporters of genotoxic assault in the soma.

Replication inhibitors modulate instability of an expanded trinucleotide repeat at the myotonic dystrophy type 1 disease locus in human cells

Gene-specific CTG/CAG repeat expansion is associated with at least 14 human diseases, including myotonic dystrophy type 1 (DM1). Most of our understanding of trinucleotide instability is from nonhuman models, which have presented mixed results, supporting replication errors or processes independent of cell division as causes. Nevertheless, the mechanism occurring at the disease loci in patient cells is poorly understood. Using primary fibroblasts derived from a fetus with DM1, we have shown that spontaneous expansion of the diseased (CTG)(216) allele occurred in proliferating cells but not in quiescent cells. Expansions were "synchronous," with mutation frequencies approaching 100%. Furthermore, cells were treated with agents known to alter DNA synthesis but not to directly damage DNA. Inhibiting replication initiation with mimosine had no effect upon instability. Inhibiting both leading- and lagging-strand synthesis with aphidicolin or blocking only lagging strand synthesis with emetine significantly enhanced CTG expansions. It was striking that only the expanded DM1 allele was altered, leaving the normal allele, (CTG)(12), and other repeat loci unaffected. Standard and small-pool polymerase chain reaction revealed that inhibitors enhanced the magnitude of short expansions in most cells threefold, whereas 11%-25% of cells experienced gains of 122-170 repeats, to sizes of (CTG)(338)-(CTG)(386). Similar results were observed for an adult DM1 cell line. Our results support a role for the perturbation of replication fork dynamics in DM1 CTG expansions within patient fibroblasts. This is the first report that repeat-length alterations specific to a disease allele can be modulated by exogenously added compounds.

Myotonic syndromes

PURPOSE OF REVIEW

To highlight recent advances in understanding the clinical manifestations and molecular genetics of myotonic syndromes, with particular emphasis on the myotonic dystrophies.

RECENT FINDINGS

Myotonic syndromes include the non-dystrophic myotonias, caused by mutations in genes encoding the chloride or sodium channels that are specific to skeletal muscle, and the myotonic dystrophies. Previous studies have shown that myotonic dystrophy type 1 is caused by the expansion of a CTG repeat in the gene. Recently, it was discovered that myotonic dystrophy type 2 (proximal myotonic myopathy) is also caused by a DNA expansion mutation. In both types of myotonic dystrophy the expanded repeat is transcribed and the RNA produced from the mutant allele is retained in nuclear inclusions. Recent studies suggest that the mutant RNA has a toxic effect on muscle fibers by interfering with the essential functions of the myonucleus, such as RNA processing.

SUMMARY

It now appears likely that myotonic dystrophy is the first instance of a genetic disease in which the harmful effect of a mutation involves the production of a pathogenic RNA. However, the exact mechanism is not understood, and it is unclear whether this RNA-mediated disease process is also responsible for the manifestations of myotonic dystrophy in non-muscle tissues.

Molecular basis for impaired muscle differentiation in myotonic dystrophy

Differentiation of skeletal muscle is affected in myotonic dystrophy (DM) patients. Analysis of cultured myoblasts from DM patients shows that DM myoblasts lose the capability to withdraw from the cell cycle during differentiation. Our data demonstrate that the expression and activity of the proteins responsible for cell cycle withdrawal are altered in DM muscle cells. Skeletal muscle cells from DM patients fail to induce cytoplasmic levels of a CUG RNA binding protein, CUGBP1, while normal differentiated cells accumulate CUGBP1 in the cytoplasm. In cells from normal patients, CUGBP1 up-regulates p21 protein during differentiation. Several lines of evidence show that CUGBP1 induces the translation of p21 via binding to a GC-rich sequence located within the 5' region of p21 mRNA. Failure of DM cells to accumulate CUGBP1 in the cytoplasm leads to a significant reduction of p21 and to alterations of other proteins responsible for the cell cycle withdrawal. The activity of cdk4 declines during differentiation of cells from control patients, while in DM cells cdk4 is highly active during all stages of differentiation. In addition, DM cells do not form Rb/E2F repressor complexes that are abundant in differentiated cells from normal patients. Our data provide evidence for an impaired cell cycle withdrawal in DM muscle cells and suggest that alterations in the activity of CUGBP1 causes disruption of p21-dependent control of cell cycle arrest.