Myotonic dystrophy (DM) is the most common form of adult-onset muscular dystrophy and is characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. There are two distinct entities currently known: DM type 1 (DM1) and type 2 (DM2). DM2 is caused by the expansion of a tetranucleotide CCTG repeat in the first intron of the zinc finger protein 9 (ZNF9) gene on chromosome 3q21,1 whereas DM1 is caused by a CTG repeat expansion in the 3′-untranslated region of the dystrophia myotonica-protein kinase gene (DMPK).2 In the normal allele for ZNF9, the repeat sequence is a complex motif with an overall configuration of (TG)n(TCTG)n(CCTG)n. The number of CCTG repeats is <30 in the normal allele, with interruptions by GCTG and/or TCTG motifs, and this allele is stably transmitted from one generation to the next.1, 3 However, in the expanded allele only the CCTG tract elongates and no GCTG and TCTG interruptions occur. The expanded ZNF9 allele is extremely unstable and the size is highly variable, ranging from 75 to 11 000 repeats, with a mean of 5000 CCTG repeats. This unprecedented repeat size and somatic heterogeneity make the molecular diagnosis of DM2 difficult, and explain why the expansion yields variable clinical phenotypes.4

To date, DM2 mutations have been identified predominantly in European Caucasians.3, 5 Although a small number of DM2 mutations have been reported in non-European populations, including families in Morocco, Algeria, Lebanon, Afghanistan and Sri Lanka,6, 7 all reported that DM2 patients had been considered to originate from a single common founder because they shared an identical haplotype.3, 5, 7 However, in 2008 we identified the first case of DM2 in an East-Asian population, in a Japanese patient with a disease haplotype distinct from that shared among Caucasians, indicating that DM2 exists in non-Caucasian populations and that there may have been separate founders.8

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