Confirmation of the Type 2 Myotonic Dystrophy (CCTG) Expansion Mutation in Patients with Proximal Myotonic Myopathy/Proximal Myotonic Dystrophy of Different European Origins: A Single Shared Haplotype Indicates an Ancestral Founder Effect

Abstract

Myotonic dystrophy (DM), the most common form of muscular dystrophy in adults, is a clinically and genetically heterogeneous neuromuscular disorder. DM is characterized by autosomal dominant inheritance, muscular dystrophy, myotonia, and multisystem involvement. Type 1 DM (DM1) is caused by a (CTG)(n) expansion in the 3' untranslated region of DMPK in 19q13.3. Multiple families, predominantly of German descent and with clinically variable presentation that included proximal myotonic myopathy (PROMM) and type 2 DM (DM2) but without the DM1 mutation, showed linkage to the 3q21 region and were recently shown to segregate a (CCTG)(n) expansion mutation in intron 1 of ZNF9. Here, we present linkage to 3q21 and mutational confirmation in 17 kindreds of European origin with PROMM and proximal myotonic dystrophy, from geographically distinct populations. All patients have the DM2 (CCTG)(n) expansion. To study the evolution of this mutation, we constructed a comprehensive physical map of the DM2 region around ZNF9. High-resolution haplotype analysis of disease chromosomes with five microsatellite and 22 single-nucleotide polymorphism markers around the DM2 mutation identified extensive linkage disequilibrium and a single shared haplotype of at least 132 kb among patients from the different populations. With the exception of the (CCTG)(n) expansion, the available markers indicate that the DM2 haplotype is identical to the most common haplotype in normal individuals. This situation is reminiscent of that seen in DM1. Taken together, these data suggest a single founding mutation in DM2 patients of European origin. We estimate the age of the founding haplotype and of the DM2 (CCTG) expansion mutation to be approximately 200-540 generations.

New insights into the genetic instability in CCTG repeats

“…In DM2, the CCTG tract is located in the ZNF9 gene and is generally interrupted in healthy range alleles by GCTG, TCTG or ACTG motifs [3,31,32]. The largest identified allele contained 32 CCTG repeats [31].…”

Population frequency of myotonic dystrophy: higher than expected frequency of myotonic dystrophy type 2 (DM2) mutation in Finland

“…Myotonic dystrophy type 1 (DM1, Steinert's disease, MIM #160900) is caused by a (CTG) n expansion mutation in the 3¢ UTR of dystrophia myotonica protein kinase (DMPK) in chromosome 19q13.3; 1-3 DM type 2 (DM2, MIM #602668) is caused by a (CCTG) n expansion mutation in the first intron of zinc-finger protein 9 (ZNF9) in chromosome 3q21. 4,5 Similar clinical features of DM1 and DM2 include myotonia and limb muscle weakness, pronounced distal in DM1 and proximal in DM2, and multi-organ involvement including cataracts, insulin resistance, elevated liver enzyme levels, male hypogonadism and cardiac conduction defects. Muscle atrophy, facial weakness, ptosis and frontal baldness are very prevalent in DM1, whereas muscle pain and hypertrophy of calf muscles are more characteristic for DM2.…”

“…4,5,8,9 It has been proposed that only uninterrupted (CCTG) n repeats are associated with the DM2 phenotype. 5,8,9 The range of (CCTG) n repeats in patients varies between 55 and 11 000, but, unlike DM1, the threshold defining the difference between normal and disease-causing repeats has not yet been firmly established.…”

Myotonic dystrophy type 2 is rare in the Japanese population

“…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.…”

“…Both the Finnish and Italian population are expected to be a relatively representative European population with regard to the DM2 mutation, because of a single European founder haplotype. 3,5,7 Genomic DNA was extracted from blood leukocytes or muscle biopsy samples according to the standard protocols. The CCTG repeat size was determined by PCR, using primers flanking the repeat.…”