Mutation of ribonucleotide reductase small subunit p53R2 affects deoxynucleotide metabolism in quiescent but not in cycling human fibroblasts

Ribonucleotide reductase catalyzes the key step in the de novo synthesis of DNA precursors (dNTPs). For activity the enzyme requires the large subunit R1 and a small subunit, either R2 or p53R2. R2 is induced in S phase and degraded during mitosis, p53R2 is stable and constitutively expressed at low level in cycling and quiescent cells. p53R2 is upregulated by p53 after DNA damage. Genetic inactivation of p53R2 causes severe mitochondrial DNA depletion in humans and ko mice. The role of p53R2 in dNTP metabolism is debated and so far has been studied in transformed cells that cannot be made quiescent. In fibroblasts from a patient with a lethal homozygous mutation in p53R2 we compared dNTP metabolism during proliferation and quiescence, obtained by prolonged serum starvation. When cycling, mutant fibroblasts showed normal growth, ribonucleotide reduction, dNTP pool sizes and mitochondrial DNA levels. During quiescence they strongly decreased ribonucleotide reduction, dCTP and dGTP content and virtually abolished dCTP catabolism. Mitochondrial DNA was unaffected. We conclude that loss of p53R2 affects ribonucleotide reduction only in resting cells where downregulation of dNTP catabolism counterbalances the loss of anabolic activity. Supported by Italian Telethon (GGP09019), Italian Association for Cancer Research and the University of Padova .