CYP3A7*1C allele is associated with reduced levels of 2-hydroxylation pathway oestrogen metabolites

Abstract

Background: Endogenous sex hormones are well-established risk factors for breast cancer; the contribution of specific oestrogen metabolites (EMs) and/or ratios of specific EMs is less clear. We have previously identified a CYP3A7*1C allele that is associated with lower urinary oestrone (E₁) levels in premenopausal women. The purpose of this analysis was to determine whether this allele was associated with specific pathway EMs.

Methods: We measured successfully 12 EMs in mid-follicular phase urine samples from 30 CYP3A7*1C carriers and 30 non-carriers using HPLC-MS/MS.

Results: In addition to having lower urinary E₁ levels, CYP3A7*1C carriers had significantly lower levels of four of the 2-hydroxylation pathway EMs that we measured (2-hydroxyestrone, P=1.1 × 10^-12; 2-hydroxyestradiol, P=2.7 × 10^-7; 2-methoxyestrone, P=1.9 × 10^-12; and 2-methoxyestradiol, P=0.0009). By contrast, 16α-hydroxylation pathway EMs were slightly higher in carriers and significantly so for 17-epiestriol (P=0.002).

Conclusions: The CYP3A7*1C allele is associated with a lower urinary E₁ levels, a more pronounced reduction in 2-hydroxylation pathway EMs and a lower ratio of 2-hydroxylation:16α-hydroxylation EMs in premenopausal women. To further characterise the association between parent oestrogens, EMs and subsequent risk of breast cancer, characterisation of additional genetic variants that influence oestrogen metabolism and large prospective studies of a broad spectrum of EMs will be required.

Figure 1

Steroid hormone synthesis and endogenous oestrogen metabolism in humans. The 14 oestrogen metabolites (EMs) formed by hydroxylation of parent oestrogens (E₁ and E₂), at the 2-, 4- and 16α-positions of the carbon ring that were measured are shown within the red box. Enzymes (cytochrome P450 (CYPs) and catechol O-methyltransferase (COMT)) involved in oestrogen metabolism are in red. E₁G, measured in our previous analysis (Johnson et al, 2012), is an E₁ conjugate present in urine. The E₁ measured in this analysis (after hydrolysis of glucuronide and sulphate conjugates in the first step of the LC-MS/MS protocol) is highly correlated with E₁G (Spearman's correlation, ρ=0.70, P<0.0001). Abbreviations: E₁=oestrone; E₂=estradiol; 2-OHE₁=2-hydroxyestrone; 2-OHE₂= 2-hydroxyestradiol; 2-MeOE₁=2-methoxyestrone; 2-MeOE₂=2-methoxyestradiol; 3-MeoE₁=2-hydroxyestrone-3-methyl ether; 4-OHE₁=4-hydroxyestrone; 4-MeOE₁=4-methoxyestrone; 4-MeOE₂=4-methoxyestradiol; 16α-OHE₁=16α-hydroxyestrone; E₃=estriol; 16-KetoE₂=16-ketoestradiol; 17-epiE₃=17-epiestriol and 17β-HSD=17β-hydroxysteroid.

Figure 2

Geometric mean levels (pmol mg⁻¹ creatinine) of 12 EMs that we measured in urine samples from 60 premenopausal women; 30 carriers of the CYP3A7*1C allele (light grey) and 30 non-carriers (dark grey). Estimates are the average of two samples per woman, taken on sequential days, calculated to be at or around ovulation based on the woman's usual menstrual cycle length. Error bars represent s.e. Levels of two of the 4-hydroxylation pathway EMs (4-MeOE₁ and 4-MeOE₂) were below detection in 92 (4-MeOE₁) and 107 (4-MeOE₂) of the samples analysed. These two EMs were, therefore, excluded.