Mitochondrial DNA Repair Processes In Oxidative Stress And Aging

碱基切除修复途径是由一类称为DNA糖基酶的酶启动的，它识别并释放受损的碱基，从而赋予DNA修复过程特异性。哺乳动物细胞携带两种主要的DNA糖基酶，用于修复氧化碱基，氧鸟嘌呤DNA糖基酶(OGG1)和核酸内切酶III同源酶(NTH1)。我们发现OGG1在线粒体氧化损伤的修复中起着至关重要的作用，并且可能是这些细胞器中唯一能清除8-oxoG的DNA糖基酶。所有的BER酶都在细胞核中编码，并运输到线粒体；然而，关于线粒体BER的调控信息非常有限。在哺乳动物的线粒体中，线粒体DNA存在于一个被称为类核的大型蛋白质-DNA复合体中。哺乳动物类核中最丰富的蛋白质组分之一是转录因子TFAM，它被认为具有将mtDNA压缩成类核结构的结构功能。以前，我们发现TFAM可以抑制BER蛋白和线粒体Pol-γ。我们认为TFAM的功能可能类似于核组蛋白，因此我们认为TFAM重塑蛋白必须存在于线粒体中，以支持mtDNA的代谢。在单独的研究中，我们发现RECQL4和CSB存在于线粒体中，因此我们评估了每种蛋白质是否可以解除TFAM的抑制。我们观察到CSB，而不是RECQL4，可以显示TFAM并减轻其抑制作用。我们正在继续寻找和询问与TFAM的蛋白质相互作用，试图更全面地描述mtDNA修复和代谢。 我们创建了一个数据库，专门为线粒体参与的疾病评分。根据CS和其他DNA修复缺陷疾病如毛细血管扩张性共济失调(AT)和A组着色性干皮病(XPA)的体征和症状，我们将这些疾病归类为可能含有线粒体成分的疾病。A组着色性干皮病(XPA)是一种典型的DNA修复缺陷疾病，患者对阳光敏感，对癌症易感性。XPA患者还表现出神经变性，导致小脑萎缩、神经病变和听力损失，其机制仍然难以捉摸。我们描述了一种线粒体应激反应现象，这种现象可能在经历慢性DNA损伤和PARP1过度激活的细胞中很常见。具体地说，我们发现XPA中有丝分裂的缺陷是由于PARP1的过度激活和NAD+(从而，SIRT1)的耗尽。这导致线粒体膜超极化、PINK1断裂和有丝分裂缺陷。这项研究强调了有丝分裂在促进线粒体池健康和防止神经退化和过早衰老方面的重要性。

The base excision repair pathway is initiated by the action of a class of enzymes known as DNA glycosylases, which recognize and release the damaged base, and thus give specificity to the DNA repair process. Mammalian cells carry two major DNA glycosylases for the repair of oxidized bases, oxoguanine DNA glycosylase (OGG1) and Endonuclease III homologue (NTH1). We found that OGG1 plays a crucial role in the repair of oxidized lesions in mitochondria and is probably the only DNA glycosylase for 8-oxoG removal in these organelles. All BER enzymes are encoded in the nucleus and transported to mitochondria; however there is very limited information on the regulation of mitochondrial BER. In mammalian mitochondria the mtDNA is found in a large protein-DNA complex known as the nucleoid. One of the most abundant protein components of mammalian nucleoids is the transcription factor TFAM, which has been postulated to have a structural function in compacting mtDNA into the nucleoid structure. Previously, we found that TFAM could inhibit BER proteins and mitochondrial pol gamma. We proposed that TFAM may be functioning like nuclear histones and therefore proposed that a TFAM remodeling protein must exit in mitochondria to allow for mtDNA metabolism. In separate studies, we documented that RECQL4 and CSB were present in mitochondria, thus we evaluated if each protein could relieve TFAM inhibition. We observed CSB, but not RECQL4, could display TFAM and alleviate its inhibition. We are continuing to search for and interrogate protein-interaction with TFAM in an attempt to more fully characterize mtDNA repair and metabolism We generated a database dedicated to scoring diseases for mitochondrial involvement. Based on the signs and symptoms seen in CS and other DNA repair deficient disorders like Ataxia Telangiectasia (AT) and Xeroderma Pigmentosum group A (XPA), we have classified these disorders as likely having a mitochondrial component. Xeroderma pigmentosum group A (XPA) is a classic DNA repair-deficient disorder with patients displaying sun sensitivity and cancer susceptibility. XPA patients also exhibit neurodegeneration, leading to cerebellar atrophy, neuropathy, and hearing loss, through a mechanism that has remained elusive. We describe a mitochondrial stress response phenomenon which may be common to cells undergoing chronic DNA damage and hyperactivation of PARP1. Specifically, we discovered defective mitophagy in XPA due to PARP1 hyperactivation and NAD+ (and thus, SIRT1) depletion. This leads to mitochondrial membrane hyper-polarization, PINK1 cleavage and defective mitophagy. This study underscores the importance of mitophagy in promoting a healthy pool of mitochondria and in preventing neurodegeneration and premature aging.