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Open Access Research article

Molecular and biochemical characterisation of a novel mutation in POLG associated with Alpers syndrome

André Schaller1*, Dagmar Hahn2, Christopher B Jackson1, Ilse Kern3, Christophe Chardot45, Dominique C Belli3, Sabina Gallati1 and Jean-Marc Nuoffer2

Author Affiliations

1 Division of Human Genetics, University Hospital Bern, Bern, Switzerland

2 Institute of Clinical Chemistry, University Hospital Bern, Bern, Switzerland

3 Department of Paediatrics, University of Geneva Children's Hospital, Geneva, Switzerland

4 Paediatric Surgery Unit, University of Geneva Children's Hospital, Geneva, Switzerland

5 Paediatric Surgery Unit, Hôpital Necker-Enfants malades, Paris, France

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BMC Neurology 2011, 11:4  doi:10.1186/1471-2377-11-4

Published: 14 January 2011

Abstract

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.