title: Global DNA methylation measured by liquid chromatography-tandem mass spectrometry: analytical technique, reference values and determinants in healthy subjects.
abstract: BACKGROUND
  Alterations in global DNA methylation are implicated in various pathobiological processes. We describe a liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) method for determination of cytosine and 5-methylcytosine in DNA.
  
  
  METHODS
  DNA was hydrolyzed using formic acid. Cytosine and 5-methylcytosine were separated by gradient-elution reversed-phase chromatography with a mobile phase containing nonafluoropentanoic acid (NFPA) as ion-pairing reagent and quantified using stable isotope dilution LC-ESI-MS/MS. The method was applied to DNA isolated from leukocytes of healthy volunteers.
  
  
  RESULTS
  Linear calibration curves were obtained in the range 0.111-4.422 ng/microL [mean correlation co-efficient 0.9983 (SD=0.0011), n=9] for cytosine and 0.0048-0.1936 ng/microL [mean correlation coefficient 0.9991 (SD=0.0010), n=9] for 5-methylcytosine. The intra- and inter-assay CVs for the 5-methylcytosine/total cytosine ratio (mCyt/tCyt) was 1.7% (n=9) and 3.5% (n=8) for calf thymus DNA (mean mCyt/tCyt ratio 6.5%), and 4.5% (n=6) and 6.5% (n=14), respectively for pBR322 DNA (mean mCyt/tCyt ratio 0.48%). The limit of detection (signal-to-noise ratio 3) was 2 pg on-column for cytosine and 5-methylcytosine. In healthy subjects (n=109), the mCyt/tCyt ratio varied from 2.6% to 4.8% (median 4.1%). DNA methylation was negatively correlated to age, but only in subjects with the methylenetetrahydrofolate reductase (MTHFR) 677 TT genotype (p=0.046). No association with B-vitamin status was observed.
  
  
  CONCLUSIONS
  This LC-ESI-MS/MS method is easy to perform and offers reproducibility, selectivity and sensitivity for studying DNA methylation. The method allows a sample throughput of approximately 200 samples/week. The MTHFR C677T genotype influences age-related changes in DNA methylation.
authors.affiliation: Department of Clinical Chemistry, VU University Medical Center, Amsterdam, The Netherlands.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Kok
authors.forename: Desirée E C
authors.initials: DE
authors.lastname: Smith
authors.forename: Rob
authors.initials: R
authors.lastname: Barto
authors.forename: Annemieke M W
authors.initials: AM
authors.lastname: Spijkerman
authors.forename: Tom
authors.initials: T
authors.lastname: Teerlink
authors.forename: Henk J
authors.initials: HJ
authors.lastname: Gellekink
authors.forename: Cornelis
authors.initials: C
authors.lastname: Jakobs
authors.forename: Yvo M
authors.initials: YM
authors.lastname: Smulders
mesh_terms: D000293:Adolescent; D000328:Adult; D000818:Animals; D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D004247:DNA / Q000378:metabolism; D019175:DNA Methylation; D005260:Female; D005492:Folic Acid / Q000097:blood; D005838:Genotype; D006801:Humans; D007962:Leukocytes / Q000378:metabolism; D008297:Male; D042965:Methylenetetrahydrofolate Reductase (NADPH2) / Q000235:genetics; D008875:Middle Aged; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D004247:DNA; C427203:calf thymus DNA; D005492:Folic Acid; D042965:Methylenetetrahydrofolate Reductase (NADPH2)