ATM-mediated serine 72 phosphorylation stabilizes ribonucleotide reductase small subunit p53R2 protein against MDM2 to DNA damage

Nucleotide metabolism: a pan-cancer metabolic dependency

Metabolic alterations are a key hallmark of cancer cells, and the augmented synthesis and use of nucleotide triphosphates is a critical and universal metabolic dependency of cancer cells across different cancer types and genetic backgrounds. Many of the aggressive behaviours of cancer cells, including uncontrolled proliferation, chemotherapy resistance, immune evasion and metastasis, rely heavily on augmented nucleotide metabolism. Furthermore, most of the known oncogenic drivers upregulate nucleotide biosynthetic capacity, suggesting that this phenotype is a prerequisite for cancer initiation and progression. Despite the wealth of data demonstrating the efficacy of nucleotide synthesis inhibitors in preclinical cancer models and the well-established clinical use of these drugs in certain cancer settings, the full potential of these agents remains unrealized. In this Review, we discuss recent studies that have generated mechanistic insights into the diverse biological roles of hyperactive cancer cell nucleotide metabolism. We explore opportunities for combination therapies that are highlighted by these recent advances and detail key questions that remain to be answered, with the goal of informing urgently warranted future studies.

Deoxyribonucleoside triphosphate pools in mammalian cells are expandable upon DNA damage

The rapid increase of dNTP pools in mammalian cells upon DNA damage has been previously documented. Alterations in protein modifications or interactions can rapidly modulate the activity and protein stability of mammalian RNR, and activation of PRPS1/2-dependent generation of PRPP enhances the production of the indispensable ribose sugar for nucleotide biosynthesis.

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.

DNA Damage Repair in Huntington's Disease and Other Neurodegenerative Diseases

Recent genome-wide association studies of Huntington's disease (HD) primarily highlighted genes involved in DNA damage repair mechanisms as modifiers of age at onset and disease severity, consistent with evidence that more DNA repair genes are being implicated in late age-onset neurodegenerative diseases. This provides an exciting opportunity to advance therapeutic development in HD, as these pathways have already been under intense investigation in cancer research. Also emerging are the roles of other polyglutamine disease proteins in DNA damage repair mechanisms. A potential universal trigger of oxidative DNA damage shared in these late age-onset diseases is the increase of reactive oxygen species (ROS) in human aging, defining an age-related mechanism that has defied other hypotheses of neurodegeneration. We discuss the potential commonality of DNA damage repair pathways in HD and other neurodegenerative diseases. Potential targets for therapy that may prove beneficial across many of these diseases are also identified, defining nodes in the ataxia telangiectasia-mutated (ATM) complex, mismatch repair, and poly ADP-ribose polymerases (PARPs).

Hypoxia Mediates Tumor Malignancy and Therapy Resistance

Hypoxia is a hallmark of the tumor microenvironment and contributes to tumor malignant phenotypes. Hypoxia-inducible factor (HIF) is a master regulator of intratumoral hypoxia and controls hypoxia-mediated pathological processes in tumors, including angiogenesis, metabolic reprogramming, epigenetic reprogramming, immune evasion, pH homeostasis, cell migration/invasion, stem cell pluripotency, and therapy resistance. In this book chapter, we reviewed the causes and types of intratumoral hypoxia, hypoxia detection methods, and the oncogenic role of HIF in tumorigenesis and chemo- and radio-therapy resistance.

Protective effects of ginsenoside Rg2 and astaxanthin mixture against UVB-induced DNA damage

Ultraviolet B (UVB) radiation induces skin damage, skin matrix degradation, and wrinkle formation through photochemical reaction and oxidative stress. Therefore, protecting the skin from UVB can prevent skin aging. In this study, we investigated the effects of a mixture (RA) of Rg2, a ginsenoside, and astaxanthin, an antioxidant, on the responses of HaCaT cells exposed to UVB (700 J/m²). The cells were incubated for 24 h after UVB exposure and cell viability was determined by MTT assay. UVB decreased cell viability by 60% compared to that of untreated control cells, whereas RA increased cell viability in a concentration-dependent manner, and this increase was significantly higher than that in the single treatment groups. Further, UVB increased the levels of DNA lesions such as cyclobutane pyrimidine dimer (CPD) and 8-hydroxyguanine (8-OHdG). Conversely, RA decreased both CPD and 8-OHdG levels in a concentration-dependent manner. UVB exposure also increased phosphorylation of ataxia-telangiectasia mutated (ATM) protein kinase and p53 and subsequently increased the levels of GADD45α, p21, and matrix metalloproteinases (MMPs)-3, -9, and -13. Additionally, UVB exposure decreased the level of COL1A1. However, RA treatment decreased the levels of p-ATM, p-p53, GADD45α, p21, MMP-3, -9, and -13 and increased the level of COL1A1 in a concentration-dependent manner. These results suggest that RA reduces UVB-induced cytotoxicity and genotoxicity through up-regulation of DNA repair via the combined effects of Rg2 and astaxanthin.

Challenges to DNA replication in hypoxic conditions

The term hypoxia refers to any condition where insufficient oxygen is available and therefore encompasses a range of actual oxygen concentrations. The regions of tumours adjacent to necrotic areas are at almost anoxic levels and are known to be extremely therapy resistant (radiobiological hypoxia). The biological response to radiobiological hypoxia includes the rapid accumulation of replication stress and subsequent DNA damage response, including both ATR- and ATM-mediated signalling, despite the absence of detectable DNA damage. The causes and consequences of hypoxia-induced replication stress will be discussed.

Clb6-Cdc28 Promotes Ribonucleotide Reductase Subcellular Redistribution during S Phase

A tightly controlled cellular deoxyribonucleotide (deoxynucleoside triphosphate [dNTP]) pool is critical for maintenance of genome integrity. One mode of dNTP pool regulation is through subcellular localization of ribonucleotide reductase (RNR), the enzyme that catalyzes the rate-limiting step of dNTP biosynthesis. In Saccharomyces cerevisiae, the RNR small subunit, Rnr2-Rnr4, is localized to the nucleus, whereas the large subunit, Rnr1, is cytoplasmic. As cells enter S phase or encounter DNA damage, Rnr2-Rnr4 relocalizes to the cytoplasm to form an active holoenzyme complex with Rnr1. Although the DNA damage-induced relocalization requires the checkpoint kinases Mec1-Rad53-Dun1, the S-phase-specific redistribution does not. Here, we report that the S-phase cyclin-cyclin-dependent kinase (CDK) complex Clb6-Cdc28 controls Rnr2-Rnr4 relocalization in S phase. Rnr2 contains a consensus CDK site and exhibits Clb6-dependent phosphorylation in S phase. Deletion of CLB6 or removal of the CDK site results in an increased association of Rnr2 with its nuclear anchor Wtm1, nuclear retention of Rnr2-Rnr4, and an enhanced sensitivity to the RNR inhibitor hydroxyurea. Thus, we propose that Rnr2-Rnr4 redistribution in S phase is triggered by Clb6-Cdc28-mediated phosphorylation of Rnr2, which disrupts the Rnr2-Wtm1 interaction and promotes the release of Rnr2-Rnr4 from the nucleus.

p53R2 as a novel prognostic biomarker in nasopharyngeal carcinoma

BACKGROUND

p53R2 is a target of p53 gene, which is essential for DNA repair, mitochondrial DNA synthesis, protection against oxidative stress, chromosomal instability, chronic inflammation and tumorigenesis. This study is aimed to investigate the expression of ribonucleotide reductase (RR) subunit p53R2 in nasopharyngeal carcinoma and its significance in the prognosis.

METHODS

The expression levels of p53R2 in 201 patients with NPC were examined by immunohistochemical assay. The correlations of p53R2 expression and clinicopathological features of nasopharyngeal carcinoma patient were analysed by chi-square test. The Kaplan-Meier survival analysis and Cox multivariate regression model were used to analyze the prognostic significance of the patients with NPC.

RESULTS

Immunohistochemical results showed that p53R2 was positively expressed in 92.5% (186/201) of nasopharyngeal carcinoma and the high expression rate was 38.3% (77/201). Further analysis observed that the negative correlation between expression of p53R2 and pT status had statistical significance (P < 0.05). Kaplan-Meier survival analysis found that the mean survival time of patients with high expression of p53R2 was 143.32 months, while the patients with low expression level of p53R2 was 121.63 months (P < 0.05). Cox regression analysis suggested that p53R2 protein expression could be used as an independent prognostic factor for nasopharyngeal carcinoma (P < 0.05).

CONCLUSIONS

This study drew a conclusion that p53R2 could be used as a prognostic biomarker indicative of the favorable outcome for patients with nasopharyngeal carcinoma.

4E-BP1, a multifactor regulated multifunctional protein

Eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) is a member of a family of translation repressor proteins, and a well-known substrate of mechanistic target of rapamycin (mTOR) signaling pathway. Phosphorylation of 4E-BP1 causes its release from eIF4E to allow cap-dependent translation to proceed. Recently, 4E-BP1 was shown to be phosphorylated by other kinases besides mTOR, and overexpression of 4E-BP1 was found in different human carcinomas. In this review, we summarize the novel findings on mTOR independent 4E-BP1 phosphorylation in carcinomas. The implications of overexpression and possible multi-function of 4E-BP1 are also discussed.

Multiple functions of p21 in cell cycle, apoptosis and transcriptional regulation after DNA damage

An appropriate control over cell cycle progression depends on many factors. Cyclin-dependent kinase (CDK) inhibitor p21 (also known as p21(WAF1/Cip1)) is one of these factors that promote cell cycle arrest in response to a variety of stimuli. The inhibitory effect of P21 on cell cycle progression correlates with its nuclear localization. P21 can be induced by both p53-dependent and p53-independent mechanisms. Some other important functions attributed to p21 include transcriptional regulation, modulation or inhibition of apoptosis. These functions are largely dependent on direct p21/protein interactions and also on p21 subcellular localizations. In addition, p21 can play a role in DNA repair by interacting with proliferating cell nuclear antigen (PCNA). In this review, we will focus on the multiple functions of p21 in cell cycle regulation, apoptosis and gene transcription after DNA damage and briefly discuss the pathways and factors that have critical roles in p21 expression and activity.

PYCR1 and PYCR2 Interact and Collaborate with RRM2B to Protect Cells from Overt Oxidative Stress

Ribonucleotide reductase small subunit B (RRM2B) is a stress response protein that protects normal human fibroblasts from oxidative stress. However, the underlying mechanism that governs this function is not entirely understood. To identify factors that interact with RRM2B and mediate anti-oxidation function, large-scale purification of human Flag-tagged RRM2B complexes was performed. Pyrroline-5-carboxylate reductase 1 and 2 (PYCR1, PYCR2) were identified by mass spectrometry analysis as components of RRM2B complexes. Silencing of both PYCR1 and PYCR2 by expressing short hairpin RNAs induced defects in cell proliferation, partial fragmentation of the mitochondrial network, and hypersensitivity to oxidative stress in hTERT-immortalized human foreskin fibroblasts (HFF-hTERT). Moderate overexpression of RRM2B, comparable to stress-induced level, protected cells from oxidative stress. Silencing of both PYCR1 and PYCR2 completely abolished anti-oxidation activity of RRM2B, demonstrating a functional collaboration of these metabolic enzymes in response to oxidative stress.

The MEK1/2 Inhibitor Pimasertib Enhances Gemcitabine Efficacy in Pancreatic Cancer Models by Altering Ribonucleotide Reductase Subunit-1 (RRM1)

PURPOSE

Gemcitabine, a nucleoside analogue, is an important treatment for locally advanced and metastatic pancreatic ductal adenocarcinoma (PDAC) but provides only modest survival benefit. Targeting downstream effectors of the RAS/ERK signaling pathway by direct inhibition of MEK1/2 proteins is a promising therapeutic strategy, as aberrant activation of this pathway occurs frequently in PDAC. In this study, the ability of pimasertib, a selective allosteric MEK1/2 inhibitor, to enhance gemcitabine efficacy was tested and the molecular mechanism of their interaction was investigated.

EXPERIMENTAL DESIGN

Cell survival and apoptosis were assessed by MTT and Caspase 3/7 Glo assays in human pancreatic cancer cell lines. Protein expression was detected by immunoblotting. The in vivo sensitivity of gemcitabine with pimasertib was evaluated in an orthotopic model of pancreatic tumor.

RESULTS

Synergistic activity was observed when gemcitabine was combined sequentially with pimasertib, in human pancreatic cancer cells. In particular, pimasertib reduced ribonucleotide reductase subunit 1 (RRM1) protein, and this was associated with sensitivity to gemcitabine. Pretreatment with MG132 impaired reduction of RRM1 protein induced by pimasertib, suggesting that RRM1 is degraded posttranslationally. Immunoprecipitation indicated enhanced MDM2-mediated polyubiquitination of RRM1 through Lys-48-mediated linkage following pimasertib treatment, an effect mediated, in part, by AKT. Finally, the combination treatment with pimasertib and gemcitabine caused significant tumor growth delays in an orthotopic pancreatic cancer model, with RRM1 downregulation in pimasertib-treated mice.

CONCLUSIONS

These results confirm an important role of RRM1 in gemcitabine response and indicate MEK as a potential target to sensitize gemcitabine therapy for PDAC. Clin Cancer Res; 21(24); 5563-77. ©2015 AACR.

Clofarabine Has Apoptotic Effect on T47D Breast Cancer Cell Line via P53R2 Gene Expression

PURPOSE

Clofarabine, a purine nucleoside analogue and inhibitor of Ribonucleotide Reductase (RR), is used for treatment of leukemia. Clofarabine-induced defect in DNA replication, induces p53 and subsequently P53R2 genes as subunit of RR. clofarabine deregulated P53R2 gene expression leading to the elevated levels of P53R2 which impose resistance to DNA damaging drugs. In this study the apoptotic and cytotoxic effects of clofarabine has been investigated on breast cancer cell line.

METHODS

Cofarabine cytotoxicity on T47D cells has been studied by MTT assay. T47D cells were exposed to the different concentrations of clofarabine for 24, 48 and 72 hours intervals. Relative expression of P53R2 gene has been studied using real-time PCR. Moreover, after treating with clofarabine the apoptotic and necrotic cells were detected using Annexin V and propodium iodide (PI) reagents by flowcytometry technique.

RESULTS

MTT assay results showed that the clofarabine IC50 on T47D cell line were 3 and 2.5µM after 48 and 72 h exposure, respectively. Clofarabine did not show any significant cytotoxic effect after 24 h exposure. The analysis of qRT-PCR showed a significant increase in P53R2 gene expression in treated cells with both 2.5 and 3 μM doses and also, the results of flowcytometry revealed 26.91 and 74.46 percent apoptosis induction in 48 and 72h treatments respectively in comparison to the control groups.

CONCLUSION

Our results showed that apoptotic and cytotoxic effects of clofarabine on T47D cell line were in time and dose dependent manner; therefore it could be considered a new candidate in breast cancer therapy.

Protein phosphorylation during Plasmodium berghei gametogenesis

Plasmodium gametogenesis within the mosquito midgut is a complex differentiation process involving signaling mediated by phosphorylation, which modulate metabolic routes and protein synthesis required to complete this development. However, the mechanisms leading to gametogenesis activation are poorly understood. We analyzed protein phosphorylation during Plasmodium berghei gametogenesis in vitro in serum-free medium using bidimensional electrophoresis (2-DE) combined with immunoblotting (IB) and antibodies specific to phosphorylated serine, threonine and tyrosine. Approximately 75 protein exhibited phosphorylation changes, of which 23 were identified by mass spectrometry. These included components of the cytoskeleton, heat shock proteins, and proteins involved in DNA synthesis and signaling pathways among others. Novel phosphorylation events support a role for these proteins during gametogenesis. The phosphorylation sites of six of the identified proteins, HSP70, WD40 repeat protein msi1, enolase, actin-1 and two isoforms of large subunit of ribonucleoside reductase were investigated using TiO2 phosphopeptides enrichment and tandem mass spectrometry. In addition, transient exposure to hydroxyurea, an inhibitor of ribonucleoside reductase, impaired male gametocytes exflagellation in a dose-dependent manner, and provides a resource for functional studies.

Tumor suppressor FOXO3 regulates ribonucleotide reductase subunit RRM2B and impacts on survival of cancer patients

The role of Ribonucleotide reductase (RR) subunits in different cancers has been intensively studied in our laboratory. RRM2B was identified as a p53-inducible RR subunit that involves in various critical cellular mechanisms such as cell cycle regulation, DNA repair and replication, and mitochondrial homeostasis, etc. However, little is known about the p53-independent regulation of RRM2B in cancer pathology. In this study, we discovered tumor suppressor FOXO3 as the novel regulator of RRM2B. FOXO3 directly bound to and transcriptionally activated the promoter of RRM2B, and induced the expression of RRM2B at RNA and protein levels. Moreover, Overexpression of RRM2B and/or FOXO3 inhibited the proliferation of cancer cells. The cancer tissue microarray data also demonstrated a strong correlation between the co-expression of FOXO3 plus RRM2B and increased disease survival and reduced recurrence or metastasis in lung cancer patients. Our results suggest a novel regulatory control of RRM2B function, and imply the importance of FOXO signaling pathway in DNA replication modulation. This study provides the first time evidence that RRM2B is transcriptionally and functionally regulated independent of p53 pathway by FOXO3, and it establishes that FOXO3 and RRM2B could be used as predictive biomarkers for cancer progression.

Checkpoint-dependent RNR induction promotes fork restart after replicative stress

The checkpoint kinase Rad53 is crucial to regulate DNA replication in the presence of replicative stress. Under conditions that interfere with the progression of replication forks, Rad53 prevents Exo1-dependent fork degradation. However, although EXO1 deletion avoids fork degradation in rad53 mutants, it does not suppress their sensitivity to the ribonucleotide reductase (RNR) inhibitor hydroxyurea (HU). In this case, the inability to restart stalled forks is likely to account for the lethality of rad53 mutant cells after replication blocks. Here we show that Rad53 regulates replication restart through the checkpoint-dependent transcriptional response, and more specifically, through RNR induction. Thus, in addition to preventing fork degradation, Rad53 prevents cell death in the presence of HU by regulating RNR-expression and localization. When RNR is induced in the absence of Exo1 and RNR negative regulators, cell viability of rad53 mutants treated with HU is increased and the ability of replication forks to restart after replicative stress is restored.

Deoxycytidine kinase augments ATM-Mediated DNA repair and contributes to radiation resistance

Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [(18)F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

Akt and p53R2, partners that dictate the progression and invasiveness of cancer

The serine/threonine kinase or the so-called "Akt" is a key regulatory molecule of signaling pathway that regulates various cellular processes. Many intracellular proteins are involved in the activation or inhibition of Akt signaling and the hyperactivation of Akt signaling pathway is found to be frequently involved in various types of human cancers. Furthermore, while p53R2, a p53-inducible peptide involved in the synthesis of dNTPs normally works toward suppression of cancer through elimination of reactive oxygen species (ROS), inhibition of MAPK/ERK pathway and providing dNTPs for DNA repair, the overexpression of p53R2 is reported to be associated with cancer progression and resistance to therapy. In this review article, we will discuss the situation in which cancer cells with hyperactive PI3K/Akt signaling can recruit p53R2 in favor of cancer progression and resistance to therapy. In the hyperactive state of PI3K/Akt signaling (which happens in the absence of deactivation or excess of activation), p53R2 can be used by cancer cells to promote proliferation. Therefore, the hyperactivity of PI3K/Akt pathway and elevated levels of p53R2 can give rise to highly invasive cancers.

Ribonucleotide reductase and cancer: biological mechanisms and targeted therapies

Accurate DNA replication and repair is essential for proper development, growth and tumor-free survival in all multicellular organisms. A key requirement for the maintenance of genomic integrity is the availability of adequate and balanced pools of deoxyribonucleoside triphosphates (dNTPs), the building blocks of DNA. Notably, dNTP pool alterations lead to genomic instability and have been linked to multiple human diseases, including mitochondrial disorders, susceptibility to viral infection and cancer. In this review, we discuss how a key regulator of dNTP biosynthesis in mammals, the enzyme ribonucleotide reductase (RNR), impacts cancer susceptibility and serves as a target for anti-cancer therapies. Because RNR-regulated dNTP production can influence DNA replication fidelity while also supporting genome-protecting DNA repair, RNR has complex and stage-specific roles in carcinogenesis. Nevertheless, cancer cells are dependent on RNR for de novo dNTP biosynthesis. Therefore, elevated RNR expression is a characteristic of many cancers, and an array of mechanistically distinct RNR inhibitors serve as effective agents for cancer treatment. The dNTP metabolism machinery, including RNR, has been exploited for therapeutic benefit for decades and remains an important target for cancer drug development.

Targeting ribonucleotide reductase for cancer therapy

INTRODUCTION

Ribonucleotide reductase (RR) is a unique enzyme, because it is responsible for reducing ribonucleotides to their corresponding deoxyribonucleotides, which are the building blocks required for DNA replication and repair. Dysregulated RR activity is associated with genomic instability, malignant transformation and cancer development. The use of RR inhibitors, either as a single agent or combined with other therapies, has proven to be a promising approach for treating solid tumors and hematological malignancies.

AREAS COVERED

This review covers recent publications in the area of RR, which include: i) the structure, function and regulation of RR; ii) the roles of RR in cancer development; iii) the classification, mechanisms and clinical application of RR inhibitors for cancer therapy and iv) strategies for developing novel RR inhibitors in the future.

EXPERT OPINION

Exploring the possible nonenzymatic roles of RR subunit proteins in carcinogenesis may lead to new rationales for developing novel anticancer drugs. Updated information about the structure and holoenzyme models of RR will help in identifying potential sites in the protein that could be targets for novel RR inhibitors. Determining RR activity and subunit levels in clinical samples will provide a rational platform for developing personalized cancer therapies that use RR inhibitors.

DNA damage response: three levels of DNA repair regulation

Genome integrity is challenged by DNA damage from both endogenous and environmental sources. This damage must be repaired to allow both RNA and DNA polymerases to accurately read and duplicate the information in the genome. Multiple repair enzymes scan the DNA for problems, remove the offending damage, and restore the DNA duplex. These repair mechanisms are regulated by DNA damage response kinases including DNA-PKcs, ATM, and ATR that are activated at DNA lesions. These kinases improve the efficiency of DNA repair by phosphorylating repair proteins to modify their activities, by initiating a complex series of changes in the local chromatin structure near the damage site, and by altering the overall cellular environment to make it more conducive to repair. In this review, we focus on these three levels of regulation to illustrate how the DNA damage kinases promote efficient repair to maintain genome integrity and prevent disease.

DNA damage-induced activation of ATM promotes β-TRCP-mediated Mdm2 ubiquitination and destruction

The Mdm2 oncoprotein promotes p53 ubiquitination and destruction. Yet, exact molecular mechanisms of Mdm2 destruction itself, under DNA damaging conditions, remain unclear. Recently, we identified SCF^β-TRCP as a novel E3 ligase that targets Mdm2 for ubiquitination and destruction in a Casein Kinase Iδ (CKIδ)-dependent manner. However, it remains elusive how the β-TRCP/CKIδ/Mdm2 signaling axis is regulated by DNA damage signals to govern p53 activity. Consistent with previous studies, we found that inactivation of the Ataxia Telangiectasia Mutated (ATM) kinase, in turn, impaired DNA damage-induced Mdm2 destruction. Although phosphorylation of Mdm2 at Ser395 (an ATM phosphorylation site) facilitated Mdm2 interaction with β-TRCP, Ser395A-Mdm2 was degraded non-distinguishably from WT-Mdm2 by SCF^β-TRCP upon DNA damaging treatments. This indicates that in addition to phosphorylating Mdm2 at Ser395, ATM may govern Mdm2 stability through other unknown mechanisms. We further demonstrated that DNA damage-induced activation of ATM directly phosphorylated CKIδ at two well-conserved S/TQ sites, which promotes CKIδ nuclear localization to increase CKIδ-mediated phosphorylation of Mdm2, thereby facilitating subsequent Mdm2 ubiquitination by SCF^β-TRCP. Our studies provide a molecular mechanism of how ATM could govern DNA damage-induced destruction of Mdm2 in part by phosphorylating both Mdm2 and CKIδ to modulate SCF^β-TRCP–mediated Mdm2 ubiquitination. Given the pivotal role of Mdm2 in the negative regulation of p53, this work will also provide a rationale for developing CKIδ or ATM agonists as anti-cancer agents.

Chromosomal instability triggered by Rrm2b loss leads to IL-6 secretion and plasmacytic neoplasms

Chronic inflammation has a tight cause-and-effect relationship with DNA damage by inflicting tissue damage and increasing cancer risk. Rrm2b, a key enzyme in de novo deoxyribonucleotide synthesis, is involved in DNA damage repair, but its role in cancer development has yet to be demonstrated. In this work, Rrm2b gene loss led to severe numerical and structural chromosome abnormalities that caused ATM activation, inducing p-Ser85 IKKγ/NEMO and IκB kinase (IKK). NF-κB consequently induced by IKK triggered sustained IL-6 expression that constitutively activated STAT3 in Rrm2b-deficient cells. High plasma interleukin-6 (IL-6) and associated hematologic disorders were observed in Rrm2b-/- mice, and 30%-40% of aged Rrm2b heterozygous knockout mice developed plasma cell neoplasms and suffered from progressive splenomegaly and ascites. The genetic ablation of IL-6 suppressed STAT3 induction and delayed disease onset in Rrm2b-/- mice, extending their lifespan. Thus, Rrm2b plays a crucial role in maintaining chromosomal stability and preventing chronic-inflammation-associated tumorigenesis.

Ribonucleotide reductase small subunit M2 serves as a prognostic biomarker and predicts poor survival of colorectal cancers

The overexpression of RRM2 [RR (ribonucleotide reductase) small subunit M2] dramatically enhances the ability of the cancer cell to proliferate and to invade. To investigate further the relevance of RRM2 and CRCs (colorectal cancers), we correlated the expression of RRM2 with the clinical outcome of CRCs. A retrospective outcome study was conducted on CRCs collected from the COH [(City of Hope) National Medical Center, 217 cases] and ZJU (Zhejiang University, 220 cases). IHC (immunohistochemistry) was employed to determine the protein expression level of RRM2, and quantitative real-time PCR was employed to validate. Multivariate logistic analysis indicated that the adjusted ORs (odds ratios) of RRM2-high for distant metastases were 2.06 [95% CI (confidence interval), 1.01-4.30] and 5.89 (95% CI, 1.51-39.13) in the COH and ZJU sets respectively. The Kaplan-Meier analysis displayed that high expression of RRM2 had a negative impact on the OS (overall survival) and PFS (progress-free survival) of CRC in both sets significantly. The multivariate Cox analysis further demonstrated that HRs (hazard ratios) of RRM2-high for OS were 1.88 (95% CI, 1.03-3.36) and 2.06 (95% CI, 1.10-4.00) in the COH and ZJU sets respectively. Stratification analysis demonstrated that the HR of RRM2 dramatically increased to 12.22 (95% CI, 1.62-258.31) in the MMR (mismatch repair) gene-deficient subgroup in the COH set. Meanwhile, a real-time study demonstrated that down-regulation of RRM2 by siRNA (small interfering RNA) could significantly and specifically reduce the cell growth and adhesion ability in HT-29 and HCT-8 cells. Therefore RRM2 is an independent prognostic factor and predicts poor survival of CRCs. It is also a potential predictor for identifying good responders to chemotherapy for CRCs.

RRM2B suppresses activation of the oxidative stress pathway and is up-regulated by p53 during senescence

RRM2B is the DNA damage-inducible small subunit of ribonucleotide reductase, the rate-limiting enzyme in de novo deoxyribonucleoside triphosphate synthesis. Although RRM2B is implicated in DNA repair and the maintenance of mitochondrial DNA content, the regulation and function of RRM2B in senescence have not been previously established. Here, we show that RRM2B is highly induced in a p53-dependent manner during senescence in primary human fibroblast IMR90 cells and is expressed at higher levels in senescent precancerous human prostatic intraepithelial neoplasm lesions compared to adjacent normal prostate glands. Paradoxically, silencing RRM2B expression leads to an increase in the level of reactive oxygen species, mitochondrial membrane depolarization, and premature senescence in a p38MAPK- and p53-dependent manner in young fibroblasts. Consistently, induction of senescence is accelerated in Rrm2b deficient mouse embryo fibroblasts. Our data demonstrate that RRM2B is induced by stress signals prior to the onset of senescence and prevents premature oxidative stress-induced senescence.

MEK2 regulates ribonucleotide reductase activity through functional interaction with ribonucleotide reductase small subunit p53R2

The p53R2 protein, a newly identified member of the ribonucleotide reductase family that provides nucleotides for DNA damage repair, is directly regulated by p53. We show that p53R2 is also regulated by a MEK2 (ERK kinase 2/MAP kinase kinase 2)-dependent pathway. Increased MEK1/2 phosphorylation by serum stimulation coincided with an increase in the RNR activity in U2OS and H1299 cells. The inhibition of MEK2 activity, either by treatment with a MEK inhibitor or by transfection with MEK2 siRNA, dramatically decreased the serum-stimulated RNR activity. Moreover, p53R2 siRNA, but not R2 siRNA, significantly inhibits serum-stimulated RNR activity, indicating that p53R2 is specifically regulated by a MEK2-dependent pathway. Co-immunoprecipitation analyses revealed that the MEK2 segment comprising amino acids 65-171 is critical for p53R2-MEK2 interaction, and the binding domain of MEK2 is required for MEK2-mediated increased RNR activity. Phosphorylation of MEK1/2 was greatly augmented by ionizing radiation, and RNR activity was concurrently increased. Ionizing radiation-induced RNR activity was markedly attenuated by transfection of MEK2 or p53R2 siRNA, but not R2 siRNA. These data show that MEK2 is an endogenous regulator of p53R2 and suggest that MEK2 may associate with p53R2 and upregulate its activity.

Proton Coupled Electron Transfer and Redox Active Tyrosines: Structure and Function of the Tyrosyl Radicals in Ribonucleotide Reductase and Photosystem II

Proton coupled electron transfer (PCET) reactions are important in many biological processes. Tyrosine oxidation/reduction can play a critical role in facilitating these reactions. Two examples are photosystem II (PSII) and ribonucleotide reductase (RNR). RNR is essential in DNA synthesis in all organisms. In E. coli RNR, a tyrosyl radical, Y122(•), is required as a radical initiator. Photosystem II (PSII) generates molecular oxygen from water. In PSII, an essential tyrosyl radical, YZ(•), oxidizes the oxygen evolving center. However, the mechanisms, by which the extraordinary oxidizing power of the tyrosyl radical is controlled, are not well understood. This is due to the difficulty in acquiring high-resolution structural information about the radical state. Spectroscopic approaches, such as EPR and UV resonance Raman (UVRR), can give new information. Here, we discuss EPR studies of PCET and the PSII YZ radical. We also present UVRR results, which support the conclusion that Y122 undergoes an alteration in ring and backbone dihedral angle when it is oxidized. This conformational change results in a loss of hydrogen bonding to the phenolic oxygen. Our analysis suggests that access of water is an important factor in determining tyrosyl radical lifetime and function. TOC graphic.

Identification and characterization of alternative promoters, transcripts and protein isoforms of zebrafish R2 gene

Ribonucleotide reductase (RNR) is the rate-limiting enzyme in the de novo synthesis of deoxyribonucleoside triphosphates. Expression of RNR subunits is closely associated with DNA replication and repair. Mammalian RNR M2 subunit (R2) functions exclusively in DNA replication of normal cells due to its S phase-specific expression and late mitotic degradation. Herein, we demonstrate the control of R2 expression through alternative promoters, splicing and polyadenylation sites in zebrafish. Three functional R2 promoters were identified to generate six transcript variants with distinct 5′ termini. The proximal promoter contains a conserved E2F binding site and two CCAAT boxes, which are crucial for the transcription of R2 gene during cell cycle. Activity of the distal promoter can be induced by DNA damage to generate four transcript variants through alternative splicing. In addition, two novel splice variants were found to encode distinct N-truncated R2 isoforms containing residues for enzymatic activity but no KEN box essential for its proteolysis. These two N-truncated R2 isoforms remained in the cytoplasm and were able to interact with RNR M1 subunit (R1). Thus, our results suggest that multilayered mechanisms control the differential expression and function of zebrafish R2 gene during cell cycle and under genotoxic stress.

Ribonucleotide reductase subunit p53R2 regulates mitochondria homeostasis and function in KB and PC-3 cancer cells

Ribonucleotide reductase (RR) is a rate-limiting enzyme that catalyzes de novo conversion of ribonucleotide 5'-diphosphates to the corresponding 2'-deoxynucleotide, essential for DNA synthesis and replication. The mutations or knockout of RR small subunit, p53R2, results in the depletion of mitochondrial DNA (mtDNA) in human, implying that p53R2 might play a critical role for maintaining mitochondrial homeostasis. In this study, siRNA against p53R2 knockdown approach is utilized to examine the impact of p53R2 depletion on mitochondria and to derive underlying mechanism in KB and PC-3 cancer cells. Our results reveal that the p53R2 expression not only positively correlates with mtDNA content, but also partakes in the proper mitochondria function, such as ATP synthesis, cytochrome c oxidase activity and membrane potential maintenance. Furthermore, overexpression of p53R2 reduces intracellular ROS and protects the mitochondrial membrane potential against oxidative stress. Unexpectedly, knockdown of p53R2 has a modest, if any, effect on mitochondrial and total cellular dNTP pools. Taken together, our study provides functional evidence that mitochondria is one of p53R2-targeted organelles and suggests an unexpected function of p53R2, which is beyond known RR function on dNTP synthesis, in mitochondrial homeostatic control.

Ribonucleotide reductase small subunit M2B prognoses better survival in colorectal cancer

Ribonucleotide reductase subunit RRM2B (p53R2) has been reported to suppress invasion and metastasis in colorectal cancer (CRC). Here, we report that high levels of RRM2B expression are correlated with markedly better survival in CRC patients. In a fluorescence-labeled orthotopic mouse xenograft model, we confirmed that overexpression of RRM2B in nonmetastatic CRC cells prevented lung and/or liver metastasis, relative to control cells that did metastasize. Clinical outcome studies were conducted on a training set with 103 CRCs and a validation set with 220 CRCs. All participants underwent surgery with periodic follow-up to determine survivability. A newly developed specific RRM2B antibody was employed to carry out immunohistochemistry for determining RRM2B expression levels on tissue arrays. In the training set, the Kaplan-Meier and multivariate Cox analysis revealed that RRM2B is associated with better survival of CRCs, especially in stage IV patients (HR = 0.40; 95% CI = 0.18-0.86, P = 0.016). In the validation set, RRM2B was negatively related to tumor invasion (OR = 0.45, 95% CI = 0.19-0.99, P = 0.040) and lymph node involvement (OR = 0.48, 95% CI = 0.25-0.92, P = 0.026). Furthermore, elevated expression of RRM2B was associated with better prognosis in this set as determined by multivariate analyses (HR = 0.48, 95% CI = 0.26-0.91, P = 0.030). Further investigations revealed that RRM2B was correlated with better survival of CRCs with advanced stage III and IV tumors rather than earlier stage I and II tumors. Taken together, our findings establish that RRM2B suppresses invasiveness of cancer cells and that its expression is associated with a better survival prognosis for CRC patients.

p53R2 inhibits the proliferation of human cancer cells in association with cell-cycle arrest

Deregulation of the expression of p53R2, a p53-inducible homologue of the R2 subunit of ribonucleotide reductase, has been found in various human cancer tissues; however, the roles p53R2 plays in cancer progression and malignancy remain controversial. In the present study, we examined changes in gene expression profiles associated with p53R2 in cancer cells, using the analysis of cDNA microarray. Gene set enrichment analysis identified that the gene set regulating cell-cycle progression was significantly enriched in p53R2-silencing human oropharyngeal carcinoma KB cells. Attenuation of p53R2 expression significantly reduced p21 expression and moderately increased cyclin D1 expression in both wild-type p53 cancer cells (KB and MCF-7) and mutant p53 cancer cells (PC3 and MDA-MB-231). Conversely, overexpression of p53R2-GFP resulted in an increase in the expression of p21 and decrease in the expression of cyclin D1, which correlated with reduced cell population in S-phase in vitro and suppressed growth in vivo. Furthermore, the MAP/ERK kinase inhibitor PD98059 partially abolished modulation of p21 and cyclin D1 expression by p53R2. Moreover, under the conditions of nonstress and adriamycin-induced genotoxic stress, attenuation of p53R2 in KB cells significantly increased phosphorylated H2AX, which indicates that attenuation of p53R2 may enhance DNA damage induced by adriamycin. Overall, our study shows that p53R2 may suppress cancer cell proliferation partially by upregulation of p21 and downregulation of cyclin D1; p53R2 plays critical roles not only in DNA damage repair but also in proliferation of cancer cells.

Molecular analysis and functions of p53R2 in zebrafish

p53R2 is a newly identified small subunit of ribonucleotide reductase and plays a pivotal role in the supply of dNTPs for genomic DNA repair and mitochondrial DNA synthesis, but little is known about its functions in zebrafish. Herein, we obtained the cDNA of zebrafish p53R2 that shares 72.8% and 72.5% amino acid identities with human p53R2 and zebrafish R2, respectively. Residues crucial for enzymatic activity are highly conserved among p53R2 proteins from different species. p53R2 in zebrafish was maternally expressed, its transcripts were detected in developing embryos and all adult tissues examined. A 250-bp minimal promoter upstream of the translational initiation site was identified to drive basal expression of p53R2 in a p53-independent manner. Expression of p53R2 was induced by DNA-damaging reagents CPT or MMS, but suppressed by p53-knockdown in zebrafish embryos. Moreover, p53R2 was mainly distributed in the cytoplasm of cells under normal condition and upon DNA damage. Furthermore, overexpression of p53R2 attenuated apoptosis of embryonic cells caused by CPT or MMS treatment and protected developing embryos from death. Therefore, functions of p53R2 in zebrafish are closely associated with its activity in DNA repair and synthesis.

Prkdc participates in mitochondrial genome maintenance and prevents Adriamycin-induced nephropathy in mice

Adriamycin (ADR) is a commonly used chemotherapeutic agent that also produces significant tissue damage. Mutations to mitochondrial DNA (mtDNA) and reductions in mtDNA copy number have been identified as contributors to ADR-induced injury. ADR nephropathy only occurs among specific mouse inbred strains, and this selective susceptibility to kidney injury maps as a recessive trait to chromosome 16A1-B1. Here, we found that sensitivity to ADR nephropathy in mice was produced by a mutation in the Prkdc gene, which encodes a critical nuclear DNA double-stranded break repair protein. This finding was confirmed in mice with independent Prkdc mutations. Overexpression of Prkdc in cultured mouse podocytes significantly improved cell survival after ADR treatment. While Prkdc protein was not detected in mitochondria, mice with Prkdc mutations showed marked mtDNA depletion in renal tissue upon ADR treatment. To determine whether Prkdc participates in mtDNA regulation, we tested its genetic interaction with Mpv17, which encodes a mitochondrial protein mutated in human mtDNA depletion syndromes (MDDSs). While single mutant mice were asymptomatic, Prkdc/Mpv17 double-mutant mice developed mtDNA depletion and recapitulated many MDDS and ADR injury phenotypes. These findings implicate mtDNA damage in the development of ADR toxicity and identify Prkdc as a MDDS modifier gene and a component of the mitochondrial genome maintenance pathway.

Oxygenomics in environmental stress

Environmental stressors such as chemicals and physical agents induce various oxidative stresses and affect human health. To elucidate their underlying mechanisms, etiology and risk, analyses of gene expression signatures in environmental stress-induced human diseases, including neuronal disorders, cancer and diabetes, are crucially important. Recent studies have clarified oxidative stress-induced signaling pathways in human and experimental animals. These pathways are classifiable into several categories: reactive oxygen species (ROS) metabolism and antioxidant defenses, p53 pathway signaling, nitric oxide (NO) signaling pathway, hypoxia signaling, transforming growth factor (TGF)-beta bone morphogenetic protein (BMP) signaling, tumor necrosis factor (TNF) ligand-receptor signaling, and mitochondrial function. This review describes the gene expression signatures through which environmental stressors induce oxidative stress and regulate signal transduction pathways in rodent and human tissues.

Effects of ginsenoside Rg2 on the ultraviolet B-induced DNA damage responses in HaCaT cells

Our previous study demonstrated the increase in the repair of UVB damage by mRg2, a mixture of ginsenosides containing 60% Rg2 in NIH3T3 cells. In the present study, the effects of purified Rg2 on the repair and apoptosis in ultraviolet B (UVB)-exposed HaCaT cells were investigated on gene expression levels. When cells were exposed to UVB and post-incubated in normal medium for 24 h, the cell viability decreased to about 50% of that in nontreated control. When Rg2 was post-incubated, however, the UVB-induced cytotoxicity was significantly prevented in an Rg2 concentration- and time-dependent manner. The apoptotic nuclear fragmentation resulting from UVB exposure was also significantly protected by the Rg2 post-incubation. Microarray analysis showed that the genes stimulated by the Rg2-alone treatment include those involved in p53 signaling pathway such as GADD45alpha, GADD45beta, and cell communication genes. RT-PCR analysis showed that the Rg2-alone treatment slightly upregulated the p53 and GADD45 transcript and protein levels by about 1.5-fold as compared with the nontreated control. The mRNA levels of p53 and GADD45 in cells exposed to UVB and post-incubated with Rg2 for 24 h decreased in an Rg2 concentration-dependent manner as compared with that post-incubated in normal medium. However, the mRNA level of the UVB-exposed cells post-incubated with 5 microM retinol was essentially the same as that post-incubated in normal medium. Time course experiment showed that the mRNA levels of p53 and GADD45 in UVB-exposed cells were upregulated by post-incubation with 50 microM Rg2 until 6 and 9 h, respectively, and then gradually decreased until 24 h. By Western blot analysis, it was also revealed that the Rg2 post-incubation decreases the expression of p53, phospho-p53, GADD45, and ATM in UVB-exposed cells. Time course analysis also indicated that these decreased expressions were due to the earlier upregulation of p53 and GADD45 proteins. When UVB-exposed cells were post-incubated with Rg2 for 24 h after UVB exposure, the level of remaining cyclobutane pyrimidine dimers decreased in both Rg2 concentration- and time-dependent manner. All these results suggest that Rg2 protects cells against UVB-induced genotoxicity by increasing DNA repair, in possible association with modulation of protein levels involved in p53 signaling pathway.

Metabolic genes in cancer: their roles in tumor progression and clinical implications

Re-programming of metabolic pathways is a hallmark of physiological changes in cancer cells. The expression of certain genes that directly control the rate of key metabolic pathways including glycolysis, lipogenesis and nucleotide synthesis are drastically altered at different stages of tumor progression. These alterations are generally considered as an adaptation of tumor cells; however, they also contribute to the progression of tumor cells to become more aggressive phenotypes. This review summarizes the recent information about the mechanistic link of these genes to oncogenesis and their potential utility as diagnostic markers as well as for therapeutic targets. We particularly focus on three groups of genes; GLUT1, G6PD, TKTL1 and PGI/AMF in glycolytic pathway, ACLY, ACC1 and FAS in lipogenesis and RRM2, p53R2 and TYMS for nucleotide synthesis. All these genes are highly up-regulated in a variety of tumor cells in cancer patients, and they play active roles in tumor progression rather than expressing merely as a consequence of phenotypic change of the cancer cells. Molecular dissection of their orchestrated networks and understanding the exact mechanism of their expression will provide a window of opportunity to target these genes for specific cancer therapy. We also reviewed existing database of gene microarray to validate the utility of these genes for cancer diagnosis.

Mechanism of inactivation of human ribonucleotide reductase with p53R2 by gemcitabine 5'-diphosphate

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleoside 5'-diphosphates to the corresponding deoxynucleotides supplying the dNTPs required for DNA replication and DNA repair. Class I RNRs require two subunits, alpha and beta, for activity. Humans possess two beta subunits: one involved in S phase DNA replication (beta) and a second in mitochondrial DNA replication (beta' or p53R2) and potentially DNA repair. Gemcitabine (F(2)C) is used clinically as an anticancer agent, and its phosphorylated metabolites target many enzymes involved in nucleotide metabolism, including RNR. The present investigation with alpha (specific activity of 400 nmol min(-1) mg(-1)) and beta' (0.6 Y./beta'2 and a specific activity of 420 nmol min(-1) mg(-1)) establishes that F(2)CDP is a substoichiometric inactivator of RNR. Incubation of this alpha/beta' with [1'-(3)H]-F(2)CDP or [5-(3)H]-F(2)CDP and reisolation of the protein by Sephadex G-50 chromatography resulted in recovery 0.5 equiv of covalently bound sugar and 0.03 equiv of tightly associated cytosine to alpha2. SDS-PAGE analysis (loaded without boiling) of the inactivated RNR showed that 60% of alpha migrates as a 90 kDa protein and 40% as a 120 kDa protein. Incubation of [1'-(3)H]-F(2)CDP with active site mutants C444S/A, C218S/A, and E431Q/D-alpha and the C-terminal tail C787S/A and C790S/A mutants reveals that no sugar label is bound to the active site mutants of alpha and that, in the case of C218S-alpha, alpha migrates as a 90 kDa protein. Analysis of the inactivated wt-alpha/beta' RNR by size exclusion chromatography indicates a quaternary structure of alpha6beta'6. A mechanism of inactivation common with halpha/beta is presented.

Activity of ribonucleotide reductase helps determine how cells repair DNA double strand breaks

Mammalian cells can choose either nonhomologous end joining (NHEJ) or homologous recombination (HR) for repair of chromosome breaks. Of these two pathways, HR alone requires extensive DNA synthesis and thus abundant synthesis precursors (dNTPs). We address here if this differing requirement for dNTPs helps determine how cells choose a repair pathway. Cellular dNTP pools are regulated primarily by changes in ribonucleotide reductase activity. We show that an inhibitor of ribonucleotide reductase (hydroxyurea) hypersensitizes NHEJ-deficient cells, but not wild type or HR-deficient cells, to chromosome breaks introduced by ionizing radiation. Hydroxyurea additionally reduces the frequency of irradiated cells with a marker for an early step in HR, Rad51 foci, consistent with reduced initiation of HR under these conditions. Conversely, promotion of ribonucleotide reductase activity protects NHEJ-deficient cells from ionizing radiation. Importantly, promotion of ribonucleotide reductase activity also increases usage of HR in cells proficient in both NHEJ and HR at a targeted chromosome break. Activity of ribonucleotide reductase is thus an important factor in determining how mammalian cells repair broken chromosomes. This may explain in part why G1/G0 cells, which have reduced ribonucleotide reductase activity, rely more on NHEJ for DSB repair.

A heterozygous truncating mutation in RRM2B causes autosomal-dominant progressive external ophthalmoplegia with multiple mtDNA deletions

Autosomal-dominant progressive external ophthalmoplegia (adPEO) is a mitochondrial disorder that is characterized by accumulation of multiple mitochondrial DNA (mtDNA) deletions in postmitotic tissues. The disorder is heterogeneous, with five known nuclear disease genes that encode the proteins ANT1, Twinkle, POLG, POLG2, and OPA1. Defects in these proteins affect mtDNA maintenance, probably leading to stalled replication forks, consequent mtDNA deletion formation, and progressive respiratory chain deficiency. Here we present a large adPEO family with multiple mtDNA deletions, whose disease was not explained by mutations in any of the known adPEO loci. We mapped the disease locus in this family to chromosome 8q22.1-q23.3. The critical linkage region contained the RRM2B gene, which encodes the small subunit of the ribonucleotide reductase p53R2, which has previously been shown to be essential for the maintenance of mtDNA copy number. Mutation screening of RRM2B revealed a heterozygous nonsense mutation in exon 9 (c.979C-->T [p.R327X]) in all affected individuals that was absent in 380 control chromosomes. The same mutation was found to segregate in another adPEO family. The mutant mRNA escaped nonsense-mediated decay and resulted in a protein with truncation of 25 highly conserved C-terminal amino acids essential for the interaction with the ribonucleotide reductase subunit R1. We conclude that dominant-negative or gain-of-function mutations in RRM2B are a cause of multiple mtDNA deletions and adPEO.