The clinical consequences of X-chromosome inactivation: Duchenne muscular dystrophy in one of monozygotic twins.

We have ascertained retrospectively a female patient, one of identical twins, who was diagnosed at age 23 years as having Duchenne muscular dystrophy (DMD). A muscle biopsy at that time showed a pattern in which large areas of destroyed muscle fibers replaced with adipose tissue were interspersed with normal-appearing muscle fascicles. The visualization of Barr bodies in the muscle biopsy, plus the patient's normal menstrual history served to rule out Turner's syndrome. The clinical expression of DMD in only one of monozygotic twins is strongly suggestive of uneven lyonization, with an excess of paternally derived X-chromosomes being inactivated in the patient. This view is supported by the appearance of the muscle biopsy. Twinning may conceivably predispose to uneven lyonization by reducing the size of the muscle cell anlage at the time of X-chromosome inactivation. Alternatively, lyonization may occur before the splitting of the embryonic mass, and by chance, the two embryonic centers could end up with a significantly different proportion of active maternal and paternal X-chromosomes.