Polymorphic CUG repeats in human mRNAs and their effects on gene expression

Area of Science:

• Molecular Biology
• Genetics
• Bioinformatics

Background:

• Expanded CUG repeats in the 3'-untranslated region (UTR) of the myotonic dystrophy protein kinase (DMPK) gene cause myotonic dystrophy type 1 (DM1).
• Previous studies suggest repeat expansions impede gene expression at multiple levels.
• The role and distribution of polymorphic CUG repeats in human mRNA remain largely uncharacterized.

Purpose of the Study:

• To conduct a comprehensive bioinformatic survey of human mRNA sequences for polymorphic CUG repeats.
• To investigate the functional impact of extended CUG repeats on protein synthesis and gene expression.
• To explore the mechanisms underlying CUG repeat-mediated gene expression inhibition.

Main Methods:

• Bioinformatic analysis of all human mRNA sequences to identify CUG repeats.
• Cell-free translation assays using rabbit reticulocyte lysate and wheat germ extracts.
• Gene expression studies in transfected cells, including PKR-minus cells.
• Northwestern blotting to detect CUG-binding proteins.

Main Results:

• CUG repeats are widely distributed across human mRNA regions, with a higher frequency in coding regions than UTRs.
• Approximately 30 genes contain polymorphic CUG repeats with potential for expansion/shrinkage.
• Long polymorphic repeats are predominantly found in the 3'-UTR of DMPK and the coding region of ribosomal protein L14.
• Extended CUG repeats inhibit cell-free protein synthesis and gene expression in transfected cells, independent of PKR.
• More CUG-binding proteins exist in human cells than previously identified.

Conclusions:

• Polymorphic CUG repeats are common in human mRNAs, with specific genes like DMPK and L14 harboring longer, potentially disease-relevant repeats.
• Extended CUG repeats can directly inhibit protein synthesis and gene expression through mechanisms not solely dependent on PKR.
• The findings highlight the widespread regulatory potential of CUG repeats and suggest novel protein interactions involved in CUG repeat-mediated effects.