In situ hybridization analysis of Dmpk mRNA in adult mouse tissues

Myotonic dystrophy1 (DM1) is an autosomal dominant, multi-system disorder resulting from a CTG repeat expansion located in the 3′ untranslated region of DMPK and immediately in the 5′ of SIX5. Skeletal muscle, heart and smooth muscle are prominently affected in DM1. Endocrine abnormalities, gonadal atrophy, brain, skin, skeletal, immune and respiratory defects are also features of the disorder. Both DMPK and SIX5 levels are decreased in DM1 patients. Importantly, expression of mutant mRNAs encoding expanded CUG repeats has been shown to alter the activity of CUG repeat binding proteins in DM1. Mouse models have demonstrated that decreased levels of Dmpk, Six5 and the expression of expanded CUG repeats independently contribute to the development of DM1 pathology. However, an important gap in these studies is a lack of clear understanding of the expression pattern of Dmpk. We demonstrate that Dmpk mRNA is expressed in a range of adult mouse tissues that show pathology in DM1 including skeletal muscle, heart, smooth muscle, bone, testis, pituitary, brain, eye, skin, thymus and lung. Thus DMPK loss or CUG repeat expansion could contribute to DM1 pathology to these tissues. Dmpk mRNA is not detected in the ovary, pancreas or kidney. Significantly, Dmpk mRNA is expressed in the intestinal epithelium, cartilage and liver, which have not been reported to show consistent abnormalities in Dmpk ^-/- mice or in transgenic animals expressing CUG repeats. Taken together these data suggest that Dmpk loss or CUG repeat expression per se may not be sufficient to initiate pathology and are consistent with the hypothesis that coexpression of specific CUG repeat binding proteins with the mutant Dmpk mRNA or deregulation of genes such as Six5 that flank the CTG repeat tract may be necessary for DM1 to manifest.