Phenotype and CTG-repeat size in myotonic dystrophy: A study of 26 patients and 55 relatives

The present multicenter study was carried out to look at genotype-phenotype correlations and CTG-repeat length variations between generations in patients with myotonic dystrophy (MD). We investigated 26 patients from 13 unrelated families and 55 of their relatives from 5 different neurological departments, clinically by application of the muscular disability rating score (MDRS), and the sum of symptoms score (SSS), electromyographically, and by DNA analysis. In MD patients, CTG repeat lengths ranged from 0.2 to 6.4 kb. All patients were heterozygous for the CTG repeats. The MDRS and SSS scores correlated significantly (r = 0.56, r = 0.58, p < 0.05) with the expansion size. Expansion was observed in 4 and retraction in 2 MD families. The expanded allele was transmitted in all but 2 MD families. In 2 subjects MD could be confirmed solely by DNA analysis. In one patient with clinical features of MD, the CTG repeat size was normal.