Increased risk of cancer mortality by smoking-induced aryl hydrocarbon receptor repressor DNA hypomethylation in Japanese population: A long-term cohort study

1

Introduction

The number of cancer deaths is increasing worldwide, and it reached approximately 9 million in 2019 [1] . Over the last 50 years, much evidence has indicated that lifestyle factors, such as smoking, excessive alcohol drinking, unhealthy eating habits, and physical inactivity, are the main risk factors of cancer incidence and mortality 2345 . Among them, smoking is a major risk factor for cancer, and it is involved in various types of cancers [2] . The International Agency for Research on Cancer (IARC) reported a causal inference between smoking and some cancers, such as lung cancer, esophagus cancer, kidney cancer, and liver cancer [6] . Although the molecular mechanisms underlying the carcinogenic characteristics of tobacco are not yet fully known, the elucidation of these mechanisms of cancer development associated with smoking may enable the early detection and treatment of smoking-related cancers.

Recently, it was noted that DNA methylation is involved in the mechanism by which smoking increases the risk of cancer [7] . DNA methylation is an epigenetic mechanism that regulates gene expression by adding methyl groups to cytosine at CpG sites. It is thought that a change in DNA methylation occurs in a subpopulation of normal cells, and it triggers the inhibition of key genes related to cell proliferation [8] . For example, various tumor suppressor genes (cyclin-dependent kinase inhibitor 2A, retinoblastoma B, and mutL homolog 1, etc.) are silenced by DNA hypermethylation in cancer tissue 91011 . Therefore, changes in DNA methylation levels are presumed to be useful for estimating the risk of developing smoking-induced cancers.

Polycyclic aromatic hydrocarbons, which are chemical substances of tobacco smoke, are ligands for aryl hydrocarbon receptor (AHR) [12] , and they are involved in carcinogenesis by activating AHR signaling [1314] . Aryl hydrocarbon receptor repressor (AHRR) represses the activity of AHR by a negative-feedback mechanism [15] , and the downregulation of AHRR induces several pro-oncogenic factors, such as hairy and enhancer of split-1, and casein kinase II [16] . Recent studies have reported that smoking induced the hypomethylation of the AHRR gene 171819 . In particular, methylation at cg05575921 in the AHRR gene is strongly affected by smoking. Given the consistent effect of smoking on carcinogenesis, a smoking-induced decrease in AHRR DNA methylation may be a molecular mechanism involved in cancer progression. Several epidemiological studies in recent years showed AHRR DNA hypomethylation increased risk for lung cancer among European population 202122 .

There are ethnic differences in the DNA methylation response to smoking, which may provide important insights into disease pathways [2324] . However, to our knowledge, it has not been reported that the association between AHHR DNA methylation and cancer mortality in Asian population. Therefore, we conducted a long-term prospective study that spanned more than 25 years to investigate the effects of AHRR DNA methylation on the cancer mortality risk in a Japanese population. Furthermore, we examined whether AHRR DNA methylation mediates the association between smoking and cancer mortality risk.