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Subnormal levels of POLgA cause inefficient initiation of light-strand DNA synthesis and lead to mitochondrial DNA deletions and progressive external ophthalmoplegia

Journal article
Authors Sara Roos
Bertil Macao
Javier Miralles Fusté
Christopher Lindberg
Elisabeth Jemt
Elisabeth Holme
Ali-Reza Moslemi
Anders Oldfors
Maria Falkenberg
Published in Human Molecular Genetics
Volume 22
Issue 12
Pages 2411-2422
ISSN 0964-6906
Publication year 2013
Published at Institute of Biomedicine, Department of Pathology
Institute of Biomedicine, Department of Clinical Chemistry and Transfusion Medicine
Institute of Biomedicine, Department of Medical Biochemistry and Cell Biology
Pages 2411-2422
Language en
Links dx.doi.org/10.1093/hmg/ddt094
Subject categories Medical Genetics

Abstract

The POLG1 gene encodes the catalytic subunit of mitochondrial DNA (mtDNA) polymerase γ (POLγ). We here describe a sibling pair with adult-onset progressive external ophthalmoplegia, cognitive impairment and mitochondrial myopathy characterized by DNA depletion and multiple mtDNA deletions. The phenotype is due to compound heterozygous POLG1 mutations, T914P and the intron mutation c.3104 + 3A > T. The mutant genes produce POLγ isoforms with heterozygous phenotypes that fail to synthesize longer DNA products in vitro. However, exon skipping in the c.3104 + 3A > T mutant is not complete, and the presence of low levels of wild-type POLγ explains patient survival. To better understand the underlying pathogenic mechanisms, we characterized the effects of POLγ depletion in vitro and found that leading-strand DNA synthesis is relatively undisturbed. In contrast, initiation of lagging-strand DNA synthesis is ineffective at lower POLγ concentrations that uncouples leading strand from lagging-strand DNA synthesis. In vivo, this effect leads to prolonged exposure of the heavy strand in its single-stranded conformation that in turn can cause the mtDNA deletions observed in our patients. Our findings, thus, suggest a molecular mechanism explaining how POLγ mutations can cause mtDNA deletions in vivo.

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