DNA damage by C1027 involves hydrogen atom abstraction and addition to nucleobases

Histone tails decrease N7-methyl-2'-deoxyguanosine depurination and yield DNA-protein cross-links in nucleosome core particles and cells

Monofunctional alkylating agents preferentially react at the N7 position of 2'-deoxyguanosine in duplex DNA. Methylated DNA, such as that produced by methyl methanesulfonate (MMS) and temozolomide, exists for days in organisms. The predominant consequence of N7-methyl-2'-deoxyguanosine (MdG) is widely believed to be abasic site (AP) formation via hydrolysis, a process that is slow in free DNA. Examination of MdG reactivity within nucleosome core particles (NCPs) provided two general observations. MdG depurination rate constants are reduced in NCPs compared with when the identical DNA sequence is free in solution. The magnitude of the decrease correlates with proximity to the positively charged histone tails, and experiments in NCPs containing histone variants reveal that positively charged amino acids are responsible for the decreased rate of abasic site formation from MdG. In addition, the lysine-rich histone tails form DNA-protein cross-links (DPCs) with MdG. Cross-link formation is reversible and is ascribed to nucleophilic attack at the C8 position of MdG. DPC and retarded abasic site formation are observed in NCPs randomly damaged by MMS, indicating that these are general processes. Histone-MdG cross-links were also detected by mass spectrometry in chromatin isolated from V79 Chinese hamster lung cells treated with MMS. The formation of DPCs following damage by a monofunctional alkylating agent has not been reported previously. These observations reveal the possibility that such DPCs may contribute to the cytotoxicity of monofunctional alkylating agents, such as MMS, N-methyl-N-nitrosourea, and temozolomide.

Enediynes bearing polyfluoroaryl sulfoxide as new antiproliferative agents with dual targeting of microtubules and DNA

A novel series of enediynes possessing pentafluorophenylsulfoxide have been developed. The innovative compounds possess antiproliferative activity against a broad panel of human cancer cells originating from breast, blood, lung, kidney, colon, prostate, pancreas or skin with IC₅₀ ranging from 0.6 to 3.4 μM. The antiproliferative activity of enediynes in darkness is associated to their ability to compromise microtubule network. In addition, exposure to UV leads to double-stranded DNA cleavage caused by the newly synthesized molecules reducing further their IC₅₀ in nanomolar range against human tumor cells, including chemo-resistant pancreatic cancer cells. Taken together, the examined data demonstrate that enediynes possessing pentafluorosulfoxide are promising molecules in the cancer therapy.

Pyrimidine Nucleobase Radical Reactivity in DNA and RNA

Nucleobase radicals are major products of the reactions between nucleic acids and hydroxyl radical, which is produced via the indirect effect of ionizing radiation. The nucleobase radicals also result from hydration of cation radicals that are produced via the direct effect of ionizing radiation. The role that nucleobase radicals play in strand scission has been investigated indirectly using ionizing radiation to generate them. More recently, the reactivity of nucleobase radicals resulting from formal hydrogen atom or hydroxyl radical addition to pyrimidines has been studied by independently generating the reactive intermediates via UV-photolysis of synthetic precursors. This approach has provided control over where the reactive intermediates are produced within biopolymers and facilitated studying their reactivity. The contributions to our understanding of pyrimidine nucleobase radical reactivity by this approach are summarized.

Reactivity of Nucleic Acid Radicals

Nucleic acid oxidation plays a vital role in the etiology and treatment of diseases, as well as aging. Reagents that oxidize nucleic acids are also useful probes of the biopolymers' structure and folding. Radiation scientists have contributed greatly to our understanding of nucleic acid oxidation using a variety of techniques. During the past two decades organic chemists have applied the tools of synthetic and mechanistic chemistry to independently generate and study the reactive intermediates produced by ionizing radiation and other nucleic acid damaging agents. This approach has facilitated resolving mechanistic controversies and lead to the discovery of new reactive processes.

Major Contributions towards Finding a Cure for Cancer through Chemotherapy: A Historical Review

The history of cancer chemotherapy is as old as cancer itself. With the increase in the complexities of cancer and the development of resistance towards existing anticancer agents, increased attention is now being paid to the advancement of chemotherapy. Some chemotherapeutic agents were discovered by accident or trial-and-error methods while others were found to be useful for neoplasia when they were being evaluated for some other purpose. Broadly, these agents have been classified as alkylating agents, antimetabolites, platinum compounds, antitumor antibiotics and natural products. Hormones and compounds interfering with hormone metabolism are widely used in cancer treatment, besides monoclonal antibodies and small molecules targeting angiogenesis. In this review an attempt is made to discuss the major breakthroughs that have shaped the course of cancer chemotherapy, helping to decrease the mortality as well as lessen the suffering of patients.

Double-strand breaks from a radical commonly produced by DNA-damaging agents

Double-strand breaks are widely accepted to be the most toxic form of DNA damage. Molecules that produce double-strand breaks via a single chemical event are typically very cytotoxic and far less common than those that form single-strand breaks. It was recently reported that a commonly formed C4′-radical produces double-strand breaks under aerobic conditions. Experiments described herein indicate that a peroxyl radical initiates strand damage on the complementary strand via C4′-hydrogen atom abstraction. Inferential evidence suggests that a C3′-peroxyl radical induces complementary strand damage more efficiently than does a C4′-peroxyl radical. Complementary strand hydrogen atom abstraction by the peroxyl radical is efficiently quenched by thiols. This mechanism could contribute to the higher than expected yield of double-strand breaks produced by ionizing radiation.

The radiomimetic enediyne, 20'-deschloro-C-1027 induces inter-strand DNA crosslinks in hypoxic cells and overcomes cytotoxic radioresistance

The ability of the radiomimetic anti-tumor enediyne C-1027 to induce DNA inter-strand crosslinks (ICLs), in addition to the expected DNA strand breaks, is unique among traditional DNA targeted cancer therapies. Importantly, radiation therapy and most radiomimetic drugs have diminished effect in hypoxic environments due to decreased induction of DNA strand breaks, which is an oxygen requiring process. However, C-1027's induction of ICLs is enhanced under hypoxia and it is actually more potent against hypoxic cells, overcoming this common tumor resistance mechanism. In this study, an analog of C-1027, 20'-deschloro-C-1027 was examined for its ability to induce DNA ICLs under hypoxic conditions. Deschloro-induced ICLs were detected under hypoxic cell-free conditions, with a concomitant reduction in the induction of DNA strand breaks. In cells deschloro behaved similarly, inducing cellular ICLs under hypoxic conditions with a reduction in DNA breaks. The cytotoxicity of deschloro treatment was similar in normoxic and hypoxic cells, suggesting that the ICL induction allows deschloro to retain its cytotoxic activity under hypoxia. It appears that rational engineering of the C-1027 family of radiomimetics holds promise toward overcoming the radioresistance associated with the hypoxic environment associated with solid tumors.

DNA damage by histone radicals in nucleosome core particles

Although DNA binding proteins shield the genetic material from diffusible reactive oxygen species by reacting with them, the resulting protein (peroxyl) radicals can oxidize the bound DNA. To explore this possible DNA damage by protein radicals, histone H4 proteins containing an azoalkane radical precursor at defined sites were prepared. Photolysis of a nucleosome core particle containing the modified protein produces DNA damage that is consistent with selective C4′-oxidation. The nucleotide(s) damaged is highly dependent on proximity to the protein radical. These experiments provide insight into the effects of oxidative stress on protein-bound DNA, revealing an additional layer of complexity concerning nucleic acid damage.

5,6-Dihydropyrimidine peroxyl radical reactivity in DNA

Nucleobase radicals are a major family of reactive species produced in DNA as a result of oxidative stress. Two such radicals, 5-hydroxy-5,6-dihydrothymidin-6-yl radical (1) and 5,6-dihydrouridin-6-yl radical (5), were independently generated within chemically synthesized oligonucleotides from photochemical precursors. Neither nucleobase radical produces direct strand breaks or alkali-labile lesions in single or double stranded DNA. The respective peroxyl radicals, resulting from O₂ trapping, add to 5′-adjacent nucleobases, with a preference for dG. Distal dG’s are also oxidatively damaged by the peroxyl radicals. Experiments using a variety of sequences indicate that distal damage occurs via covalent modification of the 5′-adjacent dG, but there is no evidence for electron transfer by the nucleobase peroxyl radicals.

DNA double strand cleavage via interstrand hydrogen atom abstraction

Double strand breaks (DSBs) are the most deleterious form of DNA damage. Natural products that produce them are potent cytotoxic agents. Designing molecules that produce DSBs via a single chemical event is challenging. We determined that formation of a C4'-nucleotide radical in duplex DNA under aerobic conditions gives rise to a DSB. The original radical yields a strand break containing a peroxyl radical, which initiates opposite strand cleavage via C4'-hydrogen atom abstraction. This mechanism provides the impetus to design DNA damaging agents that produce DSBs by abstracting a single hydrogen atom from the biopolymer.

Human DNA polymerase β, but not λ, can bypass a 2-deoxyribonolactone lesion together with proliferating cell nuclear antigen

The C1'-oxidized lesion 2-deoxyribonolactone (L) is induced by free radical attack of DNA. This lesion is mutagenic, inhibits base excision repair, and can lead to strand scission. In double-stranded DNA L is repaired by long-patch base excision repair, but it induces replication fork arrest in a single-strand template. Translesion synthesis requires a specialized DNA polymerase (Pol). In E. coli, Pol V is responsible for bypassing L, whereas in yeast Pol ζ has been shown to be required for efficient bypass. Very little is known about the identity of human Pols capable of bypassing L. For instance, the activity of family X enzymes has never been investigated. We examined the ability of different family X Pols: Pols β, λ, and TdT from human cells and Pol IV from S. cerevisiae to act on DNA containing an isolated 2-deoxyribonolactone, as well as when the lesion comprises the 5'-component of a tandem lesion. We show that Pol β, but not Pol λ, can bypass a single L lesion in the template, and its activity is increased by the auxiliary protein proliferating cell nuclear antigen (PCNA), whereas both enzymes were completely blocked by a tandem lesion. Yeast Pol IV was able to bypass the single L and the tandem lesion but with little nucleotide insertion specificity. Finally, L did not affect the polymerization activity of the template-independent enzyme TdT.