Korean J Lab Med.  2008 Dec;28(6):483-492. 10.3343/kjlm.2008.28.6.483.

Molecular and Clinical Characteristics of Myotonic Dystrophy Type 1 in Koreans

• Kim SY
• Kim JY
• Kim GP
• Sung JJ
• Lim KS
• Lee KW
• Chae JH
• Hong YH
• Seong MW
• Park SS

• KMID: 1781584
• DOI: http://doi.org/10.3343/kjlm.2008.28.6.483

Abstract

BACKGROUND
Myotonic dystrophy type 1 (DM1) is an autosomal-dominant muscular dystrophy caused by expansion of cytosine-thymine-guanine (CTG) trinucleotide repeats in the myotonic dystrophy protein kinase (DMPK) gene. The clinical features of DM1 are multisystemic and highly variable, and the unstable nature of CTG expansion causes wide genotypic and phenotypic presentations. The aim of this study was to characterize the molecular and clinical spectra of DM1 in Koreans. METHODS: The CTG repeats of 283 Korean individuals were tested by PCR fragment analysis and Southern blot. The following characteristics were assessed retrospectively: spectrum of CTG expansions, clinical findings, genotype-phenotype correlation, anticipation, and genetic instability. RESULTS: One-hundred twenty-four patients were confirmed as DM1 by molecular tests, and the CTG expansions ranged from 50 to 2,770 repeats (median 480 repeats). The most frequent clinical features were myotonia, muscular weakness, and family history. Patients with muscular weakness or dysfunction of the central nervous system harbored larger CTG expansions than those without each symptom (P<0.05). The age of onset was inversely correlated with the size of the CTG expansion (gamma=-0.422, P<0.001). The instability of CTG expansion representing as the maximum difference between sibships was observed from 50 to 700 repeats in nine families. Clinical anticipation and the increase in CTG repeat were significantly higher in maternally transmitted alleles (P=0.002). CONCLUSIONS: Molecular genetic tests are not only essential for diagnosis, but also helpful for suggesting the spectrum and relationship between genotype and phenotype in Korean DM1 patients.

Keyword

Myotonic dystrophy type 1; DMPK gene; CTG; Trinucleotide repeat expansion; Polymerase chain reaction; Southern blot; Anticipation; Instability; Age of onset; Korean

MeSH Terms

Blotting, Southern
Data Interpretation, Statistical
Female
Genotype
Humans
Korea
Male
Myotonic Dystrophy/*diagnosis/*genetics
Pedigree
Phenotype
Polymerase Chain Reaction
Protein-Serine-Threonine Kinases/genetics
Retrospective Studies
Trinucleotide Repeat Expansion/genetics

Figure

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  Fig. 1. The schematic map of the DMPK gene with relative location of CTG repeats, exons near the repeats, primers, and probe used in this study. The CTG repeats are detected by PCR using primers 102-F and 101-R when the fragment is less than 500 bp. To detect large expansions of more than 500 bp, genomic DNA was digested with the restriction endonucleases EcoRI or PstI. The EcoRI digestion results in ~9 kb band without Alu element, or ~10 kb band with Alu insertion. The PstI digestion results in ~1.2 kb and ~0.8 kb bands. Probe pM10M-6 is ~1.4 kb fragment containing the CTG repeats. Each fragment size is from the allele with Alu insertion and (CTG)20 repeats.

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  Fig. 2. Representative results from (A) PCR and fragment analysis and (B) Southern blot. (A) Chromatogram obtained (a) from a normal individual with heterozygous 12 and 17 CTG re- peats. (b) from a normal individual with homozygous five CTG repeats. (c) from a DM1 patient with one normal 13 CTG repeat in the upper panel and the expanded 129 repeats in the lower panel. The chromatogram in lower panel was obtained after longer time electrophoresis. (B) (a) an autoradiogram obtained from four individuals by Southern blot after EcoRI digestion, and (b) after PstI digestion. In autoradiogram (a), a normal allele of ~9 kb was observed in lanes 1, 2, and 4. Expanded mutant allele larger than ~10 kb was observed in lanes 1 and 2 (marked by asterisk). The expansion of CTG repeats was also suspected in lane 3 (marked by arrow), but not clearly discriminated from a normal band of ~10 kb length. In autoradiogram (b), two normal bands of ~0.8 kb and ~1.2 kb were observed in each lane, and an additional expanded band was observed in lanes 1, 2, and 3. The expanded alleles were estimated as ~690 repeats in lane 1 and ~980 repeats in lane 2. An expanded allele of 75 repeats observed in lane 3 was also determined by PCR and fragment analysis (data not shown). Therefore, these three patients were diagnosed as DM1. Lane 4 was an unaffected individual, heterozygous for 6 and 12 CTG repeats (data from PCR and fragment analysis, not shown).

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  Fig. 3. Pedigrees of 18 Korean DM1 families with two or more patients. Identifier number of each family is indicated above, and the CTG repeats number in the DMPK gene is below each symbol of family member.

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  Fig. 4. The distribution of 430 normal alleles with different CTG repeats in the DMPK gene. Histogram is derived from the analysis of 94 normal alleles from 47 normal control subjects, 239 normal alleles from 120 unrelated subjects suspicious as DM1 but harbored no expanded alleles of 50 or more CTG repeats, and 97 normal alleles from 97 unrelated DM1 patients diagnosed by molecular analysis.

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  Fig. 5. The CTG repeat expansion and the age of onset in 83 DM1 patients. Two factors show an inverse correlation.

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  Fig. 6. The size of CTG repeats expansion for (A) mother-offspring, and (B) father-offspring pairs. Anticipation was more evident in maternally-transmitted alleles. Contraction occurred only in paternally-transmitted alleles. Diagonal lines mean that the same allele size was expected between parent and offspring. Open circles located below the diagonal line indicate contraction of CTG repeats in offspring.

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