Correlation between CTG trinucleotide repeat length and frequency of severe congenital myotonic dystrophy

Myotonic dystrophy expanded CUG repeats disturb the expression and phosphorylation of τ in PC12 cells

Mental retardation is a main feature of the congenital form of myotonic dystrophy (DM1), however, the molecular mechanisms underlying the central nervous system symptoms of DM1 are poorly understood. We have established a PC12 cell line-based model expressing the DM1 expanded CUG repeats (CTG90 cells) to analyze the effects of this mutation on neuronal functions. Previously, we have reported that CTG90 cells displayed impaired NGF-induced neuronal differentiation. Because disruption of normal expression of the microtubule associated protein tau and neuronal aggregates of hyperphosphorylated tau have been associated with DM1, this study analyzes the behavior of tau in the CTG90 cells. Several alterations of tau were observed in the PC12 cells that express expanded CUG repeats, including a subtle but reproducible reduction in the expression of the tau mRNA splicing isoform containing exon 10, decreased expression of tau and hyperphosphorylation of both tau and high molecular weight tau as well as abnormal nuclear localization of tau phosphorylated at Ser396/404. Interestingly, phosphorylation regulates negatively the activity of tau as microtubule-associated protein. In addition, impaired activity of the Akt/GSK3beta pathway, which phosphorylates tau, was also identified in the CTG90 cells. Besides tau phosphorylation, the Akt/GSK3beta signaling pathway regulates other key processes of PC12 cells, such as apoptosis and neuronal differentiation. Our results indicate that defective neuronal differentiation exhibited by the PC12 cells expressing expanded CUG repeats could be the result of combinatory effects derived from the altered behavior of tau and the impaired activation of the Akt/GSK3beta signaling pathway.

Family, twin, adoption, and molecular genetic studies of juvenile bipolar disorder

Juvenile bipolar disorder (JBD) has been a subject of significant research and debate. Phenotypic differences between JBD and adult-onset bipolar disorder have led researchers to question whether or not similar neuropathologic mechanisms will be found. While much is known about the genetic and environmental contributions to the adult-onset phenotype, less is known about their contributions to JBD. Here, we review family, twin, adoption, and molecular genetic studies of JBD. Behavioral genetic data suggest both genetic and environmental contributions to JBD, while molecular genetic studies find linkage to age of onset of bipolar disorder to chromosomes 12p, 14q, and 15q. Additionally, changes associated with symptom age of onset have been recently reported in the brain-derived neurotrophic factor (BDNF) and glycogen synthase kinase 3-beta (GSK3-beta) genes. We contend that further progress in discovering the precise genetic and environmental contributions to JBD may depend on advances in phenotypic refinement, an increased appreciation of comorbid conditions, and more investigation of the longitudinal course of the disorder.

Congenital Myotonic Dystrophy

We describe a case of severe congenital myotonic dystrophy (CDM). A 38-year-old primigravida, who was known to suffer from mild myotonic dystrophy (DM), conceived spontaneously and booked for confinement at 11 weeks in our unit. The couple had been fully counseled about the risks of transmission of this condition to their offspring before embarking on this pregnancy. Despite being fully aware of the risks, they declined prenatal diagnosis. The pregnancy was monitored by serial ultrasound scans. The diagnosis of CDM was suspected by ultrasound markers of borderline ventriculomegaly, polyhydramnios, and reduced fetal movements. The pregnancy ended prematurely at 33 weeks in an emergency caesarean section because of severe fetal compromise. The neonate died almost immediately after birth. The genetic analysis of cord blood confirmed severe DM. This case highlights the importance of ultrasound markers for the diagnosis of CDM in the absence of definitive prenatal diagnosis.

Familial clustering of muscular and cardiac involvement in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is associated with both skeletal and cardiac muscle involvement. The aim of the present study was to determine whether familial clustering is observed in the severity of muscle involvement in DM1. We evaluated 51 sibling groups constituting 112 patients with genetically-verified DM1. The siblings were similar to each other in age, cytosine-thymine-guanine (CTG) repeat length, age at disease onset, muscular impairment rating score, and electrocardiographic markers of cardiac conduction disease. After adjusting for the similarities between siblings in age and CTG repeat length, the siblings remained similar to each other in measures of both skeletal and cardiac muscle involvement. These results suggest that factors other than CTG repeat length play a role in the severity and progression of the degenerative skeletal and cardiac muscle disease in DM1.

Neurodegenerative disorders associated with diabetes mellitus

More than 20 syndromes among the significant and increasing number of degenerative diseases of neuronal tissues are known to be associated with diabetes mellitus, increased insulin resistance and obesity, disturbed insulin sensitivity, and excessive or impaired insulin secretion. This review briefly presents such syndromes, including Alzheimer disease, ataxia-telangiectasia, Down syndrome/trisomy 21, Friedreich ataxia, Huntington disease, several disorders of mitochondria, myotonic dystrophy, Parkinson disease, Prader-Willi syndrome, Werner syndrome, Wolfram syndrome, mitochondrial disorders affecting oxidative phosphorylation, and vitamin B(1) deficiency/inherited thiamine-responsive megaloblastic anemia syndrome as well as their respective relationship to malignancies, cancer, and aging and the nature of their inheritance (including triplet repeat expansions), genetic loci, and corresponding functional biochemistry. Discussed in further detail are disturbances of glucose metabolism including impaired glucose tolerance and both insulin-dependent and non-insulin-dependent diabetes caused by neurodegeneration in humans and mice, sometimes accompanied by degeneration of pancreatic beta-cells. Concordant mouse models obtained by targeted disruption (knock-out), knock-in, or transgenic overexpression of the respective transgene are also described. Preliminary conclusions suggest that many of the diabetogenic neurodegenerative disorders are related to alterations in oxidative phosphorylation (OXPHOS) and mitochondrial nutrient metabolism, which coincide with aberrant protein precipitation in the majority of affected individuals.

Heart rate variability declines with increasing age and CTG repeat length in patients with myotonic dystrophy type 1

BACKGROUND

Cardiac myopathy manifesting as arrhythmias is common in the neurological disease, myotonic dystrophy type 1 (DM1). The purpose of the present study was to evaluate heart rate variability (HRV) in patients with DM1.

METHODS

In a multicenter study, history, ECG, and genetic testing were performed in DM1 patients.

RESULTS

In 289 patients in whom the diagnosis of DM1 was confirmed by a prolonged cytosine-thymine-guanine (CTG) repeat length the most common ambulatory ECG abnormality was frequent ventricular ectopy (16.3%). The 24-hour time domain parameters of SDNN (SD of the NN interval) and SDANN (SD of the mean NN, 5-minute interval) declined as age and CTG repeat length increased (SDNN: -8.5 ms per decade, 95% confidence intervals [CI]-12.9, -4.2, -8.7 ms per 500 CTG repeats, CI -15.7, -1.8, r=0.24, P<0.001; SDANN: -8.1 ms per decade, CI -12.4, -3.8, -8.8 ms per 500 CTG repeats, CI -15.7, -1.9, r=0.23, P<0.001). Short-term frequency domain parameters declined with age only (total power: -658 ms2 per decade, CI: -984, -331, r=0.23, P<0.001; low frequency (LF) power -287 ms2 per decade, CI: -397, -178, r=0.30, P<0.001; high frequency (HF) power: -267 ms2 per decade, CI: -386, -144, r=0.25, P<0.001). The LF/HF ratio increased as the patient aged (0.5 per decade, CI: 0.1, 0.9, r=0.13, P=0.03).

CONCLUSIONS

In DM1 patients a decline in HRV is observed as the patient ages and CTG repeat length increases.

Neurologic disorders masquerading as pediatric sleep problems

Neurologic disorders may present or masquerade as pediatric sleep problems and fool the pediatrician, which may delay diagnosis and treatment. Many of the sleep problems in children with neurologic disorders arise directly from primary dysfunction or delayed maturation of their sleep-wake regulation systems. It is important to realize that nocturnal frontal lobe seizures or cluster headaches can be mistaken for night terrors, and craniopharyngiomas or myotonic dystrophy may present as narcolepsy-cataplexy. Hypothalamic dysfunction may explain not only the impaired circadian rhythm disorders in children with profound mental retardation but also excessive sleepiness and hyperphagia in Prader-Willi and Kleine-Levin syndromes. Intellectually challenged children perform better, learn more, and are better behaved with sufficient restorative sleep.

Prevalence of structural cardiac abnormalities in patients with myotonic dystrophy type I

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a neurological disorder with known cardiac involvement, including conduction disturbances, arrhythmias, and ventricular dysfunction. We studied which clinical and electrocardiographic features are associated with structural cardiac abnormalities.

METHODS

History, physical examination, electrocardiography, and genetic testing were performed on 382 patients with DM1, and cardiac imaging was performed on 100 of these patients.

RESULTS

Clinical congestive heart failure was found in 7 of the 382 patients (1.8%). Structural cardiac abnormalities determined with cardiac imaging included left ventricular hypertrophy (19.8%), left ventricular dilatation (18.6%), left ventricular systolic dysfunction (14.0%), mitral valve prolapse (13.7%), regional wall motion abnormality (11.2%), and left atrial dilatation (6.3%). Left ventricular systolic dysfunction was associated with increasing age (relative risk [RR], 1.9 per decade; 95% CI, 1.1-3.2; P =.02), cytosine-thymine-guanine (CTG) repeat length (RR, 2.8 per 500 repeats; 95% CI, 1.3-6.3; P =.01), P-R >200 ms (RR, 14.7; 95% CI, 3.0-73.1; P =.001), and QRS >120 ms (RR, 5.7; 95% CI, 1.5-21.8; P =.01). P-R >200 ms was predictive of regional wall motion abnormalities. QRS >120 ms correlated with regional wall motion abnormalities and left atrial dilatation.

CONCLUSIONS

Several clinical and electrocardiographic findings in patients with DM1 are significantly associated with structural heart abnormalities. These results suggest an underlying genetic and pathophysiologic correlate that may lead to cardiac disease in these patients.

Spontaneous chromosome loss and colcemid resistance in lymphocytes from patients with myotonic dystrophy type 1

Myotonic Dystrophy type 1 (DM1) is one of the many inherited human diseases whose molecular defect is the expansion of a trinucleotide DNA sequence. DM1 shares with fragile X syndrome (FMR1), another "unstable triplet syndrome", several molecular features not present in the remaining triplet diseases. As FMR1 is also characterised by chromosome instability at the site of the expanded triplet, lymphocytes from DM1 patients and healthy donors were cultured for micronucleus (MN) analysis, in order to verify if DM1 is also prone to chromosome instability. A FISH analysis was also carried out to detect the presence of centromeric sequences in the observed MN. The data indicate that DM1 patients present a percentage of centromere-positive MN significantly higher than controls, suggesting that chromosome loss is the main mechanism underlying the origin of the increased spontaneous instability. To further assess the proneness to instability of cells of DM1 patients, cultures from patients and controls were treated in vitro with growing concentrations of two different mutagens: colcemid, a "pure" aneugen compound whose target is tubulin, and mytomicin C, a strong clastogen. The results show that the patient group is significantly less sensitive to colcemid. These data, together with FISH analysis, suggest the presence, in DM1 patients, of an already damaged tubulin, which becomes no more sensitive to the effect of colcemid and which could be the main defect underlying the aneugenic effects in DM1.

Molecular genetics of spinocerebellar ataxia type 8 (SCA8)

We previously reported that a transcribed but untranslated CTG expansion causes a novel form of ataxia, spinocerebellar ataxia type 8 (SCA8) (Koob et al., 1999). SCA8 was the first example of a dominant spinocerebellar ataxia that is not caused by the expansion of a CAG repeat translated into a polyglutamine tract. This slowly progressive form of ataxia is characterized by dramatic repeat instability and a high degree of reduced penetrance. The clinical and genetic features of the disease are discussed below.

The floppy infant: contribution of genetic and metabolic disorders

The floppy infant syndrome is a well-recognized entity for pediatricians and neonatologists. The condition refers to an infant with generalized hypotonia presenting at birth or in early life. The diagnostic work up in many instances is often complex, and requires multidisciplinary assessment. Advances in genetics and neurosciences have lead to recognition of newer diagnostic entities (several congenital myopathies), and rapid molecular diagnosis is now possible for several conditions such as spinal muscular atrophy (SMA), congenital muscular dystrophies (CMD), several forms of congenital myopathies and congenital myotonic dystrophy. The focus of the present review is to describe the advances in our understanding in the genetic, metabolic basis of neurological disorders, as well as the investigative work up of the floppy infant. An algorithm for the systematic evaluation of infants with hypotonia is suggested for the practicing pediatrician/neonatologist.

CAG-repeat length in exon 1 of KCNN3 does not influence risk for schizophrenia or bipolar disorder: a meta-analysis of association studies

Schizophrenia and bipolar disorder both show some evidence for genetic anticipation. In addition, significant expansion of anonymous CAG repeats throughout the genome has been detected in both of these disorders. The gene KCNN3, which codes for a small/intermediate conductance, calcium-regulated potassium channel, contains a highly polymorphic CAG-repeat array in exon 1. Initial evidence for association of both schizophrenia and bipolar disorder with increased CAG-repeat length of KCNN3 has not been consistently replicated. In the present study, we performed several meta-analyses to evaluate the pooled evidence for association with CAG-repeat length of KCNN3 derived from case-control and family-based studies of both disorders. Each group of studies was analyzed under two models, including a test for direct association with repeat length, and a test for association with dichotomized repeat-length groups. No evidence for a linear relationship between disease risk and repeat length was observed, as all pooled odds ratios approximated 1.0. Results of dichotomized allele-group analyses were more variable, especially for schizophrenia, where case-control studies found a significant association with longer repeats but family-based studies implicated shorter alleles. The results of these meta-analyses demonstrate that the risks for both schizophrenia and bipolar disorder are largely, if not entirely, independent of CAG-repeat length in exon 1 of KCNN3. This study cannot exclude the possibility that some aspect of this polymorphism, such as repeat-length disparity in heterozygotes, influences risk for these disorders. Further, it remains unknown if this polymorphism, or one in linkage disequilibrium with it, contributes to some distinct feature of the disorder, such as symptom severity or anticipation.

The genetics of pediatric-onset bipolar disorder

Although bipolar disorder in adults has been extensively studied, early-onset forms of the disorder have received less attention. We review several lines of evidence indicating that pediatric- and early adolescent-onset bipolar disorder cases may prove the most useful for identifying susceptibility genes. Family studies have consistently found a higher rate of bipolar disorder among the relatives of early-onset bipolar disorder patients than in relatives of later-onset cases, which supports the notion of a larger genetic contribution to the early-onset cases. Comorbid pediatric bipolar disorder and attention-deficit/hyperactivity disorder (ADHD) may also define a familial subtype of ADHD or bipolar disorder that is strongly influenced by genetic factors and may, therefore, be useful in molecular genetic studies. There are no twin and adoption studies of pediatric bipolar disorder, but the heritability of this subtype is expected to be high given the results from family studies. Thus, pediatric- and early adolescent-onset bipolar disorder may represent a genetically loaded and homogeneous subtype of bipolar disorder, which, if used in genetic linkage and association studies, should increase power to detect risk loci and alleles.

Changes in myotonic dystrophy protein kinase levels and muscle development in congenital myotonic dystrophy

Myotonic dystrophy (DM1) is caused by the expansion of a CTG repeat in the noncoding region of a protein kinase, DMPK, expressed in skeletal and cardiac muscles. The aim of the present study was to determine the effects of very large CTG expansions on DMPK expression and skeletal muscle development. In fetuses suffering from the severe congenital form of DM1 with large CTG expansions (1800 to 3700 repeats), the skeletal muscle level of DMPK was reduced to 57% of control levels and a similar reduction was observed in cultured DM1 muscle cells relative to control cultures. These results are consistent with greatly reduced DMPK expression from the mutant allele and normal expression from the unaffected allele in this autosomal dominant disorder. In normal fetuses, DMPK protein levels increased dramatically between 9 and 16 weeks and remained high throughout the remaining gestation period. DM1 fetuses showed impaired skeletal muscle development, characterized by a persistence of embryonic and fetal myosin heavy chains and almost total absence of slow myosin heavy chains at the end of gestation. DMPK expression, however, was similar in both fast and slow fibers from normal adult muscle. The reduced DMPK and the delayed slow fiber maturation in congenital DM1 may be two separate consequences of nuclear retention of DMPK RNA transcripts with expanded CUG repeats.

Expanded CTG repeats inhibit neuronal differentiation of the PC12 cell line

Myotonic dystrophy (DM) is a dominant neuromuscular disorder caused by the expansion of trinucleotide CTG repeats in the 3-untranslated region (3'-UTR) of the MtPK gene. Although DM-associated mental retardation suggests that neuronal functions are disturbed by the expansion mutation, the effect of this alteration in neuronal cells has not been approached. In this study we established stable transfectans of PC12 neuronal cell line expressing the reporter gene CAT alone (empty-vector clone) or fused to the MtPK 3'-UTR with 5, 60, or 90 CTG repeats (CTG5, CTG60, and CTG90 clones, respectively). CTG90 cells exhibited a suppression of NGF-induced neuronal differentiation while empty-vector, CTG5 and CTG60 clones differentiated normally. CTG90 cells displayed normal activation of early differentiation markers, ERK1/2, but the up-regulation of the late marker MAP2 was dramatically reduced. Our neuronal cell system provides the first information of how the mutant MtPK 3'-UTR mRNA affects neuronal functions.

Gastric emptying in myotonic dystrophic patients

BACKGROUND

Myotonic dystrophy is often associated with digestive symptoms that can precede the clinical appearance of skeletal muscle involvement. Although motility disorders may be observed in these patients at any level of the gastrointestinal tract, upper gastrointestinal symptoms have up to now usually been considered to be due to oesophageal rather than gastric dysmotility.

AIMS

To evaluate: a) gastric emptying in myotonic dystrophic patients without dyspeptic symptoms, and b) relationship between gastric emptying and severity and duration of the disease.

PATIENTS AND METHODS

Gastric emptying was evaluated in 11 non-dyspeptic dystrophic patients and in 22 healthy volunteers by means of computerised ultrasound scan, assessing the variation in the antral area over time after ingestion of a meal.

RESULTS

The final emptying time was higher in patients than in healthy volunteers (373' +/- 35' vs 270' +/- 47'; p < 0.001). Basal and maximal post-prandial antral areas were similar in the two groups. There was a significant correlation between gastric emptying and the duration of the disease (rs = 0.62; p = 0.04). No relationship was found between gastric emptying and severity of the disease.

CONCLUSIONS

Gastric emptying may be abnormally delayed in myotonic dystrophy patients, even in absence of dyspeptic symptoms. This delay is correlated with duration but not with severity of the disease. However there is no difference in either basal or maximal postprandial antral areas between myotonic dystrophy patients and healthy volunteers.

Trinucleotide repeats: mechanisms and pathophysiology

Within the closing decade of the twentieth century, 14 neurological disorders were shown to result from the expansion of unstable trinucleotide repeats, establishing this once unique mutational mechanism as the basis of an expanding class of diseases. Trinucleotide repeat diseases can be categorized into two subclasses based on the location of the trinucleotide repeats: diseases involving noncoding repeats (untranslated sequences) and diseases involving repeats within coding sequences (exonic). The large body of knowledge accumulating in this fast moving field has provided exciting clues and inspired many unresolved questions about the pathogenesis of diseases caused by expanded trinucleotide repeats. This review summarizes the current understanding of the molecular pathology of each of these diseases, starting with a clinical picture followed by a focused description of the disease genes, the proteins involved, and the studies that have lent insight into their pathophysiology.

Paternal transmission of the congenital form of myotonic dystrophy type 1: a new case and review of the literature

Myotonic dystrophy type 1 (DM1) is an autosomal dominant trinucleotide repeat disorder that shows anticipation. The mildest manifestations of the DM gene are usually noted in individuals with the smallest repeat sizes, while congenital myotonic dystrophy (CDM) is the most common clinical outcome of the larger expansions. For many years, it was thought that CDM could only be maternally transmitted. However, in the last few years, cases of paternal transmission of CDM have been described. We report a child with the CDM phenotype and 1, 800 CTG repeats born to an asymptomatic father with 65 repeats and compare this case to the four currently in the literature. We note that polyhydramnios was present in the majority of cases and that all fathers whose status was known had small repeat sizes and/or were asymptomatic at the time of their child's birth. Although it may be unusual, the possibility of the paternal transmission of CDM should be mentioned when counseling families with DM. The men who are at highest risk may be those who have small repeats sizes and are asymptomatic.

Does cytosine-thymine-guanine (CTG) expansion size predict cardiac events and electrocardiographic progression in myotonic dystrophy?

OBJECTIVE

To assess whether the size of the cytosine-thymine-guanine (CTG) expansion mutation in myotonic dystrophy predicts progression of conduction system disease and cardiac events.

DESIGN

Longitudinal study involving ECG and clinical follow up over (mean (SD)) 4.8 (1.8) and 6.2 (1.9) years, respectively, of patients stratified by CTG expansion size (E0 to E4).

PATIENTS

73 adult patients under annual review in a regional myotonic dystrophy clinic. Patients were grouped into E0/E1 (n = 25), E2 (n = 34), and E3/E4 (n = 14).

RESULTS

The proportion of patients with a QRS complex > 100 ms at baseline increased with the size of the CTG expansion (EO/E1, 4%; E2, 12%; E3/E4, 36%; p = 0.02). This trend was more pronounced at follow up (E0/E1, 4%; E2, 21%; E3/E4, 57%; p = 0.0004). The rate of widening of the QRS complex (ms/year) was similarly related to the size of the mutation (EO/E1, 0.4 (1.3); E2, 1.4 (2.5); E3/E4, 1.5 (1.6); p = 0.04). First degree atrioventricular block was present in 23% of patients at baseline and 34% at follow up, with no significant relation to expansion size. Seven patients suffered a cardiac event during follow up (sudden death in two, permanent pacemaker insertion in three, chronic atrial arrhythmia in two), of whom six were in CTG expansion group E2 or greater. Patients who experienced a cardiac event during follow up had more rapid rates of PR interval increase (9.9 (11.1) v 1.6 (2.9) ms/year; p = 0.008) and a trend to greater QRS complex widening (3.6 (4.5) v 0.9 (1.5) ms/year; p = 0.06) than those who did not.

CONCLUSIONS

Larger CTG expansions are associated with a higher rate of conduction disease progression and a trend to increased risk of cardiac events in myotonic dystrophy.

Coenzyme Q10, exercise lactate and CTG trinucleotide expansion in myotonic dystrophy

Steinert's myotonic dystrophy (DM) is a genetic autosomal dominant disease and the most frequent muscular dystrophy in adulthood. Although causative mutation is recognized as a CTG trinucleotide expansion on 19q13.3, pathogenic mechanisms of multisystem involvement of DM are still under debate. It has been suggested that mitochondrial abnormalities can occur in this disease and deficiency of coenzyme Q 10 (CoQ10) has been considered one possible cause for this. The aim of this investigation was to evaluate, in 35 DM patients, CoQ10 blood levels and relate them to the degree of CTG expansion as well as to the amount of lactate production in exercising muscle as indicator of mitochondrial dysfunction. CoQ10 concentrations appeared significantly reduced with respect to normal controls: 0.85 +/- 0.25 vs. 1.58 +/- 0.28 microg/ml (p < 0.05). Mean values of blood lactate were significantly higher in DM patients than controls (p < 0.05) both in resting conditions (2.9 +/- 0.55 vs. 1.44 +/- 1.11 mmol/L) and at the exercise peak (6.77 +/- 1.79 vs. 4.90 +/- 0.59 mmol/L), while exercise lactate threshold was anticipated (30-50% vs. 60-70% of the predicted normal maximal power output, p < 0.05). Statistical analysis showed that serum CoQ10 levels were significantly (p < 0.05) inversely correlated with both CTG expansion degree and lactate values at exercise lactate threshold level. Our data indicates the occurrence of reduced CoQ10 levels in DM, possibly related to disease pathogenic mechanisms associated with abnormal CTG trinucleotide amplification.

Effect of triplet repeat expansion on chromatin structure and expression of DMPK and neighboring genes, SIX5 and DMWD, in myotonic dystrophy

Myotonic dystrophy (DM), an autosomal dominant neuromuscular disease, is associated with expansion of a polymorphic (CTG)n repeat in the 3'-untranslated region of the DM protein kinase (DMPK) gene. The repeat expansion results in decreased levels of DMPK mRNA and protein, but the mechanism for this decreased expression is unknown. Loss of a nuclease-hypersensitive site in the region of the repeat expansion has been observed in muscle and skin fibroblasts from DM patients, indicating a change in local chromatin structure. This change in chromatin structure has been proposed as a mechanism whereby the expression of DMPK and neighboring genes, sine oculis homeobox (Drosophila) homolog 5 (SIX5) and dystrophia myotonica-containing WD repeat motif (DMWD), might be affected. We have developed a polymerase chain reaction (PCR)-based method to assay the chromatin sensitivity of the region adjacent to the repeat expansion in somatic cell hybrids carrying either normal or affected DMPK alleles and show that hybrids carrying expanded alleles exhibit decreased sensitivity to PvuII digestion in this region. Semiquantitative multiplex reverse transcriptase PCR (RT/PCR) assays of gene expression from the chromosomes carrying the expanded alleles showed marked reduction of DMPK mRNA, partial inhibition of SIX5 expression from a congenital DM chromosome, and no reduction of DMWD mRNA. Nested RT/PCR analysis of DMPK mRNA from somatic cell hybrids carrying the repeat expansions revealed that most of the DMPK transcripts expressed from the expanded alleles lacked exons 13 and 14, whereas full-length transcripts were expressed predominantly from the normal alleles. These results suggest that the CTG repeat expansion leads to a decrease in DMPK mRNA levels by affecting splicing at the 3' end of the DMPK pre-mRNA transcript.

CTCF-binding sites flank CTG/CAG repeats and form a methylation-sensitive insulator at the DM1 locus

An expansion of a CTG repeat at the DM1 locus causes myotonic dystrophy (DM) by altering the expression of the two adjacent genes, DMPK and SIX5, and through a toxic effect of the repeat-containing RNA. Here we identify two CTCF-binding sites that flank the CTG repeat and form an insulator element between DMPK and SIX5. Methylation of these sites prevents binding of CTCF, indicating that the DM1 locus methylation in congenital DM would disrupt insulator function. Furthermore, CTCF-binding sites are associated with CTG/CAG repeats at several other loci. We suggest a general role for CTG/CAG repeats as components of insulator elements at multiple sites in the human genome.

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.

Myotonic dystrophy protein kinase (DMPK) gene expression in lymphocytes of patients with myotonic dystrophy

BACKGROUND

Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disorder with defects in many tissues, including skeletal muscle myotonia, progressive myopathy, and abnormalities in heart, brain, and endocrine systems. It is associated with a trinucleotide repeat occurring in the 3' (UTR) untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Several studies have been carried out to determine DMPK gene expression in muscle and non-muscle tissues.

METHODS

DMPK gene expression was determined in lymphocytes of adult-onset patients with DM and normal controls. To quantitate total locus expression as well as allele-specific mRNA levels, semiquantitative RT-PCR assay was used. Allele-specific expression was analyzed using a Bpm1 polymorphism located at exon 10 of the DMPK gene.

RESULTS

In heterozygous patients with DM, we observed a fourfold difference between mRNA levels produced by the Bpm1-undigested allele (187 bp) compared to the Bpm1-digested allele (136 bp). By using (CTG) trinucleotide (with cytosine, thymine, and guanine) expansion polymorphism, it was shown that the down-regulated allele corresponds to the mutant allele. Interestingly, the reduction in the mutant allele-transcript levels is compensated by an increase of the wild-type allele, yielding no significant differences in total locus mRNA amount between patients and normal individuals.

CONCLUSIONS

These results suggest that the expression of the two alleles at the DMPK locus in lymphocytes is coordinated. The reduction in mutant-allele transcript levels is compensated by an increase in wild-type allele mRNA levels.

Occlusive hydrocephalus in congenital myotonic dystrophy

A case of congenital myotonic dystrophy is reported which was complicated by the development of a hydrocephalus that needed ventricular-peritoneal shunting at the age of 4 months. Although dilatation of cerebral ventricles is a common feature in these patients, an occlusive hydrocephalus has not so far been reported.

Gene conversion (recombination) mediates expansions of CTG[middle dot]CAG repeats

Genetic recombination is a robust mechanism for expanding CTG.CAG triplet repeats involved in the etiology of hereditary neurological diseases (Jakupciak, J. P., and Wells, R. D. (1999) J. Biol. Chem. 274, 23468-23479). This two-plasmid recombination system in Escherichia coli with derivatives of pUC19 and pACYC184 was used to investigate the effect of triplet repeat orientation on recombination and extent of expansions; tracts of 36, 50, 80, and 36, 100, and 175 repeats in length, respectively, in all possible permutations of length and in both orientations (relative to the unidirectional replication origins) revealed little or no effect of orientation of expansions. The extent of expansions was generally severalfold the length of the progenitor tract and frequently exceeded the combined length of the two tracts in the cotransformed plasmids. Expansions were much more frequent than deletions. Repeat tracts bearing two G-to-A interruptions (polymorphisms) within either 171- or 219-base pair tracts substantially reduced the expansions compared with uninterrupted repeat tracts of similar lengths. Gene conversion, rather than crossing over, was the recombination mechanism. Prior studies showed that DNA replication, repair, and tandem duplication also mediated genetic instabilities of the triplet repeat sequence. However, gene conversion (recombinational repair) is by far the most powerful expansion mechanism. Thus, we propose that gene conversion is the likely expansion mechanism for myotonic dystrophy, spinocerebellar ataxia type 8, and fragile X syndrome.

Myotonic dystrophy: the role of the CUG triplet repeats in splicing of a novel DMPK exon and altered cytoplasmic DMPK mRNA isoform ratios

The mechanism by which (CTG)n expansion in the 3' UTR of the DMPK gene causes myotonic dystrophy (DM) is unknown. We identified four RNA splicing factors--hnRNP C, U2AF (U2 auxiliary factor), PTB (polypyrimidine tract binding protein), and PSF (PTB associated splicing factor)--that bind to two short regions 3' of the (CUG)n, and found a novel 3' DMPK exon resulting in an mRNA lacking the repeats. We propose that the (CUG)n is an essential cis acting element for this splicing event. In contrast to (CUG)n containing mRNAs, the novel isoform is not retained in the nucleus in DM cells, resulting in imbalances in relative levels of cytoplasmic DMPK mRNA isoforms and a new dominant effect of the mutation on DMPK.

High germinal instability of the (CTG)n at the SCA8 locus of both expanded and normal alleles

The autosomal dominant spinocerebellar ataxias (SCAs) are a group of late-onset, neurodegenerative disorders for which 10 loci have been mapped (SCA1, SCA2, SCA4-SCA8, SCA10, MJD, and DRPLA). The mutant proteins have shown an expanded polyglutamine tract in SCA1, SCA2, MJD/SCA3, SCA6, SCA7, and DRPLA; a glycine-to-arginine substitution was found in SCA6 as well. Recently, an untranslated (CTG)n expansion on chromosome 13q was described as being the cause of SCA8. We have now (1) assessed the repeat size in a group of patients with ataxia and a large number of controls, (2) examined the intergenerational transmission of the repeat, and (3) estimated the instability of repeat size in the sperm of one patient and two healthy controls. Normal SCA8 chromosomes showed an apparently trimodal distribution, with classes of small (15-21 CTGs), intermediate (22-37 CTGs), and large (40-91 CTGs) alleles; large alleles accounted for only0.7% of all normal-size alleles. No expanded alleles (>/=100 CTGs) were found in controls. Expansion of the CTG tract was found in five families with ataxia; expanded alleles (all paternally transmitted) were characterized mostly by repeat-size contraction. There was a high germinal instability of both expanded and normal alleles: in one patient, the expanded allele (152 CTGs) had mostly contraction in size (often into the normal range); in the sperm of two normal controls, contractions were also more frequent, but occasional expansions into the upper limit of the normal size range were also seen. In conclusion, our results show (1) no overlapping between control (15-91) and pathogenic (100-152) alleles and (2) a high instability in spermatogenesis (both for expanded and normal alleles), suggesting a high mutational rate at the SCA8 locus.

Myotonic dystrophy protein kinase (DMPK) induces actin cytoskeletal reorganization and apoptotic-like blebbing in lens cells

DMPK, the product of the DM locus, is a member of the same family of serine-threonine protein kinases as the Rho-associated enzymes. In DM, membrane inclusions accumulate in lens fiber cells producing cataracts. Overexpression of DMPK in cultured lens epithelial cells led to apoptotic-like blebbing of the plasma membrane and reorganization of the actin cytoskeleton. Enzymatically active DMPK was necessary for both effects; inactive mutant DMPK protein did not produce either effect. Active RhoA but not constitutive GDP-state mutant protein produced similar effects as DMPK. The similar actions of DMPK and RhoA suggest that they may function in the same regulatory network. The observed effects of DMPK may be relevant to the removal of membrane organelles during normal lens differentiation and the retention of intracellular membranes in DM lenses.

FMR1 CGG-repeat instability in single sperm and lymphocytes of fragile-X premutation males

To determine the meiotic instability of the CGG-triplet repeat in the fragile-X gene, FMR1, we examined the size of the repeat in single sperm from four premutation males. The males had CGG-repeat sizes of 68, 75, 78, and 100, as determined in peripheral blood samples. All samples showed a broad range of variations, with expansions more common than contractions. Examination of single lymphocytes indicated that somatic cells were relatively more stable than sperm. Surprisingly, the repeats in sperm from the 75- and 78-repeat males had very different size ranges and distribution patterns despite the similarity of the repeat size and AGG interruption in their somatic cells. These results suggest that cis or trans factors may have a role in male germline repeat instability.

Genetic instabilities in (CTG.CAG) repeats occur by recombination

The expansion of triplet repeat sequences (TRS) associated with hereditary neurological diseases is believed from prior studies to be due to DNA replication. This report demonstrates that the expansion of (CTG.CAG)(n) in vivo also occurs by homologous recombination as shown by biochemical and genetic studies. A two-plasmid recombination system was established in Escherichia coli with derivatives of pUC19 (harboring the ampicillin resistance gene) and pACYC184 (harboring the tetracycline resistance gene). The derivatives contained various triplet repeat inserts ((CTG.CAG), (CGG.CCG), (GAA.TTC), (GTC.GAC), and (GTG.CAC)) of different lengths, orientations, and extents of interruptions and a control non-repetitive sequence. The availability of the two drug resistance genes and of several unique restriction sites on the plasmids enabled rigorous genetic and biochemical analyses. The requirements for recombination at the TRS include repeat lengths >30, the presence of CTG.CAG on both plasmids, and recA and recBC. Sequence analyses on a number of DNA products isolated from individual colonies directly demonstrated the crossing-over and expansion of the homologous CTG.CAG regions. Furthermore, inversion products of the type [(CTG)(13)(CAG)(67)].[(CTG)(67)(CAG)(13)] were isolated as the apparent result of "illegitimate" recombination events on intrahelical pseudoknots. This work establishes the relationships between CTG.CAG sequences, multiple fold expansions, genetic recombination, formation of new recombinant DNA products, and the presence of both drug resistance genes. Thus, if these reactions occur in humans, unequal crossing-over or gene conversion may also contribute to the expansions responsible for anticipation associated with several hereditary neurological syndromes.

Cognitive impairment and (CTG)n expansion in myotonic dystrophy patients

BACKGROUND

Myotonic dystrophy (DM) is a genetic multisystemic disease with muscular, endocrine, ocular, cardiac and cognitive impairment. The molecular basis of the disease has been identified in an unstable base triplet (CTG)n repeat located in the 3' untranslated region of the miotonin protein-kinase (MT-PK) gene on the long arm of chromosome 19. Cognitive impairment could be a direct expression of this genetic alteration at the central nervous system (CNS) level rather than a consequence of the neuromuscular impairment. To explore this hypothesis, we tested a group of genetically diagnosed, adult onset DM, of their nonaffected relatives (NAR), of patients with spinal muscle atrophy (SMA), and of normal controls using the Wechsler Adult Intelligence Scale (WAIS).

METHODS

Seventeen adult-onset DM patients, 9 NAR, 10 SMA patients and 20 unrelated normal controls (NC) were studied. Clinical, neuromuscular and neuropsychiatric evaluation, which included WAIS and the Schedule for Affective Disorders and Schizophrenia (SADS), were performed on the four groups. DM, NAR and NC were also assessed by a neurophysiological (P300) evaluation. A DNA analysis was performed in DM and in NAR to measure presence and magnitude of CTG expansion.

RESULTS

We found a statistically significant difference between verbal (p < .0003), nonverbal (p < .0001) and total (p < .0001) IQ of DM patients compared to IQs of NAR, SMA and NC. Seven out of 11 WAIS subtests were significantly and consistently lower in DM patients compared to SMA and/or NC. In DM patients there was a statistically significant negative correlation between nonverbal (r = -.68; p < .002) and total (r = .59; p < .01) IQ and (CTG)n. Patients with DM had a significantly lower P300 amplitude compared to NAR and NC.

CONCLUSIONS

Our study indicates that in DM there is a mild but significant cognitive impairment which correlates with the degree of CTG expansion and it is not dependent on the neuromuscular impairment; however further studies with larger groups of patients and controls are suggested to confirm our results, due to the small sample size and to a possible effect of educational level in our patients.

Expression of the myotonic dystrophy locus-associated homeodomain protein in congenital myotonic dystrophy

DM locus-associated homeodomain protein (DMAHP), a newly recognized homeodomain protein, is a candidate flanking gene and expressed in various tissues. We examined the expression of the DMAHP in tissues from a congenital myotonic dystrophy patient in comparison to that of control tissues by using semiquantitative reverse transcriptase-polymerase chain reaction. Reduced expression of DMAHP was observed in tissues from the patient with congenital myotonic dystrophy. This finding supports that reduced expression of DMAHP is related to the CTG repeat expansion.

An untranslated CTG expansion causes a novel form of spinocerebellar ataxia (SCA8)

Myotonic dystrophy (DM) is the only disease reported to be caused by a CTG expansion. We now report that a non-coding CTG expansion causes a novel form of spinocerebellar ataxia (SCA8). This expansion, located on chromosome 13q21, was isolated directly from the genomic DNA of an ataxia patient by RAPID cloning. SCA8 patients have expansions similar in size (107-127 CTG repeats) to those found among adult-onset DM patients. SCA8 is the first example of a dominant SCA not caused by a CAG expansion translated as a polyglutamine tract.

Excessive daytime sleepiness in myotonic dystrophy

The aim of the present study was to assess whether or not there is any correlation between magnetic resonance imaging (MRI) abnormalities and excessive daytime sleepiness (EDS) in a consecutive series of patients with myotonic dystrophy (MD). The influences of nocturnal breathing abnormalities and sleep morphology on EDS were also evaluated. Ten MD patients were studied by means of an all-night polysomnographic recording, the multiple sleep latency test (MSLT) and MRI. Diagnosis of MD was established on the basis of the clinical and electrophysiological evidence of myotonia as well as of the characteristic genetic pattern. No patient had respiratory failure. Polysomnography and MSLT were also evaluated in ten healthy age-matched controls under the same environmental conditions. The mean MSLT value was significantly lower in patients than in controls. Five of the ten patients were found to have pathological EDS. The quantitative sleep variables and the nocturnal apnoeas in these five patients were not significantly different from those of the patients without EDS. As two patients did not undergo MRI because of claustrophobia, the MRI data were considered in eight patients. Corpus callosum (CC) atrophy was detected in four patients, whereas three patients showed hyperintense areas in the white matter. No correlation was found between EDS and MRI indexes of subcortical atrophy as well as volume of the hyperintense areas. By contrast, a correlation was found between the MSLT value and the reduction in the anterior area of the CC. Our data suggest that CC atrophy might occur in MD patients, and that the size of the CC anterior area might be associated with EDS.

Decreased expression of myotonic dystrophy protein kinase and disorganization of sarcoplasmic reticulum in skeletal muscle of myotonic dystrophy

Pathological expression of myotonic'dystrophy protein kinase (DMPK) in skeletal muscle of myotonic dystrophy (DM) was studied by Western blot analysis, immunohistochemistry, and immunoelectron microscopy of DMPK. Western blot analysis showed that DMPK protein in DM skeletal muscles dramatically decreased. DMPK-positive muscle fibers showed typical DM pathological changes such as type I atrophy, central nuclei, nuclear chains, and sarcoplasmic masses. In degenerated DMPK-positive muscle fibers, cross-striated bands disappeared, and irregular granular DMPK-positive materials appeared in sarcoplasm. By immunoelectron microscopy, DMPK was localized in the terminal cisternae of the sarcoplasmic reticulum (SR) in DM muscle. Swollen DMPK-positive SRs were detected between well preserved myofibrils in the early stage of DM muscle degeneration, and degenerated intramembranous structures with DMPK and an accumulation of mitochondria were observed between disorganized myofibrils in degenerated DM muscle. We concluded that SR is the primary site of the degeneration of DM skeletal muscle and that the decreased DMPK might cause dysregulation of intracellular calcium metabolism, which is followed by DM muscle degeneration.

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.

Definition of regulatory sequence elements in the promoter region and the first intron of the myotonic dystrophy protein kinase gene

Myotonic dystrophy is the most common inherited adult neuromuscular disorder with a global frequency of 1/8000. The genetic defect is an expanding CTG trinucleotide repeat in the 3'-untranslated region of the myotonic dystrophy protein kinase gene. We present the in vitro characterization of cis regulatory elements controlling transcription of the myotonic dystrophy protein kinase gene in myoblasts and fibroblasts. The region 5' to the initiating ATG contains no consensus TATA or CCAAT box. We have mapped two transcriptional start sites by primer extension. Deletion constructs from this region fused to the bacterial chloramphenicol acetyltransferase reporter gene revealed only subtle muscle specific cis elements. The strongest promoter activity mapped to a 189-base pair fragment. This sequence contains a conserved GC box to which the transcription factor Sp1 binds. Reporter gene constructs containing a 2-kilobase pair first intron fragment of the myotonic dystrophy protein kinase gene enhances reporter activity up to 6-fold in the human rhabdomyosarcoma myoblast cell line TE32 but not in NIH 3T3 fibroblasts. Co-transfection of a MyoD expression vector with reporter constructs containing the first intron into 10 T1/2 fibroblasts resulted in a 10-20-fold enhancement of expression. Deletion analysis of four E-box elements within the first intron reveal that these elements contribute to enhancer activity similarly in TE32 myoblasts and 10 T1/2 fibroblasts. These data suggest that E-boxes within the myotonic dystrophy protein kinase first intron mediate interactions with upstream promoter elements to up-regulate transcription of this gene in myoblasts.

Ribozyme-mediated trans-splicing of a trinucleotide repeat

Trinucleotide repeat expansions (TREs) are a recently described class of mutations characterized by a change in the size of the genomic fragment due to amplification of the repeated unit. A number of diseases have been attributed to TRE, including Huntington disease and myotonic dystrophy (DM), but attempts at genetic therapy have yet to prove successful. A potential therapeutic approach would be to repair the expanded repeat using the trans-splicing ability of group I intron ribozymes. We have used DM as a model to test this hypothesis. A group I intron ribozyme (DMPK-RZ1) was designed to modify the TRE at the 3' end of the human myotonic dystrophy protein kinase (DMPK) transcripts. DMPK-RZ1 was shown to ligate a small DMPK mRNA fragment, contained within the ribozyme, to a simple DMPK-target RNA in vitro. It also modified a larger target transcript, leading to replacement of twelve repeats with five repeats, both in vitro and in mammalian cells. Finally, this ribozyme successfully replaced the 3' end of endogenous DMPK mRNA in fibroblasts with a different 3' region. Ribozyme-mediated RNA repair may thus form a novel therapeutic strategy for diseases associated with repeat expansions.

Immunolocalization of myotonic dystrophy protein kinase in corbular and junctional sarcoplasmic reticulum of human cardiac muscle

The subcellular localization of myotonic dystrophy protein kinase has been examined in human cardiac muscles with confocal laser-scanning microscopy and electron microscopy. A polyclonal antibody was produced against the synthesized peptide from a human kinase cDNA clone. We checked the antibody specificity for cardiac myotonic dystrophy protein kinase using an immunoblotting technique. Immunoblotting of extract from human cardiac muscles showed mainly 70 kDa and 55 kDa molecular weight bands. Confocal images of the protein kinase immunostaining showed striated banding patterns similar to those of skeletal muscles. In addition, the kinase was strongly detected around the intercalated disc. Immunoelectron microscopy showed that the kinase was mainly expressed in both corbular and junctional sarcoplasmic reticulum, but not in network sarcoplasmic reticulum. These results suggest that myotonic dystrophy protein kinase may be involved in the modulation of Ca2+ homeostasis in cardiac myofibres.

The DMPK gene of severely affected myotonic dystrophy patients is hypermethylated proximal to the largely expanded CTG repeat

Using methylation-sensitive restriction enzymes, we characterized the methylation pattern on the 5' side of the CTG repeat in the DMPK gene of normal individuals and of patients affected with myotonic dystrophy, showing expansions of the repetitive sequence. The gene segment analyzed corresponds to the genomic SacI-HindIII fragment carrying exons 11-15. There is constitutive methylation in intron 12 at restriction sites of SacII and HhaI, localized 1,159-1,232 bp upstream of the CTG repeat, whereas most, if not all, of the other sites of SacII, HhaI, and HpaII in this region are unmethylated, in normal individuals and most of the patients. In a number of young and severely affected patients, however, complete methylation of these restriction sites was found in the mutated allele. In most of these patients, the onset of the disease was congenital. Preliminary in vivo footprinting data gave evidence for protein-DNA contact in normal genes at an Sp1 consensus binding site upstream of the CTG repeat and for a significant reduction of this interaction in cells with a hypermethylated DMPK gene.

Instability of the (CTG)n repeat in congenital myotonic dystrophy

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