Possible gene dosage effect of glutathione-S-transferases on atopic asthma: using real-time PCR for quantification of GSTM1 and GSTT1 gene copy numbers.

Asthma is a complex genetic disorder characterized by chronic inflammation in the airways. As oxidative stress is a key component of inflammation, variations in genes involved in antioxidant defense could therefore be likely candidates for asthma. Three enzymes from the superfamily glutathione-S-transferase (GST) involved in the antioxidant defense were tested for association to asthma using 246 Danish atopic families in a family-based transmission disequilibrium test (TDT) design. A real-time PCR assay for relative quantification of gene copy number of GSTM1 and GSTT1 was developed. The assay made it possible to distinguish individuals with zero, one, and two copies and thereby to investigate whether the GST genes influenced susceptibility to asthma in a dose-dependent manner. We found that asthmatic patients with two copies of GSTM1 were significantly underrepresented (p<0.0005) and the significance increased by 10-fold when only atopic asthmatics were analyzed (p<0.00005). GSTT1 was significantly associated in an additive model to asthma, in which the alleles carrying the deletion of the gene were transmitted to affected offspring more often than expected by chance (p=0.019). The same transmission disequilibrium of the null GSTT1 allele was seen in patients with atopic asthma (p=0.021). The polymorphism c.342A>G (p.I105V) in GSTP1 has previously been suggested as a risk factor for asthma. However, significant association with asthma or related atopic phenotypes could not be established in our study. We conclude that deletions of GSTM1 and GSTT1 could be risk factors for asthma and that the genes might have a protective role in the development of atopic asthma.