Since its discovery, the aryl hydrocarbon receptor (AhR) has been recognized as an integral component of the carcinogenic process initiated by smoke-born polycyclic aromatic hydrocarbons and other carcinogens that result from incomplete combustion. The AhR regulates the expression of a group of genes encoding phase I cytochrome P450 monooxygenases, phase II conjugating enzymes, and phase III metabolite transporters that metabolize and dispose of PAH carcinogens, while also causing a limited level of reactive, mutagenic intermediates. Despite the pivotal importance of the AhR in the carcinogenic process in human lung, the underlying molecular mechanisms controlling AhR expression are not well established. Recently, we identified four novel variants of a repeat polymorphism, including two previously unreported single-nucleotide polymorphisms within the repeat, of the human AHR gene and have established a new method for analyzing them. These novel polymorphisms occur in a critical control region of the proximal promoter that contains multiple binding sites for specificity protein transcription factors. Our central hypothesis is tha these newly characterized AHR gene polymorphisms affect the transcriptional regulation of AhR expression, which in turn leads to altered expression and inducibility of the carcinogen-bioactivating enzymes as well as phase II and III pathways of deactivation.
The specific aims of our application are: 1) to determine the allelic frequencies of these novel polymorphisms of the AHR gene in normal tissue from lung cancer patients, and 2) to determine the roles of these novel polymorphisms in the transcriptional regulation of AhR expression. We will perform in vitro assays of transcription factor binding and AHR promoter activities in a lung tumor cell line and determine allelic expression of AhR mRNA in normal lung tissue from individuals heterozygous for these polymorphisms. These studies will determine the importance of novel polymorphisms in the AHR gene promoter on AhR expression and may ultimately lead to a clearer understanding of the genetic susceptibility to lung cancer.
Lung cancer continues to be a major public health concern and the leading cause of cancer-related death. Cigarette smoking has been identified as a major risk factor for the development of lung cancer, yet only about 10 to 20% of long-term smokers develop the disease. The susceptibility of some individuals to lung cancer is believed to be determined to a significant extent by the genetic makeup of the individuals. The aryl hydrocarbon receptor (AhR) is a key regulator of the deactivation of smoke-born carcinogens. Recently, we have identified novel variants of a repeat polymorphism in the AHR gene that are associated with the level of AhR expression. This project will begin the investigation of our hypothesis that these polymorphisms affect the level of AhR, the metabolism of carcinogens, and hence the genetic susceptibility to lung cancer.