The autosomal dominant mutation causing myotonic dystrophy (DM1) is a CTG repeat expansion in the 3'-UTR of the DM protein kinase (DMPK) gene. This multisystemic disorder includes myotonia, progressive weakness and wasting of skeletal muscle and extramuscular symptoms such as cataracts, testicular atrophy, endocrine and cognitive dysfunction. The mechanisms underlying its pathogenesis are complex. Recent reports have revealed that DMPK gene haploinsufficiency may account for cardiac conduction defects whereas cataracts may be due to haploinsufficiency of the neighboring gene, the DM-associated homeobox protein (DMAHP or SIX5) gene. Furthermore, mice expressing the CUG expansion in an unrelated mRNA develop myotonia and myopathy, consistent with an RNA gain of function. We demonstrated that transgenic mice carrying the CTG expansion in its human DM1 context (>45 kb) and producing abnormal DMPK mRNA with at least 300 CUG repeats, displayed clinical, histological, molecular and electrophysiological abnormalities in skeletal muscle consistent with those observed in DM1 patients. Like DM1 patients, these transgenic mice show abnormal tau expression in the brain. These results provide further evidence for the RNA trans-dominant effect of the CUG expansion, not only in muscle, but also in brain.

Myotonic dystrophy (DM) is caused by a CTG repeat expansion in the 3'UTR of the DM protein kinase (DMPK) gene. A very high level of instability is observed through successive generations and the size of the repeat is generally correlated with the severity of the disease and with age at onset. Furthermore, tissues from DM patients exhibit somatic mosaicism that increases with age. We generated transgenic mice carrying large human genomic sequences with 20, 55 or >300 CTG, cloned from patients from the same affected DM family. Using large human flanking sequences and a large amplification, we demonstrate that the intergenerational CTG repeat instability is reproduced in mice, with a strong bias towards expansions and with the same sex- and size-dependent characteristics as in humans. Moreover, a high level of instability, increasing with age, can be observed in tissues and in sperm. Although we did not observe dramatic expansions (or 'big jumps' over several hundred CTG repeats) as in congenital forms of DM, our model carrying >300 CTG is the first to show instability so close to the human DM situation. Our three models carrying different sizes of CTG repeat provide insight on the different factors modulating the CTG repeat instability.

A (CTG)(n) expansion in the 3'-untranslated region (UTR) of the DM protein kinase gene (DMPK) is responsible for causing myotonic dystrophy (DM), Major instability, with very large expansions between generations and high levels of somatic mosaicism, is observed in patients, There is a good correlation between repeat size (at least in leucocytes), clinical severity and age of onset, The trinucleotide repeat instability mechanisms involved in DM and other human genetic diseases are unknown, We studied somatic instability by measuring the CTG repeat length at several ages in various tissues of transgenic mice carrying a (CTG)(55) expansion surrounded by 45 kb of the human DM region, using small-pool PCR, These mice have been shown to reproduce the intergenerational and somatic instability of the 55 CTG repeat suggesting that surrounding sequences and the chromatin environment are involved in instability mechanisms. As observed in some of the tissues of DM patients, there is a tendency for repeat length and somatic mosaicism to increase with the age of the mouse. Furthermore, we observed no correlation between the somatic mutation rate and tissue proliferation capacity. The somatic mutation rates in different tissues were also not correlated to the relative inter-tissue difference in transcriptional levels of the three genes (DMAHP, DMPK and 59) surrounding the repeat.

Les cinq dernieres annees ont ete marquees par la decouverte d'un mecanisme moleculaire de maladies genetiques, encore inedit, celui des triplets repetes instables. Cette anomalie moleculaire qui montre une forte propension a l'expansion d'une generation a l'autre a permis d'expliquer au niveau moleculaire le phenomene d'anticipation. La dystrophie myotonique de Steinert, maladie musculaire la plus frequente de l'adulte, appartient a cette famille, mais elle est la seule a etre associee a l'expansion d'un triplet CTG, situee dans la region 3' non traduite (3'UTR) du gene DMPK sur le chromosome 19. Par-dela une correlation entre la taille du triplet et la gravite de la maladie, on distingue de multiples formes cliniques. L'abolition de l'expression de l'allele DMPK, porteur d'une amplification superieure a 700 CTG, ne semble pas capable a elle seule d'expliquer l'augmentation de la gravite au-dela de ce seuil. L'expansion de la repetition CTG pourrait perturber egalement l'expression de genes contigus, ou bien, la region 3'UTR acquerant de nouvelles proprietes, un gain de fonction pourrait deregler l'expression d'autres genes.

Myotonic dystrophy (DM) is one of a growing number of genetic disorders associated with a tripler repeat dynamic mutation discovered during the last five years. The intergenerational increase in size represents the mole cular basis of the long debated phenomenon known as anticipation, an increase of the severity through consecutive generations. Myotonic dystrophy (DM) is the most frequent autosomal dominant muscular dystrophy of adults, the symptoms of which may be numerous and diverse. The mutational el?ent causing DM is a dynamic amplification of a repeated (CTG)n DNA motif located within the 3' untranslated region (3'UTR) of the gene encoding myotonin protein kinase DMPK. Whereas the underlying mechanisms by which other expanded triplets, CAG, CCG and GAA produce the phenotype in other diseases are rather well-understood there has hardly been any progress in answering the key question: how does the DM CTG repeat in the 3'UTR exert its effect(s)? The severe and multisystemic manifestations of myotonic dystrophy may not be a simple monogenic loss-or gain-of-function effect. There remains a question mark upon whether the expanded repeat in DM influences the DMPK gene, its RNA or its protein products by interference with transcription, alternative splicing, transport or translational efficiency of mRNA or the entire cellular contest in which the DMPK is expressed. However haploinsufficiency of DMPK as a unique pathogenic mechanism has been ruled out and the mouse models clearly showed that DM is not simply due to a lack or excess of the DMPK protein since these animals lack myotonia, cataracts and the congenital form. Other neighbouring genes within this gene-dense area may also be involved. An alternative hypothesis could be an alteration in the normal cellular function of the 3'UTR of the DMPK. Finally it has been put forward that the expanded DM could interact with RNA. Other animal models al-e therefore needed to understand the pathological consequences of this mysterious type of mutation and to reproduce che human DM phenotype for future therapy.

The molecular basis of myotonic dystrophy (DM) has been characterised. All DM mutations characterised to date appear as an unstable elongation of a fragment containing a tandem repeat of a CTG motif, which can be visualised in both EcoRI and BamHI digests. It has been shown that the fragment is polymorphic in the normal population. Another 1 kb insertion/deletion polymorphism located near the unstable CTG repeat region has been identified. The 1 kb insertion allele is present in all DM patients. These different polymorphic systems can be distinguished using cDNA25 and BamHI, because this enzyme cuts between the site of the 1 kb insertion and the CTG repeat. We thus haplotyped DM patients from 72 French families and clearly showed that all chromosomes (100%) with the DM mutation carried the 1 kb insertion as well. In addition to this association, we detected significant linkage disequilibrium between the DM locus and D19S63 for which allelic frequencies were different from other European populations. Our results in the French DM population are thus in agreement with the hypothesis that the CTG expansion occurred on one or a few ancestral chromosomes carrying the large 1 kb insertion allele.

Myotonic dystrophy (DM) is a progressive neuromuscular disorder which results from elongations of an unstable (CTG)n repeat, located in the 3' untranslated region of the DM gene. A correlation has been demonstrated between the increase in the repeat number of this sequence and the severity of the disease. However, the clinical status of patients cannot be unambiguously ascertained solely on the basis of the number of CTG repeats. Moreover, the exclusive maternal inheritance of the congenital form remains unexplained. Our observation of differently sized repeats in various DM tissues from the same individual may explain why the size of the mutation observed in lymphocytes does not necessarily correlate with the severity and nature of symptoms. Through a molecular and genetic study of 142 families including 418 DM patients, we have investigated the dynamics of the CTG repeat meiotic instability. A positive correlation between the size of the repeat and the intergenerational enlargement was observed similarly through male and female meioses for < or = 0.5-kb CTG sequences. Beyond 0.5 kb, the intergenerational variation was more important through female meioses, whereas a tendency to compression was observed almost exclusively in male meioses, for > or = 1.5-kb fragments. This implies a size- and sex-dependent meiotic instability. Moreover, segregation analysis supports the hypothesis of a maternal as well as a familial predisposition for the occurrence of the congenital form. Finally, this analysis reveals a significant excess of transmitting grandfathers partially accounted for by increased fertility in affected males.

Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disease. The mutation has been identified as an unstable trinucleotide CTG repeat in a sequence encoding a putative cAMP-dependent protein kinase. The CTG repeat varies in length between affected siblings, and generally increases through generations in parallel with increasing severity of the disease. Congenital myotonic dystrophy, which represents the most severe phenotype, is exclusively maternally inherited. In this report, we show, by Northern blot analysis, that no mutated enlarged transcript is detectable in a 20-week-old DM fetus and in two congenitally affected infants. Furthermore, in skeletal and cardiac muscle of the DM fetus, we observe by RNA analysis, including Norhern blot and RT-PCR, an unexpectedly low expression of the paternal wild type allele. Varying degrees of expression of the mutant and/or the normal allele might therefore account for the characteristic features of the congenital form and the extreme variability of the disease.

We report on two cases of prenatal diagnosis of myotonic dystrophy (DM), using flanking markers APOC2 or CKMM on the proximal side and D19S51 on the distal side. By double digestion (TaqI and NcoI) of PCR amplified CKMM, the informativeness was increased from a PIC value of 0.57 to 0.69. Altogether, with a PIC value of 0.64 for APOC2, 0.69 for CKMM, and 0.27 for D19S51 (BglI), presymptomatic and prenatal diagnosis can thus be offered to approximately 24% of persons with a risk between 0.0004 and 0.0008 using these flanking markers.

High prevalence of myotonic dystrophy (DM) of 18.1 per 100,000 has been found in Croatian region Istria, a region where a great mixture of nations occurred over the last three centuries. The objective of this study was to test the hypothesis of common ancestry in Istrian DM families. Pedigrees were constructed on the basis of extensive family history obtained from the patients in all Istrian DM families. Church records were consulted in order to improve genealogical reconstruction. Additionally, we performed haplotype analyses with two intragenic and three extragenic DNA polymorphic markers. A common ancestor couple for three of nine nucleus families was found eight generations backward, which was supported by haplotype analysis. In spite of finding an evidence of common ancestry in Croatian Istria we argue that the phenomenon of founder effect is not sufficient to explain the high DM prevalence in Istria.