京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20195号 / 理博第4280号 / 新制||理||1615(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 杉山 弘, 教授 三木 邦夫, 教授 秋山 芳展 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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DNA methylation plays a pivotal role in governing cellular processes including genomic imprinting, gene expression, and development. Recently, the Tet family of methylcytosine dioxygenases(Tet1, Tet2 and Tet3) was found to catalyze the oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), an intermediate in the pathway of DNA demethylation. Tet enzymes catalyze this hydroxylation in a 2-oxoglutarate and Fe2+ dependent manner. We have recently reported significant levels of 5mC and 5hmC modification in immunoprecipitates of mammalian mitochondrial DNA(mtDNA). We provide the first evidence that a DNA Methyltransferase-1 isoform (mtDNMT1) translocates to the mitochondria using an N-terminal mitochondrial targeting sequence. mtDNMT1 expression is upregulated by NRF1 and PGC1α, master regulators of mitochondrial biogenesis and function, as well as by loss of p53. Altered mtDNMT1 expression asymmetrically affects mtDNA transcription. We are now pursuing the role of Tet proteins in generating 5hmC in mtDNA. Using an in vitro enzyme assay, we have successfully detected Tet activity in crude and percoll purified mitochondrial fractions of HCT116 cells. Mitoprot analysis on Tet family predicts that Tet1 may be translocated to the mitochondria. Immunoblot analysis indicates that a band of expected size(235kDa) is present on immunoblots of mitochondrial fraction from mouse embryonic stem cells with an antibody directed against Tet1. This band, however, is not protected from trypsin treatment of mitochondria indicating that Tet1 may not be transported to the mitochondrial matrix. The putative Tet1 mitochondrial targeting sequence (MTS) fails to carry heterologous protein to the mitochondria. Knock out of Tet1 in mouse ES cells also does not alter 5hmC signal in hydroxyMeDIP assay. We now seek to determine if Tet2/Tet3 may be involved in 5hmC generation. In the nucleus, 5hmC serves as an intermediate in the process of DNA demethylation through the combined action of cytidine deaminases and the base excision repair pathway. We plan to investigate if 5hmC holds the same functional significance in the mitochondria as it does in the nucleus. Our overall goal is to understand epigenetic regulation of normal mitochondrial function and changes that occur in diseases involving mitochondrial dysfunction such as ischemic heart disease, neurodegenerative diseases like Parkinsons disease, and cancer.

Somatic mutations are the main triggers that initiate the formation of cancer. Large sequencing data sets in recent years revealed a substantial number of mutational processes, many of which are poorly understood or of completely unknown aetiology. These mutational processes leave characteristic sequence patterns, often called "signatures", in the DNA. Characterisation of the mutational patterns observed in cancer patients with respect to different genomic features and processes can help to unravel the aetiology and mechanisms of mutagenesis. Here, we explored the effects of DNA modifications and DNA replication on mutagenesis. The most common mutation type, C>T mutations in a CpG context, is thought to result from spontaneous deamination of 5-methylcytosine (5mC), the major DNA modification. Much less is known about the mutational properties of the second most frequent modification, 5-hydroxymethylcytosine (5hmC). Integrating multiple genomic data sets, we demonstrate a twofold lower mutagenicity of 5hmC compared to 5mC, present across multiple tissues. Subsequently, we show how DNA modifications may modulate various mutational processes. In addition to spontaneous deamination of 5mC, our analysis suggests a key role of replication in CpG > TpG mutagenesis in patients deficient in post-replicative proofreading or repair, and possibly also in other cancer patients. Together with an analysis of mutation patterns observed in cancers exposed to UV light, tobacco smoke, or editing by APOBEC enzymes, the results show that the role of DNA modifications goes beyond the well-known spontaneous deamination of 5mC. Finally, we explored which of the known mutational processes might be modulated by DNA replication. We developed a novel method to quantify the magnitude of strand asymmetry of different mutational signatures in individual patients followed by evaluation of these exposures in early and late replicating regions. More than 75 % of mutational signatures exhibited a significant replication strand asymmetry or correlation with replication timing. The analysis gives new insights into mechanisms of mutagenicity in multiple signatures, particularly the so far enigmatic signature 17, where we suggest an involvement of oxidative damage in its aetiology. In conclusion, our results suggest that DNA replication or replication-associated DNA repair interacts with most mutagenic processes.

Alzheimer’s disease (AD) is the most common form of dementia and the sixth leading cause of death in the United States, with no therapeutic option to slow or halt disease progression. Development of two characteristic pathologic lesions, amyloid beta plaques and neurofibrillary tangles, in the brain are associated with synaptic dysfunction and neuron loss leading to memory impairment and cognitive decline. Although mutations in genes involved in amyloid beta processing are linked to increased plaque formation in the inherited familial form of AD, the more common idiopathic form, termed sporadic AD, develops in the absence of gene mutations. In contrast, alterations in gene expression and transcription occur in plaque and tangle susceptible brain regions of sporadic AD subjects, even in the earliest stages of development of pathologic burden, and may give insight into the pathogenesis of AD. Epigenetic modifications to cytosine are known to alter transcriptional states and gene expression in embryonic development as well as in cancer studies. With the discovery of enzymatically oxidized derivatives of 5-methylcytosine (5-mC), the most common epigenetic cytosine modification, a probable demethylation pathway has been suggested to alter transcriptional states of DNA. The most abundant 5-mC derivative, 5-hydroxymethylcytosine (5-hmC), while expressed at low concentrations throughout the body, is expressed at high concentrations in brain cells. To determine the role cytosine modifications play in AD, this study was directed at the quantification of epigenetic modifications to cytosine in several stages of AD progression using global, genome-wide, and gene-specific studies. To determine global levels of each cytosine derivative in brain regions relevant to AD progression, a gas chromatography/mass spectrometry quantitative analysis was utilized to analyze cytosine, 5-mC, and 5-hmC in tissue specimens from multiple brain regions of AD subjects, including early and late stages of AD progression. To determine the genome-wide impact of 5-hmC on biologically relevant pathways in AD, a single-base resolution sequencing analysis was used to map hydroxymethylation throughout the hippocampus of late stage AD subjects. Finally, to determine gene-specific levels of cytosine, 5-mC, and 5-hmC, a quantitative polymerase chain reaction (qPCR) protocol was paired with specific restriction enzyme digestion to analyze target sequences within exons of genes related to sporadic AD. Results from these studies show epigenetic modifications to cytosine are altered on the global, genome-wide, and gene-specific levels in AD subjects compared to normal aging, particularly in early stages of AD progression, suggesting alterations to the epigenetic landscape may play a role in the dysregulation of transcription and the pathogenesis of AD.

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