Progestogen levels, progesterone receptor gene polymorphisms, and mammographic density changes: results from the Postmenopausal Estrogen/Progestin Interventions Mammographic Density Study.

OBJECTIVE Estrogen plus progestin therapy (EPT) in postmenopausal women increases breast cancer risk and mammographic density to a higher extent than does estrogen therapy alone. Data from the randomized placebo-controlled Postmenopausal Estrogen/Progestinv Interventions trial showed that EPT-induced increases in serum estrone and estrone sulfate levels were positively correlated with increases in mammographic density. Here, after adjusting for serum estrone and estrone sulfate levels, we investigated the roles of posttreatment serum progestogen increase and of progesterone receptor gene (PGR) genetic variations on changes in mammographic density. METHODS We measured the percent mammographic density and serum progestogen levels in 280 Postmenopausal Estrogen/Progestin Interventions trial participants randomized to EPT treatment. Analyses of genetic variations in PGR were limited to 260 white women for whom we successfully obtained PGR genotypes. We used linear regression analyses to determine how an increase in progestogen levels and PGR genetic variation influenced mammographic density change after EPT. RESULTS The increase in posttreatment serum progestogen level was positively associated with greater increases in mammographic density after adjustment for covariates (P trend = 0.044). Compared with women in the lowest quartile of serum progestogen level, women in the highest quartile experienced a 3.5% greater increase in mammographic density (P = 0.046). We did not find a strong indication that genetic variation in PGR was associated with mammographic density increase or modified the association with serum progestogen; however, confidence in these null findings is constrained by our small sample size. CONCLUSIONS Our results suggest that higher serum progestogen levels resulting from EPT treatment lead to greater increases in mammographic density.