title: The effect of 5-fluorouracil/leucovorin chemotherapy on CpG methylation, or the confounding role of leukocyte heterogeneity: An illustration.
abstract: Blood-based epigenome-wide association studies that aim at comparing CpG methylation between colorectal cancer (CRC) patients and controls can lead to the discovery of diagnostic or prognostic biomarkers. Numerous confounders can lead to spurious associations. We aimed to see if 5-fluorouracil (5-FU)/leucovorin chemotherapy administered to cases prior to the collection of their blood has an effect on methylation. 304 patients who received treatment and 273 who did not were profiled on the HumanMethylation450 array. Association tests were adjusted for confounders, including proxies for leukocyte cell counts. There were substantial methylation differences between these two groups that vanished once the leukocyte heterogeneity was accounted for. We observed a significant decrease of T cells in the treatment group (CD4+: p=10(-6); CD8+: p=0.036) and significant increase of NK cells (p=0.05) and monocytes (p=0.0006). 5-FU/leucovorin has no effect on global and local blood-based methylation profiles, other than through differences in the leukocyte compositions that the treatment induced.
authors.affiliation: Ontario Institute for Cancer Research, Toronto, Ontario M5G 0A3, Canada. Electronic address: mathieu.lemire@oicr.on.ca.
authors.forename: Mathieu
authors.initials: M
authors.lastname: Lemire
authors.affiliation: Ontario Institute for Cancer Research, Toronto, Ontario M5G 0A3, Canada. Electronic address: hassan.zaidi@oicr.on.ca.
authors.forename: Syed H E
authors.initials: SH
authors.lastname: Zaidi
authors.affiliation: Department of Medicine, University of Ottawa, Ottawa Hospital Research Institute, Ottawa, Ontario K1H 8L6, Canada. Electronic address: bzanke@me.com.
authors.forename: Brent W
authors.initials: BW
authors.lastname: Zanke
authors.affiliation: Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada; Division of General Surgery, Toronto General Hospital, Toronto, Ontario M5G 2C4, Canada. Electronic address: steven.gallinger@uhn.on.ca.
authors.forename: Steven
authors.initials: S
authors.lastname: Gallinger
authors.affiliation: Ontario Institute for Cancer Research, Toronto, Ontario M5G 0A3, Canada; Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5S 1A1, Canada; Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A1, Canada. Electronic address: tom.hudson@oicr.on.ca.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Hudson
authors.affiliation: Department of Surgery, University Health Network, Toronto, Ontario M5G 2C4, Canada; Prosserman Centre for Health Research, Lunenfeld-Tanenbaum Research Institute, Toronto, Ontario M5G 1X5, Canada. Electronic address: sean.cleary@uhn.ca.
authors.forename: Sean P
authors.initials: SP
authors.lastname: Cleary
mesh_terms: D000328:Adult; D000368:Aged; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000627:therapeutic use*; D015179:Colorectal Neoplasms / Q000097:blood / Q000188:drug therapy* / Q000235:genetics*; D018899:CpG Islands / Q000235:genetics*; D019175:DNA Methylation / Q000187:drug effects*; D044127:Epigenesis, Genetic / Q000187:drug effects; D057890:Epigenomics / Q000379:methods; D005260:Female; D005472:Fluorouracil / Q000008:administration & dosage; D006801:Humans; D002955:Leucovorin / Q000008:administration & dosage; D007958:Leukocyte Count; D007962:Leukocytes / Q000187:drug effects / Q000378:metabolism; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D012907:Smoking
chemical_list: D002955:Leucovorin; D005472:Fluorouracil
keywords: 5-Fluorouracil; Chemotherapy; Colorectal cancer; Heterogeneity; Leukocytes; Methylation