GABRB1 and chronic obstructive pulmonary disease: In our genome-wide DNA methylation analysis, NOS1AP (cg26663636), TNFAIP2 (cg18620571), GABRB1 (cg15393297), and BID (cg01388022) are hyper-methylated both in smoker and COPD compared to non-smokers group (Fig. 7).