Congenital Phenotypes and DMPK CTG Repeat Number in Mothers and Children With Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is an autosomal dominant genetic disease. In DM1, the mutant allele expands during gametogenesis, and an extended CTG repeat sequence is inherited by the offspring. This often results in increased severity of DM1 symptoms in the affected offspring and may cause congenital myotonic dystrophy (CDM). This study aimed to clarify whether CTG repeat number predicts CDM in offspring. This retrospective study examined 14 women with DM1, their pregnancy and labor histories, and their 14 children diagnosed with DM1. There were 12 CDM patients and 2 non-CDM patients. Correlations between CDM onset and CTG repeat numbers of the mother and child were analyzed. Women who bore a child with CDM (infants with detected polyhydramnios during pregnancy, hypotonia, respiratory insufficiency, or suckling failure at birth) had a mean repeat number of 643 (standard deviation [SD] 436). For women who bore a child without CDM, the mean repeat number was 950 (SD 71), and no significant between-group difference was detected. The mean (SD) CTG repeat numbers observed in children with and without CDM were 1,646 (324) and 1,700 (565), respectively. CDM cannot be predicted based on the CTG repeat numbers of mothers or children.