title: Becker muscular dystrophy caused by an intronic mutation reducing the efficiency of the splice donor site of intron 26 of the dystrophin gene.
abstract: We describe an 11-year-old boy with dystrophinopathy who presented with a history of progressive proximal muscle weakness and elevated serum creatine kinase levels at age 6. Sequence analysis of the dystrophin (DMD) gene did not identify a mutation in the coding regions but revealed a nucleotide substitution in intron 26 (c.3603+3A>T). Since computer algorithms did not conclusively indicate that this sequence variant inactivated the splice site, we analyzed the DMD mRNA from a muscle biopsy of the patient to determine its functional significance. PCR and sequence analysis of the cDNA demonstrated that the mutation reduced the efficiency of the donor splice site and caused activation of a cryptic donor site 113 bp downstream. Activation of the cryptic donor site led to inclusion of 116 bp of intronic sequence containing a stop codon producing a truncated dystrophin protein. Residual wild-type splicing was also detected, which would explain the milder Becker rather than Duchenne phenotype in this patient. We highlight the importance of mRNA analysis for determination of pathogenicity in patients with ambiguous sequence variants in the DMD gene.
authors.affiliation: Division of Molecular Genetics, Department of Paediatric Laboratory Medicine, The Hospital for Sick Children, 555 University Avenue, Toronto, Ont., Canada. berivan.baskin@sickkids.ca
authors.forename: Berivan
authors.initials: B
authors.lastname: Baskin
authors.forename: Brenda
authors.initials: B
authors.lastname: Banwell
authors.forename: Reem Al
authors.initials: RA
authors.lastname: Khater
authors.forename: Cynthia
authors.initials: C
authors.lastname: Hawkins
authors.forename: Peter N
authors.initials: PN
authors.lastname: Ray
mesh_terms: D017398:Alternative Splicing / Q000235:genetics; D000595:Amino Acid Sequence / Q000235:genetics; D001483:Base Sequence / Q000235:genetics; D002648:Child; D018388:Codon, Terminator / Q000235:genetics; D003402:Creatine Kinase / Q000032:analysis / Q000097:blood; D004252:DNA Mutational Analysis; D016189:Dystrophin / Q000235:genetics*; D020022:Genetic Predisposition to Disease / Q000235:genetics*; D006801:Humans; D007438:Introns / Q000235:genetics*; D008297:Male; D018908:Muscle Weakness / Q000235:genetics; D018482:Muscle, Skeletal / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology; D020388:Muscular Dystrophy, Duchenne / Q000235:genetics*; D009154:Mutation / Q000235:genetics*; D022821:RNA Splice Sites / Q000235:genetics*; D012333:RNA, Messenger / Q000032:analysis / Q000235:genetics
chemical_list: D018388:Codon, Terminator; D016189:Dystrophin; D022821:RNA Splice Sites; D012333:RNA, Messenger; D003402:Creatine Kinase