Human Molecular Genetics Advance Access originally published online on August 29, 2007
Human Molecular Genetics 2007 16(23):2834-2843; doi:10.1093/hmg/ddm239
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Molecular mechanisms responsible for aberrant splicing of SERCA1 in myotonic dystrophy type 1
1 Division of Molecular and Cellular Biology, Department of Anatomy, Faculty of Medicine, University of Miyazaki, Kihara 5200, Kiyotake, Miyazaki 889-1692, Japan, 2 Division of Structural Cellular Biology, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan and 3 Department of Neurology, Osaka University Graduate School of Medicine, Osaka, Japan
* To whom correspondence should be addressed. Tel: +81 985851783; Fax: +81 985859851; Email: imaizumi{at}med.miyazaki-u.ac.jp
Received May 30, 2007; Revised July 26, 2007; Accepted August 21, 2007
Myotonic dystrophy type 1 (DM1) is an autosomal dominant neuromuscular disorder associated with an expansion of CTG trinucleotide repeats in the 3'-untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. The RNA gain-of-function hypothesis proposes that mutant DMPK mRNA alters the function and localization of alternative splicing regulators, which are critical for normal RNA processing. Previously, we found alternative splicing variants of sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 1 (SERCA1), which excluded exon 22, in skeletal muscle of DM1 patients. In the present study, we analyzed the molecular mechanisms responsible for the splicing dysregulation of SERCA1. Five ‘YGCU(U/G)Y’ motifs that could potentially serve as Muscleblind-like 1, (MBNL1)-binding motifs, are included downstream from the SERCA1 exon 22. Exon trapping experiments showed that MBNL1 acts on the ‘YGCU(U/G)Y’ motif, and positively regulates exon 22 splicing. Of the five MBNL1 motifs in intron 22, the second and third sites were important for regulation of exon 22 splicing, but the other three binding sites were not required. Overexpression of the CUG repeat expansion of DMPK mRNA resulted in exclusion of exon 22 of SERCA1. These results suggest that sequestration of MBNL1 into the CUG repeat expansion of DMPK mRNA could cause the exclusion of SERCA1 exon 22, and the expression of this aberrant splicing form of SERCA1 could affect the regulation of Ca2+ concentration of sarcoplasmic reticulum in DM patients.
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