Somatic mosaicism of p(CTG)n expansion in a case of myotonic dystrophy with parotid tumor

Myotonic dystrophy (MD) is an autosomal dominant systemic disorder with an unstable expansion of the CTG triplet repeat in the 3'-untranslated region of the gene encoding myotonine protein kinase (DMPK) which maps to chromosome 19q13.3. Somatic mosaicism of CTG repeats in MD has been reported; and it has been observed that CTG repeats in tumor tissues associated with MD are more expanded than the other tissues. It is not rare that parotid tumors are found in patients with MD. We performed Southern blot analysis for tissues from the parotid tumor, the normal parotid gland, the skeletal muscles, and the leukocyte from a 60-year-old patient with MD. CTG repeat was most expanded in the parotid tumor, and the normal parotid gland had longer expansion of CTG repeat than the skeletal muscles. The leukocyte had the shortest expansion of CTG repeat. The expansion of CTG repeat in the parotid tumor may be related to active cell division and may underlie the occurrence of tumors in MD.