Aktuelle Neurologie 2007; 34 - P603
DOI: 10.1055/s-2007-987874

Phenotypic variations in patients with linker-region mutations in the mitochondrial DNA polymerase gamma gene

C Kornblum 1, M Baron 1, M Bös 1, G Zsurka 1, WS Kunz 1
  • 1Bonn

Background: We studied three patients with evidence of mitochondrial disease for mutations in the nuclear mitochondrial DNA (mtDNA) polymerase gamma gene (POLG1). Two patients (pat.1, 2) presented with Alpers syndrome (refractory epilepsy, developmental delay, liver failure), while the third patient (pat. 3) had chronic progressive external ophthalmoplegia (CPEOplus) with first symptoms in early adulthood, but no liver involvement.

Methods: Skeletal muscle biopsy with histological, biochemical, and moleculargenetic mtDNA examinations. Postmortem muscle and liver tissue investigations with histology and quantitative mtDNA analysis (real-time PCR) in pat.1 and normal controls. Laser microdissection of cytochrome c-oxidase (COX)-negative and -positive areas of liver tissue of pat.1 followed by quantitative mtDNA analysis. Screening for the A467T and W748S POLG1 mutations by PCR-RFLP, followed by partial sequencing of POLG1.

Results: We identified the following linker-region POLG1 mutations: A467T/A467T in pat.1, A467T/F749S in pat.2, and A467T/R627W in pat.3.

Muscle biopsies of the Alpers patients had normal histological features. In contrast, muscle biopsy of the CPEOplus patient showed severe mitochondrial abnormalities (abundant ragged red fibers and COX-negative fibers). Long-range PCR of muscle mtDNA of the Alpers patients revealed multiple deletions at low levels (<5%). In contrast, the CPEOplus patient harbored multiple deletions with a high amount of deleted mtDNA in skeletal muscle, while total mtDNA copy numbers were unaffected. The dominant feature in the Alpers patients was a severe depletion of mtDNA, which was in skeletal muscle 28–50% of controls decreasing to 10% of controls in liver tissue. In COX-negative areas of the liver sample of pat.1, the mtDNA copy number decreased to 5% of its control value.

Conclusion: We propose that the dramatic liver involvement in Alpers syndrome is at least partly responsible for the poor clinical outcome. It is caused by massive mtDNA depletion very likely related to a lowered catalytic efficiency of the mutated enzyme. In contrast, mtDNA copy numbers were unaffected in our CPEOplus patient presenting with abundant multiple mtDNA deletions. This suggests milder effects of the mutations on polymerase activity. Compared to the genetic homogeneity in POLG1, our patients displayed highly heterogeneous clinical and histological phenotypes that correlated with distinct secondary effects on mtDNA integrity and distribution.