[Facioscapulohumeral myopathy and germinal mosaicism]

Germline mosaicism is now well known to account for recurrence of hereditary human disorders. Facioscapulohumeral muscular dystrophy is an autosomal dominant disorder; its locus has been identified in the telomeric region of chromosome 4 at the q35 band. It appears to have a high rate of mutation.

A young girl had presented from childhood signs of a severe form of facioscapulohumeral muscular dystrophy, but with no familial history. The diagnosis was ultimately confirmed at the age of 23 years by molecular studies evidencing the deletion. The same abnormality was sparsely found in the child's father who appeared to harbor the mutation as a germline mosaicism with no clinical expression.

This case illustrates the possibility of severe facioscapulohumeral muscular dystrophy and the dominant transmission of the disorder which may be clinically occult. It underlines the importance of molecular biology and the difficulties of genetic counselling.