Multi-level profiling unravels mitochondrial dysfunction in myotonic dystrophy type 2.

Area of Science:

• Neurology
• Mitochondrial Biology
• Genetics

Background:

• Myotonic dystrophy type 2 (DM2) is a multisystemic disease characterized by muscle weakness, atrophy, myotonia, and myalgia.
• The genetic cause is a CCTG tetranucleotide expansion in the CNBP gene, leading to RNA-dominant spliceopathy.
• Mitochondrial dysfunction is a therapeutic target in myotonic dystrophy type 1; its role in DM2 is unclear.

Purpose of the Study:

• To investigate mitochondrial structure and function in DM2 skeletal muscle.
• To explore potential therapeutic targets by understanding DM2 pathophysiology.
• To compare DM2 with myotonic dystrophy type 1 regarding mitochondrial involvement.

Main Methods:

• Cross-sectional study of DM2 skeletal muscle biopsy specimens from 48 patients.
• Proteomic, molecular (RNA sequencing, immunoblotting, mtDNA analysis), and morphological (light and electron microscopy) analyses.
• Assessment of respiratory chain enzymology, mitochondrial DNA deletions, and mitophagy.

Main Results:

• Proteomic and transcriptomic analyses showed downregulation of key mitochondrial proteins and RNA transcripts (respiratory chain complexes I, III, IV, translation factors).
• Light and electron microscopy revealed significant mitochondrial abnormalities, including COX-deficient fibers and ultrastructural changes.
• Impaired mitophagy was indicated by immunofluorescence studies, while immunoblotting and LR-PCR showed no significant differences; mtDNA copy number was reduced in DM2 patients.

Conclusions:

• This study reveals previously undescribed functional and structural mitochondrial abnormalities in DM2 skeletal muscle.
• Reduced mtDNA copy number and impaired mitophagy are key findings in DM2.
• The precise molecular link between the CCTG expansion and mitochondrial dysfunction requires further investigation.