IDMC-6 - The sixth International Myotonic Dystrophy Consortium Meeting

In 1992 three groups of investigators found that myotonic dystrophy of Steinert, classical myotonic dystrophy, now known as myotonic dystrophy type 1 (DM1), results from an unstable CTG repeat expansion in the non-coding 3� region of serinethreonine kinase gene DMPK at 19q13.3. Almost a decade later in 2001 collaborators in Minnesota and Germany discovered a second gene defect responsible for myotonic dystrophy type 2 (DM2), another autosomal dominant, multisystem disease, that is similar to but distinct from DM1. DM2 results from an unstable CCTG repeat expansion in intron 1 of the zinc finger protein 9 gene ZNF9 at 3q21. The exact cause for the instability of the DNA at these two different loci, and the explanation for the tissue mosaicism that exists in different tissues in DM1 and DM2 remains a mystery although part of the answer probably lies in faulty repair of DNA. Why the CTG repeat expansion in DM1 is larger in skeletal and cardiac muscle and in certain parts of the brain than in circulating leucocytes and some other tissues is not clear. Whether this tissue mosaicism accounts for or plays a major role in determining the spectrum of disease manifestations requires further investigation.Whether it is possible to use the tissue instability of DNA in refining the diagnosis and prognosis of patients with DM1 and DM2 is a question that has yet to be explored.