Myotonic dystrophy: will the real gene please step forward!

Cancer and Myotonic Dystrophy

Myotonic dystrophy (DM) is the most common muscular dystrophy in adults. Dominantly inherited CTG and CCTG repeat expansions in DMPK and CNBP genes cause DM type 1 (DM1) and 2 (DM2), respectively. These genetic defects lead to the abnormal splicing of different mRNA transcripts, which are thought to be responsible for the multiorgan involvement of these diseases. In ours and others' experience, cancer frequency in patients with DM appears to be higher than in the general population or non-DM muscular dystrophy cohorts. There are no specific guidelines regarding malignancy screening in these patients, and the general consensus is that they should undergo the same cancer screening as the general population. Here, we review the main studies that investigated cancer risk (and cancer type) in DM cohorts and those that researched potential molecular mechanisms accounting for DM carcinogenesis. We propose some evaluations to be considered as malignancy screening in patients with DM, and we discuss DM susceptibility to general anesthesia and sedatives, which are often needed for the management of cancer. This review underscores the importance of monitoring the adherence of patients with DM to malignancy screenings and the need to design studies that determine whether they would benefit from a more intensified cancer screening than the general population.

Myotonic dystrophy type 1 (DM1) clinical sub-types and CTCF site methylation status flanking the CTG expansion are mutant allele length-dependent

Myotonic dystrophy type 1 (DM1) is a complex disease with a wide spectrum of symptoms. The exact relationship between mutant CTG repeat expansion size and clinical outcome remains unclear. DM1 congenital patients (CDM) inherit the largest expanded alleles, which are associated with abnormal and increased DNA methylation flanking the CTG repeat. However, DNA methylation at the DMPK locus remains understudied. Its relationship to DM1 clinical subtypes, expansion size and age-at-onset is not yet completely understood. Using pyrosequencing-based methylation analysis on 225 blood DNA samples from Costa Rican DM1 patients, we determined that the size of the estimated progenitor allele length (ePAL) is not only a good discriminator between CDM and non-CDM cases (with an estimated threshold at 653 CTG repeats), but also for all DM1 clinical subtypes. Secondly, increased methylation at both CTCF sites upstream and downstream of the expansion was almost exclusively present in CDM cases. Thirdly, levels of abnormal methylation were associated with clinical subtype, age and ePAL, with strong correlations between these variables. Fourthly, both ePAL and the intergenerational expansion size were significantly associated with methylation status. Finally, methylation status was associated with ePAL and maternal inheritance, with almost exclusively maternal transmission of CDM. In conclusion, increased DNA methylation at the CTCF sites flanking the DM1 expansion could be linked to ePAL, and both increased methylation and the ePAL could be considered biomarkers for the CDM phenotype.

Expanded CTG repeat demarcates a boundary for abnormal CpG methylation in myotonic dystrophy patient tissues

Myotonic dystrophy (DM1) affects multiple organs, shows age-dependent progression and is caused by CTG expansions at the DM1 locus. We determined the DM1 CpG methylation profile and CTG length in tissues from DM1 foetuses, DM1 adults, non-affected individuals and transgenic DM1 mice. Analysis included CTCF binding sites upstream and downstream of the CTG tract, as methylation-sensitive CTCF binding affects chromatinization and transcription of the DM1 locus. In humans, in a given foetus, expansions were largest in heart and smallest in liver, differing by 40-400 repeats; in adults, the largest expansions were in heart and cerebral cortex and smallest in cerebellum, differing by up to 5770 repeats in the same individual. Abnormal methylation was specific to the mutant allele. In DM1 adults, heart, liver and cortex showed high-to-moderate methylation levels, whereas cerebellum, kidney and skeletal muscle were devoid of methylation. Methylation decreased between foetuses and adults. Contrary to previous findings, methylation was not restricted to individuals with congenital DM1. The expanded repeat demarcates an abrupt boundary of methylation. Upstream sequences, including the CTCF site, were methylated, whereas the repeat itself and downstream sequences were not. In DM1 mice, expansion-, tissue- and age-specific methylation patterns were similar but not identical to those in DM1 individuals; notably in mice, methylation was present up- and downstream of the repeat, but greater upstream. Thus, in humans, the CpG-free expanded CTG repeat appears to maintain a highly polarized pattern of CpG methylation at the DM1 locus, which varies markedly with age and tissues.

Repeat expansion disease: progress and puzzles in disease pathogenesis

Repeat expansion mutations cause at least 22 inherited neurological diseases. The complexity of repeat disease genetics and pathobiology has revealed unexpected shared themes and mechanistic pathways among the diseases, such as RNA toxicity. Also, investigation of the polyglutamine diseases has identified post-translational modification as a key step in the pathogenic cascade and has shown that the autophagy pathway has an important role in the degradation of misfolded proteins--two themes that are likely to be relevant to the entire neurodegeneration field. Insights from repeat disease research are catalysing new lines of study that should not only elucidate molecular mechanisms of disease but also highlight opportunities for therapeutic intervention for these currently untreatable disorders.

RNA surveillance: molecular approaches in transcript quality control and their implications in clinical diseases

Production of mature mRNAs that encode functional proteins involves highly complex pathways of synthesis, processing and surveillance. At numerous steps during the maturation process, the mRNA transcript undergoes scrutiny by cellular quality control machinery. This extensive RNA surveillance ensures that only correctly processed mature mRNAs are translated and precludes production of aberrant transcripts that could encode mutant or possibly deleterious proteins. Recent advances in elucidating the molecular mechanisms of mRNA processing have demonstrated the existence of an integrated network of events, and have revealed that a variety of human diseases are caused by disturbances in the well-coordinated molecular equilibrium of these events. From a medical perspective, both loss and gain of function are relevant, and a considerable number of different diseases exemplify the importance of the mechanistic function of RNA surveillance in a cell. Here, mechanistic hallmarks of mRNA processing steps are reviewed, highlighting the medical relevance of their deregulation and how the understanding of such mechanisms can contribute to the development of therapeutic strategies.

Functional MRI changes in the central motor system in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a multisystemic disease involving multiple organ systems including central nervous system (CNS) and muscles. Few studies have focused on the central motor system in DM1, pointing to a subclinical abnormality in the CNS. The aim of our study was to investigate patterns of cerebral activation in DM1 during a motor task using functional MRI (fMRI). Fifteen DM1 patients, aged 20 to 59 years, and 15 controls of comparable age were scanned during a self-paced sequential finger-to-thumb opposition task of their dominant right hand. Functional MRI images were analyzed using SPM99. Patients underwent clinical and genetic assessment; all subjects underwent a conventional MR study. Myotonic dystrophy type 1 patients showed greater activation than controls in bilateral sensorimotor areas and inferior parietal lobules, basal ganglia and thalami, in the ipsilateral premotor area, insula and supplementary motor area (corrected P<.05). Analysis of the interaction between disease and age showed that correlation with age was significantly greater in patients than in controls in bilateral sensorimotor areas and in contralateral parietal areas. Other clinical and MR characteristics did not correlate with fMRI. Functional changes in DM1 may represent compensatory mechanisms such as reorganization and redistribution of functional networks to compensate for ultrastructural and neurochemical changes occurring as part of the accelerated aging process.

Patients with primary cataract as a genetic pool of DMPK protomutation

Myotonic dystrophy 1 (DM1) is known to diminish reproductive fitness in its severe form. Since no de novo mutations are known for this disease, it has the tendency to become extinct from a population. To explain the preservation of DM1 in a population, a hypothesis that a pool of subjects for the mutated gene exists in the apparently healthy (non-DM1) population was tested. In order to determine the (CTG) repeat number, PCR was performed in 274 patients found to have primary cataract of adult onset who showed no DM1 symptoms, and were not related to DM1 patients. In four cataract patients (1.46%; 95% CI 0.5-3.7), a protomutation in the myotonin protein kinase gene was found which might lead to a complete mutation after transmission through the next generations. The number of (CTG) repeats in the remaining 270 cataract patients did not differ significantly from the control subjects in terms of the distribution of larger [(CTG)n > or = 19] versus smaller [(CTG)n < 19] alleles. We consider the primary cataract patients to be the pool of DMPK protomutation from which DM1 mutation is maintained in the population.

Myotonic dystrophy?no evidence for preferential transmission of the mutated allele: A prenatal analysis

Myotonic dystrophy is the commonest autosomal dominant type of muscular dystrophy in adults. It is one of the trinucleotide repeat expansion disorders, and its severity correlates with the number of CTG repeats in the myotonic dystrophy gene. It has been suggested that myotonic dystrophy exhibits the phenomenon of preferential transmission of the larger mutated alleles that has been described in other trinucleotide repeat disorders. Several authors have reported that the frequency of transmission of the mutated alleles is higher than 50%--a finding that, if true, does not comply with the Mendelian laws of segregation. However, these studies were based on data from the analysis of pedigrees with ascertainment bias. In our study, we determined the frequency of transmission of mutated alleles using data from prenatal molecular studies, which are not subject to ascertainment bias. This is the first study to examine the segregation of the mutated alleles in myotonic dystrophy in pregnancy. Eighty-three fetuses were examined, 30 of 62 mothers (48.38%) and 8 of 21 fathers (38.09%) transmitted the mutated allele, giving an overall transmission rate of 45.78%. We found no evidence of statistically significant deviation of the frequency of transmission of the mutated alleles from the 50% expected in autosomal dominant disorders. This study, unlike previous ones, excludes preferential transmission in myotonic dystrophy, a finding that may be attributable to the lack of correction for ascertainment bias in previous studies and to the use of prenatal data in this study.

Myotonic dystrophy and paediatric anaesthesia

Myotonic dystrophy is a neuromuscular condition inherited in an autosomal dominant fashion, and is most commonly diagnosed in the neonatal period. With improving levels of care, these patients are now presenting more commonly for anaesthesia. We review the clinical features of the condition, and then discuss the steps in the anaesthetic process, outlining the anaesthetic implications of myotonic dystrophy at each stage.

Effects of age and gene dose on skeletal muscle sodium channel gating in mice deficient in myotonic dystrophy protein kinase

Myotonic muscular dystrophy (DM) is characterized by abnormal skeletal muscle Na channel gating and reduced levels of myotonic dystrophy protein kinase (DMPK). Electrophysiological measurements show that mice deficient in Dmpk have reduced Na currents in muscle. We now find that the Na channel expression level is normal in mouse muscle partially or completely deficient in Dmpk. Reduced current amplitudes are not changed by age or gene dose, and the reduction is not due to changes in macroscopic or microscopic gating kinetics. The mechanism of abnormal membrane excitability in DM may in part be silencing of muscle Na channels due to Dmpk deficiency.

Myotonic dystrophy--a multigene disorder

Myotonic dystrophy (DM1) is the most common form of adult muscular dystrophy with an estimated incidence of 1/8000 births. The mutation responsible for this condition is an expanded CTG repeat within the 3' untranslated region of the protein kinase gene DMPK. Strong nucleosome positioning signals created by this expanded repeat cause a reduction in gene expression within the region. This "field effect" is further confounded by the retention of DMPK expansion containing transcripts, which acquire a toxic gain of function. Thus, the various manifestations exhibited by DM1 patients can be explained as a result of gene silencing, nuclear retention and sequestration of nuclear factors by the CUG containing transcript.

Anaesthetic management of a patient with myotonic dystrophy

A 13-year-old boy with myotonic dystrophy underwent insertion of a percutaneous gastrostomy feeding tube under general anaesthesia. We used a laryngeal mask airway and a spontaneously breathing technique with propofol total intravenous anaesthesia. Postoperative vomiting and aspiration, 12 h after the procedure, subsequently required intubation and ventilation. We discuss the anaesthetic management of this case and review the features of the disease to be considered when contemplating anaesthesia in such patients.

Myotonic dystrophy and myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) was designated as a gene responsible for myotonic dystrophy (DM) on chromosome 19, because the gene product has extensive homology to protein kinase catalytic domains. DM is the most common disease with multisystem disorders among muscular dystrophies. The genetic basis of DM is now known to include mutational expansion of a repetitive trinucleotide sequence (CTG)n in the 3'-untranslated region (UTR) of DMPK. Full-length DMPK was detected and various isoforms of DMPK have been reported in skeletal and cardiac muscles, central nervous tissues, etc. DMPK is localized predominantly in type I muscle fibers, muscle spindles, neuromuscular junctions and myotendinous tissues in skeletal muscle. In cardiac muscle it is localized in intercalated dises and Purkinje fibers. Electron microscopically it is detected in the terminal cisternae of SR in skeletal muscle and the junctional and corbular SR in cardia muscle. In central nervous system, it is located in many neurons, especially in the cytoplasm of cerebellar Purkinje cells, hippocampal interneurons and spinal motoneurons. Electron microscopically it is detected in rough endoplasmic reticulum. The functional role of DMPK is not fully understood, however, it may play an important role in Ca2+ homeostasis and signal transduction system. Diseased amount of DMPK may play an important role in the degeneration of skeletal muscle in adult type DM. However, other molecular pathogenetical mechanisms such as dysfunction of surrounding genes by structural change of the chromosome by long trinucleotide repeats, and the trans-gain of function of CUG-binding proteins might be responsible to induce multisystemic disorders of DM such as myotonia, endocrine dysfunction, etc.

The intrinsically unstable life of DNA triplet repeats associated with human hereditary disorders

Expansions of specific DNA triplet repeats are the cause of an increasing number of hereditary neurological disorders in humans. In some diseases, such as Huntington's and several spinocerebellar ataxias, the repetitive DNA sequences are translated into long tracts of the same amino acid (usually glutamine), which alters interactions with cellular constituents and leads to the development of disease. For other disorders, including common genetic disorders such as myotonic dystrophy and fragile X syndrome, the DNA repeat is located in noncoding regions of transcribed sequences and disease is probably caused by altered gene expression. In studies in lower organisms, mammalian cells, and transgenic mice, high frequencies of length changes (increases and decreases) occur in long DNA triplet repeats. These observations are similar to other types of repetitive DNA sequences, which also undergo frequent length changes at genomic loci. A variety of processes acting on DNA influence the genetic stability of DNA triplet repeats, including replication, recombination, repair, and transcription. It is not yet known how these different multienzyme systems interact to produce the genetic mutation of expanded repeats. In vitro studies have identified that DNA triplet repeats can adopt several unusual DNA structures, including hairpins, triplexes, quadruplexes, slipped structures, and highly flexible and writhed helices. The formation of stable unusual structures within the cell is likely to disturb DNA metabolism and be a critical intermediate in the molecular mechanism(s) leading to genetic instabilities of DNA repeats and, hence, to disease pathogenesis.

The expression of ion channel mRNAs in skeletal muscles from patients with myotonic muscular dystrophy

We investigated gene expression patterns of ion channels including the apamin-sensitive small-conductance Ca(2+)-activated K(+) (SK3) channel, the adult isoform of the skeletal muscle Na(+) channel (SkM1), the fetal isoform of skeletal muscle Na(+) channel (H1), and the Cl(-) channel (ClC-1) by using the semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) for muscle samples from patients with adult onset myotonic dystrophy (DM), amyotrophic lateral sclerosis, and polymyositis. Patients with DM showed a significant increase in SK3 mRNA but not in mRNAs for other ion channels. The increased expression of SK3 gene in DM did not correlate with H1, the marker of muscle denervation, or the percentage of type 2C fiber, the marker of muscle regeneration.

Six family genes--structure and function as transcription factors and their roles in development

The members of the Six gene family were identified as homologues of Drosophila sine oculis which is essential for compound-eye formation. The Six proteins are characterized by the Six domain and the Six-type homeodomain, both of which are essential for specific DNA binding and for cooperative interactions with Eya proteins. Mammals possess six Six genes which can be subdivided into three subclasses, and mutations of Six genes have been identified in human genetic disorders. Characterization of Six genes from various animal phyla revealed the antiquity of this gene family and roles of its members in several different developmental contexts. Some members retain conserved roles as components of the Pax-Six-Eya-Dach regulatory network, which may have been established in the common ancestor of all bilaterians as a toolbox controlling cell proliferation and cell movement during embryogenesis. Gene duplications and cis-regulatory changes may have provided a basis for diverse functions of Six genes in different animal lineages.

Mice deficient in Six5 develop cataracts: implications for myotonic dystrophy

Expansion of a CTG trinucleotide repeat in the 3' UTR of the gene DMPK at the DM1 locus on chromosome 19 causes myotonic dystrophy, a dominantly inherited disease characterized by skeletal muscle dystrophy and myotonia, cataracts and cardiac conduction defects. Targeted deletion of Dm15, the mouse orthologue of human DMPK, produced mice with a mild myopathy and cardiac conduction abnormalities, but without other features of myotonic dystrophy, such as myotonia and cataracts. We, and others, have demonstrated that repeat expansion decreases expression of the adjacent gene SIX5 (refs 7,8), which encodes a homeodomain transcription factor. To determine whether SIX5 deficiency contributes to the myotonic dystrophy phenotype, we disrupted mouse Six5 by replacing the first exon with a beta-galactosidase reporter. Six5-mutant mice showed reporter expression in multiple tissues, including the developing lens. Homozygous mutant mice had no apparent abnormalities of skeletal muscle function, but developed lenticular opacities at a higher rate than controls. Our results suggest that SIX5 deficiency contributes to the cataract phenotype in myotonic dystrophy, and that myotonic dystrophy represents a multigenic disorder.

CTG triplet repeat expansion in a laryngeal carcinoma from a patient with myotonic dystrophy

A 66-year-old Japanese man with myotonic dystrophy (DM) underwent total laryngectomy for laryngeal carcinoma. The size of the expanded DNA fragment (EF) from the leukocytes and normal laryngeal tissues of this patient was only slightly longer than that in normal subjects. EF, however, was markedly longer in the laryngeal carcinoma. These findings support the hypothesis that elongation of the CTG repeat in the DM kinase gene occurs during acquired cell proliferation.

Elongation of (CTG)n repeats in myotonic dystrophy protein kinase gene in tumors associated with myotonic dystrophy patients

Length of (CTG)n triplet repeats in myotonic dystrophy protein kinase gene (DMPK) was estimated in tumors, normal tissues of the same organs, muscles, and leukocytes from three myotonic dystrophy (DM) patients and a non-DM patient. Using cDNA 25 as a probe, a Southern blot analysis of EcoRI- and BglI-digested DNA from these tissues demonstrated the longest expansion of the repeats in the tumors of DM patients. In all tissues from a non-DM patient, the repeat length was confirmed to be stable by PCR analysis. Our data suggest that expanded (CTG)n repeat in tumor tissues may have increased the instability. This study emphasizes the importance of a long-term prospective study on the incidence of tumors in DM to clarify the pathological interrelation between the two entities.

Visualization of double-stranded RNAs from the myotonic dystrophy protein kinase gene and interactions with CUG-binding protein

Myotonic dystrophy (DM) is associated with a (CTG) (n) triplet repeat expansion in the 3'-untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Using electron microscopy, we visualized large RNAs containing up to 130 CUG repeats and studied the binding of purified CUG-binding protein (CUG-BP) to these RNAs. Electron microscopic examination revealed perfect double-stranded (ds)RNA segments whose lengths were that expected for duplex RNA. The RNA dominant mutation model for DM pathogenesis predicts that the expansion mutation acts at the RNA level by forming long dsRNAs that sequester certain RNA-binding proteins. To test this model, we examined the subcellular distribution and RNA-binding properties of CUG-BP. While previous studies have demonstrated that mutant DMPK transcripts accumu-late in nuclear foci, the localization pattern of CUG-BP in both normal and DM cells was similar. Although CUG-BP in nuclear extracts preferentially photocrosslinked to DMPK transcripts, this binding was not proportional to (CUG) (n) repeat size. Moreover, CUG-BP localized to the base of the RNA hairpin and not along the stem, as visualized by electron micro-scopy. These results provide the first visual evidence that the DM expansion forms an RNA hairpin structure and suggest that CUG-BP is unlikely to be a sequestered factor.

Ionizing radiation and genetic risks. X. The potential "disease phenotypes" of radiation-induced genetic damage in humans: perspectives from human molecular biology and radiation genetics

Estimates of genetic risks of radiation exposure of humans are traditionally expressed as expected increases in the frequencies of genetic diseases (single-gene, chromosomal and multifactorial) over and above those of naturally-occurring ones in the population. An important assumption in expressing risks in this manner is that gonadal radiation exposures can cause an increase in the frequency of mutations and that this would result in an increase in the frequency of genetic diseases under study. However, despite compelling evidence for radiation-induced mutations in experimental systems, no increases in the frequencies of genetic diseases of concern or other adverse effects (i.e., those which are not formally classified as genetic diseases), have been found in human studies involving parents who have sustained radiation exposures. The known differences between spontaneous mutations that underlie naturally-occurring single-gene diseases and radiation-induced mutations studied in experimental systems now permit us to address and resolve these issues to some extent. The fact that spontaneous mutations (among which are point mutations and DNA deletions generally restricted to the gene) originate through a number of different mechanisms and that the latter are intimately related to the DNA organization of the genes, are now well-documented. Further, spontaneous mutations include those that cause diseases through loss of function as well as gain of function of genes. In contrast, most radiation-induced mutations studied in experimental systems (although identified through the phenotypes of the marker genes) are predominantly multigene deletions which cause loss of function; the recoverability of an induced deletion in a livebirth seems dependent on whether the gene and the genomic region in which it is located can tolerate heterozygosity for the deletion and yet be compatible with viability. In retrospect, the successful mutation test systems (such as the mouse specific locus test) used in radiation studies have involved genes which are non-essential for survival and are also located in genomic regions, likewise non-essential for survival. In contrast, most of the human genes at which induced mutations have been looked for, do not seem to have these attributes. The inference therefore is that the failure to find induced germline mutations in humans is not due to the resistance of human genes to induced mutations but due to the structural and functional constraints associated with their recoverability in livebirths. Since the risk of inducible genetic diseases in humans is estimated using rates of "recovered" mutations in mice, there is a need to introduce appropriate correction factors to bridge the gap between these rates and the rates at which mutations causing diseases are potentially recoverable in humans. Since the whole genome is the "target" for radiation-induced genetic damage, the failure to find increases in the frequencies of specific single-gene diseases of societal concern does not imply that there are no genetic risks of radiation exposures: the problem lies in delineating the phenotypes of recoverable genetic damage that are recognizable in livebirths. Data from studies of naturally-occurring microdeletion syndromes in humans and those from mouse radiation studies are instructive in this regard. They (i) support the view that growth retardation, mental retardation and multisystem developmental abnormalities are likely to be among the quantitatively more important adverse effects of radiation-induced genetic damage than mutations in a few selected genes and (ii) underscore the need to expand the focus in risk estimation from known genetic diseases (as has been the case thus far) to include these induced adverse developmental effects although most of these are not formally classified as "genetic diseases". (ABSTRACT TRUNCATED)

Overexpression of myotonic dystrophy protein kinase in C2C12 myogenic culture involved in the expression of ferritin heavy chain and interleukin-1alpha mRNAs

The specific function of myotonic dystrophy protein kinase (DMPK) is still not known. We found that overexpression of human DMPK in C2C12 myogenic culture induces the expression of ferritin heavy chain (FN-H) mRNA using differential display analysis. The quantity of FN-H mRNA was greater in the DMPK transfectant with five CTG triplet repeats in the 3'-untranslated region, while it was lower in the transfectant with 46 CTG repeats, over that of the control clone. We also investigated the quantity of interleukin 1-alpha (IL-1alpha) mRNA in each culture, due to the fact that this cytokine is able to induce FN-H expression, regardless of the concentration of free iron. Quantitative, competitive polymerase chain reaction (PCR) analysis revealed that the quantity of IL1-alpha mRNA is higher in the transfectant with five repeats, compared to the quantity of mRNA in the control clone; however, it is markedly lower in the clone with 46 repeats. These results suggest that overexpression of DMPK in C2C12 cultures may up-regulate IL-1alpha expression, resulting in the induction of FN-H expression. However, a large number of CTG repeats in the 3'-untranslated region of the DMPK gene may affect the pathway of IL-1alpha transcription, thereby resulting in decreased expression of FN-H.

Reduction of the DM-associated homeo domain protein (DMAHP) mRNA in different brain areas of myotonic dystrophy patients

Myotonic dystrophy (DM) is a multisystemic disease caused by expansion of a CTG trinucleotide repeat in the 3' untranslated region of the DMPK protein kinase gene on chromosome 19q13.3. The mechanism by which this expansion causes disease remains unknown. It has been suggested that CTG expansion not only affects the expression of the DMPK gene, but also alters the nuclear RNA metabolism and expression of neighboring genes. DMAHP, which is expressed in various human tissues, including skeletal muscle, heart and brain, is immediately distal to the 3' end of DMPK gene, in a CpG island which contains the CTG repeat. Here we report a 4- to 5-fold reduction of the expression of the DMAHP gene in different brain areas of DM patients. Our results demonstrate that [CTG]n expansion alters the brain DMAHP mRNA expression supporting a dominant-negative effect at the cellular level of DM [CTG]n mutation. The reduced brain expression of DMAHP could explain cerebral impairment in DM patients.

Relationships among electrophysiological findings and clinical status, heart function, and extent of DNA mutation in myotonic dystrophy

BACKGROUND

Impulse-conduction abnormalities and arrhythmias are common in myotonic dystrophy (MD). This study was performed to determine whether a correlation exists between electrophysiological (EP) testing data and clinical status, heart function, or size of the DNA abnormality (cytosine-thymine-guanine sequence repeat).

METHODS AND RESULTS

Eighty-three MD patients underwent invasive EP studies prompted primarily by the presence of asymptomatic conduction abnormalities. AV conduction disturbances were common and mainly distal (HV interval, 66.2+/-14 ms). AV conduction observed from the surface ECG was generally concordant with endocardial measurements. However, 11 of 20 patients with normal surface ECGs had abnormal subhisian conduction. Atrial arrhythmias were inducible in 41% of cases and correlated with prolongation of the AH interval (P=0.02) and a shorter atrial refractory period (P=0.04). Induction of ventricular arrhythmias (18%) correlated strongly with age (P=0. 0003). After adjustment for age, the extent of DNA mutation correlated with the Walton score (P=0.0018) but not with conduction abnormalities or induction of arrhythmias.

CONCLUSIONS

Prolongation of the HV interval is the most common conduction abnormality in MD and can be reliably recognized only by invasive EP testing. It raises the issue of prophylactic pacing to limit the incidence of sudden death in MD. Atrial and ventricular arrhythmias are often inducible, although their predictive value remains to be determined. Young age emerged as the most powerful predictor of inducible ventricular tachyarrhythmias. Conversely, we found no relationship between ECG or EP abnormalities recorded during invasive testing and the DNA mutation size or severity of peripheral muscle involvement.

(CTG)n repeats markedly inhibit differentiation of the C2C12 myoblast cell line: implications for congenital myotonic dystrophy

Although the mutation for myotonic dystrophy has been identified as a (CTG)n repeat expansion located in the 3'-untranslated region of a gene located on chromosome 19, the mechanism of disease pathogenesis is not understood. The objective of this study was to assess the effect of (CTG)n repeats on the differentiation of myoblasts in cell culture. We report here that C2C12 myoblast cell lines permanently transfected with plasmid expressing 500 bases long CTG repeat sequences, exhibited a drastic reduction in their ability to fuse and differentiate into myotubes. The percentage of cells fused into myotubes in C2 C12 cells (53.4+/-4.4%) was strikingly different from those in the two CTG repeat carrying clones (1.8+/-0.4% and 3.3+/-0. 7%). Control C2C12 cells permanently transfected with vector alone did not show such an effect. This finding may have important implications in understanding the pathogenesis of congenital myotonic dystrophy.

Clinical and genetic characteristics of a five-generation family with a novel form of myotonic dystrophy (DM2)

We report the clinical and genetic characteristics of a five-generation family (MN1) with an unusual form of myotonic dystrophy (DM). Affected individuals have clinical features that are similar to DM including myotonia, distal weakness, frontal balding, polychromatic cataracts, infertility and cardiac arrhythmias. Genetic analyses reveal that affected individuals do not have the CTG expansion associated with DM, nor is the disease locus linked to the DM region of chromosome 19. We have also excluded the MN1 disease locus from the chromosomal regions containing the genes for the muscle sodium (alpha- and beta-subunits) and chloride channels, both of which are involved in other myotonic disorders. We have recently mapped the disease locus (DM2) in this family to a 10 cM region of chromosome 3q [Ranum LPW, Rasmussen PF, Benzow KA, Koob MD, Day JW. Nat Genet 1998;19:196-198]. The genetically distinct form of myotonic dystrophy in the MN1 kindred shares some of the clinical features of previously reported families with proximal myotonic myopathy (PROMM). The size of the MN1 family (25 affected individuals) makes it a unique resource for both clinical and genetic studies. This second form of myotonic dystrophy may help resolve the confusion that remains about how the CTG repeat expansion in the 3' untranslated portion of the myotonin protein kinase gene causes the multisystem involvement of DM.

Properties of Ca2+-activated K+ channels in erythrocytes from patients with myotonic muscular dystrophy

Using the single-channel patch-clamp technique, Ca2+-activated K+ channels of erythrocytes from patients with myotonic muscular dystrophy (MyD) were studied. Elementary single-channel properties--conductance, rectification, kinetics, voltage- and calcium-dependence--measured in inside-out patches of MyD erythrocytes, did not differ significantly from those of control cells. The activity of the channels, studied in patches attached to red cells from MyD patients, exhibited mean patch currents which were significantly higher than the controls. The increased mean patch current was due to a higher opening frequency, associated with a reduced mean channel closed time. These results indicate that Ca2+-activated K+ channels of erythrocytes from patients either detect a higher intracellular calcium concentration and/or express an augmented calcium-sensitivity. Since these channels are targets for phosphorylation, our findings make it possible to identify defective kinase mechanisms, in minimally disturbed cells of the patient, at a molecular level of resolution.

Androgen response to hypothalamic-pituitary-adrenal stimulation with naloxone in women with myotonic muscular dystrophy

Myotonic muscular dystrophy (MMD) is a disease of autosomal dominant inheritance characterized by multisystem disease, including myotonia, muscle-wasting and weakness of all muscular tissues, and endocrine abnormalities attributed to a genetic abnormality causing a defective cAMP-dependent kinase. We have previously reported that MMD patients demonstrate ACTH hypersecretion after endogenous CRH release stimulated by naloxone administration while manifesting a normal cortisol (F) response. Additionally, others have reported a reduced adrenal androgen (AA) response to exogenous ACTH administration in MMD patients. As ACTH stimulates the secretion of both AAs and F, it is possible that the discordant relationship of these hormones in MMD patients results from a defect of adrenocortical ACTH receptor function or postreceptor signaling or subsequent biochemical events. Furthermore, the molecular abnormality seen in MMD patients may suggest that the mechanism underlying the frequently observed discordances in the secretion of glucocorticoids and AAs (e.g. adrenarche, surgical trauma, severe burns, or intermittent glucocorticoid administration) are explainable solely via an alteration in the function of the ACTH receptor or postreceptor signaling. To ascertain whether the responses of F and AAs to endogenous ACTH diverged in this disorder, we prospectively studied the responses of these hormones to naloxone-stimulated CRH release in nine premenopausal women with MMD and seven healthy age and weight-matched control women. After naloxone infusion (125 micrograms/kg, i.v.), blood sampling was performed at baseline (i.e. -5 min) and at 30 and 60 min. In addition to the absolute hormone level at each time, we calculated the net increment (i.e. change) at 30 and 60 min and the area under the curve (AUC) for F, ACTH, dehydroepiandrosterone (DHA), and androstenedione (A4). Consistent with our previous study, MMD patients demonstrated higher ACTH levels at all sampling times except [minud]5 min. AUC analysis revealed the ACTHAUC values were significantly higher in MMD than in control women (457 +/- 346 vs. 157 +/- 123 pmol/min.L; P < 0.03), whereas the FAUC response did not differ between MMD and controls (13860 +/- 3473 vs. 13375 +/- 3465 nmol/min.L; P > 0.5). Despite the greater ACTH secretion, the baseline circulating dehydroepiandrosterone sulfate levels were significantly lower in MMD compared with control women (18 +/- 23 vs. 61 +/- 23 mumol/L; P < 0.002). The serum concentrations of A4 at baseline, 30 min, and 60 min and DHA levels at 30 and 60 min were also significantly lower in MMD vs. control women. Additionally, the A4AUC and DHAAUC values were significantly lower in MMD patients than in controls. Furthermore, the net response of DHA at 60 min to the endogenous ACTH increase was also reduced in MMD patients compared with that in control subjects (2.3 +/- 2.1 vs. 5.6 +/- 2.6 nmol/L; P < 0.02). In conclusion, in addition to ACTH hypersecretion to CRH-mediated stimuli, these data suggest that MMD patients have a defect in the adrenocortical response to ACTH, reflected in normal F and reduced DHA and A4 secretion. Whether this defect is inherent to the disease or simply reflects adaptive changes to chronic disease remains to be demonstrated. However, it is possible that further studies of the response of MMD patients to ACTH may reveal a mechanism that explains the frequently observed dichotomy in the secretion of glucocorticoids and AAs.

A global haplotype analysis of the myotonic dystrophy locus: implications for the evolution of modern humans and for the origin of myotonic dystrophy mutations

Haplotypes consisting of the (CTG)n repeat, as well as several flanking markers at the myotonic dystrophy (DM) locus, were analyzed in normal individuals from 25 human populations (5 African, 2 Middle Eastern, 3 European, 6 East Asian, 3 Pacific/Australo-Melanesian, and 6 Amerindian) and in five nonhuman primate species. Non-African populations have a subset of the haplotype diversity present in Africa, as well as a shared pattern of allelic association. (CTG)18-35 alleles (large normal) were observed only in northeastern African and non-African populations and exhibit strong linkage disequilibrium with three markers flanking the (CTG)n repeat. The pattern of haplotype diversity and linkage disequilibrium observed supports a recent African-origin model of modern human evolution and suggests that the original mutation event that gave rise to DM-causing alleles arose in a population ancestral to non-Africans prior to migration of modern humans out of Africa.

Over expression of the murine myotonic dystrophy protein kinase in the mouse myogenic C2C12 cell line leads to inhibition of terminal differentiation

Myotonic dystrophy (DM) is an autosomal dominant human disorder, caused by the abnormal expansion of a CTG trinucleotide repeat in the 3' untranslated region of a protein kinase gene (DMPK). Muscle symptoms are a common feature of the disorder and in the adult onset cases there are increased patterns of muscle fibre degeneration and regeneration. In the congenitally affected infants there is a failure of muscle maturation, with the histological presence of numerous immature fibres. However, the pathological mechanism in both forms of the disease is unclear. We report that over-expression of the murine dmpk gene, in a murine myogenic cell line, leads to markedly reduced levels of fusion to the terminally differentiated state. These findings complement recently published data using a heterologous expression/cell system and may have implications for the understanding of the disease process in this disorder.

Inhibition of naloxone-stimulated adrenocorticotropin release by alprazolam in myotonic dystrophy patients

Myotonic dystrophy (DM) is an autosomal dominant disorder causing myotonia, progressive muscle weakness, and endocrine abnormalities including hypothalamic-pituitary-adrenal (HPA) axis hyperresponsiveness to CRH-mediated stimuli. This ACTH hyperresponsiveness appears directly related to the underlying genetic abnormality. Naloxone (Nal)-mediated CRH release causes ACTH release in normal humans and an ACTH hyperresponse in DM. Alprazolam (APZ) attenuates the ACTH release in response to Nal in normal individuals, probably by inhibiting CRH release. This study investigates the effects of APZ on Nal-induced HPA axis stimulation in DM. The ACTH response to Nal in DM subjects was significantly reduced by APZ. Despite this DM patients have a relative resistance to APZ inhibition of Nal-induced ACTH/cortisol release. APZ caused a smaller percentage reduction in AUC for ACTH in DM compared with controls. These findings provide further insight into the mechanism(s) of the HPA axis abnormalities in DM. In DM, there may be an increase in tonic opioid inhibition to CRH release with compensatory increases in stimulatory pathways. Alternatively, these patients may have a basal increase in pituitary vasopressin levels or an enhanced AVP/CRH synergistic mechanism at the level of the corticotroph.

Fragile-X syndrome and myotonic dystrophy: parallels and paradoxes

Fragile-X syndrome and myotonic dystrophy are caused by triplet repeat expansions embedded in CpG islands in the transcribed non-coding regions of the FMR1 and the DMPK genes, respectively. Although initial reports emphasized differences in the mechanisms by which the expanded triplet repeats caused these diseases, results published in the past year highlight remarkable parallels in the likely molecular etiologies. At both loci, expansion is associated with altered chromatin, aberrant methylation, and suppressed expression of the adjacent FMR1 and DMAHP genes, implicating epigenetic mediation of these genetic diseases.

Polymorphisms of the glucocorticoid receptor gene in laboratory and wild rats: steroid binding properties of trinucleotide CAG repeat length variants

The polyglutamine tract, beginning at codon 75 in the N-terminal modulatory domain of rat glucocorticoid receptor (rGR), was analyzed in 61 inbred strains and 155 wild caught Rattus norvegicus. A discontinuous distribution of repeat lengths was found (7, 17-23 repeats). To investigate the possible significance of this distribution, full-length rGR cDNAs with 7, 18, 20, and 21 CAG repeats were expressed in CV-1 cells, and the resulting GR protein analyzed by Western blots and extensive Scatchard analyses. The quantity and steroid binding capacity of GR, together with the binding affinities for dexamethasone and corticosterone, were found to be indistinguishable for the four repeat alleles. From the sequencing of four inbred strains CAG repeat variants were found to be flanked by silent allelic substitutions at nucleotide positions 198, 531, and 711. The four variable sites extended over 471-519 bp of coding sequence, forming six Grl haplotypes. The results are discussed in the light of genetic studies on the Milan hypertensive and normotensive strains of rat. Codon sequence of rat GR required amendment at the following residues: D98, G226, D260, P600, and F602.

CUG repeats present in myotonin kinase RNA form metastable "slippery" hairpins

We show that CUG repeats form "slippery" hairpins in their natural sequence context of the myotonin kinase gene transcript. This novel type of RNA structure is characterized by strong S1 and T1 nuclease and lead cleavages in the terminal loop and by mild lead cleavages in the hairpin stem. The latter effect indicates a relaxed metastable structure of the stem. (CUG)5 repeats do not form any detectable secondary structure, whereas hairpins of increasing stability are formed by (CUG)11, (CUG)21, and (CUG)49. The potential role of the RNA hairpin structure in the pathogenesis of myotonic dystrophy is discussed.

Dynamic mutation: possible mechanisms and significance in human disease

Increases in repeat-DNA copy number are the molecular basis of a growing list of human genetic diseases, including fragile X syndrome, myotonic dystrophy, Huntington disease and a form of epilepsy. Repeat-DNA sequences undergo a unique process of dynamic mutation, the common properties of which probably reflect common molecular events. This form of mutation is no longer restricted to trinucleotide repeats, because repeats of different length have been found to undergo expansion.

Myotonic dystrophy kinase modulates skeletal muscle but not cardiac voltage-gated sodium channels

Altered modulation of skeletal muscle voltage-gated sodium channels by myotonic dystrophy kinase (DMPK) has been proposed as a possible mechanism underlying myotonia in this disease. We examined the effect of a recombinant mouse DMPK on the functional properties of human skeletal muscle (hSkM1) and cardiac (hH1) voltage-gated sodium channels in the Xenopus oocyte expression system. Co-expression of DMPK with hSkM1 in oocytes resulted in significantly lower peak sodium current amplitude as compared to cells expressing hSkM1 alone in agreement with a previous report. By contrast, DMPK had no effect on the level of expressed sodium current in cells expressing hH1. Similarly, there were no measurable effects of the kinase on the kinetics or steady-state properties of activation or inactivation. Our findings support the previous observations made with rat muscle sodium channels and demonstrate that the effect of DMPK on sodium channels is isoform specific despite conservation of a putative phosphorylation site between the two isoforms.

Trinucleotide repeat expansion at the myotonic dystrophy locus reduces expression of DMAHP

Myotonic dystrophy, or dystrophia myotonica (DM), is an autosomal dominant multisystem disorder caused by the expansion of a CTG trinucleotide repeat in the 3' untranslated region of the DMPK protein kinase gene on chromosome 19q13.3 (refs 1-3). Although the DM mutation was identified more than five years ago, the pathogenic mechanisms underlying this most prevalent form of hereditary adult neuromuscular disease remain elusive. Previous work from our laboratory demonstrated that a DNase l-hypersensitive site located adjacent to the repeats on the wild-type allele is eliminated by repeat expansion, indicating that large CTG-repeat arrays may be associated with a local chromatin environment that represses gene expression. Here we report that the hypersensitive site contains an enhancer element that regulates transcription of the adjacent DMAHP homeobox gene. Analysis of DMAHP expression in the cells of DM patients with loss of the hypersensitive site revealed a two- to fourfold reduction in steady-state DMAHP transcript levels relative to wild-type controls. Allele-specific analysis of DMAHP expression showed that steady-state transcript levels from the expanded allele were greatly reduced in comparison to those from the wild-type allele. Together, these results demonstrate that CTG-repeat expansions can suppress local gene expression and implicate DMAHP in DM pathogenesis.

Transcriptional abnormality in myotonic dystrophy affects DMPK but not neighboring genes

Myotonic dystrophy (DM) is caused by the expansion of a trinucleotide repeat, CTG, in the 3' untranslated region of a protein kinase gene, DMPK. We set out to determine what effect this expanded repeat has on RNA processing. The subcellular fractionation of RNA and the separate analysis of DMPK transcripts from each allele reveals that transcripts from expanded DMPK alleles are retained within the nucleus and are absent from the cytoplasm of DM cell lines. The nuclear retention of DMPK transcripts occurs above a critical threshold between 80 and 400 CTGs. Further analysis of the nuclear RNA reveals an apparent reduction in the proportion of expansion-derived DMPK transcripts after poly(A)+ selection. Quantitative analysis of RNA also indicates that although the level of cytoplasmic DMPK transcript is altered in DM patients, the levels of transcripts from 59 and DMAHP, two genes that immediately flank DMPK, are unaffected in DM cell lines.

Survey of maximum CTG/CAG repeat lengths in humans and non-human primates: total genome scan in populations using the Repeat Expansion Detection method

Repeat Expansion Detection (RED) is an efficient and simple method for detecting repeat expansions in the human genome, including expansion mutations resulting in disease. Here we report the first population survey of CTG/CAG repeat lengths in humans using the RED method; we have determined maximum CTG/CAG repeat length in 244 individuals from six human populations: Danes, Chinese, Japanese, Rondonian Surui, Maya and Mbuti/Biaka Pygmies. We have also sampled a number of non-human primates including eight orang-utans (Pongo pygmaeus), seven gorillas (Gorilla gorilla), seven pygmy chimpanzees (Pan paniscus), 13 common chimpanzees (Pan troglodytes) and three Hylobatidae (one Hylobates lar, one H.klossii, and one H.syndactylus). Our results demonstrate the existence of significant variation in the sizes and frequencies of the longest CTG/CAG repeat length seen per individual both within and between human populations. The population differences argue that overall mutation rates at CTG/CAG repeat loci are sufficiently low that mutation does not obliterate the effect of random genetic drift and clearly indicate that population stratification could occur in disease association studies using the RED method. No significant differences were detected among the non-human primates sampled. Our results also show that both common chimpanzees and pygmy chimpanzees (bonobos) are polymorphic for maximum length of any CTG/CAG repeats while no variation was found for gorillas and orang-utans.

Moderate intergenerational and somatic instability of a 55-CTG repeat in transgenic mice

Myotonic dystrophy (DM) is associated with the expansion of a (CTG)n trinucleotide repeat in the 3' untranslated region (UTR) of the DM protein kinase gene (DMPK). The (CTG)n repeat is polymorphic and varies in size between 5 and 37 repeats in unaffected individuals whereas in affected patients there are between 50 and 4,000 CTGs. The size of the (CTG)n repeat, which increases through generations, generally correlates with clinical severity and age of onset. The instability of the CTG repeat appears to depend on its size as well as on the sex of the transmitting parent. Moreover, mitotic instability analysis of different human DM tissues shows length mosaicism between different cell lineages. The molecular mechanisms of triplet instability remain elusive. To investigate the role of genomic sequences in instability, we produced transgenic mice containing a 45-kb genomic segment with a 55-CTG repeat cloned from a mildly affected patient. In contrast to other mouse models containing CAG repeats within cDNAs, these mice showed both intergenerational and somatic repeat instability.