genetic analysis in patients with a clinical picture of Myotonic Dystrophy. I.C. Ciro Candiano1, P. Tarantino1, S. Carrideo1, E.V. De Marco1, D. Civitelli1, F. Annesi1, F.E. Rocca1, V. Greco1, M. Caracciolo1, C. Rodolico2, A. Toscano2, G. Vita2, G. 1) 1Intitute of Neurological Sciences, National Research Council, Mangone (Cosenza), Italy; 2) 2Institute of Neurology, University of Messina.
Myotonic dystrophy (DM) is a dominantly inherited disorder; the classic form (DM1) is caused by an expanded CTG repeat in the 3-UTR of the dystrophia myotonica-protein kinase gene (DMPK) on 19q13. Disease severity varies with the number of repeats: normal subjects have 5 to 37 repeats; individuals with premutation (38÷49 repeats) are asymptomatic but premutation alleles are unstable; mildly affected persons have 50 to 80 repeats (protomutation); severely affected patients have 2,000 or more repeats. DM can also be caused by a CCTG expansion (mean ~5,000 repeats) in intron 1 of the zinc finger protein 9 (ZNF9) gene on 3q21 (DM2). We performed a molecular study of DMPK gene in patients with clinical features of DM. We analyzed the probands of 10 families and 71 unrelated patients. All patients samples were initially screened by PCR identify unaffected individuals, demonstrating two alleles in the normal range and small expansions often observed in minimally affected cases. The patients showing only one PCR allele within the normal range require a Southern confirmation that has two possible outcomes: A) the patient is ruled homozygous for the normal allele, B) an expanded allele is detected, thus confirming the diagnosis of DM. Thirty-three subjects are wild-type; among them one presents a premutation. Fourty-seven patients are genetically confirmed DM1. Molecular analysis of DMPK gene in 81 subjects revealed the expanded CTG repeat in 47 patients with varying clinical severity and various sizes of repeat amplification; one patient from a consanguineous family DM1-linked had an expansion of CTG repeat on both alleles (65 and 933 repeats) but in DM homozygotes do not differ phenotipically from heterozygotes. Two individuals carried a protomutation (50÷80 CTG repeats). For the 33 unaffected patients other neuromuscular diseases should be considered; alternatively, many milder phenotypes may be caused by DM2 mutations.