Novel heteroplasmic mtDNA mutation in a family with heterogeneous clinical presentations

Area of Science:

• Genetics
• Mitochondrial Biology
• Neurology

Background:

• Mitochondrial disorders present with diverse clinical features due to their critical role in cellular energy production.
• Maternal inheritance of mitochondrial DNA (mtDNA) mutations can lead to complex and variable phenotypes within families.

Observation:

• A family presented with varied symptoms, including spastic paraparesis, cardiomyopathy, ataxia, hearing loss, and diabetes mellitus, linked to a novel mtDNA mutation.
• Muscle biopsies showed normal morphology in the proband, but reduced activities of respiratory chain complexes I and IV were observed in affected individuals.
• A heteroplasmic G4284A mutation in the mitochondrial tRNA(Ile) gene was identified, with mutation loads varying among affected family members.

Findings:

• The G4284A mutation in mtDNA was heteroplasmic and maternally inherited, correlating with reduced complex I and IV activities in affected individuals.
• Cybrid clones with homoplasmic mutant mtDNA demonstrated significantly decreased complex IV activity and oxygen consumption rates.
• The mutation load in muscle mtDNA was higher in more severely affected individuals, suggesting a dose-dependent effect.

Implications:

• This study identifies a novel mtDNA mutation responsible for a spectrum of mitochondrial disease phenotypes.
• Understanding the genotype-phenotype correlation is crucial for diagnosing and managing mitochondrial disorders.
• The findings highlight the importance of analyzing mtDNA in cases of unexplained multisystem disorders, even with normal muscle biopsy morphology.