SAN DIEGO – Tobacco researchers are getting one step closer to identifying young smokers particularly susceptible to the carcinogenic effects of smoking, by using large-scale epidemiology studies and DNA adducts data.
“This is not lung cancer screening; this is not early detection of lung cancer; this is detection of susceptible, high-risk individuals, so they can be targeted for cessation, surveillance, and prevention,” said Stephen S. Hecht, Ph.D., professor of cancer prevention at the University of Minnesota Masonic Cancer Center in Minneapolis.
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While nicotine itself is not a carcinogen, each puff of tobacco smoke delivers a mixture of more than 70 established carcinogens, including the potent lung-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK).
Some of these carcinogens are excreted, but many will undergo metabolic activation and interact with a patient’s DNA.
The product of this interaction, called DNA adducts, causes persistent miscoding during DNA replication, leading to the thousands of mutations we see in lung cancer, he explained at a conference on lung cancer translational science sponsored by the American Association for Cancer Research and the International Association for the Study of Lung Cancer.
The biomarkers of tobacco exposure and metabolism are very well developed, with blood and urinary biomarker panels applied in many studies and validated by mass spectrometry.
Using the Shanghai cohort study, the researchers identified three urinary biomarkers – PheT (r-1,t-2,3,c-4-tetrahydroxy-1,2,3,4-tetrahydrophenanthrene), total NNAL (a metabolite of NNK and its glucuronides), and total cotinine (a surrogate for nicotine that includes cotinine and its glucuronides) – that were significantly associated with lung cancer in 950 smokers, even after adjustment for smoking duration and intensity (Cancer Res. 2011;71:6749-57).
A more recent, deeper dive of 735 of those urinary samples, still stable about 20 years after they were taken, looked specifically at polymorphisms in cytochrome P450 2A6 (CYP2A6), the primary enzyme responsible for the oxidation of nicotine and cotinine.
Data currently in press show that CYP2A6 poor metabolizers had a lower risk of lung cancer than the combined group of normal, intermediate, or slow metabolizers (odds ratio, 0.64; P value for trend = .034).
“This is logical because poor metabolizers of nicotine have more unchanged nicotine on board, so they don’t need to smoke as intensely in order to get more nicotine because that nicotine is remaining to a greater extent in its natural form, rather than being metabolized into cotinine, which is not addictive,” Dr. Hecht said.
Recent genomewide-association studies by Dr. Hecht and his colleagues of smokers in the Multiethnic Cohort Study revealed that Japanese Americans had the highest number of low nicotine CYP2A6 metabolizing genotypes of the five ethnic groups analyzed, consistent with their low lung cancer risk.
Urinary biomarker studies showed that levels of total NNAL, 3-hydroxy phenanthrene, a biomarker of polycyclic aromatic hydrocarbon uptake, and S-phenylmercapturic acid (SPMA), a biomarker of benzene exposure, were also significantly higher among African Americans than whites and significantly lower in Japanese Americans than whites, he said.
Courtesy of S. HechtDr. Stephen Hecht
The findings are consistent with the initial study results published a decade ago (N Engl J Med. 2006;354:333-342), showing that at low levels of smoking (10 cigarettes per day), Japanese Americans and Hispanics had one-third the risk of lung cancer of African Americans or Native Hawaiians (P less than .001). The differences disappeared at higher levels of smoking (30 cigarettes per day).
Results from the new analyses for Native Hawaiians and Hispanics, however, did not fit this pattern, and “we’re not sure what’s going on there and need to do more work,” Dr. Hecht said.
Nicotine equivalent levels in Native Hawaiians were actually lower than would be expected based on their high lung cancer risk, while Hispanics had higher levels than would be expected based on their low risk.
Interestingly, urinary levels of 3-hydroxypropylmercapturic acid (3-HPMA), a metabolite of acrolein, were found to be unusually high in Native Hawaiians and relatively low in Hispanics, “which may play into their lung cancer risk because acrolein is a very strong toxicant and there is evidence to suggest it is involved in lung cancer,” he said.
While tobacco exposure and metabolism biomarkers have hit their stride, less well developed are the DNA adducts and repair biomarkers that can predict lung cancer susceptibility. This is a critical step because of the role DNA adducts play in the formation of lung cancer mutations, Dr. Hecht said.
A smoker may have a high tobacco exposure as indicated by the urinary biomarkers, but have a low metabolism to form DNA adducts or high DNA repair, which would mean their DNA adduct levels would be low. Conversely, another smoker with low exposure and poor DNA repair may have higher DNA adduct levels, and thus, a greater risk for developing lung cancer.