A genetic screen pinpoints ribonucleotide reductase residues that sustain dNTP homeostasis and specifies a highly mutagenic type of dNTP imbalance

Comprehensive multi-omics analysis reveals WEE1 as a synergistic lethal target with hyperthermia through CDK1 super-activation

Hyperthermic intraperitoneal chemotherapy's role in ovarian cancer remains controversial, hindered by limited understanding of hyperthermia-induced tumor cellular changes. This limits developing potent combinatory strategies anchored in hyperthermic intraperitoneal therapy (HIPET). Here, we perform a comprehensive multi-omics study on ovarian cancer cells under hyperthermia, unveiling a distinct molecular panorama, primarily characterized by rapid protein phosphorylation changes. Based on the phospho-signature, we pinpoint CDK1 kinase is hyperactivated during hyperthermia, influencing the global signaling landscape. We observe dynamic, reversible CDK1 activity, causing replication arrest and early mitotic entry post-hyperthermia. Subsequent drug screening shows WEE1 inhibition synergistically destroys cancer cells with hyperthermia. An in-house developed miniaturized device confirms hyperthermia and WEE1 inhibitor combination significantly reduces tumors in vivo. These findings offer additional insights into HIPET, detailing molecular mechanisms of hyperthermia and identifying precise drug combinations for targeted treatment. This research propels the concept of precise hyperthermic intraperitoneal therapy, highlighting its potential against ovarian cancer.

SAMHD1 restricts the deoxyguanosine triphosphate pool contributing to telomere stability in telomerase-positive cells

SAMHD1 (Sterile alpha motif and histidine/aspartic acid domain-containing protein 1) is a dNTP triphosphohydrolase crucial in the maintenance of balanced cellular dNTP pools, which support genome integrity. In SAMHD1 deficient fibroblasts isolated from Aicardi-Goutières Syndrome (AGS) patients, all four DNA precursors are increased and markedly imbalanced with the largest effect on dGTP, a key player in the modulation of telomerase processivity. Here, we present data showing that SAMHD1, by restricting the dGTP pool, contributes to telomere maintenance in hTERT-immortalized human fibroblasts from AGS patients as well as in telomerase positive cancer cell lines. Only in cells expressing telomerase, the lack of SAMHD1 causes excessive lengthening of telomeres and telomere fragility, whereas primary fibroblasts lacking both SAMHD1 and telomerase enter normally into senescence. Telomere lengthening observed in SAMHD1 deficient but telomerase proficient cells is a gradual process, in accordance with the intrinsic property of telomerase of adding only a few tens of nucleotides for each cycle. Therefore, only a prolonged exposure to high dGTP content causes telomere over-elongation. hTERT-immortalized AGS fibroblasts display also high fragility of chromosome ends, a marker of telomere replication stress. These results not only demonstrate the functional importance of dGTP cellular level but also reveal the critical role played by SAMHD1 in restraining telomerase processivity and safeguarding telomere stability.

Complex mutation profiles in mismatch repair and ribonucleotide reductase mutants reveal novel repair substrate specificity of MutS homolog (MSH) complexes

Determining mutation signatures is standard for understanding the etiology of human tumors and informing cancer treatment. Multiple determinants of DNA replication fidelity prevent mutagenesis that leads to carcinogenesis, including the regulation of free deoxyribonucleoside triphosphate pools by ribonucleotide reductase and repair of replication errors by the mismatch repair system. We identified genetic interactions between rnr1 alleles that skew and/or elevate deoxyribonucleoside triphosphate levels and mismatch repair gene deletions. These defects indicate that the rnr1 alleles lead to increased mutation loads that are normally acted upon by mismatch repair. We then utilized a targeted deep-sequencing approach to determine mutational profiles associated with mismatch repair pathway defects. By combining rnr1 and msh mutations to alter and/or increase deoxyribonucleoside triphosphate levels and alter the mutational load, we uncovered previously unreported specificities of Msh2-Msh3 and Msh2-Msh6. Msh2-Msh3 is uniquely able to direct the repair of G/C single-base deletions in GC runs, while Msh2-Msh6 specifically directs the repair of substitutions that occur at G/C dinucleotides. We also identified broader sequence contexts that influence variant profiles in different genetic backgrounds. Finally, we observed that the mutation profiles in double mutants were not necessarily an additive relationship of mutation profiles in single mutants. Our results have implications for interpreting mutation signatures from human tumors, particularly when mismatch repair is defective.

S Phase Duration Is Determined by Local Rate and Global Organization of Replication

Simple Summary

In order for a cell to divide into two cells, it must first copy its DNA. Although the time required for this process tends not to vary much, many examples of the importance of variability have been reported. In this review, we discuss the methods used to study this question, present some of the examples of variation, and attempt to explain the factors that determine the time required in simple terms. We will show that the overall time depends on the rate of DNA replication within a region, and on the temporal organization of the regions relative to each other.

Abstract

The duration of the cell cycle has been extensively studied and a wide degree of variability exists between cells, tissues and organisms. However, the duration of S phase has often been neglected, due to the false assumption that S phase duration is relatively constant. In this paper, we describe the methodologies to measure S phase duration, summarize the existing knowledge about its variability and discuss the key factors that control it. The local rate of replication (LRR), which is a combination of fork rate (FR) and inter-origin distance (IOD), has a limited influence on S phase duration, partially due to the compensation between FR and IOD. On the other hand, the organization of the replication program, specifically the amount of replication domains that fire simultaneously and the degree of overlap between the firing of distinct replication timing domains, is the main determinant of S phase duration. We use these principles to explain the variation in S phase length in different tissues and conditions.

Still no Rest for the Reductases: Ribonucleotide Reductase (RNR) Structure and Function: An Update

Herein we present a multidisciplinary discussion of ribonucleotide reductase (RNR), the essential enzyme uniquely responsible for conversion of ribonucleotides to deoxyribonucleotides. This chapter primarily presents an overview of this multifaceted and complex enzyme, covering RNR's role in enzymology, biochemistry, medicinal chemistry, and cell biology. It further focuses on RNR from mammals, whose interesting and often conflicting roles in health and disease are coming more into focus. We present pitfalls that we think have not always been dealt with by researchers in each area and further seek to unite some of the field-specific observations surrounding this enzyme. Our work is thus not intended to cover any one topic in extreme detail, but rather give what we consider to be the necessary broad grounding to understand this critical enzyme holistically. Although this is an approach we have advocated in many different areas of scientific research, there is arguably no other single enzyme that embodies the need for such broad study than RNR. Thus, we submit that RNR itself is a paradigm of interdisciplinary research that is of interest from the perspective of the generalist and the specialist alike. We hope that the discussions herein will thus be helpful to not only those wanting to tackle RNR-specific problems, but also those working on similar interdisciplinary projects centering around other enzymes.

A selection-based next generation sequencing approach to develop robust, genotype-specific mutation profiles in Saccharomyces cerevisiae

Distinct mutation signatures arise from environmental exposures and/or from defects in metabolic pathways that promote genome stability. The presence of a particular mutation signature can therefore predict the underlying mechanism of mutagenesis. These insults to the genome often alter dNTP pools, which itself impacts replication fidelity. Therefore, the impact of altered dNTP pools should be considered when making mechanistic predictions based on mutation signatures. We developed a targeted deep-sequencing approach on the CAN1 gene in Saccharomyces cerevisiae to define information-rich mutational profiles associated with distinct rnr1 backgrounds. Mutations in the activity and selectivity sites of rnr1 lead to elevated and/or unbalanced dNTP levels, which compromises replication fidelity and increases mutation rates. The mutation spectra of rnr1Y285F and rnr1Y285A alleles were characterized previously; our analysis was consistent with this prior work but the sequencing depth achieved in our study allowed a significantly more robust and nuanced computational analysis of the variants observed, generating profiles that integrated information about mutation spectra, position effects, and sequence context. This approach revealed previously unidentified, genotype-specific mutation profiles in the presence of even modest changes in dNTP pools. Furthermore, we identified broader sequence contexts and nucleotide motifs that influenced variant profiles in different rnr1 backgrounds, which allowed specific mechanistic predictions about the impact of altered dNTP pools on replication fidelity.

Identification of MLH2/hPMS1 dominant mutations that prevent DNA mismatch repair function

Inactivating mutations affecting key mismatch repair (MMR) components lead to microsatellite instability (MSI) and cancer. However, a number of patients with MSI-tumors do not present alterations in classical MMR genes. Here we discovered that specific missense mutations in the MutL homolog MLH2, which is dispensable for MMR, confer a dominant mutator phenotype in S. cerevisiae. MLH2 mutations elevated frameshift mutation rates, and caused accumulation of long-lasting nuclear MMR foci. Both aspects of this phenotype were suppressed by mutations predicted to prevent the binding of Mlh2 to DNA. Genetic analysis revealed that mlh2 dominant mutations interfere with both Exonuclease 1 (Exo1)-dependent and Exo1-independent MMR. Lastly, we demonstrate that a homolog mutation in human hPMS1 results in a dominant mutator phenotype. Our data support a model in which yeast Mlh1-Mlh2 or hMLH1-hPMS1 mutant complexes act as roadblocks on DNA preventing MMR, unraveling a novel mechanism that can account for MSI in human cancer.

High density of unrepaired genomic ribonucleotides leads to Topoisomerase 1-mediated severe growth defects in absence of ribonucleotide reductase

Cellular levels of ribonucleoside triphosphates (rNTPs) are much higher than those of deoxyribonucleoside triphosphates (dNTPs), thereby influencing the frequency of incorporation of ribonucleoside monophosphates (rNMPs) by DNA polymerases (Pol) into DNA. RNase H2-initiated ribonucleotide excision repair (RER) efficiently removes single rNMPs in genomic DNA. However, processing of rNMPs by Topoisomerase 1 (Top1) in absence of RER induces mutations and genome instability. Here, we greatly increased the abundance of genomic rNMPs in Saccharomyces cerevisiae by depleting Rnr1, the major subunit of ribonucleotide reductase, which converts ribonucleotides to deoxyribonucleotides. We found that in strains that are depleted of Rnr1, RER-deficient, and harbor an rNTP-permissive replicative Pol mutant, excessive accumulation of single genomic rNMPs severely compromised growth, but this was reversed in absence of Top1. Thus, under Rnr1 depletion, limited dNTP pools slow DNA synthesis by replicative Pols and provoke the incorporation of high levels of rNMPs in genomic DNA. If a threshold of single genomic rNMPs is exceeded in absence of RER and presence of limited dNTP pools, Top1-mediated genome instability leads to severe growth defects. Finally, we provide evidence showing that accumulation of RNA/DNA hybrids in absence of RNase H1 and RNase H2 leads to cell lethality under Rnr1 depletion.

Cell-cycle-dependent phosphorylation of RRM1 ensures efficient DNA replication and regulates cancer vulnerability to ATR inhibition

Ribonucleotide reductase (RNR) catalyzes the rate-limiting step of de novo synthesis of deoxyribonucleotide triphosphates (dNTPs) building blocks for DNA synthesis, and is a well-recognized target for cancer therapy. RNR is a heterotetramer consisting of two large RRM1 subunits and two small RRM2 subunits. RNR activity is greatly stimulated by transcriptional activation of RRM2 during S/G2 phase to ensure adequate dNTP supply for DNA replication. However, little is known about the cell-cycle-dependent regulation of RNR activity through RRM1. Here, we report that RRM1 is phosphorylated at Ser 559 by CDK2/cyclin A during S/G2 phase. And this S559 phosphorylation of RRM1enhances RNR enzymatic activity and is required for maintaining sufficient dNTPs during normal DNA replication. Defective RRM1 S559 phosphorylation causes DNA replication stress, double-strand break, and genomic instability. Moreover, combined targeting of RRM1 S559 phosphorylation and ATR triggers lethal replication stress and profound antitumor effects. Thus, this posttranslational phosphorylation of RRM1 provides an alternative mechanism to finely regulating RNR and therapeutic opportunities for cancer treatment.

Inactivation of folylpolyglutamate synthetase Met7 results in genome instability driven by an increased dUTP/dTTP ratio

The accumulation of mutations is frequently associated with alterations in gene function leading to the onset of diseases, including cancer. Aiming to find novel genes that contribute to the stability of the genome, we screened the Saccharomyces cerevisiae deletion collection for increased mutator phenotypes. Among the identified genes, we discovered MET7, which encodes folylpolyglutamate synthetase (FPGS), an enzyme that facilitates several folate-dependent reactions including the synthesis of purines, thymidylate (dTMP) and DNA methylation. Here, we found that Met7-deficient strains show elevated mutation rates, but also increased levels of endogenous DNA damage resulting in gross chromosomal rearrangements (GCRs). Quantification of deoxyribonucleotide (dNTP) pools in cell extracts from met7Δ mutant revealed reductions in dTTP and dGTP that cause a constitutively active DNA damage checkpoint. In addition, we found that the absence of Met7 leads to dUTP accumulation, at levels that allowed its detection in yeast extracts for the first time. Consequently, a high dUTP/dTTP ratio promotes uracil incorporation into DNA, followed by futile repair cycles that compromise both mitochondrial and nuclear DNA integrity. In summary, this work highlights the importance of folate polyglutamylation in the maintenance of nucleotide homeostasis and genome stability.

Convergent allostery in ribonucleotide reductase

Ribonucleotide reductases (RNRs) use a conserved radical-based mechanism to catalyze the conversion of ribonucleotides to deoxyribonucleotides. Within the RNR family, class Ib RNRs are notable for being largely restricted to bacteria, including many pathogens, and for lacking an evolutionarily mobile ATP-cone domain that allosterically controls overall activity. In this study, we report the emergence of a distinct and unexpected mechanism of activity regulation in the sole RNR of the model organism Bacillus subtilis. Using a hypothesis-driven structural approach that combines the strengths of small-angle X-ray scattering (SAXS), crystallography, and cryo-electron microscopy (cryo-EM), we describe the reversible interconversion of six unique structures, including a flexible active tetramer and two inhibited helical filaments. These structures reveal the conformational gymnastics necessary for RNR activity and the molecular basis for its control via an evolutionarily convergent form of allostery.

Ribonucleotide reductase (RNR) catalyzes the conversion of ribonucleotides to deoxyribonucleotides, which is an essential step in DNA synthesis. Here the authors use small-angle X-ray scattering, X-ray crystallography, and cryo-electron microscopy to capture active and inactive forms of the Bacillus subtilis RNR and provide mechanistic insights into a convergent form of allosteric regulation.