Effects of the sex of myotonic dystrophy patients on the unstable triplet repeat in their affected offspring.

The mutation responsible for myotonic dystrophy (DM) is an unstable expansion of the CTG repeat within the myotonin protein kinase gene. To examine whether the parental origin of the expanded repeat influences the repeat size in offspring, we studied 51 father-child and 59 mother-child pairs with DM. Small expansions in fathers resulted in larger size expansions in their offspring, while large paternal expansions resulted in less size change in their offspring. However, there was no correlation between maternal size expansion and size increase in offspring for either congenital or noncongenital DM. These data suggest that the sex of the affected parent influences the unstable expansion of the repeat in DM offspring. While some evidence suggests that DNA methylation status cannot explain this observation, the mechanism for differential maternal/paternal transmission expansion is currently unknown.