Chapter Six - Bulky DNA Adducts, Tobacco Smoking, Genetic Susceptibility, and Lung Cancer Risk
Introduction
The generation of bulky DNA adducts consists of conjugates formed between large reactive electrophiles and DNA-binding sites [1], [2]. The term “bulky DNA adducts” comes from early experiments that employed a 32P-DNA postlabeling approach for damage analysis and refers to all the relatively large aromatic and nonpolar carcinogen DNA adducts, such as those induced from polycyclic aromatic hydrocarbons (PAH) and aromatic amines [3], [4], [5], [6]. These are well-known carcinogens present in cigarette smoke and motor vehicle exhaust and fumes from industrial processes and residential heating [7], [8], [9].
Interest in the analysis of the bulky DNA adducts is derived from early experimental animal models that demonstrated that DNA damage is necessary but insufficient for cancer development [2], [10], [11]. At the beginning of the 21st century, most evidence supported the notion that exposure to environmental carcinogens [3], [5], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], including cigarette smoke [1], [3], [5], [29], [30], [31], [32], [33], [34], [35], resulted in alterations to the structural integrity of DNA, i.e., bulky DNA adducts and oxidative DNA damage [1], [12], [19], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. Unless repaired, DNA lesions may lead to mutations in oncogenes and tumor suppressor genes [42], [47], thereby initiating carcinogenesis [48]. As such, bulky DNA adducts are widely considered as biomarkers that reflect carcinogen exposure [1], [15], [30], [37], [49], [50], [51], [52], [53] and cancer risk [1], [29], [33], [50], [54], [55], [56], [57], [58], [59], [60].
This chapter focuses on measurement of bulky DNA adducts by 32P-postlabeling, an assay long used to assess the association between genetic damage and carcinogen exposure and the predictive value of adducts for cancer risk (Fig. 1). Included is a description of the cigarette smoke constituents responsible for the bulky DNA adduct generation. Underlying mechanisms by which tobacco smoke carcinogens are metabolically activated to reactive electrophiles capable of interacting with DNA are also discussed. The chapter subsequently focuses on the history of chemical carcinogenesis and the description of 32P-postlabeling studies as well as the influence of inherited and acquired susceptibilities, especially dietary habits and DNA polymorphisms. Finally, the dose–response relationship with lung cancer, including by pooled and meta-analysis, are reviewed.
Section snippets
DNA Adduct Detection
Over time, methods to assess DNA damage have been developed for experimental animal and human studies [1], [3], [20], [61], [62], [63], [64]. Historically, administration of labeled chemicals was the method of choice whereas the study of DNA adducts was extremely rare. Chronic carcinogen exposure was difficult to model due to the high cost of synthetic radiolabeled (3H and 14C) compounds and the complexity of procedures used to assess DNA damage. In 1981, Randerath et al.[65] developed the 32
Epidemiology
In the past century, lung cancer was extremely rare and represented < 1% of all cancer cases [82], [83]. In the late 19th century, widespread cigarette use resulted in a tremendous increase in lung cancer incidence in Western countries [84]. A 1950 study of British physicians by Doll and Hill [85] provided the first evidence linking smoking and lung cancer. Smokers died about 10 years earlier than nonsmokers. The role of tobacco smoking in cancer etiology has accumulated [84], [86] including
Chemical Carcinogenesis
In 1947, the process of carcinogenesis was studied by Berenblum and Shubik in experimental animals [94]. PAH were sequentially and topically applied to rodents, followed by repeated treatment with croton oil, causing an irreversible transformation of normal cells into tumor. These authors conclusively demonstrated increased cancer frequency in treated vs untreated animals. In 1947, Miller and Miller [95] showed for the first time the binding of chemical carcinogens in rodent liver nuclear
DNA Adducts by 32P-Postlabeling
In 1986, Randerath et al.[106], demonstrated the production of bulky DNA adducts in various tissues of smokers by 32P-postlabeling. The occurrence of genotoxic events with smoking habits prompted the investigation of bulky DNA adducts in the respiratory tract of smokers [1], i.e., the main site of cigarette smoking-related cancer [8], [9]. A large number of studies (n = 25) has been performed thus far (Table 1) [31], [33], [55], [77], [80], [81], [107], [108], [109], [110], [111], [112], [113],
DNA Adduct Variability
High variability in bulky DNA adducts was found among persons exposed to similar amounts of carcinogens. Variation was associated with various parameters including gender, age, genetic susceptibilities, and dietary habit [21], [22], [32], [52], [116], [120], [137], [138], [139], [140], [141], [142], [143], [144], [145]. An earlier age of onset of smoking was reported to influence the increase and the persistence of adducts in exsmokers [117]. Polymorphisms in the 15q25 locus was linked with the
Dose–Response Relationships
Dose–response is a concept fundamental to our understanding of chemical carcinogenesis. Paracelsus, a great medieval toxicologist, told that it “is the dose that makes the poison” establishing that all the chemicals have toxic properties.
Until the development of highly sensitive methods, such as 32P-postlabeling in 1981 [65], chronic carcinogen exposure in experimental model studies has been difficult to perform due to the high cost of radiolabeled chemicals. Despite this, a number of long-term
Bulky DNA Adducts and Lung Cancer
With the exception of Cheng et al.[121], several 32P-postlabeling investigations have reported that bulky DNA adducts in peripheral blood predicted lung cancer risk. Among these were four case-controlled studies [127], [204], [205], [206] and four that employed a prospective approach [57], [203], [207], [208]. Although three (of the former) showed increased risk of lung cancer in subjects with high adduct levels, others found no association.
Using the prospective approach, a nested
Conclusion
Bulky adducts are increased in the respiratory tract of smokers with the generation of DNA damage influenced by inherited and acquired susceptibilities including vitamin intake.
Multiple genetic polymorphism may increase the effects of environmental factors and confer a greater likelihood of increased adducts. Dose–response of adducts and carcinogens, including tobacco smoke constituents, tended to be linear at low exposure, but reached a steady state at high exposure levels possibly due to
Acknowledgments
This work has received financial support from the “Istituto Toscano Tumori,” Florence, Italy; the Tuscany Region, Italy; the “Ente Cassa di Risparmio di Firenze,” Florence, Italy; the “Associazione Italiana per la Ricerca sul Cancro;” and the NIEHS Grant P42ES017198, USA.
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