PARIS – Air pollution has been recognized as a risk factor for lung cancer for about 2 decades, and already present in normal lung cells to cause cancer.
Think of it as “smoking gun–level” evidence that may explain why many nonsmokers still develop non–small cell lung cancer, said Charles Swanton, PhD, from the Francis Crick Institute and Cancer Research UK Chief Clinician, London.
“What this work shows is that air pollution is directly causing lung cancer but through a slightly unexpected pathway,” he said at a briefing prior to his presentation of the data in a presidential symposium held earlier this month in Paris at the European Society for Medical Oncology Congress 2022.
Importantly, he and his team also propose a mechanism for blocking the effects of air pollution with monoclonal antibodies directed against the inflammatory cytokine interleukein-1 beta.
Lung cancer in never-smokers has a low mutational burden, with about 5- to 10-fold fewer mutations in a nonsmoker, compared with an ever smoker or current smoker, Dr. Swanton noted.
“The other thing to say about never-smokers is that they don’t have a clear environmental carcinogenic signature. So how do you square the circle? You’ve got the problem that you know that air pollution is associated with lung cancer – we don’t know if it causes it – but we also see that we’ve got no DNA mutations due to an environmental carcinogen,” he said during his symposium presentation.
The traditional model proposed to explain how carcinogens cause cancer holds that exposure to a carcinogen causes DNA mutations that lead to clonal expansion and tumor growth.
“But there are some major problems with this model,” Dr. Swanton said.
For example, normal skin contains a “patchwork of mutant clones,” but skin cancer is still uncommon, he said, and in studies in mice, 17 of 20 environmental carcinogens did not induce DNA mutations. He also noted that a common melanoma driver mutation, BRAF V600E, is not induced by exposure to a ultraviolet light.
“Any explanation for never-smoking lung cancer would have to fulfill three criteria: one, you have to explain why geographic variation exists; two, you have to prove causation; and three, you have to explain how cancers can be initiated without directly causing DNA mutations,” he said.
Normal lung tissues in nonsmoking adults can harbor pre-existing mutations, with the number of mutations increasing likely as a consequence of aging. In fact, more than 50% of normal lung biopsy tissues have been shown to harbor driver KRAS and/or EGFR mutations, Dr. Swanton said.
“In our research, these mutations alone only weakly potentiated cancer in laboratory models. However, when lung cells with these mutations were exposed to air pollutants, we saw more cancers and these occurred more quickly than when lung cells with these mutations were not exposed to pollutants, suggesting that air pollution promotes the initiation of lung cancer in cells harboring driver gene mutations. The next step is to discover why some lung cells with mutations become cancerous when exposed to pollutants while others don’t,” he said.