Aberrant promoter methylation in lung cancer from never-smokers with and without tobacco smoke exposure.

A210

Aberrant DNA methylation in promoter regions of tumour suppressor genes (TSGs) is an early and frequent alteration in human tumorigenesis, leading to gene inactivation. Second-hand tobacco smoke (SHS) is a widespread indoor air contaminant in environments where smoking occurs; it is also a significant cause of lung cancer (LC) in non-smokers. We analysed aberrant methylation in promoter regions of five TSGs (p16, RARb, RASSF1A, DAPK, MGMT) in order to evaluate the role of aberrant methylation in lung carcinogenesis in never-smokers (NS) with and without documented exposure to SHS. In 78 cases of LC from smokers (n = 35) and NS (n = 43), aberrant methylation of TSGs was analysed by methylation-specific PCR (MSP). For the NS group, self-reported data on SHS exposure were collected in a previous epidemiological study. The overall frequencies of hypermethylation for the LCs investigated were 40% for RARb, 24% for RASSF1A and DAPK, 14% for p16 and 11.5% for MGMT genes. Hypermethylation in any of the genes analysed was detected in about 70% of cases, and the mean methylation index (MI; number of genes methylated vs number of genes analysed) was 0.23. In general, no major difference in hypermethylation frequencies was detected between LCs from smokers and non-smokers. Comparison of tumours from NS with (n = 34) and without (n = 9) SHS exposure revealed higher rate of hypermethylation in RARb; also p16 and DAPK genes showed somewhat higher frequencies in tumours from the SHS-exposed NS cases but the numbers of cases remained small. Also the MI was higher in NS with SHS exposure than in those without exposure; it was close to that detected in active smokers. Interestingly, 50% of the NS cases with SHS exposure exhibited hypermethylation in RARb gene, as compared to 22 % of the NS cases without documented SHS exposure, and 34 % of smokers. In summary, our preliminary analysis of promoter hypermethylation suggests involvement of epigenetic changes in p16, RARb and DAPK genes in lung carcinogenesis in never-smokers exposed to SHS. The study was financially supported by grants to KHP and SJ from the Finnish Cancer Foundation, and the Yrjö Jahnsson Foundation (grant 5447).