ABSTRACT: Testican 3 (coded for by SPOCK3), is an extracellular matrix heparan/chondroitin sulphate proteoglycan that possesses serine and cysteine protease inhibitor-like domains Based on the knowledge that serine proteases contribute to the destruction of the lung in cigarette smokers, but that only a fraction of smokers develop smoking-induced lung disease, we hypothesized that smokers expressed SPOCK3 at lower levels in the small airway epithelium, the initial site of smoking-induced disease, and further, that genetic variability modulates the expression of SPOCK3 in the airway epithelium. Assessment of gene expression in the small airway epithelium (10th -12th order bronchi) of healthy non-smokers (n=38) and healthy smokers (n=42), demonstrated that the expression levels of SPOCK3 were significantly lower in healthy smokers compared to healthy nonsmokers (p<0.04). Affymetrix Human SNP array 5.0 was used to assess genome wide single nucleotide polymorphisms (SNPs) within 100 kbp of the SPOCK3 gene in the same nonsmokers and smokers, and these SNPs were correlated with small airway gene expression of SPOCK3, with correction for variation in genetic ancestry. There was a significant correlation of SPOCK3 small airway epithelial gene expression with 13 adjacent SNPs in the SPOCK3 gene (p<10-3, all comparisons, Wald test). For example, the TT allele of rs13124292, located in intron 3, was associated with a small airway epithelial expression levels of 0.56 ± 0.07, and the AA genotype with expression levels of 2.31 ± 0.26 (p<10-6, pairwise t test). Interestingly, smoking appeared to lessen the degree to which genotype associated with SPOCK3 expression level, i.e., smoking to some extent overrode the influence of genetic variation. The observation that SPOCK3 gene expression in the small airway epithelium is reduced in smokers, and that smoking interacts with cis-genomic variations to determine the levels of SPOCK3 small airway epithelial gene expression, is consistent with the concept that everyone is at risk for smoking-induced lung disease, but that inherited genetic variations contribute to the pathogenesis of susceptibility to smoking-induced disease.