Heterogeneity in white blood cells has potential to confound DNA methylation measurements.
Epigenetic studies are commonly conducted on DNA from tissue samples. However, tissues are ensembles of cells that may each have their own epigenetic profile, and therefore inter-individual cellular heterogeneity may compromise these studies. Here, we explore the potential for such confounding on DNA methylation measurement outcomes when using DNA from whole blood. DNA methylation was measured using pyrosequencing-based methodology in whole blood (n = 50-179) and in two white blood cell fractions (n = 20), isolated using density gradient centrifugation, in four CGIs (CpG Islands) located in genes HHEX (10 CpG sites assayed), KCNJ11 (8 CpGs), KCNQ1 (4 CpGs) and PM20D1 (7 CpGs). Cellular heterogeneity (variation in proportional white blood cell counts of neutrophils, lymphocytes, monocytes, eosinophils and basophils, counted by an automated cell counter) explained up to 40% (p<0.0001) of the inter-individual variation in whole blood DNA methylation levels in the HHEX CGI, but not a significant proportion of the variation in the other three CGIs tested. DNA methylation levels in the two cell fractions, polymorphonuclear and mononuclear cells, differed significantly in the HHEX CGI; specifically the average absolute difference ranged between 3.4-15.7 percentage points per CpG site. In the other three CGIs tested, methylation levels in the two fractions did not differ significantly, and/or the difference was more moderate. In the examined CGIs, methylation levels were highly correlated between cell fractions. In summary, our analysis detects region-specific differential DNA methylation between white blood cell subtypes, which can confound the outcome of whole blood DNA methylation measurements. Finally, by demonstrating the high correlation between methylation levels in cell fractions, our results suggest a possibility to use a proportional number of a single white blood cell type to correct for this confounding effect in analyses.
Icelandic Heart Association, Kopavogur, Iceland.
Bjorn T
BT
Adalsteinsson
Haukur
H
Gudnason
Thor
T
Aspelund
Tamara B
TB
Harris
Lenore J
LJ
Launer
Gudny
G
Eiriksdottir
Albert V
AV
Smith
Vilmundur
V
Gudnason
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018899:CpG Islands; D019175:DNA Methylation; D044127:Epigenesis, Genetic; D005260:Female; D014644:Genetic Variation; D018398:Homeodomain Proteins / Q000235:genetics; D006801:Humans; D051657:KCNQ1 Potassium Channel / Q000235:genetics; D007963:Leukocytes, Mononuclear / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D009504:Neutrophils / Q000378:metabolism*; D024661:Potassium Channels, Inwardly Rectifying / Q000235:genetics; D017422:Sequence Analysis, DNA; D014157:Transcription Factors / Q000235:genetics; D000099161:Kcnj11 Channel
D018398:Homeodomain Proteins; D051657:KCNQ1 Potassium Channel; D024661:Potassium Channels, Inwardly Rectifying; D014157:Transcription Factors; D000099161:Kcnj11 Channel; C078492:HHEX protein, human; C496909:KCNQ1 protein, human