Chromodomain helicase DNA binding protein 5 (CHD5) is a potent tumor suppressor 
that serves as a master regulator of a tumor-suppressive network. Examination of 
the role played by CHD5 in a wide range of human cancers is warranted. In this 
study, we focused on the epigenetic modification and tumor-suppressive role of 
CHD5 in lung cancer. We measured CHD5 mRNA and protein expression in lung cancer 
cells, lung cancer tissues, and their corresponding noncancerous lung tissues 
using real-time PCR and Western blot analysis. We then determined the 
methylation status of the CHD5 promoter in these samples using 
methylation-specific sequencing and analyzed CHD5 re-expression in lung cancer 
cells treated with or without 5-aza-2-deoxycytidine, an inhibitor of DNA 
methylation. Next, the lung cancer cell clones stably expressing EGFP-CHD5 
protein or EGFP protein, respectively, were obtained and the effects of restored 
CHD5 expression on cell proliferation, colony formation, and tumorigenicity were 
assessed. CHD5 expression ranged from low to absent in the lung cancer cell 
lines and tissues examined; the CHD5 promoter was hyperethylated in these 
samples. Treatment with 5-aza-dC resulted in a localized decrease in methylation 
density and an increase in CHD5 expression. Clonogenicity and tumor growth were 
abrogated in A549 and H1299 cells upon restoration of CHD5 expression. A 
significant reduction in clonogenicity was observed; an average of 47.83 ± 4.6% 
reduction for A549-EGFP-CHD5 was observed compared to A549-EGFP, and an average 
of 56.39 ± 5.3% reduction for H1299-EGFP-CHD5 was observed compared to 
H1299-EGFP. A549-EGFP exhibited an average tumor size of 452.3 ± 36.5 mm(3), 
whereas A549-EGFP-CHD5 exhibited an average tumor size of only 57.7 ± 18.5 
mm(3). Thus, our findings indicate that CHD5 is a potential tumor suppressor 
gene that is inactivated via an epigenetic mechanism in lung cancer.