Nitrous Oxide Genotoxicity

1258 June 2013 NITROUS oxide has long been known to be an impotent and unpredictable human anesthetic.1 In this issue of the journal, Chen et al.2 report that nitrous oxide in clinical use is a potent and predictable human genotoxin. The authors observe that maintenance of anesthesia with 70% nitrous oxide and sevoflurane in patients undergoing colorectal surgery doubles the incidence of DNA damage assessed by the in vitro alkaline single cell gel electrophoresis (i.e., “SCGE” or “comet”) assay compared or 80% oxygen and sevoflurane. Genotoxicity using the comet assay has been previously reported with chronic occupational exposure to low concentrations of nitrous oxide, after isoflurane anesthesia with and without nitrous oxide, and after sevoflurane anesthesia without nitrous oxide.3–6 The current clinical trial therefore fills an important gap with regard to anesthetics often used in combination. The author’s observation that nitrous oxide-induced genotoxicity is associated with postoperative wound infection is original and warrants further consideration and confirmation. The conventional alkaline comet assay detects DNA damage that occurs rapidly on exposure, and that may lead to fixed mutations in DNA including doubleand single-strand DNA breaks, alkaline labile sites, DNA-protein and DNA– DNA cross-linking. On discontinuation of exposure, the primary lesions detected by the comet assay are most often correctly repaired in minutes to hours without persistent genetic alterations. Because it also detects single-strand breaks generated during the repair of initial damage including alkylated bases, bulky base adducts, and pyrimidine dimers, the comet assay provides an index of the kinetics of DNA strand break repair and break excision repair if serially performed. Although the kinetics and fidelity of DNA repair may vary with the type of lesion and tissue origin of target cells, little is known about interindividual differences in the capacity for DNA repair in humans. Numerous attractive features of the comet assay have sustained its development and validation in the 30 yr since its introduction.7,8 As an “early warning system,” the comet assay detects very low levels of DNA damage (i.e., in 1 × 107 base pairs), provides few negative results on exposure to known genotoxins and fewer false positive results than other cellular assays of genotoxicity including the micronucleus test, the chromosomal aberration test, and sister chromatid exchange. The comet assay is rapid, inexpensive and safe to perform, quantitative, generally free of artifacts, and is available in standardized kit formats that provide high interlaboratory replicability. Because low levels of DNA strand breaks and genetic instability detected by the comet assay after chemical exposures contribute to mutations and cancer in laboratory animals, its most prominent use at present is as a screening tool after in vivo exposure in research or regulatory test batteries.* The primary shortcoming of the comet assay is its uncertain predictive value in the absence of clear-cut and proven causal associations with specific human phenotypes. Because the clinical relevance of the comet assay’s primary endpoint, i.e., temporary strand breakage, is not known with precision, the conventional comet assay is not configured or validated to assess individual risk of disease phenotypes. Doubts about the comet assay’s analytical validity, clinical validity, and clinical utility have restrained widespread applications in medical and surgical settings. Given its many advantages, it is surprising that the comet assay has rarely been investigated in prospective cohort studies such as that described by Chen et al. Investigations designed to test for shared mechanisms of genotoxicity and deleterious outcomes, and to control for confounders such as reverse causality or association by chance alone, are fewer still. To further complicate the interpretation of comet assay results in humans, reports describing the Nitrous Oxide Genotoxicity