Molecular and biochemical characterisation of a novel mutation in POLG associated with Alpers syndrome
André Schaller1, Dagmar Hahn, Christopher B Jackson
1Division of Human Genetics, University Hospital Bern, Bern, Switzerland. andre.schaller@insel.ch
Background:
DNA polymerase γ (POLG) is the only known mitochondrial DNA (mtDNA) polymerase. It mediates mtDNA replication and base excision repair. Mutations in the POLG gene lead to reduction of functional mtDNA (mtDNA depletion and/or deletions) and are therefore predicted to result in defective oxidative phosphorylation (OXPHOS). Many mutations map to the polymerase and exonuclease domains of the enzyme and produce a broad clinical spectrum. The most frequent mutation p.A467T is localised in the linker region between these domains. In compound heterozygote patients the p.A467T mutation has been described to be associated amongst others with fatal childhood encephalopathy. These patients have a poorer survival rate compared to homozygotes.
Methods:
mtDNA content in various tissues (fibroblasts, muscle and liver) was quantified using quantitative PCR (qPCR). OXPHOS activities in the same tissues were assessed using spectrophotometric methods and catalytic stain of BN-PAGE.
Results:
We characterise a novel splice site mutation in POLG found in trans with the p.A467T mutation in a 3.5 years old boy with valproic acid induced acute liver failure (Alpers-Huttenlocher syndrome). These mutations result in a tissue specific depletion of the mtDNA which correlates with the OXPHOS-activities.
Conclusions:
mtDNA depletion can be expressed in a high tissue-specific manner and confirms the need to analyse primary tissue. Furthermore, POLG analysis optimises clinical management in the early stages of disease and reinforces the need for its evaluation before starting valproic acid treatment.
Insights
Mutations in the DNA polymerase gamma (POLG) gene cause mitochondrial DNA (mtDNA) depletion, leading to tissue-specific oxidative phosphorylation defects. Early POLG analysis is crucial for managing Alpers-Huttenlocher syndrome and preventing valproic acid toxicity.
Area of Science:
- Genetics
- Molecular Biology
- Biochemistry
Background:
- DNA polymerase gamma (POLG) is essential for mitochondrial DNA (mtDNA) replication and repair.
- Mutations in POLG cause mtDNA depletion and oxidative phosphorylation (OXPHOS) dysfunction, leading to a spectrum of clinical disorders.
- The common p.A467T mutation, located in the linker region, is associated with severe outcomes, particularly in compound heterozygotes.
Observation:
- A novel splice site mutation in POLG was identified in a child with valproic acid-induced liver failure (Alpers-Huttenlocher syndrome).
- This mutation was found in *trans* with the common p.A467T mutation.
- The combined mutations resulted in tissue-specific mtDNA depletion that correlated with reduced OXPHOS activity.
Findings:
- Mitochondrial DNA depletion exhibits significant tissue specificity.
- Quantitative PCR and OXPHOS assays demonstrated a correlation between mtDNA content and enzyme activity.
- The study characterized the functional consequences of a novel POLG mutation in conjunction with a known one.
Implications:
- Analyzing primary tissue is essential for accurate diagnosis of POLG-related disorders due to tissue-specific mtDNA depletion.
- Early POLG gene analysis can optimize clinical management and guide treatment decisions.
- Evaluating POLG status before initiating valproic acid treatment is critical to prevent severe adverse events.
