EDITORIALS

Molecular Dosimeters of Smoking Damage in the Lung

Margaret R. Spitz, Mariza de Andrade, John Di Giovanni

Affiliation of authors: Departments of Epidemiology and Carcinogenesis, The University of Texas M. D. Anderson Cancer Center, Houston.

Correspondence to: Margaret R. Spitz, M.D., Department of Epidemiology, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Box 189, Houston, TX 77030 (e-mail: mspitz{at}notes.mdacc.tmc.edu).

Traditional epidemiologic research relied on self-reported measures of "external exposure" in defining the association between cigarette smoking and cancer risk. With elucidation of carcinogen-induced genetic and epigenetic events, knowledge of the molecular etiology of smoking-related cancers has expanded rapidly and has facilitated the study of tobacco carcinogenesis.

The dose of tobacco carcinogens to which lung tissue is exposed is modulated by genetic polymorphisms in the enzymes responsible for activation (phase I) and detoxification (phase II) of tobacco carcinogens. For example, the cytochrome(s) P450 multigene superfamily of enzymes is involved in phase I oxidative processes that may create intermediates that are more reactive than the parent compounds and can be carcinogenic or mutagenic. Phase II metabolic processes generally inactivate these genotoxic compounds through conjugation that promotes cellular excretion. The activated intermediates covalently bind to DNA and form carcinogen-macromolecular adducts. Indispensable for maintaining genomic integrity and fidelity are the DNA repair mechanisms that protect cellular DNA from the deleterious effect of mutagens, by excising the adduct and repairing the genetic damage. There are interindividual differences in DNA repair capacity that have been shown to influence susceptibility to carcinogenesis. At the poor end of the repair spectrum are patients with xeroderma pigmentosum who have defects in excision repair of UV photoproducts and extreme sensitivity to sunlight-induced skin carcinogenesis.

Lung cancer risk may thus be defined by a variety of factors, including the balance between metabolic activation and detoxification of tobacco carcinogen compounds, as well as by the efficiency of DNA repair. The net effect of these processes is the carcinogen-DNA adduct. Adducts then should be a valuable and relevant marker of biologically effective exposure over time. As Wiencke et al. (1) state, they represent an integrated measure of carcinogen exposure and metabolism, DNA repair capacity, and cell turnover.

The report by Wiencke et al. (1) provides interesting and thought-provoking data, suggesting an inverse association between age at smoking initiation and aromatic DNA adduct levels in the lung tissue of former smokers. In contrast, such a relationship was not evident in current smokers in whom smoking intensity was the variable best associated with adduct level. These differences were attributed to masking of the age effect in current smokers by their approximately twofold higher adduct levels. The data in former smokers are in contrast with the profile that fits that of the genetically susceptible individual, who could be expected to develop cancer at earlier ages and/or with fewer pack-years of exposure than nonsusceptible individuals. In fact, Ryberg et al. (2) found an inverse correlation between years of smoking and adduct levels in patients with lung cancer; i.e., patients with higher adduct levels generally had a shorter duration of smoking and/or lower smoking dose. It would have been interesting to evaluate these patterns in a smoking population without lung cancer.

The methods for measuring smoking-related aromatic DNA adducts are those used by many other laboratories; however, no thin-layer chromatographic autoradiograms are shown to enable comparison of the chromatograms. This is particularly important because it is not clear whether the same adducts are being measured in current versus former smokers. Schoket et al. (3) estimated the apparent half-life for bulky DNA adducts in the bronchial tissue to be 1.7 years in former smokers. In the study by Wiencke et al. (1), the former smokers had stopped smoking for an average of 11.9 years. Genetic alterations from tobacco carcinogen exposure may persist in former smokers, even in the presence of histologically normal appearing epithelium (4,5). Thus, an area for more research is further elucidation of the kinetics of adduct removal.

The potential mechanisms associated with the observation that DNA adduct levels are higher in those patients who started smoking earlier are logical and based on knowledge of the fate of carcinogen-DNA adducts studied in human cells and tissues and in animal model systems (6,7). If DNA adduct accumulation is the mechanism for the observed age at smoking initiation effect, then it would be very important to know in which cells of the tissue the DNA adducts have accumulated. One might not expect DNA adducts to accumulate in surrogate populations of cells to the same extent as in lung tissue.

The authors show an excellent correlation between smoking-related aromatic DNA adducts in target (lung) tissue and blood mononuclear cells. These results are consistent with the authors' earlier work and suggest that such cells may serve as a useful surrogate for the lung target tissue. This is an important finding, although the use of blood monocytes as a surrogate remains controversial.

It can be hypothesized that DNA repair capacity and level of cytogenetic damage in the lymphocytes reflect processes occurring in the target organ. This hypothesis needs to be further evaluated if we are to develop quantitative risk assessment models for lung cancer. The conduct of large-scale population studies requires us to utilize readily accessible noninvasive tissues. Analysis of the DNA adduct levels in blood mononuclear cells, in relation to age at initiation of smoking, would have been interesting and might contribute to a mechanistic understanding of the process. For example, does the relationship between age at initiation of smoking and level of smoking-related aromatic DNA adducts hold for DNA adducts in these cells? If so, this would strengthen the argument that alterations in DNA repair as a result of early age at initiation of smoking might be important for the current observations in former smokers.

Another lesson learned from this report concerns the choice of statistical methodology for biomarker assessment. The authors selected the negative binomial distribution to evaluate their adduct data. Their reasoning was that this model is one generalization of the Poisson model that allows the variance to be larger than the mean (i.e., in the presence of overdispersion of the data). This model, indeed, is very useful in situations when there is interindividual variability. We have similarly shown that negative binomial regression models were appropriate for analysis of other cytogenetic risk biomarkers for a variety of cancers in case-control studies, including mutagen sensitivity as measured by bleomycin-induced chromosomal breaks and sister chromatid exchanges (8). Wiencke et al. (1) also performed analyses with log-normal models due to the skewness of the distribution of the transformed adduct level. They stated that there was no clear violation of model assumptions for either negative binomial or log-normal distributions. Unfortunately, they did not show their results. However, their approach of choosing the model that best specified the adduct count data was well executed. Since their goals were to find predictors for the adduct levels, by not transforming (again), they made understanding of the predictors much easier for the readers.

It is most likely that multiple susceptibility factors must be accounted for to represent the true dimensions of gene-environment interactions. The ability to identify current and former smokers with the highest risks of developing cancer has substantial preventive implications. These subgroups could be targeted for the most intensive screening and smoking cessation interventions and could be enrolled into chemoprevention trials. This report perhaps fits one more piece into the lung cancer risk assessment puzzle.

REFERENCES

1 Wiencke JK, Thurston SW, Kelsey KT, Varkonyi A, Wain JC, Mark EJ, et al. Early age at smoking initiation and tobacco carcinogen DNA damage in the lung. J Natl Cancer Inst 1999;91:614-9.[Abstract/Free Full Text]

2 Ryberg D, Hewer A, Phillips DH, Haugen A. Different susceptibility to smoking-induced DNA damage among male and female lung cancer patients. Cancer Res 1994;54:5801-3.[Abstract]

3 Schoket B, Phillips DH, Kostic S, Vincze I. Smoking-associated bulky DNA adducts in bronchial tissue related to CYP1A1 MspI and GSTM1 genotypes in lung patients. Carcinogenesis 1998;19:841-6.[Abstract]

4 Mao L, Lee JS, Kurie JM, Fan YH, Lippman SM, Lee JJ, et al. Clonal genetic alterations in the lungs of current and former smokers. J Natl Cancer Inst 1997;89:857-62.[Abstract/Free Full Text]

5 Wistuba II, Lam S, Behrens C, Virmani AK, Fong KM, LeRiche J, et al. Molecular damage in the bronchial epithelium of current and former smokers. J Natl Cancer Inst 1997;89:1366-73.[Abstract/Free Full Text]

6 Garner RC. The role of DNA adducts in chemical carcinogenesis.Mutat Res 1998;402:67-75.[Medline]

7 La DK, Swenberg JA. DNA adducts: biological markers of exposure and potential applications to risk assessment. Mutat Res 1996;365:129-46.[Medline]

8 de Andrade M. Statistical approaches to analyze cytogenetic biomarkers in epidemiological studies. Am J Hum Genet 1996;59 [abstract 1003].



             
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