Molecular and clinical study of two myotonic dystrophy homozygotes.

EDITOR—Myotonic dystrophy (DM) is the most common form of inherited neuromuscular disease in adults and is characterised by progressive muscle wasting and myotonia. The mutation responsible for DM has been identified as the amplification of a polymorphic (CTG)n repeat in the 3' untranslated region of a gene encoding a serine/threonine kinase (DMPK). The DM trinucleotide repeat is highly polymorphic in normal subjects, ranging from 5 to 37 CTG repeats, while 1000 or more CTG repeats have been observed in congenital DM cases. It is now generally accepted that the CTG repeat length is correlated with clinical severity and the age at onset of the disease; therefore, genetic tests are essential in the monitoring and management of DM patients and their family members. Homozygous DM cases have very rarely been published. 8 It was assumed that the homozygous state was lethal in DM or at least so severe that it would lead to intrauterine death. Cobo et al studied a consanguineous French-Canadian family in which two sisters were homozygous for the “at risk” haplotype but were asymptomatic and showed no evidence of DM on extensive clinical examination. Both sisters possessed two alleles with repeat sizes normally seen in minimally affected patients. Both parents were aVected. Martorell et al described three unrelated homozygous myotonic dystrophy patients. One patient had the classical form of myotonic dystrophy and the other two were mildly aVected. A remarkable feature was the mildness of the phenotype in the homozygous patients; one, for example, had late onset cataract as the only manifestation. Along with the observations of Cobo et al and Martorell et al, this led Zlotogora, to conclude that in myotonic dystrophy homozygotes do not diVer from heterozygotes and that, like Huntington’s disease (HD), DM is a “true dominant”.