Polycyclic aromatic hydrocarbons (PAHs) are formed during the incomplete combustion of carbon, and several are classified as human carcinogens by IARC and the US National Toxicology Program (35
). Urine 1-hydroxypyrene glucuronide reflects recent PAH exposure, and using this marker we have shown that several populations at high risk for ESCC in China, Iran, and Brazil are highly exposed to PAHs (15
). Consequently, studies on the biological effects of this exposure in high-risk populations and its potential relationship with precursor lesions and invasive cancers are needed to further elucidate the association between PAH exposure and ESCC.
The observation that a positive family history of cancer is associated with increased cancer risk is well accepted, and it is particularly evident in gastrointestinal, geniturinary, and gynecologic cancers (2
). This predisposition may be attributable to an inherited genetic susceptibility that, in some instances, is modified by the environment, i.e., a gene-environment interaction (4
). One such example is the association between tobacco use and specific Cytochrome P450-related enzyme genotypes that predispose the esophagus of smokers to an even higher risk for DNA damage or cancer (37
). A recent 15-year prospective study in Linxian has shown a significant association between a positive family history of UGI cancer and an increased risk of both esophageal and gastric cancer (5
), and a separate case-control study showed an association between family history of these cancers and risk of esophageal squamous dysplasia (6
The current study evaluated the expression of genes coding for proteins that are related to the carcinogenic effect of PAH exposure, including aryl hydrocarbon receptor (AhR), CYP1a1, CYP1b1, and aryl hydrocarbon receptor repressor (AhRR), which acts as an AhR antagonist (18
). The expression of these genes was evaluated in epithelial cell samples from the esophagus, the target organ of interest in the Linxian population, which has known environmental PAH exposure and high rates of ESCC. Our data showed that median AhR expression was significantly higher in individuals with a positive family history of UGI cancer, but was not associated with apparently higher levels of PAH exposure or the presence of high grade esophageal squamous dysplasia, the precursor lesion of ESCC. Expression levels of AhRR, CYP1a1 and CYP1b1 were all low. These results suggest that individuals in this population with a family history of UGI cancer may be more susceptible to the detrimental effects of PAH exposure, including PAH-induced cancer.
Median AhR expression was not higher among individuals who heated their homes without a chimney or among tobacco smokers, groups that appeared to be exposed to more PAHs. However, our assessments of these exposures were relatively simply and a more accurate measurement of environmental PAH exposure at the time of sample collection may be needed to test this association more completely. Median AhR expression was also not higher among subjects with high grade squamous dysplasia, possibly because the esophageal balloons sample the entire esophageal epithelium and may not accurately detect increased expression that is found only in focal mucosal lesions. In contrast, differences in AhR expression related to familial predisposition would be expected to be present in all cells, so it should be more easily seen in balloon samples.
The current study was limited by the number of samples with expression below our limit of detection. This was most significant for AhRR and the Cytochrome p450 genes. The lack of detectable expression for these genes of interest in the setting of detectable levels of a housekeeping gene is indicative of minimal to no expression (39
), and is consistent with previous reports that these genes are variably expressed in the esophagus (40
). The interpretation that this is a real finding is supported by the fact that the efficiency of our short qRT-PCR products should be relatively “independent” of RNA quality (39
), and any potential effect of suboptimal RNA quality would have been minimized after normalization of the target gene expression results to the 18s endogenous control and the Universal Human Reference sample. Finally, the positive association which we found between AhR expression and family history of cancer persisted with a variety of statistical approaches.
Advantageous aspects of our cell sampling method also deserve mention. This minimally invasive approach samples the entire mucosal epithelium of the target organ of interest and theoretically provides an integrated assessment of its exposure and biological potential. Sampling the target organ of interest is important because induction of gene expression is frequently cell and tissue-type specific (42
). Our cell sampling method also avoids problems that may occur when sampling is limited to neoplastic tissue, such as the inhibition of CYP enzyme expression by inflammatory cytokines associated with such lesional tissue (44
) and the fact that PAHs may induce xenobiotic metabolizing enzymes more potently in normal squamous cells than in premalignant or tumor cells (45
). The cell sampling method used in our study avoids these potential pitfalls and may be suitable for inclusion in other epidemiological studies, especially those looking for variation in constitutive traits.
In summary, we measured gene expression of AhR and related genes in esophageal epithelial cell samples from subjects in a Chinese population at high risk for ESCC, and we compared expression levels in subjects who differed by apparent PAH exposure, by the presence or absence of esophageal squamous dysplasia, the precursor lesion of ESCC, and by family history of upper gastrointestinal tract cancer. We found significantly higher AhR expression in subjects with a family history of upper gastrointestinal tract cancer but no difference in this expression between subjects with different levels of indoor air pollution or the presence or absence of esophageal squamous dysplasia. This is the first report of an association between AhR expression and family history of cancer in humans, and it supports the idea of a familial predisposition to the deleterious effects of PAH exposure in this high-risk population.
This project was funded in part by federal funds from the National Cancer Institute, National Institutes of Health, under contracts N01-CO-12400 and N01-RC-91019. It was also supported in part by the Intramural Research Program of the NIH, National Cancer Institute, Division of Cancer Epidemiology and Genetics. The content of this publication does not necessarily reflect the views of policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.