rs1056836 — CYP1B1 Leu432Val

Phase I detoxification enzyme that hydroxylates estradiol to potentially genotoxic 4-hydroxyestradiol and activates environmental procarcinogens including PAHs

See your personal result for CYP1B1

Upload your DNA data to find out which genotype you carry and what it means for you.

Upload your DNA data

Works with 23andMe, AncestryDNA, and other DNA test exports. Results in under 60 seconds.

CYP1B1 Leu432Val — The Estrogen and Toxin Activator

CYP1B1 is a Phase I cytochrome P450 enzyme11 Phase I cytochrome P450 enzyme
Phase I enzymes add reactive groups (usually hydroxyl -OH) to molecules, making them more water-soluble and preparing them for Phase II conjugation and excretion with a dual role that makes it uniquely important in cancer biology. First, it converts estradiol into 4-hydroxyestradiol (4-OH-E2)22 4-hydroxyestradiol (4-OH-E2)
A catechol estrogen metabolite that can be further oxidized to reactive quinones capable of forming depurinating DNA adducts — direct chemical damage to DNA, the most genotoxic of the estrogen metabolites. Second, it activates environmental procarcinogens including polycyclic aromatic hydrocarbons (PAHs) from tobacco smoke and charred foods, and heterocyclic amines from cooked meat. The Leu432Val variant (rs1056836) sits in the heme-binding domain33 heme-binding domain
The catalytic core of the enzyme where the iron-containing heme group binds substrates and performs oxidation reactions and alters the enzyme's catalytic properties toward both substrates.

Unlike most liver-dominant CYP450 enzymes, CYP1B1 is primarily expressed in extrahepatic tissues — breast, uterus, ovary, prostate, lung, and kidney — precisely the organs where its estrogen-metabolizing and carcinogen-activating roles matter most. Its expression is controlled by the aryl hydrocarbon receptor (AHR)44 aryl hydrocarbon receptor (AHR)
A ligand-activated transcription factor that responds to environmental pollutants, dietary compounds from cruciferous vegetables, and tryptophan metabolites, meaning that exposure to dioxins, PAHs, or cruciferous vegetable compounds like DIM and I3C directly upregulates CYP1B1 activity.

The Mechanism

The rs1056836 variant causes a leucine-to-valine substitution at position 432 in the heme-binding domain. On the genomic plus strand (as reported by 23andMe), the G allele encodes leucine (wild-type) and the C allele encodes valine (variant). The amino acid change alters the active site geometry, shifting the enzyme's preference between competing hydroxylation pathways.

Enzyme kinetics studies55 Enzyme kinetics studies
Shimada T et al. Catalytic properties of polymorphic human cytochrome P450 1B1 variants. Carcinogenesis, 1999 showed that Val432 forms of CYP1B1 produce a higher ratio of 4-hydroxyestradiol to 2-hydroxyestradiol compared to Leu432 forms. The 4-hydroxylation pathway is concerning because 4-OH-E2 can be oxidized to semiquinones and quinones66 semiquinones and quinones
Reactive electrophiles that form covalent bonds with DNA bases, creating unstable depurinating adducts that leave behind mutagenic apurinic sites that directly damage DNA. The 2-hydroxylation pathway, by contrast, produces less genotoxic metabolites.

A separate study by Li et al.77 study by Li et al.
Li DN et al. Polymorphisms in P450 CYP1B1 affect the conversion of estradiol to the potentially carcinogenic metabolite 4-hydroxyestradiol. Pharmacogenetics, 2000 found that the Val432-to-Leu change increases the Km (reduces binding affinity) for estradiol hydroxylation at least 3-fold, meaning the Leu432 form is less efficient at metabolizing estradiol overall. The net effect of the Val432 variant is both greater throughput and a more dangerous product ratio.

The safety of CYP1B1's reactive metabolites depends entirely on downstream Phase II enzymes — GSTP1, GSTM1, and NQO1 — which conjugate and neutralize the catechol estrogen quinones before they can damage DNA. When Phase II capacity is insufficient to handle the Phase I output, oxidative damage accumulates.

The Evidence

Endometrial cancer. A meta-analysis of 12 studies88 meta-analysis of 12 studies
Wang F et al. Association of CYP1B1 gene polymorphisms with susceptibility to endometrial cancer: a meta-analysis. Eur J Cancer Prev, 2011 encompassing 3,605 cases and 5,692 controls found that the Val432 allele significantly increases endometrial cancer risk (OR 1.23, 95% CI 1.06-1.43). This is biologically coherent: the endometrium is an estrogen-responsive tissue where CYP1B1 is expressed, and increased 4-OH-E2 production would create local genotoxic exposure.

Lung cancer. A meta-analysis of 10 studies99 meta-analysis of 10 studies
Xu W et al. Current evidence on the relationship between CYP1B1 polymorphisms and lung cancer risk: a meta-analysis. Mol Biol Rep, 2012 with 7,067 cases and 9,374 controls found that individuals homozygous for Val432 had a 39.7% higher lung cancer risk compared to Leu432 homozygotes. This likely reflects CYP1B1's role in activating PAHs from tobacco smoke rather than estrogen metabolism.

Breast cancer. Despite the strong mechanistic rationale, epidemiological evidence for breast cancer has been inconsistent. A comprehensive meta-analysis1010 comprehensive meta-analysis
Liu JY et al. Association between the CYP1B1 polymorphisms and risk of cancer: a meta-analysis. Mol Genet Genomics, 2015 found the Leu432Val variant associated with endometrial and lung cancer risk but not consistently with breast cancer across populations. Gene-environment interactions — particularly smoking status and Phase II enzyme capacity — may explain the inconsistent breast cancer findings.

Bone density. A study in postmenopausal women1111 study in postmenopausal women
Napoli N et al. The Val432Leu polymorphism of the CYP1B1 gene is associated with differences in estrogen metabolism and bone density. Bone, 2009 found that Leu432 allele carriers (on the coding strand) had significantly higher urinary estrogen metabolites and lower bone mineral density at the lumbar spine (0.931 vs 1.009 g/cm2, p=0.03) and femoral neck (0.693 vs 0.748 g/cm2, p=0.03) compared to Val/Val homozygotes. This paradoxical finding — where higher estrogen catabolism leads to a hypoestrogenic state — suggests the overall rate of estrogen metabolism matters for bone health alongside the specific pathway balance.

Practical Implications

The actionable message for Val432 carriers centers on supporting Phase II detoxification to safely neutralize the increased 4-hydroxyestradiol output. Cruciferous vegetables (broccoli, Brussels sprouts, cauliflower, kale) contain indole-3-carbinol (I3C) and sulforaphane1212 indole-3-carbinol (I3C) and sulforaphane
I3C is converted to DIM in the stomach; sulforaphane activates Nrf2, the master regulator of Phase II enzyme expression that both modulate CYP1B1 activity and upregulate Phase II enzymes including glutathione S-transferases and NQO1. Diindolylmethane (DIM)1313 Diindolylmethane (DIM)
The acid-catalyzed dimer of I3C formed in the gut; available as a supplement shifts estrogen metabolism toward the protective 2-hydroxylation pathway and away from the genotoxic 4-hydroxylation pathway.

Minimizing exposure to PAH-rich environments (tobacco smoke, heavily charred foods, industrial pollutants) is particularly important for Val432 carriers, since CYP1B1 both responds to AHR activation by these compounds and more efficiently converts them to DNA-damaging metabolites.

For women, monitoring estrogen-related health markers becomes more relevant with this variant, especially in the context of hormone replacement therapy or conditions associated with estrogen exposure.

Interactions

The most critical interaction is with Phase II conjugation enzymes. GSTP1 (rs1695), GSTM1 (null/present), and NQO1 (rs1800566) detoxify the reactive catechol estrogen quinones produced by CYP1B1. A Val432 carrier with compromised Phase II capacity (e.g., GSTM1 null deletion or NQO1*2 homozygosity) faces a compounded risk: increased production of reactive metabolites with decreased capacity to neutralize them. Published studies have confirmed that combined CYP1B1/GSTM1/GSTP1 genotypes modify cancer risk more than any single variant alone.

The AHR variant rs2066853 is also relevant because AHR controls CYP1B1 transcription. Altered AHR signaling could modify the degree to which environmental exposures induce CYP1B1 expression, affecting the overall burden of Phase I metabolite production.

COMT (catechol-O-methyltransferase) provides another detoxification route for catechol estrogens via methylation. Carriers of both CYP1B1 Val432 and slow COMT variants may have a more unfavorable estrogen metabolite profile, as both increased 4-OH-E2 production and decreased methylation clearance compound the genotoxic burden.

Nutrient Interactions

cruciferous vegetables altered_metabolism
DIM altered_metabolism

Genotype Interpretations

What each possible genotype means for this variant.

GG Standard Activity Normal

Normal CYP1B1 catalytic activity with standard estrogen metabolism profile

The Leu432 form of CYP1B1 has a lower Vmax-to-Km ratio for estradiol 4-hydroxylation compared to the Val432 form, based on recombinant enzyme studies by Shimada et al. (1999) and Li et al. (2000). This means your CYP1B1 enzyme converts less estradiol to the potentially genotoxic 4-hydroxyestradiol metabolite per unit time.

In the meta-analyses of cancer risk, the Leu/Leu genotype serves as the reference group against which increased risk is measured. In the lung cancer meta-analysis (Xu et al. 2012), Leu/Leu homozygotes had the lowest risk. In the endometrial cancer meta-analysis (Wang et al. 2011), the CC genotype (Leu/Leu on the coding strand) was similarly the lowest risk group.

Your Phase I estrogen metabolism through CYP1B1 favors the less genotoxic pathway, reducing the burden on downstream Phase II conjugation enzymes (GSTP1, GSTM1, NQO1) to neutralize reactive catechol estrogen quinones.

CG Increased Activity Intermediate

One copy of the Val432 variant — moderately increased estradiol 4-hydroxylation

With one Val432 allele, your CYP1B1 activity is intermediate between the Leu/Leu and Val/Val genotypes. The Val432 form shows a higher ratio of estradiol 4-hydroxylation to 2-hydroxylation (Shimada et al. 1999, 2001), and the heterozygous state produces a mix of both enzyme forms.

In the endometrial cancer meta-analysis (Wang et al. 2011), heterozygous carriers were included in the risk group (CG+CC vs GG on the coding strand: OR 1.24, 95% CI 1.08-1.43). The lung cancer meta-analysis (Xu et al. 2012) found an intermediate risk elevation for carriers of the Val allele (26.3% increased risk per allele copy).

The clinical significance of the heterozygous state depends substantially on your Phase II enzyme capacity. If your GSTP1, GSTM1, and NQO1 pathways are intact, the moderately increased 4-OH-E2 production is likely adequately conjugated and neutralized. The risk becomes more meaningful when Phase II capacity is also compromised.

CC High Activity High

Two copies of the Val432 variant — significantly increased estradiol 4-hydroxylation and procarcinogen activation

With two Val432 alleles, both of your CYP1B1 enzyme copies have the altered heme-binding domain geometry that favors 4-hydroxylation over 2-hydroxylation of estradiol. Enzyme kinetics studies (Shimada et al. 1999, 2001) confirmed that Val432 forms consistently produce a higher 4-OH-E2 to 2-OH-E2 ratio. Li et al. (2000) showed the Val432 form has a lower Km (higher affinity) for estradiol, meaning it binds and metabolizes estradiol more efficiently.

In the lung cancer meta-analysis (Xu et al. 2012, 7,067 cases/9,374 controls), Val/Val homozygotes had a 39.7% higher risk compared to Leu/Leu homozygotes. In the endometrial cancer meta-analysis (Wang et al. 2011), the Val allele increased risk with OR 1.23 per allele copy. The comprehensive meta-analysis by Liu et al. (2015) confirmed associations with endometrial and lung cancer while also finding a reduced ovarian cancer risk — illustrating the tissue-specific nature of CYP1B1's effects.

The bone density study by Napoli et al. (2009) provides an interesting contrast: postmenopausal women with the Leu allele (not Val) had lower BMD, likely because overall increased estrogen catabolism (regardless of pathway) creates a hypoestrogenic state. This highlights that CYP1B1 variant effects differ between cancer risk (pathway ratio matters) and bone health (total metabolism rate matters).

The critical modifier is Phase II enzyme status. If you also carry variants in GSTP1 (rs1695), have GSTM1 null deletion, or carry NQO1*2 (rs1800566), the reactive catechol estrogen quinones produced by CYP1B1 may not be adequately neutralized, compounding genotoxic risk.