Clinical characteristics of myotonic dystrophy type 1 patients with small CTG expansions

Myotonic dystrophy type 1 (DM1) is the most common adult form of muscular dystrophy. This autosomal dominant disorder results from an unstable cytosine, thymine, guanine (CTG) repeat expansion in the 3' untranslated region of a dystrophia myotonia protein kinase gene at 19q13.3.(1) The CTG repeat number ranges between 5 and 37 in normal subjects, whereas in DM1 patients, it is increased up to several thousands units. As a general rule, patients with larger CTG repeat expansions in their circulating leukocytes have an earlier age at onset and more severe symptoms. (2-6) Measurement of the CTG amplification is therefore considered valuable for prognostic assessment. To further improve the accuracy of prognostic assessment in predictive DNA testing of DM1, we performed a genotype-phenotype correlation study based on the clinical findings in patients carrying small CTG expansions.