Identification of DMD Mutation in Korean Siblings Using Full Gene Sequencing

Duchenne Muscular dystrophy (DMD) is an X-linked recessive disorder caused by mutations in the dystrophin gene, which is located in Xp21. The majority of the identified mutations are large deletions and duplications, and gene dosage assays were developed for quantitative genomic screening of copy number variations. However, remaining 25% of the DMD are due to point mutations and require direct full gene sequencing. We report Korean siblings with novel small intragenic duplication in an exon 41 (c.5756dupT) which was detected by direct sequencing of whole dystrophin (DMD) gene exons. This 1-bp duplication is a novel frameshift mutation and induces premature termination (p.Leu1919Phefs13). Gene therapy in DMD has been developed and it is important to know the exact mutation site and type to predict prognosis and to prepare further therapy. Therefore, in DMD patients with normal DMD gene dosage, direct sequencing of DMD gene is essential to detect small intragenic deletions/insertions and missense, nonsense, and splicing mutations. Address for correspondence: Dr. Myungshin Kim, M.D. Department of Laboratory Medicine, Seoul St. Mary’s Hospital, The Catholic University of Korea 505 Banpo-dong, Seocho-gu, 137-701 Korea Telephone: +82-2-2258-1645, Fax: +82-2-2258-1719 E-mail: [email protected] INTRODUCTION Duchenne muscular dystrophy (DMD) is the most common inherited muscle disease, characterized by dystrophin deficiency caused by mutations in the dystrophin (DMD) gene (Davies et al. 1988). The DMD gene is located at Xp21 with 2.2 million base pairs, with 79 exons and several promotors, comprising 0.6% of entire human genome. Dystrophin is 427kDa cytoskeletal protein that is a member of the β-spectrin/α-actinin protein family. This protein is important for the strength and flexibility of the muscle fiber menbranes. Dystrophin can be organized into four separate regions. These are the actin-binding domain at the NH 2 terminus, the central rod domain, the cysteine-rich domain, and the COOH-terminal domain. (Blake et al. 2002). In previous studies, Multiplex ligation-dependent probe amplification (MLPA) has been shown as a useful method for quantitatively detecting mutations in the DMD gene, not only for identifying deletions but also for duplications and female carriers (Yang et al. 2013). However, about 25% of the DMD are due to point mutations and require direct full gene sequencing. In this study, the researchers report Korean male siblings with novel small intragenic duplication which was detected by direct sequencing of whole DMD gene. Like our case, in patient with negative MLPA, Sanger direct sequencing is essential for the diagnosis of the DMD. The role of direct sequencing cannot be overemphasized in DMD studies.