Myotonic dystrophy patients have larger CTG expansions in skeletal muscle than in leukocytes

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.

CTG-repeat length in distal and proximal leg muscles of symptomatic and non-symptomatic patients with myotonic dystrophy: relation to muscle strength and degree of histopathological abnormalities

Myotonic dystrophy (DM) is an autosomal dominant, multisystemic disorder with a variable phenotypic expression including muscle weakness and myotonia. The muscle wasting is most marked in distal limbs and in facial and neck muscles, although proximal limb muscles become affected as the disease progresses. The CTG-trinucleotide-repeat expansion associated with myotonic dystrophy is usually larger in muscle tissue than in leukocytes. It is unclear whether the repeat length itself bears any relation to the differences in the degree of weakness and atrophy between different muscles. We therefore analysed CTG-repeat lengths in blood and in proximal (m. vastus lateralis) and distal (m. tibialis anterior) muscles of patients with DM (n = 4) and non-symptomatic carriers of the mutant DM allele (n = 2) using conventional Southern blot hybridization. Muscle strength and histopathological abnormalities were evaluated for each muscle. In patients with clinical symptoms, the degree of paresis and morphological abnormalities was markedly more pronounced in m. tibialis anterior than in m. vastus lateralis. In these individuals, the CTG-repeat length was larger in muscles than in leukocytes, whereas in the two non-symptomatic carriers no difference could be detected. Furthermore, there was no clear difference in the repeat length between the two muscles in any of the patients. In conclusion, the selective muscular weakness and atrophy in DM do not seem to be related to differences in CTG-repeat length between different muscles.

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.

Myotonic dystrophy: molecular windows on a complex etiology

Myotonic dystrophy (DM) is the most common form of adult onset muscular dystrophy, with an incidence of approximately 1 in 8500 adults. DM is caused by an expanded number of trinucleotide repeats in the 3'-untranslated region (UTR) of a cAMP-dependent protein kinase (DM protein kinase, DMPK). Although a large number of transgenic animals have been generated with different gene constructions and knock-outs, none of them faithfully recapitulates the multisystemic and often severe phenotype seen in human patients. The transgenic data suggest that myotonic dystrophy is not caused simply by a biochemical deficiency or abnormality in the DM kinase gene product. Emerging studies suggest that two novel pathogenetic mechanisms may play a role in the disease: the expanded repeats appear to cause haploinsufficiency of a neighboring homeobox gene and also abnormal DMPK RNA appears to have a detrimental effect on RNA homeostasis. The complex, multisystemic phenotype may reflect an underlying multifaceted molecular pathophysiology: the facial dysmorphology may be due to pattern defects caused by haploinsufficiency of the homeobox gene, while the muscle disease and endocrine abnormalities may be due to both altered RNA metabolism and deficiency of the cAMP DMPK protein.

Impaired cerebral glucose metabolism in myotonic dystrophy: a triplet-size dependent phenomenon

Myotonic dystrophy (DM) is caused by an expansion of a CTG triplet repeat sequence in the 3'-noncoding region of a protein kinase gene, yet the mechanism by which the triplet repeat expansion causes disease remains unknown. Impaired glucose penetration into brain tissues has been described in DM patients and is a phenomenon that remains unexplained. The present study shows that altered brain glucose metabolism is triplet repeat dependent. We studied brain glucose metabolism (CMRGlu, mumol/100 g/min) by the use of positron emission tomography and 18F-fluoro-2-deoxy-D-glucose in 11 ambulatory non-obese DM patients and in 11 age and sex matched healthy subjects. All subjects underwent a glucose tolerance test with plasma insulin determinations. The expansion of CTG triplet repeats was analyzed in patients with the probe cDNA25 after EcoRI digestion. As compared to controls, in DM patients, the CMRGlu was significantly decreased (26.26 +/- 5.05 vs. 33.43 +/- 2.18, mumol/100 g/min, P = 0.004), and after oral glucose loading, plasma insulin levels were significantly higher and plasma glucose levels remained unchanged (respectively, F = 11.21, P = 0.004 and F = 0.20, P = 0.66). Subsequently, the glucose/insulin ratio was significantly lower in DM patients (F = 6.25, P = 0.02). The length of the expansion of the CTG repeats correlated negatively with the CMRGlu (r2 = 0.63, P = 0.003) and positively with the area under the curve for insulin changes over time after oral glucose (r2 = 0.49, P = 0.016). We conclude that, in DM patients, the brain metabolism of glucose is impaired in a repeat dependent manner.

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

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A novel, heritable, expanding CTG repeat in an intron of the SEF2-1 gene on chromosome 18q21.1

There are currently 13 diseases known to be caused by unstable triplet repeat mutations; however, there are some instances (as with FRAXF and FRA16) when these mutations appear to be asymptomatic. In a search for polymorphic CTG repeats as candidate genes for bipolar disorder, we screened a genomic human chromosome 18-specific library and identified a 1.6 kb clone (7,6A) with a CTG24 repeat that maps to 18q21.1. The CTG repeat locus, termed CTG18.1, is located within an intron of human SEF2-1, a gene encoding a basic hellx-loop-hellx DNA binding protein involved in transcriptional regulation. The CTGn repeat is highly polymorphic and very enlarged alleles, consistent with expansions of up to CTG2100, were identified. PCR and Southern blot analysis in pedigrees ascertained for a Johns Hopkins University bipolar disorder linkage study and in CEPH reference pedigrees revealed a tripartite distribution of CTG18.1 alleles with stable alleles (CTG10-CTG37), moderately enlarged and unstable alleles (CTG53-CTG250), and very enlarged, unstable alleles (CTG800-CTG2100). Moderately enlarged alleles were not associated with an abnormal phenotype and have a combined enlarged allele frequency of 3% in the CEPH and bipolar populations. Very enlarged alleles, detectable only by Southern blot analysis of genomic digests, have thus far been found in only three individuals from our bipolar pedigrees, and to date, have not been found in any of the CEPH reference pedigrees. These enlarged alleles may arise, at least in part, via somatic mutation.

Expansion of the myotonic dystrophy CTG repeat reduces expression of the flanking DMAHP gene

Myotonic dystrophy, or dystrophia myotonica (DM), is a highly variable multisystem disease in which the classic adult-onset form displays progressive muscle wasting, cataracts, heart block, gonadal atrophy, insulin resistance and neuropsychiatric impairment. Its genetic basis is an expansion of CTG trinucleotide repeats in the DMPK protein kinase gene. Among the triplet repeat expansion disorders, DM is distinguished by the extended length of the repeat tract (5-13 kb in postmortem tissue) and its location in the 3' untranslated region of the gene that contains it. The pathophysiological mechanism for multisystem degeneration in DM is not understood. In contrast to the profound muscle wasting that characterizes advanced DM, only minor histopathological abnormalities have occurred in DMPK knockout mice or in mice that overexpress a human DMPK transgene, making it unlikely that changes in DMPK activity provide a unitary explanation for the disease. A DNAse hypersensitive site that maps 0.7 kb downstream (centromeric) from the CTG repeats is eliminated on DM chromosomes. This finding indicates that the repeat expansion may alter the adjacent chromatin structure and raises the possibility that it may also affect the expression of flanking genes. An interesting candidate flanking gene is DMAHP, a recently discovered homeodomain-encoding gene. We show here that DMAHP expression in myoblasts, muscle and myocardium is reduced by the DM mutation is cis, and the magnitude of this effect depends on the extent of CTG repeat expansion. These observations support the hypothesis that DMAHP participates in the pathophysiology of DM.

Myotonic dystrophy: molecular and cellular consequences of expanded DNA repeats are elusive

The mutation in the myotonic dystrophy (DM) gene is an expansion in a triplet (CTG) repeat in the 3' untranslated region of a novel gene that partially encodes a serine-threonine protein kinase (DMPK), with closest sequence homology to a small subgroup of protein kinases involved in the control of proliferation and cell shape. Expansion of the repeat correlates reasonably well with disease severity and offers a plausible molecular explanation for the previously contentious issue of anticipation. There is considerable heterogeneity in CTG expansion size in different tissues of affected individuals. The consensus of data from many laboratories indicates that DMPK mRNA is most probably downregulated as a consequence of the repeat expansion. Two polypeptides (68/78 kDa) have been shown to be absent in mouse knockout mutants and therefore can be considered as bona fide gene products. Previous data suggesting that 52-55 kDa polypeptides were likely candidates, have been firmly ruled out at the same time. Further results from studies of knockout and overexpressing transgenic mice indicate that neither simple loss nor gain of DMPK expression is sufficient to account for the DM clinical phenotype. One of the most pressing questions now being addressed is how expansion of the CTG repeat within the DMPK gene affects gene expression, not only of DMPK, but of all genes at the 19q13.3 locus: is DMPK actually responsible for the clinical phenotype seen in DM? The identification of both immediate upstream and downstream human genes (59 and DMRHP, respectively) has been an important first step to answering these questions. Only when these matters have been dealt with can one reasonably expect to start to delineate the different metabolic and signalling pathways responsible for the diverse phenotypes that make up the complex clinical picture of DM.

Phenotypic variability in Friedreich ataxia: role of the associated GAA triplet repeat expansion

We studied genotype-phenotype correlations in a group of 100 patients with typical Friedreich ataxia (FRDA), and in three groups of patients with atypical clinical presentations, including 44 Acadian FRDA, 8 late-onset FRDA (LOFA), and 6 FRDA with retained reflexes (FARR). All patients, except 3 with typical FRDA, carried two copies of the FRDA-associated GAA triplet repeat expansion. Overall, the phenotypic spectrum of FRDA appeared to be wider than defined by the currently used diagnostic criteria. Our study indicated the existence of several sources of variability in FRDA. Patients with larger GAA expansions tended to have earlier onset and were more likely to show additional manifestations of the disease. Mitotic instability of the expanded GAA repeats may partially account for the limited degree of correlation between expansion sizes as determined in lymphocytes and clinical parameters. Some clinical variants associated with specific FRDA haplotypes, such as Acadian FRDA and FARR, turned out to be unrelated to expansion sizes. No polymorphism in the frataxin coding sequence could be associated with these clinical variants.

Carrell-Krusen Symposium Invited Lecture-1997. Myotonic disorders in childhood: diagnosis and treatment

The recent discoveries that mutations in the genes for the skeletal muscle sodium and chloride channels are responsible, respectively, for paramyotonia/hyperkalemic periodic paralysis and for myotonia congenita of Thomsen have made the classification, diagnosis, and treatment of these disorders much easier. The discovery that myotonic dystrophy results from an unstable [CTG]n trinucleotide expansion has permitted the accurate diagnosis of both symptomatic and asymptomatic individuals, and has led to major advances in preventive treatment, including prenatal and genetic counseling. Diseases that resemble the inherited myotonic disorders are now easier to identify, and as a result of genetic testing a new clinical disorder that is similar to but distinct from myotonic dystrophy has emerged. This new disorder, proximal myotonic myopathy, does not appear to be linked to the genes for the sodium or chloride channels, and has cataracts, myotonia, weakness, and no abnormal expansion of the [CTG]n repeat in the gene for myotonic dystrophy. This review discusses the diagnosis and treatment of these myotonic disorders.

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.

Prediction of myotonic dystrophy clinical severity based on the number of intragenic [CTG]n trinucleotide repeats

We carried out a genotype-phenotype correlation study, based on clinical findings in 465 patients with myotonic dystrophy (DM), in order to assess [CTG] repeat number as a predictive test of disease severity. Our analysis showed that the DM subtypes defined by strict clinical criteria fall into three different classes with a log-normal distribution. This distribution is useful in predicting the probability of specific DM phenotypes based on triplet [CTG] number. This study demonstrates that measurement of triplet expansions in patients' lymphocyte DNA is highly valuable and accurate for prognostic assessment.

CTG repeat size and histologic findings of skeletal muscle from patients with congenital myotonic dystrophy

An approximate correlation has been demonstrated between the degree of CTG repeat expansion and clinical severity among myotonic dystrophy patients. Congenital myotonic dystrophy, which is the most severe form of the disease, has the largest size of CTG repeat. Muscle immaturity is a characteristic finding in congenital myotonic dystrophy muscle. We compared the CTG repeat size and histologic findings of skeletal muscle from patients with congenital myotonic dystrophy. An 8.6 kb or 9.8 kb plus an expanding band ranging from 15 kb to 17.5 kb was observed in muscle from five patients with congenital myotonic dystrophy by Southern blot analysis using EcoRI-digested DNAs probed with p5B1.4. There was no correlation between immaturity of skeletal muscle and the degree of CTG repeat expansion on skeletal muscle. Undetermined maternal factors may have an important role in the cause of immaturity of muscle in congenital myotonic dystrophy patients.

CTG repeat analysis in lymphocytes, muscles and fibroblasts in patients with myotonic dystrophy

The mutation responsible for DM has been identified as the amplification of a polymorphic (CTG)n repeat in the 3' untranslated region of the myotonin proteinase gene. To examine somatic instability of the repeat, we studied tissue variability of the CTG expansion of three mesodermally derived tissues: lymphocytes, cultured fibroblasts and muscle cells. In six patients with adult onset DM, the repeat region was larger in skeletal muscles and fibrolasts as compared to lymphocytes. Our findings indicate that somatic CTG instability between examined tissues might take place postnatally by a selection mechanism in lymphocytes.

Myotonic dystrophy: will the real gene please step forward!

The mutation underlying myotonic dystrophy (DM) was identified at the end of 1991 amidst great rejoicing from the patients supporting the research and, not least, from those who spent so long searching for it. Subsequently, the molecular genetic phenomena associated with DM have been clearly explained by the transmission behaviour of the expanding repeat, which remains the only mutation that has been described in patients. We understand the molecular basis of anticipation, why the severe congenital form is almost exclusively transmitted by affected mothers and we have widely accepted models of the population genetics of DM. Yet, despite all these clearly explained molecular events, we appear to be hardly any closer to understanding the molecular pathology of DM than when the mutation was first identified. To understand the reason for this, we have to look in detail at the mutation itself, and in particular at the locus and its complex nuances. In doing so, we begin to realise that DM is unique amongst the Mendelianly inherited disorders, in that the mutation, because of its location in a very gene-rich region of the genome, probably simultaneously renders several genes dysfunctional. The somatic heterogeneity of the repeat, coupled with the involvement of several genes, accounts for the pleiotropy observed in the phenotype. Added to this complexity is the uncertainty of the level at which gene dysfunction or gain of function is occurring. It is possibly at the level of DNA/chromatin structure and/or RNA regulation/processing, and all of these pathways may, in different tissues, contribute to the final phenotype.

A family with an unusual myotonic and myopathic phenotype and no CTG expansion (proximal myotonic myopathy syndrome): a challenge for future molecular studies

Myotonic dystrophy (DM) is a well-defined autosomal dominant disorder characterized by myotonia, muscle weakness, cardiac conduction defects, cataracts, and endocrine abnormalities. Recently a newly recognized disorder, similar to but distinct from DM, has been observed with multisystem findings including intermittent myotonia, proximal myopathy, and occasional cardiac conduction disturbances. This disorder has been called proximal myotonic myopathy (PROMM). No history of anticipation is present and there is no linkage to the gene locus for DM or to the loci for the muscle sodium or chloride channels. This report describes a family with a normal size of the CTG trinucleotide repeat expansion of the DM gene in which affected individuals have myotonia (intermittent, exacerbated by cold), bilateral cataracts, mild hypogonadism and mild temporal atrophy. Affected individuals also have proximal muscle weakness, facial involvement, nonspecific abnormalities on muscle biopsy, normal cardiac conduction, and no glucose intolerance. The absence of trinucleotide repeat expansion in the DM gene is consistent with this family being affected by a disorder distinct from DM, possibly a form of PROMM.

Monozygotic twins with 22q11 deletion and discordant phenotypes

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CTG repeat length in muscle from patients affected with myotonic dystrophy (DM)

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Somatic cell heterogeneity between DNA extracted from lymphocytes and skeletal muscle in congenital myotonic dystrophy

Myotonic dystrophy (DM) results from the expansion of an unstable CTG trinucleotide repeat in the 3' untranslated region of mRNA encoding a putative serine/threonine protein kinase. The degree of the CTG repeat amplification in genomic DNAs extracted from lymphocytes correlates with disease severity. We have analyzed the amplification of the CTG repeat of DNAs extracted from skeletal muscles and lymphocytes in five congenital DM patients. The amplification from skeletal muscles showed an increase of about 1.5 kb to 3.5 kb larger than that from lymphocytes in all patients. Furthermore, we have investigated the somatic instability of the CTG repeat in various tissues from a severe congenital DM patient.

Comparison of CTG repeat length expansion and clinical progression of myotonic dystrophy over a five year period

Myotonic dystrophy (DM) is associated with an underlying CTG trinucleotide repeat expansion at a locus on chromosome 19q13.3. We have determined the repeat length in 23 DM patients with varying clinical severity of symptoms and various sizes of repeat amplification. We confirm that as in previous studies there is no strong correlation between repeat length and clinical symptoms but find that the repeat length in peripheral blood cells of patients increases over a time span of five years indicating continuing mitotic instability of the repeat throughout life. Repeat length progression does not appear to be indicative of clinical progression but age probably is. The degree of expansion correlates with the initial repeat size and 50% of the patients with continuing expansions showed clinical progression of their disease symptoms over the five year study period.

Direct molecular analysis of myotonic dystrophy in the German population: important considerations in genetic counselling

Myotonic dystrophy (DM) is associated with the expansion and instability of a trinucleotide (CTG) repeat at the DM locus on chromosome 19. Direct genomic analysis in the German population was carried out on 18 DM families, six families with equivocal diagnosis, 69 subjects with equivocal clinical diagnosis, and 100 controls using the polymerase chain reaction (PCR) and a refined Southern protocol. In the majority of the cases molecular analysis confirmed the clinical diagnosis. These included seven cases of congenital DM (CDM) with widely differing gene expansions and instabilities. In most DM families the expanded fragment became larger in successive generations, but we also identified four families with contractions and two families that showed stability of the enlarged fragment during transmission. In four clinically defined DM patients we were unable to detect enlarged CTG repeats. Sequencing of each exon of the DM gene in two of these patients failed to show any mutations. Our cases have important implications for genetic counselling of DM families, highlighting both the diagnostic value of direct genomic analysis and its limitations.

Postzygotic instability of the myotonic dystrophy p[AGC] in repeat supported by larger expansions in muscle and reduced amplifications in sperm

We have analysed the [AGC] expansion in leucocytes, muscle and sperm from 17 individuals affected by myotonic dystrophy (DM). Skeletal muscle showed a larger repeat number than leucocytes in the same patient. A similar degree of expansion was detected in differently affected muscles of a single patient. The germline mutation (< or = 350 repeats) was expanded in somatic cells of the progeny in all patients examined. Our results provide evidence of an early postzygotic instability of the [AGC] repeat in DM.

Problems arising in correlating clinical and molecular data in myotonic dystrophy

The number of copies of CTG trinucleotide repeats in the myotonic dystrophy gene correlates to a certain degree with the clinical symptoms in the patient. Routine molecular analysis of myotonic dystrophy is performed on peripheral blood cells detecting the size of the expansion in leukocytes. However, in some cases somatic mosaicism is responsible for the presence of differently sized myotonic dystrophy alleles in different tissues of the same affected individual, complicating diagnosis and prognosis. Here we report two cases in which the correlation between molecular and clinical analysis performed with standard procedures posed some interpretative problems. The first individual was affected by an atypical clinical picture of myotonic dystrophy, the severity of which was not correlated with the low number of triplet repeats detected in his leukocyte DNA. The second case illustrates a prognostic problem in the presence of a low degree expansion in leukocytes. These examples outline the limits of standard molecular and clinical analysis in myotonic dystrophy.

Triplet repeats in neuromuscular disorders

SBMA, SCA1 and DM are all neuromuscular disorders which have very different presentation with varying degrees of severity but with a common feature of muscular weakness. In addition they all show anticipation correlating with the length of an unstable trinucleotide repeat. It is possible that trinucleotide repeat expansions will underlie other hereditary neuromuscular disorders, for example spinocerebellar ataxia type 2 (mapped to 12q and with similar symptoms to SCA1 including anticipation) is a prime candidate (30). The biochemical effects of these expansions have not yet been elucidated and therefore effective therapeutic interventions cannot currently be designed for affected individuals. However, the relative ease of diagnosis and the availability of accurate prenatal testing have already had a significant impact on the patient groups.

Myotonic dystrophy: correlation of clinical symptoms with the size of the CTG trinucleotide repeat

An unstable DNA sequence of a gene encoding a protein kinase has been identified as the molecular basis of myotonic dystrophy. The correlation between different symptoms of myotonic dystrophy and the size of this unstable base triplet (CTG)n repeat was investigated in 14 patients. DNA was prepared from whole blood by standard procedures. Detailed clinical, psychological, electrophysiological (quantified measurement of myotonia, electrocardiography) and other laboratory examinations (muscle biopsy in 4 patients, slit lamp examination) were performed. Triplet size correlated significantly with muscular disability and inversely with age at onset of the disease. A greater frequency of mental and gonadal dysfunction could be observed in patients with a larger repeat size. Other symptoms, however, such as cataract, myotonia, gastrointestinal dysfunction and cardiac abnormalities were not correlated with repeat size. Somatic mosaicism with different amplification rates in various tissues might be one possible explanation for the variable phenotypes. Furthermore, other factors such as different expression of the myotonic dystrophy gene might contribute to the clinical variability of the disease at a given triplet size.

Correlation between decreased myocardial glucose phosphorylation and the DNA mutation size in myotonic dystrophy

BACKGROUND

Myotonic dystrophy, the most common form of adult dystrophy, has been shown to be caused by amplification of CTG triplet repeat in the 3' untranslated region of a protein kinase gene located on chromosome 19. Impaired glucose metabolism has been suggested as a possible explanation of brain and skeletal muscle involvement in this multisystem disease. We investigated whether myocardial glucose metabolism is impaired in myotonic dystrophy and whether this impairment is related to the size of the mutation.

METHODS AND RESULTS

The myocardial metabolic rate for glucose (MMRGlu, mumol.min-1.g-1), K1 (blood-to-tissue transfer constant), k2 (tissue-to-blood transfer constant), and k3 (phosphorylation rate constant) were determined in 7 control subjects and 12 patients with myotonic dystrophy by using parametric images generated from dynamic cardiac positron emission tomography (PET) and 18F-fluoro-2-deoxy-glucose studies. The expansion of the CTG triplet repeats was analyzed in patients with the probe cDNA25 after EcoRI digestion. Nonparametric tests were used to compare quantitative variables between control subjects and patients. The correlations between the size of the mutation and PET parameters were studied by linear regression. MMRGlu and k3 were significantly decreased in patients compared with control subjects (0.39 +/- 0.20 versus 0.64 +/- 0.25, P = .03, and 0.09 +/- 0.07 versus 0.24 +/- 0.21, P = .03, respectively), whereas K1 and k2 were not statistically different between control subjects and patients. MMRGlu and k3 correlate inversely with the length of the CTG triplet repeat (r = -.65 and P = .03 for MMRGlu, and r = -.85 and P = .001 for k3, respectively).

CONCLUSIONS

In myotonic dystrophy, the observed reductions in MMRGlu and phosphorylation are inversely linked to the length of the mutation. This observation suggests that impaired modulation of a protein kinase involved in myocardial hexokinase activation may give a pathophysiological schema to relate the molecular defect and the abnormal myocardial metabolism in myotonic dystrophy.

Presentation, clinical course, and outcome of the congenital form of myotonic dystrophy

We report the clinical experience of 18 patients with the congenital form of myotonic dystrophy, the majority of whom were diagnosed during the neonatal period and monitored from 5 to 14 years. Prematurity associated with congenital myotonic dystrophy gives rise to the severest clinical manifestations. Among them, respiratory involvement is common and is the leading cause of death in the neonatal period. Weakness and foot deformities secondary to muscle involvement are the predominant clinical features of this group of patients from birth to age 3 or 4 years. Once muscle strength improves, learning disabilities and behavioral disturbances become the main clinical problems. All our patients, when tested after 5 years of age, had intelligence quotients under 65, clearly below the average intelligence quotient of their mothers (IQ = 80). There is no relationship between the degree of mothers' and patients' disease. No patient has presented problems with routine immunizations, and no complications were observed in the 7 patients who underwent surgery under general anesthesia. Among the surviving patients, no correlation can be established between severity of disease in the neonatal period and the magnitude of sequelae as teenagers. Mental and behavioral disturbances are the factors which mainly influence the long-term management and prognosis of this cohort of individuals.

Myotonic dystrophy with no trinucleotide repeat expansion

We report 3 patients from 2 families with myotonic dystrophy who do not show an abnormal expansion of CTG trinucleotide repeats within the myotonic dystrophy gene. Characteristic features of myotonic dystrophy in these patients were frontal balding, cataracts, cardiac conduction abnormalities, and testicular atrophy with myotonia and muscle weakness. Results of muscle histopathology were consistent with myotonic dystrophy. Genetic analysis of leukocyte and muscle DNA showed a normal number of CTG repeats. The demonstration of normal CTG repeat number for the myotonic dystrophy gene does not exclude the diagnosis of myotonic dystrophy.