Environmental Factor, May 2007, National Institute of Environmental Health Sciences
Biochemist Speaks on DNA Precursor Metabolism
By Eddy Ball
Christopher Mathews, Ph.D., gave the most recent talk in the Laboratory of Molecular Genetics (LMG) Fellows Invited Guest Lecture Series on April 2. Mathews is a Distinguished Professor Emeritus of the Department of Biochemistry and Biophysics and a member of the Environmental Mutagenesis and Carcinogenesis Research Core at Oregon State University. His lecture was titled "Maintaining Precursor Pools for Mitochondrial DNA Replication."
Mathews' talk focused on the imbalances in intracellular concentrations of the four deoxyribonucleoside triphosphates (dNTPs) that serve as the building blocks of DNA and their involvement in mutagenesis and, ultimately, cancer. His research suggests the intriguing possibility that alterations in the sizes of or ratios among these four precursor pools may be one of the earliest biochemical changes leading to mutations responsible for some mitochondria-linked diseases and cancer - that "pool size changes could be related to actual mutational pathways occurring in vivo," he explained.
The term "pool" refers to the amount of each dNTP contained in a cell. Mathews has conducted laboratory experiments to determine the size and ratios of the four mitochondrial (mt) dNTP pools under normal conditions and conditions of induced imbalance. His lab has also explored the metabolic and genetic control and properties of the enzymes that generate dNTPs, ribonucleoside diphosphate (NDP) reductase and ribonucleotide reductase (RNR).
Mathews has found that normal cells have naturally unbalanced pool sizes and that deviation from this delicate balance can induce replication errors, stimulating genomic instability and mutagenesis. Specifically, Mathews has discovered that mitochondrial dNTP pools can vary dramatically from cytosolic pools. Because mtDNA replication is a continuous process in the cell, replication errors, including point mutations and deletions, can have a cumulative effect.
The high spontaneous mutation rate of mtDNA thus may help account for the large number of genetic differences between a tumor cell and the normal cell where it originated. A better understanding of nucleic acid enzymology could lead to identification of novel targets for antiviral and anticancer drugs.
In the course of researching dNTP pools, Mathews' lab has developed methodologies for studying specific processes of DNA biosynthesis that were previously not well understood and has challenged some of the common wisdom about them. He has explored the hypothesis that damaged nucleotides may be pre-mutagenic lesions in part because of their oxidation by free radical species prior to incorporation into DNA.
The lab developed an HPLC assay to measure pool sizes of one of the oxidized nucleotides, 8-oxo-dGTP, in cell extracts. Although he found dGTP to be the least abundant dNTP in mammalian or bacterial cells, his data called into question the widespread assumption that dGTP oxidation is a significant contributor to mutagenesis induced by reactive oxygen species.
Several times during his lecture, Mathews acknowledged that many questions still need to be answered about the role of precursor pools in the cancer cascade. "We obviously have a lot of work ahead of us," he said at one point about the technical difficulties he has encountered. In addition, there is the fundamental question that Mathews and other researchers continue to face: is the mutagenic accumulation of dNTPs, as it seems, an important step in the process of carcinogenesis - or could it be an accidental by-product?
LMG Fellow Stephanie Nick- McElhinny was host for the lecture.