Reproductive factors have been linked to both breast cancer and DNA
methylation, suggesting methylation as an important mechanism by which
reproductive factors impact on disease risk. However, few studies have
investigated the link between reproductive factors and DNA methylation
in humans. Genome-wide methylation in peripheral blood lymphocytes of
376 healthy women from the prospective EPIC study was investigated using
LUminometric Methylation Assay (LUMA). Also, methylation of 458877 CpG
sites was additionally investigated in an independent group of 332
participants of the EPIC-Italy sub-cohort, using the Infinium
HumanMethylation 450 BeadChip. Multivariate logistic regression and
linear models were used to investigate the association between
reproductive risk factors and genome wide and CpG-specific DNA
methylation, respectively. Menarcheal age was inversely associated with
global DNA methylation as measured with LUMA. For each yearly increase
in age at menarche, the risk of having genome wide methylation below
median level was increased by 32% (OR: 1.32, 95% CI: 1.14-1.53). When
age at menarche was treated as a categorical variable, there was an
inverse dose-response relationship with LUMA methylation levels
(OR12-14vs.<= 11 yrs: 1.78, 95% CI: 1.01-3.17 and OR >= 15vs.<= 11 yrs:
4.59, 95% CI: 2.04-10.33; P for trend<0.0001). However, average levels
of global methylation as measured by the Illumina technology were not
significantly associated with menarcheal age. In locus by locus
comparative analyses, only one CpG site had significantly different
methylation depending on the menarcheal age category examined, but this
finding was not replicated by pyrosequencing in an independent data set.
This study suggests a link between age at menarche and genome wide DNA
methylation, and the difference in results between the two arrays
suggests that repetitive element methylation has a role in the
association. Epigenetic changes may be modulated by menarcheal age, or
the association may be a mirror of other important changes in early life
that have a detectable effect on both methylation levels and menarcheal
age