Anesthesiology 2008; 109:36 – 43 Copyright © 2008, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. Influence of Methylenetetrahydrofolate Reductase Gene Polymorphisms on Homocysteine Concentrations after Nitrous Oxide Anesthesia Peter Nagele, M.D.,* Barbara Zeugswetter, B.S.,† Caspar Wiener, B.S.,† Hansjo ¨ rg Burger, M.D.,‡ Michael Hu ¨ pfl, M.D.,‡ Martina Mittlbo ¨ ck, Ph.D.,§ Manuela Fo ¨ dinger, M.D. Background: Mutations in the methylenetetrahydrofolate re- ductase (MTHFR) gene (677C>T, 1298A>C) cause elevated plasma homocysteine concentrations and have been linked to fatal outcomes after nitrous oxide anesthesia. This study tested the hypothesis that patients with common MTHFR 677C>T or 1298A>C mutations develop higher plasma homocysteine concentrations after nitrous oxide anesthesia than wild-type patients. Methods: In this prospective, observational cohort study with blinded, mendelian randomization, the authors included 140 healthy patients undergoing elective surgery. All patients received 66% nitrous oxide for at least 2 h. The main outcome variable, plasma total homocysteine, and folate, vitamin B 12 , and holotranscobalamin II were measured before, during, and after surgery. After completion of the study, all patients were tested for their MTHFR 677C>T or 1298A>C genotype. Results: Patients with a homozygous MTHFR 677C>T or 1298A>C mutation (n  25) developed higher plasma homo- cysteine concentrations (median [interquartile range], 14.9 [10.0 –26.4] M) than wild-type or heterozygous patients (9.3 [7.5–15.5] M;n  115). The change in homocysteine after nitrous oxide anesthesia was tripled in homozygous patients compared with wild-type (5.6 M [60%] vs. 1.8 M [22%]). Only homozygous patients reached average homocysteine levels considered abnormal (> 15 M). Plasma 5-methyl-tet- rahydrofolate concentrations increased uniformly by 20% after nitrous oxide anesthesia, indicating the inactivation of methionine synthase and subsequent folate trapping. Holo- transcobalamin II concentrations remained unchanged, indi- cating no effect of nitrous oxide on vitamin B 12 plasma concentrations. Conclusions: This study shows that patients with a homozy- gous MTHFR 677C>T or 1298A>C mutation are at a higher risk of developing abnormal plasma homocysteine concentrations after nitrous oxide anesthesia. NITROUS oxide inhibits vitamin B 12 (cobalamin) by ir- reversibly oxidizing the cobalt atom of cobalamin. 1,2 This leads to a subsequent inhibition of enzymes requir- ing cobalamin in its coenzyme form. Inhibition of vita- min B 12 lasts several days because of the irreversible nature of the chemical reaction. 3,4 Among the enzymes that require active vitamin B 12 as a cofactor, methionine synthase (gene symbol MTR, EC 2.1.1.13) is crucial be- cause it is located at the juncture of two pathways: homocysteine remethylation and the folate cycle (fig. 1). Therefore, inhibition of methionine synthase via oxi- dized cobalamin results in a sustained increase of plasma homocysteine concentrations and lack of biologically active folate (“folate trapping”) that can be used for the conversion of homocysteine to methionine. 5,6 In the methylenetetrahydrofolate reductase (MTHFR) gene, which is operant in the folate cycle, two common single nucleotide polymorphisms have been described (MTHFR 677CT, MTHFR 1298AC) that are associated with a reduced enzyme activity. 7,8 Together, both poly- morphisms have a combined prevalence of approxi- mately 20% in the Western European population. 9 MTHFR 677CT results in decreased formation of active folate, methyltetrahydrofolate, 10 which in turn leads to an increase in plasma total homocysteine concentra- tions. 7 The MTHFR 677CT polymorphism is consid- ered the single most important genetic determinant of plasma homocysteine. 11 Recently, two reports describing children carrying sev- eral polymorphisms and mutations in the MTHFR gene were published. The children developed catastrophic neurologic outcomes after being anesthetized with ni- trous oxide. 12,13 Based on these reports and the appar- ent importance of the MTHFR genotype for plasma ho- mocysteine, we hypothesized that patients carrying MTHFR 677CT and/or 1298AC mutations develop higher plasma homocysteine concentrations compared with noncarriers. To test this hypothesis, we conducted a prospective cohort study in which all patients received 66% nitrous oxide. Patients were unaware of their MTHFR genotype before the study. We measured plasma total homocys- This article is accompanied by an Editorial View. Please see: Hogan K: Pharmacogenetics of nitrous oxide: Standing at the crossroads. ANESTHESIOLOGY 2008; 109:5– 6.  * Associate Professor, Department of Anesthesiology, Critical Care Medicine and Pain Therapy, Medical University of Vienna. Current position: Assistant Professor, Department of Anesthesiology, Washington University School of Med- icine, St. Louis, Missouri, † Medical Student, ‡ Staff Anesthesiologist, Department of Anesthesiology, Critical Care Medicine and Pain Therapy, § Associate Profes- sor, Core Unit for Medical Statistics and Informatics, Section of Clinical Biomet- rics,  Associate Professor, Clinical Institute of Medical and Chemical Laboratory Diagnostics, Medical University of Vienna. Received from the Department of Anesthesiology, Critical Care Medicine and Pain Therapy, Medical University of Vienna, Vienna, Austria, and the Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri. Submitted for publication November 27, 2007. Accepted for publication March 5, 2008. Supported by a Research Grant from the European Society of Anaesthe- siology, Brussels, Belgium, and registered at ClinicalTrials.gov (identifier: NCT00482456). Presented at the 16th Annual Meeting of the International Society for Anesthetic Pharmacology, San Francisco, California, October 13–17, 2007, and the Annual Meeting of the American Society of Anesthesiologists, San Francisco, California, October 13–17, 2007. Address correspondence to Dr. Nagele: Department of Anesthesiology, Wash- ington University School of Medicine, 660 South Euclid Avenue, Box 8054, St. Louis, Missouri 63110. nagelep@wustl.edu. Information on purchasing reprints may be found at www.anesthesiology.org or on the masthead page at the beginning of this issue. ANESTHESIOLOGY’s articles are made freely accessible to all readers, for personal use only, 6 months from the cover date of the issue. Anesthesiology, V 109, No 1, Jul 2008 36 Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/109/1/36/655923/0000542-200807000-00008.pdf by guest on 12 June 2022