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.


OBJECTIVE
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.


CONCLUSIONS
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.
Botta
A
A
Tor Vergata University of Rome, Department of Biopathology, Via Montpellier, 1, 00133 Rome, Italy. abottait@yahoo.it
Rinaldi
F
F
Catalli
C
C
Vergani
L
L
Bonifazi
E
E
Romeo
V
V
Loro
E
E
Viola
A
A
Angelini
C
C
Novelli
G
G
D000293:Adolescent; D000328:Adult; D005091:Exons; D015870:Gene Expression; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism*; D009223:Myotonic Dystrophy / Q000235:genetics*; D065636:Myotonin-Protein Kinase; D017346:Protein Serine-Threonine Kinases / Q000235:genetics* / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D012326:RNA Splicing; D020133:Reverse Transcriptase Polymerase Chain Reaction; D019680:Trinucleotide Repeat Expansion
C585517:DMPK protein, human; D012313:RNA; D065636:Myotonin-Protein Kinase; D017346:Protein Serine-Threonine Kinases