title: Variant CCG and GGC repeats within the CTG expansion dramatically modify mutational dynamics and likely contribute toward unusual symptoms in some myotonic dystrophy type 1 patients.
abstract: Myotonic dystrophy type 1 (DM1) is one of the most variable inherited human disorders. It is characterized by the involvement of multiple tissues and is caused by the expansion of a highly unstable CTG repeat. Variation in disease severity is partially accounted for by the number of CTG repeats inherited. However, the basis of the variable tissue-specific symptoms is unknown. We have determined that an unusual Dutch family co-segregating DM1, Charcot-Marie-Tooth neuropathy, encephalopathic attacks and early hearing loss, carries a complex variant repeat at the DM1 locus. The mutation comprises an expanded CTG tract at the 5'-end and a complex array of CTG repeats interspersed with multiple GGC and CCG repeats at the 3'-end. The complex variant repeat tract at the 3'-end of the array is relatively stable in both blood DNA and the maternal germ line, although the 5'-CTG tract remains genetically unstable and prone to expansion. Surprisingly though, even the pure 5'-CTG tract is more stable in blood DNA and the maternal germ line than archetypal DM1 alleles of a similar size. Complex variant repeats were also identified at the 3'-end of the CTG array of approximately 3-4% of unrelated DM1 patients. The observed polarity and the stabilizing effect of the variant repeats implicate a cis-acting modifier of mutational dynamics in the 3'-flanking DNA. The presence of such variant repeats very likely contributes toward the unusual symptoms in the Dutch family and additional symptomatic variation in DM1 via affects on both RNA toxicity and somatic instability.
authors.affiliation: Molecular Genetics, Faculty of Biomedical and Life Sciences, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK.
authors.forename: Claudia
authors.initials: C
authors.lastname: Braida
authors.forename: Rhoda K A
authors.initials: RK
authors.lastname: Stefanatos
authors.forename: Berit
authors.initials: B
authors.lastname: Adam
authors.forename: Navdeep
authors.initials: N
authors.lastname: Mahajan
authors.forename: Hubert J M
authors.initials: HJ
authors.lastname: Smeets
authors.forename: Florence
authors.initials: F
authors.lastname: Niel
authors.forename: Cyril
authors.initials: C
authors.lastname: Goizet
authors.forename: Benoit
authors.initials: B
authors.lastname: Arveiler
authors.forename: Michel
authors.initials: M
authors.lastname: Koenig
authors.forename: Clotilde
authors.initials: C
authors.lastname: Lagier-Tourenne
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Mandel
authors.forename: Catharina G
authors.initials: CG
authors.lastname: Faber
authors.forename: Christine E M
authors.initials: CE
authors.lastname: de Die-Smulders
authors.forename: Frank
authors.initials: F
authors.lastname: Spaans
authors.forename: Darren G
authors.initials: DG
authors.lastname: Monckton
mesh_terms: D000483:Alleles; D005260:Female; D006801:Humans; D008297:Male; D009154:Mutation; D009223:Myotonic Dystrophy / Q000235:genetics*; D010375:Pedigree; D019680:Trinucleotide Repeat Expansion; D018911:Trinucleotide Repeats