Polymorphisms of phase II xenobiotic-metabolizing and DNA repair genes and in vitro N-ethyl-N-nitrosourea-induced O⁶-ethylguanine levels in human lymphocytes

This study tested the hypothesis that genetic variants of phase II detoxification enzymes and DNA repair proteins affect individual response to DNA damage from alkylating agents. In 171 healthy individuals, an immunoslot blot assay was used to measure O⁶-ethylguanosine (O⁶-EtGua) adduct levels in peripheral blood lymphocytes treated with N-ethyl-N-nitrosourea (ENU) in vitro. The genotypes of GSTM1, GSTT1, GSTP1 I¹⁰⁵V and A¹¹⁴V, MGMT L⁸⁴F and I¹⁴³V, XPD D³¹²N and K⁷⁵¹Q, and XRCC3 T²⁴¹M were determined. Demographic and exposure information was collected by in-person interview. Student's t-test, analysis of (co)variance, and multiple linear regression models were used in statistical analyses. The mean and median (range) O⁶-EtGua levels were 94.6 and 84.8 (3.2-508.1) fmol/g DNA, respectively. The adduct level was significantly lower in people who smoked ≥25 years than that in never-smokers (square-root transformed mean values 8.20 versus 9.37, P = 0.03). Multiple linear regression models revealed that GSTT1 (β = -2.36, P = 0.009) polymorphism was a significant predictor of the level of adducts in 82 never-smokers, whereas the number of years smoked (β = -0.08, P = 0.005) and XRCC3 T²⁴¹M (β = 2.22, P = 0.007) in 89 ever-smokers. The association between GSTP1 I¹⁰⁵V, MGMT I¹⁴³V, and XPD D³¹²N with the level of adducts was not conclusive. Each polymorphism could explain 2-10% of the variation of the adduct level. These observations suggest that GSTT1 null and XRCC3 T²⁴¹M polymorphism may have some functional significance in modulating the level of ENU-induced DNA damage and these effects are smoking-dependent. Results from this exploratory study need to be confirmed in other experimental systems.