MITDNA MUTAGENESIS BY ENVIRONMENTAL CARCINOGENS

Johns Hopkins University, Baltimore MD

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Abstract

Human mitochondrial DNA (mitDNA) is extremely vulnerable to attack by environmental carcinogens because it contains no introns, has no protective histones and is continuously exposed to reactive free radicals produced within the mitochondria. In the past decade it has been clearly recognized that mutations in mitDNA lead to pathogenesis of a variety of mitochondrial diseases. It is estimated that of the 4 million children born each year in the United States, up to 4,000 develop mitochondrial disease. However, there is very little information available on the effects of environmental carcinogens, and the molecular mechanisms of mitDNA mutagenesis important in the pathology of mitochondrial diseases. Many environmental carcinogens deaminate bases in DNA. Of these bases, deamination of cytosine to uracil is of particular importance because human mitDNA is rich in cytosine and uracil mispairs with guanine in DNA, thus causing cytosine to thymine mutations during replication. Deamination, therefore, can be an important mechanism for generating mutations in mitDNA. In fact, cytosine to thymine mutations in certain mitochondrial genes cause mitochondrial myopathy, fatal infantile cardiomyopathy minus, progressive external ophthalmoplegia and other multisystem disorders. A key enzyme of the base excision repair pathway, uracil-DNA glycosylase (UDG) removes uracil from nuclear DNA and protects cells from cytosine to thymine mutations in the nucleus. Interestingly, UDG is present in mitochondria as well. However, UDG's exact biological function in mitochondria is unknown. Furthermore, nothing is known about the impact of deamination on mitDNA and on the induction of mitochondrial disease. Our previous work has established first, that human UDG physically associates, both in vivo and in vitro, with a multifunctional nuclear protein, replication protein A (RPA), and second, the aminoterminal sequence that targets UDG to mitochondria is required for its interaction with RPA. These data suggest that UDG's interaction with RPA may be important for repairing damaged mitDNA. Based upon our results, we hypothesize that mitDNA of human cells are susceptible to mutations by deaminating carcinogens, and that UDG protects mitDNA from mutation induced by these carcinogens. To test this hypothesis, we will: 1. Determine whether deaminating carcinogens cause mutations in mitDNA and subsequently lead to mitochondrial dysfunction in human cells. 2. Determine the ability of a major DNA repair protein, UDG, to protect human mitDNA from deamination-induced mutations. 3. Characterize the nature of UDG-RPA interaction in repairing damaged human mitDNA. The proposed study will fill a major gap in our understanding of the mechanisms of mitDNA mutagenesis induced by environmental carcinogens which has direct relevance in on the induction of mitochondrial diseases.