Molecular Clues to Preventing Tobacco-Related Lung Cancer

Almost every article on lung cancer opens with the grim statistics: over 172,000 new cases of cancer expected each year in the US, over 163,000 expected deaths; 1.35 million new cases and 1.18 million deaths each year worldwide; the leading cause of cancer deaths for both men and women; an overall 5-year relative survival rate of 15%.

Although active tobacco smoking accounts for an estimated 85% of lung cancer and passive smoking for 3-5%, carcinogens in the workplace and ambient environment such as radon, polycyclic aromatic hydrocarbons (PAHs), and asbestos also cause lung cancer, alone or through interactions with smoking. A dose-response relationship with smoking has been reported for all major histologic types of lung cancer: small cell lung carcinoma (SCLC) which represents about 13% of primary lung cancers and non-SCLC (NSCLC) 87%, comprising adenocarcinoma, squamous cell carcinoma, and large cell carcinoma. The increase in smoking among youth worldwide and the fact that lung cancer risk persists even 15 years after smoking cessation mean that lung cancer will remain a national and global health priority for many decades.

There is no mystery as to the major cause of this disease, but there is still much to be learned about factors that render certain individuals more susceptible and therefore in need of special surveillance and intervention. And there is more to be learned about detecting lung cancer in the early premalignant stages--in time to block the progression to malignancy. These key issues in prevention are discussed below.


Understanding Mechanisms and Individual Susceptibility

The first step in prevention is to better understand the mechanisms that lead to lung cancer and especially the interactions between tobacco smoke and host susceptibility factors. Molecular epidemiology-studies using biomarkers to quantify carcinogen dose, preclinical effects and susceptibility-has been a useful tool in this regard. In fact, molecular epidemiology has focused intensively on tobacco smoking, not only because it is the number one public health problem today but because it is a "model" environmental carcinogen offering the opportunity to validate biomarkers that are broadly relevant to the carcinogenic process. Moreover, tobacco smoke contains fifty-five known carcinogens, including PAHs (e.g., benzo[a]pyrene [BaP]), 4-aminobiphenyl (4-ABP), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a nitrosamine derived from nicotine. Many of these chemicals are also found in ambient air from motor vehicles and other combustion sources, so knowledge gained from studying smokers is applicable to prevention of cancer in other populations.

Molecular epidemiologic research over the past 10-20 years has identified a number of biomarkers that are considered "risk markers" at the group level; but none are able to precisely predict individual risk. Examples include a measure of procarcinogenic genetic damage from PAHs known as PAH-DNA adducts and polymorphisms (variants) in genes that affect individual ability to metabolize carcinogens and repair their damage to DNA.

Many carcinogens bind to DNA and form adducts that can lead to mutation and, ultimately, to cancer. Thus, adducts reflect chemical-specific genetic damage that is mechanistically relevant to carcinogenesis. In 1982 PAH-DNA adducts were first detected in human subjects in vivo, specifically in white blood cells (WBCs) and lung tissue from lung cancer patients, most of whom were smokers, and controls. A number of subsequent studies have found that PAH-DNA adduct levels are elevated in WBCs from lung cancer cases compared with control subjects, after adjusting for amount of smoking and other potential confounders. More definitive evidence of causality has come from a case-control study "nested" within the Harvard Physicians Health Study that found that "healthy" current smokers who had high levels of DNA adducts in WBC were approximately three times more likely to be ldiagnosed later with lung cancer as compared to current smokers with low concentrations of this biomarker.

Supportive evidence that the adducts play an important role in lung cancer is the observation that the pattern of mutations in the p53 tumor suppressor gene (mutated in about 50% of lung tumors) is consistent with the type of DNA adducts and mutations induced by the representative PAH, B[a]P. Specifically, in smoking-associated lung cancer, the occurrence of G:C to T:A transversions at hotspot codons (157, 248, 249, and 273) is markedly elevated. G®T transversions are detected in approximately 30% of smokers' lung cancer and in only 12-16% of lung cancers of nonsmokers.

These findings suggest that the ability to activate and bind PAHs is a risk factor in lung cancer and that DNA adducts have the potential to identify "high responders." While their sensitivity and specificity to lung cancer are too low for use in early detection, adducts can be useful in understanding susceptibility to lung cancer.

Susceptibility factors that can modulate environmental risks include genetic predisposition, ethnicity, nutritional status and age. The most extensively studied susceptibility factors are the relatively common genetic polymorphisms that determine the metabolic fate of carcinogens and the level of DNA damage they exert. Polymorphisms in certain cytochrome P450 enzymes increase the oxidative metabolism--hence activation--of diverse endogenous and exogenous chemicals to their carcinogenic intermediates, while genetic variants in enzymes such as glutathione S-transferases (GSTs) and epoxide hydrolase detoxify certain carcinogenic metabolites. Genetically determined variation in DNA repair modulates risks from agents that directly or indirectly damage the DNA. Genetic polymorphisms also appear to play a role in nicotine dependence but will not be discussed here.

The genotypes most consistently identified as "at risk" for tobacco-related lung cancer include polymorphisms in the GST genes, the CYP1A1 gene (involved in the metabolic activation of carcinogens such as PAHs), and the myeloperoxidase gene (increases generation of reactive oxygen species). The amount of risk modulation by these polymorphisms is however moderate. Variants in genes coding for DNA repair enzymes, signal transduction molecules, and molecules involved in cell growth regulation appear to be more important in lung cancer susceptibility. Again, these biomarkers are interpretable on the group level only and individual risk estimation is premature.

In addition to genetic factors, ethnicity also affects lung cancer risk from smoking. The higher rates of various smoking-related cancers in blacks may be partially explained by the finding that, in black smokers, urinary concentrations of NNK metabolites and serum concentrations of cotinine, a nicotine metabolite, exceeded those in white smokers with similar exposure to tobacco smoke. Nutritional deficits resulting in low levels of retinol and antioxidants such as vitamin E can heighten susceptibility to lung cancer. Susceptibility to carcinogens is also increased in utero and in the early years. Molecular epidemiologic studies indicate that babies in the womb and young children exposed to tobacco smoke or PAHs experience a higher internal dose of certain toxicants or greater genetic damage than adults who are similarly exposed. The likely mechanisms causing this difference include greater absorption or retention of toxicants, reduced detoxification and DNA repair, the higher rate of cell proliferation during early stages of development, and the fact that cancers initiated in the womb and in the early years have the opportunity to develop over many decades. Adolescence and young adulthood are also viewed as sensitive stages of life because of greater proliferative activity in epithelial cells of certain tissues. Thus initiation of smoking at an early age confers a higher risk of lung cancer compared to initiation later on.


Primary Prevention

The best ways to reduce risk of tobacco-related lung cancer are prevention of smoking in the first place and smoking cessation. A study of participants in a smoking cessation program demonstrated the immediate benefits of quitting: levels of PAH-DNA and 4-ABP-hemoglobin adducts in blood were significantly reduced after cessation. It is unclear whether cigarettes purported to have lower exposure to carcinogens are in fact reducing risk, since some smokers may decide not to quit and others may take up smoking based on the promise of harm reduction. Given that environmental tobacco smoke is a known risk factor for lung cancer and that children are inherently more susceptible to DNA damage, public health policies should focus on reduction of smoking in the home.

Another primary prevention strategy being intensively studied is the use of natural or chemical chemopreventive agents to prevent, inhibit, or reverse the process of carcinogenesis. This approach keys off the fact that lung carcinogenesis is known to be a diffuse field-wide carcinogenic process that takes place over decades and can lead to the development of multiple malignant lesions in diverse areas (this is known as "field cancerization"). In theory, this process affords major opportunities for intervention during the premalignant stages of disease. There is some evidence that agents such as certain antioxidant vitamins, dietary isothiocyanates, 9-cis-retinoic acid, green tea, and nonsteroidal anti-inflammatory drugs may be protective against lung cancer, but effects appear to differ between smokers, former smokers, and never smokers. More research is needed to identify the effective agents.


Secondary Prevention

There is a need for validated biomarkers for early detection of tobacco-related lung cancer. The 5-year survival rate is 49% when disease is localized; but only 16% of lung cancers are diagnosed at that stage. More than 60% of patients with lung cancer have hidden occult metastases at presentation and conventional therapies are therefore limited in their success. In light of the rapid course of the disease and the limitations of available screening methods (chest X-ray, analysis of cells in sputum, tumor markers in serum, fiberoptic examination of bronchial airways, and complex imaging approaches such as spiral CT), additional methods are needed to detect lung cancer at its earliest, most curable stage. Biomarkers such as alterations in target genes or patterns of protein expression in sera or plasma have potential in early detection and offer a potentially feasible and lower cost screening method than imaging technologies. Among the promising candidates for further large-scale validation studies are: mutation in the p53 tumor suppressor gene, genetic alterations in 3p, 9p, p16, hnRNP A2/B1, h-TERT (a gene controlling telomerase expression) and multiplex assays of microsatellite markers. Proteomic approaches may prove useful in identifying proteins that discriminate between normal sera and sera from lung cancer patients. To date, none of these biomarkers has been validated as having adequate sensitivity, specificity and predictive value for screening.

In conclusion, during the last decades there has been considerable progress in understanding mechanisms of lung carcinogenesis and especially in identifying factors that increase susceptibility. In addition, chemoprevention and early detection methods now in the validation stage promise to reduce the burden of lung cancer in the future. At the present time, however, the only known way to prevent tobacco-related cancer-- which affects nonsmokers as well as former and current smokers-is avoidance of tobacco smoking.

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