Although DNA is the carrier of genetic information, it has limited chemical stability. Hydrolysis, oxidation and nonenzymatic methylation of DNA occur at significant rates in vivo, and are counteracted by specific DNA repair processes. The spontaneous decay of DNA is likely to be a major factor in mutagenesis, carcinogenesis and ageing, and also sets limits for the recovery of DNA fragments from fossils.

Instability and decay of the primary structure of DNA

The clinical use of anthracyclines like doxorubicin and daunorubicin can be viewed as a sort of double-edged sword. On the one hand, anthracyclines play an undisputed key role in the treatment of many neoplastic diseases; on the other hand, chronic administration of anthracyclines induces cardiomyopathy and congestive heart failure usually refractory to common medications. Second-generation analogs like epirubicin or idarubicin exhibit improvements in their therapeutic index, but the risk of inducing cardiomyopathy is not abated. It is because of their janus behavior (activity in tumors vis-a-vis toxicity in cardiomyocytes) that anthracyclines continue to attract the interest of preclinical and clinical investigations despite their longer-than-40-year record of longevity. Here we review recent progresses that may serve as a framework for reappraising the activity and toxicity of anthracyclines on basic and clinical pharmacology grounds. We review 1) new aspects of anthracycline-induced DNA damage in cancer cells; 2) the role of iron and free radicals as causative factors of apoptosis or other forms of cardiac damage; 3) molecular mechanisms of cardiotoxic synergism between anthracyclines and other anticancer agents; 4) the pharmacologic rationale and clinical recommendations for using cardioprotectants while not interfering with tumor response; 5) the development of tumor-targeted anthracycline formulations; and 6) the designing of third-generation analogs and their assessment in preclinical or clinical settings. An overview of these issues confirms that anthracyclines remain "evergreen" drugs with broad clinical indications but have still an improvable therapeutic index.

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Anthracyclines: Molecular Advances and Pharmacologic Developments in Antitumor Activity and Cardiotoxicity

Oxidative DNA damage is an inevitable consequence of cellular metabolism, with a propensity for increased levels following toxic insult. Although more than 20 base lesions have been identified, only a fraction of these have received appreciable study, most notably 8-oxo-2'deoxyguanosine. This lesion has been the focus of intense research interest and been ascribed much importance, largely to the detriment of other lesions. The present work reviews the basis for the biological significance of oxidative DNA damage, drawing attention to the multiplicity of proteins with repair activities along with a number of poorly considered effects of damage. Given the plethora of (often contradictory) reports describing pathological conditions in which levels of oxidative DNA damage have been measured, this review critically addresses the extent to which the in vitro significance of such damage has relevance for the pathogenesis of disease. It is suggested that some shortcomings associated with biomarkers, along with gaps in our knowledge, may be responsible for the failure to produce consistent and definitive results when applied to understanding the role of DNA damage in disease, highlighting the need for further studies.

Oxidative DNA damage: mechanisms, mutation, and disease

Endospores of Bacillus spp., especially Bacillus subtilis, have served as experimental models for exploring the molecular mechanisms underlying the incredible longevity of spores and their resistance to environmental insults. In this review we summarize the molecular laboratory model of spore resistance mechanisms and attempt to use the model as a basis for exploration of the resistance of spores to environmental extremes both on Earth and during postulated interplanetary transfer through space as a result of natural impact processes.

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Resistance of Bacillus endospores to extreme terrestrial and extraterrestrial environments.

Mechanisms in photodynamic therapy: part one—-photosensitizers, photochemistry and cellular localization

A survey of the recent aspects of the main photoreactions induced by far-UV radiation in cellular DNA is reported. This mostly includes the formation of cyclobutadipyrimidines, pyrimidine(6-4)pyrimidone photoadducts and related Dewar valence isomers in various eukaryotic and prokaryotic cells, as monitored by using either specific or more general assays. Information is also provided on mechanistic aspects regarding the formation of 5,6-dihydro-5-(α-thyminyl) thymine, the so-called “spore photoproduct” within far-UV-irradiated bacterial spores. The second major topic of the review deals with the effects of near-UV radiation and visible light on cellular DNA which are mostly mediated by photosensitizers. The main photoreactions of furocoumarins with DNA, one major class of photosensitizers used in the phototherapy of skin diseases, involve a [2+2] cycloaddition to the thymine bases according to an oxygen-independent mechanism. In contrast a second type of photosensitized reaction which appears to play a major role in the genotoxic effects of both near-UV and visible light requires the presence of oxygen. The photodynamic effects which are mediated by either still unindentified endogenous photosensitizers or defined exogenous photosensitizers lead to the formation of a wide spectrum of DNA modifications including base damage, oligonucleotide strand breaks and DNA—protein cross-links.

Photochemistry of nucleic acids in cells.

Several mechanisms are likely to be involved in the solar radiation-mediated modifications of cellular DNA. Direct excitation of DNA bases by the UVB component (290-320 nm) of solar light gives rise, mostly through oxygen independent reactions, to the formation of dimeric pyrimidine lesions including cyclobutadipyrimidines, pyrimidine (6-4) pyrimidone photoproducts and related valence Dewar isomers. In addition, photoexcitation of cytosine and guanine may lead to the formation in relatively minor yields of 6-hydroxy-5,6-dihydrocytosine and 8-oxo-7,8-dihydroguanine, respectively. A second mechanism that requires the participation of endogenous photosensitizers together with oxygen is at the origin of most of the DNA damage generated by the UVA (320-400 nm) and visible light. Singlet oxygen, which arises from a type II mechanism, is likely to be mostly involved in the formation of 8-oxo-7,8-dihydroguanine that was observed within both isolated and cellular DNA. However, it may be expected that the latter oxidized purine lesion together with DNA strand breaks and pyrimidine base oxidation products are also generated with a lower efficiency through Fenton type reactions. A more definitive assessment of these mechanisms would require further studies aimed at the identification and quantification of the different DNA photolesions including both dimeric pyrimidine photoproducts and photooxidized lesions.

Effects of UV and visible radiation on DNA-final base damage.

Isotope dilution high-performance liquid chromatography–electrospray tandem mass spectrometry assay for the measurement of 8-oxo-7,8-dihydro-2′-deoxyguanosine in biological samples

Photosensitized reactions of nucleic acids

Gas chromatography coupled to electron impact mass spectrometry (GC-EIMS) analysis following hydrolysis of DNA is a widely used assay for the detection of oxidized nucleobases and nucleosides. However, evidence was recently provided for an oxidation of guanine residues of hydrolysed DNA during the silylation prior to GC-EIMS analysis. This reaction accounts for the overestimation of the yield of 8-oxo-7,8-dihydroguanine by GC-EIMS. In the present work, we showed that adenine, cytosine, thymine and thymidine also give rise to oxidized derivatives during the derivatization. This was inferred from the measurement of the amount of 5-formyluracil, 5-hydroxymethyluracil, 5-hydroxycytosine (5-OHCyt), 8-oxo-7,8-dihydroadenine (8-OxoAde) and 5-hydroxymethyl-2'-deoxyuridine (5-HMdUrd) in a series of experiments based on the use of purified bases and nucleosides. Isotopically labelled oxidized bases and 5-HMdUrd were used as internal standards to control the quantitative aspect of the silylation reaction. Support for an artefactual oxidation of hydrolysed DNA was provided by the comparison of the amount of 8-OxoAde and 5-OHCyt detected within native and gamma-irradiated DNA by HPLC-EC and GC-EIMS. To prevent the artefactual formation of oxidized bases during the silylation, an approach based on an HPLC prepurification was developed to remove the precursors of the oxidized bases measured in the DNA sample. The HPLC/GC-EIMS assay was successfully applied to the quantitation of 8-OxoAde and 5-OHCyt in calf thymus DNA. In addition, the detection of the dose-dependent formation of 5-HMdUrd in isolated DNA exposed to ionizing radiation was achieved using the same approach.

Jean Cadet

Papers

Photochemistry of nucleic acids in cells.

Effects of UV and visible radiation on DNA-final base damage.

Isotope dilution high-performance liquid chromatography–electrospray tandem mass spectrometry assay for the measurement of 8-oxo-7,8-dihydro-2′-deoxyguanosine in biological samples

Photosensitized reactions of nucleic acids

Observation and prevention of an artefactual formation of oxidized DNA bases and nucleosides in the GC-EMS method