[Genotype-phenotype correlation in myotonic dystrophy and prediction of clinical seriousness].

INTRODUCTION: Since description of the expansion of the number of CTG trinucleotides on the long arm of chromosome 19 in the 19 q 13.2-13.3 interval as being responsible for myotonic dystrophy (DM), many studies have established a direct relationship between the size of the expansion and the severity of the manifestations. OBJECTIVES: To evaluate the clinico-genetic correlation in a population with DM in Eastern Andalucia and to establish the predictive value of the increase in number of CTG repetitions with regard to clinical gravity. PATIENTS AND METHODS: A transverse study of persons from families known to have members with DM, classified with regard to the clinical gravity of their illness. Diagnosis was by means of clinical neurological, ophthalmological and neurophysiological examination and subsequently by genetico-molecular study, Southern blot, PCR, oligonucleotide CTC hybridization and Northern blot. RESULTS: Genetic studies confirmed the previous clinical diagnosis of DM in 78 persons, of the 145 studied, who came from 32 families. The average size of the mutation was 1-5 kb. There was close correlation between the size of the expansion and the clinical condition. Logistic repression studies permitted adequate classification in function of the size of the expansion in 87.23% of the cases of clinically relevant illness and in 90% of the cases in which the condition was not clinically relevant. CONCLUSIONS: There is a strong association between the clinical features of DM and the magnitude of the mutation. The size of the expansion has considerable predictive value in prognosis of the disorder. This may be useful when making decisions during prenatal diagnosis.