Umeå University's logo

Mitochondrial DNA (mtDNA) replication requires a steady supply of deoxyribonucleotides (dNTPs), synthesized de novo by ribonucleotide reductase (RNR). In nondividing cells, RNR consists of RRM1 and RRM2B subunits. Mutations in RRM2B cause mtDNA depletion syndrome, linked to muscle weakness, neurological decline, and early mortality. The impact of RRM2B deficiency on dNTP pools in nondividing tissues remains unclear. Using a mouse knockout model, we demonstrate that RRM2B deficiency selectively depletes dATP and dGTP, while dCTP and dTTP levels remain stable or increase. This depletion pattern resembles the effects of hydroxyurea, an inhibitor that reduces overall RNR activity. Mechanistically, we propose that the depletion of dATP and dGTP arises from their preferred degradation by the dNTPase SAMHD1 and the lower production rate of dATP by RNR. Identifying dATP and dGTP depletion as a hallmark of RRM2B deficiency provides insights for developing nucleoside bypass therapies to alleviate the effects of RRM2B mutations.

Faithful replication of mitochondrial DNA (mtDNA) depends on a continuous supply of deoxyribonucleoside triphosphates (dNTPs) generated by ribonucleotide reductase (RNR). In post-mitotic tissues, RNR activity relies on the small subunit RRM2B. Loss of RRM2B impairs dNTP synthesis and causes mitochondrial DNA depletion syndrome (MDDS), a multisystem disorder characterized by progressive muscular and neurological decline.We have previously demonstrated that RRM2B deficiency disproportionately reduces dATP and dGTP, likely due to their enhanced breakdown by the dNTPase SAMHD1. Here, we asked whether deleting SAMHD1 could correct this imbalance and alleviate the associated pathology.To test this hypothesis, we generated mice lacking both RRM2B and SAMHD1. Genetic inactivation of SAMHD1 increased purine dNTP levels in several tissues, albeit incompletely and in a tissue-dependent manner, partially restored mtDNA copy number, improved organ morphology, and extended survival by ~25%.These findings show that loss of SAMHD1 partly reverses the biochemical and physiological defects caused by RRM2B deficiency, revealing that dNTP degradation can be targeted to rebalance mitochondrial nucleotide homeostasis in specific forms of MDDS.