Noninvasive methods for measuring DNA alkylation in experimental animals and humans

Alkylpurines are liberated from alkylated DNA by glycosylase repair enzymes and, in most cases, excreted in urine without further metabolism. This phenomenon forms the basis of noninvasive methods to measure DNA alkylation in vivo. In the case of methyl adducts, such as 7-methylguanine (7-MeGua), natural backgrounds exist due to RNA turnover. However, deuterated (d₃) methylating agents or precursors give rise to d₃-7-MeGua and d₃-3-methyladenine (3-MeAde), which can be readily quantitated using gas chromatography-mass spectrometry (GC-MS). A deuterated probe drug, such as d₆-aminopyrine, can be used to measure endogenous nitrosation levels in experimental animals. In contrast, for higher alkyl homologues of alkylpurines, natural backgrounds are low or nonexistent and can be directly measured by GC-MS using stable isotope labeled internal standards. For example, increased levels of urinary 3-ethyladenine were observed in cigarette smokers. Due to recent advances in analytical methodology notably immunoaffinity cleanup of urine, measurements of excreted DNA adducts can be used in studies in human populations exposed to low levels of alkylating carcinogens.