Background: Myotonic dystrophy type 1 is caused by an unstable (CTG)n repetition located in the 3′UTR of the DM protein kinase gene (DMPK). Untranslated expanded DMPK transcripts are retained in ribonuclear foci which sequester CUG-binding proteins essential for the maturation of pre-mRNAs.
Aim: To investigate the effects of CTG expansion length on three molecular parameters associated with the DM1 muscle pathology: (1) the expression level of the DMPK gene; (2) the degree of splicing misregulation; and (3) the number of ribonuclear foci.
Methods: Splicing analysis of the IR, MBNL1, c-TNT and CLCN1 genes, RNA-FISH experiments and determination of the DMPK expression on muscle samples from DM1 patients with an expansion below 500 repetitions (n = 6), DM1 patients carrying a mutation above 1000 CTGs (n = 6), and from controls (n = 6).
Results: The level of aberrant splicing of the IR, MBNL1, c-TNT and CLCN1 genes is different between the two groups of DM1 muscle samples and correlates with the CTG repeat length. RNA-FISH analysis revealed that the number of ribonuclear foci in DM1 muscle sections increases in patients with a higher (CTG)n number. No relationships were found between the expression level of the DMPK gene transcript and average expansion sizes.
Conclusion: The CTG repeat length plays a key role in the extent of splicing misregulation and foci formation, thus providing a useful link between the genotype and the molecular cellular phenotype in DM1.
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▸ Additional figure is published online only at http://jmg.bmj.com/content/vol45/issue10
Funding: Study supported by PRIN grant #2005064759, Telethon grant #GPP07250 and AFM grant #13360. Muscle samples were provided by Telethon Biobank N° GTB07001.
Competing interests: None.
Patient consent: Obtained.