Early abnormalities in a mouse model for myotonic dystrophy : a model for Congenital Myotonic Dystrophy ? – MODELCDM

Myotonic dystrophy type I (DM1) is a dominantly inherited disorder with an incidence of 1 in 8000 adults. Clinically, it is highly variable. The adult onset form typically presents distal muscle weakness, myotonia, presenile cataracts, cardio-respiratory problems, hypersomnia, hyperinsulinism, behavioural changes and cognitive dysfunction. The more severe congenital form of DM1 (CDM) is characterised at birth by general hypotonia, respiratory distress, difficulty in sucking and swallowing, facial weakness and later on by a delayed motor development and mental retardation. The genetic mutation causing DM1 and CDM is the expansion of an unstable CTG repeat, in the 3’ UTR of the DMPK gene (DM protein kinase). The normal DMPK gene contains 5-37 CTG repeats, while all DM1 patients have repeats expanding from 50 to several thousand CTG repeat in CDM. The size of the CTG repeat, which increases from generation to generation, is generally correlated with clinical severity and age at onset. It is clear now that the majority of the symptoms observed in DM1 are due to a dominant effect of the toxic RNA containing CUG expansion. The mutant transcripts are trapped in the nucleus forming nuclear foci and alter the function of CUG binding proteins usually involved in splicing regulation. Indeed, splicing deregulation of at least 20 RNA have been observed in DM1 patients and some of them are known to be involved in the pathology. Despite the tremendous progress achieved so far to explain the molecular basis of DM1 features, the CDM form of the disease remains a mystery.
We have created a transgenic mouse model to study the CTG repeat instability, to evaluate the consequences of expression of RNA carrying CUG expansion and to develop gene therapy at preclinical level. We used large 45-kb fragments of genomic DNA cloned directly from DM1 patients. We obtained control mice with 20 CTG and mice carrying over 300 CTG repeats. Due to high level of instability similar to that observed in DM1 patients, we obtained more recently mice carrying over 1000 CTG (“DMSXL”). Homozygous mice showed high number of nuclear RNA foci, splicing abnormalities and a robust phenotype. Furthermore molecular, histological and physiological analyses in adult mice demonstrate heart, muscle and brain defects. Therefore, DMSXL mice represent a multisystemic model for DM1 that will be useful to decipher the mechanism underlying this disease, but also for systemic preclinical assays. Our mouse model is the only one expressing the DMPK gene under the regulation of its own promoter and major regulatory sequences, during development and in various tissues.
Our new project is focused particularly on the very early stage of life in the DMSXL. We have observed a very high level of mortality early in life (60%) in the first 3 weeks of age. Homozygous neonates of 1 day generally display reduced movements, pale skin colour and absence or small quantity of milk in the stomach, compared to their wild-type littermates. Interestingly, in patients, the congenital form is associated with hypotonia, respiratory distress, facial weakness and sucking and swallowing defects resulting in a very high mortality in the neonatal period. These observations lead us to think that we may have reproduced CDM in these mice. Based on these results, we propose that pathophysiological processes induced by CTG expansions originate during early development in the DMSXL mice. We will study at early stages the molecular, histological and physiological consequences of large CTG repeats. Having the only existing mouse model expressing the largest CTG repeats obtained in mice so far (up to 1700 CTG), we have in our hand a unique and powerful tool to decipher the molecular basis of at least some features of the severe congenital myotonic dystrophy.