[Molecular genetics diagnosis of Steinert's myotonic dystrophy].

Myotonic dystrophy (DM) is the most common neuromuscular disease with adult onset (incidence 1 in 8000). The biochemical basis of this autosomal dominantly inherited disease is still unknown. The most striking features are myotonia and progressive muscular wasting. There is high variability of disease severity in patients from different families, but also within the same family. For practical reasons three subtypes can be defined: The classical adult onset form of the disease, a mild form with late onset and/or very moderate symptoms, eg. cataracts only, and the most severe congenital form which is transmitted by affected females. Furthermore, the progression of DM in affected families may exhibit an increase in the severity of the disease in successive generations. This observation is called anticipation. Very recently the DM gene has been cloned and an unstable DNA sequence specific for the disease has been characterized. Detection of an enlarged DNA fragment due to the expansion of a trinucleotide (CTG) repeat within the DM gene can be used for direct DNA diagnosis in affected individuals and persons at risk. Furthermore, there is a strong correlation between the length of fragment expansion and the degree of disease severity in gene carriers. We report here our preliminary results of the investigation of over 70 patients and demonstrate the clinical usefulness of this new method by the findings in three families.