Is There a Link Between Genome-Wide Hypomethylation in Blood and Cancer Risk?

Abstract

Cancer cells display widespread genetic and epigenetic abnormalities, but the contribution to disease risk, particularly in normal tissue before disease, is not yet established. Genome-wide hypomethylation occurs frequently in tumors and may facilitate chromosome instability, aberrant transcription and transposable elements reactivation. Several epidemiologic case-control studies have reported genomic hypomethylation in peripheral blood of cancer patients, suggesting a systemic effect of hypomethylation on disease predisposition, which may be exploited for biomarker development. However, more recent studies have failed to reproduce this. Here, we report a meta-analysis, indicating a consistent inverse association between genomic 5-methylcytosine levels and cancer risk [95% confidence interval (CI), 1.2-6.1], but no overall risk association for studies using surrogates for genomic methylation, including methylation at the LINE-1 repetitive element (95% CI, 0.8-1.7). However, studies have been highly heterogeneous in terms of experimental design, assay type, and analytical methods. We discuss the limitations of the current approaches, including the low interindividual variability of surrogate assays such as LINE1 and the importance of using prospective studies to investigate DNA methylation in disease risk. Insights into genomic location of hypomethylation, from recent whole genome, highresolution methylome maps, will help address this interesting and clinically important question. Cancer Prev Res; 5(12); 1345-57. Ó2012 AACR.

LINE1methylation levels associated with increased bladder cancer risk in pre-diagnostic blood DNA among US (PLCO) and European (ATBC) cohort study participants

“…[10][11][12][13] For example, if a particular blood cell subtype were associated with both LINE1%5mC level and bladder cancer risk, then the varying proportions could either confound or modify the association, resulting in different pre-and post-bladder cancermethylation associations. In bladder cancer, alterations have been observed primarily in the number of neutrophils, and in some cases, leukocytosis (increased leukocyte/white blood cell count) has been associated with poor prognosis, but not in the earliest stages of bladder cancer.…”

“…13 A second meta-analysis did not, with the exception of studies using a total 5mC assay. 12 Included in this meta-analysis were two retrospective studies, one of breast 18 and one of renal cancer, 19 that showed increased cancer risk among those with higher methylation levels using the luminometric methylation assay (LUMA) and LINE1 assays, respectively. Although the Xu et al, 18 study did not find associations with breast cancer risk using the LINE1 assay, the inconsistency was considered to be reflecting the different genomic areas examined by each assay: the LINE1 assay measures CpG sites throughout LINE1 sequences spanning the genome; the LUMA assay measures 5mC levels in the C m CGG motif, a sequence over-represented in gene promoter regions.…”

“…4 In this study, LINE1%5mC levels were higher among participants in PLCO compared with ATBC, although similar comparative associations were observed in both studies and the levels observed are within the range of those observed in other studies. [4][5][6][7]12,13 One reason for the different levels in each study may be the ethnic variation in methylation levels between the study populations (US vs. Finnish). Also, PLCO includes both smokers and non-smokers of both sexes, whereas ATBC is comprised of male ever smokers (although the methylation difference was still apparent among male smokers).…”

“…25 Although the assay used in most recent studies to quantify LINE1 methylation (bisulfite treatment and pyrosequencing) does not measure total CpG content (as does the HPCE analysis), the accuracy, high-throughput and quantitative nature of this surrogate biomarker makes it attractive for use in large epidemiologic studies. 25,26 Quantification of LINE1 for both PLCO and ATBC was conducted at the same laboratory using the same methods. Bisulfite DNA modification (0.25-0.5 μg/μl) was conducted using the EZ DNA Methylation Kit (Zymo Research) according to the manufacturer's instructions.…”

“…7 These retrospective findings support an association between DNA hypomethylation and bladder carcinogenesis; however, since global methylation at CpG loci throughout the genome may change in response to environmental exposures, immune response, and the carcinogenic process itself, findings LINE1 methylation levels associated with increased bladder cancer risk in pre-diagnostic blood DNA among US (PLCO) and European (ATBC) cohort study participants observed retrospectively could be due to epigenetic changes that occur subsequent to cancer diagnosis. [10][11][12] Additionally, questionnaire data collected post-diagnosis could be subject to recall bias. We therefore designed a nested case-control study within the Prostate, Lung, Colorectal, Ovarian Cancer Screening Trial (PLCO), a prospective cohort conducted in the United States, selecting male and female bladder cancer cases who were cancer free and provided blood samples prior to diagnosis.…”