Smoking plays a role in lung cancer development, and now scientists have shown that smoking also affects the way genes are expressed, leading to alterations in cell division and regulation of immune response.
Notably, some of the changes in gene expression persisted in people who had quit smoking many years earlier. These findings by researchers at the National Cancer Institute (NCI), part of the National Institutes of Health, appeared in the Feb. 20, 2008, issue of "PLoS ONE".
"Smoking, we are well aware, is the leading cause of lung cancer worldwide," said NCI Director John E. Niederhuber, M.D. "Yet, a mechanistic understanding of the effects of smoking on the cells of the lung remains incomplete. This study demonstrates an important piece of this complicated puzzle. Greater understanding of the genetic alterations that occur with smoking should provide greater insight into the development of cellular targets for treating, and possibly preventing, lung cancer."
"We were able to look at actual lung tissue, tumor and non-tumor, taking into account the differences by gender, verifying the smoking status by measuring levels of cotinine, a metabolite of nicotine, in participants' plasma, and confirming results in independent samples," said Maria Teresa Landi, M.D., Ph.D., in NCI's Division of Cancer Epidemiology and Genetics, the first author of the study report.
To investigate the effects of smoking on gene activity in lung tissue, the researchers examined the gene expression profiles - patterns of gene activity - in early-stage lung tumors and non-tumor lung tissue of smokers, former smokers, and people who had never smoked cigarettes.
Gene expression was measured in 58 fresh-frozen tumor and 49 fresh-frozen non-tumor samples from 74 participants of the Environment And Genetics in Lung cancer Etiology (EAGLE) study, a large lung cancer study that was conducted in the Lombardy region of Italy.
Adenocarcinoma tumor samples were evaluated in this study because adenocarcinoma is the most common type of lung cancer, and it occurs in both smokers and people with no history of smoking. The participants were 44 to 79 years of age, and 28 were current smokers, 26 were former smokers, and 20 had never smoked.
The researchers also obtained detailed medical information about the participants (for example, whether individuals had previous lung diseases or chemotherapy) and biochemically confirmed participants' smoking status.
Using microarray techniques, which allow researchers to look at the activity of thousands of genes simultaneously, they identified 135 genes that were differently expressed in tumors of smokers vs. people who had never smoked. Among these genes, 81 showed decreased expression and 54 showed increased expression in tumor tissue.
Most of the genes showing significantly increased expression, e.g., TTK, NEK2, and PRC1, are involved in cell cycle regulation and mitosis. The cell cycle is a step-wise sequence of events in which a cell grows and ultimately divides to produce two progeny, or daughter, cells.
During the cell cycle, the chromosomes of the parent cell are duplicated and then, in a step called mitosis, divided equally between the daughter cells, ensuring that each daughter cell inherits a complete set of chromosomes. The cell apparatus responsible for the proper division of chromosomes is called the mitotic spindle.
"Our results indicate that smoking causes changes in genes that control mitotic spindle formation," said Jin Jen, Ph.D., in NCI's Center for Cancer Research, a senior author of the study report. "Irregular division of chromosomes and chromosome instability are two common abnormalities that occur in cancer cells when the chromosomes do not separate equally between the daughter cells. Therefore, changes in the mitotic process are very relevant in the development of cancer." Several of the identified genes have been suggested in the past as potential targets for cancer treatment.
The researchers also found similar expression of many genes among current smokers and former smokers in tumor tissue. Several of these genes, such as STOM, SSX2IP, and APLP2, remained altered in participants who had quit smoking more than 20 years before the study. Therefore, smoking seems to cause long-lasting changes in gene expression, which can contribute to lung cancer development long after cessation.
Looking at non-tumor lung tissues, the team found decreased activity for 73 genes and increased activity for 25 genes in current smokers. The genes most affected by smoking play a role in immune response-related processes, possibly as a lung defense mechanism against the acute toxic effects of smoking. However, non-tumor tissues seem to be able to recover from the effects of smoking. The researchers did not identify significant changes in the immune response-related genes in former smokers.
To gain a better understanding of the impact of smoking-related changes in gene expression on lung cancer survival, the researchers compared the overall gene expression smoking profile in lung tumor and non-tumor tissues with survival. They found that the altered expression of the cell cycle-related genes NEK2 and TTK in non-tumor tissues was associated with a three-fold increased risk of lung cancer mortality in smokers.
"Our data provide clues on how cigarette smoking affects the development of lung cancer, indicating that the very same mitotic genes known to be involved in cancer development are altered by smoking and affect survival. More studies are needed to confirm that the gene expression changes are due to smoking and affect tumor development or progression," said Landi. "If confirmed, these genes could become important targets for preventing and treating lung cancer."