Cytochrome P450 1B1
CYP1B1 C>G Val432Leu
The 1000 Genomes Project Database and the genome Aggregation Database (gnomAD) reports global frequencies of 61.5% and 50.12% respectively for the variant G-allele (NIH). The G allele frequencies vary widely among different ethnicities, for example 18.3% in African populations versus 90.9% in East Asian populations. (NIH)
CYP1B1 Gene Details
Cytochrome P450 1B1 (CYP1B1) is a member of the cytochrome P450 (CYP450) gene family and one of the major enzymes involved in oestrogen hydroxylation, a key reaction in hormonal carcinogenesis. Therefore, CYP1B1 is commonly implicated in hormone-mediated tumours, such as prostate, breast, endometrial, and ovarian cancer. CYB1B1, the 4-hydroxylation catalyst, has been reportedly expressed at particularly high levels in these cancers and is responsible for hormone metabolism and the formation of toxic metabolites from both endogenous and exogenous molecules.
Studies investigating possible associations between CYP1B1 polymorphisms and cancer risk have been inconsistent. It is suggested that a possible role of ethnic differences in genetic backgrounds and the environment may modify the effect of polymorphisms on cancer susceptibility.
Research suggests that the Val432Leu polymorphism in the CYP1B1 gene might modify the risk for cancers as the variant G-allele increases enzymatic activity involved in transforming oestrogens or pro-carcinogens into DNA-reactive electrophiles that may act as cancer-initiating agents.
As a preventative strategy, it is recommended that G-allele carriers reduce exposure to dietary and environmental procarcinogens i.e. PAHs, aromatic amines, nitrates and smoking.
CYP1B1 C>G Val432Leu
The human CYP1B1 gene, located at the 2p21–22 region, consists of three exons. The encoded CYP1B1 enzyme, a member of the larger CYP450 family, is involved in oestrogen and xenobiotic metabolism. Oestrogen hydroxylation and carcinogen activation, mediated by CYP450 enzymes, may occur in the liver, breast and prostate tissue, kidneys or other tissues.
CYP1B1 has been shown to catalyse 4-hydroxy oestrogen (4-OHE) formation in the breast, while it also bio-activates environmental pro-carcinogens such as polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic amines (HAAs). The enzyme also appears to play a role in the metabolism of certain anticancer agents used in the treatment of hormone-induced cancers. CYP1B1 is transcriptionally induced by compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin or dioxin and is regulated by several key transcriptional factors including the oestrogen receptor and aryl hydrocarbon receptor (AhR).
Previous reports have suggested that oestrogens exert carcinogenic effects by stimulating cell proliferation, which results in an increase in DNA replication errors. However, more recent studies have revealed another important mechanism namely that the oxidative metabolism of oestrogens can form DNA-damaging species. This predominantly refers to the 3,4-quinone, which can react with DNA to cause the mutations and therefore leading to the initiation of cancer.
It is known that 4-hydroxy-oestrogens are subsequently oxidized to form oestradiol-3,4-quinone, accompanied by the generation of reactive oxygen species (ROS). Oestradiol-3,4-quinone reacts with DNA, mainly forming unstable N3-adenine and N7-guanine DNA adducts and generating apurinic sites thought to give rise to mutations and initiate breast cancer. Hydroxy-oestrogens and quinones are detoxified by conjugation reactions catalysed by phase II metabolizing enzymes such as catechol-O-methyltransferase (COMT) and glutathione S-transferases (GSTs).
The rs1056836 SNP (Val432Leu), a non-synonymous polymorphism occurring in the third exon of CYP1B1, entails a C-to-G mutation that results in the substitution of Leu for Val. The mutation occurs within the heme-binding domain of the enzyme, potentially influencing its function. Studies have demonstrated a remarkable 3-fold increase in enzymatic activity in Val-allele carriers.
Several family-based and case-controlled studies have demonstrated that individuals with the hyperactive GG genotype are at higher risk for breast cancer, although research seem contradicting.
A more recent prospective case-control study of 427 female cases and 536 case controls in a Chinese population found that women homozygous for the variant genotype (GG) exhibited a significantly reduced risk of breast cancer compared to those with the wild-type genotype (CC).
The heterozygous genotype (CG) was not associated with breast cancer development. The precise mechanism underlying the protective effects of the variant allele of CYP1B1 remain unknown, although the authors (Qiu et al. 2018) suggested that an individual’s risk of developing breast cancer appeared to also be influenced by a combination of risk-associated alleles of COMT and GSTP1.
In support of these findings, a case-control study from Jiao et al. (2010) in a Chinese Han population (152 sporadic breast cancer cases and 156 healthy controls) revealed that the risk of breast cancer in individuals with the Leu/Leu (CC) genotype and Val/Leu (GC) genotype was increased 2.79- and 1.69-fold, respectively, compared to that of individuals homozygous for the Val-allele (GG). This also agrees with results from a study in Caucasians, in which Listgarten et al. (2004) found that the homozygous Leu/Leu (CC) and heterozygous Val/Leu (GC) genotypes conferred a 3.30- and 2.15-fold higher risk of breast cancer. In a study of a Shanghai population, Zheng et al. (2000) found that women with the Leu/Leu (CC) genotype had a 2.3-fold higher risk of breast cancer.
Another case-control study that analysed data from three populations, namely Japanese living in Nagano, Japan (JJ), Japanese Brazilians living in Sao Paulo (JB) and non-Japanese Brazilians living in Sao Paulo (NJB) found no statistically significant association between rs1056836 and breast cancer risk among the three populations combined. The subgroup analysis, however, revealed a statistically significant decreased risk of breast cancer in Val (G) allele carriers of Japanese descent. A prior meta-analysis from 2007 also concluded that Val (G) allele carriers have a lower risk of breast cancer than the Leu/Leu (CC) genotype, but only among women of mixed/African ethnicity, and no difference found in Asians. Previous studies have also demonstrated that the Val (G) allele contributes to an increase in 4-hydroxylation activity and a lower urinary 2-OHEs/16a-OHEs ratio, whereas the Leu (C) allele contributes to an increase in procarcinogen activation.
Some studies in Caucasian and Asian populations have however found no significant association between the CYP1B1 Val/Leu genotype and breast cancer risk. One such case-control study of 800 postmenopausal Caucasian women found no contribution of CYP1B1 rs1056836 to increased breast cancer risk when analysed individually. The study did however confirm its hypothesis that individual susceptibility to breast cancer may be increased by the combined effects of the risk genotypes in oestrogen metabolic genes: the concurrent presence of CYP1B1 (rs1056836) and COMT (rs4680) high-risk genotypes (CG/GG and GA/AA, respectively) posed a 2-fold risk of breast cancer. Women with three high-risk genotypes CYP1B1 (rs1056836), COMT (rs4680), MnSOD (rs4880 – TC/CC) were at a 12.2-fold increased breast cancer risk. Their results suggest that individual susceptibility to breast cancer incidence may be increased by combined effects of the high-risk genotypes in oestrogen metabolic genes.
It is believed that the tissue-specific hydroxylation and carcinogen activation initiated by CYP1B1 may affect prostate cancer risk. Yang et al. (2012) performed a meta-analysis, including 2788 cases and 2968 controls, to determine prostate cancer risk and aggressiveness associated with the rs1056836 SNP. No significant association was found with overall prostate cancer risk, but the subgroup analyses revealed that the G-allele was significantly associated with an increased prostate cancer risk in Asians and in sporadic cancer subjects. No association was observed in Caucasians or mixed population subjects, neither was the rs1056836 SNP associated with prostate cancer aggressiveness.
These results are supported by a subsequent meta-analysis (5999 cases and 5438 controls) from Zhang et al. (2013) in which, although there was no evidence that the Val432Leu polymorphism held significant association with prostate cancer in the overall population, the subgroup analysis by ethnicity revealed that the Val432Leu polymorphism was significantly associated with prostate cancer risk in Asians.
The G-allele was found significantly associated with prostate cancer risk in a population-based case-control study involving Caucasian men (1304 cases and 1266 controls). This association remained significant even after adjustment for multiple comparisons. A meta-analysis investigating the association between the Val432Leu polymorphism and urinary cancers (prostate, bladder and kidney cancer) from 17 studies (7944 cases and 7389 controls) revealed a significant association with overall urinary cancer risk. Furthermore, the authors identified reduced risk for CC genotypes in both prostate and overall urinary cancers, when studies were limited to Caucasian or Asian patients.
A case-control study of the Han Chinese population demonstrated that rs1056836 is closely related to the risk of laryngeal cancer. After analysing 300 cases and 300 controls, the authors found that the variant G-allele had a seemingly protective effect by reducing the risk of laryngeal cancer. The significantly lower risk of laryngeal cancer in G-allele carriers remained statistically significant even after adjusting for factors such as age, sex, smoking, and drinking.
Several studies have shown that CYP1B1 could also be associated with drug response. Laroche-Clary et al. (2010) have investigated the Val432Leu polymorphism in the human tumour cell lines panels of the National Cancer Institute (NCI) and the Japanese Foundation for Cancer Research (JFCR) to detect any association between this SNP and drug sensitivity. The Val432Leu allelic variants were significantly associated with reduced sensitivity to DNA-interacting anticancer agents, alkylators, camptothecins, topoisomerase II inhibitors, and some antimetabolites. In the NCI panel, cell lines homozygous for the Val-allele were globally 2-fold resistant to alkylating agents and 4.5-fold to camptothecins than cell lines homozygous for the Leu-allele. The authors stated that the association found between CYP1B1 genotypes and chemosensitivity seemed very strong as it is reinforced by the fact that two independent cell line collections behave quite similarly despite differences in genetic background.
Stop Smoking and/or Minimise Contact With Cigarette Smoke
The CYP1B1 metabolic enzymes can metabolize the pro-carcinogens in tobacco, such as aromatic ring hydrocarbons, HAAs and PAHs, into carcinogens, which in turn may activate oncogenes. A case-control study in a Chinese Han population from Yu et al. (2015) indicated that subjects who smoke or consume alcohol, especially heavy smokers and heavy drinkers, had a significantly increased risk of laryngeal cancer.
One study has found a significant increase in the breast cancer risk for smoking women in a Finnish population, especially for smokers homozygous for the Leu (C) allele.
Avoid Red Meat And High Cooking Temperatures
High temperature cooking methods such as pan-frying, grilling, oven-broiling or barbequing have been demonstrated to generate mutagens such as HCAs and PAHs. PAHs are deposited on the surface of smoked or grilled meat due to the pyrolysis of fat droppings, whereas HCAs are formed from the interaction of creatine/creatinine, amino acids and reducing sugars, which occurs at high temperatures. HCA formation increases as cooking temperature and duration increases.
These compounds are speculated to contribute to cancer risk; it may increase mutagenic frequency and has specifically been reported to induce damage to prostatic epithelium cells while PAH-DNA adducts have also been detected in human prostate cells. Native HCAs, PAHs and metabolites can reach the prostate gland via blood circulation where PAHs are activated by CYP1B1 to form PAH epoxides, which can be further activated to carcinogens or it can be detoxified by Phase II enzymes.
A multi-ethnic population-based case-study (1433 cases and 760 controls) by Catsburg et al. (2012) found a significant association between the Val-allele and an increased risk of localised prostate cancer. PAHs that enter the prostate induce CYP1B1 expression; CYP1B1 has been found to be over-expressed in prostate carcinomas and can activate PAHs into mutagenic metabolites that form DNA adducts. The CYP1B1 protein coded by the Val-allele is reportedly more active than the one coded by the Leu-allele; therefore, supporting these authors’ finding of an association between the Valine (Val) allele and higher PCA risk.
Within the same multi-ethnic population, the authors observed that a high intake of red meat as such was not associated with prostate cancer risk. Only when cooking practices were considered, did they find an association between a high intake of red meats cooked at high temperatures, especially pan-fried red meats, and advanced prostate cancer. The intake of well-done red meat was also associated with advanced prostate cancer risk. Interestingly, the intake of baked poultry showed an unpredicted inverse association with advanced prostate cancer risk, whereas the intake of pan-fried poultry showed a positive association.
The oil used in pan-frying acts as an efficient heat transfer medium between the pan and the surface of the meat, and therefore high surface temperatures are reached. Pan-frying does not expose meats to open flames and fats from the meats do not have an opportunity to drip on the flames undergoing incomplete combustion. Thus, pan-frying is typically not associated with accumulation of PAHs and the most relevant carcinogens generated by pan-frying seem to be HCAs.
Take Careful Consideration With HRT
The association between long-term hormone replacement therapy (HRT) and increased risk of breast cancer is still under debate.
As part of a case-control study of 800 postmenopausal Caucasian women, Cerne et al. (2011) found that HRT use (1 to <5 years as well as ≥5 years) was associated with a decrease in breast cancer risk. This is reportedly inconsistent with previous studies suggesting that HRT use for >5 years is associated with a small but significant increase in the risk of breast cancer.
Previously, Rebbeck et al. (2007) observed no statistically significant modification of the effect of HRT use on breast cancer risk by the rs1056836 SNP whereas, on the other hand, Diergaarde et al. (2008) found statistically significant interactions between the rs1056836 SNP and HRT use. Diergaarde et al. found that the risk increased slightly with increasing duration of HRT use among women with at least one G-allele, whereas a large increase in risk was noted among women homozygous for the C-allele (CC). Cerne et al. (2011) included both combined oestrogen-plus-progestin, and oestrogen-only HRT. In contrast, Rebbeck et al. and Diergaarde et al. included only users of combined HRT in their analyses.
Association between polymorphisms in estrogen metabolism genes and breast cancer development in Chinese women
Qiu et al, 2018.