A Clinical, Neuropathological and Genetic Study of Homozygous A467T POLG-Related Mitochondrial Disease

Abstract

Mutations in the nuclear gene POLG (encoding the catalytic subunit of DNA polymerase gamma) are an important cause of mitochondrial disease. The most common POLG mutation, A467T, appears to exhibit considerable phenotypic heterogeneity. The mechanism by which this single genetic defect results in such clinical diversity remains unclear. In this study we evaluate the clinical, neuropathological and mitochondrial genetic features of four unrelated patients with homozygous A467T mutations. One patient presented with the severe and lethal Alpers-Huttenlocher syndrome, which was confirmed on neuropathology, and was found to have a depletion of mitochondrial DNA (mtDNA). Of the remaining three patients, one presented with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS), one with a phenotype in the Myoclonic Epilepsy, Myopathy and Sensory Ataxia (MEMSA) spectrum and one with Sensory Ataxic Neuropathy, Dysarthria and Ophthalmoplegia (SANDO). All three had secondary accumulation of multiple mtDNA deletions. Complete sequence analysis of muscle mtDNA using the MitoChip resequencing chip in all four cases demonstrated significant variation in mtDNA, including a pathogenic MT-ND5 mutation in one patient. These data highlight the variable and overlapping clinical and neuropathological phenotypes and downstream molecular defects caused by the A467T mutation, which may result from factors such as the mtDNA genetic background, nuclear genetic modifiers and environmental stressors.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Liver and brain pathology for Patient 1.

Post-mortem liver samples from patient 1 (a and b) showed perivenular foci of enlarged hepatocytes with fine vacuolation (arrows). On lipid staining with oil red O (b) of frozen sections there was diffuse lipid deposition. Sample of the cerebral cortex from the occipital lobe showed full thickness neuronal loss with vacuolation and astrocytosis (c and d). Samples of the hippocampi (e and f) showed segmental neuronal loss, most marked from CA1 (arrow) and gliosis in a similar pattern (f-GFAP). Scale bars, a and b = 100 μm; c and d = 200 μm; e and f = 2 mm.

Fig 2. Brain and muscle pathology for patient 2.

A brain biopsy from patient 2 showed a little neuropil vacuolation (a) and cortical gliosis (b) but no specific diagnostic features. A muscle biopsy showed scattered cytochrome oxidase (COX)-negative fibres (f—arrows) but no other myopathic features (c) and no ragged red (d) or blue (e) fibres. Scale bars = 100 μm.

Fig 3. Muscle pathology for patient 3.

Haematoxylin-Eosin stained section (A) showed mild variation in fibre size and several fibres with peripheral accumulation of mitochondria (arrowhead). Gomori trichrome preparation (B) accentuated ragged red fibres (arrowhead) and that for Succinic dehydrogenase (C) showed many ragged blue fibres (arrowheads). COX histochemical preparation (D) revealed frequent COX-deficient fibres (arrowheads) in keeping with mitochondrial myopathy. Scale bar = 50 μm.

Fig 4. Muscle pathology for patient 4.

Succinic dehydrogenase revealed frequent ragged blue fibres (A—red arrowheads) and there were numerous COX-deficient fibres (B—red arrowheads) in keeping with mitochondrial myopathy. Scale bar = 50 μm.

Fig 5. Long range PCR and Southern Blot analysis.

A. Long PCR of muscle mt DNA from the four patients. 1, 1kb ladder; 2, Muscle negative control; 3, Patient 2; 4, Patient 3; 5, water control; 6, Patient 4; 7, patient 1. B. Depletion of mtDNA obtained from liver of patient 1. 1, control; 2, control; 3, patient 1.