Altered mitochondrial function in fibroblasts containing MELAS or MERRF mitochondrial DNA mutations

Area of Science:

• Biochemistry
• Cell Biology
• Genetics

Background:

• Inherited mitochondrial DNA (mtDNA) mutations are linked to human diseases.
• MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) and MERRF (myoclonic epilepsy and ragged-red fibres) are common mtDNA diseases caused by tRNA gene point mutations.

Purpose of the Study:

• To investigate the impact of specific mtDNA tRNA mutations on mitochondrial function in patient-derived fibroblasts.
• To determine if these mutations affect mitochondrial quantity or bioenergetic capacity.

Main Methods:

• Primary fibroblast cultures from MELAS (3243 A>G tRNA(Leu(UUR))) and MERRF (8344 A>G tRNA(Lys)) patients were used.
• Mitochondrial membrane potential and respiration were measured in permeabilized and intact cells.
• Mitochondrial content was assessed using stereological analysis and marker enzyme assays.
• Cell volume and lysosomal/residual body content were quantified.

Main Results:

• Both MELAS and MERRF fibroblasts exhibited significantly decreased mitochondrial membrane potential and respiration rates.
• Mitochondrial content was comparable between control, MELAS, and MERRF fibroblasts.
• Cell volume was 3-4 fold larger in MELAS and MERRF fibroblasts compared to controls.
• Increased lysosomal and residual body volume suggested enhanced mitochondrial degradation.

Conclusions:

• mtDNA tRNA gene mutations lead to bioenergetically incompetent mitochondria, not changes in mitochondrial amount, in cultured fibroblasts.
• Increased cell size and evidence of mitochondrial degradation are associated with these mutations.
• Findings provide insights into the cellular pathology of MELAS and MERRF diseases.