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Even small variations in dNTP concentrations decrease DNA replication fidelity, and this observation prompted us to analyze genomic cancer data for mutations in enzymes involved in dNTP metabolism. We found that sterile alpha motif and histidine-aspartate domain-containing protein 1 (SAMHD1), a deoxyribonucleoside triphosphate triphosphohydrolase that decreases dNTP pools, is frequently mutated in colon cancers, that these mutations negatively affect SAMHD1 activity, and that severalSAMHD1mutations are found in tumors with defective mismatch repair. We show that minor changes in dNTP pools in combination with inactivated mismatch repair dramatically increase mutation rates. Determination of dNTP pools in mouse embryos revealed that inactivation of oneSAMHD1allele is sufficient to elevate dNTP pools. These observations suggest that heterozygous cancer-associatedSAMHD1mutations increase mutation rates in cancer cells.

The high accuracy of DNA replication is achieved through the nucleotide selectivity of DNA polymerases, polymerase proofreading, and the mismatch repair (MMR) system that act in series. While defects in proofreading and MMR are strongly associated with the development of cancers, decreased nucleotide selectivity due to mutations in replicative DNA polymerases is an uncommon driver of cancer development. Because nucleotide selectivity can also be decreased by imbalanced dNTP pools, we investigated to what extent imbalanced dNTP pools can induce cancers. To this end we developed a mouse model with a mutation in the allosteric specificity site of ribonucleotide reductase, which is responsible for the balanced production of dNTPs. These mice had ~2-fold increased dCTP and dTTP levels and normal dATP and dGTP levels. Despite this mild dNTP pool imbalance, mutant mice had a higher incidence and an earlier onset of cancers, and these were different from the cancers that developed in wild-type controls. Because dNTP pool imbalances can be caused by defects in a plethora of genes, we propose that decreased nucleotide selectivity might be a major factor contributing to the development of spontaneous cancers.

The building blocks of DNA, dNTPs, can be produced de novo or can be salvaged from deoxyribonucleosides. However, to what extent the absence of de novo dNTP production can be compensated for by the salvage pathway is unknown. Here, we eliminated de novo dNTP synthesis in the mouse heart and skeletal muscle by inactivating ribonucleotide reductase (RNR), a key enzyme for the de novo production of dNTPs, at embryonic day 13. All other tissues had normal de novo dNTP synthesis and theoretically could supply heart and skeletal muscle with deoxyribonucleosides needed for dNTP production by salvage. We observed that the dNTP and NTP pools in wild-type postnatal hearts are unexpectedly asymmetric, with unusually high dGTP and GTP levels compared with those in whole mouse embryos or murine cell cultures. We found that RNR inactivation in heart led to strongly decreased dGTP and increased dCTP, dTTP, and dATP pools; aberrant DNA replication; defective expression of muscle-specific proteins; progressive heart abnormalities; disturbance of the cardiac conduction system; and lethality between the second and fourth weeks after birth. We conclude that dNTP salvage cannot substitute for de novo dNTP synthesis in the heart and that cardiomyocytes and myocytes initiate DNA replication despite an inadequate dNTP supply. We discuss the possible reasons for the observed asymmetry in dNTP and NTP pools in wildtype hearts.