title: Temporal stability and determinants of white blood cell DNA methylation in the breakthrough generations study.
abstract: BACKGROUND
  Epigenome-wide association studies (EWAS) using measurements of blood DNA methylation are performed to identify associations of methylation changes with environmental and lifestyle exposures and disease risk. However, little is known about the variation of methylation markers in the population and their stability over time, both important factors in the design and interpretation of EWAS. We aimed to identify stable variable methylated probes (VMP), i.e., markers that are variable in the population, yet stable over time.
  
  
  METHODS
  We estimated the intraclass correlation coefficient (ICC) for each probe on the Illumina 450K methylation array in paired samples collected approximately 6 years apart from 92 participants in the Breakthrough Generations Study. We also evaluated relationships with age, reproductive and hormonal history, weight, alcohol intake, and smoking.
  
  
  RESULTS
  Approximately 17% of probes had an ICC > 0.50 and were considered stable VMPs (stable-VMPs). Stable-VMPs were enriched for probes located in "shores" bordering CpG islands, and at approximately 1.3 kb downstream from the transcription start site in the transition between the unmethylated promoter and methylated gene body. Both cross-sectional and longitudinal data analyses provided strong evidence for associations between changes in methylation levels and aging. Smoking-related probes at 2q37.1 and AHRR were stable-VMPs and related to time since quitting. We also observed associations between methylation and weight changes.
  
  
  CONCLUSIONS
  Our results provide support for the use of white blood cell DNA methylation as a biomarker of exposure in EWAS.
  
  
  CONCLUSIONS
  Larger studies, preferably with repeated measures over time, will be required to establish associations between specific probes and exposures.
authors.affiliation: Epigenetics Unit, Department of Surgery and Cancer, Imperial College London, London, United Kingdom. j.flanagan@imperial.ac.uk.
authors.forename: James M
authors.initials: JM
authors.lastname: Flanagan
authors.affiliation: Division of Genetics and Epidemiology, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Mark N
authors.initials: MN
authors.lastname: Brook
authors.affiliation: Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Nick
authors.initials: N
authors.lastname: Orr
authors.affiliation: Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Tomczyk
authors.affiliation: Division of Genetics and Epidemiology, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Penny
authors.initials: P
authors.lastname: Coulson
authors.affiliation: Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Olivia
authors.initials: O
authors.lastname: Fletcher
authors.affiliation: Division of Genetics and Epidemiology, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Jones
authors.affiliation: Division of Genetics and Epidemiology, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Minouk J
authors.initials: MJ
authors.lastname: Schoemaker
authors.affiliation: Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Alan
authors.initials: A
authors.lastname: Ashworth
authors.affiliation: Division of Genetics and Epidemiology, The Institute of Cancer Research, London, United Kingdom. Division of Breast Cancer Research, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Anthony
authors.initials: A
authors.lastname: Swerdlow
authors.affiliation: Epigenetics Unit, Department of Surgery and Cancer, Imperial College London, London, United Kingdom. Division of Medicine, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Robert
authors.initials: R
authors.lastname: Brown
authors.affiliation: Division of Genetics and Epidemiology, The Institute of Cancer Research, London, United Kingdom. Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, London, United Kingdom.
authors.forename: Montserrat
authors.initials: M
authors.lastname: Garcia-Closas
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019175:DNA Methylation / Q000235:genetics*; D005260:Female; D055106:Genome-Wide Association Study / Q000379:methods*; D006801:Humans; D007962:Leukocytes / Q000378:metabolism*; D008297:Male; D008875:Middle Aged