Improved understanding of the etiology of estrogen receptor-α (ERα)-negative and progesterone receptor (PR)-negative breast cancers may permit improved risk prediction. In vitro studies implicate DNA hypermethylation of the ERα and PR promoters in the pathogenesis of ERα-negative and PR-negative tumors, but results are not definitive. We evaluated 200 invasive breast cancers selected from a population-based casecontrol study. DNA extracted from fixed tumor tissue cores was tested using MethyLight to assess DNA methylation at four CpG islands: ESR1 promoters A and B; PGR promoters A and B; and a CpG shore, ESR1 promoter C. DNA methylation results were compared with levels of ERα and PR, tumor characteristics, and breast cancer riskfactors. We observed mild to moderate DNA methylation levels in most tumors for ESR1 promoters A and B and PGR promoter B, and a few tumors showed mild methylation in PGR promoter A. In contrast, ESR1 promoter C showed a wide range of methylation and was weakly correlated with lower expression levels of ERα (β = -0.26; P < 0.0001) and PR (β = -0.25; P < 0.0001). The percentage of tumors with methylated PGR promoters A and B was significantly higher for tumors with low ERα (A, Fisher's test P = 0.0001; B, P = 0.033) and PR levels (A, P = 0.0006; B, P = 0.001). Our data suggest that the relationships between DNA methylation of ESR1 and PGR promoters and protein expression are weak and unlikely to represent a predominant mechanism of receptor silencing. In contrast to CpG islands, ESR1 promoter C showed a wider range of methylation levels and inverse associations with ERα and PR expression.
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