Regulation by degradation, a cellular defense against deoxyribonucleotide pool imbalances

Deoxyribonucleoside triphosphates (dNTPs) are the precursors used by DNA polymerases for replication and repair of nuclear and mitochondrial DNA in animal cells. Accurate DNA synthesis requires adequate amounts of each dNTP and appropriately balanced dNTP pools. Total cellular pool sizes are in the range of 10-100pmoles of each dNTP/million cells during S phase, with mitochondrial pools representing at most 10% of the total. In quiescent or differentiated cells pools are about 10-fold lower both in the cytosol and mitochondria. Contrary to what may be expected on the basis of the roughly equimolar abundance of the 4 nitrogen bases in DNA, the four dNTPs are present in the pools in different ratios, with pyrimidines often exceeding purines. Individual cell lines may exhibit different pool compositions even if they are derived from the same animal species. It has been known for several decades that imbalance of dNTP pools has mutagenic and cytotoxic effects, and leads to "mutator" phenotypes characterized by increased mutation frequencies. Until 10 years ago this phenomenon was considered to affect exclusively the nuclear genome. With the discovery that thymidine phosphorylase deficiency causes destabilization of mitochondrial DNA and a severe multisystemic syndrome the importance of dNTP pool balance was extended to mitochondria. Following that first discovery, mutations in other genes coding for mitochondrial or cytosolic enzymes of dNTP metabolism have been associated with mitochondrial DNA depletion syndromes. Both excess and deficiency of one dNTP may be detrimental. We study the mechanisms that in mammalian cells keep the dNTP pools in balance, and are particularly interested in the enzymes that, similar to thymidine phosphorylase, contribute to pool regulation by degrading dNTP precursors. The role of some relevant enzymes is illustrated with data obtained by chemical or genetic manipulation of their expression in cultured mammalian cells.