The CTG repeat expansion size correlates with the splicing defects observed in muscles from myotonic dystrophy type 1 patients.

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.

Analysis of Single Nucleotide Polymorphisms (SNPs) of the small-conductance calcium activated potassium channel (SK3) gene as genetic modifier of the cardiac phenotype in myotonic dystrophy type 1 patients.

Myotonic dystrophy type 1 (DM1) is the most frequently inherited neuromuscular disease in adults. It is a multisystemic disorder with major cardiac involvement most commonly represented by first-degree atrioventricular heart block (AVB), followed by different degrees of bundle-branch and intraventricular blocks In search for candidate genes, modifiers of the AVB phenotype in DM1, the expression of the small-conductance calcium activated potassium channel (SK3) gene was analysed in muscle biopsies from DM1 patients. The association between SK3 polymorphisms and the AVB phenotype was then studied analyzing 40 DM1 patients with AVB and 40 age-matched DM1 affected individuals with no ECG abnormalities. [CTG]n repeat length and cardiac clinical picture were also assessed for correlation. QRT-PCR experiments showed an over-expression of the SK3 transcript in DM1 muscle biopsies compared to healthy controls. However, no statistical association between the AVB phenotype and either the [CTG]n expansion length or the presence of specific SNPs in the SK3 gene were detected. These findings suggest that modifier genes, other than SK3, should be identified in order to explain the cardiac phenotypic variability among DM1 patients.