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Cytarabine (ara-C) and fludarabine (F-ara-A) are key drugs in leukaemia treatment. SAMHD1 is known to confer resistance to ara-C and F-ara-A, and we previously identified ribonucleotide reductase inhibitors as indirect SAMHD1 inhibitors in a phenotypic screen. The inosine monophosphate dehydrogenase (IMPDH) inhibitor mycophenolic acid (MPA) was also a hit in this screen. IMPDH inhibitors (IMPDHi) have previously shown efficacy against KMT2A-rearranged (KMT2Ar) acute myeloid leukaemia (AML). We investigated whether IMPDH inhibition could enhance the effect of ara-C and F-ara-A in AML cell lines and primary AML samples, and whether this effect was linked to KMT2A status. We found that sensitivity to IMPDHi was independent of KMT2A status. IMPDHi synergized with ara-C and F-ara-A in a SAMHD1-dependent manner in a subset of AML cells, but not in acute lymphoblastic leukaemia cell lines. Mechanistically, IMPDHi depleted allosteric SAMHD1 activators GTP and dGTP, thereby increasing active triphosphate metabolites in SAMHD1-proficient, but not SAMHD1-deficient, cells. Our findings suggest that the addition of IMPDHi to ara-C and F-ara-A may have therapeutic benefits in some AML cases.

Mutant phenotypes often vary across genetically distinct individuals. To identify the causes of such genetic background effects, we studied differences in gene essentiality across 18 genetically diverse natural yeast strains. We identified 39 genes that were essential in the laboratory reference strain but not in at least one other genetic background, and we mapped and validated the genetic variants that were responsible for the differences in gene essentiality. These variants typically occurred in single modifier genes that tended to differ between genetic backgrounds. The affected genes often indirectly compensated for the loss of the essential gene and identified naturally occurring evolutionary trajectories. Overall, our results highlight the prevalence of changes in gene essentiality in natural populations, as well as the underlying mechanisms. A thorough understanding of the causes of genetic background effects is crucial for the interpretation of genotype-to-phenotype relationships, including those associated with human disease.

In Saccharomyces cerevisiae, glucose depletion induces metabolic reprogramming through widespread transcriptional and translational reorganization. We report that initial, very rapid translational silencing is driven by a specialized metabolic mechanism. Following glucose withdrawal, intracellular NTP levels drop drastically over 30 s before stabilizing at a regulated, post-stress set point. Programmed translational control results from the differential NTP affinities of key enzymes; ATP falls below the (high) binding constants for DEAD-box helicase initiation factors, including eIF4A, driving mRNA release and blocking 80S assembly. Contrastingly, guanosine triphosphate (GTP) levels always greatly exceed the (low) binding constants for elongation factors, allowing ribosome run-off and orderly translation shutdown. Translation initiation is immediately lost on all pre-existing mRNAs before being preferentially re-established on newly synthesized, upregulated stress-response transcripts. We conclude that enzymatic constants are tuned for metabolic remodeling. This response counters energy depletion rather than being glucose specific, allowing hierarchical inhibition of energy-consuming processes on very rapid timescales.

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.

Alterations in deoxyribonucleoside triphosphate (dNTP) pools have been linked to increased mutation rates and genome instability in unicellular organisms and cell cultures. However, the role of dNTP pool changes in tumor development in mammals remains unclear. In this study, we present a mouse model with a point mutation at the allosteric specificity site of ribonucleotide reductase, RRM1-Y285A. This mutation reduced ribonucleotide reductase activity, impairing the synthesis of deoxyadenosine triphosphate (dATP) and deoxyguanosine triphosphate (dGTP). Heterozygous Rrm1^+/Y285A mice exhibited distinct alterations in dNTP pools across various organs, shorter lifespans and earlier tumor onset compared with wild-type controls. Mutational spectrum analysis of tumors revealed two distinct signatures, one resembling a signature extracted from a human cancer harboring a mutation of the same amino acid residue in ribonucleotide reductase, RRM1^Y285C. Our findings suggest that mutations in enzymes involved in dNTP metabolism can serve as drivers of cancer development.

The helicase MCM and the ribonucleotide reductase RNR are the complexes that provide the substrates (ssDNA templates and dNTPs, respectively) for DNA replication. Here, we demonstrate that MCM interacts physically with RNR and some of its regulators, including the kinase Dun1. These physical interactions encompass small subpopulations of MCM and RNR, are independent of the major subcellular locations of these two complexes, augment in response to DNA damage and, in the case of the Rnr2 and Rnr4 subunits of RNR, depend on Dun1. Partial disruption of the MCM/RNR interactions impairs the release of Rad52 -but not RPA-from the DNA repair centers despite the lesions are repaired, a phenotype that is associated with hypermutagenesis but not with alterations in the levels of dNTPs. These results suggest that a specifically regulated pool of MCM and RNR complexes plays non-canonical roles in genetic stability preventing persistent Rad52 centers and hypermutagenesis.

The generally accepted model assumes that leading strand synthesis is performed by Pol ε, while lagging-strand synthesis is catalyzed by Pol δ. Pol ε has been shown to target the leading strand by interacting with the CMG helicase [Cdc45 Mcm2–7 GINS(Psf1–3, Sld5)]. Proper functioning of the CMG-Pol ɛ, the helicase-polymerase complex is essential for its progression and the fidelity of DNA replication. Dpb2p, the essential non-catalytic subunit of Pol ε plays a key role in maintaining the correct architecture of the replisome by acting as a link between Pol ε and the CMG complex. Using a temperature-sensitive dpb2–100 mutant previously isolated in our laboratory, and a genetic system which takes advantage of a distinct mutational signature of the Pol δ-L612M variant which allows detection of the involvement of Pol δ in the replication of particular DNA strands we show that in yeast cells with an impaired Dpb2 subunit, the contribution of Pol δ to the replication of the leading strand is significantly increased.

Defects in deoxyribonucleoside triphosphate (dNTP) metabolism are associated with a number of mitochondrial DNA (mtDNA) depletion syndromes (MDS). These disorders affect the muscles, liver, and brain, and the concentrations of dNTPs in these tissues are already normally low and are, therefore, difficult to measure. Thus, information about the concentrations of dNTPs in tissues of healthy animals and animals with MDS are important for mechanistic studies of mtDNA replication, analysis of disease progression, and the development of therapeutic interventions. Here, we present a sensitive method for the simultaneous analysis of all four dNTPs as well as all four ribonucleoside triphosphates (NTPs) in mouse muscles using hydrophilic interaction liquid chromatography coupled with triple quadrupole mass spectrometry. The simultaneous detection of NTPs allows them to be used as internal standards for the normalization of dNTP concentrations. The method can be applied for measuring dNTP and NTP pools in other tissues and organisms.

Eukaryotic cells have evolved a replication stress response that helps to overcome stalled/collapsed replication forks and ensure proper DNA replication. The replication checkpoint protein Mrc1 plays important roles in these processes, although its functional interactions are not fully understood. Here, we show that MRC1 negatively interacts with CHL1, which encodes the helicase protein Chl1, suggesting distinct roles for these factors during the replication stress response. Indeed, whereas Mrc1 is known to facilitate the restart of stalled replication forks, we uncovered that Chl1 controls replication fork rate under replication stress conditions. Chl1 loss leads to increased RNR1 gene expression and dNTP levels at the onset of S phase likely without activating the DNA damage response. This in turn impairs the formation of RPA-coated ssDNA and subsequent checkpoint activation. Thus, the Chl1 helicase affects RPA-dependent checkpoint activation in response to replication fork arrest by ensuring proper intracellular dNTP levels, thereby controlling replication fork progression under replication stress conditions.