Department of Pathology, Institute of Biomedicine, The Sahlgrenska Academy at the University of Gothenburg, Gula Stråket 8, Gothenburg SE-413 45, Sweden.
Hum Mol Genet. 2013 Jun 15;22(12):2411-22. doi: 10.1093/hmg/ddt094. Epub 2013 Feb 27.
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.
POLG1 基因编码线粒体 DNA（mtDNA）聚合酶 γ（POLγ）的催化亚基。我们在此描述了一对患有成年起病的进行性眼外肌麻痹、认知障碍和线粒体肌病的同胞，其特征为 DNA 耗竭和多种 mtDNA 缺失。该表型归因于复合杂合 POLG1 突变，T914P 和内含子突变 c.3104 + 3A > T。突变基因产生具有杂合表型的 POLγ 同工型，无法在体外合成更长的 DNA 产物。然而，c.3104 + 3A > T 突变的外显子跳跃并不完全，并且存在低水平的野生型 POLγ 解释了患者的存活。为了更好地理解潜在的发病机制，我们在体外表征了 POLγ 耗竭的影响，发现前导链 DNA 合成相对不受干扰。相比之下，在较低的 POLγ 浓度下，滞后链 DNA 合成的起始无效，从而使前导链与滞后链 DNA 合成解偶联。在体内，这种效应导致重链在其单链构象中暴露时间延长，进而导致我们患者中观察到的 mtDNA 缺失。因此，我们的研究结果提出了一种分子机制，解释了 POLγ 突变如何在体内导致 mtDNA 缺失。
