Mutagenesis and deoxyribonucleotide pool imbalance

Fragile sites, chromosomal lesions, tandem repeats, and disease

Expanded tandem repeat DNAs are associated with various unusual chromosomal lesions, despiralizations, multi-branched inter-chromosomal associations, and fragile sites. Fragile sites cytogenetically manifest as localized gaps or discontinuities in chromosome structure and are an important genetic, biological, and health-related phenomena. Common fragile sites (∼230), present in most individuals, are induced by aphidicolin and can be associated with cancer; of the 27 molecularly-mapped common sites, none are associated with a particular DNA sequence motif. Rare fragile sites ( ≳ 40 known), ≤ 5% of the population (may be as few as a single individual), can be associated with neurodevelopmental disease. All 10 molecularly-mapped folate-sensitive fragile sites, the largest category of rare fragile sites, are caused by gene-specific CGG/CCG tandem repeat expansions that are aberrantly CpG methylated and include FRAXA, FRAXE, FRAXF, FRA2A, FRA7A, FRA10A, FRA11A, FRA11B, FRA12A, and FRA16A. The minisatellite-associated rare fragile sites, FRA10B, FRA16B, can be induced by AT-rich DNA-ligands or nucleotide analogs. Despiralized lesions and multi-branched inter-chromosomal associations at the heterochromatic satellite repeats of chromosomes 1, 9, 16 are inducible by de-methylating agents like 5-azadeoxycytidine and can spontaneously arise in patients with ICF syndrome (Immunodeficiency Centromeric instability and Facial anomalies) with mutations in genes regulating DNA methylation. ICF individuals have hypomethylated satellites I-III, alpha-satellites, and subtelomeric repeats. Ribosomal repeats and subtelomeric D4Z4 megasatellites/macrosatellites, are associated with chromosome location, fragility, and disease. Telomere repeats can also assume fragile sites. Dietary deficiencies of folate or vitamin B12, or drug insults are associated with megaloblastic and/or pernicious anemia, that display chromosomes with fragile sites. The recent discovery of many new tandem repeat expansion loci, with varied repeat motifs, where motif lengths can range from mono-nucleotides to megabase units, could be the molecular cause of new fragile sites, or other chromosomal lesions. This review focuses on repeat-associated fragility, covering their induction, cytogenetics, epigenetics, cell type specificity, genetic instability (repeat instability, micronuclei, deletions/rearrangements, and sister chromatid exchange), unusual heritability, disease association, and penetrance. Understanding tandem repeat-associated chromosomal fragile sites provides insight to chromosome structure, genome packaging, genetic instability, and disease.

A comprehensive weight of evidence assessment of published acetaminophen genotoxicity data: Implications for its carcinogenic hazard potential

In 2019, the California Office of Environmental Health Hazard Assessment initiated a review of the carcinogenic hazard potential of acetaminophen, including an assessment of its genotoxicity. The objective of this analysis was to inform this review process with a weight-of-evidence assessment of more than 65 acetaminophen genetic toxicology studies that are of widely varying quality and conformance to accepted standards and relevance to humans. In these studies, acetaminophen showed no evidence of induction of point or gene mutations in bacterial and mammalian cell systems or in in vivo studies. In reliable, well-controlled test systems, clastogenic effects were only observed in unstable, p53-deficient cell systems or at toxic and/or excessively high concentrations that adversely affect cellular processes (e.g., mitochondrial respiration) and cause cytotoxicity. Across the studies, there was no clear evidence that acetaminophen causes DNA damage in the absence of toxicity. In well-controlled clinical studies, there was no meaningful evidence of chromosomal damage. Based on this weight-of-evidence assessment, acetaminophen overwhelmingly produces negative results (i.e., is not a genotoxic hazard) in reliable, robust high-weight studies. Its mode of action produces cytotoxic effects before it can induce the stable, genetic damage that would be indicative of a genotoxic or carcinogenic hazard.

Serine Biosynthesis Is a Metabolic Vulnerability in FLT3-ITD-Driven Acute Myeloid Leukemia

Internal tandem duplication of the FMS-like tyrosine kinase 3 gene (FLT3-ITD) occurs in 30% of all acute myeloid leukemias (AML). Limited clinical efficacy of FLT3 inhibitors highlights the need for alternative therapeutic modalities in this subset of disease. Using human and murine models of FLT3-ITD-driven AML, we demonstrate that FLT3-ITD promotes serine synthesis and uptake via ATF4-dependent transcriptional regulation of genes in the de novo serine biosynthesis pathway and neutral amino acid transport. Genetic or pharmacologic inhibition of PHGDH, the rate-limiting enzyme of de novo serine biosynthesis, selectively inhibited proliferation of FLT3-ITD AMLs in vitro and in vivo. Moreover, pharmacologic inhibition of PHGDH sensitized FLT3-ITD AMLs to the standard-of-care chemotherapeutic cytarabine. Collectively, these data reveal novel insights into FLT3-ITD-induced metabolic reprogramming and reveal a targetable vulnerability in FLT3-ITD AML. SIGNIFICANCE: FLT3-ITD mutations are common in AML and are associated with poor prognosis. We show that FLT3-ITD stimulates serine biosynthesis, thereby rendering FLT3-ITD-driven leukemias dependent upon serine for proliferation and survival. This metabolic dependency can be exploited pharmacologically to sensitize FLT3-ITD-driven AMLs to chemotherapy.This article is highlighted in the In This Issue feature, p. 1307.

Changes in the Concentrations of Deoxynucleotides in Human Lymphocytes TreatedIn Vitrowith Genotoxic Agents

We studied the toxic and genotoxic effects of UV light and an alkylating agent (methyl methanesulphonate; MMS) in human peripheral blood lymphocytes (PBL) in Goand after PHA stimulation. We measured DNA repair synthesis and replication in treated PBL together with deoxynucleoside triphosphate pool and ATP concentrations. In addition to its genotoxic action, MMS induced a strong toxic effect in cycling cells, as shown by a marked depletion of nucleotide pools.

From Powerhouse to Perpetrator-Mitochondria in Health and Disease

In this review we discuss the interaction between metabolic stress, mitochondrial dysfunction, and genomic instability. Unrepaired DNA damage in the nucleus resulting from excess accumulation of DNA damages and stalled replication can initiate cellular signaling responses that negatively affect metabolism and mitochondrial function. On the other hand, mitochondrial pathologies can also lead to stress in the nucleus, and cause sensitivity to DNA-damaging agents. These are examples of how hallmarks of cancer and aging are connected and influenced by each other to protect humans from disease.

Identification of Inhibitors of the dNTP Triphosphohydrolase SAMHD1 Using a Novel and Direct High-Throughput Assay

The dNTP triphosphohydrolase SAMHD1 is a regulator of cellular dNTP pools. Given its central role in nucleotide metabolism, SAMHD1 performs important functions in cellular homeostasis, cell cycle regulation, and innate immunity. It therefore represents a high-profile target for small molecule drug design. SAMHD1 has a complex mechanism of catalytic activation that makes the design of an activating compound challenging. However, an inhibitor of SAMHD1 could serve multiple therapeutic roles, including the potentiation of antiviral and anticancer drug regimens. The lack of high-throughput screens that directly measure SAMHD1 catalytic activity has impeded efforts to identify inhibitors of SAMHD1. Here we describe a novel high-throughput screen that directly measures SAMHD1 catalytic activity. This assay results in a colorimetric end point that can be read spectrophotometrically and utilizes bis(4-nitrophenyl) phosphate as the substrate and Mn²⁺ as the activating cation that facilitates catalysis. When used to screen a library of Food and Drug Administration-approved drugs, this HTS identified multiple novel compounds that inhibited SAMHD1 dNTPase activity at micromolar concentrations.

SAMHD1: Recurring roles in cell cycle, viral restriction, cancer, and innate immunity

Sterile alpha motif and histidine-aspartic acid domain-containing protein 1 (SAMHD1) is a deoxynucleotide triphosphate (dNTP) hydrolase that plays an important role in the homeostatic balance of cellular dNTPs. Its emerging role as an effector of innate immunity is affirmed by mutations in the SAMHD1 gene that cause the severe autoimmune disease, Aicardi-Goutieres syndrome (AGS) and that are linked to cancer. Additionally, SAMHD1 functions as a restriction factor for retroviruses, such as HIV. Here, we review the current biochemical and biological properties of the enzyme including its structure, activity, and regulation by post-translational modifications in the context of its cellular function. We outline open questions regarding the biology of SAMHD1 whose answers will be important for understanding its function as a regulator of cell cycle progression, genomic integrity, and in autoimmunity.

Differential control of dNTP biosynthesis and genome integrity maintenance by the dUTPase superfamily enzymes

dUTPase superfamily enzymes generate dUMP, the obligate precursor for de novo dTTP biosynthesis, from either dUTP (monofunctional dUTPase, Dut) or dCTP (bifunctional dCTP deaminase/dUTPase, Dcd:dut). In addition, the elimination of dUTP by these enzymes prevents harmful uracil incorporation into DNA. These two beneficial outcomes have been thought to be related. Here we determined the relationship between dTTP biosynthesis (dTTP/dCTP balance) and the prevention of DNA uracilation in a mycobacterial model that encodes both the Dut and Dcd:dut enzymes, and has no other ways to produce dUMP. We show that, in dut mutant mycobacteria, the dTTP/dCTP balance remained unchanged, but the uracil content of DNA increased in parallel with the in vitro activity-loss of Dut accompanied with a considerable increase in the mutation rate. Conversely, dcd:dut inactivation resulted in perturbed dTTP/dCTP balance and two-fold increased mutation rate, but did not increase the uracil content of DNA. Thus, unexpectedly, the regulation of dNTP balance and the prevention of DNA uracilation are decoupled and separately brought about by the Dcd:dut and Dut enzymes, respectively. Available evidence suggests that the discovered functional separation is conserved in humans and other organisms.

With me or against me: Tumor suppressor and drug resistance activities of SAMHD1

Sterile alpha motif and histidine/aspartic acid domain-containing protein 1 (SAMHD1) is a (deoxy)guanosine triphosphate (dGTP/GTP)-activated deoxyribonucleoside triphosphate (dNTP) triphosphohydrolase involved in cellular dNTP homoeostasis. Mutations in SAMHD1 have been associated with the hyperinflammatory disease Aicardi-Goutières syndrome (AGS). SAMHD1 also limits cells' permissiveness to infection with diverse viruses, including human immunodeficiency virus (HIV-1), and controls endogenous retroviruses. Increasing evidence supports the role of SAMHD1 as a tumor suppressor. However, SAMHD1 also can act as a resistance factor to nucleoside-based chemotherapies by hydrolyzing their active triphosphate metabolites, thereby reducing response of various malignancies to these anticancer drugs. Hence, informed cancer therapies must take into account the ambiguous properties of SAMHD1 as both an inhibitor of uncontrolled proliferation and a resistance factor limiting the efficacy of anticancer treatments. Here, we provide evidence that SAMHD1 is a double-edged sword for patients with acute myelogenous leukemia (AML). Our time-dependent analyses of The Cancer Genome Atlas (TCGA) AML cohort indicate that high expression of SAMHD1, even though it critically limits the efficacy of high-dose ara-C therapy, might be associated with more favorable disease progression.

Heterozygous colon cancer-associated mutations of SAMHD1 have functional significance

Even small variations in dNTP concentrations decrease DNA replication fidelity, and this observation prompted us to analyze genomic cancer data for mutations in enzymes involved in dNTP metabolism. We found that sterile alpha motif and histidine-aspartate domain-containing protein 1 (SAMHD1), a deoxyribonucleoside triphosphate triphosphohydrolase that decreases dNTP pools, is frequently mutated in colon cancers, that these mutations negatively affect SAMHD1 activity, and that several SAMHD1 mutations are found in tumors with defective mismatch repair. We show that minor changes in dNTP pools in combination with inactivated mismatch repair dramatically increase mutation rates. Determination of dNTP pools in mouse embryos revealed that inactivation of one SAMHD1 allele is sufficient to elevate dNTP pools. These observations suggest that heterozygous cancer-associated SAMHD1 mutations increase mutation rates in cancer cells.

Metabolic dysregulation in monogenic disorders and cancer - finding method in madness

Cancer is a prime example of a disease process in which carcinogenic and metabolic changes are intertwined to promote cell survival and growth. One approach to unravel this complex relationship is by studying rare, monogenic disorders caused by mutations in genes encoding metabolic enzymes or regulators. There are hundreds of these diseases, most of which manifest in childhood and are collectively termed 'inborn errors of metabolism' (IEMs). Several IEMs demonstrate the consequences of chronic, systemic loss of a particular metabolic activity that can result in malignancy. In this Opinion article, we present a conceptual categorization of IEMs associated with cancer and discuss how assessment of these rare diseases might inform us about the biological foundations of common types of cancer and opportunities for cancer diagnosis and therapy.

Cross-species inhibition of dUTPase via the Staphylococcal Stl protein perturbs dNTP pool and colony formation in Mycobacterium

Proteins responsible for the integrity of the genome are often used targets in drug therapies against various diseases. The inhibitors of these proteins are also important to study the pathways in genome integrity maintenance. A prominent example is Ugi, a well known cross-species inhibitor protein of the enzyme uracil-DNA glycosylase, responsible for uracil excision from DNA. Here, we report that a Staphylococcus pathogenicity island repressor protein called StlSaPIbov1 (Stl) exhibits potent dUTPase inhibition in Mycobacteria. To our knowledge, this is the first indication of a cross-species inhibitor protein for any dUTPase. We demonstrate that the Staphylococcus aureus Stl and the Mycobacterium tuberculosis dUTPase form a stable complex and that in this complex, the enzymatic activity of dUTPase is strongly inhibited. We also found that the expression of the Stl protein in Mycobacterium smegmatis led to highly increased cellular dUTP levels in the mycobacterial cell, this effect being in agreement with its dUTPase inhibitory role. In addition, Stl expression in M. smegmatis drastically decreased colony forming ability, as well, indicating significant perturbation of the phenotype. Therefore, we propose that Stl can be considered to be a cross-species dUTPase inhibitor and may be used as an important reagent in dUTPase inhibition experiments either in vitro/in situ or in vivo.

Intracellular deoxyribonucleotide pool imbalance and DNA damage in cells treated with hydroxyurea, an inhibitor of ribonucleotide reductase

Imbalance in the nucleotide pool of mammalian cells has been shown to result in genotoxic damage. The goal of this study was to devise a sensitive, reproducible and simple method for detection of nucleotide pool changes in mammalian cells that could be used for problem-solving activities in drug development, e.g. mechanistic explanation of a positive response in a mammalian in vitro genotoxicity test. The method evaluated in this study is based on ethanol extraction of the total nucleotide pool, heat treatment and filtration, treatment with calf intestine alkaline phosphatase to convert nucleotides to nucleosides and analysis of the nucleosides by high-performance liquid chromatography with ultraviolet detection. The method was applied to measure the intracellular levels of deoxyribonucleotides in mouse lymphoma (ML) L5178Y cells treated with various concentrations of a model compound, hydroxyurea (HU), a ribonucleotide reductase inhibitor. DNA strand breakage and micronuclei formation were assessed in the same experiments. Imbalance of nucleotide pool (i.e. changes in the relative ratios between individual nucleotide pools) in HU-treated ML cells has been observed already at a concentration of 0.01 mmol/l, whereas genotoxic effects became apparent only at higher concentrations of HU (i.e. 0.25 mmol/l and higher) as indicated by formation of DNA strand breaks and micronuclei.

Cooperative and allosterically controlled nucleotide binding regulates the DNA binding activity of NrdR

Ribonucleotide reductases (RNRs) are required for the synthesis of deoxyribonucleoside triphosphates (dNTPs) from ribonucleotides. In Escherichia coli, regulation of RNR expression is co-ordinated with the cell cycle, and involves several regulatory proteins. One of these, NrdR, has recently been shown to regulate all three nrd operons that encode RNR isoenzymes. Repression by NrdR is believed to be stimulated by elevated dNTPs, although there is no direct evidence for this model. Here, we sought to elucidate the mechanism by which NrdR regulates nrd expression according to the abundance of (d)NTPs. We determined that ATP and dATP bind to NrdR in a negatively cooperative fashion, such that neither can fully occupy the protein. Both nucleotides also appear to act as positive heterotropic effectors, since the binding of one stimulates binding of the other. Nucleotide binding stimulates self-association of NrdR, with tri- and diphosphates stimulating oligomerization more effectively than monophosphates. As-prepared NrdR contains (deoxy)nucleoside monophosphates, diphosphates and triphosphates, and its DNA binding activity is inhibited by triphosphates and diphosphates but not by monophosphates. We propose a model in which NrdR selectively binds (deoxy)nucleoside triphosphates, which are hydrolysed to their monophosphate counterparts in order to regulate DNA binding.

Hepatitis C virus non-structural protein 3 interacts with cytosolic 5'(3')-deoxyribonucleotidase and partially inhibits its activity

Infection with hepatitis C virus (HCV) is etiologically involved in liver cirrhosis, hepatocellular carcinoma and B-cell lymphomas. It has been demonstrated previously that HCV non-structural protein 3 (NS3) is involved in cell transformation. In this study, a yeast two-hybrid screening experiment was conducted to identify cellular proteins interacting with HCV NS3 protein. Cytosolic 5′(3′)-deoxyribonucleotidase (cdN, dNT-1) was found to interact with HCV NS3 protein. Binding domains of HCV NS3 and cellular cdN proteins were also determined using the yeast two-hybrid system. Interactions between HCV NS3 and cdN proteins were further demonstrated by co-immunoprecipitation and confocal analysis in cultured cells. The cellular cdN activity was partially repressed by NS3 protein in both the transiently-transfected and the stably-transfected systems. Furthermore, HCV partially repressed the cdN activity while had no effect on its protein expression in the systems of HCV sub-genomic replicons and infectious HCV virions. Deoxyribonucleotidases are present in most mammalian cells and involve in the regulation of intracellular deoxyribonucleotides pools by substrate cycles. Control of DNA precursor concentration is essential for the maintenance of genetic stability. Reduction of cdN activity would result in the imbalance of DNA precursor concentrations. Thus, our results suggested that HCV partially reduced the cdN activity via its NS3 protein and this may in turn cause diseases.

Modulation of mutagenesis in eukaryotes by DNA replication fork dynamics and quality of nucleotide pools

The rate of mutations in eukaryotes depends on a plethora of factors and is not immediately derived from the fidelity of DNA polymerases (Pols). Replication of chromosomes containing the anti-parallel strands of duplex DNA occurs through the copying of leading and lagging strand templates by a trio of Pols α, δ and ϵ, with the assistance of Pol ζ and Y-family Pols at difficult DNA template structures or sites of DNA damage. The parameters of the synthesis at a given location are dictated by the quality and quantity of nucleotides in the pools, replication fork architecture, transcription status, regulation of Pol switches, and structure of chromatin. The result of these transactions is a subject of survey and editing by DNA repair.

Cyclin F-mediated degradation of ribonucleotide reductase M2 controls genome integrity and DNA repair

F-box proteins are the substrate binding subunits of SCF (Skp1-Cul1-F-box protein) ubiquitin ligase complexes. Using affinity purifications and mass spectrometry, we identified RRM2 (the ribonucleotide reductase family member 2) as an interactor of the F-box protein cyclin F. Ribonucleotide reductase (RNR) catalyzes the conversion of ribonucleotides to deoxyribonucleotides (dNTPs), which are necessary for both replicative and repair DNA synthesis. We found that, during G2, following CDK-mediated phosphorylation of Thr33, RRM2 is degraded via SCF(cyclin F) to maintain balanced dNTP pools and genome stability. After DNA damage, cyclin F is downregulated in an ATR-dependent manner to allow accumulation of RRM2. Defective elimination of cyclin F delays DNA repair and sensitizes cells to DNA damage, a phenotype that is reverted by expressing a nondegradable RRM2 mutant. In summary, we have identified a biochemical pathway that controls the abundance of dNTPs and ensures efficient DNA repair in response to genotoxic stress.

Novel mutator mutants of E. coli nrdAB ribonucleotide reductase: insight into allosteric regulation and control of mutation rates

Ribonucleotide reductase (RNR) is the enzyme critically responsible for the production of the 5'-deoxynucleoside-triphosphates (dNTPs), the direct precursors for DNA synthesis. The dNTP levels are tightly controlled to permit high efficiency and fidelity of DNA synthesis. Much of this control occurs at the level of the RNR by feedback processes, but a detailed understanding of these mechanisms is still lacking. Using a genetic approach in the bacterium Escherichia coli, a paradigm for the class Ia RNRs, we isolated 23 novel RNR mutants displaying elevated mutation rates along with altered dNTP levels. The responsible amino-acid substitutions in RNR reside in three different regions: (i) the (d)ATP-binding activity domain, (ii) a novel region in the small subunit adjacent to the activity domain, and (iii) the dNTP-binding specificity site, several of which are associated with different dNTP pool alterations and different mutational outcomes. These mutants provide new insight into the precise mechanisms by which RNR is regulated and how dNTP pool disturbances resulting from defects in RNR can lead to increased mutation.

Pyrimidine pool imbalance induced by BLM helicase deficiency contributes to genetic instability in Bloom syndrome

Defects in DNA replication are associated with genetic instability and cancer development, as illustrated in Bloom syndrome. Features of this syndrome include a slowdown in replication speed, defective fork reactivation and high rates of sister chromatid exchange, with a general predisposition to cancer. Bloom syndrome is caused by mutations in the BLM gene encoding a RecQ helicase. Here we report that BLM deficiency is associated with a strong cytidine deaminase defect, leading to pyrimidine pool disequilibrium. In BLM-deficient cells, pyrimidine pool normalization leads to reduction of sister chromatid exchange frequency and is sufficient for full restoration of replication fork velocity but not the fork restart defect, thus identifying the part of the Bloom syndrome phenotype because of pyrimidine pool imbalance. This study provides new insights into the molecular basis of control of replication speed and the genetic instability associated with Bloom syndrome. Nucleotide pool disequilibrium could be a general phenomenon in a large spectrum of precancerous and cancer cells.

Insights into the pathogenesis and treatment of cancer from inborn errors of metabolism

Mutations in genes that play fundamental roles in metabolic pathways have been found to also play a role in tumor development and susceptibility to cancer. At the same time, significant progress has been made in the treatment of patients with inborn errors of metabolism (IEM),(1) resulting in increased longevity and the unmasking of cancer predisposition, frequently hepatocellular carcinoma, in these conditions. These patients offer a potential opportunity to deepen our understanding of how intermediary metabolism impacts tumorigenesis. We provide an overview from the perspective of cancers in patients affected with IEM and discuss how dysregulation of these specific metabolic pathways might contribute to the mechanisms of cancer development and treatment.

Insights into the pathogenesis and treatment of cancer from inborn errors of metabolism

Mutations in genes that play fundamental roles in metabolic pathways have been found to also play a role in tumor development and susceptibility to cancer. At the same time, significant progress has been made in the treatment of patients with inborn errors of metabolism (IEM),(1) resulting in increased longevity and the unmasking of cancer predisposition, frequently hepatocellular carcinoma, in these conditions. These patients offer a potential opportunity to deepen our understanding of how intermediary metabolism impacts tumorigenesis. We provide an overview from the perspective of cancers in patients affected with IEM and discuss how dysregulation of these specific metabolic pathways might contribute to the mechanisms of cancer development and treatment.

Hyperhomocysteinemia and the role of B vitamins in cancer

BACKGROUND

Patients suffering from malignancies have increased complications due to corresponding cardiovascular diseases and risk factor for the development of venous thromboembolism. Epidemiological studies have shown that increased homocysteine plasma concentration (hyperhomocysteinemia) is related to a higher risk of coronary heart disease, stroke, peripheral vascular disease and malignancies. Homocysteine (tHcy) is an intermediate sulfur-containing amino acid produced from methionine during processing of dietary proteins. The plasma homocysteine levels are strongly influenced by diet, as well as by genetic factors. Folic acid, vitamins B6 and B12 are dietary components which influence the plasma homocysteine levels the most. Several studies have found that high blood levels of B vitamins are related to the integrity and function of DNA, and, are at least related to lower concentration of homocysteine. Folate depletion has been found to change DNA methylation and DNA synthesis in both animal and human studies. Because of this critical role of folate, most studies including homocysteine have focused on these two actions.

CONCLUSIONS

Hyperhomocysteinemia proves to be the most common condition highly associated with both venous and arterial thrombosis in many cancer patients, while the associated pathophysiology has not been precisely established yet. Therefore, of current interest is the possible role of folate metabolism developing into a cancer initiating hyperhomocysteinemia. This review will discuss this possibility.

Abnormal neurological features predict poor survival and should preclude liver transplantation in patients with deoxyguanosine kinase deficiency

Deoxyguanosine kinase (DGUOK) deficiency is the commonest type of mitochondrial DNA depletion associated with a hepatocerebral phenotype. In this article, we evaluate predictors of survival and therapeutic options in patients with DGUOK deficiency. A systematic search of MEDLINE, LILAC, and SCIELO was carried out to identify peer-reviewed clinical trials, randomized controlled trials, meta-analyses, and other studies with clinical pertinence. DGUOK deficiency was searched with the terms dGK, DGUOK, mitochondrial DNA depletion, mtDNA, and hepatocerebral. Bibliographies of identified articles were reviewed for additional references. Thirteen identified studies met the inclusion criteria and were used in this study. The analysis revealed that DGUOK deficiency is associated with a variable clinical phenotype. Long-term survival is best predicted by the absence of profound hypotonia, significant psychomotor retardation, or nystagmus. In the presence of these features, there is increased mortality, and liver transplantation does not confer increased survival. In summary, liver transplantation appears to be futile in the presence of specific neurological signs or symptoms in patients affected with DGUOK deficiency. Conversely, in the absence of these neurological features, liver transplantation may be considered a potential treatment.

In vitro approaches to develop weight of evidence (WoE) and mode of action (MoA) discussions with positive in vitro genotoxicity results

A recent analysis by Kirkland et al. [Kirkland, D., Aardema, M., Henderson, L. and Müller, L. (2005) Evaluation of the ability of a battery of 3 in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity. Mutat. Res. 584, 1-256] demonstrated an extremely high false positive rate for in vitro genotoxicity tests when compared with carcinogenicity in rodents. In many industries, decisions have to be made on the safety of new substances, and health risk to humans, without rodent carcinogenicity data being available. In such cases, the usual way to determine whether a positive in vitro genotoxicity result is relevant (i.e. indicates a hazard) for humans is to develop weight of evidence (WoE) or mode of action (MoA) arguments. These are based partly on further in vitro investigations, but usually rely heavily on tests for genotoxicity in one or more in vivo assays. However, for certain product types in the European Union, the use of animals for genotoxicity testing (as well as for other endpoints) will be prohibited within the next few years. Many different examples have been described that indicate DNA damage and genotoxic responses in vitro can arise through non-relevant in vitro events that are a result of the test systems and conditions used. The majority of these non-relevant in vitro events can be grouped under a category of 'overload of normal physiology' that would not be expected to occur in exposed humans. However, obtaining evidence in support of such MoAs is not easy, particularly for those industries prohibited from carrying out in vivo testing. It will become necessary to focus on in vitro studies to provide evidence of non-DNA, threshold or in vitro-specific processes and to discuss the potential for such genotoxic effects to occur in exposed humans. Toward this end, we surveyed the published literature for in vitro approaches that may be followed to determine whether a genotoxic effect observed in vitro will occur in humans. Unfortunately, many of the approaches we found are based on only a few published examples and validated approaches with consensus recommendations often do not exist. This analysis highlights the urgent need for developing consensus approaches that do not rely on animal studies for dealing with in vitro genotoxins.

Roles of DNA Polymerases in Replication, Repair, and Recombination in Eukaryotes

The functioning of the eukaryotic genome depends on efficient and accurate DNA replication and repair. The process of replication is complicated by the ongoing decomposition of DNA and damage of the genome by endogenous and exogenous factors. DNA damage can alter base coding potential resulting in mutations, or block DNA replication, which can lead to double-strand breaks (DSB) and to subsequent chromosome loss. Replication is coordinated with DNA repair systems that operate in cells to remove or tolerate DNA lesions. DNA polymerases can serve as sensors in the cell cycle checkpoint pathways that delay cell division until damaged DNA is repaired and replication is completed. Eukaryotic DNA template-dependent DNA polymerases have different properties adapted to perform an amazingly wide spectrum of DNA transactions. In this review, we discuss the structure, the mechanism, and the evolutionary relationships of DNA polymerases and their possible functions in the replication of intact and damaged chromosomes, DNA damage repair, and recombination.

Control of dTTP pool size by anaphase promoting complex/cyclosome is essential for the maintenance of genetic stability

Anaphase promoting complex/cyclosome (APC/C)-mediated proteolysis is essential for chromosome segregation, mitotic exit, and G1 entry. Here, we show the importance of APC/C in the control of dTTP pool size in mammalian cells. Two enzymes, thymidine kinase 1 (TK1) and thymidylate kinase (TMPK), involved in dTTP formation are the targets of the APC/C pathway. We demonstrate that TMPK is recognized and degraded by APC/C-Cdc20/Cdh1-mediated pathways from mitosis to the early G1 phase, whereas TK1 is targeted for degradation by APC/C-Cdh1 after mitotic exit. Overexpression of wild-type TK1 and TMPK induces a four- to fivefold increase in the cellular dTTP pool without promoting spontaneous mutations in the hprt (hypoxanthine-guanine phosphoribosyl transferase) gene. In contrast, coexpression of nondegradable TK1 and TMPK expands the dTTP pool size 10-fold accompanied by a drastic dNTP pool imbalance. Most interestingly, disruption of APC/C proteolysis of TK1 and TMPK leads to growth retardation and a striking increase in gene mutation rate. We conclude that down-regulation of dTTP pool size by the APC/C pathway during mitosis and the G1 phase is an essential means to maintain a balanced dNTP pool and to avoid genetic instability.

Genotoxic evaluation of the antimalarial drug, fansidar, in cultured human lymphocytes

Fansidar (pyrimethamine-sulfadoxine) has been used extensively worldwide for the treatment of chloroquine resistant Plasmodium falciparum malaria, toxoplasmosis and Pneumocystis carinii pneumonia in patients with the acquired immunodeficiency syndrome. Because of the wide usage of pyrimethamine-sulfadoxine in developing countries and the lake of information from open literature and reports from manufacturers about the genotoxicity of such antimalarial drug, the present work was suggested. The possible genetic toxicity of fansidar has been evaluated in human peripheral blood lymphocyte cultures. The frequencies of sister-chromatid exchanges (SCE) and micronuclei (MN) were scored as genetic endpoints. Both tests covering a wide range of induced genetic damage as primary DNA damage, clastogenicity and aneugenicity. Cultures were set up by using blood samples from two healthy donors and the treatment was done using different fansidar concentrations ranging from 1:20 to 10:200 microg/ml. From our results, it appears that this drug is able to induce moderate genotoxic effects, as revealed by the increases found in SCE and MN frequencies in cultures from the two donors at the two highest concentrations tested (5:100 and 10:200 microg/ml). In addition, cyotoxic/cytostatic effects of fansidar were revealed by a decrease in the proliferative rate index (PRI) and in the cytokinesis block proliferation index (CBPI). Our findings suggest that the use of this drug should be restricted to situations where other antimalarial drugs cannot be used. The drug should never be given to pregnant women.

Common Gene Polymorphisms in the Metabolic Folate and Methylation Pathway and the Risk of Acute Lymphoblastic Leukemia and non-Hodgkin's Lymphoma in Adults

Folate and methionine metabolism is involved in DNA synthesis and methylation processes. Polymorphisms in the genes of folate metabolism enzymes have been associated with some forms of cancer. In a case-control study, we evaluated whether four common polymorphisms in methylenetetrahydrofolate reductase (MTHFR C677T and A1298C), methionine synthase (MS A2756G), and methionine synthase reductase (MTRR A66G) genes may have a role in altering susceptibility to adult acute lymphoblastic leukemia (ALL) and non-Hodgkin's lymphoma (NHL). We analyzed DNA of 120 adult ALL, 200 NHL, and 257 healthy control subjects. Individual carrying the MTHFR 677TT genotype showed a 3.6-fold decreased ALL risk [odds ratio (OR) 0.28, 95% confidence interval (95% CI) 0.12–0.72] than wild-types. Similarly, MS 2756GG individuals showed a 5.0-fold decreased ALL risk (OR 0.20, 95% CI 0.02–1.45) than wild-types. In combined results, subjects with the MTHFR 677CT/TT and MS 2756AG/GG genotypes revealed a 3.6-fold ALL risk reduction (OR 0.28, 95% CI 0.14–0.58) and those with the MTHFR 677TT and MTRR 66AG genotypes revealed a 4.2-fold ALL risk reduction (OR 0.24, 95% CI 0.06–0.81). Finally, those with the MS 2756AG/GG and MTRR 66AG/GG genotypes revealed a 2.2-fold ALL risk reduction (OR 0.45, 95% CI 0.10–0.85). Single analysis for NHL did not show any significant difference for all the polymorphisms investigated, but in the low-grade NHL subgroup, we found a 2.0-fold risk reduction for the MTRR 66GG homozygous genotype (OR 0.50, 95% CI 0.25–0.99), which was higher (OR 0.37, 95% CI 0.14–0.85) when analyzed in combination with MS 2756AA genotype. These data are in accordance with the hypothesis that polymorphisms in the genes for folate and methionine metabolism might play a greater role in the occurrence of ALL than NHL by influencing DNA synthesis and/or DNA methylation.

Mechanisms of DNA damage, DNA hypomethylation, and tumor progression in the folate/methyl-deficient rat model of hepatocarcinogenesis

Using the folate/methyl-deficient rat model of hepatocarcinogenesis, we obtained evidence that may provide new insights into a major unresolved paradox in DNA methylation and cancer research: the mechanistic basis for genome-wide hypomethylation despite an increase in DNA methyltransferase activity and gene-specific regional hypermethylation. Previous studies revealed that the methyltransferase binds with higher affinity to DNA strand breaks, gaps, abasic sites, and uracil than it does to its cognate hemimethylated CpG sites, consistent with its ancestral function as a DNA repair enzyme. These same DNA lesions are an early occurrence in models of folate and methyl deficiency and are often present in human preneoplastic cells. We hypothesized that the high-affinity binding of the maintenance DNA methyltransferase to unrepaired lesions in DNA could sequester available enzyme away from the replication fork and promote passive replication-dependent demethylation. In support of this possibility, we found that lesion-containing DNA is less efficiently methylated than lesion-free DNA from folate/methyl-deficient rats and that an increase in DNA strand breaks precedes DNA hypomethylation. Despite an adaptive increase in DNA methyltransferase activity, hemimethylated DNA from folate/methyl-deficient rats is progressively replaced by double-stranded unmethylated DNA that is resistant to remethylation with dietary methyl repletion. In promoter regions, the inappropriate binding of the DNA methyltransferase to unrepaired lesions or mispairs may promote local histone deacetylation, methylation, and regional hypermethylation associated with tumor suppressor gene silencing. These insights in an experimental model are consistent with the possibility that DNA lesions may be a necessary prerequisite for the disruption of normal DNA methylation patterns in preneoplastic and neoplastic cells.

Mitochondrial DNA depletion can be prevented by dGMP and dAMP supplementation in a resting culture of deoxyguanosine kinase-deficient fibroblasts

Deoxyguanosine kinase is a constitutively expressed, mitochondrial enzyme of the deoxyribonucleoside salvage pathway. Deficiency of deoxyguanosine kinase causes early-onset, hepatocerebral mitochondrial DNA (mtDNA) depletion syndrome. To clarify the molecular mechanism of the disease, a skin fibroblast culture was studied from a patient carrying a homozygous nonsense mutation in the gene for deoxyguanosine kinase. In situ examination of DNA synthesis demonstrated that, although mtDNA synthesis is cell cycle independent in control fibroblasts, mtDNA synthesis occurs mainly during the S-phase in deoxyguanosine kinase-deficient cells. Consistent with this observation, it was found that the mtDNA content of exponentially growing, deoxyguanosine kinase-deficient cells is only mildly affected. When cycling is inhibited by serum-deprivation and cells are in a resting state, however, the mtDNA content drops considerably in deoxyguanosine kinase-deficient cells, yet remains stable in control fibroblasts. The decline in mtDNA content in resting, deoxyguanosine kinase-deficient cells can be prevented by dGMP and dAMP supplementation, providing conclusive evidence that substrate limitation triggers mtDNA depletion in deoxyguanosine kinase-deficient cells.

AID mediates hypermutation by deaminating single stranded DNA

Activation-induced deaminase (AID) is a protein indispensable for the diversification of immunoglobulin (Ig) genes by somatic hypermutation (SHM), class switch recombination (CSR), and gene conversion. To date, the precise role of AID in these processes has not been determined. Here we demonstrate that purified, tetrameric AID can deaminate cytidine residues in DNA, but not in RNA. Furthermore, we show that AID will bind and deaminate only single-stranded DNA, which implies a direct, functional link between hypermutation and transcription. Finally, AID does not target mutational hotspots, thus mutational targeting to specific residues must be attributed to different factors.

Biochemical and molecular aberrations in the rat colon due to folate depletion are age-specific

Elder adulthood and diminished folate status are each associated with an enhanced risk of colorectal carcinogenesis. We therefore examined whether these two factors are mechanistically related. Weanling male Sprague-Dawley rats (n = 44) and 1-y-old rats (n = 44) were each divided into three groups and fed diets containing 0, 4.5 or 18 micro mol folic acid/kg (deplete, replete and supplemented groups, respectively). Rats were killed at 0, 8 and 20 wk. The folate concentrations, the distribution of the different coenzymatic forms of folate, uracil incorporation into DNA and genomic DNA methylation were measured in the colonic mucosa. Folate-deplete and folate-replete elder rats had 30-45% lower colonic folate concentrations than young rats. Furthermore, 5-methyltetrahydrofolate was uniformly depleted in colons of the elder, folate-deplete rats, whereas this depletion occurred in only a minority of the younger rats. By the end of the experiment, the folate-deplete and folate-replete elder rats had approximately 50% more uracil incorporated into their colonic DNA than the corresponding young groups (P < 0.05). In elder rats, this uracil misincorporation was incremental across the three diet groups (P-test for trend < 0.05), whereas no excess uracil incorporation was observed in young rats. Neither age nor dietary folate affected genomic DNA methylation in the colon. In conclusion, the colon of elder rats is more susceptible to biochemical and molecular consequences of folate depletion than that of young rats. However, folate supplementation is as effective at sustaining adequate colonic folate status in elder rats as it is in the young.

Polymorphisms in the thymidylate synthase and serine hydroxymethyltransferase genes and risk of adult acute lymphocytic leukemia

We previously reported that 2 polymorphisms in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene at positions C677T and A1298C were associated with lower risk of adult acute lymphocytic leukemia (ALL). In the present study, we have examined whether polymorphisms in other folate-metabolizing genes play a role in ALL susceptibility. Polymorphisms in methionine synthase (MS A2756G), cytosolic serine hydroxymethyltransferase (SHMT1 C1420T), and a double (2R2R) or triple (3R3R) 28-bp tandem repeat in the promoter region of thymidylate synthase (TS) were studied and found to modulate ALL risk. In a univariate analysis, SHMT1 1420CT individuals exhibited a 2.1-fold decrease in ALL risk (odds ratio [OR] = 0.48; 95% confidence interval [CI], 0.25-0.91), whereas the 1420TT genotype conferred a 3.3-fold reduction in risk (OR = 0.31; 95% CI, 0.10-0.90). Similarly, TS 2R3R individuals exhibited a 2.8-fold reduction in ALL risk (OR = 0.36; 95% CI: 0.16-0.83), while the TS 3R3R genotype conferred an even greater level of protection (OR = 0.25; 95% CI, 0.08-0.78). However, no significant associations were evident for the MS 2756AG polymorphism (OR = 0.79; 95% CI, 0.38-1.7). In addition, potential interactions between the SHMT1 and TS or MS genes were observed. TS 3R3R individuals who were SHMT1 1420CT/TT had a 13.9-fold decreased ALL risk (OR = 0.072; 95% CI, 0.0067-0.77). Further, MS 2756AG individuals who were SHMT1 1420CT/TT had a 5.6-fold reduction in ALL risk (OR = 0.18; 95% CI, 0.05-0.63). This study suggests an important role for uracil misincorporation and resultant chromosomal damage in the pathogenesis of ALL, and that genetic interactions involving low penetrance polymorphisms in folate-metabolizing genes may increase ALL risk.

Structure and function of the radical enzyme ribonucleotide reductase

Ribonucleotide reductases (RNRs) catalyze all new production in nature of deoxyribonucleotides for DNA synthesis by reducing the corresponding ribonucleotides. The reaction involves the action of a radical that is produced differently for different classes of the enzyme. Class I enzymes, which are present in eukaryotes and microorganisms, use an iron center to produce a stable tyrosyl radical that is stored in one of the subunits of the enzyme. The other classes are only present in microorganisms. Class II enzymes use cobalamin for radical generation and class III enzymes, which are found only in anaerobic organisms, use a glycyl radical. The reductase activity is in all three classes contained in enzyme subunits that have similar structures containing active site cysteines. The initiation of the reaction by removal of the 3'-hydrogen of the ribose by a transient cysteinyl radical is a common feature of the different classes of RNR. This cysteine is in all RNRs located on the tip of a finger loop inserted into the center of a special barrel structure. A wealth of structural and functional information on the class I and class III enzymes can now give detailed views on how these enzymes perform their task. The class I enzymes demonstrate a sophisticated pattern as to how the free radical is used in the reaction, in that it is only delivered to the active site at exactly the right moment. RNRs are also allosterically regulated, for which the structural molecular background is now starting to be revealed.

Nucleoside analogues: mechanisms of drug resistance and reversal strategies

Nucleoside analogues (NA) are essential components of AML induction therapy (cytosine arabinoside), effective treatments of lymphoproliferative disorders (fludarabine, cladribine) and are also used in the treatment of some solid tumors (gemcitabine). These important compounds share some general common characteristics, namely in terms of requiring transport by specific membrane transporters, metabolism and interaction with intracellular targets. However, these compounds differ in regard to the types of transporters that most efficiently transport a given compound, and their preferential interaction with certain targets which may explain why some compounds are more effective against rapidly proliferating tumors and others on neoplasia with a more protracted evolution. In this review, we analyze the available data concerning mechanisms of action of and resistance to NA, with particular emphasis on recent advances in the characterization of nucleoside transporters and on the potential role of activating or inactivating enzymes in the induction of clinical resistance to these compounds. We performed an extensive search of published in vitro and clinical data in which the levels of expression of nucleoside-activating or inactivating enzymes have been correlated with tumor response or patient outcome. Strategies aiming to increase the intracellular concentrations of active compounds are presented.

Protection by various deoxynucleosides against deoxyadenosine-induced DNA damage in adenosine deaminase-inactivated lymphocytes

Adenosine deaminase deficiency is an inborn error resulting in immunodeficiency. The pathogenesis of the lymphopenia is not fully understood. Intracellular increases in dATP in the absence of deamination retard DNA repair in human resting lymphocytes and results in the slow accumulation of DNA strand breaks. We focused on the relationship between DNA damage and DNA precursor pools in cultures of deoxycoformycin-treated, ADA-inhibited resting lymphocytes. The addition of 10 microM deoxyadenosine led to a substantial number of DNA strand breaks within 12 h, breaks equivalent to those which occur with about 190 rad irradiation. Addition of any of the other deoxynucleosides used partially prevented this dAdo-induced DNA damage and promoted DNA repair. However, the preventive effects did not correlate inversely with intracellular dATP levels. Resting lymphocytes have very small dNTP pools. Treatment with dAdo slightly reduced dTTP and dCTP. Three kinds of deoxynucleosides, other than dAdo, restored or raised the corresponding dNTP level but the pool imbalance was only minimally corrected. Regarding the toxic effects of dAdo in ADA deficiency, not only dATP levels but also dNTP pool balance has a crucial role in the pathogenesis. Pool sizes of dTTP, dCTP, and possibly dGTP must be maintained at normal levels, if dAdo-induced DNA damage is to be avoided.

Genotoxic evaluation of the antimicrobial drug, trimethoprim, in cultured human lymphocytes

The antimicrobial drug, trimethoprim, was evaluated for genotoxicity in human peripheral blood lymphocyte cultures set-up from two healthy donors. Sister-chromatid exchanges (SCE) and micronuclei (MN) were scored as genetic endpoints. The treatment was done using different trimethoprim concentrations ranging from 1 to 100 microg/ml. From our results, we can conclude that this drug is able to induce both cytotoxic and moderate genotoxic effects, as revealed by the increases seen in SCE and MN frequencies in cultures from the two donors and, at least, at one of the concentrations tested.

Effects of dNTPs on the SCE frequency in plant root tip cells

The effects of deoxynucleoside triphosphate (dNTPs) on the frequency of sister-chromatid exchange (SCE) have been studied in plant root tip cells. Treatment with dATP caused a dose-dependent decrease in SCE frequency, while dGTP, caused a dose-dependent increase in SCE frequency both in Hordeum vulgare and in Vicia faba root tip cells. Treatment with dCTP and dTTP significantly increased SCE frequency in H. vulgare root tip cells, but significantly decreased SCE frequency in V. faba root tip cells. The SCE induced by treatment with dGTP could be totally reversed by treatment with an equal concentration of any of the other three dNTPs (dATP, dCTP and dTTP) both in H. vulgare and in V. faba. Treatment with a mixed equal concentration of dATP, dCTP, dGTP and dTTP did not alter the SCE frequency. These results suggest that dNTP pool imbalance is an SCE-affecting factor in plant cells.

Binding of allosteric effectors to ribonucleotide reductase protein R1: reduction of active-site cysteines promotes substrate binding

BACKGROUND

Ribonucleotide reductase (RNR) is an essential enzyme in DNA synthesis, catalyzing all de novo synthesis of deoxyribonucleotides. The enzyme comprises two dimers, termed R1 and R2, and contains the redox active cysteine residues, Cys462 and Cys225. The reduction of ribonucleotides to deoxyribonucleotides involves the transfer of free radicals. The pathway for the radical has previously been suggested from crystallographic results, and is supported by site-directed mutagenesis studies. Most RNRs are allosterically regulated through two different nucleotide-binding sites: one site controls general activity and the other controls substrate specificity. Our aim has been to crystallographically demonstrate substrate binding and to locate the two effector-binding sites.

RESULTS

We report here the first crystal structure of RNR R1 in a reduced form. The structure shows that upon reduction of the redox active cysteines, the sulfur atom of Cys462 becomes deeply buried. The more accessible Cys225 moves to the former position of Cys462 making room for the substrate. In addition, the structures of R1 in complexes with effector, effector analog and effector plus substrate provide information about these binding sites. The substrate GDP binds in a cleft between two domains with its beta-phosphate bound to the N termini of two helices; the ribose forms hydrogen bonds to conserved residues. Binding of dTTP at the allosteric substrate specificity site stabilizes three loops close to the dimer interface and the active site, whereas the general allosteric binding site is positioned far from the active site.

CONCLUSIONS

Binding of substrate at the active site of the enzyme is structurally regulated in two ways: binding of the correct substrate is regulated by the binding of allosteric effectors and binding of the actual substrate occurs primarily when the active-site cysteines are reduced. One of the loops stabilized upon binding of dTTP participates in the formation of the substrate-binding site through direct interaction with the nucleotide base. The general allosteric effector site, located far from the active site, appears to regulate subunit interactions within the holoenzyme.

Vitamins as antimutagens: advantages and some possible mechanisms of antimutagenic action

Dietary natural inhibitors of mutagenesis and carcinogenesis are of particular importance because they may be useful for human cancer prevention and do not have undesirable xenobiotic effects on living organisms. As was shown in numerous experiments, many endogenous substances, usually obtained in food or synthesized by cells, possess some inhibitory activity towards natural or man-made environmental mutagens which often induce increased frequency of cancer. Among such substances are vitamins, thiol compounds, porphyrin derivatives, polyphenols and others, the antigenotoxicity of which is well established in various genetic tests. Probably a number of these compounds are included in the defense systems of organisms protecting them from harmful exogenous influences continuously affecting genetic material and other components of cells. Some vitamins show protective effects; for example, E, A and C vitamins are active against well-known mutagens both in vitro and in vivo. Genetic properties of other vitamins have been insufficiently explored, but positive results were obtained for a number of them suggesting the desirability of further studies in this field. Synergism of some vitamins activity, both with other vitamins and non-vitamin substances, is of particular interest because clarifying some of their mechanisms of action could be important for understanding the functions of our defense systems.

Effects of dNTPs on the sister-chromatid exchange (SCE) frequency in human lymphocytes

The effects of deoxynucloside triphosphates (dNTPs) on the frequency of sister-chromatid exchange (SCE) was studied in human peripheral lymphocytes. Treatment with each dCTP and dTTP did not change the SCE frequency; however, dGTP caused a significant dose-dependent increase in SCE frequency, whereas dATP caused a significantly decreased in SCE frequency. The SCE-increasing effect of the treatment with dGTP could be totally reversed by treatment with equal concentrations of dATP or dCTP. Treatment with a mixture of four equal concentrations of dATP, dGTP, dCTP, dTTP did not alter the SCE frequency. These results suggest that the dNTP pool imbalance was a SCE-effecting factor in human lymphocytes, and that dGTP may be mainly responsible for this effect.

Inhibitors of thymine nucleotide biosynthesis: antimetabolites that provoke genetic change via primary non-DNA targets

Folate antagonists and direct-acting inhibitors of thymidylate synthase are potent genotoxic antimetabolites. These agents induce genetic change not by attacking DNA, but by interfering with the control of DNA precursor metabolism. This review surveys the genetic effects attributable to selected representatives of this class of antimetabolites.

On the phosphorylation of 2-chlorodeoxyadenosine (CdA) and its correlation with clinical response in leukemia treatment

The nucleoside analog 2-chlorodeoxyadenosine (CdA, Cladribine) is a chemotherapeutic agent for treatment of leukemias and lymphomas, most successfully used in hairy cell leukemia and B-cell chronic lymphocytic leukemia. CdA is phosphorylated intracellularly to its monophosphate derivative by the enzymes deoxycytidine kinase and deoxyguanosine kinase. Cell lines deficient in deoxycytidine kinase were shown to be resistant to CdA and a high deoxycytidine kinase level in combination with low 5'-nucleotidase has been proposed to partly explain the selectivity in CdA toxicity for lymphoid cells. In this report biochemical properties in CdA phosphorylation mediated by deoxycytidine kinase and deoxyguanosine kinase are reviewed and discussed in relation to the further metabolism of CdA 5'-monophosphate, the different possible mechanisms of action and the correlation with clinical response. It is concluded that much is known about the metabolism and mechanisms of action of CdA, but that the remarkable therapeutic effect in hairy cell leukemia has yet to be explicitly explained.

Thymidine kinase deficient cells with decreased TTP pools are hypersensitive to DNA alkylating agents

The effect of mutational loss of thymidine kinase (TK) on the sensitivity to alkylating agents was investigated in promyelocytic, HL-60, and T-lymphoblastoid, Molt-3, human leukemia cell lines. Although both cell lines exhibited approx. 1% residual TK activity, only HL-60 TK deficient cells had a decreased intracellular TTP pool, i.e., 20% of that of the wild-type. When treated with N-methyl-N'-nitronitrosoguanidine or ethyl methanesulfonate, HL-60 TK deficient cells showed significantly increased killing and mutation frequencies at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus relative than did wild-type. Pretreatment of cells with O6-benzylguanine, an inhibitor of O6-alkylguanine-DNA alkyltransferase, partially abolished those differences. Molt-3 wild-type and TK deficient cells had similar cell survivals and HGPRT mutation frequencies following treatment with alkylating agents. These results indicate that TK deficiency, only when a concomitant decrease of TTP pool is detected, plays a pivotal role in the sensitivity to the cytotoxic and mutagenic effects of alkylating agents.

Absence of misincorporation of pyrimidines in DNA after treatment with a combination of cisplatin (CIS-diammine-dichloro-platinum) amd 5-fluorouracil of mouse sarcoma cells

The effects on incorporation into DNA of the deoxyribonucleotides dCTP and dTTP and the DNA synthesis rate after treatment with cisplatin (CDDP), 5-fluorouracil (5-FU) or a combination of CDDP and 5-FU were studied in ascites sarcoma (Bp8) growing in mice. Single administration of CDDP gave an early (1 h) transient increase in the DNA-synthesis followed by a decrease. 5-FU as single agent did increase the rate of DNA synthesis after 6 h with a maximum at 10 h. The combination of CDDP and 5-FU markedly increased the rate of DNA synthesis up to 6 h as compared to single drug treatment. Although the dCTP pool increased after combined treatment, while the dTTP pool was unchanged, no alterations in the proportions of dTTP and dCTP incorporated into DNA could be detected. Hence, misincorporation of pyrimidines is not the mechanism for the synergistic effect of the combination of CDDP and 5-FU.